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 18O 16O ratios of the rocks. NH 4 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 NH 4 in biotites is a result of the interaction between the granitic magma and surrounding sedimentary rocks. In metasedimentary rocks, the high NH 4 content in biotites is due to inheritance from original organic material in sedimentary rocks. Biotites from migmatites of the Ryoke belt contain more NH 4 (average, 475 ppm) than those from metasedimentary rocks. This suggests the existence of a metamorphic fluid or anatectic magma enriched in NH 4.