SPECIATION REACTIONS OF ACIDIC BORON, PHOSPHORUS, FLUORINE, AND OF HYDROGEN IN GRANITIC MELT AS DEDUCED FROM COUPLED DIFFUSION EXPERIMENTS

Interactions between aqueous solutions of H₃BO₃, H₃PO₄, or HF and haplogranitic glass (Ab₃₉Or₂₈Qz₃₃) were measured at 800°C, 200 MPa by diffusion of the acid solution into anhydrous glass cylinders. In all cases, the diffusion profile for H advanced farther and hence faster into the glass, such that the starting glasses were fully hydrated at the margins of the diffusion profiles for B, P, or F.

Phosphorus: The diffusion of P is accompanied by an equimolar decrease in the Si content of melt, which was accomplished by the precipitation of quartz along the P diffusion path. In addition, the solubility of H increases (over that of the P-absent melt at the same P-T conditions) by 2 H per 1 P. Aluminum and Na diffuse toward P in a proportion that results in a small net decrease in ASI (= mol Al₂O₃/[Na₂O + K₂O]); K does not migrate, so the melt beyond the P diffusion aureole becomes peraluminous and potassic.

Boron. The diffusion of B is accompanied by an increase of 1 H per 1 B (over that of the B-absent melt at the same P-T conditions). There are no changes in the concentration profiles of the melt components other than dilution, nor any changes in elemental ratios (e.g., ASI, K* = mol K/[Na + K]).

Fluorine. The diffusion of F is accompanied by an increase of 1 H per 1 F (over that of the F-absent melt at the same P-T conditions). Changes in ASI and K* along diffusion profiles result from the advance of H into the glass, and do not correlate in any way with F.

Discussion. Phosphorus displaces Si from the T sites in the melt structure, and increases the solubility of H. Migration of Na and K, where it occurs, is coupled and takes place over the entire volume of melt. In comparison to other studies, however, the addition of B, P, and F as acids, rather than as neutral components of the glass, results in a greater affinity of each added ligand of B, P, and F for H. In the most simplified fashion, therefore, the melt speciation reaction M⁺A (A = anions of B, P, or F) + H⁺ = H⁺A + M⁺ goes to the right as the activity of H⁺ in melt increases. In addition, the diffusivity of H increases dramatically with decreasing pH from 5 (neutral) to 2 (25 wt% HF) of added solution. Upon diffusion of H₂¹⁸O into glass, H and ¹⁸O are decoupled, and H diffuses well beyond the ¹⁸O aureole. These results signify that H diffuses through melt as protons, H⁺, and not as molecular (H₂O) or radical (OH^-, H₃O⁺) species.