Initiation of melting beneath the Oslo rift: a melt inclusion perspective

This study of the most primitive magmas in the Permo–Carboniferous Oslo magmatic system aims at understanding the initial phases of magmatism in a rift-related environment. Primary melt inclusions are investigated from two compositional types of pyroxene: Cr-diopside and Ti-augite, found within the alkaline Skien basalts, southern Norway. Data from the analyses of melt inclusions in this tectonic setting indicate that the melting process is complex. In both types of pyroxene, two groups of melt inclusions can be identified, together with rare CO2 fluid inclusions. In the Cr-diopsides, these are termed the ‘CD-high Fe’ (FeO>9 wt.%) and ‘CD-low Fe’ (FeO<7.5 wt.%) groups. The melt inclusions that are part of these groups are the most primitive analysed and appear to have distinct sources in the garnet and spinel stability fields. These melts are considered to have been trapped by the developing Cr-diopside pyroxene crystals near the Mohorovicic discontinuity during lithospheric extension. o groups of melt inclusions found in the Ti-augites are called ‘TA-high Na’ (Na2O>2 wt.%) and ‘TA-low Na’ (Na2O<2 wt.%). These melts were trapped by the later (over)growths of Ti-augite at shallower depths. The two groups record evidence of pyroxene crystallisation, and also crucially of magma degassing. This latter event is considered to give rise to the variation in alkali (and P2O5) contents.