Strontium, hydrothermal systems and steady-state chemical weathering in active mountain belts

Hydrothermal systems are a common feature of active collisional orogens. These systems can have a profound effect on the Sr fluxes and Sr isotopic composition of riverine waters through the direct input of Sr from hot springs and groundwaters and the chemical weathering of hydrothermal calcite. In an effort to quantify the role of calcite weathering we present an analytical model that examines the relationship between rock uplift and steady-state chemical weathering of granites and greywackes that contain trace amounts (< 3%) of calcite. We also tested the affect of different mineral dissolution rates and production of secondary minerals on Sr fluxes. This model predicts that in areas of high rock uplift such as collisional mountain belts as much as 60% of the Sr released during chemical weathering comes from the weathering of small amounts of hydrothermal calcite. Moreover, the Sr isotopic composition of weathered material remains constant over a wide range of tectonic environments and is insensitive to the proportion of calcite to silicate weathering. As such, the Sr isotopic composition of river waters will not be an adequate proxy for silicate weathering in tectonically active orogens if calcite is present, and it is these very orogens that have the highest chemical weathering rates. We compare model results with Sr fluxes and isotopic composition of riverine waters from the Southern Alps of New Zealand, an area that is frequently used as an example of a steady-state collisional orogen. As our model predicts, roughly 50% of Sr flux is from the weathering of calcite in the areas of highest uplift along the west-flank of the Southern Alps. This proportion decreases substantially (< 20% from calcite) in the areas of lower uplift east of the Southern Alps. Because chemical weathering rates are approximately a factor of two higher in the rapidly uplifting areas as compared to the more stable regions a significant portion of strontium flux from rivers on South Island of New Zealand is from the weathering of calcite.