THE ENIGMA OF ANKERITE WITH DOLOMITE CORES IN REGIONALLY METAMORPHOSED CLASTIC SEDIMENTS FROM NORTHERN NEW ENGLAND, USA

Ankerite grains with a dolomite core occur in marls, pelites, and psammites from a Buchan terrain in Maine and a Barrovian terrain in Vermont. Dolomite cores are typically <20 μm in diameter, have sharp but irregular contacts with ankerite, and have the same crystallographic orientation as ankerite. Dolomite cores are common in the chlorite zone, present but less abundant in the biotite and garnet zones, and rare at higher grades. The texture and crystallographic orientation of dolomite and ankerite are consistent with partial replacement of detrital dolomite by ankerite by solution-reprecipitation. Fe-Mg exchange equilibrium between biotite and ankerite (but not dolomite) implies that ankerite formed before biotite (biotite has no phlogopite cores). A possible mechanism is bacterial iron reduction during diagenesis. Dolomite cores were destroyed during metamorphism as ankerites along with their dolomite cores recrystallized and were consumed by biotite-forming reactions.

The sharp ankerite-dolomite contact constrains time scales of metamorphic process. Analysis of Fe/Mg across the contact with the Cameca NanoSIMS 50L at Caltech and the Phillips CM300 FEG TEM at Johns Hopkins gives a thickness of transition <~1 μm and <~0.5 μm, respectively. Analysis of BSE gray scale contrast with the FEI CrossBeam FEG SEM/FIB at the University of Wisconsin gives a thickness ~100 nm. Fit of the gray scale profile to a model of 1-D diffusion across an infinite plane gives Dt = 10^-15 m² (± a factor of 5), where D is the Fe-Mg interdiffusion coefficient and t is the duration of diffusion. Using the experimental determination of D (Müller et al., GCA, 2012), upper bounds on residence time of dolomite cores at peak T = 400-500°C, on duration of linear cooling from peak T to 100°C, and on duration of linear heating from 100°C to peak T followed by linear cooling to 100°C are all <1 year. For linear heating and cooling lasting 10⁶ years, peak T could not have been >100°C. The enigma is how the ultrasteep chemical gradient between dolomite and ankerite could be preserved during regional metamorphism. If the measurement of D is in error, it would have to err by >6 orders of magnitude. No barrier to diffusion at the dolomite-ankerite contact was observed in TEM images. The steep gradient possibly was preserved by P gradients (Tajcmanova et al., JMG, 2014).