OXYGEN FUGACITY OF PERIDOTITES FROM THE VEMA FRACTURE ZONE

The oxygen fugacity (f_O2) of Earth’s mantle provides crucial insight into the relationship between the mantle, crust, and atmosphere. Although mid-ocean ridges are one of few tectonic settings where geologists can access mantle samples, few ridges have densely sampled measurements of peridotite f_O2. In this study, we investigated peridotites from the Vema fracture zone, a densely sampled region along the Mid-Atlantic Ridge, to constrain the relationship between f_O2 and previously published parameters such as isotopic data and sample age [1]. We identified spinel, olivine, orthopyroxene and clinopyroxene utilizing a petrographic microscope and used a scanning electron microscope to choose samples/regions of interest. We then determined precise and accurate Fe³⁺/(Fe²⁺+Fe³⁺) ratios in spinel for 12 samples using electron microprobe analysis. Oxygen fugacity values calculated from these ferric iron concentrations range from -0.143 to +0.858 log units relative to the QFM buffer (average: 0.474 ±0.193). We found no correlation between previously published Nd isotopic data and f_O2. We did, however, find a weak correlation between longitude and f_O2, consistent with trends previously observed in spinel Cr#, a proxy for melt extraction. Moving eastward along the fracture zone towards the ridge axis, f_O2 values broadly decrease as Cr#s broadly increase though more f_O2 data are needed at easter longitudes (youngest ages) and at the boundary between ‘old’ and ‘young’ samples [2] to determine whether this observation is robust. Further data is also needed to determine whether the observed variation in f_O2 is tied to sample age, melt extraction, or other ridge-related processes.

[1] Brunelli et al., 2018

[2] Cipriani et al., 2009