Origin of the lunar highlands Mg-suite: An integrated petrology, geochemistry, chronology, and remote sensing perspective

Abstract

The Mg-suite represents an enigmatic episode of lunar highlands magmatism that presumably represents the first stage of crustal building following primordial differentiation. This review examines the mineralogy, geochemistry, petrology, chronology, and the planetary-scale distribution of this suite of highlands plutonic rocks, presents models for their origin, examines petrogenetic relationships to other highlands rocks, and explores the link between this style of magmatism and early stages of lunar differentiation. Of the models considered for the origin of the parent magmas for the Mg-suite, the data best fit a process in which hot (solidus temperature at ≥2 GPa = 1600 to 1800 °C) and less dense (r ~3100 kg/m 3 ) early lunar magma ocean cumulates rise to the base of the crust during cumulate pile overturn. Some decompressional melting would occur, but placing a hot cumulate horizon adjacent to the plagioclase-rich primordial crust and KREEP-rich lithologies (at temperatures of <1300 °C) would result in the hybridization of these divergent primordial lithologies, producing Mg-suite parent magmas. As urKREEP (primeval KREEP) is not the "petrologic driver" of this style of magmatism, outside of the Procellarum KREEP Terrane (PKT), Mg-suite magmas are not required to have a KREEP signature. Evaluation of the chronology of this episode of highlands evolution indicates that Mg-suite magmatism was initiated soon after primordial differentiation (<10 m.y.). Alternatively, the thermal event associated with the mantle overturn may have disrupted the chronometers utilized to date the primordial crust. Petrogenetic relationships between the Mg-suite and other highlands suites (e.g., alkali-suite and magnesian anorthositic granulites) are consistent with both fractional crystallization processes and melting of distinctly different hybrid sources.

Gravitational search for cryptovolcanism on the Moon: Evidence for large volumes of early igneous activity

“…For gravity calculations, we need the bulk density, so we assume a typical porosity of 7% (similar to that of mare basalts, ( Kiefer et al, 2012 )) and so derive a bulk density of 2790 kg m −3 and a density contrast of 230 kg m −3 with respect to the average feldspathic crust. Because the Mg-suite rocks have typical ages of 4.2-4.4 Ga ( Carlson et al, 2014;Shearer et al, 2015 ) and thus probably experienced a higher level of impact processing than most mare basalts, their porosities might be higher than assumed here. In turn, this would reduce the density contrast between the Mg-suite and the feldspathic crust, which would increase the thickness and volume estimates relative to those derived here.…”

“…Such rocks are typically less iron-rich and thus are expected to be less dense than mare basalts ( Shearer et al, 2015;Prissel et al, 2015 ). Because there are no laboratory measurements of the density of lunar Mg-suite rocks, we calculate densities from the CIPW normative mineralogy.…”

“…The term cryptomaria was introduced by Head and Wilson (1992) to denote ancient lunar mare deposits buried below high-albedo crater and basin ejecta. Other earlier volcanic deposits, such as the extrusive equivalent of the magnesian-suite or Mg-suite basalts (e.g, Shearer et al, 2015;Prissel et al, 2013;2015 ) may not have the distinctive mineralogy and albedo contrast of the mare basalts. Thus, such deposits may not be as readily recognized in image and spectral reflectance data (e.g., no dark halo craters to reveal their presence and distinctive mineralogy), but near-infrared spectroscopy from the Moon Mineralogy Mapper has identified Mg-rich rocks on the lunar surface ( Pieters et al , 2011 ).…”

Water in evolved lunar rocks: Evidence for multiple reservoirs

“…However, it is not a simple path from late-stage magma ocean products to KREEP-rich or KREEP-related magmas, which formed either by the assimilation of the urKREEP component by rising Mg-rich diapirs at the base of the lunar crust, or by the partial melting of hybrid mantle sources, formed by sinking urKREEP (and other dense components) mixing with Mg-rich olivine-orthopyroxene cumulates (Shearer and Floss, 1999;Shearer and Papike, 2005;Elardo et al, 2011). Regardless of the formation mechanism, these magmas gave rise to what we now know as the KREEP basalts, and the norites and troctolites of the magnesian (Mg) suite (recently reviewed by Shearer et al, 2015). Fractional crystallization of magmas resembling KREEP basalts is thought to have produced geochemically evolved rocks such as quartz monzodiorites (Ryder, 1976;Ryder and Martinez, 1991;Jolliff, 1991).…”

“…Fractional crystallization of magmas resembling KREEP basalts is thought to have produced geochemically evolved rocks such as quartz monzodiorites (Ryder, 1976;Ryder and Martinez, 1991;Jolliff, 1991). Alternatively, silicate liquid immiscibility (SLI) is proposed to have played a role in the formation of the lunar felsites (e.g., Hess et al, 1975;Warner et al, 1978;Ryder and Martinez, 1991;Snyder et al, 1995;Shearer et al, 2015). Most importantly, any petrologic processing took place inside the Moon at pressures high enough to inhibit water loss from magmas or mantle rocks.…”