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Despite the abundance of garnet in deforming regions of the Earth, such as subduction zones, its rheological properties are not well defined by laboratory measurements. Here we report measurements of steady-state plastic properties of pyrope in its stability field (temperature up to 1573 K, pressure up to 6.8 GPa, strain rate ~[10.sup.-5] [s.sup.-1]) using a Deformation-DIA apparatus (D-DIA) coupled with synchrotron radiation. Synthetic pyrope ([Py.sub.100]) and natural pyrope ([Py.sub.70][Alm.sub.16][Gr.sub.14]) are both studied in a dry environment. Transmission electron microscopy (TEM) investigation of the run products indicates that dislocation glide, assisted by climb within grains and dynamic recrystallization for grain-boundary strain accommodation, is the dominant deformation process in pyrope. Both synthetic-and natural-pyropes' stress and strain-rate data, as measured in situ by X-ray diffraction and imaging, are best fitted with the single flow law: [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] where [??] is the strain rate, [sigma] = [absolute value of [sigma.sub.1] - [sigma.sub.3]] is the differential stress, R is the gas constant and T the absolute temperature. Synthetic forsterite and synthetic pyrope were stacked along the uniaxial compression direction in the same cell assembly during deformation to compare their strength at mantle condition. Forsterite is observed to be stronger than pyrope, deforming at a rate about 10% slower than the pyrope at 5.2 GPa and 1573 K. San Carlos olivine and natural pyrope were compared in a similar fashion at 6.8 GPa and 1473 K. In this case, San Carlos olivine deformed 2~3 times faster than natural pyrope. The experimental data suggest that pyrope is stronger (by more than a factor of 4) than the dominant mineral (olivine) in the upper mantle when temperatures exceed 1273 K. Keywords: Garnet, high pressure, high temperature, synchrotron X-ray imaging, stress, strain

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.earscirev.2005.03.004 Byline: Michele Lustrino Keywords: delamination; detachment; lithosphere; mantle plume; petrology; lower crust; eclogite; pyroxenite Abstract: The Earth's lithosphere can focus basaltic magmatism along pre-existing weakness zones or discontinuities. However, apart from the influence on the geochemistry of magmas emplaced in subduction tectonic settings (mantle wedge metasomatism related to dehydration of the subducting plates) the role of lithosphere as a magma source for intra-plate (both oceanic and continental), continental margin, and mid-ocean ridge magmatism is not yet fully understood. In many cases intra-plate magmatism has been explained with the existence of deep thermal anomalies (mantle plumes) whose origin has been placed near the upper-lower mantle transition zone (660 km discontinuity) or even deeper, near the mantle-core boundary ([approximately equal to]2900 km). Also in many continental flood basalt provinces (mostly initiated at craton margins) an active role for mantle plumes has been invoked to explain the high melt productivity. In these cases, no active role for melt production has been attributed to the lithospheric mantle. Potential contaminations of asthenospheric or even deeper mantle melts are often considered the only influence of the lithosphere (both crust and mantle) in basalt petrogenesis. In other cases, an active role of the lithospheric mantle has been proposed: the thermal anomalies related to the presence of mantle plumes would trigger partial melting in the lithospheric mantle. At present there is no unequivocal geochemical tracer that reflects the relative role of lithosphere and upper/lower mantle as magma sources. In this paper another role of the lithosphere is proposed. The new model presented here is based on the role of lower crustal and lithospheric mantle recycling by delamination and detachment. This process can explain at least some geochemical peculiarities of basaltic rocks found in large and small volume igneous provinces, as well as in mid-ocean ridge basalts. Metamorphic reactions occurring in the lower continental crust as a consequence of continent-continent can lead to a density increase (up to 3.8 g/cm.sup.3) with the appearance of garnet in the metamorphic assemblage (basalt[right arrow]amphibolite[right arrow]garnet clinopyroxenite/eclogite) leading to gravitative instability of the overthickened lithospheric keel (lower crust+lithospheric mantle). This may detach from the uppermost lithosphere and sink into the upper mantle. Accordingly, metasomatic reactions between SiO.sub.2-rich lower crust partial melts and the uprising asthenospheric mantle (replacing the volume formerly occupied by the sunken lithospheric mantle and the lower crust) lead to formation of orthopyroxene-rich layers with strong crustal signatures. Such metasomatized mantle volumes may remain untapped also for several Ma before being reactivated by geological processes. Partial melts of such sources would bear strong lower crustal signatures giving rise to Enriched Mantle type 1 (EMI)-like basaltic magmatism. Basaltic magmatism with such a geochemical signature is relatively scarce but in some cases (e.g., Indian Ocean) it can be a geographically widespread and long-lasting phenomenon. Author Affiliation: Dipartimento di Scienze della Terra, Universita degli Studi di Roma La Sapienza, P.le A. Moro, 5, 00185 Rome, Italy Istituto di Geologia Ambientale e Geoingegneria (IGAG)-CNR, P.le A. Moro, 5, 00185 Rome, Italy Article History: Received 26 July 2004; Accepted 31 March 2005

We report data on the composition and crystal structure of the most K-rich (3.61 wt% [K.sub.2]O) natural clinopyroxene yet discovered. The studied crystal was found as a tiny inclusion in garnet from a garnet-clinopyroxene rock of the Kumdy-Kol microdiamond mine, Kokchetav complex, Northern Kazakhstan. Microprobe analysis yields the formula ([Ca.sub.0.61][Fe.sub.0.13] [Mg.sub.0.04][Mn.sub.0.01][K.sub.0.17][Na.sub.0.05]) (Al.sub.0.61][Mg.sub.0.39])([Si.sub.1.61][Al.sub.0.39])[O.sub.6.00]. Lattice parameters are: a = 9.773(1), b = 8.926(1), c = 5.269(1) [Angstrom], [beta] = 105.75(1)[degrees]. The structure was refined up to [R.sub.all] = 2.42% using 982 independent reflections. Substitution of K for Ca causes significant modification of the average structure. No evidence for an additional M2' position was found. Crystal-chemical characteristics are compared with published data on both natural and synthetic K-bearing clinopyroxenes.

Study of mineral inclusions in garnet in orogenic peridotites provides important information on their metamorphic history. Similar study of mineral inclusions in pyroxene is also encouraging. Edenite is found as relict inclusions in phlogopite-clinopyroxene intergrowths forming the cores of clinopyroxene porphyroblasts in a garnet lherzolite at Zhimafang in the Chinese Su-Lu ultrahigh-pressure metamorphic terrane. Texture shows that the phlogopite and clinopyroxene were products of the breakdown of edenite. The composition of the edenite is represented by K-[Ed.sub.6.0][Ed.sub.70.7] Fe-[Ed.sub.6.6][Ts.sub.8.4] Fe-[Ts.sub.1.8][Tr.sub.1.2][Cm.sub.5.4]. Those of the clinopyroxene and phlogopite are, [Ur.sub.4.5][Jd.sub.3.7][Ae.sub.6.5]Ca [Ts.sub.2.2][Di.sub.81.1][En.sub.1.9] and [Phl.sub.86.0]Na-[Phl.sub.13.3] Na-[Ann.sub.0.7], respectively. The edenite breakdown is accompanied by gains of Ca, Si, and Cr, and losses of H, Mg, Al, Fe, Na, and K. The hypothetical composition of clinopyroxene resulting from isochemical breakdown of edenite is higher in Jd, CaTs, and En but lower in Di components than the cores of clinopyroxene porphyroblasts. The calculated mode of phlogopite is much higher than measured. The gain of Si and loss of H can be explained by involvement of Si[O.sub.2]-enriched fluids during formation of the Cpx+Phl intergrowth, whereas the other gains and losses suggest the existence of an older generation clinopyroxene slightly more enriched in Ca and Cr and depleted in Na, Mg, and Al, which homogenized with the clinopyroxene produced by the breakdown of edenite. Interaction with fluids also increased the Ca/Mg ratio of the system. It is suggested that, after having been exhumed to a shallow level for the first time, the peridotite was infiltrated by crust-derived fluids resulting in the replacement of clinopyroxene by edenite. During subduction, the edenite was replaced by clinopyroxene + phlogopite. Relics of edenite in clinopyroxene were able to survive ultrahigh-pressure metamorphism.

This paper presents a novel technique for integrating yttrium iron garnets, namely Ce :YIG, onto semiconductor platforms using metal-organic chemical vapor deposition (MOCVD). Large amounts of cerium (Ce) could be incorporated into the garnet structure because of the nonequilibrium nature of the technique. The method can alloy up to 54 % Ce, thereby increasing the refractive index and enhancing the Faraday rotation of the YIG films. Faraday rotations as high as 0.4 [degrees] / [micro]m at 1.3 [micro]m were achieved in MOCVD-grown garnets, exceeding the rotations of bismuth-doped YIG films (0.15 [degrees] / [micro]m at 1.3 [micro]m) grown by liquid-phase epitaxy. The easy axis of magnetization is within the plane of the films. When the garnet films were sputtered onto (100) magnesia (MgO) buffer layers, their hysteresis loops indicated that they were isotropic. Index Terms--Ce, Faraday rotator, garnet, magneto-optic.

Raman spectra of syngenetic majoritic garnets were obtained from the Brazilian Sao-Luiz diamond suite. These garnets have a low majoritic content ( 3) have a characteristic signature in the frequency region of the Si[O.sub.4]-Si[O.sub.6] stretching vibrations. This vibration gives a broad peak between 800 and 900 [cm.sup.-1] just before an intense band (900-930 [cm.sup.-1]) classically assigned to Si-O stretching vibrations of the SiO4 tetrahedra in all known garnets. This broad band is also present in non-majoritic garnets (Si = 3) that result from the transformation, within the diamonds, of majoritic garnets into low-pressure garnet + pyroxene intergrowths. We show that the presence of significant Ti[O.sub.2] content in these normal garnets leads to Raman spectra mimicking those of majoritic garnets. The occurrence of a broad shoulder between 800 and 900 [cm.sup.-1] as well as broad bands near 960 and 1030 [cm.sup.-1] in the Raman spectra are no longer diagnostic features indicative of a majoritic garnet when the Ti[O.sub.2] content of the garnets is high (>1 wt%) as is the case for the Sao Luiz diamonds. The full width at half maximum (FWHM) of the strong Raman mode near 910 [cm.sup.-1] is the only reliable signature that discriminates Si = 3 from Si > 3 garnets. The frequency of the diamond Raman mode used as an in-situ piezometer shows that the actual pressure of the garnet inclusions is between 0.5 and 0.8 GPa.

Volume diffusion rates of Ce, Sm, Dy, and Yb have been measured in a natural pyrope-rich garnet single crystal ([Py.sub.71][Alm.sub.16][Gr.sub.13]) at a pressure of 2.8 GPa and temperatures of 1,200-1,450 [degrees] C. Pieces of a single gem-quality pyrope megacryst were polished, coated with a thin layer of polycrystalline REE oxide, then annealed in a piston cylinder device for times between 2.6 and 90 h. Diffusion profiles in the annealed samples were measured by SIMS depth profiling. The dependence of diffusion rates on temperature can be described by the following Arrhenius equations (diffusion coefficients in [m.sup.2]/s): [log.sub.10][D.sub.Yb] = (-7.73 [+ or -] 0.97) - (343 [+ or -] 30 kJ [mol.sup.-1]/2.303RT) [log.sub.10][D.sub.Dy] = (-9.04 [+ or -] 0.97) - (302 [+ or -] 30 kJ [mol.sup.-1]/2.303RT) [log.sub.10][D.sub.Sm] = (-9.21 [+ or -] 0.97) - (300 [+ or -] 30 kJ [mol.sup.-1]/2.303RT) [log.sub.10][D.sub.Ce]! = (-9.74 [+ or -] 2.84) - (284 [+ or -] 91 kJ [mol.sup.-1]/2.303RT). There is no significant influence of ionic radius on diffusion rates; at each temperature the diffusion coefficients for Ce, Sm, Dy, and Yb are indistinguishable from each other within the measurement uncertainty. However, comparison with other diffusion data suggests that there is a strong influence of ionic charge on diffusion rates in garnet, with RE[E.sup.3+] diffusion rates more than two orders of magnitude slower than divalent cation diffusion rates. This implies that the Sm-Nd isotopic chronometer may close at significantly higher temperatures than thermometers based on divalent cation exchange, such as the garnet-biotite thermometer. REE diffusion rates in pyrope are similar to Yb and Dy diffusion rates in diopside at temperatures near the solidus of garnet lherzolite (~1,450 [degrees] C at 2.8 GPa), and are an order of magnitude faster than Nd, Ce, and La in high-Ca pyroxene at these conditions. At lower temperatures relevant to the lithospheric mantle and crust, REE diffusion rates in garnet are much faster than in high-Ca pyroxene, and closure temperatures for Nd isotopes in slowly-cooled garnets are ~200 [degrees] C lower than in high-Ca pyroxene.

Garnets are present in two horizons of the schist of Stevens Spring from the Basin Creek area of the Grouse Creek Mountains in northwest Utah. The two horizons possess different bulk compositions, which resulted in garnet growth by different reactions along the same pressure-temperature (P-T) path. Garnet in the upper horizon grew from the breakdown of chlorite at upper greenschistfacies conditions and garnet in the lower horizon grew from the breakdown of staurolite at upper amphibolite-facies conditions. From the upper horizon, five garnets from three samples were analyzed. All display growth zoning, ragged morphologies associated with secondary rim consumption, and yield garnet-biotite geothermometry temperatures of ~460-490 [degrees] C. From the lower horizon, one garnet from each of two samples was analyzed. These also display growth zoning, but differ from garnet in the upper horizon in that they are dominantly idioblastic and yield garnet-biotite geothermometry temperatures of ~635 [degrees] C. Garnet-biotite geothermometry calculated for every point along detailed compositional traverses across the garnets revealed localized reequilibration along rims, cracks and inclusions in both generations of garnet. Garnets from the upper horizon display prograde reequilibration and the garnets from the lower horizon display retrograde reequilibration. Numerical simulations of garnet growth using the Gibbs method with Duhem's theorem were carried out to determine P-T paths. The P-T path defined by the modeling of five garnets from the upper horizon is an isothermal pressure increase of ~1.7 kbar. The P-T path defined by the modeling of two garnets from the lower horizon has a steep P-T trajectory (dP/dT = 32 bars/[degrees] C) and a total pressure change of ~0.9 kbar. Both paths are indicative of thrust burial; however, the two paths cannot be reconciled as products of a single thrust episode. These data are interpreted to indicate two different episodes of thrust burial during the Sevier Orogeny, separated by ~150 [degrees] C of heating and partial exhumation.

Trace element concentrations of peridotitic garnet inclusions in diamonds from two Chinese kimberlite pipes were determined using the ion microprobe. Garnet xenocrysts from the same two kimberlite pipes were also analyzed for comparison. In contrast to their extremely refractory major element compositions, all harzburgitic garnets showed enrichment in light rare earth elements (REE) relative to chondrite, resulting in sinuous REE patterns. Both normal and sinuous REE patterns were observed from the lherzolitic garnets. Concentrations of REE in garnets changed significantly from diamond to diamond and no specific correlations were observed with their major element compositions. Analyses of randomly selected two to three points within every grain of a large number of garnet inclusions by the ion microprobe demonstrated that there was no evident compositional heterogeneity, and multiple grains of one phase from a single diamond host also exhibit very similar compositions. This implies that the trace element heterogeneity within one grain or among multiple inclusions from the same diamond host, as reported from Siberian diamonds, is not a common feature for these Chinese diamonds. Concentrations of Na, Ti, and Zr tend to decrease when garnets become more refractory, but variations of Sr and Li are more complex. Compositions rich in light REE and relatively poor in high field strength elements (HFSE) of the harzburgitic garnet inclusions in diamonds are generally consistent with metasomatism by carbonatite melts. The trace element features observed from the garnet inclusions in Chinese diamonds may be caused by carbonatite melt infiltration and partial melt extraction. Spatial and temporal gradients in melt/rock ratio and temperature are the main reasons for the large variations of REE patterns and other trace element concentrations.

High temperature (> 900 [degrees] C) metamorphism affected the New Russia gneiss complex in the aureole of the Marcy anorthosite, Adirondack Highlands, New York. Dehydration melting of pargasitic hornblende and plagioclase in metagabbro during contact metamorphism produced garnet among other phases, an indicator that pressure exceeded 700 MPa during anatexis. Partial melting also occurred in mangerite and charnockite. Minerals that equilibrated during melting yield barometric estimates of 970 [+ or -] 100 MPa (garnet-orthopyroxene-plagioclase-quartz in metagabbro and mangerite) and 735 [+ or -] 100 and 985 [+ or -] 100 MPa (garnet-hornblende-plagioclase-quartz, metagabbro and mangerite, respectively). From these results we infer that the Marcy anorthosite was emplaced at a depth of at least 23 km and probably near 32 km.

Garnets have been used by humans since prehistoric times. Their decorative use is common, but they were also believed to possess properties for treating bleeding and inflammation and were regarded as symbols for fidelity and constancy. Garnets are widely distributed in sedimentary rocks and occur in 7 forms. They help sedimentologists in identifying a kind of lithology as well as indicating the presence of diamonds.

The ability of single-crystal NMR to obtain crystal structure data is shown through a 27Al single-crystal NMR analysis of the garnet Y3Al5O12 (YAG). Ninty-three 27Al spectra were obtained by rotating the YAG single crystal about three perpendicular axes, producing 3,132 resonance frequencies. A detailed study of the magnitudes and relative orientations of the quadrupole coupling tensors for the tetrahedral and octahedral aluminum sites revealed the presence of a combination of symmetry elements that is unusual to cubic crystal structures.

An experiment was conducted to analyze the iron-manganese exchange of garnet and ilmenite and to reverse equilibrium over the range of the exchange system. Electron microprobe technique was utilized to characterize reactants and products of the iron-exchange process. Results showed that phases of ilmenite grains and adjacent garnet were unzoned and artificial titanium-zoning in garnet could be seen in the garnet-ilmenite interface. Furthermore, secondary titanium-fluorescence of ilmenite resulted to high apparent titanium contents.

New research were conducted to find the saturation conditions for garnet and silicate liquid. Starting conditions were natural basalt powders ranging from komatiite to nephelinite, which were partially melted at pressures between 25 and 100 kbar. Findings showed that equations of garnet-liquid equilibrium are simple and may be easily integrated into liquid line-of-descent models. Also, at high pressure, igneous garnets will not always contain Mj as the dominant mineral fraction.

The self-diffusion coefficients of Fe and Mg in Mn-poor ferromagnesian garnets were determined by employing diffusion couples made from natural pyrope and almandine garnet crystals. Results showed that the self-diffusion of Fe and Mn are substantially improved with the increase in Mn/Mg ratio. However, the improvement effect on the diffusion coefficient of Mg is relatively small. Proper use of the self-diffusion data to compute interdiffusion coefficient or diffusion matrix elements for modeling diffusion processes in natural garnets must consider these compositional effects on self-diffusion.

An experiment was conducted to determine the magneto-acoustic surface acoustic wave interaction of yttrium-iron garnet films which are grown on gallium gadolinium garnet substrates. A standard measurement technique which utilizes an oscillograph and an amplifier was used. Results showed that surface acoustic wave velocity can be controlled by an external magnetic field and the development of signal processing units are dependent on the film thickness and field orientation.

We report 207Pb/206Pb stepwise-leaching (PbSL) ages of 1762 [+ or -] 15 Ma, 1759 [+ or -] 8 Ma, and 1760 [+ or -] 7 Ma for almandine garnet, staurolite, and garnet-staurolite (combined) in an Early Proterozoic metapelite from the Black Hills collisional orogen, South Dakota. PbSL-derived 208Pb/206Pb (Th/U) trends reveal that staurolite contains inclusions of both cogenetic monazite (ca. 1760 [+ or -] 7 Ma) and older detrital zircon ([greater than or equal to]2040 Ma), whereas coexisting garnet contains only the zircon. These results and petrologic data indicate prograde sequential growth of garnet, monazite, and staurolite during tectonic burial at temperatures between [approximately]400 [degrees] C and [approximately]550 [degrees] C (i.e., well below Pb closure temperatures). We interpret the 1760 [+ or -] 7 Ma date as a maximum age for Wyoming-Superior continental collision in the Black Hills, which apparently postdated the 1800-1900 Ma Trans-Hudson orogeny in Canada, but was nearly synchronous with north-directed accretion of the Central Plains orogen in southeastern Wyoming.

The structures of synthetic [Mn.sub.3][Al.sub.2][Si.sub.3][O.sub.12] spessartine and [Ca.sub.3][Al.sub.2][Si.sub.3][O.sub.12] grossular garnet have been refined using single-crystal X-ray diffraction methods at 100 K, 293 K, and 500-550 K. The divalent X-site cations, located in large dodecahedral sites, show measurable anisotropic dynamic disorder in contrast to the rigid vibrational behavior of the SiO4 tetrahedra and Al[O.sub.6] octahedra. The amplitudes of vibration of [Mn.sup.2+] in spessartine are similar to those of [Fe.sup.2+] of almandine, in the plane of the longer X-O(4) bonds, and both are about twice that of [Ca.sup.2+] in grossular, despite the lighter mass of the latter. Heat capacities measured between 300 and 1000 K on synthetic polycrystalline spessartine and two natural nearly end-member spessartine crystals are similar to those of almandine. In addition, the IR active modes of spessartine at low frequencies are very similar to those of almandine suggesting that their heat capacities are also similar at lower temperatures. The low-energy phonon spectra of pyrope and grossular are probably considerably distinct from the two transition metal-containing garnets as suggested by their different low frequency IR active modes, reflecting the different bonding properties for Mg and Ca in garnet. The large pressure-temperature stability field of spessartine, relative to the other aluminosilicate garnets, does not appear to be due to any sort of intrinsic entropy stabilization.

Single-crystal 71 Ga nuclear magnetic resonance analysis of the garnet Y3Ga5O12(YGG) led to the determination of the first chemical shielding tensors. Analysis of the quadrupole coupling constants of the two crystallographic gallium sites in YGG indicated the presence of tetrahderally and octahedrally coordinated gallium sites with an axial symmetry. Furthermore, the compound exhibited two distinct crystallographically nonequivalent gallium sites.

The ages of three samples of Sm-Nd garnet from peridotites, pyroxenites and high-pressure granulites from Lower Austria were analyzed to determine the origin of the high-temperature garnet peridotite massifs from Lower Austria. Analysis of the Sm-Nd garnet indicated the fast cooling of the granulites and peridotites during the upper Visean. Furthermore, the periodite massifs underwent near-isothermal decompression during extension in a late Devonian marginal basin.

Reversed phase-equilibrium data, collected over the P-T range 12-20 kbar at 850-1100 [degrees] C, define the solubility of [Al.sub.2][O.sub.3] in ferrosilite (Fs) in equilibrium with almandine garnet (Alm). The new data indicate significantly lower [Al.sub.2][O.sub.3] solubility in Opx in the Fe-bearing system compared with the Mg-system and extrapolate well to merge with the higher pressure bracket of Kawasaki and Matsui (1983). Orthopyroxene-garnet thermometry in crustal rocks, based on the equilibrium studied here (Alm= 3 Fs + [Al.sub.2][O.sub.3]), is considerably more robust than previous calibrations based on the equivalent equilibrium in the Mg system. Results for several granulite terrains show that Al-Opx temperatures based on the new experimental data are generally higher than results based on Fe-Mg exchange thermometry, consistent with suggestions of previous workers. For many samples, the difference in apparent closure temperature between these equilibria (generally 50-130 [degrees] C) is within the combined uncertainty of their calibration ([approximately]75 [degrees] C) and is not as extreme as differences calculated on the basis of Harley's (1984) unreversed experimental data (Begin and Pattison 1994). The lack of sensitivity of this new thermometer to late Fe-Mg exchange makes it a powerful tool for deciphering near-peak P-T conditions for high-grade rocks.

At present the most widely used models of thermodynamic mixing properties of garnet are those that are optimized to fit the experimental phase-equilibrium data along various binary garnet joins using the method of mathematical programming. We conducted a thorough weighted regression analysis of the available volumetric, calorimetric, and phase-equilibrium data. This resulted in a much improved set of a-X relationships with the following Margules parameters (in joules, kelvins, bars). [Mathematical Expression Omitted], [Mathematical Expression Omitted], [Mathematical Expression Omitted], [Mathematical Expression Omitted], [Mathematical Expression Omitted], [Mathematical Expression Omitted] (all pfu containing 12 O atoms). The robust regression analyses allowed us to obtain the uncertainties associated with each of the parameters, quantities not available by mathematical programming analysis of phase-equilibrium data. Uncertainty estimates are essential for rigorous quantification of errors associated with mineralogic thermobarometers. From the probabilistic model developed here, we present detailed formulations for ascertaining errors of activity coefficients of the end-member components in any garnet solid solution. This provides an opportunity to evaluate the uncertainties in mineralogic thermobarometers involving garnets of compositions dissimilar to those used in the experiments on which the garnet mixing models are based. The results of this study elicit several significant points. The analysis strongly suggests that there is an excess entropy of mixing along the Fe-Mg join because Fe-Mg mixing in garnet is substantially nonideal. The excess entropy of mixing along the Ca-Mg and Ca-Fe joins is asymmetric and therefore assumption of a large symmetric entropy parameter or no entropy along these joins is an oversimplification. Large uncertainty in the mixing properties exists for Ca-rich garnets. There should be a Ca-Mg-Fe ternary interaction parameter (7110 J/mol).

X-ray composition maps of Sc, Y, P, and Cr in garnet have been measured using the electron microprobe. Zoning is discontinuous and is correlated with fluid-absent crustal melting reactions. The magnitude of the discontinuity is a function of the amount of muscovite that melts, and the partition coefficients of solids versus melt. Trace element zoning is apparently not modified by diffusive processes, so the zoning can be used to monitor the extent of melting and to examine kinetics of melting and the participation of accessory phases in crustal anatexis.

Regional structural interpretation, thermobarometry, and Gibbs-method calculations were combined to determine an approximate P-T path for kyanite-grade metapelitic rocks of the Baltimore Gneiss terrane. Garnet growth along the estimated P-T path was modeled using the computer program DiffGibbs. Models were evaluated by comparing calculated profiles of garnet components with measured profiles of a large garnet rich in plagioclase inclusions in a kyanite-bearing sample from the area. Modifications in the P-T path constrained by changes in garnet components along critical portions of the profile resulted in a very satisfactory match between measured and model-garnet profiles. In the most successful model, garnet growth began at 550 [degrees] C and 6500 bars in the assemblage chlorite + biotite + garnet + muscovite + plagioclase + quartz with increasing temperature and with decreasing and then constant pressure. At 570 [degrees] C and 5724 bars, staurolite was added to the assemblage and some garnet was consumed. In the model, garnet growth resumed after chlorite was consumed along a path having a substantial increase in pressure. The region near the rim with low measured grossular content was reproduced by cessation of garnet growth as temperature increased at constant pressure to a maximum of 615 [degrees] C at 6474 bars. After kyanite was added to the assemblage, grossular increased to the constant rim value only if temperature decreased and pressure increased. Cooling and decompression affected garnet zoning only very near the rim. Measured compositions of the rims of plagioclase inclusions in the garnet are consistent with plagioclase compositions predicted by the model. This study demonstrates that complex garnet zoning can be modeled successfully, even when the reaction history includes episodes of garnet consumption, over a path that is consistent with geologic evidence. Details of garnet zoning provide surprisingly tight constraints on the P-T path.

The stability of Ca-containing garnets on the join Ca3Fe(super 3+)(sub 2)Si3O12-Fe(super 2+)(sub 3)Fe(super 3+)(sub 2)Si3O12 (andradite - 'skiagite') depends on the pressure at 1100 degree centigrade. In andradite the solubility of skiagite rises from 12 mol% at 1.7 GPa to 48 and 64 mol% at 6.0 and 7.0 GPa respectively. At 1100 degree centigrade synchronous existence of the equilibrium garnet solid solutions with spinel + clinopyroxene + quartz is observed. The pyroxenes have less Ca and are monoclinic above the pressure 7.0 GPa.

Si magic angle spinning nuclear magnetic resonance spectroscopy (Si MAS NMR) is an effective tool in unravelling the fine structure of dodecahedral units in synthetic pyrope-grossular garnet solid solutions. Si MAS NMR measurements were compared with IR spectroscopy. Si MAS NMR was able to produce up to 13 resonances in a 50-50 mixture of grossular and pyrope garnet, enabling the determination of first and second shell dodecahedral behavior. Si MAS NMR was able to determine the proportions of calcium and magnesium substitutions in each shell, and measure variations from random mixing.

Mg cation diffusion coefficient rates in natural pyrope garnet crystal were studied at temperatures ranging from 800-1000 degrees celsius. A thin layer technique with a ion microprobe was used in the study. Applications for results from diffusion rate experiments give clues to the timing of petrologic happenings such as the formation of garnet -bearing metamorphic rock.

Concentrations of H in pure separates of mantle-derived pyrope garnet, augite, and enstatite are found by first heating the specimens, extracting the hydrogen gas under vacuum, prior to using manometry. Infrared spectroscopy showed the prior presence of and subsequent removal of up to 86-100% of the intrinsic OH from the specimens. The values of integral specific absorption coefficients for the specimens required for accurate spectroscopic determination of the OH content of mantle garnets and pyroxenes to concentration levels of a few parts ppm are discussed.

The molecular structure of garnet in samples from orthogneiss metamorphic rock from eastern China were analyzed. Garnets in some pegmatites were also analyzed. The Al-rich garnets and the Fe-rich garnets were compared for their incorporation of the elements sodium (Na), ytterbium (Yb)and yttrium (Y). Results indicated that these elements were incorporated more readily at higher pressure in Al-rich garnets than in Fe-rich garnet.

The behavior of synthetic pyrope garnet molecules under a range of temperatures was studied using x-ray spectroscopy. Choice of the temperature range of 30-973 degrees kelvin, makes way for a detailed geometrical analysis of the polyhedral distortions in this range, the determination of the thermal expansion for bond distances, and the measurement of the atomic displacement parameters. Applications of various tensors used for analysing thermal effects are discussed.

Stable hysteresis characteristics of ferrimagnetic materials are critical to the RF performance of microwave ferrite toroidal phasers. Particularly troublesome are the magnetostrictive characteristics where the hysteresis properties are altered by stress. This paper presents the results of a study addressing [Mn.sup.+3] substitutions in garnets to improve the resultant magnetostrictive characteristics in order to achieve stress-insensitive performance in waveguide toroidal phasers.

Garnets from an early granulite event that have partially broken down to orthopyroxene-bearing assemblages during a second regional granulite-facies metamorphism were used to test the closure temperature of the Sm-Nd system in garnet. The garnets from Sostrene Island, Prydz Bay, Antarctica, retain a memory of both the physical conditions of their formation and their Sm-Nd age, notwithstanding an effective diameter for diffusion of only 1 mm. Because only minor isotopic resetting of these garnets has taken place during the overprinting granulite event, the closure temperature for the Sm-Nd system in garnet in the present case must be >700-750 [degrees] C. The results show that chronological constraints are essential for the unambiguous interpretation of reaction textures in terms of thermotectonic evolution. The garnet Sm-Nd dating method is particularly well suited for this purpose.

The oxygen barometer is exclusively used for garnet peridotite as suggested by the dependency of the oxidation state of Fe in mantle garnets on the oxygen fugacity. The oxygen fugacity is measured by two Pt-Fe sensors at the interface between a layer of garnet and a mixed olivine-orthopyroxene layer. A thin layer of garnet placed between layers of olivine and orthopyroxene is equilibrated at 1300 degree centigrade.

Energy Fe-end member reaction depicts a positive dp/dT slope and its position is related to the water content of the equilibrium cordierite. The anhydrous breakdown curve has been constructed based on the determination of H2O content. The P-T position of the anhydrous curve varies depending on the context of H2O, which has been maximized and experimentally verified to be unity.

Negative deviations which are characteristic of first-row transition metals and alloys of Pd and Pt are exhibited by platinum-manganese alloys at low mole fractions of manganese. This is revealed in the mixing properties of Mn-Mg garnet and Mn-Mg olivine at 1,300 degrees centigrade in Pt-Mn alloy solid solutions. The alloys are calibrated by considering the equilibrium between Pt and Mn at 1,300 degrees centigrade at fixed oxygen potentials.

Sm-Nd isotopic analysis of garnet reveals two amphibolite metamorphic periods with a difference of 16 million years. Sm-Nd comprises hornblende, garnet, plagioclase and apatite and reveals mineral growth at a metamorphic peak. The garnet contains inclusion-filled and inclusion-free fragments with three age periods.

Two garnet crystals and one staurolite mineral block obtained from a metapelitic schist, and another garnet mineral block from an amphibolite at the Cordillera Darwin metamorphic complex, Chile, South America, are microanalyzed to determine the oxygen isotope ratio. The complex garnet-biotite-staurolite-plagioclase-muscovite-quartz-ilmenite chlorite constitutes the crystals. The experimental investigation was performed at pressure-temperature conditions of 575 degrees Celsius and 7 kilobars. Laser extraction systems were used for the collection of oxygen isotopes. The core of the minerals had 3.5% more O-18 than the surfaces.

Cation ordering of Fddd or the symmetric reductions of optically anomalous garnets from Ia3D to I1 reveals that most of the vibrations are active in the infrared regions. This factor is also indicated by crystallographic investigations. The infrared reflectance spectra within 80 to 4000 lines per centimeter revealed the characteristic pattern of the isotropic and isometric garnets. The spectra were sometimes polarized. The crystallographic refining revealed the samples to be similar. The complete fundamental region appears to be resolved in the first single crystal infrared absorption spectrum.

A study on the prograde garnet zoning patterns of schists from central Vermont and western Connecticut is conducted. The results indicate that the compositional zoning patterns in both sites resulted from the metasomatic dissolution and reprecipitation of the preexisting garnets. This is shown by the chemical analyses of fluid and mineral inclusions in the garnets and its associated abrupt compositional changes. It is also shown that the mineral assemblages produced by the metasomatic processes are more hydrous than the original prograde composition.

Chemically zoned skarn garnets provide a continuous record of hydrothermal processes in lower Paleozoic sedimentary rocks within the contact aureole around the Drammen granite in the Oslo rift, southern Norway. Major and trace element zonation profiles, the latter obtained using a scanning high-resolution proton microprobe, reveal early infiltration-controlled growth of relatively grossular rich garnets, the major and trace elements compositions being buffered by local mineral dissolution. Subsequent rapid, epitaxial growth of andradite-rich garnet on grossular-rich cores marks the onset of vigorous and focused fluid flow along high-permeability zones. During this later stage, the hydrothermal fluid composition was to a large extent externally controlled, and the andradite precipitating from these fluids was characterized by high As and W contents. The zonation patterns of the andradite-rich garnets record at least lest five intermittent growth periods, with rapid andradite precipitation from fluid batches with high (Mathematical Expression Omitted) and progressively decreasing As and W contents. Thin layers, poor in Fe, As, and W, but relatively high in Al and Mn, represent periods of slow growth rates between the major pulses of hydrothermal fluids. The marked rimward decrease in the As and W contents of the garnets may reflect influx of meteoric waters or exhaustion of these elements in the hydrothermal fluid reservoir caused by boiling-controlled distillation processes at depth.

An analysis of garnet compositional zonation in peridotite xenoliths is presented. The analysis tests garnets from 42 samples taken from southern Africa. The samples were examined by using electron probe to determine possible correlations between compositional zonation and rock history. It is shown that xenoliths maybe classified into six groups. The groups are based on element enrichment and zonation levels.

The Variscan orogen in the Moldanubian Zone of the Bohemian Massif, Czechoslovakia, featured high-temperature Group A eclogites that originated from high-pressure crystallization and massive formation of garnet and clinopyroxene obtained from very deep basaltic sources. Chemical and isotopic analyses such as Rb-Sr and Sm-Nd were used to determine eclogite equilibration conditions. Results showed that the eclogites exhibited average minimum temperature and pressure indicative of a heterogenous mantle source which formed garnets and clinopyroxenes showing retrograde compositional natures.