Experimental constraints on the stability of Mg-rich tourmaline and the boron concentration of hydrothermal fluids

Assemblages of natural tourmaline and cordierite were equilibrated experimentally in the presence of albite, andalusite, quartz, and H 2 O–B 2 O 3 solutions to constrain the boron concentration of hydrothermal fluids. The experiments were carried out at constant pressure (200 MPa) and at 400, 500, and 600 °C under fO 2 -controlled conditions ranging from NNO + 0.8 to NNO + 5.3 and for durations from 2 to 8 weeks. Both forward and reversal experiments, starting from tourmaline-free and tourmaline-bearing assemblages respectively, were performed. Under moderately oxidizing conditions (NNO + 0.8 to NNO + 2.64), cordierite plus albite and andalusite react to tourmaline plus quartz for [B 2 O 3 ] fluid >3.4 wt % at 600 °C and for [B 2 O 3 ] fluid >2.1 wt % at 500 °C. At 400 °C, tourmaline appears for [B 2 O 3 ] fluid >2.0 wt %, but smectite replaces cordierite in the reacting assemblage. Experiments under strongly oxidizing conditions (NNO + 2.7 to NNO + 5.3) show that the [B 2 O 3 ] fluid necessary for tourmaline to appear does not vary significantly with f O 2 . Our determinations of [B 2 O 3 ] fluid at equilibrium with tourmaline and cordierite are broadly consistent with previous experimental tourmalinization studies, but the comparison also highlights the potential effect of other variables such as the fluid chemistry (pH and NaCl concentration) on tourmaline stability. The experimental results have been complemented by thermodynamic calculations of cordierite–tourmaline equilibria using end-member tourmaline compositions and simple solid-solution models. The calculated equilibrium [B 2 O 3 ] fluid values are in broad agreement with the experimentally determined brackets and suggest a complex effect of solid solution between dravite and schorl end-members. The low boron concentrations required to form tourmaline from cordierite are also consistent with compositional constraints from natural hydrothermal fluids and peraluminous melts.