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The petrogenesis of ocean-floor basalts : an experimental and geochemical study

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Falloon, Trevor John (1987) The petrogenesis of ocean-floor basalts : an experimental and geochemical study. PhD thesis, University of Tasmania.

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Abstract

Parts I and II are an experimental study concerned with the current
controversy regarding the evolved versus primary nature of primitive MORB
glasses.
In part I equilibrium partial melt compositions under anhydrous
conditions are presented on four peridotite compositions at 10kbar. The
four peridotite compositions are: 'MORB pyrolite', 'Hawaiian pyrolite',
Tinaquillo lherzolite and the spinel lherzolite KLB-1. In part II
equilibrium partial melt compositions are presented for MORB pyrolite from
8 to 35kbars and for Tinaquillo lherzolite at 15 to 20kbars. The
equilibrium liquids were determined by 'sandwich' experiments. The results
of the experiments are used to test a 10kbar melt model for the generation
of primitive MORB glasses. The melt compositions from the four peridotites
at 10kbar are significantly different from primitive MORB glasses in major
element chemistry and plot away from the field of primitive MORB glasses in
the CIPW molecular normative 'Basalt tetrahedron'. The results suggest
primary MORB melts segregate from source diapirs at pressures of 8 to
25kbars in equilibrium with either lherzolite or harzburgite residues. MgO
contents of primary MORB melts range from 10-17 wt% while primary melts >17
wt% MgO are of minor importance.
Parts III, IV and V consist of a petrographic, mineral chemical,
geochemical and petrological study of dredged lavas from the North Tongan
forearc recovered during the 1984 cruise of the research vessel
'Natsushima'.
In part III petrographic, wholerock major and trace element data is
presented and the geochemical affinities of all the dredge suites
discussed.
Part IV looks in detail at the chemistry of large (up to 0.2mm) glass
inclusions contained in unusually magnesian olivine phenocrysts (Fo94 )
within the high-mg lavas.

Part V is concerned with the petrogenesis of the high-mg lavas and
associated island arc tholeiites from North Tonga. The mineral chemistry of
phenocryst and groundmass phases combined with wholerock, glass and glass
inclusion major element chemistry give unequivocal evidence of mixing of
two or more distinct parental magmas. The major, trace and isotopic (Sr,
Nd) geochemistry of the lavas can be explained by partial melting of
refractory mantle peridotite (depleted after extraction of primary MORB
picrite) at shallow depths (<10kb or higher pressures if water is present),
which has been previously 'enriched' in silicate incompatible elements by
two distinct components.

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

Copyright 1987 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:

Thesis (Ph.D.)--University of Tasmania, 1988. Bibliography: p. 199-217

Date Deposited: 08 Dec 2014 23:58
Last Modified: 16 Aug 2016 00:13
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