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Geology; March 2009; v. 37; no. 3; p. 219-222; DOI: 10.1130/G25402A.1
© 2009 Geological Society of America
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Origin of basalt fire-fountain eruptions on Earth versus the Moon

Malcolm J. Rutherford1,* and Paolo Papale2

1Department of Geological Sciences, Brown University, Providence, Rhode Island 02912, USA
2Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Pisa, 56126 Pisa, Italy


Figure 01
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  		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 devitrified 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.

 

Figure 02
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  		Figure 2. Plot of log oxygen fugacity versus pressure (both in MPa) illustrating an isothermal (1350 °C) section in the C-O system and how a graphite-bearing, low-f O2 basalt (IW buffer conditions; IW = iron + wustite) would intersect the graphite-gas surface with decreasing pressure in a magma ascent. The pressure at which a CO-rich gas first forms is dependent on the intrinsic f O2, aFeO, and temperature of the basalt (Fogel and Rutherford, 1995). Assuming the basalt carries <2000 ppm C, the oxidation would be complete in a small pressure range beginning at 40 MPa (Nicholis and Rutherford, 2005).

 

Figure 03
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  		Figure 3. Results of the Conduit4 numerical simulation of A17 orange-glass magma flow from 8 km depth to the surface compared to a tholeiite magma (with 0.1 wt% H2O and 1 wt% CO2) ascent on Earth (see Table DR1). A: Gas volume for different amounts of graphite (C = 50–2000 ppm) and dike widths (w = 0.5–4 m). B: Liquid velocity for the same initial graphite contents and dike widths. C: Pressure in the conduit for the lunar cases (all equal within error) versus a Hawaiian tholeiite ascent over the same distance on Earth.

 




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