Hydrothermal adularia at Bodie, Mono County, California
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Brent P. Fabbi
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Loss of volatiles during fountaining and flowage of basaltic lava at Kilauea Volcano, Hawaii
The amount of water and sulfur in pumice erupted during periods of vigorous activity during the 1969-71 Mauna Ulu eruption varied inversely with fountain height because of degassing during the fountaining. The pumice lost about 0.05 wt percent water and 0.003 wt percent sulfur during fountaining to heights of 400-540 m. Analyses suggest that the initial volatile content of Mauna Ulu lava was greater immediately preceding periods of high fountaining than during weak activity between those periods or after the last high fountains on December 30, 1969. Water and sulfur were systematically depleted during nearly isothermal flowage in lava tubes. Rapidly quenched samples of dipped melt show losses of about 0.03-0.04 wt percent water and 0.007-0.008 wt percent sulfur during flowage for several hours through a distance of 12 km. Glassy skins on cooled pahoehoe flows contain about 0.002-0.003 wt percent less sulfur than quenched melt at comparable distances from the vent, because of continued degassing under natural cooling conditions. Chlorine shows similar but less well defined trends. Pumice erupted in high fountains becomes more strongly oxidized than the parent magma, because of mixing with air while still at high temperatures.
Hawaii↗
An estimate of the juvenile sulfur content of basalt
Sulfur analyses by X-ray fluorescence give an average content of 107 ppm for 9 samples of fresh subaerially-erupted oceanic basalt and 680 ppm for 38 samples of submarine erupted basalt. This difference is the result of retention of sulfur in basalt quenched on the sea floor and loss of sulfur in basalt by degassing at the surface. The outer glassy part of submarine erupted basalt contains 800±150 ppm sulfur, and this amount is regarded as an estimate of the juvenile sulfur content of the basalt melt from the mantle. The slower cooled interiors of basalt pillows are depleted relative to the rims owing to degassing and escape through surface fractures. Available samples of deep-sea basalts do not indicate a difference in original sulfur content between low-K tholeiite, Hawaiian tholeiite, and alkali basalt.
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