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Origin of the LLSVPs at the base of the mantle is a consequence of plate tectonics-A petrological and geochemical perspective

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Geoscience Frontiers 2018年第5期9卷 1265-1278页

作者： Yaoling Niu China University of Geosciences Beijing 100083China Department of Earth Sciences Durham UniversityDurhamDH1 3LEUK Laboratory for Marine Geology Qingdao National Laboratory for Marine Science and TechnologyQingdao 266061China Institute of Oceanology Chinese Academy of SciencesQingdao 266071China

In studying the petrogenesis of intra-plate ocean island basalts(OIB) associated with hotspots or mantle plumes, we hypothesized that the two large-low-shear-wave-velocity provinces(LLSVPs) at the base of the mantle beneath the Pacific(Jason) and Africa(Tuzo) are piles of subducted ocean crust(SOC)accumulated over Earth's history. This hypothesis was formulated using petrology, geochemistry and mineral physics in the context of plate tectonics and mantle circulation. Because the current debate on the origin of the LLSVPs is limited to the geophysical community and modelling discipline and because it is apparent that such debate cannot be resolved without considering relevant petrological and geochemical information, it is my motivation here to objectively discuss such information in a readily accessible manner with new perspectives in light of most recent discoveries. The hypothesis has the following elements:(1) subduction of the ocean crust of basaltic composition to the lower mantle is irreversible because(2) SOC is denser than the ambience of peridotitic composition under lower mantle conditions in both solid state and liquid form;(3) this understanding differs from the widespread view that OIB come from ancient SOC that returns from the lower mantle by mantle plumes, but is fully consistent with the understanding that OIB is not derived from SOC because SOC is chemically and isotopically too depleted to meet the requirement for any known OIB suite on Earth;(4) SOC is thus the best candidate for the LLSVPs, which are, in turn, the permanent graveyard of SOC;(5) the LLSVPs act as thermal insulators, making core-heating induced mantle diapirs or plumes initiated at their edges, which explains why the large igneous provinces(LIPs) are associated with the edges of the LLSVPs;(6) the antipodal positioning of Jason and Tuzo represents the optimal momentum of inertia, which explains why the LLSVPs are stable in the spinning Earth.

关键词： LLSVPs Subducted ocean crust Subducted mantle lithosphere No superplumes Mantle chemical differentiation Optimal momentum of inertia

Effects of depth-and composition-dependent thermal conductivity and the compositional viscosity ratio on the long-term evolution of large thermochemical piles of primordial material in the lower mantle of the Earth:Insights from 2-D numerical modeling

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Science China Earth Sciences 2023年第8期66卷 1865-1876页

作者： Yang LI Zhigang ZHANG Juan LI Zhidong SHI Liang ZHAO State Key Laboratory of Lithospheric Evolution Institute of Geology and GeophysicsChinese Academy of SciencesBeijing 100029China Key Laboratory of Earth and Planetary Physics Institute of Geology and GeophysicsChinese Academy of SciencesBeijing 100029China College of Earth and Planetary Science University of Chinese Academy of SciencesBeijing 100049China Southern Marine Science and Engineering Guangdong Laboratory(Zhuhai) Zhuhai 519080China

Thermal conductivity plays an important role in the thermochemical evolution of Earth’s mantle.Recent mineral physics studies suggest that the thermal conductivity of the mantle varies with pressure and composition,and this may play an important role in the evolution of the Earth’s mantle.Meanwhile,the rheology of the deep mantle is also supposed to be composition-dependent.However,the dynamic influences of these factors remain not well understood.In this study,we performed numerical experiments of thermochemical mantle convection in 2-D spherical annulus geometry to systematically investigate the effects of depth-and composition-dependent thermal conductivity and the compositional viscosity ratio on the long-term evolution of the large thermochemical structure of primordial material in Earth’s mantle.Our results show that increasing the depth-dependent thermal conductivity leads to a larger core-mantle boundary(CMB)heat flow and allows the formation of more stable large thermochemical piles(e.g.,Large Low Shear Velocity Provinces,LLSVPs);while decreasing the composition-dependent thermal conductivity would slightly destabilize the primordial thermochemical piles,increase the altitude of these piles and the temperature differences between the piles and the ambient mantle.If the primordial mantle material is compositionally more viscous(e.g.,20 times than that of the ambient mantle),the long-term stability of the thermochemical piles of primordial material decreases,and this destabilizing effect will be enhanced by decreasing the composition-dependent thermal conductivity.As a result,the thermochemical piles would be unstable in the core-mantle boundary region.Therefore,our study indicates that the combined effects of depth-and composition-dependent thermal conductivity and compositional viscosity ratio are pronounced to the thermochemical evolution of the mantle.

关键词： Mantle convection Thermal conductivity Core-mantle boundary heat flow Primordial mantle material LLSVPs

下地幔CaSiO_(3)钙钛矿的稳定性及其对LLSVPs的启示

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矿物学报 2022年第1期42卷 121-128页

作者：肖婷婷张飞武中国科学院地球化学研究所矿床地球化学国家重点实验室贵州贵阳550081 中国科学院大学北京100049

LLSVPs的成因是当前固体地球科学研究的前沿热点,对这一成因的认识仍然存在很大的争议。认识LLSVPs的成因,对了解全球地幔对流性质、下地幔热结构以及化学物质的运输和演化等具有重要的科学意义。CaSiO_(3)钙钛矿(Ca-Pv)作为下地幔最丰... 详细信息

LLSVPs的成因是当前固体地球科学研究的前沿热点,对这一成因的认识仍然存在很大的争议。认识LLSVPs的成因,对了解全球地幔对流性质、下地幔热结构以及化学物质的运输和演化等具有重要的科学意义。CaSiO_(3)钙钛矿(Ca-Pv)作为下地幔最丰富的矿物之一,也是近年来学术界关注的热点。Ca-Pv在下地幔条件下的晶体结构仍然是目前研究的主要问题之一,Ca-Pv能否与其结构和化学式都相近的含铁布里奇曼石形成固溶体?本文通过第一性原理地球化学计算,研究了Ca-Pv及固溶体的基态稳定性,弹性性质和地震波波速特性。研究结果表明,在20~120 GPa压力下,Ca-Pv最稳定的基态结构为四方结构。基态下Ca-Pv的弹性波速值低于Mg-Pv约5%。与非互溶体系相比,x(Fe)为0%~10%的固溶体体系,引起了-2%~0%的剪切波波速负异常;-1.2%~0%的纵波波速负异常;-0.45~0.15%的体波速异常。因此,推测固溶体Mg_(1-x-y)Ca_(x)Fe_(y)SiO_(3)可能作为下地幔LLSVPs的候选矿物组分之一。

关键词：下地幔 CaSiO_(3)钙钛矿固溶体 LLSVPs

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

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Science China Earth Sciences 2022年第12期65卷 2405-2416页

作者： Min HUANG Yang LI Liang ZHAO State Key Laboratory of Lithospheric Evolution Institute of Geology and GeophysicsChinese Academy of SciencesBeijing 100029China Southern Marine Science and Engineering Guangdong Laboratory(Zhuhai) Zhuhai 519080China College of Earth and Planetary Sciences University of Chinese Academy of SciencesBeijing 100049China

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.

关键词： LLSVPs Primordial material Thermochemical piles Thermochemical structure of the deep mantle

Dupal Anomaly and Identification using Nd-Hf Isotopes

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Acta Geologica Sinica(English Edition) 2022年第2期96卷 416-429页

作者： TIAN Ye XU Jifeng LIU Xijun School of Earth Science and Resources and State Key Laboratory of Geological Processes and Mineral ResourcesChina University of GeosciencesBeijing 100083China Guangxi Key Laboratory of Hidden Metallic Ore Deposits Exploration College of Earth SciencesGuilin University of TechnologyGuilinGuangxi 541004China

The Dupal anomaly has attracted widespread attention since being discovered and is regarded as the most direct manifestation of mantle inhomogeneity at present. From the initially defined anomalies limited to the southern hemisphere to the global scale, the criteria for identifying anomalies defined by Pb isotopes have also been adjusted, providing an important method and reference for the study of the mantle evolution. Pearce and Peate(1995) proposed the method of NdHf isotope and element ratio to identify the Dupal anomaly. The Nd-Hf method also offers a possible way to discriminate the mantle region of arc magmatism through the correction of Nd in the subduction process. This paper introduces the concepts and determination methods of the Dupal anomaly, and reports new Hf isotopic data of MORB-type rocks with Dupal signature in the several Tethys ophiolites. Our results of Nd-Hf method are in good agreement with those of previous Pb isotope identification. Moreover, origins and their controversy of Dupal anomaly are reviewed, and possible internal connections between Dupal anomalies and the two Large Low Shear Velocity Provinces(LLSVPs) in the lower mantle are discussed in depth. Further studies on origin and evolution of the Dupal anomaly are suggested, especially using integrated approach of Hf-Nd and Pb isotopes.

关键词： Dupal anomaly LLSVPs Pb isotope Nd-Hf isotope mantle heterogeneity

Trial by fire:Testing the paleolongitude of Pangea of competing reference frames with the African LLSVP

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Geoscience Frontiers 2020年第4期11卷 1253-1256页

作者： Ross N.Mitchell Lei Wu J.Brendan Murphy Zheng-Xiang Li State Key Laboratory of Lithospheric Evolution Institute of Geology and GeophysicsChinese Academy of SciencesBeijingChina Earth Dynamics Research Group The Institute for Geoscience Research(TIGeR)School of Earth and Planetary SciencesCurtin UniversityGPO Box U1987PerthWAAustralia Department of Earth Sciences St.Francis Xavier UniversityAntigonishNova ScotiaCanada

Paleogeography can be reconstructed using various crust-or mantle-based reference frames that make fundamentally different assumptions.The various reconstruction models differ significantly in continental paleolongitude,but it has been difficult to assess which models are more valid.We suggest here a "LLSVP test",where an assumed correlation between present-day large low velocity shear-wave provinces and the paleogeography of supercontinent Pangea at breakup ca.200 million years ago can be used to assess the relative accuracy of published reconstructions.Closest correlations between continental paleolongitude and the African LLSVP are achieved with mantle-based reference frames(moving hotspots and true polar wander),whereas shallower crustbased reference frames are shown to be invalid.The relative success of mantle-based frames,and thus the importance of the depth of reference frame,supports the notion that mantle convection is largely vertical compared to the horizontal plate motion of tectonics.

关键词： Palaeogeography Plate tectonics LLSVPs Reference frames