Asymmetric Early Post-Magma Ocean Crust Building on the Moon’s Nearside

Abstract:

The earliest episode of post-LMO crust building on Earth’s Moon is represented by the Mg-suite, a series of ~4.3 Ga plutonic igneous rocks that require complex mantle dynamics to explain. These magmas required contributions from the earliest LMO dunites, originally formed at the base of the mantle, the anorthositic crust, and the KREEP reservoir, which is the final dregs of the LMO. Bringing these components together into a magmatic source requires rapid convective overturn of the mantle. Mg-suite samples are common in the Apollo collection. However, KREEP is only found on the nearside, and Mg-suite samples are largely absent in meteorites sourced from the farside. This calls into question whether Mg-suite magmatism was a global event, or restricted to the KREEP-rich nearside. The answer to this question has implications for the extent of early crust building and mantle dynamics.

We conducted high-T experiments and thermal evolution calculations on model Mg-suite source regions to assess the contributions of two variables on melt production: melting point depression and radiogenic heating. Experiments simulated melting an LMO dunite and crustal anorthosite source region with KREEP contents from 0 – 50 wt. %. Experiments were conducted at 1 bar pressure over a range of temperatures. Thermal evolution calculations on the same compositions calculated the relative ΔT of source regions in K/Myr, balancing a range of possible lower crustal cooling rates with radiogenic heat from K, Th, and U in KREEP.

Our experiments show that the presence of KREEP in Mg-suite source regions lowers the melting temperature of those sources significantly, leading to more melt at a given temperature. At 1250 °C, for example, the KREEP-free farside-analogous system produces an 8% partial melt, yet with 50% KREEP, the melt fraction reaches 79%. Our calculations show that Mg-suite source regions heat up by 10s of K/Myr when they contain at least 25% KREEP, and that all sources with KREEP contents of <5% cool down at all possible cooling rates. Our results demonstrate the positive feedback that KREEP had on mantle melting: the more KREEP in a source, the lower the melting temperature, the higher the heat production, and the more magma produced. This likely resulted in asymmetric crust building on the lunar nearside beginning immediately after the LMO.