Ni Systematics in Olivine as Fingerprints of Magmatic Processes in Hawaiian Basalts

University Of Hawaii, Honolulu

Investigators

Abstract

The mineral olivine is of fundamental importance in understanding upper mantle and crustal processes. The nickel (Ni) content of olivine is of great interest because it is a sensitive indicator of temperature, pressure, oxygen fugacity and bulk composition. The factors that influence equilibrium partitioning of Ni in olivine have been explored through numerous experimental studies for a wide range of experimental conditions and compositions. However, the behavior in Ni and other divalent ions in olivine remains poorly known because of the complex processes in magmatic systems. Ocean island basalts, such as those from Hawaii, show large variations in olivine Ni content. There are many competing hypotheses to explain the wide range of observed variation. To evaluate these hypotheses, lavas from the ongoing eruption of Kilauea Volcano will be used. These lavas also contain a wide range in Ni and have well characterized magmatic histories. Analyzes of major and trace elements in Ni-bearing phases in rocks with known magmatic histories will be made to assess the relative importance of each magmatic process in controlling Ni in olivine. The early crystallization history of mantle-derived magmas is dominated by olivine. Usually, it is the only mineral to crystallize for >100 oC below the liquidus at low to moderate pressures in primitive Hawaiian basalts. The composition of olivine has been used extensively to evaluate various magmatic processes including crystallization history, magma mixing accumulation and contamination processes. The Ni content of olivine has been of keen interest because of its strong compatibility in mantle-derived lavas and has been considered of fundamental importance in interpreting core formation, the nature and evolution of the mantle, melt formation in the mantle, and crystal fractionation and accumulation in basaltic magmas. Thus, the Ni content in olivine provides valuable perspectives on Earth processes. Plume-related lavas like those from Hawaii show high abundances but wide variations in Ni content at the same MgO. Controversy surrounds the origin of this variation. Well characterized samples will be used to assess what magmatic processes are responsible for controlling Ni abundance in olivine from Hawaiian basalts. Tholeiitic basalts from the ongoing eruption of Kilauea Volcano and alkalic lavas from the rejuvenation stage of volcanism for Ka`ula Island are the focus of this project. Magmatic processes for the Kilauea eruption are well known including magma mixing, crystal accumulation and temporally varying parental magma composition. For the rejuvenation stage lavas, their depth of origin will be investigated using Ni in olivine. Current depth estimates are highly variable. No previous high precision analyzes have been done on olivine in rejuvenated lavas. This study will have implication for our understanding of the source lithology and formation conditions for mantle-derived basalts.