Regolith history of the aubritic meteorite parent body revealed by neutron capture effects on Sm and Gd isotopes

Abstract

Enstatite achondrites (aubrites) when compared to other stone meteorites have unusually long cosmic-ray exposure (CRE) ages. We report here the 150 Sm/ 149 Sm and 158 Gd/ 157 Gd ratios in six different structural phases, i.e., light and dark (shocked) grains and in matrix materials of Pesyanoe, in three different fragments from Pena Blanca Spring, and in one from Norton County, Shallowater, and Khor Temiki, to investigate the regolith history on the aubrite parent body. The results from phases components of Pesyanoe confirm earlier reported evidence for regolith irradiation of several aubrites. The inferred neutron fluences for six Pesyanoe separates vary between (2.13 and 2.82) · 10 16 n cm À2 . The fluences also significantly exceed those expected from cosmic-ray irradiation during transit to Earth and approach those observed in the lunar regolith. These observations confirm that the brecciated Pesyanoe meteorite, which contains solar wind (SW) gases only in dark phases, was processed in a regolith and that structural phases were differentially irradiated before compaction. On the other hand, in some aubrites (Mt. Egerton, Shallowater, Pena Blanca Spring, Norton County) neutron capture effects may entirely be due to space irradiation.

Heterogeneous Isotopic Anomalies of Sm and Gd in the Norton County Meteorite: Evidence for Irradiation From the Active Early Sun

“…These data give neutron fluences ranging from 1.89 × 10 16 to 3.54 × 10 16 n cm −2 . In particular, one of three sets of whole rock data (3.54 × 10 16 n cm −2 ) is 2.3-2.9 times higher than data reported previously (Hidaka et al 1999(Hidaka et al , 2006, although the other two sets of data are almost similar to each other and to previous data. The variation in neutron fluences can be caused by different depths in the case of a large-sized meteorite body.…”

“…The discrepancy of neutron fluences estimated from neutron-capture produced 150 Sm and 41 Ca suggests the multi-stage irradiating history of Norton County (Welten et al 2004;Herzog et al 2011). Here we found very large isotopic shifts of Sm and Gd in specific phases of the Norton County meteorite, corresponding to 5-10 times higher neutron fluences than reported previously (Eugster et al 1970;Hidaka et al 1999Hidaka et al , 2006Herzog et al 2011). In this paper, we will discuss the possible existence of preirradiation materials migrating into the meteorite body.…”

“…Cosmogenic nuclides such as long-lived radioisotopes ( 10 Be, 26 Al, 36 Cl, and so on) and stable noble gas isotopes ( 3 He, 21 Ne, 36 Ar, 38 Ar, and so on) have been widely used to determine the cosmic-ray exposure (CRE) ages of meteorites in space. Sm and Gd isotopic shifts by neutron capture reactions also provide useful information for understanding the exposure records of meteorites because 149 Sm and 157 Gd have very large thermal neutron capture cross sections and shift to 150 Sm and 158 Gd, respectively (Eugster et al 1970;Hidaka et al 1999Hidaka et al , 2006.…”

“…Aubrites are coarse-grained rocks consisting mostly of nearly FeO-free enstatite, with minor and trace minerals very heterogeneously distributed throughout the rocks. Most aubrites generally show longer CRE ages than any other stony meteorites, and some of them are known to have experienced at least two exposures to cosmic rays, on their parent body and in space (Hidaka et al 1999(Hidaka et al , 2006Lorenzetti et al 2003;Welten et al 2004;Miura et al 2007;Herzog et al 2011). Furthermore, aubrites are known to have experienced numerous collisions, and most of them are brecciated.…”

“…In particular, Pesyanoe shows significant and heterogeneous isotopic variations of N, Ne, and Xe due to cosmic-ray irradiation, even in a small fragment (Müller & Zähringer 1966;Mathew & Marti 2003). Furthermore, Sm and Gd isotopic studies have revealed that the neutron fluences of four different phases taken from a small fragment of Pesyanoe show significant variation of up to 30% relative to the matrix portion (Hidaka et al 2006). These results cannot be explained by single-stage cosmic-ray irradiation records of Pesyanoe, suggesting isotopic evidence for preirradiation effects in the record of the regolith before compaction into the meteorite parent body.…”