The role of substrate characteristics in producing anomalously young crater retention ages in volcanic deposits on the Moon: Morphology, topography, subresolution roughness, and mode of emplacement of the Sosigenes lunar irregular mare patch

Abstract

Lunar irregular mare patches (IMPs) comprise dozens of small, distinctive, and enigmatic lunar mare features. Characterized by their irregular shapes, well‐preserved state of relief, apparent optical immaturity, and few superposed impact craters, IMPs are interpreted to have been formed or modified geologically very recently (<~100 Ma; Braden et al. ). However, their apparent relatively recent formation/modification dates and emplacement mechanisms are debated. We focus in detail on one of the major IMPs, Sosigenes, located in western Mare Tranquillitatis, and dated by Braden et al. () at ~18 Ma. The Sosigenes IMP occurs on the floor of an elongate pit crater interpreted to represent the surface manifestation of magmatic dike propagation from the lunar mantle during the mare basalt emplacement era billions of years ago. The floor of the pit crater is characterized by three morphologic units typical of several other IMPs, i.e., (1) bulbous mounds 5–10 m higher than the adjacent floor units, with unusually young crater retention ages, meters thick regolith, and slightly smaller subresolution roughness than typical mature lunar regolith; (2) a lower hummocky unit mantled by a very thin regolith and significantly smaller subresolution roughness; and (3) a lower blocky unit composed of fresh boulder fields with individual meter‐scale boulders and rough subresolution surface texture. Using new volcanological interpretations for the ascent and eruption of magma in dikes, and dike degassing and extrusion behavior in the final stages of dike closure, we interpret the three units to be related to the late‐stage behavior of an ancient dike emplacement event. Following the initial dike emplacement and collapse of the pit crater, the floor of the pit crater was flooded by the latest‐stage magma. The low rise rate of the magma in the terminal stages of the dike emplacement event favored flooding of the pit crater floor to form a lava lake, and CO gas bubble coalescence initiated a strombolian phase disrupting the cooling lava lake surface. This phase produced a very rough and highly porous (with both vesicularity and macroporosity) lava lake surface as the lake surface cooled. In the terminal stage of the eruption, dike closure with no addition of magma from depth caused the last magma reaching shallow levels to produce viscous magmatic foam due to H2O gas exsolution. This magmatic foam was extruded through cracks in the lava lake crust to produce the bulbous mounds. We interpret all of these activities to have taken place in the terminal stages of the dike emplacement event billions of years ago. We attribute the unusual physical properties of the mounds and floor units (anomalously young ages, unusual morphology, relative immaturity, and blockiness) to be due to the unusual physical properties of the substrate produced during the waning stages of a dike emplacement event in a pit crater. The unique physical properties of the mounds (magmatic foams) and hummocky units (small vesicles and large void space) altered the...

Geological Characterization of the Ina Shield Volcano Summit Pit Crater on the Moon: Evidence for Extrusion of Waning‐Stage Lava Lake Magmatic Foams and Anomalously Young Crater Retention Ages

“…These processes produce volcanic deposits with very unusual physical properties, thus exerting an influence on the nature of regolith development, and crater formation and retention processes, resulting in anomalously young interpreted ages for the Ina summit pit crater floor that more plausibly formed contemporaneously with the underlying shield volcano about 3.5 Ga ago. The two other lunar IMP occurrences dated as younger than 100 Ma by Braden et al (, i.e., Sosigenes, ~18 Ma and Cauchy 5, ~58 Ma; Figure S24) also lie at the top of dikes (Qiao et al, ; Qiao, Head, Xiao, Wilson, & Dufek, ) and hence could be reinterpreted to be emplaced in a similar manner billions of years ago. Our interpreted ancient formation ages, in contrast to the geologically very recent lava extrusion hypothesis (Braden et al, ), also coincide with the climax of global volcanism between circa 3.3–3.8 Ga ago (e.g., Pasckert et al, and references therein).…”

“…Our new model of contemporaneous late‐stage shield‐building volcanism ~3.5 Ga ago thus makes a major reevaluation of the conventional theory unnecessary. Our progressive observational and numerical investigations of Ina (Qiao et al, , ; Qiao, Head, Wilson, & Ling, ; Qiao, Head, Xiao, Wilson, & Dufek, ; Wilson & Head, , ) make it a prime target candidate for future landers, rovers, and sample return missions, which show enormous potential for strengthening our knowledge of the magmatism and thermal evolution of the Moon and other terrestrial bodies (e.g., Draper et al, ; Qiao et al, ; Wagner et al, ).…”

“…The interior floor is generally flat, slopes gently (<2°) toward the center (Figure ), and mainly lies about 20–50 m below the rim (Figures and ). The interior of Ina pit crater is dominated by three morphological units typical for other major lunar IMPs (Figure b): (1) the unusual meniscus‐like mounds, rising up to ~20 m above the proximal floor units (Figure ), occupying ~50% of the total interior area, (2) topographically lower hummocky units (~44% by area) with ridged and pitted textures, and (3) topographically lower blocky materials consisting of 1‐ to 5‐m size boulders (Braden et al, ; Garry et al, ; Qiao et al, ; Qiao, Head, Xiao, Wilson, & Dufek, ; Schultz et al, ; Strain & El‐Baz, ). The mounds are generally convex upward (Figures and ), very flat at the tops, and reach steep slopes (~10–30°) at the edges (Figure c).…”

“…The apparently optically immature blocky materials within Ina are another major characteristic that has perplexed lunar scientists for decades. In earlier analyses based on relatively coarser resolution Apollo orbital photographs, the blocky units are often described as bright or white units (El‐Baz, ; Strain & El‐Baz, ); submeter LROC NAC images show unequivocally that blocky units are basically freshly exposed boulder fields (Garry et al, ), with individual boulders reaching ~5 m in dimension; these are, however, much smaller than the largest boulders at Sosigenes, another major IMP occurrence (up to ~12 m, Qiao et al, , Qiao, Head, Xiao, Wilson, & Dufek, ).…”

“…The wide range of geological peculiarities of the Ina feature, particularly the anomalously young crater retention ages, has led to various interpretations in terms of its formation mechanism (see summaries of parts of previous investigations in Elder et al, and Qiao et al, ). Preliminary characterization of Ina attributed the high reflectance of the floor materials to deposition of sublimates (Whitaker, ).…”