Hiroji Honma

Distribution of ammonium in minerals of metamorphic and granitic rocks

Abstract Sixty-nine analyses are given for NH4 in minerals of metamorphic and granitic rocks mostly from the Ryoke belt, Japan. The distribution of NH4 in coexisting minerals is quite systematic, suggesting that NH4 is one of the stable geochemical components in high temperature processes. Biotite has the highest content of NH4, followed by muscovite, K-feldspar and plagioclase. Pure quartz is almost free from NH4. Calcic plagioclase contains less NH4 than does sodic plagioclase. The partition coefficients D Pl Bi , D Kf Bi and D Kf Bi are, on the average, 0.11, 0.38 and 0.43 respectively. The fractionation of NH4 in these minerals is quite similar to that of Rb but much smaller than that of Cs. Distribution of NH4 as well as those of Rb and Cs appears to be explained by its ionic radius and the shortest cation-O distances in alkali positions of minerals.

Oxygen isotopic study of the Cretaceous granitic rocks in Japan

The (δ18O values of nine Cretaceous granitic rocks from the low P/T type regional metamorphic zone of Japan are +10.0 to +13.2‰ relative to SMOW, while ten Cretaceous granitic rocks from the non-metamorphic zone are +7.9 to +9.8‰. The 18O-enrichment in the former rocks is mainly attributed to oxygen isotopic exchange between the granitic magma and the surrounding metamorphic rocks during regional metamorphism. The assimilation of 18O-rich country rocks is also possible in the cases such as gneissose granite and migmatite.The oxygen isotopic ratios of quartz-biotite pairs in the granitic rocks indicate that they are isotopically in near-equilibrium with each other. The quartz-biotite isotopic equilibrium temperatures estimated for these rocks range from 550° to 670° C. Feldspar is occasionally isotopically in disequilibrium with other minerals. This may be caused by exchange of oxygen isotopes between feldspar and hydrothermal or meteoric water after crystallization.

Ammonium in biotite from metamorphic and granitic rocks of Japan

Abstract Ammonium contents of biotites from metamorphic and granitic rocks of Japan have been determined, and correlated with the 18 O 16 O ratios of the rocks. NH4 contents of biotites averaged 22 ppm in granitic rocks of non-metamorphic terranes, 67 ppm in granitic rocks in the Ryoke metamorphic belt, and 279 ppm in metamorphosed sedimentary rocks of the Ryoke belt. In granitic rocks, enrichment of NH4 in biotites is a result of the interaction between the granitic magma and surrounding sedimentary rocks. In metasedimentary rocks, the high NH4 content in biotites is due to inheritance from original organic material in sedimentary rocks. Biotites from migmatites of the Ryoke belt contain more NH4 (average, 475 ppm) than those from metasedimentary rocks. This suggests the existence of a metamorphic fluid or anatectic magma enriched in NH4.

Oxygen Isotope Study of Metamorphic and Granitic Rocks of the Yanai District in the Ryoke Belt, Japan

Abstract18O/16O ratios have been obtained for 134 whole-rocks and minerals from metamorphic and granitic rocks of the Yanai district in the Ryoke belt, Southwest Japan. The 18O/16O ratios of pelitic rocks of the marginal metamorphic zone decrease progressively with increasing metamorphic grade. In the gneiss-granite complex (zone of migmatite [1]), the most characteristic feature of the rocks is that oxygen isotopic homogenization proceeds on both local and regional scales in parallel with “granitization” or chemical homogenization. Granitic rocks of various origin are fairly uniform in isotopic composition with δ18O of quartz of 12 to 14‰ (SMOW) and δ18O of biotite of 7 to 9‰ and are about 3 to 4‰ enriched in 18O compared to other Cretaceous granites of non-metamorphic terranes in Japan. The high 18O/16O ratios of granitic rocks of this district were discussed in relation to the 18O-depletion in metasediments.Oxygen isotopic fractionations among coexisting minerals from various rock-types of the gneiss-granite complex indicate that these minerals were formed under near isotopic equilibrium at a temperature of about 600 to 700° C. Some abnormal fractionations of quartz-biotite pairs also were obtained for rocks which had undergone a progressive 18O-depletion or 18O-enrichment. This is due to high resistivity of quartz and contrastive susceptibility of biotite to isotopic exchange during metamorphism and “granitization”.