流体-岩石相互作用形成的海洋岩石圈中榴辉岩的极重Ca同位素组成：对轻Mg同位素组成鉴别的启示

IF 3.6 2区地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS

Chemical Geology Pub Date : 2025-02-20 DOI:10.1016/j.chemgeo.2025.122697

Mei-Shan Zhao , Jia-Le Xu , Yi-Xiang Chen , Jia-Wei Xiong , Xin-Yue Qiao , Tatsuki Tsujimori , Marco Scambelluri

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引用次数: 0

摘要

罗丁岩是蚀变洋岩石圈的重要组成部分，是一种极富钙的岩石（CaO含量约为30% wt%）。然而，它的钙同位素组成仍不清楚。此外，岩斑岩和碳酸盐岩是海洋岩石圈中常见的两种富钙储层，其特征是轻镁同位素组成。目前尚不清楚这两种储层如何影响某些火成岩的轻Mg同位素组成。本文研究了藏南日喀则蛇绿岩中绿辉岩及其原岩辉长岩的Ca同位素组成。辉长岩δ44/40Ca值相对均匀，介于0.70 ~ 0.87‰之间，与MORB组成相当，而辉长岩δ44/40Ca值明显偏高，介于0.92 ~ 1.89‰之间。根据地层温度条件和矿物组合，可将其分为两类，它们在87Sr/86Sr同位素比值、总体Sr含量和Sr/Ca比值方面表现出截然不同的相关性。I组δ44/40Ca值与（87Sr/86Sr） I呈正相关，与Sr/Ca比值和Sr含量呈负相关。II族δ44/40Ca值与（87Sr/86Sr）i呈负相关，与Sr/Ca比值和Sr含量呈正相关。ⅰ组和ⅱ组岩石的δ44/40Ca值与地球化学组成呈相反关系，表明成矿过程中不同流体来源和流体组成的Ca添加量不同。岩化初期交代流体以海水为主，但随着流体-岩石反应的进行，溶解斜辉石和正辉石的蛇纹石化流体占主导地位。结果表明，岩化过程中发生的流体-岩石反应控制了岩化产物的Ca同位素组成。罗丁岩是蚀变洋岩石圈中一种未被发现的高Ca含量和δ44/40Ca值储层，可能通过洋板俯冲作用对地幔Ca同位素组成产生显著影响。此外，由于镁镁岩与碳酸盐岩相关组分的δ44/40Ca值不同，钙同位素可以作为区分海洋岩石圈中轻Mg同位素组分的有效工具，这对于利用CaMg同位素作为壳幔相互作用和碳酸盐岩再循环过程的有力示踪剂至关重要。

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Extremely heavy Ca isotope compositions of rodingites in the oceanic lithosphere formed by fluid-rock interaction: Implications for the discrimination of light Mg isotope components

Rodingite is a crucial component of the altered oceanic lithosphere and is an extremely Ca-rich rock (CaO content reaching ca. 30 wt%). However, its Ca isotope composition remains unclear. Furthermore, rodingite and carbonate represent two common Ca-rich reservoirs in the oceanic lithosphere characterized by light Mg isotope compositions. Currently, it remains unclear how these two rock reservoirs contribute to the light Mg isotope composition of some igneous rocks. In this study, we present the Ca isotope compositions of rodingites and their protolith gabbros from the Xigaze ophiolites in southern Tibet. The gabbros exhibit relatively homogeneous δ^44/40Ca values ranging from 0.70 to 0.87 ‰, which are comparable to the MORB composition, while rodingites display significantly higher δ^44/40Ca values ranging from 0.92 to 1.89 ‰. Based on formation temperature conditions and mineral assemblages, rodingites can be categorized into two groups that exhibit contrasting correlations with respect to the ⁸⁷Sr/⁸⁶Sr isotope ratios, bulk Sr contents and Sr/Ca ratios. Group I δ^44/40Ca values show a positive correlation with (⁸⁷Sr/⁸⁶Sr)_i but negative correlations with Sr/Ca ratios and Sr contents. Group II δ^44/40Ca values exhibit a negative correlation with (⁸⁷Sr/⁸⁶Sr)_i but positive correlations with Sr/Ca ratios and Sr contents. The opposing relationships between δ^44/40Ca values and geochemical compositions for Group I and II rodingites suggest Ca additions from different fluid sources and fluid compositions during rodingitization. Initially, seawater predominates the metasomatic fluids during the early stages of rodingitization, however, serpentinizing fluid that dissolves clinopyroxene and orthopyroxene becomes dominating as the fluid-rock reaction progresses. The results indicate that fluid-rock reactions occurring during rodingitization control the Ca isotope composition of the resulting rodingites. Rodingite represents a previously unrecognized reservoir of high Ca contents and δ^44/40Ca values in the altered oceanic lithosphere, which could significantly influence mantle Ca isotope compositions through oceanic slab subduction. In addition, due to the distinct δ^44/40Ca values between rodingite and carbonate-related components, Ca isotopes may serve as an effective tool for distinguishing these light Mg isotope components in the oceanic lithosphere, which is critical for utilizing CaMg isotopes as powerful tracers in crust-mantle interaction and carbonate recycling processes.

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来源期刊

Chemical Geology 地学-地球化学与地球物理

CiteScore

7.20

自引率

10.30%

发文量

374

审稿时长

3.6 months

期刊介绍： Chemical Geology is an international journal that publishes original research papers on isotopic and elemental geochemistry, geochronology and cosmochemistry. The Journal focuses on chemical processes in igneous, metamorphic, and sedimentary petrology, low- and high-temperature aqueous solutions, biogeochemistry, the environment and cosmochemistry. Papers that are field, experimentally, or computationally based are appropriate if they are of broad international interest. The Journal generally does not publish papers that are primarily of regional or local interest, or which are primarily focused on remediation and applied geochemistry. The Journal also welcomes innovative papers dealing with significant analytical advances that are of wide interest in the community and extend significantly beyond the scope of what would be included in the methods section of a standard research paper.