dating, Ar diffusion properties, and cooling rate determinations of severely shocked chondrites

Bogard, D. D.; Hirsch, W. C.

doi:10.1016/0016-7037(80)90219-7

Cited by 88 publications

(48 citation statements)

References 26 publications

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“…Some investigators assumed (Bogard and Hirsch, 1980;Marti and Graf, 1992;Bogard, 1995) that the different ages could be due to several catastrophic collisions. However, the small number of shock-metamorphosed meteorites analyzed by the 40Ar-39Ar technique does limit this conclusion.…”

Section: Resultsmentioning

confidence: 99%

Parent bodies of L and H chondrites: Times of catastrophic events

Alexeev

1998

Meteorit & Planetary Scien

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Abstract-An analysis of the distribution of 3He and 4He in L and H chondrites has shown that the parent body of L chondrites underwent a catastrophic collision in space 340 f 50 Ma ago. This age differs considerably from the collision age of 520 2 60 Ma given previously (Heymann, 1967). The parent body of H chondrites may also have undergone local heating and degassing -200 Ma ago. Data for L chondrites argue in favour of Antarctic and non-Antarctic meteorites having originated from a common parent body.

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Section: Resultsmentioning

confidence: 99%

Parent bodies of L and H chondrites: Times of catastrophic events

Alexeev

1998

Meteorit & Planetary Scien

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“…On the other hand, the higher ages at lower extraction temperatures may be caused by trapped radiogenic 40 Ar mobilized by the shock event. Such "saddle-shaped" Ar age spectra are observed in some strongly shocked chondrites (Bogard and Hirsch 1980) and some terrestrial samples (McDougall and Harrison 1999). Isochron plots ( 40 Ar/ 36 Ar versus 39 Ar/ 36 Ar) for Piplia Kalan seem to support the second interpretation over the first.…”

Section: Piplia Kalanmentioning

confidence: 99%

“…6c) terrestrial samples containing excess 40 Ar (Bogard and Hirsch 1980;McDougall and Harrison 1999).…”

Section: Eet 8753121mentioning

confidence: 99%

³⁹Ar‐⁴⁰Ar ages of eucrites and thermal history of asteroid 4 Vesta

Bogard

Garrison

2003

Meteorit & Planetary Scien

109

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available online at http://meteoritics.org 669 Abstract-Eucrite meteorites are igneous rocks that derived from a large asteroid, probably 4 Vesta. Past studies have shown that after most eucrites formed, they underwent metamorphism in temperatures up to ≥800°C. Much later, many were brecciated and heated by large impacts into the parent body surface. The less common basaltic, unbrecciated eucrites also formed near the surface but, presumably, escaped later brecciation, while the cumulate eucrites formed at depths where metamorphism may have persisted for a considerable period. To further understand the complex HED parent body thermal history, we determined new 39 Ar-40 Ar ages for 9 eucrites classified as basaltic but unbrecciated, 6 eucrites classified as cumulate, and several basaltic-brecciated eucrites. Precise Ar-Ar ages of 2 cumulate eucrites (Moama and EET 87520) and 4 unbrecciated eucrites give a tight cluster at 4.48 ± 0.02 Gyr (not including any uncertainties in the flux monitor age). Ar-Ar ages of 6 additional unbrecciated eucrites are consistent with this age within their relatively larger age uncertainties. By contrast, available literature data on Pb-Pb isochron ages of 4 cumulate eucrites and 1 unbrecciated eucrite vary over 4.4-4.515 Gyr, and 147 Sm-143 Nd isochron ages of 4 cumulate and 3 unbrecciated eucrites vary over 4.41-4.55 Gyr. Similar Ar-Ar ages for cumulate and unbrecciated eucrites imply that cumulate eucrites do not have a younger formation age than basaltic eucrites, as was previously proposed. We suggest that these cumulate and unbrecciated eucrites resided at a depth where parent body temperatures were sufficiently high to cause the K-Ar and some other chronometers to remain as open diffusion systems. From the strong clustering of Ar-Ar ages at ~4.48 Gyr, we propose that these meteorites were excavated from depth in a single large impact event ~4.48 Gyr ago, which quickly cooled the samples and started the K-Ar chronometer. A large (~460 km) crater postulated to exist on Vesta may be the source of these eucrites and of many smaller asteroids thought to be spectrally or physically associated with Vesta. Some Pb-Pb and Sm-Nd ages of cumulate and unbrecciated eucrites are consistent with the Ar-Ar age of 4.48 Gyr, and the few older Pb-Pb and Sm-Nd ages may reflect an isotopic closure before the large cratering event. One cumulate eucrite gives an Ar-Ar age of 4.25 Gyr; 3 additional cumulate eucrites give Ar-Ar ages of 3.4-3.7 Gyr; and 2 unbrecciated eucrites give Ar-Ar ages of ~3.55 Gyr. We attribute these younger ages to a later impact heating. Furthermore, the Ar-Ar impact-reset ages of several brecciated eucrites and eucritic clasts in howardites fall within the range of 3.5-4.

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“…We calculated the diffusivities, D/a 2 , from the rate of release of 128 Xe as a function of extraction temperature and plotted these against inverse temperature in Arrhenius plots (Fechtig and Kalbitzer 1966;Bogard and Hirsch 1980). Because the data indicate that iodine was located in two sites of quite different Xe diffusivities, we calculated D/a 2 values considering only the 128 Xe released above intermediate temperatures.…”

Section: Xe and Ar Diffusionmentioning

confidence: 99%

Ar-Ar and I-Xe ages and the thermal history of IAB meteorites

Bogard¹,

Garrison²,

Takeda³

2005

Meteoritics &Amp; Planetary Science

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Ar-Ar and I-Xe ages and the thermal history of IAB meteoritesWe interpret these older Ar ages to represent cooling after the time of last significant metamorphism on the parent body and the younger ages to represent later 40 Ar diffusion loss. The older Ar-Ar ages for IABs are similar to Sm-Nd and Rb-Sr isochron ages reported in the literature for Caddo County. Considering the possibility that IAB parent body formation was followed by impact disruption, reassembly, and metamorphism (e.g., Benedix et al. 2000), the Ar-Ar ages and IAB cooling rates deduced from Ni concentration profiles in IAB metal (Herpfer et al. 1994) are consistent if the time of the postassembly metamorphism was as late as about 4.53 Gyr ago. However, I-Xe ages reported for some IABs define much older ages of about 4558-4566 Myr, which cannot easily be reconciled with the much younger Ar-Ar and Sm-Nd ages. An explanation for the difference in radiometric ages of IABs may reside in combinations of the following: a) I-Xe ages have very high closure temperatures and were not reset during metamorphism about 4.53 Gyr ago; b) a bias exists in the 40 K decay constants which makes these Ar-Ar ages approximately 30 Myr too young; c) the reported Sm-Nd and Rb-Sr ages for Caddo are in error by amounts equal to or exceeding their reported 2-sigma uncertainties; and d) about 30 Myr after the initial heating that produced differentiation of Caddo silicate and mixing of silicate and metal, a mild metamorphism of the IAB parent body reset the Ar-Ar ages.

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dating, Ar diffusion properties, and cooling rate determinations of severely shocked chondrites

Cited by 88 publications

References 26 publications

Parent bodies of L and H chondrites: Times of catastrophic events

Parent bodies of L and H chondrites: Times of catastrophic events

³⁹Ar‐⁴⁰Ar ages of eucrites and thermal history of asteroid 4 Vesta

Ar-Ar and I-Xe ages and the thermal history of IAB meteorites

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dating, Ar diffusion properties, and cooling rate determinations of severely shocked chondrites

Cited by 88 publications

References 26 publications

Parent bodies of L and H chondrites: Times of catastrophic events

Parent bodies of L and H chondrites: Times of catastrophic events

39Ar‐40Ar ages of eucrites and thermal history of asteroid 4 Vesta

Ar-Ar and I-Xe ages and the thermal history of IAB meteorites

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³⁹Ar‐⁴⁰Ar ages of eucrites and thermal history of asteroid 4 Vesta