hide
Free keywords:
Phase equilibria; high-Ti picrite; Flood basalt; Pyroxenite; Mantle metasomatism
Abstract:
Various models have been invoked to explain the petrogenesis and Ti-content bimodality of lavas in large igneous provinces. We performed phase equilibrium experiments on an extremely high-Ti (5.4 wt% TiO2) near-primitive picrite from the base of the Paleogene (~ 55 Ma old) East Greenland Flood Basalt Province to constrain the mantle melting source. This sample has low An content, a steep rare earth element (REE) profile, and is enriched in incompatible trace elements. Near-liquidus phase relations determined over pressures of 1 atm to 1.5 GPa and temperatures from 1094 to 1400 °C reveal that the high-Ti picritic glasses are multi-saturated with olivine (Ol) + orthopyroxene (Opx) at pressures around 1 GPa, but have only Ol or Opx on the liquidus at lower and higher pressures, respectively. This indicates that the primitive melt was last equilibrated with olivine-free rocks, e.g., pyroxenite before it left the lithospheric mantle. Considering the high CaO/Al2O3 ratio (1.14) of our starting material and the presence of low-Ca pyroxene (pigeonite) at high pressure in our experiments, we propose that garnet pyroxenite was present in the mantle source. Residual garnet in the source could adequately explain the low Al2O3 content (7.92 wt%) and steep heavy REE patterns of the picrite. The high Ni content of the picritic lava (ca. 460 ppm) at 13 wt% MgO also supports a source depleted in olivine. We suggest that the high degree of enrichment in incompatible elements results from melting of a metasomatized mantle. Based on geochemical modelling, we propose that the mantle source contains 5% Ti-enriched amphibole and that moderate partial melting (15–20%) of this source can produce the high-Ti picritic lavas of East Greenland.
