Mathematics – Logic
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27..229h&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 229
Mathematics
Logic
2
Scientific paper
The 3.6-Gy ^39Ar-^40Ar age of mesosiderites has been interpreted in terms of a major impact around the time of major impacts on the Moon and the HED parent body [1;2]. We suggest instead that the young age of mesosiderites reflects lengthy primary cooling of the largest sampled asteroid; we calculate that its radius was 200-400 km. This conclusion is more consistent with the petrologic properties and our revised metallographic cooling rates of the mesosiderites. If correct, this would present us with a unique opportunity to calibrate the metallographic cooling rates. We have measured metallographic cooling rates of 5 mesosiderites (Vaca Muerta, Emery, Lowicz, Esterville, and Pinnaroo, see also [3]) using the revised phase-diagrams and diffusion constant of [4] and the local-bulk Ni technique of [5]. The metal structure indicates that the mesosiderites cooled slowly from at least 700 degrees C, where kamacite begins to precipitate, to below 320 degrees C, where tetrataenite forms. Cooling of the mesosiderites from 700 degrees C to below 300 degrees C in 900 My would require a cooling rate of more than 0.4 K/My. However, our preliminary results indicate metallographic cooling rates of approximately 0.05 K/My or almost an order of magnitude lower than the 0.4 K/My required to obtain Ar closure before 3.6 Gy ago. We find that the ^39Ar-^40Ar dates represent primary cooling for the following three reasons: 1) The metallographic cooling rates are the lowest known and--if taken at face value--inconsistent with cooling to Ar closure temperatures within 900 My. 2) Fresh shock features in the metal and silicates of mesosiderites are lacking but should be abundant if the parent body experienced a major impact 3.6 Gy ago. 3) The ^39Ar-^40Ar ages [1] are typically better defined than documented shock-ages of randomly sampled impact products since only small amounts of target material are heated sufficiently to remove Ar completely. Under the assumption that the Ar age represents primary cooling of the mesosiderites we have an independent measure of the cooling rate: 0.4 K/My. This would indicate that our revised mesosiderite cooling rates are too low by a factor of approximately 10. If, on the other hand, the Ar age represents a resetting of the Ar clock, then the cooling rate must have been even higher in order to obtain Ar closure prior to 3.6 Gy ago. A cooling rate of 0.4 K/My would require a parent body with a radius of 200 to 400 km. These radii are calculated assuming that the asteroid was covered with an insulating regolith with a thickness of between 600 and 0 m [6]. References: [1] Bogard D.D., Garrison D.H., Jordan J.L., and Mittlefehldt D. (1990) GCA, 54, 2549-2564 [2] Bogard D.D. and Garrison D.H. (1992) LPSC XXIII, 131-132. [3] Rasmussen K.L., Delaney J.S., and Prinz M. (1985) Meteoritics, 20, 738-739. [4] Saikumar V. and Goldstein J.I. (1988) GCA, 52, 715-726 [5] Rasmussen K.L. (1981) Icarus, 52, 564-576 [6] Haack H., Rasmussen, K.L., and Warren, P.H. (1990) JGR, 95, 5111-5124.
Haack Henning
Rasmussen Lars K.
Scott Edward R. D.
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