Abstract
Transformation of aragonite, a mineral phase metastable at Earth’s surface, to calcite widely occurs in both sedimentary and metamorphic systems with the presence of an aqueous phase. The transformation process can affect geochemical signatures of aragonite (a protolith). This study focused on quantification of the retention of Mg/Ca and Sr/Ca ratios, and δ18O during the transformation process as well as evaluation of the transformation rate. To investigate the effect of transformation from aragonite to calcite on elemental and stable isotope ratios, we conducted a series of experiments in NaCl solutions at temperatures between 120 and 184 °C. Two additional experiments at 250 °C were conducted to estimate the transformation rate of aragonite to calcite. Protolith materials consist of (1) synthetic (Mg; Sr-bearing or non-Mg; Sr bearing) needle-shaped microcrystals of aragonite (<5 µm in size) and (2) larger chips (>100 µm in size) of natural aragonite. X-ray diffraction (XRD) showed that microcrystals successfully transformed to calcite within 30 h and scanning electron microscopy (SEM) yielded a change in the crystal size to >10 µm in rhombohedral shape. Electron backscatter diffraction (EBSD) of the larger aragonite chips showed that transformation to randomly oriented calcite occurred at the rims and along the cracks while the core retained an aragonite crystal structure. Isotope-ratio mass spectrometry (IRMS) analyses showed that calcite δ18O was controlled by temperature and δ18O of the solution. The obtained calibration curve of isotope fractionation factor versus temperature is consistent with other studies. Inductively coupled plasma optical emission spectroscopy (ICP-OES) analyses showed that calcite partially or completely retained Mg/Ca and Sr/Ca ratios through the transformation.
Highlights
Aragonite and calcite are widespread calcium carbonate (CaCO3) polymorphic minerals that transform their crystalline structure but retain the same composition under different conditions within the Earth’s crust
This study investigates the retention of δ18O, and Sr/Ca and Mg/Ca ratios of aragonite during its transformation into calcite under the laboratory conditions
The crystallization method was based on the transformation of metastable aragonite to calcite under hydrothermal conditions at a range of temperatures
Summary
Aragonite and calcite are widespread calcium carbonate (CaCO3) polymorphic minerals that transform their crystalline structure but retain the same composition under different conditions within the Earth’s crust. The transformation of aragonite, which is metastable at standard conditions (25 ◦C, 1 bar), to calcite can be accelerated by heating at temperatures that are common for the diagenesis and metamorphism of limestone [2]. As marine carbonates are widely used in the reconstruction of the paleoenvironment, the effect of metamorphism on trace element and isotopic signatures in the transformation product (e.g., calcite) needs to be evaluated. It has been reported that the transformation process causes textural changes, as micritic limestone experienced inversion from acicular crystals, probably aragonite, to equant crystals of calcite [3]. No systematic orientational relationship between the calcite and the aragonite crystallographic directions was identified, and calcite nucleated in apparently random crystallographic orientations [5]
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