On ammonium in upper-amphibolite facies cordierite-orthoamphibole-bearing rocks from Rød, Bamble Sector, south Norway
pp. 385-388

NH4+: values of 1100-3000 ppm and 250-458 ppm have been measured in biotites and whole rock samples respectively, from cordierite-orthoamphibole-bearing rocks at Rød, Bamble Sector, south Norway. The results indicate that given: (a) suitable (Mg, Fe, Al)-rich and (Ca, Na)-poor bulk compositions, (b) no high-grade (breakdown-)reactions involving biotite, (c) reducing conditions defined by graphite-bearing assemblages, and ( d) an ammonium-rich protolith, significant amounts of ammonium in biotite can survive high-grade metamorphism. The high ammonium contents are considered supplementary evidence for a sedimentary origin of the cordierite-orthoamphibole-bearing rocks. The significant quantities of N2 previously reported in quartz from this locality are thought to be genetically related to the retrogression of the ammonium-bearing biotites.

D. Visser, Department of Geochemistry, Institute of Earth Sciences, University of Utrecht, PO Box 80.021, 3508 TA Utrecht, The Netherlands.