Anett Lázár , Attila Demény , István Hegyi , László Előd Aradi , Laurence A.J. Garvie , Péter Németh
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引用次数: 0
Abstract
Glendonites are calcite pseudomorphs of ikaite (CaCO3·6H2O), a mineral that typically forms below 4 °C. Ikaite is found in marine clastic sediments, caves and lacustrine sediments, and its presence is correlated with cold paleo-environments. However, it is uncertain whether the isotopic composition of glendonite preserves the low-temperature paleoclimate conditions. Here we study the ikaite to calcite transition by following the evolution of solutions containing ikaite, prepared at pH 12.1, 10.3 and 9.1, and filtered materials exposed to air. Ikaite transformation starts with the formation of 100–200 nm-sized amorphous calcium carbonate (ACC) on the surface of ikaite grains, which transforms to carbonates dominated by calcite. In Mg-free solutions, vaterite also forms, whereas low concentrations of Mg (e.g., 26 mg/l) favor aragonite formation. Our data suggest that synthetic ikaites are formed with strong 16O-enrichment relative to other carbonate species due to kinetic fractionation. The ikaite-derived carbonates formed in contact with ambient air show a minor δ13C change (−3.0 ± 0.8 ‰) relative to ikaite (−4.2 ± 0.3 ‰), but their oxygen isotope compositions remain constant. In contrast, transformation in solutions results in significant oxygen isotopic changes (from 17.7 to 20.5 ‰). Calcite-water oxygen isotopic equilibrium is approached within one week at 8 °C, and one day at 15 °C. Our experiments suggest that glendonite calcites formed by ikaite dehydration during early diagenesis undergo oxygen isotope exchange with pore waters and can record temperature and water composition conditions similar to those of the ikaite formation.
期刊介绍:
Chemical Geology is an international journal that publishes original research papers on isotopic and elemental geochemistry, geochronology and cosmochemistry.
The Journal focuses on chemical processes in igneous, metamorphic, and sedimentary petrology, low- and high-temperature aqueous solutions, biogeochemistry, the environment and cosmochemistry.
Papers that are field, experimentally, or computationally based are appropriate if they are of broad international interest. The Journal generally does not publish papers that are primarily of regional or local interest, or which are primarily focused on remediation and applied geochemistry.
The Journal also welcomes innovative papers dealing with significant analytical advances that are of wide interest in the community and extend significantly beyond the scope of what would be included in the methods section of a standard research paper.