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Oxygen isotope sector zoning in natural hydrothermal quartz

A.-L. Jourdan^1,,*, T. W. Vennemann¹, J. Mullis² and K. Ramseyer³

¹ Institut de Minéralogie et Géochimie, Université de Lausanne, L'Anthropole, 1015 Lausanne, Switzerland
² Mineralogisch- Petrographisches Institut, Universität Basel, Bernoullistrasse 30, 4056 Basel, Switzerland
³ Institut für Geologie, Universität Bern, Baltzerstrasse 1-3, 3012 Bern, Switzerland

^* E-mail: Jourdan{at}lewebmail.com

Oxygen isotope measurements using SIMS and laser-fluorination methods confirm the presence of concentric and sector zoning in low-temperature (200°C to <400°C) hydrothermal quartz from Alpine veins. While concentric zoning is most readily explained by changes in the chemical composition of the fluid or temperature of crystallization, the reasons for sector zoning are more difficult to explain. Relative enrichment in ¹⁸O for crystallographically different sectors of quartz corresponds to m >r >z. Sector zoning is, however, largely limited to the exterior zones of crystals and/or to crystals with large Al (>1000 ppm) and trace element contents, probably formed at temperatures <250°C. Differences in ¹⁸O between the prismatic (m) relative to the rhombohedral (r and z) growth sectors of up to 2 can be explained by a combination of a face-related crystallographic and/or a growth rate control. In contrast, isotopic sector zoning of up to about 1.5 amongst the different rhombohedral faces increases in parallel with the trace element content and is likely to represent disequilibrium growth. This is indicated by non-systematic, up to 2, differences within single growth zones and the irregular, larger or smaller, ¹⁸O values (of several permil) of the exterior compared to the inner zones of the same crystals. Disequilibrium growth may be related to the large trace element content incorporated into the growing quartz at lower temperatures (<250°C) and/or be related to fluid-vapour separation, allowing crystal growth from both a vapour as well as a liquid phase.

KEYWORDS: hydrothermal quartz, oxygen isotope, sector zoning, SIMS, laser-fluorination, Alpine veins

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