Crystal size distribution (CSD) of plagioclase and amphibole from
Soufriere
Hills volcano, Montserrat: Evidence for dynamic crystallisation /
textural
coarsening cycles
The Island of Montserrat from the Road before the Town.
1775-1776
Thomas Hearne. Whitworth art gallery, Manchester, UK
View of Soufriere Hills Volcano from Plymouth, March
1998.
The Crystal Size Distributions (CSD) of plagioclase and
amphibole
were
determined from andesites of the Soufriere Hills volcano, Montserrat.
Plagioclase
occurs as separate crystals and as chadocrysts in large amphibole
oikocrysts.
The chadocrysts represent an earlier stage of textural development
preserved
by growth of the oikocryst. Seventeen rock and eight chadocryst
plagioclase
CSDs are considered together as a series of samples of textural
development.
All are curved, concave up, and coincident, differing only in their
maximum
crystal size. Three amphibole CSDs have a similar shape and behavior,
but
at a different position from the plagioclase CSDs. A dynamic model is
proposed
for the origin of textures in these rocks. Crystallisation of
plagioclase
started following emplacement of andesite magma at a depth of at least
5 km. A steep, straight CSD developed by nucleation and growth. This
process
was interrupted by the injection of mafic magma into the chamber, or
convective
overturn of hotter magma. The magma temperature rose until it was
buffered,
initially by plagioclase solution and later by crystallisation. During
this period textural coarsening (Ostwald Ripening) of plagioclase and
amphibole
occurred: small crystals dissolved simultaneously with the growth of
large
crystals. The CSD became less steep and extended to larger crystal
sizes.
Early stages of this process are preserved in coarsened amphibole
oikocrysts.
Repetitions of this cycle generated the observed family of CSDs.
Textural
coarsening followed the ‘Communicating Neighbours’ model. Hence, each
crystal
has its own, unique growth/solution history, without appealing to
mixing
of magmas that crystallised in different environments.
Higgins, M.D. and Meilleur, D., 2009, Development and emplacement of the Inyo Domes Magmatic Suite,
California: evidence from geological, textural (CSD) and geochemical
observations of ash and lava. Journal of Volcanology and Geothermal Research 186: 280-292. doi:10.1016/j.jvolgeores.2009.07.004