Other
Scientific paper
Dec 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agufm.v22c1059m&link_type=abstract
American Geophysical Union, Fall Meeting 2001, abstract #V22C-1059
Other
1035 Geochronology, 1749 Volcanology, Geochemistry, And Petrology, 4860 Radioactivity And Radioisotopes
Scientific paper
Acapulco meteorite is thought to have cooled extremely fast ({ ~}100oC/Ma) at high T ({ ~}1000oC) followed by very slow cooling ({ ~}2oC/Ma) without any significant later-stage reheating. The timing, temperature and cause of the dramatic change of cooling rate are still controversial. The most important evidence of slow cooling at low T is from extinct-Pu fission track dating for orthopyroxene, whitlockite and apatite crystals. Because the parent 244Pu is extinct, this method yields only relative ages that may differ for different minerals as a result of differential track retentivity. Determining absolute ages from the fission track method is hampered by several fundamental uncertainties in estimating U and Pu concentrations, and in distinguishing fission tracks from other tracks produced by cosmic ray and spallation/recoil processes. One way to anchor the drifting (i.e. relative) fission track ages is dating the same sample with other chronometers which can give absolute ages in the same T range. For this purpose, we have applied single grain (U-Th)He dating techniques to apatite grains which have almost identical closure T as that of fission track retention. Four apatite grains with 150 micrometer { ~} 230 micrometer size were analyzed for U, Th (with ICP-MS) and 4He (laser extraction Quadrapole MS) concentrations, and one grain ca. 200 micrometer \times 200 micrometer was step-heated to determine diffusion parameters. Imaging of the sample shows anhedral crystal shapes such that for most apatites it's almost impossible to reconstruct the original crystal morphology from the mineral separates, preventing precise estimation of alpha particle ejection loss (FT) for each sample. The uncorrected ages, therefore minimum ages, are: 4462 \pm 51 (analytical error only; 1\sigma) Ma, 4326 \pm 51 Ma, 4386 \pm 52 Ma and 2718 \pm 39 Ma. The first three ages are indistinguishable when considering possible large variation of FT values for those samples. The oldest age of 4462 \pm 51 Ma is interpreted as the minimum age for helium closure in the Acapulco apatites. The step heating analyses revealed a simple volume diffusion at temperatures below 300oC, yielding closure T of 90oC at a cooling rate of 2oC Ma. The 110 Ma age difference between apatite (closure T ={ ~}90oC) and orthopyroxene ({ ~}280oC) fission track is combined with the new (U-Th)/He data (> 4462 \pm 51 Ma at 90oC). According to this simple comparison, significant retention of fission tracks in orthopyroxene began as early as approximately 4570 \pm 50 Ma which is identical with PbPb isochron age of 4557 Ma. This calculation suggests rapid cooling down to 300oC , around the closure T of plagioclase Ar diffusion, then followed by slow cooling. This interpretation suggests that the younger 40Ar/39Ar age (4507 \pm 9 Ma) obtained from Acapulco plagioclase, which should reflect cooling through { ~}300\degC, is spuriously young due to systematic errors (i.e., decay constants and/or standard data) in the 40Ar/39Ar method, as suggested by comparison between high-precision 40Ar/39Ar and U/Pb ages for terrestrial volcanic rocks.
Farley Ken A.
Min Kyoung
Renne Paul R.
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