BERYLLIUM IN METAMORPHIC ENVIRONMENTS (EMPHASIS ON ALUMINOUS COMPOSITIONS)

Abstract

Beryllium is an element thought to be largely associated with igneous and hydrothermal processes and the world’s major economic Be deposits are largely igneous or hydrothermal. However, metamorphic rocks also play a major role in the Be budget of the Earth’s crust. Beryllium enrichments in pegmatites and hydrothermal deposits are associated with granitic systems that many geoscientists think are derived from melting of metasedimentary rocks, with metapelites being the most fertile for Be (e.g., London and Evensen, this volume). Consequently, metamorphism plays an important role in the cycling beryllium from sediments to granitic systems. The emphasis of my chapter is on rocks of pelitic composition or otherwise relatively rich in aluminum such as bauxite and metamorphosed peraluminous pegmatites (see also Franz and Morteani, this volume) and on the minerals making up these rocks. It concerns beryllium in metamorphic environments where beryllium was not introduced during metamorphism. Two major questions are addressed: (1) average and range of Be contents of metamorphic rocks, and (2) the effect of metamorphism on their beryllium content. Addressing both questions requires not only extensive data on metamorphic rocks, but also a comparable database of beryllium contents of unmetamorphosed precursor sediments. In general, beryllium is analyzed much less often than other trace elements, and as far as I am aware, there have been no systematic large-scale studies of Be in pelitic rocks or any other rock type. For example, Terry Plank (pers. comm. 2001) finds that there still is not enough high quality data to consider its behavior in marine sediments as has been done for other trace elements (Plank and Langmuir 1998). Beryllium minerals, including both silicates and oxides, are rare in most metamorphic rocks, although a surprisingly large number of species has been found. This scarcity undoubtedly reflects the low abundance of Be in …