A glass spherule of questionable impact origin from the Apollo 15 landing site: Unique target mare basalt

Abstract

A 6 mm-diameter dark spherule, 15434,28, from the regolith on the Apennine Front at the Apollo 15 landing site has a homogeneous glass interior with a 200 ??m-thick rind of devitrified or crystallized melt. The rind contains abundant small fragments of Apollo 15 olivine-normative mare basalt and rare volcanic Apollo 15 green glass. The glass interior of the spherule has the chemical composition, including a high FeO content and high CaO/Al2O3, of a mare basalt. Whereas the major element and Sc, Ni, and Co abundances are similar to those of low-Ti mare basalts, the incompatible elements and Sr abundances are similar to those of high-Ti mare basalts. The relative abundance patterns of the incompatible trace elements are distinct from any other lunar mare basalts or KREEP; among these distinctions are a much steeper slope of the heavy rare earth elements. The 15434,28 glass has abundances of the volatile element Zn consistent with both impact glasses and crystalline mare basalts, but much lower than in glasses of mare volcanic origin. The glass contains siderophile elements such as Ir in abundances only slightly higher than accepted lunar indigenous levels, and some, such as Au, are just below such upper limits. The age of the glass, determined by the 40Ar/39Ar laser incremental heating technique, is 1647 ?? 11 Ma (2 ??); it is expressed as an age spectrum of seventeen steps over 96% of the 39Ar released, unusual for an impact glass. Trapped argon is negligible. The undamaged nature of the sphere demonstrates that it must have spent most of its life buried in regolith; 38Ar cosmic ray exposure data suggest that it was buried at less than 2m but more than a few centimeters if a single depth is appropriate. That the spherule solidified to a glass is surprising; for such a mare composition, cooling at about 50??C s-1 is required to avoid crystallization, and barely attainable in such a large spherule. The low volatile abundances, slightly high siderophile abundances, and the young age are perhaps all most consistent with an impact origin, but nonetheless not absolutely definitive. The 15434,28 glass is distinct from the common yellow impact glasses at the Apollo 15 landing site, in particular in its lower abundances of incompatible elements and much younger age. If we accept an impact origin, then the trace element relative abundances preclude both typical KREEP and the common Apollo 15 yellow impact glass from contributing more than a few percent of the incompatible elements to potential mixtures. The melted part of any target must have consisted almost entirely of a variety (or varieties) of mare basalt or glass distinct from any known mare basalts or glasses, including Apollo 15 yellow volcanic glass, or mixtures of them. However, the rind inclusions, similar to materials of local origin, do suggest a source near the Apollo 15 landing site. An impact melt cannot have dissolved much, if any, of such inclusions. A lack of regolith materials in the rind and in the melt component suggest an immature source terrain. Thus, even for an impact origin, there is the possibility (though not requirement) that the volcanic target is younger than most mare plains. The crater Hadley C, 25 km away, is a potential source. If the 15434,28 glass is instead directly of volcanic origin, it represents an extremely young mare magma of a type previously undiscovered on the Moon.