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Water content of granitic melts from Cornwall and Erzgebirge

¹ Natural History Museum, Department of Mineralogy, Cromwell Road, London SW7 5BD, UK
² GeoForschungsZentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germany
³ Czech Geological Survey Prague, Geologicka 6,15200 Prague 5, Czech Republic

^* Corresponding author, present address: Norges geologiske undersøkelse, 7491 Trondheim, Norway, e-mail: Axel.Muller{at}ngu.no

Melt inclusions (MIs) occurring in quartz of late-Variscan Sn-specialized granites from the Land's End pluton in SW England and from the eastern Erzgebirge volcano-plutonic complex in Germany were analyzed by Raman spectroscopy, secondary ion mass spectrometry and electron microprobe. Crystallized MIs were homogenized using cold-sealed autoclaves operating at 850°C and 2 kbar for 24 hours. The H₂O concentration of homogenized MIs from the Land's End granites determined by confocal Raman spectroscopy range between 1.5 and 5.4 wt.%. Several MIs from the Land's End granites contain a hypersaline fluid with 18.2 to 38.6 wt.% H₂O. Such mixed fluid and silicate-MIs are typical for magmas that were oversaturated in volatiles. The ratio of silicate glass/saline phase decreases with increasing degree of differentiation of the granite host. The H₂O content of MIs from the Niederbobritzsch granite, Schönfeld rhyodacite, Teplice rhyolite, Altenberg-Frauenstein microgranite and Schellerhau granite in the eastern Erzgebirge varies between 0.7 and 11.9 wt.%. The MIs from the volcanic rocks have more variable concentrations than the MIs from the granites. The high chemical discrepancies between MIs and whole rock suggest that the quartz phenocrysts in the Schönfeld rhyodacite were injected into a stratified magma chamber during the course of multiple recharge events at the chamber's base. MIs from granites from the eastern Erzgebirge do not contain hypersaline fluids, however they have F concentrations of up to 11.2 wt.%. The Li, Be and B contents of representative homogenized MIs were determined by SIMS. The light lithophile element ratios of MIs are constant for each magmatic province despite different fractionation degrees of the host rocks. MIs from rocks of the eastern Erzgebirge volcano-plutonic complex are relatively enriched in Li and Be, whereas MIs in granites of the Land's End pluton have higher B contents. The distinctive ratio of light lithophile elements of the silicate melt is also reflected in the light lithophile element ratio of the magmatic host quartz.

Key-words: melt inclusion, Raman spectroscopy, SIMS, granite, Cornwall, Erzgebirge.