Abstract-We investigated the ballistically dispersed melts from Meteor Crater, Arizona, USA to determine the stratigraphic extent of its melt zone from the compositional relationship of melts and target rocks. Most melt particles are crystallized, hydrated, and oxidized; pristine glasses are rare. Hydration and oxidation occurred at ambient temperatures long after the impact. The preserved glasses are generally clear and texturally homogeneous, but unlike typical impact melts, they have unusually heterogeneous compositions, both within individual particles and from sample to sample. For example, the average Si0 2 for individual particles ranges from 43 to 65%. The projectile content is unusually high and it is distributed bimodally, with specific samples containing either 5-10% or 20-30% FeO. These compositional heterogeneities most likely reflect the high carbonate content of the target rocks and the release of copious COz that dispersed the melts, thereby terminating melt flow and mixing. The high projectile content and the COz depleted residue of purely sedimentary rocks produced mafic melts that crystallized fine-grained olivine and pyroxene.The melts fall into three compositional groups reflecting variable proportions of the major target formations, Moenkopi, Kaibab, and Coconino. Least-square mixing calculations revealed one group to contain 55% Moenkopi, 40% quartz-rich, upper Kaibab, and 5% meteorite, suggesting a source depth of <30 m from the pre-impact surface. The other two melt groups have higher contents of meteorite (15-20%) and Kaibab (50-70%) and contain more SiOz than average Kaibab. The additional quartz may have been derived from Coconino or the upper Kaibab, implying melt depths >90 m or <30 m, respectively. Additional studies, especially hydrocode calculations, are needed to better understand the source depth ofthese melts and their exceptionally high projectile content.
The mare basalt component in two lunar meteorites ALHA81005 and Y791197 has been useful in constraining their ejection sites. Although Apollo 16 regolith breccias with glassy matrices have been proposed to be the best analogs for the lunar meteorites, no mare basalt component has been found in such breccias studied so far (e.g., 60019). However, a basaltic clast, Ba‐2, was found on the cut surface of a new slab of 60019, so we have studied Ba‐2 using mineralogical techniques for comparison with the lunar meteorites. Ba‐2 is a coarse‐grained, granular basalt clast, 2.5 × 5 mm. It consists of plagioclase, high‐Ca pyroxene, ilmenite, silica, and mesostasis. The pyroxene and plagioclase crystals show moderate chemical zoning. Most of the pyroxene crystals are augite, with minor iron‐rich rims adjacent to the mesostases. The An content of plagioclase ranges from 95 to 75. Compared to Apollo and Luna mare basalts, the Ba‐2 mineral chemical trends are most like high‐alumina, moderate‐TiO2 basalts found in the Apollo 16 rake samples and to Luna 16 basalt fragments; however, olivine is not present in Ba‐2. The bulk chemical composition of Ba‐2 estimated by modal combination supports the similarity. The highland pyroxene types in the lunar meteorites bear a strong resemblance to one another and to those in 60019, but 60019 appears to have significantly fewer pyroxenes with the extremely low Mg contents of the VLT basalt component in the lunar meteorites. This difference can be explained by the fact that the amount of mare basalt in 60019 is extremely small (less than 0.1%) and the fact that Fe enrichment of pyroxene in the basalt is minor.
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