Processes in high-mg, high-T magmas: Evidence from olivine, chromite and glass in palaeogene picrites from West Greenland

Uncontaminated volcanic rocks from the 60 Ma Vaigat Formation, West Greenla nd, contain 6.5 30 wt % MgO, averaging 15.5 wt % MgO. Olivine (mg-number 77 .4 93.3) forms diverse assemblages of zoned phenocrysts and xenocrysts show ing evidence for equilibrium and fractional crystallization, oxidation, par tial to complete re-equilibrium, as well as magma mixing. The olivine cryst als contain glass inclusions and have high contents of Ca and Cr, indicatin g that all olivines with up to mg-number 93.0 crystallized from melts. Asso ciated chromites (mg-number 45.4-77.2) are essentially unzoned and in equil ibrium with the olivines. Matrix glasses from pillow breccias have 6.7-8.8 wt% MgO and quenched close to 1200 degrees C with oxidation states one log- unit above the NNO (nickel-nickel oxide) buffer. Compositional differences between the glasses from different volcanic members are inherited from the primary melts. The magmas erupted as crystal-charged melts, and liquids wit h more than similar to 14 wt % MgO were not erupted. The compositions of th e unerupted parental melts were calculated by stepwise addition of equilibr ium olivine to the matrix glasses, and these melts had 20-21 wt % MgO and l iquidus temperatures of 1515-1560 degrees C. They had lower FeO* than the e rupted rocks with 20-21 wt % MgO because the rocks contain more iron-rich o livine than the melts would have led. The accumulated primary melts ascende d through a lithospheric lid of similar to 100 km thickness, and we envisag e that major crystallization took place from similar to 45 km depth and to the surface. Magma batches ascended in narrow dyke-like conduits and fracti onated high-Mg olivine and chromite at deep levels and less magnesian cryst als at shallower levels. At high ascent numerous olivine crystals were carr ied in suspension to the surface, even some from deep levels, whereas durin g slower ascent most olivines settled out before eruption. Pulsating ascent rates to mixing of magma batches in various stages of fractionation. On av erage, 13 wt % olivine was left within the crust, presumably as olivine-pla ted conduit walls. The conduit systems are similar to the crystal-rich narr ow magma chambers suggested for mid-ocean ridges but are of much greater ve rtical extent.