Ammonium in biotite from metamorphic and granitic rocks of Japan

Ammonium contents of biotites from metamorphic and granitic rocks of Japan have been determined, and correlated with the ${}^{18}\text{O}{}^{16}\text{O}$ 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.