Equilibrium Exchange Reactions
29
The National Bureau of Standards now distributes another carbonate: NBS No.20. This is Solenhofen limestone, which has a c)-value of
-4.14%0 relative to POB (CRAIG, 1957). As shown by BAUSCH and
HOEFS (unpublished data), the Solenhofen limestone is rather inhomogeneous within a single sedimentary layer. The differences in c)-values are
as much as 2.5%oin 18 0. Therefore, a better standard is urgently required.
All data given in the following are reported relative to SMOW.
3. Fractionation Mechanisms
The 18 0/ 6 0 ratio in nature exhibits variations ranging up to about
10%. These variations result from both equilibrium and kinetic fractionation effects. Fig. 10 presents a schematic diagram on the naturally occurring variations of oxygen isotopes.
a) Equilibrium Exchange Reactions
An excellent consistency in the relative 19O contents of different minerals can be found in nature. GARLICK (1969) made an attempt to arrange
coexisting minerals according to their relative tendencies to concentrate
18 0.
quartz, dolomite (anhydrite)
alkali feldspar, calcite, aragonite
leucite
muscovite, nepheline
anorthite (kyanite)
glaucophane (staurolite)
lawsonite
garnet, common pyroxenes, and amphiboles
biotite
olivine (sphene)
chlorite
ilmenite (rutile)
magnetite (hematite)
pyrochlore
This order of decreasing l80-content is due to a crystal-chemical
relationship associated with the relative affinity for 18 0. TAYLOR and
EpSTEIN (1962b) have pointed out that those minerals in which the Si-OSi bond predominates are richest in 18 0. At igneous temperatures the SiO-AI types contain approximately 4%0 less 18 0, and the Si-O-Mg and SiO-Fe types contain 2%0 less 18 0. The OH-bearing minerals have abnormally low l80rO ratios, suggesting that the OH-group may be lower in
18 0 than the rest of the oxygen in the silicate structure.
29
The National Bureau of Standards now distributes another carbonate: NBS No.20. This is Solenhofen limestone, which has a c)-value of
-4.14%0 relative to POB (CRAIG, 1957). As shown by BAUSCH and
HOEFS (unpublished data), the Solenhofen limestone is rather inhomogeneous within a single sedimentary layer. The differences in c)-values are
as much as 2.5%oin 18 0. Therefore, a better standard is urgently required.
All data given in the following are reported relative to SMOW.
3. Fractionation Mechanisms
The 18 0/ 6 0 ratio in nature exhibits variations ranging up to about
10%. These variations result from both equilibrium and kinetic fractionation effects. Fig. 10 presents a schematic diagram on the naturally occurring variations of oxygen isotopes.
a) Equilibrium Exchange Reactions
An excellent consistency in the relative 19O contents of different minerals can be found in nature. GARLICK (1969) made an attempt to arrange
coexisting minerals according to their relative tendencies to concentrate
18 0.
quartz, dolomite (anhydrite)
alkali feldspar, calcite, aragonite
leucite
muscovite, nepheline
anorthite (kyanite)
glaucophane (staurolite)
lawsonite
garnet, common pyroxenes, and amphiboles
biotite
olivine (sphene)
chlorite
ilmenite (rutile)
magnetite (hematite)
pyrochlore
This order of decreasing l80-content is due to a crystal-chemical
relationship associated with the relative affinity for 18 0. TAYLOR and
EpSTEIN (1962b) have pointed out that those minerals in which the Si-OSi bond predominates are richest in 18 0. At igneous temperatures the SiO-AI types contain approximately 4%0 less 18 0, and the Si-O-Mg and SiO-Fe types contain 2%0 less 18 0. The OH-bearing minerals have abnormally low l80rO ratios, suggesting that the OH-group may be lower in
18 0 than the rest of the oxygen in the silicate structure.