J. A. Barr

Oxygen fugacity, temperature reproducibility, and H2O contents of nominally anhydrous piston-cylinder experiments using graphite capsules

Abstract The Pt-graphite double-capsule technique is a very commonly used method in high-temperature, high-pressure experimental petrology, particularly for anhydrous experiments relevant to primitive basaltic magmas and mantle melting. We have performed a series of experiments that place better constraints on the range of oxygen fugacity imposed by this capsule material, on the Fe3+/Fe2+ ratios in experimentally produced melts and minerals, and on the temperature reproducibility in Pt-graphite capsules. Oxygen fugacity in our piston-cylinder experiments using Pt-graphite capsules is CCO-0.7 (IW+1.5, QFM-2.2) at 1.5 GPa and 1360 °C. Comparison with other estimates and thermodynamic calculations indicate that a value of CCO-0.8 ± 0.3 can be used as a first approximation at least over the P-T range relevant for MORB and OIB magma generation (0.5-3.0 GPa, 1100-1500 °C). Under those conditions, the amount of Fe3+ in silicate phases (pyroxenes, olivine, glass) and spinel is negligible (Fe3+/ ΣFe < 0.05) and would not significantly affect thermodynamic properties. Significantly higher values of fO₂ cannot be achieved using Pt-graphite or graphite only capsules, but fO₂ can be tuned to lower values by using small pieces of PtFe alloys. The potential range of fO₂ that can be reached in graphite or Pt-graphite capsules is CCO to CCO-4. Temperature reproducibility in piston-cylinder experiments has been examined and can be as low as ±10 °C. Finally, unless capsules are dried overnight at 400 °C before the experiment, small amounts of H2O are always present in nominally dry experiments. These small amounts of H2O should not, however, significantly change phase relations.

High-magnesian andesite from Mount Shasta: A product of magma mixing and contamination, not a primitive melt: COMMENT AND REPLY COMMENT

[Streck et al. (2007)][1] concluded that the high-magnesian andesite (HMA) from Mt. Shasta represents a mix of dacite, basalt, and underlying Trinity ophiolite. The authors present two mixing models calculated to reproduce the major element composition of the HMA (average of samples 85–41a–d; [