Olivine Stability Research Articles

Effect of oxygen fugacity on the storage of water in wadsleyite and olivine in H and H–C fluids and implications for melting atop the transition zone

Abstract. This study aims to experimentally constrain the water storage capacities of olivine and wadsleyite at a depth near 410 km (12–14 GPa) under water-saturated conditions, as a function of temperature, oxygen fugacity, and the presence of carbon (molar H / C of 2). Experiments have been conducted in the multi-anvil press, with sealed double capsules to preserve fluids, at 1200 to 1400 ∘C and three different oxygen fugacities fixed at the rhenium–rhenium oxide buffer (RRO), nickel–nickel oxide buffer (NNO), and iron-wüstite (IW) for oxidizing, intermediate, and reducing conditions, respectively. The water contents of minerals were measured by Raman spectroscopy that allows a very small beam size to be used and were cross-checked on a few samples with NanoSIMS analyses. We observe an effect, although slight, of fO2 on the water storage capacity of both wadsleyite and olivine and also on their solidus temperatures. At 1200 ∘C, the storage capacity of the nominally anhydrous minerals (NAMS) increases with increasing oxygen fugacity (from the IW to the RRO buffer) from 1 wt % to 1.5 wt % H2O in wadsleyite and from 0.1 wt % to 0.2 wt % in olivine, owing to the increase in H2O / H2 speciation in the fluid, whereas at 1400 ∘C the storage capacity decreases from 1 wt % to 0.75 wt % H2O in wadsleyite and down to 0.03 wt % for olivine. At high temperature, the water storage capacity is lowered due to melting, and the more oxidized the conditions are the more the solidus is depressed. Still, at 1400 ∘C and IW, wadsleyite can store substantial amounts of water: 0.8 wt % to 1 wt % H2O. The effect of carbon is to decrease water storage capacity in both wadsleyite and olivine by an average factor 2 at 1300–1400 ∘C. The trends in water storage as a function of fO2 and C presence are confirmed by NanoSIMS measurements. The solidus at IW without C is located between 1300 and 1400 ∘C in the wadsleyite stability field and drops to temperatures below 1300 ∘C in the olivine stability field. With the addition of C, the solidus is found between 1200 and 1300 ∘C in both olivine and wadsleyite stability fields.

Complex Effects of Assimilation on Sulfide Saturation Revealed by Modeling with the Magma Chamber Simulator: A Case Study on the Duluth Complex, Minnesota, USA

Abstract Wall-rock assimilation can cause effective sulfide saturation in magmas and lead to the formation of base and precious metal sulfide deposits. Detailed assessments of how assimilation affects the sulfur content at sulfide saturation (SCSS) in magmas have been scarce because of the lack of suitable thermodynamic modeling tools. The Magma Chamber Simulator (MCS) is the first geochemical modeling software that accounts for thermodynamic wall-rock phase equilibrium in open magmatic systems experiencing recharge-assimilation-fractional crystallization. We used the MCS to model SCSS in a magmatic system corresponding to the parental melt of the Partridge River intrusion of the Duluth Complex, Minnesota. This intrusion hosts several Cu-Ni deposits in troctolitic and noritic rocks, which both show evidence of assimilation of the adjacent Virginia Formation black shale. Our simulations show that the dominantly troctolitic rocks can form via fractional crystallization if the parental melt is hydrous (≥ 1 wt % H2O), while gabbroic rocks dominate when the parental melt is H2O poor (≤ 0.14 wt % H2O). Formation of norite from the hydrous parental melt requires ~20–30% of selective assimilation of black shale partial melts or bulk assimilation of stoped blocks. In the fractional crystallization simulations, increasing the H2O content of the parental melt lowers SCSS. In the hydrous fractional crystallization scenarios, SCSS is lowered further by the depletion of FeO from the residual melt, owing to enhanced olivine stability. In the assimilation simulations, the residual melt in the magma subsystem becomes enriched in SiO2, Al2O3, K2O, and H2O with simultaneous depletion in FeO, MgO, CaO, and Na2O. These compositional changes promote sulfide saturation—an effect that is more pronounced in selective rather than in bulk assimilation scenarios. Trace element models, used as a proxy for the efficiency of sulfur assimilation, show that sulfur should behave as an incompatible element (DWR (S) ≤ 1) to wall rock in the selective assimilation simulations, i.e., enriched in early-assimilated wall-rock fluids and/or partial melts, in order to fulfill the natural sulfur isotope criteria of the Duluth Complex. Bulk assimilation may also be efficient enough to modify the sulfur isotope composition, but it requires a large amount of crystallization in the magma and is, hence, considered less likely to be the main process for sulfur assimilation. If wall-rock sulfur is effectively transported to the magma, in situ precipitation of sulfides without notable subsequent upgrading by dynamic processes could produce the sulfide grade of an average Cu-Ni deposit in the Partridge River intrusion.

Experimental study on the effects of H2O upon crystallization in the Lower and Critical Zones of the Bushveld Complex with an emphasis on chromitite formation

Mafic–ultramafic layered rocks of the Lower and Critical Zones of the Bushveld Complex are believed to have formed from a parental magma usually labelled B1 with SiO2 content at 55 wt.%, MgO at about 12 wt.% and Cr2O3 concentration at 0.15 wt.%. Previous experimental study at 300 MPa and nominally dry conditions showed that this composition had orthopyroxene on liquidus between 1250 and 1300 °C, and the mineral was joined by plagioclase and clinopyroxene at and below 1166 °C. The objective of this study was to investigate the effects of variable additions of H2O (2, 4 and 6 wt.% in starting compositions) on liquidus and sub-liquidus phase equilibria. Main attention was given to the solubility of Cr-spinel and Cr distribution between melt and crystalline phases. Six experiments were carried out in the internally heated pressure vessel (IHPV) at 300 MPa and in the temperature interval 1100–1300 °C. Redox conditions were not controlled and thermal dissociation of the H2O component followed by hydrogen diffusion through container walls resulted in a noticeable oxidation of run products. The effective values of fO2 in experiments were calculated using SPINMELT 2.0 software bases on the regression of experimental data on the spinel-melt equilibrium. According to the calculations, H2O additions in isothermal series of experiments increased fO2 by 0.8–1.6 log-units from the level in a nominally dry run. Overall, the experiments showed that H2O additions suppressed crystallization of silicates, shifted olivine-orthopyroxene peritectic equilibrium towards higher olivine stability (olivine first appears in H2O-bearing compositions at 1175 °C) and increased the proportion of magnetite component in spinel solid solution. The addition of 4 wt.% H2O to the starting composition at 1200 °C resulted in complete melting of orthopyroxene, which left Cr-spinel as the only liquidus phase. Equal amount of H2O at 1125 °C leads to complete melting of plagioclase, orthopyroxene and clinopyroxene, which leaves only olivine and spinel stable and increases the liquid mass fraction from 0.62 in the nominally dry compositions to 0.9. The results of this study appear to support the idea that chromitite layers in the Critical Zone of the Bushveld Complex were formed by episodes of hydration melting of silicate cumulates.

Transport, survival and modification of xenoliths and xenocrysts from source to surface

A wide variety of magmas entrain, transport and erupt mantle material in the form of xenoliths and xenocrysts. The host magmas are often low viscosity in nature and range from basalt to more esoteric compositions such as kimberlite, nephelinite and basanite. Here we focus on kimberlite magmas which are particularly successful at transporting deep mantle cargo to the surface, including economically important quantities of diamond. Collections of mantle-derived xenoliths and xenocrysts are critical to our understanding of the structure, stability, composition, thermal state, age, and origin of the lithosphere. However, they also inform on magma transport conditions. Through a series of scaled analogue experiments, we document the relative mechanical stability of olivine, garnet, orthopyroxene, clinopyroxene and diamond xenocrysts during magma ascent. Our experiments fluidized these mantle minerals at a constant gas flux for variable amounts of time approximating transport in a high velocity, turbulent, fluid-rich (supercritical fluid or gas, depending on depth) magma. The evolution of mineral surface features, morphology and grain size distributions is analyzed as a function of residence time. We show that on timescales consistent with magma ascent, each mantle mineral is subject to mechanical modification resulting in mass loss and reshaping (rounding) by grain size reduction and surface pitting. We further discuss the chemical consequences of producing fine particle chips that are highly susceptible to dissolution. Lastly, we utilize an empirical model that relates textural observations (e.g. impact pit size) on xenocrysts to differential particle velocities. Our approach applied to natural kimberlitic olivine and garnet xenocrysts indicates differential velocities of ∼4ms−1 – the first direct estimate for velocity in an ascending kimberlite magma.

Petrological and geochemical modeling of magmas erupted at Vulcano Island in the period 54–8 ka: Quantitative constraints on the sub-volcanic architecture of the plumbing system

In this study, we present new mineralogical and petrological data from fifteen eruptive products erupted at Vulcano Island (Aeolian Arc, Italy) over a period of time from 54 to 8 ka and representative of the Eruptive Epochs from 5 to 8 of this volcanic system. These rocks show shoshonitic (SHO) to high-K calc-alkaline (HKCA) affinity, with compositions changing from primitive basalts-shoshonites (Mg#35–60) to intermediate latites (Mg#32–54) to evolved trachytes-rhyolites (Mg#23–40). The intensive variables driving the crystallization path of magmas were reconstructed through mineral-melt equilibrium and thermodynamic models, as well as barometers, thermometers, hygrometers and oxygen barometers. The stability of olivine (Fo59–91), as first phase on the liquidus, is more favored at low-P (100–300 MPa) and high-H2O (4 wt%) contents dissolved in the melt. Afterwards, the melt is co-saturated with clinopyroxene (Mg#92, diopside), whose composition progressively evolves (Mg#71, augite) as the temperature decreases to 1100 °C. The crystallization pressure recorded by clinopyroxene decreases from basalts (550–750 MPa) to shoshonites-latites-trachytes (100–450 MPa) to rhyolites (~50 MPa). The melt-H2O content (0.5–4.2 wt%) is sensitive to either pressure or melt composition, thereby controlling the plagioclase stability and chemistry (An13–77) within a thermal path of ~860–1100 °C. Titanomagnetite (Usp11–39) equilibrates with progressively more oxidized melts as the magma composition changes from basalt (ΔQFM+1.5) to rhyolite (ΔQFM+3). Mass balance calculations and trace element modeling indicate that basaltic to trachytic magmas are prevalently controlled by fractional crystallization processes, in concert with variable degrees of assimilation of crustal rocks. Conversely, rhyolitic and highly differentiated trachytic magmas are generated by extraction of interstitial melts from shallow mush zones dominated by feldspar and titanomagnetite saturation. We conclude that the architecture of the plumbing system at Vulcano Island is characterized by multiple reservoirs in which compositionally distinct magmas pond and undergo polybaric-polythermal differentiation, before erupting to the surface.

Phosphorus and aluminum zoning in olivine: contrasting behavior of two nominally incompatible trace elements

Phosphorus zoning in olivine is receiving considerable attention for its capacity to preserve key information about rates and mechanisms of crystal growth. Its concentration can vary significantly over sub-micron spatial scales and form intricate, snowflake-like patterns that are generally attributed to fast crystal growth. Ostensibly similar aluminum enrichment patterns have also been observed, suggesting comparable incorporation and partitioning behavior for both elements. We perform 1-atm crystallization experiments on a primitive Kīlauea basalt to examine the formation of P and Al zoning as a function of undercooling − ΔT (− ΔT = Tliquidus − Tcrystallization) during olivine growth. After 24 h spent at Tinitial = 1290 °C (10 °C above olivine stability), charges are rapidly cooled to final temperatures Tfinal = 1220–1270 °C, corresponding to undercoolings − ΔT = 10–60 °C (with Tliquidus = 1280 °C). Compositional X-ray maps of experimental olivine reveal that only a small undercooling (≤ 25 °C) is required to produce the fine-scale enrichments in P and Al associated with skeletal growth. Concentration profiles indicate that despite qualitatively similar enrichment patterns in olivine, P and Al have contrasting apparent crystal/melt mass distribution coefficients of $$K_{\text{P}}^{{{\text{ol}}/{\text{melt}}}}$$ = 0.01‒1 and $$K_{\text{P}}^{{{\text{ol}}/{\text{melt}}}}$$ = 0.002‒0.006. Phosphorus can be enriched by a factor > 40-fold in the same crystal, whereas Al enrichment never exceed factors of 2. Glass in the vicinity of synthetic and natural olivine is usually enriched in Al, but, within analytical uncertainty, not in P. Thus, we find no direct evidence for a compositional boundary layer enriched in P that would suffice to produce P enrichments in natural and synthetic olivine. Numerical models combining growth and diffusion resolve the conditions at which Al-rich boundary layers produce the observed enrichment patterns in olivine. In contrast, the same models fail to reproduce the observed P enrichments, consistent with our observation that P-rich boundary layers are insignificant. If instead, P olivine/melt partitioning is made to depend on growth rate, models adequately reproduce our observations of 40-fold enrichment without boundary layer formation. We surmise that near-partitionless behavior ( $$K_{\text{P}}^{{{\text{ol}}/{\text{melt}}}}$$ close to 1) of P is related to the olivine lattice being perhaps less stiff in accommodating P during rapid crystallization, and/or to enhanced formation of vacancy defects during fast growth. Our results confirm that P is a robust marker of initial rapid growth, but reveal that the undercooling necessary to induce these enrichments is not particularly large. The near-ubiquitous process of magma mixing under volcanoes, for instance, is likely sufficient to induce low-to-moderate degrees of undercooling required for skeletal growth.

Effect of water on the phase relations of primitive K-basalts: Implications for high-pressure differentiation in the Phlegraean Volcanic District magmatic system

Phase relations, compositions, and modes of two natural primitive alkaline basalts were determined through experiments performed at 0.8 GPa, temperatures between 1000 and 1310 °C, and nominal water (H2Oi) contents varying from <1 (natural water rock content) to 6 wt%. The two natural samples used as starting material are high-Mg, moderate-K, alkaline basalt scoria clasts (Mg# = 0.68–0.66, Σalkalis = 4.4 wt%) dispersed within the hydromagmatic tuff of the Solchiaro eruption (Procida Island, southern Italy). These samples are considered representative of parental magmas feeding the Phlegraean Volcanic District, which includes the Campi Flegrei caldera and the Procida and Ischia islands. The composition of the experimental phases has been compared with that of natural rocks to constrain the early stages of crystallization of poorly differentiated magmas from the Phlegraean Volcanic District. Our study shows that the liquidus temperature at H2Oi < 1 wt% is ~1300 °C whereas with water contents between 1.5 and 3 wt%, and between 4 and 6 wt%, the apparent liquidus temperature decreases of about 50–100 °C, respectively. Under natural water conditions (<1 wt%), the liquidus phase is clinopyroxene, followed by olivine and Cr-spinel and, then, by plagioclase. The increase of H2O content in the system enhances olivine stability along with clinopyroxene at the liquidus temperature. The 4–6 wt% water bearing experiments show evidence of H2O saturation at 1100 °C, with olivine being replaced by orthopyroxene later joined by pargasitic amphibole and minor amounts of oxide starting from 1080 °C. The composition of glasses in the experiments with low H2Oi content is consistent with the trachybasaltic products of Procida; glasses obtained in the runs with 1.5–3 wt% water resemble the trend formed by the shoshonitic products of Campi Flegrei. The water-saturated experiments produced variably alkali-depleted residual glasses whose composition converges toward the subalkaline field. The compositions of the experimental clinopyroxenes match well those found in the Campi Flegrei products for which we have estimated crystallization conditions from 1140 to 1200 °C of temperature and from 0.6 to 0.9 GPa of pressure (22–33 km). Phase equilibria constraints and clinopyroxene compositions suggest that the parental magmas of Procida likely contained <2 wt% of H2O and were affected by an early stage of fractionation at a depth corresponding to the local Moho (~ 25 km corresponding to ~ 0.8 GPa assuming an average crustal density of 2.8 g/cm3). Based on our results, an early magmatic storage at this level could have caused a deep degassing process able to remove variable proportions of Na and K from the melt.

Slow slip rate and excitation efficiency of deep low-frequency tremors beneath southwest Japan

We estimated the long-term average slip rate on the plate interface across the Nankai subduction zone during 2002–2013 using deep low-frequency tremors as a proxy for short-term slow slip events based on empirical relations between the seismic moment of short-term slow slip events and tremor activities. The slip rate in each region is likely to compensate for differences between the convergence rate and the slip deficit rate of the subducting Philippine Sea plate estimated geodetically, although the uncertainty is large. This implies that the strain because of the subduction of the plate is partially stored as the slip deficit and partially released by slow slip events during the interseismic period. The excitation efficiency of the tremors for the slow slip events differs among regions: it is high in the northern Kii region. Some events in the western Shikoku region show a somewhat large value. Antigorite serpentinite of two types exists in the mantle wedge beneath southwest Japan. Slips with more effective excitation of tremors presumably occur in high-temperature conditions in the antigorite+olivine stability field. Other slip events with low excitation efficiency are distributed in the antigorite+brucite stability field. Considering the formation reactions of these minerals and their characteristic structures, events with high excitation efficiency can be correlated with a high pore fluid pressure condition. This result suggests that variation in pore fluid pressure on the plate interface affects the magnitude of tremors excited by slow slip events.

Experimental investigation of condensation predictions for dust-enriched systems

Condensation models describe the equilibrium distribution of elements between coexisting phases (mineral solid solutions, silicate liquid, and vapor) in a closed chemical system, where the vapor phase is always present, using equations of state of the phases involved at a fixed total pressure (<1bar) and temperature (T). The VAPORS code uses a CaO–MgO–Al2O3–SiO2 (CMAS) liquid model at T above the stability field of olivine, and the MELTS thermodynamics algorithm at lower T. Quenched high-T crystal+liquid assemblages are preserved in meteorites as Type B Ca-, Al-rich inclusions (CAIs), and olivine-rich ferromagnesian chondrules. Experimental tests of compositional regions within 100K of the predicted T of olivine stability may clarify the nature of the phases present, the phase boundaries, and the partition of trace elements among these phases. Twenty-three Pt-loop equilibrium experiments in seven phase fields on twelve bulk compositions at specific T and dust enrichment factors tested the predicted stability fields of forsteritic olivine (Mg2SiO4), enstatite (MgSiO3), Cr-bearing spinel (MgAl2O4), perovskite (CaTiO3), melilite (Ca2Al2SiO7–Ca2Mg2Si2O7) and/or grossite (CaAl4O7) crystallizing from liquid. Experimental results for forsterite, enstatite, and grossite are in very good agreement with predictions, both in chemistry and phase abundances. On the other hand the stability of spinel with olivine, and stability of perovskite and gehlenite are quite different from predictions. Perovskite is absent in all experiments. Even at low oxygen fugacity (IW-3.4), the most TiO2-rich experiments do not crystallize Al-, Ti-bearing calcic pyroxene. The stability of spinel and olivine together is limited to a smaller phase field than is predicted. The melilite stability field is much larger than predicted, indicating a deficiency of current liquid or melilite activity models. In that respect, these experiments contribute to improving the data for calibrating thermodynamic models including MELTS.

Two types of antigorite serpentinite controlling heterogeneous slow-slip behaviours of slab–mantle interface

It is known that plate boundaries in subduction zones show heterogeneous slip nature with strongly coupled seismogenic zones and various types of episodic tremor and slip (ETS) zones. In order to examine the petrological controls on the large-scale structure, we compared recent geophysical observations in the Shikoku area, southwest Japan with petrological models of the hanging wall mantle wedge. As a result, we found a close relationship between mineral assemblages in the mantle wedge and the characteristics of slow slip behaviour recorded in the Shikoku area: Short-term ETSs take place in the antigorite + olivine stability field and silent long-term slow slip events (SSEs) take place in the antigorite+brucite stability field. Based on observations of natural antigorite serpentinites, we propose a model that the dominant serpentinization reaction in the mantle wedge changes with increasing depth resulting in variable extents of pore fluid pressures along slip planes. The serpentinization reaction in the antigorite+brucite stability field (olivine + H2O → antigorite + brucite) proceeds at the expense of water. This is consistent with moderately elevated pore pressures inferred for long-term SSEs. The existence of weak brucite enhances the development of shear zones oblique to the main foliation. The resultant anastomosing network provides fluid pathways that may help reduce pore pressures on slip planes. In contrast, progress of the serpentinization reaction in the antigorite + olivine stability field (olivine + H2O + SiO2 → antigorite) results in a large amount of residual water that contributes to further increase pore fluid pressures on slip planes of short-term SSEs. Our results imply that understanding of serpentinization reactions and their contributions to fluid networks in mantle wedge is important in constructing quantitative 3-D models for strain evolutions along plate boundaries.

Leaching of S, Cu, and Fe from disseminated Ni-(Fe)-(Cu) sulphide ore during serpentinization of dunite host rocks at Mount Keith, Agnew-Wiluna belt, Western Australia

Komatiite-hosted disseminated Ni sulphide deposits in the Agnew-Wiluna greenstone belt occur both above and below the olivine isograd that was imposed on the greenstone sequence during the M2 metamorphic/deformation event. Deposits in the northern and central part of the belt and that are located below the isograd (Mount Keith, Honeymoon Well and West Jordan) have complex sulphide mineralogy and strongly zoned sulphide assemblages. These range from least-altered assemblages of pentlandite-pyrrhotite-chalcopyrite±pyrite to altered assemblages of pentlandite±chalcopyrite, pentlandite-heazlewoodite (or millerite), heazlewoodite (or millerite), and rarely to heazlewoodite-native Ni. Deposits to the south and that are above of the olivine isograd (Six Mile, Goliath North) are dominated by less complex magmatic assemblages with a lower proportion of weakly altered pentlandite±chalcopyrite assemblages. More altered assemblages are uncommon in these deposits and occur as isolated patches around the periphery of the deposits. The sulphide zonation is reflected by whole-rock reductions in S, Cu, Fe and Zn, whereas Ni, Pt and Pd and, with some exceptions, Co are conservative. The leaching of S, Cu, Fe and Zn from sulphide assemblages and the whole rock was initiated by highly reduced conditions that were produced during low fluid/rock ratio serpentinization. Consumption of H2O resulted in Cl, a component of the fluid, being concentrated sufficiently to stabilise iowaite as part of lizardite-rich assemblages. Once the rate of olivine hydration reactions declined and during and after expansion and associated fracturing of the ultramafic sequence allowed higher fluid access, a more fluid-dominated environment formed and new carbonate-bearing fluid gained access to varying extents to the ultramafic rock sequence. This drove Cl from iowaite (to form pyroaurite) and caused the sulphide assemblages to be altered from the original magmatic assemblages and compositions to those stable at the prevailing fO2 and fS2 conditions. Mass transfer was made possible via metal chloride complexes and H2S with fluids driven by deformation associated with the M2 metamorphism. Disseminated deposits in higher metamorphic grade terrains where olivine was stable during peak metamorphism did not undergo the metasomatism seen in the deposits in areas of lower metamorphic grade. Some minor leaching of S, Fe and Cu occurred around the periphery of the deposits during early, pre-M2 peak metamorphism, but once olivine stability was reached the driving force for the series of leaching reactions was exhausted. The effect of this process on the original magmatic sulphides is to induce significant variability in texture, mineralogy and bulk composition and to markedly reduce the Fe and S contents of the sulphide fraction (in extreme cases to zero for both elements), and to reduce the volume of the sulphide fraction per unit of Ni. These changes impact unfavourably on Ni sulphide recoveries and metallurgical characteristics of these Ni ores.

Oxidised phase relations of a primitive basalt from Grenada, Lesser Antilles

A series of liquidus determinations is reported for a primitive arc basalt (15.4 wt % MgO, 45.5 wt % SiO2) from Grenada, Lesser Antilles, at anhydrous, H2O-undersaturated and H2O-saturated conditions in the pressure range 1 atm to 1.7 GPa. \(\hbox{Fe}^{3+}/\Upsigma\hbox{Fe}\) of high-pressure experimental glasses as measured by μXANES ranges from 0.44 to 0.86, corresponding to oxygen fugacities (fO2) between 3.2 and 7.8 log units above the nickel–nickel oxide redox buffer (NNO). 1-atm experiments conducted from NNO − 2.5 to + 3.8 show that increasing fO2 mainly increases the forsterite content (Fo) of olivine and has little effect on phase relations. The crystallisation sequence at lower crustal pressures for all water contents is forsteritic olivine + Cr-rich spinel followed by clinopyroxene. The anhydrous liquidus is depressed by 100 and 120 °C in the presence of 2.9 and 3.8 wt % H2O, respectively. H2O-undersaturated experiments at NNO + 3.2 to + 4.5 produce olivine of equivalent composition to the most primitive olivine phenocrysts in Grenadan picrites (Fo91.4). We conclude that direct mantle melts originating beneath Grenada could be as oxidised as ~NNO + 3, consistent with the uppermost estimates from olivine–spinel oxybarometry of high Mg basalts. μXANES analyses of olivine-bearing experimental glasses are used to develop a semi-empirical oxybarometer based on the value of \({{K}_{D}}_{\rm ol-melt}^{\rm Fe-Mg}\) when all Fe is assumed to be in the Fe2+ state (\({K}_{D}^{{\rm Fe}_T}\)). The oxybarometer is tested on an independent data set and is able to reproduce experimental fO2 to ≤1.2 log units. Experiments also show that the geochemically and petrographically distinct M- and C-series lavas on the island can be produced from hydrous melting of a common picritic source. Low pressures expand the olivine stability field at the expense of clinopyroxene, enriching an evolving melt in CaO and forcing differentiation to take place along a C-series liquid line of descent. Higher pressure conditions allow early and abundant clinopyroxene crystallisation, rapidly depleting the melt in both CaO and MgO, and thus creating the M-series.

The stability of ortho- and clinopyroxenes, olivine, and garnet in kimberlitic magma

It is generally accepted that the composition of ultrabasic nodules and their quantitative proportions do not significantly change during their transportation with kimberlitic magma to the Earth’s surface. We performed an experimental study of the relative stability of olivine, garnet, and pyroxenes in kimberlite melt at high pressure and temperatures (4 GPa, 1300–1500 °C). The study has shown that the loss in weight of minerals and, correspondingly, the rate of their dissolution in kimberlite melt differ considerably. The following sequence of the dissolution rates of minerals has been established: Cpx ≥ Opx > Gar > Ol. Pyroxenes are characterized by the most rapid dissolution, and olivine is the most stable mineral. The assumption is made that clinopyroxenites and websterites disintegrate more rapidly than dunites and lherzolites in kimberlitic magma.

Olivine as tar removal catalyst in biomass gasification: Catalyst dynamics under model conditions

Olivine ((Mg,Fe)2SiO4) has been extensively explored as an active bed material for catalytic cracking of tars during gasification of biomass in dual fluidized bed reactors. It is known that both the elemental composition, addition of Fe and high temperature calcinations influence the catalytic properties of this mineral. However, it is not clear how olivine responds to the fairly hostile environments present during gasification or what chemical state Fe takes during operation. We have investigated the stability of Austrian olivine under model conditions, resembling those in a gasifier. Powder samples were heated to 750°C in a quartz-tube flow-reactor and sequentially exposed to oxidizing (O2, H2O, CO2) or reducing gases (CO, H2) or mixtures thereof, for various durations of time. Significant changes in phase composition of the material, depending on the gas composition and the duration of the treatments, were found using X-ray photo-electron spectroscopy (XPS), X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS) and scanning electron microscopy (SEM). A large fraction of the Fe in the investigated material is present as free Fe-phases, which are sensitive to changes in the gas environment. After exposure to oxidizing gases, the free Fe phases are: Fe2O3 and Fe3O4 or MgFe2O4. Upon exposure to reducing gases, the iron oxides are converted into Fe0 and Fe3C and formation of graphitic carbon is observed. In addition, the elemental composition of the surface changes dramatically depending on the gas composition. After exposure to oxidizing environments, the amount of Fe at the surface is twice as high as after reduction. Both the change in chemical state of the Fe-phases, the amount of surface Fe and the build-up of surface carbon are fast processes under the applied conditions and significant changes are observed on the time scale of one minute. These observations have important implications for olivine as a tar cracking catalyst, especially when used in dual fluidized bed gasifiers. The fast reduction of the iron oxides upon switching from oxidizing to reducing conditions shows that olivine transports oxygen from the combustor into the gasifier. Furthermore, the catalytic properties of Fe depend strongly on its chemical state. Therefore, the catalytic function of olivine depends strongly on the gas environment and on the catalysts residence time in the gasifier. Finally, both the decreasing amount of surface-Fe and the carbon deposition observed after exposing olivine to reducing conditions can result in significant catalyst deactivation.

H2O-rich melt inclusions in fayalitic olivine from Hekla volcano: Implications for phase relationships in silicic systems and driving forces of explosive volcanism on Iceland

Silicic Icelandic magmas are widely believed to contain low to moderate H2O content prior to degassing, and that their high explosivity mostly results from the interaction of the magmas with ice or meteoric water. Here we report the compositions of glass inclusions (SiO2=57–72wt%, K2O=1.3–2.6wt%) in Fe-rich olivines (Fo2–42) from the largest Holocene eruptions of Hekla volcano (H3 and H4) on Iceland, which preserved quenched melts with very high primary H2O contents (3.3–6.2wt%). The silicic Hekla melts originate primarily by extensive (∼90%) crystal fractionation of H2O-poor (∼0.6wt%) basalts and represent an end member in the systematics of terrestrial magmas because they originate at low fO2 (ΔQFM ∼−0.1 to −0.4) and have as high H2O contents as significantly more oxidized island-arc magmas (ΔQFM≥1). This demonstrates that H2O and ΔQFM do not correlate in silicic magmas from different tectonic settings, and that fO2, not H2O content, shows a major difference between silicic ocean-island (e.g., Icelandic) and island-arc magmas. Analysis of available experimental data suggests that high H2O activity and low fO2 expand the field of olivine stability in silicic melts. Low fO2 and low MgO content could also suppress crystallization of amphibole. On the basis of these results we propose that an anhydrous mineral assemblage bearing Fe-rich olivine in evolved volcanic and Skaergaard-type intrusive rocks does not imply low H2O in magmas prior to degassing but, in contrast to the commonly held view, is an indicator of H2O-rich silicic parental magmas crystallized at low fO2. Finally, the high H2O content in magma was a major driving force of the largest explosive eruptions of Hekla volcano and must be at least as important for driving silicic explosive volcanism on Iceland as magma–ice interaction.

Thermal Stability of Binary Olivine Li1-yFe1-xMnxPO4 [0 ≤ x, y ≤ 1] for Li Rechargeable Battery

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Hydrous, Low-carbon Melting of Garnet Peridotite

The presence of volatile species in the Earth's upper mantle drives the formation of low-degree melts at pressures and temperatures at which volatile-free mantle rocks would be subsolidus. The two most abundant volatile species, given the oxidation state of the Earth's upper mantle, are carbon dioxide and water; each species has a distinct effect on the melting process. We present experimental melting results from 3 GPa and 1375°C on hydrous systems with controlled water contents and rigorously minimized carbon contamination that constrain the independent effects of these volatiles. The hydrous melts in these experiments are in equilibrium with garnet peridotite at pressures reasonable for hydrous melting under mid-ocean ridges. Compared with anhydrous experiments or carbon-rich silicate melting, the addition of water produces a melt with increased SiO_2 content relative to MgO and FeO, tantamount to an increase in the stability of olivine at the solidus relative to the other crystalline phases. We also report a substantial and unexpected change in the composition of clinopyroxene in equilibrium with the melt; the clinopyroxene stability field contracts when water is added to the system, producing clinopyroxenes with higher CaO and lower Al_2O_3 than found at the same pressure without water. The contraction of the clinopyroxene field decreases the bulk partition coefficients of TiO_2, Na_2O, heavy rare earth elements, U, and H_2O, with important implications for hydrous melting of the mantle; for example, initiating hydrous melting deeper in the garnet lherzolite stability field.

Experimental deformation of olivine single crystals at mantle pressures and temperatures

Deformation experiments were carried out in a deformation-DIA high-pressure apparatus (D-DIA) on oriented San Carlos olivine single crystals, at pressure ( P) ranging from 3.5 to 8.5 GPa, temperature ( T) from 1373 to 1673 K, and in poor water condition. Oxygen fugacity ( fO 2) was maintained within the olivine stability field and contact with enstatite powder ensured an orthopyroxene activity a opx = 1. Two compression directions were tested, promoting either [1 0 0] slip alone or [0 0 1] slip alone in (0 1 0) crystallographic plane, here called, respectively, a-slip and c-slip. Constant applied stress ( σ) and specimen strain rates ( ε ˙ ) were monitored in situ using time-resolved X-ray synchrotron diffraction and radiography, respectively. Transmission electron microscopy (TEM) investigation of run products revealed that dislocation creep was responsible for sample deformation. Comparison of the obtained high- P deformation data with the data obtained at room- P by Bai et al. [Bai, Q., Mackwell, S.L., Kohlstedt D.L., 1991, High-temperature creep of olivine single crystals. 1. Mechanical results for buffered samples, Journal of Geophysical Research, 96, 2441–2463] – on identical materials deformed at comparable T– σ– fO 2– a opx conditions – allowed quantifying the P effect on a-slip and c-slip rheological laws. A slip transition with increasing pressure, from dominant a-slip to dominant c-slip, is documented. a-slip appears sensitive to pressure, which translates into the high activation volume V a * = 12 ± 4 c m 3 /mol in the corresponding rheological law, while pressure has little effect on c-slip with V c * = 3 ± 4 c m 3 /mol . These results may explain the discrepancy between olivine low- P and high- P deformation data which has been debated in the literature for more than a decade.

Simultaneous measurements of thermal conductivity and thermal diffusivity for garnet and olivine under high pressure

Thermal conductivity λ and thermal diffusivity κ have been measured simultaneously for garnet (Al 73Py 25Gr 1Sp 1) and olivine (Fo 93Fa 7) single crystals up to 8.3 GPa and 1100 K by using a pulse heating method. Anisotropy of thermal conduction is investigated along the three crystallographic axes, [1 0 0], [0 1 0], and [0 0 1] in olivine. The pressure dependence of κ, or dln κ/d P, is determined to be 0.03–0.04 GPa −1 for the three crystallographic direction in olivine and 0.03 GPa −1 for garnet, while dln λ/d P is determined to be ∼0.04 GPa −1 for olivine and 0.03 GPa −1 for garnet. The anisotropy in thermal diffusivity or thermal conductivity of olivine is clearly observed in the present experimental range of pressure and temperature. It is likely that the anisotropy in thermal conduction would be maintained throughout the olivine stability field in the mantle down to 410 km depth. Heat capacities of olivine and garnet are calculated from the present κ and λ. Both the present heat capacities of olivine and garnet are consistent with the previously reported values within 5%. Those obtained for the three crystallographic direction in olivine are also consistent with each other within 6%. Further, the effect of basalt–eclogite transition on heat transportation is discussed based on the present results for garnet. It implies that the eclogite layer of the slab transmits thermal energy more effectively than the upper basaltic layer.

Structures, compressibilities and relative stabilities of the , and phases of Mg2SiO4 deduced from an electron-gas ionic Hamiltonian

The properties of forsterite (α-Mg2SiO4) and its high-pressure polymorphs wadsleyite (β-Mg2SiO4) and ringwoodite (γ-Mg2SiO4) play an important role in the chemical, dynamic and elastic behaviour of the upper 660 km of the Earth's mantle. Mineralogists have tended to rationalize their structures in terms of empirical ionic models, guided by Pauling's rules. Modern electron-gas theory of ionic interactions is accurate enough to encourage a detailed study of the structure and properties of these minerals within an intuitively appealing ionic model that is independent of empirical data. We therefore undertook to obtain the minimum-energy lattice structures, and their corresponding equations of state, of electron-gas models of the Mg2SiO4 polymorphs. We find that the model densities compare very well with actual observations, while compressibilities are somewhat underestimated. Unit cell geometries are also accurately represented, but coordination polyhedra tend to be more distorted than those observed, mainly because of excessive bond-angle variance. We also found that Pauling's rules do not offer a complete accounting for the observed stability of olivine at low pressures. Instead, in the model crystals it is a difference in the magnitude of the Mg–O repulsions that give olivine the energetic advantage. The higher density of ringwoodite, and a rapidly increasing Mg–O repulsion energy in olivine stabilize ringwoodite at high pressure.