Widespread melt percolation in the subcontinental mantle: Perspectives from chromium stable isotopes

IF 3.6 2区地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS

Chemical Geology Pub Date : 2025-02-19 DOI:10.1016/j.chemgeo.2025.122699

Qiuxia Wang , Jianping Zheng , Xianquan Ping , Xiangli Wang

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Abstract

Peridotites xenoliths from the subcontinental mantle exhibit a notably broad range of chromium isotope compositions in the literature, with refractory harzburgites demonstrating slightly higher δ⁵³Cr values (mean = −0.04 ± 0.06 ‰, 2SE, N = 49) compared to fertile lherzolites (mean = −0.12 ± 0.03 ‰, 2SE, N = 80). Such discrepancy cannot be explained by the partial melting of lherzolite. To better understand the observation, we analyzed the Cr isotope compositions of twenty-two lherzolites and fifteen harzburgites in Cenozoic basalts from eastern China. Taking the literature data together, the lherzolites yield an average δ⁵³Cr of −0.12 ± 0.05 ‰ (2SE, N = 99), similar to that estimated for the BSE (−0.12 ± 0.04 ‰, 2SD). Moreover, their δ⁵³Cr show no correlations with indicators of melt extraction (such as Al₂O₃ and CaO in whole-rock) or metasomatism (such as (La/Yb)_N in whole-rock and Ca/Al ratios in clinopyroxene), suggesting negligible effects of partial melting or metasomatism. Therefore, the Cr isotopic composition of the studied lherzolites may represent the original mantle signal. In contrast, the harzburgite display significantly higher δ⁵³Cr values (average − 0.04 ± 0.05 ‰, 2SE, N = 60) compared to the lherzolites. They also show no correlation between δ⁵³Cr values and melt extraction indices (such as Al₂O₃ and CaO concentrations of in whole-rocks) or metasomatism indicators (such as (La/Yb)_N in whole-rock and Ca/Al ratios in clinopyroxene). Chromium isotope fractionation during batch or fractional melting can only induce <0.01 ‰ Cr isotope variation, and thus cannot explain the elevated δ⁵³Cr values observed in harzburgites. Instead, kinetic isotope fractionation during melt percolation is more likely responsible for causing such Cr isotope offset between lherzolites and harzburgites in the subcontinental mantle. The isotopically heavy Cr pool harzburgites completements the isotopically light Cr pool in pyroxenite recognized previously.

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次大陆地幔中广泛的熔体渗透：铬稳定同位素的观点

在文献中，来自次大陆地幔的橄榄岩捕虏体显示出明显的广泛的铬同位素组成，其中难熔的哈尔兹布尔岩的δ53Cr值（平均值= - 0.04±0.06‰，2SE, N = 49）略高于肥沃的伊尔黑橄榄岩（平均值= - 0.12±0.03‰，2SE, N = 80）。这种差异不能用热橄榄岩的部分熔融来解释。为了更好地理解这一观测结果，我们分析了中国东部新生代玄武岩中22个黑斑岩和15个黑斑岩的Cr同位素组成。综合文献资料，伊尔闪石的δ53Cr平均值为- 0.12±0.05‰（2SE, N = 99），与BSE的δ53Cr平均值（- 0.12±0.04‰，2SD）相近。此外，它们的δ53Cr与熔体萃取指标（如全岩Al2O3和CaO）或交代指标（如全岩（La/Yb）N和斜辉石Ca/Al比值）没有相关性，表明部分熔融或交代的影响可以忽略不计。因此，研究的热橄榄岩的Cr同位素组成可能代表了原始的地幔信号。辉石岩的δ53Cr值（平均−0.04±0.05‰，2SE, N = 60）明显高于辉石岩。δ53Cr值与熔体萃取指标（如全岩中Al2O3和CaO浓度）或交代指标（如全岩中的（La/Yb）N和斜辉石中的Ca/Al比值）之间也没有相关性。分批熔融或分馏熔融过程中铬同位素分馏只能引起<；0.01‰的Cr同位素变化，不能解释哈茨堡土中δ53Cr值升高的原因。相反，熔体渗透过程中的动力学同位素分馏更有可能导致次大陆地幔中伊热橄榄岩和哈尔茨伯基岩之间的Cr同位素偏移。辉石岩中同位素重铬矿是对先前发现的同位素轻铬矿的补充。

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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.