--> Abstract: Assessing the Petrographical and Geochemical Record of Change in Porewater Chemistry in Radiaxial Fibrous Calcite in the Capitan Formation, West Texas and New Mexico, by M. A. Rahnis and B. L. Kirkland; #90937 (1998).

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Abstract: Assessing the Petrographical and Geochemical Record of Change in Porewater Chemistry in Radiaxial Fibrous Calcite in the Capitan Formation, West Texas and New Mexico

RAHNIS, MICHAEL A., and BRENDA L. KIRKLAND, The University of Texas at Austin

In the Capitan Formation isopachous marine calcite forms a complex 1-4 mm thick zone that consists first of mixed radiaxial-fibrous and fascicular-optic calcite, followed by radiaxial-prismatic calcite, and dolomite. Calcite in this zone contains 10-100 µm euhedral to anhedral dolomite microinclusions where luminescent dolomite overgrowth is present, and <1-10 µm dolomite microinclusions where overgrowth is absent. Carbon and oxygen stable isotopic ratios covary between values typical of marine and meteoric cements, but trace elements do not. Between the upper and lower Capitan average Mn and MgCO3 concentrations increase (25±3 to 62±31 ppm Mn; 1.5±0.8 to 2.4±1.4 mole% MgCO3), whereas average Sr concentration decreases (210±72 to 63±22 ppm). Point traverses from crystal substrate to termination show increases from 400 to 1000 ppm Sr and 1 to 4 mole% MgCO3. Sr and MgCO3 do not covary. Kaolinite microinclusions occur at the transition between radiaxial-fibrous/fasicular-optic to radiaxial-prismatic calcite, and are most abundant in radiaxial-prismatic calcite.

Small, anhedral dolomite microinclusions may result from dissolution-reprecipitation, whereas larger, euhedral dolomite inclusions and layers are probably primary cements. The range of isotopic compositions may reflect sampling of marine calcite and syntaxially overgrown meteoric calcite. Trace elements in Capitan radiaxial calcite do not covary between marine and meteoric endmember compositions, but vary stratigraphically, indicating diagenetic trace element alteration. Sr increase in the crystal growth direction parallels increase in dolomite microinclusion abundance, which may reflect primary trace element distribution. These increases are probably related to influx of inner shelf or basinal brines. Kaolinite microinclusions may reflect feldspar dissolution during the final stage of calcite overgrowth on radiaxial calcite. Capitan radiaxial calcite, despite alteration, records fundamental shifts in porewater chemistry.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah