Introduction The compositional ranges of eruptive products generated at continental arc volcanoes often display linear compositional arrays that extend from basalt to dacite and, occasionally, rhyolite (e.g., Hildreth et al. 2003 ; Price et al. 2005 ; Hora et al. 2007 ; Singer et al. 2008 ; Hildreth and Fierstein 2012 ). The time-sequenced geochemical variations of eruptive products are typically non-systematic, however, and do not necessarily reflect the long-term evolution of a common magma source by simple progressive fractionation and assimilation (cf
Experiments in high silica systems at temperatures close to the solidus often produce crystals and melt pools that are too small for in situ analysis. Oscillating the temperature during an experimental run speeds up recrystallization of magma by dissolving small and increasing the size of larger crystals, dramatically changing the crystal size distribution. This principle of periodic heating and cooling, caused for example by repeated injection of hot magma, is also a potential acceleration for the formation of phenocrystic textures in natural rocks.
Here we show that temperature cycling has the potential to significantly enlarge melt pools and crystals in a fluid saturated dacitic system. Using a natural dacite dredged from the Pacific-Antarctic Rise as starting material, we performed crystallization experiments applying temperature cycling systematically for two different temperatures and different water activities at 200 MPa. For experiments at 950 °C (with aH2O ~1, ~0.3, and <0.1) an internally heated pressure vessel was used, experiments at 800 °C (with aH2O ~1, ~0.5) were performed in a cold-seal pressure vessel. Comparative experiments at equilibrium conditions with constant temperature were performed for both approaches. For all other experiments temperature was cycled with amplitudes of 20 K for different time intervals but constant total run duration after initial equilibration at constant temperature. Additionally, for one experiment at 800 °C, the temperature was increased several times by 50 K to study the potential of dissolving tiny crystals in the matrix.
As a result of the temperature cycling, tiny crystals in the matrix were preferentially dissolved, leading to large melt pools with only rare mineral inclusions enabling microprobe analysis using a defocused beam. With regard to the area of the 10 largest crystals of each cycling experiment, clinopyroxene crystals were up to 19 times larger, and plagioclase crystals even up to 69 times when comparing to experiments performed at constant temperature. Grain sizes of FeTi-oxide phases are less influenced by this technique. Essential requirements for applying temperature cycling routinely are identical phase relations and compositions in runs with constant and cycled temperature. For all studied temperatures and water activities, the phase assemblage was the same and compositions of all phases are identical within the analytical error. Thus, the temperature cycling technique opens interesting perspectives, especially in facilitating in situ analysis in near solidus systems.
American Mineralogist, Volume 94, pages 995–1004, 2009
0003-004X/09/0007–995$05.00/DOI: 10.2138/am.2009.3129 995
Origins of Mount St. Helens cataclasites: Experimental insights
Lori A. Kennedy,* JAmes K. russeLL, And edwArd neLLes
Centre for Experimental Studies of the Lithosphere, Department of Earth and Ocean Sciences, University of British Columbia, Vancouver,
V6T 1Z4, Canada
The 2004–2006 eruption of Mount St. Helens produced a sequence of lava domes characterized
by a 1–3 m thick outer carapace of highly brecciated and comminuted dacite
The Efate Pumice Formation (EPF) is a trachydacitic volcaniclastic succession widespread in the central part of Efate Island and also present on Hat and Lelepa islands to the north. The volcanic succession has been inferred to result from a major, entirely subaqueous explosive event north of Efate Island. The accumulated pumice-rich units were previously interpreted to be subaqueous pyroclastic density current deposits on the basis of their bedding, componentry and stratigraphic characteristics. Here we suggest an alternative eruptive scenario for this widespread succession. The major part of the EPF is distributed in central Efate, where pumiceous pyroclastic rock units several hundred meters thick are found within fault scarp cliffs elevated about 800 m above sea level. The basal 200 m of the pumiceous succession is composed of massive to weakly bedded pumiceous lapilli units, each 2-3 m thick. This succession is interbedded with wavy, undulatory and dune bedded pumiceous ash and fine lapilli units with characteristics of co-ignimbrite surges and ground surges. The presence of the surge beds implies that the intervening units comprise a subaerial ignimbrite-dominated succession. There are no sedimentary indicators in the basal units examined that are consistent with water-supported transportation and/or deposition. The subaerial ignimbrite sequence of the EPF is overlain by a shallow marine volcaniclastic Rentanbau Tuffs. The EPF is topped by reef limestone, which presumably preserved the underlying EPF from erosion. We here propose that the EPF was formed by a combination of initial subaerial ignimbrite-forming eruptions, followed by caldera subsidence. The upper volcaniclastic successions in our model represent intra-caldera pumiceous volcaniclastic deposits accumulated in a shallow marine environment in the resultant caldera. The present day elevated position of the succession is a result of a combination of possible caldera resurgence and ongoing arc-related uplift in the region.
of the Eyring equation
DAVID TINKER,1,* CHARLES E. LESHER,1 GREGORY M. BAXTER,1 TAKEYUKI UCHIDA,2
AND YANBIN WANG2
1Department of Geology, University of California, One Shields Avenue, Davis, California 95616, U.S.A.
2Consortium for Advanced Radiation Sources, University of Chicago, Chicago, Illinois 60637, U.S.A.
In situ falling-sphere measurements of viscosity have been performed to determine the viscosity of
dacite melt (68 wt% SiO2) from 1.5 to 7.1 GPa at temperatures between 1730 and 1950 K, using the
T-25 MA8 multianvil apparatus at the GSECARS 13
Complex, Taupo Vol-
canic Zone, including crystal-rich basalt-dacite pumice from post-collapse deposits, reveals several
pre- and syn-eruption magmatic processes. (1) Amphibole phenocrysts in the basaltic-andesite and
andesite crystallized at the highest pressures and temperatures (P to 0.6 ± 0.06 GPa and T to 950 °C),
equivalent to mid-crustal depths (13–22 km). Inter- and intra-crystalline compositions range from Ti-
magnesiohastingsite → Ti-tschermakite → tschermakite → magnesiohornblende and some display
gradual decreases in T from core to rim, both consistent
spectrometry from plagioclase crystals, together with
textural observations from interference contrast microscopy, are consistent with contrast-
ing magma dynamics in two subvolcanic reservoirs from which silicic lavas erupted at the
Tatara-San Pedro volcanic complex, Chilean Andes. The 1 km3 late Pleistocene (68 ka)
Tatara dacite is chemically homogeneous, phenocryst-poor, and contains crystals of opa-
citized hornblende, orthopyroxene, and titanomagnetite, plus 2 mm euhedral plagioclase
phenocrysts with simple zoning patterns. Except at their rims, analyzed phenocrysts
Institutul Geologic al Romaniei, Bucuresti 78344, Romania
5Sabba S. Stefanescu Institute of Geodynamics of the Romanian Academy 19-21, Jean Louis Calderon str., Bucuresti 37, RO-70201, Romania
A Late Pan-African calc-alkaline dike swarm (basalt-andesite-dacite-rhyolite) has been investigated
in a region of over 2000 km2 in the Alpine Danubian window, South Carpathians (Romania). Amphibole
phenocrysts and microphenocrysts have been investigated by wavelength-dispersive microprobe analy-
sis and BSE imaging. The Ca-amphibole population, represented in all the
1Department of Ocean and Earth Sciences, The University of British Columbia, Vancouver, British Columbia, V6T 1Z4, Canada
2Department of Geological Science and Geological Engineering, Queen s̓ University, Kingston, Ontario, K7L 3N6, Canada
The dacite of the Pebble Creek Formation (Mount Meager, BC, Canada) is an extraordinary occur-
rence of lavas containing coarse, sieve-textured plagioclase phenocrysts that appear to have reacted
extensively with the melt. We record this unusual occurrence of a naturally reacted crystal/melt pair