Melt variability in percolated peridotite: an experimental study applied to reactive migration of tholeiitic basalt in the upper mantle

Melt-rock reaction in the upper mantle is recorded in a variety of ultramafic rocks and is an important process in modifying melt composition on its way from the source region towards the surface. This experimental study evaluates the compositional variability of tholeiitic basalts upon reaction with depleted peridotite at uppermost-mantle conditions. Infiltration-reaction processes are simulated by employing a three-layered set-up: primitive basaltic powder (‘melt layer') is overlain by a ‘peridotite layer' and a layer of vitreous carbon spheres (‘melt trap'). Melt from the melt layer is forced to move through the peridotite layer into the melt trap. Experiments were conducted at 0.65 and 0.8GPa in the temperature range 1,170-1,290°C. In this P-T range, representing conditions encountered in the transition zone (thermal boundary layer) between the asthenosphere and the lithosphere underneath oceanic spreading centres, the melt is subjected to fractionation, and the peridotite is partially melting (T s~1,260°C). The effect of reaction between melt and peridotite on the melt composition was investigated across each experimental charge. Quenched melts in the peridotite layers display larger compositional variations than melt layer glasses. A difference between glasses in the melt and peridotite layer becomes more important at decreasing temperature through a combination of enrichment in incompatible elements in the melt layer and less efficient diffusive equilibration in the melt phase. At 1,290°C, preferential dissolution of pyroxenes enriches the melt in silica and dilutes it in incompatible elements. Moreover, liquids become increasingly enriched in Cr2O3 at higher temperatures due to the dissolution of spinel. Silica contents of liquids decrease at 1,260°C, whereas incompatible elements start to concentrate in the melt due to increasing levels of crystallization. At the lowest temperatures investigated, increasing alkali contents cause silica to increase as a consequence of reactive fractionation. Pervasive percolation of tholeiitic basalt through an upper-mantle thermal boundary layer can thus impose a high-Si ‘low-pressure' signature on MORB. This could explain opx+plag enrichment in shallow plagioclase peridotites and prolonged formation of olivine gabbro

Reaction Processes between Tholeiitic Melt and Residual Peridotite in the Uppermost Mantle: an Experimental Study at 0·8 GPa

We present the results of a series of anhydrous piston cylinder experiments that illustrate the mechanisms and implications of reaction between tholeiitic melt and depleted peridotite in the uppermost mantle. To simulate infiltration-reaction processes we have applied a three-layer setup in which a layer of primitive basaltic powder (‘melt layer') is consecutively overlain by a ‘peridotite layer' and a layer of vitreous carbon spheres (‘melt trap'). The peridotite layer is mixed from pure separates of orthopyroxene, clinopyroxene and spinel (Balmuccia peridotite), and San Carlos olivine. Two tholeiitic melt compositions, respectively with compositions in equilibrium with lherzolitic (ol, opx, cpx) and harzburgitic (ol, opx) residues after partial melting at 1·5 GPa, were employed. Melt from the melt layer is forced to move through the peridotite layer into the melt trap. Experiments were conducted at 0·8 GPa with peridotite of variable grain size, in the temperature range 1200-1320°C and for run durations of 10 min to 92 h. In this P-T range, representing conditions encountered in the transition zone between the thermal boundary layer and the top of the asthenosphere below oceanic spreading centers, the melt is subjected to fractionation and the peridotite is partially melting (Ts ∼1260°C). Modal observations indicate a strong dependence between phase relations in the melt layer and changes in the modal abundances of the peridotite layer, as a function of both temperature and melt composition. Textural and compositional evidence, as well as modeling of Fe-Mg profiles in olivine, demonstrates that reaction between percolating melt and peridotite occurs by a combination of dissolution-reprecipitation and solid-state diffusion. Dissolution-reprecipitation leads to well-equilibrated phases whereas diffusional equilibration introduces zoning at experimental timescales. We discuss the observed reaction mechanisms and the consequent compositional changes in the light of local chemical equilibria and reaction kinetics. The results have direct implications for melt migration in upper-mantle thermal boundary layer

Experimentally determined distribution of fluorine and chlorine upon hydrous slab melting, and implications for F–Cl cycling through subduction zones

International audienceFluorine and chlorine are volatile elements known to be enriched in primitive arc magmas, and variations of F/Cl ratios can carry information about slab devolatilization processes. Recent experiments on the fractionations of these elements suggest that aqueous fluid has limited capacity to enrich the magma source region in F. Hence, it is difficult to explain observations of primitive arc magmas particularly rich in F. To complement previous experimental studies, we examined the fractionation of fluorine and chlorine during hydrous partial melting of subducting slab. Element-doped phase equilibria experiments were carried out in a complex chemical system at conditions equivalent to potential slab melting temperatures (750–1000 °C) across the amphibolite to eclogite facies transition (1.3–3 GPa). Partition coefficients of F and Cl between hydrous silicic melts and minerals were determined by electron microprobe and/or ion probe. Fluorine is compatible in amphibole (DFamp/glass = 1.18–1.85), and incompatible in garnet (0.034–0.140), clinopyroxene (0.059–0.505), and allanite (0.205–0.504). Hence, amphibole is an important F host, and can retain significant quantities of F in the solid residue of partial melting. On the contrary, Cl is incompatible, with DClmineral/glass generally decreasing from amphibole (0.079–0.625; one outlier at 1.87) to allanite (0.163), clinopyroxene (0.066–0.158), and garnet (0.031–0.153; outlier at 0.492). As a result, Cl is easily mobilized during partial melting. Fluorine and chlorine release during slab melting have been quantified by applying our partition coefficients to a non-modal batch melting model. The model shows that amphibole plays a key role in F/Cl fractionation during partial melting, while F/Cl is close to that of source for the melting of amphibole free eclogite. Moreover, the results from a flux-melting model employing several source compositions are compared to F and Cl abundances in primitive arc magmas. The observed variations are best described by variable additions of slab-derived aqueous fluids and hydrous melts to the mantle wedge melting region

Volatile (Li, B, F and Cl) mobility during amphibole breakdown in subduction zones

International audienceAmphiboles are ubiquitous minerals in the altered oceanic crust. During subduction, their breakdown is governed by continuous reactions up to eclogitic facies conditions. Amphiboles thus contribute to slab-derived fluid throughout prograde metamorphism and continuously record information about volatile exchanges occurring between the slab and the mantle wedge. However, the fate of volatile elements and especially halogens, such as F and Cl, in amphibole during subduction is poorly constrained. We studied metagabbros from three different localities in the Western Alps: the Chenaillet ophiolite, the Queyras Schistes Lustrés and the Monviso meta-ophiolitic complexes. These samples record different metamorphic conditions, from greenschist to eclogite facies, and have interacted with different lithologies (e.g. sedimentary rocks, serpentinites) from their formation at mid-oceanic ridge, up to their devolatilization during subduction. In the oceanic crust, the initial halogen budget is mostly stored in magmatic amphibole (F = 300–7000 ppm; Cl = 20–1200 ppm) or in amphibole corona (F = 100–7000 ppm; Cl = 80–2000 ppm) and titanite (F = 200–1500 ppm; Cl < 200 ppm) formed during hydrothermal seafloor alteration. It is thus the fate of these phases that govern the halogen fluxes between the crust and the overlying mantle and/or the plate interface in subduction zones. Li and B are poorly stored in the oceanic crust (< 5 ppm). In subduction zones, prograde metamorphism of metagabbros is first marked by the crystallization of glaucophane at the expense of magmatic and amphibole coronas. This episode is accompanied with a decrease of halogen concentrations in amphiboles (< 200 ppm of F and Cl) suggesting that these elements can be transferred to the mantle wedge by fluids. In the Queyras Schistes Lustrés complex, the intense deformation and the abundant devolatilization of metasedimentary rocks produce large fluid flows that promote rock chemical hybridization (metasomatic mixing with hybrid composition between metasedimentary rock and metagabbro) at the metasedimentary rock/metagabbro contacts. Such fluid/rock interactions result in a strong addition of Li in glaucophane (up to 600 ppm) whereas halogen concentrations are unaffected. At eclogite facies conditions, metagabbros display low halogens concentrations (< 20 ppm of F and < 100 ppm of Cl) relative to altered oceanic crust (F = 40–650 ppm; Cl = 40–1400 ppm) suggesting that these elements are continuously released by fluids during the first 30–80 km of subduction whatever the tectonic environment (e.g. slab, plate interface) and the considered fluid/rock interactions

Melt variability in percolated peridotite: an experimental study applied to reactive migration of tholeiitic basalt in the upper mantle

Melt-rock reaction in the upper mantle is recorded in a variety of ultramafic rocks and is an important process in modifying melt composition on its way from the source region towards the surface. This experimental study evaluates the compositional variability of tholeiitic basalts upon reaction with depleted peridotite at uppermost-mantle conditions. Infiltration-reaction processes are simulated by employing a three-layered set-up: primitive basaltic powder ('melt layer') is overlain by a 'peridotite layer' and a layer of vitreous carbon spheres ('melt trap'). Melt from the melt layer is forced to move through the peridotite layer into the melt trap. Experiments were conducted at 0.65 and 0.8 GPa in the temperature range 1,170-1,290 degrees C. In this P-T range, representing conditions encountered in the transition zone (thermal boundary layer) between the asthenosphere and the lithosphere underneath oceanic spreading centres, the melt is subjected to fractionation, and the peridotite is partially melting (T (s) similar to 1,260 degrees C). The effect of reaction between melt and peridotite on the melt composition was investigated across each experimental charge. Quenched melts in the peridotite layers display larger compositional variations than melt layer glasses. A difference between glasses in the melt and peridotite layer becomes more important at decreasing temperature through a combination of enrichment in incompatible elements in the melt layer and less efficient diffusive equilibration in the melt phase. At 1,290A degrees C, preferential dissolution of pyroxenes enriches the melt in silica and dilutes it in incompatible elements. Moreover, liquids become increasingly enriched in Cr(2)O(3) at higher temperatures due to the dissolution of spinel. Silica contents of liquids decrease at 1,260 degrees C, whereas incompatible elements start to concentrate in the melt due to increasing levels of crystallization. At the lowest temperatures investigated, increasing alkali contents cause silica to increase as a consequence of reactive fractionation. Pervasive percolation of tholeiitic basalt through an upper-mantle thermal boundary layer can thus impose a high-Si 'low-pressure' signature on MORB. This could explain opx + plag enrichment in shallow plagioclase peridotites and prolonged formation of olivine gabbros