INSIGHTS INTO PLANETARY PROCESSES FROM ENIGMATIC 3-PHASE SYMPLECTITES (OLIVINE-CLINOPYROXENE-SILICA) IN BASALTIC LUNAR BRECCIAS

Symplectite is a textural term used to describe the fine-grained intergrowth of ≥2 minerals, examples of which are numerous throughout the metamorphic (and igneous) rock record of Earth. From Sederholm (1916) these mineral intergrowths were proposed to occur via solid-state reaction, particularly where two minerals were in contact and became unstable due pressure ± temperature changes. Multiple examples of symplectites in extraterrestrial materials also exist, e.g., spinel symplectites in lunar dunites 72415-72418, Cr-rich symplectites in 76535. In all contexts, symplectites record a component of sample petrogenesis and thus provide insights into an object's geological evolution.

In this work, enigmatic 3-phase symplectites composed of mm to sub-mm clinopyroxene, Fe-rich olivine, and silica in basaltic lunar breccias collected from the Dominion Range (DOM) during the 2018-2019 Antarctic Search for Meteorites (ANSMET) are investigated: DOM 18543 and DOM 18666. Characterization via SEM-EDS documents grains that are wholly symplectic and grains that are interpreted to record a range of breakdown stages. Grains are subhedral to anhedral and vary in size from 20µm to 400µm. Major element analysis via EPMA constrains the olivines to Fo_1-52 and the clinopyroxenes to En_6-22Fs_52-57Wo_21-42. Collectively, these textures and mineral compositions are similar to those documented for lunar samples and martian meteorites (e.g., Shergotty).

Several scenarios have been proposed to explain the formation of these 3-phase symplectites in extraterrestrial samples: i) directly via late-stage crystallization, ii) metastable pyroxferroite breakdown (pyroxferroite formed at low pressure and under non-equilibrium conditions), iii) breakdown of pyroxferroite formed at high pressure (equilibrium conditions), iv) breakdown of metastable pyroxferroite via shock and, v) exsolution of primary metastable pyroxene (to augite and pigeonite), followed by oxide-pyroxene interaction. Through a detailed textural, mineralogical, and geochemical analyses of DOM 18543 and 18666, these scenarios will consider the role of cooling history, metastability, and shock processes on symplectite formation in extraterrestrial samples evaluated.

Sederholm, J. J. (1916). Bulletin de la Commission Géologique de Finlande, 48.