OXYGEN FUGACITY RECORDED BY XENOLITHS FROM PACIFIC OCEANIC ISLANDS

Oxygen fugacity (f_O2) plays a vital role in determining mineral stability and depth of melting in the mantle. Several studies have used the spinel peridotite oxybarometer to estimate f_O2; yet few data exist from ocean islands, despite the importance of f_O2 to understanding ocean island basalt petrogenesis (Herzberg and Asimow, 2008).

We report f_O2 recorded by peridotite xenoliths from three ocean islands: Savai'i (average f_O2 = QFM -1.4 to +0.9), Tahiti (QFM +0.6 to +0.7) and Tubuai (QFM -1.1 to +0.2). We calculate f_O2 using methods and standards from Wood and Virgo (1989) and Wood (RiMG, 1990). Oxygen fugacities span a similar range to those reported for El Hierro, Oahu, and Tahiti by Ballhaus (1993): more reduced than arc peridotites, but more oxidized than abyssal peridotites. Spinels in several of the xenoliths are heterogeneous and record a range of apparent f_O2 at the mm scale. We propose two distinct mechanisms for introducing f_O2 heterogeneity: melt refertilization (Tubuai) and diffusive reequilibration (Savai’i and Tubuai).

Spinels in one Tubuai sample record increasing f_O2 from QFM-0.6 in the xenolith interior to +1.1 at the basalt interface. Apparent f_O2 recorded by these spinels correlate with TiO₂, an indicator of melt refertilization (Pearce et al., 2000). We suggest that spinels from the xenolith interior record the relatively low f_O2 conditions of the lithospheric mantle, while host basalt has oxidized near-interface spinels. Uniformly high TiO₂, f_O2, and low olivine Mg# in Tahitian xenoliths from this study may indicate that refertilization has reset the f_O2 recorded by these rocks.

Closed-system diffusive reequilibration, caused by changes in temperature, can also change the f_O2 recorded by a peridotite. In samples from Savai'i and Tubuai with multiple spinel habits, fine intergrowth spinels and the rims of large, equant spinels record higher apparent f_O2 and lower Al₂O₃ than cores of large grains. Canil and O'Neill (1996) suggest that the MgAl₂O₄ component in spinel dissolves into pyroxenes as a function of increasing temperature, leaving a lower modal proportion of Fe³⁺-rich spinel. Because we find no evidence for melt refertilization, we suggest that thermal interaction with a plume caused subsolidus, partial reequilibration that increased the f_O2 recorded by these peridotites prior to eruption.