Christy B. Till

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.

A mechanism for low-extent melts at the lithosphere-asthenosphere boundary

Recent studies have imaged sharp vertical drops in shear wave velocity at the lithosphere-asthenosphere boundary (LAB). In some regions, the magnitude of the negative velocity gradient at the LAB is too large to be explained by changes in temperature alone. This study demonstrates that small amounts of partial melt in the shallow asthenosphere are a viable model for this sharp seismic boundary. In particular, we examine melting in the upper asthenosphere at the edge of thick cratonic lithosphere, using the example of eastern North America where a sharp LAB velocity gradient has been observed. Finite element modeling of asthenospheric flow at an abrupt lateral decrease in lithosphere thickness indicates that this geometry, together with lateral plate motions, produces edge-driven convection and asthenospheric upwelling at the continental margin. A key component of this work is a comparison of the locations and extents of melting produced by using different models for the depression of the peridotite solidus with varying H2O content. In addition, we develop a simplified parameterization of the H2O-undersaturated peridotite solidus for a constant degree of H2O saturation in nominally anhydrous minerals. The patterns of mantle flow produced by our numerical modeling and various solidus parameterizations predict less than 0.1 wt % to 2.8 wt % (0.01–3.3 vol %) melting at depths between 102 and 126 km for an asthenosphere with a mantle potential temperature of 1350°C and 150 ppm H2O, or between 91 km and a maximum of 200 km for a mantle at 1350°C and 450 ppm H2O. If the asthenosphere has a mantle potential temperature ≤1340°C or contains less than 150 ppm H2O at 3 GPa, no melting will occur. This process of generating melt in the asthenosphere to produce a sharp vertical velocity gradient at the LAB is viable in other locations where convective upwelling occurs in the shallow asthenosphere although it is dependent on asthenospheric potential temperature, composition, and H2O content. Because the asthenosphere may be heterogeneous in composition and H2O content, the onset of melting below the LAB may fluctuate with time and space, as may the magnitude of the shear velocity drop at the LAB.

Partitioning of trace elements among coexisting crystals, melt, and supercritical fluid during isobaric crystallization and melting

Abstract The distribution of trace elements among coexisting crystals, melt, and supercritical fluid during melting and crystallization is a critical constraint for understanding the evolution of magmatic systems, including the origin and development of continental and oceanic crust. Although trace-element partitioning between crystals and melt during Rayleigh fractional crystallization or melting is well-known, partitioning among co-existing melt, crystals, and supercritical fluid during anatexis or crystallization is less explored despite the ubiquity of magmatic fluids. Here we develop the trace-element differential equations governing solid-melt-fluid equilibria for melting and crystallization under fluid-present conditions and provide analytical solutions for fractional and equilibrium crystallization and melting. A compilation of solid-fluid and melt-fluid distribution coefficients for about 30 trace elements in olivine, clinopyroxene, garnet, plagioclase, alkali feldspar, biotite, amphibole, apatite, and silicic melts is provided. Forward modeling demonstrates the conditions under which fluid-meltsolid partitioning will impact trace-element signatures in magmatic systems. We show that for trace elements soluble in aqueous fluids, the composition of a melt derived by fluid-present fractional crystallization or by fluid-present fractional melting will be significantly different than in otherwise comparable fluid-absent systems. Ignoring the partitioning of soluble elements into the fluid phase leads to large errors in concentrations (over 100%) and ratios and consequent misinterpretation of the trace-element character of source material and/or the processes of fractional crystallization and melting. Although significant in any setting involving fluid-present equilibria, this analysis may have a most profound influence on fluid-present subduction zone magma generation and the evolution of shallow level fluid-saturated silicic magmatic systems.

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.

A landslide in Tertiary marine shale with superheated fumaroles, Coast Ranges, California

In August 2004, a National Forest fire crew extinguished a 1.2 ha fire in a wilderness area ~40 km northeast of Santa Barbara, California. Examination revealed that the fire originated on a landslide dotted with superheated fumaroles. A 4 m borehole punched near the hottest (262 °C) fumarole had a maximum temperature of 307 °C. Temperatures in this borehole have been decreasing by ~0.1 °C/d, although the cooling rate is higher when the slide is dry. Gas from the fumaroles and boreholes is mostly air with 3–8 vol% carbon dioxide and trace amounts of carbon monoxide, methane, ethane, and propane. The carbon dioxide is 14 C-dead. The ratios of methane to ethane plus propane [C 1 /(C 2 + C 3 )] range from 3.6 to 14. Carbon isotope values for the CO 2 range from −14‰ to −23‰ δ 13 C. 3 He/ 4 He values range from 0.96 to 0.97 times that of air. The anomalous heat is interpreted to be due to rapid oxidation of iron sulfide augmented by combustion of carbonaceous matter within the formation.