Insights into the origin of low-delta O-18 basaltic magmas in Hawaii revealed from in situ measurements of oxygen isotope compositions of olivines

摘要

In situ measurements of oxygen isotope and elemental compositions of olivines from subaerial Mauna Kea lavas reveal that their delta O-18 values correlate positively with their forsterite contents, consistent with addition of one or more low-delta O-18 components into magmas from which they grew over the course of their crystallization-differentiation histories. This result supports previous suggestions that low-delta O-18 components to Mauna Kea lavas are contaminants derived from hydrothermally-altered rocks in the volcanic edifice or lithosphere, rather than components of the underlying mantle sources of these lavas. The slope of the correlation between delta O-18 values and forsterite contents of olivines is steeper for subaerial Mauna Kea lavas than for submarine Mauna Kea lavas, and olivines from Mauna Loa lavas exhibit negligible changes in delta O-18 values over a similar range of forsterite contents. Models of assimilation-fractional crystallization (AFC) processes can explain our observations if the delta O-18 values of crustal contaminants decrease sharply at the submarine-subaerial transition in Mauna Kea volcano, and if Mauna Loa lavas are either uncontaminated or contaminated only by rocks that have delta O-18 values similar to that of primary Mauna Loa magmas. We suggest that the differences in oxygen isotope systematics among Mauna Loa, submarine Mauna Kea and subaerial Mauna Kea lavas principally reflect the sources and amounts of water available to hydrothermal systems in the volcanic edifice.