Untangling the History of Oceanic Peridotites Using Spinel Oxybarometry

Abstract:

Comprehensive knowledge of the oxygen fugacity of the upper mantle is critical to understanding the processes associated with melt production, interaction, and extraction. Thus, it is important to understand how fO₂ changes during a peridotite’s thermal and petrologic history in the asthenospheric and lithospheric mantle, as metamorphic subsolidus reequilibration can result in changes to recorded fO₂.

A case study of Tongan forearc peridotites highlights the heterogeneity seen in mantle peridotites. We analyzed two dredges located 250 km apart along the trench: one dredge ranges in fO₂ from 0.5 to 1 log unit above the QFM buffer, similar to analyses of supra-subduction zone xenoliths (e.g. Brandon and Draper, 1996; Wood and Virgo, 1989) while the other dredge ranges from QFM-0.75 to QFM+0.25 and exhibits high spinel Cr# (ranging from 0.45 to 0.75). Systematics between fO₂, Ti concentration, olivine forsterite content, and Cr# within each dredge allow us to differentiate between the effects of melt extraction, melt interaction, and cooling.

Because the spinel oxybarometry equation is dependent on temperature, it is important to be able to accurately determine the temperature recorded by peridotites. Though many geothermometers are available for mantle rocks, we assert that geothermometers based on Fe-Mg exchange between olivine and spinel are the most applicable to fO₂ calculations, because the oxygen fugacity recorded by a mantle assemblage is primarily controlled by this exchange. Additionally, preliminary analyses of diffusion profiles across olivine-spinel grain boundaries provide insight into the cooling of peridotite in the oceanic lithosphere and its effects on oxygen fugacity. Mg-Fe exchange between olivine and spinel is controlled by the distribution coefficient, K_D, which is dependent on both temperature and the proportion of Cr to other trivalent cations in spinel. We see an increase in olivine forsterite content towards the olivine-spinel interface, consistent with an increase in K_D as cooling occurs. Limited data indicate that while spinel Cr# decreases as the interface is approached, no change is seen in spinel Fe³⁺/ΣFe ratios. As a result, the increased Fo# in olivine dominates the oxybarometry equation, resulting in higher oxygen fugacity values near the interface as cooling occurs.