Roles of magmatic oxygen fugacity and water content in generating signatures of continental crust in the Alaska-Aleutian arc

Abstract:

Early depletion of Fe during magmatic differentiation is a characteristic of many arc magmas, and this may drive them towards the bulk composition of continental crust. In the Alaska-Aleutian arc, magmas are strongly Fe-depleted both in the east, where the arc sits atop pre-existing continental crust, and in the west, where the system is oceanic but convergence is highly oblique. Primary basaltic arc magmas may achieve early Fe depletion through a combination of high magmatic H₂O, which delays silicate saturation, and high oxygen fugacity (fO₂), which promotes early onset of Fe-oxide crystallization. Alternatively, low-Fe, high Mg# magmas may emerge directly from the arc mantle, possibly due to slab melting, driving mixing with Fe-rich basaltic magmas. Yet, the relative importance of H₂O, fO₂, and magmatic bulk composition in generating Fe-depletion is not clearly resolved. Here, we present new measurements of the oxidation state of Fe (Fe³⁺/∑Fe ratio; a proxy for magmatic fO₂), in combination with major element and volatile data, of olivine-hosted melt inclusions from four Alaska-Aleutian arc volcanoes (Okmok, Seguam, Korovin, Augustine), acquired using XANES spectroscopy. We use the Tholeiitic Index (THI) of Zimmer et al., 2010 to quantify the behavior of Fe in each volcano magma series (<1 is Fe-depleted, >1 is Fe-enriched). These volcanoes span a range of THI, from 0.9-0.65. The Fe³⁺/∑Fe ratios of Aleutian basalts, corrected for fractional crystallization to 6 wt.% MgO (i.e., Fe³⁺/∑Fe_6.0) range from 0.22-0.31 and correlate strongly with THI (r²>0.99), such that more Fe-depleted magmas contain a greater proportion of oxidized Fe. The maximum dissolved H₂O contents of basaltic melt inclusions from these volcanoes also strongly correlate with THI (r²>0.96), and with measured Fe³⁺/∑Fe ratios (although H₂O is not the direct cause of oxidation). These links point to a slab-derived origin of both H₂O and oxidation and thus relate slab fluxes to the Fe-depletion trends of arc magmas. These correlations also illustrate the difficulty of separating the effects of H₂O and fO₂ on arc magmatic differentiation, as the two are challenging to isolate in nature. Analysis of experimental data, however, suggest that fO₂ exhibits stronger control than H₂O on the relative appearance of spinel and silicates on the liquidus.