Light noble gases in Jilin: More of the same and something new

Abstract

Abstract— Results are reported for documented samples from two drill cores and for specimens from the strewn field of the largest known stone meteorite, the H chondrite Jilin. Core B was found to have been parallel to the surface of Jilin during its first stage of irradiation, in 2π geometry. Core A was normal to the 2π surface; in it the mean attenuation length for the production by galactic cosmic rays of 38Ar from metal and of 21Ne from Mg, Al, Si in silicates was found to be the same. A numerical value for the mean attenuation length of (71 ± 4) cm or (246 ± 14) g × cm−2 follows if corrections for the contribution from the second stage of exposure are based on T2 = 0.32 Ma; agreement with the lower values of ∼180 g × cm−2 obtained from lunar studies and target data requires T2 to be about twice as long. Previous results are confirmed that in specimens with high contents of stable spallogenic gases the ratio 21Nebulk/38Armetal is low. The suggestion had been, and is, mat this is a transition effect in near‐surface samples where the secondary cascade of nuclear‐active particles, and hence the production of 21Ne, was not yet fully developed. This suggestion is borne out by the present results on two samples that, based on cosmic‐ray tracks and 60Co content, are certified near‐surface samples (although, strictly speaking, this is true only for the depth of burial during the second stage of irradiation). Cosmic‐ray produced 60Co is positively correlated with 4He content, indicating that significant losses of 4He occurred when the Jilin meteoroid had acquired already its final size and shape and that the losses were more severe for near‐surface samples than for such from the 4π interior. Presumably, the losses were caused by a thermal spike associated with the excavation of Jilin from its parent body. The same event caused losses of part of the 3He produced during the first irradiation stage. From the systematics of the 3He/21Ne vs. 4He correlation, we derive for the second 4π irradiation a production ratio 3He/21Ne = 3.2 ± 0.4.