APPLICATION OF THE TRACE-ELEMENT AND ISOTOPE GEOCHEMISTRY OF STRONTIUM TO STUDIES OF CARBONATE DIAGENESIS

Carbonate rocks and natural waters exhibit a wide range in the concent ration and isotopic composition of strontium. This wide range and the quantifiable covariation of these parameters can provide diagnostic to ols for understanding processes of fluid-rock interaction. Careful con sideration of the uncertainties associated with trace element partitio ning, sample heterogeneity and fluid-rock interaction mechanisms is re quired to advance the application of the trace element and isotope geo chemistry of strontium to studies of diagenesis, goundwater evolution, ancient seawater chemistry and isotope stratigraphy. A principal unce rtainty involved in the application of Sr concentration variations to carbonate systems is the large range of experimental and empirical res ults for trace element partitioning of Sr between mineral and solution . This variation may be a function of precipitation rate, mineral stoi chiometry, crystal growth mechanism, fluid composition and temperature . Calcite and dolomite in ancient limestones commonly have significant ly lower Sr concentrations (20-70 p.p.m.) than would be expected from published trace element distribution coefficient values and Sr/Ca rati os of most modern sedimentary pore waters. This discrepancy probably r eflects the uncertainties associated with determining distribution coe fficient values. As techniques improve for the analytical measurement and theoretical modelling of Sr concentration and isotopic variations, the petrological analysis of carbonate samples becomes increasingly i mportant. The presence of even small percentages of non-carbonate phas es with high Rb concentrations and high Sr-87/Sr-86 values, such as cl ay minerals, can have significant effects on the measured Sr-87/Sr-86 values of carbonate rocks, due to the decay of Rb-87 to Sr-87. For exa mple, a Permian marine limestone with 50 p.p.m. Sr and 1 p.p.m. Rb wil l have a present-day Sr-87/Sr-86 value that is >2 x 10(-4) higher than its original value. This difference is an order of magnitude greater than the analytical uncertainty, and illustrates the importance of ass essing the need for and accuracy of such corrections. A quantitative e valuation of the effects of water-rock interaction on Sr concentration s and isotope compositions in carbonates strengthens the application o f these geochemical tracers. Geochemical modelling that combines the u se of trace elements and isotopes can be used to distinguish between d ifferent mechanisms of water-rock interaction, including diffusive and advective transport of diagenetic constituents in meteoric pore fluid s during the recrystallization of carbonate minerals. Quantitative mod elling may also be used to construct diagnostic fluid-rock interaction trends that are independent of distribution coefficient values, and t o distinguish between mixing of mineral end-members and fluid-rock int eraction.