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Single grain (U-Th)/He ages from phosphates in Acapulco meteorite and implications for thermal history

Min, Kyoungwon and Farley, Kenneth A. and Renne, Paul R. and Marti, Kurt (2003) Single grain (U-Th)/He ages from phosphates in Acapulco meteorite and implications for thermal history. Earth and Planetary Science Letters, 209 (3-4). pp. 323-336. ISSN 0012-821X. http://resolver.caltech.edu/CaltechAUTHORS:20121115-095120262

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Abstract

The cooling history of the Acapulco meteorite for >400°C is well established using various chronometers suggesting extremely fast cooling (>1000°C/Ma). In contrast, the thermal history for low temperatures (<400°C) is poorly understood because of large uncertainties in the chronometers applicable to this temperature range. To better constrain the cooling history for the low-temperature range, we applied (U–Th)/He dating techniques to individual phosphate grains. One whitlockite and 11 apatite grains yielded (U–Th)/He ages ranging from 1272±22 (1σ, analytical error only) Ma to 4584±51 Ma, with tight clustering at ∼4.55 Ga. The weighted mean of the five oldest ages (4538±32 Ma, 1σ uncertainty including systematic error) is suggested to be the minimum age representing primary cooling of the Acapulco body passing through ∼120°C. Although it is impossible to precisely quantify the effects of energetic α particle ejection from the outermost ∼20 μm of the phosphates, petrographic evidence suggests that most dated samples are fragments likely derived from the interior of larger grains, thus greatly reducing this source of error. Indeed the five oldest samples cannot have suffered substantial ejection since the uncorrected ages are identical with the crystallization age of the Acapulco meteorite. The new (U–Th)/He data suggest rapid cooling of Acapulco down to ∼120°C. This evidence suggests that the younger ^(40)Ar/^(39)Ar age (4507±9 (1σ) Ma) obtained from Acapulco plagioclase, which should reflect cooling through ∼300°C, is spuriously young due to systematic errors (i.e., decay constants and/or standard data) in the ^(40)Ar/^(39)Ar method, as suggested by comparison between high-precision ^(40)Ar/^(39)Ar and U/Pb ages for terrestrial volcanic rocks. The scattered He age distribution <4.0 Ga implies very heterogeneous thermal disturbances after the primary cooling of the body.


Item Type:Article
Related URLs:
URLURL TypeDescription
http://dx.doi.org/10.1016/S0012-821X(03)00080-3DOIUNSPECIFIED
http://www.sciencedirect.com/science/article/pii/S0012821X03000803PublisherUNSPECIFIED
Additional Information:© 2003 Elsevier Science B.V. Received 9 September 2002; received in revised form 28 January 2003; accepted 12 February 2003. We thank John Donovan for assistance with EMP analyses, Lindsey Hedges for support in (U-Th)/He analyses, Yoosook Kim for mineral separations, Don DePaolo and Ken Ludwig for helpful discussion and suggestions. Constructive reviews of Grenville Turner and Edward Young greatly improved the original manuscript. This work was supported by NSF Grant EAR-9814378 (P.R.R.).[BOYLE]
Funders:
Funding AgencyGrant Number
NSFEAR-9814378
Subject Keywords:(U–Th)/He dating; Acapulco meteorite; apatite; whitlockite; thermal history
Record Number:CaltechAUTHORS:20121115-095120262
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:20121115-095120262
Official Citation:Kyoungwon Min, Kenneth A. Farley, Paul R. Renne, Kurt Marti, Single grain (U–Th)/He ages from phosphates in Acapulco meteorite and implications for thermal history, Earth and Planetary Science Letters, Volume 209, Issues 3–4, 30 April 2003, Pages 323-336, ISSN 0012-821X, 10.1016/S0012-821X(03)00080-3. (http://www.sciencedirect.com/science/article/pii/S0012821X03000803)
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:35479
Collection:CaltechAUTHORS
Deposited By: Ruth Sustaita
Deposited On:15 Nov 2012 18:10
Last Modified:27 Dec 2012 03:01

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