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| Titel |
Stoping of chamber margins into mafic intrusions: Magma mingling at Soufriere Hills volcano, Montserrat |
| VerfasserIn |
Kimberly Genareau, Amanda B. Clarke |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250037062
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| Zusammenfassung |
| In July 2003, the Soufrière Hills volcano (Montserrat, British West Indies) experienced the
largest dome collapse event within the history of the current eruption (1995–present). During
this event, there were two syn-collapse Vulcanian explosions that deposited tephra throughout
the island of Montserrat, and clasts from these fall deposits have been examined texturally
and chemically to deduce information concerning conduit dynamics during the dome
collapse and contemporaneous explosions. One clast, erupted from Soufrière Hills
volcano during the syn-collapse Vulcanian explosions of July 13, 2003 reveals several
incorporated silicic zones within a vesiculated, more mafic host. In hand sample,
this clast appears to show evidence of oxidation possibly resulting from extended
residence time within the extruding lava dome. However, closer examination of clast
microtextures using scanning electron microscopy (SEM) shows that, in some cases, these
characteristics may actually result from processes occurring at depth in the chamber or lower
conduit.
One particular silicic inclusion (zone A) shows evidence of heating in several
mineralogical features, including oxidized magnetites and pyroxenes, highly fractured quartz
and plagioclase phenocrysts, and completely pseudomorphed hornblende crystals that broke
down over a period of 37–78 days. The phase assemblage of zone A suggests that, prior to
heating, the inclusion initially crystallized within the chamber at a temperature low enough to
allow quartz precipitation, as there are approximately 15 quartz phenocrysts and
microphenocrysts present in an area roughly 6 mm in diameter. Secondary ion mass
spectrometry (SIMS) analyses of the plagioclase shows consistent compositions throughout
individual crystals, averaging 53 (± 4) mol% anorthite. Assuming an H2O pressure of 130
MPa, these anorthite values allow calculation of the magma temperature during
crystallization, and result in an average temperature of 833 (± 21) Ë C. This range safely
brackets the temperature required for quartz precipitation at equivalent pressures (less than
825 Ë C). Within the groundmass, no pyroxene microlites are visible, suggesting a
crystallization pressure of more than 100 MPa, based upon decompression experiments on
a representative magma. Additionally, plagioclase phenocrysts show no obvious
zoning in their anorthite compositions as would result from ascent through the
conduit. SEM X-ray element maps reveal percolation of calcium-rich melt into the
margins of zone A, suggesting sustained contact between this silicic inclusion and the
intruding mafic magma that resulted in isolation and heating of a stoped chamber
margin.
The characteristics of this single tephra sample suggest that magma mingling at
Soufrière Hills may not only result from disaggregation of mafic inclusions during
transport through the conduit, but also from stoping of the crystallized chamber
margins into the intruding mafic magma. The more mafic portions of the erupted
clast display high vesicularity, indicating that a volatile-rich portion of the mafic
intrusion (perhaps a foam layer at the mafic/resident magma interface) ascended
or convected into the chamber and incorporated portions of the chamber margins
37–78 days prior to the July 2003 dome collapse. Although initial examination
of the tephra in hand sample shows evidence of oxidation typically attributed to
sustained residence in the lava dome, in some cases this oxidation may actually
result from heating within the system prior to ascent through the conduit. Thus,
effects of mafic magma intrusion at Soufrière Hills may not only involve heating of
the resident magma, but also disaggregation and incorporation of stoped chamber
margins, indicating another possible method of magma mingling at this active volcano. |
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