Si in the core? New high-pressure and high-temperature experimental data

High-pressure high-temperature experiments have been carried out up to 25 GPa and 2200°C in a multianvil press on assemblages made of silicates and iron-silicon alloys. At 20 GPa, silicon is extracted from the metal phase, forming stishovite reaction rims around metal grains. The silicon content in metal has been measured by analytical electron microscopy and electron microprobe. In contrast with earlier experiments, the present data were obtained by using silicon-rich metal alloys as starting materials instead of studying incorporation of silicon in initially silicon-free metal. As in most of previous studies carried out below 25 GPa, the silicon content in liquid metal increases with increasing pressure and with decreasing oxygen fugacity. The oxygen fugacity in most experiments was calculated by using two independent buffers: iron/?stite (IW) and SiO₂/Si, allowing to link consistently the Fe contents in silicates, the Si contents in metal and the temperatures of the experiments. At oxygen fugacities 4 log units below IW, silicates are in equilibrium with Si-rich metallic alloys (up to 17 wt% of Si in metal at 20 GPa and 2200°C). Extrapolation to 2 log units below IW leads to less than 0.1 wt% Si in the metal phase. Presence of several wt% of silicon in the Earth’s core thus requires highly reduced initial materials that, if equilibrated at conditions relevant to small planets, should already contain significant amount of silicon dissolved in metal.     

Tracing coalbed natural gas-coproduced water using stable isotopes of carbon

Recovery of hydrocarbons commonly is associated with coproduction of water. This water may be put to beneficial use or may be reinjected into subsurface aquifers. In either case, it would be helpful to establish a fingerprint for that coproduced water so that it may be tracked following discharge on the surface or reintroduction to geologic reservoirs. This study explores the potential of using δ¹³C of dissolved inorganic carbon (DIC) of coalbed natural gas (CBNG)–coproduced water as a fingerprint of its origin and to trace its fate once it is disposed on the surface. Our initial results for water samples coproduced with CBNG from the Powder River Basin show that this water has strongly positive δ¹³C_DIC (12‰ to 22‰) that is readily distinguished from the negative δ¹³C of most surface and ground water (−8‰ to −11‰). Furthermore, the DIC concentrations in coproduced water samples are also high (more than 100 mg C/L) compared to the 20 to 50 mg C/L in ambient surface and ground water of the region. The distinctively high δ¹³C and DIC concentrations allow us to identify surface and ground water that have incorporated CBNG-coproduced water. Accordingly, we suggest that the δ¹³C_DIC and DIC concentrations of water can be used for long-term monitoring of infiltration of CBNG-coproduced water into ground water and streams. Our results also show that the δ¹³C_DIC of CBNG-coproduced water from two different coal zones are distinct leading to the possibility of using δ¹³C_DIC to distinguish water produced from different coal zones.     

Plastic deformation of wadsleyite: II. High-pressure deformation in shear

 We have studied the dislocation microstructures that develop in (Mg_0.9Fe_0.1)₂SiO₄ wadsleyite deformed by simple shear at high pressure. The experiments were performed in a multianvil apparatus with the shear assembly designed by Karato and Rubie (1997). The samples were synthesized in a separate experiment from high-purity oxides. The deformation experiments were carried out at 14 GPa and 1300 °C with time durations ranging from 1 to 8 h leading to plastic shear strains of 60 and 73%, respectively. The microstructures investigated by transmission electron microscopy (TEM) show that dislocation glide is activated under these conditions over the whole experimental time. The easy slip systems at 1300 °C involve 1/2<111> dislocations gliding in {101} as well as [100] dislocations gliding in (010) and {011}. Received: 15 July 2002 / Accepted: 14 February 2003 Acknowledgements High-pressure experiments were performed at the Bayerisches Geoinstitut under the EU IHP — Access to Research Infrastructures Programme (Contract no. HPRI-1999-CT-00004 to D.C. Rubie). The quality of the preparation of the TEM specimens by H. Schultze is greatly appreciated.     

The high PT stability of apatite and Cl partitioning between apatite and hydrous potassic phases in peridotite: an experimental study to 19 GPa with implications for the transport of P,Cl and K in the upper mantle

High PT experiments were performed in the range 2.5–19 GPa and 800–1,500°C using a synthetic peridotite doped with trace elements and OH-apatite or with Cl-apatite + phlogopite. The aim of the study was (1) to investigate the stability and phase relations of apatite and its high PT breakdown products, (2) to study the compositional evolution with P and T of phosphate and coexisting silicate phases and (3) to measure the Cl-OH partitioning between apatite and coexisting calcic amphibole, phlogopite and K-richterite. Apatite is stable in a garnet-lherzolite assemblage in the range 2.5–8.7 GPa and 800–1,100°C. The high-P breakdown product of apatite is tuite γ-Ca₃ (PO₄)₂, which is stable in the range 8–15 GPa and 1,100–1,300°C. Coexisting apatite and tuite were observed at 8 GPa/1,050°C and 8.7 GPa/1,000°C. MgO in apatite increases with P from 0.8 wt% at 2.5 GPa to 3.2 wt% at 8.7 GPa. Both apatite and tuite may contain significant Na, Sr and REE with a correlation indicating 2 Ca²⁺=Na⁺ + REE³⁺. Tuite has always higher Sr and REE and lower Fe and Mg than apatite. Phosphorus in the peridotite phases decreases in the order P_melt ≫ P_grt ≫ P_Mg2SiO4 > P_cpx > P_opx. The phosphate-saturated P₂O₅ content of garnet increases from 0.07 wt% at 2.5 GPa to 1.5 wt% at 12.8 GPa. Due to the low bulk Na content of the peridotite, ^[8]Na^[4]P^[8]M²⁺ ₋₁ ^[4]Si₋₁ only plays a minor role in controlling the phosphorus content of garnet. Instead, element correlations indicate a major contribution of ^[6]M^2+[4]P^[6]M³⁺ ₋₁ ^[4]Si₋₁. Pyroxenes contain ~200–500 ppm P and olivine has 0.14–0.23 wt% P₂O₅ in the P range 4–8.7 GPa without correlation with P, T or X_Mg. At ≥12.7 GPa, all Mg₂SiO₄ polymorphs have <200 ppm P. Coexisting olivine and wadsleyite show an equal preference for phosphorus. In case of coexisting wadsleyite and ringwoodite, the latter fractionates phosphorus. Although garnet shows by far the highest phosphorus concentrations of any peridotite silicate phase, olivine is no less important as phosphorus carrier and could store the entire bulk phosphorus budget of primitive mantle. In the Cl-apatite + phlogopite-doped peridotite, apatite contains 0.65–1.35 wt% Cl in the PT range 2.5–8.7 GPa/800–1,000°C. Apatite coexists with calcic amphibole at 2.5 GPa, phlogopite at 2.5–5 GPa and K-richterite at 7 GPa, and all silicates contain between 0.2 and 0.6 wt% Cl. No solid potassic phase is stable between 5 and 8.7 GPa. Cl strongly increases the solubility of K in hydrous fluids. This may lead to the breakdown of phlogopite and give rise to the local presence in the mantle of fluids strongly enriched in K, Cl, P and incompatible trace elements. Such fluids may get trapped as micro-inclusions in diamonds and provide bulk compositions suitable for the formation of unusual phases such as KCl or hypersilicic Cl-rich mica.     

Statistical analysis of friction sleeve length effects on soil classification

The cone penetration test (CPT) provides profiles of the tip resistance, sleeve friction, and pore water pressure encountered while penetrating the subsurface. These parameters are used either directly or indirectly to classify the soil types present and to obtain geotechnical design parameters. However, fundamental discrepancies exist in the manner by which these parameters are measured. This paper describes the results of a study that shows the sleeve friction measurement introduces unnecessary redundancy due to the length of the standard friction sleeve compared to the measurement increment. Further, the high sleeve length to measurement increment ratio results in filtering and smoothing of the friction data, thereby causing the variability of the friction between the soil and the cone sleeve to be underestimated. The importance of understanding the role of the sleeve length on measurements is demonstrated using synthetically generated friction profiles and estimating the profiles that would be measured using sleeves of different lengths. Differences in how the soils are classified as a function of the sleeve length used to obtain each profile are illustrated. Solutions are presented to validate the synthetic sleeve friction profiles, to demonstrate the filtering and smoothing effects of the friction sleeve on the data, and to explain the implications of the sleeve length on soil classification. Copyright © 2004 John Wiley & Sons, Ltd.     

Thermal decomposition of Ferrian chamosite: an infrared emission spectroscopic study

The thermal behaviour of ripidolite, an iron-rich chlorite, has been studied in situ by infrared emission spectroscopy up to 800 °C. The more di,trioctahedral nature due to significant amounts of Fe³⁺ is reflected, in addition to the two bands around 3420 and 3560 cm⁻¹, by an extra band around 3345 cm⁻¹. This extra band is absent in pure dioctahedral chlorites without Fe³⁺. These bands have been assigned to (AlAl)O-OH, (SiAl)O-OH and (SiSi)O-OH stretching modes with increasing frequencies. The bands disappear upon dehydroxylation around 650 °C. A similar behaviour is observed for the corresponding libration modes around 716, 759 and 802 cm⁻¹. The stretching and bending modes of the inner-OH of the octahedral sheet in the 2:1 clay-like layer are observed around 3645, 943 and 904 cm⁻¹. Although the bands decrease in intensity, they remain present up to 800 °C as dehydroxylation of the octahedral sheet is not yet complete at this temperature. The presence of two bending modes is explained as being due to a differentiation between Mg-OH and Fe-OH modes. At 650 °C a new sharp band is observed around 502 cm⁻¹ assigned to a (Fe,Mg)-O-Al bending mode caused by the formation of a spinel-like interlayer phase after dehydroxylation. Received: 4 June 1999 / Accepted: 6 August 1999     

Mid-Pleistocene lavas from the Seguam volcanic center,central Aleutian arc: closed-system fractional crystallization of a basalt to rhyodacite eruptive suite

In contrast to adjacent volcanic centers of the modern central Aleutian arc, Seguam Island developed on strongly extended arc crust. K-Ar dates indicate that mid-Pleistocene, late-Pleistocene, and Holocene eruptive phases constitute Seguam. This study focuses on the petrology of the mid-Pleistocene, 1.07–07 Ma, Turf Point Formation (TPF) which is dominated by an unusual suite of porphyritic basalt and basaltic andesite lavas with subordinate phenocryst-poor andesite to rhyodacite lavas. Increasing whole-rock FeO^*/MgO from basalt to dacite, the anhydrous Plag+Ol+Cpx±Opx±Mt phenocryst assemblage, groundmass pigeonite, and the reaction Ol+Liq=Opx preserved in the mafic lavas indicate a tholeiitic affinity. Thermometry and comparison to published phase equilibria suggests that most TPF basalts crystallized Plag+Ol+Cpx±Mt at 1160°C between about 3–5 kb (±1–2% H₂O), andesites crystallized Plag+Cpx+Opx±Mt at 1000°C between 3–4 kb with 3–5% H₂O, and dacites crystallized Plag +Cpx±Opx±Mt at 1000°C between 1–2 kb with 2–3% H₂O. All lavas crystallized at f _{o 2} close to the NNO buffer. Mineral compositions and textures indicate equilibrium crystallization of the evolved lavas; petrographic evidence of open-system mixing or assimilation is rare. MgO, CaO, Al₂O₃, Cr, Ni, and Sr abundances decrease and K₂O, Na₂O, Rb, Ba, Zr, and Pb increase with increasing SiO₂ (50–71%). LREE enrichment [(Ce/Yb)_n=1.7±0.2] characterizes most TPF lavas; total REE contents increase and Eu anomalies become more negative with increasing SiO₂. Relative to other Aleutian volcanic centers, TPF basalts and basaltic andesites have lower K₂O, Na₂O, TiO₂, Rb, Ba, Sr, Zr, Y, and LREE abundances. ⁸⁷Sr/⁸⁶Sr ratios (0.70361–0.70375) and ratios of ²⁰⁶Pb/²⁰⁴Pb (18.88–18.97), ²⁰⁷Pb/²⁰⁴Pb (15.58–15.62), ²⁰⁸Pb/²⁰⁴Pb (38.46–38.55) are the highest measured for any suite of lavas in the oceanic portion of the Aleutian arc. Conversely, Nd values (+5.8 to+6.7) are among the lowest from the Aleutians. Sr, Nd, and Pb ratios are virtually constant from basalt through rhyodacite, whereas detectable isotopic heterogenity is observed at most other Aleutian volcanic centers. Major and trace element, REE, and Sr, Nd, and Pb isotopic compositions are consistent with the basaltic andesitic, andesitic, dacitic, and rhyodacitic liquids evolving from TPF basaltic magma via closed-system fractional crystallization alone. Fractionation models suggest that removal of 80 wt% cumulate (61% Plag, 17% Cpx, 12% Opx, 7% Ol, and 3% Mt) can produce 20 wt% rhyodacitic residual liquid per unit mass of parental basaltic liquid. Petrologic and physical constraints favor segregation of small batches of basalt from a larger mid-crustal reservoir trapped below a low-density upper crustal lid. In these small magma batches, the degree of cooling, crystallization, and fractionation are functions of the initial mass of basaltic magma segregated, the thermal state of the upper crust, and the magnitude of extension. Tholeiitic magmas erupted at Seguam evolved by substantially different mechanisms than did calc-alkaline lavas erupted at the adjacent volcanic centers of Kanaga and Adak on unextended arc crust. These variable differentiation mechanisms and liquid lines of descent reflect contrasting thermal and mechanical conditions imposed by the different tectonic environments in which these centers grew. At Seguam, intra-arc extension promoted eruption of voluminous basalt and its differentiates, unmodified by interaction with lower crustal or upper mantle wallrocks.     

Highly reducing conditions during Alpine metamorphism of the Malenco peridotite (Sondrio,northern Italy) indicated by mineral paragenesis and H2 in fluid inclusions

During regional metamorphism of the Malenco serpentinized peridotite (Sondrio, northern Italy), the mineral assemblage pentlandite-awaruite-magnetite-native copper-antigorite-brucite-olivine-diopside is formed. The opaque assemblage indicates very reduced fluids with fO₂ values 4 log units below QFM. Primary fluid inclusions were trapped in diopside overgrowth, contemporaneous with the opaque assemblage. These metamorphic fluids are saline aqueous solutions (about 10.4 mol% NaCl equivalent) and contain molecular H₂ of approximately 1 mol%, as shown by micro-Raman analysis and microthermometry. The fluids are interpreted to have been formed during deserpentinization at the olivine-in isograd under strong reducing conditions.     

Local relaxation around [6]Cr 3+ in synthetic pyrope–knorringite garnets, [8]Mg 3 [6](Al 1-X Cr X 3+) 2 [4]Si 3O 12, from electronic absorption spectra

Pyrope-knorringite garnets, Mg₃(Al_1-X Cr³⁺_X)₂Si₃O₁₂ with X=0.25, 0.50, and 1.00, were synthesized between 9 and 16 GPa and 1300 and 1600 °C, using multianvil high-pressure techniques. The garnets with X=0.25 and 0.50 are fine-grained, pink and violet in color. The end-member knorringites with X=1.00 are black when compact and gray when coarse-grained. The fine powder is greenish gray in natural light and pale pink under a tungsten lamp. Powder remission spectra in the wavenumber range 30 000–10 000 cm^–1 on finely powdered crystals were measured by two different methods: (I.) by the use of a small integrating sphere for small samples or (II.) microscope-spectrometric measurement using diffusely reflected radiation from a 45° illuminated microsample. Both methods yielded similar diffuse reflectance spectra. The following crystal-field parameters of ^[6]Cr³⁺ were determined for garnets with X=0.25, 0.50, 1.00: 10 Dq=17 856, 17 596, 17 286 cm^–1; and B=654, 677, 706 cm^–1; nephelauxetic ratio =(B_field/B_free)= 0.71, 0.74, 0.77. The -values indicate decreasing covalency of the Cr–O bond with increasing Cr content. The 10 Dq value for together with the mean Cr–O distance in end-member knorringite, 1.96 Å (Novak and Gibbs 1971), were used to calculate from the spectral data, local mean Cr–O distances (Langer 2001a) as a function of composition. The results indicate relatively strong local site relaxation with a value of =0.77.     

Observations on the boundary between zeolite facies and prehnite-pumpellyite facies

Using graphical analysis of the system CaO-Al₂O₃-SiO₂-H₂O-CO₂, this paper derives a topology relating the minerals calcite, laumontite, wairakite, prehnite, quartz, and zoisite. Simple thermodynamic reasoning allows this system to be applied to natural rocks and indicates that the first appearance of the assemblage epidote-chlorite-quartz (±albite) should mark the upper boundary of zeolite facies. This assemblage forms at the expense of laumontite+prehnite, laumontite+calcite, or laumontite+pumpellyite, with wairakite likely to replace laumontite as the stable zeolite at low pressures. In natural systems this proposed facies boundary is multivariant and, hence, it is likely to be strongly sensitive to compositional variables. For example, Na-bearing wairakite will be more stable than pure Ca-wairakite and increasing abundance of Fe³⁺ will tend to stabilize epidote+quartz at the expense of the zeolites. Because of this, monitoring the composition of minerals such as epidote, prehnite, or wairakite from lowvariance assemblages may provide a more-sensitive indicator of metamorphic grade than the presence or absence of any particular mineral assemblage.