STRONTIUM ISOTOPES AS INDICATORS OF MINERAL WEATHERING IN CATCHMENTS

Determination of the Sr-87/Sr-86 ratio in rain-water, stream-water, so ils and rocks in two Scottish catchments has provided useful informati on on mineral weathering. Over a period of 26 months, the Sr-87/Sr-86 ratio in rain-water had mean values of 0.7095 and 0.7098 (n = 12) wher eas stream-water had mean values of 0.7080 (n = 16, in catchment in De vonian andesites with strontium isotope ratios of 0.7117-0.7232 in the rocks and soils) and 0.7184 (n = 16, in catchment of Cambrian metamor phic rocks with strontium isotope ratios of 0.7360-0.8218 in the rocks and soils), respectively. The mineralogy consists predominantly of va rying proportions of quartz, plagioclase feldspar and K-feldspar with some mica and chlorite. The fact that the Sr-87/Sr-86 ratios in the st ream in the andesitic catchment are lower than for the rain-water prob ably indicates preferential weathering of plagioclase feldspar releasi ng Sr-86, rather than K-feldspar and mica which contain greater amount s of Sr-87. The Sr-87/Sr-86 ratio acts as a tracer for the origin of t he solutes in the stream-water. Long-term monitoring of the strontium isotope ratio in stream-water could be a potential means of detecting relative changes in soil weathering rates due to environmental change.