Images on this page were acquired using the UMass Dept. of Geosciences'
Cameca SX-50 electron microprobe.
Image 7:
Local Fe-Mg reequilibration between garnet and biotite
This a Mg map of a garnet bearing schist from the Upper Granite Gorge of the Grand
Canyon (mile 86.7). The assemblage includes garnet, biotite, quartz, plagioclase,
hornblende, and ilmentite, and peak conditions were near 600 C and 6 kbar. The
map shows the extent of Mg-Fe retrograde exchange between biotite and garnet.
Note that Mg decreases on all margins of the garnet (blue) and increases in biotite
(green) only adjacent to the garnet (see red areas). Some reequilibration has
also occurred on narrow fractures and cleavage surfaces in biotite away from garnet.
Selection of compositions for quantitative analysis is straightforward. Near peak
biotite compositions can be obtained with a minimum of analyses away from the
zone of reequilibration. Near peak garnet compositions occur just inside of the
low-Mg rim. Area (volume) calculations of the amount of net transfer of Mg from
garnet to biotite can be made with analyses from within the zone of reequilibration.
In this case, biotite and garnet seem to show near closed system behavior.
For more information about this image contact: Michael
L. Williams