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Nd-Hf Isotopic Decoupling of the Silurian–Devonian Granitoids in the Chinese Altai: A Consequence of crustal recycling of the Ordovician Accretionary Wedge?

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Journal of Earth Science 2020年第1期31卷 102-114页

作者： Yanqiong Huang Yingd Jiang Yang Yu Stephen Collett Sheng Wang Tan Shu Kang Xu State Key Laboratory of Isotope Geochemistry Guangzhou Institute of GeochemistryChinese Academy of SciencesGuangzhou 510640China University of Chinese Academy of Sciences Beijing 100049China Center for Lithospheric Research Czech Geological SurveyPragueCzech Republic

Voluminous Silurian–Devonian granitoids intruded a greywacke-dominated Ordovician accretionary wedge in the Chinese Altai. These granitoids are characterized by significant Nd-Hf isotopic decoupling, the underlying mechanism of which, so far, has been poorly understood. This issue is addressed in this study by the integration of our new and regional published geological and geochemical data. Geological studies indicated a close spatial relationship between the regional anatexis of the Ordovician wedge and the formation of the granitoids, which is characterized by a gradual textural evolution from the partial molten Ordovician wedge sedimentary rocks(the Habahe Group) to the granitoid bodies. Compositionally, these granitoids and the Ordovician Habahe Group rocks displayed close geochemical similarities, in the form of arclike trace elemental signatures as well as comparable Nd isotopic characteristics. Combined with regional available data, we suggest that the Silurian–Devonian granitoids originated from the immature and chemically primitive Habahe Group rocks. Since Nd and Hf isotopic data for the Habahe Group rocks show significant Nd-Hf isotopic decoupling, we propose that the Silurian–Devonian granitoids inherited the Nd and Hf isotopic signatures from their sources, i.e., the Habahe Group rocks. In other words, the Nd-Hf decoupling in the Habahe Group rocks is the primary causative factor leading to the prevailing Nd-Hf isotopic decoupling of the Silurian–Devonian granitoids in the Chinese Altai.

关键词： granitoids crustal recycling Ordovician wedge sediments partial melting Nd-Hf decoupling Chinese Altai geochemistry

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Mantle geochemistry: Insights from ocean island basalts

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Science China Earth Sciences 2017年第11期60卷 1976-2000页

作者： HUANG ShiChun ZHENG YongFei Department of Geoscience University of Nevada CAS Key Laboratory of Crust-Mantle Material and Environment School of Earth and Space Sciences University of Science and Technology of China

The geochemical study of the Earth's mantle provides important constraints on our understanding of the formation and evolution of Earth, its internal structure, and the mantle dynamics. The bulk Earth composition is inferred by comparing terrestrial mantle rocks with chondrites, which leads to the chondritic Earth model. That is, Earth has the same relative proportions of refractory elements as that in chondrites, but it is depleted in volatiles. Ocean island basalts(OIB) may be produced by mantle plumes with possible deep origins; consequently, they provide unique opportunity to study the deep Earth. Isotopic variations within OIB can be described using a limited number of mantle endmembers, such as EM1, EM2 and HIMU, and they have been used to decipher important mantle processes. Introduction of crustal material into the deep mantle via subduction and delamination is important in generating mantle heterogeneity; however, there is active debate on how they were sampled by mantle melting, i.e.,the role of olivine-poor lithologies in the OIB petrogenesis. The origin and location of high 3He/4He mantle remain controversial,ranging from unprocessed(or less processed) primitive material in the lower mantle to highly processed materials with shallow origins, including ancient melting residues, mafic cumulates under arcs, and recycled hydrous minerals. Possible core-mantle interaction was hypothesized to introduce distinctive geochemical signatures such as radiogenic 186 Os and Fe and Ni enrichment in the OIB. Small but important variations in some short-lived nuclides, including 142 Nd, 182 W and several Xe isotopes, have been reported in ancient and modern terrestrial rocks, implying that the Earth's mantle must have been differentiated within the first 100 Myr of its formation, and the mantle is not efficiently homogenized by mantle convection.

关键词： Mantle structure Mantle composition crustal components Crust-mantle interaction crustal recycling

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Molybdenum isotope composition of the upper mantle and its origin:insight from mid-ocean ridge basalt

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Journal of Oceanology and Limnology 2024年第3期42卷 705-708页

作者： Shuo CHEN Key Laboratory of Marine Geology and Environment Institute of OceanologyChinese Academy of SciencesQingdao 266071China

The molybdenum(Mo)isotope system is pivotal in reconstructing marine redox changes throughout Earth’s history and has emerged as a promising tracer for igneous and metamorphic *** its composition and variation across major geochemical reservoirs is essential for its application in investigating high-temperature ***,there is debate regarding theδ^(98/95)Mo value of the Earth’s mantle,with estimates ranging from sub-chondritic to super-chondritic *** analyses of global mid-ocean ridge basalt(MORB)glasses revealed significantδ^(98/95)Mo variations attributed to mantle heterogeneity,proposing a two-component mixing model to explain the observed *** studies confirmed the sub-chondriticδ^(98/95)Mo of the depleted upper mantle,suggesting remixing of subduction-modified oceanic crust as a plausible *** findings underscore the role of Mo isotopes as effective tracers for understanding dynamic processes associated with mantle-crustal recycling.

关键词： molybdenum(Mo)isotope mantle crustal recycling mid-ocean ridge basalt(MORB)

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Conjecture with water and rheological control for subducting slab in the mantle transition zone

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Geoscience Frontiers 2015年第1期6卷 79-93页

作者： Fumiko Tajima Masaki Yoshida Eiji Ohtani University of California at Irvine 4164 Frederick Reines Hall4IrvineCA 92697USA Institute for Research on Earth Evolution(IFREE) Japan Agency for Marine-Earth Science and Technology(JAMSTEC) Department of Earth and Materials Science Graduate School of ScienceTohoku University

Seismic observations have shown structural variation near the base of the mantle transition zone （MTZ） where subducted cold slabs, as visualized with high seismic speed anomalies （HSSAs）, flatten to form stagnant slabs or sink further into the lower mantle. The different slab behaviors were also accompanied by variation of the ＂660 kin＂ discontinuity depths and low viscosity layers （LVLs） beneath the MTZ that are suggested by geoid inversion studies. We address that deep water transport by subducted slabs and dehydration from hydrous slabs could affect the physical properties of mantle minerals and govern slab dynamics. A systematic series of three-dimensional numerical simulation has been conducted to examine the effects of viscosity reduction or contrast between slab materials on slab behaviors near the base of the MTZ. We found that the viscosity reduction of subducted crustal material leads to a sepa- ration of crustal material from the slab main body and its transient stagnation in the MTZ. The once trapped crustal materials in the MTZ eventually sink into the lower mantle within 20 30 My from the start of the plate subduction. The results suggest crustal material recycle in the whole mantle that is consistent with evidence from mantle geochemistry as opposed to a two-layer mantle convection model. Because of the smaller capacity of water content in lower mantle minerals than in MTZ minerals, dehydration should occur at the phase transformation depth, ~660 kin. The variation of the disconti- nuity depths and highly localized low seismic speed anomaly （LSSA） zones observed from seismic P waveforms in a relatively high frequency band （~ 1 Hz） support the hypothesis of dehydration from hydrous slabs at the phase boundary. The LSSAs which correspond to dehydration induced fluids are likely to be very local, given very small hydrogen （H＋） diffusivity associated with subducted slabs. The image of such local LSSA zones embedded in HSSAs may not be necessarily captured

关键词： Stagnant slab Discontinuity depth variation Deep water transport by subduction crustal separation crustal recycling

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