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Experimentelle und Analytische Planetologie 40Ar-39Ar and 244Pu-fission track cooling ages reveal 26Al heating of the H-chondrite parent asteroid   Synopsis: 40Ar-39Ar and 244Pu-fission track thermochronometry was applied to a suite of unshocked H chondrites of different petrologic types. Cooling ages indicate significantly longer cooling of higher metamorphosed types in deep parent body layers than of low petrologic types with faster cooling in near-surface regions [1]. Our results highly favor 26Al as heating source for planetesimals in the early solar system. They also imply fast accretion within a few Ma and support currently considered lifetimes of protoplanetary disks of <6 Ma [2]. Accretion and planetesimal heating: Fast (within a few Ma) accretion, heating, and differentiation of asteroidal sized planetesimals in the early solar system are constrained by Pb-Pb ages of Allende CAIs and basaltic achondrites [3,4]. Studies demonstrating in situ decay of short-lived 26Al [5,6] advanced this nuclide as candidate heat source for planetesimal differentiation and for heating chondritic parent bodies. Model calculations of a chondritic parent body [1] heated by 26Al, predict slower cooling for higher metamorphosed petrologic types in deeper parent body layers. Thermochronological results: Based on the different closure temperatures of radiogenic 40Ar and 244Pu fission tracks, we determined cooling curves for H chondrites of different metamorphic grades that were not disturbed by later collisional events. Our results, together with available Pb-Pb data [7], strikingly agree with a 26Al heating model, if accretion of the ~100 km sized H chondrite parent body was completed 3 Ma after Allende CAIs [1]. References: [1] Trieloff M. et al. (2003) Nature 422, 502-506. [2] Haisch K. E. jr. et al. (2001) Astrophys. J. 553, L153-L156. [3] Chen J. H. & Wasserburg G. J. (1981) Earth Planet. Sci. Lett. 5, 15-21. [4] Lugmair G. W. & Galer S. J. G. (1992) Geochim. Cosmochim. Acta 56, 1673-1694. [5] Lee T. et al. (1976) Geophys. Res. Lett. 3, 109-112. [6] Zinner E. & Göpel C. (2002) Meteorit. Planet. Sci. 37, 1001-1013. [7] Göpel C. et al. (1994) Earth Planet. Sci. Lett. 121, 153-171. Beteiligte Wissenschaftler: M. Trieloff1, E.K. Jessberger, I. Herrwerth1, J. Hopp1, C. Fiéni2, M. Ghélis2, M. Bourot-Denise2 and P. Pellas2 1Mineralogisches Institut, D-69120 Heidelberg, Germany 2Laboratoire de Minéralogie du Muséum, 75005 Paris, France Veröffentlichungen: Trieloff M., Jessberger E. K., Herrwerth I., Hopp J., Fiéni C., Ghélis M., Bourot-Denise M. and Pellas P. (2003) 40Ar-39Ar and 244Pu-fission track cooling ages reveal 26Al heating of the H chondrite parent asteroid. Meteorit. Planet. Sci. 38, A27. Trieloff M., Jessberger E. K., Herrwerth I., Hopp J., Fiéni C., Ghélis M., Bourot-Denise M. and Pellas P. (2003) Structure and thermal history of the H-chondrite parent asteroid revealed by thermochronometry. Nature 422, 502-506.