A gamma‐ray spectroscopy survey of Omani meteorites

Abstract

The gamma-ray activities of 33 meteorite samples (30 ordinary chondrites, 1 Mars meteorite, 1 iron, 1 howardite) collected during Omani-Swiss meteorite search campaigns 2001-2008 were nondestructively measured using an ultralow background gamma-ray detector. The results provide several types of information: Potassium and thorium concentrations were found to range within typical values for the meteorite types. Similar mean 26 Al activities in groups of ordinary chondrites with (1) weathering degrees W0-1 and low 14 C terrestrial age and (2) weathering degree W3-4 and high 14 C terrestrial age are mostly consistent with activities observed in recent falls. The older group shows no significant depletion in 26 Al. Among the least weathered samples, one meteorite (SaU 424) was found to contain detectable 22 Na identifying it as a recent fall close to the year 2000. Based on an estimate of the surface area searched, the corresponding fall rate is~120 events/10 6 km 2 *a, consistent with other estimations. Twelve samples from the large JaH 091 strewn field (total mass~4.5 t) show significant variations of 26 Al activities, including the highest values measured, consistent with a meteoroid radius of~115 cm. Activities of 238 U daughter elements demonstrate terrestrial contamination with 226 Ra and possible loss of 222 Rn. Recent contamination with small amounts of 137 Cs is ubiquitous. We conclude that gamma-ray spectroscopy of a selection of meteorites with low degrees of weathering is particularly useful to detect recent falls among meteorites collected in hot deserts.

The effects of terrestrial weathering on samarium‑neodymium isotopic composition of ordinary chondrites

“…Uranium enrichment is the other weathering proxy that has been observed in hot desert meteorites which is suggested to form as a result of contamination by soil fragments and interaction with fluids originating from the soil (e.g., Hezel et al, 2011). As a consequence of these processes, Lu/Hf and Th/U ratios in our analyzed meteorites, in particular in hot desert samples have been Pb isotopic systems which also have been reported by and Weber et al (2017).…”

“…Observing these effects on a fall meteorite emphasizes the importance of this matter. Isotopic changes caused by meteorite weathering are also reported in H (Stephant et al, 2018), O (Stelzner and Heide, 1996), noble gases (Cartwright et al, 2010;Kaneoka, 1983;Schultz et al, 2005), Sr (Borg et al, 2016(Borg et al, , 2003Brandon et al, 2004;Elardo et al, 2014;Shih et al, 2007), Hf (Sokol et al, 2008;Tatsumoto et al, 1981), Os (Borg et al, 2003;Brandon et al, 2012), Fe (Saunier et al, 2010), U-and Th-series as well as Cs (Weber et al, 2017), and Pb (Tatsumoto et al, 1981) isotopes.…”

Radionuclide activities in recent chondrite falls determined by gamma‐ray spectrometry: Implications for terrestrial age estimates

“…In Oman hot desert meteorite finds, 137 Cs has been detected, but the determined activities were only <0.2 to 10.3 dpm per kg (median 1.25 dpm per kg) and with lower bulk activities in large masses (Weber et al. ). In this context, it should be noted that contamination of 137 Cs likely is concentrated to the exposed surface of meteorites, so activity values obtained with gamma‐ray spectrometry are not fully representative for concentrations in the analyzed bulk material.…”

“…Potassium concentrations have often been used as a quality control when assessing the precision of gamma spectrometry measurements on ordinary chondrites (e.g., Weber et al 2017). Occasionally, studies have adopted published average bulk values of K in corresponding chondrite groups for internal standardization.…”

Meteorite terrestrial ages in Oman based on gamma spectrometry and sediment dating, focusing on the Ramlat Fasad dense collection area

“…However, the 14 C method cannot resolve the relative age distribution in the youngest samples, so it has so far been unclear to what extent the least weathered chondrites from Oman represent relatively recent falls and whether the proportion of relatively recent falls within the whole population is consistent with their terrestrial age distribution. Prior studies using gamma-ray spectrometry have identified two meteorites from Oman, SaU 424 (Weber et al, 2017) and SaU 606 (Rosén et al, 2020), that fell during the two decades covered by 22 Na measurements of these studies (~1986-2018). In the present study, we continue the search for chondrites with young terrestrial ages among the least weathered meteorites in the Omani-Swiss meteorite search collection using the previously successful method of estimating terrestrial ages by measuring 22 Na/ 26 Al activity ratios in samples which were so far not analyzed.…”