Search

The Mélange in Lamphun Province, situated in the southern region of the Chiang Mai-Chiang Rai Suture Zone (CMCRSZ), remains inadequately explored. This study employs field investigations and petrographic analysis to delineate the characteristics of the CMCRSZ Mélange, identifying nine distinct rock units. These units include Carboniferous quartz arenite interbedded with fine-grained sedimentary rocks, Permian lithic graywacke, Permian limestone hosting Pseudodaliolina sp. and Schwagerinid sp., Triassic chert containing Anisian radiolarian fossils, and ocean island basalt. This area exhibits complex folding and thrusting, accompanied by granitic intrusions and low-grade metamorphic rocks within a finer-grained sedimentary matrix. Stratigraphic assessments trace the primary material of the accretionary complex to the upper strata of the oceanic plate stratigraphy. Geochemical analyses indicate the mafic rocks as alkaline to subalkaline basalts, indicative of formation through an alkaline magma series, a conclusion supported by rare-earth element patterns. Sr/Nd isotope classification suggests ocean island tectonics for these rocks. The formation of the CMCRSZ Mélange occurred during the Paleo-Tethys oceanic crust subduction beneath the Indochina Terrane at the Permian-Triassic Boundary, accumulating diverse materials in a dynamically evolving tectonic context. Furthermore, S-type Triassic granites emerged post-collision during the Indosinian Orogeny following the thrusting of the CMCRSZ onto Sibumasu.

The valence state of Fe plays a vital role in setting and recording the oxidation state of magmas, commonly expressed in terms of oxygen fugacity ( f O 2 ). However, our knowledge of how and why f O 2 varies within and between magmatic systems remains patchy because of diverse challenges associated with estimating the valence state of Fe in glasses and minerals routinely. Here we investigate Fe valence systematics in magmatic clinopyroxene crystals from ocean island basalts (OIBs) erupted in Iceland and the Azores to explore whether they record information about magma Fe 3 + contents and magmatic f O 2 conditions. Although many studies assume that all Fe in augitic clinopyroxene crystals from OIBs occurs as Fe 2 + , we find that up to half of the total Fe present can occur as Fe 3 + , with crystals from alkali systems typically containing more Fe 3 + than those from tholeiitic systems. Thus, Fe 3 + is a major if under-appreciated constituent of augitic clinopyroxene crystals erupted from ocean island volcanoes. Most Fe 3 + in these crystals is hosted within esseneite component (CaFe 3 + AlSiO 6 ), though some may be hosted in aegirine component (NaFe 3 + Si 2 O 6 ) in crystals from alkali systems. Observations from samples containing quenched matrix glasses suggest that the incorporation of Fe 3 + is related to the abundance of tetrahedrally coordinated Al ( IV Al), implying some steric constraints over Fe 3 + partitioning between clinopyroxene and liquid (i.e., D Fe 2 O 3 cpx - liq values), though this may not be an equilibrium relationship. For example, IV Al-rich { h k 0 } prism sectors of sector-zoned crystals contain more Fe 3 + than IV Al-poor { 1 ¯ 11 } hourglass sectors. Moreover, IV Al-rich compositions formed during disequilibrium crystallisation are enriched in Fe 3 + . Apparent clinopyroxene-liquid Fe 2 + –Mg exchange equilibria (i.e., K D , Fe 2 + - Mg cpx - liq values) are similarly affected by disequilibrium crystallisation in our samples. Nonetheless, it is possible to reconcile our observed clinopyroxene compositions with glass Fe valence systematics estimated from olivine-liquid equilibria if we assume that K D , Fe 2 + - Mg cpx - liq values lies closer to experimentally reported values of 0.24 - 0.26 than values of ∼ 0.28 returned from a general model. In this case, olivine-liquid and clinopyroxene-liquid equilibria record equivalent narratives, with one of our glassy samples from Iceland recording evolution under f O 2 conditions about one log unit above fayalite-magnetite-quartz (FMQ) equilibrium (i.e., ∼ FMQ+1) and our glassy Azorean sample recording evolution under significantly more oxidising conditions (≥ FMQ+2.5) before experiencing syn-eruptive reduction, likely as a result of SO 2 degassing; our other glassy sample from Iceland was also affected by reductive SO 2 degassing. Overall, our findings demonstrate that the Fe valence systematics of clinopyroxene crystals can record information about the conditions under which OIBs evolve, but that further experimental work is required to properly disentangle the effects of magma composition, disequilibrium and f O 2 conditions on clinopyroxene-liquid equilibria involving Fe 2 + and Fe 3 + . [ABSTRACT FROM AUTHOR]

Ocean island basalts (OIBs) are generated by mantle plumes, with their geochemistry controlled by a combination of source composition, temperature, and thickness of overlying lithosphere. For example, OIBs erupting onto thicker, older oceanic lithosphere are expected to exhibit signatures indicative of higher average melting pressures. Here, we quantitatively investigate this relationship using a global data set of Neogene and younger OIB compositions. Local lithospheric thicknesses are estimated using theoretical plate‐cooling models and Bayes factors are applied to identify trends. Our findings provide compelling evidence for a correlation between OIB geochemistry and lithospheric thickness, with some variables (SiO2, Al2O3, FeO, Lu) showing linear trends that can be attributed to increasing average melting pressure, whereas others (CaO, La, λ0, and λ1) require a bi‐linear fit with a change in gradient at ∼55 km. Observed variations in highly incompatible elements are consistent with degrees of melting that decrease with increasing lithospheric thickness, as expected. Nevertheless, at thicknesses beyond ∼55 km, the implied degree of melting does not decrease as rapidly as is suggested by theoretical expectations. This observation is robust across different lithospheric thickness estimates, including those derived from seismic constraints. We infer that at thicknesses exceeding ∼55 km, weak plumes fail to effectively thin overlying lithosphere and/or produce insufficient melt to erupt. This is supported by independent estimates of plume buoyancy flux, indicating that OIB magmatism on older lithosphere may be biased toward hotter plumes. In addition, we find evidence for a "memory effect" of incomplete homogenization of melts during their ascent. Plain Language Summary: Most of Earth's volcanoes occur at tectonic plate boundaries, but some emerge within plate interiors in so‐called intra‐plate settings. These volcanoes are believed to mark the surface expression of mantle plumes: hot, buoyant columns that rise from the core‐mantle‐boundary toward the surface. As they rise, lower pressures near the surface facilitate melting. However, the lithosphere—Earth's rigid outermost shell—limits plume ascent, and therefore controls the final (lowest) melting pressure of mantle plumes (the "lid effect"). Here, we collate and analyze a global geochemical data set of oceanic island basalts—the products of plume melting—to test this hypothesis. Using a range of diagnostics and a novel probabilistic analytical approach, we find that some geochemical parameters either linearly increase or decrease with lithospheric thickness, whereas other trends exhibit abrupt changes. We propose potential explanations for these patterns, focusing on factors such as the degree of melting (which is sensitive to temperature and pressure) and variations in mantle mineralogy at different depths. Notably, we suggest that there is a higher chance of observing volcanism above hotter plumes in regions of thicker lithosphere and identify a "memory effect", whereby their geochemistry to some extent preserves information from the initial melting process. Key Points: We quantify the relationship between lithospheric thickness and ocean island basalt geochemistry: the so‐called lid effectObserved trends are controlled by pressure‐related changes in the degree of melting, mineral assemblage, and spinel‐garnet phase transitionMagmatism beneath older lithosphere may be biased toward hotter plumes that more effectively thin and penetrate overlying lithosphere [ABSTRACT FROM AUTHOR]

The Nagaland–Manipur Hill Ophiolite (NMHO) is an NNE-SSW trending linear ophiolite zone exposed in the northeastern states of Nagaland and Manipur in India. Basaltic rocks of NMHO are geochemically divisible into two broad groups in the Zr/Ti versus Nb/Y classification diagram. Samples with TiO2 < 2 wt% and Zr = 38–118 ppm plot within the basalt field, whereas samples with TiO2 > 2 wt% plot within the alkali basalt field. The latter can be subdivided into the alkali basalt group-1 (AB-1) with Zr = ~ 200 ppm and the alkali basalt group-2 (AB-2) with Zr = ~ 400 ppm. In a normal mid-ocean ridge basalt (N-MORB) normalized trace element pattern, basalt displays a near-horizontal trend from Lu to Pr at rock/N-MORB = ~ 1 and then increases slightly from Pr to Rb, whereas alkali basalt is relatively more enriched than basalt. In the chondrite-normalized rare earth element (REE) pattern, basalt displays near-horizontal trends with (La/Yb)CN ranging between 0.78 and 1.89. On the other hand, alkali basalt displays parallel and steadily increasing enrichment trends from Lu to La [(La/Yb)CN = 11.07–17.61], with a slight drop at Eu and Sm. Mantle melting models suggest: (1) partial melting of N-MORB-like sources at degrees of melting (F) = 4–8% for basalts; and (2) partial melting of ocean island basalt (OIB)-like sources at F = 7.5–17.8% for alkali basalt. Occurrences of basalts with N-MORB-like and and alkali basalt with OIB-like signatures within the NMHO complex suggest their origin from distinct magma batches with direct or indirect involvement of the Kerguelen plume. Chondrite normalised pattern for elements Nb, Zr and REE showing estimated compositions of source S1 and residue R1 for alkali basalt (estimated using DMI modeling) and source S2 for basalt (estimated using NBM modeling) in this study. DMM and OIB (Workman and Hart 2005) values are also shown for comparison. Average abyssal peridotite value is from Niu (2004) database. [ABSTRACT FROM AUTHOR]

Abstract Ocean island basalts (OIBs) are generated by mantle plumes, with their geochemistry controlled by a combination of source composition, temperature, and thickness of overlying lithosphere. For example, OIBs erupting onto thicker, older oceanic lithosphere are expected to exhibit signatures indicative of higher average melting pressures. Here, we quantitatively investigate this relationship using a global data set of Neogene and younger OIB compositions. Local lithospheric thicknesses are estimated using theoretical plate‐cooling models and Bayes factors are applied to identify trends. Our findings provide compelling evidence for a correlation between OIB geochemistry and lithospheric thickness, with some variables (SiO2, Al2O3, FeO, Lu) showing linear trends that can be attributed to increasing average melting pressure, whereas others (CaO, La, λ0, and λ1) require a bi‐linear fit with a change in gradient at ∼55 km. Observed variations in highly incompatible elements are consistent with degrees of melting that decrease with increasing lithospheric thickness, as expected. Nevertheless, at thicknesses beyond ∼55 km, the implied degree of melting does not decrease as rapidly as is suggested by theoretical expectations. This observation is robust across different lithospheric thickness estimates, including those derived from seismic constraints. We infer that at thicknesses exceeding ∼55 km, weak plumes fail to effectively thin overlying lithosphere and/or produce insufficient melt to erupt. This is supported by independent estimates of plume buoyancy flux, indicating that OIB magmatism on older lithosphere may be biased toward hotter plumes. In addition, we find evidence for a “memory effect” of incomplete homogenization of melts during their ascent.

Mantle plumes contain heterogenous chemical components and sample variable depths of the mantle, enabling glimpses into the compositional structure of Earth's interior. In this study, we evaluated ocean island basalts (OIB) from nine plume locations to provide a global and systematic assessment of the relationship between fO2 and He‐Sr‐Nd‐Pb‐W‐Os isotopic compositions. Ocean island basalts from the Pacific (Austral Islands, Hawaii, Mangaia, Samoa, Pitcairn), Atlantic (Azores, Canary Islands, St. Helena), and Indian Oceans (La Réunion) reveal that fO2 in OIB is heterogeneous both within and among hotspots. Taken together with previous studies, global OIB have elevated and heterogenous fO2 (average = +0.5 ∆FMQ; 2SD = 1.5) relative to prior estimates of global mid‐ocean ridge basalts (MORB; average = −0.1 ∆FMQ; 2SD = 0.6), though many individual OIB overlap MORB. Specific mantle components, such as HIMU and enriched mantle 2 (EM2), defined by radiogenic Pb and Sr isotopic compositions compared to other OIB, respectively, have distinctly high fO2 based on statistical analysis. Elevated fO2 in OIB samples of these components is associated with higher whole‐rock CaO/Al2O3 and olivine CaO content, which may be linked to recycled carbonated oceanic crust. EM1‐type and geochemically depleted OIB are generally not as oxidized, possibly due to limited oxidizing potential of the recycled material in the enriched mantle 1 (EM1) component (e.g., sediment) or lack of recycled materials in geochemically depleted OIB. Despite systematic offset of the fO2 among EM1‐, EM2‐, and HIMU‐type OIB, geochemical indices of lithospheric recycling, such as Sr‐Nd‐Pb‐Os isotopic systems, generally do not correlate with fO2. Plain Language Summary: Rocks from Earth's surface are mixed back into the interior during crustal recycling as a result of plate tectonics and subduction. For example, plate tectonics results in subduction of oceanic crust back into the mantle. Recycling of surface materials might oxidize the interior of the Earth. Mantle plumes, which are buoyantly rising portions of the mantle that create ocean islands such as Hawaii, Iceland, and Samoa, have the chemical and isotopic characteristics associated with recycled materials in their sources. Here we investigate rocks from mantle plumes that have heterogeneous isotopic compositions as a result of incorporating different types of recycled material to test whether their oxygen fugacity varies systematically with the type of recycled crust in their source. We show that some types of mantle plume‐derived rocks, called HIMU and enriched mantle 2 characterized by their extreme isotopic compositions, are more oxidized than the enriched mantle 1 or typical geochemically depleted rocks from mantle plumes and from spreading centers in the oceans. These results link certain recycled materials to oxidation of Earth's mantle. Key Points: Oxygen fugacity generally does not correlate with radiogenic isotopic compositions that trace recycled material in mantle‐derived rocksHIMU and enriched mantle 2 ocean island basalts are more oxidized than enriched mantle 1 or geochemically depleted ocean island basalts and mid‐ocean ridge basalts [ABSTRACT FROM AUTHOR]

The evolution and final closure of the Neo-Tethys Ocean are one of the most important geological events that have occurred on Earth since the Mesozoic. However, the evolution of the Neo-Tethys is not well constrained, in particular whether its opening occurred in the Permian or the Triassic and whether a plume was involved with its opening or not. In this study, we present geochronological and geochemical data for mafic igneous rocks in mélanges along the Yarlung Zangbo suture zone (YZSZ) in southern Tibet to constrain the timing and mechanism of opening the Neo-Tethys Ocean. Based on field observations, the YZSZ mélanges can be divided into three segments. The western (west of Zhongba) and eastern (Sangsang-Renbu) segments are composed of ocean plate stratigraphy representing accretionary complexes that formed during subduction of Neo-Tethyan oceanic lithosphere beneath the southern margin of the Asian continent. Mélanges in the central segment (Zhongba-eastern Saga) typically have a siliciclastic matrix, and represent Tethyan Himalayan strata that were structurally mixed with the southern margin of the Asian continent. Based on our and previously published geochemical data, the mafic rocks in the YZSZ mélanges are ocean island basalt (OIB)-like, with ages in the Late Permian-Middle Triassic, the Middle-Late Jurassic, and the Early Cretaceous, respectively. An OIB-like block with an age of ca. 253 Ma is identified from the Zhongba mélanges in the western segment, and it is the oldest OIB lithology yet identified in the YZSZ mélanges related to the evolution of the Neo-Tethys Ocean. Geochemical features indicate that this OIB-like block is distinct from typical OIBs and would be formed during continental rifting to incipient seafloor spreading. In the framework of plate divergent-convergent coupling systems and based on literature data for early Middle Triassic seamounts, radiolarian cherts, and normal mid-ocean ridge basalt-like oceanic crust, we conclude that opening of the Yarlung Zangbo Neo-Tethys Ocean would mainly occur at ~250–243 Ma in the Early Triassic, not later than the early phase of Middle Triassic. In addition, a mantle plume was not involved in opening the Yarlung Zangbo Neo-Tethys Ocean. On the other hand, we have also identified a suite of ca. 160 Ma OIB-like basaltic sills from the Bainang mélanges in the eastern segment, which is the same age as the OIB lithologies previously reported in the Zhongba mélanges. Based on the sill-like occurrence and absence of plume-related rock associations in this region, the Bainang OIB-like rocks might result from Middle-Late Jurassic continental rifting in northern Gondwana. Magmatism related to this tectonic event is preserved in both the YZSZ mélanges and Himalayan strata, but its tectonic significance requires further investigation. Based on this study of the YZSZ mélanges and the previous studies of YZSZ ophiolites, Gangdese belt igneous rocks, and sedimentary rocks, we have reconstructed the entire Wilson Cycle of the Yarlung Zangbo Neo-Tethys Ocean, mainly involving continental rifting and ocean opening, subduction initiation, ultraslow-spreading ridge-trench conversion, subduction re-initiation, and oceanic closure and initial India-Asia collision for the tectonic emplacement of ophiolites. These processes were associated not only with magmatic flare-ups and lulls in the Gangdese belt but also with two stages of ophiolite obduction. Our data therefore provide new insights into the evolution of the Yarlung Zangbo Neo-Tethys Ocean and related Tethyan geodynamics. [ABSTRACT FROM AUTHOR]

The geochemistry of ocean island basalts (OIB) provides important information about the composition of the mantle, its temporal evolution, and insights into crustal recycling and magma genesis. We present a new ocean island basalt isotope database (OIBID) from 48 oceanic hotspots that includes isotope data (87Sr/86Sr, 143Nd/144Nd, 176Hf/177Hf, 206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb, 187Os/188Os, 3He/4He and δ18O) from over 1250 publications. The OIBID includes >13,000 samples and provides major and trace element concentrations and relevant metadata (sample location, methods, etc.), where available. In OIBID there are >41,000 individual isotope analyses that, when combined with individual major element and trace elements data, represent nearly 370,000 individual data entries in the database. Specific criteria are provided to identify the highest quality geochemical OIB data and filter the dataset for presentation here. After filtering, the OIBID is used to identify relationships between radiogenic isotopes and incompatible trace element ratios that resolve enriched mantle (EM) from HIMU (high 238U/204Pb) mantle domains. We also show that EM lavas have only low MgO, while HIMU and high 3He/4He lavas range from low to high MgO. Additionally, we show that many oceanic hotspots still remain uncharacterized for 187Os/188Os (N = 28 oceanic hotspots lack robust analyses), 3He/4He (N = 16), and 176Hf/177Hf (N = 15). We also demonstrate that the existing published isotope data are highly biased: < 25 % of the hotspots (11 of 48 total oceanic hotspots) in the OIBID account for ≥ 75 % of the published Sr-Nd-Pb-Hf-Os-He isotope data. Just two of the 48 oceanic hotspots—Hawaiʻi and Iceland—represent 35 to 45 % of the total published analyses for these isotope systems. To correct for this considerable sampling bias, OIB isotope histograms are reweighted based on hotspot's fractional contribution to the global oceanic hotspot buoyancy flux. After reweighting, the contribution from each oceanic hotspot is in proportion to the mass of upwelling material in the plume. Our results demonstrate that very few OIB (just 7 % of all OIB samples with Nd isotopes analyzed) are geochemically enriched (143Nd/144Nd < 0.51263) while the majority of OIB are geochemically depleted (93 % of OIB samples). HIMU OIB (206Pb/204Pb > 21) are rare (only 2 % of OIB samples), and high 3He/4He OIB (3He/4He > 20 R A) are uncommon (just 9 % of OIB samples). The reweighted dataset shifts global OIB to more radiogenic lead isotope compositions than the unweighted dataset, and more than half of OIB have MORB-like 3He/4He (8 ± 2 R A) in the reweighted dataset. These observations have implications for the dominance of geochemically depleted lavas in the mantle, the formation of the HIMU mantle by subduction of oceanic crust, and the presence of MORB-like 3He/4He throughout the upper and lower mantle. • A new Sr-Nd-Pb-Hf-Os-He isotope database for 48 oceanic hotspots is presented. • Major and trace element concentrations and metadata are provided, where available. • <25 % of the hotspots account for ≥75 % of the isotope data, indicating sampling bias. • Hawaii & Iceland account for 35–45 % (depending on isotope) of global OIB isotope data. • Histograms reweighted for hotspot buoyancy fluxes are used to correct for this bias. [ABSTRACT FROM AUTHOR]

Research on the Samoan volcanic lineament has focussed on shield volcanism and the volcanic origins of the island chain, and has largely ignored the middle and late Quaternary lavas that dominate the islands of Savai'i and Upolu. Yet these rocks may hold clues to the timing, origins and nature of rejuvenation volcanicity and to important questions of ocean island geochemistry and geodynamics. Upolu is particularly interesting for it possesses both shield-stage and rejuvenation-stage lavas. Although the chronology of shield volcanism is well-established, little is known of its history of rejuvenation volcanicity. The 40Ar/39Ar ages of 131.5 ± 7.3 ka (±2 s) and 138.7 ± 2.7 ka (±2 s) from the compound volcano of Mount Lanotō in southeast Upolu represent the first geochronometric determinations on the rejuvenation-stage lavas of the island. These cast doubt on the last glacial age assigned to the Mulifanua Volcanics formation and provide the first ages for the lavas erupted from the axial rift zone that dominates the island's geomorphology. These ages offer cautious endorsement of the proposition that rejuvenation-stage volcanicity was initiated on the westernmost of the Samoan islands and propagated east. Support for this belief comes from the systematic westward increase in the incidence of rejuvenation-stage lavas along the island chain. [ABSTRACT FROM AUTHOR]

Noble gas isotope systematics of ocean island basalts (OIB) provide evidence for relatively undegassed and primitive mantle sources. These OIB sources partly derive from the deep mantle by virtue of their distinctiveness from mid-ocean ridge basalts (MORB), which dominantly sample upper mantle. New helium, neon and argon isotope data are presented for Canary Islands lavas, carbonatites and cumulate and mantle xenoliths confirming 3He/4He ratios that are the same or lower than MORB, but that are heterogeneous (∼3–9.5R A) within and between islands in the archipelago. Neon and Ar isotope systematics for lavas are mostly within the range of air compositions. Harzburgite xenoliths from Lanzarote, which are interpreted to represent ancient refractory mantle residues, have distinct He-Ne-Ar isotope systematics from lavas or cumulate xenoliths. The harzburgites are characterized by forsteritic olivine (>Fo 91) with uniformly low-3He/4He (6.6 ± 0.2R A), high 40Ar/36Ar (630–4900), and have Ne isotope compositions that range between air values or that are similar or more nucleogenic than depleted MORB mantle (DMM). Similar refractory mantle peridotites have been discovered as xenoliths at other OIB localities and are likely to be distinct from continental lithospheric mantle. Refractory mantle (RM), which by virtue of its high melting temperature is difficult to partially melt, may be a significant component in the convecting mantle and has potential to impart a cryptic noble gas signature to partial melts in intraplate, divergent and convergent margin settings. In this sense, RM may represent a 'sixth mantle component' after DMM, high-µ (high 238U/204Pb; HIMU), enriched mantle endmembers (EMI, EMII) and the 'focus zone' (FOZO). An RM-type component may be presented in the older eastern Canary Islands and possibly in recent rejuvenated volcanism from Teide (Tenerife). Canary Island intraplate volcanism samples multiple mantle components, confirming that the most gas-rich mantle sources involved in magmatism dominate OIB noble gas compositions. [ABSTRACT FROM AUTHOR]

The igneous rock of Manamas Formation in the Bihati River, Baun is one of the igneous rock outcrops in Timor Island that has not been widely studied based on its geochemical characteristic. This study aims to determine the genesis and processes that occur in the igneous rocks of the Manamas Formation within tectonic framework of Timor Island based on petrographic and geochemical analysis. X-ray Fluorescence (XRF) and Inductively Coupled Plasma-mass Spectrometery (ICP-MS) were used to determine the major elements, trace elements, and rare earth elements. The igneous rock of the Manamas Formation is a basalt intrusion with an alkaline affinity which shown an enrichment pattern of rare earth elements identical to Ocean Island Basalt (OIB). This study proves the existence of two different mechanisms of elemental enrichment, fluid related enrichment which related to the subduction activity of the Indian Ocean plate under the Banda Arc and also melt related enrichment which originated from the broken Indian Ocean plate which enters the OIB reservoir zone. The two different magmas then mix and underplating beneath the Banda Arc.

The compositional variability of ocean island basalts (OIBs) is thought to reflect partial melting of a lithologically-heterogeneous mantle source dominated by either pyroxenite or peridotite. The Pohnpei Island in Micronesia, which is associated with the Caroline hotspot, is suggested to have been generated from partial melting of a pyroxenite-rich mantle. To examine this hypothesis, we present new major- and trace-element compositions of olivine phenocrysts in basalts from the island. The olivines exhibit large systematic inter- and intra-crystalline compositional variability. In Sample DS1, olivines record compositional zonation, in which cores have relatively high Fo (77–85), Ni (550×10−6–2 392×10−6), and Fe/Mn ratios (66–82), whereas rims have lower Fo (71–78), Ni (526×10−6–1 537×10−6), and Fe/Mn ratios (51–62). By contrast, olivines within other samples preserve no clear compositional zonation, exhibiting similar or slightly lower Fo values (66–78), Ni contents (401×10−6–1 268×10−6), and Fe/Mn ratios (53–69) as the rims of zoned crystals. The distinct chemical contrast between the two different types of olivine suggests they formed in magma chambers at different depths. Analysis using forward petrological modeling and multi-element indicators (Fe/Mn, Zn/Fe, FC3MS (FeOT/CaO-(3×MgO/SiO2)), Mn/Zn, and Ni/(Mg/Fe)) of whole-rock samples and high-Fo olivines is inconsistent with a pyroxenite-rich mantle source. We suggest these inconsistencies reflect an influence on the partition coefficients of Ni and Mn between olivine and liquid during melting at variable pressures and temperatures. In addition, magma recharge and mixing within the magmatic plumbing system can change the composition of olivine. We suggest that identification of the mantle source of OIBs in volcanic islands such as the Pohnpei Island using olivine geochemistry should be treated with caution. [ABSTRACT FROM AUTHOR]

Seamount accretion is one of the most significant accretionary orogenic processes in the Central Asian Orogenic Belt, but there are few paleo‐seamounts reported from and debate on the tectonic evolution of the Junggar Ocean still exists. In this study, we present geochronological, mineralogical, geochemical and isotopic data for basalts from the Chagantaolegai ophiolitic mélanges in Junggar. Zircon U‐Pb dating on one basalt yielded a weighted mean 206Pb/238U age of 469 ± 7 Ma, which suggests that it formed in the Middle Ordovician. All rock samples belong to alkaline basalt and show similar geochemical characteristics, displaying high TiO2 (∼3 wt%), (La/Yb)N (17.6–19.0), ΣREE (232–289 ppm) and enrichment in Nb and Ta, which implies an ocean island basalt (OIB) affinity. Based upon positive εNd(t) (+4.16 to +4.23), ΔNb (0.20–0.22) and low initial 87Sr/86Sr (0.70425 to 0.70452) and Zr/Nb (3.35–3.57), we suggest that the Chagantaolegai OIB samples were likely derived from a fertile mantle source related to plume. The OIB rock assemblage, chert and marble in the southern part of the Chagantaolegai ophiolitic mélange indicates that a Middle Ordovician seamount was accreted to the Boshchekul‐Chingiz arc due to the northward subduction of the Junggar‐Balkhash Ocean. [ABSTRACT FROM AUTHOR]

Abstract Basalts from the Samoan volcanoes sample contributions from all of the classical mantle endmembers, including extreme EM II and high 3He/4He components, as well as dilute contributions from the HIMU, EM I, and DM components. Here, we present multiple sulfur isotope data on sulfide extracted from subaerial and submarine whole rocks (N = 16) associated with several Samoan volcanoes—Vailulu‘u, Malumalu, Malutut, Upolu, Savai‘i, and Tutuila—that sample the full range of geochemical heterogeneity at Samoa and upon exhaustive compilation of S‐isotope data for Samoan lavas, allow for an assessment of the S‐isotope compositions associated with the different mantle components sampled by the Samoan hotspot. We observe variable S concentrations (10–1,000 ppm) and δ34S values (−0.29‰ ± 0.30 to +4.84‰ ± 0.30, 2σ). The observed variable S concentrations are likely due to sulfide segregation and degassing processes. The range in δ34S reflects mixing between the mantle origin and recycled components, and isotope fractionations associated with degassing. The majority of samples reveal Δ33S within uncertainty of Δ33S = 0‰ ± 0.008. Important exceptions to this observation include: (a) a negative Δ33S (−0.018‰ ± 0.008, 2σ) from a rejuvenated basalt on Upolu island (associated with a diluted EM I component) and (b) previously documented small (but resolvable) Δ33S values (up to +0.027 ± 0.016) associated with the Vai Trend (associated with a diluted HIMU component). The variability we observed in Δ33S is interpreted to reflect contributions of sulfur of different origins and likely multiple crustal protoliths. Δ36S versus Δ33S relationships suggest all recycled S is of post‐Archean origin.

Basalts from the Samoan volcanoes sample contributions from all of the classical mantle endmembers, including extreme EM II and high 3He/4He components, as well as dilute contributions from the HIMU, EM I, and DM components. Here, we present multiple sulfur isotope data on sulfide extracted from subaerial and submarine whole rocks (N = 16) associated with several Samoan volcanoes—Vailulu‘u, Malumalu, Malutut, Upolu, Savai‘i, and Tutuila—that sample the full range of geochemical heterogeneity at Samoa and upon exhaustive compilation of S‐isotope data for Samoan lavas, allow for an assessment of the S‐isotope compositions associated with the different mantle components sampled by the Samoan hotspot. We observe variable S concentrations (10–1,000 ppm) and δ34S values (−0.29‰ ± 0.30 to +4.84‰ ± 0.30, 2σ). The observed variable S concentrations are likely due to sulfide segregation and degassing processes. The range in δ34S reflects mixing between the mantle origin and recycled components, and isotope fractionations associated with degassing. The majority of samples reveal Δ33S within uncertainty of Δ33S = 0‰ ± 0.008. Important exceptions to this observation include: (a) a negative Δ33S (−0.018‰ ± 0.008, 2σ) from a rejuvenated basalt on Upolu island (associated with a diluted EM I component) and (b) previously documented small (but resolvable) Δ33S values (up to +0.027 ± 0.016) associated with the Vai Trend (associated with a diluted HIMU component). The variability we observed in Δ33S is interpreted to reflect contributions of sulfur of different origins and likely multiple crustal protoliths. Δ36S versus Δ33S relationships suggest all recycled S is of post‐Archean origin.Plain Language Summary: Samoa is a group of volcanoes as islands and seamounts that reside in the south Pacific Ocean. This group of volcanoes are unique in that they erupt material from the deep mantle that was once at the surface, suggesting large scale crustal recycling was at play. Furthermore, distinct groupings of volcanoes and seamounts appear to erupt distinct recycled components. However, the nature and relative age of these components are not well understood. We use sulfur isotopes to gain insights into the materials erupted at Samoa. We provide evidence that the mantle plume beneath Samoa is geochemically zoned, as S‐isotopes are distinct at different groups of islands. We also argue that the identified recycled materials are best linked to Proterozoic sulfur and in some cases may represent recycled S reservoirs that are distributed among multiple Pacific Ocean island basalts.Key Points: Basalts from Samoan volcanoes preserve heterogeneous sulfur isotope signaturesWe observe unique S‐isotope compositions associated with distinct groups of Samoan volcanoesSulfur isotope variability is associated with multiple distinct recycled components [ABSTRACT FROM AUTHOR]

The Anatolide-Tauride block in the Tethyan belt represented passive northern continental margin of Gondwana throughout Paleozoic time. In this paper, we report on a relatively large amphibolite-facies metaquartz-syenite to -granite complex (~ 60 km2) in the Eastern Anatolia, the easternmost part of the Anatolide-Tauride block. Igneous textures in the metaintrusive complex are variably preserved. The preserved igneous minerals include mesoperthite + quartz ± aegirine-augite ± sodic amphibole ± calcic amphibole ± biotite and ± allanite. Mesoperthite is the sole feldspar in well-preserved igneous textures, suggesting hypersolvus nature of the original quartz syenite to granite. In the well-recrystallized samples, mesoperthite is partially or totally replaced by discrete grains of plagioclase and K-feldspar. U–Pb dating on igneous zircons from three samples yielded Silurian igneous crystallization ages (430–440 Ma). All rock types have typical geochemical characteristics of A-type syenite to granite with relatively high abundances of FeO*, Na 2 O, K 2 O, Nb, Ga, Zr, Ce, and Zn, and low abundances of MgO, CaO and Sr, resulting in high ratios of Fe/Mg and Ga/Al. Geochemical features such as (i) the absence of negative Nb–Ta anomalies on multielement variation diagrams, (ii) the enrichment of Nb and Ce relative to Y, and (iii) Y/Nb, Yb/Ta and Ce/Nb ratios similar to those in ocean island basalts point to A 1 -type syenite and granite which are thought to be differentiates of ocean island basaltic magmas. Estimated magma temperatures based on zirconium saturation range from 850 to 990 °C, indicating derivation of the original quartz syenite and granite from high-temperature juvenile magmas. Initial εHf values of the igneous zircons from two samples are 2.59 ± 1.14 and 2.30 ± 1.29, while δ18O values of the zircons range from 5.41 to 7.32‰ with a median of 6.50 ± 0.48‰. All these isotopic characteristics suggest that the quartz syenite to granite were derived from high-temperature mantle magmas with insignificant crustal assimilation. The Late Ordovician-Silurian A-type igneous rocks in the northern part of the Anatolide-Tauride block were probably related to a rifting event at the northern margin of Gondwana, leading to the opening of the Paleo-Tethys. This interpretation is also supported from (i) regional geological features of the Early Paleozoic sedimentary successions suggestive of extensional setting during the Late Ordovician-Silurian, (ii) the presence of Silurian anorogenic magmatism in continental blocks detached from Gondwana during Early Paleozoic time, and (iii) the presence of Silurian deep sea sedimentary blocks in Paleo-Tethyan accretionary complexes. Unlabelled Image • The Anatolide-Tauride block was passive continental margin throughout the Paleozoic. • A metaintrusive complex in the Anatolide-Tauride block displays anorogenic affinity. • It formed during Silurian from differentiation of OIB-type melts. • Late Ordovician-Silurian time represents a rifting period at the northern Gondwana. [ABSTRACT FROM AUTHOR]

The noble gas isotope systematics of ocean island basalts suggest the existence of primordial mantle signatures in the deep mantle. Yet, the isotopic compositions of lithophile elements (Sr, Nd, Hf) in these lavas require derivation from a mantle source that is geochemically depleted by melt extraction rather than primitive. Here, this apparent contradiction is resolved by employing a compilation of the Sr, Nd, and Hf isotope composition of kimberlites--volcanic rocks that originate at great depth beneath continents. This compilation includes kimberlites as old as 2.06 billion years and shows that kimberlites do not derive from a primitive mantle source but sample the same geochemically depleted component (where geochemical depletion refers to ancient melt extraction) common to most oceanic island basalts, previously called PREMA (prevalent mantle) or FOZO (focal zone). Extrapolation of the Nd and Hf isotopic compositions of the kimberlite source to the age of Earth formation yields a 143Nd/144Nd-176Hf/177Hf composition within error of chondrite meteorites, which include the likely parent bodies of Earth. This supports a hypothesis where the source of kimberlites and ocean island basalts contains a long-lived component that formed by melt extraction from a domain with chondritic 143Nd/ 144Nd and 176Hf/177Hf shortly after Earth accretion. The geographic distribution of kimberlites containing the PREMA component suggests that these remnants of early Earth differentiation are located in large seismically anomalous regions corresponding to thermochemical piles above the core-mantle boundary. PREMA could have been stored in these structures for most of Earth's history, partially shielded from convective homogenization. [ABSTRACT FROM AUTHOR]

Lithological heterogeneity is a widely accepted feature of the Earth's mantle, with recycled crustal material accounting for a significant part of heterogeneity in ocean island basalt (OIB) geochemistry. Fe isotopes have been used to link geochemical heterogeneity in OIB sources to distinct mantle lithologies due to their mineral-specific equilibrium fractionation effects, or their composition, such as incorporation of kinetically-fractionated core liquids entrained from the core-mantle boundary. Here we present Fe isotope data for Samoan shield, and Azores volcanoes, together with a combined phase-equilibria and equilibrium mineral-melt isotope fractionation model. These OIB lavas allow us to study the roles of core-derived and recycled mantle components in generating heavy δ 57Fe melts. Heavy δ 57Fe correlates with radiogenic isotope signatures of enrichment by a crustal component and not with Fe/Mn or any indicator of core involvement. However, single-stage melting of a MORB-like eclogitic pyroxenite cannot generate the heavy δ 57Fe seen in Pitcairn, Azores, and rejuvenated Samoa lavas. Melts of a reaction-zone pyroxenite (commonly suggested to form part of the OIB source), derived from eclogite melts hybridised with peridotite, also fail to generate the heaviest Fe isotopic compositions seen in OIB. Instead, the generation of heavy δ 57Fe melts in OIB requires: (1) processes that make subducted eclogite isotopically heavier than its pristine precursor MORB (e.g., hydrothermal alteration, metamorphism, sediment input); (2) lithospheric processing, such as remobilisation of previously frozen small-degree melts, or a contribution from lithospheric material metasomatised by silicate melts; and/or (3) melting conditions that limit the dilution of melts with heavy δ 57Fe by ambient lower δ 57Fe materials. No single process we consider can generate the heavy δ 57Fe seen in the Azores, Pitcairn, and rejuvenated Samoan lavas. Therefore, it cannot be assumed that a pyroxenite lithology derived from recycled crustal material is the sole producer of heavy δ 57Fe melts in OIB, nor can these signatures be related to contributions from the Earth's core. Instead, the observation of heavy δ 57Fe OIB melts cannot be ascribed to a unique source or process. This ambiguity reflects the multitude of processes operating from the generation of recycled lithologies through to their mantle melting: from MORB generation, its low temperature alteration, through mantle heterogeneity development and lithospheric processing, to eruption at ocean islands. [ABSTRACT FROM AUTHOR]

Basalts from the islands of Mangaia, Tubuai, Rurutu (old stage), and Raivavae (Rairua stage) in the South Pacific, and from St. Helena Island in the South Atlantic, are characterized by extremely radiogenic Pb isotopic compositions (e.g., 206Pb/204Pb > 20.5). These ocean island basalts (OIBs) define the famous HIMU (high μ, where μ = 238U/204Pb) component in the mantle 'zoo', which has traditionally been thought to represent ancient recycled oceanic crust with highly fractionated U/Pb and Th/Pb ratios. However, recent high-precision analyses of minor and trace elements in olivine phenocrysts from HIMU OIBs suggest a peridotitic mantle source rather than pyroxenitic/eclogitic remnants of subducted oceanic crust. To better constrain the lithology of the HIMU source is crucial for further understanding the nature of the HIMU component. Here we utilize stable iron (Fe) isotopes, a novel tool for identifying the mantle source lithology of basalts, to further investigate the lithology of the HIMU source by examining classic HIMU OIBs from the Cook-Austral volcanic chain in the South Pacific. The results show that these OIBs have δ57Fe values varying from 0.09‰ to 0.18‰, which are similar to those of normal mid-ocean ridge basalts (N-MORBs, δ57Fe = 0.15 ± 0.05‰, 2SD). Quantitative calculations indicate that the fractional-crystallization-corrected δ57Fe corr values (0.06–0.15‰) of these basalts can well be explained by partial melting of garnet peridotite with δ57Fe values ranging from 0.05‰ to 0.09‰. Such low δ57Fe corr values, however, are difficult to be reproduced by partial melting of eclogite with a MORB-like δ57Fe value (δ57Fe = 0.15‰). Our new Fe isotope data, combined with previously reported heavy whole-rock zinc isotopes (δ66Zn = 0.38 ± 0.03‰) as well as high Mn/Fe (100Mn/Fe = 1.5–1.7) and Ca/Al (mostly >15) ratios of olivine phenocrysts in the HIMU OIBs, further confirm that the lithology of the HIMU mantle source is carbonated peridotite. • OIBs from the type localities for the HIMU component show MORB-like δ57Fe values. • The MORB-like Fe isotopic compositions cannot be attributed to magma evolution. • Partial melting of peridotitic HIMU source can explain the observed δ57Fe values. [ABSTRACT FROM AUTHOR]

Ocean island basalts (OIBs) and associated sedimentary units form part of the ocean plate stratigraphy accreted onto island arcs or continental arcs and provide important insights into convergent margin processes. Here, we investigate some of the typical localities of Sanpozan, Shirakidani, and Kamiananai within the Chichibu composite belt in Kochi, SW Japan, where the accretionary complexes incorporate OIBs and sedimentary rocks that preserve the travelogue of an oceanic plate from a mid‐oceanic ridge (MOR) to subduction zone. We present petrology and geochemistry, as well as zircon U–Pb geochronology and REE data of the accreted OIB and sedimentary formations from Sanpozan, Shirakidani, and Kamiananai. The OIBs are enriched in LILE and LREE and depleted in HREE. The chondrite‐normalised REE patterns of zircon grains from OIBs and sedimentary rocks display LREE depletion and HREE enrichment, sharply positive Ce and Sm anomalies and negative Pr and Eu. LA‐ICP‐MS U–Pb geochronology of zircon grains from the OIBs shows three major populations with 206Pb/238U mean ages of 164–456, 724–955, and 1,638–2,202 Ma. The diverse ages suggest heterogeneous recycled source components in the plume‐related upwellings that generated the OIBs, as well as different times of formation and accretion of the Palaeozoic and Mesozoic oceanic islands. The dominant population of zircon grains from the sedimentary units shows 206Pb/238U mean ages of 147 and 288 Ma and indicate a major change in the provenance regime with subduction and the disappearance of older arcs attesting to the destruction of continental crust in convergent margins. [ABSTRACT FROM AUTHOR]

A multi-disciplinary study of the KR1 segment of the Australian–Antarctic Ridge has been conducted since 2011. We present geochemical and age dating results for samples dredged from three sites on the KR1 seamount trail. The majority of the samples are alkaline ocean island basalts with subdominant enriched tholeiites. The samples from the DG05 bathymetric depression include ice-rafted erratics from Antarctica, which consist of gabbro, diabase, various granitoids, volcanic rocks such as trachyte and rhyolite and deformed or undeformed sedimentary rocks. The main provenance of glacial erratics is considered to be the Ross Sea region. However, Carboniferous to Cretaceous ages of erratics indicate that some of these may originate from the western regions of West Antarctica. Based on the size and topography of the volcanic features and geochemical characteristics of the alkaline ocean island basalts (La/SmN = 2.62–3.88; Tb/YbN = 1.54–2.67) and the enriched tholeiites, the KR1 seamount trail is interpreted to be a submarine hotspot chain that is the product of alkaline volcanic eruption and seafloor spreading. [ABSTRACT FROM AUTHOR]

West Junggar is located at the southwest margin of the Central Asian Orogenic Belt and includes Silurian pillow basalts of the Mayilashan Formation. The petrogenesis and tectonic setting of these pillow basalts are important for the understanding of the tectonic evolution and metallogeny of the West Junggar area. This paper presents geochronological, geochemical, and whole‐rock Sr–Nd–Pb isotope data from the pillow basalts of the Mayilashan Formation. Zircon LA‐ICP‐MS U–Pb dating of a pillow basalt, which is in conformable contact with the chert, suggests that they were erupted at 437.2 ± 2.2 Ma marking the timing of generation of these rocks as Middle Silurian. Geochemically, all the pillow basalts bear the signature of ocean island basalt (OIB), and are characterized by alkaline affinity with high concentrations of TiO2 (3.28–4.12 wt.%), LILE and LREE enrichment and HREE depletion ((La/Yb)N = 5.5–7.3), with very weak Eu anomalies (Eu/Eu* = 0.96–1.06), and no obvious Nb, Ta, or Ti negative anomalies. Their Sr–Nd–Pb isotopic compositions ((87Sr/86Sr)I = 0.7037–0.7051, εNd(t) = 1.9–2.9, 206Pb/204Pbi = 17.74–18.22, 207Pb/204Pbi = 15.48–15.52, and 208Pb/204Pbi = 36.49–37.86) show Dupal‐like isotopic signature of ophiolites in the southern Paleo‐Asian Ocean. These characteristics indicate that the magmas were derived from a deep OIB reservoir, that is, a depleted but slightly heterogeneous asthenospheric mantle source with ~5–15% partial melting of garnet and spinel lherzolite. Our obtained results, in conjunction with previous published data, allow us to suggest that the alkaline pillow basalts formed in a seamount within an intraoceanic setting, where a larger number of seamounts with different ages occurred in the Paleo‐Asian Ocean. [ABSTRACT FROM AUTHOR]

Abstract Thallium (Tl) isotope compositions of ocean island basalts (OIBs) have been proposed as a novel tracer of subducted oceanic crust and sediments in ocean island basalt sources, which could act as direct confirmation that deep mantle recycling eventually resurfaces through mantle upwelling to form ocean island basalt magmatism. However, it is unknown if oceanic crust that went through an active subduction zone would retain the Tl isotope compositions recorded in hydrothermally altered oceanic crust and authigenic marine sediments. In this study we present Tl isotope and concentration data for samples of subducted oceanic crust from five different locations: Zambezi Belt, Zambia; Cabo Ortegal complex, Iberian Massif, Spain; Raspas Complex, southwest Ecuador; Syros island, Cyclades, Greece; Tian Shan, northwest China. Thallium concentrations in most samples follow strong linear relationships with K, Rb, Cs and Ba, which strongly suggest that the mineral phengite is the primary control of Tl abundances in subducted oceanic crust. This conclusion is consistent with recent Tl data sets for arc lavas that imply residual phengite in the arc lava source regions as a strong control of Tl recycling. We find that Tl isotope compositions vary widely and systematically in each location depending on the protolith, metamorphic and metasomatic history of the samples. Samples from Cabo Ortegal and Raspas Complex reveal Tl isotope compositions similar to their protoliths, which were comprised of low-temperature altered oceanic crust. Tian Shan metamorphic rocks and Zambian eclogites reveal invariant Tl isotope values indistinguishable from average mantle, which is best explained by overprinting by metamorphic fluids that contained high concentrations of Tl and other alkali metals. Samples from Syros reveal a range of Tl isotope compositions from normal mantle towards values for pelagic clay sediments. The sediment-like values in Syros likely arose from fluids released from the surrounding mélange matrix that consists of serpentinite, metagabbros and metasediments. Each of the three Tl isotope ranges observed for subducted oceanic crust samples here are mirrored by individual OIB locations. Cabo Ortegal and Raspas Complex display Tl isotope compositions identical to St. Helena, suggesting that the HIMU component likely comprises subduction modified low-temperature altered oceanic crust. Thallium isotope ratios in Zambia and Tian Shan eclogites and blueschists are identical to lavas from Iceland, whereas Syros metamorphic rocks overlap almost exactly with lavas from Hawaii. Our data, therefore, show that Tl isotope compositions of oceanic crust and sediments can be traced through the subduction process and eventually is expressed largely unmodified in ocean island basalts. [ABSTRACT FROM AUTHOR]

Crustal recycling can induce significant chemical and lithological heterogeneity of the mantle. Ocean island basalts (OIBs) are the surface expressions of rising mantle plumes from the Earth's deep interior and thus can be a probe of the compositional heterogeneity in the deep mantle. Endmember OIBs sampled the most isotopically extreme mantle components including EM1 (Enriched mantle 1), EM2 (Enriched mantle 2), HIMU (high μ ; μ = 238U/204Pb), and FOZO (Focus zone), but the lithological properties of their source regions are still not well understood. Here we explore the potential of stable calcium (Ca) isotopes in identifying mantle source lithology by investigating OIBs from the type localities for the several mantle components. We find that the δ44/40Ca values of classic HIMU OIBs (0.77 ± 0.09‰; 2SD, N = 15) and Louisville FOZO OIBs (0.78 ± 0.06‰; 2SD, N = 4) are indistinguishable from the average MORB (mid-ocean ridge basalt) value (0.84 ± 0.09‰; 2SD, N = 31), while the δ44/40Ca values of the Pitcairn EM1 (0.65–0.72‰) and the Samoan (Tutuila) EM2 OIBs (0.61–0.71‰) are distinctly lower than those of MORBs. Such Ca isotopic distinction cannot be attributed to magmatic differentiation because the EM1 and EM2 OIBs have obviously lower δ44/40Ca values than the HIMU and FOZO OIBs even at a same MgO content. Equilibrium fractionation during partial melting of garnet peridotite with bulk silicate Earth (BSE) δ44/40Ca (0.94 ± 0.05‰) can explain the Ca isotopic compositions of the HIMU and FOZO OIBs but is unable to reproduce the Ca isotopic variation of all OIBs in this study. Quantitative modeling suggests that the coupling of low δ44/40Ca (< 0.7‰) and high Gd/Yb (> 3.5) ratios in the EM1 and EM2 OIBs most likely points to an eclogitic source component with MORB-like or even lower δ44/40Ca (e.g., < 0.8‰), possibly reflecting contributions from recycled oceanic crust and sediments. A negative correlation between average δ44/40Ca and δ57Fe values (a useful proxy for mantle source lithology) of all endmember OIBs further confirms this proposal and highlights the significant contribution of distinct source lithologies to the Ca isotopic variations in these basalts. Therefore, our study indicates that Ca isotopes of basalts can be another promising tracer of lithological heterogeneity in the mantle. • OIBs from the type localities for the mantle endmembers show heterogeneous δ44/40Ca. • MORB-like δ44/40Ca of the HIMU and Louisville FOZO OIBs points to peridotitic sources. • Low δ44/40Ca values of the EM1 and EM2 OIBs require an eclogitic source component. [ABSTRACT FROM AUTHOR]

• Indonesia contains arc volcanoes that are spatially and/or geochemically anomalous. • Giant porphyry deposits are spatio-temporally associated with anomalous volcanism. • We suggest these anomalous volcanoes and ore deposits are linked to slab tearing. • Slab tearing may also facilitate extremely explosive eruptions in island arcs. Most Holocene volcanoes in Indonesia display geochemical and geophysical properties characteristic of formation via mantle wedge metasomatism. However, some volcanoes (Toba, Ungaran, Muriah, Rinjani, Tambora, Sangeang Api, and Batu Tara) are anomalous, showing alkaline geochemical affinities, anomalously violent recent eruptions, and/or positioning above sections of slab generally considered too deep to induce mantle melting via slab dehydration. These anomalous volcanoes are spatially and temporally associated with giant Pliocene–recent porphyry Cu deposits (in thin continental or oceanic crust), contrary to global occurrences of porphyry Cu deposits that are typically restricted to thick (>45 km crustal thickness) continental arcs. Using geophysical data to model slab geometry and geochemical data to evaluate source compositions and melting mechanisms, we demonstrate that anomalous Indonesian volcanoes and giant porphyry Cu deposits are linked to slab segmentation due to slab tearing. In the slab beneath Toba, the mechanically weak Investigator Fracture Zone, which separates a more buoyant Cenozoic segment of the Indo-Australian plate from older, denser, and more steeply dipping Jurassic–Cretaceous lithosphere to the east, likely created a sub-vertical slab tear feeding volcanism via poloidal mantle flow. The alkaline volcanoes Muriah, Ungaran, Tambora, and Sangeang Api (as well as the sub-alkaline volcano Rinjani) are positioned along edges of slab segments defined by negative seismic velocity anomalies at depths of ∼185–400 km, which we interpret as elongate slab windows formed during localised choking of subduction by anomalously buoyant oceanic lithosphere. Although no negative velocity anomalies are associated with Batu Tara, we speculate that this alkaline volcano is located above a small-scale slab tear, potentially associated with the subducted Scott Plateau. We propose that poloidal asthenospheric flow along slab segment boundaries has triggered deep, low-degree partial melting with varying contributions of slab melt, sediment, and fluid. Products of this melting regime are sub-alkaline and alkaline lavas with trace element patterns enriched in incompatible elements relative to the main arc volcanoes. Based on spatial, temporal, and geochemical associations of giant Pliocene–recent porphyry Cu deposits with proposed slab tears (and an absence of evidence for slab melting in the formation of these deposits), we suggest that slab tears may have generated oxidised magmas by facilitating melting in the garnet stability field, emulating the petrogenetic effects of thick continental crust and slab melting. We further hypothesise that slab tearing locally enhanced magma flux, leading to extremely violent Quaternary eruptions at Toba, Rinjani, and Tambora. [ABSTRACT FROM AUTHOR]

The origins of differences in the Mn and Ni contents of olivine phenocrysts from ocean island basalts (OIB) and mid-ocean ridge basalts (MORB) are disputed. One prevalent hypothesis is mixing between peridotite melts and melts of an olivine-free pyroxenite that forms by reaction between large extents of melting of recycled ocean crust and peridotite. A second is peridotite melting at various pressures without contributions from recycled materials, and a third is mixing of recycled crust into peridotite sources. All models purport to account for olivine data, but an additional requirement is that the models account for corresponding liquid compositions. Here we demonstrate that none of the proposed models alone are able to account for both olivine and liquid data. The pyroxenite model predicts variations in liquid Mn to account for high Fe/Mn ratios, in contrast to the data where the high ratios result primarily from Fe variations. Using an updated model of Ni partitioning, combined with quantitative modeling of polybaric melting, confirms the important role of pressure on olivine compositions, and in addition shows that extent of melting and mantle fertility are also important. Most MORB are consistent with ∼10% -15% melting of peridotite at lower pressures, but low Ni olivines found on ridges adjacent to hot spots require a depleted mantle source. High Ni olivines can be produced by low degree melting of peridotite at high pressure, but such models do not produce the high Ni liquids that are also observed, nor the trace element abundances such as high TiO 2. Mixing basalt and peridotite sources is not successful because it fails to account for the low Na/Ti ratios of OIB. A successful model for OIB involves adding ∼3–5% of a low degree eclogite melt with high trace element abundances and low Na/Ti to mantle peridotite, and then melting this source to small extents at high pressure. The need for a low degree melt of eclogite also rules out models that call upon high degrees of melting of eclogite that react with peridotite to produce OIB sources, and shows that this melting does not occur during upwelling beneath hot spots, where high degree melts of eclogite would occur. A firm conclusion of this work is that olivine compositions cannot be used to calculate the amount of recycled crust in mantle sources. Further work to explore variations within OIB and MORB will benefit from the realization that olivine compositions alone cannot be interpreted reliably without quantitative polybaric melting models and attention to co-existing liquids and trace element abundances. • Develop an improved equation for Ni partitioning between olivine and liquid. • Ni and Mn are affected by melting pressures, extents and peridotite fertility. • Peridotite fertilized by 3–5% low degree eclogite melt is the source for Hawaii. • High pressure-low extent fertilized peridotite melting form Hawaii olivine and lava. • Two-stage pyroxenite model cannot simultaneously explain Hawaii olivine and lava. [ABSTRACT FROM AUTHOR]

Abstract Iron isotopic compositions are demonstrably powerful tracers of foundational planetary processes, including crust and core formation. In many volcanic environments, however, geochemical vestiges of these processes are obscured by the effects of magmatic differentiation on Fe isotopic compositions. Recent decades have witnessed continued refinement of observational and experimental approaches to Fe isotope fractionation during silicate differentiation. In contrast, the influence of sulfide fractionation on Fe isotopic compositions in terrestrial environments is known only from theoretical approaches and limited experimental data for relatively siliceous magmatic systems. One reason for this may be that sulfide fractionation is difficult to definitively trace using traditional major and minor element variation patterns. We utilize well-characterized lavas and cumulate xenoliths from Piton de la Fournaise and Piton des Neiges, Réunion Island, that have previously been examined for their highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re) contents to investigate the effect of sulfide fractionation on Fe isotopes. The Fe isotopic compositions of the basalts range from δ56Fe values of 0.04 to 0.15‰ (average: 0.10‰) and the compositions of the cumulate xenoliths range from δ56Fe values of −0.07 to 0.08‰ (average: 0‰). In the absence of metal, HSE preferentially partition into sulfide phases, making them important tracers of sulfide segregation during magmatic differentiation. We find that commonly-observed co-variations between Fe isotopic compositions and major element oxide abundances are relatively underdeveloped for Réunion lavas. The correlation between Fe isotopic composition and MgO, for example, has a similar statistical significance to the correlation between Fe isotopic composition and Pd/Ir ratios, suggesting an important role of sulfides during Fe isotopic fractionation. After accounting for sulfide segregation, we determine that the parental magma Fe isotopic composition calculated for Piton de la Fournaise would be overestimated by 0.04‰ (within propagated error, 0.01–0.06‰) when considering silicate differentiation alone. An analogous calculation for Kilauea Iki basalts, for which there is available Fe isotopic and HSE data, yields a somewhat smaller difference of 0.02‰ (0–0.03‰). These differences may partially explain Fe isotopic compositions in other settings that could not previously be reconciled with a dominantly peridotitic and/or chondritic mantle source. This discovery may warrant discussion of the apparent decoupling between Fe and radiogenic isotopes in ocean island basalts, where the latter shows significant global variations and the former may show little or none. Our work highlights the need for additional constraints on the behavior of Fe isotopes during crustal recycling processes and reinforces the notion that consideration must be given to the effect of magmatic differentiation on Fe isotopic compositions. [ABSTRACT FROM AUTHOR]

Of the isotopically distinctive mantle domains, the so-called HIMU (“high-μ”; μ =  238 U/ 204 Pb) source is the most extreme, and its genesis continues to be debated. We report very strong U enrichment at unchanged Th concentrations in Cretaceous oceanic serpentinites with exceptionally high 206 Pb/ 204 Pb (reaching 56) but unchanged 208 Pb/ 204 Pb. Similar, but less extreme, features are found in 1.9 billion years old altered oceanic crust (AOC). Forward modelling demonstrates that mantle, if metasomatised by supercritical liquids derived from AOC and serpentinites, evolves to the HIMU Pb isotope signatures, while satisfying experimental and empirical constraints on subduction zone element processing. By contrast, no model solutions for the conventional proposal of the HIMU source representing residual igneous altered oceanic crust can be reconciled with 208 Pb/ 204 Pb, strengthening the need for a paradigm shift regarding HIMU OIB genesis. Over time, the net U addition to the convecting mantle via deeply subducted serpentinite has expressed itself as the so-called second terrestrial Pb isotope paradox, or kappa conundrum. [ABSTRACT FROM AUTHOR]

The South Tianshan Orogenic Belt (STOB) is situated in the southwest margin of the Central Asian Orogenic Belt and is characterized by the distribution of ophiolitic mélanges and metamorphic rocks. In this study, we focus on 5 ophiolitic mélanges with ocean island basalt (OIB) and aim to discuss the tectonic evolution of the South Tianshan Ocean. The Kynda, Kainar, Uchkel, Heiyingshan, and Guluogou ophiolitic mélanges in STOB mainly consist of metamorphic peridotite, gabbros, basaltic lavas (pillows), abyssal radiolarian cherts, and limestones. The basaltic rocks mainly occur as tectonic blocks within the mélanges. These basaltic rocks are characterized by LREE enrichment and HREE depletion, very weak or no Eu anomalies, and no obvious Nb, Ta, and Ti negative anomalies, positive εNd(t) values, relatively high initial 87Sr/86Sr ratios, and wide 206Pb/204Pb ratios, showing the signature of OIB. The OIB‐type rocks from Kynda, Kainar, and Guluogou are likely to be derived from garnet lherzolite by relatively high degrees of melting (10% to 20%). But the basaltic rocks of Uchkel and Heiyingshan formed at significantly lower degrees of partial melting (1% to 5%) of spinel and/or garnet lherzolite. The OIB‐type rocks in these ophiolitic mélanges are considered as accreted seamount fragments and may represent mantle plume‐related magmatism within the South Tianshan Ocean. In combination with previous work, we suggest an updated model, which is bidirectional oceanic subduction with seamounts accretion and subduction system, is suitable for Palaeozoic tectonic evolution of the South Tianshan Ocean. [ABSTRACT FROM AUTHOR]

The Meso-Cenozoic geodynamic evolution of the Eastern Pontides Orogenic Belt, a mountain chain extending parallel to the southeastern margin of the Black Sea, has been controversial for the last forty years. Here we present data for a newly discovered alkaline gabbro body and its surrounding basaltic rocks in the northern part of the Eastern Pontides Orogenic Belt. We also provide a comprehensive assessment of the Late Mesozoic-Cenozoic geodynamic evolution of the Eastern Mediterranean region. The gabbroic body is bounded by reverse faults along its northern and southern borders and is surrounded by vesicular, pillow-fragment breccias and pillow basalts. Mineral compositions suggest that crystallization of the gabbros began at about 1170 °C, and the lowest preserved crystallization T is near 1000 °C. Estimated pressure at the beginning of crystallization is 5.7–7.4 kb. The 40 Ar/ 39 Ar dating of kaersutite and plagioclase and U Pb dating of titanite indicated that the Hayrat gabbro crystallized at ~67 Ma (Late Maastrichtian). Whole rock major-trace-rare earth element and Sr-Nd-Pb isotope data indicate that the gabbros and basalts have different origins. The gabbros are alkaline and exhibit the geochemical features of OIB, whereas the basalts are tholeiitic and reveal depletions of HFSE that are similar to those of arc rocks. The gabbros are strongly fractionated, and derive from an enriched, lithospheric mantle source, with partial melting occurring in a garnet-stable environment. The basalts are less fractionated, and probably derive from a shallower source in which spinel peridotite was the predominant lithology. Considering all new and old geological, geochemical, geochronological and geophysical data from the Black Sea Basin and the Eastern Pontides-Lesser Caucasus-Alborz Orogenic Belt, we suggest that the alkaline Hayrat gabbro formed in an oceanic intraplate setting, and was accreted to the forearc region of the Eastern Pontides Orogenic Belt during southward subduction of Paleotethyan lithosphere. It was later tectonically juxtaposed with subaqueously erupted, arc-related basalts. [ABSTRACT FROM AUTHOR]

The tectonic setting in which Jurassic igneous rocks of the Sanandaj-Sirjan Zone (SaSZ) of Iran formed is controversial. SaSZ igneous rocks are mainly intrusive granodiorite to gabbroic bodies, which intrude Early to Middle Jurassic metamorphic basement; Jurassic volcanic rocks are rare. Here, we report the age and petrology of volcanic rocks from the Panjeh basaltic-andesitic rocks complex in the northern SaSZ, southwest of Ghorveh city. The Panjeh magmatic complex consists of pillowed and massive basalts, andesites and microdioritic dykes and is associated with intrusive gabbros; the overall sequence and relations with surrounding sediments indicate that this is an unusually well preserved submarine volcanic complex. Igneous rocks belong to a metaluminous sub-alkaline, medium-K to high-K calc-alkaline mafic suite characterized by moderate Al 2 O 3 (13.7–17.6 wt%) and variable Fe 2 O 3 (6.0–12.6 wt%) and MgO (0.9–11.1 wt%) contents. Zircon U-Pb ages (145–149 Ma) define a Late Jurassic (Tithonian) age for magma crystallization and emplacement. Whole rock compositions are enriched in Th, U and light rare earth elements (LREEs) and are slightly depleted in Nb, Ta and Ti. The initial ratios of 87 Sr/ 86 Sr (0.7039–0.7076) and εNd(t) values (−1.8 to +4.3) lie along the mantle array in the field of ocean island basalts and subcontinental metasomatized mantle. Immobile trace element (Ti, V, Zr, Y, Nb, Yb, Th and Co) behavior suggests that the mantle source was enriched by fluids released from a subducting slab (i.e. deep-crustal recycling) with some contribution from continental crust for andesitic rocks. Based the chemical composition of Panjeh mafic and intermediate rocks in combination with data for other gabbroic to dioritic bodies in the Ghorveh area we offer two interpretations for these (and other Jurassic igneous rocks of the SaSZ) as reflecting melts from a) subduction-modified OIB-type source above a Neo-Tethys subduction zone or b) plume or rift tectonics involving upwelling metasomatized mantle (mostly reflecting the ~550 Ma Cadomian crust-forming event). [ABSTRACT FROM AUTHOR]

The short-lived Sm 146 → 142 Nd isotope system ( t 1 / 2 = 103 Ma ) provides constraints on the timing and processes of terrestrial silicate fractionation during the early Hadean. Although some Archean terranes preserve variability in 142 Nd/ 144 Nd, no anomalies have been resolved previously in young rocks. This study provides high precision 142 Nd/ 144 Nd data on a suite of ocean island basalts from Samoa and Hawaii previously shown to have variable depletions in 182 W/ 184 W that are inversely correlated with 3 He/ 4 He ratios. Improved analytical techniques and multiple replicate analyses of Nd show a variation in μ 142 Nd values between −1.3 and +2.7 in the suite, relative to the JNdi standard. Given the reproducibility of the standard (±2.9 ppm, 2 SD), two Samoan samples exhibit resolved variability in their 142 Nd/ 144 Nd ratios outside of their 95% confidence intervals, suggesting minor variability in the Samoan hotspot. One sample from Samoa has a higher μ 142 Nd of +2.7, outside the 95% confidence interval (±1.0 ppm) of the average of the JNdi standard. Limited, but resolved, variation in 142 Nd/ 144 Nd within the suite suggests the preservation of early Hadean silicate differentiation in the sources of at least some basalts from Samoa. Larger variations of 182 W/ 184 W and 3 He/ 4 He ratios in the same samples suggest that metal–silicate separation and mantle outgassing left a more persistent imprint on the accessible mantle compared to 142 Nd/ 144 Nd ratios which are impacted by early silicate differentiation. [ABSTRACT FROM AUTHOR]

Chemical Geodynamics relies on a paradigm that the isotopic composition of ocean island basalt (OIB) represents equilibrium with its primary mantle sources. However, the discovery of huge isotopic heterogeneity within olivine-hosted melt inclusions in primitive basalts from Kerguelen, Iceland, Hawaii and South Pacific Polynesia islands implies open-system behavior of OIBs, where during magma residence and transport, basaltic melts are contaminated by surrounding lithosphere. To constrain the processes of crustal assimilation by OIBs, we employed the Magma Chamber Simulator (MCS), an energy-constrained thermodynamic model of recharge, assimilation and fractional crystallization. For a case study of the 21-19 Ma basaltic series, the most primitive series ever found among the Kerguelen OIBs, we performed sixty-seven simulations in the pressure range from 0.2 to 1.0 GPa using compositions of olivine-hosted melt inclusions as parental magmas, and metagabbro xenoliths from the Kerguelen Archipelago as wallrock. MCS modeling requires that the assimilant is anatectic crustal melts (P2O5 ≤ 0.4 wt.% contents) derived from the Kerguelen oceanic metagabbro wallrock. To best fit the phenocryst assemblage observed in the investigated basaltic series, recharge of relatively large masses of hydrous primitive basaltic melts (H2O = 2-3 wt%; MgO = 7-10 wt.%) into a middle crustal chamber at 0.2 to 0.3 GPa is required. Our results thus highlight the important impact that crustal gabbro assimilation and mantle recharge can have on the geochemistry of mantle-derived olivine-phyric OIBs. The importance of crustal assimilation affecting primitive plume-derived basaltic melts underscores that isotopic and chemical equilibrium between ocean island basalts and associated deep plume mantle source(s) may be the exception rather than the rule. [ABSTRACT FROM AUTHOR]

Trace elements are widely used to interpret the origin of mantle-derived magmas, yet we lack detailed understanding of how trace elements behave during melting of mantle source components. Here, we present new data on trace element distribution and partitioning between phases from high pressure (3.0 to 5.0 GPa), high-temperature (1230 to 1550 °C) melting experiments on starting compositions that represent altered oceanic crust and metasedimentary protoliths. These compositions are expected to be recycled into the mantle via subduction or delamination to form heterogeneous mantle domains that are implicated in the genesis of intraplate and/or ocean floor magmas. In most of the experiments, the investigated trace elements behave incompatibly, expect for HREE and Y, which are compatible in garnet, and V, Cr and Zn, which partition into both garnet and clinopyroxene. Relative to Nd, P is more compatible in garnet than clinopyroxene, leading to fractionation of P/Nd with melting in some cases. Melt compositions in some experiments with low melt fractions feature distinctive negative anomalies for Nb, and for Sr, Ba and Eu, due to retention of these elements in minor/accessory rutile and feldspar, respectively. We also show that highly incompatible trace element (e.g., Cs, Th, U, LREE) concentrations in melts are strongly controlled by melt fraction, whereas moderately incompatible (M-HREE, Zr) to compatible (Cr, V) element concentrations are controlled by temperature and/or phase composition. Pressure has relatively little influence on trace element behaviour at the investigated conditions. Based on our results, we suggest that partial melting of eclogitic components of mantle domains may ultimately produce magmas with trace element compositions that are unlike peridotite-sourced magmas. Therefore, the trace element systematics of mantle-derived magmas should not only be interpreted in terms of mantle source compositions, but also with consideration to source petrology (e.g., mineral compositions and accessory phase stability) and melting conditions (e.g., melt fraction, pressure, temperature). [ABSTRACT FROM AUTHOR]