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An experimental study of liquid compositions in equilibrium with plagioclase + spinel lherzolite at low pressures (0.75 GPa)


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Chalot-Prat, F, Falloon, TJ, Green, DH and Hibberson, WO 2010 , 'An experimental study of liquid compositions in equilibrium with plagioclase + spinel lherzolite at low pressures (0.75 GPa)' , Journal of Petrology, vol. 51, No. 11 , pp. 2349-2376 , doi: 10.1093/petrology/egq060.

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Models of formation of basaltic crust at mid-ocean ridges by adiabatic
upwelling of fertile mantle lherzolite require knowledge of
phase relations and phase compositions during melting at appropriate
pressures. Spinelþplagioclase lherzolites are found among peridotite
samples from the ocean floor and ophiolitic exposures. At low
pressure (51·2 GPa) the five-phase assemblage (olivineþorthopyroxeneþclinopyroxeneþplagioclaseþspinel)
is present at the
anhydrous lherzolite solidus. New experimental data on mineral
and melt compositions, at 0·75 GPa and 1140^12608C in the
(CrþNaþFeþCaþMgþAlþSi) system, demonstrate
smooth covariant relationships between oxides for melt compositions
and in partition relationships both between mineral pairs and between
minerals and melts. Molecular normative projections demonstrate
that liquids on the five-phaseþliquid cotectic occupy a
narrow compositional range. Of the mineral solid solutions that control
the liquid composition, the [Ca/(CaþNa)] or anorthite/
albite content of the plagioclase is dominant and liquids vary from
silica undersaturated and nepheline-normative at the sodic (oligoclase)
end to orthopyroxene and quartz-normative at the calcic
(anorthite) end of the cotectic. Spinel (Cr^Al) solid solution has
limited variation on the five-phaseþliquid cotectic. It is very
Cr-rich at the sodic end and has limited compositional variation
from50 to 20 in Cr/(CrþAl) at the anorthitic end.With fixed
plagioclase composition on the cotectic, melt compositions show
small compositional shifts with Fe^Mg (at Mg# between
85 and 95) and with Cr/(CrþAl). The compositional vectors are
consistent with effects observed in the end-member simple systems
(FCMAS) and (CrCMAS). In comparing liquids at the anorthite
end of the five-phaseþliquid cotectic with those on the Cr-free
CMAS four-phaseþliquid cotectic at 0·75 GPa, it is evident that
the presence of Al-rich Cr^Al spinel shifts liquid compositions to
more silica-rich and silica-oversaturated composition.These experimentally
defined melt compositions in equilibrium with plagioclaseþspinel
lherzolite are unlike quenched glasses from mid-ocean
ridge settings.The data do not support models of mantle upwelling
at low potential temperature (12808C) that produces low melt fractions
at low pressures, leaving residual plagioclaseþspinel lherzolite.
The detailed mineral compositional data at the solidus provide
a template for comparison with natural plagioclaseþspinel
lherzolites refertilized by porous reactive flow.

Item Type: Article
Authors/Creators:Chalot-Prat, F and Falloon, TJ and Green, DH and Hibberson, WO
Journal or Publication Title: Journal of Petrology
DOI / ID Number: 10.1093/petrology/egq060
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