PHASE EQUILIBRIA AND CONDITIONS OF SILICATE LIQUID IMMISCIBILITY IN SILICIC LUNAR MAGMAS AT MID-LOWER CRUSTAL PRESSURES AND VARIOUS H2O CONTENTS; ORIGIN OF LUNAR RED SPOT VOLCANISM

The Apollo missions returned cm-size fragments of silicic materials from the Moon. The global relevance of these so-called quartz monzodiorites (QMD), felsites, and other samples has been questioned, but recent remote sensing data suggests that the lunar “red spots” are Fe-poor and contain high-Si phases, potentially representing surface expressions of silicic magmatism. We build on previous work to further investigate the behavior of Si-rich lunar materials, extending experimental studies to higher pressures. Beginning with synthetic compositions akin to natural QMD’s and a Fe-poor derivative, we explore silicic phase equilibrium in the mid-lower lunar crust. Can evolved lunar magmas reach silicate liquid immiscibility (SLI) in this environment? How do temperature, pressure, and dissolved H₂O content affect the compositions of such melts crystallizing pyroxene(s) plagioclase, and ilmenite? We performed carbon- and iron-saturated static experiments at 100 and 200 MPa, from nominally dry up to 2 wt% starting H₂O, in the range of 1025-1165 °C. Clinopyroxene is on the liquidus (1150°C dry), followed by plagioclase at ~1090 °C and ilmenite and a phosphate at ~1055 °C. Silica crystallizes at ~1040 °C, in experiments with less than 1 wt% H₂O. SLI, generally expressed as Fe-rich blebs within a Si-rich, Fe-poor parent liquid, occurs in experiments doped with 0.3 wt% H₂O, at both pressures. In QMD experiments, the SLI temperature threshold is ~1040 °C, whereas in Fe-poor experiments it is ~1050 °C. Volatile species (Cl, S) concentrate in areas containing the Fe-rich immiscible blebs, as do Ti, Mg, P, and Ca. Immiscible liquids are less common in equivalent dry experiments and not present in experiments with high starting H₂O contents (>1 wt%), suggesting that small amounts of dissolved H₂O do not prevent development of SLI in lunar magmas. Debate persists regarding the importance of SLI in forming large amounts of Si-rich material on the Moon. We find that KREEP-enriched melts in equilibrium with plagioclase, two pyroxenes and ilmenite are constrained to lie close to the boundary of the immiscible melt field, and that evolution of such a magma may or may not produce SLI depending on concentrations of minor elements (P, Ti, etc) as well as H₂O. This finding may explain why some lunar red spots have a high Th content while others do not.