Light noble gas records and cosmic ray exposure histories of recent ordinary chondrite falls

Abstract

We measured noble gas concentrations and isotopic ratios (He, Ne, and Ar isotopes) in six recent ordinary chondrite falls: Mangui (L6), Viñales (L6), Ozerki (L6), Tamdakht (H5), Kheneg Ljouâd (LL5/6), and Katol (L6). Among them, the three L6 chondrites Mangui, Viñales, and Ozerki fell in only a few months' interval; their apparent similar petrographic and mineralogic characteristics might indicate source crater pairing. To test this hypothesis, we have investigated those meteorites for their cosmic ray exposure (CRE) histories, using the cosmogenic noble gases 3He, 21Ne, and 38Ar. We systematically (re)calculated the CRE ages as well as the gas retention ages of these meteorites. The CRE age of the Mangui is, based on noble gases, <1 Ma, which is unusually short for an L chondrite. Indeed, the range of exposure ages for L chondrites is generally distributed between ˜1 and ˜60 Ma, with major peaks occurring around ˜5, ˜30, and ˜40 Ma. In addition, the cosmogenic 3Hecos data of two Mangui duplicates are consistent with a remarkably high loss of helium by diffusion due to heating by solar radiation. Such a short parent body‐Earth transfer time (<1 Ma) can be explained by a delivery from an Earth‐crossing object. Regarding the other L6 chondrites, Viñales has a nominal CRE age of ˜9.4 Ma, whereas the Ozerki meteorite has a nominal CRE age of ˜1.2 Ma, which is consistent with Korochantseva et al. (2019). Based on their CRE ages as well as on their gas retention ages, it appears that none of these three recent L6 chondrite falls are source crater paired, and therefore, all three originate from different meteoroids. The nominal exposure ages of Tamdakht, Kheneg Ljouâd, and Katol are ˜3.2, ˜11, and ˜30 Ma, respectively, and are consistent with identified age peaks on the exposure age histogram of H, LL, and L chondrites, respectively. The nominal CRE age of Tamdakht is consistent with previous observations for H chondrites and implies that they are dominated by small impact events occurring in several parent bodies.

Light noble gases in 11 achondrites: Cosmic ray exposure ages, gas retention ages, and preatmospheric sizes

“…The noble gas concentrations and isotopic ratios ( 3,4 He, 20,21,22 Ne, and 36,38,40 Ar) were measured in multicollection mode using the noble gas mass spectrometer Noblesse © from Nu Instruments company. A detailed description of the analytical procedures is available in, for example, Smith et al (2021). Briefly, bulk samples with masses ranging from $3.2 to $5.3 mg were first cleaned with ethanol in an ultrasonic bath, dried, weighted, and then loaded into the laser sample chamber of the noble gas extraction and purification line.…”

“…A detailed description of the analytical procedures is available in, for example, Smith et al. (2021). Briefly, bulk samples with masses ranging from ~3.2 to ~5.3 mg were first cleaned with ethanol in an ultrasonic bath, dried, weighted, and then loaded into the laser sample chamber of the noble gas extraction and purification line.…”

“…The released gases were first cleaned using a series of getters, operating at room temperature and 200°C, to remove all reactive gases which would otherwise compromise the measurements (e.g., CO 2 , H 2 O, hydrocarbons, etc. ; Smith et al, 2021). The He-Ne fraction was separated from the Ar fraction using an activated charcoal trap held at LN 2 temperature for 20 min.…”

“…Cosmic ray exposure (CRE) ages are useful in establishing histograms in which peaks represent the major collisions in the parent bodies responsible for the ejection of meteorites of each type. The determination of CRE ages not only provides information about the delivery mechanisms of small bodies in the solar system but also helps to identify possible source crater pairing (e.g., Eugster et al., 2006; Herzog & Caffee, 2014; Smith et al., 2021; Stephenson et al., 2017; Zeigler et al., 2005). Gas retention ages based on radiogenic 4 He and 40 Ar provide valuable information about the thermal and alteration history of meteorites.…”

“…Gas retention ages based on radiogenic 4 He and 40 Ar provide valuable information about the thermal and alteration history of meteorites. The combination of both CRE ages and gas retention ages is a good way to determine whether two meteorites are source crater paired (e.g., Eugster et al., 2006; Herzog & Caffee, 2014; Smith et al., 2021; Stephenson et al., 2017). Particularly, CRE age histograms, which represent records of CRE ages for a same meteorite class, are useful to investigate catastrophic impact events in the asteroid belt.…”