Extending Fe-Mg Olivine-liquid Partitioning to Picritic Compositions at One Atmosphere

Iron-magnesium partitioning between olivine and silicate melt exerts a fundamental control on the FeO and MgO contents of olivine-saturated liquids undergoing low-pressure fractionation as well as the compositions of liquids produced during partial melting of peridotite. Surprisingly, very little low-pressure olivine-liquid experimental data has been published on natural melt compositions with > 16 wt% MgO. Results from high-pressure experiments [1] are scattered but they hint at an increasing value of the exchange coefficient, K_D=(FeO/MgO)^ol/(FeO/MgO)^liq, as melt MgO contents increase from 20 to 30 wt%; the K_D values increase by as much as 30% relative over this range. Whether one uses K_D = 0.30 or 0.38 has a dramatic effect on the compositions of reconstructed primary magmas, and it is therefore of interest to better determine the appropriate value(s). We conducted one-atmosphere experiments on a synthetic, alkali-free, Hawaiian picrite composition (25.7 wt% MgO, 45.7 wt% SiO₂) at QFM and temperatures ranging from 1300° -1500° C. Samples were suspended on pre-saturated Pt-Fe wire loops and quenched after ∼12 to 72 hours. Time series experiments were conducted at 1300° and 1450° C. Melt and olivine are present in all experiments; chrome spinel occurs in the 1300° and 1350° C experiments. Over the temperature range investigated, wt% MgO in the glass varies from 13.4 to 23.8, while olivine forsterite contents increases from 88 to 92 (mole%). The weight fraction of glass shows a strong positive linear correlation with temperature, while the weight fraction of olivine is inversely correlated with temperature. The K_D values calculated from the olivine-liquid pairs (after first calculating liquid FeO/Fe₂O₃ using [2]), are independent of temperature and melt composition (the mean value is 0.32±0.01, 1 sigma). Our mean value is the same as that calculated by [3] from a large body of lower temperature one-atmosphere experiments from the literature, and supports the results of the olivine-addition calculations on Mauna Kea glasses reported in [3]. [1] Kushiro & Walter (1998) Geophys. Res. Letters 25, 2337-2340. [2] Kress & Carmichael (1991) Contrib. Mineral. Petrol. 108, 82-92. [3] Stolper et al. (2004) GGG 5, doi 10.1029/2003GC000553.