Joint 56th Annual North-Central/ 71st Annual Southeastern Section Meeting - 2022

Paper No. 9-12
Presentation Time: 8:00 AM-12:00 PM


AMES, Joshua and BIRNER, Suzanne, Chemistry Department, Berea College, 101 Chestnut St., Berea, KY 40403

The oxygen fugacity (fO2) 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 fO2. 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 fO2 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 fO2. We did, however, find a weak correlation between longitude and fO2, consistent with trends previously observed in spinel Cr#, a proxy for melt extraction. Moving eastward along the fracture zone towards the ridge axis, fO2 values broadly decrease as Cr#s broadly increase though more fO2 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 fO2 is tied to sample age, melt extraction, or other ridge-related processes.

[1] Brunelli et al., 2018

[2] Cipriani et al., 2009