< ? & Search for a Meteoritic Component at the Beaverhead Impact Structure , Montana

The Beaverhead impact structure, in southwestern Montana (44°36-N;112°58'W), was identified recently by the presence of shatter cones and impactitcs in outcrops of Proterozoic sandstones of the Belt Supergroup [1]. The cones occur over an area > 100 km. Because the geologic and tectonic history of this region is long and complex, the outline of the original impact crater is no longer identifiable. The extent of the area over which shatter cones occur suggests, however, that the feature may have been at least 60 km in diameter. The absence of shatter cones in younger sedimentary units suggests that the impact event occurred in late Precambrian or early Paleozoic time. We have collected samples of shocked sandstone from the socalled "Main Site" of dark-matrix breccias, and of impact breccias and melts from the south end of Island Butte. The melts, occurring often as veins through brecciated sandstone, exhibit a distinctive fluidal texture, a greenish color, and a cryptocrysUuline matrix, with small inclusions of deformed sandstone. Samples of the same type, along with country rock, were analyzed previously for majorand trace-element abundances [2]. It was found that, although the majorelement composition was relatively uniform, the trace-element composition showed variations between the melt material and the adjacent sandstone. These variations were attributed to extensive weathering and hydrothermal alteration. In a more specific search for a possible meteoritic signature in the breccia and the melt material we have conducted a new series of trace-element analyses on powders of our own samples by thermal neutron activation analysis. Our results indicate that IT abundances in the breccia, the melts, and the adjacent sandstone clasts are no greater than about 0.1 ppb, suggesting no IT enrichment of the breccia or the melts relative to the country rock. However, both the breccia and the melt material exhibit notable enrichments in Cr (8and 10-fold), in U (9and 5-fold), and in the heavy REEs (1.5 and 3-fold), respectively (normalization carried out relative to La) (see Table 1). Such enhancements relative to die country sandstone suggest that weathering and hydrothermal alteration alone might not be sufficient to account for the observed variations in traceelement abundances, and that the incorporation of a Cr. U, and HREE-rich component in the breccia and the melts is likely. Because both the breccia and the melts show no Ir or Ni enrichments