Petrography and C and O isotope geochemistry of crinoids, Burlington-Keokuk Formation, southeastern Iowa and Western Illinois

Conference · Fri Jan 01 04:00:00 EST 1988

OSTI ID:6339343

Kohrt, K A; Meyers, W J; Hanson, G N

Petrographic observations and C and O isotopic compositions of crinoids from the Mississippian (Osagean) Burlington-Keokuk formation constrain the compositional evolution of early phreatic zone pore waters. Crinoid skeletal calcites exhibit a different luminescence than itraskeletal calcite cements, implying that these two phases were precipitated from fluids with different redox potential. Regional isotopic variations of petrographically identical crinoids suggest pore water isotopic compositions change, while redox potentials remain constant. A positive covariance is found between delta/sup 18/O and delta/sup 13/C of bulk crinoids at each of 10 localities in the study area. The covariant trends fan out from the estimated Osagean marine calcite composition (delta/sup 18/O = -2.19 o/oo PDB, 2s = 0.72; delta/sup 13/C = 4.45 o/oo PDB, 2s = 0.62) intersecting an invariant delta/sup 18/O (-4.5 o/oo PDB) meteoric calcite line at delta/sup 13/C values that increase at localities distal from proposed meteoric recharge. This covariance results from physical mixing of crinoid skeletal calcite and intraseketal calcite cements that are interpreted to have precipitated from mixed evolved phreatic zone pore waters. Increasing mean /sup 13/C values of bulk crinoids at localities away from proposed sites of meteoric recharge suggests enrichment of pore water /sup 13/C due to increasing rock-water interaction is downflow areas of the aquifer. Increasing linear covariance at localities down the hydrologic gradient is attributed to convergence of fluid end members to similar total dissolved carbon concentrates.

• OSTI ID: 6339343
• Report Number(s): CONF-880301-
• Country of Publication: United States
• Language: English

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