Magmatic Water
59
which is tolerated by some silicate melts at water pressures of a few
kilobars, corresponds to a molar concentration of approximately 30%.
Magmatic water is a solution that has thoroughly exchanged chemically and isotopically at magmatic temperatures with a magma system
or an igneous rock body. The ultimate origin of the water - either
juvenile, in the sense that it is coming from degassing within the lower
crust or upper mantle, recycled meteoric water, or water from sedimentary rock that has subsequently exchanged both chemically and isotopically with the magma system - is not inherent in this definition.
The concept of juvenile water is a widespread one and has influenced
various independent fields in igneous petrology and oregenesis. Juvenile
water is commonly defined as water that has never been in contact with
surface waters. Testing this concept of juvenile water with D/H in combination with 180rO measurements is a highly interesting topic.
One way of looking for water of possible juvenile origin is by analyzing the D/H ratios of fluid inclusions. KOKUBU et al. (1961) found t5Dvalues of waters from liquid inclusions in basalts to be between - 33 and
- 60%0. RYE (1966) reported D/H ratios of fluid inclusions in calcite,
quartz, and sphalerite within a narrow range of - 68 to - 83%0.
RYE and O'NEIL (1968) analyzed the 180-content in fluid inclusions.
They found that the 180rO ratio of the water present in oxygen-bearing
minerals such as quartz and calcite indicates that the inclusions have
exchanged 18 0 with their host minerals. Only the t5 18 0-values of water in
sphalerites with a very narrow 15 18 0 range between 5.8 to 6.2 seems to
represent the primary isotopic composition.
Another way to search for juvenile water might be through measuring the D/H ratio of hydroxyl minerals of possible mantle or lowercrustal origin. SHEPPARD and EpSTEIN (1970) have determined the D/H
ratio of unaltered phlogopites, having relatively uniform D/H ratios
(t5D = -58 ± 18%0). These authors estimated the t5D-value of juvenile
wa ter to be - 48 ± 20%0.
Based on analyses of OH-bearing igneous minerals, primary magmatic waters have been defined by TAYLOR (1967) as waters that have
15 18 0 between 7.0 and 9.5%0 and t5D between -50 and -80%0. TAYLOR
(1967) has shown that water that has isotopically equilibrated with a
large reservoir of igneous silicates at magmatic temperatures has a welldefined oxygen-isotope composition between 7.0 and 9.5%0. A magmatic
fluid will retain this t5 18 0-value only as long as the isotopic composition
is controlled by silicate exchange at magmatic temperatures. Lower exchange temperatures (both inside and outside the igneous body) can
modify this t5 18 0-value.
The hydrogen isotope composition of magmatic waters are more
variable than their 180rO ratios. The average t5D of biotite and horn-
59
which is tolerated by some silicate melts at water pressures of a few
kilobars, corresponds to a molar concentration of approximately 30%.
Magmatic water is a solution that has thoroughly exchanged chemically and isotopically at magmatic temperatures with a magma system
or an igneous rock body. The ultimate origin of the water - either
juvenile, in the sense that it is coming from degassing within the lower
crust or upper mantle, recycled meteoric water, or water from sedimentary rock that has subsequently exchanged both chemically and isotopically with the magma system - is not inherent in this definition.
The concept of juvenile water is a widespread one and has influenced
various independent fields in igneous petrology and oregenesis. Juvenile
water is commonly defined as water that has never been in contact with
surface waters. Testing this concept of juvenile water with D/H in combination with 180rO measurements is a highly interesting topic.
One way of looking for water of possible juvenile origin is by analyzing the D/H ratios of fluid inclusions. KOKUBU et al. (1961) found t5Dvalues of waters from liquid inclusions in basalts to be between - 33 and
- 60%0. RYE (1966) reported D/H ratios of fluid inclusions in calcite,
quartz, and sphalerite within a narrow range of - 68 to - 83%0.
RYE and O'NEIL (1968) analyzed the 180-content in fluid inclusions.
They found that the 180rO ratio of the water present in oxygen-bearing
minerals such as quartz and calcite indicates that the inclusions have
exchanged 18 0 with their host minerals. Only the t5 18 0-values of water in
sphalerites with a very narrow 15 18 0 range between 5.8 to 6.2 seems to
represent the primary isotopic composition.
Another way to search for juvenile water might be through measuring the D/H ratio of hydroxyl minerals of possible mantle or lowercrustal origin. SHEPPARD and EpSTEIN (1970) have determined the D/H
ratio of unaltered phlogopites, having relatively uniform D/H ratios
(t5D = -58 ± 18%0). These authors estimated the t5D-value of juvenile
wa ter to be - 48 ± 20%0.
Based on analyses of OH-bearing igneous minerals, primary magmatic waters have been defined by TAYLOR (1967) as waters that have
15 18 0 between 7.0 and 9.5%0 and t5D between -50 and -80%0. TAYLOR
(1967) has shown that water that has isotopically equilibrated with a
large reservoir of igneous silicates at magmatic temperatures has a welldefined oxygen-isotope composition between 7.0 and 9.5%0. A magmatic
fluid will retain this t5 18 0-value only as long as the isotopic composition
is controlled by silicate exchange at magmatic temperatures. Lower exchange temperatures (both inside and outside the igneous body) can
modify this t5 18 0-value.
The hydrogen isotope composition of magmatic waters are more
variable than their 180rO ratios. The average t5D of biotite and horn-