FLUID INFILTRATION AND THE PRESERVATION OF COESITE

The survival of coesite in UHP rocks has been linked most commonly to rapid exhumation and inclusion in strong phases, such as garnet, which can sustain a high internal over-pressure during decompression. The clearest case for rapid exhumation is made for the Dora Maira massif, where rates on the order of 2-3 cm/year have been constrained. Extrapolation of experimental kinetic data indicates, however, that even such high rates may be insufficient to allow the preservation of coesite. Moreover, the case for such rapid exhumation in other UHP terranes is not yet clear. Furthermore, rare occurrences of intergranular coesite as well as inclusion in relatively weak phases such as dolomite and epidote indicate that other factors must also play a role.

 

In this study we address the role of fluid infiltration in the preservation of coesite by employing FTIR spectroscopy and cathodoluminescence (CL) microscopy to examine coesite and associated phases in UHP-metamorphic rocks from the Dora Maira, the Dabie Shan, and Western Norway, and from a kimberlite (Roberts Victor Mine). In all cases, OH concentrations in coesite are below the detection limit (~1 ppm H₂O), consistent with experimental investigations that show that OH solubility in coesite is low (~10-20 ppm H₂O) at the peak UHP pressures. The silica phases surrounding the coesite, however, show varying amounts of H₂O. This is most spectacularly seen in rocks from the Dora Maira that contain at least three phases of silica replacing coesite: "palisade"-textured quartz (1-100 ppm H₂O, red-violet CL), "mosaic quartz", which may be related to chalcedony (up to 3000 ppm H₂O, yellow/brown CL), and opal (~7 wt% H₂O, dark blue CL). The quartz in other UHP samples typically contains 100-500 ppm H₂O, with rare occurrences of wetter quartz comparable to the mosaic quartz found in the Dora Maira.

 

We infer that palisade quartz forms under nearly dry conditions, at modest to high temperatures as a result of dilation of the host phase. The formation of hydrous silica phases such as chalcedony and opal, however, must take place at much lower temperatures, as a result of fluid infiltration following cracking of the host phase. Delay of fluid infiltration to low temperatures, where kinetics may be slow even in the presence of water, may thus be one of the most critical factors in the preservation of coesite.