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The Urucum iron- and manganese formation (IF-MnF) in the Santa Cruz Formation, Brazil, was deposited in a glacially influenced, late Neoproterozoic depositional environment. It has proven to be a reliable and robust archive for the late Neoproterozoic, allowing unique insights into the composition of seawater during an interval that is characterized by dramatic changes in the Earth's litho-, hydros-, atmo-, and biosphere, including several episodes of low-latitude glaciations. Here we present highly siderophile element data of the Urucum IF-MnF to evaluate elemental sources that affected the Neoproterozoic seawater from which the Urucum IF-MnF precipitated. High uncertainties associated with current dating attempts of this formation overlapped with the Marinoan glaciation (∼650–635 Ma) as well as the Gaskiers glaciation event (∼580 Ma). A Re-Os regression line defined by iron-, manganese-, and chert-rich samples of the Urucum formation is interpreted as an isochron, yielding a refined age estimate of 577 ± 38 Ma and an initial 187Os/188Os ratio of 0.122 ± 0.003. The proposed depositional age overlaps with previous published age data of the Urucum IF-MnF, but relates the Urucum IF-MnF to the Ediacaran Gaskiers glaciation and overlaps with the age of the Shuram CIE (∼567 Ma). An initial 187Os/188Os ratio of 0.122 ± 0.003 is interpreted to represent pristine Neoproterozoic seawater and supports strong hydrothermal input, probably related to rifting, that promoted ferruginous conditions in the Urucum seawater. In an alternative scenario, Re-Os isotope data for the Urucum IF-MnF could also be interpreted in a multi-component mixing scenario, defined by hydrothermally influenced water masses that sourced most metals within the Urucum basin and a crustal component that entered the Urucum seawater either as detrital admixture or via riverine loads. Although none of the scenarios can be ruled out, we consider the isochron scenario as more plausible. [ABSTRACT FROM AUTHOR]

To gain insight into the source of ore material, this study presents new Re–Os and Pt–Os isotopic and highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re) abundance data for chromitite and native osmium from the Guli massif of ultramafic alkaline rocks and carbonatites located in the Maimecha–Kotui province, Polar Siberia. The study utilized a number of analytical techniques, including electron probe microanalysis (EPMA), negative thermal ionization mass spectrometry (N-TIMS), and high pressure asher digestion and isotope dilution inductively-coupled plasma mass-spectrometry (ID ICP-MS). The HSE concentrations in chromitite samples range from 191 to 866 ppb with a predominance of Ir-group platinum-group elements (PGE) (Os, Ir, and Ru) over Pt-group PGE (Rh, Pt, and Pd) and Re, which is consistent with the platinum-group mineral control (i.e., Os–Ir alloys and laurite, RuS2) within the chromitite. The Re–Os and Pt–Os isotope data indicate that the HSE budget of the chromitite and native osmium from the Guli massif was largely controlled by that of their mantle source, which evolved with long-term near-chondritic Re/Os and Pt/Os ratios; this source had time-integrated Re/Os and Pt/Os within the range of those for the majority of komatiites and abyssal peridotites worldwide. [ABSTRACT FROM AUTHOR]

Base metal sulfides (Fe–Ni–Cu–S) are ubiquitous phases in mantle and subduction-related lithologies. Sulfides in the mantle often melt incongruently, which leads to the production of a Cu–Ni-rich sulfide melt and a solid residue called monosulfide solid solution (mss). Even though peridotite-hosted sulfides, which tend to be more Ni-rich, are likely completely molten at mantle potential temperatures, the same is not true for eclogite-hosted Ni-poor, Fe-rich sulfides. Because of this, solid crystalline mss may persist at higher pressures and equilibrate with co-existing sulfide melt along colder geotherms, like those associated with subduction zones. Because highly siderophile elements (HSE—Pt, Pd, Rh, Ru, Os, Ir, and Re) are known to fractionate as a result of mss/sulfide-melt equilibrium, the persistence of an mss/sulfide-melt assemblage to higher pressures may lead to the fractionation of these elements during the subduction process. In this contribution, we carried out an experimental investigation of the partitioning behavior of the HSE, as well as Cu and Ni, between mss and sulfide melt over a pressure and temperature range relevant to equilibration between Earth's surface and transition zone depths (0.1 MPa to 14 GPa; 930–1530 ∘ C), and variable Ni contents in sulfide. Results show that at higher pressures, the HSE are considerably less fractionated as a result of mss and sulfide melt equilibrium compared to lower pressure conditions. This is exemplified by a lowering of the D i mss / melt for the more compatible HSE (Ru, Os, Ir, Rh and Re) from around 10 at 0.1 MPa to just above or below unity at 14 GPa. Moreover, the higher the Ni content of the bulk sulfide assemblage, the larger the degree of change in the magnitude of HSE fractionation seen over the pressure range studied. The exchange coefficient ( K D Ru - Pt ) between highly compatible HSE (Ru) and less compatible Pt illustrates a notable contrast. In the Ni-poor composition (E1), K D Ru - Pt changes from 27 at 0.1 MPa to 6 at 14 GPa. In contrast, the Ni-rich composition exhibits a broader range, with K D Ru - Pt ranging from 150 to 17 across the same pressure interval. Our results highlight key differences between experimental data obtained at lower and higher pressure, and how composition, namely the Ni content of sulfide, affects HSE partitioning behavior. [ABSTRACT FROM AUTHOR]

Major and trace element abundances, including highly siderophile elements, and 187Os and 182W isotopic compositions were determined for ca. 89 Ma mafic and ultramafic rocks from the islands of Gorgona (Colombia) and Curaçao (Dutch Caribbean). The volcanic systems of both islands were likely associated with a mantle plume that generated the Caribbean Large Igneous Provence. The major and lithophile trace element characteristics of the rocks examined are consistent with the results of prior studies, and indicate derivation from both a chemically highly-depleted mantle component, and an enriched, or less highly-depleted mantle component. Highly siderophile element abundances for these rocks are generally similar to rocks with comparable MgO globally, indicating that the major source components were not substantially enriched or depleted in these elements. Rhenium-Os isotopic systematics of most rocks of both islands indicate derivation from a mantle source with an initial 187Os/188Os ratio between that of the contemporaneous average depleted mid-ocean ridge mantle and bulk silicate Earth. The composition may reflect either an average lower mantle signature, or global-scale Os isotopic heterogeneity in the upper mantle. Some of the basalts, as well as two of the komatiites, are characterized by calculated initial 187Os/188Os ratios 10–15 % higher than the chondritic reference. These more radiogenic Os isotopic compositions do not correlate with major or trace element systematics, and indicate a mantle source component that was most likely produced by either sulfide metasomatism or ancient Re/Os fractionation. Tungsten-182 isotopic compositions measured for rocks from both islands are characterized by variable μ182W values ranging from modern bulk silicate Earth-like to strongly negative values. The μ182W values do not correlate with major/trace element abundances or initial 187Os/188Os compositions. As with some modern ocean island basalt systems, however, the lowest μ182W value (−53) measured, for a Gorgona olivine gabbro, corresponds with the highest 3He/4He previously measured from the suite (15.8 R/R A). Given the lack of correlation with other chemical/isotopic compositions, the mantle component characterized by negative μ182W and possibly high 3He/4He is most parsimoniously explained to have formed as a result of isotopic equilibration between the mantle and core at the core-mantle boundary. [ABSTRACT FROM AUTHOR]

The ca. 1.95-Ga Jormua Ophiolite Complex (JOC), Finland, is a rare Paleoproterozoic ophiolite that preserves a record of diverse upper mantle materials and melting processes. Meter-scale grid sampling of four JOC outcrops, as well as non-grid samples, permits evaluation of meter- to kilometer-scale mantle heterogeneity within the JOC. Significant heterogeneity is observed between the four grids, and also among a number of the non-grid samples examined. Variations in the concentrations of fluid-mobile elements are particularly large among different samples and locations. New whole-rock major, lithophile trace, and highly siderophile element data (HSE: Os, Ir, Ru, Pt, Pd, Re), including 187Re–187Os isotopic data, for serpentinized harzburgites indicate the presence of two distinct compositional types and probable modes of origin within the JOC. This is consistent with prior findings. Type 1 is similar to modern refractory abyssal-type mantle. Type 2 is more highly refractory than Type 1, and most likely represents samples from sub-continental lithospheric mantle (SCLM). Type 1 mantle is moderately heterogeneous with respect to major and trace element and Os isotopic compositions at both the meter and kilometer scales. By contrast, Type 2 mantle is considerably more homogeneous than Type 1 grids at the meter scale, but is more heterogeneous at the kilometer scale. The median initial γOs value for Type 1 mantle, calculated for 1.95 Ga, is ~ −2.0 (where γOs is the % deviation in 187Os/188Os relative to a chondritic reference calculated for a specified time). This isotopic composition is consistent with a moderate, long-term decrease in Re/Os relative to the estimate for primitive mantle, prior to JOC formation. The similarity in this γOs value to the value for the modern abyssal mantle, as well as the initial values for several Phanerozoic ophiolites, suggests that the upper mantle achieved a Re/Os ratio similar to the chondritic reference by ~2 Ga, then evolved along a subparallel trajectory to the chondritic reference since then. For this to occur, only limited Re could have been permanently removed from the upper mantle since at least the time the JOC formed. A localized secondary metasomatic event at ~2 Ga, concurrent with the estimated obduction age for the JOC and subsequent Svecofennian Orogeny, affected the HSE systematics of some Type 1 samples. By contrast, late Archean Os TRD model ages for Type 2 rocks indicate a depletion event superimposed upon the long-term Re depletion of the abyssal mantle. This event was established no later than ~2.6 Ga and may have occurred during a period of significant, well-documented crustal production in the Karelia craton at ~2.7 Ga. [ABSTRACT FROM AUTHOR]

Chemical and isotopic data were obtained for ten iron meteorites classified as members of the IIIE group. Nine of the IIIE irons exhibit broadly similar bulk siderophile element characteristics. Modeling of highly siderophile element abundances suggests that they can be related to one another through simple crystal-liquid fractionation of a parent melt. Our preferred model suggests initial S, P, and C concentrations of approximately 12 wt%, 0.8 wt%, and 0.08 wt%, respectively. The modeled IIIE parent melt composition is ∼4 times more enriched in highly siderophile elements than a non-carbonaceous (NC) chondrite-like parent body, suggesting a core comprising ∼22% of the mass of the parent body. Although chemically distinct from the other IIIE irons, formation of the anomalous IIIE iron Aletai can potentially be accounted for under the conditions of this model through the non-equilibrium mixing of an evolved liquid and early formed solid. Cosmic ray exposure-corrected nucleosynthetic Mo, Ru, and W isotopic compositions of four of the bona fide IIIE irons and Aletai indicate that they originated from the non-carbonaceous (NC) isotopic domain. Tungsten-182 isotopic data for the IIIE irons and Aletai yield similar model metal-silicate segregation ages of 1.6 ± 0.8 Myr and 1.2 ± 0.8 Myr, respectively, after calcium aluminum-rich inclusion (CAI) formation. These ages are consistent with those reported for other NC-type iron meteorite parent bodies. The IIIE irons are chemically and isotopically similar to the much larger IIIAB group. Despite some textural, mineralogical, and chemical differences, such as higher C content, the new results suggest they may have originated from a different crystallization sequence on the same or closely-related parent body. [ABSTRACT FROM AUTHOR]

Metal-silicate segregation is one of the most fundamental mechanisms in planetary differentiation, with primitive achondrites offering important constraints on this process. Brachinites and brachinite-like achondrites (BLA) are olivine-dominated primitive achondrites that experienced up to ∼20% partial melt removal under relatively oxidized (ΔIW ∼ −1) conditions within an initially chondritic parent body and represent residues after inefficient metal-loss. We present bulk rock and in situ lithophile and highly siderophile element (HSE) abundance systematics as well as 187Re-187Os data for five olivine-rich primitive achondrites. These new data confirm classification of Reid 013 as a brachinite, three of the samples as BLA (Northwest Africa [NWA] 6874, NWA 7499, and Miller Range 090805), and the final sample as an ungrouped primitive olivine-rich achondrite (NWA 7680). An aliquot of MIL 090805 shows amongst the highest total HSE contents (>35 ppm) and the highest Pt content (∼23 ppm) of any primitive achondrite. Compiled HSE data for brachinites and BLA show correlations between total HSE abundance, Pt enrichment, and average olivine Fo. This correlation can be explained by variable melting (∼10–20%) of an H ordinary chondrite-like protolith, with retention of both Fe-metal and a Pt-rich alloy phase distinct from the observed Fe-metal phases in more depleted residues. Such a Pt-alloy phase is likely stabilized by elevated abundances of the HSE during chondrite melting and the low solubility of HSE in melts. Rhenium-Os isotope data in the studied samples has been modified by recent mobilization of Re during terrestrial weathering, with the limited range of measured 187Os/188Os in brachinites and BLA supporting minor fractionation of Re/Os during melting and an ancient (∼4.5 Ga) partial melting event to explain their compositions. These results indicate that models of planetary differentiation should consider the low solubility of Pt in chondrite melts and the potential for alloy formation to modify HSE abundances of silicate mantles. [ABSTRACT FROM AUTHOR]

The abundances of highly siderophile elements (HSE: Re, Os, Ir, Ru, Pt, Pd), as well as 187Re-187Os and 182Hf-182W isotopic systematics were determined for separated metal, slightly magnetic, and nonmagnetic fractions from seven H4 to H6 ordinary chondrites. The HSE are too abundant in nonmagnetic fractions to reflect metal-silicate equilibration. The disequilibrium was likely a primary feature, as 187Re-187Os data indicate only minor open-system behavior of the HSE in the slightly and non-magnetic fractions. 182Hf-182W data for slightly magnetic and nonmagnetic fractions define precise isochrons for most meteorites that range from 5.2 ± 1.6 Ma to 15.2 ± 1.0 Ma after calcium aluminum inclusion (CAI) formation. By contrast, 182W model ages for the metal fractions are typically 2-5 Ma older than the slope-derived isochron ages for their respective, slightly magnetic and nonmagnetic fractions, with model ages ranging from 1.4 ± 0.8 Ma to 12.6 ± 0.9 Ma after CAI formation. This indicates that the W present in the silicates and oxides was not fully equilibrated with the metal when diffusive transport among components ceased, consistent with the HSE data. Further, the W isotopic compositions of size-sorted metal fractions from some of the H chondrites also differ, indicating disequilibrium among some metal grains. The chemical/isotopic disequilibrium of siderophile elements among H chondrite components is likely the result of inefficient diffusion of siderophile elements from silicates and oxides to some metal and/or localized equilibration as H chondrites cooled towards their respective Hf-W closure temperatures. The tendency of 182Hf-182W isochron ages to young from H5 to H6 chondrites may indicate derivation of these meteorites from a slowly cooled, undisturbed, concentrically-zoned parent body, consistent with models that have been commonly invoked for H chondrites. Overlap of isochron ages for H4 and H5 chondrites, by contrast, appear to be more consistent with shallow impact disruption models. The W isotopic composition of metal from one CR chondrite was examined to compare with H chondrite metals. In contrast to the H chondrites, the CR chondrite metal is characterized by an enrichment in 183W that is consistent with nucleosynthetic s-process depletion. Once corrected for the correlative nucleosynthetic effect on 182W, the 182W model age for this meteorite of 7.0 ± 3.6 Ma is within the range of model ages of most metal fractions from H chondrites. The metal is therefore too young to be a direct nebular condensate, as proposed by some prior studies.

Northwest Africa (NWA) 6704 is a unique achondrite characterized by a near-chondritic major element composition with a remarkably intact igneous texture. To investigate the origin of this unique achondrite, we have conducted a combined petrologic, chemical, and 187Re-187Os, O, and Ti isotopic study. The meteorite consists of orthopyroxene megacrysts (En55-57Wo3-4Fs40-42; Fe/Mn = 1.4) up to 1.7 cm in length with finer interstices of olivine (Fa50-53; Fe/Mn = 1.1-2.1), chromite (Cr# ~ 0.94), awaruite, sulfides, plagioclase (Ab92An5Or3) and merrillite. The results of morphology, lattice orientation analysis, and mineral chemistry indicate that orthopyroxene megacrysts were originally hollow dendrites that most likely crystallized under high super-saturation and super-cooling conditions (1-102 °C/h), whereas the other phases crystallized between branches of the dendrites in the order of awaruite, chromite → olivine → merrillite → plagioclase. In spite of the inferred high supersaturation, the remarkably large size of orthopyroxene can be explained as a result of crystallization from a melt containing a limited number of nuclei that are preserved as orthopyroxene megacryst cores having high Mg# or including vermicular olivine. The Re-Os isotope data for bulk and metal fractions yield an isochron age of 4576 ± 250 Ma, consistent with only limited open system behavior of highly siderophile elements (HSE) since formation. The bulk chemical composition is characterized by broadly chondritic absolute abundances and only weakly fractionated chondrite-normalized patterns for HSE and rare earth elements (REE), together with substantial depletion of highly volatile elements relative to chondrites. The HSE and REE characteristics indicate that the parental melt and its protolith had not undergone significant segregation of metals, sulfides, or silicate minerals. These combined results suggest that a chondritic precursor to NWA 6704 was heated well above its liquidus temperature so that highly volatile elements were lost and the generated melt initially contained few nuclei of relict orthopyroxene, but the melting and subsequent crystallization took place on a timescale too short to allow magmatic differentiation. Such rapid melting and crystallization might occur as a result of impact on an undifferentiated asteroid. The O-Ti isotope systematics (Δ17O = -1.052 ± 0.004, 2 SD; ε50Ti = 2.28 ± 0.23, 2 SD) indicate that the NWA 6704 parent body sampled the same isotopic reservoirs in the solar nebula as the carbonaceous chondrite parent bodies. This is consistent with carbonaceous chondrite-like refractory element abundances and oxygen fugacity (FMQ = -2.6) in NWA 6704. Yet, the Si/Mg ratio of NWA 6704 is remarkably higher than those of carbonaceous chondrites, suggesting significant nebular fractionation of forsterite in its provenance.

The IC iron meteorite group is characterized utilizing nucleosynthetic mass-independent isotopic compositions and 182W age constraints, coupled with siderophile element concentration measurements and modeling of crystal-liquid fractionation processes. The six IC irons analyzed, Arispe, Bendego, Chihuahua City, Nocoleche, NWA 2743, and Winburg have indistinguishable Mo and W genetic isotopic compositions and are consistent with derivation from the same parent body, which formed in the non-carbonaceous (NC) nebular reservoir. A pre-exposure µ182W value (parts-per-million deviations in isotopic ratios from terrestrial standards) for the six IC irons of −337 ± 5 corresponds to a metal-silicate segregation age of 1.0 ± 0.4 Myr after calcium-aluminum-rich inclusion (CAI) formation. This age is similar to those determined for other NC iron groups. Siderophile element abundances of the IC irons are generally similar and characterized by minor depletions in the more volatile siderophile elements. Highly siderophile element (HSE) distributions among the IC group suggest that the initial parent body core was S-rich, with preferred model results indicating an initial melt composition with ∼ 18 wt% S, 2 wt% P and 0.03 wt% C. Processes in addition to fractional crystallization, such as late-stage parent body modification, possibly as a result of impacts, and subsequent metal-melt mixing, are required within the first 100 Myr of Solar System history to explain the range of HSE abundances. [ABSTRACT FROM AUTHOR]

Rhenium-Os isotope and highly siderophile element (HSE, including Re, Os, Ir, Ru, Pt, and Pd) abundance systematics of Archean komatiites can be used to estimate the stirring rates of the mantle for the HSE and the timing of homogenization of late accreted materials within the mantle. In this study, we report Re-Os isotope and HSE abundance data for ~2.9 Ga komatiites and basalts from the Sumozero-Kenozero greenstone belt in the SE Fennoscandian Shield. The lavas are characterized by excellent preservation of the primary textural, chemical, and Re-Os isotope characteristics. The Re-Os isotopic data for spinifex-textured and cumulate komatiite and massive basalt samples from the lowermost sequences define a precise 10-point isochron (MSWD = 2.6) with an age of 2904 ± 18 Ma and an initial 187Os/188Os = 0.10758 ± 18 (γ187Os(2904) = +0.45 ± 0.17). This is the first direct age determination for the Sumozero-Kenozero lower komatiite-basalt sequences. Our modeling indicates that the mantle source of the komatiites and basalts evolved with a time-integrated 187Re/188Os = 0.418 ± 6. This ratio is well within the uncertainty of the bulk chondritic average 187Re/188Os = 0.410 ± 51 (2SD), also consistent with the chondritic evolution of the majority of komatiite mantle sources observed globally. The mantle source of the Sumozero-Kenozero komatiites has been calculated to contain the total HSE abundances of 58 ± 7% of those in the estimates for modern Bulk Silicate Earth (BSE). This estimate is in the middle of the range for other late Archean and Proterozoic komatiite systems. Using the estimated HSE abundances in the sources of komatiite systems as a function of their ages and ISOPLOT regression analysis, we calculated the average time in the past by which late accreted materials have been completely homogenized within the mantle to be 2.48 ± 0.23 Ga. These data require that the residence times of the late accreted planetesimals within the mantle, before complete homogenization, were on average 1.92 ± 0.23 Ga. This estimate represents a constraint on the average mixing rates of the mantle in terms of the HSE abundances in the Hadean and the Archean. [ABSTRACT FROM AUTHOR]

The abundances of highly siderophile elements (HSE) in planetary mantles and achondrites potentially provide important constraints on several aspects of planet formation, including the nature and composition of late accreted materials. Here, we experimentally and systematically assess the distribution of the HSE between silicate melts, sulfide and/or metal liquids at the highly to moderately reduced conditions thought to have characterized Earth accretion. The results show that the chalcophile behavior of all elements, except for Re, is strongly decreased at low FeO and/or high S concentrations in the silicate melt. There are considerable differences between how FeO and/or S contents of the silicate melt affect the D values of the various HSE, with the largest effects observed for Pd, Pt, Ir and Au. If liquid metal is Si-rich and S-poor, the siderophile behavior of the HSE mimics that in the presence of sulfide liquids, but with an offset due to differences in HSE activities in metal and sulfide liquids. Using our new experimental data, we quantify the relative effects of O in sulfide and S in silicate melt on the sulfide liquid-silicate melt partitioning behavior of the HSE using a thermodynamic approach. The resulting expressions were used to model the distribution of the HSE in highly reduced and differentiated EH- and EL chondritic parent bodies and during differentiation of the aubrite parent body. Our results show that even with their strongly decreased chalcophile and siderophile behavior at highly reduced conditions, HSE abundances in the mantles of these parent bodies remain extremely low. However, if such bodies accreted to Earth, any residual metal present in the parent body mantle and subsequently retained in Earth's mantle would dramatically affect HSE abundances and produce chondritic ratios, making it impossible to track the potential accretion of a large reduced impactor to the BSE using HSE abundance systematics. In terms of the aubrite parent body, our results confirm previous hypotheses related to the importance of (un)differentiated core forming metals in establishing the HSE contents of unbrecciated aubrites. Finally, our results confirm that sulfides are likely a minor source of HSE abundances in aubrites, particularly for Re, consistent with sample observations. [ABSTRACT FROM AUTHOR]

Continental formation models invoke subduction or plume‐related processes to create the buoyant, refractory character of continental lithospheric mantle (CLM). From similarities in melt depletion, major element composition, modal clinopyroxene, and Os isotope systematics it has been proposed that oceanic mantle lithosphere is the likely protolith to non‐cratonic CLM, however, a direct link between the two has been difficult to ascertain. Using dredged mantle peridotite xenoliths from the Ferrel Seamount, off the west coast of Baja California, Mexico, we show that tectonic isolation of an oceanic plate may lead to formation of non‐cratonic CLM. Ferrel xenoliths are coarse‐grained spinel lherzolite, or rare harzburgite. Bulk‐rock and clinopyroxene trace element compositions reveal two‐stages of melt refertilization following melt depletion, with infiltration by mid‐ocean ridge basalt‐type melts, followed by melt addition from host alkali basalt. Melt depletion correlations with 187Os/188Os and highly siderophile element abundances indicate preserved melt depletion and refertilization processes are ancient. From these observations, the Ferrel xenoliths represent lithosphere from the abandoned Pacific‐Farallon ridge. The history of melt depletion, followed by MORB‐melt refertilization is consistent with the peridotites representing oceanic mantle lithosphere that was subsequently incorporated into the Baja‐Guadalupe microplate during "ridge jump." These peridotites demonstrate that isolation of oceanic lithosphere that is rafted onto a continental margin provides a viable means for producing non‐cratonic CLM. We suggest that continuation of late‐stage, low degree melt refertilization may provide a link between oceanic lithosphere and non‐cratonic CLM and propose a tectonic model to preserve and facilitate this continued evolution. Key Points: Ancient melt depletion can be preserved despite metasomatism and suggest Ferrel Seamount peridotite was formed by mid‐ocean ridge processesFerrel Seamount peridotites likely represent preserved Pacific‐Farralon lithosphereA tectonic process, ridge jump, can isolate oceanic lithosphere and provides a starting point for the formation of continental lithosphere [ABSTRACT FROM AUTHOR]

Abstract Continental formation models invoke subduction or plume‐related processes to create the buoyant, refractory character of continental lithospheric mantle (CLM). From similarities in melt depletion, major element composition, modal clinopyroxene, and Os isotope systematics it has been proposed that oceanic mantle lithosphere is the likely protolith to non‐cratonic CLM, however, a direct link between the two has been difficult to ascertain. Using dredged mantle peridotite xenoliths from the Ferrel Seamount, off the west coast of Baja California, Mexico, we show that tectonic isolation of an oceanic plate may lead to formation of non‐cratonic CLM. Ferrel xenoliths are coarse‐grained spinel lherzolite, or rare harzburgite. Bulk‐rock and clinopyroxene trace element compositions reveal two‐stages of melt refertilization following melt depletion, with infiltration by mid‐ocean ridge basalt‐type melts, followed by melt addition from host alkali basalt. Melt depletion correlations with 187Os/188Os and highly siderophile element abundances indicate preserved melt depletion and refertilization processes are ancient. From these observations, the Ferrel xenoliths represent lithosphere from the abandoned Pacific‐Farallon ridge. The history of melt depletion, followed by MORB‐melt refertilization is consistent with the peridotites representing oceanic mantle lithosphere that was subsequently incorporated into the Baja‐Guadalupe microplate during “ridge jump.” These peridotites demonstrate that isolation of oceanic lithosphere that is rafted onto a continental margin provides a viable means for producing non‐cratonic CLM. We suggest that continuation of late‐stage, low degree melt refertilization may provide a link between oceanic lithosphere and non‐cratonic CLM and propose a tectonic model to preserve and facilitate this continued evolution.

Ungrouped iron meteorites Tishomingo, Willow Grove, and Chinga, and group IVB iron meteorites, are Ni-rich. Similarities include enrichments of 10–100 × CI for some refractory siderophile elements, and equivalent depletions in more volatile siderophile elements. Superimposed on the overall enrichment/depletion trend, certain siderophile elements (P, W, Fe, Mo) are depleted relative to elements of similar volatility. All three ungrouped irons derive from parent bodies formed in the early Solar System. Willow Grove and Chinga are characterized by cosmic ray exposure corrected 182W/184W consistent with metal-silicate segregation on their parent bodies within 1–3 Myr of Solar System formation, within the age range determined for segregation of magmatic iron meteorite parent bodies, including group IVB irons. Tishomingo is characterized by a younger model age 4–5 Myr subsequent to Solar System formation, reflecting either late stage melting resulting from 26Al decay, or an impact resetting. The discovery of stishovite in Tishomingo, indicating exposure to a minimum shock pressure of 8–9 GPa, is consistent with the latter. Stishovite in Tishomingo and chromite included in troilite-daubréelite in IVB irons allows oxygen isotopic composition comparison between these meteorites. Different mass independent oxygen isotopic compositions of IVB irons and Tishomingo indicate genetically distinct parent bodies. By contrast, mass independent Mo isotopic compositions overlap within analytical uncertainties, indicating a similar, carbonaceous chondrite (CC) type genetic heritage. Molybdenum and 183W isotopic data for Chinga and Willow Grove indicate derivation from CC type parent bodies. Willow Grove shares Mo and 183W isotopic compositions with the Ni-rich South Byron Trio (SBT) grouplet and the Milton pallasite. These Ni-rich meteorites likely formed in the same general nebular environment as other CC planetesimals, likely the outer Solar System. Highly siderophile element (HSE) abundances of Willow Grove and Tishomingo are similar to some IVB meteorites, consistent with formation by moderate degrees of fractional crystallization from initial metallic melts with low S and P, and modestly fractionated HSE. The comparable HSE abundances of Tishomingo and Willow Grove to some IVB irons, yet substantially higher Ni concentrations, indicate formation on parent bodies with lower bulk HSE abundances or HSE concentration in proportionally smaller volumes of metal. HSE abundances in Chinga are considerably lower than in IVB irons, highly fractionated, and processes responsible for these remain elusive. For IVB irons and these ungrouped irons, high temperature condensation likely dominated the enrichment and depletion of the refractory and volatile siderophile elements, respectively. Parent body degassing may have also played a role. Relative depletion of volatile siderophile elements is not, however, a universal feature of high-Ni meteorites. The SBT and Milton pallasite are Ni-rich, but less depleted in the more volatile siderophile elements. Nickel enrichment was likely driven by oxidation of Fe metal during parent body accretion or core segregation. Oxidation of the Tishomingo and Willow Grove parent bodies may have occurred at ∼ IW + 1, indicated by relative Mo and W depletions due to metal/water reaction during differentiation. Late-stage reduction, indicated by the presence of Cr-bearing sulfides in Tishomingo and IVB irons, may have resulted from exhaustion of the oxidant. [ABSTRACT FROM AUTHOR]

Late veneer is an important paradigm in early Earth and planetary studies. It refers to the late addition of extraterrestrial materials to the Earth's mantle after the core formation, which leads to the overabundances of highly siderophile elements in the primitive upper mantle. In this review, the origin, evolution, and expansion of the late veneer hypothesis are summarized, including some unresolved problems. I hope this review would be helpful for the new entrants to this field. [ABSTRACT FROM AUTHOR]

• HSE data for high-3He/4He picrites formed during opening of north and south Atlantic. • Large degree partial melts from peridotite mantle with chondritic Os isotope character. • Atlantic LIP show evidence for deep mantle contributions during ocean basin formation. • Recycled components in high-3He/4He OIB consistent with lower degrees of partial melting. Osmium isotope and highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re) abundance data are reported for picrites and basalts from the ∼132 Ma Etendeka large igneous province (LIP) and the ∼60 Ma North Atlantic Igneous Province (NAIP). Picrite dykes of the Etendeka LIP have HSE abundances and 187Os/188Os (0.1276 to 0.1323; γOs i = -0.5 to +3.1) consistent with derivation from high-degree partial melting (>20 %) of a peridotite source with chondritic to modestly supra-chondritic long-term Re/Os. High-3He/4He NAIP picrites from West Greenland represent large-degree partial melts with similarly elevated HSE abundances and 187Os/188Os (0.1273 to 0.1332; γOs i = -0.2 to +3.9). Broadly chondritic Os isotope ratios have also been reported for the ∼132 Ma Paraná LIP and the ∼201 Ma Central Atlantic Magmatic Province (CAMP). Consequently, LIP associated with Atlantic Ocean opening derive, at least in part, from partial melting of peridotite mantle distinct from the depleted mantle associated with mid-ocean ridge basalt volcanism. Modern locations with high-3He/4He (>25R A) include ocean island basalts (OIB) from Ofu (Samoa), Loihi (Hawaii) and Fernandina (Galapagos) in the Pacific Ocean, and from Iceland, which is considered the modern manifestation of NAIP magmatism. Unlike Etendeka and NAIP picrites, these modern OIB have Sr-Nd-Pb-Os isotopes consistent with contributions of recycled oceanic or continental crust. The lower degree of partial melting responsible for modern high-3He/4He OIB gives higher proportions of fusible recycled crustal components to the magmas, with radiogenic 187Os/188Os and low-3He/4He. The high-3He/4He, incompatible trace element-depleted mantle component in both LIP and OIB therefore also has long-term chondritic Re/Os, which is consistent with an early-formed reservoir that experienced late accretion. Atlantic LIP (CAMP; Paraná-Etendeka; NAIP) provide geochemical evidence for a prominent role for mantle plume contributions during continental break-up and formation of the Atlantic Ocean, a feature hitherto unrecognized in other ocean basin-forming events. [ABSTRACT FROM AUTHOR]

Despite being volumetrically minor components, carbonate-rich ultramafic magmas like aillikites represent good candidates to investigate the compositional variations in plume and/or lithospheric mantle sources because they represent low-degree melts which preferentially sample highly fusible components including recycled crustal material. To gain new insights into the composition of the plume-related magmas and, more broadly, the petrogenesis of ultramafic lamprophyres, we have undertaken the first comprehensive study of bulk rock and mineral (olivine and Ti-magnetite) highly siderophile element (HSE) abundances and Re-Os isotopes combined with in situ major-, trace-element and Sr-Nd isotope analyses of apatite and perovskite from the Permian Wajilitag aillikites of the Tarim large igneous province, China. The Wajilitag aillikites have high PPGE (Pt and Pd) contents relative to IPGE (Os, Ir and Ru), which can be ascribed to low-degree partial melting and/or fractionation of olivine and laurite. Measured 187Os/188Os ratios are moderately to highly radiogenic (0.186–0.313) with age-corrected γOs values up to +113. In situ Sr and Nd isotope analyses of apatite phenocrysts (87Sr/86Sr (i) = 0.70349–0.70384; εNd (i) = +1.3 to +4.9) and fresh perovskite grains (87Sr/86Sr (i) = 0.70340–0.70390; εNd (i) = +1.3 to +3.8) exhibit limited variability both within and across samples from different aillikite dykes and the only volcanic pipe in the area. These Nd isotopic values resemble those from bulk-rock samples (εNd (i) = +1.9 to +5.2), whereas Sr in apatite and perovskite extends to marginally less radiogenic values than the bulk-rock compositions (87Sr/86Sr (i) = 0.70362–0.70432). The moderately depleted Sr-Nd isotope compositions of magmatic apatite and perovskite, and the previously reported mantle-like C isotope values of these samples suggest that the aillikites and their carbon probably derived from a sub-lithospheric (plume) source with minimal contribution of deeply subducted material. Conversely, the radiogenic Os isotope compositions of the Tarim aillikites and separated minerals require some contribution from recycled crustal material in the plume source. Mass balance calculations suggest that the radiogenic Os isotopes and moderately depleted Sr-Nd isotopes can be reproduced by less than one third of eclogite component addition to a moderately depleted mantle source. We conclude that the combination of complementary isotopic systems can enlighten contributions from different components to mantle-derived magmas and, in this case, clarifies the occurrence of carbon-free subducted oceanic crust in the Tarim plume. [ABSTRACT FROM AUTHOR]

Constraining the degree of preservation of a meteoritic signature within an impact structure provides vital insights in the complex pathways and processes that occur during and after an impact cratering event, providing information on the fate of the projectile. The IODP-ICDP Expedition 364 drilling recovered a ∼829 m continuous core (M0077A) of impactites and basement rocks within the ∼200-km diameter Chicxulub impact structure peak ring. No highly siderophile element (HSE) data have been reported for any of the impact melt rocks of this drill core to date. Previous work has shown that most Chicxulub impactites contain less than 0.1% of a chondritic component. Only few impact melt rock samples in previous drill cores recovered from the Chicxulub might contain such a signal. Therefore, we analyzed impact melt rock and suevite samples, as well as pre-impact lithologies of the Chicxulub peak ring, with a focus on the HSE concentrations and Re–Os isotopic compositions. Similar to the concentrations of the other major and trace elements, those of the moderately siderophile elements (Cr, Co, Ni) of impact melt rock samples primarily reflect mixing between a mafic (dolerite) and felsic (granite) components, with the incorporation of carbonate material in the upper impact melt rock unit (from 715.60 to 747.02 meters below seafloor). The HSE concentrations of the impact melt rocks and suevites are generally low (<39 ppt Ir, <96 ppt Os, <149 ppt Pt), comparable to the values of the average upper continental crust, yet three impact melt rock samples exhibit an enrichment in Os (125–410 ppt) and two of them also in Ir (250–324 ppt) by one order of magnitude relative to the other investigated samples. The 187Os/188Os ratios of the impact melt rocks are highly variable, ranging from 0.18 to 2.09, probably reflecting heterogenous target rock contributions to the impact melt rocks. The significant amount of mafic dolerite (mainly ∼20–60% and up to 80–90%), which is less radiogenic (187Os/188Os ratio of 0.17), within the impact melt rocks makes an unambiguous identification of an extraterrestrial admixture challenging. Granite samples have unusually low 187Os/188Os ratios (0.16 on average), while impact melt rocks and suevites broadly follow a mixing trend between upper continental crust and chondritic/mantle material. Only one of the investigated samples of the upper impact melt rock unit could also be interpreted in terms of a highly diluted (∼0.01–0.05%) meteoritic component. Importantly, the impact melt rocks and pre-impact lithologies were affected by post-impact hydrothermal alteration processes, probably remobilizing Re and Os. The mafic contribution, explaining the least radiogenic 187Os/188Os values, is rather likely. The low amount of meteoritic material preserved within impactites of the Chicxulub impact structure may result from a combination of the assumed steeply-inclined trajectory of the Chicxulub impactor (enhanced vaporization, and incorporation of projectile material within the expansion plume), the impact velocity, and the volatile-rich target lithologies. [ABSTRACT FROM AUTHOR]

The projected relative abundances of the highly siderophile elements (HSE; Re, Os, Ir, Ru, Pt, and Pd) for bulk parent bodies of 10 magmatic iron meteorite groups/grouplet (IC, IIAB, IIC, IID, IIF, IIIAB, IIIF, IVA, IVB, and South Byron Trio) are broadly similar and show no resolvable differences between noncarbonaceous (NC) and carbonaceous (CC) genetic heritage. The processes driving genetic isotopic heterogeneity in the early Solar System, therefore, evidently did not leave discernable chemical fingerprints with respect to HSE relative abundances on the bulk planetesimal scale. By contrast, the absolute abundances of HSE projected for parent body cores, which reflect core size, are more variable and, on average, higher in CC bodies compared to NC bodies. Overall, bulk core chemical compositions, as well as core size, are linked to the distribution of Fe within a parent body, which is controlled by its oxidation state. The CC parent bodies are constrained to have formed under heterogeneous oxidizing conditions which were, on average, more oxidizing than those of the NC environment. [ABSTRACT FROM AUTHOR]

New 187Re-187Os, 87Rb-87Sr, triple O-isotope isotope, bulk rock highly siderophile- (HSE: Os, Ir, Ru, Pt, Pd, Re), major- and trace-element abundance data are reported for a variety of carbonaceous, ordinary and enstatite chondrite meteorites. In addition, new mineral chemical data are reported for the Chelyabinsk LL5 ordinary chondrite fall for comparison with existing chondrite data and to investigate element sequestration into metal and mineral phases within some chondrites. The focus of the study is to link the variations observed in the HSE abundances and Re-Os isotopes with other isotopic and elemental data to explore the relative roles of sample sizes, terrestrial alteration and parent body processes more fully on chondrite meteorite compositions. Trace element variations in Chelyabinsk silicate, oxide and metal grains highlight the importance of geochemical heterogeneity imparted by mineralogical variations and mode effects, as well as sample size. Using a range of sample powder aliquot sizes, it is possible to show that this becomes significant for the HSE at <0.1 g. Variations in high field strength elements relative abundances (HFSE: Ti, Zr, Nb, Ta, Hf) are also identified within individual aliquots of carbonaceous chondrite Ivuna, emphasizing the importance of complete dissolution of refractory phases. The range of fall and find meteorites examined here demonstrates that terrestrial alteration effects revealed for trace elements (e.g., Ba, U, Sr) do not correlate particularly well with Re/Os variations. Instead, the Re/Os ratios of carbonaceous chondrites are susceptible to disturbance, more so than indicated by incompatible trace element systematics, with the Murchison CM2 carbonaceous chondrite showing significant Re/Os fractionation between sample aliquots. For sample aliquots measured that do not show significant mode or terrestrial alteration effects, parent body processes appear to be largely restricted to thermal metamorphism and dehydration. Including data for this study, the combined published dataset for Re-Os isotope and HSE abundances now extends to 33 ordinary, 39 carbonaceous, 27 enstatite and 6 Rumuruti chondrites. The range in absolute HSE abundances among these meteorite groups is ∼30%, with all chondrites having, within uncertainties, the same average Os, Ir, Ru, Pt and Pd abundances. Notably, carbonaceous chondrites have long-term Re/Os ∼ 8% lower than for the other chondrite groups. If chondrite groups are representative of early planetary feedstocks, then the measured 187Os/188Os of ordinary chondrites make them a close match to the composition of the bulk silicate Earth. Assuming ∼0.5% late accretion of ordinary chondrites to Earth, this would result in a long-term Rb/Sr ratio ∼0.6% higher than from late accretion of carbonaceous chondrites, indicating that ordinary chondrites are a potentially attractive source for moderately volatile enrichment. [ABSTRACT FROM AUTHOR]

Late accretion occurred through addition of massive impactors to Earth, leading to potential heterogeneity in the distribution of highly siderophile elements (HSE: Os, Ir, Ru, Pt, Pd, Re) within the mantle. Abyssal peridotites sample the present-day convecting mantle, which make them useful for examining the distribution of the HSE within the mantle. Here we report new HSE abundance data and 187Os/188Os ratios, in conjunction with mineral chemistry and bulk rock major- and trace-element compositions for abyssal peridotites from the fast-spreading Pacific Antarctic Ridge (PAR) and East Pacific Rise (Hess Deep), and for slow to intermediate spreading ridges from the Southwest Indian Ridge, Central Indian Ridge and Mid-Atlantic Ridge. These analyses expand the global abyssal peridotite Os isotope and HSE database, enabling evaluation of potential variations with spreading rate, from ultraslow (<20 mm/yr, full spreading rate) to fast (135–150 mm/yr). Accounting for likely effects from seawater modification and serpentinization, the Pacific data reveals heterogeneous and sometimes significant melt depletion for PAR (3–23% melt depletion; 187Os/188Os from 0.1189 to 0.1336, average = 0.1267 ± 0.0065; 2SD) and Hess Deep abyssal peridotites (15–20% melt depletion; 0.1247 ± 0.0027). Abyssal peridotites from fast to intermediate spreading ridges reveal no systematic differences in the distribution and behavior of the HSE or Os isotopes, or in degrees of melt depletion, compared with slow to ultraslow spreading ridges. These observations arise despite significant differences in melt generation processes at mid-ocean ridges, suggesting that the effects of ancient melt depletion are more profound on HSE compositions in abyssal peridotites than modern melting beneath ridges. Using global abyssal peridotites with Al 2 O 3 content > 2 wt.%, the average composition of the primitive mantle is 0.3 ppb Re, 4.9 ppb Pd, 7.1 ppb Pt, 7.2 ppb Ru, 3.8 ppb Ir and Os, showing no Pd/Ir, but a positive Ru/Ir anomaly, relative to chondrites. There is ∼50% variation of the HSE abundances in the oceanic mantle, with much of this variation being observed at small length scales (<1 km) and due entirely to both modern and more ancient partial melting effects. Consequently, any significant HSE heterogeneities formed during late accretion or early Earth differentiation processes are no longer recognizable in the mantle sampled within ocean basins, implying generally efficient mixing of Earth's mantle for these elements. By contrast, relatively ancient heterogeneity in Os and other radiogenic isotopes has been effectively preserved in the convecting mantle over the last ∼2 Ga, through recycling processes and through preservation and isolation of melt-depleted refractory residues. [ABSTRACT FROM AUTHOR]

• Absence of Os isotope and highly siderophile element evidence of extraterrestrial input for the Younger Dryas sediments. • Unradiogenic Os isotope ratios and highly siderophile element concentrations fingerprint volcanic aerosols. • Correlated unradiogenic Os peaks supports the proposed distant and episodic volcanic mechanisms for the Younger Dryas event. One of the prevailing hypotheses for the origin of the Younger Dryas (YD) cooling event is that it resulted from a bolide impact or airburst. Purported impact markers peak at or near the YD basal boundary layer at Northern Hemisphere locations. In this study, the 187Os/188Os ratios and highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re) abundances in a well-dated sediment section through the Younger Dryas at the Debra L. Friedkin site, Texas are reported. Unradiogenic 187Os/188Os peaks, which could be mantle-derived or extraterrestrial, have been found above, within, and below the YD basal boundary layer. Mass balance mixing models using chondrites or iron meteorites with upper continental crust fail to duplicate the chondrite-normalized HSE patterns of the sediment samples. These HSE signatures in the Friedkin site section replicate those found in Hall's Cave, Texas. The new results here thus independently confirm that the HSE abundances in the unradiogenic Os layers are likely a fingerprint of volcanic gas aerosols derived from large Plinian eruptions and not extra-terrestrial materials. To better constrain the lithological origins of YD sediments from the Friedkin and Hall's Cave sites, Texas, trace elements are presented here. The rare earth elements (REE) patterns and Ir, Ni, Ti and Zr abundances are also characterized with terrestrial signatures as opposed to impact melt rocks. An age profile correlation between the two study sites, further shows that three unradiogenic Os peaks overlap in time. The results are inconsistent with the extraterrestrial hypothesis and support instead an episodic and volcanic origin for the observed geochemical anomalies at the Debra L. Friedkin and Hall's Cave sites, Texas. [ABSTRACT FROM AUTHOR]

Dunites in ophiolites form by pyroxene dissolution and olivine precipitation during melt‐peridotite interaction. We present structural and geochemical data on peridotites from the Batin region (Wadi Tayin massif) of the Oman ophiolite, where an exceptionally large (∼9.5 km long) dunite body was sampled by the ICDP Oman Drilling Project (BA4A borehole). 900–1,200 m beneath the petrological Moho, this dunite is overlain by harzburgite hosting pyroxene‐depleted and pyroxene‐rich bands. Highly siderophile elements (HSEs) and Os isotopes, excellent tracers of melt flow through peridotites, were measured in dunites and interspersed harzburgites from BA4A borehole. The Batin dunite is structurally and chemically distinct from dunites from the Moho Transition Zone and basal section of the ophiolite, resembling instead sparse dunite veins in the main mantle section. Batin dunites have fairly uniform Os, Ir, and Ru abundances, but Pd and Pt contents increasing with depth. One deep dunite sample has initial 187Os/188Os more radiogenic than MORB. Though the limited number of data demands prudence, we suggest that the Batin dunite formed from a large pulse of radiogenic melts, whose flow was impeded ∼1,200 m below the Moho. As these melts ascended, they may have lost their radiogenic character and relative Pt and Pd enrichment through interaction with peridotites, which have much higher HSE contents than melts. Such interaction would also diminish the under‐saturation in pyroxene of the melts, eliminating their capacity to sufficiently dissolve the pyroxene of the host harzburgite to form dunite, thus explaining the upper limit of the dunite at ∼900 m. Plain Language Summary: Dunites in ophiolites form by pyroxene dissolution and olivine precipitation during melt‐peridotite interaction. Highly siderophile elements (HSEs) and Os isotopes are excellent tracers of this process. We present structural and geochemical data on dunites and interspersed harzburgites from borehole BA4A of the ICDP Oman Drilling Project in the Batin region (Wadi Tayin massif) of the Oman ophiolite. This region features an exceptionally large dunite body, included in the harzburgitic mantle section. This exploratory study of the Batin dunite shows that it contrasts chemically and structurally with dunites from the Moho Transition Zone and basal section of the ophiolite, but presents similarities with dunitic veins of the main mantle section. Nevertheless, its composition changes systematically with height, becoming progressively less fertile, and depleted in the incompatible HSE, platinum, and palladium. We suggest that the Batin dunite formed by the interaction of the preexisting harzburgite with a large melt pulse that stagnated at ∼1,200 m below the Moho. As these melts ascended through the overlying ∼300 m, their composition was modified through interaction with the harzburgite, eliminating their undersaturation in pyroxene and thus their capacity to form dunite. This may explain the upper limit of the dunite at ∼900 m beneath the Moho. Key Points: The Batin dunite is an exceptionally large dunite, interspersed in the mantle section, discordant to asthenospheric flowIts major and highly siderophile element composition becomes progressively less fertile with increasing heightMagmas were injected in a late melt pulse at unusual depth, and progressively lost their reactive capacity as they migrated upward [ABSTRACT FROM AUTHOR]

The Huoyanshan iron meteorite shower, recently found in the Gobi Desert of Hami, Xinjiang, China, has very high Ni (21.1 wt%) content and low Au (2.0 ppm), Ir (0.02 ppm), Ge (1.7 ppm), and Ga (1.1 ppm) contents, and was classified into IAB‐sLH subgroup. The iron has a finest octahedrite structure of Widmanstätten pattern (the intergrowth of kamacite [α] and taenite [γ]) with plessite matrix, and euhedral schreibersite (Sch) crystals exclusively enclosed in kamacite bands. The textural features suggest the following formation process: γ→γ + Sch→γ + Sch + α, and then γ→α2 + γ. The metallographic cooling rate of Huoyanshan iron was determined to be 3–50°C/Myr using both the taenite Ni profile‐matching and taenite central Ni content methods, with the bandwidths corrected for crystallographic orientation by electron backscatter diffraction (EBSD). The cooling rate of Huoyanshan is consistent with other sLH irons and confirms the slow cooling history of the IAB low‐Au subgroups. These slow cooling rates require immediate re‐accretion with a thick brecciated fragments layer in the parent body after the impact melting event. The depleted but unfractionated Re, Os, Ir, Ru, and Pt and the enriched Pd and Au abundances of Huoyanshan iron and other sLH subgroup irons show complementary feature to that of refractory metal nuggets in Ca‐, Al‐rich inclusions (CAIs), which could be explained by extracting the metallic Fe‐Ni with HSE predominantly remained in CAIs from a CAI‐bearing asteroid. The very high Ni content of sLH subgroup and its Mo isotope evidence (Worsham et al., 2017, https://doi.org/10.1016/j.epsl.2017.02.044) suggest it has an oxidized parent body with non‐carbonaceous chondrite‐like precursor composition. We propose that the sLH subgroup was produced by impact melting of a LL like and CAI‐bearing asteroid, followed by fast burying of thick and porous silicate breccia. Plain Language Summary: Iron meteorites are the metal remnants of asteroids, and they were classified as magmatic and non‐magmatic. IAB complex is the major non‐magmatic irons and was divided into several subgroups. The metal‐silicate fractionation of the IAB complex was likely related with impact events in the early stage of the solar system. Here, we present the detailed petrography, trace element composition, and metallographic cooling rate of the Huoyanshan iron meteorite found recently, a new member of the IAB‐sLH subgroup, in order to reveal its petrogenesis, post‐impact thermal history and the geochemical features of its precursor parent body. Our results suggest that IAB low‐Au subgroups were buried by thick silicate breccia layers immediately after the impact events, in order to hold their slow metallographic cooling rates. The very high Ni contents, the highly siderophile element patterns, and the Mo isotopes of IAB‐sLH irons suggest a genetic relationship with an oxidized non‐carbonaceous chondritic asteroid. Key Points: Petrography and trace elements of Huoyanshan iron, a new member of the IAB‐sLH subgroup and recently found were reportedSlow metallographic cooling rates of IAB low‐Au subgroups suggest immediate re‐accretion with silicate breccia layer after impact eventsThe high‐Ni endmember parent body of IAB complex was probably contributed mainly from an oxidized (LL‐like) and Ca‐, Al‐rich inclusion (CAI)‐bearing asteroid [ABSTRACT FROM AUTHOR]

The Lesvos ophiolite, Greece, was recently identified as an analog to the External Liguride Unit formed at an ocean‐continent‐transition (OCT) setting. To further study the evolutionary history of this body, we analyzed highly siderophile element abundances and Re‐Os isotopic compositions of 14 Lesvos peridotites, in combination with petrology, bulk‐rock major, and trace element geochemistry. Almost all the Lesvos peridotites fall within the field of global OCT and mid‐ocean‐ridge (MOR) peridotites, indicating that these rocks have not experienced subduction‐related processes. The near‐horizontal 187Os/188Os versus Al2O3 trend over a wide range of Al2O3 (0.45–3.66 wt. %) may have been caused by recent melt depletion during rifting and thinning of the Lesvos lithosphere. Combining data from available global ophiolitic peridotites, we find that a large proportion of the more Al‐depleted supra‐subduction‐zone (SSZ) peridotites show lower Os/Ir, higher Pd/Ir and remarkably elevated radiogenic Os relative to other tectonic environments (OCT and MOR). By linking this kind of geochemical evolution to the Wilson Cycle, a complete picture emerges: (a) In the OCT to MOR stages, the extensional rifting environment may lead to mild to moderate melt depletion, followed by, or associated with, infiltration of S‐saturated (high fS2) basaltic magmas; (b) when progressing into the SSZ stage, more extreme degrees of mantle melt depletion may be driven by aqueous fluids in the sub‐arc mantle. During this stage, high‐fO2 slab‐derived fluids and/or S‐undersaturated (low fS2) boninitic magmas may infiltrate the sub‐arc mantle, followed by subsequent S‐saturated forearc basaltic magma infiltration. Plain Language Summary: Establishing the relationship between the geochemical evolution and geodynamic mechanism of the Earth's upper mantle is an important objective of the study of ophiolites. There are two major aims of this study: (a) Using highly siderophile elemental (HSE) and Re‐Os isotopic geochemical tools to evaluate the tectonic setting for the Greek Lesvos peridotites; (b) on the basis of geochemical and isotopic comparisons among available global ophiolitic peridotites from various tectonic settings to reconstruct the Earth's upper mantle evolution. First, we find that although the Lesvos peridotites underwent S‐saturated melt overprinting after primary melt extraction that was related to the Mesozoic Pangea breakup, they did not experience subduction‐related processes. Second, we summarize that supra‐subduction‐zone (SSZ) peridotites in other ophiolites have not only recorded extremely high degrees of melt depletion but also show significantly higher Pd/Ir ratios and more radiogenic 187Os/188Os, compared with ocean‐continent‐transition (OCT) and mid‐ocean‐ridge (MOR) peridotites that formed beneath spreading centers. We believe that episodic infiltrations by S‐saturated and/or undersaturated melts/fluids may be an important driving force for these mantle compositional changes during the Wilson Cycle, which can also cause HSE‐bearing sulfides/alloys to be abnormally enriched, and possibly lead to formation of platinum group elements (PGE)‐rich chromite ore deposits. Key Points: The Lesvos peridotites belong to ocean‐continent‐transition (OCT) mantle tectonites with relatively unfractionated highly siderophile elemental (HSE) patternsThe Lesvos peridotites were formed recently, synchronous with Pangea breakup that led to formation of embryonic Neo‐Tethyan oceanic basinEpisodic infiltration by S‐saturated or undersaturated melts/fluids, dominates the evolution of lithosphere during the Wilson Cycle [ABSTRACT FROM AUTHOR]

Primitive achondrites derive from the residual mantle of incompletely differentiated planetesimals, from which partial silicate and metallic melts were extracted. As such, primitive achondrites are uniquely useful to examine the early stages of planetesimal melting and differentiation. To better understand this early melting and melt segregation as well as the nature of the melts involved, we obtained mass-dependent Ru isotopic compositions of 17 primitive achondrites, including winonaites, acapulcoite-lodranites, ureilites, brachinites, and two ungrouped samples. Most primitive achondrites with subchondritic Ru concentrations are characterized by heavy Ru isotopic compositions relative to chondrites, likely reflecting the extraction of isotopically light partial metallic melts. While the segregation of early-formed partial Fe-Ni-S melts likely had no effect on the Ru isotope compositions, the subsequent extraction of S-free partial metallic melts at higher temperatures provides a viable mechanism for producing the observed Ru isotopic fractionation and fractionated highly siderophile element ratios among primitive achondrites. Together, these observations indicate that differentiation of primitive achondrite parent bodies involved the segregation of distinct partial metallic melts over a range of temperatures, and that these melts ultimately formed a partial core with fractionated and light Ru isotopic composition. This contrasts with the unfractionated Ru isotope signatures previously estimated for bulk iron meteorite cores, which therefore indicates quantitative metal segregation during core formation in the iron meteorite parent bodies. The less efficient metal segregation in primitive achondrite parent bodies most likely reflects lower initial amounts of heat-producing 26Al due to later accretion or impact disruption of the parent bodies during differentiation. [ABSTRACT FROM AUTHOR]

Widespread Cenozoic intraplate basalts from eastern China offer the opportunity to investigate the consequences of interaction between the stagnant Pacific slab and overlying asthenosphere and chemical heterogeneity within this "big mantle wedge". We present and compile a comprehensive study of highly siderophile elements and Mg-Zn isotopes of this magmatic suite (60 samples including nephelinites, basanites, alkali basalts and tholeiites). The large-scale Mg-Zn isotopic anomalies documented in these basalts have been ascribed to mantle hybridization by recycled Mg-carbonates from the stagnant western Pacific plate. Our results reveal that the nephelinites and basanites are characterized by unfractionated platinum-group element (PGE) patterns normalized to primitive upper mantle (PUM) (e.g., Pd N /Ir N normalized to PUM = 1.1 ± 0.8, 1σ), relatively high total PGE contents (e.g., Ir = 0.25 ± 0.14 ppb) and modern mantle-like 187Os/188Os (0.142 ± 0.020). These characteristics are coupled with lighter Mg isotope (δ26Mg = −0.48 ± 0.07‰) and heavier Zn isotope (δ66Zn = +0.46 ± 0.06‰) compositions compared to the mantle values (δ26Mg: −0.25 ± 0.07‰; δ66Zn: +0.18 ± 0.05‰). Together, these data are interpreted to reflect the oxidative breakdown of low proportions of mantle sulfides in the sources of these small-degree melts, likely caused by recycled carbonates, which then release chalcophile-siderophile elements into carbonatitic melts. By contrast, the contemporaneous alkali basalts and tholeiites are characterized by highly fractionated PGE patterns (e.g., Pd N /Ir N = 4.4 ± 3.3; Ir = 0.037 ± 0.027 ppb) and radiogenic 187Os/188Os (0.279 ± 0.115) coupled with less fractionated Mg-Zn isotope compositions (δ26Mg: −0.39 ± 0.05‰; δ66Zn: +0.35 ± 0.03‰). In combination with other isotopic (e.g., Sr-Nd) and chemical (SiO 2 , Ce/Pb, Ba/Th, Fe/Mn) constraints, the alkali basalts and tholeiites were derived from higher degree melting of ancient pyroxenite-bearing mantle in addition to mixing with the aforementioned nephelinitic and basanitic melts. Collectively, we suggest that deep recycled carbonates promoted melting within the "big mantle wedge" leading to the generation of Cenozoic intraplate basalts across eastern China and the "redox freezing of carbonates" may cause the oxidation of Fe0 and S2-. This process may provide an important mechanism to oxidize mantle sulfides and transfer precious metals from deep mantle to crust. [ABSTRACT FROM AUTHOR]

Shergottite meteorites are ultramafic to mafic igneous rocks derived from partial melting of distinct regions of the martian mantle. As such, they trace magmatic processes, including fractional crystallization and mixing processes in Mars. New chalcophile (Cu, Se, Zn, Pb), siderophile (Ni, Co, W), and highly siderophile element (HSE: Au, Re, Pd, Rh, Pt, Ru, Ir, Os) abundance data are reported for sulfide assemblages in a suite of thirteen incompatible trace-element depleted, intermediate and enriched shergottites, along with new whole-rock HSE abundance and 187Os/188Os data for seven shergottites. Sulfide grains in depleted and intermediate shergottites typically have the highest absolute abundances of HSE, with broadly flat CI-chondrite normalized patterns. Enriched shergottite sulfide grains typically have highly variable Au, elevated Pd and Rh and are relatively depleted in Zn, Ir and Os. The new HSE whole-rock data for enriched (Northwest Africa [NWA] 7397, NWA 7755, NWA 11043), and intermediate shergottites (NWA 10961, NWA 11065, NWA 12241, and NWA 12536) are generally consistent with existing 187Os/188Os and HSE abundance data for these geochemical groupings. Enriched shergottites with >1 ppb bulk rock Os have measured 187Os/188Os ranging between 0.1296 and 0.1471, with variable Pd and Pt contents. Intermediate shergottites with >1 ppb bulk rock Os have chondrite-relative proportions of the HSE at ∼0.01 to 0.001 × CI chondrites and 187Os/188Os from 0.1284 and 0.1295. Sulfides are the major host of the HSE, and they control the behavior of the HSE during petrogenetic processes in shergottite magmas, enabling the determination of HSE compatibility for martian magmatism in the order: Os > Ir ≥ Ru ≥≥ Rh ≥ Pd ≥ Re ≥ Pt ≥ Au. Fractionation models of removal of an olivine-dominated cumulate recreate HSE patterns for the whole-rock shergottites. Enriched shergottites are best reproduced by 25 to 30% of fractionation from a degassed parent melt (250 ± 50 ppm of S), whereas depleted and intermediate shergottites can be explained by slightly lower fractionation (10 to 15%) from higher S content parent melts (350 ± 100 ppm of S). Sulfur contents in the melt that are ∼50% higher than these estimates yield earlier S-saturation during fractional crystallization, leading to an abrupt decrease of the more compatible HSE (Ru, Ir, Os), which is not observed. These results indicate that the martian mantle and partial melts derived from it, are probably not anomalously enriched in S, and instead are similar to slightly higher than those of the terrestrial mantle and its partial melts. [ABSTRACT FROM AUTHOR]

The eight iron meteorites currently classified as belonging to the IIC group were characterized with respect to the compositions of 21 siderophile elements. Several of these meteorites were also characterized for mass independent isotopic compositions of Mo, Ru and W. Chemical and isotopic data for one, Wiley, indicate that it is not a IIC iron meteorite and should be reclassified as ungrouped. The remaining seven IIC iron meteorites exhibit broadly similar bulk chemical and isotopic characteristics, consistent with an origin from a common parent body. Variations in highly siderophile element (HSE) abundances among the members of the group can be well accounted for by a fractional crystallization model with all the meteorites crystallizing between ∼10 and ∼26% of the original melt, assuming initial S and P concentrations of 8 wt.% and 2 wt.%, respectively. Abundances of HSE estimated for the parental melt suggest a composition with chondritic relative abundances of HSE ∼6 times higher than in bulk carbonaceous chondrites, consistent with the IIC irons sampling a parent body core comprising ∼17% of the mass of the body. Radiogenic 182W abundances of two group IIC irons, corrected for a nucleosynthetic component, indicate a metal-silicate segregation age of 3.2 ± 0.5 Myr subsequent to the formation of Calcium-Aluminum-rich Inclusions (CAI). When this age is coupled with thermal modeling, and assumptions about the Hf/W of precursor materials, a parent body accretion age of 1.4 ± 0.5 Myr (post-CAI) is obtained. The IIC irons and Wiley have 100Ru mass independent "genetic" isotopic compositions that are identical to other irons with so-called carbonaceous chondrite (CC) type genetic affinities, but enrichments in 94,95,97Mo and 183W that indicate greater s- process deficits relative to most known CC iron meteorites. If the IIC irons and Wiley are of the CC type, this indicates variable s- process deficits within the CC reservoir, similar to the s- process variability within the NC reservoir observed for iron meteorites. Nucleosynthetic models indicate that Mo and 183W s- process variability should correlate with Ru isotopic variability, which is not observed. This may indicate the IIC irons and Wiley experienced selective thermal processing of nucleosynthetic carriers, or are genetically distinct from the CC and NC precursor materials. [ABSTRACT FROM AUTHOR]

Cratonic lithospheric mantle is believed to have been formed in the Archean, but kimberlite-hosted coarse peridotites from Udachnaya in the central Siberian craton typically yield Paleoproterozoic Re-depletion Os isotope ages (T RD). By comparison, olivine megacrysts from Udachnaya, sometimes called "megacrystalline peridotites", often yield Archean T RD ages, but the nature of these rare materials remains enigmatic. We provide whole-rock (WR) Re-Os isotope and PGE analyses for 24 olivine-rich xenoliths from Udachnaya as well as modal and petrographic data, WR and mineral major and trace element compositions. The samples were selected based on (a) high olivine abundances in hand specimens and (b) sufficient freshness and size to yield representative WR powders. They comprise medium- to coarse-grained (olivine < 1 cm) dunites, a megacrystalline (olivine > 1 cm) dunite, olivine megacrysts and low-orthopyroxene (11–21% opx) harzburgites equilibrated at 783–1154 °C and 3.9–6.5 GPa; coarse dunites have not been previously reported from Udachnaya; two xenoliths contain ilmenite. The harzburgites and dunites have similar WR variation ranges of Ca, Al, Fe, Cr and Mg# (0.917–0.934) typical of refractory cratonic peridotites, but the dunites tend to have higher MgO, NiO and Mg/Si. Mineral abundances and those of Ca and Al are not correlated with Mg# WR ; they are not due to differences in melting degrees but are linked to metasomatism. Several samples with high 187Re/188Os show a positive linear correlation with 187Os/188Os with an apparent age of 0.37 Ga, same as eruption age of host kimberlite. Robust T RD ages were obtained for 16 xenoliths with low 187Re/188Os (0.02–0.13). T RD ages for low-opx harzburgites (1.9–2.1 Ga; average 2.0 ± 0.1 Ga, 1 σ) are manifestly lower than for dunites and megacrysts (2.4–3.1 Ga); the latter define two subsets with average T RD of 2.6 ± 0.1 Ga and 3.0 ± 0.1 Ga, and T MA of 3.0 ± 0.2 Ga and 3.3 ± 0.1 Ga, respectively. Differences in olivine grain size (coarse vs. megacrystalline) are not related to age. The age relations suggest that the dunites and megacrysts could not be produced by re-melting of harzburgites, e.g. in arc settings, nor be melt channel materials in harzburgites. Instead, they are relict fragments of lithospheric mantle formed in the Archean (likely in two events at or after 2.6 Ga and 3.0 Ga) that were incorporated into cratonic lithosphere during the final assembly of the Siberian craton in the Paleoproterozoic. A multi-stage formation of the Siberian lithospheric mantle is consistent with crustal basement ages from U-Pb dating of zircons from crustal xenoliths at Udachnaya and detrital zircons from the northern Siberian craton (1.8–2.0, 2.4–2.8 and 3.0–3.4 Ga). The new data from the Siberian and other cratons suggest that the formation of strongly melt-depleted cratonic lithosphere (e.g. Mg# ≥0.92) did not stop at the Archean-Proterozoic boundary as is commonly thought, but continued in the Paleoproterozoic. The same may be valid for the transition from the 'Archean' (4–2.5 Ga) to modern tectonic regimes. [ABSTRACT FROM AUTHOR]

Highly depleted Archean peridotites have proven very amenable to Re-Os model age dating. In contrast, due to the increasing heterogeneity of mantle Os isotope compositions with time, the Re-Os system has not been as effective in dating post-Archean peridotites. The timing of depletion and accretion of post-Archean lithospheric mantle around cratons is important to understand within the context of the evolution of the continents. In an attempt to precisely date post-Archean peridotite xenoliths, we present a study of the petrology, mineralogy and geochemistry, including whole-rock Re-Os isotopes, highly siderophile elements and clinopyroxene-orthopyroxene Sr-Nd-Hf isotopes of peridotite xenoliths from Lake Nyos in the Cameroon Volcanic Line (CVL). Eight Nyos peridotite xenoliths, all fresh spinel lherzolites, are characterized by low to moderate olivine Fo contents (88.9–91.2) and low spinel Cr# (8.4–19.3), together with moderate to high whole-rock Al 2 O 3 contents (2.0–3.7%). These chemical characteristics indicate that they are mantle residues of a few percent to <20% partial melting. However, trace element patterns of both clinopyroxene and orthopyroxene are not a pristine reflection of melt depletion but instead show various extents of evidence of metasomatic enrichment. Some of the samples contain orthopyroxene with 143Nd/144Nd lower than its coexisting clinopyroxene, which is best explained by recent short-timescale alteration, most likely by infiltration of the host basalt. Because of these metasomatic effects, the Sr-Nd isotope systematics in pyroxenes cannot sufficiently reflect melt depletion signatures. Unlike Sr-Nd isotopes, the Lu-Hf isotope system is less sensitive to recent metasomatic overprinting. Given that orthopyroxene hosts up to 33% of the Lu and 14% of the Hf in the whole rock budget of these rocks and has 176Hf/177Hf similar to, or higher than, coexisting clinopyroxene, it is necessary to reconstruct a whole-rock Lu-Hf isochron in order to constrain the melt depletion age of peridotites. The reconstructed Nyos Lu-Hf isochron from ortho- and clinopyroxenes gives an age of 2.01 ± 0.18 Ga (1σ), and when olivine and spinel are considered, is 1.82 ± 0.14 Ga (1σ). Both ages are identical within error, and they are within error of the alumina-187Os/188Os pseudo-isochron ages (1.2–2.4 Ga) produced on the peridotites from Lake Nyos, consistent with their oldest rhenium depletion Os model ages (2.0 Ga). We conclude that the Nyos peridotites, and the lithospheric mantle that they represent, were formed at ∼2.0 Ga, indicating that the reconstructed whole-rock Lu-Hf isotope system can be a powerful radiometric dating tool that is complementary to and in some instances, more precise than the Re-Os isotope system in dating well-preserved post-Archean peridotites. The recognition of ∼2.0 Ga subcontinental lithospheric mantle (SCLM) in the Nyos area suggests that the Nyos region was assembled as a Paleoproterozoic block, or that it represents fragments of the SCLM from the nearby Paleoproterozoic domain juxtaposed through collisional emplacement during the Pan African Orogeny. With regards to the origin of the CVL, our data reveal that the Hf isotopic compositions of the Nyos peridotites are too radiogenic to be the main source of the CVL basalts. [ABSTRACT FROM AUTHOR]

Impactors striking the Moon since the formation of its crust have left an indelible imprint on the lunar surface, in the guise of craters and associated impact rocks. The lunar crust has low intrinsic abundances of the highly siderophile elements (HSE: Re, Os, Ir, Ru, Pt, Rh, Pd, Au), at greater than 3000 times lower than in chondrite meteorites. Consequently, during impact, bolides with chondritic or differentiated iron-rich compositions should impart elevated HSE signatures to the lunar crust. Here we examine glassy lunar impact melt coats (IMC) from the outside rims of Apollo 16 cataclastic anorthosites (60015, 65325) and breccias (65035), as well as both fragments and powders of Antarctic anorthositic regolith breccia (ARB) meteorites (Miller Range 090034/36/70/75 and MacAlpine Hills 88105) for their petrography, mineral chemistry and bulk-rock compositions. The HSE concentrations for IMC range from ∼0.001 to 0.1 × CI chondrite, with measured 187Os/188Os between 0.1189 and 0.1366. Anorthositic regolith breccia meteorites, which have components with 2.6–4.1 Ga ages, have similar HSE concentrations to IMC, but typically have lower 187Os/188Os (0.1164–0.1284). These latter Os ratios are generally less radiogenic than those measured in ∼3.8–3.9 Ga Apollo impact melt breccias. The Apollo 16 IMC are not well-dated, but their KREEPy trace-element signatures and associated ages of 3.7 to 3.8 Ga for Apollo 16 glasses might imply, at least in part, an origin from the Imbrium or Serenitatis basin-forming impacts. Within the IMC, metal-schreibersite-troilite assemblages record significant inter-element HSE fractionation which is also reflected in bulk HSE patterns for both IMC and ARB meteorites. Variations in relative and absolute HSE compositions directly reflect the control of metal and sulfide segregation within and between impact melt and breccia lithologies. Collectively, IMC and ARB meteorites exhibit approximately 50% of the variation in Ru/Ir and 187Os/188Os observed in lunar impact melt breccias. These results imply that significant variations in inter-element compositions can occur during impact brecciation and melting and so some impact melt rock HSE compositions may not record the compositions of impactors that struck the Moon with fidelity. Nonetheless, the generally low 187Re/188Os of lunar impact melt rocks means that osmium isotope ratios provide evidence for impact composition, and a change from ordinary to carbonaceous-like impactors either with time – or location – striking the Moon. [ABSTRACT FROM AUTHOR]

New highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re) abundance and Os isotopic compositions, along with major- and trace-element abundance and Sr-Nd-Hf-Pb isotope data are reported for high-MgO (>8–17 wt.%) lavas from the islands of São Miguel, Terceira, Pico and Faial in the Azores archipelago. The lavas span the range of Sr-Nd-Hf-Pb isotope and trace-element abundance variability reported previously for the islands. Pico and Faial lavas preserve distinct absolute and relative HSE abundances and 187Os/188Os (Ʃ HSE = 2 ± 2 ng g−1; 2SD; 187Os/188Os in lavas with > 50 pg g−1 Os = 0.1255 ± 31; 2SD) compared with São Miguel and Terceira lavas (Ʃ HSE = 5 ± 3 ng g−1; 187Os/188Os 50pg = 0.1284 ± 48). Although HSE abundance variations in volcanic rocks from individual Azorean islands can be explained by sulfide crystallization, those between Pico and Faial and São Miguel and Terceira can only reasonably be attributed to partial melting of distinct mantle sources. Pico and Faial lava compositions are consistent with high degrees of sulfide melting (90–100%) from a sulfide-depleted mantle source (<100 ppm) with relatively unradiogenic 187Os/188Os (<0.126). São Miguel and Terceira lavas likely derive from a sulfide-enriched (300–600 ppm) mantle source that underwent lesser degrees of sulfide melting (1–15%), with more radiogenic 187Os/188Os (>0.128). Radiogenic sulfides are not solely linked to the enriched lithophile isotope signature found in eastern São Miguel. Instead, São Miguel and Terceira magmatism may collectively result from metasomatism associated with the recycling of old subducted oceanic lithosphere. Abundances of sulfide-hosted HSE, and 187Os/188Os, are decoupled from silicate-hosted Sr-Nd-Hf-Pb isotope compositions, in the mantle. Compositional variations of the HSE among global OIB suggest that, while distinct mantle domains are represented by heterogeneous sulfide mineralogy, comprising common metasomatic (enriched) and magmatic (depleted) compositions, these sulfide populations are distinct beneath different OIB due to their origin in different lithospheric materials (e.g., oceanic crust versus pelagic sediment), and to the degree of partial melting caused by variable mantle potential temperature or lithospheric thickness. [ABSTRACT FROM AUTHOR]

Sample decomposition using inverse aqua regia at elevated temperatures and pressures (e.g., Carius tube or high‐pressure asher) is the most common method used to extract highly siderophile elements (HSEs: Ru, Rh, Pd, Re, Os, Ir, Pt and Au) from geological samples. Recently, it has been recognised that additional HF desilicification is necessary to better recover HSEs, potentially contained within silicate or oxide minerals in mafic samples, which cannot be dissolved solely by inverse aqua regia. However, the abundance of interfering elements tends to increase in the eluent when conventional ion‐exchange purification procedures are applied to desilicified samples. In this study, we developed an improved purification method to determine HSEs in desilicified samples. This method enables the reduction of the ratios of isobaric and polyatomic interferences, relative to the measured intensities of HSE isotope masses, to less than a few hundred parts per million. Furthermore, the total procedural blanks are either comparable to or lower than conventional methods. Thus, this method allows accurate and precise HSE measurements in mafic and ultramafic geological samples, without the need for interference corrections. Moreover, the problem of increased interfering elements, such as Zr for Pd and Cr for Ru, is circumvented for the desilicified samples. [ABSTRACT FROM AUTHOR]

Traverses across amphibole-chlorite schist metamorphic reaction zones at contacts between meta-igneous and ultramafic rocks in high-pressure mélange zones were analyzed for highly siderophile element (HSE) concentrations and 187Os/188Os in two mélange zones (Catalina Schist, Santa Catalina Island, CA, and Attic-Cycladic Complex, Syros, Greece) in order to investigate the relative effects of mechanical mixing and diffusive and advective transport by fluids in subduction-related reaction zones. Results from Catalina Schist traverses show co-varying elevated concentrations of Os, Ir, Ru, Cr, and MgO and inversely correlated initial 187Os/188Os that display irregular patterns across the chlorite-actinolite schist reaction zones. These elements are strongly enriched in peridotite relative to mafic crust, and are relatively fluid immobile. The observations suggest that these components were added via mechanical mixing of mantle peridotitic material with material derived from mafic blocks. In a traverse across a Syros mafic eclogite block with a narrow chlorite schist reaction zone and broader glaucophane-rich reaction zone, the Os, Ir, Ru, and Cr concentrations in the chlorite schist are just barely higher than those in the eclogite block, while the initial 187Os/188Os ratio falls between the eclogite core and adjacent serpentinite. These compositions show a relatively minor metasomatic exchange between eclogite block and serpentinite. A second sample set from Syros was chosen across a contact zone between a meta-tuffite and a serpentinite because previous studies suggest it was not affected by mechanical mixing. This traverse contains a ~1 m wide complex metasomatic reaction zone that is used to study the relative contributions of diffusive and advective transport. The traverse reveals strong mobilization of Os, Ir, Ru and Pt beyond the scale of the outcrop, and minor advective enrichment of Re and Pd. This indicates that all HSE may be mobilized by fluid advection beyond the local scale in some subduction zone mélange zones. • Mechanical mixing in Catalina Schist reaction zones creates variability in HSE with no evidence for fluid mobility. • All HSE were mobilized in the Syros mélange zone through fluid advection beyond the local scale • The HSE are differentially affected by the diffusive and advective transport processes acting on the rocks. [ABSTRACT FROM AUTHOR]

Highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re), major and trace element abundances, and 187Re–187Os systematics are reported for xenoliths and lavas from Aitutaki (Cook Islands), to investigate the composition of Pacific lithosphere. The xenolith suite comprises spinel-bearing lherzolites, dunite, and harzburgite, along with olivine websterite and pyroxenite. The xenoliths are hosted within nephelinite and alkali basalt volcanic rocks (187Os/188Os ∼0·1363 ± 13; 2SD; ΣHSE = 3–4 ppb). The volcanic host rocks are low-degree (2–5%) partial melts from the garnet stability field and an enriched mantle (EM) source. Pyroxenites have similar HSE abundances and Os isotope compositions (Al2O3 = 5·7–8·3 wt %; ΣHSE = 2–4 ppb; 187Os/187Os = 0·1263–0·1469) to the lavas. The pyroxenite and olivine websterite xenoliths directly formed from—or experienced extensive melt–rock interaction with—melts similar in composition to the volcanic rocks that host the xenoliths. Conversely, the Aitutaki lherzolites, harzburgites and dunites are similar in composition to abyssal peridotites with respect to their 187Os/188Os ratios (0·1264 ± 82), total HSE abundances (ΣHSE = 8–28 ppb) and major element abundances, forsterite contents (Fo89·9±1·2), and estimated extents of melt depletion (<10 to >15%). These peridotites are interpreted to sample relatively shallow Pacific mantle lithosphere that experienced limited melt–rock reaction and melting during ridge processes at ∼90 Ma. A survey of maximum time of rhenium depletion ages of Pacific mantle lithosphere from the Cook (Aitutaki ∼1·5 Ga), Austral (Tubuai'i ∼1·8 Ga), Samoan (Savai'i ∼1·5 Ga) and Hawaiian (Oa'hu ∼2 Ga) island groups shows that Mesoproterozoic to Neoproterozoic depletion ages are preserved in the xenolith suites. The variable timing and extent of mantle depletion preserved by the peridotites is, in some instances, superimposed by extensive and recent melt depletion as well as melt refertilization. Collectively, Pacific Ocean island mantle xenolith suites have similar distributions and variations of 187Os/188Os and HSE abundances to global abyssal peridotites. These observations indicate that Pacific mantle lithosphere is typical of oceanic lithosphere in general, and that this lithosphere is composed of peridotites that have experienced both recent melt depletion at ridges and prior and sometimes extensive melt depletion across several Wilson cycles spanning periods in excess of two billion years. [ABSTRACT FROM AUTHOR]

Mantle lithosphere influences rift system tectonic evolution, yet the age and composition of rifted lithosphere is typically difficult to constrain due to limited sampling. In the West Antarctic Rift System (WARS), Cenozoic to recent alkaline volcanic rocks yield a variety of peridotite and pyroxenite xenoliths that allow sampling of lithosphere. We report osmium and helium isotope data, elemental abundances, and petrology, for a suite of xenoliths and lavas from the Hut Point Peninsula of Ross Island. Recently (<1.3 Ma) erupted basanites yield fresh dunite and harzburgite (olivine forsterite [Fo] 90.1–88.2), lherzolite (Fo 90.6–87.4), and pyroxenite xenoliths (Fo 89.3–87.3). The basanite lavas contain abundant large olivine xenocrysts (Fo 89.7–88.0), with more ferroan matrix olivine grains (Fo 83.7–81.2) and have HIMU-like incompatible trace-element signatures. The 3He/4He ratios (6.8 ± 0.3R A ; 2SD) defined by co-existing He-rich xenoliths indicate a mantle source distinct from high-3He/4He plume mantle. Pyroxenite and lherzolite xenoliths have similar relative abundances of incompatible trace elements to host lavas, whereas dunite xenoliths have refractory compositions. Melt-rock reaction occurring in the xenoliths is demonstrated by replacement by amphibole or clinopyroxene to form pyroxenite and lherzolite lithologies, or as amphibole-impregnated dunites. The 187Re 187Os systematics of the lavas, pyroxenites and lherzolites define an apparent isochron, with an initial 187Os/188Os ratio of 0.1286 ± 0.0001. The initial 187Os/188Os ratio is within uncertainty of dunite and harzburgite xenolith Os isotope compositions (0.1279–0.1303). Pervasive evidence for melt-rock interaction indicates that the straight-line relationship in 187Re/188Os-187Os/188Os space is a mixing line between high Re/Os lavas with radiogenic 187Os/188Os, and dunite and harzburgite. Petrological and geochemical evidence indicates that dunite and harzburgite xenoliths represent young lithosphere, with rhenium depletion ages up to ~250 Ma. The timing of formation and composition of the Hut Point Peninsula xenoliths are consistent with both destruction and creation of mantle lithosphere during or after subduction along the Gondwana margin, prior to WARS formation. Modification of mantle lithosphere by subduction is also consistent with generation of HIMU-like metasomatized mantle reservoirs that fed Cenozoic to recent alkali volcanism of Mount Erebus and the WARS. The presence of young lithosphere within the WARS has collateral implications for rift dynamics and melting processes, especially beneath Mount Erebus, contrasting with older lithospheric mantle beneath the Trans-Antarctic Mountains and Marie Byrd Land. Unlabelled Image • Melt-rock reaction between lithosphere and basanite melts at Hut Point Peninsula • Straight-line relationship in 187Re/188Os-187Os/188Os space represents mixing of melts and lithosphere • Dunite and harzburgite xenoliths are lithosphere formed during or after subduction along Gondwana margin • Subduction-generated mantle likely source of HIMU signatures, influencing West Antarctic Rift System dynamics and melting processes [ABSTRACT FROM AUTHOR]

Determination of highly siderophile element (HSE; Au, Pt, Pd, Ir, Os, Rh and Ru) concentrations in relatively unweathered and unaltered marine shales from the Barberton, Witwatersrand and Transvaal supergroups in the Kaapvaal Craton revealed systematic differences, interpreted to reflect secular changes in the HSE content of Mesoarchaean to Palaeoproterozoic seawater. Most of the studied marine shales have HSE concentrations in the range given for average Archaean crustal rocks (0.5–5 ppb), with the exception of the shales in the Witwatersrand Supergroup. These shales contain up to three times more HSE, independent of source rock lithology in the granitoid-greenstone-dominated hinterland. Although sedimentary pyrite incorporated gold from synsedimentary to early diagenetic waters, its modal proportion (<3 vol.%) is too small to account for the total amount of Au and PGE in the studied marine shales. Instead, our results suggest that in addition to contributions from pyrite, some colloidal gold was attached to clay-sized sediments during source area weathering. Probably, colloidal gold and some of the platinum group elements were mechanically aggregated during sediment suspension and deposited synchronously with the host marine sediments. [ABSTRACT FROM AUTHOR]

Abstract Extremely low and variable concentrations of osmium (Os) and other highly siderophile elements (HSE) in most tektites make it challenging to establish direct links between these impact-related materials and their possible extraterrestrial contribution. New Os concentrations (2–43 ppt) and 187Os/188Os ratios (0.131–0.68) in a suite of fifteen well-characterized Australasian tektites from Laos (Muong Nong and splash-form types) with variable Ni enrichment indicate a maximum of ∼0.005% addition of a chondritic impactor. This is similar to some Australasian tektites from Vietnam with similarly low siderophile contents, but significantly lower than found in previous studies of more Ni-rich Australasian splash-form tektites and microtektites from different parts of the Australasian strewn field (e.g., Indonesia, South China Sea). The contents of HSE and Re–Os isotopic compositions of layered Muong Nong-type Australasian tektites are highly variable, suggesting mingling of crustal-derived (siderophile element-poor) and extraterrestrial (siderophile element-rich) materials. The absence of a direct correlation between HSE and Ni contents is interpreted to result from a fractionation process related to their different vaporization/condensation temperatures. The low Os abundance in most of the analyzed Australasian tektites, combined with non-radiogenic 187Os/188Os far below average upper continental crust, may provide a direct test to distinguish continental versus seawater impact scenario. In the absence of any specific low-Os target, a particular process of Os loss following impact is required. We envisage a scenario where evaporative loss of >>90% Os in the form of Os oxides from the overheated tektite melt is aided by volatile species derived from dissociated seawater and/or saline pore water embedded in sediments off-shore Indochina, consistent with elevated contents of halogens in Australasian tektites. This water-assisted Os loss could also play significant role for Central European tektites, while the continental surface with limited amount of water would prevent from more efficient HSE loss as could be the case for Ivory Coast tektites. [ABSTRACT FROM AUTHOR]

Abstract We present major- and trace-element concentrations, Sr–Nd–Hf isotopic compositions, Re Os isotopes, and highly siderophile element (HSE) abundances of 17 xenolithic mantle peridotites from the Nuomin volcanic field to constrain the assembly and evolution of the Xing'an–Mongolia Orogenic Belt (XMOB), Northeast China. The Nuomin peridotites are divided into five groups (harzburgite groups Haz-1, Haz-2, and Haz-3, and lherzolite groups Lhz-1 and Lhz-2). The Haz-1 peridotites are characterized by refractory chemical compositions (Al 2 O 3 contents of 1.03–2.30 wt% and olivine forsterite (Fo) numbers of 90.7–92.5) that are best interpreted as residues after high degrees of mantle partial melting. The Haz-2 and Haz-3 peridotites exhibit slightly more fertile compositions (Al 2 O 3 of 1.72–1.80 wt% and Fo of 89.0–89.9 for the Haz-2, and Al 2 O 3 of 2.86 wt% and Fo of 91.2 for the Haz-3), resulting from refertilization by silicate melts. The Lhz-1 samples are more fertile (Al 2 O 3 of 2.90–3.64 wt% and Fo of 89.1–90.3) than the Lhz-2 samples (Al 2 O 3 of 1.64–2.16 wt% and Fo of 90.9–91.3). The combination of trace-element data (Ti/Eu ratio) and Sr–Nd–Hf isotopic compositions indicates the presence of at least three peridotite end-members (Haz-1, Haz-2, and Lhz-1–Lhz-2) beneath the Nuomin area, which can be explained by the infiltration of two distinct metasomatic agents (MA1: composition similar to potassic basaltic rocks in the Wudalianchi–Erkeshan–Keluo area, and MA2: Fe–Ti-enriched melt). HSE abundances and Os isotopic data show that the Os isotopic compositions of the Haz-1, Lhz-1, and Lhz-2 peridotites were not significantly affected by lithophile-element-dominated metasomatism and can be used to constrain the melt extraction ages of these peridotites. The lower 187Os/188Os ratios of Haz-1 peridotites define a prominent rhenium depletion age (T RD) of ~1.6 Ga and represent fragments of the Paleo–Mesoproterozoic lithospheric mantle rather than reflecting the heterogeneous characteristics of the asthenosphere or extraneous emplacement. In contrast, the Lhz-1 and Lhz-2 lherzolite groups together have a narrow range of 187Os/188Os ratios that give younger T RD ages (~0.5 to 1.0 Ga). Our data demonstrate that both the Paleo–Mesoproterozoic and Neoproterozoic subcontinental lithospheric mantle coexist beneath the XMOB and that they correspond respectively to the bimodal age peaks of crustal growth of ~1.6 Ga and ~0.8 Ga defined by the XMOB granitoids. The XMOB was assembled primarily during the Phanerozoic by multiple blocks that had formed from the Paleoproterozoic to Neoproterozoic. The similar chemical and Sr–Nd–Hf isotopic signatures of both young and old lithospheric mantle indicate that the large-scale metasomatism event involving potassic melts most likely occurred during the evolution of the Paleo–Asian Ocean. The crustal materials carried to the mantle by multiple subduction events could be an effective source of potassium for the origin of potassic basaltic rocks. Highlights • Both the Paleo-Mesoproterozoic and Neoproterozoic SCLM coexist beneath the XMOB. • Mantle ages match well the two age peaks of crustal growth defined by granitoids. • XMOB was assembled by numerous Paleoproterozoic to Neoproterozoic blocks. • Subducted crustal materials could be an effective source of potassium in the mantle. [ABSTRACT FROM AUTHOR]

Abstract The abundances of highly siderophile elements (HSE: Re, Os, Ir, Ru, Pt, Pd), as well as 187Re-187Os and 182Hf-182W isotopic systematics were determined for separated metal, slightly magnetic, and nonmagnetic fractions from seven H4 to H6 ordinary chondrites. The HSE are too abundant in nonmagnetic fractions to reflect metal-silicate equilibration. The disequilibrium was likely a primary feature, as 187Re-187Os data indicate only minor open-system behavior of the HSE in the slightly and non-magnetic fractions. 182Hf-182W data for slightly magnetic and nonmagnetic fractions define precise isochrons for most meteorites that range from 5.2 ± 1.6 Ma to 15.2 ± 1.0 Ma after calcium aluminum inclusion (CAI) formation. By contrast, 182W model ages for the metal fractions are typically 2–5 Ma older than the slope-derived isochron ages for their respective, slightly magnetic and nonmagnetic fractions, with model ages ranging from 1.4 ± 0.8 Ma to 12.6 ± 0.9 Ma after CAI formation. This indicates that the W present in the silicates and oxides was not fully equilibrated with the metal when diffusive transport among components ceased, consistent with the HSE data. Further, the W isotopic compositions of size-sorted metal fractions from some of the H chondrites also differ, indicating disequilibrium among some metal grains. The chemical/isotopic disequilibrium of siderophile elements among H chondrite components is likely the result of inefficient diffusion of siderophile elements from silicates and oxides to some metal and/or localized equilibration as H chondrites cooled towards their respective Hf-W closure temperatures. The tendency of 182Hf-182W isochron ages to young from H5 to H6 chondrites may indicate derivation of these meteorites from a slowly cooled, undisturbed, concentrically-zoned parent body, consistent with models that have been commonly invoked for H chondrites. Overlap of isochron ages for H4 and H5 chondrites, by contrast, appear to be more consistent with shallow impact disruption models. The W isotopic composition of metal from one CR chondrite was examined to compare with H chondrite metals. In contrast to the H chondrites, the CR chondrite metal is characterized by an enrichment in 183W that is consistent with nucleosynthetic s -process depletion. Once corrected for the correlative nucleosynthetic effect on 182W, the 182W model age for this meteorite of 7.0 ± 3.6 Ma is within the range of model ages of most metal fractions from H chondrites. The metal is therefore too young to be a direct nebular condensate, as proposed by some prior studies. [ABSTRACT FROM AUTHOR]

The late Norian "chaotic carbon episode" in the Late Triassic has been identified in North America, Italy, and China, reflecting a global carbon isotope perturbation. Although it has been suggested that catastrophic events, such as the emplacement of large igneous provinces, were the cause of this perturbation, direct evidence for volcanic forcing is lacking. In order to investigate the cause of the global "chaotic carbon episode" in the late Norian, we obtained osmium and organic carbon isotope data (187Os/188Os and δ13C org) and major, trace, and highly siderophile element data and documented changes in radiolarian assemblages in a middle–upper Norian bedded chert succession in the Sakahogi section, Japan, deposited in a low-latitude zone of the Panthalassa Ocean. The δ13C org data exhibit a significant carbon isotope excursion (CIE) during the late Norian (Sevatian 1) Mockina bidentata conodont zone. High-resolution 187Os/188Os data exhibit two negative shifts during the CIE, which are interpreted as related to volcanic activity. The biostratigraphic age and Os isotopic mass balance considerations for the ocean suggest that the onset of Angayucham volcanism was the source of unradiogenic Os that caused the decrease in seawater 187Os/188Os ratios during the Sevatian. A remarkable diversification of radiolarians with a large Mesosaturnalis bloom occurred during the low-187Os/188Os interval. This suggests that the Sevatian Mesosaturnalis species are likely indicators of the onset of Angayucham volcanism. • The Re-Os isotope profile of the Norian chert succession in Japan was determined. • Os isotope data suggest the onset of the Angayucham volcanism during the Sevatian 1. • A negative δ13C org excursion was detected in the Sevatian 1 in the study section. • Abundant Mesosaturnalis radiolarians occurred during the low 187Os/188Os interval. [ABSTRACT FROM AUTHOR]

Abstract While the mantle roots directly beneath Archean cratons have been relatively well studied because of their economic importance, much less is known about the genesis, age, composition and thickness of the mantle lithosphere beneath the regions that surround the cratons. Despite this knowledge gap, it is fundamentally important to establish the nature of relationships between this circum-cratonic mantle and that beneath the cratons, including the diamond potential of circum-cratonic regions. Here we present mineral and bulk elemental and isotopic compositions for kimberlite-borne mantle xenoliths from the Parry Peninsula and Central Victoria Island, Arctic Canada. These xenoliths provide key windows into the lithospheric mantle underpinning regions to the North and Northwest of the Archean Slave craton, where the presence of cratonic material has been proposed. The mantle xenolith data are supplemented by mineral concentrate data obtained during diamond exploration. The mineral and whole rock chemistry of peridotites from both localities is indistinguishable from that of typical cratonic mantle lithosphere. The cool mantle paleogeotherms defined by mineral thermobarometry reveal that the lithospheric mantle beneath the Parry Peninsula and Central Victoria Island terranes extended well into the diamond stability field at the time of kimberlite eruption, and this is consistent with the recovery of diamonds from both kimberlite fields. Bulk xenolith Se and Te contents, and highly siderophile element (including Os, Ir, Pt, Pd and Re) abundance systematics, plus corresponding depletion ages derived from Re-Os isotope data suggest that the mantle beneath these parts of Arctic Canada formed in the Paleoproterozoic Era, at ∼2 Ga, rather than in the Archean. The presence of a diamondiferous Paleoproterozoic mantle root is part of the growing body of global evidence for diamond generation in mantle roots that stabilized well after the Archean. In the context of regional tectonics, we interpret the highly depleted mantle compositions beneath both studied regions as formed by mantle melting associated with hydrous metasomatism in the major Paleoproterozoic Wopmay-Great Bear-Hottah arc systems. These ∼2 Ga arc systems were subsequently accreted along the margin of the Slave craton to form a craton-like thick lithosphere with diamond potential thereby demonstrating the importance of subduction accretion in building up Earth’s long-lived continental terranes. [ABSTRACT FROM AUTHOR]