The complex exposure history of the Jiddat al Harasis 073 L-chondrite shower

Abstract

We measured the concentrations and isotopic compositions of He, Ne, and Ar in 29 bulk samples from 11 different strewn field fragments of the large Jiddat al Harasis (JaH) 073 L6 chondrite shower, including 7 samples from known locations within the main mass. In addition, we measured the concentrations of cosmogenic 10Be, 26Al, 36Cl, and 41Ca in 10 samples. All fragments of this shower are characterized by low 10Be concentrations (7.6-12.8 dpm/kg), high 26Al/10Be ratios (3.55), large contributions of neutron capture 41Ca (200-1800 dpm/kgCa), low 3He/21Ne ratios (1.5-3.0), large variations in cosmogenic 21Ne (1.2-12) x 10^(-8) cm^3 STP/g, and significant contributions of neutron-capture 36Ar. Stepwise heating experiments show that neutron-capture produced 36Ar is predominantly released between 1000-1200 degrees C. All these results are consistent with a first-stage exposure of ~65 Ma within ~20 cm of the surface of the L-chondrite parent body, followed by ejection of a 1.52 m large object, which was then delivered to Earth within about 0.5 and 0.7 Ma. The cosmogenic nuclide data in JaH 073 thus corroborate the trend that many of the large chondrites studied so far experienced a complex exposure history. The observed 3He/21Ne ratios of 2.53.0 in the most shielded samples (including those of the main mass) are lower than predicted by model calculations, but similar to the lowest values found in the large Gold Basin L-chondrite shower. The Bern plot, which gives a linear correlation for 3He/21Ne versus 22Ne/21Ne, is evidently not valid for very high shielding. Some of our measured 22Ne/21Ne ratios in JaH 073 are lower than 1.06, which is not well understood, but might be explained by loss of cosmogenic neon from shocked sodium-rich plagioclase during terrestrial weathering. The amount of trapped atmospheric argon in the JaH 073 fragments varies by almost two orders of magnitude and shows only a weak correlation with the size of the fragments, which range from <100 g to >50 kg. Finally, low concentrations of radiogenic 4He and 40Ar indicate incomplete degassing <1 Ga ago, probably at the main collision event on the L-chondrite parent body ~480 Ma ago.