Graphite-(Mo,W)S 2 intergrowth as a palaeoenvironmental proxy in metasedimentary rocks
Graphite-(Mo,W)S 2 intergrowth as a palaeoenvironmental proxy in metasedimentary rocks
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DOI：
10.1016/j.lithos.2017.10.007
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发表时间：
2017-12
期刊：
影响因子：
3.5
通讯作者：
A. Cabral;A. Zeh;Nívea Cristina da Silva Viana;T. Schirmer;B. Lehmann
中科院分区：
文献类型：
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作者：
A. Cabral;A. Zeh;Nívea Cristina da Silva Viana;T. Schirmer;B. Lehmann
原始富有机碳沉积岩中钼的富集构成了推断海水中溶解氧存在的基础。有机物从海水中去除溶解的六价钼，在缺氧和缺氧条件下实现。然而，目前还不清楚这种钼基代用品是否保留在变质条件下，有机碳不再保留。在这里，我们描述了石墨和含高达21质量%的W的辉钨矿（WS 2）在固溶体中的聚集体。这些聚集体散布在富含硫化物的锰硅酸盐碳酸盐岩（queluzite），变质角闪岩相条件下的巴西米纳斯吉拉斯的Barbacena绿岩带内。我们的发现表明：（i）W和Mo一样，是古环境代用指标;（ii）W代用指标对高fS 2/fO 2环境敏感;（iii）Mo和W代用指标都以（Mo，W）S2与石墨共生的形式在角闪岩相叠印中保存下来。太古代绿岩以（Mo，W）S2-石墨共生体的形式存储古环境信息。
Molybdenum enrichment in pristine organic-C-rich sedimentary rocks forms the basis for inferring the presence of dissolved oxygen in seawater. Organic matter removes dissolved hexavalent Mo from seawater where anoxic and euxinic conditions are attained. However, it is unknown whether this Mo-based proxy is retained under metamorphic conditions where organic C is no longer preserved. Here, we describe aggregates of graphite and molybdenite (MoS2) containing up to 21 mass per cent of W as tungstenite (WS2) in solid solution. These aggregates are disseminated in a sulfide-rich Mn-silicate-carbonate rock (queluzite), metamorphosed under amphibolite-facies conditions within the Archaean Barbacena greenstone belt in Minas Gerais, Brazil. Our finding indicates that: (i) W is, like Mo, a palaeoenvironmental proxy; (ii) the W proxy is sensitive to highfS2/fO2environments; (iii) both Mo and W proxies survive amphibolite-facies overprint as (Mo,W)S2intergrown with graphite. Archaean greenstones are potential candidates for storing palaeoenvironmental information as (Mo,W)S2–graphite intergrowths.