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Journal of Petrology | Volume 41 | Number 3 | Pages 357-362 | 2000
© Oxford University Press 2000

The Merzbacher & Eggler (1984) Geohygrometer: a Cautionary Note on its Suitability for High-K Suites

A. K. SCHMITT1,* and S. L. DE SILVA2

1GEOFORSCHUNGSZENTRUM POTSDAM, PB 4.2, TELEGRAFENBERG B124, 14473 POTSDAM, GERMANY
2DEPARTMENT OF GEOGRAPHY, GEOLOGY AND ANTHROPOLOGY, INDIANA STATE UNIVERSITY, TERRE HAUTE, IN 47809, USA

Received November 13, 1998; Revised typescript accepted September 2, 1999


  Abstract

The use of the popular Merzbacher & Eggler (1984, Geology 12, 587–590) experimental geohygrometer for calcalkaline andesites and dacites is critically evaluated and two pitfalls are found. First, calculation of the correct projection parameters is problematic because two endmember calculation schemes are found in the literature; Baker & Eggler (1983, Journal of Volcanology and Geothermal Research 18, 387–404) and Walker et al. (1979, Contributions to Mineralogy and Petrology 70, 111–125). Although related, these two schemes have crucial differences that can result in very different projection parameters for the plagioclase component. This is most crucial for high-K compositions; H2O contents estimated using the Walker et al. (1979) scheme can be as much as 100% higher than those estimated using the Baker & Eggler (1983) projection. Incorrect projection parameter calculation has led to overestimation of water contents in high-K andesites and dacites from the Central Andes. Second, for medium-K and high-K andesitic–dacitic compositions water contents derived using the Merzbacher & Eggler (1984) geohygrometer deviate considerably from water contents estimated using other methods. Experimental data from the literature, and our studies of water contents inferred from melt inclusions and plagioclase–melt equilibrium for dacites from the Altiplano–Puna Volcanic Complex of the Central Andes indicate that the Merzbacher & Eggler (1984) geohygrometer should not be applied to compositions with K2O >1·9 wt %, as originally calibrated.

KEY WORDS: magmatic volatiles; pre-eruptive water; experimental geohygrometer; high-K suites; melt inclusion


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