92
C. Variations of Stable Isotope Ratios in Nature
VIII. Sedimentary Rocks
Sediments are the weathering products and residues of magmatic,
metamorphic and sedimentary rocks after transport and accumulation
in water and air. The recycling of sediments, incorporated into new
sedimentary rocks, has been increased during the earth's history.
Classification of sedimentary rocks is based on easily recognizable
characteristics that reflect something about the mode of transport and
the environment of deposition. It is customary to consider sedimentary
rocks in two categories: clastic and chemical. Transported fragmental
debris of all kinds - sands, gravel, shell fragments - make up the clastic
component of the rock. Inorganic precipitates from water (e.g., gypsum
and silica) obviously belong to the chemical category. But the chemical
components also include biogenic material extracted from waters or
partly from air and secreted as skeletons of living organisms. One of the
most serious problems in this classification is the possible mingling of
the two classes.
1. Silicates
Chemical weathering of high-temperature silicates proceeds mainly
by reactions with water, carbon dioxide, and oxygen. Compared with the
composition of the primary magmatic rocks, average sediments are distinctly higher in water, carbon, and chlorine, and also in the ratio of
ferric to ferrous iron.
The 180rO and DjH ratios of weathering products will be mainly
controlled by 1) the isotopic composition of the coexisting waters, 2) by
the isotopic fractionation factors between the water involved and each of
the minerals formed during weathering and 3) by the temperature
(SAVIN and EpSTEIN, 1970a; LAWRENCE and TAYLOR, 1971). Because the
weathering normally involves large amounts of meteoric water relative
to the amount of parent rock, the isotopic composition of the parent
rock has little influence on the isotopic composition of the weathering
products. LAWRENCE and TAYLOR (1971) were able to confirm that the
isotopic composition of weathered rocks mainly reflects the isotopic
variations of meteoric waters. From this follows that clay minerals which
originated in fresh waters or underwent diagenetic isotopic exchange
have isotope ratios that are smaller than those minerals whose isotopic
compositions have been established in a marine environment.
Most of the soils found on earth have b l8 0-values from + 15 to
+ 25%0 and bD-values from - 30%0 to -100%0. Soils rich in gibbsite or
other AI-Fe hydoxides are isotopically lighter in 18 0 than clay-rich soils.
SAVIN and EpSTEIN (l970a) and LAWRENCE and TAYLOR (1971) estimated that kaolinites and montmorillonites are about 27%0 richer in 18 0
C. Variations of Stable Isotope Ratios in Nature
VIII. Sedimentary Rocks
Sediments are the weathering products and residues of magmatic,
metamorphic and sedimentary rocks after transport and accumulation
in water and air. The recycling of sediments, incorporated into new
sedimentary rocks, has been increased during the earth's history.
Classification of sedimentary rocks is based on easily recognizable
characteristics that reflect something about the mode of transport and
the environment of deposition. It is customary to consider sedimentary
rocks in two categories: clastic and chemical. Transported fragmental
debris of all kinds - sands, gravel, shell fragments - make up the clastic
component of the rock. Inorganic precipitates from water (e.g., gypsum
and silica) obviously belong to the chemical category. But the chemical
components also include biogenic material extracted from waters or
partly from air and secreted as skeletons of living organisms. One of the
most serious problems in this classification is the possible mingling of
the two classes.
1. Silicates
Chemical weathering of high-temperature silicates proceeds mainly
by reactions with water, carbon dioxide, and oxygen. Compared with the
composition of the primary magmatic rocks, average sediments are distinctly higher in water, carbon, and chlorine, and also in the ratio of
ferric to ferrous iron.
The 180rO and DjH ratios of weathering products will be mainly
controlled by 1) the isotopic composition of the coexisting waters, 2) by
the isotopic fractionation factors between the water involved and each of
the minerals formed during weathering and 3) by the temperature
(SAVIN and EpSTEIN, 1970a; LAWRENCE and TAYLOR, 1971). Because the
weathering normally involves large amounts of meteoric water relative
to the amount of parent rock, the isotopic composition of the parent
rock has little influence on the isotopic composition of the weathering
products. LAWRENCE and TAYLOR (1971) were able to confirm that the
isotopic composition of weathered rocks mainly reflects the isotopic
variations of meteoric waters. From this follows that clay minerals which
originated in fresh waters or underwent diagenetic isotopic exchange
have isotope ratios that are smaller than those minerals whose isotopic
compositions have been established in a marine environment.
Most of the soils found on earth have b l8 0-values from + 15 to
+ 25%0 and bD-values from - 30%0 to -100%0. Soils rich in gibbsite or
other AI-Fe hydoxides are isotopically lighter in 18 0 than clay-rich soils.
SAVIN and EpSTEIN (l970a) and LAWRENCE and TAYLOR (1971) estimated that kaolinites and montmorillonites are about 27%0 richer in 18 0