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

Tad M. Smith, Steve L. Dorobek

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 ^dgr18O values from all sections vary from -11.2 to +7.5^pmil. ^dgr13C ratios vary from +0.4 to +5.5^pmil but are relatively consistent within individual sections. Stoichiometry of the dolomite varies from 49.9 to 55.7 mole % CaCO3.

Measured sections from the northern portion of the study area dominantly contain 18O-depleted dolomite (+0.8 to -11 2^pmil ^dgr18O). The most 18O-depleted dolomite, however, typically occurs in the upper 100 m of these sections. These 18O-depleted dolomites have homogeneous ^dgr13C 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

rock ratio conditions. Decreasing amounts of water-rock interaction with depth is suggested by more heterogeneous ^dgr18O and ^dgr13C ratios downsection.

Measured sections in the southern portion of the field area contain greater amounts of 18O-enriched dolomite (+7.5 to -7.1^pmil ^dgr18O). Dolomite from these sections also has more heterogeneous ^dgr13C values and variable CaCO3 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. Regional differences in the isotopic composition of the dolomite probably is the result of a northward increase in the duration of subaerial exposure after deposition of Mission Canyon Formation.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990