Hydrogen Isotope Investigation of Amphibole and Glass in Dacite Magmas Erupted in 1980–1986 and 2005 at Mount St. Helens, Washington

JOURNAL OF PETROLOGY VOLUME 54 NUMBER 6 PAGES 1047^1070 2013 doi:10.1093/petrology/egt005 Hydrogen Isotope Investigation of Amphibole and Glass in Dacite Magmas Erupted in 1980^1986 and 2005 at Mount St. Helens, Washington S. J. UNDERWOOD1*, T. C. FEELEY1 AND M. A. CLYNNE2 1 DEPARTMENT OF EARTH SCIENCES, MONTANA STATE UNIVERSITY, BOZEMAN, MT 59717, USA 2 UNITED STATES GEOLOGICAL SURVEY, 345 MIDDLEFIELD ROAD, MAIL STOP 910, MENLO PARK, CA 94025, USA RECEIVED SEPTEMBER 16, 2009; ACCEPTED JANUARY 9, 2013 ADVANCE ACCESS PUBLICATION FEBRUARY 28, 2013 In active, shallow, sub-volcanic magma conduits the extent of the dehydrogenation^oxidation reaction in amphibole phenocrysts is controlled by energetic processes that cause crystal lattice damage or conditions that increase hydrogen diffusivity in magmatic phases. Amphibole phenocrysts separated from dacitic volcanic rocks erupted from 1980 to 1986 and in 2005 at Mount St. Helens (MSH) were analyzed for dD, water content and Fe3þ/Fe2þ, and fragments of glassy groundmass were analyzed for dD and water content. Changes in amphibole dD values through time are evaluated within the context of carefully observed volcanic eruption behavior and published petrological and geochemical investigations. Driving forces for amphibole dehydrogenation include increase in magma oxygen fugacity, decrease in amphibole hydrogen fugacity, or both. The phenocryst amphibole (dD value c. ^57ø and 2 wt % H2O) in the white fallout pumice of the May 18, 1980 plinian eruptive phase is probably little modified during rapid magma ascent up an 7 km conduit. Younger volcanic rocks incorporate some shallowly degassed dacitic magma from earlier pulses, based on amphibole phenocryst populations that exhibit varying degrees of dehydrogenation. Pyroclastic rocks from explosive eruptions in June^October 1980 have elevated abundances of mottled amphibole phenocrysts (peaking in some pyroclastic rocks erupted on July 22, 1980), and extensive amphibole dehydrogenation is linked to crystal damage from vesiculation and pyroclastic fountain collapse that increased effective hydrogen diffusion in amphibole. Multiple amphibole dD populations in many 1980 pyroclastic rocks combined with their groundmass characteristics (e.g. mixed pumice textures) support models of shallow mixing prior to, or during, eruption as new, volatile-rich magma pulses blended with more oxidized, degassed magma. Amphibole dehydrogenation is quenched at the top surface of MSH dacite lava lobes, but the diversity in the dDamph populations in original fresh lava flow surfaces may occur from blending magma domains with different ascent histories in the sub-volcanic environment immediately before eruption. Multi-stage open-system magma degassing operated in each parcel of magma rising toward the surface, whereas the magma below 7 km was a relatively closed system, at least to the October 1986 eruption based on the large population of minimally dehydrogenated, rim-free amphibole in the lavas. Magma degassing and possibly H isotope exchange with low-dD fluids around the roof zone may have accompanied the 1·5 km upward migration of the 1980 magma body. The low-dDamph (c. ^188 to ^122ø) oxy-amphibole phenocrysts in lava spines extruded in May 2005 reflect dehydrogenation as ascending viscous magma degassed and crystallized, and fractures that admitted oxygen into the hot solidified lava spine interior facilitated additional iron oxidation. *Corresponding author. Present address: 50 Red Cloud Place, Bozeman, MT 59715-8727, USA. Telephone: (406) 522-3996. E-mail:  The Author 2013. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@ oup.com KEY WORDS: amphibole dehydrogenation; hydrogen diffusion; hydrogen isotopes; magma degassing; Mount St. Helens I N T RO D U C T I O N Magma degassing is known to fractionate protium (1H) and deuterium (2H or D) in explosive to effusive volcanic eruption sequences of crystal-poor rhyolites (Newman et al., 1988; Taylor, 1991). Explosive volcanic eruptions can result from disruption of active hydrothermal systems JOURNAL OF PETROLOGY VOLUME 54 through poorly sealed volcanic conduits (see Sheridan et al., 1981; Sheridan & Wohletz, 1983; Wohletz, 1986). Distinguishing magma degassing from hydromagmatic processes in porphyritic arc volcanic rocks, although challenging (see Kuroda et al., 1988), may be possible where hydrogen isotope signals (i.e. a D/H shift) in hydrous phenocrysts and in glassy groundmass are distinct (see O’Neil & Taylor, 1985; Westrich et al., 1985; Taran et al., 1997), and by considering rock textures within the eruption context (see Heiken et al., 1988; Taylor, 1988). Amphibole and biotite are the most common hydrous phenocrysts in volcanic rocks used for H isotope studies; rapidly quenched, fresh volcanic rocks can preserve magmatic dD values in such phenocrysts (Graham et al., 1984; Hoblitt & Harmon, 1993; Miyagi & Matsubaya, 2003). Amphibole phenocrysts in volcanic rocks generally are little affected by degassing during eruption because the H isotope re-equilibration rate in amphibole is comparatively slow in silicic magmas (Hildreth & Drake, 1992; Hoblitt & Harmon, 1993; Kusakabe et al., 1999; Harford & Sparks, 2001). However, H isotope fractionation in Fe-rich calcic amphibole may occur in certain high-temperature sub-volcanic environments (Clowe et al., 1988; Kuroda et al., 1988; Phillips et al.,1988; Miyagi et al.,1998; Popp et al., 2006). In contrast to the swiftness of the reactions observed in experiments, natural samples of volcanic rocks typically contain variably hydrated oxy-amphibole crystals with a range of dDamph values (see Hildreth & Drake, 1992; Hoblitt & Harmon, 1993; Kusakabe et al., 1999; Harford & Sparks, 2001). Identification of shallow conditions or processes in the sub-volcanic magmatic conduits that may influence dehydrogenation in hydrous phenocrysts is needed (see Dyar et al., 1993; King et al., 1999). For example, the common occurrence of dehydrogenated hydrous minerals noted in some pumiceous rocks from pyroclastic eruptions (see Miyagi & Matsubaya, 2003) could be related to physical damage to phenocrysts, which can significantly increase the effective diffusion of hydrogen (Underwood et al., 2012). To investigate D/H shifts in amphibole phenocrysts and enveloping glass, a well-documented modern eruption sequence from an intermediate to silicic volcano is needed. We selected the 1980^1986 eruptions at Mount St. Helens (MSH) because they attracted intense scientific scrutiny, including studies focused on the formation and stability of amphibole phenocrysts in the magmas (e.g. Rutherford & Devine, 1988; Rutherford & Hill, 1993), and whole-rock D/H values from crater dome lavas (Anderson & Fink, 1989, 1990; Anderson et al., 1995). The radical change in eruptive style in subsequent 2004^2008 eruptions (i.e. lava spines and whalebacks) was caref (...truncated)