Caffee MW

Folco L

Hillegonds D

Nishiizumi K

Welten KC

FOLCO, LUIGI

Archivio della Ricerca - Università di Pisa

COSMOGENIC RADIONUCLIDE EVIDENCE OF A LARGE AND HETEROGENEOUS H3-6 CHONDRITE SHOWER: FRONTIER MOUNTAIN 90174. K. C. Welten1, K. Nishiizumi1, D. Hillegonds2, M. W. Caffee3 and L. Folco4. 1Space Sciences Laboratory, University of California, Berkeley, CA 94720. E-mail: kcwelten@berkeley.edu. 2CAMS, Lawrence Livermore National Laboratory, Livermore, CA 94550, 3Department of Physics, Purdue University, West Lafayette, IN 47907. 3Museo Nazionale dell’Antartide, Siena, Italy.  Introduction: Identifying large chondrite showers in the Ant-arctic meteorite collection is a challenging task driven by two ob-jectives: it removes the bias of these showers for determining me-teorite influx rates and it identifies possible candidates of complex exposure histories, which seem more frequent among large objects. As part of a study of the terrestrial ages and exposure history of H5 and H6-chondrites from Frontier Mountain (FRO), we previously identified a large shower, FRO 90174 [1]. The concentrations of cosmogenic 10Be, 26Al, 36Cl and 41Ca in the stone and metal frac-tion of nine members belonging to this shower constrained its pre-atmospheric radius to 80-100 cm [2]. We now present cosmogenic radionuclide data in 14 additional FRO H-chondrites of all petro-logic types. Our results provide key evidence that the FRO 90174 shower is much more heterogeneous than previously assumed and dominates the FRO meteorite population. Radionuclide concentrations. We separated and purified the metal fraction of 14 FRO H-chondrites. Cosmogenic 10Be, 26Al, 36Cl and 41Ca in the metal fractions were measured by accelerator mass spectrometry (AMS) at Lawrence Livermore National Labo-ratory. For 7 FRO H-chondrites, we analyzed cosmogenic radionu-clides in the stone fraction at the AMS facility of Purdue Univer-sity. The noble gas concentrations and detailed exposure history of this shower is discussed in [3]. Ten of the 14 FRO H-chondrites measured in this study show low concentrations of cosmogenic radionuclides in the metal phase, indicating they have the same terrestrial age, 100±20 kyr, and ex-perienced similarly high shielding conditions as the FRO 90174 shower [1,2]. The 26Al concentrations in the stone fraction of 7 H-chondrites also overlap with those previously measured for the FRO 90174 shower [2], confirming that all these specimens came from the same large object. The FRO 90174 shower now includes samples of all petrologic types (3-6) as well as fragments with a brecciated nature. The cosmogenic nuclide results thus corroborate that the specimens of the FRO 90174 shower are small samples (the largest one is merely 44 g) of a heterogeneous lithology which should be classified as an H3-6 breccia. This heterogeneous shower confirms a recent hypothesis that many brecciated ordinary chon-drite showers may have gone unnoticed in the Antarctic meteorite collections [4]. Mass distribution: The cumulative mass distribution of 467 FRO H-chondrites is not smooth like that of the FRO L-chondrites, but shows a steep increase at ~30 g. This feature can be attributed to the FRO 90174 shower, which is dominated by many fragments <30 g. From the mass distribution we estimate that more than 300 of the 467 H-chondrites are part of the FRO 90174 shower. This large shower thus explains the high H/L-chondrite ratio (3.8) at FRO. References: [1] Welten K. C. et al. 1999. Antarctic Meteorite Research 12:94-107. [2] Welten K. C. et al. 2001 Meteoritics & Planetary Science 36:301-317. [3] Leya I. et al. (this issue). [4] Welzenbach L. et al. 2005. Abstract #1425, 35th Lunar and Plane-tary Science Conference. 68th Annual Meteoritical Society Meeting (2005) 5310.pdf

Cosmogenic radionuclide evidence of a large and heterogeneous H3-6 chondrite shower: Frontier Mountain 90174

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