High Pressure Temperature Experiments Research Articles

A diamond-bearing core-mantle boundary on Mercury

Abundant carbon was identified on Mercury by MESSENGER, which is interpreted as the remnant of a primordial graphite flotation crust, suggesting that the magma ocean and core were saturated in carbon. We re-evaluate carbon speciation in Mercury’s interior in light of the high pressure-temperature experiments, thermodynamic models and the most recent geophysical models of the internal structure of the planet. Although a sulfur-free melt would have been in the stability field of graphite, sulfur dissolution in the melt under the unique reduced conditions depressed the sulfur-rich liquidus to temperatures spanning the graphite-diamond transition. Here we show it is possible, though statistically unlikely, that diamond was stable in the magma ocean. However, the formation of a solid inner core caused diamond to crystallize from the cooling molten core and formation of a diamond layer becoming thicker with time.

Electron Microscopy Characterization of Minerals in the K-Na-Al-Si-O System Recovered from High Pressure-Temperature Experiments.

Electron Microscopy Characterization of Minerals in the K-Na-Al-Si-O System Recovered from High Pressure-Temperature Experiments.

Limited nitrogen isotopic fractionation during core-mantle differentiation in rocky protoplanets and planets

15N/14N ratios of meteorites are a powerful tool for tracing the journey of life-essential volatiles like nitrogen (N), carbon and water from nebular solids to the present-day rocky planets, including Earth. The utility of 15N/14N ratios of samples originating from differentiated protoplanets (e.g., iron meteorites) and planets (e.g., Earth’s mantle) for tracing this journey could be affected by the fractionation of N isotopes during core-mantle differentiation, which would overprint their primitive compositions. The extent of N isotopic fractionation during core-mantle differentiation and its effect on the 15N/14N ratios of resulting metallic and silicate reservoirs is, however, poorly understood. Using high pressure–temperature experiments, here we show that equilibrium N isotopic fractionation between metallic and silicate melts (Δ15Nalloy–silicate = δ15Nalloy – δ15Nsilicate = –3.3 ‰ to –1.0 ‰) is limited across a wide range of oxygen fugacity and is much smaller than previous estimates. Also, we present ab initio calculations based on the relevant N speciation in metallic and silicate melts confirming both the magnitude and direction of equilibrium N isotopic fractionation predicted by our experimental results. Limited N isotopic fractionation during core-mantle differentiation suggests that the core and mantle relicts largely preserve the N isotopic compositions of their bulk bodies. Based on the δ15N values of non-carbonaceous iron meteorites (as low as –95 ‰), we predict that the extent of variations in the N isotopic compositions of inner solar system protoplanets was larger than that recorded by enstatite chondrites (δ15N = –29 ‰ to –6‰). As most of the Earth grew primarily via the accretion of similar inner solar system protoplanets, a relatively high δ15N value of present-day Earth’s primitive mantle (–5‰) cannot be explained by the accretion of enstatite chondrite-like materials alone and necessitates a significant contribution of 15N-rich materials to the Earth’s interior.

A New Approach Determining a Phase Transition Boundary Strictly Following a Definition of Phase Equilibrium: An Example of the Post-Spinel Transition in Mg2SiO4 System

The Clapeyron slope is the slope of a phase boundary in P–T space and is essential for understanding mantle dynamics and evolution. The phase boundary is delineating instead of balancing a phase transition’s normal and reverse reactions. Many previous high pressure–temperature experiments determining the phase boundaries of major mantle minerals experienced severe problems due to instantaneous pressure increase by thermal pressure, pressure drop during heating, and sluggish transition kinetics. These complex pressure changes underestimate the transition pressure, while the sluggish kinetics require excess pressures to initiate or proceed with the transition, misinterpreting the phase stability and preventing tight bracketing of the phase boundary. Our recent study developed a novel approach to strictly determine phase stability based on the phase equilibrium definition. Here, we explain the details of this technique, using the post-spinel transition in Mg2SiO4 determined by our recent work as an example. An essential technique is to observe the change in X-ray diffraction intensity between ringwoodite and bridgmanite + periclase during the spontaneous pressure drop at a constant temperature and press load with the coexistence of both phases. This observation removes the complicated pressure change upon heating and kinetic problem, providing an accurate and precise phase boundary.

Internal resistive heating of non-metallic samples to 3000K and >60GPa in the diamond anvil cell.

High pressure-temperature experiments provide information on the phase diagrams and physical characteristics of matter at extreme conditions and offer a synthesis pathway for novel materials with useful properties. Experiments recreating the conditions of planetary interiors provide important constraints on the physical properties of constituent phases and are key to developing models of planetary processes and interpreting geophysical observations. The laser-heated diamond anvil cell (DAC) is currently the only technique capable of routinely accessing the Earth's lower-mantle geotherm for experiments on non-metallic samples, but large temperature uncertainties and poor temperature stability limit the accuracy of measured data and prohibits analyses requiring long acquisition times. We have developed a novel internal resistive heating (IRH) technique for the DAC and demonstrate stable heating of non-metallic samples up to 3000K and 64GPa, as confirmed by in situ synchrotron x-ray diffraction and simultaneous spectroradiometric temperature measurement. The temperature generated in our IRH-DAC can be precisely controlled and is extremely stable, with less than 20K variation over several hours without any user intervention, resulting in temperature uncertainties an order of magnitude smaller than those in typical laser-heating experiments. Our IRH-DAC design, with its simple geometry, provides a new and highly accessible tool for investigating materials at extreme conditions. It is well suited for the rapid collection of high-resolution P-V-T data, precise demarcation of phase boundaries, and experiments requiring long acquisition times at high temperature. Our IRH technique is ideally placed to exploit the move toward coherent nano-focused x-ray beams at next-generation synchrotron sources.

Rates of protoplanetary accretion and differentiation set nitrogen budget of rocky planets.

The effect of protoplanetary differentiation on the fate of life-essential volatiles like nitrogen and carbon and its subsequent effect on the dynamics of planetary growth is unknown. Because the dissolution of nitrogen in magma oceans depends on its partial pressure and oxygen fugacity, it is an ideal proxy to track volatile re-distribution in protoplanets as a function of their sizes and growth zones. Using high pressure-temperature experiments in graphite-undersaturated conditions, here we show that the siderophile (iron-loving) character of nitrogen is an order of magnitude higher than previous estimates across a wide range of oxygen fugacity. The experimental data combined with metal-silicate-atmosphere fractionation models suggest that asteroid-sized protoplanets, and planetary embryos that grew from them, were nitrogen-depleted. However, protoplanets that grew to planetary embryo-size before undergoing differentiation had nitrogen-rich cores and nitrogen-poor silicate reservoirs. Bulk silicate reservoirs of large Earth-like planets attained nitrogen from the cores of latter type of planetary embryos. Therefore, to satisfy the volatile budgets of Earth-like planets during the main stage of their growth, the timescales of planetary embryo accretion had to be shorter than their differentiation timescales, i.e., Moon- to Mars-sized planetary embryos grew rapidly within ~1-2 Myrs of the Solar System's formation.

Mineralogy of the deep lower mantle in the presence of H2O.

Understanding the mineralogy of the Earth's interior is a prerequisite for unravelling the evolution and dynamics of our planet. Here, we conducted high pressure-temperature experiments mimicking the conditions of the deep lower mantle (DLM, 1800–2890 km in depth) and observed surprising mineralogical transformations in the presence of water. Ferropericlase, (Mg, Fe)O, which is the most abundant oxide mineral in Earth, reacts with H2O to form a previously unknown (Mg, Fe)O2Hx (x ≤ 1) phase. The (Mg, Fe)O2Hx has a pyrite structure and it coexists with the dominant silicate phases, bridgmanite and post-perovskite. Depending on Mg content and geotherm temperatures, the transformation may occur at 1800 km for (Mg0.6Fe0.4)O or beyond 2300 km for (Mg0.7Fe0.3)O. The (Mg, Fe)O2Hx is an oxygen excess phase that stores an excessive amount of oxygen beyond the charge balance of maximum cation valences (Mg2+, Fe3+ and H+). This important phase has a number of far-reaching implications including extreme redox inhomogeneity, deep-oxygen reservoirs in the DLM and an internal source for modulating oxygen in the atmosphere.

Eoarchaean tectonics: New constraints from high pressure-temperature experiments and mass balance modelling

Four billion years ago the formation and preservation of the Earth’s first continental crust marked a new stage in the long-term geochemical evolution of our planet. This continental material is predominantly composed of plagioclase-rich granitoids that form Eoarchaean Tonalite and Trondjhemite (ETT) rock suites. There are two plausible overarching models for generating ETT: by a variety of intraplate melting mechanisms or through the onset of primitive subduction. Here we use mass balance modelling to show that ETT can only be derived from the hydrous melting of mafic rocks in convergent margins. These mafic source regions could be composed of relatively undepleted (oceanic plateau-like) non-arc metabasalts with or without residual rutile. Melts derived from these metabasalts subsequently mix with a slab-related component to form ETT magmas. Additionally, ETT can be formed from partial melting of metabasalts with geochemical affinities similar to modern island arc basalts. We propose that a primitive form of plate tectonics began on the surface of Hadean magma ocean(s), in a similar way to the plate-like motions observed on modern lava lakes, and subsequently evolved into the plate tectonics operating today. Hydrous melting of subducted basaltic crust and the formation of the proto-continental ETT crust would have followed the onset of accumulation of liquid water on the Earth’s surface.

Delivery of carbon, nitrogen, and sulfur to the silicate Earth by a giant impact.

Earth's status as the only life-sustaining planet is a result of the timing and delivery mechanism of carbon (C), nitrogen (N), sulfur (S), and hydrogen (H). On the basis of their isotopic signatures, terrestrial volatiles are thought to have derived from carbonaceous chondrites, while the isotopic compositions of nonvolatile major and trace elements suggest that enstatite chondrite-like materials are the primary building blocks of Earth. However, the C/N ratio of the bulk silicate Earth (BSE) is superchondritic, which rules out volatile delivery by a chondritic late veneer. In addition, if delivered during the main phase of Earth's accretion, then, owing to the greater siderophile (metal loving) nature of C relative to N, core formation should have left behind a subchondritic C/N ratio in the BSE. Here, we present high pressure-temperature experiments to constrain the fate of mixed C-N-S volatiles during core-mantle segregation in the planetary embryo magma oceans and show that C becomes much less siderophile in N-bearing and S-rich alloys, while the siderophile character of N remains largely unaffected in the presence of S. Using the new data and inverse Monte Carlo simulations, we show that the impact of a Mars-sized planet, having minimal contributions from carbonaceous chondrite-like material and coinciding with the Moon-forming event, can be the source of major volatiles in the BSE.

New Generation of Hoek Cells

A new generation of Hoek cells was designed and built to conduct triaxial testing on rock-like materials. The new design has the advantages of both conventional Hoek cells (e.g., minimal preparation and setup time) and conventional triaxial systems (e.g., on specimen radial deformation measurement), and it is capable of measuring the permeability at a controlled temperature. This quick release triaxial cell is robust, fast, and cost efficient and can be used for high pressure–high temperature experiments. In order to assess the robustness of the designed cell, a number of triaxial tests were carried out on different rock specimens (sandstone and marble) at different confining pressures (10, 30, and 50 MPa) and controlled temperatures (30°C and 60°C). In addition, the permeability measurement was conducted on the sandstone specimen under triaxial loading having 10 MPa confining pressure and 30°C temperature.

Diamond anvils with a round table designed for high pressure experiments in DAC

ABSTRACTHere, we present new Diamond Anvils with a Round Table (DART-anvils) designed for applications in the diamond anvil cell (DAC) technique. The main features of the new DART-anvil design are a spherical shape of both the crown and the table of a diamond and the position of the centre of the culet exactly in the centre of the sphere. The performance of DART-anvils was tested in a number of high pressure high-temperature experiments at different synchrotron beamlines. These experiments demonstrated a number of advantages, which are unavailable with any of the hitherto known anvil designs. Use of DART-anvils enables to realise in situ single-crystal X-ray diffraction experiments with laser heating using stationary laser-heating setups; eliminating flat-plate design of conventional anvils, DART-anvils make the cell alignment easier; working as solid immersion lenses, they provide additional magnification of the sample in a DAC and improve the image resolution.

Experimental and Modelling Investigations of Asphaltene Precipitation During Pressure Depletion and Gas Injection Operations

Asphaltene precipitation problems manifest themselves in different stages of oil reservoirs production. Experimental and modeling investigations are, therefore, employed as promising tools to assist in predictions of asphaltene precipitation problems and selection of proper production facilities. This study concerns experimental and modeling investigations of asphaltene precipitation during natural production and gas injection operations for a heavy Iranian crude oil at reservoir conditions. First, with design and performance of high pressure–high temperature experiments, asphaltene precipitation behavior is comprehensively investigated; the effects of pressure and temperature are fully studied during pressure depletion tests and the role of injection gas composition on precipitation is described in gas injection experiments. In the next stage, the obtained experimental results are fed into a commercial simulator to develop the asphaltene precipitation model. The results for the pressure depletion experiments indicate that the maximum amount of asphaltene precipitation takes place at fluid bubble point pressure. Increase in the temperature, as seen, causes to reduce the amount of precipitation for the entire range of pressures. For gas injection experiments, the onset of precipitation for CO2, associated, and N2 gases takes place at around 0.20, 0.28, and 0.50 gas to mixture mole ratios, respectively. Carbon dioxide shows the highest asphaltene precipitation values and nitrogen has the lowest amounts for the whole range of gas mole fractions. Finally, the results for modeling indicate successful asphaltene precipitation predictions for both pressure depletion and gas injection processes.

In situ high pressure high temperature experiments in multi-anvil assemblies with bixbyite-type In2 O3 and synthesis of corundum-type and orthorhombic In2 O3 polymorphs

Our in situ high pressure high temperature experiments in multi-anvil assemblies unambiguously evidence the stability of bixbyite-type c-In2O3 at 6 GPa from room temperature to ca. 600°C. At 5.5 GPa and ca. 1100°C, c-In2O3 reacts with free carbon from the amorphous Si‒B‒C‒N capsule being reduced to metallic indium. The material recovered from the ex situ multi-anvil experiment at 6 GPa and 1100°C using the Mo capsule is inhomogeneous, thereby its phase composition depends on the specimen position from the furnace midline that in turn is characterized by the inhomogeneous temperatures. In the midpoint of the furnace, at the highest temperature point, c-In2O3 completely transforms into a corundum-type rh-In2O3 polymorph that is recovered under ambient conditions, as confirmed by X-ray powder and electron diffraction and Raman spectroscopy. Transmission electron microscopic characterization indicates the growth of single crystals of corundum-type rh-In2O3 with an average crystal size of ∼3 μm in the specimen part away from the furnace midline. The automated electron diffraction tomography analysis and X-ray powder-diffraction point out at the possible formation of orthorhombic In2O3 polymorphs.

Rapid ascent conditions of diamond-bearing kimberlitic magmas: Findings from high pressure–temperature experiments and finite element modeling

This paper deals with the problem of rapid ascent mechanism of kimberlite magmas with a multi-directional approach: 1) the kinetics of diamond–graphite transition; 2) settling velocity of diamond phenocrysts in magmas and 3) formation of ruptures required for magma ascent with a high speed. Based on the diamond–graphite transition, we present an estimate of the ascent rates from high pressure and temperature experiments using Walker-type multi-anvil apparatus. The experiments were conducted with diamond, placed within a synthetic kimberlitic assemblage, keeping an initial pressure and temperature of 6GPa and 1350°C, respectively. It was observed that the volume fraction of diamond to graphite conversion strongly depended on the ascent rates. Using electrical resistivity and X-ray diffraction studies, we measured the degree of graphitization as a function of the ascent rate (u). For u<3m/s, diamond underwent almost complete graphitization (conversion>90%), whereas it remained nearly intact (conversion<10%) when u>10m/s. Our theoretical calculations of the settling velocity of mantle xenoliths again confirm that diamond can exist when u>3m/s. We performed numerical experiments with finite element (visco-elastic) models to analyze the dynamics of tensile failure at the tip of magma pools, leading to dilatational vertical fractures for magma transport. Considering the tensile strength of mantle in the order of 0.5kb, our models show this failure process as a function of the critical shape (Ar: ratio of vertical and horizontal dimensions) and density contrast (Δρ) of magma pools. The critical Δρ is estimated nearly 200kg/m3 when Ar is very large (>4).

Twin chamber sample assembly in DAC and HPHT studies on GaN nano-particles

In this paper, we have suggested a novel idea of twin chamber sample assembly for separating ruby from the sample to overcome certain problems during high pressure–high temperature experiments using diamond anvil cell. Two holes of diameter 70μm were drilled symmetrically about the centre of the preindented area (500 μm diameter) in a stainless steel gasket using EDM. Using ruby pressure calibration, good pressure correlation between these two holes was established up to about 15GPa. Also, high pressure – high temperature (HPHT) experiments on the III-V compound semiconductor GaN were performed up to ∼ 3GPa and 2000K using the two chamber sample assembly and a laser heated diamond anvil cell facility. NaCl, both hydrous and anhydrous, were used as the pressure transmitting media in two separate experiments. Micro-Raman spectroscopy was used to characterise the HPHT treated samples. While GaN remained stable in its wurtzite phase when heated in anhydrous NaCl medium, it transformed to Ga2O3 with rocksalt structure in the hydrous NaCl medium.

Experimental pressure-temperature phase diagram of boron: resolving the long-standing enigma

Boron, discovered as an element in 1808 and produced in pure form in 1909, has still remained the last elemental material, having stable natural isotopes, with the ground state crystal phase to be unknown. It has been a subject of long-standing controversy, if α-B or β-B is the thermodynamically stable phase at ambient pressure and temperature. In the present work this enigma has been resolved based on the α-B-to- β-B phase boundary line which we experimentally established in the pressure interval of ∼4 GPa to 8 GPa and linearly extrapolated down to ambient pressure. In a series of high pressure high temperature experiments we synthesised single crystals of the three boron phases (α-B, β-B, and γ-B) and provided evidence of higher thermodynamic stability of α-B. Our work opens a way for reproducible synthesis of α-boron, an optically transparent direct band gap semiconductor with very high hardness, thermal and chemical stability.

Parental media for diamonds and primary inclusions by evidence of physicochemical experiment

The mantle-carbonatite conception of genesis of diamond (Litvin, 2007, 2009) is briefly outlined. A generalized of compositions of the multi-component heterogeneous parental medium for diamond and primary inclusions therein is first developed. Boundary compositions of the parental medium are represented by mineral components of the peridotitic and eclogitic parageneses, mantle carbonatites and C-O-H-fluids, phases of accessory type both soluble in the carbonate-silicate melts (chlorides, phosphates, etc.) and insoluble in them (sulfides), as well as carbon. Phase relations of the multi-component eclogite-carbonatite-sulfide-diamond are studied at 7 GPa. Syngenesis diagram for diamond, paragenetic and xenogenetic minerals (including the solubility curves of diamond in carbonate-silicate and sulfide melts) is constructed. Physicochemical mechanism of formation of diamond and paragenetic minerals is revealed. Genetic classification of primary inclusions in natural diamonds is worked out. Analysis of physicochemical history of diamond formation in the mantle chamber of carbonatite magma is accomplished. Introduction .The mantle-carbonatite conception of genesis of diamond of kimberlite deposits (Litvin, 2007 a, b , 2009) is justified by the complex of mineralogical, experimental and theoretical data. Completely miscible carbonate-silicate growth melts with dissolved elemental carbon (carbon source for diamond) form a basis for the parental media for the dominating mass of natural diamonds and paragenetic minerals. The parental carbonate-silicate-carbon melts are multi-component heterogeneous media with strongly changeable compositions. So, silicate constituents are presented by minerals and components both peridotite and eclogite parageneses (Sobolev, 1974). It is established in the high- pressure high-temperature experiments that both peridotite-carbonatite-carbon and eclogite-carbonatite- carbon melts are represented as highly efficient diamond-forming media. An example of diamond crystallization in melt oversaturated in dissolved carbon of the eclogite-carbonatite system is demonstrated in the fig. 1. Composition of parental medium for diamond and primary inclusions. A generalized of compositions of the multi-component heterogeneous parental medium for diamond and primary inclusions therein is developed (Fig. 2).The major composition tetrahedron of the system peridotite-eclogite-carbonatite-soluble admixture componens is its basic element. Belonging of the mineral assemblage of syngenetic inclusions in natural diamonds to two major parageneses, - peridotitic and eclogitic, - is took into consideration. The boundary compositions of the parental medium are determined with mineral components of the auxiliary tetrahedrons - peridotitic (Ol-Opx-Cpx-Grt), eclogitic (Cpx-Grt-Crn-Coes), mantle carbonatite (Mg, Fe, Ca, K, Na, etc. carbonates), phases and components of admixtured accessory sort - soluble in carbonate-silicate melts (chlorides, phosphates, etc.) including the components of C-O-H-volatile compounds, and carbon too. All the components of the boundary tetrahedrons mentioned above are responsible for formation of paragenetic mineral phases together with diamond and, correspondingly, for capture of the paragenetic inclusions by natural diamonds. At the same time due to physicochemical experiment it turned out that xenogenetic mineral phases are presented within the assemblage of so called syngenetic inclusions. Sulfide minerals which are not soluble in diamond-forming carbonate-silicate melts as well as sulfide melts which are completely immiscible with these are among the xenogenetic phases. In the fig. 2, the auxiliary boundary tetrahedron for sulfide components is positioned out of the major tetrahedron and is

High-pressure melting relations in Fe–C–S systems: Implications for formation, evolution, and structure of metallic cores in planetary bodies

We present new high-pressure temperature experiments on melting phase relations of Fe–C–S systems with applications to metallic core formation in planetary interiors. Experiments were performed on Fe–5 wt% C–5 wt% S and Fe–5 wt% C–15 wt% S at 2–6 GPa and 1050–2000 °C in MgO capsules and on Fe–13 wt% S, Fe–5 wt% S, and Fe–1.4 wt% S at 2 GPa and 1600 °C in graphite capsules. Our experiments show that: (a) At a given P– T, the solubility of carbon in iron-rich metallic melt decreases modestly with increasing sulfur content and at sufficiently high concentration, the interaction between carbon and sulfur can cause formation of two immiscible melts, one rich in Fe-carbide and the other rich in Fe-sulfide. (b) The mutual solubility of carbon and sulfur increases with increasing pressure and no super-liquidus immiscibility in Fe-rich compositions is likely expected at pressures greater than 5–6 GPa even for bulk compositions that are volatile-rich. (c) The liquidus temperature in the Fe–C–S ternary is significantly different compared to the binary liquidus in the Fe–C and Fe–S systems. At 6 GPa, the liquidus of Fe–5 wt% C–5 wt% S is 150–200 °C lower than the Fe–5 wt% S. (d) For Fe–C–S bulk compositions with modest concentration of carbon, the sole liquidus phase is iron carbide, Fe 3C at 2 GPa and Fe 7C 3 at 6 GPa and metallic iron crystallizes only with further cooling as sulfur is concentrated in the late crystallizing liquid. Our results suggest that for carbon and sulfur-rich core compositions, immiscibility induced core stratification can be expected for planets with core pressure less than ∼6 GPa. Thus planetary bodies in the outer solar system such as Ganymede, Europa, and Io with present day core–mantle boundary (CMB) pressures of ∼8, ∼5, and 7 GPa, respectively, if sufficiently volatile-rich, may either have a stratified core or may have experienced core stratification owing to liquid immiscibility at some stage of their accretion. A similar argument can be made for terrestrial planetary bodies such as Mercury and Earth’s Moon, but no such stratification is predicted for cores of terrestrial planets such as Earth, Venus, and Mars with the present day core pressure in the order ⩾136 GPa, ⩾100 GPa, and ⩾23 GPa. (e) Owing to different expected densities of Fe-rich (and carbon-bearing) and sulfur-rich metallic melts, their settling velocities are likely different; thus core formation in terrestrial planets may involve rain of more than one metallic melt through silicate magma ocean. (f) For small planetary bodies that have core pressures <6 GPa and have a molten core or outer core, settling of denser carbide-rich liquid or flotation of lighter, sulfide-rich melt may contribute to an early, short-lived geodynamo.

Cryogenic gas loading in a Mao–Bell-type diamond anvil cell for high pressure-high temperature investigations

A simple system for loading argon fluid at cryogenic temperatures in a Mao-Bell-type diamond anvil cell (DAC) has been developed. It is done in a two step process in which the piston-cylinder assembly alone is submerged in the cryogenic chamber for trapping the liquefied inert gas. Liquid nitrogen is used for condensing the argon gas. This system is now being efficiently used for loading liquid argon in the DAC for high pressure-high temperature experiments. The success rate of trapping liquefied argon in the sample chamber is about 75%. The performance of the gas loading system is successfully tested by carrying out direct conversion of pyrolitic graphite to diamond under high pressure-high temperature using laser heated DAC facility.

A thermobarometer for sphene (titanite)

Sphene and zircon are common accessory minerals in metamorphic and igneous rocks of very different composition from many different geological environments. Their essential structural constituents, Ti and Zr, are capable of replacing each other to some degree. In this paper we detail the results of high pressure–temperature experiments as well as analyses of natural sphene crystals that establish a systematic relationship between temperature, pressure and Zr concentration in sphene. Calibrations of the temperature and pressure relationships are presented as a thermobarometer. Synthetic sphene crystals were crystallized in the presence of zircon, quartz and rutile at 1–2.4 GPa and 800–1,000°C from hydrothermal solutions. Crystals were analyzed for Zr by electron microprobe (EMP). The experimental results define a log-linear relationship between equilibrium Zr content (ppm by weight), pressure (GPa) and reciprocal absolute temperature: \( {\text{log}}({\text{Zr}}^{{{\text{sphene}}}} ,{\text{ppm}}) = 10.52( \pm 0.10) - \frac{{7708( \pm 101)}} {{T(K)}} - 960( \pm 10)\frac{{P({\text{GPa}})}} {{T(K)}} - {\text{log}}(a_{{{\text{TiO}}_{2} }} ) - \log (a_{{{\text{SiO}}_{2} }} ). \)The incorporation of Zr into sphene was found to be rather sensitive to pressure effects and also to the effects of kinetic disequilibrium and growth entrapment that result in sector zoning. The Zr content of sphene is relatively insensitive to the presence of both REEs and F-Al substitutions in sphene. To supplement and test the experimental data, sphenes from seven rocks of well-constrained origin were analyzed for Zr by both EMP and ion microprobe (IMP). The sphene thermobarometer records crystallization temperatures that are consistent with independent thermometry. When applied to natural sphene of unknown origin or growth conditions, this thermobarometer has the potential to estimate temperatures with an approximate uncertainty of ±20°C over the temperature range of interest (600–1,000°C). The Zr-in-sphene thermobarometer can also be used in conjunction with the Zr-in-rutile thermobarometer to estimate both pressure and temperature of crystallization.