UNRAVELING THE ORIGIN STORY OF FELSIC ROCK FRAGMENTS CONTAINED IN APOLLO BRECCIAS AND SOILS

In the absence of plate-tectonics and/or a significant water-rock cycle the mechanism(s) that generates silicic volcanism on the Moon remains elusive. Though relatively abundant, and large silicic features have been identified via remote sensing, our only samples of nominally granitic materials are rare felsite clasts in Apollo 12, 14, 15, and 17 breccias and lunar soils. For more than a decade we have conducted coordinated microstructural, mineralogical, compositional, and geochronological investigation of these fragments to explore the formation mechanism(s) and timing of lunar felsic magmatism.

Felsic clasts can be broadly divided into those with granophyric/micrographic textures, and those with a more diverse population of complex intergrowths of plagioclase, Ba-enriched alkali feldspar, ferroan low-Ca pyroxene and silica. Deciphering primary magmatic clasts from impact generated/modified clasts is required to interpret isotopic data for chronology and identify the primary source(s) of felsic melts and volatiles on the Moon. It is notable that felsic clasts studied to date generally exhibit older ages than the measured crater-counted surface ages of silicic features.

Recent results show that felsic clasts in double-drive tube 73001/2 contain either two polymorphs of silica (quartz and tridymite) or solely quartz together with alkali feldspar, e-plagioclase, and minor pyroxene. Tridymite points to a high-temperature crystallization history, while quartz reflects a protracted thermal cooling period. This is supported by quartz microstructures including Dauphiné twins and hackle textured fracture patterns, indicative of a solid-state phase transformation. Additionally, Ti-in-quartz thermometry supports the temperatures inferred from the stability of each silica polymorph. The discovery of e-plagioclase superstructures along with the thermal history for quartz-bearing samples are consistent with a cooling magma body at depth and not a near-surface impact melt.

These data provide some of the first concrete evidence for an intrusive origin for felsite rock fragments within the Apollo sample collection. These fragments may therefore represent the mid- to lower-crustal plumbing system of the silicic features on the lunar surface such as the Gruithuisen Domes, the focus of the upcoming NASA Commercial Lunar Payload Services mission Lunar-VISE.