Vapor Undersaturated Ultradepleted MORB Liquids in Plagioclase-hosted Melt Inclusions from Garrett Fracture Zone Lavas

Abstract

Melt inclusions (MI) entrapped in minerals that form at depth provide an opportunity to estimate relatively unmodified volatile concentrations in undegassed magmas. In this study, we report on the major, trace, and volatile element concentrations in plagioclase-hosted MI and compare those values to previously reported data from olivine-hosted MI and volcanic glasses erupted from the Garrett Fracture Zone (GFZ). Our new data were derived from two plagioclase ultraphyric basalt (PUB) samples, one of which contains two compositionally distinct populations of plagioclase-hosted MI. One population of MI is compositionally similar to olivine-hosted MI and the associated lavas from the same ridge segment. The other population of plagioclase-hosted MI in that sample is characterized by comparatively high MgO and Al₂O₃, extremely low incompatible trace element concentrations, and high CO₂ concentrations that are equivalent to minimum pressures of entrapment of 2-10 kbar. The major element compositions of those high-MgO plagioclase-hosted MI are similar to liquid compositions previously produced experimentally at 5-10 kbar. Few of the plagioclase-hosted MI in either of GFZ sample contain vapor bubbles – consistent with them being vapor undersaturated at the time of entrapment. Taken together, these observations imply that some of the MI represent the product of melting a depleted source that is either CO₂-rich or where CO₂ undersaturated magmas have scavenged a CO₂-rich component during transport from the melting regime to the surface.