Ca/Al of plagioclase-hosted melt inclusions as an indicator for post-entrapment processes at mid-ocean ridges?

Abstract

The composition of melt inclusions in basalts erupted at mid-ocean ridges may be modified by post-entrapment processes, so the present composition of melt inclusions may not represent their original composition at the time of entrapment. By combining the melt inclusion composition in samples from the South Mid-Atlantic Ridge at 19ºS analyzed in this study, and from the Petrological Database, we found that post-entrapment crystallization processes resulted in higher Ca/Al, Mg#[100°—atomic Mg²⁺/(Mg²⁺+Fe²⁺)], MgO and FeO contents, and lower CaO and Al₂O₃ contents of plagioclase-hosted melt inclusions relative to those hosted in olivine. In addition, melt inclusions hosted in plagioclase with anorthite content larger than 80mol.% had been modified more readily than others. By discussing the relationships between Ca/Al and fractional crystallization, post-entrapment crystallization, and the original melt composition, we propose that Ca/Al can be regarded as an indicator of the effect of post-entrapment processes on melt inclusion composition. Specifically, i) when Ca/Al<0.78, melt inclusion compositions corrected for fractional crystallization to Mg#=72 can represent the primary magma at mid-ocean ridges; ii) when 0.78<Ca/Al<1.0, melt inclusions are mainly modified by post-entrapment crystallization effects, and can reveal the original melt composition after correcting for these effects; iii) when Ca/Al>1.0, the compositions of melt inclusions do not reflect the original melt composition nor preserve information about the mantle source. According to these criteria, plagioclase-hosted melt inclusions with Ca/Al>1.0 in basalts from the South Mid-Atlantic Ridge at 19ºS cannot represent the composition of the melt at the moment of their entrapment.