58
C. Variations of Stable Isotope Ratios in Nature
complexes. High 180rO ratios accompany those trends having the
lower Fe/Mg ratios, whereas ferrogabbro trends are associated with
depletion in 18 0. Variations in oxygen fugacity may be responsible for
these effects, since early precipitation of magnetite leads to both
iSO-enrichment and Fe-depletion in later differentiates.
Fractional crystallization may produce larger variations in the latecrystallizing minerals. Therefore, granitic pegmatites show a greater variability than do plutonic granites. For instance, quartz in pegmatites
ranges from 9 to 18%0 (after TAYLOR and EpSTEIN, 1961).
Because sedimentary and metamorphic rocks have higher £5 18 0-values than igneous rocks, assimilation of country rocks into a magma
chamber can affect the oxygen isotope composition of the magma. However, processes related with assimilation are very complex (e.g., dehydration and decarbonation), that is nearly impossible to predict to which
extent the 180-content ofa magma may be changed.
The general aspects of the 180rO fractionations connected with the
emplacement of igneous plutonic bodies have been discussed by SHIEH
and TAYLOR (1969a) and TURI and TAYLOR (1971 a, b).
These investigations showed that a single pluton is not as homogenous as might perhaps be expected from the constant initial £5180-value
before emplacement. A marginal 180-enrichment of a pluton emplaced
into 180_rich country rocks is a common phenomena. Assimilation of
sialic crust is suggested as the reason why these rocks have abnormally
high JI80-values. On the other hand, studies by TAYLOR (1968a, 1971)
and TAYLOR and FORESTER (1971) have demonstrated that some of those
igneous instrusions emplaced at relatively shallow depths in the earth's
crust have interacted with meteoric groundwaters during their crystallization and cooling history. Igneous rocks that have exchanged with
large quantities of heated groundwaters at high temperatures can have
their 180rO ratios lowered by as much as 10 to 15%0. The amounts of
H 20 involved are estimated to be about equal in volume to that of the
exchanged rock. Therefore, we may conclude that much alteration of
igneous rocks is probably caused by meteoric waters rather than by H 20
released during magmatic crystallization.
The major conclusion is that there are wide variations in the manner
in which granitic plutons undergo 180-interactions with their country
rocks and also large variations in the magnitudes of these isotopic effects.
1. Magmatic Water
In the gas and fluid phase of magmatic melts, H20 is the main
constituent, followed by CO2 • At high pressures silicate melts can contain significant amounts of water. A water content of 5% by weight,
C. Variations of Stable Isotope Ratios in Nature
complexes. High 180rO ratios accompany those trends having the
lower Fe/Mg ratios, whereas ferrogabbro trends are associated with
depletion in 18 0. Variations in oxygen fugacity may be responsible for
these effects, since early precipitation of magnetite leads to both
iSO-enrichment and Fe-depletion in later differentiates.
Fractional crystallization may produce larger variations in the latecrystallizing minerals. Therefore, granitic pegmatites show a greater variability than do plutonic granites. For instance, quartz in pegmatites
ranges from 9 to 18%0 (after TAYLOR and EpSTEIN, 1961).
Because sedimentary and metamorphic rocks have higher £5 18 0-values than igneous rocks, assimilation of country rocks into a magma
chamber can affect the oxygen isotope composition of the magma. However, processes related with assimilation are very complex (e.g., dehydration and decarbonation), that is nearly impossible to predict to which
extent the 180-content ofa magma may be changed.
The general aspects of the 180rO fractionations connected with the
emplacement of igneous plutonic bodies have been discussed by SHIEH
and TAYLOR (1969a) and TURI and TAYLOR (1971 a, b).
These investigations showed that a single pluton is not as homogenous as might perhaps be expected from the constant initial £5180-value
before emplacement. A marginal 180-enrichment of a pluton emplaced
into 180_rich country rocks is a common phenomena. Assimilation of
sialic crust is suggested as the reason why these rocks have abnormally
high JI80-values. On the other hand, studies by TAYLOR (1968a, 1971)
and TAYLOR and FORESTER (1971) have demonstrated that some of those
igneous instrusions emplaced at relatively shallow depths in the earth's
crust have interacted with meteoric groundwaters during their crystallization and cooling history. Igneous rocks that have exchanged with
large quantities of heated groundwaters at high temperatures can have
their 180rO ratios lowered by as much as 10 to 15%0. The amounts of
H 20 involved are estimated to be about equal in volume to that of the
exchanged rock. Therefore, we may conclude that much alteration of
igneous rocks is probably caused by meteoric waters rather than by H 20
released during magmatic crystallization.
The major conclusion is that there are wide variations in the manner
in which granitic plutons undergo 180-interactions with their country
rocks and also large variations in the magnitudes of these isotopic effects.
1. Magmatic Water
In the gas and fluid phase of magmatic melts, H20 is the main
constituent, followed by CO2 • At high pressures silicate melts can contain significant amounts of water. A water content of 5% by weight,