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Excess argon in Mount St. Helens plagioclase as a recorder of magmatic processes
Author(s) -
Layer Paul W.,
Gardner James E.
Publication year - 2001
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/2001gl013855
Subject(s) - plagioclase , felsic , mafic , geology , dacite , geochemistry , argon , magma , basalt , volcano , volcanic rock , andesite , chemistry , quartz , paleontology , organic chemistry
Excess argon in plagioclase crystals from young (<4000 ybp) tephra layers from Mount St. Helens, Washington, illustrates the importance of argon isotopes in understanding magmatic processes. 40 Ar/ 39 Ar step‐heating identifies two distinct argon reservoirs in the plagioclase; a high Cl/K, low Ca/K reservoir with atmospheric 40 Ar/ 36 Ar and a low Cl/K, higher Ca/K reservoir with variable 40 Ar/ 36 Ar. The first is probably glass, whereas the second is “true” plagioclase. Felsic dacite plagioclases have little or no excess argon, but those from more mafic layers have significantly higher 40 Ar/ 36 Ar, indicating a non‐atmospheric source. These variations are seen within a layer that contains both mafic and felsic dacite, and are inconsistent with either xenocrystic or restitic origin for plagioclase. The magma chamber exhibits long‐term open‐system degassing behavior punctuated by short‐term fluctuations from influx of high 40 Ar/ 36 Ar basalt. The relative timing between mixing and eruption can affect the amount of excess argon recorded in plagioclase.