Investigating the water content of magmas from Mount St. Helens

On May 18, 1980, Mount St. Helens underwent a catastrophic, explosive eruption that sent material from a blast cloud up to 17 miles from the volcano on the north side and sent ash 15 miles into the air. Pyroclastic flows from the volcano reached as far as 5 kilometers away. Ash from the explosion reached the east coast of United States in 3 days and circled the globe in 15 days. This explosion produced ash that covered an estimated 22,000 square miles of land. 56 people and numerous animals were killed in the explosion. [1]

Experimental constraints on the presence of amphibole crystals in the erupted pumices and measured water contents of melt inclusions [2,3] show that the magma from the May 18, 1980 eruption contained at least 4.6 wt% H2O prior to this eruption, and originally came from a depth of at least 7.2 km.

A series of eruptions followed the May 18, 1980 event. Most of the early eruptions were pumice-forming explosive eruptions, whereas most events after April 1981 were effusive, dacite dome-forming eruptions. It has been shown that the water content of the magmas decreased over time [4], even though the temperature and degree of oxidation (oxygen fugacity) of the magma remained constant from 1980 through 1981[2].

A thick section of a plagioclase phenocryst from the May 18, 1980 eruption of mount St. Helens. Yellow squares mark the areas of analysis with the infrared microscope.

This work focuses on two new projects undertaken to evaluate the feasibility of using plagioclase feldspar phenocrysts as “time capsules” that preserve magmatic water content during and after eruption:

1. Characterization of the rate and mechanism of diffusion of hydrogen in OH-bearing feldspars at magmatic temperatures (800-1000°C) through modeling of experimental diffusion data. The diffusion rate of hydrogen at magmatic temperatures determines the amount of OH preserved in a feldspar phenocryst during a typical eruptive event, and the extent to which zoning of OH in feldspar phenocrysts with complex magmatic histories should be preserved.

2. Evaluation of OH concentrations in plagioclase phenocrysts from the 1980-1981 eruption sequence of Mount St. Helens . The water content of successive eruptions decreased from 4.6 wt% H2O for the Plinian eruption on May 18, 1980, to less than 1 wt% H2O for the later dome-forming dacites. This decrease in magmatic water content is reflected in the OH concentrations of plagioclase phenocrysts.

Results from these two studies show that in the absence of large changes in oxygen fugacity or eruption temperature, the water contents of silicic melts immediately prior to eruption are recorded in the OH concentrations of volcanic plagioclase.

The infrared spectra of OH in plagioclase from the May 18, 1980 eruption. The area under the OH bands is used to calculate the concentration of OH in the feldspar structure.

The OH concentration in the plagioclase is correlated to the water content of the magma for each eruption (as recorded in melt inclusions). This relationship may provide a way to determine water contents of historic eruptions from Mount St. Helens and from other volcanos, by measuring OH concnetrations in feldspar phenocrysts.
   

References

[1] http://geopubs.wr.usgs.gov/fact-sheet/fs036-00/

[2] Rutherford et al. (1985) JGR 90(B4), 2929-2947.

[3] Rutherford and Devine (1988) JGR 93(B10), 11949-11959.

[4] Melson (1983) Science 221, 1387-1391.

 

 

 

This material is based upon work supported by the National Science Foundation under Grant no. EAR-0409883. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author and do not necessarily reflect the views of the National Science Foundation.