Ca/Sr and 87Sr/86Sr geochemistry of disseminated calcite in Himalayan silicate rocks from Nanga Parbat: Influence on river-water chemistry

Abstract

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 is ≤ 0.29 wt% of silicate rocks sampled in the Raikhot watershed and has Ca/Sr (µmol/nmol) and 87Sr/86Sr 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 ≤ 3.6 wt% of the silicate rock samples and has Ca/Sr (µmol/nmol) and 87Sr/86Sr ratios that range from 0.535 to 3.33 and from 0.715 757 to 0.771 244, respectively. For all samples, the 87Sr/86Sr ratios of disseminated calcite are similar to the 87Sr/86Sr ratios measured in the silicate host rock. Within the partially glaciated Raikhot watershed, the rapid weathering of disseminated calcite with high Ca/Sr and 87Sr/86Sr 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.