Noble gas and oxygen isotope studies of aubrites: A clue to origin and histories

Abstract

Noble gas measurements were performed for nine aubrites: Bishopville, Cumberland Falls, Mayo Belwa, Mount Egerton, Norton County, Peñ a Blanca Spring, Shallowater, ALHA 78113 and LAP 02233. These data clarify the origins and histories, particularly cosmic-ray exposure and regolith histories, of the aubrites and their parent body(ies). Accurate cosmic-ray exposure ages were obtained using the 81 Kr-Kr method for three meteorites: 52 ± 3, 49 ± 10 and 117 ± 14 Ma for Bishopville, Cumberland Falls and Mayo Belwa, respectively. Mayo Belwa shows the longest cosmic-ray exposure age determined by the 81 Kr-Kr method so far, close to the age of 121 Ma for Norton County. These are the longest ages among stony meteorites. Distribution of cosmic-ray exposure ages of aubrites implies 4-9 break-up events (except anomalous aubrites) on the parent body. Six aubrites show ''exposure at the surface'' on their parent body(ies): (i) neutron capture 36 Ar, 80 Kr, 82 Kr and/or 128 Xe probably produced on the respective parent body (Bishopville, Cumberland Falls, Mayo Belwa, Peñ a Blanca Spring, Shallowater and ALHA 78113); and/or (ii) chondritic trapped noble gases, which were likely released from chondritic inclusions preserved in the aubrite hosts (Cumberland Falls, Peñ a Blanca Spring and ALHA 78113). The concentrations of 128 Xe from neutron capture on 127 I vary among four measured specimens of Cumberland Falls (0.5-76 · 10 À14 cm 3 STP/g), but are correlated with those of radiogenic 129 Xe, implying that the concentrations of ( 128 Xe) n and ( 129 Xe) rad reflect variable abundances of iodine among specimens. The ratios of ( 128 Xe) n /( 129 Xe) rad obtained in this work are different for Mayo Belwa (0.045), Cumberland Falls (0.015) and Shallowater (0.001), meaning that neutron fluences, radiogenic 129 Xe retention ages, or both, are different among these aubrites. Shallowater contains abundant trapped Ar, Kr and Xe (2.2 · 10 À7 , 9.4 · 10 À10 and 2.8 · 10 À10 cm 3 STP/g, respectively) as reported previously (Busemann and Eugster, 2002). Isotopic compositions of Kr and Xe in Shallowater are consistent with those of Q (a primordial noble gas component trapped in chondrites). The Ar/Kr/Xe compositions are somewhat fractionated from Q, favoring lighter elements. Because of the unbrecciated nature of Shallowater, Q-like noble gases are considered to be primordial in origin. Fission Xe is found in Cumberland Falls, Mayo Belwa, Peñ a Blanca Spring, ALHA 78113 and LAP 02233. The majority of fission Xe is most likely 244 Pu-derived, and about 10-20% seems to be 238 U-derived at 136 Xe. The observed ( 136 Xe) Pu corresponds to 0.019-0.16 ppb of 244 Pu, from which the 244 Pu/U ratios are calculated as 0.002-0.009. These ratios resemble those of chondrites and other achondrites like eucrites, suggesting that no thermal resetting of the Pu-Xe system occurred after $4.5 Ga ago. We also determined oxygen isotopic compositions for four aubrites with chondritic noble gases and a new aubrite LAP 02233. In spite of their chondritic noble gas sign...

Cosmic-Ray Exposure Ages of Meteorites

“…It is a curiosity often remarked that this atypically large CRE age was also the first one ever reported for a stone (Begemann et al, 1957). Even after 40 years of analyses, Norton County, along with one other aubrite, Mayo Belwa (Miura et al, 2007), ranks among the stones with the longest CRE ages. At the other extreme, the smallest CRE age for aubrites, $12 Ma, belongs to the type specimen for the group, the Aubres fall.…”

“…Eberhardt et al (1965) noted two features important in interpreting the irradiation histories of aubrites, namely, the presence of solar noble gases and of isotopic anomalies associated with irradiation by thermal neutrons. Six of the aubrites contain solar noble gases (Lorenzetti et al, 2003, Miura et al, 2007, a fact that has significance for their CRE ages (Caffee et al, 1988). First, the presence of solar noble gases in interior samples of meteorites implies a two-stage (at least) irradiation by GCRs.…”

The origin of aubrites: Evidence from lithophile trace element abundances and oxygen isotope compositions

“…Aubrites and enstatite chondrites clearly formed under similar, extremely reducing formation conditions and in addition display almost identical isotopic compositions for many elements (e.g., O, Ti, Cr, S, Si, Clayton and Mayeda, 1996;Newton et al, 2000;Miura et al, 2007;Trinquier et al, 2007;Zhang et al, 2012;Savage and Moynier, 2013;Defouilloy et al, 2016). This suggests that their parent body (or bodies) accreted from materials which condensed in similar regions of the solar nebula.…”

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

“…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.…”

“…Among the stony meteorites, the Norton County aubrite has one of the longest CRE ages: 111-115 Ma (Lorenzetti et al 2003;Miura et al 2007;Herzog et al 2011). Considering that the recovery of the main mass is over 1000 kg, the size of the meteoroid is large enough to accumulate many kinds of cosmogenic nuclides.…”