Kameni volcano, Thera, Greece.
1) Chemical and textural diversity of
Kameni (Greece) dacites:
Role of vesiculation in juvenile and mature basal crystal masses
Dacite lavas erupted from Kameni Islands volcanic centre (Greece) during
the last 2000 years have a limited range in chemical composition (SiO2 =
64.0-68.5%) which contrasts with their wide range in plagioclase
abundance (3-22%) and crystal size distributions. Most plagioclase
crystals have simple zoning and occur independently or in loose clusters
with finer-grained cores. We propose that magmatic diversity was
produced by the interaction between crystals that formed at the base of
a magma reservoir and bubbles produced by injection and vesiculation of
more mafic magma. Two end-member situations can be identified: In
juvenile systems, the basal crystal mass is loosely connected and
readily disrupted by bubble formation. The crystal-bubble couples
accumulate at the top of the reservoir, from where they can enter the
sub-volcanic plumbing system to produce high-crystal content, chemically
unevolved magmas. In the mature system, the crystal mass is well
connected so bubbles displace the evolved, interstitial magma and
liberate only a smaller number of crystals from the crystal mass. This
process produces chemically evolved magmas, with lower crystal contents.
The oldest lavas seem to have been produced from mature systems, whereas
the youngest eruptions were of lavas produced from juvenile systems.
This progression may reflect an overall reduction in repose times during
the last two thousand years.
Higgins, M.D., Debecq, A., Vander Auwera,
J., Nomoikou, P., Chemical and textural diversity of Kameni (Greece)
dacites: role of vesiculation in juvenile and mature basal crystal
masses.
Contrib Mineral Petrol 176, 13 (2021).
https://doi.org/10.1007/s00410-020-01764-3
2) Magma dynamics beneath Kameni volcano, Thera, Greece,
as revealed by crystal size and shape measurements.
The eruption of 1866. The Kameni island lie at the centre of the
caldera. |
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Sizes distributions of plagioclase crystals in series of recent
porphyritic dacite lavas from Kameni volcano, Greece, can be modelled by
mixing two populations of crystals, each with overlapping linear crystal
size distributions (CSD) - termed microlites and megacrysts. The magmas
bearing the microlites and megacrysts started to crystallise 6-13 and
24-96 years respectively before each eruption. The dates of initiation of
crystallisation of the megacrysts indicates that they are left-overs of
earlier injections of new magma into a shallow chamber: Some magma remains
after each eruption and continues to crystallise. New magma with few or no
crystals is then introduced and the microlites crystallise from the mixed
magma. Eruption followed 6-13 years after mixing. Such a model would
suggest that some porphyritic magmas are products of a shallow magma
chamber that is never completely emptied, just topped up from time to
time.
Higgins, M.D., 1996, Magma dynamics beneath Kameni volcano, Greece, as
revealed by crystal size and shape measurements: Journal of Volcanology
and Geothermal Research, v. 70, p. 37-48. https://doi.org/10.1016/0377-0273(95)00045-3