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首页> 外文期刊>Earth and Planetary Science Letters: A Letter Journal Devoted to the Development in Time of the Earth and Planetary System >Pb, O, and C isotopes in silicified Mooidraai dolomite (Transvaal Supergroup, South Africa): implications for the composition of Paleoproterozoic seawater and 'dating' the increase of oxygen in the Precambrian atmosphere
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Pb, O, and C isotopes in silicified Mooidraai dolomite (Transvaal Supergroup, South Africa): implications for the composition of Paleoproterozoic seawater and 'dating' the increase of oxygen in the Precambrian atmosphere

机译：硅化的Mooidraai白云岩中的Pb，O和C同位素（南非德兰士瓦超级集团）：对古元古代海水成分的影响和“约会”前寒武纪大气中氧气的增加

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摘要

The Mooidraai Dolomite Formation is a unit of marine sedimentary carbonates in the upper Transvaal Supergroup which recorded significant changes in the composition of the Earth's atmosphere and ocean. Previously available data had suggested that this dolomite was about 2.2 Ga old and showed #delta#~(13)C_(carb) values around 0.8% PDB, and hence was an exception to the positive excursion of #delta#~(13)C_(carb) values observed worldwide in marine carbonates deposited between 2.25 and 2.05 Ga ago. We studied the Pb-O-C isotope systematics of drill core samples from a highly silicified and aluminosilicate-free sub-unit of the Mooidraai Dolomite Formation, that yields well-preserved micritic dolomite grains. Selective leaching of the silicified dolomite revealed a significant Pb isotopic contrast between the carbonate and the quartz fraction. The former shows only a narrow range of ~(208)Pb/~(204)Pb (35.35-35.58) which does not correlate with ~(206)Pb/~(204)Pb, indicating the absence of Th-derived detrital Pb. The carbonate-bound Pb defines an isochron that corresponds to an age of 2394 +- 26 Ma (2#sigma#; MSWD =1.8, n=22) which we interpret as the diagenetic age of the Mooidraai dolomite. The #delta#~(13)C_(carb) values range from 0.51 to 0.64 per thousand PDB and confirm the previous data. The #delta#~(18)O_(carb) values range from -2.08 to +0.18 per thousand PDB and are amongst the heaviest reported yet from Early Precambrian marine sedimentary carbonates. The quartz fraction is considerably lighter than the carbonate fraction, and shows #delta#~(18)O_(qtz) values between -9.22 and -6.73 per thousand PDB (+21.40 and +23.97 per thousand SMOW). Together with evidence from fluid inclusion microthermometry suggesting that the quartz formed at minimum temperatures between 130 and 155 deg C, this indicates that the intense silicification was post-depositional. Comparison of #delta#~(18)O_(carb) values of silicified and non-silicified Mooidraai samples suggests that the post-depositional silicification had sealed the system and thereby protected the primary isotopic compositions of the micritic dolomite. The heavy #delta#~(18)O_(carb) values suggest that 2.4 Ga ago the oxygen isotopic composition of seawater differed only little from that of today. The new Pb-Pb carbonate age for the Mooidraai dolomite indicates that the upper Transvaal Supergroup is about 200 my older than previously thought. This explains the 'normal' carbon isotopic composition of the Mooidraai dolomite, and suggests that the pronounced increase of the oxygen content in the Precambrian Earth's atmosphere that is seen in strata underneath the Mooidraai dolomite occurred before 2.4 Ga ago. On this background, it appears unlikely that the positive excursion of the carbon isotopic ratios of 2.25-2.05 Ga old marine sedimentary carbonates is related to this increase in atmospheric oxygen.
机译：Mooidraai白云岩组是上德瓦瓦尔超群中海洋沉积碳酸盐的一个单元，记录了地球大气和海洋组成的重大变化。先前的可用数据表明，该白云石的年龄约为2.2 Ga，并且显示＃delta＃〜（13）C_（carb）值约为0.8％PDB，因此是＃delta＃〜（13）C_的正偏移的一个例外。全世界在2.25至2.05 Ga以前沉积的海洋碳酸盐中观测到的（carb）值。我们研究了Mooidraai白云岩层中高度硅化且不含硅铝酸盐的亚单元的钻芯样品中的Pb-O-C同位素系统，该系统可保存完好的变质白云岩晶粒。硅化白云石的选择性浸出表明碳酸盐和石英级分之间存在明显的Pb同位素对比。前者仅显示〜（208）Pb /〜（204）Pb的窄范围（35.35-35.58），与〜（206）Pb /〜（204）Pb不相关，表明不存在Th衍生的碎屑Pb 。碳酸盐结合的铅定义了一个等时线，它对应于2394±26 Ma（2＃sigma＃; MSWD = 1.8，n = 22）的年龄，我们将其解释为Mooidraai白云岩的成岩年龄。 ＃delta＃〜（13）C_（carb）值的范围为每千个PDB 0.51至0.64，并确认先前的数据。 δ＃〜（18）O_（carb）值的范围是每千个PDB -2.08至+0.18，是前寒武纪早期海洋沉积碳酸盐中迄今报道的最重的值。石英馏分比碳酸盐馏分轻得多，并且每千个PDB的＃delta ~~（18）O_（qtz）值在-9.22和-6.73之间（每千SMDB分别在+21.40和+23.97之间）。连同流体夹杂显微温度计的证据表明，石英是在130至155摄氏度之间的最低温度下形成的，这表明强烈的硅化作用是沉积后的。硅化和非硅化的Mooidraai样品的δ＃〜（18）O_（carb）值的比较表明，沉积后的硅化作用已封闭了系统，从而保护了白云岩的主要同位素组成。重的δ-（18）O_（碳水化合物）值表明2.4 Ga以前海水的氧同位素组成与今天的差别很小。 Mooidraai白云岩的新的Pb-Pb碳酸盐年龄表明，上部的瓦斯瓦瓦尔超群比以前认为的年龄大了200我。这解释了Mooidraai白云岩的“正常”碳同位素组成，并表明在Mooidraai白云岩下面的地层中看到的前寒武纪地球大气中氧含量的明显增加发生在2.4 Ga之前。在这种背景下，似乎不太可能将2.25-2.05 Ga老海洋沉积碳酸盐的碳同位素比正偏移与大气氧的这种增加相关。

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