Experimental constraints on the storage conditions of phonolites from the Kerguelen Archipelago

Phase relations of a phonolite (K1) and a tephri-phonolite (K2) from the Upper Miocene lavas in the Southeast Province of the Kerguelen Archipelago have been investigated in the P/T range 100–500 MPa and 700–900 °C at two fO₂ conditions (~ NNO and ~ NNO+2.3) to clarify the differentiation and pre-eruptive conditions of these magmas. Crystallization experiments were performed in cold seal pressure vessels (CSPV) and internally heated pressure vessels (IHPV) at various X_H2O, under reducing (log fO₂ ~ NNO) and oxidizing conditions (log fO₂ ~ NNO+2.3). Under reducing conditions, the resulting phase assemblage for K1 was: titanomagnetite, nepheline, alkali feldspar, clinopyroxene and biotite; under oxidizing conditions, the assemblage was: magnetite, plagioclase, alkali feldspar, nepheline, titanite (minerals given in the order of appearance with decreasing T at 200 MPa for 4 wt% water in the melt). It is emphasized that an effect of fO₂ on the phase stability of feldspars and feldspathoides was observed. Comparison of the natural and experimental phase assemblages shows that the pre-eruptive conditions for K1 must have been in the log fO₂ range NNO+1–NNO+2, at pressures above 200–250 MPa. Assuming a temperature of 800 °C, the water content of the melt is constrained to be between 4 and 6 wt% H₂O. The pre-eruptive fO₂ conditions for the less evolved sample K2 are more oxidizing with log fO₂ close to NNO+2.3. The experimental results show that the enrichment of alkalis in residual melts during differentiation of tephri-phonolites is enhanced at high fO₂.Editorial responsibility: J. Hoefs