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Abstract

To investigate the effect of water on phase
relations and compositions in a basaltic system, we
performed crystallization experiments at pressures of
100, 200 and 500 MPa in a temperature range of 940 to
1,220C using four different water contents. Depending
on the water activity, the oxygen fugacity varied between
1 and 4 log units above the quartz-magnetitefayalite
buffer. Addition of water to the dry system
shifts the solidus > 250C to lower temperatures and
increases the amount of melt drastically. For instance,
at 1,100C and 200 MPa, the melt fraction increases
from 12.5 wt% at a water content of 1.6 wt% to 96.3%
at a water content of 5 wt% in the melt. The compositions
of the experimental phases also show a strong
effect of water. Plagioclase is shifted to higher anorthite
contents by the addition of water. Olivine and
clinopyroxene show generally higher MgO/FeO ratios
with added water, which could also be related to the
increasing oxygen fugacity with water. Moreover, water
affects the partitioning of certain elements between
minerals and melts, e.g., the Ca partitioning between
olivine and melt. Plagioclase shows a characteristic
change in the order of crystallization with water that
may help to explain the formation of wehrlites
intruding the lower oceanic crust (e.g., in Oman,
Macquarie Island). At 100 MPa, plagioclase crystallizes
before clinopyroxene at all water contents. At
pressures > 100 MPa, plagioclase crystallizes before
clinopyroxene at low water contents (e.g. < 3 wt%),
but after clinopyroxene at H2O in the melt > 3 wt%.
This change in crystallization order indicates that a
paragenesis typical for wehrlites (olivine–clinopyroxene–
without plagioclase) is stabilized at low pressures
typical of the oceanic crust only at high water contents.
This opens the possibility that typical wehrlites in the
oceanic crust can be formed by the fractionation and
accumulation of olivine and clinopyroxene at 1,060C
and > 100 MPa in a primitive tholeiitic basaltic system
containing more than 3 wt% water. The comparison of
the experimental results with evolution trends calculated
by the thermodynamic models ‘‘MELTS’’ and
‘‘Comagmat’’ shows that neither model predicts the
experimental phase relations with sufficient accuracy.

Item Type:	Article
Authors/Creators:	Feig, ST and Koepke, J and Snow, JE
Keywords:	Basalt, Water, experiment, phase equilibria
Journal or Publication Title:	Contributions to Mineralogy and Petrology
ISSN:	0010-7999
DOI / ID Number:	https://doi.org/10.1007/s00410-006-0123-2
Additional Information:	The original publication is available at www.springerlink.com
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