Аннотация:The thermal histories of magma bodies are important to
evaluate the potential hazards of a given volcano and the
processes that build up the continental crust. Current methods
of estimating the time scales of magma storage and growth
are mainly based on zircon geochronology and chemical reequilibration of phenocrysts. Here we explore the use of
elemental exchange between touching pairs of clinopyroxene
(Cpx) and orthopyroxene (Opx), plus the formation of
exsolution lamellae as an additional means to extract
temporal and temperature information. Recent progress and
improved availability of nanoscale characterization using
NanoSIMS, Analytical Transmission Electron Microscopy
(ATEM) and Focused Ion Beam (FIB) sample preparation
enables to achieve quantitative analysis of small features and
short compositional profile (in order of a nanometer) of a
range of elements and concentration levels.
We have collected FIB cross sections and ATEM across
several Cpx-Opx pairs, and across exsolution lamellae of Cpx
of a range of volcanic rocks. The FIB sections allowed to
correct for the slope across Cpx-Opx interface which can
significantly deviate from the preferred 90°. We obtained
chemical profiles with the SEM and ATEM of the FIB foils.
SEM profiles gave a spatial resolution of 0.3μm and good
analytical precision of around 0.5% relative. ATEM allow
much better spatial resolution (0.3 nm), but analytical
precision decreased to about 5 % relative. We constrained
pyroxene crystallization temeparture and modeled the profiles
(mainly Fe and Mg exchange) using Cpx-Opx-Pigeonite
thermodynamic equilibria (QUILF) and a forward model for a
range of cooling histories and times. We found that the
ATEM data are too noisy to obtain precise time constraints
(varying between a few days and a few year of residence),
whereas the SEM data suggest maximum times of about 10
years.