Abstract
D/H ratios have been obtained for biotites and chlorites from epizonal granites of the Chugoku belt, Southwest Japan. A good correlation between oxygen isotopic features of the rock and D/H ratio of its biotite made it possible to distinguish five types of biotite. Type-O: Primary biotites. Coexisting minerals are in near oxygen isotopic equilibrium at a magmatic temperature. δD values of biotites vary only from -89 to -96‰. Type-O': Contamination of magma by some basic materials. Coexisting minerals are not in oxygen istopic equilbirium. Biotites have relatively high δ18O and δD values compared to those of type-O. Type-I: Chloritization by residual fluids. Biotites are enriched in 18O, but their δD values are well within the range for biotites of type-O. Type II: Deuteric isotope exchange with meteoric water at high temperatures. Coexisting minerals are depleted in 18O on varying degrees and except for feldspar-biotite pairs they are not in oxygen isotopic equilbirium with each other. δD values of biotites also are low. Type-III: Deuteric isotopic exchange with meteoric water at low temperatures-hydrothermal alteration. Biotites contain 3.5-4.5% water. Quartz is oxygen isotopically comparable to those in type-O, whereas biotite and many of feldspars are much depleted in 18O. Biotites have very low δD values.
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