102
C. Variations of Stable Isotope Ratios in Nature
Together with the sulfur there is often associated a calcitic limestone
matrix, which was found to be depleted in 13C - which is again convincing evidence for an organic origin ofthe carbonate and a bacterial origin
of the sulfur.
IX. Metamorphic Rocks
Metamorphic rocks are mineralogically transformed sedimentary or
igneous rocks. The temperature range of metamorphism is generally
between that of the sedimentary environment and that at which magma
is generated by rock fusion, or even above this temperature (granulite
facies). There is sometimes an overlap with low-temperature diagenetic
processes, and at the other extreme, metamorphism overlaps conditions
under which the more fusible minerals begin to melt in the presence of
water (migmatite).
Metamorphism occurs in essentially solid systems, with little or no
contribution from silicate-rich melts. However, a water vapor and carbon dioxide gas phase plays an essential role.
As has already been demonstrated, every major class of rock has a
characteristic range of isotope composition. For the oxygen isotope
composition, VERHOOGEN et al. (1970) have put it the following way: "In
igneous rocks it is determined by the source of the primary magma and
by isotopic fractionation during fractional crystallization. The oxygen of
sedimentary rocks has a composition that reflects a state of equilibrium
between weathering products and meteoric or ocean water ... The isotopic composition of oxygen in metamorphic rocks gives a good indication of the extent to which externally supplied water may have affected
the original composition inherited from the parent sedimentary or igneous rock. (For example, amphibolites formed by regional metamorphism of basalt have significantly higher 180-contents than most basalts.
Pelitic schists tend to give values somewhat lower than shales. What
seems to be implied is isotopic exchange with an external reservoir of
water.)"
Regarding the isotopic composition of metamorphic rocks, two
problems are of special interest: First the question of whether isotopic
equilibrium has been attained during metamorphism; second, the determination of a "temperature offormation".
Fractionations among coexisting minerals commonly indicate that
isotopic equilibrium has often been nearly attained in the mineral assemblages during metamorphism. A good example of large-scale oxygen
isotope equilibrium over several thousand km 2 was observed by TAYLOR (1964) in the Adirondack Mountains, New York. However, sometimes there is obviously a lack of isotope equilibrium. PERRY and BONNI-
C. Variations of Stable Isotope Ratios in Nature
Together with the sulfur there is often associated a calcitic limestone
matrix, which was found to be depleted in 13C - which is again convincing evidence for an organic origin ofthe carbonate and a bacterial origin
of the sulfur.
IX. Metamorphic Rocks
Metamorphic rocks are mineralogically transformed sedimentary or
igneous rocks. The temperature range of metamorphism is generally
between that of the sedimentary environment and that at which magma
is generated by rock fusion, or even above this temperature (granulite
facies). There is sometimes an overlap with low-temperature diagenetic
processes, and at the other extreme, metamorphism overlaps conditions
under which the more fusible minerals begin to melt in the presence of
water (migmatite).
Metamorphism occurs in essentially solid systems, with little or no
contribution from silicate-rich melts. However, a water vapor and carbon dioxide gas phase plays an essential role.
As has already been demonstrated, every major class of rock has a
characteristic range of isotope composition. For the oxygen isotope
composition, VERHOOGEN et al. (1970) have put it the following way: "In
igneous rocks it is determined by the source of the primary magma and
by isotopic fractionation during fractional crystallization. The oxygen of
sedimentary rocks has a composition that reflects a state of equilibrium
between weathering products and meteoric or ocean water ... The isotopic composition of oxygen in metamorphic rocks gives a good indication of the extent to which externally supplied water may have affected
the original composition inherited from the parent sedimentary or igneous rock. (For example, amphibolites formed by regional metamorphism of basalt have significantly higher 180-contents than most basalts.
Pelitic schists tend to give values somewhat lower than shales. What
seems to be implied is isotopic exchange with an external reservoir of
water.)"
Regarding the isotopic composition of metamorphic rocks, two
problems are of special interest: First the question of whether isotopic
equilibrium has been attained during metamorphism; second, the determination of a "temperature offormation".
Fractionations among coexisting minerals commonly indicate that
isotopic equilibrium has often been nearly attained in the mineral assemblages during metamorphism. A good example of large-scale oxygen
isotope equilibrium over several thousand km 2 was observed by TAYLOR (1964) in the Adirondack Mountains, New York. However, sometimes there is obviously a lack of isotope equilibrium. PERRY and BONNI-