RB-SR ISOTOPE SYSTEMATICS OF A GRANITIC SOIL CHRONOSEQUENCE - THE IMPORTANCE OF BIOTITE WEATHERING

The Rb-Sr isotope systematics of bedrock, soil digests, and the cation exchange fraction of soils from a granitic glacial soil chronosequenc e in the Wind River Mountains, Wyoming, USA, were investigated. Six so il profiles ranging in age from 0.4 to similar to 300 kyr were studied and revealed that the Sr-87/Sr-86 ratio of exchangeable strontium in the B-horizons decreased from 0.7947 to 0.7114 with increasing soil ag e. Soil digests of the same samples showed much smaller variation in S r-87/Sr-86 from 0.7272 to 0.7103 and also generally decreased with inc reasing soil age. Elevation of the Sr-87/Sr-86 ratios of Sr released b y weathering over the soil digest and bedrock values results from the rapid weathering of biotite to form hydrobiotite and vermiculite in th e younger soils. Biotite is estimated to weather at approximately eigh t times the rate of plagioclase (per gram of mineral) in the youngest soil profile and decreases to a rate of only similar to 20% of that of plagioclase in the oldest soil. Rb-87/Sr-86 ratios of the soil cation exchange fraction are estimated to be depleted by factors of up to 11 over the Rb-87/Sr-86 ratios released by weathering, due to ion exchan ge partitioning. This study demonstrates that the Sr-87/Sr-86 ratio re leased by weathering of crystalline rocks can deviate significantly fr om bedrock values, and that in soils less than similar to 20 kyr in ag e which contain biotite in the soil parent material, weathering-derive d Sr-87/Sr-86 values can be elevated so dramatically that this factor must be considered in estimations of weathering rates based on stronti um isotopes. Copyright (C) 1997 Elsevier Science Ltd.