LOW-PRESSURE CRYSTALLIZATION PATHS OF H2O-SATURATED BASALTIC-HAWAIITIC MELTS FROM MT ETNA - IMPLICATIONS FOR OPEN-SYSTEM DEGASSING OF BASALTIC VOLCANOS

Melt inclusions indicate that the basaltic-hawaiitic magmas from Mt Et na (Sicily) contained up to 2.3 wt% H2O dissolved in the melt, prior t o eruption. The effect of H2O degassing during magma ascent has been e xperimentally constrained between 1135 and 1009 degrees C, for P-H2O = P-total varying from 800 to 270 bars. The starting material was a pri mitive hawaiitic lava sample (MgO = 7.1 wt%) representative of the les s evolved lava emitted at Mt Etna. Experiments were conducted in TZM p ressure vessels, with Ag70Pd30 capsules in order to minimize the FeO l oss. At temperatures of 1135-1090 degrees C, P-H2O = 800 bars, with Ni NiO and FMQ buffers, olivine (Fo(83-80)) is the liquidus phase in equi librium with a residual hawaiitic melt (Mg# 0.60-0.57; CaO/Al2O3 = 0.8 2). Salitic pyroxene begins to crystallize at 1075 degrees C, plagiocl ase at 1025 degrees C and at 1009 degrees C, the magma is 33.5% crysta llized with olivine, Ca-rich pyroxene, and plagioclase (16:60:24). At P-H2O = 270 bars, FMQ buffer, olivine (Fo(79.3-80)) and salitic pyroxe ne are the main liquidus phases between 1100 and 1090 degrees C. They are in equilibrium with hawaiitic melts (Mg# 0.51; CaO/Al2O3 = 0.73). At 1070 degrees C, the experimental charges are highly crystallized (a pproximate to 51.5%) with olivine (Fo(70)) salite, and plagioclase (An (78.4-76.3)) in 14:44:42 relative proportions. Increasing the water co ntent of hawaiitic-basaltic magmas expands the stability field of the olivine (relative to the other phases), lowers the crystallization tem peratures of pyroxene and plagioclase, and results in the development of a more An-rich plagioclase. When compared to the natural samples, t hese results support a model of early and moderate crystallization of olivine from Etnean primitive hawaiitic magmas, containing close to 2. 5 wt% H2O, with NNO oxygen buffering conditions, at low pressure (P-H2 O = P-total) We propose a model of crystallization driven by decompres sion and water degassing during the emplacement of magma in the volcan ic pile itself and possibly concomitant with the opening of fractures. Copyright (C) 1998 Elsevier Science Ltd.