ESTIMATING MAGMATIC TEMPERATURES AND SOURCE LITHOLOGIES FOR OFF-AXIS ALKALINE MAGMAS – IN SNAEFELLSNES, WESTERN ICELAND

Iceland is a hot-spot influenced, subaerial segment of the Mid-Atlantic Ridge. The active rift volcanics are tholeiites magmas generated by decompression melting, enhanced by the added temperature flux and/or more fusible lithologies of a mantle plume. The Snaefellsnes Peninsula is an off-axis area that has recent (< 1 Ma) alkaline volcanism unconformably overlying older Neogene flood basalts. It is unclear what role mantle temperature and source lithology (e.g., enriched peridotite vs pyroxenite) have in generating this off-axis volcanism in an area of limited crustal extension. In order to address these questions, high precision analyses of olivine macrocrysts and spinel inclusion were carried out on representative lavas from throughout the Snaefellsnes Peninsula.

High-precision compositional analyses of olivine and spinel inclusions were carried out on the Jeol SuperProbe JXA-8230 at the University of Iowa. Trace element concentrations of olivine macrocrysts from 20 transitional-alkaline lava samples across Snaefellsnes have Mn/Fe ratios and Ni contents consistent with peridotite sources (after Sobolev et al. 2007), suggesting that the alkaline volcanism is likely ‘tapping’ an enriched peridotite source. The Al-in-olivine thermometer calculates the equilibrium crystallization temperature for olivine based on the partitioning of Al between olivine (host) and spinel (inclusion). This thermometer requires no knowledge or estimates of liquid composition and is not dependent on other mineral phases that may or may not be present in the sample. Al-in-olivine thermometry was employed on 122 olivine spinel pairs from 20 samples across the Snaefellsnes Peninsula. In order to accurately estimate analytical and computation errors, a Monte Carlo error propagation was carried out on each olivine-spinel pair.

Published olivine-spinel Al-exchange thermometry on Iceland rift lavas give temperatures 150-200°C above primitive MORB. The estimated maximum crystallization temperatures from Snaefellsnes are similarly elevated. These results show that there is not a significant temperature gradient over the 50-150 km between the rift and off-axis Snaefellsnes mantle.