Mark R. Frank

Toward an internally consistent pressure scale.

Our ability to interpret seismic observations including the seismic discontinuities and the density and velocity profiles in the earths interior is critically dependent on the accuracy of pressure measurements up to 364 GPa at high temperature. Pressure scales based on the reduced shock-wave equations of state alone may predict pressure variations up to 7% in the megabar pressure range at room temperature and even higher percentage at high temperature, leading to large uncertainties in understanding the nature of the seismic discontinuities and chemical composition of the earths interior. Here, we report compression data of gold (Au), platinum (Pt), the NaCl-B2 phase, and solid neon (Ne) at 300 K and high temperatures up to megabar pressures. Combined with existing experimental data, the compression data were used to establish internally consistent thermal equations of state of Au, Pt, NaCl-B2, and solid Ne. The internally consistent pressure scales provide a tractable, accurate baseline for comparing high pressure–temperature experimental data with theoretical calculations and the seismic observations, thereby advancing our understanding fundamental high-pressure phenomena and the chemistry and physics of the earths interior.

Gold solubility, speciation, and partitioning as a function of HCl in the brine-silicate melt-metallic gold system at 800°C and 100 MPa

A vapor-undersaturated synthetic brine was equilibrated with metallic gold and a haplogranitic melt at 800°C and 100 MPa to examine the solubility, speciation and partitioning of gold in the silicate melt-brine-metallic gold system. The starting composition of the NaCl-KCl-HCl-H2O brine was 70 wt.% NaCl (equivalent) with starting KCl/NaCl ranging from 0.5 to 1. KCl/HCl was varied from 3.2 to 104 to evaluate the solubility and partitioning of gold as a function of the concentration of HCl in the brine. Inclusions of brine were trapped in a silicate glass during quench. Inclusion-poor and inclusion-rich portions of glass were analyzed for gold and chloride by using neutron activation analysis. The inclusion-poor glass yielded an estimate of the solubility of gold and chloride in the silicate melt. The solubility of gold in the melt, at gold metal saturation, was estimated as ≈1 ppm. The solubility of gold in the brine was estimated by mass balance, given the concentration of gold and chloride in the inclusion-poor and inclusion-rich glasses. The solubility of gold metal at low-HCl concentrations in the brine, CHClb, (3 × 103 to 1.1 × 104 ppm) is ≈40 ppm (by weight) and is independent of the HCl concentration under those conditions. For CHClb of 1.1 × 104 to 4.0 × 104 ppm, the solubility of gold increased from 40 to 840 ppm, and the solubility is given by: log CAub = [2.2 · log CHClb] − 7.2(1) These data suggest that a significant amount of gold is not chloride complexed in brines at low-HCl concentrations ( 1.1 × 104 ppm). The calculated Nernst partition coefficient (DAub/m) for gold between a brine and melt varied from 40 to 830 over a range of brine HCl concentrations of 3 × 103 to 1.1 × 104 ppm. Our results indicate a significant amount of gold can be transported by a brine in the magmatic-hydrothermal environment independent of the fugacity of sulfur in the system. Thus brines provide an effective mechanism for the scavenging of gold from a crystallizing melt and transport into an associated magmatic-hydrothermal system, regardless of their sulfur contents.

Alkali exchange equilibria between a silicate melt and coexisting magmatic volatile phase: an experimental study at 800°C and 100 MPa

Abstract Many experimental studies have been performed to evaluate the composition of coexisting silicate melts and magmatic volatile phases (MVP). However, few studies have attempted to define the relationship between melt chemistry and the acidity of a chloride-bearing fluid. Here we report data on melt composition as a function of the HCl concentration of coexisting brines. We performed 35 experimental runs with a NaCl-KCl-HCl-H 2 O brine (70 wt% NaCl [equivalent])-silicate melt (starting composition of Qtz 0.38 Ab 0.33 Or 0.29 , anhydrous) assemblage at 800°C and 100 MPa. We determined an apparent equilibrium constant K ′ meas (K, Na) =( C Na m × C KCl b )/( C NaCl b × C K m ) for the equilibrium NaCl b +Σ K m =Σ Na m + KCl b , (where CKClb, CNaClb, CKm, and CNam are total concentrations of potassium and sodium chloride in the brine, and potassium and sodium in the melt, respectively) as a function of the HCl concentration in the brine (C HCl b ). Although K′ meas (K, Na) was not affected by variations in KCl/NaCl of the brine, it did vary inversely with C HCl b . The relationship is given by K ′ meas (K, Na) = K ′ ex (K, Na) + a C HCl b [where C HCl b is in wt% and a = 0.03; K′ ex (K, Na) = 0.40 ± 0.03 (1σ) and represents the exchange of model sodium and potassium between chloride components in the brine and the aluminate components (NaAlO 2 and KAlO 2 ) in the melt. This empirical result will be discussed in light of a structural hypothesis; however, validation of the model awaits determinations based on spectroscopy or transport properties–thermodynamic relations alone cannot be used as evidence of structure. The form of this equation is consistent with a model wherein sodium is present in the melt as both sodium aluminate and sodium hydroxide components, and HCl reacts with the NaOH component in the melt to produce NaCl and H 2 O. The correlation between fugacity of H 2 O ( f H 2 O sys), model NaOH m /ΣNa m , aluminum saturation index (ASI), and the ratio (HCl/NaCl) b of an exsolving MVP is complex. f H 2 O sys and the ASI are the main controls on model NaOH m /ΣNa m in the system, with model NaOH m /ΣNa m increasing with increasing f H 2 O sys. This relationship can be used to estimate the C HCl b in subaluminous systems, an improvement over previous models. Data for metal partitioning between a volatile phase and melt are commonly presented in the literature as metal–sodium exchange equilibria (i.e., K Cu,Na for the exchange of copper and sodium). However, the variation in K′ meas (K, Na) observed in this study implies that the treatment of metal partitioning between a volatile phase and melt as metal–alkali exchange equilibria is complex because alkali partitioning is not constant and suggests that experimental partitioning studies need to carefully control the HCl/NaCl in experimental vapors and brines. This effect may explain discrepancies in metal–alkali exchange equilibria presented in the literature. Therefore, metal–alkali exchange cannot be described fully by a single metal–alkali equilibrium but must be examined by multiple equilibria.

Low-pressure decomposition of chrysotile as a function of time and temperature

Abstract Chrysotile from Thetford, Quebec, was heated at constant temperature in quench furnaces from 200 to 1000 °C for 4 to 720 h, and the products were analyzed by XRD and optical microscopy. XRD patterns for chrysotile from Thedford, Jeffrey Quebec, and New Idria, California, heated for up to 8 h at constant temperatures from 400 to 800 °C in hydrothermal diamond-anvil cells, were obtained at 300 s intervals by using synchrotron radiation. The studies show that differences in the decomposition temperatures of chrysotile reported in the literature can be explained by differences in the temperatures, time at temperature, particle size, and partial pressure of water. Chrysotile heated for 30 days is destroyed between 475 and 500 °C, whereas chrysotile heated to 800 °C survives for only minutes. A prograde heating experiment, consistent with the literature, shows chrysotile destruction beginning at 600 °C and completed at 750 °C, demonstrating that such experiments should not be used to establish temperatures of thermal stability. Forsterite forms readily at 600 °C when water is retained transiently in the decomposition products, but does not form after 4 h of heating samples of small fiber size and high surface area from which water is readily lost. When water is retained in the sample chamber during heating to 800 °C over 4 h, a rapid destruction of chrysotile and formation of forsterite was observed, followed by the appearance and later destruction of the intermediate reaction products talc, a tridymite-like phase, and anthophyllite; enstatite appearance coincides with the destruction of the tridymite-like phase and talc. In quench experiments, a 10+ Å phase is observed between 500 and 600 °C, and talc is observed between 587 and 800 °C. The optical properties of the run products between 600 and 900 °C are highly variable, with a range in the index of refraction perpendicular to length of up to 0.06 and a minimum index of refraction 0.03 below the starting material in fibers in the 650 °C run product; the variability reflects the inhomogeneity in products and reactants in this temperature range. In quench experiments, the intensity of the XRD patterns of chrysotile decreased on heating between 200 and 400 °C, and increased at 450 °C, demonstrating recrystallization of chrysotile. A gradual heating experiment showed a similar pattern with the lowest intensities at 250 °C and maximum intensities at 550 °C. The two types of chrysotile reported in the literature may reflect simply the presence of chrysotile of different particle sizes. Chrysotile may also possess a spectrum of stabilities due to variable strain energies of curvature. The presence of a tridymite-like phase supports the formation of a Si-rich dehydroxylate II. The absence of forsterite and chrysotile in brake-wear debris reflects the high temperature and disaggregated fiber bundles resulting from friction.

K-feldspar-muscovite-andalusite-quartz-brine phase equilibria: An experimental study at 25 to 60 MPa and 400 to 550 C

Abstract Felsic magmas may evolve one or more water or chlorine-rich fluid phases which can transport heat and solutes into associated hydrothermal systems and can contribute to alteration and ore deposition. To understand the role of a high-salinity aqueous phase in the magmatic hydrothermal environment, the composition of a subcritical, vapor-undersaturated high-salinity liquid phase (brine) in equilibrium with K-feldspar-muscovite-quartz and muscovite-andalusite-quartz was determined for pressures and temperatures ranging from 25 MPa and 400°C to 60 MPa and 550°C, with total Cl (NaCl + KCl + HCl) concentrations ranging from 3.42 to 8.56 (moles of solute/kg solution). Values of log 10 (KCl/HCl) have been obtained for the equilibria: 1.5 K-feldspar + HCl = 0.5 muscovite + 3 quartz + KCland muscovite + HCl = 1.5 andalusite + 1.5 quartz + 1.5 H 2 O + KCl. For the K-feldspar-muscovite-quartz-brine equilibrium, log 10 (KCl/HCl) = 1.6 ± 0.1, 0.81 ± 0.06, 0.54 ± 0.04 and 0.42 ± 0.08 at 25 MPa and 400°C, 40 MPa and 450°C, 50 MPa and 500°C, and 60 MPa and 550°C (pressures and temperatures of the experiments), respectively. For the muscovite-andalusite-quartz-brine equilibrium, log 10 (KCl/HCl) = 0.63 ± 0.1, −0.063 ± 0.06, 0.17 ± 0.05, and 0.25 ± 0.08 at the pressures and temperatures of the experiments, respectively. Comparison of our results with previous studies conducted at higher pressures and with lower-salinity aqueous phases show that the mineral stability fields in the K-feldspar-muscovite-andalusite-quartz system shift to lower KCl/HCl values with increasing salinity and decreasing pressure.

An evaluation of synthetic fluid inclusions for the purpose of trapping equilibrated, coexisting, immiscible fluid phases at magmatic conditions

Abstract We report data that allow us to evaluate the method of trapping immiscible, saline aqueous fluids (i.e., vapor and brine in the NaCl-KCl-HCl-FeCl2-AuHCl2-H2O system) as synthetic fluid inclusions in pre-fractured quartz cores in order to quantify the concentrations of Au, Fe, K, and Na, among coexisting three-phase, immiscible fluids (i.e., haplogranite melt, brine, and vapor) at magmatic conditions. Coexisting vapor and brine were trapped experimentally at 800 °C and 100-110 MPa as synthetic fluid inclusions in both quartz microfractures and quenched silicate melt (i.e., glass), and also sampled indirectly using the recovered quenched aqueous fluid. Quartz-hosted and glass-hosted brine inclusions were analyzed by laser-ablation inductively-coupled-plasma mass spectrometry (LAICPMS) and instrumental neutron activation analysis (INAA), respectively. Quenched aqueous fluid from each experiment containing a quartz core was recovered and analyzed by atomic absorption spectrophotometry (AAS). The composition of aqueous fluids trapped as quartz-hosted inclusions, glass-hosted inclusions, and those recovered after quench yield consistent and precise data, at the 2σ uncertainty level, for the elements of interest. The overlapping Au, Fe, K, and Na concentrations in aqueous fluids trapped and analyzed via three entirely different instrumental techniques (i.e., LAICPMS, INAA, and AAS) suggest strongly that quartz microfractures heal on a slow enough time scale to permit entrapment of fully equilibrated aqueous fluids at our experimental PTX conditions. The data evince clearly that the chemical composition of fluids in quartz microfractures at the time of self-healing represents equilibrium conditions; hence, synthetic fluid inclusions in experiments with low thermal gradients across the charge provide a reasonable estimate of fluid composition at least at the experimental conditions examined in this study.

Magnesite formation from MgO and CO2 at the pressures and temperatures of Earth's mantle

Abstract Magnesite (MgCO3) is an important phase for the carbon cycle in and out of the Earth’s mantle. Its comparably large P-T stability has been inferred for several years based on the absence of its decomposition in experiments. Here we report the first experimental evidence for synthesis of magnesite out of its oxide components (MgO and CO2) at P-T conditions relevant to the Earth’s mantle. Magnesite formation was observed in situ using synchrotron X-ray diffraction, coupled with laserheated diamond-anvil cells (DACs), at pressures and temperatures of Earth’s mantle. Despite the existence of multiple high-pressure CO2 polymorphs, the magnesite-forming reaction was observed to proceed at pressures ranging from 5 to 40 GPa and temperatures between 1400 and 1800 K. No other pressure-quenchable materials were observed to form via the MgO + CO2 = MgCO3 reaction. This work further strengthens the notion that magnesite may indeed be the primary host phase for oxidized carbon in the deep Earth

Solubility of xenotime in a 2 M HCl aqueous fluid from 1.2 to 2.6 GPa and 300 to 500 °C

Abstract Constraining mass transfer of the rare earth elements (REE) and high field strength elements (HFSE) from subducted oceanic crust and metasediments to the mantle wedge is fundamental toward interpreting processes that affect trace element mobility in subduction zone environments. Experimental studies of the partitioning of trace elements involving aqueous fluids at P-T conditions appropriate for slab-mantle wedge conditions are complicated by the difficulties in retrieving the fluid. Here we present the results from an application of an in situ technique that permits quantitative determination of element concentrations in aqueous fluid at geologically relevant supercritical conditions. We focus on pressures and temperatures appropriate for devolatilization-induced element transfer in subduction zone environments, and conditions obtained during regional metamorphism. In this study, we used a hydrothermal diamond-anvil cell (HDAC) and in situ synchrotron X-ray fluorescence (SXRF) to quantify the concentration of Y, an important trace element often used as a proxy for the heavy REE in geologic systems, in a xenotime-saturated 2 M HCl-aqueous fluid at 1.19 to 2.6 GPa and 300-500 °C. At these pressures and temperatures the solubility of yttrium ranges from 2400 to 2850 ppm. We find that the concentration of Y decreases with increasing fluid density. These new data, combined with published data generated from experiments done at lower pressure, in fluids of nearly identical composition and also NaCl-H2O fluids, constrain the effects of pressure and temperature on the ability of aqueous fluid containing Cl to scavenge and transport Y and, by analogy, the HREE. Although the physical properties of Y are similar to the high field strength elements, Y exhibits geochemical behavior that is analogous to the heavy rare earth elements (HREE).

P–V equation of state for Fe2P and pressure-induced phase transition in Fe3P

As part of our ongoing investigations of elasticity and high-pressure stability in the Fe–P system, we have measured the room-temperature bulk modulus (K 0T) of Fe2P, barringerite, to 8 GPa using in situ synchrotron X-ray diffraction and diamond anvil cells. A second-order fit (i.e. dK/dP fixed at 4) to our experimental data using the Birch–Murnaghan equation of state produces a K 0T of 165±3 GPa. This value is ∼4% less than the experimental values for Fe3P. For comparison with the experimental data, we have also performed first-principle theoretical calculations on this phase. For ferromagnetic Fe2P at zero pressure, we find that the magnetic moments increase rapidly for a Hubbard U>1 eV and are significantly higher than observed experimentally. Thus, our results support previous findings that magnetism in Fe2P is largely itinerant with at most a minor component due to on-site correlation in the iron-3d shell. Additionally, we present new high-pressure diffraction data for a natural Fe3P, schreibersite, sample which conclusively demonstrate that a first-order phase transformation occurs between 15 and 20 GPa.