Oxygen fugacity at the base of the Talkeetna arc, Alaska

Abstract

The origin of the more oxidized nature of arc magmas as compared to that of mid-ocean ridge basalts (MORB) is debated, considered to be either a feature of their mantle source, or produced during crustal transit and eruption. Fe^(3+)/Fe^T ratios (Fe^(3+)/[Fe^(3+) + Fe^(2+)]) in arc volcanic rocks and glasses and thermodynamic oxybarometry on mantle xenoliths from arc lavas indicate elevated magmatic oxygen fugacity (fO_2), whereas, redox-sensitive trace elements ratios and abundances in arc volcanic rocks have been used to suggest that arcs have source regions with fO_2 similar to the MORB source. Here, we take an alternative approach by calculating the fO_2 of the uppermost mantle and lowermost ultramafic cumulates from the accreted Jurassic Talkeetna arc (Alaska). This approach allows us to quantify the fO_2 of the sub-arc mantle and of primary arc magmas crystallizing at the base of an island arc, which have not been affected by processes during crustal transit and eruption which could affect their fO_2. Implementing olivine–spinel oxybarometry, we find that the upper mantle (harzburgites and lherzolites) and ultramafic cumulates (clinopyroxenites and dunites) crystallized between + 0.4 and + 2.3 log units above the fayalite-magnetite-quartz buffer, consistent with previous studies suggesting that the sub-arc mantle is oxidized relative to that of MORB. In addition, the Talkeetna paleo-arc allows us to examine coeval lavas and their redox-sensitive trace element ratios (e.g., V/Sc). The average V/Sc ratios of high MgO (> 6 wt%) lavas are 6.7 ± 1.6 (2σ), similar to that of MORB. However, V/Sc ratios must be interpretted in terms the degree of partial melting, as well as, the initial V/Sc ratio of the mantle source in order to derive information about fO_2 of their mantle source. The V/Sc ratios of Talkeetna lavas are consistent with the elevated fO_2 recorded in the sub-arc mantle and primitive cumulates (olivine Mg# [Mg/(Mg + Fe)] × 100 > 82) if a depleted mantle source underwent 15–20% melting. Our results suggest that the arc mantle is, on average, more oxidized than the MORB source and that V/Sc ratios must be interpreted in the context of a partial melting model where all model parameters are appropriate for arc magma genesis. This study reconciles V/Sc ratios in arc volcanic rocks with fO_2 of primary arc basalts and the sub-arc mantle from the same locality.

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