Mn-Cr Isotope Systematics of Angrite Northwest Africa 4801

Introduction: Chronometers based on the decay of short-lived radionuclides give the opportunity to resolve time differences of less than a million years for early events in the solar system. However, with shortlived radionuclides only relative ages can be obtained. These relative ages have to be mapped onto an absolute time scale. The absolute age is provided by the PbPb chronometer which is capable of producing ages with the precision comparable to that of short-lived chronometers. For more than a decade for the Mn-Cr relative chronometer we used the angrite LEW 86010 as an absolute time marker. The Pb-Pb age of this meteorite is 4557.8±0.4 Ma [1] and the corresponding Mn/Mn ratio is (1.25±0.07) x 10 [2]. The angrites are especially suitable as time markers because they are early equilibrated planetary differentiates that mostly are undisturbed by later events. Since then a series of new angrites was discovered and highresolution Pb-Pb ages have been obtained [3]. However, small but systematic differences between the “absolute” Mn-Cr ages and the Pb-Pb ages can be observed: the latter are slightly older, if LEW 86010 is used as a time marker (e.g. [4]). Moreover, a recently re-determined Pb-Pb age of LEW 86010 is 4558.55±0.15 Ma that is ~0.8 Ma older than the age we previously used [3]. Therefore, the goal of this work is an attempt to conduct a thorough investigation of the Mn-Cr isotope system in another angrite, NWA 4801, that is abundant in a high Mn/Cr phase (olivine) and for which a precise Pb-Pb age is now available: 4557.99±0.12 Ma [3]. For the purpose of simplicity we adopt here a value of 4558.0±0.13 Ma. Northwest Africa 4801: The recently discovered (May 2007) meteorite NWA 4801 was classified by Irving and Kuehner [5] as a plutonic igneous cumulate angrite. This angrite has a metamorphosed texture and is coarse-grained (grain size 0.1-1.2 mm). Although it has an overall cumulus texture, there is evidence for subsequent annealing. The meteorite consists mostly of Al-Ti clinopyroxene and anorthite (some as polycrystalline aggregates), calcic olivine, pleonaste, merrillite and minor troilite and altered metal. Anorthite includes infrequent Cr-pleonaste grains (Cr2O3 = 6.3 wt.%). Some clinopyroxene and olivine are poikilitically enclosed in Cr-poor pleonaste mantles around large Crpleonaste grains. Merrillite is more abundant than in most other known angrites [5]. Experimental, Results, and Discussion: To obtain phases with different Mn/Cr ratios we first applied our usual differential dissolution procedure that allows separating chromites from silicates. Silicates (Sil) were dissolved in an HF/HNO3 mixture at room temperature, the residue (Res 1) (mostly chromite with some remaining silicates) was dissolved in the same mixture at ~180o C, and the remaining tiny grains (Res 2) (chromite) were dissolved at ~180o C in HClO4. From a separate piece of the meteorite the olivine phase (Olv 1 and Olv 2) was separated by handpicking at the University of Washington. We have measured Cr/Cr ratios and Mn and Cr abundances in all these fractions and in a total rock fraction (TR). Because of the high Fe/Cr ratios in olivine (~860) and the high cosmic ray exposure age (31.6 My [6]) the Cr isotopic compositions in Olv 1 and Olv 2 were corrected for a spallation contribution (~ 7 ppm). As in the past, in order to achieve higher precision on the Mn/Mn ratio, we have applied the second order fractionation correction to the Cr/Cr data based on the Cr/Cr ratios [2]. This procedure assumes no excesses or deficits of Cr. However, it was shown recently [7], that bulk samples of some meteorite classes, including the angrite Angra dos Reis, possess relative deficits of Cr. Our data for NWA 4801 also show a deficit of Cr: -0.35±0.06 ε. This is essentially the same value that was obtained for Angra dos Reis: -0.36±0.07 ε [7]. Here we present both the second order corrected data and the ‘raw’ data (i.e. with no second order fractionation correction applied). The results are presented in Figures 1 and 2.