39Ar40Ar age and petrology of Chico: Large-scale impact melting on the L chondrite parent body

Bogard, D. D.; Garrison, D. H.; Norman, Marc D.; Scott, E. R. D.; Keil, K.

doi:10.1016/0016-7037(95)00051-z

Cited by 118 publications

(118 citation statements)

References 32 publications

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“…Ages were calculated relative to the age of the Nl-25 hornblende (2.65 Ga) using the K decay parameters reported by Steiger and Jäger (1977). Characteristics of the NL-25 hornblende standard and experimental details are given in Bogard et al (1995Bogard et al ( , 2000. Nakhlite ArAr data are reported in Electronic annex EA-2.…”

Section: Samples and Experimental Methodsmentioning

confidence: 99%

39Ar–40Ar ages of martian nakhlites

Park

Garrison

Bogard

2009

Geochimica et Cosmochimica Acta

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We report 39 Ar- 40 Ar ages of whole rock (WR) and plagioclase and pyroxene mineral separates of nakhlites MIL 03346 and Y-000593, and of WR samples of nakhlites NWA 998 and Nakhla. All age spectra are complex and indicate variable degrees of 39 Ar recoil and variable amounts of trapped 40 Ar in the samples. Thus, we examine possible Ar-Ar ages in several ways. From consideration of both limited plateau ages and isochron ages, we prefer Ar-Ar ages of NWA 998 = 1334 ± 11 Ma, MIL 03346 = 1368 ± 83 Ma (mesostasis) and 1334 ± 54 Ma (pyroxene), Y-000593 = 1367 ± 7 Ma, and Nakhla = 1357 ± 11 Ma, (2r errors). For NWA 998 and MIL 03346 the Ar-Ar ages are within uncertainties of preliminary Rb-Sr isochron ages reported in the literature. These Ar-Ar ages for Y-000593 and Nakhla are several Ma older than Sm-Nd ages reported in the literature. We conclude that the major factor in producing Ar-Ar ages slightly too old is the presence of small amounts of trapped martian or terrestrial 40 Ar on weathered grain surfaces that was degassed along with the first several percent of 39 Ar. A total K-40 Ar isochron for WR and mineral data from five nakhlites analyzed by us, plus Lafayette data in the literature, gives an isochron age of 1325 ± 18 Ma (2r). We emphasize the precision of this isochron over the value of the isochron age. Our Ar-Ar data are consistent with a common formation age for nakhlites. The cosmic-ray exposure (CRE) age for NWA 998 of $12 Ma is also similar to CRE ages for other nakhlites. Published by Elsevier Ltd.

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Section: Samples and Experimental Methodsmentioning

confidence: 99%

39Ar–40Ar ages of martian nakhlites

Park

Garrison

Bogard

2009

Geochimica et Cosmochimica Acta

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“…The irradiation constant (J-value) was 0.02111 ± 0.00008, and was determined from multiple irradiated samples of the hornblende age standard, NL-25 (Bogard et al, 1995). Argon was released by stepwise temperature extraction and its isotopic composition measured on a VG-3600 mass spectrometer.…”

Section: Ar-ar Analysesmentioning

confidence: 99%

Concordant Rb–Sr, Sm–Nd, and Ar–Ar ages for Northwest Africa 1460: A 346Ma old basaltic shergottite related to “lherzolitic” shergottites

Nyquist¹,

Bogard²,

Shih³

et al. 2009

Geochimica et Cosmochimica Acta

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Multiple lines of evidence show that the Rb-Sr, Sm-Nd, and Ar-Ar isotopic systems individually give robust crystallization ages for basaltic (or diabasic) shergottite Northwest Africa (NWA) 1460. In contrast to other shergottites, NWA 1460 exhibits minimal evidence of excess 40 Ar, thus allowing an unambiguous determination of its Ar-Ar age. The concordant RbSr, Sm-Nd, and Ar-Ar results for NWA 1460 define its crystallization age to be 346 ± 17 Ma (2r). In combination with petrographic and trace element data for this specimen and paired meteorite NWA 480, these results strongly refute the suggestion by others that the shergottites are $4.1 Ga old. Current crystallization and cosmic-ray exposure (CRE) age data permit identification of a maximum of nine ejection events for Martian meteorites (numbering more than 50 unpaired specimens as of 2008) and plausibly as few as five such events. Although recent high resolution imaging of the Martian surface has identified limited areas of sparsely cratered terrains, the meteorite data suggest that either these areas are representative of larger areas from which the meteorites might come, or that the cratering chronology needs recalibration. Time-averaged 87 Rb/ 86 Sr = 0.16 for the mantle source of the parent magma of NWA 1460/480 over the $4.56 Ga age of the planet is consistent with previously estimated values for bulk silicate Mars in the range 0.13-0.16, and similar to values of $0.18 for the ''lherzolitic" shergottites. Initial e Nd for NWA 1460/480 at 350 ± 16 Ma ago was +10.6 ± 0.5, which implies a time-averaged 147 Sm/ 144 Nd of 0.217 in the Martian mantle prior to mafic melt extraction, similar to values of 0.211-0.216 for the ''lherzolitic" shergottites. These time-averaged values do not imply a simple two-stage mantle/melt evolution, but must result from multiple episodes of melt extractions from the source regions. Much higher ''late-stage" e Nd values for the depleted shergottites imply similar processes carried to a greater degree. Thus, NWA 1460/480, the ''lherzolitic" shergottites and perhaps EET 79001 give the best (albeit imperfect) estimate of the Sr-and Nd-isotopic characteristics of bulk silicate Mars. Published by Elsevier Ltd.

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“…A 47.6 mg matrix sample of GRA 95209 was irradiated with fast neutrons to convert part of the 39 K to 39 Ar. The irradiation constant (J = 0.02843 ± 0.00005) was determined from samples of the NL-25 hornblende (Bogard et al, 1995) irradiated along with GRA 95209. The J values for three NL-25 samples in each of two parallel irradiation tubes defined linear trends of identical slope, from which the J value for GRA 95209 was deduced from its relative irradiated position.…”

Section: Ar-40 Ar Datingmentioning

confidence: 99%