SR-87 SR-86 RATIOS OF SIERRA-NEVADA STREAM WATERS - IMPLICATIONS FOR RELATIVE MINERAL WEATHERING RATES/

The Sr-87/Sr-86 ratios of dissolved strontium in six Sierra Nevada str eams at base flow are 0.00070 to 0.00175 higher than the average whole -rock value for the granodiorite bedrock underlaying the drainage basi ns. We suggest that the strontium isotope ratios and major cation comp ositions of the stream waters are controlled predominantly by the weat hering reactions: plagioclase double right arrow kaolinite and biotite double right arrow vermiculite. Stream drainages that were glaciated similar to 10 kyr ago (exposing fresh bedrock surfaces) have Sr-87/Sr- 86 ratios 0.00036 to 0.00105 higher than drainages that were not glaci ated in the past similar to 100 kyr. We interpret the Sr-87/Sr-86 rati os of the stream water to indicate that biotite is weathering similar to 6 times and similar to 4 times more rapidly than plagioclase in the recently glaciated and nonglaciated drainages, respectively. Our resu lts suggest that continental glaciation may have the effect of acceler ating biotite weathering, significantly elevating riverine Sr-87/Sr-86 ratios in regions draining silicate bedrock for periods of similar to 100 kyr.