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| Titel |
Fe-Mg diffusion chronometry in orthopyroxene from the Minoan eruption of Santorini, Greece |
| VerfasserIn |
Taya Flaherty, Tim Druitt, Gareth Fabbro, Fidel Costa, Katie Preece, Chad Deering |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250130660
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| Publikation (Nr.) |
 EGU/EGU2016-10949.pdf |
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| Zusammenfassung |
| Constraining the timescales governing magma ascent and storage prior to ignimbrite
eruptions is crucial for understanding the behaviour of caldera volcanoes. Diffusion
chronometry provides estimates of the pre-eruptive residence times of crystals at magmatic
temperatures, and hence of the longevities of bodies of crystal-bearing magma that are finally
discharged. We have used Fe-Mg diffusion chronometry in orthopyroxene (opx) crystals from
the Minoan eruption of Santorini in order to calculate the pre-eruptive residence times of
these crystals. The Minoan eruption occurred in the late 17th century BCE, and discharged
30-80 km3 of rhyodacitic magma containing about 10 vol.% of plagioclase, opx, cpx and
Fe-Ti oxides. The orthopyroxenes have compositions of Wo2−3En52−70Fs28−45 (#Mg =
0.53-0.65) with Al2O3 contents typically <0.5 wt.%, and include normally zoned,
reversely zoned and unzoned types. Compositional images reveal sector zoned
morphologies with Al-rich prismatic zones and Al-poor terminations, possibly
indicative of rapid growth. Representative opx crystals were extracted from pumices and
mounted in epoxy grain mounts. High-resolution backscattered electron images
of zoned crystals with greyscale values calibrated for Mg# were used to identify
Fe-Mg gradients across zone boundaries, which were then modelled as diffusion
gradients using published diffusion coefficients for Mg-Fe interdiffusion within the
a-b plane of opx, a magmatic temperature (855 ± 25 ˚ C) and fO2 determined
from touching magnetite-ilmenite pairs in the same rock. Our models assumed
initial step functions in Mg and Fe concentrations, and that any non-zero width is a
result of diffusion. The time required for to reach the observed width of diffusion at
855˚ C was taken to be a maximum residence time, and was calculated for a total
of 22 zone boundaries from 13 crystals. Profiles were taken perpendicular to the
crystal length, within the a-b crystallographic plane. Only zone boundaries with
bounding plateaus in Mg and Fe concentrations (implicit in the diffusion model) were
used.
Resulting timescales ranged from 28 years to <0.1 y, with 70 % less than 1 y. No
significant differences of timescale spectra from the four eruptive units were observed. There
is some suggestion in the data that timescales from individual crystals are highest near the
centre and decrease towards the rim, which would be consistent with crystal growth on the
timescales concerned. Placed in the context of published melt inclusion barometry,
Mg-in-plagioclase diffusion chronometry and phase equilibria, we infer that the opx crystals
grew during the ascent of batches of volatile-saturated rhyodacitic melt from the
middle crust, and their subsequent injection into the shallow pre-eruptive magma
chamber. The opx timescales are in excellent agreement with those obtained previously
using Mg diffusion in plagioclase, reinforcing the conclusion that final assembly of
the shallow magma body took place only years to months prior to the eruption. |
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