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NOBLE GASES IN ALLENDE FLUFFY AND COMPACT

The Ne and Ar isotopic composition of Allende CAIs can consistently be explained by cosmogenically produced Ne and Ar. Thereby, the extraordinary chemistry of CAIs and the large preatmospheric size of Allende, which may cause significant cosmogenic production of Ar from Cl, were taken into account. In contrast to earlier workers, we conclude that no trapped noble gas components are needed to explain the Ne and Ar signatures of Allende CAIs. Introduction: Calcium-aluminum-rich inclusions, CAIs, represent the oldest available material that formed in the solar system [1]. Due to their extraordinarily refractory chemistry CAIs are either interpreted as the first condensates from a cooling nebula of solar composition, or as the most refractory residues left over from an intense fractional evaporation process [2]. CAIs experienced higher peak temperatures than chondrules [3], which lost most of their primordial Ne and Ar due to the melting event, e.g., [4]. Thus, it could be expected that CAIs lost all of the primordial Ne and Ar. Nevertheless, [5] reported a trapped Ne component in fine-grained Allende CAIs, and [6] reported Ne-E consisting of nearly pure Ne [7] in an Efremovka CAI. If CAIs were formed close to the young sun as proposed by [3] they could be expected to contain at least remnants of solar-like noble gases. We measured Ne and Ar in fine-grained fluffy and coarser-grained compact Allende CAIs to re-examine the presence of a trapped component besides cosmogenic noble gases, and to rule on differences in the formation of fluffy and compact CAIs. Experimental: 22 samples of 5 fluffy and 4 compact Allende CAIs were separated under a binocular microscope. After a 24 hour bake out in vacuum to remove adsorbed atmospheric gases, the noble gases were extracted by melting the samples with an IR laser (CW mode). This provides very low blanks of ~1x10 and 2x10 cmSTP for Ne and Ar, respectively, contributing <6% to both, the measured Ne and Ar signals. Results and Discussion: Neon isotopes: In contrast to matrix-like samples (grey triangles in Fig. 1) the compact and fluffy CAIs (black solid and open circles, respectively) plot on a line between chondritic cosmogenic Ne and Ne-E. However, also calculated cosmogenic Ne-isotopic compositions of mineral phases common in CAIs as well as pure Na and Al (production rates from [8] for a body of 50 cm radius and a mean shielding depth of 25 cm) plot on this line. Thus, the Ne-data can be explained straightforwardly by cosmogenic Ne produced from Na and Al bearing phases present in CAIs, although they do not contradict the presence of Ne-E in the CAIs. None of the CAI data points plots above this line, which would indicate a trapped component as reported by [5]. Fig. 1. Neon isotopic composition of fluffy and compact CAIs (open and black solid circles, respectively.). For comparison, also Allende matrix data are plotted (grey triangles). Fas=fassaite, Spi=spinel, Mel=melilite, An=anorthite, Gr=grossite, Co=corundum, Hi=hibonite, Nep=nepheline, Sod=sodalite. See text for further