Laser ablation ICP-MS analysis of olivine-hosted melt inclusions from the Mull plateau group lavas, Mull, Scotland

The 840 km lava field of the Isle of Mull, Scotland, represents only a small part of the North Atlantic Tertiary Igneous Province. The cause of this igneous activity at approximately 60 Ma, was the arrival and decompression melting of a hot plume. Previous work on the Mull lavas has focused on the whole rock geochemistry. The aim of this study is the analysis of olivine-hosted melt inclusions by Laser-Ablation Inductively-Coupled-Plasma Mass-Spectrometry (LA-ICP-MS). Melt inclusions are small aliquots of melt trapped by growing crystals (i.e. olivine, plagioclase) during magma evolution prior to and during eruptions. Therefore, melt inclusions analysis can provide valuable information on crystallisation temperatures, the sequence of phase changes in the host magma during cooling, cooling rates, the composition and volatile content of the host magma and liquid lines of decent. In the context of Continental Flood Basalts, melt inclusions also have the potential to provide information on the timing, extent and geochemical effects of crustal contamination. Whole rock data indicated that the parental magmas of Mull Plateau Group lavas represent 4 to 13% melting of a depleted mantle source, located in the spinel-garnet transition zone. Whereas, melt inclusion data show that these parental magmas are mixtures of partial melts of between 2 and 20%, and that the whole rock values only reflect the mean extent of melting. These parental magmas rose through the lithosphere, with little interaction, before ponding near the lithospheric-crust boundary. Fractionation of spinel, olivine and clinopyroxene occurred concurrently with contamination of up to 8% Lewisian granulite by both Assimilation during Turbulent Assent and Assimilation during Fractional Crystallisation processes, with the hottest, most voluminous mantle melts suffering the greatest contamination. Many of the MPG lavas contain olivine phenocrysts enclosing decrepitated melt inclusions, which decrepitate in response to a drop in pressure, but without a corresponding drop in temperature. Therefore, the parental magmas of the MPG lavas rose rapidly from the lower crust, before being emplaced on Mull