Lunar Mg suite: Thermobarometry and petrogenesis of parental magmas

Abstract

Using compilations of internally consistent thermodynamic data on minerals and advances in quantifying the energetics of crystalline solid solutions, we have recalculated the pressures and temperatures of equilibration of troctolite sample 76535 and a number of spinel cataclasites. The five phase assemblage (olivine + orthopyroxene + clinopyroxene + Cr‐spinel + plagioclase), which occurs in symplectite assemblages in 76535, gives equilibration temperatures from 800°–900°C and pressures from 200 to 250 MPa, indicating a deep (40–50 km) crustal source. Spinel cataclasite assemblages give minimum pressures of ∼100 MPa. Mg suite magmas were apparently intruded throughout the lunar crust early in lunar history. The textures and equilibration temperatures in many Mg suite samples are consistent with subsolidus recrystallization and the Mg‐rich minerals of some refractory troctolites and dunites may be residues of a crustal melting process or products of subsolidus elemental redistribution. The Sm‐Nd age of 4.26±0.06 Ga of troctolite 76535 most likely represents the time of subsolidus recrystallization and/or partial melting. In an attempt to identify the primary source material for the Mg suite magmas, a variety of models were tested using the MELTS and MAGFOX codes. A good fit between mineral compositions and crystallization sequences is obtained by ∼20% batch melting at 0.5 GPa of an average whole Moon composition with an Mg # of 0.85. Melts formed by batch melting at higher pressures and melts formed by polybaric partial fusion over the pressure range from 2.0 to 0.3 GPa were either too poor in Mg and/or crystallized clinopyroxene before orthopyroxene. Mixing with 10–20% urKREEP, which is required to satisfy the REE constraints of the Mg suite, reduces the Mg # of the parental magmas and requires a whole Moon Mg # closer to that of the terrestrial mantle.