Redox Heterogeneity of the Mantle Inferred from Hotspots

Abstract:

Hotspots provide a unique opportunity to sample mantle heterogeneities and thereby gain insight into the geodynamic history of our planet. Well-documented excursions in isotopic ratios from values typical of normal mid-ocean ridge basalt (MORB) classsically underpin our understanding of geochemical heterogeneity. Major elements too appear to record litholoigcal heterogeneity and thus deepen our understanding of the processes responsible for creating heterogeneity. Here we report the Fe³⁺/∑Fe ratios of submarine glasses dredged from ocean islands (Samoa, Hawai’i, Pitcairn, Societies) and plume-affected ridge segments (Azores, Galapagos). When corrected for low-pressure crystal fractionation to 8 wt.% MgO, Fe³⁺/∑Fe₍₈₎ratios of ocean island basalts (OIB) from Samoa, Loihi, and Pitcairn extend to higher values than normal MORB (0.16±0.01) and, with one exeption, overlap with the range observed in back arc basin basalts (0.16 to 0.21 at Samoa, 0.17 to 0.19 at Loihi, and 0.19 to 0.27 at Pitcairn). Plume-affected samples are among the most oxidized MORBs (~0.168 at Azores and ~0.175 at Galapagos).

Basalt Fe³⁺/∑Fe ratios provide one proxy for the oxygen fugacity (fO₂) of the mantle from which they derive. The higher oxidation state of Fe in OIB relative to MORB may reflect higher fO₂ in the source mantle of OIB relative to MORB, and may further explain major element systematics in OIBs as well as their more calc-alkaline nature. We also report that Fe³⁺/∑Fe₍₈₎ ratios correlate with isotopic ratios, such as ⁸⁷Sr/⁸⁶Sr, which may trace recycled continental crust (Samoa: R=0.57, n=8; Galapagos: R = 0.94, n=8). Strikingly, these positive correlations are opposite to those observed in MORB away from plumes, where Fe³⁺/∑Fe₍₈₎ ratios correlate negatively with indicies of enrichment (e.g. ⁸⁷Sr/⁸⁶Sr, R = -0.71, n=31 and Ba/La, R = -0.72, n=19, Cottrell and Kelley, Science, 2013 and unpub.). Yet, these new OIB data support conclusions drawn from MORB: redox heterogeneities reflect large-scale geodynamic processes acting over long time periods and are not overprinted by melting and differentiation (C&K, Science, 2013). We infer that a variety of procesess, including recycling of surface-derived materials, generate redox heterogeneities in the mantle that persist and influence OIB petrogenesis.