Geochemical investigation of Gabbroic Xenoliths from Hualalai Volcano: Implications for lower oceanic crust accretion and Hualalai Volcano magma storage system

- The lower oceanic crust (LOC) beneath the Hawaiian Islands has undergone minimal hydrothermal alteration.
- The hydrothermal alteration history and composition of LOC can provide constraints on the mechanisms of LOC accretion.
- A deep magma reservoir existed within or at the base of the LOC during the shield stage of Hualalai Volcano.
- Isotopic heterogeneity observed in Hawaiian melts appears to derive from the heterogeneous plume source(s).

The patterns of axial hydrothermal circulation at mid-ocean ridges both affect and are influenced by the styles of magma plumbing. Therefore, the intensity and distribution of hydrothermal alteration in the lower oceanic crust (LOC) can provide constraints on LOC accretion models (e.g., “gabbro glacier” vs. “multiple sills”). Gabbroic xenoliths from Hualalai Volcano, Hawaii include rare fragments of in situ Pacific lower oceanic crust. Oxygen and strontium isotope compositions of 16 LOC-derived Hualalai gabbros are primarily within the range of fresh MORB, indicating minimal hydrothermal alteration of the in situ Pacific LOC, in contrast to pervasive alteration recorded in LOC xenoliths from the Canary Islands. This difference may reflect less hydrothermal alteration of LOC formed at fast ridges than at slow ridges. Mid-ocean ridge magmas from slow ridges also pond on average at greater and more variable depths and undergo less homogenization than those from fast ridges. These features are consistent with LOC accretion resembling the “multiple sills” model at slow ridges. In contrast, shallow magma ponding and limited hydrothermal alteration in LOC at fast ridges are consistent with the presence of a long-lived shallow magma lens, which limits the penetration of hydrothermal circulation into the LOC.Most Hualalai gabbros have geochemical and petrologic characteristics indicating derivation from Hualalai shield-stage and post-shield-stage cumulates. These xenoliths provide information on the evolution of Hawaiian magmas and magma storage systems. MELTS modeling and equilibration temperatures constrain the crystallization pressures of 7 Hualalai shield-stage-related gabbros to be ∼2.5-5 kbar, generally consistent with inferred local LOC depth. Therefore a deep magma reservoir existed within or at the base of the LOC during the shield stage of Hualalai Volcano. Melt-crust interaction between Hawaiian melts and in situ Pacific crust during magma storage partially overprinted clinopyroxene Sr and Nd isotope compositions of LOC-derived gabbros. Although minor assimilation of Pacific crust by Hawaiian melts cannot be excluded, the range of oxygen isotope compositions recorded in Hawaiian lavas and cumulates cannot be generated by assimilation of the in situ LOC gabbros, which have relatively uniform and MORB-like δ18O values. To first order, the isotopic heterogeneity observed in Hawaiian melts appears to derive from the heterogeneous plume source(s), rather than assimilation of local oceanic crust.