106
C. Variations of Stable Isotope Ratios in Nature
reasonably well with temperatures estimated from heat flow calculations,
from mineral parageneses, and from melting experiments.
Since metamorphic reactions commonly involve hydration and dehydration, water must be able to diffuse into or out of the reacting
system. If, for example, the metasedimentary rocks have undergone isotopic exchange with significant quantities of pore-fluids in communication with or derived from plutonic igneous rocks during metamorphism,
one should expect a decrease in 18 0/ 6 0 ratios with increasing metamorphic grade. If metamorphism involves only simple loss of water
through dehydration reactions, then the 180jI60 ratios of metasedimentary rocks should increase during progressive metamorphism.
However, as we have demonstrated, contact metamorphic rocks
seem to preserve the original b-values of the parent sedimentary materials. This should mean that they themselves have supplied the metamorphic water.
In the case of regional metamorphism the situation seems to be
entirely different. Only exchange with an extensive external oxygen reservoir can account for systematic, observed 180-changes. SHIEH and
TAYLOR (1969a) postulate exchange with fluids that are ultimately derived from deep-seated plutonic igneous bodies. However, this view
leaves many questions unanswered.
Summarizing, regional metamorphism usually involves exchange
with an external oxygen reservoir, whereas contact metamorphism generally does not.
In addition to the dominating importance of determining "temperatures of formation" of metamorphic rocks, there are broad possibilities
for applying 180jI60 studies in metamorphic petrology. Some of these
possibilities are cited in the following.
Since metamorphic rocks are transformed sedimentary or igneous
rocks, both of which show very different initial isotopic compositions,
18 0/ 6 0 studies might be useful in distinguishing ortho-rocks from pararocks based on the idea that the effects of initial isotopic differences in
the precursor rocks have been to some extent retained (SCHWARCZ and
CLAYTON, 1965). VOGEL and GARLICK (1970) determined b l8 0-values in
metamorphic eclogites and found two types: those with anomalously
low b-values and those with high b-values. They concluded that the
isotopically light eclogites originated from basaltic rocks that interacted
with light meteoric waters at high temperatures, and that the isotopically
heavy eclogites are possibly derived from dolomitic pelites.
TAYLOR and COLEMAN (1968) have shown that glaucophane-bearing
metamorphic rocks form over a very wide temperature range probably
from 200 to 550
0
C. These authors suggested that blue-schist-facies rocks
may be subdivided into two separate facies: a low-temperature lawson-
C. Variations of Stable Isotope Ratios in Nature
reasonably well with temperatures estimated from heat flow calculations,
from mineral parageneses, and from melting experiments.
Since metamorphic reactions commonly involve hydration and dehydration, water must be able to diffuse into or out of the reacting
system. If, for example, the metasedimentary rocks have undergone isotopic exchange with significant quantities of pore-fluids in communication with or derived from plutonic igneous rocks during metamorphism,
one should expect a decrease in 18 0/ 6 0 ratios with increasing metamorphic grade. If metamorphism involves only simple loss of water
through dehydration reactions, then the 180jI60 ratios of metasedimentary rocks should increase during progressive metamorphism.
However, as we have demonstrated, contact metamorphic rocks
seem to preserve the original b-values of the parent sedimentary materials. This should mean that they themselves have supplied the metamorphic water.
In the case of regional metamorphism the situation seems to be
entirely different. Only exchange with an extensive external oxygen reservoir can account for systematic, observed 180-changes. SHIEH and
TAYLOR (1969a) postulate exchange with fluids that are ultimately derived from deep-seated plutonic igneous bodies. However, this view
leaves many questions unanswered.
Summarizing, regional metamorphism usually involves exchange
with an external oxygen reservoir, whereas contact metamorphism generally does not.
In addition to the dominating importance of determining "temperatures of formation" of metamorphic rocks, there are broad possibilities
for applying 180jI60 studies in metamorphic petrology. Some of these
possibilities are cited in the following.
Since metamorphic rocks are transformed sedimentary or igneous
rocks, both of which show very different initial isotopic compositions,
18 0/ 6 0 studies might be useful in distinguishing ortho-rocks from pararocks based on the idea that the effects of initial isotopic differences in
the precursor rocks have been to some extent retained (SCHWARCZ and
CLAYTON, 1965). VOGEL and GARLICK (1970) determined b l8 0-values in
metamorphic eclogites and found two types: those with anomalously
low b-values and those with high b-values. They concluded that the
isotopically light eclogites originated from basaltic rocks that interacted
with light meteoric waters at high temperatures, and that the isotopically
heavy eclogites are possibly derived from dolomitic pelites.
TAYLOR and COLEMAN (1968) have shown that glaucophane-bearing
metamorphic rocks form over a very wide temperature range probably
from 200 to 550
0
C. These authors suggested that blue-schist-facies rocks
may be subdivided into two separate facies: a low-temperature lawson-