Mathematics – Logic
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27q.274p&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 274
Mathematics
Logic
Scientific paper
We are actually witnessing an exciting transition between our past ignorance on asteroid prehistories and a better understanding of their early thermal evolution. This is the result of data from absolute (U/Pb, Ar-Ar) and relative (^244Pu, ^26Al) chronologies obtained on one object having the simplest post-accretional thermal history: the H-asteroid. The motor of this deciphering has been the recent confirmation of the previously suggested "onion-shell" model (Pellas and Storzer, 1981), with U/Pb absolute ages anticorrelated with petrological types (Gopel et al., 1991). The very fast cooling of three (unshocked) H4 chondrites (Ste Marguerite: SM, Forest Vale and Beaver Creek: BC) from their peak metamorphic temperature (700 K) was confirmed both by metallography and Pu fission track thermometry (Lipschutz et al., 1989). This observation, first made on BC, led to predict the possible presence of live ^26Al in feldspars of this chondrite (Pellas and Storzer, 1981). This prediction is confirmed today by the discovery of live ^26Al in SM feldspars (Zinner and Gopel, this conf.), with an abundance sufficient to explain the mild metamorphism of petrological type 4, together with the fact that the more metamorphosed H6 chondrites did not melt. From this observation an upper limit of 3 Ma can be inferred between the condensation-accretion, the heating spike due to ^26Al decay, and the cooling which has frozen the feldspar phase of SM. Such an upper limit corresponds to the formation time of the H-asteroid. The fast cooling of some H4 materials does also preclude the existence of an insulating regolith of petrologic type 3, which would have drastically slowed down the cooling process, as already suggested (Pellas and Storzer, 1981). Consequently, accretion of type 3 materials must have occurred later. This result appears to discard the hypothesis of electromagnetic induction in the protosolar wind as the effective heat source (Herbert et al., 1991). The ^26Al heating also has important implications for our understanding of the compositional gradient in the asteroid belt (Gradie and Tedesco, 1982), which must be an inherent property of the condensing nebula (as is the case for the planets). This confirms the conclusions of Wood and Pellas (1991). From the above observations, and provided that a phase with high Fe/Ni ratios is found, it seems safe to predict that live ^60Fe could also be detected in those H4 chondrites that cooled fast, and which therefore must present a much higher ^60Fe/^56Fe ratio than that measured in Chervony Kut eucrite (Shukolyukov and Lugmair, 1992). Thus it seems that we are close to finally understanding the motor(s) of metamorphism and differentiation to which planetesimals were subjected around 4.565 Ga ago, in full agreement with Urey's earlier suggestions (1955). REFERENCES: Gopel C., Manhes G., and Allegre C. (1991) Meteoritics 26, 338. Gradie J.C. and Tedesco E. (1982) Science 216, 1405-1407. Herbert F., Sonett C.P., and Gaffey M.J. (1991) in The Sun in Time (eds. C.P. Sonett, M.S. Giampapa, and M.S. Matthews) pp. 710-739. Univ. Arizona Press, Tucson, Arizona. Lipschutz M.E., Gaffey M.J., and Pellas P. (1989) in Asteroids II (eds. R.P. Binzel, T. Gehrels and M.S. Matthews) pp. 740-777. Pellas P. and Storzer D. (1981) Proc. R. Soc. Lond. A374, 253-270. Shukolyukov A. and Lugmair G.W. (1992) Lunar Planet. Sci. (abstract) 23, 1295. Wood J.A. and Pellas P. (1991) in The Sun in Time (eds. C.P. Sonett, M.S. Giampapa, and M.S. Matthews) pp. 740-760. Urey H.C. (1955) Proc. Acad. Sci. U.S. 41, 127-144. Zinner E. and Gopel C. (1992) Abstract 55th Annual Meeting Meteoritical Soc. (this volume).
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