DETECTION OF JUVENILE MATERIAL FROM MARCH 1980 TEPHRA FROM MOUNT ST. HELENS

Early phreatic eruptions may contain juvenile material that provides information about the ascending magma, and thus serve as precursors of impending magmatic eruptions. Samples of early tephra erupted in late March, 1980, from Mount St. Helens, were collected before the lateral blast and climactic plinian eruption that followed on May 18, more than seven weeks later. We have separated and analyzed sparse, vesicular glass shards that were found in these early tephra samples.

We concentrated light density fractions from several samples of early tephra, and detected fresh vesicular glass shards in tephra erupted on 28 March 1980, but not in the tephras of April 8 and 16. Results of electron microprobe analysis indicate that the composition of these glasses are the same as or very similar to matrix glass in pumices erupted on 18 and 25 May 1980. The composition of the vesicular glass erupted in March is different from that of glass shards from older, pre-1980 tephra layers of Mount. St. Helens. Furthermore, micro-FTIR analysis of the March 1980 shards indicates they have low H2O/OH ratios compared to hydrated glasses. These lines of evidence strongly suggest that the March glass represents juvenile material.