OXYBAROMETRY OF PERIDOTITES FROM VARIOUS TECTONIC SETTINGS

Much of what we understand today about mantle oxygen fugacity (fO₂) originated with Bernard Wood. We require knowledge of both the dynamic range and absolute value of mantle fO₂ to interpret petrological, geochemical, and geophysical data. The community has spent decades developing proxies for mantle fO₂; yet today we have reached little consensus about the processes that modify fO₂, nor even general agreement about the fO₂s recorded by mantle-derived rocks. Here we focus on recent work stemming from just one of Wood’s many contributions to the theory, quantification, and interpretation of mantle fO₂in space and time: spinel peridotite oxygen barometry.

Using electron microprobe techniques based on those developed by Wood and Virgo (1989) and Mössbauer-characterized spinel standards provided by Wood, we have improved both the precision and accuracy of spinel Fe³⁺/ΣFe ratio determinations. We report on these newly honed techniques as well as their application to peridotites from a range of tectonic settings including ridges, subduction zones, and ocean islands. Taken together, this internally consistent suite of samples allows us to characterize several petrological processes that bear on the apparent fO₂recorded by peridotites.

We find that forearc peridotites (Tonga, Marianas) typically record fO₂ values (QFM+0.5 to QFM+2) that generally scale with the extent of melt depletion (Cr#), but that pockets of melt-depleted forearc peridotite record fO₂s indistinguishable from abyssal peridotite (<QFM), indicating that the mantle wedge is highly heterogeneous. The fO₂s recorded within these suites appear immune to sea-floor alteration/serpentinization.

Ocean island xenoliths (Savai’i, Hawai’i, Tahiti, and Tubuai) record fO₂s ranging from QFM-1 to QFM+1 as well as heterogeneity on the thin section scale. The apparent fO₂s record the oxidizing influence of open system melt infiltration as well as the reducing influence of closed system subsolidus metamorphic reequilibration (Wall et al., this meeting).

Peridotites therefore record a host of fO₂-sensitive processes, operating over a range of spatial scales, which may be important to, but become obscured by, the basalts that sample them.