A Volcanological Paradox in a Thin-Section: Large Explosive Eruptions of High-Mg Magmas Explained Through a Vein of Silicate Glass in a Serpentinized Peridotite Xenolith (Hyblean Area, Sicily)

Abstract

Ultramafic magmas (MgO ≥ 18 wt%) are generally thought to be primary mantle melts formed at temperatures in excess of 1600 °C. Volatile contents are expected to be low, and accordingly, high-Mg magmas generally do not yield large explosive eruptions. However, there are important exceptions to low explosivity that require an explanation. Here we show that hydrous (hence, potentially explosive) ultramafic magmas can also form at crustal depths at temperatures even lower than 1000 °C. Such a conclusion arose from the study of a silicate glass vein, ~1 mm in thickness, cross-cutting a mantle-derived harzburgite xenolith from the Valle Guffari nephelinite diatreme (Hyblean area, Sicily). The glass vein postdates a number of serpentine veins already existing in the host harzburgite, thus reasonably excluding that the melt infiltrated in the rock at mantle depths. The glass is highly porous at the sub-micron scale, it also bears vesicles filled by secondary minerals. The distribution of some major elements corresponds to a meimechite composition (MgO = 20.35 wt%; Na2O + K2O < 1 wt%; and TiO2 > 1 wt%). On the other hand, trace element distribution in the vein glass nearly matches the nephelinite juvenile clasts in the xenolith-bearing tuff-breccia. These data strongly support the hypothesis that an upwelling nephelinite melt (MgO = 7–9 wt%; 1100 ≤ T ≤ 1250 °C) intersected fractured serpentinites (T ≤ 500 °C) buried in the aged oceanic crust. The consequent dehydroxilization of the serpentine minerals gave rise to a supercritical aqueous fluid, bearing finely dispersed, hydrated cationic complexes such as [Mg2+(H2O)n]. The high-Mg, hydrothermal solution "flushed" into the nephelinite magma producing an ultramafic, hydrous (hence, potentially explosive), hybrid magma. This hypothesis explains the volcanological paradox of large explosive eruptions produced by ultramafic magmas.