Synthesized polycrystalline samples composed of enstatite and olivine with different volumetric ratios were deformed in compression under anhydrous conditions in a Paterson gas-medium apparatus at 1150-1300 degrees C, an oxygen fugacity buffered at Ni/NiO, and confining pressures of 300 or 450 MPa (protoenstatite or orthoenstatite fields). Mechanical data suggest a transition from diffusion to dislocation creep with increasing differential stress for all compositions. Microstructural analyses by optical and scanning electron microscopy reveal well-mixed aggregates and homogeneous deformation. Crystallographic preferred orientations measured by electron backscatter diffraction are consistent with activation of the slip systems (010)[100] and (010)[001] for olivine and (100)[001] and (010)[001] for enstatite, as expected at these conditions. Nonlinear least-squares fitting to the full data set from each experiment allowed the determination of dislocation creep flow laws for the different mixtures. The stress exponent is 3.5 for all compositions, and the apparent activation energies increase slightly as a function of enstatite volume fraction. Within the limits of experimental uncertainties, all two-phase aggregates have strengths that lie between the uniform strain rate (Taylor) and the uniform stress (Sachs) bounds calculated using the dislocation creep flow laws for olivine and enstatite. Calculation of the Taylor and Sachs bounds at strain rate and temperature conditions expected in nature (but not extrapolating in pressure) indicates that using the dislocation creep flow law for monomineralic olivine aggregates provides a good estimate of the viscosity of olivine-orthopyroxene rocks deforming by dislocation creep in the deeper lithosphere and asthenosphere. The rheology of Earth's upper mantle is generally modeled using mechanical flow laws determined for aggregates composed only of olivine minerals, in spite of the polyphase nature of mantle rocks. In this study, we investigated the effect of phase volume proportions on the high-temperature deformation properties of aggregates composed of the two most abundant minerals in the upper mantle, olivine and enstatite. The samples were deformed under dry conditions in triaxial compression at 1150-1300 degrees C, under oxygen fugacity fixed at the Ni/NiO solid buffer, and confining pressures of 300 or 450 MPa, at conditions where enstatite has two different crystallographic structures. At both pressures, in the dislocation creep regime, where deformation occurs mostly by the motion of dislocations along slip planes within mineral grains, the strengths of all the two-phase mixtures lie between the uniform strain rate and the uniform stress bounds, which assume iso-strain and iso-stress conditions, respectively, in all the grains comprising each aggregate. Extrapolating these bounds to temperatures and strain rates expected in nature indicates that the viscosity of mantle rocks can be modeled adequately with the dislocation creep flow law for olivine. Polycrystalline samples of enstatite and olivine with different volumetric ratios were deformed in compression at high temperature All two-phase aggregates have strengths in dislocation creep that lie between the uniform stress and uniform strain rate bounds In dislocation creep the flow law for olivine provides a good estimate of the viscosity of olivine-pyroxene rocks under natural conditions

Brachinites are igneous, ultramafic and unbrecciated primitive achondrites mainly composed of olivine and pyroxene, with a partial melt residue or cumulate origin that is still debated. This study presents a petrological and microstructural study of 10 brachinite meteorites to identify igneous and deformation processes responsible for their formation. Detailed microstructural analyses were performed using secondary electron microscopy, electron probe microanalysis, and electron backscatter diffraction. The olivine-spinel and two-pyroxene closure temperatures of these brachinites ranged from 814 +/- 27 to 909 +/- 20 degrees C. Olivine [001] axes coincide with the elongation of olivine grains and the crystallographic preferred orientations (CPO) of clinopyroxene [001] axes, which led us to infer lineation as olivine [001] axes parallel to X for all samples. Olivine CPO displays a strong or medium concentration of [001] axes parallel to X and [010] axes parallel to Z pointing to a B-type fabric. Limited internal deformation of olivine grains and slip systems identified from low-angle boundary misorientation analyses are inconsistent with the CPO and grain misorientation distributions are close to those found in untextured rocks. Olivine B-type fabrics may therefore result not from plastic deformation but rather from rigid crystal rotation or preferential crystal growth during compaction. Olivine subgrain boundary misorientation axes imply limited activation of [001](100) and [100](001) slip systems, which could be due to primary plastic deformation during or after crystal settling. These new results reveal that brachinites developed a cumulate texture and accommodated only weak plastic deformation during early differentiation processes in the parent body. Brachinite meteorites, rich in olivine and pyroxene, are classified as ultramafic and unbrecciated primitive achondrites. However, the precise way in which they formed is still a subject of debate. This study conducted a thorough petrological and microstructural examination of 10 brachinites using scanning electron microscopy, microprobe, and electron backscatter diffraction. The findings reveal that the closure temperatures of the brachinites are between 814 +/- 27 and 909 +/- 20 degrees C. Olivine crystal preferred orientations (CPO) indicate a B-type fabric [001](010), characterized by a strong to moderate concentration of [001] axes (slip direction) parallel to the lineation and [010] axes (slip plane) perpendicular to the foliation. Grain misorientation distributions resemble those found in untextured rocks (e.g., undeformed granular peridotite), implying that the B-type fabrics in olivine were a result of rigid crystal rotation or preferential crystal growth during cumulate formation or melt flow alignment and weak compaction. Olivine subgrain boundary misorientation axes suggest limited activation of [001](100) C-type and [100](001) E-type slip systems, which could be the result of compaction and primary plastic deformation during or after crystal settling. These new results indicate that brachinites developed a cumulate texture and accommodated only weak plastic deformation during early differentiation processes in the parent body. Petrology and microstructural analyses allow classification of the studied brachinites into two groups suggesting different igneous processes The brachinites display cumulate textures with magma flow and variable weak compaction intensities Olivine underwent early plastic deformation, and moderate to weak intensities of [010] axes normal to compaction point to relative brachinite locations

Constraining the thermal and compositional state of the mantle is crucial for deciphering the formation and evolution of Mars. Mineral physics predicts that Mars’ deep mantle is demarcated by a seismic discontinuity arising from the pressure-induced phase transformation of the mineral olivine to its higher-pressure polymorphs, making the depth of this boundary sensitive to both mantle temperature and composition. Here, we report on the seismic detection of a midmantle discontinuity using the data collected by NASA’s InSight Mission to Mars that matches the expected depth and sharpness of the postolivine transition. In five teleseismic events, we observed triplicated P and S waves and constrained the depth of this discontinuity to be 1,006 ± 40 km by modeling the triplicated waveforms. From this depth range, we infer a mantle potential temperature of 1,605 ± 100 K, a result consistent with a crust that is 10 to 15 times more enriched in heat-producing elements than the underlying mantle. Our waveform fits to the data indicate a broad gradient across the boundary, implying that the Martian mantle is more enriched in iron compared to Earth. Through modeling of thermochemical evolution of Mars, we observe that only two out of the five proposed composition models are compatible with the observed boundary depth. Our geodynamic simulations suggest that the Martian mantle was relatively cold 4.5 Gyr ago (1,720 to 1,860 K) and are consistent with a present-day surface heat flow of 21 to 24 mW/m 2 .

The textures of solid and molten metal in the presence of varying fractions of silicate melt at high temperature have been investigated to shed light on differentiation processes occurring in magma oceans formed on rocky bodies of the early solar system. Analogue experiments have been performed in a three-phase system (composed of coexisting metal, forsterite and silicate melt) in both static (1 GPa, 1723 K) and dynamic (i.e. agitated, at 1 bar, 1713 K and 1743 K) conditions. Micro-textures were analyzed with SEM and EBSD techniques, while mesotextures of the metallic phase were analyzed using ex-situ 3D microtomography. Although all samples exhibit the same micro-scale organization consistent with the minimization of local interfacial energies, their meso-scale textures differ significantly. Static conditions produce metal grains that have shapes close to spherical, corresponding to the state predicted by the grain-scale minimization of interfacial energies. In contrast, under dynamic conditions and in the presence of high silicate melt fractions (>= 50 vol%), molten metal coalesces to form pools with sizes that are several orders of magnitude larger than those predicted by grain growth mechanisms. Furthermore, in agreement with expectations based upon an interfacial energy budget, images show that nickel grains, whether solid or molten, do not occur surrounded entirely by silicate melt, but rather in contact with both forsterite crystals and silicate melt, leading to the formation of composite aggregates. Assuming that a magma ocean has less than 50 vol% of crystals (the upper limit that permits convective motion), thermodynamic calculations indicate that at the necessary temperatures, the metallic subsystem (Fe-NiS) of the planetesimal is entirely molten and the silicate residue is only composed of olivine. Convective motions in such a body will drive agitation, promoting the formation of composite aggregates of olivine and molten ironsulfide, their initial coalescence and subsequent fragmentation. In detail, these composite aggregates have a reduced density contrast with the surrounding silicate melt that reduces their settling velocities compared to pure metal. They also entrain olivine during the downward migration of iron-sulfide pools. Olivine grains concentrate at the surface of the metallic pools, hindering coalescence between pools or with a pre-existing core. An alternative differentiation scenario for core formation is explored in which the simple compaction of partially molten mixtures in the basal non-convecting layer of the magma ocean expels the interstitial silicate melt upward, such that the local fraction of iron-sulfide increases by mass-balance, reaching its percolation threshold and allowing core formation. This process is not only limited to early accreted planetesimals but may also occur in terrestrial bodies.

The elastic properties of a pure, synthetic fayalite aggregate were studied by coupled synchrotron X-ray diffraction and ultrasonic interferometry in a DIA-type multi-anvil press. Measurements at pressures up to about 7 GPa and temperatures up to 873 K yielded an adiabatic bulk modulus, KS0=127.2±0.3 GPa with (∂KS/∂P)T0=6.5±0.1, and a shear modulus, G0=53.3±0.4 GPa with (∂G/∂P)T0=1.25±0.05. When fixing (∂KS/∂P)T0=5.3 (after (∂KT/∂P)T0 from Nestola et al., 2011), KS0 increases to about 130 GPa. These estimates of (KS0,(∂KS/∂P)T0) follow a general linear trend, K=f(dK/dP), for fayalite. We define limited ranges for both bulk and shear moduli from previous studies, and we discuss how these variations affect seismic velocities and the determination of a mineralogical model in the context of the Mars InSight SEIS (Seismic Experiment for Interior Structure) experiment.

Understanding metal-silicate differentiation in small rocky bodies that accreted early in solar system history requires quantification of the effects of variable amounts of silicate melt and molten metal on the connectivity of metal-rich liquids. To shed light on this question, the equilibrium geometry and textural ripening of metal grains in the vicinity of the metal interconnection threshold have been determined experimentally. High pressure and temperature experiments were performed in the three-phase system forsterite + silicate melt + nickel at conditions of 1 GPa and up to 2080 K using piston-cylinder and Paris-Edinburgh presses. Sample textures were analyzed by 3D X-Ray microtomography either in-situ at the PSICHE beamline of the SOLEIL synchrotron, or on quenched samples using a laboratory Computed Tomography scan. Although dihedral angles point to textural equilibrium at the scale of individual grains at the end of each experiment, the attainment of textural equilibrium at sample scale is not straightforward. Depending on the relative proportions of the phases, different states of textural maturation are revealed. A particularly important issue is that time-resolved in-situ microtomography data show that cold (subsolidus) compression at the beginning of the experiment leads to soft metal grains being squeezed between silicates, leading to a forced interconnectivity of nickel. High temperature experiments with metal contents <= 20 vol% resulted in disruption of these forced networks, while the networks persisted in time for metal contents >= 25 vol%. This is taken to indicate that the stable interconnection threshold of pure nickel in a partially molten silicate matrix lies between 20 and 25 vol%. Therefore, we conclude that care must be taken when defining the interconnection threshold: not only should there be existence of a network of touching grains, but this network must persist in time in the absence of external forces and at pressure-temperature conditions that permit grain-boundary movement (i.e. excluding kinetically arrested systems). Growth of silicate grains is identified as the process driving textural maturation, and may explain the variability of interconnection thresholds reported in the literature. In addition, these considerations shed light on the diversity of textures observed in natural meteoritic samples (e.g. carbonaceous and ordinary chondrites and primitive achondrites), providing textural arguments to constrain the processes that affected these meteorites.

We have studied by high-resolution X-ray Computed Tomography the effect of crystal clustering on the Shape-Preferred Orientation (SPO) development in synthetic magmas experimentally deformed at 300 MPa and 475-550 degrees C. A fully connected solid network that could potentially induce the onset of yield strength is not achieved in these suspensions containing 16 vol% of crystals. The alumina grain population exhibits a glomeroporphyritic texture made of isolated grains (59.6%) and clusters of touching grains (40.4%). The SPO of the sub-population of isolated grains exhibits foliation and lineation, which are closely parallel to the plane and direction of shear, respectively. The SPO of clustered grains has little influence overall shape fabric. The angular relationships between the average foliation and cluster elongation provide a good indicator of the shear sense. Finally, we highlight the strain partitioning between nearly non-deformed large clusters acting as rigid glomerocrysts and highly sheared zones in low concentrated suspensions. (C) 2019 Academie des sciences. Published by Elsevier Masson SAS.

In situ X-ray diffraction was used to measure the isothermal bulk modulus at room conditions (KT0) of synthetic olivines with different iron contents. The chemical formulae of the olivine samples were ðFex; Mg1 xÞ2SiO4 with x = 0.45; 0.64; 0.82; 1, with 1% standard deviation (referenced as Fa45, Fa64, Fa82 and Fa100, respectively). All experiments were performed in the multi-anvil apparatus installed at NSLS beamline X17B2, to pressures up to about 7 GPa. Unit-cell volumes under hydrostatic conditions and differential stresses present in the samples were calculated using the method developed by Singh et al. (1998), and pressures measured using NaCl as a standard were then corrected for these stresses. Using a second-order Birch–Murnaghan equation of state, we obtained the isothermal bulk modulus of each composition: KFa45 T0 1⁄4 131:4 2:6 GPa, KFa64 T0 1⁄4 132:1 3:1 GPa, KFa82 T0 1⁄4 136:3 1:7 GPa and KFa100 T0 1⁄4 134:8 1:4 GPa. These values combined with data available in the literature show that the KT0 of Fe-rich olivines increases very slowly with the Fe content, but possibly not in a simple linear trend. C 2018 Published by Elsevier Masson SAS on behalf of Académie des sciences. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/ by-nc-nd/4.0/). Corresponding author. E-mail address: frederic.bejina@irap.omp.eu (F. Béjina).

This study is based on an integrated approach combining results from petrology, geophysics and modeling to explain the origin of continental alkaline magmatism in the Turkish–Iranian plateau (TIP). Synthesis of the petrological and chemical characteristics of the alkaline magmatism of the TIP, extending from 80 Ma to the present, shows that the alkaline lavas can be classified as ultrapotassic (UK), transitional potassic to sodic (TK) and sodic-high-potassium (HK) and sodic-low-potassium (LK) lavas, all derived from a heterogeneous mantle source. Synthesis of the most recent seismic tomography images for the region shows the presence of a large set of low-velocity elliptical bodies, ∼100 km in size, referred to as ‘Compaction Pockets’ (CP), scattered from the top of the mantle transition zone (MTZ) to the base of the lithosphere beneath the TIP. A model is developed, which shows that the low velocities in these Compaction Pockets result from the percolation and concentration of volatile-rich melts liberated from the MTZ. These volatile-rich melts interact with their surrounding mantle, which has a temperature ∼100°C lower than the usual subcontinental mantle adiabat. It is argued that this results in the precipitation of hydrated and carbonated mineral phases (at ∼8–6·5 GPa and <4 GPa, respectively), and partial melting of the resulting heterogeneous mantle (6·5–4 GPa) at critical depths. Melt extraction via dykes occurs when the top of the Compaction Pocket successively crosses these critical depths. The three groups of UK, sodic-LK and sodic-HK lavas may be linked to distinct melt extraction events. The chemical composition of TK melts may result from the mixing of UK and sodic melts, or partial melting of metasomatized mantle at lithospheric depths. The ‘Compaction Pocket’ model offers a robust new concept to explain alkaline magmatism in the context of continental orogeny.

In situ X-ray diffraction was used to measure the isothermal bulk modulus at room conditions (K-T0) of synthetic olivines with different iron contents. The chemical formulae of the olivine samples were (Fe-x, Mg1-x)(2)SiO4 with x = 0.45; 0.64; 0.82; 1, with 1% standard deviation (referenced as Fa45, Fa64, Fa82 and Fa100, respectively). All experiments were performed in the multi-anvil apparatus installed at NSLS beamline X17B2, to pressures up to about 7 GPa. Unit-cell volumes under hydrostatic conditions and differential stresses present in the samples were calculated using the method developed by Singh et al. (1998), and pressures measured using NaCl as a standard were then corrected for these stresses. Using a second-order Birch-Murnaghan equation of state, we obtained the isothermal bulk modulus of each composition: K-T0(Fa45) = 131.4 +/- 2.6 GPa, K-T0(Fa64) = 132.1 +/- 3.1 GPa, K-T0(Fa82) = 136.3 +/- 1.7 GPa and K-T0(Fa100) = 134.8 +/- 1.4 GPa. These values combined with data available in the literature show that the K-T0 of Fe-rich olivines increases very slowly with the Fe content, but possibly not in a simple linear trend. (C) 2018 Published by Elsevier Masson SAS on behalf of Academie des sciences.

Surface volcanism at la Reunion and Hawaii occurs with an offset of 150-180 km upstream to the plume axis with respect to the plate motion. This striking observation raises questions about the forcing of plume-lithosphere thermo-mechanical interactions on melt trajectories beneath these islands. Based on visco-elasto-plastic numerical models handled at kilometric resolution, we propose to explain this offset by the development of compressional stresses at the base of the lithosphere, that result from elastic plate bending above the upward load exerted by the plume head. This horizontal compression adopts a disc shape centered around the plume axis: (i) it is 20 km thick, (ii) it has a 150 km radius, (iii) it lays at the base of the elastic part of the lithosphere, i.e., around, similar to 50-70 km depth where the temperature varies from similar to 600 degrees C to similar to 750 degrees C, (iv) it lasts for 5 to 10 My in an oceanic plate of age greater than 70 My, and (vi) it is controlled by the visco-elastic relaxation time at 50-70 km depth. This period of time exceeds the time during which both the Somalian/East-African and Pacific plates drift over the Reunion and Hawaii plumes, respectively. This indicates that this basal compression is actually a persistent feature. It is inferred that the buoyant melts percolating in the plume head pond below this zone of compression and eventually spread laterally until the most compressive principal elastic stresses reverse to the vertical, i.e., similar to 150 km away from the plume head. There, melts propagate through dikes upwards to similar to 35 km depth, where the plate curvature reverses and ambient compression diminishes. This 30-35 km depth may thus host a magmatic reservoir where melts transported by dykes pond. Only after further magmatic differentiation can dykes resume their ascension up to the surface and begin forming a volcanic edifice. As the volcano grows because of melt accumulation at the top of the plate, the lithosphere is flexed downwards, inducing extra tensile stress at 30-35 km depth and compression at 15 km depth (induced by the edifice load). It implies that now the melts pond at similar to 15 km and form another magmatic reservoir lying just underneath the crust. These processes explain the ponding of primary (shield) melts at similar to 35 km and similar to 15 km depths as recorded below La Reunion, Mauritius or Hawaii volcanoes, all shifted by similar to 150 km with respect to the plume axis. (C) 2017 Elsevier B.V. All rights reserved.

A dense polycrystalline aggregate of synthetic fayalite (Fe2SiO4) was deformed up to 8.5 GPa at room temperature in the D-DIA press installed at the European Synchrotron Radiation Facility beamline ID06. Five successive shortening–lengthening cycles were performed at different pressures and up to a final strain of approximately 25% at a typical strain rate of about 10−5 s−1. Lattice stresses were quantified from (hkl) reflections accessible with a 55-keV monochromatic beam. Combined stress and strain data show that during each cycle, fayalite deforms elastically before yielding at an axial strain close to 2%. This yielding occurs at a macroscopic stress (taken as the average of the estimated lattice stresses) of 1.5–2 GPa, irrespective of pressure. Very moderate stress hardening takes place beyond the yield point, and the average stress becomes almost constant after a strain of 5–6%, suggesting a low-temperature plastic regime. Lattice stresses estimated with (131), (130), and (022) reflections are always higher than stresses estimated with (111) and (112) by a factor of about 1.5. In addition, the (131) lattice stress becomes progressively lower than the (130) and (022) lattice stresses with increasing pressure, which suggests a possible change in dominant slip systems around 5–6 GPa. Combining our results with data from Chen et al. (Phys Earth Planet Inter 143–144:347–356, 2004), we determined a low-temperature plasticity flow law with an activation energy of 217 ± 25 kJ mol−1 and a Peierls stress at 0 GPa, σ p0 = 3.92 ± 0.02 GPa, that is consistent with dislocation motion being limited by discrete obstacles. The pressure dependence is almost entirely accounted for by the Peierls stress, with dσ p/dP = G′/G 0, where G′ is the derivative of G 0, the shear modulus. Our results suggest that fayalite has a smaller pressure dependence of low-temperature plasticity than (Mg0.9Fe0.1)2SiO4 and that the transition between low-temperature plasticity and high-temperature creep occurs at lower temperatures and lower stresses in fayalite than in Mg-rich olivines. An increase in iron content in olivine may therefore enhance ductility and lower the effect of pressure on creep, resulting in a viscosity contrast of up to 50 between fayalite and (Mg0.9Fe0.1)2SiO4 at pressures and temperatures of the lithospheric mantle.

Grain growth experiments in the system forsterite (Fo) + nickel (Ni) have been performed on two analogue mixtures of ordinary chondrites, with volume % of Fo:Ni (95:5) and (80:20). These two mixtures have been studied at temperatures of 1390 degrees C and 1340 degrees C, at an oxygen fugacity (102) three orders of magnitude below the Ni-NiO buffer, for durations between 2 h and 10 days. Microstructures and grain size distributions show that grain growth is normal and that for durations >10 h the Zener relation is verified (i.e., the ratio of Fo and Ni grain size is independent of time). Comparison with results previously obtained at 1440 degrees C shows a similar grain growth exponent (n similar to 5) for both phases, consistent with growth of forsterite by grain boundary migration, limited by the growth rate of nickel. The details of size distribution frequencies and the value of grain-growth exponent indicate that the nickel grains, which pin forsterite grain boundaries, grow by diffusion along one-dimensional paths (i.e., along forsterite triple junctions). The derived activation energies for nickel and forsterite are 235 +/- 33 kJ/mol and 400 +/- 48 kJ/mol respectively. Within the framework of the Zener relation, this unexpected difference of activation energy is shown to be related to temperature-dependent variations in the ratio of Ni and Fo grain-size that are consistent with observed variations in Fo-Ni-Fo dihedral angle. These data thus indicate that the presence of all phases should be taken into account when considering the activation energy of growth rate of individual phases. As an application, the experimentally derived growth law for metal has been used in conjunction with temperature-time paths taken from models of the thermal history of the H-chondrite parent body to estimate the grain size evolution of metal in H-chondrites. A remarkably self-consistent picture emerges from experimentally derived grain-growth laws, textural data of metal grains in well characterised H-chondrite samples, and geochemically constrained temperature-time paths, providing the potential to use textural data of metal as a window into the thermal history of chondritic samples. (C) 2016 Elsevier B.V. All rights reserved.

A fine grained fully-dense olivine aggregate was deformed in a D-DIA press at room temperature and pressures ranging from 3.5 to 6.8GPa, at constant strain rates between 6×10-6 and 2.2×10-5s−1. A weighted non-linear least square fit of a dataset including our results and data from other high-pressure studies to a low-temperature plasticity flow law yields a Peierls stress σP0=7.4(0.5)GPa and an activation energy E∗=232(60)kJ.mol−1. The dependence of the Peierls stress to pressure, σP=σP0(1+0.09P), appears to be larger than the value predicted by the formulation proposed by Frost and Ashby (1982). With such a dependence, the activation volume is very small (V*=1.6(1.7)cm3.mol−1). Extrapolation to natural conditions yields a viscosity of ~1023-1024Pa.s for a cold subducting slab at depths of 50–100km.

We report an experimental study on the grain growth kinetics and microstructural evolution of olivine-metal systems. 32 experiments have been performed on four different mixtures of forsterite (Fo) and nickel (Ni) (respectively in vol.%, 95:5, 80:20,30:70 and 10:90). All experiments were carried out at high temperature (1440 degrees C), at oxygen fugacity (fO(2)) similar to 3 log units below the Ni-NiO buffer and for durations between 2 h and 21 days. In aggregates containing isolated forsterite grains in a nickel matrix (Fo + Ni (10:90)) forsterite did not grow, and grain growth of nickel was abnormal. In all other cases, features of normal grain growth were observed and the data were fit to the classic grain growth law d(n) - d(0)(n) = kt. For all these aggregates, grain growth exponents are in the range 4.0 < n < 5.7 and are not correlated with the phase proportion in the mixture. On the other hand, grain growth of forsterite is slower and that of nickel faster as the proportion of nickel increases. Details of the normalised grain size distributions (NGSD) are used to infer that grain growth of forsterite occurred by grain boundary migration rate-limited by nickel grains. For nickel, two types of behaviour were observed. In aggregates containing 5 or 20 vol.% nickel, grain growth occurred by "coalescence" strongly rate-limited by grain boundary migration of forsterite. In nickel-rich aggregates (Fo + Ni (70:30)), grain growth occurred by grain boundary migration. Comparison with studies from the literature shows that our experimental data are similar to those of Nichols and Mackwell (1991) for porous olivine aggregates, both Ni and pores apparently behaving as mobile minor phases. Extrapolation to time-scales relevant to the formation of ordinary chondrite parent bodies shows that mean grain sizes reached (Fo and Ni) are of the same order of magnitude (100 mu m to 1 mm) as those observed in several types of meteorites. Iron rich phases (kama-cite-taenite and/or troilite) in several H-chondrites are shown to present normal grain growth features and NGSD shapes similar to those in our experimental samples where nickel grains are not interconnected, i.e., Fo:Ni (95:5) and (80:20). The grain growth mechanism for the iron-rich phases in these meteorites may thus also occur by "coalescence" rate-limited by grain boundary migration of olivine. (C) 2012 Elsevier B.V. All rights reserved.

We have fabricated dense polyphase mixtures (silicate and oxide minerals, metal, and silicate melt) by Spark Plasma Sintering (SPS), a technique that has only recently been applied to silicates. We describe the SPS method and some of the characteristics of sintered specimens of forsterite, forsterite + MgO, forsterite + metal (Ni or Fe) and forsterite + metal + silicate melt. We show that SPS is a very efficient method to quickly prepare dense (>99 %) polyphase aggregates with homogeneous microstructures (well distributed phases, narrow grain size distributions, polygonal grain boundaries ... ) and small grain sizes with little or no grain growth during sintering. We observe the development of slight Shape Preferred Orientations (SPO), especially for the metal phases, due to the very simple setup we used here. Overall, SPS offers substantial advantages over traditional sintering techniques, making it perfectly suitable for experimental studies in Earth sciences, in particular those involving kinetic processes.