Search

Deep, carbonate-rich melts are key constituents of kimberlites and are crucial for understanding the cycle of volatile elements in the mantle. On the Siberian craton, the Udachnaya-East kimberlite hosts extremely well-preserved nodules composed of chlorides + carbonates + sulfates, that do not present any relict sedimentary textures. These salty nodules display textures that are commonly observed in quenched liquids and may thus represent the very last stage liquid of the kimberlite. Alternatively, they could represent assimilated sedimentary material, or even post-magmatic hydrothermal alteration, because kimberlites are known to ascend through the lithosphere while assimilating material from their wall rocks. Here we focus specifically on those chloride-carbonate nodules, which are composed of 70% chloride + 30% alkali-carbonate and sulfate, and used two radiogenic systems (Rb-Sr, Sm-Nd) and the isotopic composition of sulfur, in addition to their major and trace element compositions (n = 3). We then compared the results with the same geochemical data on host kimberlites (n = 4), sedimentary cover (n = 3) and hydrothermal veins (n = 3). Taken together, our results show that the nodules are not the product of a contamination by the Cambrian sedimentary cover. Trace element patterns of the nodules display extreme enrichments in the same elements that are relatively depleted in the host kimberlite but also in kimberlites worldwide (K, Rb, Sr, Pb), suggesting that chloride-carbonate nodules are snapshots of the latest stage liquid present in the kimberlite system. Their isotopic compositions (Rb-Sr, Sm-Nd and δ34S) are consistent with a common magmatic source with their host kimberlite. We propose that chloride-carbonate nodules record a missing compositional endmember, which could explain the trend towards more radiogenic Sr isotope ratios at nearly constant Nd signatures observed in their host kimberlite, as well as in other kimberlites worldwide. This observed trend suggests the presence of a recycled component with high Rb/Sr (such as salts or terrigenous sediments) in the mantle sampled by some kimberlites, either in the lithosphere or the asthenosphere. This study highlights that the role of alkalies and halogens may have been underestimated in the genesis of kimberlites at depths where diamonds are stable, as well as in more evolved magmatic stages. Segregations of chlorides and carbonates occur specifically in sulfate-bearing kimberlites, which may thus sample a mantle domain in which sulfates with δ34S > 0‰ are dominant. The existence of such a reservoir could explain the apparent imbalance observed between the chondritic value (δ34S of 0‰) and the negative S isotopic compositions of mantle sulfides (MORB and peridotites).

The asthenosphere beneath both ancient and modern oceans is highly heterogeneous owing to the recycling of continental, sub-arc, or sub-oceanic mantle domains and subducted oceanic crust and pelagic sediments. The identification and discrimination of those “old” mantle domains are important for understanding of the generation of oceanic crust and the tectonic setting of ophiolites. Here we report geochemical data including Re–Os and Lu–Hf isotopic compositions for mantle peridotites from the Kop Mountain ophiolite in the western Neo-Tethys. These peridotites have whole-rock and mineral compositions comparable with those of abyssal peridotites. Trace element modeling of clinopyroxenes indicates that they were subjected to anhydrous melting, and in this respect the rocks are distinguishable from supra-subduction zone peridotites. Re–Os and Lu–Hf isotopic compositions further indicate that these mantle peridotites underwent ancient (at or before 1.52 Ga) melt extraction and Jurassic melt metasomatism. The Kop Mountain mantle peridotites thus represent ancient sub-oceanic mantle domains recycled into the Neo-Tethyan asthenosphere, undergoing earlier melt depletion before being entrained at the Neo-Tethyan ridge axis. Our results, together with data published for other ophiolites, indicate that the Os isotopic distribution of the Neo-Tethyan ophiolites is identical to that of global abyssal peridotites, suggesting similar melting histories. The Neo-Tethyan asthenosphere was highly heterogeneous, probably with numerous recycled continental, sub-arc, and sub-oceanic mantle inputs. The recycling of ancient mantle in the asthenosphere could be an alternative explanation for the generation of anomalous compositions in both modern and ophiolitic oceanic crusts such as K-rich lavas, enriched mid-ocean ridge basalts, and arc-like basalts. The use of basalt geochemical data to discriminate between tectonic settings (i.e., mid-ocean ridge or supra-subduction zone) of ophiolites should thus be approached with caution.

Research subject. The Verkhoyano-Kolymsky areal of ore mineralization in the Far East of Russia.Data and methods. We used the state metallogenic map of Russia, Sc. 1: 2 500 000 (2000) and the gravity map of Russia Sc. 1: 2 500 000 (2001). Modeling was conducted by studying the deep structure of the earth’s crust and upper mantle from the anomalies of the density contrast of geological media in the intervals between the centers of density inhomogeneities and the surfaces of equivalent spheres.Results. 3D-distributions of density contrast (µz-parameter) in the crust and upper mantle of the Verkhoyano-Kolymsky region related to the rheological properties of geological media were analyzed. In the gravity models designed without attraction of external information, the structures of thrust, splitting, stretching, as well as the structures of central type (CTS) of the plume nature, were identified. In the regional stretching zone, at the border of lithospheric segments, the revealed Indigiro-Kolymsky and Verkhoyansk CTSs were described in 3D space. These structures are characterized by a mushroom-like upwelling of the asthenosphere, associated with heat flow anomalies. The identified structures differ in terms of asthenosphere depth, age and ore mineralization. The location of ore clusters and regions in the zones of CTS obeys concentric ore-magmatic zoning, typical for this type of structures. The central (trunk-like) zone of Indigiro-Kolymsky CTS features mainly high-temperature gold-quarts mineralization. On the periphery, along with gold areas, there are tin-tungsten, tin and complex ore mineralization areas. The majority of gold fields with low-temperature gold-sulfide, tin and polymetallic mineralization are attributed to the flanks of Indigiro-Kolymsky CTS. In the Verkhoyanska CTS, the majority of ore regions are characterized by multi-formation ore mineralization. In the central part of this structure, areas with mainly low-temperature tin, mercury-antimony and gold-silver ore mineralization are located. On the flanks, gold mineralization is either absent or subsidiary.Conclusions. As a result of a simple procedure, implying generalization of multiple decisions of the elementary inverse problem of gravity potential, main features of the deep structure of the Verkhyano-Kolima region were defined. In the regional stretch zone, at the boundary of lithospheric segments, the Indigiro-Kolimskaya and Verkhoyanskaya CTSs of the plume nature that control the location of ore deposits were identified and described in 3D space.

Research subject. Experimental modelling of the transformation of complex hydrocarbon systems under extreme thermobaric conditions was carried out. The results obtained were compared with geological observations in the Urals, Kamchatka and other regions.Material and methods. The materials for the research were a model hydrocarbon system similar in composition to natural gas condensate and a system consisting of a mixture of saturated hydrocarbons and various iron-containing minerals enriched in 57Fe. Two types of high-pressure equipment were used: a diamond anvils cell and a Toroid-type high-pressure chamber. The experiments were carried out at pressures up to 8.8 GPa in the temperature range 593–1600 K.Results. According to the obtained results, hydrocarbon systems submerged in a subduction slab can maintain their stability down to a depth of 50 km. Upon further immersion, during contact of the hydrocarbon fluid with the surrounding iron-bearing minerals, iron hydrides and carbides are formed. When iron carbides react with water under the thermobaric conditions of the asthenosphere, a water-hydrocarbon fluid is formed. Geological observations, such as methane finds in olivines from ultramafic rocks unaffected by serpentinization, the presence of polycyclic aromatic and heavy saturated hydrocarbons in ophiolite allochthons and ultramafic rocks squeezed out from the paleo-subduction zone of the Urals, are in good agreement with the experimental data.Conclusion. The obtained experimental results and presented geological observations made it possible to propose a concept of deep hydrocarbon cycle. Upon the contact of hydrocarbon systems immersed in a subduction slab with iron-bearing minerals, iron hydrides and carbides are formed. Iron carbides carried in the asthenosphere by convective flows can react with hydrogen contained in the hydroxyl group of some minerals or with water present in the asthenosphere and form a water-hydrocarbon fluid. The mantle fluid can migrate along deep faults into the Earth’s crust and form multilayer oil and gas deposits in rocks of any lithological composition, genesis and age. In addition to iron carbide coming from the subduction slab, the asthenosphere contains other carbon donors. These donors can serve as a source of deep hydrocarbons, also participating in the deep hydrocarbon cycle, being an additional recharge of the total upward flow of a water-hydrocarbon fluid. The described deep hydrocarbon cycle appears to be part of a more general deep carbon cycle.

We present new major and trace element and Sr, Nd, Pb, Hf, and Os isotope data for Quaternary basaltic lavas and tephra from the Itasy volcanic field, in order to assess the origin and sources of recent volcanism in the central highlands of Madagascar. Our age determinations provide further evidence for Pleistocene volcanism on the island, with ages between ∼31 and 104 ka. Mafic magmas from Itasy exhibit significant geochemical heterogeneity, and they are chemically distinct from those of the neighboring Ankaratra volcanic complex. Trace element signatures of the Itasy volcanic field are very similar to those of ocean island basalts (OIB), with significant enrichment in high field strength elements (e.g. Nb and Ta) and depletion in large ion lithophile elements (e.g. Rb, Cs, and K), and relatively high Nb/U (>45) and Ce/Pb (>25) ratios. Significant intra-volcanic field isotopic heterogeneity is observed, with well-defined negative correlations of 87Sr/86Sr with 206Pb/204Pb and 143Nd/144Nd. These correlations, along with highly radiogenic Os isotopic signatures in many Itasy samples, could be attributed to assimilation of the Precambrian basement. However, with the exception of Os, the elemental and isotopic characteristics of the Itasy magmas are inconsistent with significant crustal assimilation, and instead are interpreted to represent features of their mantle sources. The Itasy ΔeHf and Δ7/4 values fall within the range of MORB and OIB, and indicate derivation primarily from an asthenospheric mantle source. The absence of significantly negative ΔeHf in the Itasy magmas argues against the involvement of ancient recycled oceanic crust or stranded lower crust in their sources, but the presence of a low µ (LOMU) component in some Itasy magmas can be attributed to pollution of the asthenosphere by delaminated subcontinental lithospheric mantle. A simple two-stage, three-component mixing model indicates that a mixture of mantle containing 90% depleted asthenosphere (DMM) and 10% foundered SCLM, which is further mixed with 30 to 50% of a plume-type mantle component, can reproduce the isotopic compositions of the Itasy magmas. Based on our model, it is possible that easily fusible superplume materials might be widely distributed in the upper mantle underneath the EARS, from the Indian Ocean to the Red Sea-Gulf of Aden, and mixed with shallow asthenosphere that has itself incorporated eroded SCLM. These components are heterogeneously distributed at a small spatial scale, similar to that observed recently in other mafic monogenetic volcanic fields in both intraplate and subduction settings.

SUMMARYWe determine a new 3-D shear wave velocity (Vs) model down to 400 km depth beneath the Cape Verde hotspot that is far from plate boundaries. This Vs model is obtained by using a new method of jointly inverting P- and S-wave receiver functions, Rayleigh-wave phase-velocity data and S-wave arrival times of teleseismic events. Two Vs discontinuities at ∼15 and ∼60 km depths are revealed beneath volcanic islands, which are interpreted as the Moho discontinuity and the Gutenberg (G) discontinuity. Between the north and south islands, obvious high-Vs anomalies exist in the uppermost mantle down to a depth of ∼100–150 km beneath the Atlantic Ocean, whereas obvious low-Vs anomalies exist in the uppermost mantle beneath the volcanic islands including the active Fogo volcano. These low-Vs anomalies merge into a significant column-like low-Vs zone at depths of ∼150–400 km beneath the Cape Verde swell. We propose that these features in the upper mantle reflect a plume-modified oceanic lithosphere–asthenosphere system beneath the Cape Verde hotspot.

The South Okhotsk Sea province, which includes the Sakhalin, Kunashir, Iturup, Urup islands and surrounding sea areas, comprises numerous occurrences of rare, noble metals and other mineralization, as well as petroleum fields, gas hydrate accumulations, and, locally, active emission of water–hydrocarbon gases. The occurrences and deposits of solid, liquid, and gaseous mineral resources are controlled by hidden deep-seated transform-type fault zones: Nosappu (Tuscarora), Iturup, and Urup. These long-lived extended (over 1000 km) zones are distinguished on the NW Pacific megaplate margin, near the SE flank of the Kuril–Kamchatka trough. Using seismotomographic methods we have established their extension to the west from the seismic focal zone in the oceanic slab subsided into mantle transition zone. In the areas of pull-apart extension, the faults provided active formation of drainage channels for seawater penetration in the lithosphere with subsequent serpentinization of its ultramafic rocks, and for decompressional generation of ascending mantle-derived abiogenic fluid flows. The latter penetrated from the subslab asthenosphere into the suprasubduction mantle wedge and sublithospheric mantle and caused metasomatism. The subsequent migration of the flows led to the generation of the primary magma reservoirs in the lower parts of the continental lithosphere, and intermediate and peripheral chambers in the Earth’s crust. The injection of melts from the chambers in the consolidated crust led to the formation of abyssal and hypabyssal intrusive massifs, arch-dome uplifts, and magmatogenic-ore systems predominantly among the rocks of the pre-Pliocene basement. The concentration of oil and gas accumulations from the mantle-derived abiogenic hydrocarbons bearing mercury, gold, rhenium, and PGE in reservoirs beneath fluid-impermeable beds among Cenozoic sedimentary basins also was related to deep sub- and supraslab fluid flows.

A model of the geological evolution of the Japan and Okhotsk seas was developed based on radioisotope age, mineralogical, and isotope-geochemical study of the Late Mesozoic–Cenozoic volcanic rocks. The geodynamic settings of volcanic stages were determined as follows: (1) the Late Cretaceous continental-margin (calc-alkaline), (2) the Eocene transform-margin (adakite) in the Sea of Okhotsk, (3) the Miocene–Pliocene marginal sea (alkaline basaltic) in the Sea of Japan, and (4) the Pliocene–Pleistocene island arc (calc-alkaline) in the southern Sea of Okhotsk. It has been established that parental magmas were derived from subcontinental lithosphere, oceanic asthenosphere, and lower-mantle plume-continental (CAB) and plume-oceanic (OIB) sources. Geodynamic settings changed from the Late Cretaceous subduction to the Maastrichtian–Pliocene transform margin. The transform margin setting involved destruction and extension, maximum oceanic-margin spreading (end of the Early Miocene–beginning of the Middle Miocene), and post-spreading lower mantle plume upwelling (the Middle Miocene – Pliocene). It was completed by the resumption of the Pliocene–Pleistocene subduction of the Pacific plate beneath the Eurasian continent.

Viscoelastic behaviour of materials at the lithosphere-asthenosphere boundary is observed in this paper. The observed post-seismic and aseismic deformation help us to understand the rheological properties of the mantle and asthenosphere. A quasi-static model having homogeneous, isotropic, elastic material overlying viscoelastic material of a standard linear solid (SLS) is considered. A long strike-slip fault of finite width inclined to the free surface due to a sudden movement has been studied in this work. Analytical solutions for displacement, stresses and strains are obtained before and after fault movement using a technique involving the use of Green’s functions and integral transforms, assuming that tectonic forces maintain a shear strain far away from the fault. The effect of aseismic fault movement across it is found to depend on distance, dimension, relative position and other characteristics of the fault. Also, the effect of different inclinations, elastic layer thickness and slip magnitude have been studied. The study of such earthquake fault dynamical model helps us to understand the mechanism of the lithosphere-asthenosphere boundary.

The occurrence of post-subduction magmatism in continental collision zones is a ubiquitous feature of plate tectonics, but its relation with geodynamic processes remains enigmatic. The nature of mantle sources in these settings, and their interaction with subduction-related components, are difficult to constrain using bulk rocks when magmas are subject to mixing and assimilation within the crust. Here we examine post-collisional magma sources in space and time through the chemistry of olivine-hosted melt inclusions and early-formed minerals (spinel, olivine and clinopyroxene) in primitive volcanic rocks from the Neogene–Quaternary East Carpathian volcanic range in Călimani (calc-alkaline; 10.1–6.7 Ma), Southern Harghita (calc-alkaline to shoshonitic; 5.3–0.03 Ma) and the Perșani Mountains (alkali basaltic; 1.2–0.6 Ma). Călimani calc-alkaline parental magma compositions indicate a lithospheric mantle source metasomatised by ~ 2% sediment-derived melts, and are best reproduced by ~ 2–12% melting. Mafic K-alkaline melts in Southern Harghita originate from a melt- and fluid-metasomatised lithospheric mantle source containing amphibole (± phlogopite), by ~ 5% melting. Intraplate Na-alkaline basalts from Racoș (Perșani) reflect small-degree (1–2%) asthenosphere-derived parental melts which experienced minor interaction with metasomatic components in the lithosphere. An important feature of the East Carpathian post-collisional volcanism is that the lithospheric source regions are located in the lower plate (distal Europe-Moesia), rather than the overriding plate (Tisza-Dacia). The volcanism appears to have been caused by (1) asthenospheric uprise following slab sinking and possibly south-eastward propagating delamination and breakoff, which induced melting of the subduction-modified lithospheric mantle (Călimani to Southern Harghita); and (2) decompression melting as a consequence of minor asthenospheric upwelling (Perșani).

Deep seismic studies of the upper mantle conducted in Russia with the use of nuclear explosions and laboratory studies of mantle materials at high pressure and temperature have revealed new structural and petrophysical features of the upper mantle beneath Northern Eurasia. These features are hard to explain within the framework of current understanding of the structure of the continental lithosphere. For example, in the region of the asthenosphere distinguished according to the thermal field, no corresponding decrease in seismic velocities has been detected, but instead low-velocity layers have been identified at a depth of 100–150 km within the lithosphere. However, the asthenosphere may be distinguished by structural features of the seismic boundaries. This means that it is represented by a layer of elevated plasticity without partial melting. The laboratory studies also indicate that seismic velocities do not depend on the composition of mantle rocks and are controlled primarily by their temperature. This made it possible to infer the temperature regime of the upper mantle from seismic data. Calculations carried out on this basis have shown that the thickness of the lithosphere beneath the Siberian Craton does not vary and is 300–350 km. These data are inconsistent with evaluations of the lithosphere bottom depth based on the heat flow at the surface. This discrepancy may be explained by a stronger effect of deep fluids on the heat flows and on petrophysical parameters of mantle rocks. At depths where mechanical properties of rocks drastically change and their porosity increases, the density of the deep fluids decreases, and the fluids release much heat. As a result, low-velocity domains (plumes), which may contain partial melts, are formed and the surface heat flow increases. This may explain how low-velocity layers can be formed at a depth of 100–150 km and why the temperatures determined from seismic data differ from those derived from the heat flow data. Deep fluids also initiate physical and chemical transformations in mantle rocks, for instance, produce depleted material that has a reduced density but is characterized by the unvarying seismic velocity. Deviations from linear relationship between seismic velocity in a material and its density is also detected at integrated interpretations of seismic and gravimetric data. Physicochemical transformations of rocks in areas where deep fluid flows are focused can also account for the origin of complex reflective boundaries identified in the lithosphere.

SUMMARY While seismic anisotropy can potentially provide crucial insights into mantle dynamics, 3-D imaging of seismic anisotropy is still a challenging problem. Here, we present an extension of our regional full-waveform inversion method to image seismic anisotropy in the lithosphere and asthenosphere from teleseismic P and S waveforms. The models are parametrized in terms of density and the 21 elastic coefficients of the fourth-order elasticity tensor. The inversion method makes no a priori assumptions on the symmetry class or on the orientation of the symmetry axes. Instead, the elasticity tensors in the final models are decomposed with the projection method. This method allows us to determine the orientation of the symmetry axes and to extract the contributions of each symmetry class. From simple synthetic experiments, we demonstrate that our full-waveform inversion method is able to image complex 3-D anisotropic structures. In particular, the method is able to almost perfectly recover the general orientation of the symmetry axis or complex layered anisotropic models, which are both extremely challenging problems. We attribute this success to the joint exploitation of both P and S teleseismic waves, which constrain different parts of the elasticity tensor. Another key ingredient is the pre-conditioning of the gradient with an approximate inverse Hessian computed with scattering integrals. The inverse Hessian is crucial for mitigating the artefacts resulting from the uneven (mostly vertical) illumination of teleseismic acquisitions.

The late Cenozoic Anatolian Metallogenic Trend in Turkey is a manifestation of westerly Aegean subduction and easterly Arabian continental collision along the Western Tethyan Orogenic Belt. The 1500 × 200 km late Cenozoic Anatolian magmatism hosts many gold-rich porphyry and epithermal deposits and prospects (~ 27 Moz Au) that formed during the tearing of the Aegean slab and Arabian slab break-off events that initiated at 15 Ma in western Anatolia and 25 Ma in eastern Anatolia. Although the temporal and spatial correlations between slab segmentation, late Cenozoic magmatism, and associated gold-rich porphyry and epithermal mineralization has been intuitively proposed, the genetic relationships between asthenospheric upwelling, subcontinental lithospheric mantle instability, and magma fertility have not been well established. Herein, new geochemical and radiogenic Sr, Nd, and Pb isotope data from Miocene to Pliocene magmatic rocks that are genetically related to porphyry, epithermal, and skarn systems from western, central, and eastern Anatolia are presented and interpreted with previously published geochemical data from late Cenozoic intrusion-related mineral systems throughout Anatolia. The magma fertility of the late Cenozoic post-subduction igneous suites along the late Cenozoic Anatolian Metallogenic Trend is assessed and is best indicated by the SiO2 (61.8 ± 5.5 wt%) content, Sr/Y (> 20) and Ba/Ta ratios (> 428), and (eNd)i values (

Based on statistical processing of gravity anomalies and tectonic interpretation of 3D distributions of density contrast in the crust and upper mantle of Southeast China, new features of rheological layering of the tectonosphere are revealed. Relations of near-surface geological structures with a deep tectonosphere structure in this region are defined. New data on deep relationship of lithospheric segments of a different rank are obtained: splitting, underthrusting, thrusting, and strike-slip fault in the different depth ranges of geological space. Hidden tension zones in the lower crust and subcrustal mantle have been revealed. New assessments independent of the previous were obtained on the lithosphere thickness, subcrustal and asthenosphere layers of lowered viscosity, and their location in geological space. Rheological layering of the crust and upper mantle of Southeast China produces new features of the collision and subduction of the lithosphere segments, which are universal of the West Pacific continental margin.

A model of the geological evolution of the Sea of Okhotsk is presented based on data from radioisotope age determinations and the mineral and isotope-geochemical composition of Late Mesozoic–Cenozoic volcanic rocks. We discuss the probable interrelationship between gas geochemical manifestations of gas fluxes with anomalous methane concentrations and the formation of gas hydrates, along with the occurrence of volcanic processes in the Sea of Okhotsk, fault zones, and different geological structures of basement and sedimentary deposits, landslides, and earthquakes. As a result of the studies, the nature of each volcanic stage has been determined. These include the Late Cretaceous continental-marginal belt stage (calc-alkaline), the Eocene transform-marginal (adakite) stage, and the Pliocene-Pleistocene island arc stage in the southern part of the Sea of Okhotsk. The sources of magma generation have been recognized, namely, the lithosphere subcontinental, asthenosphere oceanic, and plume-oceanic (OIB). The change in geodynamic regimens was traced back from the Late Cretaceous subduction regimen to the Maastrichtian–Datian transform-marginal, which continued as far as the Pliocene and ended at the resumption of the Pliocene–Pleistocene subduction of the Pacific Plate beneath the Eurasian continent. This involved the processes of destruction of the subduction plate, lithosphere and asthenosphere diapirism, and lower mantle plume upwelling. In the periods of geodynamic, seismotectonic and volcanic activities, gas migrated along the fault zones from the depth to the surface, together with different volcanic substrates ascending through the upper and lower mantle. Gas contained CO2, CH4, H2, He, N2, O2, and superheated steam (Н2О). Gas plays an important dynamic and physicochemical role in the evolution of the Sea of Okhotsk. At the present stage, gas fluxes migrating from the depth to the surface manifest as gas bubbles penetrating from the bottom sediments into the water column, with some portion of gas penetrating into the atmosphere. In the areas with gas fluxes, the fields containing anomalous concentrations of hydrocarbons, gas hydrate, carbon dioxide, hydrogen, and helium result in the formation of gas hydrates and associations of authigenic minerals and various geochemical elements.

A detailed study on petrology and mineral chemistry of 12 mantle xenoliths from Late Cretaceous basaltic lava flows at Daxizhuang has been conducted to constrain the nature and secular evolution of the Mesozoic lithospheric mantle beneath the eastern North China Craton (NCC). The Daxizhuang mantle xenoliths are mainly anhydrous spinel lherzolite, with subordinate olivine websterite and rare spinel wehrlite. Based on petrographic and geochemical characteristics, the Daxizhuang spinel lherzolite can be subdivided into two groups. Group 1 lherzolite has olivine with Fo contents from 89.4 to 90.6 and LREE-depleted through spoon-shaped to LREE-enriched REE (rare earth element) patterns in clinopyroxene grains, reflecting variable degrees of partial melt extraction (up to 18%) overprinted by later incipient metasomatism. In contrast, Group 2 lherzolite is strongly metasomatized and enriched in iron, as evidenced by relatively lower Fo contents of olivine (84.9 to 88.2) than that from the Group 1 lherzolite and convex-upward trace element patterns in clinopyroxene grains. The Daxizhuang olivine websterite and spinel wehrlite are rich in Fe, Al and low Mg# in olivine and pyroxenes, and slight Ti-enrichment in spinel. Numerical modeling of the Mg# shows that the Daxizhuang olivine websterite and wehrlite may have resulted from the reaction between host residual lherzolite and evolved, silicate melts at high melt/rock ratios. The result of this study indicates that the lithospheric mantle beneath Daxizhuang underwent multiple metasomatism through asthenosphere-lithosphere interaction, which plays an important role in the transformation of the lithospheric mantle of the NCC at least during Late Cretaceous time.

We present geochemical and 40Ar∕39Ar age data for a peridotite xenolith-bearing basalt dredged from the Seifu Seamount (SSM basalt) in the northeast Tsushima Basin, southwest Sea of Japan. An 40Ar∕39Ar plateau age of 8.33±0.15 Ma (2σ) was obtained for the SSM basalt, indicating that it erupted shortly after the termination of back-arc spreading in the Sea of Japan. The SSM basalt is a high-K to shoshonitic alkali basalt that is characterized by light rare earth element enrichment. The trace element features of the basalt are similar to those of ocean island basalt, although the Yb content is much higher, indicating formation by the low-degree partial melting of spinel peridotite. The Nd, Sr, and Pb isotopic compositions of the SSM basalt differ from those of back-arc basin basalts in the Sea of Japan. The Sr–Nd isotopic composition of the SSM basalt suggests its source was depleted mid-ocean ridge mantle containing an enriched mantle (EM1) component. The SSM basalt was formed in a post-back-arc extension setting by the low-degree partial melting of an upwelling asthenosphere that had previously been associated with the main phase of back-arc magmatism.

The genesis of hypozonal orogenic gold deposits is highly controversial, with both crustal metamorphic fluids and variously derived sub-crustal fluids suggested as fluid and metal sources. The Lower Jurassic Danba gold deposit, a rare Phanerozoic hypozonal orogenic gold deposit on the western margin of the Yangtze Craton, China, provides an opportunity to resolve these controversies. Danba is characterized by biotite–amphibole alteration and pyrrhotite-dominant ore assemblages formed at ~ 500–600 °C. The δ34S values of ore-related pyrrhotite range from + 3.1 to + 9.9‰, in sharp contrast to those of syn-sedimentary pyrrhotite from Devonian host rocks which are between − 6.8 and − 9.5‰. The Pb isotope compositions of ore-related pyrrhotite, with 206Pb/204Pb = 17.85–18.25 and 207Pb/204Pb = 15.48–15.67, are less radiogenic and more variable than those of unaltered host rocks. These differences, combined with the hypozonal feature of the deposit, indicate that ore metals and sulfur were derived from a sub-crustal, rather than a local crustal source. The PGE patterns of ore-related pyrrhotites are similar to those of sulfides in Baltic mantle lithosphere that was metasomatized by aqueous fluids in terms of marked enrichment of Pd and Ru but significantly differ from those of magmatic intrusions and related magmatic–hydrothermal deposits. This suggests that the metals that formed the Danba deposit were transported via aqueous fluid from metasomatized mantle lithosphere rather than from a magmatic source. The δ18O of mantle fluid calculated from hydrothermal quartz and biotite ranges from 10 to 12‰. The anomalously high S and O isotope ratios and variable Pb isotope ratios, together with PGE data, support a model in which the inferred mantle lithosphere source was significantly hydrated, metasomatized, and fertilized by fluid derived from subducted oceanic sediments with elevated δ34S and δ18O values. Geological evidence suggests that this subduction event was Neoproterozoic in age. This study defines a model in which hydrated and metasomatized mantle lithosphere was formed during early subduction, but it was over 500 million years later, during later Lower Jurassic asthenosphere upwelling, that metal- and sulfur-rich fluid was released into the crust to form the Danba gold deposit.

The 190Pt-4He age of isoferroplatinum from andesitic fluid-explosion breccias (fluidolites, a new type of platinum-bearing rock) of the Poperechnoye deposit (Malyi Khingan, Russia) was estimated as 125 ± 21 Ma. This age correlates well with peak magmatic activity in the eastern Central Asian orogenic belt caused by upwelling of slab-window asthenosphere in a transform continental margin setting.

Isostasy is a key concept in geoscience in interpreting the state of mass balance between the Earth’s lithosphere and viscous asthenosphere. A more satisfactory test of isostasy is to determine the depth to and density contrast between crust and mantle at the Moho discontinuity (Moho). Generally, the Moho can be mapped by seismic information, but the limited coverage of such data over large portions of the world (in particular at seas) and economic considerations make a combined gravimetric-seismic method a more realistic approach. The determination of a high-resolution of the Moho constituents for marine areas requires the combination of gravimetric and seismic data to diminish substantially the seismic data gaps. In this study, we estimate the Moho constituents globally for ocean regions to a resolution of 1° × 1° by applying the Vening Meinesz-Moritz method from gravimetric data and combine it with estimates derived from seismic data in a new model named COMHV19. The data files of GMG14 satellite altimetry-derived marine gravity field, the Earth2014 Earth topographic/bathymetric model, CRUST1.0 and CRUST19 crustal seismic models are used in a least-squares procedure. The numerical computations show that the Moho depths range from 7.3 km (in Kolbeinsey Ridge) to 52.6 km (in the Gulf of Bothnia) with a global average of 16.4 km and standard deviation of the order of 7.5 km. Estimated Moho density contrasts vary between 20 kg m-3 (north of Iceland) to 570 kg m-3 (in Baltic Sea), with a global average of 313.7 kg m-3 and standard deviation of the order of 77.4 kg m-3. When comparing the computed Moho depths with current knowledge of crustal structure, they are generally found to be in good agreement with other crustal models. However, in certain regions, such as oceanic spreading ridges and hot spots, we generally obtain thinner crust than proposed by other models, which is likely the result of improvements in the new model. We also see evidence for thickening of oceanic crust with increasing age. Hence, the new combined Moho model is able to image rather reliable information in most of the oceanic areas, in particular in ocean ridges, which are important features in ocean basins.

The distribution of water in the upper mantle plays a crucial role in the Earth's deep water cycle, magmatism, and plate tectonics. To better constrain how these large-scale geochemical systems operate, peridotite and pyroxenite mantle xenoliths from Kilbourne Hole (KH) and Rio Puerco (RP) along the Rio Grande Rift (NM, USA) were analyzed for water, and major and trace element contents. These xenoliths sample a lithosphere whose composition was influenced by subduction and rifting, and can be used to examine the effects of melting, metasomatism, and sub-solidus equilibration on the behavior of water. The first result is that in KH xenoliths, olivines underwent negligible H loss during xenolith ascent, i.e. preserved their mantle water contents. These olivine water contents are used to calculate mantle viscosities of 0.5–184 · 1021 Pa·s. These viscosity values are more than 40 times higher than those of the asthenosphere and show that KH peridotites represent samples from the lithosphere. The preservation of olivine water contents is exceptional for off-cratonic xenoliths, and the KH peridotites provide the first estimate of the average concentration of water in Phanerozoic continental mantle lithosphere at 81 ± 30 ppm H2O. The mantle lithosphere beneath the Rio Grande rift is nevertheless heterogeneous with water contents ranging from

We report experimental measurements of volume and grain boundary diffusion of 26Mg in Mg2SiO4 bi-crystals at asthenosphere temperatures as a ground reference for olivine. By analysis of literature and combination with previous data, we provide Arrhenius laws D = D0 exp(− E/RT) at ambient pressure for volume diffusion of Mg in Mg2SiO4 in the intrinsic regime along the three crystallographic axes as well as grain boundary diffusion. Parameters for average volume diffusion, calculated as geometrical mean of the three crystallographic axes, are $$D_{0} = 10^{-2.12\,\pm\, 0.91}\,{\text{m}^{2}/{\text{s}}}\, {\text{and}}\, E = 443 \pm 42 {\text{kJ}}/{\text{mol}}.$$ Parameters for average grain boundary diffusion in aggregate are $$D_{0} = 10^{-1.16\, \pm\, 0.49}\,{\text{m}^{2}/{\text{s}}}\, {\text{and}}\, E = 359 \pm 14 {\text{kJ}}/{\text{mol}}.$$ In the asthenosphere, the decrease of both volume and grain boundary diffusion coefficients as a function of pressure should remain negligible up to 1 GPa, and ~ 1 log unit at 10 GPa, while the increase in iron- and hydrogen-bearing olivine should range from ~ 1 to 2 log unit. The equilibration of Mg in olivine grains can be considered instantaneous with respect to geological timescales in the asthenosphere. However, the transport of Mg remains below km-scale even after 1 Gy at 1600 °C of volume or grain boundary diffusion. Long-range transport of major elements in the upper mantle is not possible by solid-state diffusion. Equilibration of long-range heterogeneities and large mass transport is rather controlled by diffusion in intergranular fluid or melt phase, liquid percolation and mantle convection.

The style of convective force transmission to plates and strain-localization within and underneath plate boundaries remain debated. To address some of the related issues, we analyze a range of deformation indicators in southern California from the surface to the asthenosphere. Present-day surface strain rates can be inferred from geodesy. At seismogenic crustal depths, stress can be inferred from focal mechanisms and splitting of shear waves from local earthquakes via crack-dependent seismic velocities. At greater depths, constraints on rock fabrics are obtained from receiver function anisotropy, Pn and P tomography, surface wave tomography, and splitting of SKS and other teleseismic core phases. We construct a synthesis of deformation-related observations focusing on quantitative comparisons of deformation style. We find consistency with roughly N-S compression and E-W extension near the surface and in the asthenospheric mantle. However, all lithospheric anisotropy indicators show deviations from this pattern. Pn fast axes and dipping foliations from receiver functions are fault-parallel with no localization to fault traces and match post-Farallon block rotations in the Western Transverse Ranges. Local shear wave splitting orientations deviate from the stress orientations inferred from focal mechanisms in significant portions of the area. We interpret these observations as an indication that lithospheric fabric, developed during Farallon subduction and subsequent extension, has not been completely reset by present-day transform motion and may influence the current deformation behavior. This provides a new perspective on the timescales of deformation memory and lithosphere-asthenosphere interactions., Geochemistry, Geophysics, Geosystems, 22 (11), ISSN:1525-2027

Unconventional hydrocarbon resources such as shale oil/gas and coal-bed methane have become an increasingly important source of energy over the past decade. The Vaca Muerta Shale (Neuquen Basin, Argentina) contains the second largest technically recoverable quantity of shale gas in the world. Exploitation of the play has been complicated by elevated concentrations of CO2 in several fields, the origin of which is currently poorly understood. Elevated CO2 levels are consistently encountered when deep-rooted faults in the Auquilco Evaporite Formation, present below the Vaca Muerta Shale, overlap with shallower faults that propagate from the top of evaporites into the shale, indicating a sub-evaporate origin of the CO2. Here we report new isotopic analysis of CO2-rich gases from two producing fields. CO2 concentrations increase with C1/(C2 + C3) values (4.8–33.5) and fractionation of δ13CCO2 (−0.9 to −7.7‰), suggest that CH4 have been displaced by CO2 which entered the shale after hydrocarbon maturation. The noble gas composition (3He/4He of 3.43–3.95 RA, where RA is the atmospheric ratio of 1.399 × 10−6, 21Ne/22Ne of 0.0310–0.0455, 20Ne/22Ne of 9.89–10.52, 40Ar/36Ar of 2432–3674 and CO2/3He 6.8–20.2 × 107) of the gases is consistent with mixing of magmatic CO2 with crustal hydrocarbon-rich gases and provides evidence for the loss of significant CO2. Using inverse modelling techniques, we determine that the magmatic gas has a 3He/4He of 3.95–4.08 RA, CO2/3He of 8.8–16 × 108 and 20Ne/22Ne of 12.13−0.10+0.08, 21Ne/22Ne of 0.074−0.003+0.004. Based on the radiogenic He and Ne this is consistent with a depleted asthenosphere mantle source, which has been trapped in the crust since 6.0–22.8 Ma. This is significantly younger than Late Cretaceous maturation of the hydrocarbon source rocks. Mantle melting as a result of asthenosphere upwelling induced by the collision of the South Chile Ridge and the Chile Trench at ~14 Ma is the most likely source of the CO2. Gases from below the shale contain two air saturated water-derived noble gas components, distinguished on the basis of 20Ne†/36Ar, 84Kr/36Ar, 132Xe/36Ar ratios. These are consistent with early and late stage open system Rayleigh fractionation of groundwater-derived noble gases. We find evidence that these mix with the magmatic component prior to entering the Vaca Muerta and mixing with an adsorption derived gas retained in the source kerogen.

Quaternary alkali basalts and related rocks are located in the northern portion of the Sanandaj–Sirjan zone in Iran (SaSZ). These rocks, also known as the Salmas Quaternary volcanic rocks (SQVR), consist primarily of basalts and trachy basalts (mafic parts) and basaltic trachy andesites and trachy andesites (intermediate parts), which are highly sodic alkaline and high-K calc-alkaline in character, respectively. These mafic to intermediate samples contain variable proportions of pyroxene, olivine and plagioclase, alongside ubiquitous accessory levels of biotite, amphibolte and oxides. Based on the negative Nb, Ta, Zr, and Hf, and also positive Th and U anomalies in some samples, the SQVR formed in a widely-recognized syn-collision environment following the termination of oblique Neotethys subduction beneath the Central Iranian microplate. Following slab break-off of the remnants of the oceanic lithosphere under the Arabia-Eurasia collision zone, a combination of asthenospheric upwelling, small-scale mantle convection, and localized crustal extension promoted magmatism. We model that decompression melting occurred on the boundary of the lithosphere and asthenosphere at around 65–70 km. The magma of the SQVR resulted from relatively small degrees of partial melting (

The ~ 120 Ma Jiaodong province in the North China Craton, composed of abundant Mesozoic granites that intrude Precambrian metamorphic basement, is bordered by the Sulu orogenic belt, which formed through the collision between the North China and Yangtze cratons in the Triassic. Insights on the geodynamic setting and source of gold and fluids in the Jiaodong gold province are based on a detailed study of the Sanshandao deposit. The δ13CPDB and δ18O values of hydrothermal calcite from this deposit range from − 4.3 to − 6.5‰ and 11.4 to 15.1‰, respectively, which are compatible with a mantle source. Pre-ore mafic dykes in the deposit show typical arc-like geochemical features, suggesting that the mantle source was metasomatized prior to the basic magmatism. The δ34S values of gold-related pyrite from Sanshandao and other large deposits in the province range from 10.9 to 11.5‰ and are higher than those of the country rocks, but consistent with the signature of orogenic gold deposits sourced from a Neoproterozoic sedimentary reservoir. These high positive δ34S values are interpreted to be inherited from auriferous Neoproterozoic sedimentary rocks of the northern Yangtze Craton, which were subducted into the mantle lithosphere beneath the North China Craton during the formation of eclogite in the adjacent Sulu orogenic belt in the Triassic. The syn-ore Lower Cretaceous mafic dykes display oceanic island basalt-like geochemical features, suggesting that asthenosphere upwelling triggered the release of gold and sulfur from an enriched and fertilized mantle lithosphere, contributing to the auriferous fluid which formed the widespread gold mineralization in the Jiaodong province at ca. 120 Ma.

Crustal recycling is an important cause of mantle heterogeneity and can have significant control on basalt compositions. Recycled components from the subducted (Paleo-) Pacific slab have frequently been recognized in Cenozoic basalts from the eastern North China Craton (NCC). However, it still remains unclear if the subducted Paleo-Asian oceanic slab contributed to intraplate basalts in this Craton. In a search of evidence for the recycled components from this slab, we have studied the Ar Ar age and elemental and Sr-Nd-Pb isotope compositions of newly-discovered basalts from the Langshan area and compiled a regional synthesis of Cenozoic alkali basalts from the northwest NCC. This region is far from the Pacific domain but near the suture zone of the Paleo-Asian Ocean. With a Late Cretaceous eruption age (~89 Ma), Langshan basalts have low silica and high FeO, MgO and alkali contents, high incompatible elemental concentrations, positive Sr, Eu, Ba, Nb, Ta and negative Pb and Ti anomalies with high Ce/Pb, Nb/U, Ba/Rb and low Rb/Sr ratios, superchondritic Zr/Hf ratios, and uniform radiogenic isotopes (ɛNd(t) = 2.20–2.51, 87Sr/86Sri = 0.703807–0.704348, 206Pb/204Pbi = 18.200–18.384, 207Pb/204Pbi = 15.470–15.515, 208Pb/204Pbi = 38.007–39.430). These compositional characteristics are shared by the compiled Cenozoic alkali basalts from northwest NCC. The low silica and high MgO, FeO and alkali contents together with the positive Nb, Ta and negative Ti anomalies were probably controlled by silica-deficient garnet pyroxenite in the mantle source. The partial melting is estimated to have occurred in the asthenosphere under an average mantle potential temperature of 1300–1450 °C and a pressure of ~2.5 GPa. The positive Sr, Eu, Ba, Nb, Ta anomalies and the canonical indices (high Ce/Pb, Nb/U, Ba/Rb and low Rb/Sr ratios) indicate the involvement of a subducted oceanic igneous slab. Considering 1) the particular tectonic locality of the study region, 2) the inferred northward increase of silica-deficient pyroxenite in the mantle source of Cenozoic alkali basalts, 3) the evidence for strong lithospheric modification beneath the northwest NCC induced by slab-derived components, and 4) the longevity of subducted slabs in convecting mantle, the subducted Paleo-Asian oceanic materials could have introduced ubiquitous mantle heterogeneity beneath the northwest NCC and played a significant role in the generation of the Late Cretaceous to Cenozoic intraplate alkali basalts there.

The large ore clusters and uranium deposits detected in southeast Russia (eastern Transbaikal Region), eastern Mongolia, and northern China constitutes a large area, specialised in U, with a wide range of geological and mineral ore types. Amongst various types of uranium ore, hydrothermal deposits related to volcanic rocks, infiltration deposits of uranium sandstone and polygenous mineralisation in leucogranites represent main commercial interest. Large uranium clusters are located on the outer, side and peripheral parts of the Great Xing'an volcano-plutonic belt as well as in the framing taphrogenic depressions. The clusters formation timed to late Mesozoic (I-K) geodynamic processes. Deep geodynamics allowed the fluid flows of asthenosphere which were concentrating around the periphery of the slab stagnated in the transition mantle zone to penetrate into the above lithosphere and earth's crust with formation of pockets of magma, and ore-bearing volcano-tectonic depressions eventually. Therefore, the presence of igneous stocks, extrusions, and volcanic necks within the volcano-plutonic belts and depressions can be indicative of mineral potential of the area.

The Neoproterozoic magmatism in South China may have been related to assembly and breakup of the supercontinent Rodinia. The ca. 815-Ma mafic intrusions in the Dahongshan region at the northern Yangtze Block are composed of fine- to medium-grained gabbro and diabase, which show variable major and trace element compositions with (La/Yb)N ratios of 0.53–3.43. The mafic rocks also have large ranges of initial 87Sr/86Sr ratios (0.70318–0.70810), ƐNd values (−2.8 to +4.3) and Pb isotopes (206Pb/204Pb = 17.57–18.46, 207Pb/204Pb = 15.43–15.56, 208Pb/204Pb = 37.45–38.12). These lines of evidence suggest that the Dahongshan mafic rocks underwent significant crustal contamination during the magma emplacement. AFC calculations reveal that 10–20% crustal materials were involved in their petrogenesis. Two primary samples from the Dahongshan intrusions display low U/Th (0.36–0.37), Th/Zr (0.001–0.002), and Nb/Y (0.02–0.03) ratios, and show DMM-like ƐNd and Pb isotopes, indicating that their primary melts were derived from an asthenosphere mantle which had not been modified by slab-derived materials. In the northern margin of the Yangtze Block, the Neoproterozoic magmatism changed from arc-affinity to MORB- and OIB-like characters at around 815 Ma. Such transition is proposed to have resulted from slab tearing and detachment that induced upwelling of the asthenosphere mantle.

The Gangdese magmatic belt, located along the southern margin of the Lhasa terrane, plays a critical role in understanding the tectonic framework associated with the Indian–Asian slab collision. In this paper, a chronology of zircon U–Pb and geochemical analysis of the rock of the Cuobulaguo granitic mass in the southern of the Gangdese, Tibet, revealed a series of results. The results show that the LA-ICP-MS monzonitic granite zircon U–Pb ages are 61–59 Ma, which corresponds to the same period as the magmatic arc of the southern limit of Gangdese. In terms of geochemical composition, the granite is rich in ω(SiO2) 70.09% to 72.64%, with a high ω(Al2O3) 14.40% to 15.99%, a low ω(TiO2) 0.08% to 0.24%, ω(MgO) 0.41% to 0.76%, ω(Fe2O 3 T ) 6.82% to 29.9%, ω(P2O5) 0.07% to 0.12%, and ω(CaO) 1.06% to 1.75%. The granite mainly belongs to the high-K calc-alkaline series. The light rare earth element (LREE) content of skeletal granite is between 133.69 × 10−6 ~ 226.64 × 10−6 and the heavy rare earth element (HREE) content is between 17.36 × 10−6 and 32.11 × 10−6. LREE/HREE is between 5.05 and 7.83. It is enriched in light rare earth elements (LREE) and large ion lithophile elements, such as Rb, K, U, etc., depletes high field strength elements, such as P, Nb, and Ta, and has the geochemical composition of arc magmatic rocks. In addition, the aluminum saturation index (A/CNK = 1.06 to 1.11) of Cuobulaguo granite, belongs to I-type granite. The comprehensive analysis showed that with the beginning of the collision between the Indian–Asian continental, the subduction ocean plate was separated from the continental plate due to gravity, resulting in an increase in the asthenosphere, which made partial fusion of the lithospheric mantle. It invaded the bottom of the lower crust, which in turn induced a partial melting of the lower crust to form granite.

The East Kunlun Orogen is located in the northern part of the Tibetan Plateau and has received significant attention in tracing the evolution of Paleo-Tethys Ocean. In this study, we present U-Pb zircon geochronology, geochemistry and Sr-Nd isotopic data on the Kengdenongshe tholeiitic gabbro in the eastern part of the orogen. The gabbro is characterised by high and variable MgO and Fe2O3, as well as low K2O and TiO2 contents, typical of low-K tholeiitic series. The rock shows enrichment in large-ion lithophile elements, depletion in high field strength elements, with rare earth element signature similar to that of a back-arc basin basalt. The rock displays relatively low and uniform (87Sr/86Sr)i ratios and high eNd(t) values. These geochemical and isotopic features are interpreted to represent magma derivation from partial melting of an enriched asthenospheric source. Our model envisages that the initial melting started in the garnet-facies (~4%) and continued in the spinel-facies (~6%). The geochemical and isotopic systematics of the gabbro are consistent with magma generation in a back-arc basin setting triggered by the subduction of the Paleo-Tethys Ocean. In conjunction with the data on mafic magmatic suites in the region, we trace a progressive change in source from asthenosphere to increasingly enriched mantle wedge. Our study proposes a transition in the geodynamic setting of the east Paleo-Tethys Ocean from back-arc extension during the Late Permian to continued subduction during the Late Permian and Middle Triassic.

New K–Ar ages, major and trace element concentrations, and Sr–Nd–Pb isotope data are presented for Oligocene to recent mafic volcanic rocks from the Ethiopian Plateau, the Main Ethiopian Rift (MER), and the Afar depression. Chronological and geochemical data from this study are combined with previously published datasets to reveal secular variations in magmatism throughout the entire Ethiopian volcanic region. The mafic lavas in these regions show variability in terms of silica-saturation (i.e. alkaline and sub-alkaline series) and extent of differentiation (mafic through intermediate to felsic). The P–T conditions of melting, estimated using the least differentiated basalts, reveal a secular decrease in the mantle potential temperature, from when the flood basalt magmas erupted (up to 1600 °C) to the time of the rift-related magmatism (

The continental lithosphere is created and modified at convergent plate boundaries through complex, and interdependent, tectonic and magmatic processes. Evolution of the continental lithosphere modifies conditions within the mantle wedge, and therefore, the generation and composition of magmas at subduction zones. One approach to unraveling interactions between the continental lithosphere and asthenosphere in subduction zones is through the evolution of pre-, syn- and post-collisional mafic volcanism (i.e., MgO ≥ 6 wt%). Such lavas are most likely to contain characteristic geochemical fingerprints of the composition and physical condition of the mantle source at the time of melting. Here, we combine new analyses of thirty-one samples from the Hoh Xil Basin of the northern Tibetan Plateau with a compilation of published geochemical and geochronological data for late Mesozoic and Cenozoic magmas from the Indo-Eurasian collisional orogen. Geochronology of the Hoh Xil lavas reveals unrecognized early (∼27 Ma) alkali basaltic magmatism in northern Tibet that likely triggered melting of the mafic lower and metasedimentary upper continental crust, generating trachyanadesites-trachydacites and rhyolites, respectively. Based on a combined analysis of new and published compositional, age and spatial patterns of mafic lavas (MgO ≥ 6 wt%), we delineate two distinct periods of Tibetan mafic magmatism, with a major compositional change occurring at roughly 46 Ma. Early (pre-46 Ma) mafic lavas erupted only in central and southern Tibet, and were generated by melting within the mantle wedge during the northward subduction of the Neo-Tethyan oceanic slab. Late (post-46 Ma) mafic lavas were produced by the partial melting of metasomatized subcontinental lithospheric mantle. The latter group is spatially restricted to central Tibet between 46-30 Ma. After 30 Ma, lavas derived from melting of the lithospheric mantle expanded both northward and southward, eventually encompassing the entire Tibetan Plateau by 20 Ma. These observations suggest that melting of the lithospheric mantle beneath Tibet initiated earlier, and was more widespread, than previously thought. Our results also suggest that widespread melting of the mantle lithosphere across Tibet post-dates the cessation of upper crustal shortening in northern Tibet at ∼30 Ma, and precedes the initiation of crustal extension at ∼15-10 Ma. Hence, we suggest that plateau-wide melting of metasomatized subcontinental mantle lithosphere initiated following the collision of India with Eurasia, at ∼46 Ma. This melting was induced by detachment of the Neo-Tethyan slab concomitant with thinning of the continental mantle lithosphere. Such melting has continued until the present, with the exception of volcanic quiescence in south-central Tibet since ∼10 Ma. This quiescence likely arose from the underthrusting of Indian lithosphere beneath Eurasia. Although melting of the mantle lithosphere beneath Asia has previously been interpreted as an abrupt event, we propose that mantle lithosphere thinning and removal has been part of a continuum melting process during the Indo-Asian orogen that reflects an evolution from mantle wedge to mantle lithosphere melting accompanying the transition from subduction to collision.

Systematic shear wave splitting (SWS) study was conducted to illuminate the mantle deformation and dynamics in the Carpathian-Pannonian region (CPR) where its tectonic mechanism is still enigmatic and actively debated. We have obtained a total of 1170 SWS measurements using seismic data recorded by 59 broadband seismic stations. The resulting SWS measurements mostly possess NW-SE fast orientations and are mostly oblique or even orthogonal to the trends of major geological features and the absolute plate motions. Spatial coherency of the splitting parameters reveals that the center of the observed anisotropy is most likely originated from the asthenosphere at the depth of about 250 km. Thus, the uniformly NW-SE fast orientations is more likely to represent an asthenospheric flow, which has partially or completely overprinted the anisotropic fabrics developed during the formation of the Pannonian basin. Such an asthenospheric flow is regionally deflected along the edges of continental keels beneath the South Carpathians and the Vrancea zone where complex anisotropy is well-determined and range-parallel fast orientations are revealed based on two-layered fitting of the SWS measurements. Our results combined with previous SWS measurements suggest that the revealed NW-SE asthenospheric flow is actually a key component of the toroidal flow induced by the Adriatic subduction, which dominantly promotes the current upper mantle deformations beneath the CPR and its adjacent regions.

The passive margin of the eastern coast of the United States is known to be geologically active, with recently rejuvenated topography, intraplate seismicity, and volcanism of Eocene age. This study uses seismic data from the Mid-Atlantic Geophysical Integrative Collaboration (MAGIC) experiment to constrain lateral variations in the attenuation of teleseismic P waves beneath the central Appalachian Mountains to shed light on the structure and dynamics of the upper mantle at this “active” passive margin. We use a Monte Carlo approach to estimate variations in attenuation along with both data and model uncertainties. The quality factor of the upper mantle dramatically decreases over a distance of less than 50 km on the western side of the central Appalachian Mountains, where a low-velocity anomaly has been previously inferred. Extrinsic factors such as scattering or focusing are rejected as explanations for the observations on the basis of finite-difference waveform modeling experiments. The peak in attenuation beneath the crest of the Appalachian Mountains requires that near- to super-solidus conditions occur in the upper mantle and is co-located with volcanism of Eocene age. Our preferred interpretation is that the attenuation reflects the removal of the mantle lithosphere via delamination beneath the mountains, followed by ongoing small-scale convection.

SUMMARY The electromagnetic (EM) field generated by ocean tidal flow is readily detectable in both satellite magnetic field data, and in ocean-bottom measurements of electric and magnetic fields. The availability of accurate charts of tidal currents, constrained by assimilation of modern satellite altimetry data, opens the possibility of using tidal EM fields as a source to image mantle electrical resistivity beneath the ocean basins, as highlighted by the recent success in defining the globally averaged lithosphere–asthenosphere boundary (LAB) with satellite data. In fact, seafloor EM data would be expected to provide better constraints on the structure of resistive oceanic lithosphere, since the toroidal magnetic mode, which can constrain resistive features, is a significant component of the tidal EM field within the ocean, but is absent above the surface (in particular in satellite data). Here we consider this issue in more detail, using a combination of simplified theoretical analysis and 1-D and 3-D numerical modelling to provide a thorough discussion of the sensitivity of satellite and seafloor data to subsurface electrical structure. As part of this effort, and as a step toward 3-D inversion of seafloor tidal data, we have developed a new flexible 3-D spherical-coordinate finite difference scheme for both global and regional scale modelling, with higher resolution models nested in larger scale solutions. We use the new 3-D model, together with Monte Carlo simulations of errors in tidal current estimates, to provide a quantitative assessment of errors in the computed tidal EM signal caused by uncertainty in the tidal source. Over the open ocean this component of error is below 0.01 nT in Bz at satellite height and 0.05 nT in Bx on the seafloor, well below typical signal levels. However, as coastlines are approached error levels can increase substantially. Both analytical and 3-D modelling demonstrate that the seafloor magnetic field is most sensitive to the lithospheric resistance (the product of resistivity and thickness), and is more weakly influenced (primarily in the phase) by resistivity of the underlying asthenosphere. Satellite data, which contain only the poloidal magnetic mode, are more sensitive to the conductive asthenosphere, but have little sensitivity to lithospheric resistance. For both seafloor and satellite data’s changes due to plausible variations in Earth parameters are well above error levels associated with source uncertainty, at least in the ocean interior. Although the 3-D modelling results are qualitatively consistent with theoretical analysis, the presence of coastlines and bathymetric variations generates a complex response, confirming that quantitative interpretation of ocean tidal EM fields will require a 3-D treatment. As an illustration of the nested 3-D scheme, seafloor data at five magnetic and seven electric stations in the northeastern Pacific (41○N, 165○W) are fit with trial-and-error forward modelling of a local domain. The simulation results indicate that the lithospheric resistance is roughly 7 × 108 Ωm2. The phase of the seafloor data in this region are inconsistent with a sharp transition between the resistive lithosphere and conductive asthenosphere.

Indentation of the Arabian platform into the eastern domain of the Anatolide-Tauride Block (ATB) led to sub-horizontal rupture and break-off of the Arabian segment of the subducting southern Neotethyan oceanic slab beneath ATB in the late Cenozoic. Although this rupture has been well-defined by seismic tomography and numerical modeling, its relationship with concomitant magmatism in central and eastern Anatolia has not been firmly demonstrated. To address this issue, we compiled, integrated and interpreted an extensive database of previously-published age and geochemical data. Our analysis shows that late Cenozoic magmatism in both eastern and central Anatolia was nearly continuous over a distance of ~1000 km, and is herein named the Eastern Anatolian Magmatic Belt. Magmatism initiated at 23 Ma in response to rupturing of the north-dipping subducting oceanic slab below eastern Anatolia and upwelling of hot asthenospheric mantle. The onset of initial rupture-related magmatism is ~12 Ma older than previously proposed. The magmatic belt migrated westward from eastern Anatolia (23-15 Ma) to central Anatolia (12 Ma-present) and is a proxy for the westward progression of the slab rupture and related asthenospheric infiltration. Magmatism generated from the shallow melting of the previously-metasomatized Anatolian subcontinental lithospheric mantle and asthenosphere by decompression due to the impingement of the Arabian sub-slab asthenospheric mantle. In eastern Anatolia, slab-rupture magmatism was followed by a second phase of magmatism that formed in response to destabilization and convective removal of the thick Anatolian lithospheric root (13 Ma-present). This new magmatic model emphasizes that the geochemical signature of post-subduction magmatism, as recorded by primitive basalts that constantly evolved through time and space. Magmatic sources recorded the shifting of mantle melt source domains that were controlled by slab segmentation and induced mantle flow.

Three parameters: temperature, hydrogen content, and the presence of partial melt, are the dominant controls on the rheology of the convecting upper mantle. As such, they determine the dynamics that control plate tectonics and continental evolution. Since hydrogen depresses the peridotite solidus temperature, these parameters are strongly linked petrologically. We have developed a genetic algorithm code to statistically assess the likelihood that a section of upper mantle contains partial melt. This code uses magnetotelluric observations and petrological constraints on composition and solidus temperatures and allows for uncertainties in the geotherm and the electrical conductivity structure. We have applied this code to the convecting upper mantle beneath (1) a stable continent (the Superior Craton); (2) a hot spot (Tristan da Cunha); (3) stable, old oceanic lithosphere (the northwest Pacific Ocean); and (4) young oceanic lithosphere (adjacent to the East Pacific Rise). Results show that the volume of melt in the convecting upper mantle is heterogeneous. The highest melt proportions are beneath the hot spot while little to no melt is required in the other regions. All regions show low water contents (generally

This study analyzes the microstructures and deformational characteristics of spinel peridotite xenoliths from the Nógrád-Gömör Volcanic Field (NGVF), located on the northern margin of a young extensional basin presently affected by compression. The xenoliths show a wide range of microstructures, bearing the imprints of heterogeneous deformation and variable degrees of subsequent annealing. Olivine crystal preferred orientations (CPOs) have dominantly [010]-fiber and orthorhombic patterns. Orthopyroxene CPOs indicate coeval deformation with olivine. Olivine J indices correlate positively with equilibration temperatures, suggesting that the CPO strength increases with depth. In contrast, the intensity of intragranular deformation in olivine varies as a function of the sampling locality. We interpret the microstructures and CPO patterns as recording deformation by dislocation creep in a transpressional regime, which is consistent with recent tectonic evolution in the Carpathian-Pannonian region due to the convergence between the Adria microplate and the European platform. Postkinematic annealing is probably linked to percolation of metasomatism by mafic melts through the upper mantle of the NGVF prior to the eruption of the host alkali basalt. Elevated equilibration temperatures in xenoliths from the central part of the volcanic field are interpreted to be associated with the last metasomatic event, which only shortly preceded the ascent of the host magma. Despite well-developed olivine CPOs in the xenoliths, which imply a strong seismic anisotropy, the lithospheric mantle alone cannot account for the shear wave splitting delay times measured in the NGVF, indicating that deformation in both the lithosphere and the asthenosphere contributes to the observed shear wave splitting., The authors would like to thank the people who contributed to this work. We owe thanks to Fabrice Barou and David Adams for their help with EBSD-SEM analyses at Geosciences Montpellier and at CCFS Macquarie University, respectively. L?szl? Aradi is acknowledged for his help with field work, petrography and MTEX application. We are grateful for the constructive criticism of Sandra Piazolo and Jacques Pr?cigout and two anonymous reviewers, as well as for the editorial handling of Stephen Parman. Our research received financial support from a Marie Curie International Reintegration Grant (grant NAMS-230937), a postdoctoral grant (grant PD101683) of the Hungarian Scientific Research Fund (OTKA), and a Bolyai J?nos Postdoctoral Research Fellowship of the Hungarian Academy of Sciences to I. J. K., as well as from the Lend?let Pannon LitH2Oscope Research Group (Research Centre for Astronomy and Earth Sciences, Hungarian Academy of Sciences). N. L. received support from Macquarie University international PhD scholarship and project and travel funding from ARC Centre of Excellence for Core to Crust Fluid Systems (CCFS). K. H. acknowledges funding from Ministry of Economy, Industry and Competitiveness (MINECO, Spain) and the State Research Agency (AEI, Spain; grants FPDI-2013-16253 and CGL2016-81085-R). Instruments used at Macquarie University are funded by DEST Systemic Infrastructure Grants, ARC LIEF, NCRIS/AuScope, industry partners, and Macquarie University. The data used in this paper are listed in the references, tables, and supporting information. The raw EBSD data are available from the corresponding author upon request. This is the 92nd publication of the Lithosphere Fluid Research Lab (LRG), contribution 1361 from the ARC Centre of Excellence for Core to Crust Fluid Systems (www.ccfs.mq.edu.au) and 1320 from the GEMOC Key Centre (www.gemoc.mq.edu.au).

Flattening of old seafloor as a result of the time evolution of oceanic lithosphere and asthenosphere has been studied for several decades and remains one of the unresolved fundamental problems of geodynamics. The phenomenon of seafloor flattening at ages greater than 70-80 Ma can be explained by a simple model of a cooling plate with a fixed thickness, but the physical cause of flattening is not fully understood. Here, a simple model of lithosphere rejuvenation is considered as an alternative mechanism and is also shown to represent similar features of seafloor flattening. We suggest that constraints on mantle structure from use of data such as variations of seafloor depth and heat flow are weak, but that first order differences in mantle thermal structure are introduced by these two models. This suggests that probing of the mantle by geophysical arrays on the seafloor, particularly those sensitive to mantle temperature such as magnetotelluric imaging, should be able to constrain the cause of seafloor flattening. In the present study, recent results of seafloor electromagnetic explorations are compiled and estimated depths to the high conductivity layer (HCL) are used as a proxy for 1,300 °C isotherm depths. For the data obtained at sites well away from plate boundaries in the Pacific Ocean, a model of lithosphere rejuvenation was found to better explain the correlation between HCL depths and seafloor depths than a standard plate cooling model.

Throughout the oceanic asthenosphere there exists a zone of seismic velocities that are lower than would be predicted. The cause of this low velocity zone (LVZ), whether partial melt or solid-state mechanisms, has been debated for decades. We investigate the LVZ by considering seismic and magnetotelluric data from tectonically stable, ∼70 Myr old lithosphere in the central Pacific Ocean. We utilise recent experimental advances on the influence of partial melt and seismic attenuation. Results show that the LVZ is characterised by a small volume of interconnected melt and by low hydrogen contents. Beneath the LVZ, the asthenosphere does not contain partial melt but has high hydrogen contents and low attenuation values, indicating large grain sizes and/or low oxygen fugacities. To explain these observations, we propose that a small amount of unextractable melt is trapped in the asthenosphere after melting at mid-ocean ridges.

A large number of Eocene–Oligocene alkaline/alkali-rich igneous rocks were developed in the Tuotuohe region of the Qinghai-Tibetan Plateau. In this study, we present zircon U–Pb ages, Hf isotope data, and major and trace element compositions of the Cenozoic alkaline rocks from the Tuotuohe region in order to constraint the petrogenesis and tectonic evolution history of Qiangtang Block. Zircon U–Pb ages were measured via LA–ICP–MS to be 39.6, 37.6 and 32.0 Ma. The 39.6 Ma trachyte was characterized by low SiO2 and high K2O and MgO contents. The 37.6 and 32.0 Ma orthophyres show enrichment in SiO2 and K2O, but deficient in MgO. All of the samples belong to the alkaline rocks. These rocks display enrichment in REE, LREE, and LILE, depletion in HFSE, and no obvious Eu anomalies. Geological and geochemical features suggest that there were two possible mechanisms for the origin of the alkaline rocks in the Tuotuohe region: (1) the removed mafic lower crust dropped into the asthenosphere, forming the mix magma (Nariniya trachyte); (2) the upwelling asthenosphere triggered the crustal melting (Nariniya and Zamaqu orthophyre). The Eocene–Oligocene alkaline rocks in the study and adjacent areas are likely to be the result of the same tectonic–magmatic event of deep lithospheric evolution that is the crustal material melting triggered by lithospheric delamination. This conclusion extends the influence scope of lithospheric delamination eastward to the Tuotuohe region (~ 92°E) from Banda Co (~ 82°E).

The paper presents newly acquired data on concentrations of major and trace elements and on Sr, Nd, and Pb isotope composition in Pliocene and Late Pleistocene–Holocene mafic volcanic rocks of the Uksichan volcanic center, one of the largest in the Sredinnyi Range of Kamchatka. Based on these data, the mafic Pliocene volcanics are thought to be produced by the melting of heterogenized mantle material, which had been hybridized by subduction and asthenospheric processes. The behavior of HFSE and Pb isotopic systematics provide evidence of the melting of subducted sediment and origin of pyroxenite segregations in the peridotite matrix. The low ∆8/4Pb values of the Pliocene lavas of Uksichan shield volcano and in modern large volcanic centers in the Central Kamchatka Depression are correlated with the magmatic productivity, which indicates, when considered together with HFSE and HREE behavior, that the Pacific asthenosphere was involved in the magma-generating processes. The Late Pleistocene–Holocene basalt volcanism, which was spatially constrained to the peripheries of the Pliocene shield edifice, developed in an extensional environment as a result of the melting of an enriched mantle source. The attenuation and then complete termination of volcanic activity in the Sredinnyi Range in the Late Pleistocene–Holocene was associated with an increase in the ∆8/4Pb of the mafic lavas, which indicates that the center of the activity related to the oceanic asthenosphere shifted eastward toward the Central Kamchatka Depression. The influence of the oceanic asthenosphere on subduction-related magmatism is not unique to convergence zones alone and should be taken into consideration when models are constructed for the origin of juvenile continental crust.

Shear-wave splitting provides insight into geodynamic processes such as lithospheric deformation and upper mantle flow. This study presents shear wave splitting parameters determined from XKS (SKS, SKKS and PKS) and Pms phases (from receiver functions) recorded by the 2D seismic array, SANDWICH, deployed in central Tibet from November 2013 to April 2016. The XKS splitting measurements show a generally strong anisotropy with an average of 1.3 s, that even includes 17 stations with delay times no less than 2.0 s. Interestingly, significantly weak splitting is also observed between Nam Tso and Siling Tso near 90°E. Spatial coherence analysis of the splitting parameters indicates an upper asthenospheric source as the primary cause of seismic anisotropy in the study region. The regionally dominant ENE-WSW oriented upper-mantle anisotropy can emerge from the corner flow in the overlying mantle wedge induced by the subduction of the Indian lithospheric slab. The crustal anisotropy obtained through Pms splitting analysis might reflect lattice preferred orientation of anisotropic minerals formed by the ENE oriented middle-lower crustal flow, with a minor contribution to the strong anisotropy. The weak splitting observations in the east are likely caused by the nearly vertical α-axis of olivine induced by upwelling asthenosphere at a slab tear in the Indian lithosphere.

High Mg diorites provide important insights into crust-mantle interaction during tectonic evolution of orogenic belt. An integrated study including zircon U-Pb dating, whole-rock geochemistry and isotopic compositions of Sr and Nd was carried out on high Mg diorites of the Wulong pluton located in the narrowest part of the South Qinling Belt. Crystallized ages of 218-210 Ma are obtained in combination with the published data. These rocks are characterized with relatively high MgO (2.76–7.17%), Cr (68.7-310 ppm) and Ni (21.3–76.3 ppm) contents. Most diorites exhibit high Sr/Y and La/Yb ratios with negligible Eu anomaly. Isotopic compositions of whole-rock show enriched attributes (87Sr/86Sr(i) = 0.7050–0.7071, eNd(t) = −7.56 to −3.45). Geochemical models indicate that these rocks did not result from melting of subducted slab, magma mixing, fractional crystallization or melts derived from delaminated lower crust reacted with mantle. Instead, high Mg diorites were generated through basaltic melts derived from asthenosphere mantle assimilating crustal components with crystallized phase of garnet-bearing amphibolite (AFC). Together with regional geological setting of the South Qinling Belt, we propose that the parental magma of these high Mg diorites was probably originated from upwelled asthenosphere mantle at the base of thinned crust after delamination of thickened crust during Late Triassic. The continental crust of the South Qinling Belt was thickened as a result of collision with the Yangtze Block during Middle Triassic. Then eclogitized lower crust collapsed and foundered into asthenosphere mantle due to partial melting of thickened crust during Late Triassic. In response, the upwelling and underplating of asthenosphere mantle took place at the base of continental crust. Moreover, upwelling of hot mantle may provide the heat source for migmatites and initiate the doming of Foping area.

The East Sikhote Alin volcanic belt (~1500 km long) is commonly regarded as a tectonomagmatic structure formed in the Late Cretaceous in a subduction environment, which was followed by the destruction of the oceanic slab and active asthenospheric diapirism in the Cenozoic. However, the nature of the lateral zonality of the Late Cretaceous volcanic complexes, which is distinctly expressed in a number of geological and metallogenic parameters, remains poorly studied. In this paper, this issue is considered on the example of the poorly studied Late Cretaceous Bolbinsk Formation of northern Sikhote Alin. Analysis of the published geological information in combination with geochronological, trace-element, and isotopic data indicates that the structure was developed in a non-subduction geodynamic setting. The predominance of magnesian (Mg# = 26–40) adakites (La/Yb = 19–34) and high-niobium basalts and low lead (Δ8/4Pb = 30–46) and high neodymium (143Nd/144Nd 0.51279–0.51281) isotopic ratios suggest an elevated permeability of the subducted slab and injection of a “hot” oceanic asthenosphere into the mantle wedge. Consequently, the lateral geological, geochemical, and metallogenic zonality of the East Sikhote Alin volcanogenic belt was formed at the early stages of its evolution as a result of the specific configuration of the Late Cretaceous continental margin.

Spatial isotope variations in oceanic island basalts hold clues to the geochemical structure of mantle plumes. This signal is however susceptible to contributions from non-plume sources and modifications by physical processing in the upper mantle, i.e. source hybridization and segregation through gradients in melting conditions. A comprehensive survey of lava isotope systematics is therefore necessary to reveal spatiotemporal patterns at the scale of a single hotspot and disentangle plume sources from upper mantle physical and chemical contributions. We present Hf and Pb isotope data on 72 previously characterized onland and submarine lavas from the islands of Ni‘ihau and Kaua‘i, and from the North Arch Volcanic Field. These lavas cover the entire ∼ 5 Ma eruptive history of Ni‘ihau and Kaua‘i, allowing for a detailed assessment of the temporal geochemical evolution of Hawaiian volcanism. Furthermore, the broad lateral coverage of volcanic activity offered by these lavas constrains the spatial variability of sources across the volcanic chain, allowing investigation of the role of an upper asthenosphere contribution to plume volcanism. Early, shield-building (tholeiitic) lavas from Kaua‘i and Ni‘ihau overlap in Nd–Hf–Sr–Pb isotopes, contrasting with the across-plume asymmetry seen in younger shield lavas (i.e. the LOA-KEA double chain), and their isotope systematics suggest sampling of both LOA and KEA components. In contrast, late Kaua‘i and Ni‘ihau rejuvenated lavas do not overlap in isotope composition. They share common isotope characteristics with rejuvenated lavas from North Arch, Ka‘ula and O‘ahu, and together form two isotopically distinct, southwestern and northeastern groups. The two groups have steep slopes (>1.7) in a eNd − eHf plot and require distinct depleted components. The high eHf for a given eNd, high 87Sr/86Sr, and trace element systematics of the southwestern group (Ni‘ihau, Ka‘ula and O‘ahu) indicate a depleted component intrinsic to the plume. In contrast, lavas from the northeastern group (Kaua‘i and North Arch) indicate a source component isotopically identical to Pacific MORB. The shift from a spatially broad and homogeneous plume source in the shield stage of Kaua‘i and Ni‘ihau (6–3.5 Ma) to a bilaterally heterogeneous source during the rejuvenated stage (

There is debate to what degree mid-ocean ridge basalts (MORB) can be used to infer the composition of the sub-ridge mantle and whether the upper mantle contains components that are difficult to recognize by observations on MORB alone. Here we report trace element and isotope data on a fossil mantle section exposed in the ophiolitic Lanzo South massif, Italy. Our findings show the existence of clinopyroxenes in geochemically depleted melt-infiltrated harzburgites with an extreme radiogenic Hf-isotopic composition but MORB-like Nd isotopic compositions. The simplest way of explaining the depleted trace elements in combination with the decoupled isotopic compositions is that the harzburgites were infiltrated by a melt with isotopic compositions inherited from an anciently (>1 Ga) depleted source. The existence of melts from ultra-depleted sectors of the asthenosphere allows constraining of the configuration of the heterogeneities in the sub-ridge mantle. The parallel arrays in Hf–Nd isotope composition space of different Mid-Ocean ridge segments imply the ubiquitous presence of anciently depleted residual mantle as the matrix in which more enriched components occur in pockets.

The Samoan islands are an archipelago hosting a quarter million people mostly residing inthree major islands, Savai'i and Upolu (Samoa), and Tutuila (American Samoa). The islands haveexperienced sea level rise by 2-3 mm/year during the last half century. The rate, however, has dramaticallyincreased following the Mw 8.1 SamoaTonga earthquake doublet (megathrust + normal faulting) inSeptember 2009. Since the earthquake, we found largescale gravity increase (0.5 Gal/year) around theislands and ongoing subsidence (8-16 mm/year) of the islands from our analysis of Gravity Recovery AndClimate Experiment gravity and GPS displacement data. The postseismic horizontal displacement is faster inSamoa, while the postseismic subsidence rate is considerably larger in American Samoa. The analysis oflocal tide gauge records and satellite altimeter data also identified that the relative sea level rise becomesfaster by 7-9 mm/year in American Samoa than Samoa. A simple viscoelastic model with a Maxwellviscosity of 2–3×10(exp 18) Pa s for the asthenosphere explained postseismic deformation at nearby GPS sites aswell as Gravity Recovery And Climate Experiment gravity change. It is found that the constructiveinterference of viscoelastic relaxation from both megathrust and normal faulting has intensified thepostseismic subsidence at American Samoa, causing ~5 times faster sea level rise than the global average.Our model indicates that this trend is likely to continue for decades and result in sea level rise of 30-40 cm,which is independent of and in addition to anticipated climaterelated sea level rise. It will worsen coastalflooding on the islands leading to regular nuisance flooding.

The Earth’s mantle has a complex history of partial melting and melt-peridotite reaction that have redistributed Ca and other elements between residues and melting products. Given the considerable Ca isotopic variation reported in mantle rocks, evaluation of the fractionation of stable Ca isotopes in magmatic processes in the mantle is critical to decode mantle evolution and the effect of recycled materials. We have performed precise and accurate Ca isotopic analyses on a series of well-characterized spinel-facies mantle peridotites (lherzolite, harzburgite and dunite, n = 29), pyroxenites (websterite, clinopyroxenite and orthopyroxenite, n = 15) and their mineral constituents (n = 8) from the Balmuccia and Baldissero peridotite massifs of the Ivrea Zone in the Italian Alps. These peridotites underwent variable degrees of melting and melt-peridotite reaction, whereas the pyroxenites are mainly the products of melt-peridotite reaction and crystallization of migrating basic melts from the asthenosphere. The lherzolites from Balmuccia and Baldissero show δ44/40Ca values of 0.94 ± 0.11‰ (2sd, n = 22), which are uniform within long-term external reproducibility (±0.14‰, 2sd). The δ44/40Ca values of the harzburgites (0.83‰ to 0.92‰) do not differ from those of lherzolites, including those with a history of intensive melt-peridotite reaction to form replacive dunites. The websterites and spinel clinopyroxenites display δ44/40Ca of 0.86 ± 0.10‰ (n = 14), within the range of the lherzolites and harzburgites. The indistinguishable δ44/40Ca among these very diverse mantle rocks is the consequence of the overwhelming control of stable Ca isotopes by clinopyroxene in the magmatic processes involved, because clinopyroxene dominates the budget of Ca (>90% for harzburgites; 93% to 99% for lherzolites, websterites and clinopyroxenites). Only the clinopyroxene-poor ( 25% melting and replacive dunites tend to display slightly heavier Ca isotopic compositions. Consequently, irrespective of their magmatic history, most fertile mantle rocks from different geological settings display a homogenous Ca isotope composition, summarized as, δ44/40Ca of 0.94 ± 0.10‰ (2sd, n = 47) for the Earth’s mantle. The deviations in Ca isotopic variations observed in other mantle rocks may be attributed to kinetic isotope fractionation and metasomatism by melts with isotopic compositions influenced by recycled crustal materials.