Olivine-hosted melt inclusions provide insights into primitive magma compositions driving young eruptions from Red Crater, Tongariro (New Zealand)

Abstract

Volcanism at continental arcs is ultimately driven by mantle-derived magmas; however, erupted compositions are typically overprinted by crustal interactions during ascent, obscuring deeper signatures. We target olivine and their melt inclusions (MI) in basaltic andesite scoria erupted from Red Crater (Tongariro, New Zealand) to capture primitive melt compositions prior to upper crustal processing. Olivine crystals comprise several, mostly antecrystic populations, with forsterite contents from Fo_70–91 and variable textures indicative of complex crystallisation histories within a dynamic, transcrustal magmatic system. MI in forsteritic (≥ Fo₈₄) olivine cores contain primitive basaltic melts and record early fractionation (~10–20%) of olivine, spinel ± clinopyroxene at sulfur-saturated, moderately oxidised conditions during entrapment. Incompatible trace elements display variable enrichments in fluid-mobile and light rare earth elements that can be explained by ~3–10% partial melting of a shallow mantle source with variable contributions of slab-derived fluid and/or sediment components and minor to no crustal assimilation. Similarities with caldera-associated olivine-hosted MI in the central Taupō Volcanic Zone (TVZ) suggest a common source, with differences such as higher calcium and chlorine at Red Crater and trace element variations potentially related to distinct slab-derived contributions, differences in mantle mineralogy, degree of melting or crustal interactions. Offsets in incompatible trace element abundances between Red Crater olivine-hosted MI, groundmass glass and whole-rock compositions imply 10–20% crustal contamination with metasedimentary basement rocks during magma ascent and shallow storage. Whole-rock data are considered less suitable for investigating primitive magma compositions and mantle sources, with olivine-hosted MI providing a more robust basis for interpretations.