Calcitization of aragonite speleothems in limestone caves in Korea: Diagenetic process in a semiclosed system

초록

A coordinated textural, stable isotopic, and elemental investigation of calcitized speleothems was carried out to delineate the diagenetic transformation of unstable aragonite to stable low-Mg calcite. Many aragonite speleothems in certain limestone caves in Korea (Baekgol Cave, Jungteogeori Cave, and Chilsong Cave) have been partially calcitized. The neomorphic calcite crystals vary in size from tens of microns to a few millimeters. Numerous relic aragonite crystals and relic growth lines are present within the neomorphic calcites. Original calcite speleothems (i.e., initially deposited as calcite) tend to be more depleted in Sr and enriched in Mg than the original aragonite, and trace-element compositions of the neomorphic calcites approach their original aragonites. Also, O- and C-isotope compositions of the neomorphic calcite show similar ranges to those of the original aragonite. This suggests that the calcitization within the speleothems occurred in a semiclosed diagenetic system with respect to trace elements and oxygen isotopes. Diagenetic trends strongly suggest that stable isotopes and trace elements of the original aragonite were remobilized during calcitization. Unaltered aragonite speleothems in the Jungteogeori Cave are enriched in (13)C compared to original calcite speleothems, which may indicate that calcite speleothems were influenced more by soil-derived meteoric water during speleothem growth or that aragonite speleothems were deposited from cave water that was more buffered by carbon derived from the surrounding limestone. The (18)O enrichment observed in original aragonite speleothems compared to original calcite speleothems may imply that the aragonite speleothems were formed by evaporation under less humid conditions.

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