LOW-TEMPERATURE AQUEOUS ALTERATION ON THE CR CHONDRITE PARENT BODY: IMPLICATIONS FROM IN SITU OXYGEN-ISOTOPE ANALYSES

摘要: The presence of hydrated minerals in chondrites indicates that water played an important role the geologic evolution early Solar System; however, process aqueous alteration is still poorly understood. Renazzo-like carbonaceous (CR) are particularly well-suited for study alteration. Samples range from being nearly anhydrous to fully altered, essentially representing snapshots through time. We studied oxygen isotopes secondary-minerals six CR varying hydration states determine how fluid conditions (including composition and temperature) evolved on parent body. Secondary analyzed included calcite, dolomite, magnetite. O-isotope calcites ranged δ18O ≈ 9 35 ‰, dolomites 23 27 magnetites -18 5 ‰. Calcite less-altered samples showed more evidence compared heavily altered samples, likely reflecting lower water/rock ratios. Most magnetite plotted a single trend, with exception grains extensively chondrite MIL 090292. 090292 diverges this possibly indicating anomalous origin meteorite. If calcite formed equilibrium, then relative 18O fractionation between them can be used extract temperature co-precipitation. Isotopic Al Rais carbonate-magnetite assemblages revealed low precipitation temperatures (~60°C). Assuming body experienced closed-system alteration, similar exercise parallel arrays yields "global" ~55 88 °C. These secondary mineral indicate O-isotopic altering upon progressive beginning near Δ17O ~ 1 ‰ 10 becoming increasingly 16O-enriched toward final -1.2 -15