87SR/86SR INVESTIGATION OF WEATHERING REACTIONS IN A TROPICAL GRANITOID WATERSHED

We studied strontium isotope systematics in the Rio Icacos watershed in Puerto Rico in order to trace the relative contributions of individual minerals to weathering fluxes. This study compliments work done with 234U/238U disequilibria and Ge/Si ratios in the same system, providing critical information about the mechanisms of biotite weathering in particular, and allowing us to trace changing sources of Sr to streamwater during storm events.

We analyzed 87Sr/86Sr ratios in primary plagioclase, biotite, and hornblende from the Tertiary quartz diorite bedrock, as well as secondary kaolinite and altered biotite mineral separates from the soil and saprolite profile. We also measured 87Sr/86Sr ratios of 10 porewater samples taken from tension lysimeters at a range of depths, from 853 cm near the bedrock-saprolite interface to 15 cm near the soil surface. Streamwater samples from the catchment were also analyzed, under both baseflow and stormflow conditions.

Streamwaters and the deepest porewaters have low 87Sr/86Sr ratios (0.7052 to 0.7063) and high [Sr], primarily reflecting plagioclase (0.7042, 560 ppm Sr) and hornblende (0.7058, 80 ppm Sr) weathering at the bedrock-saprolite interface. Soil porewater 87Sr/86Sr ratios steadily become increasingly radiogenic (higher value) with decreasing depth, reaching a value of 0.712 near the surface. This reflects a combination of the increasing contribution of biotite weathering, and the contribution of sea salt aerosol Sr, which is assumed to have the seawater 87Sr/86Sr ratio of 0.7092. Primary bedrock biotite has a highly radiogenic 87Sr/86Sr ratio of 0.7827, altered biotite from the saprolite (505 cm) has an intermediate value 0.7307, and the altered biotite present in the soil (66 cm) has a 87Sr/86Sr ratio of 0.7055. We suggest that the radiogenic interlayer-site Sr in the altered biotite is gradually replaced with unradiogenic local porewater Sr during weathering. Kaolinite has a 87Sr/86Sr ratio of 0.7065, similar to the porewater value near the saprolite-bedrock interface, where the majority of the kaolinite is formed from the weathering of the plagioclase. A sequence of 10 samples measured through a stormflow event showed increasing and then decreasing 87Sr/86Sr ratios, reflecting an increase in shallower soil and saprolite-derived Sr during peak flow.