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Bubble formation during decompression of andesitic melts

  • Adrian Fiege EMAIL logo , François Holtz and Sarah B. Cichy
Published/Copyright: May 15, 2014
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American Mineralogist
From the journal American Mineralogist Volume 99 Issue 5-6

Abstract

Bubble formation during continuous decompression from ~400 to ~70 MPa was investigated experimentally in hydrous andesitic melts at T = 1030 °C and at an oxygen fugacity (fO2) of about log(fO2/bar) = QFM+1 (QFM: quartz-fayalite-magnetite buffer). Experiments were carried out at variable decompression rates (r), ranging from 0.0005 to 0.1 MPa/s. The samples were directly quenched after decompression, allowing the investigation of the influence of r on the bubble formation. The effect of variable annealing times (tA) after decompression was also investigated for experiments performed at a decompression rate of 0.1 MPa/s. These samples were annealed for tA = 0 to 72 h at final pressure (70 MPa) to study changes in vesiculation during magma storage at shallow depths after fast ascent. Backscattered electron (BSE) images of the samples were analyzed to determine bubble number densities (BND).

The BND values increase strongly with increasing r and vary from about 102.2 mm-3 at 0.0005 MPa/s to about 104.6 mm-3 at 0.1 MPa/s. After fast decompression (r ~ 0.1 MPa/s), the BND decrease significantly with tA, i.e., from ~104.6 mm-3 at tA = 0 h to ~102.9 mm-3 at tA = 72 h. A comparison of the derived BND values with recently published experimental data demonstrates the essential role of the decompression path on bubble formation. The BND are higher in experiments with multi- or single-step decompression when compared to continuous decompression. The new data show that H2O-undersaturated andesitic melts are characterized by 1 to 2 log units higher BND values than H2O saturated rhyolitic melts after decompression with the same rate, indicating a strong influence of melt composition on bubble nucleation. This compositional effect is not predicted accurately by existing models and the interpretation of the vesicularity of dacitic to andesitic melts may lead to overestimations of magma ascent rates by about an order of magnitude.

Keywords : Bubble formation; bubble number density; andesite; continuous decompression
Published Online: 2014-5-15
Published in Print: 2014-5-1

© 2014 by Walter de Gruyter Berlin/Boston

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https://doi.org/10.2138/am.2014.4719
Keywords for this article
Bubble formation; bubble number density; andesite; continuous decompression

Articles in the same Issue

  1. Highlights and Breakthroughs
  2. Absence of pressure-induced electron spin-state transition of iron in silicate glasses upon compression
  3. Dolomite discloses a hidden history of subducting slabs
  4. Crossroads in Earth and Planetary Materials
  5. Crystal structures of laihunite and intermediate phases between laihunite-1M and fayalite: Z-contrast imaging and ab initio study
  6. Minerals in the Human Body
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  8. Volcanic Rocks
  9. The cooling kinetics of plagioclase feldspar as revealed by electron-microprobe mapping
  10. Spinels Renaissance–Past, Present, and Future
  11. High-pressure behavior of thiospinel CuCr2S4
  12. Martian Rocks and Soil
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  14. A temperature-controlled sample stage for in situ micro-X-ray diffraction: Application to Mars analog mirabilite-bearing perennial cold spring precipitate mineralogy
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