PGE, Re-Os, and Mo isotope systematics in Archean and early Proterozoic sedimentary systems as proxies for redox conditions of the early Earth
PGE, Re-Os, and Mo isotope systematics in Archean and early Proterozoic sedimentary systems as proxies for redox conditions of the early Earth
复制标题
DOI：
10.1016/j.gca.2004.10.006
复制
发表时间：
2005-04
影响因子：
5
通讯作者：
C. Siebert;J. Kramers;T. Meisel;P. Morel;T. Nägler
中科院分区：
文献类型：
--
作者：
C. Siebert;J. Kramers;T. Meisel;P. Morel;T. Nägler
本文报道了南非巴伯顿绿岩带（Fig Tree和Moodies群，3.25 - 3.15 Ga）、津巴布韦贝林圭绿岩带（Manjeri组，3.25-3.15 Ga）、南非和津巴布韦的细粒碎屑沉积物和黑色页岩的Re-Os数据、PGE浓度、Mo浓度和同位素数据。2.7 Ga）和页岩，来自南非威特沃特斯兰德、文特斯多普和德兰士瓦超群，范围为2.95 - 2.2 Ga。适度的氧化条件下，需要动员Re和Mo在环境中，钼分馏只发生在溶液中，这些参数，因此有潜在的使用作为古氧化还原代理早期地球。巴伯顿绿岩带沉积物的PGE + Re丰度模式与科马提岩的PGE + Re丰度模式形状相似，分布均匀。这表明：（1）PGE来自主要由超镁铁质组成的来源，（2）PGE基本上是以颗粒形式运输和沉积的。来自年轻的贝林圭绿岩带的沉积物显示出更多的分馏PGE + Re模式，Re/Os比比巴伯顿沉积物高10至100倍。它们的铂族元素丰度模式和Re/Os比值介于中太古代页岩和新元古代到近代黑色页岩之间。它们反映了沉积环境中Re从溶液中的清除作用。各时代黑色页岩的δ98/95 Mo值与其浓度相关。巴伯顿绿岩带样品中钼的含量为0.1 -3 ppm，类似于花岗岩-玄武质来源。这个Mo的δ98/95 Mo相对于现今平均海水钼（莫莫）介于−1.9和−2.4‰之间，因此相对于这样的来源没有同位素分馏。与PGE类似，这表明以固体形式运输。贝林圭绿岩带的沉积物显示出部分钼浓度（高达6 ppm）和钼同位素分馏（δ98/95 Mo高达−1.4‰，相对于莫莫）。PGE + Re和Mo的综合数据主要显示中太古代的还原条件，并表明到2.7 Ga，大气和海洋的氧化程度变得更加严重。来自德兰士瓦超级群的相当年轻的样本（约。2.2 Ga）令人惊讶地主要具有低Mo浓度（约1 ppm），并且相对于大陆源没有显示出显著的Mo同位素分馏。其中可能的解释是，2.7 Ga后，水库的影响，或钼去除硫化物沉淀硫酸盐还原后，在早元古代海洋还原大气条件的回报。
Re-Os data and PGE concentrations as well as Mo concentrations and isotope data are reported for suites of fine clastic sediments and black shales from the Barberton Greenstone Belt, South Africa (Fig Tree and Moodies Groups, 3.25–3.15 Ga), the Belingwe Greenstone Belt, Zimbabwe (Manjeri Formation, ca. 2.7 Ga) and shales from the Witwatersrand, Ventersdorp and Transvaal Supergroups, South Africa ranging from 2.95 to 2.2 Ga. Moderately oxidizing conditions are required to mobilize Re and Mo in the environment, Mo fractionation only occurs in solution, and these parameters thus have potential use as paleoredox proxies for the early Earth. PGE + Re abundance patterns of Barberton Greenstone Belt sediments are uniform and very similar in shape to those of komatiites. This indicates (1) that the PGE came from a source of predominantly ultramafic composition and, (2) that PGE were transported and deposited essentially in particulate form. Sediments from the younger Belingwe Greenstone Belt show more fractionated PGE + Re patterns and have Re/Os ratios 10 to 100× higher than those of Barberton sediments. Their PGE abundance patterns and Re/Os ratios are intermediate between those of the mid-Archean shales and Neoproterozoic to Recent black shales. They reflect scavenging of Re from solution in the sedimentary environment. δ98/95Mo values of black shales of all ages correlate with their concentrations. The Barberton Greenstone Belt samples have ∼1–3 ppm Mo, similar to a granitoid-basaltic source. This Mo has δ98/95Mo between −1.9 and −2.4‰ relative to present day mean ocean water molybdenum, MOMO and is thus not isotopically fractionated relative to such a source. Similar to the PGE this indicates transport in solid form. Sediments from the Belingwe Greenstone Belt show in part enhanced Mo concentrations (up to 6 ppm) and Mo isotope fractionation (δ98/95Mo up to −1.4‰ relative to MOMO). The combined PGE + Re and Mo data show mainly reducing conditions in the mid-Archean and suggest that by 2.7 Ga, the atmosphere and oceans had become more oxidizing. Substantially younger samples from the Transvaal Supergroup (to ca. 2.2 Ga) surprisingly have mainly low Mo concentrations (around 1 ppm) and show no significant Mo isotope fractionation relative to the continental source. Among possible explanations for this are a return to reducing atmospheric conditions after 2.7 Ga, reservoir effects, or Mo removal by sulfide precipitation following sulfate reduction in early Proterozoic oceans.