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| Titel |
Origin and significance of the 2011 El Hierro xeno-pumice |
| VerfasserIn |
Kirsten Zaczek-Pedroza, Valentin R. Troll, Frances M. Deegan, Fiona C. Meade, Steffi Burchardt, Juan C. Carracedo, Andreas Klügel, Chris Harris, Sebastian Wiesmaier, Sylvia E. Berg, Abigail K. Barker |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250098690
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| Publikation (Nr.) |
 EGU/EGU2014-14391.pdf |
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| Zusammenfassung |
| During the first week of the 2011/2012 submarine eruption off El Hierro (Canary Islands),
peculiar light-coloured pumiceous rocks (xeno-pumice) were found floating on the sea.
The appearance of these rocks led to a potentially inappropriate response from
the authorities, because the rocks were viewed as likely indicators of high-silica
magma and possible explosive eruptive behaviour. However, the eruption remained a
relatively minor and dominantly effusive event and the origin and significance of
these peculiar xeno-pumice rocks for volcanic monitoring remains unresolved.
Three contrasting models have been put forward, describing them as: a) recycled
hydrothermally altered felsic magmatic rocks (Meletlidis et al., 2012, Geophys. Res.
Lett., 39:L17302), b) sediment-contaminated high-silica magma (Sigmarsson et al.,
2013, Contrib. Mineral. Petrol., 165:601-622) or c) frothy xenoliths from pre-island
sedimentary strata that were melted while immersed in magma (Troll et al., 2012,
Solid Earth, 3:97–110). Here, we combine the three available datasets to assess the
origin of El Hierro xeno-pumice in the light of texture, mineralogy, major and trace
element composition, and oxygen isotope characteristics in order to discuss their
significance.
We note that: 1) Sedimentary relicts occur frequently in xeno-pumice samples with
occasionally observed relict bedding. 2) Vesicle sizes are extremely variable, which
documents multiple degassing events. The vesicles are biggest especially close to
sedimentary relicts, likely the result of a complex mix of minerals and porewaters originally
present. 3) The mineral assemblage of xeno-pumice includes quartz, smectite, illite,
wollastonite, jasper and mica (XRD) and is akin to marine sedimentary rocks in the region
(Hoernle, 1998, J. Petrol.,39:859–880; Robertson & Stillman, 1979, J. Geol. Soc.,
136:47 –60; Aparicio et al., 2006, Geol. Mag. 143:181 –193). 4) CIPW norms
calculated from xeno-pumice major element compositions show the samples to
be Si-oversaturated, partly corundum-normative, but with not magmatic mineral
components in the norm. 5) Trace element concentrations of xeno-pumice are unlike any
magmatic rocks from El Hierro and La Palma, but similar to known sedimentary
rocks from the region. 6) A distinct Eu anomaly, typical for continentally derived
sediment, is present in xeno-pumice but absent in El Hierro and La Palma magmatic
rocks. Lastly, 7) Oxygen isotope values range from 9.1 to 11.6 o (n=6), which are
elevated with respect to magmatic rocks from the Canaries (5.2 to ~ 7 o), but are
characteristic for sediment derived from a mixture of continental (S- and I-type) granite
sources.
The combined datasets allow for a more thorough discussion on the origin of El
Hierro xeno-pumice and we conclude that xeno-pumice is compositionally akin to
sedimentary rocks from the region, but strikingly dissimilar to magmatic rocks
from El Hierro and La Palma. We suggest a dominantly sedimentary origin for
xeno-pumice. Xeno-pumice is therefore not an indicator for explosive felsic magma
within volcanic plumbing systems, but a reflection of magma-crust interaction during
ascent and emplacement. The occurrence of similar frothy sedimentary xenoliths
in recent and historical eruptions on Lanzarote, Gran Canaria, Tenerife and La
Palma make a case for xeno-pumice being a common phenomenon in the region. |
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