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Monazite is both partially replaced and overgrown by a ThSi[O.sub.4] phase along grain rims in a series of experiments from 300 to 900 [degrees]C and 200 to 1000 MPa. All experiments consisted of 10 mg of 100-500 [micro]m size, euhedral to subhedral crystals of a natural Th-free monazite-(Ce), 5 mg of Th[(N[O.sub.3]).sub.4]*5[H.sub.2]O, 2.5 mg of Si[O.sub.2], and 5 mg of [H.sub.2]O loaded into 3 mm wide, 1 or 1.3 cm long platinum capsules that were arc welded shut. Experimental conditions were: 300 [degrees]C at 200 and 500 MPa; 300, 400, 500, 600, and 700 [degrees]C at 500 MPa (cold seal hydrothermal autoclave); and 900[degrees]C at 1000 MPa (Catz assembly; pistoncylinder press). Back-scattered electron (BSE) imaging, electron back-scattered diffraction (EBSD) analysis, and transmission electron microscopy (TEM) indicates that in the experiments from 500 to 900[degrees]C, the ThSi[O.sub.4] phase took the form of monoclinic huttonite implying that huttonite, associated with monazite, could exist metastably over a much greater P-T range than previously thought. TEM analysis of a foil cut perpendicular to the monazite-huttonite interface from the 600 [degrees]C, 500 MPa experiment using a focused ion beam (FIB) indicates that the huttonite as well as the interface between the huttonite and monazite is characterized by fluid inclusions. High-resolution TEM analysis indicates that the huttonite-monazite interface is coherent. In the case of replacement of monazite by huttonite, fluid-aided dissolution-reprecipitation is proposed as the most likely mechanism responsible. Keywords: Monazite, huttonite, dissolution-reprecipitation, metastability, metasomatism, EBSD, TEM, pseudomorphism

The release of Zr, U, and Pb from nearly metamict zircon and its recrystallized analog and of Zr from fully crystalline and slightly radiation-damaged zircon in [H.sub.]O + HCl [+ or -] Si[O.sub.2] fluids was investigated in situ at temperatures between 200 and 500[degrees]C using a hydrothermal diamond-anvil cell and time-resolved synchrotron-radiation XRF analyses. Dissolution of nearly metamict zircon proceeded much faster than that of zircon with little or no radiation damage and resulted in a 1.5 to 2 log units higher Zr molality in 6 to 7 m HCl fluids. Extensive recrystallization of the almost fully amorphous material started at 260 to 300[degrees]C in [H.sub.2]O + HCl, and at about 360[degrees]C if quartz was added, and was coupled with a decrease of the Zr concentration in the fluid by more than an order of magnitude. Recrystallization in 7 m HCl had little effect on the aqueous U and Pb concentrations, whereas addition of quartz caused a more sluggish decrease of the Zr concentration in the fluid upon recrystallization and lowered the release of U. The data presented here support the interpretation that enhanced Zr mobility in low-grade metamorphic rocks may be related to dissolution of metamict zircon by aqueous fluids and illustrate the significance of the silica activity on the kinetics of dissolution and recrystallization during zircon-fluid interaction. Keywords: High-temperature studies, zircon-aqueous fluid interactions, hydrothermal diamond-anvil cell, in-situ SR-XRF analysis, dissolution kinetics, order-disorder, metamictization, recrystallization, fluid phase, [H.sub.2]O + HCl [+ or -] Si[O.sub.2], recrystallization kinetics

In the Monte Rosa area, Northern Italy, the assemblage garnet + phengite + quartz was formed in polymetamorphic metapelites from pre-existing biotite and plagioclase during an eclogite-facies Alpine metamorphic overprint. While phengite nucleated within plagioclase, garnet formed l0 to 20 [micro]m wide continuous rims along the original biotite-plagioclase interphase boundaries. Garnet formation involved diffusion of Ca-, Fe-, and Mg-bearing species across the growing rims. The garnet rims show an overall asymmetric compositional zoning and distinct nanometer scale chemical patterns across grain boundaries within the garnet polycrystal. The compositional patterns resulted from the interplay of diffusion along grain and interphase boundaries and volume diffusion. Individual garnet crystals are separated by low-angle grain boundaries, which are predominantly oriented perpendicular to the reaction rim giving rise to an overall palisade microstructure. Grain boundaries contain arrays of closely spaced channels about 2 nm wide, which are filled with an amorphous phase. These channels represent direct links between the garnet-biotite and garnet-plagioclase reaction fronts and provide pathways for fast diffusion. From numerical simulation of the observed chemical patterns, diffusion of Ca and Fe is inferred to have been 5-6 orders of magnitude faster within grain boundaries than through the interior of garnet grains. Although the amorphous grain boundary phase only takes up a small fraction (2-5 per rail) of the total volume of the garnet polycrystal, short-circuit diffusion along grain boundaries contributed substantially to material transfer across the growing garnet rim. Despite the presence of fast pathways, bulk diffusion was too slow to allow for chemical equilibration of the phases involved in garnet rim formation, even on a micrometer scale. Relying on published garnet volume diffusion data, [D.sub.gb] (where gb = grain boundary), is estimated to be on the order of [l0.sup.-(19-20)] [m.sup.2]/s at the estimated reaction conditions of 650 [degrees]C and 12.5 kbar, where [D.sup.Ca.sub.gb] is slower than [D.sup.Fe.sub.gb] by a factor of ten. Keywords: Garnet, high-pressure metamorphism, grain boundary, diffusion, TEM

Fluid-bearing microinclusions in diamonds ( Carbonates appear in all assemblages, together with either halides or silicates, indicative of the importance of carbonatitic high-density fluid during diamond growth and fluid evolution. The presence of halide-carbonate or silicate-carbonate assemblages is in agreement with the bulk composition of the microinclusions as detected by EPMA. The high K content of some microinclusions detected by EPMA cannot be accounted for by the solid phases analyzed by TEM. This discrepancy suggests that K is concentrated in the residual fluid that is lost during TEM sample preparation. In addition to microinclusions, large cavities containing amorphous phases were found in the inner parts of one Siberian and one Canadian diamond. An Al-rich phase is the most abundant, and it is accompanied by Ca-rich and Si-rich phases. These phases may be explained by amorphization of crystalline phases. A breakdown of a single melt into three immiscible components is less likely. Keywords: Crystal structure, high-density fluid, high-silica mica, carbonate, halide, secondary minerals, multi-phase assemblage

Metastable coesite is an important pressure indicator tier ultrahigh-pressure rocks. However, in many cases coesite does not survive exhumation, but reacts back to quartz. Although it was shown experimentally that incorporation of H in coesite increases with increasing pressure, most coesite relics found in nature are essentially dry (i.e., OH concentrations are below detection limit,

For more than 100 years mineralogists, physicists, chemists, geologists, and biologists have discussed the formation of periodic Liesegang patterns observed in natural and experimental systems. Spectacular examples of minerals showing complex periodic patterns are agate, malachite, and sphalerite. Here we report the first observation of Liesegang-like patterns in hydrothermally treated metamict (i.e., amorphous) zircon. The structures observed show curved bands, radial sets of pocket-like wave fronts or irregular curved patterns in both cathodoluminescence and backscattered electron images. They are composed of alternating zones of crystallographically well-aligned, polycrystalline zircon along with remnant amorphous pockets and a phase assemblage of randomly oriented zircon crystallites, monoclinic Zr[O.sub.2], and amorphous Si[O.sub.2], as revealed by transmission electron microscopy. Analyses by secondary ion mass spectrometry and electron microprobe reveal that the latter zones are characterized by higher hydrogen concentrations and higher Zr-Si ratios. Both zones are also distinguishable by a distinctly different crystallite size. We propose a possible pattern-forming mechanism that is based on a feedback of hydrogen diffusion, zircon nucleation, and the displacement of hydrogen atoms from growing crystallites.

A series of clinopyroxenes from the lower crust and upper mantle beneath the Siberian platform was investigated by Fourier-transform infrared (FTIR) spectroscopy and transmission electron microscopy (TEM). The IR spectra of all our samples exhibit three groups of absorption bands at (1) 3445-3465, (2) 3500-3540, and (3) 3600-3624 [cm.sup.-1]. Using synchrotron IR radiation, which utilizes a spot size of only 5 x 5 [micro]m, we realized that the intensities of the absorption bands, mostly those of group 3, showed extreme variation within one crystal. TEM as well as polarized and high-pressure IR spectroscopy indicated that the OH groups that cause the bands of group 3 were not intrinsic but due to nm-sized inclusions of sheet silicates. The intensities and peak positions of the bands of group 2 correlate with the amount of tetrahedral [Al.sup.3+] indicating that the charge-deficient substitution of Al for Si is responsible for the bands of group 2. The intensities and peak positions of the bands of group 1 correlate with the concentration of vacancies at M2 indicating that the cation vacancies at M2 control the incorporation of hydroxyl responsible for the bands of group 1. The bands of groups 1 and 2 are caused by the same type of OH dipole, however, occurring in different structural environments. The concentration of the structurally bound water of the omphacitic clinopyroxene is in the range from 31 to 514 ppm [H.sub.2]O (by weight). Surprisingly, the lowest concentration was found in clinopyroxene, that comes from the highest pressure region, i.e., the diamond-bearing eclogite xenoliths of the Mir kimberlite pipe. The highest values were obtained in omphacites of the lower-pressure grospydites of the Zagadochnaya kimberlite pipe and in omphacitic clinopyroxene of the lower pressure granulites of the Udachnaya kimberlite pipe, respectively. The low water content of clinopyroxene from the high-pressure region seems to be controlled by low water activity during crystallization. However, hydrogen loss during the uplift cannot be ruled out.

A suite of 18 zircon gemstones from placers in the Highland/Southwestern Complex, Sri Lanka, were subjected to a comprehensive study of their radiation damages and ages. The investigation included X-ray diffraction. Raman and PL spectroscopy, electron microprobe, PIXE and HRTEM analysis, as well as (U-Th)/He and SHRIMP U-Th-Pb age determinations. Zircon samples described in this study are virtually homogeneous. They cover the range from slightly metamict to nearly amorphous. Generally concordant U-Th-Pb ages averaging 555 [+ or -] 11 Ma were obtained. Late Ordovician zircon (U-Th)/He ages scattering around 443 [+ or -] 9 Ma correspond reasonably well with previously determined biotite Rb-Sr ages for rocks from the HSWC. Slightly to moderately metamict zircon has retained the radiogenic He whereas only strongly radiation-damaged zircon (calculated total fluences exceeding ~3.5 x [10.sup.18] [alpha]-events/g) has experienced significant He loss. When compared to unannealed zircon from other localities, Sri Lanka zircon is about half as metamict as would correspond to complete damage accumulation over a ~555 m.y. lasting self-irradiation period, suggesting significant annealing of the structural radiation damage. Insufficient consideration of this has often resulted in significant underestimation of radiation effects in zircon. We suggest to estimate 'effective [alpha]-doses' for Sri Lanka zircon by multiplying total [alpha]-fluences, which were calculated using the zircon U-Th-Pb age, by a correction factor of 0.55. This conversion may be applied to literature data as well, because all gem-zircon samples from Sri Lanka (this work and previous studies) seem to reveal the same general trends of property changes depending on the radiation damage. The use of 'effective [alpha]-doses' for Sri Lanka zircon contributes to more reliable quantitative estimates of radiation effects and makes possible direct comparison between natural and synthetic radiation-damaged zircon.

Aluminum orthoposphate (AlP[O.sub.4]) has polymorphs isostructural with tridymite, cristobalite, and quartz. Berlinite is the low-temperature form that corresponds to [alpha]-quartz. We report berlinite-quartz solid solutions to crystallize from a synthetic P-rich peraluminous granitic melt, similar in composition to the most volatile-rich silicate melt inclusions found in pegmatites. The crystallization took place in experiments performed in cold-seal pressure vessels at 450-700 [degrees]C and 0.1-0.2 GPa [H.sub.2]O pressure. At these conditions, the berlinite-quartz mutual solubility is limited to 5-7 mol% Si[O.sub.2] on the phosphate side of the solvus and to the maximum of 1 mol% AlP[O.sub.4] on the silica-rich side. The mutual solubility appears to decrease with falling temperature. At low T the crystals of berlinitequartz solid solutions are strongly zoned and show complex intergrowths between the P-rich and silica-rich phases. They were studied by electron microprobe, transmission electron microscopy, and Raman spectroscopy. In the light of our new experimental results, the extreme P enrichment reported earlier for some natural quartz-hosted melt inclusions may be explained as a post-entrapment contamination by the berlinite-bearing host.

Electron-microprobe (EMP) U-Th-Pb dating on polyphase and discordant monazites from polymetamorphic granulites of the Andriamena unit (north-central Madagascar) reveals inconsistent chemical ages. To explain these drastic variations, transmission electron microscopy (TEM) foils were prepared directly from thin sections by using the focused ion beam technique. The most important result of the TEM study is the demonstration of the presence of small (~50 nm) Pb-rich domains where large variations in EMP ages occur. We suggest that radiogenic Pb was partially reincorporated in monazite during the recrystallization at 790 Ma. Because the excited volume of EMP is ~4 [micro][m.sup.3], U-Th-Pb dating yielded various apparent older ages without geological significance. In addition, TEM analysis of the foils revealed the presence of an ~150-nm-wide amorphous zone along the grain boundary of monazite and its host quartz. This Fe-Si-Al-rich phase may have formed as a result of fluid activity at 500 Ma, and the phase's amorphous state may be due to the irradiation from U and Th decay in the monazite. This demonstrates for the first time the enormous potential of the TEM investigations on site-specific specimens prepared with the focused ion beam technique for the interpretation of geochronological data. Keywords: monazite, discordance, focused ion beam, transmission electron microscopy, U-Th-Pb ages, Madagascar.