James L. Gooding

Aqueous alteration on the hydrous asteroids: Results of EQ3/6 computer simulations

Abstract In order to understand the course of aqueous alteration of the hydrous asteroids we constructed two models for carbonaceous chondrite precursor material, one (model A) based upon the anhydrous mineralogy of the abundant CM chondrites and the other (model B) based upon that of the equally abundant CV chondrites. We then proceeded to calculate the likely course of aqueous alteration on asteroids composed of these materials through the use of the EQ3/6 computer algorithm. We find that the alteration mineralogy of the CM chondrites (and by extension the CM parent asteroids) may best be produced by starting with the anhydrous mineralogy of the same CM chondrites, at temperatures of 1 to approximately 25°C, and at total solution carbon concentrations which vary from as little as 10−8m up to at least 10−2m. A wide range of rock/fluid ratios is permitted by this alteration mineralogy. Under these conditions solution pH is calculated to vary from 7 to just above 12. Solution Eh is calculated to vary from −0.5 to −0.75 V. We calculate that the mineralogy of the important CI chondrites can be well-reproduced by alteration of either CM or CV anhydrous material. This alteration is calculated to occur best for temperatures of 50 to (at least) 150°C, total solution carbon concentrations varying from approximately 10−3 to (at least) 10−2m, and a wide range of rock/fluid ratios. Under these conditions solution pH is calculated to vary from 7 to between 9 and 10, and Eh from −0.3 to −0.8 V, in the direction of increasing alteration. We therefore conclude that these were the principal conditions of aqueous alteration on the CI parent asteroids. The alteration assemblage observed for the CM chondrites is not produced by alteration of the CV chondrites under the modeling conditions imposed by this study, which suggests that the CM chondrites do not necessarily share the same parent asteroids with the CV chondrites. On a purely mineralogical basis, howerver, the CI chondrites could have been produced from either (or both) CM or CV chondrite material, and therefore be present on either type of parent asteroid.

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.

Rapid Growth of Magnesium-Carbonate Weathering Products in a Stony Meteorite from Antarctica

Nesquehonite, a hydrous magnesium carbonate, occurs as a weathering product on the surface of the Antarctic meteorite LEW 85320(H5 chondrite). Antarctic meteorites have resided on the earth for periods of 104 to 106 years, but the time needed for weathering products to form has been uncertain. Isotopic measurements of δ13C and δ18O indicate that the nesquehonite formed at near freezing temperatures by reaction of meteoritic minerals with terrestrial water and carbon dioxide. Results from carbon-14 dating suggest that, although the meteorite has been in Antarctica for at least 3.2 x 104 to 3.3 x 104 years, the nesquehonite formed after A.D.1950.