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crust should almost completely dehydrate and should not cause any metamorphism.
Schmidt therefore, concluded that phengite is the principle potassium host at sub-
solidus conditions. It transports potassium and water to depths of up to 300 km and
could yield over the entire depth range K-rich
fluids or melts (depending on the
speci
c geotherm), which are likely to constitute one of the primary metasomatic
agents.
15.12 Metasomatic Fluid Transport
Navon and Stolper (1987) used the analogy between mantle processes and chro-
matographic fractionation in the laboratory to explain mantle metasomatism, and
envisaged large scale percolation. They thought that as the
fluid moves upward it
interacts and equilibrate with the matrix composition. As the column is used up, the
next batch of
fluid passes through the column without any interaction with the
matrix. Each element behaves differently and mass fractionation takes place.
According to Navon and Stolper,
the factors governing fractionation include
equilibrium partition coef
cients (K
D
, matrix to melt) and the equilibrium mass
fraction of an element in the liquid (Xf).
f
). The compatible element reacts with the
matrix before incompatible elements move through the column at a slower rate. The
compatible elements have a high K
D
value relative to peridotite and lower Xf
f
for a
speci
c melt composition.
Navon and Stolper thought that a refractory harzburgite or lherzolite, which lost
basalt fraction from previous melting, might represent the lithospheric mantle.
Silicate melts produced in the asthenosphere or
fluid produced from melting of a
subducted slab, might be enriched in LREE relative to chondrite (also see Bergman
1987). Theoretical values of K
D
and X
f
used by them predict that relatively com-
patible middle or heavy REE (HREE) will be retained in the matrix and incom-
patible LREE will move to a greater distance. They concluded that small quantities
of melt pass through the column and the fronts of the more incompatible LREE
reach the top of the mantle column ahead of the more compatible REE. Thus,
relative to other REE, the LREE should increase in the in
ltrating melt.
Nielson and Wilshire (1993) studied xenoliths from Lherz and Horoman massif.
According to them if their model is correct, even modest LREE enrichments in the
xenoliths of Lherz massif would require melt percolation for as long as
25,000 years, an unrealistic life span for mantle dyke conduits, that supply meta-
somatic
flow have been inferred in mantle peri-
dotite samples on scales of tens of meters at the most, but are well-documented only
on scales of centimetres or decimeters. In all these hypotheses porous
fluid. Effects of porous medium
flow is
controlled by proximity to magma-
lled fractures. Theoretical and experimental
studies suggest that the factors, which control the degree of permeability in partially
fused peridotites include, 1) melt fraction and dihedral (wetting) angle and the
possible presence of C
O
H
fluids in the mantle (Watson et al. 1990). Low dihedral
-
-
angles are observed in ma
c and ultrama
c melt compositions in peridotite at upper
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