Volatile characteristics of peralkaline rhyolites from Kenya; an ion microprobe, infrared spectroscopic and hydrogen isotope study

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Authors:Wilding, Martin C.; Macdonald, Ray; Davies, J. Eric; Fallick, Anthony E.
Author Affiliations:Primary:
Lancaster University, Environmental Science Division, Lancaster, United Kingdom
Other:
Scottish Universities Research and Reactor Centre, United Kingdom
Volume Title:Contributions to Mineralogy and Petrology
Source:Contributions to Mineralogy and Petrology, 114(2), p.264-275. Publisher: Springer International, Heidelberg-New York, International. ISSN: 0010-7999
Publication Date:1993
Note:In English. 44 refs.; illus. incl. 6 tables, sketch map
Summary:Peralkaline rhyolites of the Greater Olkaria volcanic complex, Kenya Rift Valley, were derived from separated, although closely related, magma chambers. Ion microprobe analyses of glass inclusions in quartz phenocrysts show pre-eruptive water contents of <= 3.4 wt.%, contrasting with previous estimates that the magmas were anhydrous. The values approach predicted solubility levels corresponding with water saturation at low crustal P (1 kbar). The glass matrices of the rhyolites have low water contents, ranging from 0.07 to 0.46 wt.%, suggesting significant degassing during, or prior to, eruption. Infrared measurements of the matrix glasses show variation in the relative proportions of the two hydrous species dissolved in the glasses. The amount of molecular water, determined semi-quantitatively, apparently increases with increased F content and peralkalinity. This suggests a competition between hydroxyl groups and F ions for similar sites within the melt structure. The mechanism of degassing has been investigated, using hydrogen isotopes. The range of deltaD values in most rocks can be produced by varied degrees of open-system degassing of rhyolite melt initially in equilibrium with water of a fixed, or limited deltaD value. There is evidence to suggest that closed-system degassing may also have been a significant component in some rhyolites. The exact mechanisms of degassing remain uncertain. Particular problems include the relative contribution of open- and closed-system degassing during eruption and the initial vapour compositions and solubility relationships. [P.Br.]
Subjects:Cenozoic; Closed systems; Complexes; D/H; Degassing; Fluoride ion; Fluorine; Framework silicates; Glasses; Halogens; Hydrogen; Igneous rocks; Inclusions; Infrared spectra; Ion probe data; Isotope ratios; Isotopes; Low pressure; Magma chambers; Magmas; Mass spectra; Melts; Peralkalic composition; Phenocrysts; Pleistocene; Pressure; Quartz; Quaternary; Rhyolites; Silica minerals; Silicates; Solubility; Spectra; Stable isotopes; Volatiles; Volcanic centers; Volcanic features; Volcanic rocks; Water; Africa; East Africa; East African Rift; Kenya; Greater Olkaria Complex; Hydroxyl group; Navaisha Complex; Southern Kenya
Abstract Numbers:94M/666
Record ID:1993023610
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom
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