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Primitive ankaramitic melts in island arcs : evidence from melt inclusions

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Della-Pasqua, Fernando N (1997) Primitive ankaramitic melts in island arcs : evidence from melt inclusions. PhD thesis, University of Tasmania.

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Abstract

Primitive ankaramite rocks differ from picrite by their richness in clinopyroxene (cpx>ol), their high
CaO/Al2O3 values (>1, wt%) and their high normative di/ol values (>0.7, mol%). Ankaramite rocks are
generally interpreted as a variety of basalt enriched in clinopyroxene crystals yet some studies suggest that
ankararnite rocks crystallized from primitive ankaramitic melts. The origin of ankaramitic melts is of
interest because their compositions differ from the picritic compositions of experimentally produced partial
melts in equilibrium with mantle peridotite, yet many of their of their characteristics are similar to those of
primitive mantle melts, i.e. high Mg# values, high Ni and Cr, and magnesian olivine phenocrysts.
The four primitive ankaramite suites studied in this thesis are from the Ulakan Formation in Bali, and
the Rinjani volcano in Lombok (Sunda arc), and from Merelava and Epi (Vanuatu arc). Primary melt
inclusions in magnesian olivine phenocrysts (Fo>90) from each suite are studied to investigate the early
stages of ankaramite magma evolution. The composition of these melt inclusions after homogenization are
quite unlike picrite and have CaO/Al203 values which range up to —1.7, and are therefore ankaramitic.
Additionally, the CaO/Al203 values of melt inclusions within the same grain are similar, but vary between
phenocrysts.
Critical assessment of the data suggests that the composition of melt inclusions trapped in olivine
can be modified by Fe-Mg re-equilibration with the host, before the magma is erupted. This process might
lower the original FeO* content of the trapped melt by several wt% ("Fe-loss"). The extent of "Fe-loss"
depends on (1) the time spent by the host olivine phenocryst in the magma before eruption (residence time)
and (2) the rate at which the re-equilibration occurred. Most of the compositions of homogenized melt
inclusions in this study are affected by "Fe-loss" and are therefore not directly representative of the original
trapped melt. A recalculation procedure is developed to reconstruct the original composition of the melt at
the moment of trapping. Both, before and after recalculation, the compositions of melt inclusions display
di/ol values >1, thus retaining their ankaramitic affinities. The CaO/Al203 values of melt inclusions are
unaffected by this procedure. The recalculated melt inclusion compositions are also more silica-undersaturated
(ne-, lc- and cs-normative) than the host ankaramite rocks, which range from ne- to hy-normative.
If the melt inclusions are aliquots of the parent melts, these results suggest that the ankaramite
magma formed by the aggregation of strongly-silica-undersaturated primitive ankaramite melts with high
CaO/Al203 and high di/o/ values. A link may therefore exist between these trapped melts, the hypothetical
primary melts and the formation of ankaramitic magmas. This link is explored experimentally at high
pressures, using the composition of a representative melt inclusion in Fosq from the Lombok ankaramite
suite as a starting mix composition. Liquidus and near-liquidus phase relations of this melt, under dry,
hydrous, CO2-H20- and CO2-bearing conditions, and between 1 to 3 GPa pressures, lack orthopyroxene near
the liquidus. Therefore a direct origin is excluded for this composition by partial melting of mantle
peridotite (i.e. not a primary melt). Instead, the trapped melt inclusions may have been derived from even
more mafic primary melts that were generated at higher temperatures and pressures than those prevailing
when the ankaramitic magma aggregated. Two mechanisms are considered for the formation of these primary melts that have high CaO/Al203 values (>1): (a) partial melting of lherzolite at high pressures (>5
GPa) and (b) partial melting of lherzolite at lower pressures (<5 GPa) in the presence of CO2-rich mantle
fluids.
The natural phenocryst assemblage and composition of the host ankaramite rock are only duplicated
experimentally under hydrous conditions. With increasing CO2 content in the fluid, the compositions of
synthesized clinopyroxene crystals become less calcic and are unlike the natural phenocrysts. Thus the
crystallization and aggregation of the ankaramite magma may have taken place under hydrous conditions at
depths where olivine and clinopyroxene are co-crystallizing as phenocryst phases. In the case of the Lombok
ankaramite suite, these conditions correspond to pressures equivalent to depths of around —35 km below the
base of the arc crust. The aggregated ankararnite magma may mix and continue to crystallize isobarically
and in-situ, thus producing the wide compositional range observed in olivine (—Fo75.91) and clinopyroxene
(—Wo45-47En50-41Fs5-12) phenocrysts, as well as the variable zoning and resorption textures. Reaction and
re-equilibration of this aggregated magma with the sub-arc mantle before eruption, may cause the silicaenrichment
that distinguishes the wholerock ankaramite compositions from the melt inclusions in their
olivine phenocrysts.
Tiny pleonastic daughter spinel crystals with up to 65 wt% Al2O3 and virtually no chromium, occur
within melt inclusions in olivine phenocrysts. These aluminous spinels could form in melt inclusions
provided the trapped melt becomes sufficiently Al-enriched and Cr-depleted, following the fractional
crystallization of olivine on the walls. Daughter aluminous spinels were found in the melt inclusions of
olivine phenocrysts from all four ankaramite suites studied and may therefore, also occur in melt inclusions
of other basaltic rocks. The occurrence of aluminous spinels in melt inclusions is therefore not evidence for
the trapping of contaminant aluminous melts.
This thesis attempts to 1) demonstrate that ankaramitic melts with CaO/Al203 values (>1) and
normative di/ol values >0.7 exist and can be parental to ankaramite rocks; 2) describe the Fe-Mg reequilibration
("Fe-loss") process that affects melt inclusions in olivine phenocrysts before magma eruption;
and 3) re-interpret the formation of aluminous spinel crystals in melt inclusions that have previously been
interpreted as evidence for contaminant aluminous melts in basaltic magma chambers.

Item Type: Thesis (PhD)
Keywords: Island arcs, Basalt, Olivine
Copyright Holders: The Author
Copyright Information:

Copyright 1997 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, 1998. Includes bibliographical references

Date Deposited: 09 Dec 2014 00:03
Last Modified: 11 Mar 2016 05:55
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