CONSTRAINTS ON EMPLACEMENT DEPTHS AND ERUPTION DYNAMICS FOR PILLOW LAVAS FROM QUARRIES IN SOUTHWESTERN ICELAND

Vatnsskarð and Undirhliðar quarries expose the central and northern parts of a Weichselian age glaciovolcanic pillow-dominated tindar on the Reykjanes Peninsula in southwest Iceland. Pollock et al. (2014) described the geochemistry and lithostratigraphy of Undirhliðar, identifying multiple lithostratigraphic packages of pillow lavas, dikes and shallow intrusions, and tuff-breccia units. The basal pillow units are petrographically and geochemically different from the dikes and upper pillow units. Geochemically, Vatnsskarð is similar to the upper pillow units/dikes from Undirhliðar, consistent with a fissure system that was simultaneously active in multiple places, with magma distributed through ridge-parallel dikes (Perpalaj et al., 2014).

Vatnsskarð quarry comprises a variety of lithofacies, but pillow lava and vitric tuff-breccia are predominant. Intrusions, both conformable and cross-cutting, are exposed in the quarry, and associated dikes occur immediately to the south. All coherent units at Vatnsskarð have Pl>>Ol>Cpx as phenocrysts and in the groundmass, with a limited range of compositions for Pl (An68-89; avg ~An80) and Ol (Fo64-83) (Perpalaj et al., 2014). Ratios of incompatible trace elements are consistent with all of the samples being cogenetic. Variations in major element geochemistry are consistent with low-pressure fractionation from the most primitive sample. Dikes immediately south of the quarry are geochemically distinct, and may represent a third, separate eruption cycle.

FTIR analyses of 30 doubly polished glass wafers from Undirhliðar yield H2O concentrations ranging from 0.23-0.40 wt.% (Hiatt, 2014). The solubility model of Papale et al. (2006) was used to estimate emplacement pressures of 2.3-4.2 MPa, which correspond to paleo-water depths of 238-442 m and minimum ice thickness estimates of 260-482 m (Hiatt, 2014). Hiatt (2014) found that reconstructed paleo-water depths decrease with stratigraphic height, from ~410 m to ~240 m. New FTIR analyses from Vatnsskarð will test hypotheses that some of the pillows in each of the quarries were emplaced simultaneously, under similar hydrostatic/glaciostatic conditions. As the samples from Vatnsskarð are from similar elevations along the ridge, we expect them to have similar volatile concentrations.