The ~12.1ka BP Vedde Ash: Pre-eruptive P-T-H2O-constraints and Identification of Primary Tephra Layers

The ~ 12.1ka BP Vedde Ash, a bimodal tephra of basaltic and rhyolitic composition linked to the Katla Volcanic System on Iceland, is an important chronostratigraphic marker for the North Atlantic Region and Northern Europe. In some depositional environments however, the use of the Vedde Ash as a marker horizon is not straightforward, as Vedde Ash deposits may get confused with those of rhyolitic Vedde-like events, and primary deposits with non-contemporaneous, reworked deposits. This study investigates methods to identify primary deposits of the Vedde Ash in three marine gravity cores (P457 886-1-GC, 886-2-GC and 888-2-GC) from the southwestern Icelandic shelf. Using visual inspection on the macroscopic and microscopic scale, grain size analysis, and electron microprobe and 2D shape parameter analysis of tephra shards, two correlating tephra layers related to the Katla Volcanic System, and two correlating tephra layers related to the Veidivötn-Bárdarbunga System, have been identified in the cores and their stratigraphic position and depositional modes constrained. Major element signatures, bimodality of basalt and rhyolite, and geochemical evidence of phase mixing between those two components identify the Katla Volcanic System layers as the Vedde Ash. Depositional histories are complex, involving processes such as fallout, rafting of fallout by sea ice and/or icebergs, and background sedimentation of remobilised shards both from the layer-forming and from other events. The results demonstrate the potential of a combined-method approach for distinguishing primary and reworked tephra. This study also presents the first P-T-H2O-constraints for the Vedde Ash, using Fsp-Liq geothermohygrometry and Cpx-Liq geothermobarometry. For the rhyolitic component, calculations were performed for a 1 to 10 kBar range of assumed pressure, yielding T of 824.7 ± 6 °C with 5.2 ± 0.3 wt% H2O at 1 kBar, and 858.7 ± 7 °C with 5.4 ± 0.3 wt% H2O at 10 kBar. Calculations for the basaltic component yielded T of 1129.2 ± 24 °C. H2O calculations were inconclusive for the basaltic, and pressure calculations inconclusive for both components. A tentative model of pre-eruptive magma processes is suggested, involving the rise of high Fe-Ti, low Mg# evolved basaltic melt mixing with and perhaps triggering the eruption of an older rhyolitic melt. The geochemistry of the basaltic Vedde Ash component indicates low degrees of partial melting, falling into an apparent trend of increasing partial melting beneath the Katla Volcanic System from the time of the Vedde eruption at ~ 12.1ka BP to historical times, that may reflect southward progressive rifting along the Icelandic Eastern Volcanic Zone.