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The origin of ultrapotassic igneous rocks

Foley, Stephen F(Stephen Francis) 1986 , 'The origin of ultrapotassic igneous rocks', PhD thesis, University of Tasmania.

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This thesis consists of a review of ultrapotassic igneous rock
occurrences and three experimental programs designed to examine the
petrogenesis of the lamproites.
A definition for ultrapotassic rocks is introduced using the
whole-rock chemical screens K20>3 wt%, MgO>3 wt% and K20/Na20>2. Three
major end-member groups are recognised; Group I (lamproites) are
characterised by low CaO, A1203 and Na 2 0, high K 2 0/A1203 and Mg-number,
and extremely high incompatible element contents; Group II have low Si0 2
and high CaO, and lower incompatible elements than group I although they
have high relatively Sr; Group III rocks occur in orogenic areas and have
high CaO and A1203, and low Ti0 2 , Nb and Ba typical of island arc rocks.
Primary magmas-for all three groups probably originate by partial melting
of mantle material enriched in incompatible elements. The chemical
signatures of the groups indicate differences in (i) source composition
prior to enrichment, (ii) the chemical nature of the enriching agent, and
(iii) pressure-temperature conditions of melting.
The liquldus mineralogy of a pristine, primary leucite lamproite from--
Gaussberg, Antarctica, was studied at .1 atm with controlled f0 2 , oxygen
fugacity at the time of crystallisation of the Gaussberg rock is shown by
ferric value [10OFe 3 /(Fe 3 +Fe2 )] of spinel, Fe 2 03 content of leucite and
Mg-number of olivine, to have been just below NNO. Application of the spinel
ferric value calibration to other lamproites indicates that they began to
crystallise at f0 2 ranging from MW to above NNO. The ferric value of
spinel is very sensitive to changes in oxygen fugacity, and may prove
useful as a diamond survivability indicator': diamonds are unlikely to
survive in the more oxidised lamproite magmas.
The effect of fluorine, an important constituent of ultrapotassic
rocks, on phase relationships in the kalsilite-forsterite-quartz system
was studied at 28kbar. Fluorphiogopite is found to be stable to 300 0C
higher than hydroxyphlogopite, and the peritectic point PHL+EN+F0+L, which
can be used to model melting of a mica-harzburgite mantle, lies at an
equally magnesian composition. Fluorine acts as a melt polymerising agent
as shown by the expansion of the enstatite phase volume relative to
forsterite and by FTIR spectroscopic studies. Fluorine forms bonds with
network modifying cations and removes KA10 2groups from the
aluminosilicate network, causing an increase in Si/(Si+Al) in the network.
However, in the presence of water fluorine will appear to depolymerise melts due to the action of OH released by HF dissolution; the viscosity
will be lowered by fluorine in either case due to the formation of
fluoride complexes.
A model is developed for the origin of lamproitic magmas by partial
melting of a mica-harzburgite mantle in a reduced environment in the
presence offluorine. Lamproitestypically carry depleted mantle nodules
and have H20-and F-rich, but CO 2-poor compositions. Primary lamproite
magmas appear to range in silica content from around 40 wt% (olivine
lamproites) to at least 52 wt% (leucite lamproites). In a reduced mantle
(f02 1W to IW+2 log units) CH4 will be the dominant carbon species in
fluids, and CO2will be very rare even in a carbon-rich environment. CH4
also acts as a depolymeriser, so that production of silicic melts will be
optimised in a reduced, fluorine-rich mantle. Olivine lainproites may be
produced by melting of a similar composition at higher pressures.
Calculations show that oxidation from the proposed reduced conditions at
source to observed surface oxidation states can be achieved by dissociation
of only 0.1 wt% H20 driven by diffusive loss of H2.
Silica-poor rocks of Group II may originate in an oxidised environment
with abundant CO2but little H 2 0. Fluorine will maintain a large phase
field for mica in these conditions so that initial melts will be magnesian
and strongly silica-undersaturated.
A technique is developed for liquidus experiments at high pressures
in the presence of reduced H20>CH 4fluids. Two lamproite compositions were
studied by this technique to test the hypothesis outlined above. The
olivine lamproite has olivine as the liquidus phase at all pressures
studied (up to 40 kbar), but the increasing stability of orthopyroxene+
mica with pressure indicates that there may be a OL+OPX+PHL point at the
liquidus between 45 and 55 kbar. This is consistent with the occurrence of
diamonds in olivine lamproitës. The leucite lamproite has liquidus fields
for olivine, mica and orthopyroxene with increasing pressure, but has no
point where the three coexist. These phase relationships can be
interpreted to fit the mica-harzburgite melting model (with melting at 20
kbar) if minor olivine fractionation occurs at high pressures, or possibly if
the water content of the source differs from that of the experiments. Thus,
pressure variation may be the principal control of lamproite chemistry.
Several experiments with variable CH4 /H2 0 or H20/CO2 fluids enable
comparison of melting behaviour at varying f0 2 . At very low f02, melting
temperatures are increased due to lowered water activity, but mica
stability is increased due to its higher F/OH.

Item Type: Thesis - PhD
Authors/Creators:Foley, Stephen F(Stephen Francis)
Keywords: Igneous rocks, Lamproite, Basalt, Petrogenesis
Copyright Holders: The Author
Copyright Information:

Copyright 1986 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, 1987. Off-print of article: The origin of Al-rich spinel inclusions in A.L. Jaques and S.F. Foley in pocket

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