Fossil ~(26)Al and ~(53)Mn in the Asuka 881394 eucrite: evidence of the earliest crust on asteroid 4 Vesta

摘要

Asuka 991394 is a unique magnesian eucrite with pyroxenes that are Mg-rich like those of cumulate eucrites, but with a granulitic texture unlike the textures of cumulate eucrites. Plagioclase compositions are ～An_(98), and are even more calcic than those in cumulate eucrites. Pyroxene does not show pigeonite-to-orthopyroxene inversion texturees, suggesting different crystallization conditions than those of cumulate eucrites. Mn-Cr isotopic analyses determined initial ~(53)Mn/~(55)Mn = (4.6±1.7) * 10~(-6) and initial ε(~(53)Cr)_I = 0.25±0.17 in A881394. This initial ~(53)Mn abundance corresponds to a formation interval Δt_(LEW) = -6±2 Ma relative to the LEW 86010 angrite, implying an 'absolute' age of 4564±2 Ma. Both the invital ~(53)Mn abundance and the initial ε(~(53)Cr)_I value for A881394 are identical to those previously determined for the HED parent body at the time of its differentiation. Al-Mg isotopic ananlyses determined initial ~(26)Al/~(27)Al = (1.18±0.14) * 10(-6), from which a formation interval Δt_(CAI) = 3.95±0.13 Ma is calculated relative to the canonical value ~(26)Al/~(27)Al=5*10~(-5) for CAI. Combining this formation interval with a recently reported Pb-Pb age of 4567.2±0.6 Ma for CAI gives 4563.2±0.6Ma as the age of A881394, in excellent agreement with the age based on the Mn-Cr formation interval. Algernatively, the ~(53)Mn and ~(26)Al formation intervals of A881394 allow the Mn-Cr and Al-Mg timescales to be intercalibrated, suggesting that an 'absolute' CAI age of 4568 Ma is most consistent with the 4558 Ma Pb-Pb age of LEW 86010. The initial ~(26)Al abundance existing in A881394 would have been insufficient to cause global melting in the HED parent body (probably asteroid 4 Vesta). nevertheless, it could have been derived by radioactive decay over only ～2 Ma from an abundance that would have been sufficient to cause global meriting. The higher value of molar Mg/(Mg+Fe) = 0.57 for A881394 than those of the ordinary (basaltic) eucrites (Mg/(Mg+Fe) = 0.30-0.42) suggests additional factors may have been important for magma genesis on the parent body. If ~(26)Al were the only heat source, partial melting would have been the major process in the interior of the parent body, and Mg/(Mg+Fe) would be lower in the melts than in the primordial source material. late=stage accretion could have supplied relatively magnesian primordial materials to the surface of the parent body, thereby increasing Mg/(Mg+Fe) in a shallow magma ocean from which A881394 crystallized, and also may have augmented ~(26)Al heating. The granulitic texture of A881394 may have been produced during residence in the thin, earliest, crust, kept hot by the magma beneath it. If ~(26)Al was, nevertheless, the major heat source for asteroidal melting, it may account for declining post-accretion heating of main belt asteroids with increasing heliocentric distance.