Diffusion of dissolved carbonate in magmas: experimental results and applications

Details Diffusion of dissolved carbonate in magmas: experimental results and applications Diffusion of dissolved carbonate in magmas: experimental results and applications

Dec 1982

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1982e%26psl..61..346w&link_type=abstract

Earth and Planetary Science Letters, Volume 61, Issue 2, p. 346-358.

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37

Scientific paper

The diffusivity of dissolved carbonate in sodium aluminosilicate (60% SiO2, 30% Na2O, 10% Al2O3) and iron-free ``basalt'' melts was measured by a 14C thin-source tracer technique at conditions of combined high temperature and high pressure, ranging from 800°C to 1350°C and 0.5 to 18 kbar for the simple melt and 1350° to 1500°C at 15 kbar for the haplobasalt melt. The two melts have similar dissolved carbonate transport properties, expressed for the simple system at 0.5 kbar by: Dcarbonate=3.559exp(-46,600/RT)
The pressure dependence at 1200°C is given by: Dcarbonate=4.13×10 -7exp(-11.02P/RT)
The activation energy of 46.6 kcal/mole is low relative to expected values for small, highly-charged cations, suggesting that the actual diffusing species is a larger carbonate anion. This interpretation is supported by the 11 cm3/mole activation volume, which corresponds to twice the molar volume of oxygen anions. Because the 14C diffusion gradients conform quite well to a constant-diffusivity, thin-source model, there can be no strong dependence of carbonate diffusivity upon dissolved carbonate content, at least in the 0-0.2 wt.% range represented by the experimental profiles.
Carbonate diffusion is markedly different from previously determined systematics for water, being 1-2 orders of magnitude slower in granitic magmas and considerably faster in basalts. This difference can result in diffusional fractionation of CO2 and H2O in magmas, which may be most significant in small-scale processes such as growth of vapor bubbles and formation of some types of glass inclusions in phenocrysts. The new data also place constraints on the rate at which CO2-dominated fluid bubbles can actually grow. Under more or less isothermal conditions at ~ 10 kbar, the growth rate is on the order of 1 mm/day, depending upon the degree of supersaturation in CO2. When combined with knowledge of initial CO2 contents of magmas and observed characteristics of dispersed bubbles (e.g., in ocean-floor basalt glasses), the diffusivity data may expedite the reconstruction of magma decompression paths.
Presently at the Department of Earth and Space Sciences, State University of New York at Stony Brook, NY 11794, U.S.A.

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