Timing of Basaltic Volcanism in Ureilite Parentbody Inferred from the 26al Ages of Plagioclase-bearing Clasts in Dag-319 Polymict Ureilite

Introduction: Ureilitic meteorites are ultramafic rocks with highly depleted REE abundance, while showing primitive signatures, such as abundant noble gases and oxygen isotopic heterogeneities. A lack of basaltic components among ureilitic meteorites is the major cause of our poor understanding of their evolution. Especially, chronological studies of ureilites are limited because of depletion of incompatible trace elements (U, K, Rb, REE) used for radiometric dating. DaG-319 polymict ureilites contains variety of plagio-clase-bearing clasts, corresponding to the products of basaltic magma generated on the ureilite parent body [1,2]. In-situ oxygen isotopic analyses of these clasts showed that they plot along CCAM line, thus confirming their ureilitic origins [3]. The SIMS trace element analyses in plagioclase indicated that (1) ureilite parent body evolved from a chondritic precursor with CI-like alkali to refractory elements ratios, and (2) low degree (1%-10%) partial melts were extracted repeatedly from solid residue to produced depleted mafic residue [4]. Here we report the results of the 26 Al-26 Mg age determination in DaG-319 plagioclase-bearing clasts. Previous studies on similar clasts in other polymict ureilites gave only the upper limits of initial 26 Al/ 27 Al ratios (<10-6 ; >3.5 m.y. after CAIs [5]). The high precision (≤1‰) SIMS Mg isotopic analyses are newly developed in order to detect a small 26 Mg excess in ureil-itic plagioclase. Our new results demonstrate a short time scale of million years or less for the igneous differentiation of their parent body. Samples: We have analyzed 10 plagioclase-bearing clasts from two thin sections of DaG-319 (AMNM 4963-1, 4963-3, named as " a " and " g " , respectively) listed in Table 1, covering a wide range of plagioclase compositions (An0-100). The previous SIMS trace element study showed that calcic plagioclase (An>40) is often depleted in incompatible trace elements such as K, Ti, and Ba, while those in albitic plagioclase (An<30) is consistent with fractional crystallization process [4]. Among the listed samples, g18, g8, and a22A are highly depleted in incompatible trace elements, while a21A, g15 and a34A are the most evolved ones by fractional crystallization process.