Origin of basalt fire-fountain eruptions on Earth versus the Moon

Fire-fountain eruptions of basaltic magma occur on Earth at centers such as Kilauea (Hawaii), and deposits from apparently similar eruptions have been found on the lunar surface. The driving force for terrestrial fi re-fountain eruptions is the exsolution of dissolved CO2 based on gases dissolved in melt inclusions trapped in olivine phenocrysts and the relatively high oxidation state of these magmas. Gases released at the vent show that SO2, and eventually H2O, are partitioned into the CO2-rich gas, adding to the gas volume. In contrast, analytical and experimental studies of lunar samples indicate that the gas phase responsible for driving the lunar eruptions was CO-rich and produced by the oxidation of C (graphite) carried in the slowly ascending low-fO2 basalt. The graphite oxidation occurs when the pressure in the ascending lunar magma reaches that of the graphite-gas reaction surface (40 ± 1 MPa or ~8 km depth for the Apollo 17 orange-glass magma). As graphite is oxidized, some FeO is reduced, potentially forming a Fe-rich metal phase, and Fe-rich metal spherules are present in beads in lunar glass deposits. Other gas species such as S, Cl, and F partitioned variously into CO-rich lunar volcanic gas, and appear in surface coatings on the glass spherules. Modeling of the magma fl ow from 8 km depth to the lunar surface shows that the gas bubble volume fraction (assuming initial C at 50–500 ppm levels) ranges from 0.5 to 0.8 at the surface, the exit velocity ranges from 15 to 35 m/s, and the low-viscosity magma fragments only as it erupts at the lunar surface. Figure 1. Optical photomicrographs. A: Thin section near top of A17 orange-glass deposit (74001/2 drill core) showing small and large beads and fragments making up the deposit. Glass beads are variably devitrifi ed to a microcrystalline mixture of cryptocrystalline olivine and ilmenite (Weitz et al., 1999). Bead at center bottom contains a euhedral olivine microphenocryst that contains two metal spherules formed by graphite oxidation prior to growth of the phenocrysts. B: Thin section made from small (2–3 mm) rounded glass spherules erupted in the 1959 Kilauea Iki eruption. Olivine phenocrysts (<5 mm) are present in this basalt , and microlites occur in the beads. The high vesicle content of these beads dominates the internal texture compared to the lunar orange glass. on February 17, 2014 geology.gsapubs.org Downloaded from