V24B-01
Numerical Modelling of Subduction Initiation at Passive Margins
Despite its crucial significance for the plate tectonics theory, subduction initiation remains a largely enigmatic open problem. It has been showed that at most passive margins elastic and frictional forces exceed gravitational instability and ridge-push forces, which preclude subduction initiation. From this point of view, the Atlantic Brazilian margin, characterized by a relatively thin lithosphere, represents a rare case where force balance is favourable for subduction initiation, which is evidenced by several tectonic/thermal features. In relation to this natural case we studied numerically physical parameters that control tectonic processes at passive margins characterized by thinned to normal continental lithosphere. The investigated parameters are thermal age of the oceanic plate, temperature structure (thickness) and density of the continental lithosphere, and the rheology of crust and mantle. Our experiments show that three different geodynamic regimes can be discriminated: (1) stable margin, (2) overthrusting and (3) subduction. Both overthrusting and subduction are driven by inherent gravitational instability of the passive margin due to the strong density contrast between the continental crust and adjacent oceanic lithosphere. This instability forces continental crust to thrust over the oceanic plate. In the case of overthrusting regime the thrusting of the continental crust over the oceanic plate is associated with deflection but not subduction of this plate, which remains attached to the continent. Transition from stable margin to either overthrusting or subduction is mainly dependent on the ductile strength of the lower crust with weak and hot crust favouring its oceanward movement. On the other hand, transition from overthrusting to subduction is crucially controlled by the ductile strength and density of the continental mantle lithosphere. Subduction is strongly favoured by rheologically weak (hot, hydrated) and depleted continental mantle characterized by lowered density compared to the oceanic lithosphere. Moreover, the present numerical experiments show that the age of the oceanic lithosphere at a passive margin does not play any significant role for the initiation of subduction.
V24B-02
Simple shear deformation of low-temperature serpentines and its consequence on the rheological behavior in the forearc mantle wedge
Water liberated by dehydration reactions within a subducting slab may induce serpentinization in the overlying forearc mantle, as indicated by regions of low seismic velocity and high Poissonfs ratio. It is well known that the transformation from peridotite (mantle rocks) into serpentinite invokes a significant decrease in physical strength. Although the frictional properties of serpentines are well investigated by low-pressure mechanical tests, deformation mechanism might be different as increasing pressure. In addition, previous deformation experiments are limited in low strain and pure shear deformation, and therefore the textural evolution of serpentine-rich rocks is poorly understood. We conducted deformation experiments in simple- shear geometry at a temperature of 200°C and a pressure of 1.0 GPa using a solid-medium apparatus (modified Griggs type), which condition corresponds to the depth for the forearc mantle wedge in cool subduction zones such as northeast (NE) Japan. We used a starting material of mesh-texture serpentinites, consisting of lizardite and/or chrysotile after olivine, from the Mineoka ophiolite, central Japan. Deformation experiments are carried out at a constant strain-rate (3.0x10-4) with different shear strains (γ=2,4,6). With increasing strain, the mesh texture is gradually transformed into ribbon texture. The rhombic or elliptical cores are surrounded by fibrous rims, and tend to rotate parallel to the strain ellipse. The highly sheared sample shows a significant lattice-preferred orientation, although the starting materials show a distinct crystallographic orientation in the core and rim. These deformation microstructures indicate ductile deformation in the low-temperature serpentines rather than brecciation or cataclastic flow. The forearc mantle wedge in NE Japan is generally anhydrous down to a depth 60 km as inferred from the occurrence of interplate thrust events, however, the low velocity anomalies are locally recognized with few large interplate earthquakes. This indicates that the presence of low-temperature serpentines in the region reduces the seismic coupling by plastic flow and prohibit the generation of great subduction earthquakes along the plate boundary.
V24B-03
Serpentinisation of the Mantle Wedge Controls the Chemistry of Arc Magmas
Arc magmas are derived from mantle wedge source regions that are more highly oxidised than other parts of the mantle but the cause of the oxidation is not well understood. Conventionally it has been attributed to influx of water derived from dehydration of the underlying slab, but this possibility has been comprehensively debunked by Frost et al. (1998). A direct link to the introduction of the fluids that flux melting fails to explain the ubiquitous high oxygen fugacity of xenoliths from this region, which point to extensive oxidation of the wedge. If it is not possible to introduce excess oxygen to the mantle wedge, it is likely that the oxidation arises from residual enrichment in oxygen due to dissociation of water and subsequent loss of hydrogen. Serpentinisation is the only geological process that generates sufficiently reducing conditions for water to dissociate, and it allows hydrogen fugacities to rise to sufficiently high levels for hydrogen to be lost as a migrating vapour phase. Because it creates magnetite, serpentinisation results in an increase in the Fe3+ content of the serpentinised rocks, which means that if subsequent heating regenerates less hydrous peridotite assemblages, the magnetite content of their spinel phase will be higher than in the precursor peridotite. For these regenerated peridotites, the greater magnetite content results in a higher oxygen fugacity than for the precursor peridotite. Hence the high Fe3+ signature generated during serpentine growth is carried down by flow in the wedge beyond the temperatures of serpentine breakdown and results in generation of high Fe3+:Fe2+ magmas. The main introduction of slab volatiles to the mantle wedge takes place at low temperatures, probably above 50km depth, where fluids are more abundant than at greater depths. This interpretation is consistent with geological and geophysical evidence for the existence of a serpentine body immediately above the slab in both ancient and modern subduction systems. Serpentinisation has implications for the budgets of S and of some metals in arc magmas, which are of significance for the major geochemical anomalies in S, Cu, Au Mo etc. associated with many arc magmas. It may generate Fe-Ni alloys with the potential to concentrate siderophile elements at shallow levels, while subsequently the more oxidised environment as the wedge melts may result in lower sulfide levels, with less entrapment of chalcophile elements in immiscible sulphide melts and mobilisation of S as sulphate. B. R. Frost, C. Ballhaus C, Geochim. Cosmochim. Acta 62, 329-331 (1998).
V24B-04
Rheology of serpentines, seismicity and mass transfer in subduction zone
Serpentinites have a lower density and lower viscosity than "dry" ultramafic rocks and it was proposed, based on numerical simulations, that they play a major role in mantle-slab decoupling, and in downward (sink) or upward (exhumation) motion of eclogites and ultra-high pressure (UHP) rocks in subduction zones. Rheological data on antigorite, the stable variety of serpentine in subduction zones, are obtained over a P-T range of 1-4 GPa and 200-500 /deg C that cover most of its stability field. The experiments were carried out in a D-DIA apparatus installed at GSECARS on the 13-BM-D line of APS. The determined stress-strain curves were fitted to a power-law equation including both temperature and pressure dependence. The results confirm that serpentinites acts as a weak layer that allows significant mass transfer along the "serpentinized channel" and dynamic processes such as mantle slab decoupling, and mantle wedge convection. Regardless of the temperature, serpentinized mantle at the slab surface has a low viscosity that allows localizing the deformation and impeding stress build-up. It will limit the downdip propagation of large earthquakes, and allow viscous relaxation as an origin of post-seismic deformations and slow earthquakes. Models of growth and transport of a serpentinized channel using available kinetic and present rheological data explain high exhumation rates of eclogites and limited thickness of the channel at great depths (above 50 km), and slower exhumation at in a thick hydrated mantle corner at shallower depths. Such channels may be difficult to detect from sismic tomography or using guided waves because of their small thickness (less than 3 km).
V24B-05 INVITED
Grain growth in peridotites: a high pressure study of Zener pinning
Transport phenomena are strongly influenced by grain-size. In subduction zones the extreme spatial variations in temperature and composition of mantle peridotites have dramatic consequences on grain size distributions. This study intends to demonstrate the relevance of Zener pinning in controlling the textural evolution of peridotites. Time-resolved, grain growth experiments have been performed on model peridotite compositions at temperatures of 1000 °C and 1100 °C, 1.0 GPa, and run durations from 103 to 106 seconds. All the runs have been carried out under nominally dry and C-saturated conditions. Run products have been characterized by XRPD (at a synchrotron beam, ESRF, Grenoble), TEM images and by EDS/WDS microprobe. In order to enhance grain boundaries and allow the quantitative textural analysis of all phases, a chemical etching has been performed. The abundances of olivine, orthopyroxene, clinopyroxene and spinel obtained by Rietveld refinement of XRPD data are constant throughout the entire annealing process, revealing that textural rearrangement is the only time-dependent process observed. Mean grain size data were fitted by the growth law Gn - Gon = k(t - to). Almost two orders of magnitude difference exists between the growth rate constant of the dispersed phase, spinel, and that of the major phases, i.e. orthopyroxene, olivine, clinopyroxene. The Crystal Size Distributions of olivine, clinpyroxene and spinels are in agreement with the Communicating Neighbours theory. The analysis of CSDs obtained for the orthopyroxene confirms the presence of two populations of grains, which have been involved in different kinetic evolution: the matrix grains and the porphyroblasts, which result from abnormal grain growth (AGG). Spatial analysis performed applying Voronoi tessellation and Delaunay triangulations shows that both matrix growth and AGG are related to the pinning process of spinel on grain boundaries. Preliminary data on the role of garnet are discussed. Large differences in coarsening rates between the dispersed particles and the matrix invalidate the assumption of a synchronous ripening of minerals constituting the mantle. The abundance and the spatial distribution of volumetrically minor minerals are expected to control grain growth via Zener pinning as the rate-limiting factor. Viscosity contrasts are therefore expected to be largely influenced by low percentages of dispersed phases. The pattern of viscosity in the mantle wedge is evaluated.
V24B-06
Solubility of Water in Basaltic Melt at Upper Mantle Conditions: a new Experimental Approach
The transport of volatiles from subducted oceanic lithosphere into the mantle wedge is recognized as a driver of arc volcanism. The solubility of H2O in basaltic silicate liquids at upper mantle conditions is an important and poorly constrained variable in quantifying the process of volatile transport within subduction zone systems. Existing experimental H2O solubility data for natural basalt compositions are limited to pressures of 600 MPa or less due to the difficulty of quenching very hydrous glasses from upper mantle conditions. We have developed a new experimental approach to determining the solubility of H2O in natural basaltic melts that circumvents this problem by monitoring the water content of olivine in equilibrium with hydrous basaltic liquid at high pressure. A series of melting experiments were conducted in a piston- cylinder device at 1 GPa and 1200 °C using San Carlos olivine and a natural basalt composition as starting materials. The initial water content of the experimental charge was systematically varied between 0 and 40 wt % H2O. Following each experiment the water content of olivine in the charge was measured by ion probe. Our initial results show that the H2O content of olivine increases linearly up to 30 wt H2O in the charge, at which point the concentrations plateau at values that are within error of olivine in equilibrium with a supercritical aqueous fluid at the same conditions. Once the bulk H2O concentrations have been corrected for the mass of melt present at the end of the run, our data indicate that the solubility of H2O in basaltic liquid is significantly larger than predicted by extrapolation of empirical models for H2O solubility calibrated from low pressure experiments.
V24B-07
Silicate-Sulfide-Oxide Phase Equilibria in Subduction Zones
Classical O2- and S2-dependent equilibria have been extrapolated to high P, showing that fO2 and fS2 in a wide variety of geological environments can be gauged by comparison to fluid buffering reactions. Many of these equilibria involve magnetite, which in intermediate to mafic rock types is eliminated with increasing P during the gabbro to eclogite transition (Green and Ringwood, 1967). It is not generally recognized that traditional buffers, such as QFM (quartz-fayalite-magnetite), are not appropriate for evaluating fluid conditions in subduction zones. This paper will present new fS2 and fO2 equilibria that are consistent with silicate, oxide, and sulfide petrography of the blueschist-eclogite belt in northern New Caledonia (Brown, 2007), and are therefore appropriate for evaluating fluid conditions consistent with subduction zone metamorphism. We evaluate reactions, which are supported by textural evidence in natural specimens, between silicates, oxides, and sulfides (stable at low P) to form garnet (at higher P). Several analogues that eliminate plagioclase +/- magnetite to produce garnet are proposed. A consequence of reacting Fe in sulfide to produce garnet is the concentration of chalcophile elements in the remaining sulfide phase. This is consistent with petrographic observations in the New Caledonian high P belt. Implications of new equilibiria demonstrate that phase relations proposed with garnet cannot be related to classical fS2-fO2 phase diagrams involving QFM. The only reactions involving mineral phases commonly present throughout the range of PT conditions in subducted crust involve ilmenite-rutile-sulfide equilibria. These equilibria can be widely applied to evaluating fluid conditions in the crust and mantle, not just subduction zones. References Brown, J.L. (2007) The Deep Sulfur Cycle: Insights from sulfide metamorphism in blueschist and eclogite, NE New Caledonia. PhD Thesis, The Australian National University. Green, D.H. and Ringwood, A.E. (1967) An Experimental Investigation of Gabbro to Eclogite Transformation and Its Petrological Applications. Geochimica Et Cosmochimica Acta 31(5), 767.
V24B-08
New Observations on the Melting Behavior of H2O-Saturated Mantle: Applications to Subduction Zones
We present new experimental data on the melting behavior of H2O-saturated fertile peridotite to improve our understanding of mantle wedge melting at subduction zones. Piston cylinder experiments were conducted over a range of temperatures (880-1200°C) above the H2O-saturated peridotite solidus at 3.2 GPa to quantify changes in mineral and melt compositions with increased degrees of melting. Earlier experiments confirm the H2O-saturated solidus is ~810°C at 3.2 GPa (Till et al., AGU, 2007). At 1125-1200°C, H2O-saturated peridotite melts are low-alkali olivine-orthopyroxene normative tholeiites with 47-49 wt% SiO2, 9-12 wt% Al2O3 and 16-20 wt% MgO. These melts are in equilibrium with a harzburgite residue consistent with intermediate-to-high degree melting of mantle peridotite. The melts become multiply saturated with a lherzolite residue (i.e., olivine, orthopyroxene, clinopyroxene and garnet) between 1125 and 1100°C. Between 880-1100°C, the melting reaction is olivine + clinopyroxene + garnet = orthopyroxene + melt. The change in melt fraction with temperature in our experiments, which contain 14.5 wt% H2O, is equivalent or greater than that measured in anhydrous peridotite melting experiments at identical conditions and significantly greater than previously inferred for peridotite melting in the presence of 6-8 wt% H2O at 1.2 GPa by Gaetani and Grove (1998). This suggests peridotite melt productivity is enhanced in the presence of large amounts of H2O.