Acapulco: Constraining the cooling history of a parent body

Details Acapulco: Constraining the cooling history of a parent body Acapulco: Constraining the cooling history of a parent body

Jul 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994metic..29r.496m&link_type=abstract

Meteoritics (ISSN 0026-1114), vol. 29, no. 4, p. 496

Computer Science

1

Cooling, Histories, Meteoritic Composition, Phase Transformations, Recrystallization, Abundance, Gases, Metallography, Trapping

Scientific paper

The high degree of recrystallization observed in the Acapulco meteorite coupled with old ages was interpreted to require fast cooling rates. There are conflicting estimates of such rates and the cooling history of this parent asteroid needs to be better constrained. Palme et al. deduced high cooling rates based on metallographic and fission track studies, while Pellas and Fieni infer low cooling rates of 1.7 C/Ma between 560 K and 380 K, based on fission track data. The latter authors suggest that the parent body cooled fast at higher temperatures and considerably slower at lower temperatures. We present new results obtained from metallographic studies and from the closed-system evolution of radiogenic Xe-129 in the trapped Xe in orthopyroxene at times when system closure was observed in different locations within the parent asteroid. In the present work the cooling rate of the Acapulco meteorite was determined using a metallographic technique, which involves the measurement of the size of the island phase, which, along with the honeycomb phase, comprises the cloudy zone structure of the retained taenite of the metal particles of meteorites. The study of the isotopic signatures of the abundant trapped Xe component in Acapulco reveals that the ratio Xe-129/Xe-130 = 1.060 did evolve from an initial value of 1.040 observed in ordinary chondrites. An extremely large trapped component is observed in acapulcoite Yamato 74063 and shows a more evolved ratio of 1.092 at the time of Xe entrapment in orthopyroxenes. For our calculations we assume that these ratios characterize the early closed-system evolution of trapped gases within the Acapulco asteroid. Using the observed Xe-129/Xe-132 ratios in Acapulco bulk samples and major phases, we can evaluate the timescale involved in the equilibrium evolution of radiogenic Xe.

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