Showing papers in "American Mineralogist in 1995"

The Effects of Temperature and ƒ O 2 on the Al-in-Hornblende Barometer

Abstract: The AI-in-hornblende barometer potentially offers a basis for estimating crystallization pressure for granitic batholiths. However, owing to the simplicity of its formulation, misuse of the barometer can occur. Many granitic intrusions are emplaced at conditions inconsistent with those of the existing experimental calibrations, including f02 2 kbar at typical crustal pressures. For the Mount Stuart batholith, consideration of temperature yields revised pressures that are in agreement with pressures obtained from wall rocks and eliminates much of the apparent domal structure. Low-f02 granites have amphibole Fe/(Fe + Mg) ratios that exceed the typical 0.40-0.65 range used in most experimental and empirical calibrations. Examples from anorogenic granitic batholiths of mid-Proterozoic age yield pressures that are too high by a factor of two to three in comparison with pressures obtained from adjacent metamorphic assemblages. Hornblende in these granites not only has high Fe/(Fe + Mg) but also low ratios of FeH to Fe2+. The anomalously high Al in Fe-rich, FeH -poor hornblende is inferred to be the result of increased [6JAIoccupancy of the M2 site not buffered by the Mg and FeH abundances typical of amphiboles in calc-alkaline and other high- f02 plutonic rocks.

Crystal chemistry of the monazite and xenotime structures

Abstract: Monazite and xenotime, the RE(P04) dimorphs, are the most ubiquitous rare earth (RE) minerals, yet accurate structure studies of the natural phases have not been reported. Here we report the results of high-precision structure studies of both the natural phases and the synthetic RE(P04) phases for all individual stable rare earth elements. Monazite is monoclinic, P2/n, and xenotime is isostructural with zircon (space group 14/amd). Both atomic arrangements are based on [001] chains of intervening phosphate tetrahedra and RE polyhedra, with a REOgpolyhedron in xenotime that accommodates the heavy lanthanides (Tb-Lu in the synthetic phases) and a RE09 polyhedron in monazite that preferentially incorporates the larger light rare earth elements (La-Gd). As the structure "transforms" from xenotime to monazite, the crystallographic properties are comparable along the [00I] chains, with structural adjustments to the different sizes of RE atoms occurring principally in (00I). There are distinct similarities between the structures that are evident when their atomic arrangements are projected down [001]. In that projection, the chains exist in (100) planes, with two planes per unit cell. In monazite the planes are offset by 2.2 A along [010], relative to those in xenotime, in order to accommodate the larger light RE atoms. The shift of the planes in monazite allows the RE atom in that phase to bond to an additional 02' atom to complete the RE09 polyhedron.

Quantitative analysis of trace OH in garnet and pyroxenes

Abstract: To calibrate infrared (IR) spectroscopy for quantitative analysis of trace structural OH in specific minerals, we have determined concentrations of H in pure separates of mantle derived pyrope garnet (56 ± 6 ppm H_2O by weight), augite (268 ± 8, ppm H_2O, and enstatite (217 ± 11 ppm H_2O) by manometry after heating the samples and extracting H_2 gas under vacuum. IR spectroscopy confirmed the presence of intrinsic OH in these samples prior to extraction and indicated between 86 and 100% removal of H during the extraction procedure. The integral specific absorption coefficients of 1.39 ± 0.14(lσ),7.09 ± 0.32, and 15.6 ± 0.94 /(ppm H_2O.cm^2) for pyrope, augite, and enstatite, respectively, allow precise spectroscopic determination of the OH content of upper mantle garnets and pyroxenes to concentration levels of a few parts per million. Uncertainties in accuracy depend on mineral composition and characteristics of the OH absorption spectrum and are estimated to range between ± 10 and ± 50%.

Comparison of microanalytical methods for estimating H2O contents of silicic volcanic glasses

Abstract: Three methods of estimating H20 contents of geologic glasses are compared: (1) ion microprobe analysis (secondary ion mass spectrometry), (2) Fourier-transform infrared spectroscopy(FTIR), and (3) electron microprobe analysis using the Na decay-curve method. Each analytical method has its own advantages under certain conditions, depending on the relative importance of analytical accuracy, precision, sensitivity, spatial resolution, and convenience, and each is capable of providing reasonably accurate estimates of the H20, or total volatile, content of geologicglasses.The accuracy of ion microprobe analyses depends critically on the availability of well-characterized hydrous standard glasses. Precision is often better than 0,2 wt% (10). The method provides good spatial resolution (-15 #m) and the capability to determine simultaneously the abundance of other volatile species of interest (e.g., F, B). FTIR spectroscopy provides excellent analytical sensitivity (-10 ppm), accuracy and precision «0.1 wt%),and the capability to determine the abundance of H20 and C02 species (H20, OH-, C02' eOj-) in analyzed glasses, although the spatial resolution (> 25-35 #m) is not as good as that of the ion microprobe. The main advantages of the estimation of H20 contents of hydrous glasses using the electron microprobe are excellent spatial resolution (- 10 #m) and analytical convenience. The disadvantages are that accuracy and precision (>0.5 wt%) are not as good as those associated with the other methods, but, for certain applications, these uncertainties may be acceptable for the estimation of H20 contents of H20-rich (> 1 wt%) samples.

A simple model for sector zoning in slowly grown crystals; implications for growth rate and lattice diffusion, with emphasis on accessory minerals in crustal rocks

Abstract: Abstract The occurrence of sector zoning in minerals of regional metamorphic or low-temperature intrusive origin implies that rapid growth is not required for the development of this particular form of homogeneous disequilibrium. It is shown here that sector zoning can be a natural consequence simply of slow lattice diffusion. Given anisotropic surface enrichment coupled with the low dilfusivities typical of highly charged elements in refractory accessory minerals such as zircon and titanite, sector zoning can arise even in cases of growth rjates as low as a few micrometers per million years. According to the proposed model, the development of sector zoning depends upon the competition between growth rate (V) and lattice diffusion (Di) within the near-surface layer (l), such that above a critical value of Vl/Di (~0.5-3), sector zoning is unavoidable in crystals that exhibit selective enrichment on some growth surfaces. Known diffusivities of rare earth elements in zircon and titanite lead to the expectation of sector zoning in these minerals with respect to REEs for reasonable geologic growth rates. Even in the case of clinopyroxene, diffusion of REEs and high field-strength elements may be slow enough to contribute to the development of sector zoning in laboratory-grown crystals.

Quartz-coesite transition revisited; reversed experimental determination at 500-1200 degrees C and retrieved thermochemical properties

Abstract: We have determined the quartz-coesite transition by reversed experiments in a pistoncylinder apparatus in the range 500-1200 0c. The differencebetween the sample pressure and apparent pressure was calibrated by (1) studying the friction decay in the hysteresis loop defined by the relationship between apparent ("nominal") pressure and piston position in the compression and decompression cycles, and (2) determining the melting temperature of LiCl by DTA in pressure cells similar to those used in the reversal experiments and comparing the results with those determined in the gas apparatus. The equilibrium transition boundary can be expressed as P (kbar) = 21.945 (:to.1855) + 0.006901 (:to.0003)T (K). It is subparallel to, but -1.5 kbar higher than, the transition boundary determined by Bohlen and Boettcher (1982). We have also retrieved the entropy [39.56 :t 0.2 J/(mol. K)] and enthalpy off ormation (-907.25 :t 0.007 kJ/mol) from elements of coesite at 1 bar, 298 K., from our phase-equilibrium data and selected thermochemical data from the literature. From the characteristics of the hysteresis loop we conclude that the often-used practice of maintaining a constant nominal pressure by repeated pressure adjustment during an experiment leads to variation of pressure on the sample.

H2o solubility in haplogranitic melts: compositional, pressure, and temperature dependence

Abstract: H20 solubility has been determined in haplogranitic melts (system Si02-NaAISi30s- KAISi30s, Qz-Ab-Or) in the range 0.5-8 kbar and 800-1350 0c. Three types of starting materials were used: dry glass cylinders, prehydrated glass pieces, or dry glass blocks surrounded by glass powder. All the starting materials gave consistent results. The H20 contents of the glasses were determined by Karl-Fischer titration. Dissolved H20 was demonstrated to be distributed homogeneously throughout the isobarically quenched melts (glasses) using infrared spectroscopy. The compositional dependence of H20 solubility was mainly determined at 0.5 kbar, 900 and 1000°C; 1 kbar, 850°C; and 4.8 kbar, 800°C. Seventeen compositions containing 25, 35, or 45 wt% normative Qz and with various Or/(Or + Ab) ratios (0.86-0.09, Ab and Or expressed as normative weight percent) were investigated. At 0.5 kbar, H20 sol- ubility was little affected by the anhydrous composition. By contrast, molar H20 solubility in alumino silicate melts was significantly dependent upon anhydrous composition between 1 and 5 kbar. The highest solubility values were obtained for the most Ab-rich melts. This alkali effect has important implications for the physical and chemical properties of granitic melts. The effect of pressure (P) on H20 solubility at P ;:: 3 kbar is greater than that reported in previous studies. Between 3 and 8 kbar at 800 OC,there is a (nearly linear) positive correlation between P and H20 solubility. The effect of temperature (T) on H20 solubility was investigated for a composition QZ2sAb3sOr34 (normative weight percent) in the P-T range 0.5-8 kbar and 800-1350 0C. Water solubility ranged from retrograde (with increas- ing T) at P ~ 4 kbar through temperature independence at approximately 4.5 kbar to prograde at P = 5 kbar. Calculated H20 solubilities using the model of Burnham and Nekvasil (1986) are slightly high at 0.5 kbar and significantly low at 5 kbar, compared with the experimental data. This implies that calculated H20 activities for haplogranitic systems using the H20 content of the melt may be overestimated at high pressure (P ;:: 5 kbar). Using the ther- modynamic model of Silver and Stolper (1985) and assuming a proportion of molecular H20 and OH groups close to that defined for albite melts by Silver and Stolper (1989), we found that the partial molar volume of H20 in a melt with a composition QZ2sAb3sOr34 has to be close to 10-12 cm3/mol to obtain a good agreement between the calculated and the experimentally determined H20 solubility curves in the pressure range 1-8 kbar at 900°C.

Textures and Sr, Ba, Mg, Fe, K, and Ti compositional profiles in volcanic plagioclase: Clues to the dynamics of calc-alkaline magma chambers

Abstract: Concentration profiles for Ca, Na, AI, K, Fe, Ti, Mg, Sr, and Ba obtained by electron microprobe and secondary ion mass spectrometry from plagioclase crystals, together with textural observations from interference contrast microscopy, are consistent with contrasting magma dynamics in two subvolcanic reservoirs from which silicic lavas erupted at the Tatara-San Pedro volcanic complex, Chilean Andes. The 1 km3 late Pleistocene (68 ka) Tatara dacite is chemically homogeneous, phenocryst-poor, and contains crystals of opacitized hornblende, orthopyroxene, and titanomagnetite, plus 2 mm euhedral plagioclase phenocrysts with simple zoning patterns. Except at their rims, analyzed phenocrysts show nearly monotonic core-to-rim changes from An52 to An31, including slight decreases in MgO and FeO' and slight increases in Ba, Sr, and K concentrations. Abrupt, but small, chemical shifts are associated with many subtle dissolution surfaces. In contrast, the 0.1 km3 of chemically and texturally heterogeneous Holocene San Pedro dacite contains crystals of clinopyroxene, orthopyroxene, hornblende, biotite, titanomagnetite, ilmenite, and abundant 2 mm euhedral plagioclase phenocrysts, plus plagioclase and olivine xenocrysts derived from fragmented quenched basaltic inclusions. Analyzed phenocrysts span the range from Ann to An32. Abrupt increases of 15 mol% An, large increases in MgO, FeO', and Sr, and decreases in Ba and K20 concentrations are associated with a few major dissolution surfaces. Plagioclase xenocrysts of An74-86have higher Sr, MgO, and FeO' and lower Ba and K concentrations than the phenocrysts, reflecting crystallization from a basaltic melt. In both dacites, phenocryst rims decrease in Sr and increase in K and Ti relatiye to equilibrium values as a consequence of rapid crystal growth at high undercooling during magma ascent and eruption. Modeling of variations in apparent D~~ag/melt implies growth rates for the ~ 100 JLm phenocryst rims of 10-9 cm/s, suggesting that magma ascent may have taken several months. Provided these kinetic effects are identified, the remainder of the concentration profiles for Sr, Ba, Mg, and K in plagioclase phenocrysts can be inverted using partition coefficient expressions to monitor the temporal evolution of melts in the preeruptive magma chambers. Monotonic melt trends and periodic thermal dissolution of Tatara dacite phenocrysts imply a magma chamber closed to inputs of new magma and heat. Repeated cycles of crystal growth and weak dissolution occurred during phenocryst retention in thermally driven convection cells characterized by steady-state laminar flow. In contrast, dissolution events associated with large, abrupt shifts in melt Sr, Mg, and K/Ba toward basaltic values indicate that in the Holocene magma chamber heating of silicic melt adjacent to basaltic inclusions during magma mingling caused plagioclase dissolution. Renewed plagioclase growth from the resulting small volumes of hybrid melt recorded highly localized chemical mixing by diffusion and accompanying cycles of fractionation. INTRODUCTION and chamberwide stratification (Hildreth, 1979; Marsh, 1989a; Druitt and Bacon, 1989). In addition, open-system behavior, including mechanical mingling, thermal equilibration, and chemical mixing of initially separate magmas (Eichelberger, 1978; Bacon, 1986), crustal assimilation (Wilcox, 1954; Bacon et aI., 1989), and eruption and degassing (e.g., Rutherford and Hill, 1993; CashProcesses in sub volcanic magma chambers playa major role in producing the spectrum of igneous rock compositions on Earth. Intrinsic phenomena include the following: cooling, crystallization, and convection of the magma, crystal fractionation or retention in residual melt, 0003-004 X/9 5/0708-077 6$02.00 776 SINGER ET AL.: VOLCANIC PLAGIOCLASE man, 1992) are well documented. Despite theoretical and experimental studies focused on such processes (e.g., Sparks et at, 1984, 1993; Sparks and Marshall, 1986; Huppert and Sparks, 1988; Marsh, 1988, 1989a, 1989b; Weinstein et at, 1988; Oldenberg et aI., 1989), extracting unambiguous information about the dynamic history of natural magmas has proved difficult (Marsh, 1990, 1991; Sparks, 1990; Huppert and Turner, 1991). Plagioclase phenocrysts preserve records of the chemical and physical evolution of magma chambers (e.g., Anderson, 1984; Pearce et aI., 1987; Blundy and Shimizu, 1991) because slow interdiffusion (Grove et aI., 1984) ensures that their chemical and textural zoning reflects primary growth. The major element composition of plagioclase (albite-anorthite) and its stability are functions of intensive and kinetic parameters, including melt composition and H20 content, temperature and pressure (Tsuchiyama, 1985; Rutherford and Devine, 1988; Housh and Luhr, 1991), decompression (Nelson and Montana, 1992), and growth rate (Lofgren, 1980). Thus, it is difficult to discriminate uniquely among rival magmatic effects on the basis of binary compositional variations. Since Homma's (1932) interpretation that oscillatory zoned plagioclase in andesitic lavas recorded thermally driven magmatic convection cycles, many petrologists have sought to correlate zoning in plagioclase with specific magmatic processes. An approach that can be used in conjunction with profiles of binary plagioclase components is to examine variations in non binary components present at minor (< 5000 ppm) or trace « 500 ppm) levels in these phenocrysts. Depending on the element, these concentrations reflect abundances in the melt, An content of the plagioclase, and kinetic factors such as diffusion in the melt and crystal growth rates. In this paper we address the behavior of nonbinary elements present at variable concentrations in coexisting melts, including the major elements Mg, Fe, Ti, and K. The elements Sr and Ba, which are present in trace amounts in the liquid but at minor (Sr) or trace (Ba) concentrations in plagioclase, are presumed (along with K) to occur as stoichiometric nonbinary feldspar substitutions. The precisions ofSr, Ba, Mg, and Fe determinations in plagioclase by EPMA are limited by low count rates, high detection limits, Na volatilization, and Fe fluorescence (e.g., Longhi et aI., 1976). Droitt and Bacon (1989) discriminate between high-Sr (> 2500 ppm) and low-Sr ( < 1500 ppm) plagioclase phenocrysts with a 2crprecision of :t 20% (300-500 ppm). Such uncertainties preclude application of EPMA data to studies of zoning in which the goal is to quantify subtle compositional variations in liquids or to separate kinetic from compositional and thermal effects. Secondary ion mass spectrometry (SIMS) using an ion microprobe permits more precise measurements of non binary element concentrations in plagioclase (Meyer et at, 1974; Steele et at, 1980). The ion probe provides the spatial resolution (~ 10 JLm) and analytical precision (:t2-50/0) necessary for inversion of zoning profiles for 777 elements such as Sr, Mg, and Ba to obtain records of melt evolution (Meyer et at, 1974; Shimizu, 1978; Blundyand Shimizu, 1991). Compositionally dependent textural features in etched plagioclase crystals may be imaged at the micrometer scale using reflected-light Nomarski differential interference contrast (NDIC) microscopy (Anderson, 1983). Internal features of plagioclase phenocrysts, such as dissolution surfaces and fine oscillatory growth zoning, which are difficult to detect in transmitted light, are readily visible with NDIC (Pearce et at, 1987; Pearce and Kolisnik, 1990). All EPMA and SIMS data in this study were obtained following thorough documentation of dissolution and growth features using NDIC imagery. Calculations of transient melt compositions from solidphase compositions require partitioning expressions that accurately incorporate thermal and compositional dependencies. The types of dependencies inherent in plagioclase-melt expressions differ for Sr, Ba, Fe, Mg, and K (Blundy and Wood, 1991; Longhi et at, 1976; Nagasawa and Schnetzler, 1971). Although errors in these expressions (analytical and formulaic) apparently do not preclude the applications proposed in this paper, additional determinations on the basis of carefully designed experiments could resolve several outstanding questions (Morse, 1992; Blundy and Wood, 1992, 1994). The D~~ag/melt and D§~glmeltrelations derived by Blundy and Wood (1991) from experimental data are based on their conclusion that partitioning of Sr and Ba between plagioclase and melt is insensitive to melt composition and temperature and is principally a function of An content of the crystal. This interpretation is reinforced by experiments illustrating relations between cation charge and radius, lattice site energetics, and partition coefficients (Blundy and Wood, 1994). Experiments of Longhi et al. (1976) also suggest that partition coefficients for Mg and Fe in plagioclase are largely independent of melt composition. Thus, if crystal growth occurred under nearequilibrium conditions, measurements ofSr, Ba, Mg, and Fe concentrations in plagioclase permit calculations of concentrations in the coexisting melt. Melt compositions calculated in this manner must be viewed with caution because high rates of crystal growth at large undercoolings may induce significant departures from equilibrium (Albarede and Bottinga, 1972; Shimizu, 1983). Blundy and Shimizu (1991) argued that plagioclase growth rates in plutons are sufficiently low as to preclude departure from equilibrium. Our results demonstrate that this assumption is not always valid for volcanic rocks. The role of kinetics in minor and trace element partitioning needs careful investigation. Moreover, the thermal and chemical histories of different parts of magma chambers are different, and where convective motions occur, phenocrysts with different histories will be brought into close proximity. The goals of this study are twofold: (1) to demonstrate the efficacy of integrated NDIC-EPMA-SIMS studies in

Geochemical alteration of pyrochlore group minerals: Pyrochlore subgroup

Abstract: Primary alteration of uranpyrochlore from granitic pegmatites is characterized by the substitutions ADYD-+ ACaYO,ANaYF-+ ACaYO,and ANaYOI-I--+ ACaYO.Alteration occurred at "" 450-650 °C and 2-4 kbar with fluid-phase compositions characterized by relatively low aNa+,high aeaH, and high pH. In contrast, primary alteration of pyrochlore from nepheline syenites and carbonatites follows a different tre:nd represented by the substitutions ANaYF -+ ADYDand ACaYO-+ ADYD.In carbonatites, primary alteration of pyrochlore probably took place during and after replacement of diopside + forsterite + calcite by tremolite + dolomite :t ankerite at ""300-550 °C and 0-2 kbar under conditions of relatively low aHF,low aNa+,low aeaH, low pH, and elevated activities of Fe and Sr. Microscopic observations suggest that some altered pyrochlor1es are transitional between primary and secondary alteration. Alteration paths for these specimens scatter around the trend ANaYF-+ ADYD.Alteration probably occurred at 200-350 °C in the presence of a fluid phase similar in composition to the fluid present during primary alteration but with elevated activities of Ba and REEs. Mineral reactions in the system Na-Ca-Fe-Nb-O-H indicate that replacement of pyrochlore by fersmite and columbite occurred at similar conditions with fluid conpositions having relatively low aNa+,moderate aeaH, and moderate to high aFeH.Secondary alteration « 150 °C) is charactlerizedby the substitutions ANaYF-+ ADYD,ACaYO-+ ADYD,and ACaXO-+ ADXDtogether with moderate to extreme hydration (10-15 wt% H20 or 2-3 molecules per formula unit). Minor variations in the amounts of Mg, AI, K, Mn, Fe, Sr, Ba, and REEs are commonly observed as a result of secondary alteration. Major cation exchange for K, Sr, and Ba is a feature of samples from laterite horizons overlying carbonatites. In most cases U, Th, and B-site cations remain relatively constant. Radiogenic Pb is typically lost via long-te:rm diffusion, but in some grains of uranpyrochlore 25-90% of the Pb is lost as a result of alteration.

Atomic radii of noble gas elements in condensed phases

Abstract: Neutral atomic radii of dissolved noble gas elements in condensed phases are obtained by treating the neutral atoms as "ions" of zero oxidation state and by interpolation from a plot of radius vs. oxidation state for isoelectronic ions. The major assumption is that the radius of an ion or a neutral atom having an electronic configuration of a noble gas element depends primarily on the interaction between the nucleus and the surrounding electrons and not on the interaction between the ion and its neighbors. As expected, the values of the new set of radii are slightly greater than the univalent radii and smaller than the radii of noble gas atoms in their crystals (in which the coordination number is 12). The neutral atomic radii (in angstroms) of noble gas elements are where CN = coordination number. Errors are given at the 20"level. Values in parentheses were determined by extrapolation and have greater errors. The larger error for the He radius is due to large uncertainty in the H - radius.

High-pressure phase transition in brucite, Mg(OH)2

Abstract: Brucite, Mg(OH)2, was investigated by Raman spectroscopy to pressures of 36.6 GPa under nonhydrostatic conditions and to 19.7 GPa under quasi-hydrostatic conditions. Several new Raman lines are first observed at 4 GPa, demonstrating the existence of a high-pressure structural change. One of the new lines grows over a broad pressure interval, and this growth can be explained by a resonant interaction with the Eg translational mode. Raman data are compared with recent infrared spectroscopy, X-ray diffraction, and neutron diffraction studies of brucite. The structural change is likely to involve displacement or disordering of the H atoms, consistent with neutron diffraction results. The Ramanactive O-H stretching vibration in brucite decreases with pressure at the rate of -7 cm-II GPa, larger than the pressure dependence of the infrared-active stretching vibration by more than a factor of ten. The primary differences in the Raman spectra of Mg(OH)2 and Ca(OH)2 are that the O-H vibrational frequencies in Mg(OH)2 vary linearly with pressure, and the O-H stretching vibration band width increases with pressure at a rate that is an order of magnitude lower for Mg(OH)2 than for Ca(OH)2' INTRODUCI'ION cused on determining the brucite-peric1ase dehydration curve, which also constrains the thermodynamic propThe high-pressure behavior of brucite, Mg(OH)2, is of erties of H20 under conditions of the lower crust and considerable interest for studying such diverse phenommantle. Advances in multi-anvil press technology have ena as dehydration reactions at high pressure, compresrecently enabled the dehydration reaction to be studied sion-induced arnorphization, and the behavior of hyup to 15 GPa and 1500 K (Leinenweber et aI., 1993; drous minerals in the Earth's upper mantle. The discovJohnson and Walker, 1993). Detailed analyses of the reery of large numbers of magnesian silicates containing sults require accurate characterization of the thermodystructurally bound OH (Finger and Prewitt, 1989; Kannamic properties of brucite. zaki, 1991) has raised questions concerning the potential Pressure-induced amorphization has now been docurole of such phases in the Earth's interior. Because of its mented in many materials. Portlandite, Ca(OH)2, which structural and chemical simplicity, brucite serves as a is isomorphous with brucite, amorphizes when comuseful prototype for hydrous and layered minerals at high pressed above 11 GPa at room temperature (Meade and pressures. Thermodynamic properties of brucite have been Jeanloz, 1990). In addition to the loss of X-ray diffraction investigated in several recent studies. The equation of peaks, significant changes in the Raman and infrared state has been measured under shock compression (Sispectra of this material have been observed (Kruger et makov et aI., 1974; Duffy et aI., 1991) and under high al., 1989; Meade et aI., 1992; Duffy, in preparation). In static pressures (Fei and Mao, 1993; Catti et aI., 1995; contrast, brucite has been found to be stable to 78 GPa Duffy et aI., 1995). Measurements of the thermal expanby X,ray diffraction (Fei and Mao, 1993) and to 34 GPa sivity at both ambient and high pressure have also been by infrared spectroscopy (Kruger et aI., 1989). As appears reported (Redfern and Wood, 1992; Fei and Mao, 1993). to be the general case for materials undergoing these tranNeutron diffraction has been conducted at elevated pressitions, the amorphization of Ca(OH)2 has been intersures on both normal (Catti et aI., 1995) and deuterated preted as the result of a frustrated phase transition. Howsamples (Parise et aI., 1994). The structure and bonding ever, no such phase transition in brucite has been deof Mg(OH)2 has been investigated theoretically using the tected by shock compression, static compression, or inHartree-Fock approximation (Sherman, 1991; D'Arco et frared spectroscopy experiments over a broad pressure aI., 1993). and temperature range. The phase equilibria of brucite have been the subject Brucite crystallizes in the trigonal CdI2 structure (P3m1) of high-pressure experimental investigations from the (Bernal and Megaw, 1935; Petch and Megaw, 1954; Elle1940s to the present. Such studies (e.g., Bowen and Tutman and Williams, 1956; Zigan and Rothbauer, 1967). tle, 1949; Kennedy, 1956; Irving et aI., 1977) have foThis is a layered structure in which each Mg ion is sur0003-004X/95/0304-0222$02.00 222 TABLE 1. Pressure dependence of the Raman modes of brucite . iMiJP' " .. ". . Mode (em-I) (em-'/GPa) (em-I) E.,(T) 280.0 5.40 0.15P 280 359.6 0.60 + 0.04P 383.8 2.18 408.1 4.21 0.20P A,.(T) 444.7 6.93 0.15P 443 E.,(R) 727.5 725 A,.(I) 3652.0 -7.68 3652 3661.3 -5.34 DUFFY ET AL.: HIGH-P PHASE TRANSITION IN BRUCITE 223 Fig. 1. Crystal structure of brucite. The large spheres are 0 atoms, the intermediate spheres are Mg atoms, and the small spheres are H atoms. The O-H bonds are directed along the c axis. rounded by a distorted octahedron of 0 atoms (Fig. 1). The Mg ions lie in planes with the 0 ions above and below them in a sandwich arrangement. The O-H bonds are perpendicular to these planes. The brucite layers are stacked along the c direction and held together by weak interlayer forces. There is conflicting evidence regarding the nature of these forces. On the basis of the interlayer o distances, Bernal and Megaw (1935) concluded the layers were held together by weak dipole forces. X-rayemission spectra, however, support the existence of some H bonding between the layers (Haycock et aI., 1978). Ab initio Hartree-Fock studies, however, have failed to find evidence for H bonding at ambient or elevated pressure (Sherman, 1991; D'Arco et aI., 1993). Factor group analysis indicates there are six allowed lattice modes for brucite: three of these are infrared active, and three are Raman active (Mitra, 1962). In addition, there are Ramanand infrared-active internal modes. The lattice vibrations consist of translational modes that correspond to vibrations of the O-H units that are either parallel [A,.(T) and A2u(T)] or perpendicular [E.(T) and Eu(T)] to the c axis. There are also rotational vibrations of the OH ions: E.(R) and Eu(R). The internal modes are symmetric (Raman-active) and antisymmetric (infrared-active) O-H stretching vibrations [A,.(I) and A2u(I)]. Ambient-pressure polarized Raman and infrared spectra (Dawson et aI., 1973) from normal and deuterated samples have been used to make mode assignments (Table 1). -...-....------.Data from this study. .. Data from Dawson et al. (1973). EXPERIMENTAL TECHNIQUE Brucite was synthesized in a piston-cylinder apparatus at 1.5 GPa and 1073 K under H20-saturated conditions. The samples were produced in the same experiment as those used in high-pressure X-ray diffraction studies (Fei and Mao, 1993; Duffy et aI., 1995). The crystals were transparent platelets with lateral dimensions up to 50 #m and thicknesses of < 10 #m. Ambient-pressure Raman spectra and X-ray diffraction confirmed that the samples were brucite and no impurity phases were detectable. Raman experiments were carried out in a Mao-Bell diamond-anvil cell with 600-#m culet type I diamonds. The sample was loaded into a hole 300 #m in diameter and 75 #m thick in a T301 steel gasket. Brucite samples were compressed both nonhydrostatically (with no pressure medium) and under quasi-hydrostatic conditions using Ne as a pressure medium. Raman spectra were recorded with a multichannel Raman microprobe (Dilor XY) in a backscattering configuration using a charge-coupled device (CCD) detector with 1024 x 298 channels (e.g., Hemley, 1987). The excitation source was Ar+ laser operated at either 488.0 or 514.5 nm at powers of 100 m W or less. The laser light was focused onto the sample and Raman signal collected using a Leitz L25 objective. Data accumulation times ranged ITom 100 to 3000 s. Peak positions and widths were determined by fitting the spectra to Lorentzian line shapes with background subtraction. Pressures were determined from the fluorescence of small ruby chips (1-5 #m) distributed through the sample volume (Mao et aI., 1986). Pressure was measured both before and after collecting Raman spectra at several positions within the chamber. In experiments with no pressure medium, the R, and R2 ruby fluorescence peaks were not always well resolved, and pressure differences of -1 GPa could develop across the sample chamber. For the Ne medium experiments, the pressure varied by <0.1 GPa across the sample, and the two ruby fluorescence peaks were always well resolved. For decompression experiments, the pressure before and after taking Raman measurements varied by up to 2.5 GPa.

Incongruent dissolution of REE- and Sr-rich apatite in peraluminous granitic liquids: Differential apatite, monazite, and xenotime solubilities during anatexis

Abstract: The relative solubilities of monazite (Mnz), xenotime (Xno), and apatite (Ap: REE- and Sr-rich, and REE- and Sr-poor) have been studied in peraluminous granitic liquids in month-long experiments at 750 °C and 200 MPa (PH20). In contrast with the high solubility of the apatite (0.7 wtOjoP205) in strongly peraluminous liquids, monazite and xenotime have much lower solubilities «0.05 wt°joP205). In mildly to strongly peraluminous compositions, P205 in the liquid is 0.03-0.04 wt°joat xenotime saturation and 0.02-0.05 wt°jo at monazite saturation; in keeping with the low P in the glasses, RE203 contents are below EMP detection thresholds (:50.08 wt°jo) for all conditions of experiments (saturation of liquid at equilibrium and local saturation around apatite). Apatite dissolves incongruently, crystallizing REE-rich monazite on its surface (1-4 ~m-Iong grains), resulting in similar low REE contents in liquids. Monazite precipitation occurs along the margins of dissolving apatite crystals, even though the bulk liquid is not monazite-saturated. The abundance of monazite microcrystals increases with the REE content of the apatite and the degree of apatite dissolution. The reaction relationship (Ap + LiqI -+ Mnz + Liq2), stemming from differences in relative solubilities (greater than an order of magnitude) between apatite and monazite, results in the dissolution of much smaller amounts of REE into peraluminous liquids than expected by simple evaluation of apatite REE contents. The amount of REE contributed from apatite directly to peraluminous granitic liquid is related to the amount of apatite dissolved by simple mass balance only if the total REE content of the apatite is sufficiently low that monazite saturation in liquid (50-100 ppm RE203 at 750 °C) is avoided. During dissolution of Sr-rich apatite, the Sr partitions into the liquid, and, at 750 °C and 200 MPa (PH20),the diffusion coefficient of Sr in liquid is ---2 x 10-10 cm2js (R2 = 0.659). The reaction relationships described above may have application to some textural features observed in natural igneous rocks. For example, clusters of monazite microcrystals might be indicators of dissolved apatite; monazite morphology can be used to distinguish the source of its REE and the general petrological process (rock anatexis or magma crystallization) under which the monazite formed. In addition, monazite microcrystals could serve as nucleation sites for other minerals, which might explain their common inclusion in biotite and amphibole within granitoids.

Site preference of rare earth elements in fluorapatite

Abstract: Crystals ofLa-, Gd-, and Dy-bearing fluorapatite [La-FAp, Gd-FAp, Dy-FAp; CalO-X-2YNayREEx+y (P1_xSixO.)6A2, with x = 0.24-0.29, Y = 0,32-0.36; P6/m] have been synthesized hydrothermally, and their structures refined at room temperature with single-crystal X-ray intensities to R = 0.015-0.018. Na is essentially restricted to the Cal position in La-FAp, Gd-FAp, and Dy-FAp, in contrast to Nd-FAp, which was synthesized under slightly different conditions and has appreciable Na in Ca2 as well. Site occupancies for REE in Cal and Ca2, respectively, are 0.023(1) and 0.093(1) in LaFAp, 0.038(1) and 0.111(1) in Nd-FAp, 0.038(0) and 0.077(0) in Gd-FAp, and 0.039(1) and 0.060(1) in DyFAp. The REE site occupancy ratio (REE-Ca2 to REE-Cal) appears to decrease monotonically for REE3+cations through the 4f transition-metal series. With this assumption, site occupancy ratios (REE-Ca2 to REE-Cal) for some other REE in natural apatite are estimated to be: La 4.04, Ce 3.67, Pr 3.30, Nd 2.92, Sm 2.47, Eu 2.25, Gd 2.03, Dy 1.54, Y 1.29, Er 1.05. These single-REEsite occupancyratios may not be transferrable to natural apatite, The Cal and Ca2 site occupancies are generallyconsistent with site preferences deduced from bond-valence calculations, which show that the substitutions for Ca lead to equalization of Cal and Ca2 bond valences. Also, the REE site occupancy ratio correlates inversely with F bond valence.

The influence of excess alkalis on the viscosity of a haplogranitic melt

Abstract: The effect of the addition of 5, 10, and 20 wt% of the alkali oxides on the viscosity of a haplogranitic melt composition has been investigated at I atm and in the temperature range of 400-1650 0c. The high-temperature viscosity data were obtained with concentric cylinder viscometry and the low-temperature viscosity data using micropenetration viscometry. The combined data sets for low- and high-temperature viscosities have been fitted for each composition using the Tamann-Vogel-Fulcher (TVF) equation. The effect of alkali oxide on the viscosity of a haplogranite melt is extreme. The viscosity decreases with added alkali oxide content in a nonlinear fashion. The first few mole percent of alkali oxide added decreases viscosity several orders of magnitude, whereas subsequent addition of alkali oxide has a much smaller effect. The effects of each of the alkalis are broadly similar, implying that the structural role of the alkalis is common to all. In detail however, the viscosity of the strongly peralkaline melts investigated here increases with the size of the added alkali cation in the order Li < Na < K,Rb,Cs. This trend probably reflects a minor influence ofthe alkali-O bond strengths on the melt viscosity. This distinction of a dominant depolymerizing influence and a minor alkali specific bond-strength influence has important implications for the comparison of these data with those for the addition of other depolymerizing agents on the viscosity ofhaplogranitic melt (e.g., H20, F20-1).

H in rutile-type compounds: I. Single-crystal neutron and X-ray diffraction study of H in rutile

Abstract: The crystal structure of natural hydrous rutile from a mantle eclogite nodule has been refined from single-crystal neutron diffraction data collected at 24 K. The chemical composition was 95.52 wt% Ti02, 0.74 wt% Fe203, 0.68 wt% A1203, 1.16 wt% Cr203, 1.86 wt% Nb205, and 0.04 wt% MnO, with approximately 13000 ppm OH-, as estimated by IR spectroscopy. The position of the H atom was located by examining the negative residuals in the difference Fourier maps. The refined position is near the shared edge of the cation octahedron at xl a = 0.42(1), ylb = 0.50(1), and zlc = 0 with a site occupancy of 2.7%, which is consistent with the H concentration estimated by IR spectroscopy. The O-H bond distance is 1.05 A and the OH vector is in the (001) plane, which is consistent with the strong w-polarization of the OH absorption observed in the IR spectra. Single-crystal X-ray data were collected from a synthetic anhydrous rutile crystal and a hydrous rutile crystal from the same mantle nodule as the sample used in the neutron study. The detailed X-ray structures of both rutiles were compared and no significant differences were found; the ale ratios and the 0 position were identical within error. Therefore, we conclude that the addition of H into rutile for the purpose of charge-balancing excess 3 + cations does not significantly change the structure.

The high-pressure stability of talc and 10 Å phase: potential storage sites for H2O in subduction zones

Abstract: The pressure-temperature conditions of the reactions governing the high-pressure stability of talc were investigated in experiments on the bulk composition Mg3Si40IO(OH)2 + H20 at 2.9-6.8 GPa, 650-820 °C, using piston-cylinder and multianvil apparatus. The reaction talc = enstatite + coesite + vapor was bracketed between 800 and 820°C at 2.90-2.95 GPa and between 770 and 780°C at 3.77-4.02 GPa. The lower-pressure bracket, which is just above the quartz-coesite phase transition, is consistent with some of the previous brackets on the reaction talc = enstatite + quartz + vapor and with the position of the talc dehydration reaction calculated using THERMOCALCv2.4 (Powell and Holland, 1988; Holland and Powell, 1990; Holland, personal communication). This revised version of THERMOCALC incorporates new compressibility and thermal expansivity data for talc (Pawley, Redfern, and Wood, in preparation). Agreement between experimental and calculated curves continues up to 4 GPa, but at higher pressures the talc dehydration reaction occurs at lower temperatures than calculated, so that by 4.6 GPa the thermal stability of talc is at <730°C. At -5 GPa, 710 °C, there is an invariant point involving talc, 10 A phase, enstatite, coesite, and vapor. This point marks the highest pressure at which talc is stable. Above it, the thermal stability of 10 A phase expands with increasing pressure. Its maximum stability is unknown. Talc is a common hydrothermal alteration product of peridotite and may transport H20 in subducting slabs from shallow depths to depths of -150 km. It may also crystallize in overlying mantle-wedgeperidotite after infiltration of fluid from the slab, and its dehydration in the mantle wedge may lead to partial melting. The 10 Aphase may transport H20 in silica-enriched hydrated peridotite to depths of at least 200 km.

Impedance spectra of hot, dry silicate minerals and rock; qualitative interpretation of spectra

Abstract: Abstract Impedance spectroscopy helps distinguish the contributions that grain interiors and grain boundaries make to electrical resistance of silicate minerals and rocks. The technique also distinguishes the low-frequency response due to the presence of instrument electrodes. We measured olivine, orthopyroxene, clinopyroxenes, and both natural and synthetic clinopyroxenite. Measurements were made at 1 bar, from 750 to 1150 °C, and over a frequency range from < 10-4 to > 106 Hz; some measurements were also made at 300-850 °C and 10-20 kbar. The grain-interior response lies at highest frequency, the sample- electrode response at low frequencies, and the grain boundary response at mid-frequencies. Grain interiors show as semicircular impedance arcs when plotted on the complex plane, and sample-electrode responses of hot single crystals and of hot dry rocks are exhibited as depressed arcs. In comparison, monofrequency measurements contain no information to identify the source of the response; at 1 kHz they detect only the resistance sum of grain interiors and grain boundaries and at low frequency (≤ 1 Hz) are likely to sense all three components. The major experimental problem is to find electrodes that make good contact with the sample and that are stable with time. The effect of pressure (10 kbar, 300-800 °C) is to diminish the resistance associated with grain boundaries and the sample-electrode interface, in the laboratory and presumably in nature. Monofrequency measurements at 1 bar may underestimate the conductivity of rocks at similar temperature but higher pressure. A network of electrical elements is presented for use in interpreting impedance spectra and conductive paths in hot or cold, wet or dry, minerals and rocks at any pressure. In dry rocks, a series network path predominates; in wet rocks, aqueous pore fluid and crystals both conduct. Finite resistance across the sample-electrode interface is evidence that electronic charge carriers are present at the surface, and presumably within, the silicate minerals and rocks measured.

Al K-edge XANES spectra of aluminosilicate minerals

Abstract: AI K-edge X-ray absorption near-edge structure (XANES) spectra of a range of aluminosilicate and aluminum oxide minerals were collected using synchrotron radiation. The AI K-edge spectra of aluminosilicates containing fourfold-coordinated Al ([4IAl)and sixfold-coordinated AI ([6IAl)are qualitatively interpreted on the basis of a comparison with the Si K-edge spectra of exquartz and stishovite and MO calculations for tetrahedral and octahedral clusters. Some near-edge features are attributed to the multiple scattering (MS) effect from the more distant shell atoms. The Al K-edge (peak C) shifts toward higher energy with an increase in the coordination number (CN) of AI, from 1566.7 eV for [41Al (averaged for eight samples) to 1567.8 eV for [SIAIand to 1568.3 eV for [6JAl(averaged for 17 samples). For [41AI and [6JAlaluminosilicates, respectively, the Al K-edge shifts to higher energy with increase in the AI-O bond distance (dAl.a), distortion of the Al polyhedron (.:iAl.a),and decrease in the AI-O bond valence (SAl.a).AIso for [4JAland [6JAlaluminosilicates, the relative intensity of the Al K-edge is correlated with the content (in weight percent) of AI in tetrahedral and octahedral sites, respectively. This correlation therefore establishes AI K-edge spectroscopy as a potential technique for semiquantitatively determining the distribution of Al between fourfold- and sixfold-coordinated sites.

Kinetics of the reaction H2O+O = 2OH in rhyolitic and albitic glasses; preliminary results

Abstract: The kinetics of homogeneous reactions are important in understanding the cooling history of rocks and in understanding experimental speciation data. We have experimentally studied the kinetics of the interconversion reaction between H_2O molecules and OH groups in natural rhyolitic glasses (0.5-2.3% total water) and a synthetic albitic glass (l% total water) at 400-600°C. The reaction rate increases with temperature and total water content. Equilibrium is not always approached monotonically; the speciation may first depart from equilibrium and then come back to equilibrium. Experimental reaction rates agree with those inferred from previous speciation data of rhyolitic glasses quenched from 850°C. The experimental data are modeled successfully by considering both the reaction and the diffusion of OH that brings OH groups together to react. This study shows that species concentrations in glasses quenched from ≤ 600°C reflect those at experimental temperatures unless the water content is higher than that used in the present study. Species concentrations in glasses with total water contents ≥0.8 wt% and which were rapidly quenched in water from 850°C do not represent their equilibrium concentrations in the melt at 850°C, but record a lower apparent equilibrium temperature that depends on water content and quench rate. Natural rhyolitic glasses and glass inclusions do not record preeruptive melt speciation, though total water content may be conserved. The experimental data are used to infer cooling rates for natural obsidian glasses. Pyroclastic glass fragments from the bb site of Mono Craters have cooling rates similar to air-cooled experimental charges (~3°C/s). Different types of glasses from the Mono Craters have different cooling rates, which cover four orders of magnitude. Some natural obsidians appear to have had complex cooling histories. The wide range of cooling rates and thermal histories is consistent with previous inferences that some obsidian clasts at the Mono Craters formed as glass selvages lining volcanic conduits or dikes that were subsequently caught up in the explosive eruption, which led to variable degrees of transient heating followed by rapid cooling and deposition. These experimental data reveal surprisingly rich detail in water speciation in volcanic glasses and show how, at least in principle, quantitative constraints on thermal histories can be extracted by experimentation and application of kinetic models.

Archean mantle heterogeneity and the origin of diamondiferous eclogites, Siberia: Evidence from stable isotopes and hydroxyl in garnet

Abstract: Data are presented for the O isotopic composition of clinopyroxene and garnet, the C isotopic composition of diamond, and the OH- content of garnet from eclogite xenoliths brought to the surface by the Udachnaya kimberlite pipe, Yakutia, Siberia. Radiogenic isotopic data suggest that the eclogites could have been derived from an ultradepleted mantle at approximately 2.9 Ga (Pearson et al., 1995; Snyder et al., in preparation). O isotopic compositions generally show equilibration between the eclogitic minerals (Δ_(cpx-Grt) = 0.11-0.41‰) and have δ^(18)O_(SMOW) for both garnet and clinopyroxene that lie near the range of accepted mantle values of 5.7±0.7‰. However, several eclogites indicate significant deviations from this range, at higher values of 6.8-7.0‰. Also, two eclogites lie at the lower end of the mantle range, at values of 4.8 and 5.0‰ (all in clinopyroxene). C isotopic compositions of diamonds all have δ^(13)C_(PDB) in the range of -1 to -7‰ and are centered at approximately -5‰, also within the range of accepted mantle values. The OH- contents of the garnet are generally between 0 and 22 ppm (as H_(2)0), although two samples exceed 70 ppm. This range of OH- is similar to eclogitic garnet from the Kaapvaal craton of southern Africa. The mantle C isotopic values of associated diamonds, the majority of O isotopic data, and the low OH- content of the minerals, although suggesting a general lack of crustal participation in the formation of the Udachnaya eclogites, do not rule out the participation of some ancient crustal material. That these eclogites include both ^(18)O-enriched and ^(18)O-depleted types suggests that the protoliths may have been overprinted by both low- and high-temperature hydrothermal events (cf. Jacob et aI., 1994). A positive correlation between δ^(18)O and ^(87)Sr/^(86)Sr allows the interpretation of these eclogites as representing a cross section of an Archean ophiolite. However, the lack of a single coherent grouping on a plot of δ^(18)O vs. ^(87)Sr/^(86)Sr suggests that, if the Udachnaya eclogites were derived from oceanic crust, they cannot be cogenetic and must represent at least two separate ophiolite sequences. Conversely, if the eclogites are found to be cogenetic, then a totally different process affected their formation and a probable metasomatic mechanism was operative. Because of the total lack of correlation of δ^(18)O with other geochemical parameters, we find no compelling evidence that all eclogites are derived ultimately from oceanic crust.

Study of the cubic to tetragonal transition in Mg2TiO4 and Zn2TiO4 spinels by 17O MAS NMR and Rietveld refinement of X-ray diffraction data

Abstract: Cation ordering and structural changes in synthetic MgzTiO. and Zn2TiO. spinels at temperatures across the polymorphic transition from the high-temperature cubic (Fd3m) to the low-temperature tetragonal (P4,22) structure are examined by 170 magic-angle spinning (MAS) NMR (9.4 T) and Rietveld structure refinement of powder X-ray diffraction data. The '70 NMR spectra of cubic MgzTiO. and ZnzTiO. are similar, each showing one broad peak, positioned at 303 and 301 ppm, respectively. At the transition to the tetragonal phase, spectra of both Mg2TiO. and ZnzTiO. show significant narrowing because of the onset oflong-range cation ordering in the tetragonal structure. The '70 NMR spectrum of tetragonal Zn2TiO. shows two narrow peaks, at 301 and 273 ppm, corresponding to the two crystallographically distinct 0 sites in the tetragonally distorted spinel, showing that '70 chemical shift is sensitive to octahedral Zn- Ti substitution in Zn2TiO.. In contrast, the '70 NMR spectrum of tetragonal Mg2TiO. shows only one peak, at 298 ppm. The structures of cubic and tetragonal Mg2TiO. and Zn2TiO. are compared. Tetragonal Zn2TiO. exhibits greater distortion than MgzTiO. at the M I, 0 I, and 02 sites. These subtle structural differences do not explain differences in the '70 NMR spectra. The '70 NMR spectra of the cubic MgzTiO. and ZnzTiO. show no change with quench temperature above the transition to the cubic phase, suggesting that short-range ordering does not occur in cubic MgzTiO. and ZnzTiO.. A two-phase region is observed for both MgzTiO. and Zn2TiO., below 664 and 561°C, respectively, where the cubic and tetragonal phases are shown to be at equilibrium. The 170 peak position of MgTi03 is observed at 398 ppm. This chemical-shift displacement of 100 ppm to high frequency of MgzTiO. is related to increased distortion in MgTi03.

Reaction of orthopyroxene in peridotite xenoliths with alkali basalt melt and its implication for genesis of Alpine-type chromitite

Abstract: The Kawashimo alkali basalt of the southwest Japan arc has peridotite xenoliths with a wide lithological range, from lherzolite {Fo of olivine, 89; Cr' [=Cr/(Cr + AI) atomic ratio] of spinel O.IO}to harzburgite (Fo of olivine, 91; Cr' of spinel, 0.54). Reaction zones between orthopyroxene and alkali-basalt melt are low-pressure analogues of mantle-melt interaction products and consist of two subzones: a fme-grained inner subzone (adjacent to orthopyroxene) and a relatively coarse-grained outer subzone. In both xenolith types the reaction products are olivine + diopsidic clinopyroxene :t spinel :t glass, but the spinel concentrations are remarkably different around lherzolite from those around harzburgite. Cr-bearing spinel is concentrated only in the outer subzone on harzburgite orthopyroxene (Cr' > 0.14); the inner subzone on harzburgite orthopyroxene and both the inner and outer subzones on lherzolite orthopyroxene (Cr' = 0.05) are almost free of spinel. The remarkable enrichment of spinel in the outer subzone on the harzburgite orthopyroxene suggests a mechanism of spinel concentration, Le., the origin of podiform chromitite is related to interaction between Cr-rich orthopyroxene and basaltic melt. This observation for the Kawashimo xenoliths is concordant with the near absence ofpodiform chromitite in Iherzolitic mantle. Chromian spinel could be concentrated if a relatively silica-rich secondary melt, produced by interaction between pyroxene-undersaturated magma and harzburgite orthopyroxene, is mixed with a primitive magma in the upper mantle.

X-ray single-crystal diffraction study of pyrope in the temperature range 30-973 K

Abstract: X-ray single-crystal diffraction data are presented for synthetic pyrope garnet over the temperature range 30-973 K, which extends the range previously studied The structure refinements allow a detailed geometrical analysis of the polyhedral distortions over this temperature range, the determination of the thermal expansion for bond distances, and the determination of atomic displacement parameters The problem of static or dynamic disorder of the Mg site is discussed on the basis of the temperature behavior of the atomic displacement tensors, which show no evidence of static disorder on this site The hightemperature anharmonic contribution to the thermal vibrations is investigated by the use of Gram-Charlier high-order expansion terms of the displacement parameter tensor