Crystallization, recrystallization, and impact-metamorphic ages of eucrites Y792510 and Y791186

Abstract

Monomict eucrites Y792510 and Y791186 of pyroxene metamorphic type 6 contain areas of recrystallized mesostasis and granoblastic pyroxene. Textural comparisons suggest that the mesostasis areas in these eucrites originally resembled analogous areas in the pristine basaltic eucrite clast Y75011,84. 39Ar 40Ar ages were determined for microbreccias Y791186,87 and Y792510,62 and crystalline clast Y792510,65. The 39Ar 40Ar ages for Y791186,87 and Y792510,62 are identical within error limits at 3.46 ± 0.18 Ga and 3.40 ± 0.16 Ga (2σ), 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 apparently older 39Ar 40Ar age of 3.60 ± 0.04 Ga (2σ) suggesting that it was degassed in a separate event, or to a lesser degree. Rb Sr and Sm Nd ages were determined for both Y792510 clasts. The Rb Sr age of 3.26 ± 0.24 Ga (2σ), for Y792510,65 was totally reset by the impact event(s), whereas the Rb Sr age of 4.26 ± 0.62 Ga (2σ) for Y792510,62 either was partially reset by the same event(s), or by earlier recrystallization of mesostasis. The 147Sm 143Nd age calculated for Y792510,65 including all data is 4.67 ± 0.22 Ga (2σ), but is likely to have been affected by weathering in the Antarctic. It becomes 4.57 ± 0.09 Ga if only data for the major minerals plagioclase and pyroxene are included in the isochron. The 147Sm 143Nd age of Y792510,62 is 4.34 ± 0.06 Ga (2σ) for the same mineral phases but may be biased by the presence of weathered Ca-phosphates in the pyroxene separate. The 146Sm 142Nd age of Y792510,65 is 4.42 −0.06 +0.04 Ga corresponding to initial 146Sm/ 144Sm= 0.0030 ± 0.0010 relative to ( 146Sm/ 144Sm) lew = 0.0076 ± 0.0009 for the LEW86010 angrite at 4.558 Ga ago. The Y792510,65 ages from the two Sm Nd chronometers are concordant at ∼4.48 Ga, which may be the crystallization age of this eucritic basalt (late crystallization). Alternatively, these Sm Nd ages may have been affected by the event in which Ca-phosphate and augite in mesostasis were recrystallized and augite and chromite in granoblastic areas were derived from cloudy pyroxenes (late recrystallization). The subsolidus recrystallization may be a consequence of a thermal spike in the midst of an otherwise monotonically cooling environment resulting from impact cratering of the hot crust of the newly formed eucrite parent body. Pyroxene homogenization probably occurred prior to formation of granoblastic pyroxene and close to the time of initial crystallization. Mn Cr data provide only a hint of live 53Mn ( t 1/2 = 3.7Ma) and imply either chromium isotopic equilibration after decay of 53Mn or late crystallization. Impact brecciation and lithification probably accompanied thermal annealing ∼3.4–3.6 Ga ago as given by the 39Ar- 40Ar ages; the Rb Sr age of Y792510,65 is consistent with this interpretation. The 39Ar- 40Ar and Rb Sr systematics of minute K-rich inclusions in plagioclase and pyroxene formed by previous impact and thermal annealing events probably were reset then. These young impact-metamorphic ages demonstrate that impact resetting of radiometric ages on the HED parent body (asteroid 4 Vesta?) occurred over an interval of at least 0.7 Ga.