OXYGEN FUGACITY OF ABYSSAL PERIDOTITES ALONG THE GAKKEL RIDGE

The oxygen budget of the Earth’s mantle is important in understanding how our planet evolves chemically over time. The Gakkel Ridge is the world’s slowest spreading ridge [1], and exposes peridotites along its axis that record the activity of oxygen in the upper mantle. Our samples comprise relatively fertile lherzolites and harzburgites (Cr#=0.13-0.17, 3.1-8.3% modal cpx [2]) as well as refractory harzburgites (Cr#=0.43-0.55, 0.2-1.0% modal cpx [2]). Using spinel peridotite oxygen barometry [3], we calculated the oxygen fugacity (fO₂) of a suite of 10 peridotites from the Gakkel Ridge in order to investigate how melt processes affect the oxygen budget of the Earth’s interior.

We show that the low-Cr# lherzolites and harzburgites range from -0.1 to +0.6 log units relative to the QFM buffer, consistent with the global abyssal peridotite array, whereas high-Cr# refractory harzburgites have low fO₂ values, ranging from -0.7 to -2.7 log units below QFM, with the most refractory samples falling significantly lower than the global array.

Because D’Errico et al. (submitted) interprets the refractory samples as recording ancient melt extraction, the low fO₂ recorded by these samples may originate in the geologic past, perhaps even in a different tectonic setting. While LREE enrichment in the refractory harzburgites [2] provides evidence for refertilization by an infiltrating melt that could have recently imprinted reducing conditions, we see no corresponding increase in TiO₂ content in the spinels, which weakens this hypothesis. Further research on additional refractory harzburgites is needed to constrain whether the reduced nature of these samples is telling us something about the effect of extreme melt extraction on fO₂ at ridges, or whether these samples record a unique history that obscures processes operating at ridges today.

[1] Coakley and Cochran, EPSL (1998), [2] D’Errico et al., submitted, [3] Bryndzia and Wood, American Journal of Science (1990)