ISOTOPIC-RATIOS AND RELEASE RATES OF STRONTIUM MEASURED FROM WEATHERING FELDSPARS

Based upon dissolution of feldspars under controlled laboratory condit ions, we conclude that Sr release, at pH 3, is neither consistently st oichiometric nor constant for the feldspars measured. Bytownite, micro cline, and albite all initially release Sr at rates which are 5 (bytow nite) to 160 (microcline) times faster than steady-state release rates . The Sr/Si ratios in the early effluents are significantly elevated c ompared to the bulk mineral values. The Sr-87/Sr-86 measured in efflue nt early in dissolution is higher than the bulk mineral Sr-87/Sr-86 fo r bytownite, but lower than bulk mineral ratios for microcline and alb ite. Sr-87/Sr-86 ratios for the feldspar powders also changed markedly during dissolution of the three phases. Ln part, nonstoichiometric re lease of Sr can be explained by the presence of secondary phases (exso lution lamellae or minute quantities of accessory phases) or by surfac e leaching. Although we infer that these feldspars eventually release Sr with isotopic composition roughly equal to that of the bulk mineral at steady-state, the feldspars dissolve at extremely different rates (bytownite releases Sr at a steady-state rate similar to 10(2) to 10(3 ) times faster than albite and microcline, at pH 3). Therefore, a mixt ure of these feldspars, or of other minerals exhibiting vast differenc es in dissolution rate, will release Sr-87/Sr-86 ratios distinctly dif ferent from the bulk whole rock. In addition, initial Sr release rates of the minerals (bytownite > microcline > albite) differ from steady- state release rates (bytownite > albite > microcline), complicating an alysis of weathering solutions. Log (rate constants) for bytownite, al bite, and microcline decrease from -13.5 to -16.4 to -17.2 (mol Sr cm( -2) s(-1)). Interpretation of catchment scale riverine Sr-87/Sr-86 rat ios on the basis of whole-rock Sr isotopes is, therefore, problematic at best, and would require normalization of bulk isotopic ratios by re lative rates of dissolution of Sr-contributing phases. We also argue t hat abraded feldspar particles formed naturally, for example, during g laciation, will show this initial transient nonstoichiometric release. However, once the transient release is completed (perhaps 10(2) to 10 (3) yr after abrasion), as long as the solution chemistry remains rela tively constant, stoichiometric release of cations from feldspars, inc luding Sr, is expected. The most likely way, therefore, to increase ri verine fluxes of major cations or radiogenic Sr-87/Sr-86 is to create highly flushed land, therefore, far from equilibrium) water-rock syste ms such as glacial sediments and soils, with reactive minerals (e.g., carbonate, plagioclase, or biotite) containing significant radiogenic strontium. Copyright (C) 1998 Elsevier Science Ltd.