AbstractsEarth & Environmental Science

The origin of primitive ocean island and island arc basalts

by Stephen Eggins

Institution: University of Tasmania
Year: 1989
Keywords: Basalt
Record ID: 1032134
Full text PDF: http://eprints.utas.edu.au/19508/1/whole_EgginsStephen1991_thesis.pdf


Fundamental aspects concerning the origin of ocean island basalts and primitive island arc magmas are addressed using examples from the Vanuatu Arc, Hawaii, and the Tasmantid Seamounts. A suite of alkali-olivine to tholefitic basalts, newly dredged from the Tasmantid Seamounts, are possible primary and near-primary compositions (e.g. Mg#'s 61-70, Ni = 221-322 ppm). Their bulk compositions correspond with those produced by experimental melting of peridotite between -1.0GPa (for tholeiites) and -2.5GPa (for alkali-olivine basalts), leaving residual mineralogies of (spine!) lherzolite and harzburgite. The inferred absence of residual garnet necessitates magma generation from sources with middle/heavy REE values >chondrites to account for the fractionated REE patterns of the Tasmantid basalts. An experimental liquidus study on a new Kilauea primary melt estimate (16wt% MgO), based on the most Mg-rich olivine phenocrysts occurring in Hawaiian lavas (i.e. Mg# 90.5), demonstrates equilibrium with mantle peridotite (harzburgite) at 2.0GPa and -14500C. Garnet is not a liquidus phase below -3.5GPa, reaffirming previous interpretations based on experimental studies, for shallow garnet-absent generation of Hawaiian olivine tholeiite and picrite primary magma estimates. In an effort to reconcile phase equilibria evidence for shallow melt segregation and trace element geochemistry arguments for deep garnet-present melting, geochemical models for dynamic melt segregation from an upwelling mantle plume have been assessed. These models are found to have little or no capacity to reproduce the geochemical characteristics of Hawaiian, or other ocean island tholeiites, if melting proceeds beyond the garnet peridotite stability field to shallower levels. Two possible models may account for the geochemical characteristics of ocean island tholeiites: (1) melting occurs entirely within the presence of residual garnet, requiring the generation of ultramagmesian primary melts (>20wt% MgO) that are capable of equilibrating at high temperature and pressure (>3.0GPa) with garnet peridotite; (2) melting of an incompatible element enriched source, bearing a "residual garnet" geochemical signature, occurs at relatively shallow levels (-1-2GPa) to produce olivine tholeiitepicritic primary melts. A suitable source enriched in incompatible elements is the oceanic lithosphere, fertilised by small melt fractions migrating from the underlying mantle, as is consistent with peridotite-C-H-0 phase equilibria and melt segregation considerations. Ambae is a site of voluminous eruptions of primitive olivine and clinopyroxene phenocryst-rich lavas in the Vanuatu Arc. Three distinct lava suites, all erupted in the previous 100Ka, can be identified on the basis of stratigraphy, phenocryst mineralogy, and geochemistry. The youngest suite, which mantles much of the island, ranges from highalumina basalt through to picritic compositions with up to -20 wt% MgO. Geochemical variation in this suite is controlled by fractional…