Cosmogenic Radionuclides in Ureilites from Frontier Mountain, Antarctica: Evidence for a Polymict Breccia

Introduction. The Frontier Mountain (FRO) meteorite collection includes 15 small ureilites with masses of 0.5-35 g. The relatively large number of ureilites (compared to only 2 HED meteorites) suggests that many of these specimens are probably part of a single fall. However, the FRO ureilites show several different lithologies with differences in olivine compositions, olivine/pyroxene ratios, shock stage and bulk carbon content [1-3]. Based on similar mineralogy, some of the FRO ureilites were recognized as pairs [1,2], whereas other specimens show two different lithologies. This could link many of the FRO ureilites together, as small fragments of a polymict breccia [2,3]. However, due to the different lithologies and the difficulty of recognizing brecciated textures in small samples, the extent of pairing among FRO ureilites is not very clear. In addition, Ne cosmic-ray exposure (CRE) ages of 1.4 Myr (90228), 6.0 Myr (90036) and 12.8 Myr (90054) were reported for three FRO specimens [4-6]. Since a previous study has shown that cosmogenic radionuclides provide a powerful tool to identify paired specimens of a heterogeneous Antarctic meteorite shower [7], we measured concentrations of Be (t1/2=1.5 Myr), Al (t1/2=0.705 Myr) and Ca (t1/2=0.104 Myr) in 11 FRO ureilites. In this work, we constrain their exposure history and terrestrial age and evaluate whether they are small fragments of a heterogenous ureilite breccia. Experimental. Ureilite samples of 100-170 mg were crushed and dissolved in HF/HNO3. Small aliquots of the dissolved samples were taken for chemical analysis by atomic absorption spectrometry. We separated Be, Al, Cl and Ca from the dissolved samples for analysis by accelerator mass spectrometry. Concentrations of Be and Al were then measured at Purdue University, those of Ca at LLNL. Results and Discussion. Ten of 11 FRO ureilites show relatively constant Be and Al concentrations of 20±2 dpm/kg and 41±4 dpm/kg, respectively; whereas FRO 97013 shows much lower Be and Al concentrations and most likely represents a distinct fall. Based on the low Be and Al, we estimate a CRE age of 1.0-1.5 Myr for FRO 97013. The high Be concentrations and low Al/Be ratios of 2.02.5 in the remaining ureilites suggest that these were exposed in space long enough (>6 Myr) to reach saturation levels for Be and Al. For FRO 90036 and 90054 this is consistent with the Ne CRE ages reported [5,6], but for FRO 90228 the radionuclide age is much higher than the Ne age of ~1.4 Myr, which seems inexplicably low. In the following discussion our null hypothesis states that all ureilites except FRO 97013 represent a single fall. We will test this hypothesis using our radionuclide results (Table 1). The variations in Be and Al are much larger than the experimental uncertainties of 2-5%, so they must be due to differences in shielding conditions and/or chemical composition. Since variations in Al are not correlated with those in Be (Fig. 1), they are not due to different shielding depths in a single object. Instead, the variations in Be and Al are probably due to differences in bulk compositions, as we found large variations in Al (0.10.6 wt%), Ca (0.5-3.6 wt%), Fe (9.8-16.9 wt%) and in undissolved C-rich material.