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Titel |
Magma mixing enhanced by bubble segregation |
VerfasserIn |
S. Wiesmaier, D. Morgavi, C. J. Renggli, D. Perugini, C. P. De Campos, K.-U. Hess, W. Ertel-Ingrisch, Y. Lavallée, D. B. Dingwell |
Medientyp |
Artikel
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Sprache |
Englisch
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ISSN |
1869-9510
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Digitales Dokument |
URL |
Erschienen |
In: Solid Earth ; 6, no. 3 ; Nr. 6, no. 3 (2015-08-21), S.1007-1023 |
Datensatznummer |
250115509
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Publikation (Nr.) |
copernicus.org/se-6-1007-2015.pdf |
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Zusammenfassung |
In order to explore the materials' complexity induced by bubbles rising
through mixing magmas, bubble-advection experiments have been performed,
employing natural silicate melts at magmatic temperatures. A cylinder of
basaltic glass was placed below a cylinder of rhyolitic glass. Upon melting,
bubbles formed from interstitial air. During the course of the experimental
runs, those bubbles rose via buoyancy forces into the rhyolitic melt, thereby
entraining tails of basaltic liquid. In the experimental run products, these
plume-like filaments of advected basalt within rhyolite were clearly visible
and were characterised by microCT and high-resolution EMP analyses.
The entrained filaments of mafic material have been hybridised. Their
post-experimental compositions range from the originally basaltic
composition through andesitic to rhyolitic composition. Rheological
modelling of the compositions of these hybridised filaments yield
viscosities up to 2 orders of magnitude lower than that of the host
rhyolitic liquid. Importantly, such lowered viscosities inside the filaments
implies that rising bubbles can ascend more efficiently through pre-existing
filaments that have been generated by earlier ascending bubbles. MicroCT
imaging of the run products provides textural confirmation of the phenomenon
of bubbles trailing one another through filaments. This phenomenon enhances
the relevance of bubble advection in magma mixing scenarios, implying as it
does so, an acceleration of bubble ascent due to the decreased viscous
resistance facing bubbles inside filaments and yielding enhanced mass flux
of mafic melt into felsic melt via entrainment. In magma mixing events
involving melts of high volatile content, bubbles may be an essential
catalyst for magma mixing.
Moreover, the reduced viscosity contrast within filaments implies repeated
replenishment of filaments with fresh end-member melt. As a result, complex
compositional gradients and therefore diffusion systematics can be expected
at the filament–host melt interface, due to the repetitive nature of the
process. However, previously magmatic filaments were tacitly assumed to be
of single-pulse origin. Consequently, the potential for multi-pulse
filaments has to be considered in outcrop analyses. As compositional
profiles alone may remain ambiguous for constraining the origin of
filaments, and as 3-D visual evidence demonstrates that filaments may have
experienced multiple bubbles passages even when featuring standard diffusion
gradients, therefore, the calculation of diffusive timescales may be
inadequate for constraining timescales in cases where bubbles have played an
essential role in magma mixing. Data analysis employing concentration
variance relaxation in natural samples can distinguish conventional
single-pulse filaments from advection via multiple bubble ascent advection
in natural samples, raising the prospect of yet another powerful application
of this novel petrological tool. |
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