Olivine-hosted melt inclusions and melting processes beneath the FAMOUS zone (Mid-Atlantic Ridge)

Abstract

Major- and trace-element abundances have been obtained for olivine-hosted melt inclusions from a picritic basalt (ARP73–10–03) dredged in the FAMOUS locality along the Northern Mid-Atlantic Ridge. Isolated in homogeneous, highly primitive olivine phenocrysts (Fo 87.2–91.3), melt inclusions provide important information about the processes of melt generation beneath mid-ocean ridges. They contain near-primary magmas that display a large range of major- and trace-element compositions and form a primitive continuation of the FAMOUS whole rock suites. Both slightly depleted and enriched melt inclusions can be distinguished on the basis of their trace-element compositions, with (La/Sm) N ratios ranging from 0.58 to 1.52. Melt inclusions display chemical trends on both major- and trace-element variation diagrams, indicating that they constitute a suite of genetically-related, highly primitive magmas. The evolution of the melt inclusions is inconsistent with a model of mantle-melt chemical exchange during reactive transport, suggesting that the trapped melts are able to preserve pristine information about the melting process and/or source composition. Comparison of these trends with predicted curves for models of peridotite melting indicates that compositional variations are best reproduced by polybaric partial melting of a relatively homogeneous mantle source and subsequent mixing in various proportions of the melt batches produced at different degrees of melting and/or in different parts of the melting system. These observations require that transport of melts from the melting region to the site of olivine crystallization occurs without significant chemical exchange with the surrounding mantle.