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

Abstract

Radionuclide activities were measured in the low-background gamma-ray spectrometry facility GeMSE in eight meteorite falls (Lost City, Tamdakht, Huaxi, Boumdeid, Xining, Kamargaon, Degtevo, and Ouidiyat Sbaa) and two finds (SaU 606 and M€ urtschenstock) to evaluate the use of radionuclides for terrestrial age estimates. Results indicate that these meteorites were all derived from small-(r < 25 cm) to medium-sized (r < 65 cm) meteoroids. Short-lived 48 V (t 1/2 = 16.0 d) and 51 Cr (t 1/2 = 27.7 d) were only detected in Oudiyat Sbaa (EH), while 7 Be (t 1/2 = 53.1 d) was also detected in Degtevo (H) and Kamargaon (L), in agreement with reported fall dates. The 22 Na/ 26 Al activity ratio in Huaxi agrees with the previously reported short cosmic-ray exposure age of this meteorite while 22 Na/ 26 Al in Kamargaon likely records a complex exposure history. Bayesian statistical analysis verifies the detection of very low activities of 44 Ti (t 1/2 = 60 a) in the relatively large H chondrites (>100 g) Degtevo, Huaxi, Tamdakht, Lost City, and SaU 606. Additionally, large samples from Oudiyat Sbaa (EH) and Kamargaon (L) gave positive detections. For H chondrite target compositions, detected 44 Ti(Fe+Ni)/ 26 Al averaged 0.055 AE 0.013. Activities of 22 Na and 54 Mn in SaU 606 show that this meteorite fell between July and September 2012, making SaU 606 the second recent fall from Oman identified using gamma-ray spectrometry. The upper activity limit of 22 Na in the M€ urtschenstock meteorite shows that it fell prior to 1999 and is not related to a bolide observation in 2015. M€ urtschenstock shows 137 Cs~109 higher than previously determined in Oman meteorites, likely due to Chernobyl fallout.

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

“…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. Furthermore, recent results showing relatively constant 44 Ti/ 26 Al activity ratios in observed chondrite falls present an opportunity to extend the low range terrestrial age estimates (approximately 20-300 a) of suitable meteorites from the collection (Rosén et al, 2020).…”

“…Figure 6b shows the integrated count rates at 1156.2-1157.8 keV (1157 keV AE 1σ) in samples which were analyzed for the current study compared to count rates in previously analyzed samples from recent meteorite falls. Detection efficiencies are consistently higher for small sample masses, but the total detected counts from (Rosén et al, 2020). Falls (2008-2017) >100 g (green symbols) show higher count rate than meteorite finds (red symbols) with similar mass.…”

“…In order to maintain an efficient screening schedule, the sample spectra were continuously evaluated during ongoing analyses. The measurement times (Table 1) were adjusted accordingly by extending the measurements when apparent peaks from short-lived cosmogenic radionuclides were observed, as in the previously described case of SaU 606 (Rosén et al, 2020) Ti by concentrating the main target elements of these isotopes (Co and Fe + Ni, respectively) in a smaller mass and thereby increasing detection efficiencies while decreasing the contributions from 40 K and 26 Al to the Compton background during analyses. The starting mass amounted to approximately 78 g after trimming off rust and fusion crust.…”

“…After leaching and cleaning the material amounted to approximately 17 g. The final concentrate was wrapped in cling wrap, folded to an oblate-shaped package, and measured in the GeMSE facility during several measurement sessions amounting to a total screening time of 61 days. All counting efficiencies were simulated using GEANT4 v9.6p03 (Agostinelli et al, 2003) according to similar procedures as have previously been described elsewhere (Rosén et al, 2020;von Sivers et al, 2016). The detection efficiencies for the RaF 033 metal concentrate measurements were simulated using the bulk density given by the sample mass and measured dimensions.…”

“…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.…”