BLUESCHIST-GREENSCHIST FACIES TRANSITION MODELED BY THE REACTION GLAUCOPHANE + PARAGONITE + QUARTZ = VERMICULITE + ALBITE

Although the transition of the blueschist-greenschist facies has been proposed to be modeled by the reaction glaucophane (Gl) + paragonite (Pg) + water (V) = clinochlore (Chl) + albite (Ab) + quartz (Qtz) (1), we have found in the system Na₂O-MgO-Al₂O₃-SiO₂-H₂O that vermiculite (Vr) forms repeatedly and persistently. We have, therefore, investigated the reaction Gl + Pg + Qtz ± V = Vr + Ab (2), as a possible alternative reaction.

Reaction (2) was studied at P-T ranges of 600°C-800°C and 14-20 kbars in a ½-inch diameter piston-cylinder press. For reversal experiments water content was kept in the range of 0-4 wt%. Experiments included (a) a reversal mixture for reaction (1), (b) a mixture of synthetic Gl and Chl (c) a mixture of natural Gl ((Na_1.93Ca_0.07)(Mg_1.85Fe_1.14Fe³⁺_0.43Al_1.65)(Al_0.08Si_7.92)O₂₂(OH)₂) and natural Chl ((Mg_4.36Fe_0.48Al_1.15)(Al_1.17Si_2.85)O₁₀(OH)₈), (d) a reversal mixture for reaction (2), and (e) treatments of Vr (Na_0.50(Mg_1.50Al_1.00)(Al_0.50Si_3.50)O₁₀(OH)₂•nH₂O) as a single phase at various P-T conditions.

The location of reaction (1) could not be determined because of the consistent loss of Chl and the appearance of Vr at low pressures. Reaction (2) was found to lie at 600°C/15.5 ± 1.5 kbars; 700°C/17 ± 0.5 kbars; 800°C broadly bracketed between 17 and 20 kbars, but with partial melting at18-19 kbars. A strong reaction occurs between synthetic Gl and Chl and natural Gl and Chl yielding Vr. X-ray diffraction heating stage analysis was done to detect changes in the interlayer cation composition (Mg for Na) by looking for an increase in the d₀₀₁ reflection at 200°C (0% relative humidity). A possible decrease of Na content from 0.5 to 0.1 cations (increase in Mg cations) may occur with increasing temperature. A probable miscibility gap exists for the single Vr phase as evidenced by the nucleation of Pg and Qtz at temperatures below 800°C.

Vermiculite appears to be a stable high temperature metamorphic phase, as suggested earlier by Velde (1978, Cont. Min. Pet.), evidenced by its reversible growth and breakdown via reaction (2). It appears the Vr forms especially at the expense Gl and Chl.