Geocheminar spring-2012

Although CR chondrites have experienced little or no thermal metamorphism, they have undergone varying degrees of parent-body aqueous alteration. O-isotopic measurements of CR chondrites show increasing ?17O and ?18O in more aqueously altered meteorites but these measurements have limitations because of terrestrial weathering and sample heterogeneity. Needed is a petrologic scale for aqueous alteration in CR chondrites analogous to the one constructed for CM chondrites. We examined 30 CR chondrites and used petrographic and compositional parameters to create a CR alteration index. At the onset of alteration mesostasis is converted to phyllosilicate. As alteration continues metal grains and sulfides are oxidized and then alteration of phenocrysts begins until all mafic silicates have been converted to phyllosilicates. The CRs show the full range of aqueous alteration with type 3.0 being the least altered and type 2.0 being the most altered; most have had minimal interaction with water and belong to type 2.9 or 3.0. ++++++++++++++++++++++++++++++++++ It is generally agreed that there was a steep decline in the impact rate on the Moon after 3.9 Ga, but there the nature of bombardment before this time is controversial. Zircons are ideal for investigating the early lunar bombardment because (1) low initial Pb results in high precision U-Pb age measurements, (2) the crystallization ages of lunar zircons all predate the proposed cataclysm, (3) zircons incorporate both U and Pu, so we can measure fissiongenic Xe degassing ages and Pb-Pb crystallization ages for the same crystal. We measured Xe isotopic abundances of three large (~300 ?m) individual Apollo 14 zircons using the University of Manchester Refrigerator Enhanced Laser Analyser for Xenon. Two of the samples produced sufficient xenon for precise xenon isotope ratios to be determined. All releases from these samples are consistent with the 238U end member suggesting little or no Xe contributed from 244Pu fission, which places a upper limit on the xenon degassing age of ~3.9 Ga.