Carbonaceous and unequilibrated ordinary chondrites reveal mineralogical, chemical, and isotopic signs of aqueous alteration on their parent bodies (asteroids). A likely alteration scenario starts with accretion of asteroids as a mixture of reduced and anhydrous rocky fragments (chondrules, fine grained materials), condensed organic material, and water ice. Solids consisted of solar nebula products (Fe-Ni metal, FeS, Mg-rich and Ca-silicates, amorphous silicate materials, and Ca-Al-oxides) and minor presolar grains. Decay of short-lived radionuclides caused ice melting, and the composition of primary aqueous fluids was affected by uneven dissolution of minerals, glasses and organic compounds, as well as precipitation of secondary minerals. As alteration progressed, water was consumed in competitive oxidation and hydration reactions. Phyllosilicates (cronstedtite, serpentine, saponite, etc.) and tochilinite formed through hydration. Oxidation led to formation of ferrous (serpentine, olivine) and ferric (magnetite, cronstedtite, andradite) minerals, secondary sulfides (tochilinite, pyrrhotite, pentlandite), Ni-rich alloys, chromite, phosphates, carbonates and sulfates. Other salts (e.g., alkali halides) precipitated from aqueous solutions at latest stages of aqueous alteration. In some cases, local chemical equilibration of individual minerals with solutions have occurred. In many other cases, insufficiencies of water and/or limited time led to incomplete alteration. It follows that aqueous solutions in parent bodies rarely reached equilibrium with the whole rock, and reaction rates played important roles. In contrast to solid products of aqueous alteration in chondrites, a determination of fluid chemistry is difficult. Rare fluid inclusions in some secondary minerals are too small to be analyzed with current methods. Although some characteristics of fluids (pH, redox state) could be deduced from specific secondary mineralogy assemblages, better evaluations of concentrations and activities of solutes requires modeling approach. Until recently, physical-chemical modeling of aqueous alteration have been focused on secondary mineralogy, and aqueous chemistry has not been considered in details. To understand chemical evolution of asteroidal fluids, we propose to link theoretical models of aqueous alteration with observations in chondrites.
|Effective start/end date||5/1/10 → 4/30/14|
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