Joan Andújar

On the conditions of magma mixing and its bearing on andesite production in the crust.

Mixing between magmas is thought to affect a variety of processes, from the growth of continental crust to the triggering of volcanic eruptions, but its thermophysical viability remains unclear. Here, by using high-pressure mixing experiments and thermal calculations, we show that hybridization during single-intrusive events requires injection of high proportions of the replenishing magma during short periods, producing magmas with 55-58 wt% SiO2 when the mafic end-member is basaltic. High strain rates and gas-rich conditions may produce more felsic hybrids. The incremental growth of crustal reservoirs limits the production of hybrids to the waning stage of pluton assembly and to small portions of it. Large-scale mixing appears to be more efficient at lower crustal conditions, but requires higher proportions of mafic melt, producing more mafic hybrids than in shallow reservoirs. Altogether, our results show that hybrid arc magmas correspond to periods of enhanced magma production at depth.

Structure of the plumbing system at Tungurahua volcano, Ecuador: Insights from phase equilibrium experiments on July-August 2006 eruption products

Understanding the plumbing system structure below volcanoes and the storage conditions (temperature, pressure, volatile content and oxygen fugacity) of erupted magmas is of paramount importance for eruption forecasting and understanding of the factors controlling eruptive dynamics. Phase equilibria experiments have been performed on a Tungurahua andesite (Ecuador) to shed light on the magmatic conditions that lead to the July-August 2006 eruptions and the parameters that controlled the eruptive dynamics. Crystallization experiments were performed on a representative August 2006 mafic andesite product between 950-1025oC, at 100, 200 and 400 MPa and NNO+1 and +2, and water mole fractions in the fluid (XH2O) from 0.3 to 1 (water-saturation). Comparison of the natural phenocryst assemblage, proportions and phenocryst compositions with our experimental data indicates that the natural andesite experienced two levels of ponding prior to the eruption. During the first step, the magma was stored at 400 MPa (15-16 km), 1000oC, and contained ca. 6 wt % dissolved H2O. In the second step, the magma rose to a confining pressure of 200 MPa (8-10 km), where subsequent cooling (down to 975oC) and water-degassing of the magma led to the crystallization of reversely zoned rims on pre-existing phenocrysts. The combination of these processes induced oxidation of the system and overpressure of the reservoir, triggering the July 2006 eruption. The injection of a new, hot, volatile-rich andesitic magma from 15-16 km into the 200 MPa reservoir shortly before the eruption, was responsible for the August 2006 explosive event. Our results highlight the complexity of the Tungurahua plumbing system in which different magmatic reservoirs can co-exist and interact in time and are the main controlling factors of the eruptive dynamics.

Experimental Simulations of Magma Storage and Ascent

One of the key issues in utilizing precursor signals of volcanic eruption is to reliably interpret geophysical and geochemical data in terms of magma movement towards the surface. An important first step is to identify where the magma is stored prior to ascent. This can be studied through phase-equilibrium experiments designed to replicate the phase assemblage and compositions of natural pyroclasts or by measuring volatiles in melt inclusions from previous eruptions. The second crucial step is to characterize the magmatic conditions and processes that will guide the eruption style. This may be addressed through controlled dynamic decompression or deformation experiments to examine the different rates that govern the kinetics of syn-eruptive degassing, crystallization, and strain. Comparing the compositional and textural characteristics of these experimental products with the natural samples can be used to retrieve magma ascent conditions. These experimental simulations allow interpretation of direct observations and in situ measurements of syn-eruptive processes leading to more accurate forecasting of future eruptive scenarios.

Phase Equilibria of Pantelleria Trachytes (Italy): Constraints on Pre-eruptive Conditions and on the Metaluminous to Peralkaline Transition in Silicic Magmas

&NA; The relationships between trachytes and peralkaline rhyolites (i.e. pantellerites and comendites), which occur in many continental rift systems, oceanic islands and continental intraplate settings, is unclear. To fill this gap, we have performed phase equilibrium experiments on two representative metaluminous trachytes from Pantelleria to determine both their pre‐eruptive equilibration conditions (pressure, temperature, H2O content and redox state) and liquid lines of descent. Experiments were performed in the temperature range 750–950°C, pressure 0·5–1·5 kbar and fluid saturation conditions with XH2O [= H2O/(H2O + CO2)] ranging between zero and unity. Redox conditions were fixed below the nickel‐nickel oxide buffer (NNO). The results show that at 950°C and melt water contents (H2Omelt) close to saturation, trachytes are at liquidus conditions at all pressures. Clinopyroxene is the liquidus phase, being followed by iron‐rich olivine and alkali feldspar. Comparison of experimental and natural phases (abundances and compositions) yields the following pre‐eruptive conditions: P = 1 ± 0·5 kbar, T = 925±25°C, H2Omelt = 2 ± 1 wt %, and fO2 between NNO ‐ 0·5 and NNO ‐ 2. A decrease in temperature from 950°C to 750°C, as well as of H2Omelt, promotes a massive crystallization of alkali feldspar to over 80 wt %. Iron‐bearing minerals show gradual iron enrichment when T and fO2 decrease, trending towards the compositions of the phenocrysts of natural pantellerites. Despite the metaluminous character of the bulk‐rock compositions, residual glasses obtained after 80 wt % crystallization evolve toward comenditic compositions, owing to profuse alkali feldspar crystallization, which decreases the Al2O3 of the melt, leading to a consequent increase in the peralkalinity index [PI = molar (Na2O + K2O)/Al2O3]. This is the first experimental demonstration that peralkaline felsic derivatives can be produced by low‐pressure fractional crystallization of metaluminous mafic magmas. Our results show that the pantelleritic magmas of basalt‐trachyte‐rhyolite igneous suites require at least 95 wt % of parental basalt crystallization, consistent with trace element evidence. Redox conditions, through their effect on Fe‐Ti oxide stabilities, control the final iron content of the evolving melt.