Ascent and eruption of a lunar high-titanium magma as inferred from the petrology of the 74001/2 drill core

An analysis of the orange grasses and crystallized beads from the 68 cm dee p 74001/2 core has been conducted to understand the processes occurring dur ing ascent and eruption of the Apollo 17 orange glass magma. Equilibrium be tween melt and metal blebs (Fe85Ni14Co1) within the core, along with Cr con tents in olivine phenocrysts, suggest there was an oxidation of C and a red uction of the melt at an O fugacity of IW-1.3 and 1320 degrees C to form CO gas at 200 bars or similar to 4 lan depth. This was followed by developmen t of more oxidized conditions during ascent. Also during ascent, there was formation of euhedral, homogeneous Fo(81) olivine crystals and spinel cryst als with higher Al and Mg contents than the smaller spinels in the crystall ized beads. Both the metal blebs and Al-rich spinels were trapped inside th e Fo(81) olivine phenocrysts as they grew prior to eruption. The compositio n of the orange glasses are homogeneous throughout the core, except for a f ew distinct glasses at the top that appear to have been mixed in by microme teorite reworking. A few glassy melt inclusions of orange glass composition trapped in the Fo(81) phenocrysts contain 600 +/- 100 ppm S and similar to 50 ppm CI compared to the 200 ppm S and 50 ppm Cl in the orange glass melt when quenched. These inclusions therefore document the addition of 400 ppm S to the GO-rich volcanic gas during the eruption. The size and distribution of different volcanic beads in the Apollo 17 depo sit indicate a mode of eruption in which the orange glasses and partially c rystallized beads formed further away from the volcanic vent where cooling rates were faster. Progressively larger and more numerous crystals in the b lack beads reflect slower cooling rates at higher optical densities in the volcanic plume. The development of a brown texture in the orange glasses at the bottom of the core, where the black beads dominate, is interpreted to result from devitrification by subsolidus heating either as the orange glas ses fell back through the hot plume or after deposition on the surface. The change from domination by orange glasses to black beads in the core probab ly reflects a decrease in gas content over time, which consequently would i ncrease the plume optical density and favor slower cooling rates.