Tracking mineral replacement in metamorphic rocks using optical and chemical fingerprints of pre-existing minerals: an example of retrograde Al2 SiO5 transformation

The study of an intensely hydrated, retrograde metapelite revealed unusual reaction textures involving the replacement of sillimanite by andalusite. Shapes of sillimanite crystals are preserved within andalusite as domains of increased birefringence. Where sillimanite relics are preserved, they are continuous with this pseudomorphic texture in andalusite. The sillimanite “ghosts” can also be observed in the CL microscope due their different brightness. Microprobe analyses show that Fe contents control the difference in optical parameters between the optical pseudomorphs and the bulk of the andalusite, Fe being distinctly higher in the latter. The low Fe contents of the sillimanite ghosts were obviously inherited from the pre-existing sillimanite. The chemical signature of sillimanite preserved after polymorphic transformation further involves Mg and possibly Ti, showing a distribution similar to Fe, albeit at much lower concentrations. All these elements are low in both sillimanite and andalusite pseudomorphs after sillimanite, but have higher concentrations in the adjacent, non-pseudomorphic andalusite. Thus, the optical pseudomorphs are essentially a chemical copy of pre-existing sillimanite. As this example shows, chemical fingerprints of pre-existing minerals may be preserved in newly grown ones, providing direct evidence for mineral growth sequences and corresponding metamorphic reactions.