Mid-ocean ridges are the largest active volcanic systems on Earth, forming a global network of plate boundaries where new oceanic crust is continuously generated from the upwelling of the mantle. The mid-ocean ridge basalts (MORBs) erupted here serve as vital ‘geochemical windows’ into the composition, thermal state, and redox conditions of the upper mantle.
Because iron (Fe) is the most abundant redox-flexible element in basalts, its speciation into ferric (Fe3+) and ferrous (Fe2+) iron is used as the primary indicator of redox in the mantle, and is often expressed as the ratio of Fe3+ against total iron (∑Fe).
There is a lack of consensus on the absolute Fe3+/∑Fe value for global MORB, due to differing analytical methods and interpretations. Resolving this ratio is essential for maintaining an internally consistent paradigm for the formation of the Earth’s oceanic crust; a change of only 0.04 in Fe3+/∑Fe corresponds to a change of ~1 km of predicted oceanic crust thickness. These redox constraints also influence ocean biogeochemistry, where iron speciation influences nutrient cycling, microbial activity, and the chemical evolution of seawater.
We propose a novel, quantitative method of elucidating Fe3+/∑Fe in MORB by ‘counting electrons’ - using electron paramagnetic resonance spectroscopy.