Abstract

The neomorphism of aragonite to calcite involves complex textural and chemical alterations that are in part controlled by the degree of communication between diagenetic and ambient formation fluids. Two taxa, Montastrea annularis and Strombus gigas, from the vadose zone of the Pleistocene Key Largo Formation in southern Florida, were examined in order to evaluate the effect of initial skeletal porosity on system closure during neomorphism. Fabrics of replacement calcite in both taxa are similar, consisting of blocky crystals largely restricted to areas of former skeletal aragonite. Oriented aragonite and organic inclusions result in the preservation of relic fabrics in both taxa. The chemical signatures of coralline and molluscan aragonite and calcite are strongly dependent on initial skeletal porosity. Compared to original aragonite, Montastrea calcite is enriched in magnesium (1,151 + or - 64 ppm versus 2,114 + or - 619 ppm) and depleted in strontium (7,397 + or - 376 ppm versus 2,510 + or - 43 ppm). These data, in conjunction with large calcite oxygen depletions (-3.8 + or - 0.2% per thousand versus -5.7 + or - 0.3% per thousand ) and variable calcite carbon depletions (-0.6 + or - 0.6% per thousand versus -1.3% per thousand to -7.3% per thousand ), demonstrate significant exchange between neomorphic front and formational fluids. In contrast, Strombus calcite composition is largely inherited from original aragonite and is only slightly enriched in strontium and magnesium (2,299 + or - 316 ppm versus 2,519 + or - 759 ppm and 96 + or - 23 ppm versus 173 + or - 79 ppm), while carbon exhibits a small depletion (+5.3 + or - 0.5% per thousand versus +4.4 + or - 0.8% per thousand ). Similarity of Strombus aragonite and calcite compositions reflects high rock/water ratios in relatively closed diagenetic systems. Calcite oxygen depletion (-0.5 + or - 0.5% per thousand versus -3.6 + or - 0.4% per thousand ) observed in Strombus, however, requires some exchange between formation and neomorphic front fluids. Differences in the thickness of replacement calcite rims between taxa reflect variable skeletal porosities and rates of migration of neomorphic fronts. Neomorphism was six times faster (by volume) in porous Montastrea than in dense Strombus skeletal structures. Thus, skeletal porosity influences degrees of system openness during aragonite neomorphism.

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