Deep Mantle Material Research Articles

Mechanisms of Crustal-Mantle Material and Energy Exchanges: Impacts on Hydrocarbon and Geothermal Resources.

The exchange of matter and energy between crust and mantle significantly influences the formation and development of oil, gas, and geothermal resources. Understanding how these exchanges impact these resources is crucial in geological science. In many oil-rich basins in China, significant accumulations of H2, CO2, geothermal energy, and other associated resources linked to deep mantle materials or geological processes have been discovered. Therefore, investigating the effects of crust-mantle material and energy exchanges on these resources is of utmost importance. This paper aims to systematically analyze the effects of mantle materials (e.g., H2, CO2, catalytic elements) and energy upwelling into basins. It synthesizes the impacts of various organic-inorganic interactions on hydrocarbon generation, evolution of organic source rocks, reservoir development, and the formation and accumulation of oil and gas. Finally, it proposes a mechanism detailing how interactions between mantle matter and energy influence specific resources. Simultaneously, this study employs numerical simulations to uncover the specific impact of magma intrusions on the geothermal field of surrounding rock formations. It demonstrates that magma chambers, continuously supplied with mantle energy, create regional thermal anomalies and facilitate the development of high-temperature geothermal resources. This underscores that mantle materials and energy not only significantly influence the generation of oil and gas but also govern the formation of geothermal resources and the evolution of thermal reservoirs. This research provides a theoretical foundation for understanding the subsequent formation of oil and gas resources under the influence of deep geological processes. Moreover, it furnishes a scientific basis for evaluating and exploring the symbiotic relationship between oil and gas resources and geothermal reservoirs.

Melting at the base of a terrestrial magma ocean controlled by oxygen fugacity

Heat delivered from accretionary impacts is thought to have led to extensive melting of early Earth’s silicate mantle, resulting in a deep magma ocean covering the surface. The mantle’s oxygen fugacity is thought to have increased over accretion and core formation due to increasingly oxidated impactors and lower mantle self-oxidation, but the influence of this on the solidus of deep primitive mantle materials has not been well constrained. Here we assess the effect of oxygen fugacity on conditions at the bottom of a magma ocean by experimentally determining the solidus of mantle pyrolite at pressures of 16–26 GPa at high oxygen fugacities. We find that over this pressure range, the solidus in experiments conducted under oxidizing conditions is at least 230–450 °C lower than in experiments conducted under more reducing conditions. Assuming constant magma ocean temperature, this would imply a magma ocean floor that deepens by about 60 km for each log unit increase in mantle oxygen fugacity. The strong influence of oxygen fugacity on mantle melting suggests that models of early Earth thermal evolution and geochemical models of core formation should be reassessed.

Origin of the Site U1504 alkaline basalts in the South China Sea continental margin: Insights on deep mantle diversity and subduction dynamics under continental arcs

Alkaline basalts produced in continental arcs should contain information different from the arc tholeiite-calc-alkaline-series magmas, and their origin could provide unique constraints on deep mantle composition and material cycling. However, due to their sparse occurrence, alkaline basalts in continental arcs have not been studied thoroughly, which hinders our understanding of the mantle diversity and subduction dynamics under continental arcs. In this study, we present new 40Ar/39Ar ages, major and trace elements, and Sr-Nd-Hf isotopic data for the International Ocean Discovery Program Site U1504 alkaline basalts in the continental arc developed on the South China Block (SCB). These alkaline basalts were generated at ca. 121 Ma and display typical oceanic-island basalt geochemical characteristics. Their relatively high εNd(t) (3.5−3.7) and low (87Sr/86Sr)i (0.7034−0.7040) and La/Nb (0.5−1.0) values suggest that they were mainly derived from an asthenosphere mantle source. Compared to alkaline basalts in the SCB inland, U1504 alkaline basalts exhibit lower K2O/Na2O, Zr/Sm, Zr/Y, εNd(t), and εHf(t) values, indicating the addition of minor sub-continental lithospheric mantle. The enrichment of Nb, Ta, light rare earth elements, and slight depletion of Zr, Hf, and Ti, as well as elevated Fe/Mn and Sm/Yb and low CaO, indicate that their mantle lithology was mainly garnet pyroxenite. Based upon these findings and previous studies, the garnet pyroxenite was probably formed by the interaction of upwelling asthenosphere with slab edges in the scenario of break-off of the Paleo-Pacific Plate, and such interaction diversified the mantle chemistry beneath continental arcs. In conjunction with other reported alkaline basalt data, it is proposed that the enriched asthenosphere mantle beneath the SCB had formed sequentially from inland to coastal since the late Mesozoic, and this may be related to lateral and vertical flow in the deep asthenosphere controlled by the break-off of subducted plates.

Stable Zirconium Isotopic Compositions of the Metasomatized Mantle

AbstractMantle metasomatism, which affects the geochemical and lithological properties of the Earth, is crucial to the evolution and modification of the Earth's interior. Variations in chemical proxy of metasomatism, including high‐field strength elements (HFSE, e.g., Zr‐Ti isotopes and isovalent Zr‐Hf), have been commonly utilized as a tool to unravel the processes associated with the evolution of the Earth. However, the impact of mantle metasomatism by diverse agents on the Zr isotopic compositions of metasomatized mantle remains poorly understood. Here we carried out high‐precision double‐spike δ94/90Zr measurements for a suite of basalts, which were derived from the mantle sources metasomatized by typical metasomatic agents of fluid, silicate melt and carbonate melt originating from the deep mantle or recycled crustal materials. Mantle metasomatism results in lithological diversities within the mantle, ranging from hybridized peridotites to pyroxenite/eclogite and carbonated eclogite, which can significantly fractionate the isovalent HFSEs. However, our data show that these basalts have primitive mantle‐like δ94ZrNIST values (−0.033–0.043 ‰), regardless of their superchondritic Zr/Hf. The primitive mantle‐like δ94ZrNIST, irrelevant to any proxy of metasomatism (e.g., Sr‐Nd isotopes, Zr/Hf and La/Sm), indicate that mantle metasomatism and subsequence melting produce no measurable Zr isotopic variations in metasomatized mantle‐derived basalts. The δ94ZrNIST of basalts thus can be representative of the mean metasomatized mantle compositions. Combined with previously reported komatiites and oceanic basalts, we show that the Earth's mantle displays consistent δ94ZrNIST (−0.013 ± 0.053 ‰), even if it has a secular dynamic evolution with the interaction between Earth's exterior and interior.

Dynamics of medium-deep seismicity parameters before large earthquakes in South Asian seismic focal zones

The study was continued to reveal the spatio-temporal relationship between strong crustal shocks and the moderate, medium-deep seismicity that precedes them. In the area of preparation of these shocks, the following regularities were revealed: in space – the presence of a high creepex gradient (in particular, the boundary transition between its positive and negative anomalies), in time – a direct correlation of the graphs MS(t) and Cr(t), indicating the creation in the period of preparation of the source under the conditions of a strictly determined influence of the source-size on creepex. Since the proportional dependence of the creepex on the geometric size of the source is usually associated with an increased heterogeneity of the medium in the source, it can be assumed that the processes of brittle fracture of blocks of the undercutting crust coexist here with the processes of inflow of deep mantle material, which ensures the heterogeneity of the properties of the medium in the area of preparation of large earthquakes in the South Asian subduction zones. This conclusion is confirmed on the average creepex anomaly maps by the presence of their increased gradient in the deep region of the preparation of these events.

Geothermal gradient and heat flow of the Erlian Basin and adjacent areas, Northern China: Geodynamic implication

The Erlian Basin, located in the eastern Central Asian Orogenic Belt (CAOB) between North China and the Siberian cratons, is a Mesozoic basin developed on a fold basement. A good understanding of the thermal regime in this area allows for greater insight into the geodynamic processes of the CAOB. In this study, geothermal gradient and heat flow of the Erlian Basin were assessed using temperature logs from 14 boreholes and rock thermal conductivity data. The results show that the geothermal gradients of the Erlian Basin range from 29.6 ℃/km to 51.4 ℃/km with an average of 39.1 ± 5.4 ℃/km. The heat flow varies between 63 mW/m2 and 109.5 mW/m2 with a mean of 81.4 ± 12.0 mW/m2. High anomalous zones occur in the Manite, Uliastai and Chuanjing depressions, with geothermal gradient values of approximately 40–60 °C/km and heat flow of approximately 80–110 mW/m2. The heat flow anomalies are consistent with the distribution of Cenozoic volcanoes, which may provide hot material from the deep mantle. The contributions of the excessive heat from the intruded bodies to the present-day surface heat flow anomaly are estimated to be about 7–9 mW/m2 in the Uliastai Depression and 17–20 mW/m2 in the Manite Depression, respectively. The characteristics of the geothermal regime of the Erlian Basin is a consequence that the subduction and rollback of the Pacific Plate had triggered regional lithospheric extension and upwelling of deep mantle material, basaltic melts entered the lithosphere along the fragile zone resulted from the closure of the Paleo-Asian Ocean.

Slab weakening during the olivine to ringwoodite transition in the mantle

The strength of subducted slabs in the mantle transition zone influences the style of mantle convection. Intense deformation is observed particularly in relatively old subducted slabs in the deep mantle transition zone. Understanding the cause of this regional and depth variation in slab deformation requires constraint of the rheological properties of deep mantle materials. Here, we report results of in situ deformation experiments during the olivine to ringwoodite phase transformation, from which we infer the deformation process under the conditions of cold slabs deep in the mantle transition zone. We find that newly transformed fine-grained ringwoodite deforms by diffusion creep and that its strength is substantially smaller than that of coarser-grained minerals but increases with time. Scaling analysis, based on a model of transformation kinetics and grain-size evolution during a phase transformation, suggests that a cold slab will be made of a mixture of weak, fine-grained and strong, coarse-grained materials in the deep transition zone, whereas a warm slab remains strong because of its large grain size. We propose that this temperature dependence of grain size may explain extensive deformation of cold slabs in the deep transition zone but limited deformation of relatively warm slabs. In relatively cold subducted slabs, olivine may transform to finer-grained, weaker ringwoodite than in warm slabs, according to deformation experiments under conditions analogous to those in the mantle transition zone and scaling analysis.

Tracing deep fluids in the Bozhong Sag: Evidence from integrated petrography and geochemistry data

Abstract The Bozhong Sag is a mature exploration area for petroleum in the Bohai Bay basin, China, which is a typical oil-bearing basin and hosts the Penglai (PL) 19-3, and Qinhuangdao (QHD) 32-6 giant oil fields (Xue, 2018). Despite 60 years of hydrocarbon exploration in the basin, only few large gas fields have been discovered. The discovery of the Bozhong (BZ) 19-6 gas field not only indicated that the Bozhong Sag has great potential for natural gas exploration, but also raised questions regarding the formation of large-scale natural gas fields in typical oil-prone basins and in sapropelic source rocks. The present study therefore performs a detailed investigation of the sources and migration pathways of the deep fluids in the Bozhong Sag area as a means of shedding light into this scientific challenge. The Bozhong Sag is located in the thinnest crust and tectonic transition zone of the Bohai Bay basin. Intense extension and thinning of lithosphere led to the upward ejection of deep crust and mantle materials under diapirism. Improvements in this tectonic setting and the fluid migration path occurred as a result of early faults that were formed by compressive thrust in the Indosinian-Yanshan orogenies in addition to a process of extensional development and reactivation of Himalayan faults. We obtained cores from 13 wells in the Bozhong Sag, and subsequently observed their petrography and analyzed samples for fluid inclusions. Also, volcanic rock samples were analyzed for major and trace elements, and carbonate rock samples were analyzed for δ13C, δ18O, and 87Sr/86Sr. Previously reported δ13CCO2 and He isotopic data were are also discussed. Our results showed that petrography was characterized by thermal fading and hydrothermal breccia around fractures filled with quartz and calcite. Deep fluid activity was mainly reflected by petrographic assemblages of quartz, ankerite, pyrite, plus calcite and volcanic eruptive rocks that were i) enriched with large ion-lithophile elements (LILE), ii) enriched with light rare-earth elements (LREE), iii) relatively deficient in high-field strength elements (HFSE), and iv) relatively deficient heavy rare-earth elements (HREE). Thermal anomaly events were reflected by the homogenization temperature of fluid inclusions, and characteristics of magmatic hydrothermal fluids were exhibited by δ13C, δ18O, and 87Sr/86Sr. Our findings indicated that deep fluids were mainly derived from the deep parts of the upper mantle, and that they were slightly contaminated by crustal materials. Combining our results with those of previous studies on deep xenoliths and xenolithic fluid compositions in eruptive rocks of eastern China (Zhang et al., 1999; Han et al., 2008), we interpreted that deep fluid properties in the study region were characterized by high H2 and low total volatile gases. We conclude that the deep fluids in the Bozhong Sag was mainly derived from deep material in the upper mantle, and that the fluid activity pattern was aided by the deep fracture-fault systems in addition to central and fissure-type wide-area eruption activities.

Helium Isotopic Composition of the Songduo Eclogites in the Lhasa Terrane, Tibet: Information from the Deep Mantle

Helium isotopic compositions are considered to be ideal tracers to identify whether mantle materials have been added to crustal rocks or fluids. In this paper, we present the helium isotopic compositions of the Songduo eclogites in the Lhasa terrane, Tibet. We found that garnet and omphacite in the eclogites have different helium retention characteristics. The 4He content of most omphacite grains are about 10-20 times of that of garnet, suggesting that omphacite has a higher ability to capture 4He than garnet. Similarly, there is about 10-20 times difference in 3He content between omphacite and garnet in the same eclogite samples. The 3He/4He ratios of garnet and omphacite in these rocks range from 0.27 to 0.60 Ra (relative to the modern air 3He/4He ratio, 1.4×10-6). These ratios are within the range of both mantle-and crust-derived helium, suggesting mixed sources. The Songduo eclogites have much higher 3 He/4He ratios than those observed in the Dabie eclogites of eastern China. Such high ratios are typically thought to be associated with deep mantle sources. We cautiously conclude that deep mantle materials might have been involved during the formation of the Songduo eclogites.

New Constraints on the Origin of the EM‐1 Component Revealed by the Measurement of the La‐Ce Isotope Systematics in Gough Island Lavas

AbstractHot spot lavas show a large diversity of isotope compositions resulting from the recycling of surface material into the convective mantle. Amongst the mantle end‐members, EM‐1 (enriched mantle) is widely debated and scenarios involving either old pelagic sediments subducted into the deep mantle or subcontinental lithospheric material incorporated at shallow depths are commonly evoked. We selected 12 lavas from Gough Island (south Atlantic Ocean) for the measurement of 138La‐138Ce, 147Sm‐143Nd, and 176Lu‐177Hf isotope systems. Results show limited ranges for ε143Nd, ε138Ce, and ε176Hf values, and Ce/Ce* do not correlate with measured isotope ratios. Cerium isotope compositions allow us to exclude the contribution of old sedimentary material carrying a negative, elemental cerium anomaly in the mantle source. Pelagic sediments are indeed characterized by strongly negative, elemental cerium anomalies and also high La/Ce ratios. Modeling a primitive mantle source contaminated by 0.4% to 2.2% of different 2.5 Ga‐old pelagic components is able to reproduce the lowest Ce/Ce* values. However, the cerium isotope measurements show ε138Ce values between −0.39 and 0.15, too low to give support to the incorporation of recycled pelagic sediments in the mantle source of the lavas. Our results suggest that the incorporation of subcontinental lithospheric material at shallow depths during the plume ascent is a more suitable model to explain the formation of the EM‐1 component. Hafnium and Nd isotopes also support this scenario. Subcontinental lithosphere sampled via kimberlites and lamproites has isotopic compositions that plot generally below the mantle array, a signature that is also seen in Gough lavas.

Mantle Transition Zone Structure Beneath Northeast Asia From 2‐D Triplicated Waveform Modeling: Implication for a Segmented Stagnant Slab

AbstractThe structure of the mantle transition zone (MTZ) in subduction zones is essential for understanding subduction dynamics in the deep mantle and its surface responses. We constructed the P (Vp) and SH velocity (Vs) structure images of the MTZ beneath Northeast Asia based on two‐dimensional (2‐D) triplicated waveform modeling. In the upper MTZ, a normal Vp but 2.5% low Vs layer compared with IASP91 are required by the triplication data. In the lower MTZ, our results show a relatively higher‐velocity layer (+2% Vp and −0.5% Vs compared to IASP91) with a thickness of ~140 km and length of ~1,200 km atop the 660‐km discontinuity. Taking this anomaly as the stagnant slab and considering the plate convergence rate of 7–10 cm/year in the western Pacific region during the late Cenozoic, we deduced that the stagnant slab has a subduction age of less than 30 Ma. This suggests that the observed stagnancy of the slab in the MTZ beneath Northeast Asia may have occurred no earlier than the Early Oligocene. From the constraints derived individually on Vp and Vs structures, high Vp/Vs ratios are obtained for the entire MTZ beneath Northeast Asia, which may imply a water‐rich and/or carbonated environment. Within the overall higher‐velocity stagnant slab, a low‐velocity anomaly was further detected, with a width of ~150 km, Vp and Vs reductions of 1% and 3% relative to IASP91. Such a gap may have provided a passage for hot deep mantle materials to penetrate through the thick slab and feed the Changbaishan volcano.

Correlation of lithospheric “de‐rooting” of the Sulu‐Dabie Orogen to tectonic‐sedimentary process of the Hefei Basin: Constraints from Mesozoic coupling of basin and orogen

The lithospheric “de‐rooting” of the Sulu‐Dabie Orogen (SDO) is an important tectonic mechanism for post‐orogenic tectono‐magmatism, which influenced the sedimentation, evolution, and development of oil–gas accumulation in the Hefei Basin (HB). Combined with seismic, geological, and sedimentary data of the HB and the SDO, this paper studies the coupling of the HB and the SDO since the Late Mesozoic. While the North China Block (NCB) indented south‐eastwards under the Greater South China Block (GSCB) since the Mesozoic, the SDO underwent three intense tectonic stages, i.e., subduction, exhumation, and uplift. The upwelling of deep mantle material causes the exhumation of the ultrahigh‐pressure metamorphic (UHPM) rocks to the upper crust or surface, forming the UHPM. Influenced by the indentation of the NCB, the “de‐rooting” of the SDO lithosphere, caused the delamination and thinning of the lithosphere, accompanied by the continuous flexural subsidence in the HB. The basin underwent at least four stages of tectonic evolution, involving sequentially: the flexural basin, the foreland basin, the strike‐slip‐related basin, and the faulted graben development periods. The tectonic evolution of the HB was closely coupled with the SDO orogeny. The “de‐rooting” of the SDO lithosphere resulted in a strong uplift and erosion of the orogen, providing abundant source of material for the HB and causing the successively northwards migration of basin depocenters at its early stage. Finally, influenced by the subduction retreat of the (Palaeo‐) Pacific Plate and the strike‐slip movement of the Tanlu Fault, the depocenters migrated to the east.

Origin of Earth's Water: Chondritic Inheritance Plus Nebular Ingassing and Storage of Hydrogen in the Core

AbstractRecent developments in planet formation theory and measurements of low D/H in deep mantle material support a solar nebula source for some of Earth's hydrogen. Here we present a new model for the origin of Earth's water that considers both chondritic water and nebular ingassing of hydrogen. The largest embryo that formed Earth likely had a magma ocean while the solar nebula persisted and could have ingassed nebular gases. The model considers iron hydrogenation reactions during Earth's core formation as a mechanism for both sequestering hydrogen in the core and simultaneously fractionating hydrogen isotopes. By parameterizing the isotopic fractionation factor and initial bulk D/H ratio of Earth's chondritic material, we explore the combined effects of elemental dissolution and isotopic fractionation of hydrogen in iron. By fitting to the two key constraints (three oceans' worth of water in Earth's mantle and on its surface; and D/H in the bulk silicate Earth close to 150 × 10−6), the model searches for best solutions among ~10,000 different combinations of chondritic and nebular contributions. We find that ingassing of a small amount, typically >0–0.5 oceans of nebular hydrogen, is generally demanded, supplementing seven to eight oceans from chondritic contributions. About 60% of the total hydrogen enters the core, and attendant isotopic fractionation plausibly lowers the core's D/H to ~130 × 10−6. Crystallized magma ocean material may have D/H ≈ 110 × 10−6. These modeling results readily explain the low D/H in core‐mantle boundary material and account for Earth's inventory of solar neon and helium.

Geochronology and geochemistry of the Carboniferous felsic rocks in the central Great Xing'an Range, NE China: Implications for the amalgamation history of Xing'an and Songliao–Xilinhot blocks

The tectonic evolutionary history between the Xing'an Block (XB) and Songliao–Xilinhot Block (SXB) has been hotly debated. Here, we present precise geochronological ages and whole‐rock geochemical data in the central Great Xing'an Range for the Carboniferous felsic rocks to reveal this tectonic history, as the petrogenesis of these rocks were closely related to tectonic activity between the XB and SXB. The age data with detailed geochemical analyses indicate three important stages of magmatism: early Carboniferous, early Late Carboniferous, and middle Late Carboniferous. The early Carboniferous granodiorites formed from the partial melting of metavolcanic rocks at mantle depths and show I‐type features. The early Late Carboniferous monzogranites were generated by the partial melting of metagreywackes at middle to lower crustal depths, and they are S‐type granites. The middle Late Carboniferous rhyolites formed from the partial melting of a thinned lower crust, and they display A‐type characteristics. The data reveal three tectonic evolutionary stages between the XB and SXB during the Carboniferous era: from early Carboniferous subduction to early Late Carboniferous syn‐collision, and to middle Late Carboniferous post‐collisional extension. The accretionary mechanism of each stage featured horizontal accretion above subduction zones, amalgamation of different blocks, and vertical accretion fed by deep mantle materials, respectively.

Slab narrowing in the Central Mediterranean: the Calabro-Ionian subduction zone as imaged by high resolution seismic tomography

A detailed 3D image of the Calabro-Ionian subduction system in the central Mediterranean was obtained by means of a seismic tomography, exploiting a large dataset of local earthquakes and computing algorithms able to build a dense grid of measure nodes. Results show that the slab is continuous below the southern sector of the Calabro-Peloritan Arc, but the deformation processes developing at its edges are leading to its progressive narrowing, influencing tectonics and magmatism at the surface, and with possible stress concentration in the tip zones. In the southwest, the deformation occurring at a free slab edge lead to propagation of a vertical lithospheric tear in the overriding plate, which extends along a NW-SE fault system (Aeolian-Tindari-Letojanni) up to about 30 km into the Ionian Sea; further southeast, the lithosphere appears only flexed and not broken yet. In the northeast, the slab seems to break progressively, parallel to the trench. Finally, northwest of Mt. Etna, the tomography highlights low VP that can be related to an upwelling of deep mantle material likely flowing laterally through a window opened by the complete slab detachment.

A Bayesian method to quantify azimuthal anisotropy model uncertainties: application to global azimuthal anisotropy in the upper mantle and transition zone

Seismic anisotropy is a powerful tool to constrain mantle deformation, but its existence in the deep upper mantle and topmost lower mantle is still uncertain. Recent results from higher mode Rayleigh waves have, however, revealed the presence of 1% azimuthal anisotropy between 300 km and 800 km depth, and changes in azimuthal anisotropy across the mantle transition zone boundaries. This has important consequences for our understanding of mantle convection patterns and deformation of deep mantle material. Here, we propose a Bayesian method to model depth variations in azimuthal anisotropy and to obtain quantitative uncertainties on the fast seismic direction and anisotropy amplitude from phase velocity dispersion maps. We applied this new method to existing global fundamental and higher mode Rayleigh wave phase velocity maps to assess the likelihood of azimuthal anisotropy in the deep upper mantle and to determine whether previously detected changes in anisotropy at the transition zone boundaries are robustly constrained by those data. Our results confirm that deep upper mantle azimuthal anisotropy is favored and well-constrained by the higher mode data employed. The fast seismic directions are in agreement with our previously published model. The data favor a model characterized, on average, by changes in azimuthal anisotropy at the top and bottom of the transition zone. However, this change in fast axes is not a global feature as there are regions of the model where the azimuthal anisotropy direction is unlikely to change across depths in the deep upper mantle. We were, however, unable to detect any clear pattern or connection with surface tectonics. Future studies will be needed to further improve the lateral resolution of this type of model at transition zone depths

Deep mantle origin and ultra-reducing conditions in podiform chromitite: Diamond, moissanite, and other unusual minerals in podiform chromitites from the Pozanti-Karsanti ophiolite, southern Turkey

The Pozanti-Karsanti ophiolite situated in the eastern Tauride belt, southern Turkey, is a well-preserved oceanic lithosphere remnant comprising, in ascending order, mantle peridotite, ultramafic and mafic cumulates, isotropic gabbros, sheeted dikes, and basaltic pillow lavas. Two types of chromitites are observed in the Pozanti-Karsanti ophiolite. One type of chromitites occurs in the cumulate dunites around the Moho, and the other type of chromitites is hosted by the mantle harzburgites below the Moho. The second type of chromitites has massive, nodular, and disseminated textures. We have conducted the mineral separation work on the podiform chromitites hosted by harzburgites. So far, more than 100 grains of microdiamond and moissanite (SiC) have been recovered from the podiform chromitite. The diamonds and moissanite are accompanied by large amounts of rutile. Besides zircon, monazite and sulfide are also very common phases within the separated minerals. The discovery of diamond, moissanite, and the other unusual minerals from podiform chromitite of the Pozanti-Karsanti ophiolite provides new evidences for the common occurrences of these unusual minerals in ophiolitic peridotites and chromitites. This discovery also suggests that deep mantle processes and materials have been involved in the formation of podiform chromitite.

Fault distribution and formation mechanism of a magnetic quiet zone in the northern South China Sea

A magnetic quiet zone (MQZ) is located in the ocean–continent transition zone in the northern South China Sea (SCS), according to ΔT magnetic anomaly data. Features of the gravity and magnetic anomalies in the MQZ are discussed by the multi‐scale wavelet decomposition method. Comprehensive boundary identification and fault analysis by analytic signal and normalized total horizontal derivative methods achieve some good effects in the northern SCS. The inversion results suggest a deep magnetic basement, a shallow Curie isothermal interface and thin magnetic layers. The average depth of the Curie isothermal interface is about 14–28 km in the northern SCS, and the thickness of the magnetic layer is about 6–10 km in the MQZ. The formation mechanism of the MQZ is discussed, and the thinning of the magnetic layer is considered as the origin of the MQZ in the northern SCS. The deep hot mantle materials were upwelled during the extension of the SCS, and the magnetic layer was demagnetized. In addition, the magnetic anomaly caused by thick Mesozoic sediments was very weak, and the NW–SE‐trending faults made the magnetic bodies fragmental. The distribution of major faults is consistent with the fluctuating changes of the Curie isothermal interface, and the faults in the MQZ are the upward intrusion channels of the deep hot materials. Copyright © 2016 John Wiley & Sons, Ltd.

Constraining the source of mantle plumes

In order to link the geochemical signature of hot spot basalts to Earth's deep interior, it is first necessary to understand how plumes sample different regions of the mantle. Here, we investigate the relative amounts of deep and shallow mantle material that are entrained by an ascending plume and constrain its source region. The plumes are generated in a viscous syrup using an isolated heater for a range of Rayleigh numbers. The velocity fields are measured using stereoscopic Particle-Image Velocimetry, and the concept of the ‘vortex ring bubble’ is used to provide an objective definition of the plume geometry. Using this plume geometry, the plume composition can be analysed in terms of the proportion of material that has been entrained from different depths. We show that the plume composition can be well described using a simple empirical relationship, which depends only on a single parameter, the sampling coefficient, sc. High-sc plumes are composed of material which originated from very deep in the fluid domain, while low-sc plumes contain material entrained from a range of depths. The analysis is also used to show that the geometry of the plume can be described using a similarity solution, in agreement with previous studies. Finally, numerical simulations are used to vary both the Rayleigh number and viscosity contrast independently. The simulations allow us to predict the value of the sampling coefficient for mantle plumes; we find that as a plume reaches the lithosphere, 90% of its composition has been derived from the lowermost 260–750 km in the mantle, and negligible amounts are derived from the shallow half of the lower mantle. This result implies that isotope geochemistry cannot provide direct information about this unsampled region, and that the various known geochemical reservoirs must lie in the deepest few hundred kilometres of the mantle.

Episodic entrainment of deep primordial mantle material into ocean island basalts.

Chemical differences between mid-ocean ridge basalts (MORBs) and ocean island basalts (OIBs) provide critical evidence that the Earth's mantle is compositionally heterogeneous. MORBs generally exhibit a relatively low and narrow range of 3He/4He ratios on a global scale, whereas OIBs display larger variability in both time and space. The primordial origin of 3He in OIBs has motivated hypotheses that high 3He/4He ratios are the product of mantle plumes sampling chemically distinct material, but do not account for lower MORB-like 3He/4He ratios in OIBs, nor their observed spatial and temporal variability. Here we perform thermochemical convection calculations which show the variable 3He/4He signature of OIBs can be reproduced by deep isolated mantle reservoirs of primordial material that are viscously entrained by thermal plumes. Entrainment is highly time-dependent, producing a wide range of 3He/4He ratios similar to that observed in OIBs worldwide and indicate MORB-like 3He/4He ratios in OIBs cannot be used to preclude deep mantle-sourced hotspots.