Ar-Ar EVIDENCE FOR AN ~4.26 GA IMPACT HEATING EVENT ON THE LL PARENT BODY

Introduction: Miller Range 99301 is a type 6, unbrecciated LL chondrite [1]. MIL 99301 is of interest because some compositional and petrographic features suggest it experienced rather high shock grades, whereas other features suggest it is relatively unshocked. Inconsistent shock indicators could be explained if MIL 99301 was shocked but then partly annealed by heat produced by impacts on the parent body [1 and references therein]. The hypothesis that MIL 99301 experienced high temperature metamorphism (type 6) followed by a later shock event that heated, but did not melt, the constituent feldspar [1] can be evaluated using Ar-Ar chronology. This is because Ar-Ar ages of shocked ordinary chondrites are generally <4.2 Ga [2-5], whereas Ar-Ar ages of unshocked meteorites are generally older, and between 4.52 4.38 Ga [6]. Petrography Evidence to suggest MIL99301 experienced high shock grades includes: (1) extensive silicate darkening, typical of shock grades S3-S6; (2) chromite-plagioclase assemblages and thin chromite veins, characteristic of S4-S6; and (3) low-Ca clinopyroxene grains, characteristic of shock grades S3-S6 [1, and references therein]. Contrasting evidence from plagioclase and olivine grains suggests MIL 99301 experienced much lower shock grades. There are two feldspar phases in MIL 99301: a coarsely crystalline variety (mean composition of Ab83.1, Or5.8), and a finer-grained variety, which is only slightly more CaO-rich [1]. The crystalline plagioclase grains in MIL 99301 have sharp optical extinction. According to Stöffler et al. [7], plagioclase develops undulatory extinction at shock grades of ~S2 and becomes partially isotropic at S4. At higher grades of S5 or S6, plagioclase should have transformed into maskelynite, which is optically istropic [7]. The plagioclase textures appear to suggest that MIL 99301 experienced very low, ~S1 shock levels [1]. Olivine also appears to have experienced low shock grades. Most ordinary S3 chondrites exhibit planar fractures and undulatory extinction. At higher levels (S4), mosaicism begins to develop, and at still higher shock grades (S5-6), planar fractures and planar deformation features form [7]. However, nearly every olivine grain in MIL 99301 exhibits sharp optical extinction [1]. Thus, the plagioclase and olivine appear to have shock grades of S1 according to the criteria of Stöffler et al. [7]. Because the Ar-Ar method dates the age of K-bearing phases, if the plagioclase grains have been annealed by heat produced during impact events to type 4 metamorphic conditions [1], a relatively young Ar-Ar age may be expected. Ar-Ar Chronology. A plot of Ar-Ar age and K/Ca ratios versus the cumulative fraction of Ar released during stepwise temperature extractions of a whole-rock sample of MIL99301 is shown in Fig. 1. A substantial decrease in Ar/Ar and Ar/Ar ratios (not shown) over the first several extractions indicate the release of adsorbed terrestrial Ar, which may account for the higher Ar-Ar ages at low fractional Ar release. Between ~6% and 78% of the Ar released the age uniformly increases from 4.16 Ga to 4.26 Ga and is indicative of a small amount of diffusive loss of Ar, possibly from Antarctic weathering. A slight decrease in age for two extractions at ~80% Ar released is probably due to release of Ar that was recoil-implanted into grain surfaces of pyroxene during irradiation. Above 83% Ar released the Ar-Ar age rapidly increases to a quasi-age plateau of ~4.52 Ga for four extractions releasing 11% of the total Ar. Changes in the rate of release of Ar with extraction temperature suggests that those extractions releasing above 83% Ar release constitute a distinct K-bearing phase possessing different Ar diffusion properties. The entire Ar age spectrum does not resemble that expected for Ar loss from a single K-bearing phase.