The Cauchy 5 Small, Low-Volume Lunar Shield Volcano: Evidence for Volatile Exsolution - Eruption Patterns and Type 1/Type 2 Hybrid Irregular Mare Patch Formation

The lunar shield volcano Cauchy 5, sitting at the low diameter-height-volume end of the population, is the only known example containing two different types of Irregular Mare Patches (IMPs) in very close association: (1) the pit crater interior Type 1 IMP composed of bleb-like mounds surrounded by a hummocky and blocky floor unit and (2) Type 2 IMPs, small, often optically immature pits less than similar to 5 m deep, located on the generally block-deficient shield flanks. A four-phase lunar magma ascent/eruption model predicts that during a relatively brief eruption, low magma rise rates maximize volatile exsolution in lava filling the pit crater. Bubble-rich magmas overtop the pit crater and form extremely vesicular flows on the shield flanks. Exposure of the flanking flows to vacuum produces a fragmental layer of exploded glassy bubble walls. Subsequent second boiling upon cooling of the flanking flow interiors releases additional volatiles which migrate and collect, forming magmatic foams and gas pockets. As magma rise rates slow, trapped gas and magmatic foam build up below the cooling pit crater floor. Magmatic foams are extruded to form Type 1 IMP deposits. Type 2 IMPs on the flanks are interpreted to be due primarily to subsequent impacts causing collapse of the flow surface layer into the extremely vesicle-and void-rich flow interior. Anomalously young pit crater floor/shield flank crater retention ages compared with surrounding maria ages may be due to effects of Cauchy 5 substrate characteristics (extreme micro- and macroporosity, foamy nature, and glassy auto-regolith) on superposed crater formation and retention. Plain Language Summary A group of distinctive and unusual features in the lunar maria known as irregular mare Patches (IMPs) are of two types: Type 1 (mound + floor) usually occurring in volcanic pit craters and related depressions, and dated to less than 100 Ma old and Type 2 (pit only) occurring as scattered pits in localized areas of the lunar maria and too small to obtain ages. We investigated Cauchy 5, a small lava shield that is anomalous in that both Type 1 and Type 2 IMPs occur in very close association. Models of magma ascent and eruption in small-volume, low-volume-flux mare basalt eruptions show that gas exsolution is optimized. Gas release patterns and pit crater lava lake behavior produce Type 1 IMPs on the lava lake floor and Type 2 IMPs on the shield volcano flanks from void collapse and subsequent impacts. The extremely vesicular, void-rich, and foam-like nature of the lava lake floor and shield flank flows forms a substrate whose characteristics are predicted to significantly influence the formation and degradation of superposed impact craters. This potentially causes the IMPs to appear to be much younger than the adjacent mare units.