TRACE ELEMENT FRACTIONATION ON GROWTH SURFACES OF SPELEOTHEMS

Stalagmites grow as drip water slowly flows down over their surface and precipitates calcite or aragonite. As calcite is gradually extracted from the drip water, trace elements (TE’s) are fractionated between the calcite and residual solution. This should lead to Rayleigh fractionation of these elements between calcite and solution, governed by the partition coefficient, Kd, between crystal and solution. Kd values for Sr, Mg and other ions are dependent on temperature (T). For constant initial concentration of element M in drip water, M/Ca should vary as a function of Kd(T) and extent of calcite depletion, X. Using lengthwise sections of stalagmites, it is possible to measure variations in Sr/Ca, Mg/Ca, etc., along single growth layers, and use data to evaluate X and thus Kd values. We determined Mg, Sr values on multiple traverses down the growth layers of a 60 cm stalagmite from W. Virginia. Surprisingly, while Mg/Ca increased as expected, Sr/Ca decreased, and Sr/Ca correlates with 1/[Mg/Ca]. Sr uptake may be governed by kinetic factors not apparent in slow growth experiments such as those of Huang & Fairchild (2001). Correlation between Sr/Ca and Mg/Ca is consistent with control of both extent of calcite depletion.