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 chronosequence in the Wind River Mountains, Wyoming, USA, were investigated. Six soil profiles ranging in age from 0.4 to ∼300 kyr were studied and revealed that the 87Sr86Sr ratio of exchangeable strontium in the B-horizons decreased from 0.7947 to 0.7114 with increasing soil age. Soil digests of the same samples showed much smaller variation in 87Sr86Sr from 0.7272 to 0.7103 and also generally decreased with increasing soil age. Elevation of the 87Sr86Sr ratios of Sr released by weathering over the soil digest and bedrock values results from the rapid weathering of biotite to form hydrobiotite and vermiculite in the younger soils. Biotite is estimated to weather at aaproximately eight times the rate of plagioclase (per gram of mineral) in the youngest soil profile and decreases to a rate of only ∼20% of that of plagioclase in the oldest soil. 87Rb86Sr ratios of the soil cation exchange fraction are estimated to be depleted by factors of up to 11 over the 87Rb86Sr ratios released by weathering, due to ion exchange partitioning. This study demonstrates that the 87Sr86Sr ratio released by weathering of crystalline rocks can deviate significantly from bedrock values, and that in soils less than ∼20 kyr in age which contain biotite in the soil parent material, weathering-derived 87Sr86Sr values can be elevated so dramatically that this factor must be considered in estimations of weathering rates based on strontium isotopes.