Meteoric Alteration of Early Formed Dolomite During Long-Term Subaerial Exposure: Mississippian Mission Canyon Formation: ABSTRACT

Abstract

Dolomite from the Mississippian Mission Canyon Formation in south central Montana has been petrographically and geochemically analyzed to constrain the effects of long-term subaerial exposure on the formation of well-ordered and stoichiometric dolomite. Samples from five measured stratigraphic sections were analyzed for stoichiometry and for carbon and oxygen isotope composition. Dolomite {delta}{sup 18}O values from all sections vary from {minus}11.2 to +7.5{per thousand}. {delta}{sup 13}C ratios vary from +0.4 to +5.5{per thousand} but are relatively consistent within individual sections. Stoichiometry of the dolomite varies from 49.9 to 55.7 mole % CaCO{sub 3}. Measured sections from the northern portion of the study area dominantly contain {sup 18}O-depleted dolomite (+0.8 to {minus}11 2{per thousand} {delta}{sup 18}O). The most {sup 18}O-depleted dolomite, however. typically occurs in the upper 100 m of these sections. These {sup 18}O-depleted dolomites have homogeneous {delta}{sup 13}C ratios and are stoichiometric. Dolomite from the upper 100 m of these sections predates early calcite cement and locally is calcitized or partially replaced by Fe-oxides. These data suggest that early formed dolomite may have been recrystallized by meteoric water under very high water-rock ratio conditions. Decreasing amounts of water-rock interaction with depth is suggested by more heterogeneous {delta}{sup 18}O and {delta}{supmore » 13}C ratios downsection. Measured sections in the southern portion of the field area contain greater amounts of {sup 18}O-enriched dolomite (+7.5 to -7.1{per thousand} {delta}{sup 18}O). Dolomite from these sections also has more heterogeneous {delta}{sup 13}C values and variable CaCO{sup 3} content. These data suggest that the dolomite probably has not been extensively altered and may more closely represent the original geochemistry of the early formed dolomite.« less