¹Nicolas P. Walte,^1,2Christopher M. Howard,³Gregor J. Golabek
Earth and Planetary Science Letters (in Press) Link to Article [https://doi.org/10.1016/j.epsl.2023.118247]
¹Heinz Maier-Leibnitz Center for Neutron Science (MLZ), Technical University Munich, 85748 Garching, Germany
²ISIS Neutron & Muon Spallation Facility, Rutherford Appleton Laboratory, Harwell Science and Innovation Campus, Chilton, Oxfordshire OX11 0QX, United Kingdom
³Bayerisches Geoinstitut (BGI), University of Bayreuth, 95447 Bayreuth, Germany
Copyright Elsevier

Main group pallasites were likely formed by the collision of their parent body with a smaller impactor that caused a mixing of mantle and core material from the target and impactor, respectively. In order to better constrain the collision, we present particle size distribution (PSD) analyses of olivines in seven main group pallasites and of the Eagle station pallasite. The PSDs of two fragmental pallasites (Admire and Huckitta) contain linear segments in bi-logarithmic particle number versus size diagrams that are similar to terrestrial cataclasites or impact-related rocks. On the other hand, the PSDs of four angular pallasites only display linear segments above their respective average grain sizes. We argue that fragmental pallasites record brittle rock deformation close to the impact site of the collision, while angular pallasites represent deeper-lying mantle rocks of the target body that were disintegrated by the downward percolation of core metal from the impactor. High strain-rate deformation experiments with the system olivine – FeS melt ± Au melt produced microstructures and PSDs that are broadly similar to these two textural groups. The experiments also suggest that de-localized metal melt percolation and concomitant mantle disintegration as evidenced in angular pallasites is facilitated by weak grain boundaries caused by a small fraction of previously present metal melt in the mantle as opposed to localized diking that is dominant in a melt-free mantle. The former mechanism is expected to prevent efficient core-merging and instead causes mantle-impregnation with metal melt, which could be active when planetesimal mantles were still warm due to short-lived radiogenic heating. In addition to the parent body of the PMG, asteroid 16 Psyche may be an example of this inefficient core-merging mechanism.

¹Stephen M. Elardo,¹Daniel F. Astudillo Manosalva
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2023.05.020]
¹The Florida Planets Lab, Department of Geological Sciences, University of Florida, Gainesville, FL 32611, USA
Copyright Elsevier

Clasts of feldspathic lithologies such as magnesian anorthosites, anorthositic troctolites, and granulites in lunar meteorites have greatly expanded knowledge of the lithologic diversity of the lunar crust. However, their origins and relationships to other major crustal lithologies such as the ferroan anorthosites, magnesian-suite, and alkali-suite are not fully understood. Here we present the results of phase equilibrium modeling using the MELTS and MAGFOX programs designed to understand the origins of lunar crustal lithologies and petrologic connections between them. We show that the major and trace element compositions of the Mg- and alkali-suites are consistent with partial melting of hybridized sources and are inconsistent with decompression melting + assimilation models. Our results also show that the vertical trend in Mg# in mafic silicates vs. An# in plagioclase characteristic of feldspathic meteoritic lithologies and ferroan anorthosites can be produced by fractional crystallization of KREEP-free Mg-suite melts, but that these melts are not likely to contribute significantly to the composition of the global crust. Furthermore, we calculated potential parental melt compositions for these crustal lithologies using the abundances of REEs in plagioclase in FANs, the Mg- and alkali-suites, and clasts in feldspathic meteorites. Our results show that despite low bulk rock abundances of incompatible trace elements in many feldspathic lunar meteorites, parental melts with overall REE abundances similar to or in excess of KREEP are needed to reproduce the REE abundances in plagioclase in clasts from feldspathic lunar meteorites. However, the lack of a Na enrichment trend in their plagioclase compositions with decreasing Mg# requires very Na-depleted melts without a KREEP component. The available data regarding magnesian anorthosites, anorthositic troctolites, and granulites in lunar meteorites, and inferences made here regarding their parental melt compositions, lead to contradictory and ambiguous conclusions regarding their origins.