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 Greenland, 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 showing evidence for equilibrium and fractional crystallization, oxidation, partial to complete re-equilibration, as well as magma mixing. The olivine crystals contain glass inclusions and have high contents of Ca and Cr, indicating that all olivines with up to mg-number 93·0 crystallized from melts. Associated chromites (mg-number 45·4-77·2) are essentially unzoned and in equilibrium with the olivines. Matrix glasses from pillow breccias have 6·7-8·8 wt % MgO and quenched close to 1200°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 with more than ~14 wt % MgO were not erupted. The compositions of the unerupted parental melts were calculated by stepwise addition of equilibrium olivine to the matrix glasses, and these melts had 20-21 wt % MgO and liquidus temperatures of 1515-1560°C. They had lower FeO(*) than the erupted rocks with 20-21 wt % MgO because the rocks contain more iron-rich olivine than the melts would have had. The accumulated primary melts ascended through a lithospheric lid of ~100 km thickness, and we envisage that major crystallization took place from ~45 km depth and to the surface. Magma batches ascended in narrow dyke-like conduits and fractionated high-Mg olivine and chromite at deep levels and less magnesian crystals at shallower levels. At high ascent velocities numerous olivine crystals were carried in suspension to the surface, even some from deep levels, whereas during slower ascent most olivines settled out before eruption. Pulsating ascent rates led to mixing of magma batches in various stages of fractionation. On average, 13 wt % olivine was left within the crust, presumably as olivine-plated conduit walls. The conduit systems are similar to the crystal-rich narrow magma chambers suggested for mid-ocean ridges but are of much greater vertical extent.