Trachytic Melt Inclusions Hosted in Clinopyroxene Offer a Glimpse Into Samoan EM2‐Endmember Melts

Abstract

AbstractWe present major and trace element data on clinopyroxene‐hosted, glassy melt inclusions (MIs) from a highly enriched (high 87Sr/86Sr = 0.71856) lava (ALIA‐D115‐18) from the Samoan hot spot. Following correction for post‐entrapment crystallization, the MIs are trachytic (62.6–65.4 wt% SiO2, 1.5–3.0 wt% MgO, and 7.2–10.6 wt% total alkalis (Na2O + K2O)) ALIA‐D115 lavas dredged off the submarine flanks of Savai'i Island, western Samoa have previously been suggested to exhibit binary endmember magma mixing between a mafic, low 87Sr/86Sr magma and an evolved, high 87Sr/86Sr magma (the Enriched Mantle 2 endmember; EM2). In major and trace element space, these ALIA‐D115 lavas form a mixing trend between more primitive Samoan shield lavas and the MIs of this study, suggesting the MIs could represent the EM2‐derived mixing endmember. Recent efforts to constrain the EM2 mixing endmember composition shows remarkable overlap with the MI compositions, supporting the argument they could represent the 87Sr/86Sr mixing endmember to the ALIA‐D115 lavas and are derived from the EM2 endmember. Additionally, one melt inclusion contains Fo73 olivines not in equilibrium with the trachytic melt, suggesting the melt interacted with a mafic magma prior to entrapment within host clinopyroxene. Projecting the ALIA‐D115 mixing trend in 87Sr/86Sr versus SiO2 and K2O space shows that the trend intersects an average MI composition at 87Sr/86Sr values of ∼0.725, thereby offering a window into more extreme compositions associated with the EM2 endmember.