Low pressure crystallization paths of H_2O-saturated basaltic-hawaiitic melts from Mt Etna: Implications for open-system degassing of basaltic volcanoes

摘要

Melt inclusions indicate that the basaltic-hawaiitic magmas from Mt Etna (Sicily) contained up to 2.3 wt% H_2O dissolved in the melt, prior to eruption. The effect of H_2O degassing during magma ascent has been experimentally constrained between 1135 and 1009 ℃, for P_(H_2O) = P_(total) varying from 800 to 270 bars. The starting material was a primitive hawaiitic lava sample (MgO = 7.1 wt%) representative of the less evolved lava emitted at Mt Etna. Experiments were conducted in TZM pressure vessels, with Ag_(70)Pd_(30) capsules in order to minimize the FeO loss. At temperatures of 1135-1090 ℃, P_(H_2O) = 800 bars, with NiNiO and FMQ buffers, olivine (Fo_(83-80)) is the liquidus phase in equilibrium with a residual hawaiitic melt (Mg# 0.60-0.57; CaO/Al_2O_3 = 0.82). Salitic pyroxene begins to crystallize at 1075 ℃, plagioclase at 1025 ℃ and at 1009 ℃, the magma is 33.5% crystallized with olivine, Ca-rich pyroxene, and plagioclase (16:60:24). At P_(H_2O) = 270 bars, FMQ buffer, olivine (Fo_(79.3-80)) and salitic pyroxene are the main liquidus phases between 1100 and 1090 ℃. They are in equilibrium with hawaiitic melts (Mg# 0.51; CaO/Al_2O_3 = 0.73). At 1070 ℃, the experimental charges are highly crystallized (≈51.5%) with olivine (Fo_(70)), salite, and plagioclase (An_(78.4-76.3)) in 14:44:42 relative proportions. Increasing the water content of hawaiitic-basaltic magmas expands the stability field of the olivine (relative to the other phases), lowers the crystallization temperatures of pyroxene and plagioclase, and results in the development of a more An-rich plagioclase. When compared to the natural samples, these results support a model of early and moderate crystallization of olivine from Etnean primitive hawaiitic magmas, containing close to 2.5 wt% H_2O, with NNO oxygen buffering conditions, at low pressure (P_(H_2O) = P_(total)). We propose a model of crystallization driven by decompression and water degassing during the emplacement of magma in the volcanic pile itself and possibly concomitant with the opening of fractures.