Ultrasteep composition gradients within ankerite grains from regionally metamorphosed marls

Ankerite (Ank) grains frequently contain cores of almost pure dolomite (Dol) in the garnet zone and at lower grades of three areally extensive stratigraphic units from both Buchan and Barrovian terrains in northern New England, USA. Dol cores and Ank rims have Fe/(Fe+Mg) in the range 0.006-0.011 and 0.16-0.23, respectively. Ank grains with dolomite cores have many features closely analogous to K-feldspar rims around albite cores produced experimentally by incomplete hydrothermal reaction between albite and KCl solution [1]: (a) Ank-Dol contacts are irregular but sharp at the micron scale, (b) The orientations of the Ank and Dol crystal lattices are identical, (c) Fe/(Fe+Mg) of Ank is often slightly greater at the Ank-Dol contact than further away, (d) The Ank-Dol contact is often decorated with open pores. Based on these similarities, Ank appears to have replaced detrital Dol by a dissolution-reprecipitation mechanism during diagnesis or very low grade regional metamorphism.