Heterogeneous isotopic release rates during mineral weathering
BES-supported investigator Prof. Susan Brantley (Pennsylvania State University) and collaborators Dr. J. T. Chesley (University of Arizona) and Dr. L. L. Stillings (University of Wyoming) have shown that the rate of mineral weathering processes, recorded over geologic timescales, is sensitive to small variations in mineral chemistry and structure, which govern the mechanisms of selective release and uptake of elements and isotopes. The quantities and isotopic ratios of trace elements, first released from precursors during weathering and then sequestered in the rock record during mineral precipitation from groundwater, can provide a record of weathering rates spanning millions of years. Brantley and coworkers have measured the isotopic ratios of trace quantities of Sr that were released in laboratory experiments on the dissolution of feldspar, an aluminosilicate mineral prevalent in the Earth's crust (Geochimica et Cosmochimica Acta, V 62, pp 1493-1500). Their laboratory experiments show that the signature 87Sr/ 86Sr ratio varies significantly from that released during early dissolution to values for later steady-state or bulk mineral dissolution. The investigators also observed that small compositional changes and the presence of secondary phases and surface precipitates resulted in signficant deviations of dissolution rates and 87Sr/ 86Sr ratios, demonstrating that minor variations in mineral chemistry and structure have a large effect on the release rates of trace-element isotopes. On this basis, they conclude that interpreting the sources of catchment-scale radiogenic Sr-isotopic values should include not just whole-rock values-- as customarily done, but also consideration of the composition and dissolution rates of representative minor and secondary phases.
Dr. Brantley's research was supported by OBES/Geosciences; Dr. Chesley's research by an NSF Post-doctoral fellowship and by the Keck foundation.
