¹Amanda C. Stadermann,²Bradley L. Jolliff,²Michael J. Krawczynski,¹Christopher W. Hamilton,¹Jessica J. Barnes
Meteoritics & Planetary Sciences (in Press) Link to Article [https://doi.org/10.1111/maps.13795]
¹Lunar and Planetary Laboratory, University of Arizona, 1629 E. University Blvd, Tucson, Arizona, 85721 USA
²Department of Earth and Planetary Sciences & McDonnell Center for Space Sciences, Washington University in St. Louis, 1 Brookings Dr, St. Louis, Missouri, 63130 USA
Published by arrangement with John Wiley & Sons

Sample 12032,366-18 is a 41.2 mg basaltic rock fragment collected during the Apollo 12 mission to the Moon. It is enriched in incompatible trace elements (e.g., 7 ppm Th), but does not have a bulk composition that would be considered a KREEP (enriched in potassium, rare earth elements, and phosphorous) basalt. The sample is of particular interest because it may be representative of some of the mare basalts within Oceanus Procellarum that are inferred to be Th-rich, based on remote sensing data. The major mineral assemblage of 12032,366-18 is pyroxene, plagioclase, olivine, and ilmenite, and the bulk composition has 4.2 wt% TiO2, 11.7 wt% Al2O3, and 0.25 wt% K2O. The sample contains regions of late-stage crystallized minerals and glass (collectively termed mesostasis), including K-feldspar, apatite, rare earth (RE) merrillite, ilmenite, troilite, silica, and relatively sodic plagioclase adjacent to ferroan pyroxene. The mesostasis also occurs in several areas that are highly enriched in silica and intergrown with K-feldspar and very fine-grained, high-mean-atomic-number phases. We explore the petrology of this sample, including the origin of the Si-K-rich mesostasis to assess whether the mesostasis had formed by silicate liquid immiscibility (SLI). We used experiments to determine if the bulk composition of 12032,366-18 is representative of a bulk liquid composition, how the residual liquid evolves, and to investigate the partitioning of elements between phases as the melt evolves. Experiments support that the mesostasis formed by SLI after crystallization of minerals closely matches the major-mineral assemblage of 12032,366-18. Experiments bracket the onset of SLI and merrillite saturation between 1024 and 1002 °C. Some high field strength elements, such as Zr and P, partition preferentially into the Fe-rich liquid. From the experiments, we infer that the bulk composition of 12032,366-18 represents the magma from which it crystallized. Based on the Th-rich and KREEP-bearing chemistry of this sample, along with experimental evidence showing that the sample is representative of a bulk liquid composition and not a cumulate, we conclude that basalt fragment 12032,366-18 was delivered to the Apollo 12 landing site as ejecta from a distant impact and could represent an Oceanus Procellarum basalt. Missions to Oceanus Procellarum, such as Chang’E 5, have the potential to confirm whether some of those basalts are indeed enriched in Th and other incompatible trace elements as indicated by remote sensing.

¹Jie-Jun Jing,²Yanhao Lin,³Jurrien S.Knibbe,¹Wim van Westrenen
Earth and Planetary Science Letters 584, 117491 Link to Article [https://doi.org/10.1016/j.epsl.2022.117491]
¹Faculty of Science, Vrije Universiteit Amsterdam, De Boelelaan 1085, 1081 HV Amsterdam, the Netherlands
²Center for High Pressure Science and Technology Advanced Research, Beijing, 100094, People’s Republic of China
³Royal Observatory of Belgium, Ringlaan 3, 1180 Ukkel, Belgium
Copyright Elsevier

High-pressure, high-temperature experiments have been conducted at deep lunar mantle conditions to constrain the garnet stability field. Using the Taylor Whole Moon composition, garnet is found to be stable at pressures above 3 GPa and temperatures below 1700 °C, yielding a smaller stability field than previously suggested on the basis of thermodynamic calculations. Experimental data are used to model equilibrium crystallization in a ‘two-stage’ model of lunar magma ocean (LMO) crystallization starting from a fully molten Moon. In the first stage, isothermal (1600 °C) equilibrium crystallization of the LMO would produce garnet-bearing lherzolite cumulates (containing up to ∼20 wt.% garnet) in the lowermost lunar mantle. Garnet in the deep lunar mantle would significantly decrease the Al2O3 content of the residual LMO and impact HREE/MREE fractionation. Numerical modeling of the second stage (residual LMO fractional crystallization) shows a delay in plagioclase saturation compared to models of single-stage fractional crystallization of a whole-Moon LMO of the Taylor Whole Moon composition, thinning the anorthositic crust from 95 km to 75 km. To reach the upper limit of current estimates of the average lunar crustal thickness (∼45 km), the two-stage scenario needs to be accompanied by a total of 10% liquid trapped in cumulates and 70% efficiency of plagioclase flotation. We also conduct trace element evolution modeling and reproduce a REE pattern identical to high K, REE, and P (KREEP) compositions after 99.8% solidification, when starting with a CI chondritic REE abundance. The density of a garnet-bearing deep lunar mantle is significantly higher than the density of olivine/orthopyroxene mixtures without garnet. The present-day lowest mantle in the Moon could therefore be characterized by chemical interactions between the earliest (garnet-bearing) and latest (ilmenite-bearing) products of LMO crystallization.