CRYSTALLIZATION, RECRYSTALLIZATION, AND IMPACT-METAMORPHIC AGES OF EUCRITES Y792510 AND Y791186

Monomict eucrites Y792510 and Y791186 of pyroxene metamorphic type 6 c ontain areas of recrystallized mesostasis and granoblastic pyroxene. T extural comparisons suggest that the mesostasis areas in these eucrite s originally resembled analogous areas in the pristine basaltic eucrit e clast Y75011,84. (39)Ar-Ar-40 ages were determined for microbreccias Y791186,87 and Y792510,62 and crystalline clast Y792510,65. The Ar-39 -Ar-40 ages for Y791186,87 and Y792510,62 are identical within error l imits at 3.46 +/- 0.18 Ga and 3.40 +/- 0.16 Ga (2 sigma), respectively . Both these samples were involved in an impact event in which Ar was quantitatively degassed 3.43 +/- 0.10 Ga ago. Y792510,65 has an appare ntly older Ar-39-Ar-40 age of 3.60 +/- 0.16 Ga (2 sigma) suggesting th at it was degassed in a separate event, or to a lesser degree. Rb-Sr a nd Sm-Nd ages were determined for both Y792510 clasts. The Rb-Sr age o f 3.26 +/- 0.24 Ga (2 sigma), for Y792510,65 was totally reset by the impact event(s), whereas the Rb-Sr age of 4.26 +/- 0.62 Ga (2 sigma) f or Y792510,62 either was partially reset by the same event(s), or by e arlier recrystallization of mesostasis. The Sm-147-Nd-143 age calculat ed for Y792510,65 including all data is 4.67 +/- 0.22 Ga (2 sigma), bu t is likely to have been affected by weathering in the Antarctic. It b ecomes 4.57 +/- 0.09 Ga if only data for the major minerals plagioclas e and pyroxene are included in the isochron. The Sm-147-Nd-143 age of Y792510,62 is 4.34 +/- 0.06 Ga (2 sigma) for the same mineral phases b ut may be biased by the presence of weathered Ca-phosphates in the pyr oxene separate. The Sm-146-Nd-142 age of Y792510,65 is 4.32(-0.06)(+0. 04) Ga corresponding to initial Sm-146/Sm-144 = 0.0030 +/- 0.0010 rela tive to (Sm-146/Sm-144)(LEW) = 0.0076 +/- 0.0009 for the LEW86010 angr ite at 4.558 Ga ago. The Y792510,65 ages from the two Sm-Nd chronomete rs are concordant at similar to 4.48 Ga, which may be the crystallizat ion age of this eucritic basalt (late crystallization). Alternatively, these Sm-Nd ages may have been affected by the event in which Ca-phos phate and augite in mesostasis were recrystallized and augite and chro mite in granoblastic areas were derived from cloudy pyroxenes (late re crystallization). The subsolidus recrystallization may be a consequenc e of a thermal spike in the midst of an otherwise monotonically coolin g environment resulting from impact cratering of the hot crust of the newly formed eucrite parent body. Pyroxene homogenization probably occ urred prior to formation of granoblastic pyroxene and close to the tim e of initial crystallization. Mn-Cr data provide only a hint of live M n-53 (t(1/2) = 3.7 Ma) and imply either chromium isotopic equilibratio n after decay of Mn-53 or late crystallization. Impact brecciation and lithification probably accompanied thermal annealing similar to 3.4-3 .6 Ga ago as given by the Ar-39-Ar-40 ages; the Rb-Sr age of Y792510,6 5 is consistent with this interpretation. The Ar-39-Ar-40 and Rb-Sr sy stematics of minute K-rich inclusions in plagioclase and pyroxene form ed by previous impact and thermal annealing events probably were reset then. These young impact-metamorphic ages demonstrate that impact res etting of radiometric ages on the HED parent body !asteroid 4 Vesta?) occurred over an interval of at least 0.7 Ga. Copyright (C) 1997 Elsev ier Science Ltd.