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Abstract

Large variations in the perfection of order and in substitution of calcium for magnesium occur in sedimentary dolomite, and complicate the trace element and isotopie chemistry of this complex mineral. The stabilization of metastable disordered and/or Ca-rich phases is as important in forming ancient dolomitic rocks as is the stabilization of aragonite and Mg-calcite in forming ancient limestones.

The equilibrium value of Δ18O (δ18Odolomite—δ18Ocalcile) is most likely 3 ± 1% at 25°C. Redetermination of phosphoric acid fractionation factors (α=(18O/l6OCO2 )/(18O/16Ocarbonate)) yields values of 103 In α between 11.4 and 11.9 for most sedimentary doLomite. The redetermined value for calcite is 10.5. Holocene dolomite from two locahties yields values of about 12.5. Most ancient dolomite is very depleted in 18O relative to presumed Holocene analogs.

Mathematical modeling of the diagenetic behavior of the trace element Sr in conjunction with oxygen isotopie changes during the diagenesis of limestone substantiates recent suggestions that experimentally determined distribution coefficients for calcite do not apply under actual diagenetic conditions. An analogous situation probably exists with respect to dolomite. Most ancient dolomite is trace-element-depleted relative to Holocene analogs.

At present, quantitative interpretation of absolute isotopie or trace element values of dolomite is tenuous, not only due to problems in assigning distribution coefficients and fractionation factors, but also in coping with multiple recrystallizations in partly closed chemical systems. Because recrystaUization is neither completely open nor completely closed-system, chemical information from early events is incompletely erased by later ones. Qualitative interpretation of regional or stratigraphie variations in chemical or crystallographic parameters appears to be quite useful, however, given our present state of ignorance.

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