The origin of primitive ocean island and island arc basalts
Creators: name: Eggins, Stephen
Publisher: name: University Of Tasmania
Year: 2023

description: 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 &gt;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 (&gt;20wt% MgO) that are capable of equilibrating at high temperature and pressure (&gt;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 crystallisation (and accumulation) of magnesian olivine (to Mg# 93.4) and clinopyroxene (to Mg# 92) and accessory Cr-rich spinel. The liquid line of descent can be traced from an Mg-rich picritic parent (Mg#-81) to high-alumina basalts in which crystallise calcic plagioclase (to An -90) relatively Ferich olivine (Mg#-80) and clinopyroxene (Mg#-80) and titanomagnetite. The dominant basaltic lava suites erupted throughout the Vanuatu Arc are notable for their primitive phenocryst assemblages which comprise magnesian olivine and clinopyroxene Cr-rich spinel and calcic plagioclase. These assemblages enable identification of a range of Mg-rich parent magmas (Mg# 75-82) spanning low-K tholeiites to high-K alkali picrites from which the spectrum of more evolved high-alumina basalts and andesites occurring in the arc are derived by fractionation processes. The primitive nature of the parent compositions provide unequivocal evidence for melt derivation from upper mantle peridotite. They require high temperature (&gt;-13000C) melting of peridotite between -2 and -4 GPa conditions which are likely to exist only within the core of the mantle wedge. Incompatible element geochemistry in the Vanuatu Arc parent magmas is dominated by the interaction of two components which are separately responsible for LILE enrichment over LREE and HFSE depletion relative to LREE. The enrichment of LILE over LREE which dominates the low-K tholeiitic compositional end-member is consistent with the introduction into the arc source of an LlLE (and Pb) -rich fluid released from the dehydration of amphibole. The other component which dominates highK alkaline end-member is associated with strong enrichment of both 111 P. and LREE but not of HFSE characteristics which suggest contamination of the source by a small melt fraction generated in the presence of a residual HFSE-bearing phase. A third source component equivalent to the regional upper mantle (N-MORB source) peridotite is recognised from the affinity of several lava suites to the N-MORB basalts of the North Fiji Basin. The majority of lava suites in the Vanuatu Arc however have very low HFSE and HREE (and sometimes also LREE) abundances (0.2-0.5x NMORB) which require their upper mantle protoliths to be considerably more refractory and depleted in incompatible elements than an N-MORB source."