Search

Zircon grains from the metasedimentary lower crust of the Rio Grande Rift, New Mexico, preserve a metamorphic record of the transition from Laramide compression to Eocene extension. Zircon U‐Pb isotopes and trace‐element concentrations from five two‐pyroxene metaigneous granulite xenoliths define discrete populations: older zircon cores (∼15–50 Ma) that are depleted in heavy rare‐earth elements (HREE) but Ti‐rich, and younger zircon rims (∼3–15 Ma) with elevated HREE and lower Ti concentrations. Coupled phase equilibria and garnet‐melt‐zircon trace‐element partitioning calculations show that the older zircon cores equilibrated in thick (>40 km), hot (800–900°C), garnet‐bearing lower crust during the cessation of compression at the end of the Laramide orogeny. Zircon rim domains equilibrated at lower pressures, consistent with >9 km of thinning of the lower crust. Thermal‐kinematic calculations show that these pressure‐temperature‐time constraints require thinning of the lithospheric mantle prior to and during regional Cenozoic extension. Convective erosion of the mantle lithosphere over tens of millions of years, possibly facilitated by dynamics of the Farallon slab, provides a mechanism to facilitate lower crustal heating and extension. Key Points: Zircon U‐Pb dates from five granulite xenoliths, Rio Grande Rift, range between ∼50 and ∼3 MaZircon cores equilibrated in thick (>40 km), hot (800–900°C), garnet‐bearing lower crust during the Laramide orogenyP‐T‐time constraints require thinning of the lithospheric mantle prior to and during regional Cenozoic extension [ABSTRACT FROM AUTHOR]

Rare-Earth Elements (REE) are key geochemical tracers of crust–mantle differentiation, but there are few direct data on REE-rich minerals in mantle rocks. Here, we report the combined petrography and comprehensive chemical and isotopic characterization of three coesite- and kyanite-bearing eclogite xenoliths from the Udachnaya kimberlite pipe (Siberian craton), which are unusual in that two xenoliths (one with diamond and graphite) contain discrete, idiomorphic crystals of allanite at the grain boundaries of garnet and omphacite. Another xenolith contains allanite as part of a complex aggregate of calcite, apatite, barite, and celestine hosted by serpentine, which is a low-temperature secondary minerals likely result from metasomatic reaction at shallower depths during the transport of eclogite by the erupting kimberlite melt. The bulk rock composition reconstructed from the trace element composition of garnet and omphacite show marked depletion in LREE, precluding equilibration with kimberlite melt, whereas the measured bulk compositions show chondrite-normalized REE patterns with conspicuous depletions of Ce–Pr–Nd relative to La and Sm. The presence of 0.005 to 0.008 wt % of allanite, texturally and chemically out of equilibrium with the rock-forming minerals, allows balancing the LREE and Sm–Nd budget of the rock, whereas Th and U require additional hosts. This not only highlights the utility of measuring bulk eclogite xenoliths in bringing this unusual component to light, but also demonstrates that the long-known incompatible element enrichment in bulk eclogites reflects the deposition of discrete phases rather than merely bulk kimberlite melt addition. Although allanite is stable in metabasalts at the pressure–temperature conditions of 1025°C to 1080°C and 3.6 to 4.8 GPa recorded by the eclogite xenoliths, its association with Ba-Sr minerals suggests its formation via reaction of the host eclogites with kimberlite melt. This is supported by the similarity in 143Nd/144Nd ratios between bulk eclogite (0.51227–0.51249) and the host kimberlite at eruption, whereas clinopyroxene in part retains unradiogenic Sr (87Sr/86Sr = 0.70205 ± 0.00011) related to ancient depletion. The discovery of allanite in the Udachnaya eclogites demonstrates that this REE mineral can form when omphacite and grossular-rich garnet in eclogite breakdown in contact with REE- and alkali-rich carbonatite/kimberlite melt, and may be more common than hitherto recognized. Crystallization of allanite in the cratonic mantle eclogite reservoir may also help explain the difference in LREE abundances between the more strongly enriched carbonatite/kimberlite at depth and the final erupted product. It is likely that allanite is overlooked at eclogites xenoliths, while it is common accessory mineral, hosting REE in orogenic UHP/HP eclogites. Further studies are required to deciphered the peculiarities in metamorphic history recorded in eclogites xenoliths and orogenic eclogites, as well as the differences ancient (Archean/Proterozoic) and Phanerozoic subduction processes. [ABSTRACT FROM AUTHOR]

Pressure–temperature estimates of a xenolith found within a post-obduction granodiorite in southern New Caledonia provide evidence for subcrustal, granulite facies, peak crystallisation conditions (ca. 850 °C—8.5 ± 1.0 kbar), followed by isobaric cooling to 700 °C, and final decompression with partial rehydration at ca. 650 °C—3.5 kbar. The xenolith, dated at 24.7 Ma (U-Pb zircon), i.e., the same age as the granodiorite host rock, has low SiO2 (35.5 wt%) and high Al2O3 (33.2 wt%) contents, suggesting that it is the restite of a previous melting episode, while the elevated Ca (Ba and Sr) contents suggest mantle metasomatism. Although the concentrations of Rb, K, Ca, Ba, and Sr have been strongly modified, some geochemical (REE patterns and some "immobile" trace element ratios) and isotopic (Sr and Nd isotopic ratios, U-Pb zircon age) characteristics of the granulite facies xenolith are similar to those of the xenoliths found in other Late Oligocene intrusions in southern New Caledonia; therefore, this rock is interpreted to be related to an early magmatic episode. The rock protolith was emplaced and equilibrated at the base of the crust where it underwent ductile deformation. Younger ascending magma picked it up and they eventually crystallised together at a shallow crustal level, near the tectonic sole of the ophiolite. The recrystallisation and ductile deformation at ~8.5 kbar suggest that a rheological discontinuity existed at about 25–28 km, probably representing the Moho. It is concluded that a continental crust of normal thickness must have existed beneath New Caledonia at about 24 Ma, i.e., 10 Ma after obduction. [ABSTRACT FROM AUTHOR]

Sapphires crystals were identified in the corundum-bearing xenoliths in Miocene (Upper to Middle Sarmathian) andesites of the Slanské vrchy Mountains, eastern Slovakia at the Záhradné, Hubošovce and Vechec localities. The sapphire crystals occur in (1) micaceous xenoliths, which are built mostly of dark mica from annite-phlogopite series (biotite), K-feldspars, plagioclase with abundant inclusions of hercynite, ilmenite and Ti-rich magnetite, locally with pyroxenes of enstatite-ferosilite series at the localities Záhradné and Hubošovce. Less abundant are sapphires occurring in cordieritic xenoliths (2), dominantly consisting of cordierite, plagioclase and sillimanite with minor hercynite and ilmenite inclusions discovered in the Vechec quarry. Sapphires are dark blue to light blue with vitreous to diamond lustre and no visible pleochroic colour change forming mostly pseudohexagonal tabular, locally more complex euhedral to subhedral crystals up to 2.0 mm in size with triangular-shaped patterns on crystal faces. Raman spectroscopy showed characteristic corundum peaks at 419 cm-1 A1g mode and 384 cm-1 Eg mode with other peaks assigned to Al2O3 crystal vibrations. Chemical composition (EPMA, LA-ICP-MS) shows typical content of 98.12-99.60 wt. % Al2O3 with increased concentrations of Fe, Ti, Cr, V, Mg and Ga, locally also Na, Ca, K, B and Li. Genesis of corundum-bearing xenoliths interpreted from paragenetic observations and geochemical data shows clear metamorphic trend. Formation of sapphires was caused by incorporation of Al-rich precursor metasediments depleted in silica into the magmatic reservoir, which caused thermal overprint of the precursor mineral assemblage and led to the formation of high temperature mineral association suitable for corundum crystallization. Furthermore, sapphire crystals from the Hubošovce quarry exhibit spinel coronas, which are typically developed from destabilization of corundum during their retrograde development. [ABSTRACT FROM AUTHOR]

Clinopyroxenite xenoliths comprising cumulus clinopyroxene ± amphibole together with intercumulus phlogopite ± olivine ± apatite ± titanite form a large part of the xenolith load in the oldest deposits of the Rockeskyllerkopf Volcanic Complex (RVC) in the West Eifel volcanic field. The xenoliths also contain xenocrysts of olivine and clinopyroxene derived from mantle peridotite and clinopyroxene from lower crustal granulite. The clinopyroxenite xenoliths are divided into five groups on the basis of their modal mineralogy and mineral compositions. Groups 1 to 4 define a continuous compositional trend indicative of fractionation of a mafic alkaline magma. Group 5 xenoliths are compositionally distinct and have been tentatively linked to high pressure crystallization of phonolitic magma within the RVC system. Thermobarometry of the group 1 to 4 xenoliths indicates that they crystallized between 1 and 4 kilobars, equivalent to a depth of 4 to 14 km. Group 1 to 3 xenoliths all crystallized at between 1050°C and 1150°C, whereas the amphibole-rich group 4 xenoliths give temperature estimates of ~900°C. The clinopyroxenites share a common parent magma with clinopyroxene–phlogopite veins found in subcontinental lithospheric mantle xenoliths. However, the vein forming mama was richer in incompatible elements, in particular Zr and Hf and is interpreted to be an early formed batch of magma with the clinopyroxenites crystallizing from magma derived from the same mantle, which had been depleted by the earlier phase of melting. Intrusion of magma began around 155 ky prior to the eruption of the RVC. Fe–Mg interdiffusion profiles in zoned clinopyroxene show that the magma that formed the xenoliths was present in the crust for up to 28 ky prior to the eruption. However, most samples give interdiffusion times between 1.5 and 9.9 ky. Based on xenocryst residence times and the calculated P–T conditions for clinopyroxene, there were at least seven separate batches of magma emplaced below Rockeskyllerkopf, probably as sills. [ABSTRACT FROM AUTHOR]

Mantle rocks usually contain rare earth elements (REEs) in very low concentrations. Here, we document an occurrence of monazite associated with REE-rich apatites in a carbonate-bearing wehrlite xenolith from central Vietnam. The xenolith displays an equigranular matrix of rounded olivine grains while texturally primary orthopyroxene, clinopyroxene and spinel are notably absent. Scattered within the olivine matrix, two types of domains are present: domain-I contains blocky clinopyroxene grains within a matrix of quenched silicate melt and is associated with a second generation of olivine, small euhedral spinel, and rare grains of carbonates. Domain-II contains irregularly shaped patches of carbonate associated with silicate glass, secondary olivine, spinel, and clinopyroxene. Monazite and apatite occur only in domain-I: very small rounded to elongate monazite I grains are included in primary olivine, partly crosscut by fine glass veinlets, monazite II as large grains up to 300 × 200 μm in size and monazite III as small euhedral and needle-like crystals in silicate glass pools. Apatite I forms lath-shaped to rounded crystals up to 200 × 50 μm in size, whereas apatite II is present within silicate melt pools where it forms euhedral needle-like to equant grains. Monazites show compositional variation mainly with respect to ∑REE2O3 (63–69 wt %) and ThO2 (1.1–5.3 wt %) and only minor variations in P2O5 (29–32 wt %), SiO2 (<0.05–0.4 wt %), and CaO (0.2–0.4 wt %). Apatites are characterized by strongly variable and high REE2O3 and SiO2 contents (4–27 wt % ∑REE2O3, 0.6–6.8 wt % SiO2) as well as with significant Na2O (0.3–1.5 wt %), FeO (0.1–1.8 wt %), MgO (0.2–0.6 wt %) and SrO (0.2–0.9 wt %) contents. F and Cl contents are in the range 1.9–3.0 wt % and 0.2–0.8 wt %, respectively. The textures observed in this wehrlite xenolith are thought to be the result of an interaction of depleted (harzburgitic) mantle with cogenetic silicate and carbonatite melts formed by fractionation-driven immiscibility within a parental SiO2 undersaturated melt characterized by high P, CO2, and REE contents. The immiscibility occurred in the shallow subcontinental lithosphere at T of 700–800 °C and a depth of ~30 km and the melt–rock interaction occurred in two successive and most likely nearly simultaneous events: an initial stage of metasomatism was triggered by the P-REE-CO2-rich agent with low aH2O resulting in the co-precipitation of carbonates as patches and along micro-veins and of phosphates in a peridotitic assemblage. A second stage is characterized by pervasive infiltration of an alkali-rich basaltic melt into the carbonate + phosphate-bearing assemblage. Based on 232Th and 208Pb contents of monazite, a young age of ~2 Ma can be calculated for the timing of the monazite-forming metasomatic imprint. Based on 39Ar-40Ar extrusion ages of the xenolith-hosting alkali basalts of 2.6–5.4 Ma, this indicates that both carbonatite and basaltic melt infiltration must have occurred no more than a few hundred thousand years before extraction of the xenolith to the surface. [ABSTRACT FROM AUTHOR]

Abstract Zircon grains from the metasedimentary lower crust of the Rio Grande Rift, New Mexico, preserve a metamorphic record of the transition from Laramide compression to Eocene extension. Zircon U‐Pb isotopes and trace‐element concentrations from five two‐pyroxene metaigneous granulite xenoliths define discrete populations: older zircon cores (∼15–50 Ma) that are depleted in heavy rare‐earth elements (HREE) but Ti‐rich, and younger zircon rims (∼3–15 Ma) with elevated HREE and lower Ti concentrations. Coupled phase equilibria and garnet‐melt‐zircon trace‐element partitioning calculations show that the older zircon cores equilibrated in thick (>40 km), hot (800–900°C), garnet‐bearing lower crust during the cessation of compression at the end of the Laramide orogeny. Zircon rim domains equilibrated at lower pressures, consistent with >9 km of thinning of the lower crust. Thermal‐kinematic calculations show that these pressure‐temperature‐time constraints require thinning of the lithospheric mantle prior to and during regional Cenozoic extension. Convective erosion of the mantle lithosphere over tens of millions of years, possibly facilitated by dynamics of the Farallon slab, provides a mechanism to facilitate lower crustal heating and extension.

The detection and quantification of metal sulfides in host rocks by electrical measurements have been priorities for field and laboratory studies, motivated by mineral prospecting and fundamental interest in the mantle structure or core/mantle differentiation, among other reasons. Here, we reanalyze electrical data for a dunite host with added FeS or Fe-S-Ni (Saxena et al. 2021), and report additional experimental runs along with electron microprobe analyses. The applied pressure was 2 GPa; impedance spectra were acquired while annealing at 1023 K (below the metal-sulfide solidus), and while varying temperature from 570 to 1650 K. Addition of 6.5 or 18 vol% FeS strongly enhances conductivity of the bulk sample compared with that of the dunite host, though values are 100–100 000 times less than those of pure FeS. These results indicate that most metal sulfide content is not part of a viable conductive path, even for the 18 vol% quantity. Nevertheless, the relatively high conductivity and weak temperature dependence of the 18 vol% sample reveal that contiguous paths of solid or molten FeS span the electrodes. The sample with 6.5 vol% sulfide also exceeds the percolation threshold for temperatures as low as ~100 K below the eutectic melting point, likely because FeS softens. Conductivity is nearly unchanged upon crossing the eutectic temperature, however a decline over 1400–1500 K reveals that the 6.5 vol% molten FeS forms a fragile electrical network in dunite. Samples with Fe50S40Ni10 or Fe40S40Ni20 (at%) are less conductive than pure dunite at temperatures below ~1450 K. This surprising result, likely caused by a reducing influence of Fe or Ni metal, does not support the use of FeS as an analog for compositions with nickel or excess metal. Our findings suggest that probing the electrical network of metal sulfides as solids complements other studies focused on connectivity of molten metal sulfides. [ABSTRACT FROM AUTHOR]

We present constraints on the hydration state and rheology of the lithospheric mantle beneath the North Anatolian fault zone (NAFZ). Peridotite xenoliths from the Biyikali and Çorlu volcanic centers record deformational microstructures consistent with shearing in a lithosphere‐scale transcurrent fault system. Analysis by Fourier transform infrared spectroscopy indicates that nominally anhydrous phases retain some OH−, but bulk rock concentrations are generally restricted to <50 ppm H2O by weight. From the rock microstructure, we determined differential stress magnitude and active deformation mechanism(s); combined with estimates of hydration state, we constrained the rheology. Recrystallized grain size piezometry shows that the mantle beneath the NAFZ sustained differential stresses of 10–20 MPa, largely independent of depth. The dominant deformation mechanism(s) change with depth; xenoliths extracted from shallower depths record evidence for grain size‐sensitive creep possibly in the presence of melt. At intermediate depths, both dislocation creep and grain size‐sensitive mechanisms were active, and we did not observe evidence for deformation in the presence of melt. The deepest samples were dominated by dislocation creep. The strong temperature sensitivity of creep mechanisms, combined with the low variability in differential stress, contributes to a stratified viscosity profile ranging from 1018 Pa s for the deepest samples to >1022 Pa s at shallower depths (assuming a melt‐free rheology). Although difficult to quantify from the rock record, melt likely reduced the viscosity of the shallow lithospheric mantle. Vertical stratification in viscosity beneath the NAFZ, the result of melt‐present deformation and/or transitions in the dominant deformation mechanism, has important consequences for the seismic cycle of strike‐slip fault systems. Plain Language Summary: Plate boundary‐scale transcurrent faults, systems in which one lithospheric plate is translated laterally relative to another, may transect the entire lithosphere. However, we lack sufficient understanding of how rocks in the upper mantle beneath such fault systems deform, and the influence of these rocks on seismicity in the crust. Here, we study rocks from the lithospheric mantle beneath the North Anatolian fault zone (NAFZ) that have been entrained in ascending magma and erupted at the surface. These rocks offer a glimpse as to how the lithospheric mantle behaves beneath a seismically active plate boundary‐scale fault. We show that the hydration state (i.e., how much water is contained in the constituent minerals) and rheologic structure of the lithospheric mantle beneath the NAFZ is more complex than commonly assumed in seismic models, and that melt likely plays an important role in weakening the uppermost mantle. Key Points: Bulk rock hydration is generally restricted to <50 ppm H2O by weight and differential stresses lie within the range of 10–20 MPaDominant deformation mechanism(s) change with increasing depth from grain size‐sensitive creep, possibly in the presence of melt, to dislocation creepCalculated viscosities span several orders of magnitude. Changing deformation mechanisms and melt reduce the viscosity of the uppermost mantle [ABSTRACT FROM AUTHOR]

Abstract We present constraints on the hydration state and rheology of the lithospheric mantle beneath the North Anatolian fault zone (NAFZ). Peridotite xenoliths from the Biyikali and Çorlu volcanic centers record deformational microstructures consistent with shearing in a lithosphere‐scale transcurrent fault system. Analysis by Fourier transform infrared spectroscopy indicates that nominally anhydrous phases retain some OH−, but bulk rock concentrations are generally restricted to 1022 Pa s at shallower depths (assuming a melt‐free rheology). Although difficult to quantify from the rock record, melt likely reduced the viscosity of the shallow lithospheric mantle. Vertical stratification in viscosity beneath the NAFZ, the result of melt‐present deformation and/or transitions in the dominant deformation mechanism, has important consequences for the seismic cycle of strike‐slip fault systems.

Numerous mantle xenoliths 6–12 cm in size and sub-angular to rounded in shape occur within Mio-Pliocene basanite lavas of the monogenic volcano of Hosséré Sédé in the Adamawa plateau. Xenoliths of spinel lherzolite exhibit protogranular, equigranular or porphyroclastic texture. Microprobe chemical analyses show that olivine is highly magnesian (Fo88-90), clinopyroxene crystals are diopside and augite (Wo41.6–49.6 En45.3–53.7 Fs4.2–6.2), orthopyroxene crystals are enstatite (Wo1.4–1.5 En88.6–89.0 Fs9.6–9.9) and spinel crystals are mainly Al-spinel associated to minor Cr-spinel. Estimated temperatures and pressures through empirical formulas show that Hosséré Sédé xenoliths have equilibrated between 1085 and 1204 °C and 1.08 to 1.57 GPa, corresponding to sampling depths of 36 and 52 km. Detailed petrographical and mineral chemistry of Hosséré Sédé xenoliths evidences the complex nature and composition of the subcontinental lithosphere under the Adamawa plateau. This may involve a probable uplift of the whole area after a limited extensional event, possible metasomatism through melt infiltration during shearing of the lithospheric mantle along the Pan African strike-slip fault system. [ABSTRACT FROM AUTHOR]

Fluorine-rich granites and rhyolites occur throughout the southern Rocky Mountains, but the origin of F-enrichment has remained unclear. We test if F-enrichment could be inherited from ancient mafic lower crust by: (1) measuring amphibole compositions, including F and Cl contents, of lower crustal mafic granulite xenoliths from northern Colorado to determine if they are unusually enriched in halogens; (2) analyzing whole-rock elemental and Sr, Nd, and Pb isotopic compositions for upper crustal Cretaceous to Oligocene igneous rocks in Colorado to evaluate their sources; and (3) comparing batch melting models of mafic lower crustal source rocks to melt F and Cl abundances derived from biotite data from the F-rich silicic Never Summer batholith. This approach allows us to better determine if the mafic lower crust was pre-enriched in F, if it is concentrated enough to generate F-rich anatectic melts, and if geochemical data support an ancient lower crustal origin for the F-rich rocks in the southern Rocky Mountains. Electron microprobe analyses of amphibole in lower crustal mafic granulite xenoliths show they contain 0.56–1.38 wt% F and 0.45–0.73 wt% Cl. Titanium in calcium amphibole thermometry indicates that the amphiboles equilibrated at high to ultrahigh temperature conditions (805 to 940 °C), and semiquantitative amphibole thermobarometry indicates the amphiboles equilibrated at 0.5 to 1.0 GPa prior to entrainment in magmas during the Devonian. Mass balance calculations, based on these new measurements, indicate parts of the mafic lower crust in Colorado are at least 3.5 times more enriched in F than average mafic lower crust. Intrusions coeval with the Laramide Orogeny (75 to 38 Ma) pre-date F-rich magmatism in Colorado and have Sr and Nd isotopic compositions consistent with mafic lower crust ± mantle sources, but many of these intrusions contain elevated Sr/Y ratios (>40) that suggest amphibole was a stable phase during magma generation. The F-rich igneous rocks from the Never Summer igneous complex and Colorado Mineral Belt also have Sr and Nd isotopic compositions that overlap with the lower crustal mafic granulite xenoliths, but they have lower Sr/Y, higher Nb and Y abundances, and distinctly less radiogenic 206Pb/204Pbi compositions than preceding Laramide magmatism. Batch melt modeling indicates low-degree partial melts derived from rocks similar to the mafic lower crustal xenoliths we analyzed can yield silicic melts with >2000 ppm F, similar to estimated F melt concentrations for silicic melts that are interpreted to be parental to evolved leucogranites. We suggest that F-rich silicic melts in the southern Rocky Mountains were sourced from garnet-free mafic lower crust, and that fluid-absent breakdown of amphibole in ultrahigh temperature metamorphic rocks was a key process in their generation. Based on the composition of high-F amphibole measured from lower crustal xenoliths, the temperature of amphibole breakdown and melt generation for these F-enriched source rocks is likely >100 °C higher than similar lower crust with low or average F abundances. As such, these source rocks only melted during periods of unusually high heat flow into the lower crust, such as during an influx of mantle-derived magmas related to rifting or the post-Laramide ignimbrite flare-up in the region. These data have direct implications for the genesis of porphyry Mo mineralization, because they indicate that pre-enrichment of F in the deep crust could be a necessary condition for later anatexis and generation of F-rich magmas. [ABSTRACT FROM AUTHOR]

Pressure–temperature estimates of a xenolith found within a post-obduction granodiorite in southern New Caledonia provide evidence for subcrustal, granulite facies, peak crystallisation conditions (ca. 850 °C—8.5 ± 1.0 kbar), followed by isobaric cooling to 700 °C, and final decompression with partial rehydration at ca. 650 °C—3.5 kbar. The xenolith, dated at 24.7 Ma (U-Pb zircon), i.e., the same age as the granodiorite host rock, has low SiO2 (35.5 wt%) and high Al2O3 (33.2 wt%) contents, suggesting that it is the restite of a previous melting episode, while the elevated Ca (Ba and Sr) contents suggest mantle metasomatism. Although the concentrations of Rb, K, Ca, Ba, and Sr have been strongly modified, some geochemical (REE patterns and some “immobile” trace element ratios) and isotopic (Sr and Nd isotopic ratios, U-Pb zircon age) characteristics of the granulite facies xenolith are similar to those of the xenoliths found in other Late Oligocene intrusions in southern New Caledonia; therefore, this rock is interpreted to be related to an early magmatic episode. The rock protolith was emplaced and equilibrated at the base of the crust where it underwent ductile deformation. Younger ascending magma picked it up and they eventually crystallised together at a shallow crustal level, near the tectonic sole of the ophiolite. The recrystallisation and ductile deformation at ~8.5 kbar suggest that a rheological discontinuity existed at about 25–28 km, probably representing the Moho. It is concluded that a continental crust of normal thickness must have existed beneath New Caledonia at about 24 Ma, i.e., 10 Ma after obduction.

We present data for lithospheric mantle xenoliths sampled from two alkali basalts in south‐central Vietnam, Pleiku and Xuan Loc, including fertile spinel peridotites. To better determine the origins of the Indochinese subcontinental lithospheric mantle (SCLM), including impacts of posited tectonic extrusion, we present major and trace elements, and 87Sr/86Sr, 143Nd/144Nd, 176Hf/177Hf, 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb in xenolith mineral separates. Most peridotites from Pleiku and Xuan Loc have fertile major element compositions, "depleted" and "spoon‐shaped" rare earth element (REE) patterns, interpreted to record prior melt depletion followed by melt metasomatism, and variable but generally depleted isotopic signatures (e.g., 87Sr/86Sr = 0.70238–0.70337 and 143Nd/144Nd = 0.512921–0.514190). A small group of refractory peridotites have "enriched" REE patterns suggesting more extensive metasomatism and enriched isotope ratios (87Sr/86Sr = 0.70405 and 143Nd/144Nd = 0.512755–0.512800). The presence of both fertile and refractory xenoliths records a heterogeneous SCLM beneath Vietnam. Based on geothermobarometry calculations, fertile xenoliths have equilibrium temperatures of 923–1,034°C and pressures of 11.7–15.8 kbar, while refractory xenoliths have comparable temperatures of 923–1,006°C, but lower pressures of 7.1–10.0 kbar, suggesting refractory rocks are dominantly present at shallower depths. We suggest that the lithospheric mantle has experienced variable melt extraction around 1.0–1.3 Ga, producing heterogeneous major element compositions. While we cannot rule out partial removal and replacement of the lithosphere, large‐scale delamination is not necessary to explain observed characteristics. The entire SCLM was more recently metasomatized by melts resembling Cenozoic basalts, suggesting recent asthenospheric melting has modified the SCLM by melt infiltration. Key Points: Lithospheric mantle xenoliths in Vietnam record a heterogeneous subcontinental lithospheric mantle (SCLM) beneath Indochina, containing fertile and refractory compositionsRefractory xenoliths overall derive from shallower depths than fertile xenoliths, suggesting a chemically stratified Indochinese SCLMThe SCLM was metasomatized by melts resembling Cenozoic basalts, suggesting only one melt infiltration event that does not require delamination [ABSTRACT FROM AUTHOR]

Abstract We present data for lithospheric mantle xenoliths sampled from two alkali basalts in south‐central Vietnam, Pleiku and Xuan Loc, including fertile spinel peridotites. To better determine the origins of the Indochinese subcontinental lithospheric mantle (SCLM), including impacts of posited tectonic extrusion, we present major and trace elements, and 87Sr/86Sr, 143Nd/144Nd, 176Hf/177Hf, 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb in xenolith mineral separates. Most peridotites from Pleiku and Xuan Loc have fertile major element compositions, “depleted” and “spoon‐shaped” rare earth element (REE) patterns, interpreted to record prior melt depletion followed by melt metasomatism, and variable but generally depleted isotopic signatures (e.g., 87Sr/86Sr = 0.70238–0.70337 and 143Nd/144Nd = 0.512921–0.514190). A small group of refractory peridotites have “enriched” REE patterns suggesting more extensive metasomatism and enriched isotope ratios (87Sr/86Sr = 0.70405 and 143Nd/144Nd = 0.512755–0.512800). The presence of both fertile and refractory xenoliths records a heterogeneous SCLM beneath Vietnam. Based on geothermobarometry calculations, fertile xenoliths have equilibrium temperatures of 923–1,034°C and pressures of 11.7–15.8 kbar, while refractory xenoliths have comparable temperatures of 923–1,006°C, but lower pressures of 7.1–10.0 kbar, suggesting refractory rocks are dominantly present at shallower depths. We suggest that the lithospheric mantle has experienced variable melt extraction around 1.0–1.3 Ga, producing heterogeneous major element compositions. While we cannot rule out partial removal and replacement of the lithosphere, large‐scale delamination is not necessary to explain observed characteristics. The entire SCLM was more recently metasomatized by melts resembling Cenozoic basalts, suggesting recent asthenospheric melting has modified the SCLM by melt infiltration.

Study of xenoliths of crustal rocks from the Novinka kimberlite pipe (Upper Muna kimberlite field) showed that the crust at different levels is composed of pyroxene and garnet–pyroxene crystal schists and garnet–pyroxene gneisses. The exsolution structures in pyroxenes and amphiboles and the garnet rims around ilmenite and pyroxene grains indicate that the mineral assemblages of crystal schists formed upon cooling of rocks at a constant pressure. The PT equilibrium conditions indicate that garnet–pyroxene crystal schists are present in the middle crust (P = 7–8 kbar), whereas garnet–pyroxene gneisses could be constituent rocks of the lower crust (P = 9–10.1 kbar). Sodalite is identified for the first time in xenoliths of the crystal schists, which is evidence of the presence of highly saline brines at the final stages of cooling of rocks. The U–Pb age of zircons indicates a Neoarchean (2.7 Ga) tectono-thermal event, which was accompanied by partial melting of the crust. The stage of 1.9 Ga is weakly manifested in garnet–pyroxene gneisses. Our data support an earlier conclusion on vertical and lateral crustal heterogeneity of the Yakutsk diamondiferous province and the absence of a correlation between the reworking degree of the crust and its spatial location relatively to the main collision zones of the Siberian Craton. [ABSTRACT FROM AUTHOR]

Anorthite-diopside-spinel-forsterite xenoliths occur in basaltic lava flow in northern part of the Pannonian basin in the Novohrad-Gemer Volcanic Field (NGVF). Several lines of geochemical evidence suggest that the typical skarn assemblage originated by multiple carbonatitic and alkaline silicate metasomatism of pristine basalt to picrobasalt protolith rather than by the modification of crustal carbonates around shallow magma chambers. Interaction with mantle-derived magnesite or dolomite carbonatite melt triggered the crystallization of neoformed forsterite at the expense of primary clinopyroxene. The alkaline silicate metasomatism resulted in the replacement of primary diopside and augite by neoformed aegirine and aegirine-augite accompanied by amphiboles (richterite, katophorite, arfvedsonite). Weakly metasomatised Group 1 xenoliths retained their fore-arc magmatic geochemical signatures: flat REE patterns, low concentrations of high-field strength elements (Ti, P, Zr) and Ni compared to MORB, low ratios of highly-to-less incompatible elements (Ti/V, Zr/Y) relative to MORB, low Na 2 O, low total REE, and supra-subduction affinity of primary clinopyroxenes, being thus analogues to fore-arc basalts (FAB). In contrast, strongly metasomatised Group 2 xenoliths exhibit LREE-enriched patterns sub-parallel with those of host trachybasalts. Some Group 1 xenoliths also exhibit large positive Eu anomalies and strongly depleted signatures diagnostic of low-pressure plagioclase-pyroxene cumulates. One xenolith with complementary negative Eu, Sr and positive Rb anomalies has been identified as high-Mg basaltic andesite melt residuum. Sr and Nd isotopes plot along the OIB-MORB array. The Group 1 converges at initial 87Sr/86Sr and 143Nd/144Nd ratios of 0.7038–0.7041 and 0.5127–0.5128, respectively, thus recording a 140–190 Ma old mantle source. Thermobarometry data on metasomatically unmodified pyroxenes have been arranged along decompression cooling trend, propagating from ∼1230 °C and ∼ 1 GPa and ending at ∼1160 °C around atmospheric pressure. The maximum pressure indicates the melt generation zone ∼35 km deep, reflecting thin lithospheric mantle. The FABs identified for the first time in the AlCaPa megaunit corroborate the affinity of the NGVF lithospheric segment to Dinarides. • Skarnoid xenoliths are identified as forearc basalts that underwent metasomatism. • Sr Radiogenic isotopes are indicating Jurassic ages of formation. • The signature of the xenoliths is contrasting the back-arc position of the host. • The xenoliths were formed near the Dinarides in a forearc setting. • Their hodiern position is related to the geodynamic evolution of the area. [ABSTRACT FROM AUTHOR]

Granulite xenoliths, sourced from the lower cratonic crust and brought to the Earth's surface within volcanic rocks, offer unique insights into the formation and destruction of cratons. Age determinations are essential for constructing the thermal history of cratons. However, accurate dating is complicated by the xenoliths' extended exposure to high temperatures and their complex thermal histories. Specifically, the effects of extended lower crust residency and entrainment heating on coupled Sm–Nd and Lu–Hf systematics remain to be explored. Six xenoliths hosted in the Mesozoic dioritic porphyry from the eastern North China Craton were sampled from center to edge using micro-drilling/sawing techniques. From each xenolith, two to four concentric zones were successfully extracted for coupled Lu–Hf and Sm–Nd geochronological analysis. These xenoliths predominantly exhibit a granoblastic texture, with minerals that are mostly unzoned. Textural and chemical analyses of minerals indicate a decompression process following the granulite-facies metamorphism, with subsequent multiple reheating events. Thermobarometric calculations suggest granulite-facies equilibration at temperatures of ∼750 °C and pressures of 1.2–2.0 GPa. Consistent Lu–Hf and U–Pb ages of ca. 1.7 Ga, obtained from garnet, zircon, and titanite in chemical and textural equilibrium, are attributed to granulite-facies metamorphism. The Paleoproterozoic Lu–Hf ages and the Neoproterozoic Sm–Nd dates are indistinguishable across all concentric zones within each xenolith, whereas Sm–Nd ages are consistently ∼1.0 Ga younger than the corresponding Lu–Hf ages. This discrepancy suggests a complex history for this section of the lower cratonic crust, marked by a granulite-facies metamorphic event around 1.7 Ga, followed by prolonged cooling. A significant heating event during the Tonian, likely due to regional mantle plume activity, is inferred to have partially or completely reset the Sm–Nd isochrons. This thermal event and subsequent thermal relaxation on the samples at different residence depths thereby decoupled the Sm–Nd system from the Lu–Hf system, signifying a thermal pulse at the southern margin of North China Craton. The Mesozoic entrainment heating event appears to have had minimal impact on the Lu–Hf/Sm–Nd systematics of garnet and U–Pb of zircon/titanite. Our data highlight the potential of coupled microsampling Lu–Hf and Sm–Nd geochronology on individual xenoliths to reveal tectonic episodes within the lower crust that might be undetected by accessory mineral U–Pb geochronology. [ABSTRACT FROM AUTHOR]

• Zircon and titanite in xenoliths document Eocene metasomatism of the Wyoming craton lower crust. • Metasomatism by hydrous and carbonated fluids linked to subduction of the Shatsky conjugate and Farallon slab beneath the northern margin of the craton. • Metasomatism, slab foundering, and heating triggered partial loss of the Wyoming craton lithosphere. New zircon and titanite U-Pb and trace-element data from minette- and kimberlite-hosted lower crustal xenoliths from near the northern margin of the Wyoming craton document multiple periods of metasomatism from 62 to 50 Ma. Metasomatism was either coincident or preceded eruption of the host magmas by up to 14 Myr (eruption dated at 52 Ma and 48 Ma based on new zircon U-Th/He data). Thus, fluid/melt interactions were not solely related to magmatic entrainment and eruption. Accessory mineral trace element data reveal metasomatism by fluids from multiple sources, including water and carbonatite potentially sourced from the subducted Farallon slab. These metasomatic episodes are interpreted to reflect pan-lithospheric hydration events that weakened the deep cratonic lithosphere and facilitated partial removal following rollback and foundering of the Farallon plate. The new data corroborate previous suggestions that subduction-related metasomatism may be a key requisite for the destruction of otherwise stable cratons. [ABSTRACT FROM AUTHOR]

This study focuses on the ability of spinel crystals in ultramafic xenoliths, like zircon, to store the primary composition and structure when exposed to magma fluid flows or the thermal heating of ultramafic fragments in magma flows during volcanic eruptions. Mantle ultramafic rock xenoliths from explosives of the Avacha Volcano in Kamchatka have different facies of their metasomatic changes above the magma chamber. An alternative hypothesis of their source is from fragments of layered mafic intrusions containing spinels, which store initial petrogenetic records of the original magma melt. Experiments were conducted to prove this idea using a Budker Institute of Nuclear Physics (INP SB RAS) unit. Large scales of change in the composition and structure of spinel crystals were caused by the hot fluids from melted xenolith sites.

Complex multiphase reaction rims that form during garnet breakdown are known as kelyphite coronae and are common amongst exhumed mantle xenoliths. It has long been established that a reaction of garnet and olivine produces kelyphite corona consisting of spinel and pyroxenes, and that preservation of high-pressure garnet cores requires sufficiently rapid uplift of material through the spinel lherzolite stability field from depths of at least 60 km. We present new high-pressure, high-temperature experiments of garnet breakdown in the spinel–lherzolite stability field demonstrating that a series of cascading reactions can reproduce the multilayer, multiphase kelyphites seen in nature. In all experiments where breakdown occurred, a melt appears to have moderated the reactions towards equilibrium; we believe this to be the first experimental confirmation of the importance of such melts in garnet breakdown reactions. In our experiments at least three distinct zones of concentric kelyphite growth can occur at a single pressure, temperature condition; we suggest, therefore, that such kelyphites seen in natural samples do not have to be caused by a multistage uplift path as is often assumed. Kelyphitic coronae surrounding garnet have previously been used to estimate uplift rates; however, the lack of kinetic data for relevant exhumation reactions has limited their use for PTt pathway estimations and the understanding of emplacement mechanisms. In order to constrain accurate PTt pathways we use reaction rim thickness as a proxy for reaction progress and present preliminary results for the kinetics of garnet breakdown. [ABSTRACT FROM AUTHOR]

An alkaline volcanic activity with a relative Plio-Quaternary age (based on the succession of layers on the field) occurred in the northern part of Uromieh Dokhatar magmatic belt, Iran. Hornblendite xenoliths mostly displaying cumulitic texture occur in the trachyandesites from this magmatic episode. The thermobarometric results indicate that these xenoliths formed within the crust. Based on mineralogical and chemical characteristics, these xenoliths are divided into two groups. In Group 1, plagioclase (andesine) modal content is less than 10% while amphibole (magnesiohastingsite) and biotite are the main minerals. In Group 2, plagioclase (labradorite) modal content is higher than 20% while amphibole (pargasite) and biotite are the main minerals. Positive anomalies of U, Ba, Ti, and K in hornblendite xenoliths are probably related to the accumulation process of amphibole and biotite. The study of the chemical composition of amphiboles indicates crystallization of amphibole from Group 1 and Group 2 hornblendite xenoliths and host volcanic rock under high oxidation conditions. Hornblendite xenoliths display high LREE/HREE and LREE/MREE ratios. The $^{86}$Sr/$^{87}$Sr ratios in xenoliths of Groups 1 and 2 and host volcanic rocks are 0.706291, 0.704685, and 0.709545, respectively, while $^{143}$Nd/$^{144}$Nd ratios are 0.512580, 0.512736, and 0.512561, respectively. Group 1 xenoliths are characterized by a negative Eu anomaly while this anomaly is positive for Group 2. Considering the REE concentrations and distribution patterns, the major element compositions of amphibole, biotite, and plagioclase as well as the Sr and Nd isotopic ratios, it appears that the parental melts of Group 1 and Group 2 xenoliths differ in composition. Those two theoretical parental melts also have different compositions when compared to that of the host volcanic rock. The studied alkaline rocks are commonly located along the main faults. Therefore it seems that the movement of the main faults (especially North Tabriz Fault, North Misho and Tasuj faults) has provided a path for the lavas carrying the investigated xenoliths to penetrate the continental crust. Taken together the geochemical characteristics of the studied rocks indicate a mantle source variably metasomatized by a subduction event prior to the collision between Eurasian and Arabian continents.

The polymictic breccias, extremely rare mantle conglomerates being the fragments of rocks and large mantle minerals cemented with fine-grained mass have been long provoking interest of petrologists worldwide. This work provides a comparative analysis of two xenoliths of polymictic rocks from kimberlite pipes of different age and productivity occurring in the Siberian craton. The similarity of the chemical composition of some minerals of polymictic breccias from kimberlites within different parts of the Siberian craton assumes possible formation of these minerals caused by the same factors in the prekimberlite period of these parts of the lithospheric mantle. A wide range of compositions and chaotic zoning of minerals, the presence of exsolution textures in orthopyroxene, ilmenite, sulfide, and kelyphite rims on garnet suggest that the fragments of the studied xenoliths are unbalanced. Sample SH18/20 is the first polymictic breccia showing asthenospheric melt-fluids sampling shallow depths of the spinel facies.

The tectonic history of southern Africa includes Archean craton formation, multiple episodes of subduction and rifting and some of the world's most significant magmatic events. These processes left behind a compositional trail that can be observed in xenoliths and measured by geophysical methods. The abundance of kimberlites in southern Africa makes it an ideal place to test and calibrate mantle geophysical interpretations that can then be applied to less well‐constrained regions. Magnetotellurics (MT) is a particularly useful tool for understanding tectonic history because electrical conductivity is sensitive to temperature, bulk composition, accessory minerals, and rock fabric. We produced three‐dimensional MT models of the southern African mantle taken from the SAMTEX MT data set, mapped the properties of ∼36,000 garnet xenocrysts from Group I kimberlites, and compared the results. We found that depleted regions of the mantle are uniformly associated with high electrical resistivities. The conductivity of fertile regions is more complex and depends on the specific tectonic and metasomatic history of the region, including the compositions of metasomatic fluids or melts and the emplacement of metasomatic minerals. The mantle beneath the ∼2.05 Ga Bushveld Complex is highly conductive, probably caused by magmas flowing along a lithospheric weakness zone and precipitating interconnected, conductive accessory minerals such as graphite and sulfides. Kimberlites tend to be emplaced near the edges of the cratons where the mantle below 100 km depth is not highly resistive. Kimberlites avoid strong mantle conductors, suggesting a systematic relationship between their emplacement and mantle composition. Plain Language Summary: The present‐day composition of Earth's tectonic plates results from past geological processes. We can learn about Earth's composition from deep rock samples that are carried to the surface during volcanic eruptions and by probing its physical properties, like electrical conductivity, with geophysics. In southern Africa, there are extensive deep rock samples, which have been brought to the surface by kimberlite volcanoes that also host diamonds, and also extensive geophysical data. In this article, we compare the rock compositions with electrical conductivity to learn more about Earth's composition. Our results show that the oldest parts of the plates, which retain compositions similar to their initial composition, appear resistive. On the other hand, regions that have been intruded by deep fluids or molten rock can be resistive or conductive, depending on the types of minerals that were formed during the intrusion. The kimberlite volcanoes mostly erupted through the edges of the most resistive parts of the plates and did not erupt through the conductors. These results will help us to make more accurate interpretations about the composition of parts of the Earth where we do not have deep rock samples. Key Points: Electrical conductivities from 3D magnetotelluric models in southern Africa are compared with compositions from garnet xenocrystsDepleted regions are electrically resistive. All conductors are in metasomatized regions but the style of metasomatism affects conductivityKimberlites tend to surround resistors while avoiding strong conductors, suggesting mantle composition is a control on magmatism [ABSTRACT FROM AUTHOR]

Osmium isotopes, whole rock and mineral geochemical data from peridotite xenoliths from two Miocene McMurdo Volcanic Group cinder cones in the Transantarctic Mountains (TAM) in Southern Victoria Land, Antarctica, reveal that the underlying mantle preserves evidence for major mid-Proterozoic lithosphere formation despite the crust being dominated by late Neoproterozoic-Ordovician (~0.65–0.47 Ga) rocks. The Hooper Crags xenolith suite is dominated by harzburgites with highly refractory olivine Mg# (up to 92.3) and depleted bulk rock major and platinum group element + Re abundances, with 187Os/188Os ratios indicating depletion in the mid-Proterozoic. Pipecleaner Glacier xenoliths, 18 km distant, are lherzolites with olivine Mg# (<91) and fertile major and platinum group element abundances, with Os isotope abundances defining an aluminochron that also indicates mid-Proterozoic depletion. Although exposed crust along this portion of Antarctica reveals only minor evidence for Proterozoic magmatism, the major episode of lithosphere formation indicated by the Os isotope data is supported by published bulk rock Sm-Nd isotope and zircon εHf mantle model ages of Neoproterozoic to Ordovician plutonic rocks. The heterogeneous circum-cratonic mid-Proterozoic mantle under Southern Victoria Land has therefore persisted on a Ga timescale, including through the formation and destruction of Rodinia and Gondwana supercontinents as well as extensive crustal melting and emplacement of the Ferrar large igneous province. This longevity may be due to the thick (>250 km) East Antarctic Craton lithosphere shielding the immediately adjacent circum-cratonic mantle from being affected by convective asthenosphere-driven erosion. This contrasts with mantle lithosphere accreted distally to the East Antarctic Craton (represented by the now-detached Zealandia continent), which did not attain extreme thickness and has therefore been more susceptible to tectonic reworking and lateral translation. [ABSTRACT FROM AUTHOR]

This paper offers the first-ever study of a local chromitite stringer (~ 2 cm wide) enriched in platinum-group elements (PGE) from the Kevitsa Ni-Cu-PGE deposit. Chromite forming this chromitite is rich in Cr2O3 (29.06 ̶ 37.17 wt%) and FeO (FeOtotal = 45.88 ̶ 56.06 wt%) and low in MgO (0.04 ̶ 0.91 wt%). The chromitite host an unusual set of mineral inclusions including: (1) platinum-group minerals of the laurite-erlichmanite (RuS2-OsS2) solid solution series, irarsite (IrAsS), sperrylite (PtAs2), hollingworthite (RhAsS), anduoite [(Ru, Os)As2], kotulskite (PdTe) and As-, Te-, Os-, and Ru- bearing unidentified phases; (2) exsolution-lamellae of ilmenite and ulvöspinel along with chromite (111) and (100) planes, respectively; (3) silicates including crystallographically-oriented biotite, euhedral pyroxene and irregular-shaped amphiboles which tend to follow chromite growth planes, and (4) Fe-oxides, pyrrhotite, pentlandite, galena, barite, apatite, and native gold. Intercumulus minerals are microcrystalline Mg-rich biotite and minor amphibole. Pyroxenes (diopside) located immediately above the chromitite stringer exhibit higher CaO (23.02 ̶ 25.42 wt%) and Na2O (0.16 ̶ 0.55 wt%) than pyroxenes (augite) located below. We suggest that chromite saturation in the mafic melt was caused by local assimilation of a small pelitic xenolith. The xenolith also provided K and H2O explaining the occurrence of K-and volatile-rich intercumulus phases. Crystallization of chromite in this environment promoted preferential sequestering of Ir and Ru, and to a lesser extent Os, from the parental magma giving rise to an anomalous iridium-group-PGE (IPGE: Ir, Ru, and Os) enrichment. [ABSTRACT FROM AUTHOR]

The interpretation of low‐resistivity anomalies in the lithospheric mantle of several cratonic regions has invoked hydrogen, or connected networks of graphite with iron‐rich silicates, and/or metal sulfides. Electrical laboratory measurements are a powerful approach for exploring these alternatives. We report electrical measurements of two xenoliths (pyroxenite and dunite) from Tanzania; two metal sulfides (FeS and Fe‐S‐Ni); and several mixtures of metal sulfides (3.4–18.2 vol.%) with xenolith. A multi‐anvil press was employed to maintain a 2 GPa pressure and temperatures up to 1,627 K. The addition of 3.4 vol.% FeS to the pyroxenite or dunite matrix has little effect on bulk resistivity, particularly for T > 800 K. However, the resistivity drops dramatically–by factors of up to 1,000, depending on temperature–upon addition of 6.5 vol.% FeS in the dunite. Addition of 18.2% FeS causes a further decrease of the same magnitude relative to the 6.5% sample. Scanning electron microscope images do not reveal the formation of a connected FeS network as part of the decreased resistivity. Possible explanations for the apparently conflicting results include connections of the sulfide that are not imaged in the back‐scattered images, either because of limited resolution, or perhaps the inherent limitations of the 2‐D perspective. The complete data set (xenoliths, metal sulfides, and mixtures) was modeled with a modified version of Archie's law, and we find satisfactory agreement over a truncated temperature range. We conclude that the low‐resistivity anomalies in Tanzania, Kaapvaal, and Gawler cratons can be explained by the presence of a few vol.% of solid sulfide. Plain Language Summary: Some regions in the cold and stable parts of the Earth's upper mantle have exceptional electrical properties; they are very conductive. What causes these anomalies is not well understood, and one explanation could be the presence of iron‐sulfur compounds. Here we tested this hypothesis by performing electrical experiments under pressure and temperature on natural and synthetic mantle rocks. The amount of iron‐sulfur compound was varied to understand its effect on the electrical properties. Our results show that a small amount of iron‐sulfur compound can affect the electrical properties of the upper mantle dramatically. We then modeled our data to apply these results to the field. We conclude that the electrical anomalies in Africa and Australia can be explained with a few volume % of solid iron‐sulfur compound in the mantle rock. Key Points: The resistivity of sulfide‐xenolith samples at 2 GPa drops by a factor of up to 1,000 for an addition of FeS between 3.4 and 6.5 vol.%A fully connected network of the FeS phase is not necessary to achieve significantly lowered bulk resistivityLow‐resistivity anomalies in several cratons can be explained with a few vol.% of solid sulfide [ABSTRACT FROM AUTHOR]

In Georgia the Paleogene Adjara-Trialeti riftogenic belt (length 350 km, width 50-2 km) is dominantly composed of trachytic and trachytic-andesitic pyroclastic deposits, though plutonic rocks also play an important role in the structure. In this article, we report new data on the (LA-ICP-MS) U-Pb zircon geochronology and petrochemistry of the plutons, their xenoliths and restite from this belt. The results indicate that the magmatism in the basin began in the Early Eocene (~50 Ma) associated with the formation of pyroclastic rocks. The mafic intrusions (~46-44 Ma) led to the assimilation and contamination of sialic crust and formation of monzo-syenite melts emplaced at ~43-42 Ma. The Eocene monzo-syenite plutons contain xenoliths of Paleozoic granites (312±7 to 474±5 Ma) and tholeiitic basalts that contain inherited zircon grains ranging in age from Neo-Proterozoic (747±33 Ma, 632±29 Ma) to Cambrian (515±9 Ma). Paleozoic granite xenoliths show complete mineralogical and age similarity to the Adjara-Trialeti belt adjacent pre-Jurassic massif granites. Inherited zircon grains most likely are captured by magmas during ascent that cuts through the Gondwana-derived old continental crust. Obtained results and regional geological analysis demonstrate that the riftogenic basin of the Adjara-Trialeti belt developed on the pre-Jurassic crystalline basement, from Late Cretaceous to Eocene into a back-arc extensional regime to post-collision geodynamic setting. [ABSTRACT FROM AUTHOR]

The Cenozoic basalts exposed in Bo Phloi Gem Field, Kanchanaburi Province, western Thailand are a host to different gem materials (e.g., sapphire, black spinel, black pyroxene and zircon) as well as other xenocrysts and xenoliths from the deep-seated formations onto the earth surface. However, only felsic xenoliths have never been investigated and reported in detail though they are in fact significant evidence of ancient tectonic processes of this area. In this study, the felsic xenoliths were sampled and classified, on the basis of petrochemistry, into granite, syenogranite, and syenite. However, they contain similar mineral assemblages including essentials of quartz, K-feldspar, and plagioclase with different proportions and accessories of biotite, zircon, and opaque minerals. Moreover, large phenocrysts of K-feldspar and plagioclase commonly present as a primary texture which are frequently corroded and replaced by 'sieved texture' with secondary cumulative fringe of tiny feldspar and quartz. These secondary textures clearly indicate quenching after re-heating during transportation by basaltic magma. Geochemical analyses indicate that the alkaline and peraluminous magma show enrichment of Rb and depletion of Ba, Nb, Ta, Ti with steep slope of LREE/HREE enrichment patterns. These evidences suggest low-degree partial melting of crustal materials related to the collisional S-type granite magmatism. In addition, U-Pb dating of zircon from a felsic xenolith yields 211.6±1.3 Ma comparable to the Late Triassic magmatism of the central belt granite in this region which is resulted from the collision between Sibumasu and Indochina terranes. [ABSTRACT FROM AUTHOR]

Very few zircon-bearing, kimberlite-hosted mantle eclogite xenoliths have been identified to date; however, the zircon they contain is crucial for our understanding of subcratonic lithospheric mantle evolution and eclogite genesis. In this study, we constrain the characteristics of zircon from mantle eclogite xenoliths based on existing mineralogical and geochemical data from zircons from different geological settings, and on the inferred origin of mantle eclogites. Given the likely origin and subsequent evolution of mantle eclogites, we infer that the xenoliths can contain zircons with magmatic, metamorphic and xenogenic (i.e. kimberlitic zircon) origins. Magmatic zircon can be inherited from low-pressure mafic oceanic crust precursors, or might form during direct crystallization of eclogites from primary mantle-derived melts at mantle pressures. Metamorphic zircon within mantle eclogites has a number of possible origins, ranging from low-pressure hydrothermal alteration of oceanic crustal protoliths to metasomatism related to kimberlite magmatism. This study outlines a possible approach for the identification of inherited magmatic zircon within subduction-related mantle eclogites as well as xenogenic kimberlitic zircon within all types of mantle eclogites. We demonstrate this approach using zircon grains from kimberlite-hosted eclogite xenoliths from the Kasai Craton, which reveals that most, if not all, of these zircons were most likely incorporated as a result of laboratory-based contamination. [ABSTRACT FROM AUTHOR]

Xenolithic structures are considered to be natural defects in the igneous rocks that highly affect their physicochemical and mechanical properties. They are foreign rock fragments trapped in the magma or lava during the cooling process. They are totally unrelated to the igneous rocks, as they compose from different types of rock pieces that enclosed in the monumental granites during their emplacement. This paper aims to study and characterize the xenolith fragment trapped in the studied granitic obelisk. The analytical study was carried out using different techniques such as polarizing microscope, x-ray diffraction, scanning electron microscope, EDS elemental analysis and BET surface area. The results clarified that the studied xenolith fragment is classified as biotite schist xenolith. In addition, it was observed that the xenolith fragment not only affects the aesthetic appearance of the studied granitic obelisk but also it represents a serious weakness point in its structure. [ABSTRACT FROM AUTHOR]

Garnet-bearing xenoliths are observed within a trachytic dome extrusion in the Hisarlıkaya region (Ankara). These garnets exhibiting greenish-reddish-dark brown colour and ranging in sizes up to 1 cm were examined in terms of mineralogical, geochemical, and gemmological characteristics. Mineralogical studies indicate that these garnets (And 88-93 Grs 7-12) are linked to solid solution series, which are dominantly andradite with lower content of grossular. According to major, trace, and rare earth element (REE) analysis, the representative garnet crystal shows high CaO, Fe2O3, Al2O3, MgO, MnO, V, W, and light rare earth elements (LREE) concentrations. These high concentrations might indicate the mobility of these elements during skarn formation related with contact metamorphism, material exchange occurring via hydrothermal fluids, site geometry of the crystal, ionic radius of cations, and charge balance. The Hisarlıkaya garnets display dodecahedron-trapezohedron crystal habits with good translucency, glassy, transparent features, and also optical isotropic character. Their refractive indices are high (>1.78), and specific gravities range from 3.66 to 3.67 g/cm3. These garnets are not suitable for use as gemstones since they cannot be cut or processed considering all the mineralogical properties of garnets. [ABSTRACT FROM AUTHOR]

The temperature-dependent exchange of Ni and Mg between garnet and olivine in mantle peridotite is an important geothermometer for determining temperature variations in the upper mantle and the diamond potential of kimberlites. Existing calibrations of the Ni-in-garnet geothermometer show considerable differences in estimated temperature above and below 1100 °C hindering its confident application. In this study, we present the results from new synthesis experiments conducted on a piston cylinder apparatus at 2.25–4.5 GPa and 1100–1325 °C. Our experimental approach was to equilibrate a Ni-free Cr-pyrope-rich garnet starting mixture made from sintered oxides with natural olivine capsules (Niolv ≅ 3000 ppm) to produce an experimental charge comprised entirely of peridotitic pyrope garnet with trace abundances of Ni (10–100 s of ppm). Experimental runs products were analysed by wave-length dispersive electron probe microanalysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). We use the partition coefficient for the distribution of Ni between our garnet experimental charge and the olivine capsule lnD grt / olv Ni ; Ni grt Ni olv , the Ca mole fraction in garnet ( X grt Ca ; Ca/(Ca + Fe + Mg)), and the Cr mole fraction in garnet ( X grt Cr ; Cr/(Cr + Al)) to develop a new formulation of the Ni-in-garnet geothermometer that performs more reliably on experimental and natural datasets than existing calibrations. Our updated Ni-in-garnet geothermometer is defined here as: T ∘ C = - 8254.568 X grt Ca × 3.023 + X grt Cr × 2.307 + lnD grt olv Ni - 2.639 - 273 ± 55 where D grt / olv Ni = Ni grt Ni olv , Ni is in ppm, X grt Ca = Ca/(Ca + Fe + Mg) in garnet, and X grt Cr = Cr/(Cr + Al) in garnet. Our updated Ni-in-garnet geothermometer can be applied to garnet peridotite xenoliths or monomineralic garnet xenocrysts derived from disaggregation of a peridotite source. Our calibration can be used as a single grain geothermometer by assuming an average mantle olivine Ni concentration of 3000 ppm. To maximise the reliability of temperature estimates made from our Ni-in-garnet geothermometer, we provide users with a data quality protocol method which can be applied to all garnet EPMA and LA-ICP-MS analyses prior to Ni-in-garnet geothermometry. The temperature uncertainty of our updated calibration has been rigorously propagated by incorporating all analytical and experimental uncertainties. We have found that our Ni-in-garnet temperature estimates have a maximum associated uncertainty of ± 55 °C. The improved performance of our updated calibration is demonstrated through its application to previously published experimental datasets and on natural, well-characterised garnet peridotite xenoliths from a variety of published datasets, including the diamondiferous Diavik and Ekati kimberlite pipes from the Lac de Gras kimberlite field, Canada. Our new calibration better aligns temperature estimates using the Ni-in-garnet geothermometer with those estimated by the widely used (Nimis and Taylor, Contrib Mineral Petrol 139:541–554, 2000) enstatite-in-clinopyroxene geothermometer, and confirms an improvement in performance of the new calibration relative to existing versions of the Ni-in-garnet geothermometer. [ABSTRACT FROM AUTHOR]

Platinum-group element (PGE) + Re abundances and Os isotopic compositions were determined for metasomatized peridotite and cumulate pyroxenite xenoliths from Kharchinsky Volcano, Kamchatka. Two peridotites have moderately high PGE concentrations with 2.9–3.3 ppb Os and smoothly variable PGE patterns similar to abyssal peridotites and encompassing primitive and depleted mantle compositions. One of these has unradiogenic Os (187Os/188Os = 0.1148) at the lower limit of compositions observed in abyssal peridotite which results in a time of Re depletion (T RD) model age of ∼1.8 Ga. These contrasts Kharchinsky peridotites that are Pt-Ir enriched with sharply fractionated PGE patterns with Pt/Pd = 132–168 and Ir/Ru ∼ 4.0. This pattern of enrichment in the heavier and more highly siderophile Pt and Ir over the lighter and more siderophile-chalcophile Ru and Pd (Fleet and Stone, 1991; Fleet et al., 1991) is consistent with sulfur depletion under oxidizing metasomatic conditions that leave a residue with PGE host minerals that are dominantly alloys. A single Ru-enriched peridotite with low PGE abundances is interpreted to reflect advanced stages of paragenesis driven by metasomatism and resulting in the loss of all PGE and Re relative to Ru. All Kharchinsky peridotites included in this study have relatively unradiogenic Os (187Os/188Os = 0.1148–0.1314) but some are coupled to PGE and lithophile element characteristics that require oxidizing metasomatic conditions (Ce/Ce*≪1.0) and desulfurization leading to a general loss of PGE and data patterns that are consistent overall with low abundances of relatively radiogenic Os in arc peridotites globally (Brandon et al., 1996; Widom et al., 2003; Saha et al., 2005). Kharchinsky pyroxenites, which are relatively undeformed and only weakly metasomatized, have similarly fractionated PGE patterns with enrichments in Pt (5.7–46 ppb) and Pd (0.5–2.7 ppb) but with Os, Ir, and Ru abundances < 0.23 ppb. The patterns resemble arc basalts and may reflect preferential mobility of Pt, Pd, and Re in slab fluids. High Pt and Pt/Pd up to 33 in the pyroxenites indicate that a Pt-rich alloy with high Pt/Pd is present in the early (olivine-rich) cumulate assemblage. Several Kharchinsky samples (peridotites and pyroxenites) are Pt-enriched with high Pt/Os and Pt/Re > 20 that will produce rapid ingrowth of 186Os relative to 187Os. Deep recycling and storage of such subduction-modified rocks might preclude the need for core-mantle exchange to explain 186Os-enrichments in komatiites and Hawaiian picrites originating in the deep mantle (Brandon et al., 1998; Brandon and Walker, 2005). [ABSTRACT FROM AUTHOR]

The pressure dependence of the exchange of Cr between clinopyroxene and garnet in peridotite is applicable as a geobarometer for mantle-derived Cr-diopside xenocrysts and xenoliths. The most widely used calibration (Nimis and Taylor Contrib Miner Petrol 139: 541–554, 2000; herein NT00) performs well at pressures below 4.5 GPa, but has been shown to consistently underestimate pressures above 4.5 GPa. We have experimentally re-examined this exchange reaction over an extended pressure, temperature, and compositional range using multi-anvil, belt, and piston cylinder apparatuses. Twenty-nine experiments were completed between 3–7 GPa, and 1100–1400 °C in a variety of compositionally complex lherzolitic systems. These experiments are used in conjunction with several published experimental datasets to present a modified calibration of the widely-used NT00 Cr-in-clinopyroxene (Cr-in-cpx) single crystal geobarometer. Our updated calibration calculates P (GPa) as a function of T (K), CaCr Tschermak activity in clinopyroxene a CaCrTs cpx , and Cr/(Cr + Al) (Cr#) in clinopyroxene. Rearranging experimental results into a 2n polynomial using multiple linear regression found the following expression for pressure: P GPa = 11.03 + - T K ln (a CaCrTs cpx) × 0.001088 + 1.526 × ln Cr # cpx T K where Cr # cpx = Cr Cr + Al , a CaCrTs cpx = Cr - 0.81 · Cr # cpx · Na + K , with all mineral components calculated assuming six oxygen anions per formula unit in clinopyroxene. Temperature (K) may be calculated through a variety of geothermometers, however, we recommend the NT00 single crystal, enstatite-in-clinopyroxene (en-in-cpx) geothermometer. The pressure uncertainty of our updated calibration has been propagated by incorporating all analytical and experimental uncertainties. We have found that pressure estimates below 4 GPa, between 4–6 GPa and above 6 GPa have associated uncertainties of 0.31, 0.35, and 0.41 GPa, respectively. Pressures calculated using our calibration of the Cr-in-cpx geobarometer are in good agreement between 2–7 GPa, and 900–1400 °C with those estimated from widely-used two-phase geobarometers based on the solubility of alumina in orthopyroxene coexisting with garnet. Application of our updated calibration to suites of well-equilibrated garnet lherzolite and garnet pyroxenite xenoliths and xenocrysts from the Diavik-Ekati kimberlite and the Argyle lamproite pipes confirm the accuracy and precision of our modified geobarometer, and show that PT estimates using our revised geobarometer result in systematically steeper paleogeotherms and higher estimates of the lithosphere‒asthenosphere boundary compared with the original NT00 calibration. [ABSTRACT FROM AUTHOR]

Recent studies of serpentine-free, spinel peridotite xenoliths from the mantle lithosphere beneath the active Kamchatka and West Bismarck arcs have shown that these rocks are enriched in silica and highly depleted in incompatible elements in comparison with melting residues of either primitive or mid-ocean ridge mantle. It has been suggested that the silica-rich nature of peridotites from the intra-oceanic, fore- and sub-arc mantle lithosphere, collectively referred to as 'Supra-Subduction Zone (SSZ) peridotites', is primarily of residual origin and inherited from source processes during partial melting in the SSZ mantle asthenosphere (mantle wedge). However, quantifying the contribution of post-melting processes to the silica-rich nature of SSZ peridotites has remained challenging. Here we report petrological and major and trace element data for a new suite of spinel harzburgite xenoliths from the mantle lithosphere beneath TUBAF seamount, located in the fore-arc region of New Ireland (Papua New Guinea area). All samples are fresh peridotites displaying coarse-grained protogranular textures, and sometimes high orthopyroxene (up to ∼29 wt%) at low clinopyroxene (≤4 wt%) contents, which are typical for SSZ peridotites worldwide. TUBAF peridotites in this study have suffered very little post-melting metasomatism through the formation of ≤1 wt% amphibole, which subsequently experienced decompression-induced breakdown during the xenolith ascent. Otherwise, the rocks display a high degree of inter-mineral equilibration and melting signatures preserved through sub-solidus re-equilibration. The bulk-rock chemistry of TUBAF peridotites record a Fe-Al correlation along the 25–30% melting isopleths from ∼2 to <1 GPa, in combination with the distinctive enrichment in silica and (TiO 2 , Al 2 O 3 , Na 2 O)-depletion of SSZ peridotites. This strongly supports the melting origin of these 'residual SSZ signatures'. Bulk-rock and mineral lithophile trace element compositions of TUBAF xenoliths are similar to those of other residual SSZ peridotites, consistent with 25–30% of nearly-pure fractional melt extraction (critical mass porosity <0.001%) in the presence of a fluxing agent enriched in highly incompatible elements. We re-assess earlier interpretations of the origins of TUBAF peridotites by melting at mid-ocean ridges. Instead, we show that these rocks have experienced their last melting event in the mantle wedge, similar to samples from the Izu-Bonin-Mariana fore-arc and the Kamchatka and West Bismarck arcs. We also demonstrate that post-melting metasomatism (including fibrous orthopyroxene that are absent from the samples in this study) is unrelated to the residual SSZ mantle signatures, for which we present the results of polybaric and isothermal flux-melting models including minor element partitioning parameterizations. These models imply that residual SSZ signatures form when previously depleted mantle protoliths are hybridized by hydrous, silica-rich liquids. From the unique Fe-Al correlation in TUBAF peridotites and their low temperatures of equilibration, it appears that fractional hybridization-melting processes forming these rocks occurred in a fore-arc environment with shallow mantle decompression, likely during Oligocene to Miocene subduction along the Manus-Kilinailau trench. [ABSTRACT FROM AUTHOR]

Spectacular reactions textures developed during the non-isochemical breakdown of orthopyroxene and spinel in peridotite mantle xenoliths in intraplate alkaline basalts from the Auckland Volcanic Field and the Dunedin Volcano in New Zealand, and Jeju Island in South Korea. Mantle orthopyroxene is replaced by symplectitic intergrowths of silica-rich glass and olivine (± clinopyroxene ± orthopyroxene) with individual minerals showing large diffusion gradients at their margins proximal to the host melt. The glasses are basaltic to trachytic and formed by incongruent melting of orthopyroxene and subsequent re-equilibration by diffusion and mixing during changing boundary conditions with the host magma. Although the glass composition is strongly dependent on the alkalinity of the host magma, the distance to the reacting contact and the respective phase assemblage that develops in the reaction zone (melt + ol ± clinopyroxene ± orthopyroxene), the most evolved melts converge at trachytic composition. Diffusion-driven Fe enrichment for mantle olivine indicates residence timescales within the host magma on the order of weeks to several months. [ABSTRACT FROM AUTHOR]

Skarnoid calc-silicate xenoliths composed of anorthite, clinopyroxene and Mg-Al spinel occur in alkali basalts of the Pliocene-Pleistocene intra-plate magmatic province in the northern part of the Pannonian Basin. Randomly oriented and elongated pseudomorphs are tschermakite crystals replaced by olivine, spinel and plagioclase. The relict amphibole within the pseudomorphs is characterized by high VIAl, between 1.95 and 2.1, and very low occupancy of the A-site (3+Al)AlSiO6 endmember with an equal proportion of VIAl3+ and Fe3+. Concentrations of kushiroite CaAlAlSiO6 endmember, up to 47.5 mol%, are the highest recorded in terrestrial samples. The AlFe3+-rich pyroxenes originated at the expense of diopside-augite during the interaction with carbonate-aluminosilicate melt. Forsterite (Fo72–83) and hemoilmenite with up to 32 mol% geikielite (9.3 wt% MgO) also crystallized from the melt, leaving behind the residual calcic carbonate with minor MgO (1–3 wt%). Columnar habit of neoformed olivine growing across diopside-augite layers indicates rapid crystallization from eutectic liquid. Euhedral aragonite and apatite embedded in fine-grained calcite or aragonite groundmass indicate slow crystallization of the residual carbonatite around the calcite-aragonite stability boundary. Corundum exsolutions in rock-forming anorthite are products of superimposed low-pressure pyrometamorphic reworking during transport in alkali basalt. Concomitant alkali metasomatism produced neoformed interstitial sodalite, nepheline, sanidine, albite, biotite, Mg-poor ilmenite (10–18 mol% MgTiO3), Ti-magnetite and fluorapatite. Olivine-ilmenite-aragonite-calcite thermobarometry returned temperatures of 770–860 °C and pressures of 1.8–2.1 GPa, whereas plagioclase-amphibole thermobarometer yielded 781 ± 13 °C and 2.05 ± 0.03 GPa. The calculated pressures correspond to depths of 60–70 km. The calc-silicate xenoliths are most likely metamorphosed marbles; however, a magmatic protolith (metagabbro, metaanorthosite) cannot be ruled out owing to high Cr contents in spinels (up to 30 mol% chromite) and abundant Cu-sulfides.

Water is key to plate tectonics on Earth, which, in turn, is vital to the production of continental crust. Although arc lavas erupt in a volatile‐rich state and calc‐alkaline arc plutons are distinguished by the presence of hydrous minerals such as hornblende and biotite, the water content of arc magmas earlier in their evolution—in the deep crust—remains poorly constrained. Here, we report H2O contents in nominally anhydrous minerals measured in situ on petrographic thin sections by secondary ion mass spectrometry of Proterozoic deep crustal xenoliths from Colorado, USA. Clinopyroxene, orthopyroxene, and garnet contain average H2O contents ranging from 75–760, 233–410, and 42–139 ppm, respectively. Reconstructed bulk rock H2O contents range from ~60 to ~650 ppm. Intermineral H2O ratios overlap experimental mineral/melt D values and are used to calculate H2O of melts last in equilibrium with the xenoliths. We propose that these xenoliths represent cumulates fractionated from a primitive, hydrous (≥1 wt.% H2O) melt at high (~1 GPa) pressures, similar to conditions in modern subduction zones and potentially associated with widespread arc accretion that formed the core of North America in the Precambrian. Key Points: We analyzed H2O contents in cpx, opx, and garnet in Proterozoic lower crustal xenoliths from ColoradoMineral H2O content was used to reconstruct bulk rock and calculate melt H2OXenoliths represent metamorphosed igneous cumulates from high‐P, hydrous magmas [ABSTRACT FROM AUTHOR]

New data are presented on the quartz porphyry dikes of the Salmi batholith belonging to anorthosite–rapakivi granite rock complexes. The relationships of various igneous rocks of the Salmi batholith and metasomatites were determined along with the isotopic U–Pb age by the LA–ICP–MS technique and the temperature of zircon formation by the Ti content. [ABSTRACT FROM AUTHOR]

Carbonate-bearing assemblages in the mantle have been interpreted to be the source for Si-undersaturated, CO 2 -rich magmas, including kimberlites. However, direct evidence for carbonate in the mantle is rare in the contemporary literature. Here we present petrography, trace element, and C-O-Sr-Nd-Pb isotope composition for a suite of carbonate xenoliths from the Kimberley region kimberlites to ascertain their mantle or crustal origin and gain insight to the potential for the occurrence of carbonate in the mantle. Carbonate xenoliths were found in large kimberlite blocks from the Bultfontein kimberlite and Big Hole region. The xenoliths are characterised by pale green alteration margins made of fine-grained microlites of an unknown mineral as well as spherules surrounded by glassy material. They are generally 1–4 cm in size, coarse-grained (1–2 mm), and comprised entirely of calcite. Carbonate xenoliths from the Bultfontein kimberlite have low total REE concentrations (0.2–4.9 ppm), constant 87Sr/86Sr i (0.7047–0.7049) combined with variable ƐNd i (−0.1 to −26.2) and 206Pb/204Pb i , 207Pb/204Pb i , and 208Pb/204Pb i of 16.7–18.8, 15.3–15.6, 36.5–38.4, respectively. Xenoliths from the Big Hole sample have higher 87Sr/86Sr i (0.7088–0.7095), lower ƐNd i (−24.5 to −3.8), and 206Pb/204Pb i , 207Pb/204Pb i , and 208Pb/204Pb i of 18.9–19.9, 15.7–15.8, 38.4–38.8, respectively. The δ13C values for both Bultfontein (−5.7 to −6.6‰) and Big Hole (−4.7 to −5.4‰) carbonates are within the typical range expected for mantle-derived carbonate. The δ18O values (15.5–17.5‰) are higher than those of mantle silicate rocks, indicative of late-stage low-temperature interaction with fluids; a common feature of groundmass calcite in the Kimberley kimberlites. The Sr- and C- isotope composition of the Bultfontein xenoliths indicates a mantle origin whereas the Big Hole xenolith Sr- and C-isotopes are more ambiguous. Isotope mixing models are inconsistent with interaction between the host kimberlite and carbonate xenoliths. Correlation between ƐNd i and δ18O values for the Bultfontein xenoliths indicates late-stage interaction with low-temperature fluids, which may also be responsible for the large range in ƐNd i. This in turn indicates that the highest ƐNd i of −0.1 represents the primary carbonate xenolith signature, and this value overlaps typical Group I kimberlites. We discuss two possible origins for the carbonate xenoliths. (1) Carbonate xenoliths from the sub-continental lithospheric mantle (SCLM), where quenched margins and the large range of ƐNd i are related to formation in the mantle. (2) Carbonate xenoliths from an earlier phase of carbonatite magmatism. The similarity of isotope signatures of the Bultfontein carbonates to Group I kimberlite may further suggest a link between kimberlite and carbonatite volcanism such as observed elsewhere in the world. [ABSTRACT FROM AUTHOR]

Combined observations from natural and experimental deformation microstructures are often used to constrain the rheological properties of the upper mantle. However, relating natural and experimental deformation processes typically requires orders of magnitude extrapolation in strain rate due to vastly different time scales between nature and the lab. We examined a sheared peridotite xenolith that was deformed under strain rates comparable to laboratory shearing time scales. Microstructure analysis using an optical microscope and electron backscatter diffraction (EBSD) was done to characterize the bulk crystallographic preferred orientation (CPO), intragrain misorientations, subgrain boundaries, and spatial distribution of grains. We found that the microstructure varied between monophase (olivine) and multiphase (i.e., olivine, pyroxene, and garnet) bands. Olivine grains in the monophase bands had stronger CPO, larger grain size, and higher internal misorientations compared with olivine grains in the multiphase bands. The bulk olivine CPO suggests a dominant (010)[100] and secondary activated (001)[100] that are consistent with the experimentally observed transition of the A to E-types. The bulk CPO and intragrain misorientations of olivine and orthopyroxene suggest that a coarser-grained initial fabric was deformed by dislocation creep coeval with the reduction of grain size due to dynamic recrystallization. Comparing the deformation mechanisms inferred from the microstructure with experimental flow laws indicates that the reduction of grain size in orthopyroxene promotes activation of diffusion creep and suggests a high activation volume for wet orthopyroxene dislocation creep.

The James Ross Island Volcanic Group (JRIVG) at the northern tip of the Antarctic Peninsula is composed of various types of volcanic rocks erupted during the Late Miocene to Late Pleistocene. The sub-aerial lavas are the most abundant erupted products within the suite and are represented by alkali olivine basalts that contain significant amounts of ultramafic xenoliths. Precise determination of chemical compositions of olivine by electron microprobe and LA-ICP-MS reveals more than one compositionally distinct olivine population in the xenoliths and the host lavas. The majority of the olivine grains from the xenolith suite are remarkable for their high Mg/Fe ratios (>Fo 88) and low-Ca contents (<500 ppm) and are similar in composition to the mantle olivine, while some others, despite their similarly low-Ca abundances, are characterized by significantly lower Mg/Fe (Fo 88) from the basaltic lavas also have low Ca contents compared to MORB olivine at similar Fo, indicating an arc-melt like volatile content of the primary magma. Evaluation of minor and trace element relative abundances in olivine further indicate that the alkaline basalts in the JRIVG are the products of peridotite-dominated partial melting of a volatile-rich mantle source with signatures of mantle hydration most probably promoted by preceding subduction events. The results from olivine chemistry, when combined with the evaluation of primary melt compositions, appear to be consistent with the view that the primary magmas from which the JRIVG basalts were derived are the results of partial melting of a mantle domain that has experienced hydrous silicate melt metasomatism through interaction of peridotitic upper mantle rocks with melts of a slab-derived component, most probably generated by dehydration melting of subducted oceanic crust. • Alkali lavas of the JRIVG have intra-plate geochemical characters • Compositions of primitive olivine imply Arc-melt like volatile contents • Olivine chemistry indicate peridotite-dominated melting of a hybrid source • Source peridotite interacted with hydrous silicate melt • Recycled crustal material is the source of reacting silicate melt [ABSTRACT FROM AUTHOR]

The microstructural, petrofabric and geochronological study of deep-seated xenoliths from the La Encomienda volcano (Neogene Campo de Calatrava Volcanic Field) has helped to characterize the nature and dynamics of processes operative in the lower crust and subcontinental mantle domains of the Central Iberian Zone in the Iberian Massif (Spain). We present the finding of three new xenolith types: noritic andesine meta-anorthosites, aluminous granulites and felsic granulites. Plagioclase (andesine), hypersthene, ilmenite, apatite and zircon make up the mineral assemblage in equilibrium in the anorthosite xenoliths, whereas K-feldspar (sanidine), antiperthitic plagioclase (oligoclase-andesine), quartz, sillimanite, garnet, rutile and biotite conform the mineral assemblage in the aluminous granulites. Felsic granulites differ from the aluminous type in the smaller amount of garnet, the occurrence of perthitic microcline and the lack of sillimanite and biotite. The presence of significant fabrics in the constituent minerals indicates that they underwent high-temperature deformations representative of low crustal domains. U-Th-Pb analyses on anorthositic zircons provide a Variscan Late Carboniferous Concordia age of 309 ± 3 Ma, interpreted as the age of the igneous protolith of the granulitic anorthosite. Estimations for zircons in the felsic granulites yield three age groups at 288 ± 2 Ma, 398 ± 2 Ma and 490 ± 2 Ma. The first one would represent a late- to post-Variscan (Cimmerian) regional metamorphic event under granulite-facies conditions. The last two might be related to the age of an igneous component of the protolith at ca. 490 Ma and possibly that of a previous tectonothermal event at 400 Ma. 206Pb/238U results on the aluminous and felsic granulitic rutiles give ages of 5 ± 1 Ma that may correspond to resetting during Neogene volcanic events. A tentative scenario would contemplate the existence of Upper Carboniferous (late-Variscan) lithospheric shear zones along which mantle-derived components would have raised until the mantle-crust interface. The associated thermal budget would have acted as an important heat source responsible for heat conduction into the lower crust, along with eventual crustal melting. Removal of the mafic portions would have led to formation of low-density plagioclase-rich accumulations propagating upwards until its eventual incorporation in low crustal domains along with the formation of deep-seated granulites. The occurrence of these lithologies and the large volumes of hybrid monzogranites to granodiorites in the Central Iberian Zone might indicate, congruently, that, more than expected, mantle-derived magmas would have also played an important role during crustal growth processes related to the final stages of the Variscan orogeny in this part of the Iberian Massif. • New finding of meta-anorthosite, aluminous granulite and felsic granulite xenoliths • LPOs of the constituent minerals indicate HT deformation in low crustal domains. • Variscan Late Carboniferous (309 ± 3 Ma) for the igneous protolith of the anorthosite • Variscan regional granulite-facies metamorphism (288 ± 2 Ma) • Mantle-derived magmas involved in crustal growth during Variscan orogeny [ABSTRACT FROM AUTHOR]

Dunite, harzburgite, and clinopyroxenite xenoliths from Kharchinsky volcano, Kamchatka, have abundances and ratios of incompatible trace elements similar to those in arc volcanic rocks (elevated Ba/Th, La/Yb, Nd/Hf, and Sr/Y). All orthopyroxenes and some clinopyroxenes in the peridotites have U‐shaped rare‐earth element patterns. Negative Ce anomalies are present in orthopyroxenes with Ce/Ce* as low as 0.01 and down to 0.22 in whole‐rock peridotite data. Ce anomaly growth is linked to increasing La/Sm and enrichments in Rb, U, Pb, and Ba over La and Ce. Isotopes (Pb, Sr, Nd, and Hf) indicate pelagic sediment, and hydrothermal crusts play no role in Ce anomaly development. Instead, Ce anomalies appear to be products of fluid transport and elemental scavenging under oxidizing conditions beneath the deep forearc. Textures and compositions of aluminous green spinels indicate most of the peridotites were partially melted and recrystallized at depth. Veins and pockets of amphibole reflect impregnation late in the petrogenesis of the rocks by melts similar to Kamchatka basalts. Orthopyroxenite xenoliths are fine‐grained with fibrous orthopyroxene that has high‐Mg/Mg + Fe (up to 0.96) and generally lower CaO and Al2O3 compared to peridotite orthopyroxenes and perhaps formed by reaction of siliceous fluids with olivine. Kharchinsky xenoliths have Pb, Sr, and Nd isotopes similar to Kamchatka volcanic rocks, but Hf isotopes in clinopyroxenites and gabbros are more radiogenic by 1–3 epsilon units. Patterns in isotopic data indicate a compositional change in the source of Kamchatka volcanism within the past 20 million years. Plain Language Summary: This paper presents the results of a study of rare rock fragments (xenoliths) that were transported from the Earth's deep interior to the surface during an eruption of Kharchinsky volcano, Kamchatka. The chemical compositions, mineralogy, and textures of the samples were studied with the goal of understanding the processes that affected rocks, which may play a role in the formation of magmas in the Kamchatka subduction zone. The key process that affected the xenoliths involved the addition of fluids and dissolved elements to the samples at temperatures of 500–700 °C. These fluids are derived from seawater that was transported to 30‐ to 50‐km depths by subduction of the Pacific Plate beneath Kamchatka. Subsequent to the addition of fluid, there was a shift in the position of the Kamchatka‐Pacific Plate boundary that led to an increase in temperature and the formation of small quantities of melt that crystallized to a distinctive group of secondary minerals that are present in the samples and that postdate (overprint) the initial effects of fluid addition. The final step in the evolution of the samples was infiltration by an Fe‐ and Mg‐rich magma that crystallized principally amphibole‐group minerals. Key Points: The mantle beneath Kharchinsky was metasomatized by fluids that originated beneath the deep forearc in a highly oxidized settingA change in the locus of subduction led to in situ melting and recrystallization of fluid metasomatized mantle beneath KharchinskyThe isotopic composition of Kamchatka volcanism changed in the past 20 million years, likely reflecting a change in the subducting plate [ABSTRACT FROM AUTHOR]