Piston-cylinder Experiments Research Articles

Experimental determination of tin partitioning between titanite, ilmenite, and granitic melts using improved capsule designs

Abstract Investigating mineral/melt Sn partitioning at high temperatures and pressures is a difficult task because Sn is a redox-sensitive multivalent element and easily alloys with noble metal sample capsules. To obtain accurate Sn partition coefficients between titanite, ilmenite, and granitic melts, we developed single capsule Pt or Au and double capsule Pt95Rh5 (or Au)-Re designs to avoid significant Sn loss at a controlled oxygen fugacity (fO2). With these new capsule designs, we performed piston-cylinder experiments of Sn partitioning between titanite, ilmenite, and granitic melts. The experimental P-T-fO2 conditions were 0.5–1.0 GPa, 850–1000 °C, and ~QFM+8 to ~QFM-4 (quartz-fayalite-magnetite, QFM, buffer), with fO2 controlled by the solid buffers of Ru-RuO2, Re-ReO2, Co-CoO, graphite, and Fe-FeO. The obtained mineral/melt Sn partition coefficients (DSnmin/melt) are 0.48–184.75 for titanite and 0.03–69.45 for ilmenite at the experimental conditions. The DSnmin/melt values are largely dependent on fO2, although the effects of temperature and melt composition are also observed. DSnTtn/melt strongly decreases with decreasing fO2, from ~46–185 at the most oxidizing conditions (Ru-RuO2 buffer), to ~2–16 at moderately oxidizing to moderately reducing conditions (Re-ReO2 to Co-CoO and graphite buffers), to <1 at the most reducing conditions (Fe-FeO buffer). DSnIlm/melt exhibits a variation trend similar to DSnTtn/melt but is always lower than DSnTtn/melt at a given fO2. These DSnmin/melt values can be applied to quantitatively assess the mineralization potential of granitic magmas. Using DSnTtn/melt, we estimate that Sn contents are ~150–400 ppm in the pre-mineralization magmas of the tin-mineralized Qitianling plutons (South China).

Trace Element Partitioning Between Saline Aqueous Fluids and Subducted Metasediments, and the Origin of the “Sediment Fingerprint” in Arc Magmas

AbstractIsotopic and elemental signatures in arc magmas bear similarities to marine sediments from the forearc. Partial melts of subducted sediments were previously invoked as the predominant carrier of material between the slab and mantle wedge, whereas aqueous fluids were deemed too inefficient at mobilizing trace elements to account for arc magma compositions. However, trace element solubility in aqueous fluids may be significantly affected by the formation of chloride complexes. We introduced NaCl into the sediment‐H2O system, and re‐investigated trace element partitioning between aqueous fluids and metasediments in a series of piston cylinder experiments at 2.5–4.5 GPa and 600–700°C. The diamond trap method was employed to separate the fluids, which were analyzed in the frozen state. Trace element compositions of fluids and all solid phases were obtained by LA‐ICP‐MS. We combined our data with published literature to demonstrate that saline fluids and sediment melts have a comparable efficiency in extracting incompatible elements from subducted sediments, producing similar element fractionation during mobile phase expulsion. Accessory phases such as allanite may only be stable in particularly Rare Earth Elements‐enriched lithologies. The Sr, Pb and Nd isotopic signatures of arc magma suggest that the mantle wedge is infiltrated by mobile phases derived from both the sediment and basalt sections of the subducted oceanic crust. Up to several percent of either sediment‐derived saline fluid or melt can account for the isotopic and elemental compositions of most arc magma. Geochemical similarities between arc magma and forearc sediments are not necessarily evidence of slab melting.

Vibrational entropy of disordering in omphacite

The cations of an ordered omphacite from the Tauern window were gradually disordered in piston cylinder experiments at temperatures between 850 and 1150 °C. The samples were examined by X-ray powder diffraction and then investigated using low-temperature calorimetry and IR spectroscopy. The low-temperature heat capacity data were used to obtain the vibrational entropies, and the line broadening of the IR spectra served as a tool to investigate the disordering enthalpy. These data were then used to calculate the configurational entropy as a function of temperature. The vibrational entropy does not change during the cation ordering phase transition from space group C2/c to P2/n at 865 °C but increases with a further temperature increase due to the reduction of short-range order.

The ascent of subduction zone mélanges: Experimental constraints on mélange rock densities and solidus temperatures

Mélanges are mixtures of subducted materials and serpentinized mantle rocks that form along the slab-mantle interface in subduction zones. It has been suggested that mélange rocks may be able to ascend from the slab-top into the overlying mantle, as solid or partially molten buoyant diapirs, and transfer their compositional signatures to the source regions of arc magmas. However, their ability to buoyantly rise is in part tied to their phase equilibria during melting and residual densities after melt extraction, all of which are poorly constrained. Here, we report a series of piston-cylinder experiments performed at 1.5–2.5 GPa and 500–1050°C on three natural mélange rocks that span a range of mélange compositions. Using phase equilibria, solidus temperatures, and densities for all experiments, we show that melting of mélanges is unlikely to occur along the slab-top at pressures ≤ 2.5 GPa, so that diapirism into the hotter mantle wedge would be required for melting to initiate. For the two metaluminous mélange compositions, diapir formation is favored up to pressures of at least 2.5 GPa. For the peraluminous mélange composition investigated, diapir buoyancy is possible at 1.5 GPa but limited at 2.5 GPa due to the formation of high-density garnet, primarily at the expense of chlorite. We also evaluate whether thermodynamic modeling (Perple_X) can accurately reproduce the phase equilibria, solidus temperatures, and density evolution of mélange compositions. Our analysis shows good agreement between models and experiments in mélange compositions with low initial water contents and low-pressure (≤ 1.5 GPa) conditions. However, discrepancies between the thermodynamic models and experiments become larger at higher pressures and high-water contents, highlighting the need for an improved thermodynamic database that can model novel bulk compositions beyond the canonical subducting lithologies. This study provides experimental constraints on mélange buoyancy that can inform numerical models of mélange diapirism and influence the interpretations of both geophysical signals and geochemical characteristics of magmas in subduction zones.

Effects of H2O–CO2 Fluids, Temperature, and Peridotite Fertility on Partial Melting in Mantle Wedges and Generation of Primary Arc Basalts

Abstract Many lines of evidence from high P–T experiments, thermodynamic models, and natural observations suggest that slab-derived aqueous fluids, which flux mantle wedges contain variable amounts of dissolved carbon. However, constraints on the effects of H2O–CO2 fluids on mantle melting, particularly at mantle wedge P–T conditions, are limited. Here, we present new piston cylinder experiments on fertile and depleted peridotite compositions with 3.5 wt.% H2O and XCO2 [= molar CO2 / (CO2 + H2O)] of 0.04–0.17. Experiments were performed at 2–3 GPa and 1350°C to assess how temperature, peridotite fertility, and XCO2 of slab-derived fluid affects partial melting in mantle wedges. All experiments produce olivine + orthopyroxene +7 to 41 wt.% partial melt. Our new data, along with previous lower temperature data, show that as mantle wedge temperature increases, primary melts become richer in SiO2, FeO*, and MgO and poorer CaO, Al2O3, and alkalis when influenced by H2O–CO2 fluids. At constant P–T and bulk H2O content, the extent of melting in the mantle wedge is largely controlled by peridotite fertility and XCO2 of slab-fluid. High XCO2 depleted compositions generate ~7 wt.% melt, whereas, at identical P–T, low XCO2 fertile compositions generate ~30 to 40 wt.% melt. Additionally, peridotite fertility and XCO2 have significant effects on peridotite partial melt compositions. At a constant P–T–XCO2, fertile peridotites generate melts richer in CaO and Al2O3 and poorer in SiO2, MgO + FeO, and alkalis. Similar to previous experimental studies, at a constant P–T fertility condition, as XCO2 increases, SiO2 and CaO of melts systematically decrease and increase, respectively. Such distinctive effects of oxidized form of dissolved carbon on peridotite partial melt compositions are not observed if the carbon-bearing fluid is reduced, such as CH4-bearing. Considering the large effect of XCO2 on melt SiO2 and CaO concentrations and the relatively oxidized nature of arc magmas, we compare the SiO2/CaO of our experimental melts and melts from previous peridotite + H2O ± CO2 studies to the SiO2/CaO systematics of primitive arc basalts and ultra-calcic, silica-undersaturated arc melt inclusions. From this comparison, we demonstrate that across most P–T–fertility conditions predicted for mantle wedges, partial melts from bulk compositions with XCO2 ≥ 0.11 have lower SiO2/CaO than all primitive arc melts found globally, even when correcting for olivine fractionation, whereas partial melts from bulk compositions with XCO2 = 0.04 overlap the lower end of the SiO2/CaO field defined by natural data. These results suggest that the upper XCO2 limit of slab-fluids influencing primary arc magma formation is 0.04 < XCO2 < 0.11, and this upper limit is likely to apply globally. Lastly, we show that the anomalous SiO2/CaO and CaO/Al2O3 signatures observed in ultra-calcic arc melt inclusions can be reproduced by partial melting of either CO2-bearing hydrous fertile and depleted peridotites with 0 < XCO2 < 0.11 at 2–3 GPa, or from nominally CO2-free hydrous fertile peridotites at P > 3 GPa.

Amphibole stability, water storage in the mantle, and the nature of the lithosphere-asthenosphere boundary

Amphibole could potentially be an important host of water in the upper mantle. Moreover, the decomposition of amphibole has been invoked as a possible cause of the lithosphere-asthenosphere boundary. However, amphibole stability has been experimentally studied mostly under water-saturated conditions, which are unrealistic for most of the mantle that may contain only traces of water. Experiments with low nominal water contents yielded controversial results and were properly hampered by problems in controlling water activity. We have solved this problem with a novel experimental approach. We carried out piston cylinder experiments from 900 to 1350°C and 2 to 4.5 GPa using a peridotitic composition coexisting with an excess H2O-N2 fluid phase. The dilution by inert N2 was used to precisely control water fugacity to values realistic for the upper mantle. Numerous reversed experiments were carried out to circumvent problems with the metastable formation of amphibole. Our data show a dual effect of water fugacity on the stability of amphibole. With decreasing water activity, the stability field is simultaneously displaced to lower pressures, and expanded to higher temperatures. This behavior is due to two different decomposition reactions with dehydration involving only solid phases at low temperature, but melting at high temperature. Along a continental geotherm, amphibole will never be stable for typical upper mantle water contents. However, for a mantle containing 150 – 200 ppm of water, traces of amphibole may form in a narrow pressure interval along an oceanic geotherm. Here, amphibole may contribute significantly to bulk water storage, although most of the water still resides in nominally anhydrous minerals. However, even if amphibole is stable in a restricted depth range, it cannot account for the lithosphere-asthenosphere boundary, since decomposition proceeds through a solid-state reaction and does not involve melting for realistic mantle water contents.

Multiple Episodes of Rock-Melt Reaction at the Slab-Mantle Interface: Formation of High Silica Primary Magmas in Intermediate to Hot Subduction Zones

AbstractSiliceous slab-derived partial melts infiltrate the sub-arc mantle and cause rock-melt reactions, which govern the formation of diverse primary arc magmas and lithological heterogeneities. The effect of bulk water content, composition of reactants, and nature of melt infiltration (porous versus channelized) on the rock-melt reactions at sub-arc conditions have been investigated by previous studies. However, the effect of multiple episodes of rock-melt reactions in such scenarios has not been investigated before. Here, we explore mantle wedge modifications through serial additions of hydrous-silicic slab partial melts and whether such a process may ultimately explain the origin of high-Mg# andesites found in arcs worldwide. A series of piston-cylinder experiments simulate a serial addition of silicic slab melts in up to three stages (I through III) at 3 GPa and 800–1050°C, using rock-melt proportions of 75–25 and 50–50. A synthetic KLB-1 and a natural rhyolite (JR-1) represented the mantle and the slab components, respectively. Right from the first rock-melt interaction, the peridotite mantle transforms into olivine-free mica-rich pyroxenites ± amphibole ± quartz/coesite in equilibrium with rhyolitic-hydrous melts (72–80 wt% SiO2 and 40–90 Mg#). The formation of olivine-free pyroxenite seems to be controlled by complex functions of T, P, rock-melt ratio, wedge composition, and silica activity of the slab-melt. Remarkably, the pyroxenites approach a melt-buffered state with progressive stages of rock-melt reactions, where those rhyolitic melts inherit and preserve the major (alkalis, Fe, Mg, Ca) and trace element slab-signature. Our results demonstrate that lithological heterogeneities such as pyroxenites formed as products of rock-melt reactions in the sub-arc mantle may function as melt ‘enablers,’ implying that they may act as pathways that enable the infiltrating melt to retain their slab signature without undergoing modification. Moreover, the density contrast between the products of rock-melt reaction (melts and residues) and the average mantle wedge (~150 to 400 kg/m3) may help forming instabilities and diapiric rise of the slab components into the mantle wedge. However, the fate of the primitive slab-melts seems to be associated with the length of the pathway of mantle interaction which explains the evident wide magma spectrum as well as their degree of slab garnet-signature dilution. This work and the existence of high-Mg# Mexican-trondhjemites indicates that almost pristine slab-melts can make their way up to crustal levels and contribute to the arc magma diversity.

Chemical compaction and fluid segregation in piston cylinder experiments

Fluid segregation is a ubiquitous process in deep-seated rocks, mainly driven by buoyancy and gravitational compaction, which occur in drained systems until permeability decreases and the system become practically undrained. The mechanism and rate of fluid segregation following this stage are poorly constrained, despite the importance of these factors for fluid distribution and the physical and chemical properties in the Earth. To this end, we performed sintering experiments of quartzite, with H2O–CO2 fluids of 1.9%–18.0% added volume fraction using a piston-cylinder apparatus at 900°C under nominally isotropic pressure of 1 GPa. The subsequent chemical redistribution of silica and fluids resulted in capsule-scale fluid segregation (CFS) and the formation of dense quartzite (∼0.3%) within 192 h in pure H2O systems. Comparative experiments showed that dissolution/precipitation was not caused by the temperature gradient. Instead, we considered the fluid pressure difference within the experimental capsule between fluid-rich and fluid-poor domains caused by the different response against small pressure oscillation during experiments. Fluid-rich rock is elastically “soft” (i.e., having larger Skempton's coefficient) compared to fluid-poor rock to increase pore fluid pressure under a compressional stress change. Therefore, more silica dissolves in fluid-rich domains with higher solubility, while it precipitates in fluid-poor domains through diffusive transport, expanding porosity contrast. This chemical compaction in the capsule scale is effective as long as elasticity dominates matrix viscosity because the pressure difference relaxes with time. The calculated relaxation time of quartzite at the experimental condition was comparable to the intervals of the oil pressure addition which produced stress change. A one-dimensional model for porosity evolution showed that time-averaged SiO2 diffusivity during CFS was much larger than the grain boundary diffusivity of tightly sintered quartz aggregates, but was comparable to the diffusivity of SiO2 in aqueous fluid with ∼0.1 vol% fluid fraction. During metamorphism where the fluid pressure difference may be maintained for longer duration, the spontaneous silica cementation may play a role for formation of syn-metamorphic veins and control of the amount of pore fluids transported to deep Earth.

Carbon recycling efficiency in subduction zones constrained by the effects of H2O-CO2 fluids on partial melt compositions in the mantle wedge

The extent of CO2 transfer from subducting lithologies to the overlying mantle wedge in general and to the arc magma source regions in particular remains debated. The limit of CO2 transfer to the sub-arc mantle could be estimated if the effects of CO2 on the primary hydrous melt compositions of mantle wedge can be assessed in relation to the observed compositions of primitive arc magmas. Here we present new piston cylinder and multi-anvil experiments using Au75Pd25 and Au capsules on four depleted peridotite + H2O ± CO2 starting compositions with 3.5 wt.% H2O and XCO2 [=molar CO2/ (CO2+H2O)] of 0–0.17. Experiments were performed at 2–4 GPa and 1200°C to constrain how the presence of variable CO2 in slab-derived aqueous fluids affects the composition of peridotite partial melts. All experiments consisted of low degree melts (<10 wt.%) in equilibrium with olivine + orthopyroxene ± clinopyroxene. Melts at 2–4 GPa are basaltic for XCO2 of 0–0.10 and become SiO2-poor and CaO-rich at XCO2 > 0.10. Comparison between our experimental partial melt compositions with a global dataset of the most primitive arc magmas suggests that the upper limit of XCO2 in fluids inducing melting in mantle wedges is ∼0.10 at 2–4 GPa. We apply these new constrains to an H2O and CO2 mass balance model for subduction zones and estimate that at least 34–86% of CO2 entering subduction zones bypasses the sub-arc melt generation zone and is subducted to the convecting mantle, either carried by the slab or by the down-dragged limb of the mantle wedge directly above the slab.

Trace element partitioning during incipient melting of phlogopite-peridotite in the spinel and garnet stability fields

Potassium-rich magmatism represents subordinate magma volumes worldwide but has been observed in many geodynamic settings. Most potassic magmas are thought to derive from very-low degrees of melting of metasomatized mantle lithologies. We performed piston-cylinder experiments to determine trace element partition coefficients between incipient potassic silicate melts and phlogopite ± pargasite peridotite in the spinel (1 GPa) and garnet stability fields (3 GPa). Most of the rare Earth elements (REEs) are compatible in pargasite but incompatible in phlogopite. Although garnet remains the mineral phase most efficiently fractionating heavy from light REEs (DYbgrt/melt/DLagrt/melt > 750), orthopyroxene can also significantly fractionate REEs, e.g., achieving DYbopx/melt/DLaopx/melt > 100 at 3 GPa. Mineral-liquid partition coefficients vary by about one order of magnitude between incipient melts derived from the spinel and garnet stability fields. We thus show that trace element partitioning at the onset of melting is controlled more by pressure (through melt composition) than by the extent of melting. With increasing pressure, Rb and Ba exhibit different behaviors in phlogopite, with DBaphl/melt > DRbphl/meltat 1 GPa and the opposite at 3 GPa. At 1 GPa, a decrease of melt polymerization (lower NBO/T) with increasing melt fraction translates into a significant decrease of most phlogopite partition coefficients. Finally, we show that resolvable inter-element fractionations do occur when phlogopite- (and pargasite)-bearing peridotite are melted, indicating that trace element ratios are not always faithfully representative of that of their sources but bear the imprint of varied P–T conditions of melting and contrasted pre-metasomatic histories. This self-consistent partition coefficient dataset thus gives a new scope to understand the complex petrogenesis of K-rich magmas in orogenic settings.

Effects of phosphorus on partial melting of the Martian Mantle and compositions of the Martian Crust

Phosphorus is estimated to be ten times more enriched in the martian mantle compared to the terrestrial mantle. Yet, its effects on primary melt composition and melting phase relations in martian systems is unknown. We performed piston-cylinder experiments at a constant upper mantle pressure of 2 GPa and temperatures of 1210–1450 °C using a model martian primitive mantle composition with P2O5 content of 0 and 0.5 wt.%. All experiments produced an assemblage of olivine + orthopyroxene + melt ± pigeonite ± apatite ± spinel. Our experimental results, in combination with a previous study at similar P-T conditions and major element bulk composition but containing 0.2 wt.% bulk P2O5, show that the addition of phosphorus dramatically increases the abundance of more polymerized residual mineral such as orthopyroxene while decreasing the proportion of less polymerized residual phases such as olivine, especially at low extent of melting (∼11 wt.%). Such effects lead to lower SiO2 concentrations in the near-solidus melt by up to 10 wt.% for mantle P2O5 of 0.2 and 0.5 wt.%. Increasing bulk P2O5 to 0.5 wt.% also leads to slightly elevated CaO/Al2O3 ratio and increased FeO* concentration in mantle-derived melts with the latter likely due to formation of Fe-O-P complexes in the liquid. Our study suggests that elevated phosphorus in the martian mantle has important consequences regarding the composition and mineralogy of the crust, partly made with primary melts, and of the upper mantle. Because of elevated P, variably melt-depleted upper mantle of Mars is likely to be richer in orthopyroxene compared to the terrestrial mantle and the elevated P content is partly responsible for several geochemical attributes of martian basalts compared to those on Earth. Extrapolating our experimental results to a range of pressures, we suggest a depletion of P in the mantle through time, which likely contributed to major elemental compositional differences between ancient Gusev and Gale crater basalts and more recent martian meteorites.

COH-fluid induced metasomatism of peridotites in the forearc mantle

Devolatilization of subducting lithologies liberates COH-fluids. These may become partially sequestered in peridotites in the slab and the overlying forearc mantle, affecting the cycling of volatiles and fluid mobile elements in subduction zones. Here we assess the magnitudes, timescales and mechanism of channelized injection of COH-fluids doped with {mathrm{Ca}}_{mathrm{aq}}^{2+}, {mathrm{Sr}}_{mathrm{aq}}^{2+} and {mathrm{Ba}}_{mathrm{aq}}^{2+} into the dry forearc mantle by performing piston cylinder experiments between 1–2.5 GPa and 600–700 °C. Cylindrical cores of natural spinel-bearing harzburgites were used as starting materials. Based on mineral assemblage and composition three reaction zones are distinguishable from the rim towards the core of primary olivine and orthopyroxene grains. Zone 1 contains carbonates + quartz ± kyanite and zone 2 contains carbonates + talc ± chlorite. Olivine is further replaced in zone 3 by either antigorite + magnesite or magnesite + talc within or above antigorite stability, respectively. Orthopyroxene is replaced in zone 3 by talc + chlorite. Mineral assemblages and the compositions of secondary minerals depend on fluid composition and the replaced primary silicate. The extent of alteration depends on fluid CO2 content and fluid/rock-ratio, and is further promoted by fluid permeable reaction zones and reaction driven cracking. Our results show that COH-fluid induced metasomatism of the forearc mantle is self-perpetuating and efficient at sequestering {mathrm{Ca}}_{mathrm{aq}}^{2+}, {mathrm{Sr}}_{mathrm{aq}}^{2+}, {mathrm{Ba}}_{mathrm{aq}}^{2+} and CO2aq into newly formed carbonates. This process is fast with 90% of the available C sequestered and nearly 50% of the initial minerals altered at 650 °C, 2 GPa within 55 h. The dissolution of primary silicates under high COH-fluid/rock-ratios, as in channelized fluid flow, enriches SiO2aq in the fluid, while CO2aq is sequestered into carbonates. In an open system, the remaining CO2-depleted, Si-enriched aqueous fluid may cause Si-metasomatism in the forearc further away from the injection of the COH-fluid into peridotite.

An Experimental Study of Chlorite Stability in Varied Subduction Zone Lithologies with Implications for Fluid Production, Melting, and Diapirism in Chlorite-Rich Mélange Rocks

AbstractFour ultramafic bulk compositions comprising only natural minerals were used to constrain the stability field of chlorite in a variety of subducted, chlorite-rich rocks through an examination of key chlorite dehydration reactions relevant to the sub-arc. Seventy-four piston cylinder experiments were conducted at a range of pressures (1.0–5.0 GPa) and temperatures (500°C–1150°C).Bulk 1 represents a chlorite mélange (Mg# = 0.94) typically formed in the subduction channel. This composition was used to examine the terminal chlorite reactions to olivine, orthopyroxene, and spinel at low pressure and to olivine, garnet, and spinel at high pressure. Chlorite achieves a thermal maximum stability at 2.0 GPa, 850°C; at 3.0 GPa, 850°C; and at 5.0 GPa, 760°C. The terminal chlorite breakdown reaction rises at a much steeper Clapeyron slope than shown in previous studies. Bulk 2 contains additionally antigorite and tremolite, to constrain phase relations in more fertile compositions. Chlorite reacts with clinopyroxene at ~100°C lower temperatures and with orthopyroxene at ~20°C–60°C lower temperatures than the terminal chlorite breakdown. The reactions have a subparallel Clapeyron slope and none of the three chlorite dehydration reactions crosses the antigorite breakdown reaction up to 5 GPa. This demonstrates that chlorite is the most stable carrier of H2O to high temperatures in subducted ultramafic rocks. Chlorite mélanges that form at the subduction plate interface will dehydrate at 850°C–800°C, 80–120 km depth for intermediate to hot subduction geotherms and liberate 10–12 wt.% of H2O, triggering wet melting in associated sediments. For cold subduction geotherms, chlorite dehydration occurs at 780°C–740°C, 120–170 km depth. Interaction of such fluids with sediments will likely produce a supercritical fluid phase. No melting in the ultramafic rocks has been observed at the chlorite breakdown reactions. Wet melting of the chlorite mélange at 3 GPa occurred between 1100°C and 1150°C.The stability of chlorite in more Fe-rich mélanges (bulk Mg# = 0.50 and 0.68, respectively) were conducted at 3.0 GPa and revealed thermal maxima at 650°C and 765°C, respectively. Collectively, the thermal stability of chlorite is dependent upon the Mg# of the bulk composition and spans over 200°C at sub-arc depths. The density of run products was calculated to test the validity of the chlorite mélange diapir model. With the progressive breakdown of chlorite, ultramafic chlorite mélanges transform into garnet peridotite, thereby losing any buoyancy they initially possessed. This makes the likelihood of mélange diapirs as a major transport mechanism through the sub-arc unfeasible.

Orthopyroxene rim growth during reaction of (Co, Ni, Mn, Zn)-doped forsterite and quartz: Experimental constraints on element distribution and grain boundary diffusion

Mantle metasomatism is an important process in subduction zones in which fluids from the dehydrating oceanic slab interact with the overlying upper mantle resulting in a chemical alteration of the mantle. Consequently, this fluid-rock interaction may influence the mantle rock's physical properties such as the deformation behavior. In order to study element redistribution during mantle metasomatism in the laboratory, we used the simplified model reaction olivine + quartz = orthopyroxene, where olivine acts as representative for the upper mantle and quartz as proxy for the metasomatizing agent. We conducted piston-cylinder experiments at 1.5 GPa and 950 to 1400 °C, lasting between 48 and 288 h, on samples containing a mixture of quartz and one set of synthesized forsterite samples doped with either Co, Ni, Mn, or Zn. Additionally, we tested the influence of either nominally anhydrous or hydrous experimental conditions on the chemical distribution of the respective dopant element by using either crushable alumina or natural CaF2 as pressure medium. Results of the chemical analyses of the recovered samples show dopant specific partitioning between doped forsterite and orthopyroxene independent of the confining pressure medium; except for the runs in which Ni-doped forsterite samples were used. The observed Ni- and Co-enrichment in forsterite samples may be used to identify mantle rocks that underwent mantle metasomatism in nature.

Sanukitoid crystallization relations at 1.0 and 0.3 GPa

Sanukitoid (vaugnerites, appinites, durbachites) intrusions have been described in multiple locations around the world. Initially thought as an only basic-intermediate series, increasing interest in these rocks have pointed to the conclusion that they represent a whole differentiation series. A distinct geochemistry with high MgO, LILE and LREE (specially K, Ba and Sr) and depletion in CaO differentiates them from the standard calc-alkaline batholiths they appear associated with. While some of such features may be inherited from the metasomatized mantle source, no consistent explanation has been given for the singular geochemistry of the series. Accordingly, we propose two initial hypotheses: (1) the sanukitoid array is due to the existence of a cotectic line that determines the behaviour of the system; (2) the sanukitoid array does not represent a cotectic series, but the result of a ubiquitous process that deviates the rocks from their ideal differentiation trend. To test the thermodynamic behaviour of the system piston cylinder experiments were conducted at different temperatures in two pressure series appropriate for lower and upper crust differentiation (1.0, 0.3 GPa and 1100 °C, 1050 °C, 1000 °C, 900 °C). Two starting compositions are used: a sanukitoid enclave from Los Pedroches batholith (SW Spain) and a hydrous synthetic glass. Experimental results show that, while 1.0 GPa runs follow the calc-alkaline cotectic, 0.3 GPa runs follow a line that coincides with the sanukitoid series, proving the existence of a low pressure cotectic line. However, lack of cumulates at shallow depths and low water contents in the fractionated magmas (the granites) suggest that a complementary mechanism must be involved. The observed geochemical features may also be accounted for by contamination by entrainment of orthopyroxene from the source or early magmatic cumulates. This is supported by the showcased crystallization pattern, with orthopyroxene representing the liquidus phase, the presence of amphibole clots in these rocks, and the presence of autoliths with abundant orthopyroxene in sanukitoid intrusions. Additionally, the existence of a low pressure cotectic system induces the re-equilibration and digestion of such entrained orthopyroxene in the liquid when reaching shallow depths, leaving a seemingly homogeneous liquid. The two discussed mechanisms are valid, compatible, complementary and account for the geochemical trend shown by the series.

Effect of chlorine on water incorporation in magmatic amphibole: experimental constraints with a micro-Raman spectroscopy approach

Abstract. Amphibole represents an important repository of water (among other volatiles, e.g., chlorine and fluorine) in the lithosphere in all those environments characterized by the circulation of fluids and hydrous melts, such as subduction zones and subcontinental lithospheric mantle. Therefore, detailed knowledge of the mechanisms ruling water incorporation in amphibole is essential to assess the amount of water that can be fixed in the lithosphere by this mineral and, ultimately, gain a better insight into the deep water cycle. Water is incorporated into the structure of amphibole as hydroxyl (OH−), which is hosted in the anion site O(3), and the incorporation is mainly controlled by the oxo-substitution mechanism M(1)Ti14++O(3)O22- M(1)(Mg2+, Fe2+)-1+ O(3)(OH-)-2-. However, the fluids and melts circulating in the lithospheric mantle can be variably enriched in halogens (Cl− and F−) that can substitute OH− in the anion site O(3) of amphibole, thus potentially affecting its water budget. The aim of this study is to evaluate the effect of Cl on the oxo-substitution and the incorporation of water in amphibole. End-loaded piston cylinder experiments were conducted at pressure and temperature conditions compatible with the upper-mantle depth (1.4 GPa and 1015–1050 ∘C) in order to favor the crystallization of amphibole at equilibrium with the coexisting melt. Alkali basalt powder was used as starting material, and water doped with different contents of Cl was added to each experiment. Two ranges of oxygen fugacity (fO2) were investigated at ΔFMQ = −2.6 (log fO2 [experiment] − log fO2 [FMQ buffer]) and ΔFMQ = +1.7, where FMQ is fayalite–magnetite–quartz, in order to preliminarily identify the potential influence of the fO2 on the water budget in amphibole. In this contribution, we propose a new method to quantify water in amphiboles using confocal micro-Raman spectroscopy. The H2O contents range from 2.20 ± 0.10 wt % to 5.03 ± 0.47 wt % in glasses and from 0.93 ± 0.08 wt % to 1.50 ± 0.12 wt % in amphiboles, resulting in a partition coefficient of water between amphibole and glass (Amph/LDH2O) ranging from 0.29 ± 0.06 to 0.52 ± 0.08. Our results show a positive correlation between the Cl content of amphibole (from 0.18 wt % to 0.88 wt %) and the Amph/LDH2O. This effect is ascribed to the incorporation of Cl at the anion site O(3) that influences the oxo-substitution mechanism by impeding the entrance of Ti4+ at the M(1) sites and thus preventing the amphibole dehydrogenation. The effect of Cl reported in this study, which is related to a change in the amphibole crystal structure, highlights that high Cl concentrations in magmatic systems favor the incorporation of water in amphibole rather than in the coexisting melt, although the exchange coefficient between Amph/LDH2O and Amph/LDCl supports a preferential incorporation of water over Cl in amphibole. Therefore, the presence of abundant Cl influences the hydration state of magmas evolving from upper-mantle conditions towards crustal roots with the crystallization of amphibole.

Influence of intensive parameters and assemblies on friction evolution during piston-cylinder experiments

Abstract Piston-cylinder assemblies exhibit inhomogeneous pressure distributions and biases compared to the theoretical pressure applied to the hydraulic press because of the thermal and mechanical properties of the assembly components. Whereas these effects can partially be corrected by conventional calibration, systematic quantification of friction values remain very sparse and results vary greatly among previous studies. We performed an experimental study to investigate the behavior of the most common cell assemblies, i.e., talc [Mg3Si4O10(OH)2], NaCl, and BaCO3, during piston-cylinder experiments to estimate the effects of pressure, temperature, run duration, assembly size, and assembly materials on friction values. Our study demonstrates that friction decreases with time and also partially depends on temperature but does not depend on pressure. We determined that friction decreases from 24 to 17% as temperature increases from 900 to 1300 °C when using talc cells, indicating a friction decrease of ~2% per 100 °C increase for 24 h experiments. In contrast, friction becomes independent of time above 1300 °C. Moreover, at a fixed temperature of 900 °C, friction decreases from 29% in 6 h runs to 21% in 48 h runs, corresponding to a decrease of friction of 0.2% per hour. Similar results obtained with NaCl cell assemblies suggest that friction is constant within error, from 8% in 9 h runs to 5% in 24 h runs. At 900 °C, possible steady-state friction values are only reached after at least 48 h, indicating that friction should be considered a variable for shorter experiments. We establish that assembly materials (and their associated thermomechanical properties) influence the friction correction more than the dimensions of the assembly parts. Finally, we show that the use of polytetrafluoroethylene film instead of conventional Pb foil does not modify friction but significantly reduces the force required for sample extraction, thus increasing the lifetime of the carbide core, which in turn enhances experimental reproducibility.

The effect of fluorine on reaction-rim growth dynamics in the ternary CaO-MgO-SiO2 system

AbstractGrowth of reaction rims is mainly controlled by a change in physical parameters such as pressure and temperature, a change in the chemical composition of the system, and/or by the presence of volatiles. In particular, the effect of volatiles other than water on reaction-rim growth remains poorly understood. To accurately model metamorphic and metasomatic processes, a quantification of the effect of volatiles on reaction-rim growth dynamics is necessary but hitherto missing.In this study, reaction rims were experimentally grown in a series of piston-cylinder experiments in the ternary CaO-MgO-SiO2 system at 1000 °C and 1.5 GPa with 0–10 wt% F for 20 min. In the fluorine-free system, a rim sequence of wollastonite (Wo) | merwinite (Mer) | diopside (Di) | forsterite (Fo) | periclase (Per) formed, complying with the stable phase configuration at water-saturated conditions. As soon as 0.1 wt% F was introduced into the system, humite group minerals (HGMs) and monticellite (Mtc) appeared, resulting in the multilayer rim sequence Wo | Mer | Mtc | Fo + HGMs | Per. In experiments with fluorine concentrations ≥0.5 wt%, cuspidine (Csp) appears in the layer sequence and represents the major fluorine sink. Our data show that the addition of fluorine may stabilize the fluorine-bearing phases cuspidine and HGMs to higher temperatures, which is in agreement with previous studies (Grützner et al. 2017). However, the appearance of the nominally anhydrous minerals (NAMs) monticellite and åkermanite (Ak) at this P-T condition suggests that the addition of fluorine may also affect the stability of nominally fluorine-free minerals. This may be explained by the effect of fluorine on the Gibbs free energies of fluorine-bearing phases, which in turn affects the relative Gibbs free energies and thus the stabilities of all phases. An increase in absolute rim thickness from 11.8(21) to 105.6(22) µm (1σ standard deviations in parentheses) in fluorine free and 10 wt% F experiments, respectively, suggests that fluorine enhances absolute component mobilities and thus results in faster rim growth rates. Additionally, due to the presence of fluorine, a change in relative component mobilities results in microstructural changes such as a phase segregation of diopside and cuspidine at high-fluorine (≥3 wt% F) concentrations.These results not only imply that reaction rims may be used as a tool to infer the amount of fluorine present during metamorphic reactions but also that we need to consider the role of fluorine for a correct interpretation of the P-T-t history of metamorphic and metasomatic rocks.

The composition of subduction zone fluids and the origin of the trace element enrichment in arc magmas

The partitioning of major and trace elements between eclogite and aqueous fluids with variable salinity was studied at 700–800 °C and 4–6 GPa in piston cylinder and multi anvil experiments. Fluid compositions were determined using the diamond trap technique combined with laser ablation ICP-MS measurements in the frozen state. In addition to NaCl, SiO2 is the main solute in the fluids. The fluid/eclogite partition coefficients of the large ion lithophile elements (LILE), such as Rb, Cs, Sr, and Ba as well as those of the light rare earths (LREE), of Pb, and of U increase by up to three orders of magnitude with salinity. These elements will therefore be efficiently transported by saline fluids. On the other hand, typical high field strength elements, such as Ti, Nb, and Ta, are not mobilized even at high salinities. Increasing temperature and pressure gradually increases the partitioning into the fluid. In particular, Th is mobilized by silica-rich fluids at 6 GPa already at low salinities. We show that we can fully reproduce the trace element enrichment pattern of primitive arc basalts by adding a few percent of saline fluid (with 5–10 wt% Cl) released from the basaltic slab to the zone of melting in the mantle wedge. Assuming 2 wt% of rutile in the eclogite equilibrated with the saline fluid produces a negative Nb Ta anomaly that is larger than in most primitive arc basalts. Therefore, we conclude that the rutile fraction in the subducted eclogite below most arcs is likely < 1 wt%. In fact, saline fluids would even produce a noticeable negative Nb Ta anomaly without any rutile in the eclogite residue. Metasomatism by sediment melts alone, on the other hand, is unable to produce the enrichment pattern seen in arc basalts. We, therefore, conclude that at least for primitive arc basalts, the release of hydrous fluids from the basaltic part of the subducted slab is the trigger for melting and the main agent of trace element enrichment. The contribution of sediment melts to the petrogenesis of these magmas is likely negligible. In the supplementary material, we provide a “Subduction Calculator” in Excel format, which allows the calculation of the trace element abundance pattern in primitive arc basalts as function of fluid salinity, the amount of fluid released from the basaltic part of the subducted slab, the fluid fraction added to the source, and the degree of melting.

Experimental settling, floatation and compaction of plagioclase in basaltic melt and a revision of melt density

Centrifuge-assisted piston cylinder experiments were conducted on plagioclase in basaltic melt at 1140–1250 °C, 0.42–0.84 GPa and mostly 1000 g. One set of experiments assesses the settling velocity of a dilute plagioclase suspension; a second sinks or floats plagioclase in a MORB-type melt exploring conditions of neutral buoyancy; and a third set examines floatation of plagioclase from an evolved lunar magma ocean composition. A compaction rate for plagioclase cumulates is established. The experiments demonstrate that neutral density of plagioclase An74 in a MOR-type tholeiitic basalt occurs at 0.59 ± 0.04 GPa (1200 °C), contrasting predictions by present models on melt density which yield a density inversion pressure at 0.10–0.15 GPa. In nature, the level of neutral buoyancy depends on melt composition; nevertheless, for the onset of plagioclase crystallization in dry tholeiitic basalts, our result is robust. As the molar volume of plagioclase is well known, the experimentally determined pressure of neutral buoyancy indicates a correction of -1.6% to previous density models for silicate melts. It follows that for (tholeiitic) layered mafic intrusions, plagioclase is negatively buoyant for early, relatively primitive, parent melts. In contrast, the extreme Fe enrichment of a fractionating lunar magma ocean leads to melt densities that let anorthite always float. Compaction φ/φ0 of experimental plagioclase cumulates is quantified to φ/φ0 = − 0.0582 log (Δρ·h·a·t) + 1.284, where φ0 is the porosity after settling (67 ± 2%), h the cumulate pile height, a acceleration and φ porosity as a function of time t. Gravitational-driven compaction in tens of m-thick plagioclase cumulate in basaltic magmas reaches down to ~ 40% porosity within hundreds of years, a timescales competing with characteristic cooling times of cumulate layers of mafic intrusions. To achieve plagioclase modes > 80% due to compaction, an additional overload of ~ 100 m (layers) of mafic minerals would be required. Compaction of a lunar anorthosite crust of 35 km to 20% porosity (i.e. ~ 90% plagioclase after crystallization of the interstitial melt) would require 30 kyrs.