Evidence for Extrusive Mg-Suite Magmatism on the Moon? Fine-Grained Magnesian Clasts in an Apollo 16 Impact Melt Breccia
The magnesian suite (Mg-suite) of rocks record some of the earliest intrusive magmatism on the Moon. Studies of these Mg-suite rocks find they are plutonic or hypabyssal, formed typically kilometers under the lunar surface. Several models exist to explain the formation and evolution of the Mg-suite but distinguishing between hypotheses can be difficult given the limited sample availability. The global extent of Mg-suite magmatism remains in debate and is key to constraining models of early secondary crust building. In this study, we present magnesian clasts within Apollo impact melt rock 68815. These clasts contain olivine, plagioclase, with minor amounts of Mg-Al-spinel and pyroxene similar to spinel troctolites of the Mg-suite, but they lack plutonic textures. We provide evidence that some of the clasts may be of extrusive volcanic origin akin to terrestrial komatiites while others might represent crystalline impact melts. There exists a large breadth of evidence for Mg-suite intrusives, whereas here, we present possible evidence of Mg-rich volcanic counterparts. If valid, this would broaden the known diversity of lunar volcanism during the initial stages of secondary crust building. We anticipate this finding to provide a greater constraint onto models of Mg-suite ascent and emplacement, which only currently consider intrusive magmatism, as well as renewed motivation to examine impact melt breccias for rare and understudied lithologies. Future trace element studies or radiometric dating could be used to further interrogate the connections of these clasts to the Mg-suite.