IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

4A2_1D-O8

Room B1

Date/Time: July 24 10:45-11:00

Exsolution lamellae in volcanic pyroxenes as an indicator of cooling times and sizes ofmagma reservoirs
Fidel Costa1 , Jason S Herrin2
1

Earth Observatory of Singapore, Nanyang Technological University, Singapore, 2 Facility for Analysis,
Characterisation, Testing, and Simulation, Nanyang Technological University, Singapore
E-mail: fcosta@ntu.edu.sg

Magma residence times below active volcanoes are crucial for understanding the processes of magma evolution
and for proper assessment of volcano related hazards. Major progress has been made in the last decades by
a combined approach of modeling zoning patterns of crystals, bulk-rock and single mineral analyses of U-Th
disequilibria isotope series, and in-situ determination of crystallization ages of zircon. Here we explore anotherapproach that can potentially lead to new insights into the, sizes, residence times and internal processes of magma
bodies that is based on the existence of solvus relations in several mineral groups. Clino- and orthopyroxenes forexample may start crystallizing at high magmatic temperature, and new compositions will grow with decreasing thetemperature. But once the system intercepts the solvus, early-formed pyroxenes will tend reequilibrate throughformation of exsolution lamellae.

The size, spacing, and composition of the lamellae depend on the initial

temperature, cooling rate, and bulk composition. This means that the characteristics of exsolution lamellae arepotentially a record of the cooling history which can be related to the size, growth styles (single batch vs. pulses),and cooling rates of the magma. This type of information has been exploited in several studies of plutonic bodies.
However, exsolution lamellae have been rarely documented in volcanic pyroxenes (except for plutonic xenocrysts),
probably because of faster cooling rates and shorter magma residence times. Using a new Field Emission Gun
Electron Microprobe (EM), and electron backscattered diffraction we have found micro to submicron exsolution
lamellae subparallel to the [001] plane (e.g., high-T exsolution) on clinopyroxene from dacites and andesites fromvolcanoes in Chile and Indonesia. The size of the lamellae is too small to fully characterize with the EM so we haveused the Focused Ion Beam technique to extract cross-sectional slices of lamellae in clinopyroxenes perpendicular
to the 001 plane. We obtained element maps and traverses using energy dispersive spectrometry and electronenergy loss spectroscopy with an Analytical Transmission Electron Microscope. We found that the width of thelamellae vary from about 50 nm to a few hundred nm, and thus near the resolution of backscattered electron maps
produced by the EM. Using experimentally calibrated growth rates of exsolution lamellae in clinopyroxene, andsimplified cooling models of magma reservoirs of various sizes, we find that the observed lamellae require timescales of formation of a few thousand years from a reservoir of a few to tens of km3 . Given the small sizes ofthe lamellae that we observed, it seems likely that they have been overlooked in other volcanic systems, but theinformation they could provide is difficult to obtain by other approaches and should lead to better understanding of
the growth rates and sizes of subvolcanic magma reservoirs.

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