Dept. of Earth and Environmental
Rensselaer Polytechnic Institute
Troy, NY 12180 USA
Dept. of Geology and Geophysics, University of
Madison, WI 53706 USA
A discontinuous change in garnet composition is predicted to occur when there is a discontinuous change in the amount of melt present in a rock, as will occur during vapor-absent melting of muscovite. The magnitude of the discontinuity is a function of the amount of muscovite that melts, and the partition coefficients of solids versus melt. Correlated discontinuous changes in the concentrations of Ca, Sc, Y, P and Cr in garnets from the Rangeley Formation, Fall Mountain, New Hampshire, are consistent with a vapor absent melting process. The magnitude of these discontinuities can be used to monitor the amount of melt produced, and to examine kinetics of melting and the participation of accessory phases in crustal anatexis.
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Figure 2 caption. X-ray composition maps of several garnets
from sample K92-12D from the Rangeley Formation, Fall Mountain,
New Hampshire. Scale bars show compositions in mole fractions,
molar ratio, or ppm as determined from spot analyses. Conditions
for major elements (Grs, Sps, Fe/(Fe+Mg)): width of field = 3.6
mm, 512x512 pixels, 7 µm/pixel, faraday cup current = 200
na, dwell time = 20 ms/pixel. Conditions for trace elements (P,
Y, Sc, Cr): width of field = 8.2 mm, 512x512 pixels, 16 µm/pixel,
faraday cup current = 2000 na, dwell time = 100 ms/pixel.