Terrestrial weathering of Antarctic stone meteorites: Formation of Mg-carbonates on ordinary chondrites
Abstract
White efflorescences of weathering origin occur superposed on fusion crusts, or along fractures in the interiors, of approximately 5% of all meteorites in the US Antarctic collection. Efflorescences from equilibrated ordinary chondrites consist of the hydrous Mg-carbonates nesquehonite (±hydromagnesite). X-ray diffraction and scanning electron microscope studies of efflorescences from LEW 85320 (H5) show abundant elongate prismatic crystals of nesquehonite (idiomorphic, not pseudomorphous after lansfordite), with minor local encrustations of hydromagnesite. Abundances of Na, K, Ca, and Rb in efflorescences from LEW 85320 suggest that the observed contents of these elements would require only modest fractionation of chondritic composition, whereas extensive fractionation would be required to derive the observed cation ratios from terrestrial sea-salts. Therefore, cations in evaporite minerals on Antarctic meteorites are most likely not products of contamination by terrestrial (marine) salts. The Mg in the efflorescences probably originated from weathering of meteoritic olivine; other cations in the efflorescences are also of meteoritic provenance. Thermodynamic analysis of the reaction forsterite + water + carbon dioxide → nesquehonite + silica at Antarctic temperatures and pCO 2 indicates spontaneity for all water activities greater than 0.65, compatible with the presence of liquid water as brines and/or thin films.
- Publication:
-
Geochimica et Cosmochimica Acta
- Pub Date:
- January 1991
- DOI:
- 10.1016/0016-7037(91)90400-Y
- Bibcode:
- 1991GeCoA..55...67V
- Keywords:
-
- Antarctic Regions;
- Carbonates;
- Chondrites;
- Meteoritic Composition;
- Stony Meteorites;
- Weathering;
- Electron Microscopy;
- Magnesium Compounds;
- Olivine;
- Thermodynamic Properties;
- X Ray Diffraction;
- METEORITES;
- WEATHERING;
- ANTARCTIC METEORITES;
- STONY METEORITES;
- EOCS;
- MAGNESIUM;
- CARBONATE;
- ORDINARY CHONDRITES;
- LABORATORY STUDIES;
- XRD;
- SEM;
- SAMPLES;
- METEORITE;
- X-RAY METHODS;
- ELECTRON MICROSCOPY;
- LEW METEORITES;
- LEW 85320;
- H5 CHONDRITES;
- CHONDRITES;
- CRYSTALS;
- COMPOSITION;
- ABUNDANCE;
- SODIUM;
- POTASSIUM;
- CALCIUM;
- RUBIDIUM;
- FRACTIONATION;
- ORIGIN;
- FORMATION;
- CONTAMINATION;
- CATIONS;
- EVAPORITES;
- THERMODYNAMICS;
- IRON;
- HYDROLYSIS;
- OXIDES;
- CLAY;
- SILICATES;
- PROCEDURE;
- MINERALOGY;
- CHEMISTRY;
- OXIDATION;
- HYDRATION