Silicates
93
than the coexisting water, whereas gibbsites are only 18%0 richer in 18 0.
On the other hand, all the clay minerals and hydroxides are depleted in
deuterium relative to the coexisting water: kaolinite and montmorillonite: ~ 30%0, gibbsite: ~ 15%0.
LAWRENCE and TAYLOR (1970) have used the t:5D- and t:5 18 0-values of
ancient kaolinites to determine t:5D and t:5 18 0 of ancient meteoric waters.
Their results suggest that the isotopic composition of Mid-Tertiary waters is similar to the present-day composition.
The proportion of detrital to authigenic minerals of argillaceous
rocks has been extensively debated. For some time authigenic clay mineral formation has been assumed to produce the largest fraction of
modern sediments. But recent experimental results have demonstrated
that the predominant fraction of clays and shales must be of detrital
ongm.
18 0 rO ratios of modern ocean sediment core samples indicate that
generally the authigenic components may be distinguished from detrital
components (authigenic minerals are generally 10 to 30%0 heavier in 18 0
than detrital minerals of high-temperature origin, SA YIN and EpSTEIN,
1970b).
180rO ratios of detrital minerals appear to reflect the provenance
and mode of origin. Detrital quartz, for instance, seems to be resistant to
weathering, and it will retain its original 180-content as established in the
parent rocks. REX et al. (1969) for instance found that the 180rO ratio of
quartz isolated from Hawaiian soils, Pacific sediments, and tropospheric
dusts are remarkably uniform. The authors suggested a common aeolian
origin of this quartz from continental land masses.
On the other hand the Jl80-values of authigenic minerals suggest
formation in the marine environment under or near to isotopic equilibrium conditions. The existence of authigenic feldspars of sedimentary
or diagenetic origin has long been recognized. SA YIN and EpSTEIN
(1970c) analyzed feldspar grains consisting of secondary overgrowths on
detrital cores and found a striking difference of approximately 10%0
between the isotopic composition of the core and that of the rim. The
typical igneous t:5-value obtained for the core indicates that it did not
exchange during the formation of the authigenic feldspar.
SAVIN and EpSTEIN (1970b) extended their investigations on pure
clay minerals to the more complicated fine grained ocean sediments. The
isotope data on whole-rock ocean sediments cannot be interpreted without a knowledge of their chemical and mineralogical composition. Some
of the major minerals present in the carbonate-free fractions of ocean
sediments are the clay minerals whose isotopic compositions have been
discussed before, quartz, feldspar and the iron and manganese oxides. To
calculate the average oxygen isotope composition of sediments, we have
93
than the coexisting water, whereas gibbsites are only 18%0 richer in 18 0.
On the other hand, all the clay minerals and hydroxides are depleted in
deuterium relative to the coexisting water: kaolinite and montmorillonite: ~ 30%0, gibbsite: ~ 15%0.
LAWRENCE and TAYLOR (1970) have used the t:5D- and t:5 18 0-values of
ancient kaolinites to determine t:5D and t:5 18 0 of ancient meteoric waters.
Their results suggest that the isotopic composition of Mid-Tertiary waters is similar to the present-day composition.
The proportion of detrital to authigenic minerals of argillaceous
rocks has been extensively debated. For some time authigenic clay mineral formation has been assumed to produce the largest fraction of
modern sediments. But recent experimental results have demonstrated
that the predominant fraction of clays and shales must be of detrital
ongm.
18 0 rO ratios of modern ocean sediment core samples indicate that
generally the authigenic components may be distinguished from detrital
components (authigenic minerals are generally 10 to 30%0 heavier in 18 0
than detrital minerals of high-temperature origin, SA YIN and EpSTEIN,
1970b).
180rO ratios of detrital minerals appear to reflect the provenance
and mode of origin. Detrital quartz, for instance, seems to be resistant to
weathering, and it will retain its original 180-content as established in the
parent rocks. REX et al. (1969) for instance found that the 180rO ratio of
quartz isolated from Hawaiian soils, Pacific sediments, and tropospheric
dusts are remarkably uniform. The authors suggested a common aeolian
origin of this quartz from continental land masses.
On the other hand the Jl80-values of authigenic minerals suggest
formation in the marine environment under or near to isotopic equilibrium conditions. The existence of authigenic feldspars of sedimentary
or diagenetic origin has long been recognized. SA YIN and EpSTEIN
(1970c) analyzed feldspar grains consisting of secondary overgrowths on
detrital cores and found a striking difference of approximately 10%0
between the isotopic composition of the core and that of the rim. The
typical igneous t:5-value obtained for the core indicates that it did not
exchange during the formation of the authigenic feldspar.
SAVIN and EpSTEIN (1970b) extended their investigations on pure
clay minerals to the more complicated fine grained ocean sediments. The
isotope data on whole-rock ocean sediments cannot be interpreted without a knowledge of their chemical and mineralogical composition. Some
of the major minerals present in the carbonate-free fractions of ocean
sediments are the clay minerals whose isotopic compositions have been
discussed before, quartz, feldspar and the iron and manganese oxides. To
calculate the average oxygen isotope composition of sediments, we have