Title: Volatile evolution in Archean rare-element granitic pegmatites; evidence from the hydrogen isotopic composition of channel H 2 O in beryl
Abstract: The channel H2O in beryl constitutes a large reservoir of molecular water; concentrations attain 2.4wt.o/0. Stepwise heating experiments, carried ovt in vacuo, permitted the differential extraction of both the channel H2O and water from other minor sources (e.g., fluid inclusions) for quantification and hydrogen isotope analysis. Beryl is a ubiquitous primary mineral in the two groups of Late Archean granitic pegmatites that occur in the Dryden area, in Ontario, and the hydrogen isotope composition of its channel H2O has a range of ca. 25o/or. Igneous fractionation trends in the rare-element granitic pegmatites of the Mavis Lake group, the focus of this study, are described by: (l) systematic changes in the mineral assemblage of the pegmatites with increasing distance from the parent granire (beryl to spodumene - beryl - tantalite), (2) decreasing K,/Rb ratio in blocky K-feldspar that coexists with beryl (K/RbKrs = 59 to 16), (3) increasing concentrations of channel H2O in beryl (1.16 to 2.25 wt.t/o), and (4) increasing 6D values in beryl (-78 to -53%o). As there is no compelling field or petrographic evidence for vapor saturation during the bulk of pegmatite crystallization, these trends are interpreted to have resulted from crystallization at vapor-undersaturated conditions, with the composition of the channel H2O in the beryl approximating that of dissolved magmatic H2O. Hydrogen isotope fractionations between muscovite and the channel H2O in coexisting beryl from the Taylor Beryl No. 2 (A1,4r-1-1,o : -2lo/oi and Fairservice Property No. 1 (Ayr_g 190loo) pegmatites are analytically indistinguishable, and yield temperatures of crystallization in the range 5J0-430oC, which agree well with those based on phase-equilibrium studies. As such, the existence of molecular H2O, trapped in channels in beryl during the late stages of pegmatite crystallization, may facilitate the detailed study of volatile evolution in a wide range of Archean and younger granitic pegmatite systems.
Publication Year: 1992
Publication Date: 1992-09-01
Language: en
Type: article
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Cited By Count: 32
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