摘要
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 1.5GPa 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^55–58when 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 Ni O and Mn O 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.
基金
supported by the National Programme on Global Change and Air-Sea Interaction (No. GASI-GEOGE-02)
the National Nature Science Foundation of China (Nos. 41772040,91858104)
the Fundamental Research Funds for the Central Universities,Hohai University (No. 2013/B18020030)。
