Computer Science
Scientific paper
Jul 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994gecoa..58.3101s&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 58, Issue 14, pp.3101-3116
Computer Science
6
Scientific paper
K-Ca and Rb-Sr age determinations were made for lunar granite 12033,576. This small fragment is a subsample of soil particle 12033,507, which was collected subsurface at the north rim of Head Crater. The K-Ca mineral isochron is well-defined and yields an age of 3.62 ± 0.11 (2 ) Ga for ( 40 K ) = 0.5543 Ga -1 and an initial 40 Ca / 44 Ca of 47.160 0.006 (2 ) (normalized to 42 Ca / 44 Ca = 0.31221). The Rb-Sr mineral isochron is quite disturbed and yields an imprecise young age of 2.20 ± 0.65 Ga for ( 87 Rb ) = 0.0139 Ga -1 and a high initial 87 Sr / 86 Sr of 0.775 0.022. Assuming that the granite crystallized ~ 4.0 Ga ago (a modeled formation age), and was shock-heated 0.8 Ga ago ( 39 Ar- 40 Ar age), the intermediate Rb-Sr and K-Ca ages are interpreted as due to partial resetting by thermal diffusion of strontium and calcium. Diffusion data determined for argon in 12033,507 give a value of D / a 2 ~1 × 10 -6 s -1 for a heating temperature of 700°C; at this temperature it would require a time on the order of days to totally degas argon. The required diffusion coefficients, D , for calcium (7 ~ 9 × 10 -13 cm 2 s -1 ) and Sr (~8 × 10 -12 cm 2 s -1 ) that are predicted from the observed age resetting are in good agreement with those determined experimentally in granitic melts. The K-Ca system was about a factor of ten more resistant to resetting than the Rb-Sr system, which in turn was ~ 10 times more resistant to resetting than the K-Ar system. Diffusion data indicate that isotopic exchange of calcium and strontium was responsible for the partial resetting of the K-Ca and Rb-Sr chronometers, rather than elemental exchange of potassium and rubidium among mineral phases. Rb-Sr isotopic data also were obtained for granite 14303,206, a subsample of breccia clast 14303,204, which was collected at the Fra Mauro region about 180 km from the Apollo 12 site. The mineral isochron is quite disturbed and yields an imprecise age of 3.95 ± 0.38 Ga for ( 87 Rb ) = 0.0139 Ga -1 and initial 87 Sr / 86 Sr = 0.7046 0.0051. The Rb-Sr age is comparable to the previously determined K-Ca age of 4.04 ± 0.64 Ga. The Rb-Sr and K-Ca ages of this clast are imprecise and are statistically indistinguishable from the well-defined 39 Ar- 40 Ar degassing age of 3.83 ± 0.03 Ga or the Rb-Sr and K-Ca model ages of 4.30 ± 0.06 Ga and 4.25 ± 0.12 Ga, respectively. Because these model ages are within error of a reported zircon U-Pb age of 4.31 Ga, we assume this granitic clast crystallized ~ 4.3 Ga ago and was shock-heated ~ 3.83 Ga ago. Using the apparent resetting of the K-Ar, Rb-Sr, and K-Ca isotopic systems, and literature data for relative strontium and calcium diffusivity, we estimate a shock-reheating temperature of ~ 860°C would be necessary to partially reset the Rb-Sr and K-Ca ages to their nominal values. This reheating temperature is uncertain because of the large errors on the ages, but would be consistent with complete resetting of the K-Ar age and partial resetting of the Rb-Sr and K-Ca ages, if the hot clast was buried under ~1 m of the ejecta blanket for ~1 day.
Bogard Donald D.
Nyquist Larry E.
Shih Chia-You
Wiesmann Henry
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