Ca/Sr and 87Sr/86Sr geochemistry of disseminated calcite in Himalayan silicate rocks from Nanga Parbat:: Influence on river-water chemistry
Ca/Sr and 87Sr/86Sr geochemistry of disseminated calcite in Himalayan silicate rocks from Nanga Parbat:: Influence on river-water chemistry
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DOI：
10.1130/0091-7613(2000)28
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发表时间：
2000-05-01
期刊：
影响因子：
5.8
通讯作者：
Blum, JD
中科院分区：
文献类型：
--
作者：
Jacobson, AD;Blum, JD
从巴基斯坦北方南迦帕尔巴特峰内的Raikhot流域和其他地点取样的片麻岩、片岩和花岗岩基岩中发现了微量的浸染状方解石。方解石颗粒主要存在于单个硅酸盐矿物中，位于颗粒边界处，并作为断裂填充物横切岩石的矿物学结构，浸染状方解石占Raikhot流域硅酸盐岩样品的重量百分比小于或等于0.29%，Ca/Sr（μ mol/nmol）和Sr-87/Sr-86比值分别为0.878 - 5.33和0.794 039 - 0.930 619。在南迦帕尔巴特地区的其他地方，浸染状方解石占硅酸盐岩石样品的重量百分比小于或等于3.6%，Ca/Sr（μ mol/nmol）和Sr-87/Sr-86比值分别为0.535 - 3.33和0.715 757 - 0.771 244。对于所有的样品，Sr-87/Sr-86比率的浸染方解石是相似的Sr-87/Sr-86比率测量的硅酸盐寄主岩石。在部分冰川化的Raikhot流域，快速风化的浸染方解石与高Ca/Sr和Sr-87/Sr-86的比例有很强的影响，对河水的化学成分，并超过了硅酸盐矿物溶解的贡献。浸染方解石组合物与整个喜马拉雅地区的源水化学的比较表明，浸染方解石可能是一个更重要的组成部分喜马拉雅硅酸盐岩石比以前认识到的。因此，计算有关的Sr同位素地球化学的喜马拉雅河流，大气CO2消耗应考虑这种广泛的和成分可变的碳酸盐端成员。
Trace amounts of disseminated calcite were identified in gneiss, schist, and granite bedrock sampled from the Raikhot watershed and other locations within the Nanga Parbat massif of northern Pakistan. The calcite grains occur interstitially within individual silicate minerals, at grain boundaries, and as fracture fillings that transect the mineralogic fabric of the rock, Disseminated calcite composes less than or equal to 0.29 wt% of silicate rocks sampled in the Raikhot watershed and has Ca/Sr (mu mol/nmol) and Sr-87/Sr-86 ratios that range from 0.878 to 5.33 and from 0.794 039 to 0.930 619, respectively. Elsewhere in the Nanga Parbat region, disseminated calcite composes less than or equal to 3.6 wt% of the silicate rock samples and has Ca/Sr (mu mol/nmol) and Sr-87/Sr-86 ratios that range from 0.535 to 3.33 and from 0.715 757 to 0.771 244, respectively. For all samples, the Sr-87/Sr-86 ratios of disseminated calcite are similar to the Sr-87/Sr-86 ratios measured in the silicate host rock. Within the partially glaciated Raikhot watershed, the rapid weathering of disseminated calcite with high Ca/Sr and Sr-87/Sr-86 ratios has a strong influence on the chemical composition of stream water and exceeds contributions from silicate mineral dissolution. Comparisons of disseminated calcite compositions with source-water chemistry throughout the Himalaya suggest that disseminated calcite may be a more important component of Himalayan silicate rocks than previously recognized. Therefore, calculations relating the Sr isotope geochemistry of Himalayan rivers to atmospheric CO2 consumption should consider this widespread and compositionally variable carbonate end member.