On the potential for lunar highlands Mg-suite extrusive volcanism and implications concerning crustal evolution

• Published in: Icarus (New York, N.Y. 1962) N.Y. 1962), 2016-10, Vol.277, p.319-329
• Main Authors: Prissel, Tabb C., Whitten, Jennifer L., Parman, Stephen W., Head, James W.
• Format: Article
• Language: English
• Subjects: Crusts; Density; Geological processes; Highlands; Lunar crust; Magma; Melts; Moon; Spectroscopy; Texture; Volcanic activity; Volcanism
• ISSN: 0019-1035; 1090-2643
• DOI: 10.1016/j.icarus.2016.05.018

•Buoyancy-driven ascent of Mg-suite magmas is investigated.•Mg-suite melts are ∼200–300kg/m3 less dense than mare basalts.•The nearside southern-highlands may contain Mg-suite volcanic material.•The low-density crust measured by GRAIL may have fractured >4.1Ga.•Mare volcanism requires more than buoyancy forces alone. The lunar magnesian-suite (Mg-suite) was produced during the earliest periods of magmatic activity on the Moon. Based on the cumulate textures of the samples and a lack of evidence for Mg-suite extrusives in both the sample and remote sensing databases, several petrogenetic models deduce a predominantly intrusive magmatic history for Mg-suite lithologies. Considering that ∼18% of the lunar surface is covered by mare basalt flows, which are substantially higher in density than estimated Mg-suite magmas (∼2900 versus ∼2700kg/m3), the apparent absence of low-density Mg-suite volcanics is surprising. Were Mg-suite magmas predominantly intrusive, or have their extrusive equivalents been covered by subsequent impact ejecta and/or later stage volcanism? If Mg-suite magmas were predominantly intrusive, what prevented these melts from erupting? Or, if they are present as extrusives, what regions of the Moon are most likely to contain Mg-suite volcanic deposits? This study investigates buoyancy-driven ascent of Mg-suite parental melts and is motivated by recent measurements of crustal density from GRAIL. Mg-suite dunite, troctolite, and spinel anorthosite parental melts (2742, 2699, and 2648kg/m3, respectively) are considered, all of which have much lower melt densities relative to mare basalts and picritic glasses. Mg-suite parental melts are more dense than most of the crust and would not be expected to buoyantly erupt. However, about 10% of the lunar crust is greater in density than Mg-suite melts. These areas are primarily within the nearside southern highlands and South Pole-Aitken (SP-A) basin. Mg-suite extrusions and/or shallow intrusions were possible within these regions, assuming crustal density structure at >4.1Ga was similar to the present day crust. We review evidence for Mg-suite activity within both the southern highlands and SP-A and discuss the implications concerning crustal evolution as well as Mg-suite petrogenesis. Lower crustal densities measured by GRAIL are consistent with the lack of observed Mg-suite extrusives. If Mg-suite extrusive volcanism was prevented by the low density of the crust, it would suggest the lunar crust was f