Relative contributions of silicate and carbonate rocks to riverine Sr fluxes in the headwaters of the Ganges

摘要: Abstract Exhumation of the Himalayan-Tibetan orogen is implicated in marked rise seawater 87Sr/86Sr ratios since 40 Ma. However both silicate and carbonate rocks Himalaya have elevated there disagreement as to how much 87Sr flux derived from weathering. Most previous studies used element bedrock constrain proportions silicate- carbonate-derived Sr river waters. Here we use arrays water compositions sampled head waters Ganges Indian Nepalese end-member ratios. The tributaries draining catchments restricted a limited range geological units can be described by two-component mixing components lie on plane multicomponent composition space. Key elemental are determined intersection tributary with planes Na = 0 for constant Ca/Na silicate. fractions sources then calculated mass-balance Sr-Ca-Mg-Na Comparison implies that secondary calcite deposition may important some dissolution low-Mg trace explain discrepancies Sr-Ca-Na-Mg covariation. Alternatively, composition-dependent precipitation or incongruent reactions rotate trends cation-ratio diagrams. calculations not sensitive transformations processes provided transformed component vectors constrained. Silicates provide ∼50% dissolved assuming between Ca-Mg-Na covariation rock arise addition calcite. If rotated deposition, this estimate would increased. Moreover, when inputs considered, responsible 70 ± 16% (1σ) forcing changes Sr-isotopic composition. Since earlier predict weathering generates little 20% total Himalayan systems, study demonstrates significance greatly underestimated if decouple cation those source properly evaluated.