Diffusion of dissolved CO sub 2 and Cl in hydrous silicic to intermediate magmas

Journal Article · Mon Jul 01 00:00:00 EDT 1991 · Geochimica et Cosmochimica Acta; (United States)

DOI:https://doi.org/10.1016/0016-7037(91)90031-Y· OSTI ID:5862775

Watson, E B ^[1]

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1. Rensselaer Polytechnic Inst., Troy, NY (United States)

The bulk diffusivity of dissolved CO{sub 2} in obsidian and dacite melts containing 0-11 wt% dissolved water at 800-1,100C and 1 GPa was characterized using a {sup 14}C radiotracer technique. Five values were also obtained for Cl in melts containing 8% H{sub 2}O. Carbon dioxide diffusivity (D{sub CO{sub 2}}) in nominally dry molten obsidian is similar to previously reported values for a simple melt in the Na{sub 2}O-Al{sub 2}O{sub 3}-SiO{sub 2} system and for dry basalt, indicating little effect of melt composition on bulk CO{sub 2} diffusion. Three D{sub CO{sub 2}} values for hydrous dacite melt are consistent with this lack of a melt composition effect. Dissolved water, however, causes an increase of D{sub CO{sub 2}} in molten obsidian by about an order of magnitude for each 5% of added water. The activation energy of {approximately}18 kcal/mol is lower by a factor of {approximately}3 than that for diffusion in dry basalt. In comparison with existing data for diffusion of water in silicic melts at their typical liquidus temperatures, D{sub CO{sub 2}} is generally lower unless the melt contains more than about 6% dissolved H{sub 2}O, in which case D{sub CO{sub 2}} {approximately} D{sub H{sub 2}0}. The few data obtained from chlorine diffusion reveal the D{sub Cl} is lower by a factor of 5-10 than D{sub CO{sub 2}} in the same melt. The overall systematics of the new data on dissolved volatile diffusion indicate that diffusional fractionation effects are likely to be most significant in silicic to intermediate magmas having water contents in the 0-4% range.

• OSTI ID: 5862775
• Journal Information: Geochimica et Cosmochimica Acta; (United States), Vol. 55:7; ISSN 0016-7037
• Country of Publication: United States
• Language: English

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