Magnesium isotope composition of sabkha porewater and related (Sub-)Recent stoichiometric dolomites, Abu Dhabi (UAE)
Magnesium isotope composition of sabkha porewater and related (Sub-)Recent stoichiometric dolomites, Abu Dhabi (UAE)
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DOI：
10.1016/j.chemgeo.2014.11.020
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发表时间：
2015-01
期刊：
影响因子：
3.9
通讯作者：
A. Geske;S. Lokier;M. Dietzel;D. Richter;D. Buhl;A. Immenhauser
中科院分区：
文献类型：
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作者：
A. Geske;S. Lokier;M. Dietzel;D. Richter;D. Buhl;A. Immenhauser
最早的海相成岩的萨布哈型碳酸盐岩在地球的地质记录中广泛分布。这些碳酸盐可能作为过去海水镁同位素（δ 26 Mg）比率的档案。然而，目前，镁同位素（Δ 26 Mg）在海水（这里是蒸发的海洋孔隙水）和萨布哈白云岩之间的分馏没有受到限制。为了探索Δ 26 Mgdol-Mg（孔隙水），我们利用了海湾地区（Trucial海岸，阿拉伯联合酋长国）实际的Sabkha型白云岩沉淀。本文记录并讨论了第一个详细的含镁固体的sabkha δ 26 Mg数据集，包括化学计量的辉长岩（有序度> 0.9;平均δ 26 Mgdol = − 0.79‰ ± 0.41 2σ，n = 17）和相关的海洋孔隙沃茨。X射线衍射和扫描电子显微镜证实了在浅岩心中存在具有广泛有序性的白云石晶体，但单个晶体太小（< 10 μm），无法从其宿主沉积物中机械分离。因此，开发了一种方法，通过使用乙二胺四乙酸二钠化学分离最化学计量的白云石晶体从共存的化学计量较少的白云石和其他含镁矿物和流体相。从海沟中采集的萨布哈浅层地下水的δ 26 Mg值约为-0.59 ‰，即相对于现今海水的-0.83 ‰，仅中度富集26 Mg。蒸发孔隙水δ 26 Mg相对海水δ 26 Mg富集+0.43 ‰。在这里，我们使用术语“表观”分馏变量Δ 26 Mgdol-Mg（孔隙水）（+0.1和-0.7 ‰），无法与实验推导的分馏因子从受控实验室设置进行比较。此外，蒸发的塞卜哈孔隙水在同位素特征方面与海水不同。就目前的知识水平而言，我们的数据集不支持最早的成岩sabkha镁铁质岩代表海水δ 26 Mg值随时间长期变化的直接档案的假设。我们目前的理解是，镁同位素签名的sabkha镁橄榄石是有关复杂的动力学的前体形成，溶解/沉淀反应，包括微生物的影响，并涉及可变的镁源和汇在时间和空间上可变的微环境。
Earliest marine diagenetic sabkha type dolomites are widespread in Earth's geological record. Potentially, these carbonates may act as archives of past seawater magnesium isotope (δ26Mg) ratios. At present, however, the fractionation of magnesium isotopes (Δ26Mg) between seawater – here evaporated marine porewater – and sabkha dolomite is not constrained. In order to explore Δ26Mgdol-Mg(porewater), we make use of actualistic sabkha type dolomite precipitation in the Gulf region (Trucial coast, United Arab Emirates). This paper documents and discusses the first detailed sabkha δ26Mg data set of Mg-bearing solids including stoichiometric dolomites (degree of ordering > 0.9; mean δ26Mgdol= − 0.79‰ ± 0.41 2σ, n = 17) and related marine pore waters. The presence of dolomite crystals with a broad range of ordering in shallow cores is documented using X-ray diffraction and scanning electron microscopy, but individual crystals are too small (< 10 μm) to be mechanically separated from their host sediment. Hence, a method was developed to chemically separate the most stoichiometric dolomite crystals from coexisting less stoichiometric dolomites and other Mg-bearing minerals and fluid phases by using disodium ethylenediaminetetracetic acid. Sabkha shallow ground water collected in trenches displays δ26Mg values of about − 0.59‰, i.e. is only moderately enriched in26Mg relative to the present-day seawater signature of − 0.83‰. Conversely, the δ26Mg of evaporated porewater is enriched by + 0.43‰ relative to that of δ26Mgseawater. Here we use the term “apparent” fractionation for the variable Δ26Mgdol-Mg(porewater)(+ 0.1 and − 0.7‰) obtained that cannot be compared with experimentally deduced fractionation factors from controlled laboratory settings. Moreover, evaporated sabkha porewater differs, in terms of its isotope signature, from seawater. With regard to the current level of knowledge, the hypothesis that earliest diagenetic sabkha dolomites represent direct archives of secular changes of seawater δ26Mg values with time is not supported by our dataset. Our present understanding is that the magnesium isotope signature of sabkha dolomites is related to complex kinetics of precursor formation, dissolution/precipitation reactions including microbiological effects and involves variable Mg sources and sinks in a temporally and spatially variable microenvironment.