Exposure Ages of H Chondrites with Helium Loss

Details Exposure Ages of H Chondrites with Helium Loss Exposure Ages of H Chondrites with Helium Loss

Sep 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30q.575s&link_type=abstract

Meteoritics, vol. 30, no. 5, page 575

Astronomy and Astrophysics

Astrophysics

5

Ages, Cosmic Ray Exposure, Chondrites, H, Gases, Noble, Radiogenic, Orbits

Scientific paper

Conclusions regarding collisional destruction, stratigraphy and regolith history of meteorite parent bodies are based on the distribution and clustering of cosmic-ray exposure ages (see e.g. [1]). Furthermore, exposure age distributions are used to discuss possible relations between groups of meteorites like differences between Antarctic and non-Antarctic meteorites [2] and meteoroid streams [3]. Also information on the orbits of meteoroids can be obtained from cosmogenic nuclides because the concentrations of cosmogenic 3He and radiogenic 4He as well as of 40Ar are influenced by solar heating in orbits with small perihelia distances. We have investigated the noble gas data of 57 H-chondrite falls and finds with low 4He concentrations (< 500(10^-8cm^3/g)[4]. Several new measurements (22) have been carried out to decrease the uncertainties of exposure age calculations. Production rates used as well as procedures for shielding corrections follow those given in [2]. In most cases a deficit of radiogenic 4He is correlated with loss of cosmogenic 3He documented by ratios < 1 of exposure ages calculated from 3He and 21Ne or 38Ar. Only 6 chondrites show concordant exposure ages in spite of large losses of radiogenic gases. The loss of radiogenic gas without loss of 3He is interpreted as degassing of the meteoritic material before (or during) ejection from their parent body. An exposure age histogram of the subgroup of H-chondrites with He-loss in comparison with that of 170 well determined H-chondrites is given in Fig. 1. The two distributions are quite distinct: Chondrites with He-loss show no clustering of exposure ages around 7 or 8 Ma, generally explained as the time of one or two catastrophic break-up events; exposure ages of chondrites with He-loss are generally shorter than those of normal H-chondrites. This is demonstrated by a high percentage with exposure ages < 2 Ma (26% versus 2.4% for all H-chondrites) and a low percentage with exposure ages > 10 Ma (18% versus 48%). The lifetime of meteoroids with small perihelion distances seems to be shorter than those that were involved in the 8 Ma break-up(s). Further investigations must show whether or not this group belongs to a specific population of H-chondrites. Figure caption:Fig.1: Exposure age histogram of H-chondrites with loss of radiogenic 4He and cosmogenic 3He. The scale is logarithmic with a resolution of 10% of the age, an estimate of the uncertainties. References: [1] Marti K. and Graf T.(1992) Annu. Rev. Earth Planet. Sci., 220, 221. [2] Schultz L. et al. (1991) GCA, 55, 59. [3] Schultz L. and Weber H. W. (1995) LPS XXVI, 1247. [4] Schultz L. and Kruse H. (1989) Meteoritics, 24, 155 + supplements. _

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