Showing papers in "American Mineralogist in 2005"

Some Precambrian banded iron-formations (BIFs) from around the world: Their age, geologic setting, mineralogy, metamorphism, geochemistry, and origins

Abstract: Banded iron-formations (BIFs) occur in the Precambrian geologic record over a wide time span. Beginning at 3.8 Ga (Isua, West Greenland), they are part of Archean cratons and range in age from about 3.5 until 2.5 Ga. Their overall volume reaches a maximum at about 2.5 Ga (iron-formations in the Hamersley Basin of Western Australia) and they disappear from the geologic record at about 1.8 Ga, only to reappear between 0.8 and 0.6 Ga. The stratigraphic sequences in which BIFs occur are highly variable. Most Archean iron-formations are part of greenstone belts that have been deformed, metamorphosed, and dismembered. This makes reconstruction of the basinal setting of such BIFs very difficult. The general lack of metamorphism and deformation of extensive BIFs of the Hamersley Range of Western Australia and the Transvaal Supergroup of South Africa allow for much better evaluations of original basinal settings. Most Archean iron-formations show fine laminations and/or microbanding. Such microbanding is especially well developed in the Brockman Iron Formation of Western Australia, where it has been interpreted as chemical varves, or annual layers of sedimentation. BIFs ranging in age from 2.2 Ga to about 1.8 Ga (e.g., those of the Lake Superior region, U.S.A., Labrador Trough, Canada, and the Nabberu Basin of Western Australia) commonly exhibit granular textures and lack microbanding. The mineralogy of the least metamorphosed BIFs consists of combinations of the following minerals: chert, magnetite, hematite, carbonates (most commonly siderite and members of the dolomite-ankerite series), greenalite, stilpnomelane, and riebeckite, and locally pyrite. Minnesotaite is a common, very low-grade metamorphic reaction product. The Eh-pH stability fields of the above minerals (and/or their precursors) indicate anoxic conditions for the original depositional environment. The average bulk chemistry of BIFs, from 3.8 through 1.8 Ga in age, is very similar. They are rich in total Fe (ranging from about 20 to 40 wt%) and SiO2 (ranging from 43 to 56 wt%). CaO and MgO contents range from 1.75 to 9.0 and from 1.20 to 6.7 wt%, respectively. Al2O3 contents are very low, ranging from 0.09 to 1.8 wt%. These chemical values show that they are clean chemical sediments devoid of detrital input. Only the Neoproterozoic iron-formations (of 0.8 to 0.6 Ga in age) have very different mineralogical and chemical make-ups. They consist mainly of chert and hematite, with minor carbonates. The rare-earth element profiles of almost all BIFs,with generally pronounced positive Eu anomalies, indicate that the source of Fe and Si was the result of deep ocean hydrothermal activity admixed with sea water. The prograde metamorphism of iron-formations produces sequentially Fe-amphiboles, then Fe-pyroxenes, and finally (at highest grade) Fe-olivine-containing assemblages. Such metamorphic reactions are isochemical except for decarbonation and dehydration. The common fine lamination (and/or microbanding) as well as the lack of detrital components in most BIFs suggest that such are the result of deposition, below wave base, in the deeper parts of ocean basins. Those with granular textures are regarded as the result of deposition in shallow water, platformal areas. Carbon isotope data suggest that for a long period of time (from Archean to Early Proterozoic) the ocean basins were stratified with respect to δ13C (in carbonates) as well as organic carbon content. In Middle Proterozoic time (when granular BIFs appear) this stratification diminishes and is lost. The Neoproterozoic BIFs occur in stratigraphic sequences with glaciomarine deposits. These BIFs are the result of anoxic conditions that resulted from the stagnation in the oceans beneath a near-global ice cover, referred to as “Snowball Earth.” All of the most “primary” mineral assemblages appear to be the result of chemical precipitation under anoxic conditions. There are, as yet, no data to support that BIF precipitation was linked directly to microbial activity. The relative abundance of BIF throughout the Precambrian record is correlated with a possible curve for the evolution of the O2 content in the Precambrian atmosphere.

The Ti-saturation surface for low-to-medium pressure metapelitic biotites: Implications for geothermometry and Ti-substitution mechanisms

Abstract: The Ti content of biotite can serve as a geothermometer for graphitic, peraluminous metapelites that contain ilmenite or rutile and have equilibrated at roughly 4–6 kbar. The relationship between Ti-content, temperature, and Mg/(Mg + Fe) value was calibrated empirically using an extensive natural biotite data set (529 samples) from western Maine and south-central Massachusetts in combination with the petrogenetic grid of Spear et al. (1999). The calculated Ti-saturation surface is curved such that for a given Mg/(Mg + Fe) value, Ti concentration increases as a function of temperature in a nonlinear fashion, and for a given temperature Ti concentrations decrease with an increase in Mg/(Mg + Fe). The fit to the Ti-saturation surface can be reformulated as the geothermometric expression: T = {[ln(Ti) − a − c ( X Mg)3]/ b }0.333, in which T is temperature in degrees Celsius, Ti is the number of atoms per formula unit (apfu) normalized on the basis of 22 O atoms, X Mg is Mg/(Mg + Fe), a = −2.3594, b = 4.6482 × 10−9 and c = −1.7283. The calibration range for this expression is X Mg = 0.275–1.000, Ti = 0.04–0.60 apfu, and T = 480–800 °C. Precision of the Ti-in-biotite geothermometer is estimated to be ±24 °C at the lower temperature range and improves to ±12 °C at higher temperatures. Application of the Ti-in-biotite geothermometer to ilmenite- or rutile-bearing, graphitic, peraluminous metapelites equilibrated at 3–6 kbar is generally consistent with independent temperature determinations, but with some deviations that represent local reequilibration. Consequently, the Ti systematics in biotite can also serve as the basis of a very sensitive indicator of chemical equilibrium, or lack thereof. Application of the geothermometer to metapelites not containing the requisite mineral assemblages can lead to minor-to-significant errors in estimated temperatures. Biotite Ti-substitution mechanisms are controlled by several factors. Based on the biotite calibration data set, magnesian biotites ( X Mg > 0.65) incorporate Ti in accordance with the exchange vector TiAl2R−1Si−2, where R is the sum of the divalent cations Mg + Fe + Mn. This substitution mechanism is primarily a response to misfit of the octahedral and tetrahedral layers in magnesian biotites. Intermediate biotites ( X Mg <0.65), particularly at higher temperatures, exhibit enhanced Ti concentrations, most consistent with the Ti-deprotonation TiO2R−1(OH) −2 exchange vector. Dominance of Ti-deprotonation substitution is largely a function of reduction of H2O activity at higher metamorphic grades. Supplementary biotite data from metaluminous amphibolites and mafic granulites, metamorphosed isothermally with variable H2O activities, reveal that low-Al biotite incorporates significantly higher concentrations of Ti relative to peraluminous biotite as a result of a combination of the exchange vectors TiO2R−1(OH) −2 and RSiAl−2 substituting in roughly an 8:1 ratio.

Investigation of smectite hydration properties by modeling experimental X-ray diffraction patterns: Part I. Montmorillonite hydration properties

Abstract: Hydration of the <1 μm size fraction of SWy-1 source clay (low-charge montmorillonite) was studied by modeling of X-ray diffraction (XRD) patterns recorded under controlled relative humidity (RH) conditions on Li-, Na-, K-, Mg-, Ca-, and Sr-saturated specimens. The quantitative description of smectite hydration, based on the relative proportions of different layer types derived from the fitting of experimental XRD patterns, was consistent with previous reports of smectite hydration. However, the coexistence of smectite layer types exhibiting contrasting hydration states was systematically observed, and heterogeneity rather than homogeneity seems to be the rule for smectite hydration. This heterogeneity can be characterized qualitatively using the standard deviation of the departure from rationality of the 00 l reflection series (ξ), which is systematically larger than 0.4 A when the prevailing layer type accounts for ~70% or less of the total layers (~25% of XRD patterns examined). In addition, hydration heterogeneities are not distributed randomly within smectite crystallites, and models describing these complex structures involve two distinct contributions, each containing different layer types that are interstratifed randomly. As a result, the different layer types are partially segregated in the sample. However, these two contributions do not imply the actual presence of two populations of particles in the sample. XRD profile modeling also has allowed the refinement of structural parameters, such as the location of interlayer species and the layer thickness corresponding to the different layer types, for all interlayer cations and RH values. From the observed dependence of the latter parameter on the cation ionic potential ( v / r; v = cation valency and r = ionic radius) and on RH, the following equations were derived: \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \[Layer\ thickness\ (1W)\ =\ 12.556\ +\ 0.3525\ {\times}\ ({ u}/\mathit{r}\ {-}\ 0.241)\ {\times}\ ({ u}\ {\times}\ RH\ {-}\ 0.979)\] \end{document} \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \[Layer\ thickness\ (2W)\ =\ 15.592\ +\ 0.6472\ {\times}\ ({ u}/\mathit{r}\ {-}\ 0.839)\ {\times}\ ({ u}\ {\times}\ RH\ {-}\ 1.412)\] \end{document} which allow the quantification of the increase of layer thickness with increasing RH for both 1W (one water) and 2W (two water) layers. In addition, for 2W layers, interlayer H2O molecules are probably distributed as a unique plane on each side of the central interlayer cation. This plane of H2O molecules is located at ~1.20 A from the central interlayer cation along the c* axis.

Igneous thermometers and barometers based on plagioclase + liquid equilibria: Tests of some existing models and new calibrations

Abstract: Although many formulations of plagioclase + liquid equilibria have been calibrated in the last decade, few models specifically address the issue of temperature ( T ) prediction. Moreover, for those that do, T error is not addressed, greatly limiting their use as geothermometers. Several recent models of plagioclase-liquid equilibria are thus tested for their ability to recover T from their calibration data, and predict T from experiments not used for calibration. The models of Sugawara (2001) and Ghiorso et al. (1995, 2002) outperform earlier calibrations. These models perform reasonably well at T > 1100 °C, though recovery and prediction of T is less precise for hydrous compositions. In addition, these models cannot be integrated with geo-hygrometers, or other mineral-melt thermometers and barometers; the following expression predicts T with up to 40% greater precision: \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \begin{eqnarray*}&&\frac{10^{4}}{\mathit{T}(K)}\ =\ 6.12\ +\ 0.257\ ln\ \left(\frac{[Ab^{pl}]}{[Ca^{liq}(Al^{liq})^{2}(Si^{liq})^{2}]}\right)\ {-}\ 3.166[Ca^{liq}]\ +\ 0.2166[H_{2}O^{liq}]\\&&{-}\ 3.317\left[\frac{Al^{liq}}{Al^{liq}\ +\ Si^{liq}}\right]\ +\ 1.216[Ab^{pl}]^{2}\ {-}\ 2.475\ {\times}\ 10^{{-}2}[\mathit{P}(kbar)]\end{eqnarray*} \end{document}(1) Because these thermometers are pressure ( P ) sensitive, a temperature-sensitive barometer was also developed \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \begin{eqnarray*}&&\mathit{P}(kbar)\ =\ {-}42.2\ +\ 4.94\ {\times}\ 10^{{-}2}\ [\mathit{T}\ (K)]\ +\ 1.16\ {\times}\ 10^{{-}2}\ \mathit{T}\ (K)\ ln\left(\frac{[Ab^{pl}Al^{liq}Ca^{liq}]}{[An^{pl}Na^{liq}Si^{liq}]}\right).\\&&{-}\ 382.3[Si^{liq}]^{2}\ +\ 514.2[Si^{liq}]^{3}\ {-}\ 19.6ln[Ab^{pl}]\ {-}\ 139.8[Ca^{liq}]\\&&+\ 287.2[Na^{liq}]\ +\ 163.9\ [K^{liq}]\end{eqnarray*} \end{document}(2) In these models, T is in Kelvins and P is in kbar. Anpl and Abpl are the fractions of anorthite and albite in plagioclase, calculated as cation fractions: An = CaO/(CaO + NaO0.5 + KO0.5) and Ab = NaO0.5/(CaO+NaO0.5+KO0.5). Terms such as Alliq refer to the anhydrous cation fraction of Al in the liquid; H2O in Equation 1 is in units of wt%. Errors on these models are comparable to those for clinopyroxene thermobarometers: In Equation 1, R = 0.99 and the standard error of estimate (SEE) is 23 K; for Equation 2, R = 0.94 and the SEE is 1.8 kbar. The models successfully recover mean pressures for experimental data that are not used for calibration, and are furthermore able to recover near-1-atm P estimates for volcanic rocks from Kilauea, Hawaii, which are thought to have crystallized at or very near Earth’s surface.

Structural characterization of biogenic Mn oxides produced in seawater by the marine bacillus sp. strain SG-1

Abstract: Natural Mn-oxide nanoparticles and grain coatings are ubiquitous in the environment and profoundly impact the water quality and quality of sediments through their ability to degrade and sequester contaminants. These oxides, which are believed to form dominantly via oxidation of Mn2+ by marine and freshwater bacteria, have extremely high sorptive capacities for heavy metals. We have used XANES, EXAFS, and synchrotron (SR)-XRD techniques to study biogenic Mn oxides produced by spores of the marine Bacillus sp. strain SG-1 in seawater as a function of reaction time under in-situ conditions. An EXAFS model was developed to fully account for the structure and features in the data, providing realistic structural information. The first observed biogenic solid-phase Mn-oxide product is a layered phyllomanganate with hexagonal sheet symmetry and an Mn-oxidation state similar to that in δ-MnO2, between 3.7 and 4.0. XRD and SEM-EDS data show the biooxides to have a phyllomanganate 10 A basal plane spacing and an interlayer containing Ca. With time, a phyllomanganate oxide with pseudo-orthogonal sheet symmetry appears. Fits to these EXAFS spectra suggest the octahedral layers have relatively few Mn octahedral site vacancies in the lattice and the layers bend to accommodate Jahn-Teller distortions creating the change in symmetry. A reaction mechanism is proposed to account for the observed products. The phyllomanganate oxides observed in this study may be the same as the most abundant Mn-oxide phases suspended in the oxic and sub-oxic zones of the oceanic water column that are of global importance in trace metal and nutrient cycling.

Biotic and abiotic products of Mn(II) oxidation by spores of the marine bacillus sp. strain SG-1

Abstract: Bacterial Mn(II) oxidization by spores of Bacillus, sp. strain SG-1 has been systematically probed over the time scale 0.22 to 77 days under in-situ conditions and at differing Mn(II) concentrations. Three complementary techniques, K -edge X-ray absorption near-edge spectroscopy (XANES), X-ray emission spectroscopy (XES), and in-situ synchrotron radiation-based X-ray diffraction (SR-XRD), have been utilized to examine time-dependent changes in Mn oxidation state, local-, and long-range structure in amorphous, crystalline, cell-bound, and solute Mn species. The primary solid biogenic product of Mn(II) oxidation is an X-ray amorphous oxide similar to δ-MnO2, which has a Mn oxidation state between 3.7 and 4.0. Reaction of Mn(II) with the primary biogenic oxide results in the production of abiotic secondary products, feitknechtite or a 10 A Na phyllomanganate. The identity of the secondary product depends upon the Mn(II) concentration as described by thermodynamic relations. A decrease in the dissolved Mn(II) concentration is followed by mineralogic transformation of the secondary products. Thus, Mn(II) appears to act as a reductant toward the biogenic oxide and to control the stability of secondary reaction products. Mineralogic changes similar to these are likely to be commonplace in natural settings where bacterial Mn(II) oxidation is occurring and may liberate sorbed metal ions or alter the rates of important Mn oxide surface-mediated processes such as the degradation of organic molecules. It is plausible that microbes may exploit such mineral transformation reactions to indirectly control specific chemical conditions in the vicinity of the cell.

The visible and infrared spectral properties of jarosite and alunite

Abstract: The visible and infrared spectral properties of two natural jarosite minerals and a suite of synthetic jarosites and alunite samples are described here. The fundamental stretching and bending vibrations observed in the infrared region for SO 4 2− and OH − are compared with the near-infrared overtones and combinations of these vibrations. Shifts were observed in the SO 4 2− and OH − bands for Al 3+ vs. Fe 3+ at the octahedral sites and K + vs. Na + at the “A” (frequently monovalent) sites. Crystal-field theory bands were observed for jarosite near 435, 650, and 900–925 nm and were compared to those of iron oxides. Spectral bands near 1.76, 2.17, 2.53, 4.5, 8–10, and 15–24 μm (corresponding to ~5670, 4600, 3970–4150, 2100–2300, 1000–1225, and 420–675 cm −1 , respectively) for alunite and near 0.43, 0.91, 1.85, 2.27, 2.63, 4.9, 8–10, and 15–24 μm (corresponding to ~23 000, 10 990, 5400, 4350–4520, 3800–4150, 1950–2200, 1000–1190, and 440–675 cm −1 , respectively) for jarosite would be most useful for detecting these minerals using remote sensing on Earth or Mars. These minerals are important indicators of alteration processes, and this study contributes toward combined visible/near-infrared and mid-infrared spectral detection of these two alunite-group minerals.

Analytical perils (and progress) in electron microprobe trace element analysis applied to geochronology: Background acquisition, interferences, and beam irradiation effects

Abstract: Electron probe microanalysis (EPMA) of accessory minerals such as monazite, xenotime, and thorite for minor- and trace-element concentrations and geochronology, requires consideration of beam irradiation effects (increasing heat and charge) as higher current densities and lengthy counting times are employed, and requires careful, detailed assessment of background intensities and interferences. A carbon coat (250 A thickness) is generally inadequate for prevention of absorbed current s uctuation and beam damage when using the high current densities applied for high precision (e.g., 200 nA, focused beam). Beam irradiation effects include element mobility in monazite, resulting in P loss relative to REE. Coating materials of higher electrical and thermal conductivity are indicated, and use of gold (≥100 A) is strongly suggested. Systematic compositional and, therefore, age variability can result simply from analytical effects, requiring evaluation of all aspects of data acquisition. The spectra relevant to measurement of Y, Th, Pb, and U are complex, especially in REE-bearing minerals. Acquisition of detailed spectral wavelength scans allows recognition of background and peak interferences, as well as curvature. Background intensities can be extracted directly from scan data by regression. X-ray mapping allows delineation of domains, guiding background acquisition and detailed quantitative analysis. Minor substitution or s uorescence of unexpected elements can compromise analyses, one documented effect being the s uorescence of K in monazite adjacent to, or hosted by, K-feldspar or micas. This effect, clearly evident within 10 micrometers from K-feldspar, can result in erroneous U concentrations leading to misinterpretation of rim “ages” as younger overgrowths. Absorption edges associated with Th also become relevant to the measurement of U at high Th concentrations. Because background intensity is sensitive to variation in average atomic number, backgrounds must be acquired from each identiÞ ed domain, with particular attention being paid to Th variation. Misapplication of background intensities can result in large age discrepancies, for example, application of backgrounds obtained from a high-Th domain in monazite to measurement of a low-Th domain (1/3 of the amount in the high-Th domain) results in an overestimation of the UMβ background intensity of 0.008 cps/nA, and an overall “increase” in age of 70 m.y.

Effect of current density on the electron microprobe analysis of alkali aluminosilicate glasses

Abstract: In a previous work (Morgan and London 1996), we proposed an optimized procedure for electron microprobe analysis (EMPA) of rhyolitic glasses using a broad (20 μm diameter), low-current (2 nA) fixed beam. Some important applications for EMPA of glass, such as vitreous inclusions in minerals and experimental run products, require smaller beam diameters that produce greater area] current densities (expressed as nA/μm 2 ). For these situations, we have assessed the effect of areal current density on the migration of Na and its concomitant effects on other elements and their ratios during EMPA of granitic glasses. Anhydrous and hydrous glasses of a haplogranite composition (Ab 3 8 . 2 3 Or 2 9 . 3 1 Qtz 3 3 . 3 7 C 0 . 1 0 ) were analyzed at 20 kV accelerating potential, using 2-50 nA beam currents, fixed beam diameters of 2-20 μm, and counting times scaled to yield similar analytical uncertainty at each condition (∼2.6% relative for Na 2 O). There is almost no loss of Na (≤1.7-2.7% relative) using a current density of 0.006 nA/μm 2 , minor (7-9%) Na loss for current densities up to 0.1 nA/μm 2 , and increasing Na loss with higher current densities that becomes severe at >0.5 nA/μm 2 (e.g., 48-63% relative loss from hydrous glass at 50 nA and 2 μm during 3-6 s of irradiation). Sodium migration is more pronounced in hydrous glasses than in anhydrous ones, with significant loss from hydrous glass occurring during the first second of irradiation. The migration of Na results in increased concentrations of Al and Si, but little or no change in the concentration of K; if not fully corrected for, these effects produce systematic errors in important elemental ratios. With current densities 100% of the Na 2 O value obtained). The correlation of analytical condition (beam current and diameter) with current density and EMPA results provided here allows analysts to select beam conditions that optimize the quality of analyses. When current densities >∼0.01 nA/μm 2 must be used (e.g., with beam spot sizes <20 μm), the results can lead to improved estimates of the systematic errors due to alkali migration. Natural and some experimental glasses contain a variety of other minor components among which Ca and Fe are important, and so the discussion of analytical methods is extended to more complex compositions. For example, Na migration is accelerated as glass structures become less polymerized than those of simple tectosilicate stoichiometry (e.g., due to increasing alkalinity and/or the presence of fluxing components such as F, Cl, B). Analysis using 20 kV accelerating voltage, as opposed to 15 kV, both slightly decreases Na migration and improves limits of detection and statistical accuracies for minor components such as Fe while providing reasonable beam penetration depths.

Aluminum coordination and the densification of high-pressure aluminosilicate glasses

Abstract: To better understand the relationship between atomic-scale structures and densities of aluminosilicate glasses and liquids, we used 2 7 Al MAS NMR to determine the speciation of aluminum ions in K 3 AlSi 3 O 9 , Na 3 AlSi 3 O 9 , and Ca 3 Al 2 Si 6 O 1 8 glasses quenched from melts at 3 to 10 GPa. These data are a first approximation of high-pressure melt structure and illustrate the effects of the type of modifier cation. High field strength modifier cations (e.g., Ca) clearly induce more high-coordinated Al than lower field strength cations (e.g., Na and K). Measured glass densities show that, especially with rapid decompression, a significant portion of the total densification observed in-situ in melts is retained on return to ambient temperature and pressure. Observed increases in Al coordination are well correlated with decreased volume, which suggests that this structural change is a major part of the mechanism for recovered densification of high-pressure melts. Additionally, 2 3 Na MAS NMR, combined with the 2 7 Al MAS spectra and density determinations, reveal that other changes, such as the compression of modifier cation sites and/or decreased network bond angles, must also be significant, especially at low pressure.

Direct observations of pseudomorphism: compositional and textural evolution at a fluid-solid interface

Abstract: Solid-fluid interactions often involve the replacement of one phase by another while retaining the morphology and structural details of the parent phase, i.e, pseudomorphism. We present in situ observations of the evolution of both the solid and fluid compositions at the interface during such a replacement reaction in the model system KBr-KCl-H 2 O, in which a single crystal of KBr is replaced by a single crystal of KCl. The pseudomorphism is initiated by epitaxial growth at the fluid-mineral interface, when the dissolution of the parent phase results in an interfacial fluid layer that is supersaturated with respect to a different solid composition. The subsequent evolution of the coupled dissolution and growth can be related to local equilibrium defined by a Lippmann diagram. The reaction features, including the development of porosity in the new solid phase, share many characteristics of replacement reactions in nature as well as in technical applications.

A synchrotron Mössbauer spectroscopy study of (Mg,Fe)SiO3 perovskite up to 120 GPa

Abstract: The electronic environment of the Fe nuclei in two silicate perovskite samples, Fe_(0.05)Mg_(0.95)SiO_3 (Pv05) and Fe_(0.1)Mg_(0.9)SiO_3 (Pv10), have been measured to 120 GPa and 75 GPa, respectively, at room temperature using diamond anvil cells and synchrotron Mossbauer spectroscopy (SMS). Such investigations of extremely small and dilute ^(57)Fe-bearing samples have become possible through the development of SMS. Our results are explained in the framework of the “three-doublet” model, which assumes two Fe2+-like sites and one Fe^(3+)-like site that are well distinguishable by the hyperfine fields at the location of the Fe nuclei. At low pressures, Fe^(3+)/∑Fe is about 0.40 for both samples. Our results show that at pressures extending into the lowermost mantle the fraction of Fe^(3+) remains essentially unchanged, indicating that pressure alone does not alter the valence states of iron in (Mg,Fe)SiO_3 perovskite. The quadrupole splittings of all Fe sites first increase with increasing pressure, which suggests an increasingly distorted (noncubic) local iron environment. Above pressures of 40 GPa for Pv10 and 80 GPa for Pv05, the quadrupole splittings are relatively constant, suggesting an increasing resistance of the lattice against further distortion. Around 70 GPa, a change in the volume dependence of the isomer shift could be indicative of the endpoint of a continuous transition of Fe3+ from a high-spin to a low-spin state.

Thermodynamic properties of the Tschermak solid solution in Fe-chlorite: Application to natural examples and possible role of oxidation

Abstract: The standard-state thermodynamic properties of 14 A Fe-amesite [Fe 4 Al 4 Si 2 O 1 0 (OH) 8 ] were calculated from the reequilibration experiments of Parra et al. (2005) and natural data bearing on the Fe-Mg partitioning between chlorite and chloritoid over a wide range of pressure (P), temperature (T), and rock composition. The combination of these data with the three-site mixing model and thermodynamic properties of daphnite proposed by Vidal et al. (2001) led us to reappraise the evolution of the Margules parameter W G A l F e with pressure and temperature. The new data [H 0 f F e - A m = -7 607 460 J/mol, S 0 F e - A m = 514.8 J/(mol.K), V 0 F e - A m = 20.90 J/bar, W G A l F e = 1200 - 31 T + 0.7(P - 1)] are compatible with all the natural data and reequilibration experiments, and in fair agreement with the results of synthesis. They are used to calculate the compositional evolution of chlorite with T, P, or a O 2 at different bulk-system compositions. The calculated phase relations and chlorite compositions are compared with previous experiments conducted in the FeO-Al 2 O 3 -SiO 2 -H 2 O system. The addition of the Fe-amesite end-member increases the number of equilibria that can be calculated for any natural assemblage involving chlorite. Two independent equilibria are obtained for natural (Fe-Mg)-chlorite-quartz or diaspore assemblages. It is therefore possible to estimate metamorphic conditions for high-variance assemblages from the composition of chlorite. We discuss the reliability of such P-T estimates using natural chlorites occurring in low-pressure and low-temperature or medium- to high-pressure and temperature samples. Assuming that the proposed model and thermodynamic parameters are correct, these various natural data suggest that X F e 3 + [= Fe 3 + /(Fe 2 + + Fe 3 + )] in metamorphic chlorites is low (<0.1), whereas it can be as high as 0.3 in Si- and Mg-rich chlorites formed at low-P-T conditions.

Comparative planetary mineralogy: Valence state partitioning of Cr, Fe, Ti, and V among crystallographic sites in olivine, pyroxene, and spinel from planetary basalts

Abstract: This is a comparative planetary mineralogy study emphasizing the valence-state partitioning of Cr, Fe, Ti, and V over crystallographic sites in olivine, pyroxene, and spinel from planetary basalts. The sites that accommodate these cations are the M2 site (6 to 8-coordinated) and M1 site (6-coordinated) in pyroxene, the M2 site (6- to 8-coordinated) and Ml (6-coordinated site) in olivine, and the tetrahedral and octahedral sites in spinel. The samples we studied are basalts from Earth, Moon, and Mars, and range in oxygen fugacity conditions from IW-2 (Moon) to IW+6 (Earth), with Mars somewhere in between (IW to IW+2). The significant elemental valence-states at these f o 2 conditions are (from low to high f o 2 ): Ti 4 + , V 3 + , Fe 2 + , Cr 2 + , Cr 3 + , V 4 + , and Fe 3 + . V 2 + and Ti 3 + play a minor role in the phases considered for the Moon, and are found in very low concentrations. V 5 + plays a minor role in these phases in oxidized terrestrial basalts because it is probably lower in abundance than V 4 + , and has an ionic radius that is so small (0.054 nm, 6-coordinated), that it is almost at the lower limit for octahedral coordination, and can even be tetrahedrally coordinated. The role of Cr 2 + in the Moon is significant, as Cr 2 + predominates in basaltic melts at f o 2 less than IW-1. Lunar olivine has been found to contain mostly Cr 2 + , whereas coexisting pyroxene contains mostly Cr 3 + . Fe 3 + is very important in Earth, less so in Mars, and nonexistent in the Moon. The importance of the Fe 2 + to Fe 3 + transition cannot be overstated and, indeed, their crystal-chemical differences, in terms of behavior (based on size and charge), are similar to the differences between Mg and Al. We note that for pyroxene in six of the seven terrestrial suites we studied, Fe 3 + (in the M1 site) coupled with Al (in the tetrahedral site) is one of the two most important charge-balance substitutions. This substitution is of lesser importance in Mars and does not exist in lunar basalts.

Blueschists, eclogites, and decompression assemblages of the Zermatt-Saas ophiolite: High-pressure metamorphism of subducted Tethys lithosphere

Abstract: The Zermatt-Saas ophiolite of the Swiss Alps represents a complete sequence of Mesozoic Tethys oceanic lithosphere. The ophiolite was subducted during early phases of the Alpine orogeny and the mafic rocks were transformed to eclogites and blueschists. Metabasalts locally preserve pillow structures in which glaucophanite forms rims on eclogitic pillow cores. Omphacite-garnet-glaucophane-epidote-ferroan dolomite-Mg-chloritoid-talc-paragonite-chlorite–rutile form characteristic coeval blueschist- and eclogite-facies assemblages. Omphacite + garnet + glaucophane + epidote + rutile represents an equilibrium assemblage that formed during deformation and in the period when the rocks reached the greatest depth of subduction. In rocks containing this assemblage, an additional significant mineral pair is Mg-chloritoid + talc. Coarse chloritoid ( X Mg ~ 0.45) and talc formed in dispersed clusters after the last penetrative deformation. The assemblage may require > 2.7 GPa pressure to form. It developed at maximum pressure conditions corresponding to the return-point of the ophiolite in the subduction zone. Coarse paragonite and chlorite replaced parts of the earlier formed assemblages and removed free H 2 O from the rocks. Exhumation of the HP to UHP ophiolite rocks was accompanied by development of symplectite rims and other replacement products along grain boundaries of the eclogite minerals by decompression reactions in a fluid-deficient regime. Particularly noteworthy is the formation of margarite, paragonite, chlorite, albite, barroisite, and preiswerkite. The latter mineral, a very rare Na-biotite, formed as a result of the decomposition of chloritoid + paragonite and is associated with magnetite and hercynite. Omphacite breakdown produced diopside-albite-barroisite symplectites. Calculated equilibrium assemblage phase diagrams for metabasite compositions indicate P-T conditions of ~2.5–3.0 GPa and ~550–600 °C. The conditions of the subduction-related metamorphism denote P and T at the return-point, which coincide with the upper P-T limit of antigorite. Antigorite-serpentinites constitute the largest volume of rocks within the ophiolite. We suggest that the P-T conditions recorded by the exhumed mafic rocks are coupled to those of antigorite breakdown in the serpentinite that released large amounts of dehydration water in the subducted serpentinite slab facilitating exhumation of the Zermatt-Saas eclogites and blueschists.

Post-aragonite phase transformation in CaCO3 at 40 GPa

Abstract: CaCO3 (purity 99.9%) was loaded into a 50–100 μm diameter hole that was drilled into a rhenium gasket using an excimer laser. The CaCO3 powder was mixed with 8 wt% platinum to absorb the laser radiation to provide a heat source, and the Pt was also used as an internal pressure calibrant. These were mechanically ground in an agate mortar for several hours to ensure homogeneity and a sufÞ ciently Þ ne grain size. In some experiments, NaCl was used as a pressure transmitting medium to reduce deviatoric stress and temperature gradients in the sample. The sample thickness was 10–30 μm. The samples were heated with a TEM01-mode YLF laser or multi-mode YAG laser to overcome potential kinetic effects on possible phase transitions. The size of the heating spot was about 30–100 μm. The laser beam was not scanned to heat the sample, because scanning causes huge temperature gradients and thus promotes chemical disequilibrium in the sample. The reaction between the sample and the rhenium gasket was negligible, as the temperatures of the rhenium gasket were much lower than those of the samples during the laser heating. The sample temperature in some experiments was measured using the spectroradiometric method. The spectroradiometric system consists of a thermoelectrically cooled CCD detector (Princeton Instruments, HAM 256 × 1024) and a spectrograph (Acton Research, SpectraPro-150). The spectrometer allows us to measure a temperature proÞ le across a laser-heated spot. Temperatures were determined by Þ tting the thermal radiation spectrum between 600 and 800 nm to the Planck radiation function:

Validation of LA-ICP-MS fluid inclusion analysis with synthetic fluid inclusions

Abstract: Laser ablation—inductively coupled plasma—mass spectrometry (LA-ICP-MS) has become recognized as a sensitive, efficient, and cost-effective approach to measuring the major-, minor-, and trace-solute compositions of individual fluid inclusions in minerals. As a prerequisite for the routine analysis of natural inclusions in our laboratory, the precision and accuracy of the technique was assessed using sets of multi-element synthetic fluid inclusions. Five multi-element standard solutions were prepared, and incorporated as fluid inclusions in quartz crystals at 750 °C and 7 kbar. Fluid inclusions were ablated with a 193 nm ArF excimer laser and analyzed with a quadrupole ICP-MS, equipped with an octopole reaction cell for the removal of Ar-based interferences. The internal standard used in all cases was Na. Analytical precision for K, Rb, and Cs is typically better than 15% RSD, whereas Li, Mg, Ca, Sr, Ba, Mn, Fe, Cu, Zn, and Cl analyses are typically reproducible within 30% RSD. Measured concentrations approximate a Gaussian distribution, suggesting that analytical errors are random. Analyses for most elements are accurate within 15%. Limits of detection vary widely according to inclusion volume, but are 1 to 100 μfor most elements. These figures of merit are in excellent agreement with previous studies. We also demonstrate that, over the range investigated, precision and accuracy are insensitive to inclusion size and depth. Finally, the combination of our LA-ICP-MS analyses with microthermometric data shows that charge-balancing to NaCl-H2O equivalent chloride molality is the most valid approach to LA-ICP-MS data reduction, where chloride-dominated fluid inclusions are concerned.

Pressure and temperature-dependence of water solubility in Fe-free wadsleyite

Abstract: The effects of temperature and pressure on water solubility in wadsleyite in the system MgO-SiO 2 -H 2 O were investigated. Experiments were carried out using a 1000 ton multi-anvil press. One series of experiments was performed at a fixed pressure of 15 GPa and at various temperatures and in a second series the temperature was fixed at 1200 °C and pressure was varied from 13 to 18 GPa. The starting material was a mixture of oxides and hydroxides equivalent to the composition Mg 2 SiO 4 + 5 wt% H 2 O. Run products consisted of wadsleyite, quenched hydrous melt, and minor amounts of clinoenstatite. The water content of wadsleyite was quantified by ion probe. Results show that at 15 GPa, the water solubility in wadsleyite decreases significantly with increasing temperature from ∼2.2 wt% H 2 O at 900 °C down to ∼0.9 wt% H 2 O at 1400 °C; the corresponding Mg/Si ratios increase from 1.80 to 1.91 over this temperature range. This effect appears to be largely due to changes in the water activity in the coexisting melt. The partition coefficient of water between wadsleyite and coexisting melt is nearly independent of temperature with D w a t e r w a d s l e y i t e / m e l t 0.08. No significant effect of pressure on water solubility was observed at 1200 °C. Our data suggest that the water storage capacity of wadsleyite in the transition zone is much lower than previously suggested. Together with previous results on ringwoodite, our data imply a strong decrease of the water partition coefficient between wadsleyite and ringwoodite with temperature. This decrease could have two important consequences: (1) The width of the 520 km discontinuity may vary strongly as a function of temperature. (2) During cooling of the Earth's mantle since the Hadean, water may have increasingly partitioned from the lower into the upper part of the transition zone.

Contributions to precision and accuracy of monazite microprobe ages

Abstract: We examine the factors controlling accuracy and precision of monazite microprobe ages, using a JEOL 733 Superprobe equipped with 4 PET crystals, and both 1-atm gas flow Ar X-ray detectors and sealed Xe X-ray detectors. Multiple PET crystals allow for simultaneous determination of Pb concentration on up to 3 detectors, and the effects of different detector gases on spectral form can be addressed. Numerous factors in the X-ray production, detection, and counting sequence affect spectral form, including: choice of accelerating voltage, changes in d -spacing of the diffraction crystal, use of X-ray collimation slits, and type of detector gas. The energy difference between Ar K α X-rays and Xe L α X-rays results in, for 1-atm Ar detectors, escape peaks of second-order LREE L line X-rays that cannot be filtered using differential mode PHA. The second-order LREE energies are passed to the counter and produce, for a 140 mm Rowland circle, several problematic interferences in the Pb region of a monazite wavelength-dispersive (WD) spectrum. WD monazite spectra produced with Xe detectors are free from second-order LREE interferences in the Pb region; escape peaks of the second-order LREE are filterable with differential mode PHA if Xe detectors are employed. Silicon, Ca, Y, Ce, P, Th, U, and Pb (2 spectrometers) are measured as part of the monazite microprobe dating protocol; ±2σ variations in elements fixed for ZAF corrections do not affect the age outside of analytical uncertainty. Th M α, U M β, and Pb M α are the analyzed lines of the age components. Corrections for interference of Th M ζ1,2 and Y L γ2,3 on Pb M α are significant, but can be done precisely, and reduce the precision of the M α analysis by a trivially small amount. Th M γ, M 3- N 4, and M 5- P 3 interferences on U M β can be corrected, as well, but Th M 5 and M 4 absorption edges in high-Th samples make estimation of U M β background problematic. Background fits for U M β peaks show that linear vs. exponential fits for U M β do not, in general, produce statistically significant differences in microprobe ages. However, linear vs. exponential background fits for Pb M α peaks do produce significantly different ages, most likely because of (1) low Pb concentrations relative to U; (2) Th M ζ1 interference on backgrounds between Th M ζ1 and Pb M β; and (3) S K α and K β interference in S-bearing monazite. For 6-min analyses (3 min peak, 3 min background) at 25 keV and 200 nA, 1σ Pb precisions are approximately 3–4% at 1700 ppm and 9.5% at 750 ppm; at 15 keV, precision decreases by roughly 25% of the 25 keV value. These precisions are constant for fixed current, analysis time, and concentration, but the statistical precision of distinct populations of monazite grains (domains) is a function of the total number of analyses within the domain. Instrumental errors (current measurement, dead time, pulse shift, d -spacing change) add 1–10% to random errors, but errors caused by pulse shift and d -spacing changes can be accounted for and corrected. Decreasing accelerating voltage from 25 to 15 keV decreases ZAF correction factors by as much as 50% relative, but replicate age analyses of Trebilcock monazite at 15 and 25 keV are statistically indistinguishable. Grain orientation, miscalculated background intensity, uncorrected interferences, and surface effects also introduce systematic errors. Accurate background interpolation and interference correction reduces systematic error to approximately 5–20% in addition to random (counting) error. Microprobe ages (~420 Ma) and 208Pb/232Th SIMS ages (~430 Ma) of monazite from Vermont are in agreement to within ~10 m.y. The discrepancy between U-Th-total Pb microprobe ages and 208Pb/232Th ages is removed when the high background measurement for Pb M α is shifted to the short-wavelength side of Pb M β, removing a possible Th M ζ1 interference.

A systematic study of OH in hydrous wadsleyite from polarized FTIR spectroscopy and single-crystal X-ray diffraction: Oxygen sites for hydrogen storage in Earth∑ s interior

Abstract: The incorporation of hydrogen into wadsleyite (β-Mg 2 SiO 4 ) was investigated using poltarized FTIR spectroscopy and X-ray diffraction on oriented single crystals. The experiments were carried out with a new suite of samples containing between ~100 and ~10 000 ppm H 2 O by weight (wt ppm), encompassing the H-contents most relevant to Earth’s potentially hydrous mantle transition zone. Attempts to synthesize anhydrous wadsleyite resulted in water contents of no less than ~50 wt ppm H 2 O. An empirical relation between the b / a axial ratio against estimated wt ppm concentrations of H 2 O in wadsleyite ( C H 2 O ) was determined: ( b / a ) = 2.008(1) + 1.25(3) × 10 −6 · C H 2 O Polarized infrared absorption spectra were measured in the three orthogonal sections perpendicular to the major axes of the optical indicatrix ellipsoid and are used in concert with results from new structure refinements to place constraints on the main absorbers in the structure. All of the main bands in the O-H stretching region of the FTIR can be explained by protonation of O1, the anomalous non-silicate oxygen site. We assign the band at 3614 cm −1 to a bent hydrogen bond O1···O1 (2.887 A) along the M3 edge in the a-c plane. The band at 3581 cm −1 is assigned to a bent hydrogen bond on O1···O3 (3.016 A) of the M3 edge in the b-c plane. The absorption bands at 3360, 3326, and 3317 cm −1 are best explained by hydrogen bonds on O1···O4 (3.092 A) and O1···O4 (2.795 A) along the M3 and M2 edges with possible splitting of one band due to vacancy ordering, but we cannot rule out contributions from three other (O1···O3) edges. The broad absorption feature at 3000 cm −1 is unambiguously assigned to the O4···O4 (2.720 A) tetrahedral edge of the Si 2 O 7 group pointing along the [100] vector. On hydration to ~1 wt% H 2 O, M-site vacancies are observed exclusively at M3. A systematic shortening of several (interpreted) hydrogen bonded O···O M-site edges is attributed to reduced O-O repulsive forces on protonation in the vicinity of an M-site vacancy.

Environmentally important, poorly crystalline Fe/Mn hydrous oxides: Ferrihydrite and a possibly new vernadite-like mineral from the Clark Fork River Superfund Complex

Abstract: Ferrihydrite and a vernadite-like mineral, in samples collected from the riverbeds and floodplains of the river draining the largest mining-contaminated site in the United States (the Clark Fork River Superfund Complex), have been studied with transmission electron microscopy (TEM) and energy dispersive X-ray (EDX) analysis. These poorly crystalline minerals are environmentally important in this system because contaminant heavy metals (As, Cu, Pb, and/or Zn) are always associated with them. Both two- and six-line ferrihydrite have been identified with selected-area electron diffraction. For the vernadite-like mineral, the two d values observed are approximately between 0.1 and 0.2 A larger than those reported for vernadite, the Mn hydrous oxide that is thought to have a birnessite-like structure, but which is disordered in the layer stacking direction. In several field specimens, the ferrihydrite and vernadite-like minerals are intimately mixed on the nanoscale, but they also occur separately. It is suggested that the vernadite-like mineral, found separately, is produced biogenically by Mn-oxidizing bacteria, whereas the same mineral associated with ferrihydrite is produced abiotically via the heterogeneous oxidation of Mn 2 + a q initially on ferrihydrite surfaces. Evidence from this study demonstrates that the vernadite-like mineral sorbs considerably more toxic metals than does ferrihydrite, demonstrating that it may be a good candidate for application to heavy-metal sorption in permeable reactive barriers.

Relict coesite exsolution in omphacite from Western Tianshan eclogites, China

Abstract: Exsolution rods of relict coesite together with quartz were identified in omphacite in eclogites from western Tianshan, China. They are oriented along the c-axis of the host clinopyroxenes and have grain size up to 30 μm long and 2-3 μm wide. Raman spectra of exsolved lamellae yield consistent but weak bands at 521, 270, 181, 151, and 118 cm - 1 , typical for coesite, in addition to those of quartz and the host omphacite. Such occurrences together with textured observations suggest a two-stage evolution of SiO 2 exsolution rods in omphacite. Lamellae of coesite were apparently exsolved from supersilicic omphacite at P 5.0 GPa and the transformation from coesite to quartz occurred during retrograde metamorphism.

Relationships between SEM-cathodoluminescence response and trace-element composition of hydrothermal vein quartz

Abstract: Laser-ablation ICP-MS data of hydrothermal vein quartz with zonation in scanning electron microscope cathodoluminescence reveal two groups of trace elements, one that co-varies in concentration with luminosity and another that remains uniform throughout. Bright luminosity correlates with a high total abundance of trace elements in early quartz, including Al (up to 410 μg/g), Ti (up to 240 μg/g), K (up to 330 μg/g), Li (up to 8 μg/g), Na (up to 36 μg/g), and Fe (up to 20 μg/g). Up to 20 times lower concentrations of these elements are associated with dark luminosity in late quartz. Concentrations of P (~21 μg/g), Ga (~0.3 μg/g), Ge (~1.3 μg/g), Sn (~1.5 μg/g), Cu (~0.3 μg/g), and Ag (~0.1 μg/g) demonstrate no relationship with luminosity. Charge balance cannot be achieved for bright luminescent quartz unless the presence of up to 6 μg/g H+ (not analyzed by LA-ICPMS) or interstitial Al3+ is invoked. Interestingly, the inferred ~6 μg/g H+ remains constant for quartz containing more than 10 μmol total trace elements and might represent the solubility of H+ in hydrothermal quartz at temperatures between 450 and 700 °C. LA-ICPMS results indicate that the fluid chemistry determines the quartz trace-element pattern, which may serve as a monitor for the chemical environment from which quartz crystallized. Ratios of Na/Al and Ti/Al are uniform, whereas K/Al evolves toward lower values with decreasing luminosity. Combined evidence, including quartz vein textures, silica solubility data, and estimates of the temperature of quartz growth, suggest that early quartz crystallized fast but late quartz grew more slowly at 350 to 425 °C. It is speculated that the higher the quartz growth rate is, the more trace elements are incorporated into quartz, besides the temperature dependence known from the literature. Clearly, growth rate, temperature and fluid chemistry are important parameters affecting luminosity; however, their relative importance may vary from case to case.

Very low solubility of rutile in H2O at high pressure and temperature, and its implications for Ti mobility in subduction zones

Abstract: The solubility of rutile in H2O has been measured at 1000–1100 °C, 1–2 GPa. The data indicate that solubility is very low over the investigated range, with a maximum of 4.7 millimol/kg H2O at 1100 °C, 2 GPa. The data were fit with the equation log m Ti + 4.892–10470/ T + 0.1923 P , where m Ti is Ti molality, T is in Kelvins, and P in GPa. When compared to previous results, the new data indicate substantially lower solubility, opposite pressure dependence, and thermodynamic properties of the reaction rutile + TiO2,aq that are now consistent with other oxide hydrolysis reactions. Calculations of Ti transport during mantle metasomatism by H2O in subduction zone environments predict much lower Ti mobility at all conditions. These results offer strong support for models of Ti retention in eclogites during slab devolatilization, and require that examples of significant Ti mass transfer be explained by complexing agents in solution, most likely aluminosilicate complexes.

Raman spectroscopy and vibrational analyses of albite: From 25 °C through the melting temperature

Abstract: Raman spectra were collected for crystalline albite from 25 °C to above the 1118 °C melting temperature, where vibrational assignments for the crystal spectra were determined by lattice dynamics (LD). The Raman spectra and associated vibrational assignments are reported for triclinic albite (NaAlSi3O8) at 25 °C and monoclinic albite at 1060 °C. The 25 °C calculations determined that localized T-O stretch and O-T-O bend modes are above 900 cm –1 (where T = Si,Al), while motions from the aluminosilicate tetrahedral cage mixed with Na displacements occur in modes as high as 814 cm –1 . Vibrational modes for the most prominent peaks in the spectrum, between 350 and 550 cm –1 , are dominated by four-membered tetrahedral ring deformations. For completeness, calculated infrared mode frequencies and their atomic displacements are reported for the 25 °C structure and compared with normal mode calculation results and observed infrared mode frequencies presented by von Stengel (1977). At higher temperatures, modes above 550 cm –1 broaden and shift to lower frequencies by 15 to 27 cm –1 ; modes below 550 cm –1 broaden, but experience little, if any frequency shifts. Albite melted sluggishly, was completely liquid at 1320 °C, and remained amorphous upon cooling to room temperature. At frequencies above 550 cm –1 , the crystalline albite peaks, and possibly their vibrational assignments, can be correlated to Raman bands for albite glass. Spectral differences below 550 cm –1 between crystal and glass correspond to changes of average tetrahedral ring type upon melting, as shown by Taylor and Brown (1979).

Electron-microprobe dating as a tool for determining the closure of Th-U-Pb systems in migmatitic monazites

Abstract: High spatial resolution dating of monazite by the electron-probe microanalyzer (EPMA) enables systematic and detailed studies of small minerals. Like zircon, monazite records the complex history undergone by the host rocks. Recent improvements in the statistical treatment of many in situ data now make it possible to decipher the related thermal events and so obtain reliable and precise ages. Our work shows that a significant number of individual spot analyses is required to reach such precise information (i.e., more than 30–40 data). Using the examples of monazites from three migmatites and one granite, we show how to select the most efficient method of age calculation according to the U and Th geochemistry of the grains, or grain domains, that we are trying to date. Three situations may be met: (1) monazites exhibiting significant Th/U ratio variation, (2) monazites exhibiting a fairly constant Th/U ratio, but significant U + Th heterogeneity, and (3) monazites of constant U and Th concentrations. For the first case, a precise mean age can be calculated using a method of data reduction in the Th/Pb = f (U/Pb) diagram, whereby a precision of ±5 − 10 Ma (2σ) is commonly achieved. For the second case, an isochron age can be calculated according to the Pb = f (Th*) method, with a common precision of around 20 Ma (2σ), whereas for the third case, a simple weighted average age can be calculated. Using these approaches, coupled with a back-scattered electron image study, we demonstrate that inheritance is probably as common for monazite as for zircon. In addition, the combination of high spatial resolution and precise age determination show the limited extent of Pb diffusion in monazite. Finally, an example from a migmatite from southern French Guiana demonstrates the especially robust behavior of the Th-U-Pb system in monazite. This system remains closed during late migmatization and during the subsequent zircon crystallization and zircon overgrowth of protolith zircons. The monazite yielded exactly the same age as the protolith zircons.

Stability and equation of state of MgGeO3 post-perovskite phase

Abstract: A phase transition of MgGeO3 perovskite was examined at high-pressure and -temperature using synchrotron X-ray diffraction measurements. The results demonstrate that it transforms to a CaIrO3-type post-perovskite phase above 63 GPa at 1800 K. The density increase is 1.5% at the transition pressure. These observations confirm that MgGeO3 is a low-pressure analogue to MgSiO3, for which a similar phase transition was recently found above 125 GPa and 2500 K. The unit-cell parameters of MgGeO3 post-perovskite phase obtained at 300 K during decompression from 79 to 6 GPa show that the b -axis is significantly more compressible than are the a - and c -axes, which could be due to the GeO6-octahedral sheet stacking structure along b . The bulk modulus was determined to be K = 192(±5) GPa with a fixed pressure derivative of the bulk modulus, K ′, of 4.

First-principles studies on the elastic constants of a 1:1 layered kaolinite mineral

Abstract: The mechanical behavior of minerals under high pressure has attracted considerable interest in recent years. Clay minerals, including kaolinite, are common minerals found in sedimentary environments and their behavior under pressure is an important factor in understanding seismogenic zones. Kaolinite is a ubiquitous member of the kaolin group of 1:1 phyllosilicates that have recently been found to undergo a structural phase transition between 2.0 and 2.5 GPa. In this study, the bulk modulus and elastic constants of kaolinite have been calculated from first principles within density functional theory (DFT) for the first time. The bulk modulus is predicted to be 23 GPa for kaolinite. The calculated elastic constant tensors indicate that the a direction is slightly more flexible than the b direction. The calculated elastic constant tensor along c is much lower than the constants calculated along a and b consistent with the crystal structure of kaolinite. Elastic wave velocities for P- and S-waves were calculated to be 7.34 and 3.50 km/s, respectively. Since an agreement between the theoretical and experimental values is satisfactory, we conclude that the theoretical calculations presented here are useful in seismic research for predicting the mechanical properties of minerals that are difficult to obtain experimentally because of their small particle size (typically <2 micrometers).

Structure and luminescence characteristics of quartz from pegmatites

Abstract: Samples of pegmatite quartz from several localities in Norway and Namibia were investigated by cathodoluminescence (CL) microscopy and spectroscopy, electron paramagnetic resonance (EPR) measurements, and trace-element analysis (ICP-MS) to obtain information about their structure and luminescence characteristics. The defect structure and trace-element composition of the pegmatite quartz samples that were studied differ from those of quartz of other origin (hydrothermal, igneous and metamorphic quartz). EPR measurements reveal an almost complete lack of intrinsic lattice defects associated with O or Si vacancies (e.g., E′ center, O 3− 2 center), whereas some trace elements (Al, Ti, Ge, Li) are apparently enriched and form paramagnetic centers. The results indicate that there possibly is a redistribution of alkali ions during electron irradiation. The diamagnetic [AlO 4 /M + ] 0 center transforms into the paramagnetic [AlO 4 ] 0 center, while the compensating ions diffuse away and may be captured by the diamagnetic precursor centers of [GeO 4 ] 0 and [TiO 4 ] 0 to form paramagnetic centers ([TiO 4 /Li + ] 0 , [GeO 4 /Li + ] 0 ). These defects result in a specific luminescence behavior, which is very similar for all samples. In general, quartz from pegmatites shows homogeneous CL without growth zoning or internal structures suggesting constant physicochemical conditions during crystal growth. The CL emission is dominated by a transient bluish-green CL, which disappears after 60–100 s of electron irradiation. The two main emission bands centered at 505 nm (2.45 eV) and 390 nm (3.18 eV) are probably related to alkali-compensated, trace-element centers in the quartz structure. Other CL emission bands, which are characteristic features of igneous, metamorphic, or hydrothermal quartz (e.g., at 450 nm = 2.75 eV, 580 nm = 2.14 eV, 650 nm = 1.91 eV) are almost completely lacking. This fact indicates that the defects responsible for these CL emissions are absent in pegmatite quartz.

Sodic amphibole exsolutions in garnet from garnet-peridotite, North Qaidam UHPM belt, NW China: Implications for ultradeep-origin and hydroxyl defects in mantle garnets

Abstract: Two types of amphibole exsolution lamellae were discovered in garnet from a few garnet peridotites from the North Qaidam ultrahigh-pressure metamorphic belt, northern Tibet, NW China. The amphibole lamellae are strictly oriented in four directions corresponding to the isometric form {111} (i.e., octahedron planes) of garnet. Observations by transmission electron microscopy (TEM) show that amphibole lamellae are topotaxtially concordant with the host garnet and possess 9.8 to 10 A lattice “c”-spacing. Electron-microprobe analysis reveals that these exsolved amphibole lamellae are high in Na and Ti. The TEM observations and recalculated compositions indicate that the oriented amphibole lamellae exsolved from original supersilicic majorites with high concentrations of Na2O (0.3 wt%) and hydroxyl (up to 1000 ppm by weight). These results imply that the host garnet peridotites were formed at depths greater than 200 km and that garnet can be an important reservoir of water at such depths in the mantle.