In-situ Moissanite in Dunite: Deep Mantle Origin of Mantle Peridotite in Luobusa Ophiolite, Tibet

We report the discovery of an in-situ natural moissanite as an inclusion in the Cr-spinel from the dunite envelope of a chromitite deposit in Luobusa ophiolite,Tibet.The moissanite occurs as a twin crystal interpenetrated by two quadrilateral signal crystals with sizes of 17 pm × 10 μm and 20 μm × 7 μm,respectively.The moissanite is green with parallel extinction.The absorption peaks in its Raman spectra are at 967-971 cm-1,787-788 cm-1,and 766 cm-1.The absorption peaks in the infrared spectra are at 696 cm-1,767 cm-1,1450 cm-1,and 1551 cm-1,which are distinctly different from the peaks for synthetic silicon carbide.Moissanites have been documented to form in ultra-high pressure,high temperature,and extremely low fO2 environments and their 13C-depleted compositions indicate a lower mantle origin.Combined with previous studies about other ultra-high pressure and highly reduced minerals in Luobusa ophiolite,the in-situ natural moissanite we found indicates a deep mantle origin of some materials in the mantle sequence of Luobusa ophiolite.Further,we proposed a transformation model to explain the transfer process of UHP materials from the deep mantle to ophiolite sequence and then to the supra-subduction zone environment.Interactions between the crown of the mantle plume and mid-ocean ridge are suggested to be the dominant mechanism.

The deep mantle upwelling beneath the northwestern South China Sea:Insights from the time-varying residual subsidence in the Qiongdongnan Basin

Deep hot mantle upwelling is widely revealed around the Qiongdongnan Basin on the northwestern South China Sea margin. However, when and how it influenced the hyper-extended basin is unclear.To resolve these issues, a detailed analysis of the Cenozoic time-varying residual subsidence derived by subtracting the predicted subsidence from the backstripped subsidence was performed along a new seismic reflection line in the western Qiongdongnan Basin. For the first time, a method is proposed to calculate the time-varying strain rates constrained by the faults growth rates, on basis of which, the predicted basement subsidence is obtained with a basin-and lithosphere-scale coupled finite extension model, and the backstripped subsidence is accurately recovered with a modified technique of backstripping to eliminate the effects of later episodes of rifting on earlier sediment thickness. Results show no residual subsidence in 45–28.4 Ma. But after 28.4 Ma, negative residual subsidence occurred, reached and remained ca. -1000 m during 23–11.6 Ma, and reduced dramatically after 11.6 Ma. In the syn-rift period(45–23 Ma), the residual subsidence is ca. -1000 m, however in the post-rift period(23–0 Ma),it is positive of ca. 300 to 1300 m increasing southeastwards. These results suggest that the syn-rift subsidence deficit commenced at 28.4 Ma, while the post-rift excess subsidence occurred after 11.6 Ma.Combined with previous studies, it is inferred that the opposite residual subsidence in the syn-and post-rift periods with similar large wavelengths(>10^(2) km) and km-scale amplitudes are the results of transient dynamic topography induced by deep mantle upwelling beneath the central QDNB, which started to influence the basin at ca. 28.4 Ma, continued into the Middle Miocene, and decayed at ca.11.6 Ma. The initial mantle upwelling with significant dynamic uplift had precipitated considerable continental extension and faulting in the Late Oligocene(28.4–23 Ma). After ca. 11.6 Ma, strong mantle upwelling probably occurred beneath the Leizhou–Hainan area to form vast basaltic lava flow.

Geochemical Characteristics of Volcanic Rocks from ODP Site 794,Yamato Basin:Implications for Deep Mantle Processes of the Japan Sea

Deep mantle processes and the dynamic mechanism of magmatism in the Japan Sea Basin are important processes that have not been studied in detail. In this paper, systematic evaluation of basalt samples from the ocean drilling program Site 794 in the Japan Sea was performed, which included petrography, whole-rock major- and trace-element analysis, Sr-Nd-Pb isotopic composition, and electron microprobe analysis of plagioclase and clinopyroxene. These basalts belong to the tholeiitic series with porphyritic texture and massive Ca-rich plagioclase, clinopyroxene, and minor olivine phenocrysts. The basalts are characterized as flat rare earth elements and high-field-strength elements with remarkably low ratios of （La/Yb）N （0.75-2.51）, significant positive anomalies of Ba, Sr, and Rb and no Eu anomaly （dEn = 0.99-1.36）. The samples showed relatively high 87Sr/86Sr （0.70425- 0.70522）, 207pb/204pb （15.511-15.610）, and 208pb/204pb （38.064-38.557） values and a low 143Nd/144Nd ratio （0.51271-0.51295）. The basalts from Site 794 can be divided into upper, middle, and lower volcanic rocks （UVR, MVR, and LVR） on the basis of their stratigraphic level. The MVR was geochemically derived from the depleted mantle, whereas the UVR and LVR originated from a nondepleted and relatively enriched mantle source with contributions from subducted Pacific plate fluid and sediments. Use of geothermobarometers indicates that the crystallization pressure for the UVR and LVR （6.25-11.19 kbar） was significantly higher than that of the MVR （3.48-5.84 kbar）. The UVR and LVR may have been derived from the low-degree （5%-10%） partial melting of spinel lherzolite, while the MVR originated from a shallower mantle source with a high degree （10%-20%） of partial melting. In addition, the geochemical characteristics of the samples are consistent with a younger age （13-17 Ma） and the depleted composition of the MVR and an older age （17-23 Ma） and slightly enriched composition of the UVR and LVR. Therefore, temporal changes in the mantle source from old and enriched to young and depleted and subsequently to old and nondepleted may have been associated with progressive lithospheric extension and thinning, as well as at least two episodes of diverse asthenospheric upwelling and pull-apart tectonic motion in the Yamato Basin.

Assortment of Deep Mantle Fluids and Their Products in Kimberlites from China

Based on studies of petrography, mineralogy and mineral chemistry, deep mantle fluids and their products in kimberlites and diamonds can be assorted into the ultradeep fluid-transmitted minerals with an oxygen-free feature, the deep fluid metasomatized-minerals characterized by enrichment in TiO2, K2O, BaO, REE and Fe3+, and the deep fluid-reformed minerals. The three types show a successive descent in fluid origin depth and metasomatism strength, and they have brought forth a series of corresponding metasomatic products.

Ophiolites as Archives of Recycled Crustal Material Residing in the Deep Mantle

Deeply subducted lithospheric slabs may reach to the mantle transition zone(MTZ,410-660 km depth)or even to the core–mantle boundary(CMB)at depths of^2900km.Our knowledge of the fate of subducted surface material at the MTZ or near the CMB is poor and based mainly on the tomography data and laboratory experiments through indirect methods.Limited data come from the samples of deep mantle diamonds and their mineral inclusions obtained from kimberlites and associated rock assemblages in old cratons.We report in this presentation new data and observations from diamonds and other UHP minerals recovered from ophiolites that we consider as a new window into the life cycle of deeply subducted oceanic and continental crust.Ophiolites are fragments of ancient oceanic lithosphere tectonically accreted into continental margins,and many contain significant podiform chromitites.Our research team has investigated over the last 10 years ultrahigh-pressure and super-reducing mineral groups discovered in peridotites and/or chromitites of ophiolites around the world,including the Luobusa(Tibet),Ray-Iz(Polar Urals-Russia),and 12 other ophiolites from 8orogenic belts in 5 different countries(Albania,China,Myanmar,Russia,and Turkey).High-pressure minerals include diamond,coesite,pseudomorphic stishovite,qingsongite(BN)and Ca-Si perovskite,and the most important native and highly reduced minerals recovered to date include moissanite(Si C),Ni-Mn-Co alloys,Fe-Si and Fe-C phases.These mineral groups collectively confirm extremely high?pressures(300 km to≥660 km)and super-reducing conditions in their environment of formation in the mantle.All of the analyzed diamonds have unusually light carbon isotope compositions(δ13C=-28.7 to-18.3‰)and variable trace element contents that*d i stinguish them from most kimberlitic and UHPmetamorphic varieties.The presence of exsolution lamellae of diopside and coesite in some chromite grains suggests chromite crystallization depths around>380 km,near the mantle transition zone.The carbon isotopes and other features of the high-pressure and super-reduced mineral groups point to previously subducted surface material as their source of origin.Recycling of subducted crust in the deep mantle may proceed in three stages:Stage 1–Carbon-bearing fluids and melts may have been formed in the MTZ,in the lower mantle or even near the CMB.Stage 2–Fluids or melts may rise along with deep plumes through the lower mantle and reach the MTZ.Some minerals,such as diamond,stishovite,qingsongite and Ca-silicate perovskite can precipitate from these fluids or melts in the lower mantle during their ascent.Material transported to the MTZ would be mixed with highly reduced and UHP phases,presumably derived from zones with extremely low f O2,as required for the formation of moissanite and other native elements.Stage 3–Continued ascent above the transition of peridotites containing chromite and ultrahigh-pressure minerals transports them to shallow mantle depths,where they participate in decompressional partial melting and oceanic lithosphere formation.The widespread occurrence of ophiolite-hosted diamonds and associated UHP mineral groups suggests that they may be a common feature of in-situ oceanic mantle.Because mid-ocean ridge spreading environments are plate boundaries widely distributed around the globe,and because the magmatic accretion of oceanic plates occurs mainly along these ridges,the on-land remnants of ancient oceanic lithosphere produced at former mid-ocean ridges provide an important window into the Earth’s recycling system and a great opportunity to probe the nature of deeply recycled crustal material residing in the deep mantle

Deep mantle hydrogen in the pyrite-type FeO_(2)–FeO_(2)H system

The pyrite-type FeO_(2)and FeO_(2)H were synthesized at the pressure-temperature conditions relevant to Earth’s deep lower mantle.Through the water-iron reaction,the pyrite-phase is a good candidate to explain the chemical heterogeneities and seismological anomalies at the bottom of the mantle.The solid solution of pyrite-type FeO_(2)and FeO_(2)H,namely the FeO_(2)Hx(0≤x≤1),is particularly interesting and introduces puzzling chemical states for both the O and H atoms in the deep mantle.While the role of H in the FeO_(2)–FeO_(2)H system has been primarily investigated,discrepancies remain.In this work,we summarize recent progress on the pyrite-phase,including FeO_(2),FeO_(2)H,and FeO_(2)Hx,which is critical for understanding the water cycling,redox equilibria,and compositional heterogenicities in the deep lower mantle.

Effects of thermal, compositional and rheological properties on the long-term evolution of large thermochemical piles of primordial material in the deep mantle

Seismic tomography observations have shown that there are two large low shear velocity provinces(LLSVPs)above the core-mantle boundary beneath Africa and the Pacific.The thermal and compositional properties of these two LLSVPs may differ from those of the ambient mantle,and they are suggested to be thermochemical piles of primordial material in the lower mantle.Their evolution is of great importance to our understanding of mantle dynamics.In this study,we systematically conducted numerical experiments to investigate the effects of the buoyancy ratio(B),compositional viscosity ratio(Δη_(c)),and heat-producing ratio(Λ)of the primordial material on the long-term evolution of thermochemical piles.Our results show that the buoyancy ratio plays the most important role in the stability of these piles.When the buoyancy ratio is small,and the primordial material is enriched in heat-producing elements(Λ>1),the stability of these piles decreases with increasing compositional viscosity ratio or heat-producing ratio.For cases with homogeneous heat production(Λ=1),the stability of these piles increases with increasingΔηc.We further compare constant internal heating with radioactive decay internal heating,and find that the longterm stability of thermochemical piles slightly decreases with radioactive decay heating,but the overall differences between these two internal heating modes are relatively small.

The Deep Earth Engine Driving Major Surface Events

During Earth’s 4.6 billion-year history,its surface has experienced environmental changes that drastically impacted habitability.The changes have been mostly attributed to near-surface processes or astronomical events with little consideration of Earth’s deep interior.Recent progresses in high-pressure geochemistry and geophysics,however,indicate that deep Earth processes may have played a dominant role in the surface(Mao and Mao,2020).

The Discovery of Diamonds in Chromitites of the Hegenshan Ophiolite,Inner Mongolia,China

Diamond, moissanite and a variety of other minerals, similar to those reported from ophiolites in Tibet and northern Russia, have recently been discovered in chromitites of the Hegenshan ophiolite of the Central Asian Orogenic Belt, north China. The chromitites are small, podiform and vein-like bodies hosted in dunite, clinopyroxene-bearing peridotite, troctolite and gabbro. All of the analysed chromite grains are relatively Al-rich, with Cr^# [100Cr/（Cr＋Al）] of about 47-53. Preliminary studies of mainly disseminated chromitite from ore body No. 3756 have identified more than 30 mineral species in addition to diamond and moissanite. These include oxides （mostly hematite, magnetite, ruffle, anatase, cassiterite, and quartz）, sulfides （pyrite, marcasite and others）, silicates （magnesian olivine, enstatite, augite, diopside, uvarovite, pyrope, orthoclase, zircon, sphene, vesuvianite, chlorite and serpentine） and others （e.g., calcite, monazite, glauberite, iowaite and a range of metallic alloys）. This study demonstrates that diamond, moissanite and other exotic minerals can occur in high-Al, as well as high-Cr chromites, and significantly extends the geographic and age range of known diamond-bearing ophiolites.

East African topography and volcanism explained by a single,migrating plume

Anomalous topographic swells and Cenozoic volcanism in east Africa have been associated with mantle plumes.Several models involving one or more fixed plumes beneath the northeastward migrating African plate have been suggested to explain the space-time distribution of magmatism in east Africa.We devise paleogeographically constrained global models of mantle convection and,based on the evolution of flow in the deepest lower mantle,show that the Afar plume migrated southward throughout its lifetime.The models suggest that the mobile Afar plume provides a dynamically consistent explanation for the spatial extent of the southward propagation of the east African rift system(EARS),which is difficult to explain by the northeastward migration of Africa over one or more fixed plumes alone,over the last≈45 Myr.We further show that the age-progression of volcanism associated with the southward propagation of EARS is consistent with the apparent surface hotspot motion that results from southward motion of the modelled Afar plume beneath the northeastward migrating African plate.The models suggest that the Afar plume became weaker as it migrated southwards,consistent with trends observed in the geochemical record.

Coupled deep-mantle carbon-water cycle: Evidence from lower-mantle diamonds

Diamonds form in a variety of environments between subducted crust,lithospheric and deep mantle.Recently,deep source diamonds with inclusions of the high-pressure H2O-phase ice-VII were discovered.By correlating the pressures of ice-VII inclusions with those of other high-pressure inclusions,we assess quantitatively the pressures and temperatures of their entrapment.We show that the iceVII-bearing diamonds formed at depths down to 800±60 km but at temperatures 200–500 K below average mantle temperature that match the pressure-temperature conditions of decomposing dense hydrous mantle silicates.Our work presents strong evidence for coupled recycling of water and carbon in the deep mantle based on natural samples.

Crustal and upper mantle structure and deep tectonic genesis of large earthquakes in North China

From the 1960 s to 1970 s, North China has been hit by a series of large earthquakes. During the past half century,geophysicists have carried out numerous surveys of the crustal and upper mantle structure, and associated studies in North China.They have made significant progress on several key issues in the geosciences, such as the crustal and upper mantle structure and the seismogenic environment of strong earthquakes. Deep seismic profiling results indicate a complex tectonic setting in the strong earthquake areas of North China, where a listric normal fault and a low-angle detachment in the upper crust coexist with a high-angle deep fault passing through the lower crust to the Moho beneath the hypocenter. Seismic tomography images reveal that most of the large earthquakes occurred in the transition between the high-and low-velocity zones, and the Tangshan earthquake area is characterized by a low-velocity anomaly in the middle-lower crust. Comprehensive analysis of geophysical data identified that the deep seismogenic environment in the North China extensional tectonic region is generally characterized by a low-velocity anomalous belt beneath the hypocenter, inconsistency of the deep and shallow structures in the crust, a steep crustalal-scale fault,relative lower velocities in the uppermost mantle, and local Moho uplift, etc. This indicates that the lithospheric structure of North China has strong heterogeneities. Geologically, the North China region had been a stable craton named the North China Craton or in brief the NCC, containing crustal rocks as old as ～3.8 Ga. The present-day strong seismic activity and the lower velocity of the lower crust in the NCC are much different from typical stable cratons around the world. These findings provide significant evidence for the destruction of the NCC. Although deep seismic profiling and seismic tomography have greatly enhanced knowledge about the deep-seated structure and seismogenic environment, some fundamental issues still remain and require further work.

Subduction and retreating of the western Pacific plate resulted in lithospheric mantle replacement and coupled basin-mountain respond in the North China Craton

The North China Craton(NCC) witnessed Mesozoic vigorous tectono-thermal activities and transition in the nature of deep lithosphere. These processes took place in three periods:(1) Late Paleozoic to Early Jurassic(~170 Ma);(2) Middle Jurassic to Early Cretaceous(160-140 Ma);(3) Early Cretaceous to Cenozoic(140 Ma to present). The last two stages saw the lithospheric mantle replacement and coupled basin-mountain response within the North China Craton due to subduction and retreating of the Paleo-Pacific plate, and is the emphasis in this paper. In the first period,the subduction and closure of the PaleoAsian Ocean triggered the back-arc extension, syn-collisional compression and then post-collisional extension accompanied by ubiquitous magmatism along the northern margin of the NCC. Similar processes happened in the southern margin of the craton as the subduction of the Paleo-Tethys ocean and collision with the South China Block. These processes had caused the chemical modification and mechanical destruction of the cratonic margins. The margins could serve as conduits for the asthenosphere upwelling and had the priority for magmatism and deformation. The second period saw the closure of the Mongol-Okhotsk ocean and the shear deformation and magmatism induced by the drifting of the Paleo-Pacific slab. The former led to two pulse of N-S trending compression(Episodes A and B of the Yanshan Movement) and thus the pre-existing continental marginal basins were disintegrated into sporadically basin and range pro vince by the Mesozoic magmatic plutons and NE-SW trending faults.With the anticlockwise rotation of the Paleo-Pacific moving direction, the subduction-related magmatism migrated into the inner part of the craton and the Tanlu fault became normal fault from a sinistral one. The NCC thus turned into a back-arc extension setting at the end of this period. In the third period, the refractory subcontinental lithospheric mantle(SCLM) was firstly remarkably eroded and thinned by the subduction-induced asthenospheric upwelling, especially those beneath the weakzones(i.e.,cratonic margins and the lithospheric Tanlu fault zone). Then a slightly lithospheric thickening occurred when the upwelled asthenosphere got cool and transformed to be lithospheric mantle accreted(~125 Ma) beneath the thinned SCLM. Besides, the magmatism continuously moved southeastward and the extensional deformations preferentially developed in weak zones, which include the Early Cenozoic normal fault transformed from the Jurassic thrust in the Trans-North Orogenic Belt, the crustal detachment and the subsidence of Bohai basin caused by the continuous normal strike slip of the Tanlu fault, the Cenozoic graben basins originated from the fault depression in the Trans-North Orogenic Belt, the Bohai Basin and the Sulu Orogenic belt. With small block size, inner lithospheric weak zones and the surrounding subductions/collisions, the Mesozoic NCC was characterized by(1) lithospheric thinning and crustal detachment triggered by the subduction-induced asthenospheric upwelling.Local crustal contraction and orogenesis appeared in the Trans-North Orogenic Belt coupled with the crustal detachment;(2)then upwelled asthenosphere got cool to be newly-accreted lithospheric mantle and crustal grabens and basin subsidence happened, as a result of the subduction zone retreating. Therefore, the subduction and retreating of the western Pacific plate is the outside dynamics which resulted in mantle replacement and coupled basin-mountain respond within the North China Craton. We consider that the Mesozoic decratonization of the North China Craton,or the Yanshan Movement, is a comprehensive consequence of complex geological processes proceeding surrounding and within craton, involving both the deep lithospheric mantle and shallow continental crust.