HIGH-RESOLUTION IMAGING AND ANALYSIS OF LAMELLAR MICROSTRUCTURES IN UHP GARNET, ITALIAN ALPS

This presentation reports quantitative analytical data and compositional images (EDS and WDS X-ray intensity maps) on apparent compositional lamellar microstructures in pyrope-rich (94 mol% prp, 5 mol% alm, 1 mol% grs) garnet from an impure aluminous quartzite from the Case Ramello locality, Dora Maira Massif, Italian Alps. Thermodynamic modeling on this sample indicates maximum metamorphic P of 3.5 GPa at a peak T of 775 °C. As far as we are aware, there have been no previous reports of such lamellar microstructures in garnet, either from normal crustal rocks or from UHP rocks. Two sets of parallel lamellar structures have been observed in the garnet images, with an intersection angle of about 20 degrees between the two that produces a “moiré-like” interference effect. A quantitative analytical traverse across these microstructures shows that the amplitude of the compositional waves is about 0.6 mol% Alm or Prp and the average wavelength is 7.5 microns. The lamellar structures are not visible optically as with pigeonite lamellae in augites, but this may be because the compositional contrast is so slight. The analytical traverses for prp and alm are almost perfectly antithetical, while grs shows little or no variation (sps is very low, near detection limits). Given that we are on the edge of imaging and analytical resolution, there remains a possibility that we are seeing some kind of artifact, but the fact that the two essentially independent techniques are showing the same effect and that the effect is lamellar or wave-like rather than random gives us confidence that these compositional waves are real. It is currently unclear whether these compositional waves were established during crystal growth (thus perhaps revealing growth mechanisms) or during essentially isothermal decompression (thus recording a form of volume-induced proto-exsolution), but the magnitude of rapid pressure change during exhumation may argue for a post-growth origin during exhumation.