The role of melt/rock ratio in olivine-rich troctolite formation via basalt-dunite reaction: an experimental study at 0.5 GPa

Microstructural and chemical evidence supports the origin of olivine-rich troctolites through multistage interactions between a precursor mantle dunite and an infiltrating basalt (Drouin et al., 2010). Borghini et al. (2018) recently performed reactive dissolution and crystallization experiments juxtaposing MORB-type glasses on a melt-bearing dunite at 1300°C and then cooling to 1150°C at constant pressure (0.5 and 0.7 GPa). Their results showed that olivine-rich troctolites might form through dunite infiltration followed by reactive crystallization of interstitial melts resulting in textural relations and mineral chemistry comparable with natural rocks. Significantly they suggested that the initial melt/rock ratio strongly influences the extent of interaction and, thus, the final mineral abundances and chemistry. However, the reaction couple strategy used in their experiments did not allow a quantification of the role of melt/rock ratio. At this purpose, we are performing piston cylinder experiments at 0.5 GPa using a mixture of San Carlos olivine (Fo90) and a reacting MORB-type melt at variable proportions olivine:melt (9:1, 3:1 and 1:1). For each mixture, we perform at 0.5 GPa both isothermal run at 1300°C for 24 hours and crystallization experiment step cooled at a rate of 1°C/min from 1300°C down to 1100°C. Specific aims are to define and quantify how the melt/rock ratio controls the amount of olivine dissolution, the lithology produced by reaction, and the final mineral chemistry. Preliminary results of the isothermal experiment run at 0.5 GPa and 1300°C with 25 wt% of initial basalt powder in the starting mix (olivine:melt 3:1) show run products made of olivine and glass. Olivine occurs both as large subhedral crystals with straight and lobate curvilinear rims against the interstitial glass or as smaller rounded grains. Mineral chemistry indicates that after the high-temperature interaction reacted olivine has slightly lower XMg and higher CaO contents. Remarkably, NiO content is significantly lower than that of the starting San Carlos olivine and it is still lower than NiO content in olivine from reaction experiments with lower melt/olivine ratios. Compared to the initial melt, final glass composition is characterized by higher XMg, SiO2 and NiO contents and lower FeO, Cr2O3 and Al2O3 abundances. Borghini G., Francomme J.E., Fumagalli P. 2018. Melt-dunite interactions at 0.5 and 0.7 GPa: experimental constraints on the origin of olivine-rich troctolites. Lithos. 323, 44-57. Drouin M., Ildefonse B., Godard M. 2010. A microstructural imprint of melt impregnation in slow spreading lithosphere: olivine-rich troctolites from the Atlantis Massif, Mid-Atlantic Ridge, 30°N, IODP Hole U1309D. Geochem. Geophys. Geosyst. 11, 1-21.