Abstract
Since the 1970's, aragonite neomorphism (conversion of aragonite to calcite with partial retention of original internal structure) has been attributed to thin diagenetic films, often in meteoric water. Pleistocene material from Enewetak, the Bahamas and eastern Yucatan show the transition from aragonite to calcite "in progress". Original aragonitic material includes coral, Halimeda, molluscs (especially gastropods) and ooids. In all three areas, aragonite commonly shows partial intrafabric dissolution. In many cases, calcite crystals grew over that highly microporous aragonite to create calcite with partial retention of the original depositional fabric, creating neomorphosed (calcitized) aragonite. Geochemical data support this scenario. Carbon and oxygen isotopic compositions of the neomorphic calcites are similar to sparry meteoric calcite cements indicating crystals precipitated from similar waters as the cements. Stable carbon isotopes and Mg concentrations in neomorphic calcites are different from their aragonitic precursors and are not consistent with the thin-film model, which assumes a semi-closed system. Approximately 40 of the 95 Pleistocene Enewetak samples have aragonite with intrafabric dissolution adjacent to calcitized (neomorphic) aragonite. Neomorphosed (calcitized) aragonite was not observed immediately adjacent to unaltered aragonite (i.e., separated by what would have been a thin film) in any Enewetak samples. This supports aragonite neomorphism by a two-stage process (intrafabric dissolution followed by calcite precipitation) rather than by thin films. Variations in concentration of inclusions in neomorphosed aragonite are similar to variations of relict material in aragonite with partial intrafabric dissolution and span a continuum from abundant aragonitic inclusions in neomorphic spar to inclusion-free calcite cement.
Published Version
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