Trace element zoning in garnet as a monitor of dehydration melting in pelites


Frank S. Spear

Dept. of Earth and Environmental Sciences
Rensselaer Polytechnic Institute
Troy, NY 12180 USA



Matthew J. Kohn

Dept. of Geology and Geophysics, University of Wisconsin
Madison, WI 53706 USA



 

Abstract

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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Color X-ray maps - Figure 2

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.


Grossular map






Spessartine map






Fe/(Fe+Mg) map






Phosphorus map






Yttrium map






Scandium map






Chromium map