Pink Moon: The petrogenesis of pink spinel anorthosites and implications concerning Mg-suite magmatism

- We experimentally test spinel production via melt-wallrock interactions on the Moon.
- New lunar rock type, pink spinel anorthosite, is chemically linked to Mg-suite.
- Pink spinel anorthosite can be used as a proxy for Mg-suite magmatism on the Moon.
- Because Mg-spinel detections occur globally, Mg-suite is also a global phenomenon.
- KREEP is likely not required for Mg-suite petrogenesis.

NASA's Moon Mineralogy Mapper (M3) has identified and characterized a new lunar rock type termed pink spinel anorthosite (PSA) (Pieters et al., 2011). Dominated by anorthitic feldspar and rich in MgAl2O4 spinel, PSA appears to have an unusually low modal abundance of mafic silicates, distinguishing it from known lunar spinel-bearing samples. The interaction between basaltic melts and the lunar crust and/or assimilation of anorthitic plagioclase have been proposed as a possible mechanism for PSA formation (Gross and Treiman, 2011; Prissel et al., 2012). To test these hypotheses, we have performed laboratory experiments exploring magma-wallrock interactions within the lunar crust. Lunar basaltic melts were reacted with anorthite at 1400 °C and pressures between 0.05-1.05 GPa. Results indicate that PSA spinel compositions are best explained via the interaction between Mg-suite parental melts and anorthositic crust. Mare basalts and picritic lunar glasses produce spinels too rich in Fe and Cr to be consistent with the M3 observations.The experiments suggest that PSA represents a new member of the plutonic Mg-suite. If true, PSA can be used as a proxy for spectrally identifying areas of Mg-suite magmatism on the Moon. Moreover, the presence of PSA on both the lunar nearside and farside (Pieters et al., in press) indicates Mg-suite magmatism may have occurred on a global scale. In turn, this implies that KREEP is not required for Mg-suite petrogenesis (as KREEP is constrained to the nearside of the Moon) and is only necessary to explain the chemical make-up of nearside Mg-suite samples.