MELT INCLUSIONS IN PRIMITIVE OLIVINE PHENOCRYSTS: THE ROLE OF LOCALIZED REACTION PROCESSES IN THE ORIGIN OF ANOMALOUS COMPOSITIONS

Abstract

Melt inclusions are small portions of liquid trapped by growing crystals during magma evolution. Recent studies of melt inclusions have revealed a large range of unusual major and trace element compositions in phenocrysts from primitive mantle-derived magmatic rocks [e.g. in high-Fo olivine (Fo > 85 mol %), spinel, high-An plagioclase]. Inclusions in phenocrysts crystallized from more evolved magmas (e.g. olivine Fo < 85 mol %), are usually compositionally similar to the host lavas. This paper reviews the chemistry of melt inclusions in high-Fo olivine phenocrysts focusing on those with anomalous major and trace element contents from mid-ocean ridge and subduction-related basalts.We suggest that a significant portion of the anomalous inclusion compositions reflects localized, grainscale dissolution-reaction-mixing (DRM) processes within the magmatic plumbing system. The DRM processes occur at the margins of primitive magma bodies, where magma is in contact with cooler wall rocks and/or pre-existing semi-solidified crystal mush zones (depending on the specific environment). Injection of hotter, more primitive magma causes partial dissolution (incongruent melting) of the mush-zone phases, which are not in equilibrium with the primitive melt, and mixing of the reaction products with the primitive magma. Localized rapid crystallization of high-Fo olivines from the primitive magma may lead to entrapment of numerous large melt inclusions, which record the DRM processes in progress. In some magmatic suites melt inclusions in primitive phenocrysts may be naturally biased towards the anomalous compositions. The occurrence of melt inclusions with unusual compositions does not necessarily imply the existence of new geologically significant magma types and/or melt-generation processes, and caution should be exercised in their interpretation.