Magnesium isotopes of chlorite-rich hydrothermal sediments in the Okinawa Trough and indications for Mg cycle  

作　　者：Hebin SHAO Shouye YANG Aiguo DONG Zhongya HU 

机构地区：[1]Key Laboratory for Polar Science of the State Oceanic Administration,Polar Research Institute of China,Shanghai 200136,China [2]State Key Laboratory of Marine Geology,Tongji University,Shanghai 200092,China [3]Institute of Earth Sciences,China University of Geosciences(Beijing),Beijing 100083,China

出　　处：《Science China Earth Sciences》2023年第3期648-662,共15页中国科学（地球科学英文版）

基　　金：supported by the National Natural Science Foundation of China (Grant Nos. 41806229, 41730531, and 41991324)。

摘　　要：Seafloor hydrothermal systems play a significant role in the oceanic Mg cycle due to ubiquitous deposits of secondary Mg-rich clays during the strong fluid-rock reactions.However,the magnitude of net Mg enrichment and Mg isotopic fractionation,particularly within the medium-high temperature hydrothermal systems in felsic-hosted settings,are not well studied yet.Here we report elemental and isotopic compositions of Mg in hydrothermal chlorite-rich sediments,volcanic materials,and terrigenous sediments collected during the IODP Expedition 331 drilled to the thick sediment-covered and felsic-hosted middle Okinawa Trough(Iheya North Knoll) in the West Pacific.We investigate the sources of Mg in chlorite and Mg isotopic behavior at medium-high temperature hydrothermal alteration.After 1 mol/L HCl leaching,Mg isotopic compositions of chlorite-rich sediments present overall similar values in the residual fractions and bulk samples albeit with slightly higher values in the leachates.Mineralogical differentiation primarily determines the Mg isotopic compositions,showing that siliciclastic residues have slightly higher δ^(26) Mg values than the leachates dominated by carbonates and oxides/hydroxides.Significant Mg isotopic fractionation happened in the medium-high temperature(~150°C to 260°C) felsic-hosted hydrothermal system,with Δ^(26)MgChl-SW ranging from 0.15‰ to 0.71‰ and yielding a negative correlation with temperature.This observation suggests the preferential incorporation of heavy Mg isotopes by the secondary chlorite precipitation.We infer that the medium-high temperature hydrothermal systems can take up about 8–14% of riverine input of Mg in the arc and back-arc regions.Incomplete removal of aqueous Mg in porewater and vent fluids by the medium-high temperature hydrothermal alterations in the arc and back-arc basins provides constraints on the Mg budget and isotopic composition of seawater.

关 键 词：Hydrothermal alteration CHLORITE Okinawa Trough Magnesium(Mg)isotopes Integrated Ocean Drilling Program 

分 类 号：P736.4[天文地球—海洋地质]