著者

Weigang Peng Lifei Zhang Tingting Shen

出版者

日本地球惑星科学連合

雑誌

JpGU-AGU Joint Meeting 2017

巻号頁・発行日

2017-03-10

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Subduction zones play a significant role in regulating the carbon fluxes in solid earth by carrying shallow carbonated oceanic rocks into the deep mantle. Carbonated serpentinite / serpentinized mantle peridotite (ophicarbonate), in spite of not a very crucial part of the global carbon budget compared with the two others (carbonated sediments and carbonated basaltic oceanic crust) owing to its limited volume proportion, show great potential for CO2 capture and storage (CCS). Ophicarbonates, in southwestern Tianshan (China) HP-UHP metamorphic belt, could be divided into two different types based on their inside carbonate phases (such as magnesite and dolomite). Field occurrence, petrography, major and trace element concentrations, carbonate C and O isotopic compositions and whole-rock Sr isotopic studies indicate distinct origins of the two types of ophicarbonates: (a) CO2-bearing seawater hydrothermal alteration (S-ophimagnesite) and (b) high pressure COH-Ca fluids metasomatism (M-ophidolomite). High pressure COH fluids, produced by decarbonation reactions and (or) carbonate dissolution of subducted S-ophimagnesite during their peak metamorphic temperature (530~590℃), transfer upwards into the subduction channel and induce the blueschist-facies overprint in eclogites with the release of Ca. Then, the combined COH-Ca fluids in fluid channel, in turn, metasomatize slab - interface mantle rocks to form the M-ophidolomites and thermodynamic phase equilibrium modeling defines the pressure at 18.5~23.5kbar. This implies high pressure CO2 sequestration in serpentinite could prevent the CO2-bearing fluids from directly transporting upwards into the mantle wedge.In addition, the occurrence of through-going chrysotile + magnesite veins in serpentinites indicates that carbonic fluids could be mobilized at relatively low P and T conditions as well and the end product of metasomatism by these carbonic fluids is called listvenite, which refers to the fully carbonated serpentinite. Isotopic compositions (C, O and Sr) indicate that CO2-bearing fluids responsible for the formation of listvenite may originate from the previous S-ophimagnesite and M-ophidolomite during their retrograde exhumation. We infer that the formation of listvenite (P≈5kbar ,T≈350℃) are closely related to the process of a second serpentinization with rodingization (P=4~8.5kbar ,T=200-410℃) in southwestern Tianshan (China) based on the compilation of previous researches and our field observation, demonstrating that carbonate dissolution could occur not only at high pressure but also at a low pressure and temperature condition during the exhumation of the subduted slab.We stress that the content of initial carbon uptake in oceanic altered peridotite determined only by analogy of carbon content in exhumated ophicarbonates could be underestimated, because decarbonation and (or) carbonate dissolution exist extensively in ophicarbonates, even at low P and T conditions.