PROGRESSIVE DEPROTONATION OF STAUROLITE DURING METAMORPHISM AND ANATEXIS: NANOSIMS, TEM AND EXPERIMENTAL CONSTRAINTS

We have analyzed by NanoSIMS the H content in staurolite coexisting with melt from four experimental run products. Three samples result from the melting of a graphitic metapelite at 785°C/5 kbar (sample BC4), 835°C/8 kbar (sample 854) and 893°C/8 kbar (sample 833). The staurolite from the starting material has an X_Fe of 0.83, and an H₂O in the range 1.43 – 1.80 wt.%, corresponding to 2.7 – 3.3 OH groups per 48 oxygens.

During the experiment the assemblages melt-Qtz-Grt-Sil-Ilm-Her-St, (BC4), melt-Qtz-Grt-Sil-Rt-Bt-Kfs-St (854) and melt-Qtz-Grt-Sil-Ilm-Crn-Her-St (833) are produced, and St is rimmed by hercynitic spinel in two of three runs. The staurolite is more magnesian (X_Fe respectively 0.81; 0.58-0.66 and 0.67-0.72) and contains respectively 4400, 1500 and 1300 ppm H₂O, with a 1s error of 200 ppm. These values correspond to 0.80, 0.28 and 0.24 OH groups per 48 oxygen atoms, respectively.

The fourth sample consists of fragments of garnet with nanogranite inclusions from the migmatites around the Ronda Peridotite (Spain), which were brought to 850°C and 15 kbar in order to remelt the polycrystalline inclusions in garnet. In the remelted inclusions, thin rims of staurolite form as overgrowths on kyanite solid inclusions. The staurolite, coexisting with a melt with c. 10 wt.% H₂O, has X_Fe = 0.57, and H₂O = 1.3 wt.%, corresponding to 2.35 OH groups per 48 oxygen atoms. Also this staurolite is richer in Al (>18.5 atoms) with respect to common compositions.

In order to verify that the low-OH staurolite is not an amorphous product of the thermal decomposition of precursor crystals from the starting material, three FIB foils were obtained from sample BC4 and examined by TEM. SAED patterns show intense and sharp reflections, and d-spacings consistent with orthorhombic-pseudo orthorhombic symmetry. Both diffraction and HR images confirm high crystallinity and crystal order.

These results suggest that, similarly to other hydroxylated minerals like biotite, staurolite undergoes a progressive T-dependent deprotonation, and that this process may reach completion with the occurrence of H-free terms at (U)HT suprasolidus conditions. The thermodynamic model for staurolite should be improved to account also for this T-dependent deprotonation, but more data are needed to understand which exchange vector(s) is responsible for it.