Hydrous Mantle Research Articles

H2 Production in Subduction Zone

Convergence zones where ophiolites outcrops, such as Oman and New Caledonia, allow the generation of natural H2 by redox phenomena. Nevertheless, the role of hydration of the mantle wedge between the overriding plate and the slab is often evoked to explain the H2 emanations. In the case of active subduction, such as that of the Nazca plate beneath the Andes, the presence of H2 emanation has been noted. In this study, we modelled the processes of (de)hydration and melting within the subduction zone. The quantification of water involved in mantle wedge hydration served as a proxy for H2 production. Multiple simulations have been conducted to investigate oceanic-continental subduction, wherein various parameters, especially the subduction angle, have been varied. It appears that in the case of flat subduction, the mantle hydration zone is large, extending up to 500 km from the trench. The extend is controlled by the velocity of the overriding plate. In the case of a steep subduction, the zone is narrower, and is located between the trench and the volcanic arc. Magma formation competes with H2 generation for the use of water expelled from the subducting plate. In the transition from a steep to a flat slab, the mantle hydration zone widens and the volcanic zone moves away from the trench. The oceanic crust may undergo melting, leading to a change in magma composition, before volcanism diminishes in intensity and then disappears.

LOMU Type Alkali Basalts in East Asia Sourced from Subduction Recycling of Multiple Ancient Crustal Components

Abstract The origin of alkali basalts with distinctive time-integrated low U/Pb (low μ, LOMU) from East Asia is controversial due to the complex geochemical and tectonic signatures reported from this region. We report new data on the petrology and geochemistry of the Higashi-Matsuura and Kita-Matsuura alkali basalts from Southwest Japan, which confirm the presence of a LOMU-type mantle component below the Japanese Islands, similar to East Asian intraplate volcanoes. We use whole rock geochemistry and mineral chemistry to determine mantle melting, magma evolution conditions and isotopic evolution of the LOMU mantle source. Petrological studies show that the Higashi-Matsuura alkali basalts (~3 Ma) were derived from a hydrous mantle source with ~880 μg/g H2O, at a pressure of 1.8 to 2.3 GPa at a mantle potential temperature of ~1300°C. Trace element modelling concludes that these basalts may have been generated through the hydrous upwelling and partial melting of the asthenospheric mantle containing ~15% dehydrated eclogitic component. Higashi-Matsuura alkali basalts show 206Pb/204Pb values of 17.72 to 18.04 which are among the lowest values from Southwest Japan. Relatively older (6–8 Ma) alkali basalts from the Kita-Matsuura area showing similar physicochemical characteristics, do not show LOMU-type isotopic trends. Trace element and Pb–Sr–Nd isotopic data indicate that the Higashi-Matsuura mantle component is similar to the extreme LOMU components reported from the Northeast China alkali basalts, as well as the recently discovered Petit Spot volcanoes on the Pacific Plate. We model the origin of the LOMU signature from the lowest reported Pb isotope ratios in East Asia, from intraplate alkali basalts in Northeast China. Our model suggests that at least two separate subduction events of marine sediments, at 1.8 and 2.2 Ga, are required to explain the observed Pb isotopic variation in the East Asian region. Other LOMU type basalts from East Asia, including Southwest Japan and Petit Spot, define a linear trend between the Xiaogulihe basalts and lithospheric mantle xenoliths, with 40% to 60% Pb in Higashi-Matsuura basalts being derived from the lithospheric mantle during upwelling. This suggests that the LOMU array in East Asia may have been formed by mixing between multiple ancient, subducted sediment components accumulated at the mantle transition zone for about 2 billion years, and its recent upwelling due to dehydration of the stagnant Pacific slab and related melting of the metasomatised asthenospheric mantle.

Deep low velocity layer in the sublithospheric mantle beneath India

SUMMARY Globally, there is now a growing evidence for a low velocity layer in the deeper parts of the upper mantle, above the 410 km discontinuity (hereafter called LVL-410). The origin of this layer is primarily attributed to interaction of slabs or plumes with a hydrous mantle transition zone (MTZ) that results in dehydration melting induced by water transport upward out of the MTZ. However, the ubiquitous nature of this layer and its causative remain contentious. In this study, we use high quality receiver functions (RFs) sampling diverse tectonic units of the Indian subcontinent to identify Ps conversions from the LVL-410. Bootstrap and differential slowness stacking of RFs migrated to depth using a 3-D velocity model reveal unequivocal presence of a deep low velocity layer at depths varying from 290 to 400 km. This layer appears more pervasive and deeper beneath the Himalaya, where detached subducted slabs in the MTZ have been previously reported. Interestingly, the layer is shallower in plume affected regions like the Deccan Volcanic Province and Southern Granulite Terrane. Even though a common explanation does not appear currently feasible, our observations reaffirm deep low velocity layers in the bottom part of the upper mantle and add to the list of regions that show strong presence of such layers above the 410 km discontinuity.

Migration and accumulation of hydrous mantle incipient melt in the Earth's asthenosphere: Constraints from in-situ falling sphere viscometry measurements

The Earth's oceanic lithosphere-asthenosphere boundary (LAB) is marked by a notable decrease in seismic wave velocities and an increase in attenuation. This phenomenon is likely attributed to the accumulation of partial melt at the top of the asthenosphere. Nevertheless, the process involving the upward migration and aggregation of low-degree partial melts, highly likely to be mantle incipient melts in the asthenosphere, remains underexplored. Viscosity is a key factor controlling the flow of melts, thus in this study, we used In-situ X-ray falling sphere viscometry experiments to determine the viscosity of the mantle incipient melt containing 1.8–4.0 wt.% water at 1.5–6 GPa and 2100 K. We found that water and pressure can effectively decrease the viscosity of the mantle incipient melt. Therefore, the mantle incipient melts are highly mobile with a high segregation velocity in the deeper part of the asthenosphere. However, during ascent, their mobility diminishes due to an increase in viscosity and a decrease in water content. These mechanisms potentially contribute to the accumulation of partial melt at the top of the asthenosphere.

Hydrous mantle plume promoted the generation of continental flood basalts in the Tarim large igneous province

Recent research on the water content of large igneous provinces (LIPs) has revealed that water has a significant impact on the formation of LIPs. However, most studies focus on the water content of mafic–ultramafic rocks, while relatively little attention has been paid to the water content of continental flood basalts (CFB), which form the major part of LIPs and are characterized by huge volumes (> 1 × 105 km3) and short eruption times. Here, we determined water contents of clinopyroxene crystals from the Akesu diabase, which is co-genetic with flood basalts of the Tarim LIP in China. Based on these measurements, we obtained a water content of higher than 1.23 ± 0.49 wt.% for the parental magma to the Tarim CFB and a minimum water content of 1230 ± 490 ppm for the mantle source, thus indicating the presence of a hydrous mantle plume. Combined with previous studies, our results suggest that water plays a key role in the formation of the Tarim LIP. Additionally, the whole-rock compositions of the Akesu diabase indicate a contribution of pyroxenite in the mantle source. This is consistent with a model, in which water was brought into the Tarim mantle plume by a subducted oceanic plate that entered the deep mantle.

Structure and dynamics of southern Mariana margin: Constraints from seismicity, tomography and focal mechanisms

The southern Mariana margin is a tectonically distinctive and rapidly deforming region with the world deepest trench and unusual deformation and magmatism. However, its related deep structure and dynamics are still poorly understood. In this study, we determine robust 3-D P and S wave velocity models down to 130 km depth beneath the southern Mariana margin by using arrival-time data of local earthquakes recorded at 12 near-field seismic stations. Our tomographic results together with local seismicity and focal mechanisms reveal that the subducted Pacific slab is rapidly rolling back with a steep dip angle and narrowly but strongly coupled with the thin forearc block, which results in the deepest trench. An inferred eastward mantle flow transports the enhanced hydrous mantle melt beneath the southwest Mariana rift to the southern Mariana Trough, causing unusual deformation, magmatism and volcanism in the new oceanic crust. In addition, seismicity in the west Mariana ridge lithosphere may indicate active foundering of the arc lower crust.

Modeling the Global Water Cycle—The Effect of Mg‐Sursassite and Phase A on Deep Slab Dehydration and the Global Subduction Zone Water Budget

AbstractWe study the interplay of the thermal structure within subducted oceanic slabs together with the stability fields of hydrous phases that control slab dehydration and the amount of water transported into the deeper mantle. We implement different published thermodynamic data for phase A and Mg‐sursassite into a model of 56 subduction zones and evaluate the effect of these phases on the global water budget. We modeled vertical fluid fluxes within the slab such that dehydration‐derived fluid was allowed to react with fluid‐undersaturated rocks in other parts of the slab. The effect of Mg‐sursassite on the global water budget is limited, because the Clapeyron slope of this dehydration reaction is steeper than most pressure‐temperature trajectories in subduction zones. Two sets of published thermodynamic data for phase A yield significantly different values for the amount of deeply subducted water, ranging between 8 × 108 Tg/Ma and 1.4 × 109 Tg/Ma. In some subduction zones, the differences span several orders of magnitude. The absolute modeled amount of deeply subducted water strongly depends on the depth and intensity of slab mantle hydration, but a comparison of modeled and experimental data indicates that the thermodynamic dataset that yielded higher values is more reliable and should be implemented in future thermodynamic models. Our results show that the stable phases around the choke point as well as the slope and position of the phase A‐out reaction influence the deep water release from the slab, but the slope and position of the phase A dehydration reaction mainly control the recycling of water into the deep mantle.

Unveiling attenuation structures in the northern Taiwan volcanic zone.

This cutting-edge study delves into regional magmatism in northern Taiwan through advanced 3-D P- and S-wave frequency-dependent attenuation tomography. Positioned at the dynamic convergence boundary between the Philippine Sea Plate and the Eurasian Plate, Taiwan experiences moderate earthquakes and intriguing volcanic activity, with a focus on the Tatun volcano group. Employing the Formosa seismic array for high-resolution results, our research identifies high-attenuation anomalies (low Q) beneath the northern Taiwan volcanic zone (NTVZ) and offshore submarine volcanoes, indicative of potential hydrothermal activities and magma reservoirs at varying depths. Additionally, we explore low-attenuation anomalies (high Q) in the forearc region of the Ryukyu subduction zone, suggestive of partial saturation linked to serpentinization processes resulting from seawater infiltration or forearc mantle hydration. These findings shed light on the complex geological features and provide essential insights into the crustal properties of northern Taiwan, contributing to a deeper understanding of its magmatic evolution and tectonic processes.

Water-driven evolution of hydrous boron-bearing silicate melts at high pressures up to 8 GPa: Implications for dynamic properties and boron isotope content in hydrous mantle melts

A prominent seismic weakening has suggested the potential presence of H2O-bearing complex silicate melts in the Earth’s interiors. As boron is typically accompanied by the hydrous fluids and melts and may be transported into deep mantle, boron isotopic evolution enables us to trace the deep-water cycle. The impact of H2O on the structures of boron-bearing magmas is essential to grasp the melting processes and the seismic weakening in Earth’s mantle. In this study, we report experimental results on the structural changes in volatile-bearing, partially-depolymerized aluminoborosilicate glasses (NaAl0.5B0.5Si3O8) - an analog for boron-rich rhyolitic melts - with varying H2O content and at pressure conditions from 6 to 8 GPa (∼180–240 km depth in the mantle) via multi-nuclear (1H, 17O, 27Al, 11B, 29Si, and 23Na) magnetic resonance (NMR) spectroscopy. At isobaric conditions both at 6 GPa and 8 GPa, compared to the coordination environments of anhydrous quenched melts, adding water up to 4 wt% generally promotes the formation of [4]B and systematically suppresses the [5,6]Al fractions; however, in the glasses with 8 wt% water (at 8 GPa), a dramatic increase in both [6]Al and [4]B is observed, while Si coordination number remains unchanged. In contrast, at a constant H2O content of 4 wt%, an increase in the pressure from 6 to 8 GPa results in an increase in the [5,6]Al fractions and a decrease in the [4]B fraction. The results highlight complex water-pressure dependences on the cation coordination transformation. Upon adding water content at isobaric condition, boron is more prone to be highly-coordinated over aluminum. In contrast, with increasing pressure at a constant water content, aluminum prefers to be highly-coordinated over boron. These NMR results allow us to refine water dissolution schemes, highlighted by selective network depolymerization of B and Al-bearing bridging oxygens and subsequent annihilation of oxygen triclusters consisting primarily of Al, consistent with a water-driven evolution of B-Al coordination numbers in response to pressurization. These changes induce non-linear decrease in melt viscosity and seismic weakening in the mantle at depths of ∼180–240 km. Observed boron coordination evolutions indicate that deep mantle melts could exhibit complex pressure-water-driven changes in the tendency to enrich 10B. In particular, the effect of 8 wt% water contributes to ∼41 % of the overall reduction of δ11B in deep melts. Finally, as the deep magmatic melts could also be more mafic than those studied here, the water-driven structural changes in more mafic melts with much less boron contents under compression remain to be explored. While the details may vary with melt composition, the current probing of the structures of boron-bearing compressed melts with varying degree of hydration enables better prediction of the deep boron and water cycles.

Probing the deep mantle wedge in an active subduction zone: Xenoliths from the Mercaderes Volcanic District, Southern Colombia

In subduction zones, the mantle wedge is the domain where liquid phases released by the oceanic subducting plates cause mantle hydration, metasomatism, and partial melting, producting the arc crust. However, direct petrologic information on mantle wedges overlying active subduction zones remains scarce. Here we constrain key aspects of the petrological evolution of the supra-subduction mantle underneath the active volcanic district of Southern Colombia (Mercaderes) by studying a unique suite of peridotite and pyroxenite xenoliths hosted by andesites and lamprophyres. These garnet-bearing peridotite, websterite, clino- and orthopyroxenite xenoliths equilibrated between 2.5 and 4 GPa and 1100–1200 °C. The studied rocks display considerable heterogeneity in terms of lithotypes, textures, mineral abundance and extent of metasomatic overprint. Most samples display fine-grained mylonitic textures overprinting coarse-grained peridotite and garnet pyroxenite precursors; the EBSD study of undeformed garnet websterite samples suggests crystallization from melts and/or melt-rock reaction processes. Modal enrichment in clinopyroxene as well as in the alkalis and LREE contents in some garnet peridotite implies interaction with alkaline basaltic melts, whereas enrichment in silica, Pb and Sr in the garnet-bearing orthopyroxenites records interaction with metasomatic, slab-derived liquids. Some garnet clinopyroxenite xenoliths display marked positive Sr anomalies and minor Eu anomalies, indicating derivation of these rocks from Mg-rich, plagioclase-bearing (and garnet-absent) low-pressure cumulates. Their equilibration in the garnet field implies that after crystallization these clinopyroxenites were transported to deeper levels in the mantle, as the result of foundering into the mantle of the dense arc roots, as already documented in Mercaderes and elsewhere in the Andes. In conclusion, the Mercaderes xenoliths probe a heterogeneous and chemically active domain of the Colombian mantle wedge, where deep sinking lower crustal rocks, melt metasomatism by distinct venues of chemically different melts, and deformation predate fast exhumation by volatile-rich melts.

Integrated geophysical-petrological model of Damavand volcano, North Iran: Compositional structure of crust and upper mantle from seismic temperature profiling and gravity anomaly fitting

Our study employs well-constrained integrated geophysical-petrological modeling to investigate the compositional structure of Damavand volcano, the Alborz mountains in northern Iran. The thermochemical model utilizes high-resolution compressional velocity anomalies to determine temperature, while geochemical data from volcanic rocks further constrain mostly the lithospheric mantle model. By examining density and velocity, we deduce the upper mantle's compositional structure using observed free-air gravity anomalies and shear-wave seismic models. Our results reveal a lower crust density reduction of 150 kg/m3 due to a high-temperature regime (1080 °C) and the presence of melt indicated by a calculated Vp/Vs ≥ 1.8. The magma storage in the lithosphere is characterized by a harzburgite-melt mixture in agreement with shear-wave seismic profiles, and the lithospheric magma reservoir is positively buoyant with a density reduction of ∼75 kg/m3 compared to the ambient lithospheric mantle. The absence of a warm zone beneath the Alborz suggests that a transient mantle flow from the 660 km transition zone beneath Eurasia is responsible for the lateral heating (and possibly hydration) of the lithospheric mantle. This aligns with the high-temperature conditions observed in the Damavand (Alborz) region. Furthermore, the presence of elevated Nb (and high Nb/Yb ratio as well as radiogenic Nd isotope) concentrations in certain volcanic rocks suggests that a portion of the melt may have originated from the Earth's asthenosphere mantle. Moreover, the existence of water and/or volatiles (likely ≥0.64 wt%, as determined through a comparison of predicted and observed seismic velocities, along with previous laboratory analyses) promotes decompression melting of the lithospheric mantle at significantly lower temperatures (1230–1130 °C) in contrast to earlier estimations (1430–1180 °C). Finally, the reductions in velocity observed in the lower crust and uppermost lithospheric mantle beneath Damavand are likely attributed to thermal and compositional factors, respectively. The presence of a hydrated and melt-rich composition within the lithospheric mantle can account for the observed decrease in seismic velocity.

Heterogeneous mush-dominated plumbing system and mantle sources of alkaline lamprophyres in Tuoyun Basin, SW Central Asian Orogenic Belt

Lamprophyres are spatially and temporally widespread in continental belts, but their magmatic differentiation process and melting dynamic mechanisms remain subjects of debate. In this study, we report mineral chemistry, bulk-rock compositions, and Sr-Nd-O isotopes of the alkaline lamprophyres (including monchiquite and camptonite) from the Tuoyun Basin of the southwestern Central Asian Orogenic Belt. Based on the P-T estimates, transcrustal magmatic reservoirs from the lower crust to upper crust are revealed for monchiquite and camptonite. These magmatic reservoirs were potentially connected, and the magmatic recharge and mixing processes that occurred were recorded by zoned clinopyroxene and amphibole. The diverse crystal populations are suggested to have originated from multiple batches of magmas, which were then involved in forming the subsequent magmas. The Sr-Nd isotopic compositions are relatively depleted, with (87Sr/86Sr)i = 0.70390−0.70452 and εNd(t) = 5.46−6.49 for the monchiquite, while the values for camptonite are 0.70391−0.70463 and 4.71−5.02. Moreover, the olivine in situ O isotopes in the monchiquite show δ18OV-SMOW values of 4.81‰−5.40‰, which are comparable to those of the mantle peridotite. Compared with the experimental melts, a hybridized mantle source of hydrous peridotite + silica-depleted pyroxenite is proposed, and partial melting of this component was induced by the upwelling asthenosphere. Ultimately, our study highlights the critical role of the magmatic plumbing system and hydrous mantle source in the formation of Tuoyun lamprophyres.

Mantle hydration initiated by Ca metasomatism in a subduction zone: An example from the Chandman meta-peridotite, western Mongolia

Fluids in subduction zones contain various aqueous species and cause metasomatic reactions as well as hydration (i.e., serpentinization) at the slab–mantle interface. However, the nature of elemental transfer during fluid infiltration into the dry mantle in the subduction zone is poorly understood. In this study, we describe novel textures related to orthopyroxene (Opx) pseudomorphs and antigorite veins in the Chandman meta-peridotite body in the Alag Khadny accretionary wedge, western Mongolia. This body consists mainly of meta-harzburgite and meta-dunite, and hydration proceeded pervasively within olivine domains associated with the development of antigorite vein networks. Pseudomorphs after orthopyroxene consist of secondary clinopyroxene (S-Cpx) + tremolite (Tr) ± [secondary olivine (S-Ol) + antigorite (Atg)] whereas the primary clinopyroxene (P-Cpx) is almost unaltered. The pseudomorphs after Opx are rarely cut by Atg veins. The mineral occurrences indicate two main stages of mantle alteration: (1) Opx replacement (Ca-metasomatism) and (2) Atg formation induced by Si-bearing fluid infiltration (i.e., the main hydration event), followed by low-temperature (T) alteration that formed lizardite, chrysotile, and brucite. The mineralogical sequence (S-Cpx + Tr ± S-Ol ± Atg) related to Opx replacement can be explained by replacement at 550–650 °C (assuming a pressure of 1.0–2.0 GPa), which is broadly consistent with temperatures experienced by eclogites in the Chandman area during exhumation. Atg in P-Ol domains could have formed at lower temperatures (<500 °C; assuming a pressure of 1.0–2.0 GPa). Mass balance calculations show that Opx replacement was characterized by gains of Ca and a small amount of H2O, whereas Ol hydration was characterized by gains of Si and H2O. A modeled slab-derived fluid, which was calculated to be in equilibrium with basaltic crustal rocks, indicates the time-integrated fluid flux required for Ca metasomatism of the Chandman meta-peridotite body, with an assumption of the alteration thickness of L = 1000 m, was 0.18–1.5 × 106 m3 fluid m−2 rock, whereas the time-integrated fluid flux required for Si metasomatism was 0.3–1.7 × 103 m3 fluid m−2 rock, which is 102–103 lower than that of the Ca metasomatism. The characteristics of the Chandman meta-peridotite body suggest that: (1) the alteration type and trapped elements in the mantle derived from slab-derived fluids vary with temperature and local Si activity; and (2) intense fracture networks facilitated extensive serpentinization related to the Si metasomatism.

Quenching of basaltic melts by volatile exsolution

Normally, volatiles in silicate melts are ephemeral components that escape as gases when the melt reaches fluid saturation. When fluid saturation occurs at elevated pressure, magmatic fluids may have large amounts of oxide solute dissolved, are less volatile, and may resemble viscous gels. In Cyprus we have the rare case that solutes of a magmatic fluid coexist with H2O saturated basaltic to boninitic glasses. Quenching of the melts and fluid solutes was induced by fluid segregation. When the fluids exsolved, the liquidus temperature was raised and the melts were left supercooled, while the system temperature remained ± constant. Quenching rates deduced from the morphologies and compositions of quench crystals were high. We analyzed coexisting glasses and fluid solutes for major and trace elements. The fluid mobile trace elements (Rb, K, Pb, Sr) are enriched in both the glasses and fluid solutes. Both endmembers (melt and fluid) have a common parentage and originated within a hydrous mantle source. The glasses have 2.5 ± 0.25 wt.% H2O and record residual H2O contents left after fluid exsolution was completed. Water contents in glasses correspond to an H2O partial pressure (pH2O) of 65 ± 10 MPa and an emplacement depth on the seafloor of 6500 ± 1000 m, provided equilibrium was reached between the pH2O imposed by the melts and the seawater column. Following fluid exsolution, the degree of supercooling ∆T of the melts relative to the dry MgO-in-melt liquidus temperature was – 65 ± 10 °C. The cooling rate ∆T/t at the time of crystallization of dendritic clinopyroxene crystals can be semi-quantified from the distribution of Al2O3 between metastable clinopyroxene dendrites and melt, to at least – 50 °C h−1. Toward the end of the article we speculate if other cases exist where quenching was triggered by fluid exsolution. A possible example are spinifex textures deep inside komatiite flows where quenching rates by conductive cooling did not exceed 0.3 to 1 °C h−1. Our proposition assumes that many spinifex-textured komatiites were hydrous, that they contained H2O in quantities sufficient to reach fluid saturation at emplacement pressure, and that spinifex textures formed as a result of supersaturation by fluid loss.

Neoproterozoic tectonic transition from subduction to back-arc extension along the western Yangtze Block, South China: Petrological evidence of Nb-enriched basalts and arc-type intrusive rocks

Identifying the tectonic transition process from persistent slab subduction to back-arc extension is vital for the systematical study of arc magmatic evolution in a temporal perspective. Although Neoproterozoic subduction setting along the western Yangtze Block has been widely documented, the detailed tectonic transition from subduction to back-arc extension was not systematically constrained. The coexistence of Nb-enriched rocks and normal arc-type rocks can generally provide unique insights into back-arc extension background under subduction setting. Herein we therefore present combined data of petrology, mineralogy, zircon U-Pb geochronology, whole-rock major and trace elements, and Sr-Nd isotopes as well as zircon Lu-Hf isotopes for the Neoproterozoic Nb-enriched basalts and arc-type intrusive rocks (i.e., gabbros and diorites) from the Luding area along the western Yangtze Block (South China), to characterize back-arc extension setting of Neoproterozoic continental magmatic arc. Zircon U-Pb ages suggest that the Nb-enriched basalts and arc-type gabbros-diorites formed coevally at ca. 780 Ma. The Nb-enriched basalts have much clinopyroxene phases and display higher TiO2, P2O5, Nb, Nb/La, and Nb/U values than normal arc volcanics. They are weakly enriched in Rb and Ba, and slightly depleted in Nb, Ta, Zr, and Hf, and show decoupled Nd-Hf isotopes with highly positive whole-rocks εNd(t) (+5.79 ∼ +7.39) and relatively low zircon εHf(t) (+1.09 ∼ +9.05) values, indicating an origination of subduction slab melts-metasomatized mantle wedge source, resembling with the genesis of those typical Nb-enriched mafic rocks in a back-arc setting. Compared with Nb-enriched basalts, the arc-type gabbros and diorites contain more hornblende minerals, and are characterized by variably high MgO (3.10–5.71 wt%) contents and Mg# values (51.5–57.2), and enriched LREEs and LILEs (e.g., Rb, Ba, Sr, and K) as well as depleted Nb, Ta, and Ti. Considering their relatively depleted and coupled Nd-Hf isotopes, these arc-type gabbros and diorites were formed by partial melting of hydrous mantle wedge modified by subduction fluids. The co-occurrence of Nb-enriched basalts and arc-type gabbros-diorites studied here therefore reveal Neoproterozoic back-arc extension background under the subduction setting. Combined with ca. 810–780 Ma mafic rocks showing Nb-enriched signatures and ca. 860–740 Ma normal arc-type mafic-intermediate rocks in the western Yangtze Block, we support that Neoproterozoic geodynamic transition from continuing subduction to back-arc extension initially occurred at ca. 820–810 Ma. The Neoproterozoic back-arc extension is thus significant for geochemical changes of subduction-related metasomatized agents and arc magmatic types.

Genesis and interaction of magmas at Nishinoshima volcano in the Ogasawara arc, western Pacific: new insights from submarine deposits of the 2020 explosive eruptions

Sudden changes of eruption styles and magma compositions at arc volcanoes are enigmatic processes. Nishinoshima volcano, western Pacific, has had historical eruptions in 1973–1974 and from 2013 on and off to the present day. These eruptions were characterized by effusive Strombolian eruptions of andesite magmas until mid-June 2020, when they suddenly transitioned to violent explosive Strombolian eruptions that produced tephra fallout over a wide area. To understand this transition, we conducted marine surveys and sampling of the extensive submarine deposits of the tephra fallout. Our new data demonstrate that the full compositional range of the 2020 eruptions spans from basalt to dacite. We present evidence for magma mixing of newly injected basalt with andesite magmas. Nishinoshima consists of an andesitic main edifice surrounded by basaltic knolls: previous studies have shown that Nishinoshima andesite compositions can be generated by olivine fractionation of primary andesitic magmas that result from partial melting of hydrous mantle at relatively low pressures under the thin crust of the Ogasawara arc; knoll basalt compositions can be generated by partial melting of mantle at greater depths and were interpreted as older events of the volcano. We show that basalt magmas could have been generated throughout the entire history of Nishinoshima. In addition, we show that andesites from Nishinoshima and nearby Nishinoshima-Minami Knoll, which are only ∼8 km apart, have distinct subduction components. Together, these data improve our understanding of the diverse primary magmas responsible for the construction and continuing eruptive activity of an active island arc volcano.

Constraints on MTZ Water Content From Joint Inversion of Diurnal Variations and Magnetospheric Signals

AbstractElectrical conductivity of mantle minerals is highly sensitive to water content, so estimates of this physical parameter based on long‐period electromagnetic induction data can help to constrain mantle hydration. Reliable constraints require conductivity model with sufficient resolution at relevant depths, as well as a comprehensive uncertainty analysis. In this global induction study, we combine data from the primarily ionospheric diurnal variation and longer‐period magnetospheric ring current bands. Joint inversion of these two data sources allows significantly improved vertical resolution of radial conductivity variations, especially in the mantle transition zone (MTZ). We then evaluate how uncertainties in the resulting conductivity model, in thermal models, and in laboratory results affect water content estimates, focusing on the MTZ. Our analysis suggests that the conductivity model is best explained by a dry MTZ with pyrolitic composition. Considering the uncertainties, an upper bound on average water content in the MTZ is 0.3 wt.% of water.

The influence of mantle hydration and flexure on slab seismicity in the southern Central Andes

Knowledge of the causative dynamics of earthquakes along subduction-zone interfaces and within oceanic slabs is relevant for improving future seismic hazard assessments. Here, we combine the analysis of seismic tomography, the 3D structure of the slab and seismicity to investigate the controlling factors driving slab seismic activity beneath the southern Central Andes. We evaluate the ratio distribution between compressional and shear-wave seismic velocities (Vp/Vs) as a proxy for the hydration state of the lithospheric mantle, oceanic slab, and plate interface. Regions of high Vp/Vs, i.e. areas of hydrated mantle, are principally caused by compaction effects and dehydration reactions. In contrast, slab seismicity in areas of low Vp/Vs and inferred lower fluid content in the overriding plate is facilitated by enhanced flexural stresses due to changes in the subduction angle of the oceanic plate. Plate-interface background seismicity correlates with areas of higher Vp/Vs (hydrous interface) at depths <50 km, while areas of most pronounced plate-locking coincide with regions of low Vp/Vs (anhydrous interface). The regions of anhydrous plate interface are likely candidates for future great megathrust events due to their higher potential for elastic energy accumulation compared to more hydrated regions.

Genesis of Mesozoic high-Mg dioritic rocks from the eastern North China Craton: Implications for the evolution of continental lithosphere

Abstract Pre-Cenozoic high-Mg andesites (HMAs) are mostly present in continental interiors, but their genetic relationship with continental lithosphere evolution remains unclear because of uncertainties of their mantle source, magmatic processes, and physicochemical conditions of formation. Early Cretaceous high-Mg dioritic rocks (HMDs, analogs of HMAs) of the Jinling complex in the Luxi area are typical intra-plate intrusions of the eastern North China Craton (NCC) and can be subdivided into two groups (Group-I and -II) on the basis of their petrographic and geochemical features. Group-I HMDs show low SiO2 contents (52.47–56.10 wt%) and Sr/Y (34.5–39.6) and (La/Yb)N (10.3–13.6) ratios but high contents of MgO (7.86–9.13 wt%), Y (18.3–20.3 ppm), Yb (1.43–1.47 ppm), and compatible elements (Cr = 407–585 ppm; Ni = 117–216 ppm), classifying as sanukitic rocks. Group-II HMDs are characterized by high SiO2 contents (63.81–64.87 wt%) and Sr/Y (47.1–63.4) and (La/Yb)N (16.1–17.5) ratios with low MgO (2.90–3.08 wt%), Y (0.88–1.04 ppm), Yb (0.88–1.04 ppm), and compatible elements (Cr = 201–213 ppm; Ni = 55–57 ppm) contents, belonging to adakitic rocks. Group-I and Group-II HMDs of the Jinling complex are closely related in spatial and temporal distribution, and all have enriched Sr-Nd isotopic compositions and arc-like trace element patterns with abundant hydrous minerals. Therefore, the Jinling HMDs should share a common source of ancient sub-continental lithospheric mantle that was metasomatized by aqueous fluids derived from the subducted Paleo-Pacific slab. The Jinling HMDs were not formed from interaction between slab-derived melts and mantle-wedge peridotites but were instead derived from partial melting of hydrous mantle peridotites in the continental interior of the eastern NCC. The distinctly different petrography, geochemistry, and mineralogy of the two groups of rocks resulted mainly from differing magmatic processes at crustal depths. Thus, Pre-Cenozoic intra-plate HMAs/HMDs are genetically distinct from Cenozoic HMAs that were mostly present in arc settings and generally represent juvenile crust growth. In a way, Archean tonalitic-trondhjemiticgranodioritic rocks (TTG) and sanukitoids, geochemically similar to HMAs/HMDs, could also be derived from interaction between slab-derived melts and mantle-wedge peridotites in arc settings or partial melting of hydrous mantle peridotites in continental interiors, and thus might not always be related with continental crustal growth and the onset of plate subduction.

Mantle serpentinization and associated hydrogen flux at North Atlantic magma-poor rifted margins

AbstractMantle serpentinization influences the rheology of altered peridotites and the global fluxes of energy and volatiles, the generation of seafloor and sub-seafloor chemolithotrophic life, and the carbon cycle. As a by-product of serpentinization, molecular hydrogen (H2) is generated, which supports chemosynthetic communities, and this mechanism may have driven the origin of life on early Earth. At continent-ocean transition zones (COTs) of magma-poor rifted margins, the mantle is exposed and hydrated over hundreds of kilometers across the rift, but the H2 fluxes associated with this process are poorly known. Here, we coupled a thermomechanical model with serpentinization reaction equations to estimate associated H2 release during mantle exhumation at COTs. This reproduced a tectonic structure similar to that of the West Iberia margin, one of the best-studied magma-poor margins. We estimated the rate of H2 production from mantle hydration at (7.5 ± 2.5) × 107 mol/(yr × km). By estimating the area of exhumed mantle from wide-angle seismic profiles at North Atlantic magma-poor margins, we calculated that the accumulated H2 production could have been as high as ~4.3 × 1018 mol (~8.6 × 1012 metric tons) prior to opening of the North Atlantic Ocean, at a rate of ~1.4 × 1017 mol/m.y. This is one quarter of the total predicted flux produced by the global system of mid-ocean ridges, thus highlighting the significance of H2 generation at magma-poor margins in global H2 fluxes, to hydrogenothropic microbial life, and, perhaps, as a potential energy source.