Search

Quantification of discrete pressure–temperature domains in deformed chlorite + white mica-bearing metapelites was undertaken on mineral compositions derived by two-dimensional microprobe compositional mapping of selected areas of rock thin sections. In order to achieve compositional information at sufficient analytical precision, spatial resolution and sample coverage within a typical analysis time of 1 day, an optimization of measurement methods was necessary. The method presented here allows collection of raw counts for eight different element concentrations at an analytical precision of ∼1–2 wt%. X-ray intensity multiplane maps (one map per measured chemical element) are translated into concentration multiplane maps, utilizing selected conventionally measured spot analyses combined with the Castaing approximation for each mineral. As this step requires identification of the different minerals present in the mapped area, a statistical clustering technique to identify different groups of composition was developed, guided by simple petrographic inspection of the thin section, to delineate the important minerals in the mapped area. Finally, the compositions of each pixel are translated into a mineral structural formula thus yielding a new kind of image with a high content of petrological information. The reliability of the mineral composition images was emphasized by carrying out precision tests on the analytical data. The possible use of chemical maps to infer the P– T–deformation history of metamorphic rocks is illustrated with two samples from the Spitzbergen and the Sambagawa blueschist facies belts. In both samples, a strong correlation between structures and chemistry is observed. Qualitative estimates of P– T conditions from the Si-content of mica and chlorite are in good agreement with their location in microstructures that formed at different times. Therefore, the combination of chemical maps with microstructural observations is a very powerful approach to understand both the evolution of complex metamorphic rocks and the control by deformation of mineral reactivity. [ABSTRACT FROM AUTHOR]

Quantitative X-ray maps of composition from a chlorite, K-white mica, albite, quartz and garnet bearing thin section from a Sambagawa blueschist facies metapelite were combined with a multi-equilibrium calculation method to calculate a P– T-Fe3+/Fe2+-deformation map at the millimetre scale. The studied sample was chosen because elongated chlorite crystallization tails (pressure shadows) rimmed by phengite are present, which is an appropriate assemblage for the quantification of the P– T evolution. Chlorite temperature and Fe3+ content maps were calculated by successive iterations for each pixel analysis of Fe3+ until convergence of the four chlorite-quartz-H2O equilibria that can be written using the Fe- and Mg-amesite, clinchlore, daphnite and sudoite chlorite end-members. The calculated map of Fe2+/Fe3+ in chlorite is in good qualitative agreement with the in situ mapping of this ratio using XANES (X-ray absorption near edge structure) techniques. The temperature map indicates that high temperature chlorite zones with low Fe3+ contents alternate with lower temperature zones and higher Fe3+ contents in the crystallization tail. Late fractures perpendicular to the elongation axis of the tail are filled by very low temperature chlorite (<250 °C) showing Fe3+/Fetotal up to 0.4. Groups of chlorite and mica pixels were then identified based on compositional and structural criteria, and a P– T-deformation map was calculated using representative analyses of these groups. The calculated P– T-deformation map suggests that in contrast to chlorite, the composition of most mica did not change significantly during exhumation. Mica reequilibrated in late EW shear bands only. EW shearing was already active at 0.1 GPa, 500 °C, which corresponds to the peak temperature (and probably pressure) conditions, at reduced redox conditions. The intensity of deformation probably decreased with decrease in temperature to ∼350–400 °C. At this temperature, a second main deformation event corresponding to a further EW stretching occurred and was still active below 250 °C and more oxidizing conditions. These results indicate that the scale at which P– T data can be obtained is now close to the scale of observation of structural geologists. A close link between deformation and mineral reaction is therefore possible at the microscopic scale, which provides information about the relationship between deformation and mineral reactivity, the modalities of deformation with time and the P– T conditions at which it occurred. [ABSTRACT FROM AUTHOR]