Cretaceous seawater and hydrothermal fluid compositions recorded in abiogenic carbonates from the Troodos Ophiolite, Cyprus
Cretaceous seawater and hydrothermal fluid compositions recorded in abiogenic carbonates from the Troodos Ophiolite, Cyprus
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DOI：
10.1016/j.chemgeo.2018.07.014
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发表时间：
2018-09
期刊：
影响因子：
3.9
通讯作者：
C. Weinzierl;M. Regelous;K. Haase;W. Bach;F. Böhm;D. Garbe‐Schönberg;Yadong Sun;M. Joachimski;S. Krumm
中科院分区：
文献类型：
--
作者：
C. Weinzierl;M. Regelous;K. Haase;W. Bach;F. Böhm;D. Garbe‐Schönberg;Yadong Sun;M. Joachimski;S. Krumm
通过分析塞浦路斯91 Ma Troodos蛇绿岩洋壳内流体沉淀的非生物成因碳酸盐的化学和同位素组成，我们评估了蛇绿岩作为古海水和热液流体组成档案的潜力。蛇绿岩上段流体Mg/Ca、Sr/Ca和87sr /86Sr随温度变化的计算结果与钻探海洋地壳的结果相似，并得到了晚白垩世海水Mg/Ca、Sr/Ca和87sr /86Sr的文献值。这表明蛇绿岩中的碳酸盐可以用来估计古代海水的组成，而不是最古老的保存在原位的海洋地壳年龄。虽然从原位海洋地壳中回收的大多数碳酸盐在<60 °C的温度下沉淀，但Troodos蛇绿岩中的非生物成因碳酸盐在7 °C至218 °C的温度范围内形成。这些提供了独特的见解，了解化学和矿物学的过程，海水转化为海洋地壳内的高温热液流体。我们利用Mg/Ca、Sr/Ca、87Sr/86Sr和δ44/40Ca相对于计算温度（δ18O）的“热液变化图”来追踪Troodos洋壳内部的流体演化。研究发现，连续的流体—地壳相互作用、含镁、含钙矿物的沉淀和硬石膏的早期形成（bbb44 °C）使白垩纪海水逐渐转变为Troodos热液。Troodos数据与全球海洋地壳非生物成因碳酸盐岩数据对比表明，所有白垩纪遗址的低温流体演化化学路径相似。这些不同的遗址代表了不同的大地构造环境（洋中脊vs.俯冲带），具有不同的基底成分（玄武岩、玄武岩安山岩/博宁岩），位于不同的海洋盆地（大西洋、太平洋、地中海[特提斯]）。碳酸盐记录的相似性表明，这些差异对海底低温风化和热液蚀变影响不大。然而，来自较年轻海洋地壳的非生物成因碳酸盐岩与白垩纪趋势不同，遵循不同的流体演化路径。这表明，海水成分的时间变化可能控制着海底低温风化和热液蚀变的性质和程度。将现代和白垩纪海水加热至200 °C，并在其他条件相同的情况下依次加入平均的Troodos玄武岩安山岩的流体-地壳相互作用热力学模型预测了白垩纪海洋地壳的流体演化和蚀变与今天不同，并且在中等流体温度之前，初始海水化学影响着海底蚀变的性质和程度。例如，与现代相比，白垩纪海洋地壳蚀变过程中形成的碳酸盐数量是现代的两倍，这表明白垩纪从水圈-大气系统进入海洋地壳的二氧化碳通量比现在要大，并且在整个地质时期可能是不同的。
We evaluate the potential of ophiolites as archives of paleoseawater and hydrothermal fluid compositions by analysing the chemical and isotopic composition of abiogenic carbonates, precipitated from fluids within the oceanic crust of the 91 Ma Troodos Ophiolite, Cyprus. Calculated variations in fluid Mg/Ca, Sr/Ca and87Sr/86Sr with temperature within the upper sections of the ophiolite are similar to those from drilled oceanic crust, and yield literature values for late Cretaceous seawater Mg/Ca, Sr/Ca and87Sr/86Sr. This indicates that carbonates from ophiolites could be used to estimate the composition of ancient seawater at times before the age of the oldest preserved in-situ oceanic crust. Whereas most carbonates recovered from in-situ oceanic crust were precipitated at temperatures <60 °C, abiogenic carbonates from the Troodos Ophiolite formed over a temperature range of 7 °C to 218 °C. These provide unique insights into the chemical and mineralogical processes that transform seawater into a high temperature hydrothermal fluid within the oceanic crust. We use ‘hydrothermal variation diagrams’ of Mg/Ca, Sr/Ca,87Sr/86Sr and δ44/40Ca versus calculated temperature (δ18O) to trace this fluid evolution within the Troodos oceanic crust. We find that successive fluid-crust-interaction, the precipitation of Mg- and Ca-bearing minerals and the early formation of anhydrite (>44 °C) gradually transform Cretaceous seawater into a Troodos hydrothermal fluid. Comparison of the Troodos data with a global dataset of abiogenic carbonates from in-situ oceanic crust shows that the chemical pathways of low-temperature fluid evolution are similar for all Cretaceous sites. These different sites represent varied geotectonic settings (mid-ocean ridge vs. suprasubduction zone), with different basement composition (basalt, basaltic andesite/boninite) and situated in different ocean basins (Atlantic, Pacific, Mediterranean [Tethys]). The similarity in the carbonate record indicates that these differences do not significantly influence seafloor weathering and hydrothermal alteration at low temperatures. However, abiogenic carbonates from younger oceanic crust differ from the Cretaceous trends and follow different fluid evolution pathways. This indicates, that temporal variations in the composition of seawater may control the nature and the extent of seafloor weathering and hydrothermal alteration at low temperatures. A thermodynamic model of fluid-crust interaction, in which modern and Cretaceous seawater are heated to 200 °C while an average Troodos basaltic andesite is successively added under otherwise identical conditions predicts that fluid evolution and alteration of the oceanic crust were different in the Cretaceous than they are today, and that initial seawater chemistry affects the nature and the extent of seafloor alteration up to moderate fluid temperatures. For example, twice the amount of carbonate formed during alteration of the oceanic crust in the Cretaceous compared to modern times, indicating that the flux of CO2from the hydrosphere-atmosphere system into the oceanic crust was greater in the Cretaceous than it is nowadays, and that it probably varied throughout geologic time.