Temporal redox variation in basaltic tephra from Surtsey volcano (Iceland)
The oxidation state of magma controls and/or tracks myriad petrologic phenomena, and new insights into oxidation are now made possible by high-resolution measurements of Fe3+/∑Fe in volcanic glasses. We present new μ-XANES measurements of Fe3+/∑Fe in a time series of basaltic tephra from the 1963–1967 eruption of Surtsey (Iceland), to examine if the magma mixing between alkalic and tholeiitic basalts that is apparent in the major and trace elements of these glasses is also represented in their oxidation states. Raw Fe3+/∑Fe data show a temporal trend from oxidized to reduced glasses, and this is accompanied by decreasing indices of mantle enrichment (e.g., La/Yb, Zr/Y). When expressed as composition- and temperature-corrected fO2, the trend has a similar magnitude (~0.3 log units) to the variation in fO2 due to ridge-plume interaction along the Reykjanes Ridge. These data indicate that the oxidation state of mixed magmas can be retained through fractionation and degassing processes, and that matrix glass Fe3+/∑Fe in tephras can be used to make inferences about the relative oxidation states of parental magmas during nuanced magma mixing.
Keywordsμ-XANES Redox Surtsey Magma mixing
We thank the Diamond Light Source for access to beamline I18 (proposal number SP11497-1), and invaluable analytical support from K. Ignatyev. The Smithsonian Institution National Museum of Natural History is thanked for loan of standard NMNH 117393. We acknowledge the late S.P. Jakobsson for his critical contributions to the study of Surtsey, and JDLW for facilitating fieldwork in Iceland. We thank K.A. Kelley and an anonymous reviewer for constructive reviews of this work.
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