Aragonite neomor­phism via intrafabric dissolution and calcite pre­cipi­ta­tion, not thin films

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.