Etienne Médard

Kinematic variables and water transport control the formation and location of arc volcanoes

The processes that give rise to arc magmas at convergent plate margins have long been a subject of scientific research and debate. A consensus has developed that the mantle wedge overlying the subducting slab and fluids and/or melts from the subducting slab itself are involved in the melting process. However, the role of kinematic variables such as slab dip and convergence rate in the formation of arc magmas is still unclear. The depth to the top of the subducting slab beneath volcanic arcs, usually ∼110 ± 20 km, was previously thought to be constant among arcs. Recent studies revealed that the depth of intermediate-depth earthquakes underneath volcanic arcs, presumably marking the slab–wedge interface, varies systematically between ∼60 and 173 km and correlates with slab dip and convergence rate. Water-rich magmas (over 4–6 wt% H2O) are found in subduction zones with very different subduction parameters, including those with a shallow-dipping slab (north Japan), or steeply dipping slab (Marianas). Here we propose a simple model to address how kinematic parameters of plate subduction relate to the location of mantle melting at subduction zones. We demonstrate that the location of arc volcanoes is controlled by a combination of conditions: melting in the wedge is induced at the overlap of regions in the wedge that are hotter than the melting curve (solidus) of vapour-saturated peridotite and regions where hydrous minerals both in the wedge and in the subducting slab break down. These two limits for melt generation, when combined with the kinematic parameters of slab dip and convergence rate, provide independent constraints on the thermal structure of the wedge and accurately predict the location of mantle wedge melting and the position of arc volcanoes.

Amorphous mAteriAls: properties, structure, And durAbility† Compositional dependent compressibility of dissolved water in silicate glasses

Abstract The sound velocities and elastic properties of a series of hydrous rhyolite, andesite, and basalt glasses have been determined by Brillouin scattering spectroscopy at ambient conditions to elucidate the effect of glass composition on the compressibility of dissolved water. Both the adiabatic bulk (KS) and shear modulus (μ) of the dry glasses decrease with increasing silica content (KS,basalt > KS,andesite > KS,rhyolite and μbasalt > μandesite > μrhyolite). For each composition, the shear modulus systematically decreases with increasing water content. Although the addition of up to 14 mol% water decreases the KS of andesite and basalt glasses by up to 6%, there is no discernable effect of water on the KS of the rhyolite glasses. The partial molar KS of dissolved water (KS) in rhyolite, andesite, and basalt glasses are 37 ± 5, 19 ± 7, and 40 ± 3 GPa, corresponding to partial molar isothermal compressibilities (β̅T) of 0.029 ± 0.005, 0.042 ± 0.004, and 0.026 ± 0.002 GPa−1, respectively. These results indicate that the compressibility of dissolved water strongly depends on the bulk composition of the glass; hence, the partial molar volume of water cannot be independent of the bulk composition at elevated pressure. If the compressibility of dissolved water also depends on composition in the analog melts at high temperature and pressure, these observations will have important consequences for magmatic processes such as magma mixing/unmixing and fractional crystallization.

Grove et al. reply

Replying to England, P. C. & Katz, R. F. 468, doi:10.1038/nature09154 (2010)In their Comment England and Katz suggest that our model contains two flaws and that there are additional problems in our thermal models. This Reply points out an important part of our model that England and Katz appear to have missed, addresses their suggestion that there are flaws and discusses whether our thermal models are in error.

Fayalite oxidation processes in Obsidian Cliffs rhyolite flow, Oregon

Abstract This study investigates the oxidation of fayalite Fe22+SiO4 that is present in lithophysae from a rhyolite flow (Obsidian Cliffs, Oregon). Textural, chemical, and structural analyses of the successive oxidation zones are used to constrain: (1) the oxidation processes of olivine, and (2) the role of temperature, chemical diffusion, and meteoric infiltration. Petrologic analyses and thermodynamic modeling show that the rhyolite flow emplaced at 800-950 °C. Fayalite-bearing lithophysae formed only in the core of the lava flow. Variations in the gas composition inside the lithophysae induced the oxidation of fayalite to a laihunite-1M zone Fe12+Fe23+□1(SiO4)2. This zone is made of nano-lamellae of amorphous silica SiO2 and laihunite-3M Fe2+1.6Fe3+1.6□0.8(SiO4)2+ hematite Fe2O3. It probably formed by a nucleation and growth process in the fayalite fractures and defects and at fayalite crystal edges. The laihunite-1M zone then oxidized into an “oxyfayalite” zone with the composition Fe2+0.52Fe3+2.32□1.16(SiO4)2. This second oxidation zone is made of lamellae of amorphous silica SiO2 and hematite Fe2O3, with a possible small amount of ferrosilite Fe2+SiO3. A third and outer zone, composed exclusively of hematite, is also present. The successive oxidation zones suggest that there may be a mineral in the olivine group with higher Fe3+ content than laihunite-1M. The transformation of laihunite-1M to this “oxyfayalite” phase could occur by a reaction such as This would imply that Fe3+ can also be incorporated in the M1 site of olivine.