Experimental Constraints on Formation of Low-Cr^# Chromitite: Effect of Variable H₂O and Cr₂O₃ on Boninitic-Magma and Harzburgite Reactions

Reactions between a boninitic or basaltic magma and harzburgite at shallow mantle depths are thought to be closely related to the formation of podiform chromitites, but little experimental data is available on these reactions. In this study, a series of experiments were conducted at 1.5 GPa and 1 000-1 400 ^oC to investigate the interactions between boninitic magma and harzburgite in homogenous mixed systems with varied bulk concentrations of water (~0.7 wt.%-10 wt.%) and Cr₂O₃ (~0.2 wt.%-4 wt.%). In the experimental charges, chromite grains can be observed coexisting with orthopyroxene, clinopyroxene±olivine, and quenched melt in the Cr-bearing systems. The bulk concentration of Cr₂O₃ in the starting material has a slight effect on compositional changes in the chromites generated. However, the Cr^# (Cr^#=100×Cr/(Cr+Al)) and Mg^# (Mg^#=100×Mg/(Mg+Fe)) values for the chromites exhibit positive and negative correlations, respectively, with the bulk H₂O concentrations. At 1 100 ^oC, chromite Cr^# values range from ~33-35 to ~58-65, and chromite Mg^# values range from ~70-73 to~55-58 when bulk H₂O contents in the starting material are increased from ~0.7 wt.% to ~10 wt.%. The experimentally produced chromites have compositions (as expressed by Cr^#, Mg^#, and NiO and MnO contents) similar to natural chromites from low-Cr^# chromitite bodies. We suggest that the interactions between boninitic magmas with varied H₂O contents and harzburgite in a shallow mantle wedge could be a possible mechanism that forms the low-Cr^# chromitites found in ophiolites. We emphasize here that H₂O may play an important role in the compositional evolutions of natural chromitites.