Lithium, boron and chlorine as tracers for metasomatism in high-pressure metamorphic rocks: a case study from Syros (Greece) |
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Authors: | Horst R Marschall Rainer Altherr Katalin Gméling Zsolt Kasztovszky |
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Institution: | 1. Department of Earth Sciences, Wills Memorial Building, Queen’s Road, University of Bristol, Bristol, BS8 1RJ, UK 2. Mineralogisches Institut, Universit?t Heidelberg, Im Neuenheimer Feld 236, 69120, Heidelberg, Germany 3. Institute of Isotopes, Hungarian Academy of Sciences, P.O. Box 77, 1525, Budapest, Hungary
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Abstract: | High-pressure metamorphic (HPM) rocks (derived from igneous protoliths) and their metasomatised rinds from the island of Syros (Greece) were analysed for their B and Cl whole-rock abundances and their H2O content by prompt-gamma neutron-activation analysis (PGNAA) and for their Li and Be whole-rock abundances by ICP-OES. In the HPM rocks, B?/Be and Cl?/Be ratios correlate with H2O contents and appear to be controlled by extraction of B and Cl during dehydration and prograde metamorphism. In contrast, samples of the metasomatised rinds show no such correlation. B?/Be ratios in the rinds are solely governed by the presence or absence of tourmaline, and Cl?/Be ratios vary significantly, possibly related to fluid inclusions. Li/Be ratios do not correlate with H2O contents in the HPM rocks, which may in part be explained by a conservative behaviour of Li during dehydration. However, Li abundances exceed the vast majority of published values for Li abundances in fresh, altered, or differentiated oceanic igneous rocks and presumably result from metasomatic enrichment of Li. High Li concentrations and highly elevated Li/Be ratios in most metasomatised samples demonstrate an enrichment of Li in the Syros HP mélange during fluid infiltration. This study suggests that B and Cl abundances of HPM meta-igneous rocks can be used to trace prograde dehydration, while Li concentrations seem to be more sensitive for retrograde metasomatic processes in such lithologies. |
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