Calibration of cosmogenic noble gas production in ordinary chondrites based on 36Cl‐36Ar ages. Part 1: Refined produced rates for cosmogenic 21Ne and 38Ar

Self Cite

106

We present noble gas data for 16 shergottites, 2 nakhlites (NWA 5790, NWA 10153), and 1 angrite (NWA 7812). Noble gas exposure ages of the shergottites fall in the 1–6 Ma range found in previous studies. Three depleted olivine‐phyric shergottites (Tissint, NWA 6162, NWA 7635) have exposure ages of ~1 Ma, in agreement with published data for similar specimens. The exposure age of NWA 10153 (~12.2 Ma) falls in the range of 9–13 Ma reported for other nakhlites. Our preferred age of ~7.3 Ma for NWA 5790 is lower than this range, and it is possible that NWA 5790 represents a distinct ejection event. A Tissint glass sample contains Xe from the Martian atmosphere. Several samples show a remarkably low (21Ne/22Ne)cos ratio < 0.80, as previously observed in a many shergottites and in various other rare achondrites. This was explained by solar cosmic ray‐produced Ne (SCR Ne) in addition to the commonly found galactic cosmic ray‐produced Ne, implying very low preatmospheric shielding and ablation loss. We revisit this by comparing measured (21Ne/22Ne)cos ratios with predictions by cosmogenic nuclide production models. Indeed, several shergottites, acalpulcoites/lodranites, angrites (including NWA 7812), and the Brachina‐like meteorite LEW 88763 likely contain SCR Ne, as previously postulated for many of them. The SCR contribution may influence the calculation of exposure ages. One likely reason that SCR nuclides are predominantly detected in meteorites from rare classes is because they usually are analyzed for cosmogenic nuclides even if they had a very small (preatmospheric) mass and hence low ablation loss.

Section: Resultsmentioning

confidence: 95%

“…In such cases, the ( 21 Ne/ 22 Ne) cos ratio will also be a useful shielding indicator to calculate 21 Ne exposure ages (e.g., Dalcher et al. ). Otherwise, this ratio cannot be used for this purpose.…”

Section: Scr Noble Gases In Shergottites and Other Achondritesmentioning

confidence: 99%

Noble gases in 18 Martian meteorites and angrite Northwest Africa 7812—Exposure ages, trapped gases, and a re‐evaluation of the evidence for solar cosmic ray‐produced neon in shergottites and other achondrites

Wieler

Huber

Busemann

et al. 2016

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106

“…An alternative approach relies on empirical production rates and production rate ratios (valid only for meteoroids with a size <228 g cm −2 ; Leya and Masarik ) calculated as a function of cosmogenic 22 Ne/ 21 Ne (Dalcher et al. ). The CRE ages determined with the different methods are indistinguishable within their uncertainties (Table ).…”

Section: Resultsmentioning

confidence: 99%

Park Forest (L5) and the asteroidal source of shocked L chondrites

Meier

Welten

Riebe

et al. 2017

The Park Forest (L5) meteorite fell in a suburb of Chicago, Illinois (USA) on March 26, 2003. It is one of the currently 25 meteorites for which photographic documentation of the fireball enabled the reconstruction of the meteoroid orbit. The combination of orbits with pre‐atmospheric sizes, cosmic‐ray exposure (CRE), and radiogenic gas retention ages (“cosmic histories”) is significant because they can be used to constrain the meteoroid's “birth region,” and test models of meteoroid delivery. Using He, Ne, Ar, 10Be, and 26Al, as well as a dynamical model, we show that the Park Forest meteoroid had a pre‐atmospheric size close to 180 g cm−2, 0–40% porosity, and a pre‐atmospheric mass range of ~2–6 tons. It has a CRE age of 14 ± 2 Ma, and (U, Th)‐He and K‐Ar ages of 430 ± 90 and 490 ± 70 Ma, respectively. Of the meteorites with photographic orbits, Park Forest is the second (after Novato) that was shocked during the L chondrite parent body (LCPB) break‐up event approximately 470 Ma ago. The suggested association of this event with the formation of the Gefion family of asteroids has recently been challenged and we suggest the Ino family as a potential alternative source for the shocked L chondrites. The location of the LCPB break‐up event close to the 5:2 resonance also allows us to put some constraints on the possible orbital migration paths of the Park Forest meteoroid.

“…The 3 He cosmic ray exposure (CRE) age was calculated using Eugster () and 21 Ne and 38 Ar CRE ages were calculated using equations provided by Dalcher et al. (), which yield ages that are 6–7% higher than those of Eugster () for 21 Ne and Schultz et al. () for 38 Ar.…”

Section: Resultsmentioning

confidence: 99%

“…The 21 Ne and 38 Ar cosmic ray exposure (CRE) ages are calculated according to Dalcher et al. (), the 3 He age according to Eugster (), in all three cases using the ( 22 Ne/ 21 Ne) cos ratio of 1.19 to correct for shielding. The 21 Ne and 38 Ar ages agree with each other at 72 Ma, while the somewhat lower 3 He age of 58 Ma may indicate some loss of cosmogenic 3 He, as is also indicated by the 3 He/ 21 Ne ratio, which falls ~15% below the correlation of 3 He/ 21 Ne versus 22 Ne/ 21 Ne (Eberhardt et al.…”

Section: Resultsmentioning

confidence: 99%

The Vicência meteorite fall: A new unshocked (S1) weakly metamorphosed (3.2)LLchondrite

Keil

Zucolotto

Krot

et al. 2015

The Vicência meteorite, a stone of 1.547 kg, fell on September 21, 2013, at the village Borracha, near the city of Vicência, Pernambuco, Brazil. It was recovered immediately after the fall, and our consortium study showed it to be an unshocked (S1) LL3.2 ordinary chondrite. The LL group classification is based on the bulk density (3.13 g cm À3 ); the chondrule mean apparent diameter (0.9 mm); the bulk oxygen isotopic composition (d 17 O = 3.768 AE 0.042&, d 18 O = 5.359 AE 0.042&, D 17 O = 0.981 AE 0.020&); the content of metallic Fe,Ni (1.8 vol%); the Co content of kamacite (1.73 wt%); the bulk contents of the siderophile elements Ir and Co versus Au; and the ratios of metallic Fe 0 /total iron (0.105) versus total Fe/Mg (1.164), and of Ni/ Mg (0.057) versus total Fe/Mg. The petrologic type 3.2 classification is indicated by the beautifully developed chondritic texture, the standard deviation (~0.09) versus mean Cr 2 O 3 content (~0.14 wt%) of ferroan olivine, the TL sensitivity and the peak temperature and peak width at half maximum, the cathodoluminescence properties of chondrules, the content of trapped 132 Xe tr (0.317 9 10 À8 cm 3 STP g À1 ), and the Raman spectra for organic material in the matrix. The cosmic ray exposure age is~72 Ma, which is at the upper end of the age distribution of LL group chondrites. The meteorite is unusual in that it contains relatively large, up to nearly 100 lm in size, secondary fayalite grains, defined as olivine with Fa >75 , large enough to allow in situ measurement of oxygen and Mn-Cr isotope systematics with SIMS. Its oxygen isotopes plot along a mass-dependent fractionation line with a slope of~0.5 and D 17 O of 4.0 AE 0.3&, and are similar to those of secondary fayalite and magnetite in the unequilibrated chondrites EET 90161, MET 96503, and Ngawi. These data suggest that secondary fayalite in Vicência was in equilibrium with a fluid with a D 17 O of~4&, consistent with the composition of the fluid in equilibrium with secondary magnetite and fayalite in other unequilibrated ordinary chondrites. Secondary fayalite and the chondrule olivine phenocrysts in Vicência are not in isotopic equilibrium, consistent with low-temperature formation of fayalite during aqueous alteration on the LL parent body. That alteration, as dated by the 53 Mn-53 Cr chronology age of secondary fayalite, took place 4:0 þ1:4 À1:1 Ma after formation of CV CAIs when anchored to the quenched angrite D'Orbigny.