DISSOLUTION-REPRECIPITATION REACTIONS IN ALKALI FELDSPARS AND SUBSEQUENT COHERENT EXSOLUTION

Paul Ribbe made many seminal contributions to feldspar mineralogy, particularly at the level of crystal structure and microtextures. This contribution is concerned with complex and irregular intracrystal microtextures produced by dissolution-reprecipitation. We have used SEM and CL to investigate the way strain-controlled microperthites coarsen during deuteric alteration of alkali feldspars from the 1166 Ma Klokken intrusion, S Greenland. Cumulate alkali feldspars ~2 cm long, bulk composition ~Ab60Or40, have retained their igneous crystal shapes, flattened in (010), without modification. Coherent exsolution started at ~700 °C leading to strain-controlled braid crypto- and micro-perthites which at <450 °C were coarsened by up to 1000x to patch perthites, by interaction with deuteric fluids. The patch perthites are incoherent, unstrained, micropore-rich intergrowths of <1->100 μm subgrains of albite and irregular microcline. Coarsening was driven by coherency strain energy.

Imaging of the patch perthites by SEM CL shows that individual subgrains have both oscillatory and sector zoning. Spatial variations in both the intensity and wavelength of luminescence record the position of advancing faces on subgrains during changes in the concentration of CL activators in the fluid. Most zoning is in albite, but adjacent subgrains of albite and K-feldspar may display the same zoning pattern implying that they were growing synchronously. Individual zones can be traced for 100s of μm along subgrain faces. Fluid films were laterally extensive and rates of solute movement over mm-scales were considerably greater than rates of crystal growth. The zoned subgrains have no optical expression but sometimes make up entire cm-scale crystals, striking evidence for the pervasive character of dissolution-reprecipitation reactions.

After the deuteric reactions had ceased there was a reversion to diffusion-controlled exsolution. Some K-feldspar subgrains contain straight, semicoherent albite exsolution lamellae decorated with dislocations. From the bulk composition of the subgrains this occurred at <~350 °C, implying that the deuteric fluids had ceased to play a role below this T.