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How eclogite/pyroxenite‐derived melts evolve through the refractory lithosphere above a plume remains poorly understood. Here we conducted layered experiments of reaction between tholeiitic melts and harzburgite at 2–3 GPa, 1,400–1,500°C, with a run duration ranging from 2 to 24 hr. The resulting residual melts exhibit lower SiO2, TiO2, Al2O3, FeO, CaO, and total alkali contents, higher Ni, MgO, and Mg#, and almost constant CaO/Al2O3 compared to the initial tholeiitic melts. The compositions of the residual melts are influenced by factors such as the melt/harzburgite mass ratio, temperature, and run duration. Decreasing the melt/rock ratio or increasing temperature and run duration leads to a greater extent of assimilation. Under disequilibrated conditions (2 hr), the residual melts have higher SiO2, FeO, and MgO, and lower CaO, Al2O3, and total alkali contents compared to those under equilibrated conditions. The results suggest that interface reactions involving olivine dissolution and orthopyroxene precipitation, and chemical diffusion occur simultaneously during the interaction process. The compositions of the residual melts are largely controlled by interface reactions within 2 hr, followed by dominant chemical diffusion between the melts and refertilized harzburgite from 2 to 24 hr. Based on the experimental results, we propose a two‐stage model for the origin of Hawaiian shield stage parental magmas. Eclogite/pyroxenite‐derived tholeiitic melts first react with harzburgite, with varying melt/rock ratios, to produce residual melts in the deep lithosphere. These residual melts subsequently mix with plume peridotite‐derived melts at shallow depths, contributing to the geochemical diversity observed in Hawaiian shield stage lavas. Plain Language Summary: We conducted experiments to understand how melts derived from subducted oceanic crust interact with surrounding rocks during their journey through the Earth's lithosphere. By simulating melt‐rock reaction along wall‐rock boundary under high‐pressure and high‐temperature conditions, we found that the interaction mechanism involves olivine dissolution and orthopyroxene precipitation. The extent of interaction between the melts and rocks depends on factors such as the amount of melt present and the duration of the interaction. We also observed that certain chemical reactions took place at the interface between the melts and rocks, which affect the composition of the resulting melts. Our findings suggest that the formation of magmas with specific chemical compositions, similar to those observed in Hawaiian shield stage lavas, can be elucidated by the reaction between tholeiitic melts derived from eclogite/pyroxenite and harzburgite. The harzburgite was generated through prior high‐degree melting of peridotite influenced by the high temperatures in the plume setting. We propose a two‐stage model involving initial reactions in the deep lithosphere followed by mixing of the melts in shallower depths. This model provides insights into the processes that contribute to the diversity of magmas observed in Hawaiian volcanic islands. Key Points: Eclogite melts reacting with harzburgite produce high SiO2, MgO, and Ni tholeiitic meltsResidual melt composition is influenced by melt size and reaction time, involving interface reactions and diffusionEclogite melt interaction with harzburgite, followed by mixing with peridotite melts, accounts for Hawaiian shield stage lava generation [ABSTRACT FROM AUTHOR]

• 2.25 Ga A 2 -type granites are identified in the SW Yangtze Block. • The Paleo-Mesoproterozoic A 2 -type granites yield a secular increase in δ18O. • Diverse high-δ18O materials contribute to this secular increase. The elevation of δ18O in zircon through Earth history has been previously identified as being coeval with the assembly of post-Archean supercontinents. This was interpreted to result from the increasing incorporation of high-δ18O sediments into magmas due to enhanced crustal thickening and reworking during supercontinent assembly. However, little attention has been paid to the variations of δ18O in igneous rocks during the extensional phases of the supercontinent cycle. Given the scarcity of early Paleoproterozoic granites due to the ca. 2.3–2.2 Ga tectono-magmatic lull, we first report a suite of ca. 2.25 Ga granites from the SW Yangtze Block by providing zircon U-Pb-Hf-O isotopes, whole-rock geochemical, and Sm-Nd isotopic analyses to constrain their petrogenesis. The results demonstrate that they are A 2 -type granite and originated from the partial melting of Meso-Neoarchean igneous crust in a post-collision extensional setting. Integrating these data with a global compilation of Paleo-Mesoproterozoic A 2 -type granites allows us to further explore secular changes in elemental and isotopic compositions of these granite types. The A 2 -type granites in this study exhibit mantle-like zircon δ18O values, in accord with those from the compiled pre-1.85 Ga A 2 -type granites. Additionally, the data show a secular increase in zircon δ18O for the compiled Paleo-Mesoproterozoic A 2 -type granites. Similar increasing trends are also observed in various compositional proxies that imply the increasing addition of mafic igneous rocks into magma sources that are likely associated with the back-arc extensional settings of the Great Proterozoic Accretionary Orogen (GPAO). Considering that the compiled post-1.8 Ga A 2 -type granites have depleted Nd-Hf isotopes, we posit that they were mainly derived from mafic rocks with arc-like geochemical compositions. In this context, the secular increase in zircon δ18O for the post-1.8 Ga A 2 -type granites might reflect the increasing assimilation of altered oceanic crust into the back-arc and inboard magmatic systems of retreating accretionary orogens. In contrast, the compiled ca. 1.85 Ga A 2 -type granites possess non-radiogenic Hf and elevated O isotopes, indicative of the incorporation of ancient sediments in their magma sources. We suggest that they are most likely generated in a post-collision extensional setting associated with the very early stages of assembly of the Columbia/Nuna supercontinent. [ABSTRACT FROM AUTHOR]

How subduction-related magmatism starts at convergent plate margins is still poorly understood. Here we show that boron isotope variations in early-formed boninites from the Izu-Bonin arc, combined with radiogenic isotopes and elemental ratios document rapid (~0.5 to 1 Myr) changes in the sources and makeup of slab inputs as subduction begins. Heterogeneous hornblende-granulite facies melts from ocean crust gabbros ± basalts fluxed early melting to generate low silica boninites. Hydrous fluids from slab sediments and basalts later fluxed the low silica boninites mantle source to produce high silica boninites. Our results suggest that initially the uppermost parts of the slab were accreted near the nascent trench, perhaps related to early low-angle subduction. The rapid changes in slab inputs recorded in the boninites entail a steepening subduction angle and cooling of the plate interface, allowing for subduction of slab sediment and basalt, and generating hydrous fluids at lower slab temperatures. The geochemical record of subduction initiation is still not well understood, despite >50 years of study. Here, the authors use boron isotopes in Izu-Bonin boninites to document rapid changes in slab inputs to melting at the start of subduction, related to the steepening and cooling of the downgoing Pacific plate. [ABSTRACT FROM AUTHOR]

The origin of enriched mantle 1 (EM1) component in the mantle has long been debated. EM1 source of basalts has been variously attributed to recycled sub-continental lithosphere, recycled lower continental crust, or recycled oceanic crust plus pelagic sediments. Molybdenum stable isotope systematics has the potential to resolve this debate, because of the significant isotopic differences between mantle and crustal reservoirs. Here we present molybdenum isotope data for EM1-type continental intraplate basalts from Nuominhe (NMH), Northeast China. The NMH basalts display significant variations in both δ98/95Mo (98Mo/95Mo ratio relative to NIST SRM 3134, −0.49 to −0.15‰) and Mo/Ce (0.009–0.037), and strong correlations between δ98/95Mo, Mo/Ce and Hf isotope and trace elements. EM1 end-member of the NMH basalts shows the lightest δ98/95Mo and lowest Mo/Ce values. The involvement of continental crust and sub-continental lithospheric mantle cannot account for the light Mo isotopic compositions of the basalts. By contrast, the recycled oceanic crust with pelagic sediments, characterized by low δ98/95Mo and Mo/Ce values, is the satisfactory candidate for the EM1 component in the NMH basalts. Our results also suggest a residual slab reservoir with light δ98/95Mo values (<−0.5‰) in the mantle beneath Northeast China, representing the complementary reservoir to heavy Mo isotope continental crust. Identification of this reservoir makes the Mo isotope system a powerful tracer of crust-mantle interactions. [ABSTRACT FROM AUTHOR]

Granulite-facies xenoliths entrained within igneous rocks shed a light on poorly understood yet critical questions about age, origin and history of the lower continental crust. Here, we present the first coupled in-situ U-Pb, Lu-Hf and O isotope data for the Precambrian zircons from fourteen deep-crustal xenoliths from the North China craton. The geochronological data demonstrates that the oldest lower crustal remnants formed at ∼3.82 Ga and related magmatism continued until ∼3.55 Ga. The Eo-Paleoarchean zircons, with exception of one analysis, have subchondritic initial Hf isotopes with negative ε Hf (t) and are characterized by normal mantle to elevated δ18O values (5.37–6.90‰), indicating a development of a low Lu/Hf reservoir and hydrosphere-crust interactions in the early Earth. Magmatic zircons from lower crustal xenoliths of North China define a strongly episodic distribution of ages at 3.82–3.55 Ga, ∼2.7 Ga, ∼2.5 Ga and 1.95–1.85 Ga and a non-linear Hf isotope-age array for nearly 2 Gyr, indicating episodic crustal generation and reworking through time. The lower crustal xenoliths record a change in zircon O isotope compositions from a restricted range at 3.8–2.6 Ga to more variable after ∼2.5 Ga. Both heavier (δ18O up to 10.25‰) and lighter (δ18O low to 2.16‰) O isotope compositions than the normal mantle are observed in ∼2.5 Ga magmatic zircons. The secular change in zircon O isotopes documents an increase in recycling rate of surface-derived materials (including high-δ18O sediments, weathered and altered rocks and low-δ18O altered oceanic crust) into magma sources at the end of Archean, which, in turn, is possibly linked to modern style subduction processes and maturation of the crust at that time. [ABSTRACT FROM AUTHOR]

Understanding the subduction-driven recycling of economically important chalcophile metals (e.g., Cu, Au, Ag, and platinum-group elements, PGEs) and volatiles (e.g., sulfur) is critical for assessing models of ore formation. However, whether the sulfide remains stable in recycled altered oceanic crust (AOC) in the mantle is unclear, and the influence of AOC-derived metasomatic agents on the extraction efficiency of chalcophile elements from the deep mantle remains poorly understood. Here we report whole-rock Cu-Ag-Au-Re-PGEs of Cenozoic alkaline basalts at Hannuoba, eastern China, which were derived from the interaction of carbonated eclogite (recycled AOC)-derived melts with lithospheric peridotites. Volatile-rich primitive melts from carbonated eclogite are poor in strongly chalcophile metals (e.g., ∑PGE < 1 ng/g, Au < 1 ng/g, Cu < 30 μg/g). Remarkably, primitive melts show significant fractionation of elements with variable chalcophile affinities, such as lower Cu/Ag ratios than those in primitive mantle and MORBs. These features reflect the sequestration of these metals in residual mono-sulfide solid solution (MSS) during low-degree melting of the AOC in the deep mantle. Given the retention of sulfides in the AOC, partial melts derived from it are rich in sulfur but poor in strongly chalcophile metals. The Hannuoba alkaline melts scavenged some strongly chalcophile metals from peridotites (especially PGEs) during their ascent. However, this did not necessarily lead to the formation of chalcophile metal-enriched melts (e.g., Au < 1 ng/g and Cu < 60 μg/g). These processes appear to be common in eastern China, as evidenced by the low abundances of Cu and Au found in many primitive intraplate alkaline basalts. We propose that strongly chalcophile elements in subducted AOC are efficiently transported into, and stored in, the deep mantle, with a small proportion returning to the exosphere through volcanism. Moreover, the chalcophile metal-poor and volatile-rich melts of subducted AOC could also explain the addition of abundant sulfur (secondary sulfides), but limited chalcophile metals to the metasomatized mantle. [ABSTRACT FROM AUTHOR]

The Xiaohaizi wehrlite intrusion in the early Permian Tarim Large Igneous Province, Northwest China, is characterized by unusual high-An (up to 86) plagioclases. It has been suggested that H2O may have exerted a major control on their formation, but this interpretation requires further direct evidence. Moreover, it remains unclear where the water came from. In order to unravel these questions, we present electron microprobe analyses of minerals and melt inclusions in clinopyroxene macrocrysts in the dikes crosscutting the Xiaohaizi wehrlite intrusion and in situ oxygen isotope data of zircons from the Xiaohaizi wehrlite. The homogenized melt inclusions have restricted SiO2(45.5–48.7 wt.%) and Na2O + K2O (2.4–3.8 wt.%) contents, displaying sub-alkaline affinity. This is inconsistent with the alkaline characteristic of the parental magma of the clinopyroxenes, suggesting significant modification of melt inclusions by contamination of the host clinopyroxene due to overheating. Nevertheless, the Ca/Na ratios (2.9–4.7) of melt inclusions are the upper limit of the parental magma of the clinopyroxenes due to high CaO (21.5–23.0 wt.%) and very low Na2O (0.22–0.34 wt.%) contents in the host clinopyroxenes. Thermodynamic calculation suggests that under fixed P (2.7 kbar) and T (1000°C), and assumed H2O (~1.5 wt.%) conditions, the Ca/Na ratio of the parental magma cannot generate high-An plagioclase in the wehrlite. The results confirm that H2O exerts a major control. Zircon δ18O (VSMOW) values (2.99–3.71‰) are significantly lower than that of mantle-derived zircon (5.3 ± 0.6‰). Such low zircon δ18O values may be due to incorporation of large amounts of low-δ18O, hydrothermally altered oceanic crust. However, geochemical and Sr-Nd-Pb isotopic data do not support recycled oceanic crust in the mantle source of the Xiaohaizi intrusion. Alternatively this can be explained by incorporation of meteoritic water in the magma chamber. This will increase the H2O content of the liquid that finally crystallize high-An plagioclases. [ABSTRACT FROM AUTHOR]

The Dongfanghong hornblende gabbro is located in the western part of the Wandashan Orogen and to the east of the Jiamusi Block in NE China. It was emplaced into Early Paleozoic oceanic crust (i.e. Dongfanghong ophiolite) at ~ 275 Ma and both later collided with the eastern margin of the Jiamusi Block. The Dongfanghong gabbro is sub-alkaline with high Na 2 O contents and is characterized by enrichment in light rare earth elements (LREE), large ion lithosphile elements (LILE), Sr, Eu, and Ba, and depletion in high field strength elements (HFSE). The enriched isotopic signatures ( 87 Sr/ 86 Sr i = ~ 0.7065, εNd(t) = ~− 0.5, 208 Pb/ 204 Pb i = ~ 38.05, 207 Pb/ 204 Pb i = ~ 15.56, 206 Pb/ 204 Pb i = ~ 18.20 and zircon εHf(t) = ~+ 5.8) indicate an enriched mantle (EM2) source, with some addition of continental material. It has arc geochemical affinities similar to Permian arc igneous rocks in the eastern margin of the Jiamusi Block, the Yakuno Ophiolite in SW Japan, arc rocks along the western margin of the North America Craton, and also the Gympie Group in eastern Australia. All these features, together with information from tectonic discrimination diagrams, suggest that the Dongfanghong gabbro formed in an immature island arc. The spatial configuration of ~ 290 Ma immature continental arc rocks in the eastern part of the Jiamusi Block and the ~ 275 Ma immature island arc Dongfanghong gabbro in the Wandashan Orogen to the east is best explained by eastward arc retreat and slab roll-back of the Paleo-Pacific Plate. This model is also supported by the Carboniferous–Permian stratigraphic transition in the Jiamusi Block from oceanic carbonate rocks to coal-bearing terrestrial clastic rocks and andesites. We thus suggest that both Paleo-Pacific subduction and roll-back occurred in the Early Permian along the eastern margin of Asia. [ABSTRACT FROM AUTHOR]

Major, trace element and Sr–Nd–Pb isotopic data of basalts emplaced during 90–40 Ma in the North and Northeast China are compiled in this review, with aims of constraining their petrogenesis, and by inference the evolution of the North China Craton during the late Cretaceous and early Cenozoic. Three major components are identified in magma source, including depleted component I and II, and an enriched component. The depleted component I, which is characterized by relatively low 87 Sr/ 86 Sr (<0.7030), moderate 206 Pb/ 204 Pb (18.2), moderately high ε Nd (∼4), high Eu/Eu ∗ (>1.1) and HIMU-like trace element characteristics, is most likely derived from gabbroic cumulate of the oceanic crust. The depleted component II, which distinguishes itself by its high ε Nd (∼8) and moderate 87 Sr/ 86 Sr (∼0.7038), is probably derived from a sub-lithospheric ambient mantle. The enriched component has low ε Nd (2–3), high 87 Sr/ 86 Sr (>0.7065), low 206 Pb/ 204 Pb (17), excess Sr, Rb, Ba and a deficiency of Zr and Hf relative to the REE. This component is likely from the basaltic portion of the oceanic crust, which is variably altered by seawater and contains minor sediments. Comparison with experimental melts and trace element modeling suggest that these recycled oceanic components may be in form of garnet pyroxenite/eclogite. These components are young (<0.5 Ga) and show an Indian-MORB isotopic character. Given the share of this isotopic affinity by the extinct Izanaghi-Pacific plate, currently stagnated within the mantle transition zone, we propose that it ultimately comes from the subducted Pacific slab. Eu/Eu ∗ and 87 Sr/ 86 Sr of the 90–40 Ma magmas increases and decreases, respectively, with decreasing emplacement age, mirroring a change in magma source from upper to lower parts of subducted oceanic crust. Such secular trends are created by dynamic melting of a heterogeneous mantle containing recycled oceanic crust. Due to different melting temperature of the upper and lower ocean crust and progressive thinning of the lithosphere, the more fertile basaltic crustal component is preferentially sampled during the early stage of volcanism, whereas the more depleted gabbroic lower crust and lithospheric mantle components are preferentially sampled during a late stage. This model is consistent with a protracted destruction process of the lithosphere beneath eastern China. The presence of significant recycled oceanic crust components in the 90–40 Ma basalts highlights the influence of Pacific subduction on the deep processes in the North China Craton, which can be traced back at least to the late Cretaceous. This, along with the conjugation of crustal deformation pattern in this region with the movement of the Pacific plate, makes the Pacific subduction as a potential trigger of the destruction of the North China Craton. [ABSTRACT FROM AUTHOR]

Primitive magmas with distinct geochemical signatures that record mantle heterogeneity are obscured in continental intraplate volcanic settings owing to crustal interaction. Here, we report new geochronological and geochemical data for two adjacent Late Neogene primitive intraplate volcanoes in Northeast China: the Great and Lesser Wangqing River twin volcanoes (GWR/LWR). Our results suggest that the GWR and LWR volcanics are controlled by olivine crystal sorting and that crustal contamination is negligible, which implies these twin volcanoes provide a rare window into mantle components. The LWR tephrite is marked by moderate enrichment of light rare earth elements (REEs) with significant positive Sr anomalies. It is also enriched in radiogenic Sr-Nd-Hf-Pb isotopes but exhibits 187Os/188Os values that are comparable to those of the primitive upper mantle. The GWR volcanics (picrite, basanite and tephrite) are characterised by a steeper REE pattern and a lack of Sr anomalies, while being isotopically more radiogenic. Combined with olivine chemistry, which reveals a hybrid source lithology, we propose that the LWR and GWR sampled recycled lower and upper oceanic crust components, respectively. Both components have high 87Sr/86Sr and low 206Pb/204Pb ratios, akin to an enriched mantle I (EM1) source, indicating that the EM1 components can be formed by recycling oceanic crust with limited sediment. [Display omitted] • Neogene intra-continental twin volcanoes in two parallel faults in NE China • Continental mafic lava with olivine-only fractionation and no contamination • Asymmetric trace element and radiogenic isotopic compositions • Recycling of a complete packet of oceanic crust with limited sediments [ABSTRACT FROM AUTHOR]

Abstract: Basaltic rocks from Shuangliao, northeast China include basanite, alkali olivine basalt, transitional basalt and sub-alkaline diabase. Ar–Ar dating shows that the basanites and alkali olivine basalts formed earlier (48.5–51Ma) than the transitional basalts and diabases (43–41.6Ma). These rocks have the highest Fe2O3 contents (13.4–14.6wt.%) and lowest (87Sr/86Sr)i ratios (<0.703) among the Cenozoic basalts from eastern China. On a primitive-mantle normalized variation diagram, they show positive Eu, Sr, Nb and Ta anomalies, and depletion in very incompatible elements (Rb, Ba, Th, U), reminiscent of HIMU-type oceanic island basalts. In particular, the basanites possess noticeable negative K anomalies. Nevertheless, their Pb isotopic compositions (206Pb/204Pb=18.13–18.34) do not show the high time-integrated 238U/204Pb mantle component expected for a HIMU basalt. On a 206Pb/204Pb versus 207Pb/204Pb diagram, most samples straddle the Northern Hemisphere Reference Line (NHRL), in salient contrast to the majority of Chinese Cenozoic basalts, which plot above the NHRL. These data, as well as a comparison with high-pressure experimental melts, are consistent with the presence of young subducted oceanic crust (SOC) in the source of Shuangliao basalts. Varying (87Sr/86Sr)i, La/Nb and Eu/Eu* with rock-type suggests that the upper oceanic crust (with variable amount of lower oceanic crust) was preferentially sampled by earlier (51–48Ma), highly alkaline rocks, whereas the lower oceanic crust was predominantly sampled in later (41–43Ma) transitional basalts and diabases. This temporal trend is attributed to the differential melting of a heterogeneous source in association with lithospheric thinning, during which fusible upper oceanic crust melted earlier than lower oceanic crust and peridotites. We postulate that the SOC components may have been derived from the seismically detected stagnant Pacific slab within the mantle transition zone. This hypothesis is supported by the same Indian MORB-like isotopic composition being found in the Shuangliao basalts and in the extinct Izanaghi–Pacific plate of NW Pacific. The latter has been subducting underneath the eastern Asian continent since the early Cretaceous. [Copyright &y& Elsevier]

Despite numerous studies, the origin of intraplate basalts from eastern Asia is still elusive. This includes the relative importance of subducted oceanic crust and metasomatized lithospheric mantle in generation of these lavas. To address this important issue, we have carried out an integrated Mg-Sr-Nd isotope study of Jiaohe garnet pyroxenite xenoliths and Cenozoic basalts from the Changbaishan-baoqing volcanic belt (CVB), NE China. The Jiaohe garnet pyroxenite xenoliths exhibit low Hf/Hf* ((Hf N /(Sm N × Nd N)0.5, 0.7–0.9), depleted Sr Nd isotopic compositions (87Sr/86Sr i = 0.7035–0.7041; ε Nd = +2.9 − +8.6) and low δ26Mg (−0.61 to −0.39‰). These features reflect both protolith heterogeneity and the effect of isotopic exchange with carbonates during subduction. The CVB basalts display variable Mg isotopic compositions. Low-Si basalts are characterized by low δ26Mg (−0.27 to −0.44‰), variable Hf/Hf* (0.7–1.4) and depleted Sr Nd isotopic compositions (87Sr/86Sr i = 0.7039–0.7050; ε Nd = +1.0 − +4.7), which were attributed to partial melting of upwelling asthenospheric mantle containing recycled oceanic crustal components. In contrast, high-Si basalts display low Hf/Hf* (0.6–0.9), EM1-like Sr Nd isotopic compositions (87Sr/86Sr i = 0.7047–0.7054; ε Nd = −2.4 - -0.2) and normal mantle-like δ26Mg values (−0.18 to −0.33‰). Such features can be explained by participation of metasomatized lithospheric mantle in their source. The transition from low-Si melts with low-δ26Mg to high-Si melts with normal mantle-like δ26Mg for the CVB basalts are genetically related to continental rifting, asthenospheric mantle upwelling and mechanical-chemical erosion of basal metasomatized lithosphere induced by subduction of the Pacific oceanic slab. Recycled oceanic crust and basal lithospheric mantle both played a fundamental role in generation of the intraplate basalts in eastern Asia. [Display omitted] • The Jiaohe garnet pyroxenite xenoliths exhibit low δ26Mg values. • Low-Si basalts with low δ26Mg were derived from recycled oceanic crust. • High-Si basalts with normal δ26Mg were derived from enriched lithospheric mantle. [ABSTRACT FROM AUTHOR]

The contribution of recycled oceanic crust to the mantle source of mid-ocean ridge basalts (MORBs) remains a subject of debate, as radiogenic isotopes alone yield ambiguous insights. Here, we present comprehensive data on whole-rock major element, trace element, and Mo–Nd–Hf isotopic compositions for MORB samples from International Ocean Discovery Program Expedition 349 sites U1431E and U1433B in the eastern (ESB) and southwestern (SWB) subbasins, respectively, of the South China Sea (SCS). The δ98/95Mo values of the SCS MORBs exhibit a significant range (from −0.80‰ to +0.05‰), in contrast to the restricted composition observed in MORBs (δ98/95Mo = −0.19‰ ± 0.01‰). The ESB MORBs display extremely light Mo isotopic compositions (δ98/95Mo = −0.80‰ to −0.24‰) with Nd Hf isotopic compositions similar to those of Pacific MORB, whereas the SWB MORBs show higher δ98/95Mo values (−0.44‰ to +0.05‰) and depleted Nd Hf isotopic compositions. The δ98/95Mo values of the SCS MORBs are negatively correlated with Ce/Mo ratios and positively correlated with ε Hf values, suggesting the incorporation of dehydrated slab components into the source of the SCS MORBs. The subbasin-scale mantle heterogeneity in the SCS can be best explained by varying degrees of interaction between a mantle plume and stagnant slabs in the mantle transition zone, as suggested by mantle tomography. The rigid stagnant slab in the mantle transition zone beneath the SWB largely impeded the upwelling of the mantle plume, whereas the slab beneath the ESB was disrupted by the plume and subsequently transported into the upper mantle. This study, therefore, provides compelling evidence for the involvement of recycled oceanic slabs in generating MORB and the influence of an ascending mantle plume on seafloor spreading in a continental marginal sea. Mantle recycling processes in marginal sea basins exhibit distinct characteristics compared to those occurring in open oceans. • The SCS MORBs mainly show much lower δ98/95Mo values than global MORB. • Extremely light Mo isotopes in the SCS MORBs result from abundant ROC in the source. • The Mo isotopic compositions of the SCS MORBs exhibit subbasin-scale variations. • Plume-stagnant slab interaction leads to large-scale mantle heterogeneity in the SCS. [ABSTRACT FROM AUTHOR]

Magma plumbing systems exert strong controls on the formation and evolution of the oceanic crust during crust accretion. However, plumbing system dynamics based largely on mid-ocean ridge basalts (MORBs) are extremely difficult to constrain because the MORBs are aggregated melts and original information may have been blurred. Copper isotopic compositions of lower crustal gabbroic cumulates effectively archive early-stage histories because sulfides constitute the dominant Cu budget of gabbroic cumulates and are largely isolated from subsequent magma filtering processes. Here, we present Cu isotopes of a suite of gabbroic cumulates from the ultraslow-spreading Southwest Indian Ridge (Hole U1473A), and MORBs from the South Mid-Atlantic Ridge and East Pacific Rise. The δ65Cu of the MORBs from this study (+0.04 ‰ to +0.19 ‰) match those of previous MORB analyses, indicating uniform δ65Cu at various spreading rates. In contrast, the δ65Cu of gabbroic rocks vary significantly (−1.14 ‰ to 0.87 ‰), exceeding by far the range known for peridotites. The Cu budget of these gabbroic rocks is hosted in sulfides and the mantle-like S isotopic compositions of sulfides indicate their origin of igneous processes. These results thus suggest that magma accumulation and sulfide segregation would lead to notable Cu isotopic fractionation in the lower oceanic crust. We propose that repeated recharging of primitive melts and efficient mixing of melts within the plumbing system can buffer the removal of early 63Cu-rich sulfides from sulfide-saturated MORBs. The discrepancy in δ65Cu between lower oceanic crust and MORBs is a natural consequence of continuous replenishment that occurs concurrently with melt-rock interaction, mixing, crystallization and extraction, irrespective of the oceanic settings. The consistent δ65Cu in MORBs (0.09 ± 0.08 ‰) and komatiites (0.06 ± 0.06 ‰) further indicates that their Cu isotopes reflect the mean mantle source composition, providing a robust δ65Cu for bulk silicate Earth (BSE) of 0.08 ± 0.08 ‰ (2sd). Accordingly, we propose that in open sub-ridge plumbing systems, other incompatible element isotopes and ratios of elements with similar incompatibility in MORBs as Cu isotopes suggest, could also represent mean mantle source compositions. [ABSTRACT FROM AUTHOR]

Fore-arc crust and mantle wedge serpentinite, dragged down by the subducting slab, play a critical role in enriching the sub-arc mantle source of arc magmas. However, discriminating between these two components in arc magmas remains challenging because the fore-arc crust may contain serpentinite components inherited from previous subduction event(s). Here we present molybdenum concentration and isotope data, coupled with reanalyzed and published geochemical data, for common representatives of circum-Pacific high-Mg andesites and adakites. Elevated δ98/95Mo values (−0.13‰ to 0.00‰) in the Kamchatka and Aleutian high-Mg andesites are associated with elevated Mo/Ce (0.026 to 0.075), Ba/Th (138 to 808), and Sb/Ce (0.0026 to 0.0192) ratios, as well as Sr-Nd-Hf-Pb isotope compositions close to the depleted mantle and moderated δ18O values (+6.6‰ to +7.8‰). These characteristics are consistent with compositional contributions from slab-derived aqueous fluids via fore-arc serpentinites. In contrast, decreasing δ98/95Mo values (−0.07‰ to −0.48‰) in the Cascadia and Setouchi high-Mg andesites as well as four adakites from Fiji and the Austral Volcanic Zone, are generally accompanied by decreasing Mo/Ce (0.036 to 0.002) and Sb/Ce (0.0066 to 0.0002) ratios, increasing Sr/Y ratios (15 to 207), and Sr-Nd-Hf-Pb-O isotopic variations ranging from values reflecting altered oceanic crust to those resembling sediment input. These features can be accounted for by an increasing melt contribution from the subducted oceanic crust. Nine adakites from the Aleutians, Fiji, Panama, and the Austral Volcanic Zone exhibit δ98/95Mo values (−0.19‰ to −0.04‰) higher than the depleted mantle but lower Mo/Ce ratios (0.007 to 0.018) and low to moderate Sb/Ce ratios (0.0002 to 0.0025), indicating a potential serpentinite contribution. However, elevated Sr/Y ratios (57 to 295), unradiogenic Sr-Pb isotopes, radiogenic Nd-Hf isotopes, and moderate δ18O values (+6.3‰ to +6.5‰) suggest that these adakites are derived from fore-arc crust dragged down by the descending oceanic crust. The circum-Pacific adakites in this study show a decrease in δ98/95Mo with decreasing Mo/Ce, consistent with partial melting of oceanic crust and 98Mo pre-enriched fore-arc crust, respectively, having lost 98Mo during subduction dehydration. Thus, our study not only distinguishes the components between eroded fore-arc crust and wedge serpentinite but also indicates a major role of both dehydration and melting in facilitating material transfer in subduction zones. • High δ98/95Mo in high-Mg andesites reflect eroded fore-arc serpentinite. • Low δ98/95Mo in adakites represent the slab melt component. • High δ98/95Mo in adakites represent eroded fore-arc crust. • Mo isotopic data support a liquid-dominated mass transfer mode in subduction zones. [ABSTRACT FROM AUTHOR]

The sources and origins of the mobile element phases that enrich convergent plate margin mantle sources and trigger arc melting remain controversial. New high-precision Pb isotope data in combination with Sr isotopes of Kurile arc volcanic rocks reveal that the decomposition of serpentinite below the subducting slab Moho played an important role in generating slab fluid/melt to fertilize the subarc mantle. The Kurile arc volcanic rocks have compositional features indicative of a role for subducted slab inputs. However, the classic model of decomposition of hydrous minerals in subducting marine sediments and altered basalts to produce slab-derived fluid and/or melt phases do not explain Kurile lava compositions. The correlation between Pb and Sr isotopes in Kurile lavas requires that the slab fluids or melts must involve substantial Pb and Sr contributions from comparatively unaltered basalt/dolerite/gabbro with an isotopic signature similar to Indian Ocean mantle. We suggest that the decomposition of serpentinites from below the subducting slab Moho led to substantial fluid-driven extraction of Pb and Sr from relatively unaltered oceanic crust as these deep fluids transited upward through the subduction interface into the mantle wedge. The presence of Indian-type oceanic crust in the northwestern Pacific basin is consistent with recent plate reconstruction results showing that the Cretaceous-age Izanagi-Pacific ridge, which likely created the oceanic crust beneath the present Kurile arc, was adjacent to the mantle of the Neo-Tethys Ocean. A switch from Pacific-type to Indian-type subducting oceanic crust occurred between 50 and 42 Ma beneath the Izu-Bonin arc based on the Pb isotope systematics of erupted lavas from this period. This observation may indicate that subduction of the Izanagi-Pacific ridge along the Asian continental margin between 50 and 40 Ma reduced the velocity of the Pacific plate relative to that of the Philippine Sea Plate, and led to dextral strike-slip of the Izu-Bonin-Mariana subduction zone. • Serpentinite dehydration in the lithospheric slab is a universal phenomenon. • Indian-type oceanic crust is present in the northwestern Pacific basin. • Cretaceous Izanagi-Pacific ridge was linked to the Neo-Tethys Ocean mantle. • Mid-ocean ridge subduction induced dextral strike-slip of the IBM subduction zone. [ABSTRACT FROM AUTHOR]

The Wuda Tuff Flora in the North China Craton (NCC) is a "vegetational Pompeii" and one of the most completely preserved marsh ecosystems of the late Paleozoic Northern Hemisphere. However, the precise age, geochemistry, and petrogenesis of the thick tuff containing this peat-forming flora have not been established. We conducted a comprehensive petrographic, mineralogical, whole-rock geochemical, and zircon U Pb and Hf O isotopic investigation of five samples from a vertical section through the tuff bed. Our results confirm that the material that buried the flora is a volcanic tuff bed that has undergone intensive post-deposition alteration. Whole-rock Al 2 O 3 /TiO 2 ratios (65.8–103.5) and negative Eu/Eu* values (0.27–0.38) further suggest a felsic volcanic origin. Secondary ion mass spectrometry zircon U Pb dating of the altered tuff bed yielded a weighted mean 206Pb/238U age of 295.9 ± 1.4 Ma (1σ, MSWD = 0.94, n = 39), constraining the age of the Wuda Tuff Flora to the earliest Permian. Depletions of Nb and Ta in both the whole-rock and zircon compositions indicate an arc-related setting for the volcanism. The positive εHf(t) values (0.0 to +5.0) and δ18O values (5.05‰–6.17‰) of zircon grains from the Wuda Tuff suggest a juvenile crust origin of the volcanic magma. Geochemical comparisons with contemporaneous igneous rocks suggest that the volcanic eruption that led to the formation of the Wuda Tuff and associated vegetational Pompeii was most probably caused by arc volcanism along the western margin of the NCC related to subduction of Paleo-Asian Ocean oceanic crust, implying that the Alxa Terrane had not yet amalgamated with the NCC during the early Permian. • The Wuda Tuff was felsic arc-originated with juvenile crust. • The age of Permian Tuff Flora at Wuda Coalfield is 295.9 ± 1.4 Ma. • A magmatic arc may exist to the west of North China Craton during early Permian. [ABSTRACT FROM AUTHOR]