The influence of trace amounts of water on the viscosity of rhyolites |
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Authors: | R J Stevenson N S Bagdassarov D B Dingwell C Romano |
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Institution: | Bayerisches Geoinstitut, Universit?t Bayreuth, D-95440 Bayreuth, Germany, DE Institut für Meteorologie und Geophysik, Universit?t Frankfurt, D-60323 Franfurt, Germany, DE Dipartimento di Scienze della Terra, Terza Universitá degli Studi di Roma, 3, Largo San Murialdo 1, Rome, Italy e-mail: don.dingwell@uni-bayreuth.de ?deceased, IT
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Abstract: | As a major volatile in volcanic systems, water has a significant influence on the rheological properties of silicic magmas.
This is especially so at minor water contents relevant to the emplacement of silicic lavas. To investigate the influence of
water on the viscosity of natural rhyolitic obsidians, a novel strategy has been adopted employing parallel-plate and micropenetration
techniques. Viscosities have been determined on three types of material: (a) raw water-bearing obsidians; (b) remelted (1650 °C,
1 atm) degassed glasses of the obsidians; and (c) hydrothermally hydrated (1300 °C, 3 kbar) obsidians. Ten natural rhyolitic
obsidians (peraluminous, calc-alkaline and peralkaline) were employed: seven originated from lava flows and contained <0.2 wt.%
H2O, two samples were F-rich from pyroclastic successions, and one was an obsidian cobble with 1.5 wt.% water also associated
with pyroclastic units. Melt compositions and water contents were stable during viscometry. The measured decreases in activation
energies of viscous flow and viscosity with small amounts of water are much greater than the Shaw calculation scheme predicts.
In addition, a marked non-linear decrease in η exists with increasing water content. In contrast to the case for peralkaline
rhyolites, 0.1–0.2 wt.% water decreases activation energies significantly (up to 30%) for calc-alkaline compositions. These
results have important implications for the ease of near-surface degassing of silicic magmas during emplacement and permit
the testing of calculational models for viscosity, largely based on synthetic systems.
Received: 7 July 1997 / Accepted: 6 April 1998 |
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Keywords: | Rhyolite Calc-alkaline Peralkaline Viscosity Water Activation energy |
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