¹A. Sedaghat, ¹C. McLeod, ²M. Loocke, ³B. Shaulis
Icarus (in Press) Open Access Link to Article [DOI: 10.1016/j.icarus.2026.117331]
¹Department of Geology and Environmental Earth Science, Miami University, Oxford, OH 45056, USA
²Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA 70803, USA
³Trace Element and Radiogenic Isotope Laboratory (TRAIL), University of Arkansas, Fayetteville, AR 72701, USA
Copyright Elsevier

Through integrated textural, mineralogical, and geochemical studies, symplectites provide fundamental constraints on the evolution and stability of geological systems. The returned 2018–2019 Antarctic Search for Meteorites Dominion (DOM) Range lunar meteorites contain ubiquitous three-phase symplectites consisting of fayalitic olivine (Fa), Ca-Fe-rich pyroxene (Px), and a silica phase (Si). Here, the textures and compositions of two textural symplectite types in DOM 18509 and DOM 18543 are investigated. Typical symplectites are characterized by a relatively uniform distribution of Fa-Px-Si and fine grain sizes (≤5 μm). Fayalite is Fe-rich (Fa_91–96) with associated pyroxene at Wo_31–40En_6–11Fs_51–61. Bulk compositions determined via reconstruction and broad beam analyses yielded lunar pyroxferroite compositions. The preservation of rare pyroxferroite in lunar materials is reported for DOM 18509: Wo_12-13En_9-10Fs_77–78. In contrast, non-typical symplectites exhibit variable grain sizes and consist of a Ca-rich host pyroxene (Wo_23–40En_8-18Fs_49–68) containing blebs of olivine (Fa_86–95) and silica. Reconstruction for the bulk precursor material yields a metastable pyroxene (Wo_24-26En_12-16Fs_58–64). While both symplectite types are interpreted as subsolidus breakdown products during slow cooling, their formation is associated with different breakdown pathways from distinct precursor phases. During late-stage crystallization in highly evolved lunar basaltic systems, pyroxene compositions enter the forbidden zone. At this stage, metastable Ca-rich pyroxene forms, followed by crystallization of metastable pyroxferroite during extreme Fe-enrichment. Subsequently, non-typical symplectites form through the subsolidus multistep breakdown of metastable Ca-rich pyroxene, whereas typical symplectites form through the direct subsolidus breakdown of pyroxferroite. These two symplectite textures record the evolution of metastable phases within reduced magmatic conditions in extraterrestrial environments.