Deciphering magma plumbing processes beneath Pako seamount in the West Pacific: constraints from textural and compositional diversities of clinopyroxene phenocrysts

Abstract

Magma recharge and mixing can result in diversity of magmatic compositions and trigger the eruptions of volcanoes. Such processes can be read through the textural and compositional characterization of minerals in volcanic rocks. Lavas from the Pako guyot in the West Pacific exhibit a large compositional variation and host abundant clinopyroxene phenocrysts, providing a good opportunity to investigate the origin of whole-rock compositional diversities through mineral texture and chemistry. Here we present textural characterization and in-situ major and trace element and Sr isotopic data for clinopyroxene phenocrysts from five lava samples collected from the Pako guyot in the West Pacific. The clinopyroxene phenocrysts commonly show normal and reverse zoning or complex zoning (e.g., combination of normal, reverse and/or patchy zoning). Only a few clinopyroxene phenocrysts are in FeMg equilibrium with their respective fine-grained matrix pointing to an autocryst origin. The majority of the clinopyroxene phenocrysts are in disequilibrium with their respective fine-grained matrix both texturally (resorption texture) and chemically (higher Mg# and larger ⁸⁷Sr/⁸⁶Sr variation than that in equilibrium with the respective fine-grained matrix). These clinopyroxene phenocrysts form continuously variable major and trace element concentrations and exhibit nearly identical trace element patterns to the autocrysts, indicating that they are antecrysts that crystallized from co-genetic, progenitor magmas. The clinopyroxene phenocrysts overall display a large variation of Sm/Yb, Dy/Yb and ⁸⁷Sr/⁸⁶Sr (0.7036–0.7061) ratios at a small range of Mg# indicating that they crystallized from multiple batches of melts formed from mixing between focal zone (FOZO)- and enriched mantle 1 (EM1)-derived melts. Clinopyroxene textural and compositional diversities record complex magmatic processes that occurred in the Pako magma reservoir including fractional crystallization, recycling of early formed crystals following dissolution and resorption, and multiple magma replenishment and mixing. Deep (at a depth of ∼20 km near the crust-lithospheric mantle boundary) fractionation of clinopyroxene results in the evolved composition (MgO < 7 wt%) and multiple magma replenishment and mixing generate compositional diversities of the Pako guyot lavas.