The mineralogy, petrography and diagenesis of carbonate speleothems from caves in the Guadalupe Mountains, New Mexico
Polyak, Victor James
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The carbonate mineralogy of speleothems from caves in the Guadalupe Mountains includes: calcite, aragonite, dolomite, hydromagnesite, monohydrocalcite, and huntite. Calcite precipitates as a primary subequant and fibrous crystal fabric, and as Mg-calcite having >2 mole% Mgco3 in solid solution; calcites which contain less are suspected of having undergone recrystallization or solution. Aragonite precipitates inorganically in association with high magnesian calcite; and biogenically in speleothems generally consisting of Mg-calcite having 2-5 mole% Mgco3 which have formed near cave entrances. Mg-calcite, aragonite, and Carich dolomite dominate the mineral component in these speleothems, and hydromagnesite is probably widely distributed within most caves of the Guadalupe Mountains in minute amounts. Monohydrocalcite and huntite are rare. Speleothem fragments, used as seeds, were coated with subequant and fibrous calcite up to 0.3 mm thick during a one year exposure to cave drip waters. Calcite crystallized by nucleation on clean seed surfaces, or by optically continuous growth of the seed surface. The crystals grow as crude trigonal prisms having upon them numerous crystallites; crystal terminations are rhombohedral and poorly developed. "Growth" layers in speleothems are represented by several features, some of which are not related to growth. For instance, many white layers contain fluid inclusions and porosity which are the result of "hydration" and partial solution and/or recrystallization (when these layers existed at the surface). These layers (hydration rims) represent changes in cave environments such as flooding (by freshwater runoff) of the caves, but not submergence of the speleothems. Layers defined by crystal terminations represent periodic changes in drip water influx, and incomplete terminations may record changes in water film thickness. Layers of internal sediment also may reflect environmental changes within caves. Speleothems which formed near entrances of caves can contain well preserved fossils. Mites and various arthropod parts are the most distinctive fossils extracted. Tearshaped and filamentous fluid inclusions are inferred to be trace fossils of microorganisms; the inclusions also may be oriented along layers. Calcite and aragonite co-exist and can co-precipitate. Calcite eventually replaces aragonite generally as pseudospar, and sometimes as a fibrous fabric. Dolomite appears to form secondarily after Mg-calcite or hydromagnesite, however; some dolomite may precipitate as subequant and fibrous crystals. Dolomite pseudomorphically replaces what are inferred to have been globules of hydromagnesite on aragonite needles, and replaces hydromagnesite moonmilk as a microspar or pseudospar. Calcite in speleothems, which forms from vadose seepage, is Mg-calcite. Mg-calcites are metastable relative to stoichiometric Mg-free calcite, and therefore, they are inevitably converted to low magnesian calcite through stabilization (incongruent dissolution) or recrystallization. Many (linear) fluid inclusion-rich layers and layers containing abundant porosity are layers of low magnesian calcite. These layers seem to represent periods during which recrystallization occurred. Eventually, under normal surface conditions, speleothemic Mg-calcite will progressively recrystallize to Mg-free calcite, destroying all of the primary internal structures.