Boundary conditions for the petrogenesis of plagioclase ultraphyric basalts (PUB) from the northeast (NE) pacific ridge system: Constraints from plagioclase-hosted melt inclusions

Abstract

The crystal cargo in plagioclase ultraphyric basalts (PUB) together with their melt inclusions (MI) record changes in the major, trace, and isotopic compositions of magmas as they evolve. That geochemical signature provides evidence for the identity and relative significance of the igneous processes such as fractionation, mixing, assimilation, and degassing that modify their host magmas during transport from the melting regime to the surface. This investigation focuses on a case study quantifying the relative roles of these processes active globally at mid-ocean ridges (MOR) based on data collected from plagioclase megacrysts and their homogenized MI from a set of PUB lavas sampled at Blanco Fracture Zone (BFZ), Gorda Ridge, and Endeavor segment of Juan de Fuca (JdF) Ridge in the Northeast (NE) Pacific Ridge system. We combine results for major element phase equilibria calculations based on the MELTS algorithm and apply the major element chemistry, including phase proportions and compositions, to estimate the impact of fractional crystallization and partial melting in the generation and differentiation of these PUB lavas. Our results, in the form of a set of simulations that are based on a range of conditions, enclose the MI, megacryst, and associated lava data and suggest that much of the compositional variation exhibited can be bracketed by 8–20 % incremental batch melting of a heterogeneous mantle protolith, followed by a combined 65–75 % polybaric and isobaric fractional crystallization during ascent. Therefore, only 25–35 % of magma generated in the melting regime survive as liquids to be erupted as lavas.