Peridotite/melt partitioning experiments constraining the geochemical signature of CO2-bearing alkaline magmas from redox melting to the source of ocean island basalts

Mantle/melt partitioning of trace elements is governed by both melt composition and the chemistry of peridotite-forming minerals (olivine, orthopyroxene, clinopyroxene and garnet/spinel), which in turn are controlled by the pressure-temperature conditions in the melting column. Although several sets of mineral/melt partition coefficients are available for various mantle lithologies and P-T conditions, none constrains the partitioning behavior for realistic CO₂-H₂O-bearing silicate melts saturated with the four mantle minerals along the mantle adiabat, conditions that will determine the geochemical signatures of melts released from asthenosphere upwellings. To thus performed “forced multiple saturation experiments” on a highly Si-undersaturated primitive ocean island basanite composition from Cape Verde in which the melt is forced into equilibrium with four-phase garnet lherzolite at adiabatic temperatures (1380-1420 ^oC) at 3-7 GPa. This yields mineral and melt compositions in the melting column of a mantle upwelling from the incipient redox melts forming at 7 GPa to the oceanic lithosphere-asthenosphere boundary (LAB). In-situ analyses by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) were conducted to determine the mineral/melt partitioning of high field strength elements (HFSE: Nb, Ta, Zr, Hf), Ba, Sr, Th, U, REEs, Y, moderately siderophile elements (e.g., W, Mo), alkalis (K₂O, Na₂O) and other minor elements (TiO₂, P₂O₅) at each pressure step. These pressure-dependent partition coefficients and our melting reaction stoichiometries are then employed to model the geochemical signatures of CO₂-bearing silicate melt rising through the asthenosphere. The modeled results are then compared to primitive alkaline magmas erupted in both continental and oceanic settings to test whether peridotite/melt trace element partitioning to varying depths effectively encompasses the geochemical spectrum of intraplate magmatism.