The histories of ordinary chondrite parent bodies: U,Th‐He age distributions

Abstract-We measured the concentrations of noble gases in 32 ordinary chondrites from the Dar al Gani (DaG) region, Libya, as well as concentrations of the cosmogenic radionuclides 14 C, 10 Be, 26 Al, 36 Cl, and 41 Ca in 18 of these samples. Although the trapped noble gases in five DaG samples show ratios typical of solar or planetary gases, in all other DaG samples, they are dominated by atmospheric contamination, which increases with the degree of weathering. Cosmic ray exposure (CRE) ages of DaG chondrites range from ~1 Myr to 53 Myr. The CRE age distribution of 10 DaG L chondrites shows a cluster around 40 Myr due to four members of a large L6 chondrite shower. The CRE age distribution of 19 DaG H chondrites shows only three ages coinciding with the main H chondrite peak at ~7 Myr, while seven ages are <5 Myr. Two of these H chondrites with short CRE ages (DaG 904 and 908) show evidence of a complex exposure history. Five of the H chondrites show evidence of high shielding conditions, including low 22 Ne/ 21 Ne ratios and large contributions of neutron-capture 36 Cl and 41 Ca. These samples represent fragments of two or more large pre-atmospheric objects, which supports the hypothesis that the high H/L chondrite ratio at DaG is due to one or more large unrecognized showers.The 14 C concentrations correspond to terrestrial ages <35 kyr, similar to terrestrial ages of chondrites from other regions in the Sahara but younger than two DaG achondrites. Despite the loss of cosmogenic 36 Cl and 41 Ca during oxidation of metal and troilite, concentrations of 36 Cl and 41 Ca in the silicates are also consistent with 14 C ages <35 kyr. The only exception is DaG 343 (H4), which has a 41 Ca terrestrial age of 150 ± 40 kyr. This old age shows that not only iron meteorites and achondrites but also chondrites can survive the hot desert environment for more than 50 kyr. A possible explanation is that older meteorites were covered by soils during wetter periods and were recently exhumed by removal of these soils due to deflation during more arid periods, such as the current one, which started ~3000 years ago.Finally, based on the 26 Al/ 21 Ne and 10 Be/ 21 Ne systematics in 16 DaG meteorites, we derived more reliable estimates of the 10 Be/ 21 Ne production rate ratio, which seems more sensitive to shielding than was predicted by the semi-empirical model of Graf et al. (1990) but less sensitive than was predicted by the purely physical model of Leya et al. (2000).

Section: Gas Retention Ages and Their Relation To 3 He And 21 Ne Expomentioning

confidence: 68%

Exposure history and terrestrial ages of ordinary chondrites from the Dar al Gani region, Libya

Welten

Nishiizumi

Finkel

et al. 2004

“…These "ages" provide information about thermal and impact histories of meteoroids and their parent bodies. About 80% of T4 ages of LL- (Wasson and Wang, 1991). In our case, 85% of the calculated ages are >2.2 Ga.…”

Section: Radiogenic Gasesmentioning

confidence: 85%

Noble gases in twenty‐one Saharan LL‐chondrites: Exposure ages and possible pairings

Scherer

Herrmann

Schultz

1998

(Part of a series ofpapers dedicated to the memory of Paul Barringer)Abstract-Nineteen LL-chondrites and two LLL-chondrites (Adrar 003 and Tanezrouft 010) from the Saharan desert of Algeria and Libya have been analysed for their He, Ne and Ar composition as well as for their abundance of 84Kr and 132Xe. Calculated 21Ne cosmic-ray exposure ages vary between 2 and 35 Ma. The age distribution is consistent with that of modem LL-chondrite falls except that no dominant peaks can be observed, especially not the one related to a 15 Ma collisional event. However, the lack of young exposure ages of <8 Ma is obvious. This is a characteristic feature of LL-chondrites. Only one of the 21 LLchondrites, namely Acfer 066, contains solar gases and is thus considered a regolith breccia. Three specimens reveal considerable loss of 3He, probably due to periods of elevated temperatures in orbits with small perihelion distances. Furthermore, severe loss of 4He and 4oAr is found in two samples. Considering possible pairings, we suggest 14 individual falls basically on the basis of the noble gas data, the petrographic subclassification and by taking the find location into consideration. However, there are constraints on confirming pairings solely on the basis of our studies. Thus, we can only exclude individual samples with a unique noble gas fingerprint from paired specimens.

“…One of these falls is Chainpur, whch Wasson and Wang (1991) listed as an LlLL3 chondrite because its kamacite Co contents are outside the nominal LL-chondrite range (Rubin, 1990). Thus, although the five LL3 chondrites studied here (four falls, one fmd) represent only 7% of known LL3 chondrites, they include 50% of well-classified LL3 falls.…”

Section: Analytical Proceduresmentioning

confidence: 89%

Size‐frequency distributions of chondrules and chondrule fragments in LL3 chondrites: Implications for parent‐body fragmentation of chondrules

Nelson

Rubin

2002

Abstract-We measured the sizes and textural types of 719 intact chondrules and 1322 chondrule fragments in thin sections of Semarkona (LL3.0), Bishunpur (LL3. l), Krymka (LL3. I), Piancaldoli (LL3.4) and Lewis Cliff 88175 (LL3.8). The mean apparent diameter of chondrules in these LL3 chondrites is 0.80 q5 units or 570 pm, much smaller than the previous rough estimate of -900 pm. Chondrule fragments in the five LL3 chondrites have a mean apparent cross-section of 1.60 q5 units or 330 pm. The smallest fi-agments are isolated olivine and pyroxene grains; these are probably phenocrysts liberated from disrupted porphyritic chondrules. All five LL3 chondrites have fragment1 chondrule number ratios exceeding unity, suggesting that substantial numbers of the chondrules in these rocks were shattered. Most fragmentation probably occurred on the parent asteroid. Porphyritic chondrules (porphyritic olivine + porphyritic pyroxene + porphyritic olivine-pyroxene) are more readily broken than droplet chondrules (barred olivine + radial pyroxene + cryptocrystalline). The porphyritic fragmentkhondrule number ratio (2.0) appreciably exceeds that of droplet-textured objects (0.9). Intact droplet chondrules have a larger mean size than intact porphyritic chondrules, implying that large porphyritic chondrules are fragmented preferentially. This is consistent with the relatively low percentage of porphyritic chondrules within the set of the largest chondrules (57%) compared to that within the set of the smallest chondrules (81%). Differences in mean size among chondrule textural types may be due mainly to parent-body chondrule-fragmentation events and not to chondruleformation processes in the solar nebula.