Xenolith Constraints on “Self‐Assimilation” and the Origin of Low δ 18 O Values in Mauna Kea Basalts

Abstract

Cumulate xenoliths from Mauna Kea (MK) provide a window into magma storage, assimilation, and mixing. Sr-, Nd-, and Pb-isotope ratios in MK xenoliths are similar to late shield (< 400 kA) and post-shield MK basalts. Low equilibration pressures indicate formation within the MK volcanic edifice (< 2.5–3 kbar). This contrasts with cumulate xenoliths from Hualalai, which record equilibration pressures up to 9 kbar and include fragments of Pacific Ocean lower crust. Xenolith clinopyroxene and olivine span a large range in δ 18 O (3.3 to 5.0‰), extending to lower values than most mantle peridotites or mid-ocean ridge basalts (δ 18 O ol ≈ 5.2‰). Positive correlation between δ 18 O and mineral Mg# indicates assimilation of low-δ 18 O components coupled with magma fractionation. Lack of correlation between δ 18 O and Sr-, Nd-, or Pb-isotopes suggests assimilation of hydrothermally altered edifice material rather than Pacific Ocean crust. δ 18 O (4.7–4.8‰) lower than typical mantle values in the most primitive olivine phenocrysts (Mg# ~90) may reflect a low-δ 18 O (~4.9–5.0‰) recycled component intrinsic to the Hawaiian plume. However, most of the δ 18 O variability in MK phenocrysts and xenoliths reflects local assimilation. Although edifice assimilation produces only minor compositional shifts (except for O-isotopes), assimilation of Pacific crust may alter radiogenic isotope ratios in magmas ponding near or below the MOHO. Many post-shield basalt suites have elevated 187 Os/ 188 Os signatures suggesting assimilation of old (radiogenic) Pacific crust. Consideration of the effects of Pacific crust assimilation is necessary for accurate interpretation of temporal isotopic shifts in late-stage Hawaiian volcanoes in terms of plume structure.