Thermal Expansion of Periclase (MgO) and Tungsten (W) to Melting Temperatures

 In situ x-ray data on molar volumes of periclase and tungsten have been collected over the temperature range from 300 K to melting. We determine the temperature by combining the technique of spectroradiometry and electrical resistance wire heating. The thermal expansion (α) of periclase between 300 and 3100 K is given by α=2.6025 10⁻⁵+1.3535 10⁻⁸ T+6.5687 10⁻³ T⁻¹−1.8281 T⁻². For tungsten, we have (300 to 3600 K) α=7.862 10⁻⁶+6.392 10⁻⁹ T. The data at 298 K for periclase is: molar volume 11.246 (0.031) cm³, α=3.15 (0.07) 10⁻⁵ K⁻¹, and for tungsten: molar volume 9.55 cm³, α=9.77 (10.08) 10⁻⁶ K⁻¹. Received: July 18, 1996 / Revised, accepted: February 14, 1997     

IR active orientation of OH bending mode in topaz

Infrared (IR) and nearinfrared (NIR) absorption spectra of hydrous and F-rich topazes were measured to assign an OH bending mode of topaz. Three absorption peaks at 1165, 3650, and 4803 cm⁻¹ are assigned to OH related absorption peaks. Since a peak at 4803 cm⁻¹ can be assigned to a combination mode of 1165 and 3650 cm⁻¹, the 1165 cm⁻¹ peak is harmonic with the 3650 cm⁻¹ peak. Polarized IR absorption spectra of (100), (010), and (001) planes of the hydrous topaz were measured to examine IR active orientation of the 1165 cm⁻¹ OH related mode. Three pleochroic distributions of the absorption peak at 1165 cm⁻¹ on (100), (010), and (001) planes indicate an active orientation of the 1165 cm⁻¹ OH related mode. The IR active orientation of the 1165 cm⁻¹ OH related mode in topaz is normal to the OH dipole. The orthogonality and harmonic combination mode indicate that the 1165 cm⁻¹ peak is OH bending mode. The active orientation of OH bending mode is polarized in the plane normal to the OH dipole. The polarization suggests that anisotropic thermal vibration of protons on the hydroxyl is maximum along the IR active orientation. Received: August 16, 1996 / Revised, accepted: April 20, 1997     

The study of the kinetics and the mechanism of dehydroxylation in muscovite by ESR on Fe3+

 Dehydroxylation of muscovite in the form of small lamellae at 923 <T <1173 K was studied by Electron Spin Resonance (ESR) on Fe³⁺. The kinetics of the process has been established to be described by the model of continuous nucleation on the large surface planes of the small plates. Determined by experimental data the rate constant of the process k is shown to be that of dehydroxylation itself. The activation energy obtained by data at T<1100 K is 97.5 KJ·mol⁻¹. The nonlinear dependence of ln(k) on 1/T is explained by the theory of transitions induced by the fluctuative preparation of a potential barrier as a result of thermal oscillations of ions in the lattice. At high temperatures the potential curve of the hydroxyl's proton is transformed so that it can overcome the barrier from one potential well to the other (from one hydroxyl site to the adjacent one). Such transformations of the curve can be caused by the oscillations of large structural clusters (∼1·10⁻²² kg) with the frequency ∼4.5·10¹² s⁻¹. Received: 3 August 1995 / Accepted: 13 April 1997     

Electronic absorption and luminescence spectroscopic studies of kyanite single crystals: differentiation between excitation of FeTi charge transfer and Cr3+ dd transitions

A selected set of five different kyanite samples was analysed by electron microprobe and found to contain chromium between <0.001 and 0.055 per formula unit (pfu). Polarized electronic absorption spectroscopy on oriented single crystals, R₁, R₂-sharp line luminescence and spectra of excitation of λ₃- and λ₄-components of R₁-line of Cr³⁺-emission had the following results: (1) The Fe²⁺–Ti⁴⁺ charge transfer in c-parallel chains of edge connected M(1) and M(2) octahedra shows up in the electronic absorption spectra as an almost exclusively c(||Z′)-polarized, very strong and broad band at 16000 cm⁻¹ if <, in this case the only band in the spectrum, and at an invariably lower energy of 15400 cm⁻¹ in crystals with  ≥ . The energy difference is explained by an expansion of the O_f–O_k, and O_b–O_m edges, by which the M(1) and M(2) octahedra are interconnected (Burnham 1963), when Cr³⁺ substitutes for Al compared to the chromium-free case. (2) The Cr³⁺ is proven in two greatly differing crystal fields a and b, giving rise to two sets of bands, derived from the well known dd transitions of Cr^{3+ 4}A_2g→⁴T_2g(F)(I), →⁴T_1g(F)(II), and →⁴T_1g(P)(III). Band energies in the two sets a and b, as obtained by absorption, A, and excitation, E, agree well: I: 17300(a, A), 17200(a, E), 16000(b, A), 16200(b, E); II: 24800(a, A), 24400(a, E); 22300(b, A), 22200(b, E); III: 28800(b,A) cm⁻¹. Evaluation of crystal field parameters from the bands in the electronic spectra yield Dq(a)=1730 cm⁻¹, Dq(b)=1600 cm⁻¹, B(a)=790 cm⁻¹, B(b)=620 cm⁻¹ (errors ca. ±10 cm⁻¹), again in agreement with values extracted from the λ³, λ⁴ excitation spectra. The CF-values of set a are close to those typical of Cr³⁺ substituting for Al in octahedra of other silicate minerals without constitutional OH⁻ as for sapphirine, mantle garnets or beryl, and are, therefore, interpreted as caused by Cr³⁺ substituting for Al in some or all of the M(1) to M(4) octaheda of the kyanite structure, which are crystallographically different but close in their mean Al–O distances, ranging from 1.896 to 1.919 A (Burnham 1963), and slight degrees of distortion. Hence, band set a originates from substitutive Cr³⁺ in the kyanite structural matrix. The CF-data of Cr³⁺ type b, expecially B, resemble those of Cr³⁺ in oxides, especially of corundum type solid solutions or eskolaite. This may be interpreted by the assumption that a fraction of the total chromium contents might be allocated in a precursor of a corundum type exsolution. Received: 3 January 1997 / Revised, accepted: 2 May 1997     

Electrical properties of opal-CT

 The electrical properties of opal-CT are validated at temperatures from 600 to 840 °C and frequencies from 5 Hz to 10 MHz. The opals are hydrothermal, containing less than 11270 ppm total of Al, Fe, Ca, Na, and K, and from 1.17 to 17.63 wt% H₂O interstitial and structural. Opal-CT shows fine crystallites, measuring from 19.4 to 22.7 μm, of an ordered tridymite-M stratification with high-cristobalite, embedded in a non-crystalline matrix. When heated to 600 °C, the non-crystalline phase devitrifies to the same stacked high-cristobalite-tridymite-M crystals. Opals containing less than 2070 ppm of cationic impurities are characterized by one single high-frequency complex impedance arc corresponding to the bulk polarization of the crystalline phase, of capacitances between 25 and 30×10⁻¹² F and resistances from 132 to 890 ohm. Opals having over 6300 ppm of cationic impurities show two superimposed high- and low-frequency complex impedance arcs. The high-frequency arc corresponds to the bulk polarization of the crystalline phase, of capacitance between 8 and 15.7×10⁻¹² F and resistance from 14 to 236 ohm, less than the capacitance of 0.25 to 0.53×10⁻⁹ F and resistance from 26 to 360 ohm of the non-crystalline minor intergranular material represented by the low-frequency impedance arc. The electric module shows one single vertex, ascribed to the bulk polarization of the crystalline phase. The conductivities are from 10⁻⁷ to 10⁻⁴ ohm⁻¹cm⁻¹, in the range of poor ionic conductors, essentially constant below 1.8 kHz, rapidly increasing at higher frequencies, due to ionic and electronic charge carriers. The activation energy changes between 0.905 and 1.003 eV for the conduction mechanism in the crystalline phase and from 0.924 to 1.087 eV in the non-crystaline phase. X-ray diffraction and impedance spectroscopy confirm that opal-CT is a crystalline stacked sequence of tridymite-M and cristobalite-high, in a non-crystalline matrix. Received October 20, 1995/Revised, accepted June 15, 1996     

Reactions and reaction rates in the regional aquifer beneath the Pajarito Plateau,north-central New Mexico,USA

Reactions and reaction rates within aquifers are fundamental components of critical hydrological processes. However, reactions simulated in laboratory experiments typically demonstrate rates that are much faster than those observed in the field. Therefore, it is necessary to conduct more reaction rate analyses in natural settings. This study of geochemical reactions in the regional aquifer in the Pajarito Plateau near Los Alamos, New Mexico combines modeling with petrographic assessment to further knowledge and understanding of complex natural hydrologic systems. Groundwater geochemistry shows marked evolution along assumed flow paths. The flow path chosen for this study was evaluated using inverse mass balance modeling to calculate the mass transfer. X-ray diffraction and field emission gun scanning electron microscopy were used to identify possible reactants and products. Considering the mineralogy of the aquifer and saturation indices for the regional water refined initial interpretations. Calculations yielded dissolution rates for plagioclase on the order of 10⁻¹⁵ mol s⁻¹ m⁻² and for K-feldspar on the order of 10⁻¹⁷ mol s⁻¹ m⁻², orders of magnitude slower than laboratory rates. While these rates agree with other aquifer studies, they must be considered in the light of the uncertainty associated with geometric surface area estimates, ¹⁴C ages, and aquifer properties.     

Crystal field transitions in Co2[Al4Si5]O18 cordierite and CoAl2O4 spinel determined by neutron spectroscopy

Inelastic magnetic neutron scattering has been used to determine the energy of the ⁴ A ₂→⁴ T ₂ transition in CoAl₂O₄ spinel and the δ₁ transition in Co₂[Al₄Si₅]O₁₈ cordierite. The observed crystal field splitting in Co-spinel is 485 meV (3900 cm⁻¹), which corresponds to a crystal field stabilization energy of 56.2 kJmol⁻¹. The transition energy of the δ₁ transition in Co-cordierite has been determined to be 21 meV (170 cm⁻¹). The present data demonstrate that magnetic neutron scattering can be used to measure crystal field transitions at energies of interest in the study of 3d-containing silicates. It may be used to measure transition energies when the use of optical spectroscopy is inappropriate. Received: 30 January 1997 / Accepted: 5 July 1997     

A theoretical study of the absorption spectra of Pb+ and Pb3+ in site K+ of microcline: application to the color of amazonite

The blue-green color of amazonite has been assigned by various authors to ions Pb⁺ (6 s)² (6 p) and/or Pb³⁺ (6 s) in site of K⁺ of microcline. Owing to the complex which forms between the ion Pb³⁺ and the lone pairs of the oxygen atoms surrounding it, the peripheral electron of Pb³⁺ passes on the levels (6 p) of the latter, which results in a great similarity of the spectra of Pb⁺ and Pb³⁺ in amazonite (the transition energies are multiplied by a factor greater than 1), whereas, in the isolated state, these spectra are completely different from one another. An analytical development of the crystal field around a site K⁺ is established. Under the effect of the crystal field, the transition ² P _1/2→² P _3/2 (6 p) is split into two double transitions. The lower transition only falls in the visible domain (1.6–1.8 eV for Pb⁺), the second in U−V. The green color would arise from the ion Pb⁺, whereas the blue one would be attributed to the ion Pb³⁺. Received: 23 January 1997 / Revised, accepted: 10 September 1997     

X-ray diffraction study of magnesite at high pressure and high temperature

 P–V–T measurements on magnesite MgCO₃ have been carried out at high pressure and high temperature up to 8.6 GPa and 1285 K, using a DIA-type, cubic-anvil apparatus (SAM-85) in conjunction with in situ synchrotron X-ray powder diffraction. Precise volumes are obtained by the use of data collected above 873 K on heating and in the entire cooling cycle to minimize non-hydrostatic stress. From these data, the equation-of-state parameters are derived from various approaches based on the Birch-Murnaghan equation of state and on the relevant thermodynamic relations. With K′₀ fixed at 4, we obtain K₀=103(1) GPa, α(K⁻¹)=3.15(17)×10⁻⁵ +2.32(28)×10⁻⁸ T, (∂K_T/∂T)_P=−0.021(2) GPaK⁻¹, (dα/∂P)_T=−1.81×10⁻⁶ GPa⁻¹K⁻¹ and (∂K_T/∂T)_V= −0.007(1) GPaK⁻¹; whereas the third-order Birch-Murnaghan equation of state with K′₀ as an adjustable parameter yields the following values: K₀=108(3) GPa, K′₀=2.33(94), α(K⁻¹)=3.08(16)×10⁻⁵+2.05(27) ×10⁻⁸ T, (∂K_T/∂T)_P=−0.017(1) GPaK⁻¹, (dα/∂P)_T= −1.41×10⁻⁶ GPa⁻¹K⁻¹ and (∂K_T/∂T)_V=−0.008(1) GPaK⁻¹. Within the investigated P–T range, thermal pressure for magnesite increases linearly with temperature and is pressure (or volume) dependent. The present measurements of room-temperature bulk modulus, of its pressure derivative, and of the extrapolated zero-pressure volumes at high temperatures, are in agreement with previous single-crystal study and ultrasonic measurements, whereas (∂K_T/∂T)_P, (∂α/∂P)_T and (∂K_T/∂T)_V are determined for the first time in this compound. Using this new equation of state, thermodynamic calculations for the reactions (1) magnesite=periclase+CO₂ and (2) magnesite+enstatite=forsterite+CO₂ are consistent with existing experimental phase equilibrium data. Received September 28, 1995/Revised, accepted May 22, 1996     

Thermoluminescence and hydroxyl defects associated with broad-band infra red absorption in synthetic quartz

A study has been made of thermoluminescence from synthetic quartz with varying hydroxyl impurity concentrations up to approximately 300 H/10⁶ Si which are associated with a “broad-band” IR absorption in the range 2600–3700 cm⁻¹. These hydroxyl defects are known to be important in the hydrolytic weakening of quartz. We have found only minor differences in the glow curves of unheated crystals but significant intensity increases when “wet” crystals are heated sufficiently to cause bubble formation. It would seem that the electron traps are unaffected by the bubble formation, but the electron/luminescence centre radiative recombination probability is increased.     

Hydroxyl defects in garnets from mantle xenoliths in kimberlites of the Siberian platform

A suite of more than 200 garnet single crystals, extracted from 150 xenoliths, covering the whole range of types of garnet parageneses in mantle xenoliths so far known from kimberlites of the Siberian platform and collected from nearly all the kimberlite pipes known in that tectonic unit, as well as some garnets found as inclusions in diamonds and olivine megacrysts from such kimberlites, were studied by means of electron microprobe analysis and single-crystal IR absorption spectroscopy in the v _OH vibrational range in search of the occurrence, energy and intensity of the v _OH bands of hydroxyl defects in such garnets and its potential use in an elucidation of the nature of the fluid phase in the mantle beneath the Siberian platform. The v _OH single-crystal spectra show either one or a combination of two or more of the following major v _OH bands, I 3645–3662 cm⁻¹, II 3561–3583 cm⁻¹, III 3515–3527 cm⁻¹, and minor bands, Ia 3623–3631 cm⁻¹, IIa 3593–3607 cm⁻¹. The type of combination of such bands in the spectrum of a specific garnet depends on the type of the rock series of the host xenolith, Mg, Mg-Ca, Ca, Mg-Fe, or alkremite, on the xenolith type as well as on the chemical composition of the respective garnet. Nearly all garnets contain band systems I and II. Band system III occurs in Ti-rich garnets, with wt% TiO₂ > ca. 0.4, from xenoliths of the Mg-Ca and Mg-Fe series, only. The v _OH spectra do not correspond to those of OH⁻ defects in synthetic pyropes or natural ultra-high pressure garnets from diamondiferous metamorphics. There were no indications of v _OH from inclusions of other minerals within the selected 60 × 60 μm measuring areas in the garnets. The v _OH spectra of pyrope-knorringite- and pyrope-knorringite-uvarovite-rich garnets included in diamonds do not show band systems I to III. Instead, they exhibit one weak, broad band (Δv _OH 200–460 cm⁻¹) near 3570 cm⁻¹, a result that was also obtained on pyrope-knorringite-rich garnets extracted from two olivine megacrysts. The quantitative evaluation, on the basis of relevant existing calibrational data (Bell et al. 1995), of the sum of integral intensities of all v _OH bonds of the garnets studied yielded a wide range of “water” concentrations within the set of the different garnets, between values below the detection limit of our single-crystal IR method, near 2 × 10⁻⁴ wt%, up to 163 × 10⁻⁴ wt%. The “water” contents vary in a complex manner in garnets from different xenolith types, obviously depending on a large number of constraints, inherent in the crystal chemistry as well as the formation conditions of the garnets during the crystallization of their mantle host rocks. Secondary alteration effects during uplift of the kimberlite, play, if any, only a minor role. Despite the very complex pattern of the “water” contents of the garnets, preventing an evaluation of a straightforward correlation between “water” contents of the garnets and the composition of the mantle's fluid phase during garnet formation, at least two general conclusions could be drawn: (1) the wide variation of “water” contents in garnets is not indicative of regional or local differences in the composition of the mantle's fluid phase; (2) garnets formed in the high-pressure/high-temperature diamond-pyrope facies invariably contain significantly lower amounts of “water” than garnets formed under the conditions of the graphite-pyrope facies. This latter result (2) may point to significantly lower f _H2O and f _O2 in the former as compared to the latter facies. Received: 25 November 1997 / Accepted: 9 March 1998     

Calcium Carbonate Nucleation in an Alkaline Lake Surface Water, Pyramid Lake, Nevada, USA

Calcium concentration and calcite supersaturation (Ω) needed for calcium carbonate nucleation and crystal growth in Pyramid Lake (PL) surface water were determined during August of 1997, 2000, and 2001. PL surface water has Ω values of 10–16. Notwithstanding high Ω, calcium carbonate growth did not occur on aragonite single crystals suspended PL surface water for several months. However, calcium solution addition to PL surface-water samples caused reproducible calcium carbonate mineral nucleation and crystal growth. Mean PL surface-water calcium concentration at nucleation was 2.33 mM (n = 10), a value about nine times higher than the ambient PL surface-water calcium concentration (0.26 mM); mean Ω at nucleation (109 with a standard deviation of 8) is about eight times the PL surface-water Ω. Calcium concentration and Ω regulated the calcium carbonate formation in PL nucleation experiments and surface water. Unfiltered samples nucleated at lower Ω than filtered samples. Calcium concentration and Ω at nucleation for experiments in the presence of added particles were within one standard deviation of the mean for all samples. Calcium carbonate formation rates followed a simple rate expression of the form, rate (mM/min) = A (Ω) + B. The best fit rate equation “Rate (Δ mM/Δ min) = −0.0026 Ω + 0.0175 (r = 0.904, n = 10)” was statistically significant at greater than the 0.01 confidence level and gives, after rearrangement, Ω at zero rate of 6.7. Nucleation in PL surface water and morphology of calcium carbonate particles formed in PL nucleation experiments and in PL surface-water samples suggest crystal growth inhibition by multiple substances present in PL surface water mediates PL calcium carbonate formation, but there is insufficient information to determine the chemical nature of all inhibitors.     

The single-crystal,polarized-light,FTIR spectrum of stoppaniite,the Fe analogue of beryl

We report here a single-crystal polarized-light study of stoppaniite, ideally (Fe,Al,Mg)₄(Be₆Si₁₂O₃₆)(H₂O)₂(Na,□), from Capranica (Viterbo). Polarized-light FTIR spectra were collected on an oriented (hk0) section, doubly polished to 15 μm. The spectrum shows two main bands at 3,660 and 3,595 cm⁻¹; the former is strongly polarized for E ⊥c, while the latter is polarized for E //c. A sharp and very intense band at 1,620 cm⁻¹, plus minor features at 4,000 and 3,228 cm⁻¹ are also polarized for E //c. On the basis of literature data and considering the pleochroic behavior of the absorptions, the 3,660 cm⁻¹ band is assigned to the ν₃ stretching mode and the 1,620 cm⁻¹ (associated with an overtone 2*ν₂ at 3,230 cm⁻¹) band to the ν₂ bending mode of “type II” water molecules within the structural channels of the studied beryl. The sharp band at 3,595 cm⁻¹ is not associated with a corresponding ν₂ bending mode; thus it is assigned to the stretching vibration of O–H groups in the sample. The minor 4,000 cm⁻¹ feature can be assigned to the combination of the O–H bond parallel to c with a low-frequency metal-oxygen mode such as the Na–O stretching mode. The present results suggest that the interpretation of the FTIR spectrum of Na-rich beryl needs to be carefully reconsidered.     

Computer modelling of a pressure induced phase change in clinoenstatite pyroxenes

 Using lattice dynamic modelling of pure MgSiO₃ clinopyroxenes, we have be able to simulate the properties of both the low-clino (P2₁/c) and a high-density-clino (C2/c) phases and our results are comparable with the high pressure (HP) X-ray study of these phases (Angel et al. 1992). The transition between the two phases is predicted to occur at 6GPa. The volume variation with pressure for both phases is described by a third-order Birch-Murnaghan equation of state with the parameters V _{0  low}=31.122 cm³·mol⁻¹, K _{T0  low}= 107.42 GPa, K′ _{T0  low}=5.96, V _{0  high}=30.142 cm³·mol^–1, K _{T0  high}102.54 GPa and K′ _{T0  high}=8.21. The change in entropy between the two modelled phases at 6GPa is ΔS _{6 Gpa}=−1.335 J·mol⁻¹·K⁻¹ and the equivalent change in volume is ΔV _{6 GPa}=−0.92 cm³·mol⁻¹, from which the gradient of the phase boundary δP/δT is 0.0014 GPa·K⁻¹. The variation of the bulk modulus with pressure was also determined from the modelled elastic constants and compares very well with the EOS data. The reported Lehmann discontinuity, ∼220 km depth and pressure of 7.11Gpa, has an increase in the seismic compressional wave velocity, v _p , of 7.14% using the data given for PREM (Anderson 1989). At a pressure of 7GPa any phase transition of MgSiO₃ pyroxene would be between ortho (Pbca) and high-clino. We find the value of v _p at 7GPa, for modelled orthoenstatite (Pbca), is 8.41 km·sec⁻¹ and that for the modelled high-clino phase at 7GPa is 8.93 km·sec⁻¹, giving a dv _p /v _p of 6.18%. Received: July 26, 1996 / Revised, accepted: September 27, 1996     

Hydraulic properties of the crystalline basement

Hydraulic tests in boreholes, up to 4.5 km deep, drilled into continental crystalline basement revealed hydraulic conductivity (K) values that range over nine log-units from 10⁻¹³−10⁻⁴ m s⁻¹. However, K values for fractured basement to about 1 km depth are typically restricted to the range from 10⁻⁸ to 10⁻⁶ m s⁻¹. New data from an extended injection test at the KTB research site (part of the Continental Deep Drilling Program in Germany) at 4 km depth provide K=5 10⁻⁸ m s⁻¹. The summarized K-data show a very strong dependence on lithology and on the local deformation history of a particular area. In highly fractured regions, granite tends to be more pervious than gneiss. The fracture porosity is generally saturated with Na–Cl or Ca–Na–Cl type waters with salinities ranging from <1 to >100 g L⁻¹. The basement permeability is well within the conditions for advective fluid and heat transport. Consequently, fluid pressure is hydrostatic and a Darcy flow mechanism is possible to a great depth. Topography-related hydraulic gradients in moderately conductive basement may result in characteristic advective flow rates of up to 100 L a⁻¹ m⁻² and lead to significant advective heat and solute transfer in the upper brittle crust. An erratum to this article can be found at     

Hydrochemistry and Classification of Groundwater Resources of Ishwardi Municipal Area,Pabna District,Bangladesh

The chemical property of groundwater depends largely on the mineralogical composition of the rocks through which the water has moved and the rate of movement and these characteristics of surface water depend on organic and inorganic reactions, industrial effluents, rainfall and temperature etc. The underground water tends to contain more dissolved materials than those in surface water because of their more intimate and longer contact with organic materials of soil and rock particles. The groundwater of the studied area is dominant of alkaline earth’s (Ca²⁺ and Mg²⁺) and weak acids (HCO₃ ⁻) which may be classified as Magnesium-Bicarbonate and Calcium Carbonate types. Genetically, the groundwater of the area belongs to both “Normal Chloride” “Normal Sulphate” and “Normal Carbonate” to “Super Carbonate” group. Based on EC, SAR and RC, the groundwater of the area varies from good–excellent quality for irrigation purposes with low alkali hazard and medium salinity hazard.     

Chlorite-rich ultramafic reaction zones in Colorado Plateau xenoliths: recorders of sub-Moho hydration

 Three chlorite-rich and one garnet-pyroxenite xenolith from the diatreme at Moses Rock, Utah, document storage and transport of water and consequent metasomatism in the mantle within the stability field of garnet peridotite, probably at depths of at least 75 km. Three mineral assemblages are present in zones in one chlorite-rich xenolith: in that xenolith, an assemblage of chlorite+enstatite+diopside+ ilmenite+titanian chondrodite is separated by diop- side+“talc” from an assemblage of chlorite+diopside+ilmenite+pyrite. Euhedral grains of enstatite (0.02% Al₂O₃, 0.05% CaO)+diopside record low temperatures, and high Mn/Fe in these pyroxenes was caused by growth in chlorite-dominated rock. Derivation from garnet lherzolite is established by relict pyrope (Py₇₁Gr₁₁Alm₁₈). The “talc” has Fe/Mg unusually high relative to that of associated chlorite, and electron probe analyses of the “talc” sum low, consistent with excess water; the unusual composition may be due solely to alteration and consequent submicroscopic intergrowths of other phases, but the “talc” could be an analogue of the high-pressure synthetic 10-Å phase. Garnet pyroxenite has a retrograde assemblage of chlorite-garnet-omphacite. The chlorite-rich rocks formed at contacts between garnet peridotite and other mantle rock in response to fluid flow. Pressures ≥2.2 GPa are consistent with stability of enstatite + aqueous fluid and of diopside + talc, with the occurrence of titanian chondrodite, and with the stability of garnet lherzolite. A chlorite separate has δ¹⁸O=6.9, consistent with mantle hydration. The small-scale reaction zones could have formed in a geologically brief time, plausibly just before eruption at about 25 Ma, and the responsible fluids probably also catalyzed recrystallization of associated eclogites. The hydration may have been restricted to shear zones that traversed the lower crust and the mantle to at least 75 km depth. The chlorite-rich rocks may be from the deepest part of the mantle that was sampled by the diatreme eruption. Chlorite-garnet pairs in garnet pyroxenites and pyrope megacrysts yield temperatures in the range 410–510^° C. Low temperatures in the mantle of the Colorado Plateau are consistent with an unusually low mantle heat flux and with cooling of lithosphere by an underlying subducted slab. Received: 14 April 1994/Accepted: 23 December 1994     

Interdiffusion of Na,K,and Ca Between Granitic Melts and NaCl Liquid

This study is aimed at determining the diffusion coefficient of net-work modifiers (mainly Na, K, and Ca) in a two-phase melt-NaCl system, in which the melts are granitic and the system is NaCl-rich in composition. The diffusion coefficients of Na, K, and Ca were measured at the temperatures of 750 – 1400°C, pressures of 0.001 × 10⁸ – 2 × 10⁸ Pa, and initial H₂O contents of 0 wt% –6.9 wt% in the granitic melts. The diffusion coefficients of Fe and Mg were difficult to resolve. In all experiments a NaCl melt was present as well. In the absence of H₂O, the diffusion of net-work modifiers follows an Arrhanious equation at 1 × 10⁵ Pa: lgD_ca=−3. 88−5140/T, lgD_k =−3. 79−4040/T, and lgD_Na, =−4.99−3350/T, where D is in cm² /s andT is in K. The diffusion coefficients of Ca, Na, K, and Fe increase non-linearly with increasing H₂O content in the melt. The presence of about 2 wt% H₂O m the melt will lead to a dramatical increase in diffusivity, but higher H₂O content has only a minor effect. This change is probably the result of a change in the melt structure when H₂O is present. The diffusion coefficients measured in this study are significantly different from those in previous works. This may be understood in terms of the “transient two-liquid equilibrium” theory. Element interdiffusion depends not only on its concentration, but also on its activity co-efficient gradient, which is reflected by the distribution coefficient, of the two contacting melts.     

Melt dynamics of a layered mantle plume source

The supply rates of basaltic magma to volcanoes restrict the flow rates within their mantle sources. Layered source regions consisting of alternating layers of melt and residuum have permeabilities that are orders of magnitude larger than percolative sources. Relevant supply rates for Hawaiian volcanoes are obtained for source permeabilities within the range 10⁻⁴–10⁻² cm². The results are within the range for layered sources, suggesting that a layered source is a physically viable model for Hawaiian plume sources. Received: 4 March 1997 / Accepted: 23 April 1998     

Thermobarometry of mafic igneous rocks based on clinopyroxene-liquid equilibria, 0–30 kbar

 Models for estimating the pressure and temperature of igneous rocks from co-existing clino-pyroxene and liquid compositions are calibrated from existing data and from new data obtained from experiments performed on several mafic bulk compositions (from 8–30 kbar and 1100–1475° C). The resulting geothermobarometers involve thermodynamic expressions that relate temperature and pressure to equilibrium constants. Specifically, the jadeite (Jd; NaAlSi₂O₆)–diopside/hedenbergite (DiHd; Ca(Mg, Fe) Si₂O₆) exchange equilibrium between clinopyroxene and liquid is temperature sensitive. When compositional corrections are made to the calibrated equilibrium constant the resulting geothermometer is (i) 10⁴ T=6.73−0.26^* ln [Jd^px*Ca^liq*Fm^liqDiHd^px*Na^liq*Al^liq] −0.86^* ln [Mg^liqMg^liq+Fe^liq]+0.52^*ln [Ca^liq] an expression which estimates temperature to ±27 K. Compared to (i), the equilibrium constant for jadeite formation is more sensitive to pressure resulting in a thermobarometer (ii) P=−54.3+299^* T10⁴+36.4^* T10⁴ ln [Jd^px[Si^liq]^2*Na^liq*Al^liq] +367^*[Na^liq*Al^liq] which estimates pressure to ± 1.4 kbar. Pressure is in kbar, T is in Kelvin. Quantities such as Na^liq represent the cation fraction of the given oxide (NaO_0.5) in the liquid and Fm=MgO+FeO. The mole fractions of Jd and diopside+hedenbergite (DiHd) components are calculated from a normative scheme which assigns the lesser of Na or octahedral Al to form Jd; any excess Al^VI forms Calcium Tschermak’s component (CaTs; CaAlAlSiO₆); Ca remaining after forming CaTs and CaTiAl₂O₆ is taken as DiHd. Experimental data not included in the regressions were used to test models (i) and (ii). Error on predictions of T using model (i) is ±40 K. A pressure-dependent form of (i) reduces this error to ±30 K. Using model (ii) to predict pressures, the error on mean values of 10 isobaric data sets (0–25 kbar, 118 data) is ±0.3 kbar. Calculating thermodynamic properties from regression coefficients in (ii) gives V^Jd _f of 23.4 ±1.3 cm³/mol, close to the value anticipated from bar molar volume data (23.5 cm³/mol). Applied to clinopyroxene phenocrysts from Mauna Kea, Hawaii lavas, the expressions estimate equilibration depths as great as 40 km. This result indicates that transport was sufficiently rapid that at least some phenocrysts had insufficient time to re-equilibrate at lower pressures. Received: 16 May 1994/Accepted: 15 June 1995