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The origin of primitive ocean island and island arc basalts


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Eggins, Stephen (1989) The origin of primitive ocean island and island arc basalts. PhD thesis, University of Tasmania.

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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
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 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 (>-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.

Item Type: Thesis (PhD)
Keywords: Basalt
Copyright Holders: The Author
Copyright Information:

Copyright 1989 the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
would be pleased to hear from the copyright owner(s).

Additional Information:

Includes bibliographical references (p. 268-292a). Thesis (Ph.D.)--University of Tasmania, 1991

Date Deposited: 08 Dec 2014 23:57
Last Modified: 15 Aug 2016 03:55
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