PETROGRAPHY AND CHEMISTRY OF LUNAR IMPACT GLASSES FROM DOMINION (DOM) RANGE LUNAR METEORITES DOM 18543 AND 18666: CONSTRAINING LUNAR GEOLOGY AND LUNAR IMPACT FLUX

As can be observed from Earth, the Moon’s crust is littered with impact craters providing an extensive record of lunar surface reworking over ~4 billion years. Formed during these intense high pressure, high temperature impact events are the lunar impact glasses which represent quenching of the regolith during ballistic flight. These quenched melts have the potential to provide compositional constraints on local and regional lunar geology and information on the impact flux over time. In this study, the lunar impact glasses of two lunar meteorites, Dominion (DOM) Range 18543 and 18666, are characterized.

The glass spherules in DOM 18543 and 18666 are circular to oval in shape, glassy and unfractured to heavily fractured, and typically range in diameter from 10 μm to 100 μm. DOM 18543 spherules are mafic at <51 wt. % SiO₂ and low total alkalis (<1 wt. %, <0.2 wt. % Na2O + K2O) with an average of 42.7 wt. % SiO₂ (±9.07, 2σ) and 0.12 wt. % Na₂O + K₂O (±0.20, 2σ). The average composition of other measured major element oxides are as follows: wt. % TiO₂ at 0.61 (±0.99, 2σ); wt. % Al₂O₃ at 26.54 (±13.60, 2σ); wt. % FeO at 8.91 (±10.42, 2σ); wt.% MgO at 5.75 (±5.40, 2σ); and wt. % CaO at 15.47 (±5.42, 2σ).

It is unlikely that the glass spherules in DOM 18543 are associated with volcanic glasses based on wt. % TiO₂ contents alone as these are characterized by >4 wt. % TiO₂ (and up to 16 wt. % TiO₂). Second, a MgO/Al₂O₃ ratio of >1.25 is attributed to a volcanic, pyroclastic origin while a MgO/Al₂O₃ ratio of <1.25 is attributed to glass of an impact origin. DOM 18543 glass spherules MgO/Al₂O₃ average 0.28 (±0.84, 2σ, and a median value of 0.21). This value is thus interpreted as being consistent with an impact origin. Future work will involve in-situ major element characterization of DOM 18666 glass spherules via electron probe microanalysis (EPMA) and evaluating DOM spherule geochemistry within the context of volcanic and impact glasses throughout the lunar geological record.