AMNH EPS: Cl in magma
Document Actions
The dissolution and exsolution of chlorine in felsic, intermediate, and mafic aluminosilicate magmas
Chlorine is a volatile element that is important to a variety of geologic processes. For example, chlorine exsolves from magma to the atmosphere during volcanic degassing; as a result, it may contribute to the greenhouse gas effect, the formation of acid rain, and the degradation of the ozone layer. The release of chlorine with water and other volatiles from magma located beneath the Earth's surface is also very important in the transport and concentration of ore metals such as gold, copper, and iron to form useful mineral deposits. In our experimental petrology laboratory, we measure how much chlorine dissolves in various magmas by melting rock samples at high temperatures and pressures in the presence of chlorine-rich compounds. The amount of chlorine that enters the melt depends upon the composition of the melt, the temperature and the pressure. This information is used to understand how and when chlorine is released from natural magmas and how chlorine movement may influence the formation of ore deposits and processes occurring in the atmosphere. This research is funded by the National Science Foundation.
This photograph depicts the products resulting from one of our
experiments on a rhyolite from Spor Mountain, Utah. The image is
approximately 200 microns wide and was taken on a scanning electron
microscope. Most of the sample consists of a fluorine- and
chlorine-bearing glass. The elongate crystals are alkali feldspar, the
black areas are quartz, the extremely white rounded crystals are iron
oxides, and the black spherical areas are fluid bubbles that had
contained molten sodium and potassium chloride. During the experiment,
the sample was held at a pressure of 2,000 bars (about 2,000 times
atmospheric pressure); the intial temperature of the experiment was 950
C and the final one was 700 C.
References
- Webster, J.D. and Rebbert, C.R. (1998) Experimental investigation
of H2O and Cl- solubilities in F-enriched silicate liquids;
implications for volatile saturation of topaz rhyolite magmas.
Contributions to Mineralogy and Petrology (in review).
- Webster, J.D. (1997) Chloride solubility in felsic melts
and the role of chloride in magmatic degassing. Journal of Petrology
(in press).
- Webster, J.D. (1997) Exsolution of Cl-bearing fluids from
chlorine-enriched mineralizing granitic magmas and implications for ore
metal transport. Geochimica et Cosmochimica Acta, 61, 1017-1029.
- Webster, J.D. (1995) Early exsolution of chlorine-enriched fluids from felsic magmas:
a crucial but poorly-understood process. U.S. Geological Survey Circular
1129, 160-161.
- Carroll, M.R., and Webster, J.D. (1994) Solubilities of sulfur, noble gases, nitrogen,
chlorine, and fluorine in magmas. In M.R. Carroll and J.R. Holloway, eds.,
Volatiles in Magmas, Reviews in Mineralogy Volume 30, 231-279.
- Webster, J.D. (1992a) Fluid-melt interactions in Cl-rich granitic systems: Effects of melt
composition at 2 kbar and 800 C. Geochimica et Cosmochimica Acta 56, 659-
678.
- Webster, J.D. (1992b) Fluid-melt interactions involving Cl-rich granites: Experimental study from 2 to 8 kbar. Geochimica et Cosmochimica Acta 56, 679-687.
Return to Research topics | EPS Homepage
© 1997 The American Museum of Natural History.
