Garnet Components Research Articles

Garnet + liquid equilibrium

New experiments were performed to determine saturation conditions for garnet and silicate liquid. Starting compositions were natural basalt powders ranging from komatiite to nephelinite, which were partially melted at pressures between 25 and 100 kbar. Rounded grains of natural pyrope or grossular were added to some experiments to induce garnet saturation, and to aid the segregation of liquid pools for microprobe analysis. Simple expressions describing K eq as a function of P, T and liquid composition were calibrated by linear least squares analysis of the data from this, and other, studies. Since garnets do not often occur as phenocrysts, equations were designed to predict garnet compositions when P, T and a silicate liquid composition are given. The regression data have a pressure range of 20–270 kbar, and compositions as diverse as nephelinite and komatiite. These models should thus apply to a broad range of geological problems. The majorite component in garnet was found to increase with increasing P, but compositional effects are also important. A garnet saturation surface applied to liquids with chondritic compositions shows that such liquids crystallize garnet with Mj contents of 0.27–0.42 at 200 kbar. Models of Earth differentiation thus need to account not only for fractionation of majorite, but also for Fe-, Ca-, Na- and Ti-bearing garnet components, which occur in non-trivial quantities at high pressure. Since many models of igneous petrogenesis rely on mineral-melt partition coefficients for the minor elements Na, Ti, and Cr, partition coefficients for these elements were also examined. The K d gar/liq for Na was found to be P-sensitive; Na contents of basalts may thus potentially yield information regarding depths of partial melting.

An Appraisal of Endmember Energy and Mixing Properties of Rare Earth Garnets

The thermodynamic properties of rare earth aluminum (REE 3Al 5O 12), iron (REE 3Fe 5O 12), and gallium (REE 3Ga 5O 12) garnets are assessed by means of critical evaluation of the existing experimental data and thermodynamic treatment of their vibrational, static and volumetric properties. The mixing properties of the various REE garnet components are calculated from these endmember thermodynamic data and the interionic static potential model developed earlier for major silicate garnet components. These results permit evaluation of the solubility behavior for REE in natural phases. The results suggest that the mixing of REE garnet components at trace level (i.e., below about 10 2 ppm) with major silicate garnet components is virtually ideal, but strong positive deviations, associated with generalized polyhedral distortion of the host phase at higher concentration level of the solute, prevent bulk REE solubility from exceeding a few hundreds ppm. The computed rare earth element fractionations between garnet and supercritical fluid at various P, T conditions put in evidence the main control operated by the intrinsic energy properties of REE-garnet end members in determining the light REE/heavy REE (LREE/HREE) fractionation observed in natural garnet specimens.

Pressure-temperature-reaction history of metapelitic rocks from the Maryland Piedmont on the basis of correlated garnet zoning and plagioclase-inclusion composition

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°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 °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°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.

Paragonite eclogites from Dabie Shan, China: re‐equilibration during exhumation?

Abstract Paragonite in textural equilibrium with garnet, omphacite and kyanite is found in two eclogites in the ultrahigh‐pressure metamorphic terrane in Dabie Shan, China. Equilibrium reactions between paragonite, omphacite and kyanite indicate a pressure of about 19 kbar at c. 700° C. However, one of the paragonite eclogites also contains clear quartz pseudomorphs after coesite as inclusions in garnet, suggesting minimum pressures of 27 kbar at the same temperature. The disparate pressure estimates from the same rock suggest that the matrix minerals in the ultrahigh‐pressure eclogites have recrystallized at lower pressures and do not represent the peak ultrahigh‐pressure assemblages. This hypothesis is tested by calibrating a garnet + zoisite/clinozoisite + kyanite + quartz/coesite geobarometer and applying it to the appropriate eclogite facies rocks from ultrahigh‐ and high‐pressure terranes. These four minerals coexist from 10 to 60 kbar and in this wide pressure range the grossular content of garnet reflects the equilibrium pressure on the basis of the reaction zoisite/clinozoisite = grossular + kyanite + quartz/coesite + H2O. The results of the geobarometer agree well with independent pressure estimates from eclogites from other orogenic belts. For the paragonite eclogites in Dabie Shan the geobarometer indicates pressures in the quartz stability field, confirming that the former coesite‐bearing paragonite‐eclogite has re‐equilibrated at lower pressures. On the other hand, garnets from other coesite‐bearing but paragonite‐free kyanite‐zoisite eclogites show a very wide variation in grossular content, corresponding to a pressure variation from coesite into the quartz field. This wide variation, partly due to a rimward decrease in grossular component in garnet, is caused by partial equilibration of the mineral assemblage during the exhumation.

Effect of melt composition on Bi incorporation in iron garnets

The saturation temperature, the growth rate and the crystal composition of rare-earth iron garnet films grown from a PbOBi 2O 3B 2O 3 flux were investigated. The concentrations of the garnet component oxides, the proportions of the flux oxides, B 2O 3 and Bi 2O 3, and the ratio Fe 2O 3/RE 2O 3 in the melt were systematically varied to determine their effect. The melts generally obey Van Erk's theory of solubility and growth rate. However, an increase of either the B or Bi concentration in the melt linearly decreases the rare earth activity, which results in an increase in both the garnet solubility and the effective segregation coefficient of Bi into growing garnet films. Using the rare earth activity calculated from solubility data, a Bi segregation coefficient of 0.0010 is observed that is independent of melt composition and growth temperature.

Formula omitted] ratio and composition of the Earth's upper mantle

Undifferentiated meteorites (chondrites) have the same relative abundances of refractory lithophile elements (Ca, Al, Ti, Sc, REE, etc.), despite variable absolute concentrations. The reasonable assumption of chondritic ratios among refractory elements in the bulk Earth is used to constrain the chemical composition of the upper mantle in the following way: Correlations of the compatible refractory elements Ca, Al, Ti, Sc and Yb with MgO are worldwide very similar in suites of spinel-lherzolite xenoliths from basaltic rocks. Such suites represent upper mantle material depleted to differing degrees by extraction of partial melts. From these refractory elements vs. MgO correlations, ratios of pairs of refractory elements were calculated at various MgO contents. Chondritic Al Ti and Sc Ti ratios were only obtained for MgO contents below 36%. A chrondritic Sc Yb ratio requires an MgO content above 35%. We therefore accept 35.5% as the most reasonable MgO content of undepleted upper mantle. This MgO content is slightly below the spinel-lherzolite with the lowest measured MgO content (36.22%). The corresponding Al 2O 3 content of 4.75% is higher than in previous estimates of upper mantle composition. The concentrations of other elements were obtained from similar correlations at a MgO content of 35.5%. The resulting present upper mantle composition is enriched in refractory elements by a factor of 1.49 relative to Si and Cl and by a factor of 1.12 for Mg relative to Si and Cl. These enrichments are in the same range as those for the Vigarano type carbonaceous chondrites. The Mg/Mg + Fe ratio of 89 is slightly lower than previous estimates. The Ca Al ratio in spinel lherzolite suites is, however, uniformly higher worldwide than the chondritic ratio by about 15%. Orogenic peridotites as well as komatiites appear to have similar non-chondritic Ca Al ratios. It is therefore suggested that this non-chondritic Ca Al ratio is a characteristic of the upper mantle, possibly since the Archean. A minor fractionation of about 4% of garnet in an early, global melting event (deep magma ocean?) is presented as the most likely cause for the high Ca Al - ratio . In this case the addition of 4% of such a garnet component to the undepleted present upper mantle would be required to obtain the composition of the primordial upper mantle. The Ca Al - ratio of this primordial mantle would be 15% higher than that of the undepleted present upper mantle, resulting in an enrichment of refractory elements of 1.70 ( Al Si relative to Cl) for the primordial upper mantle.

Thermodynamics of minerals of variable composition: Andradite-grossularite and pistacite-clinozoisite solid solutions

Exchange-mineral equilibria with Al and Fe3+ aqueous chloride solutions (aq.), Andr + AlCl aq 3 = FeCl aq 3 + Gros, (1) Psc + AlCl aq 3 = FeCl aq 3 + Czo, (2) were studied under the following experimental conditions: 500°;C and 580°;C and 1 and 2 kbar, respectively, with an overall concentration of metals in the aqueous solutions of about 0.5 M and pH 3. The mixing functions of the components in garnet and epidote were calculated from the experimental data. Thermodynamic treatment of experimental evidence for reaction (1) led to the conclusion that, within the accuracy of experiment, garnet in the andradite-grossularite series was an ideal solid solution. However, epidote solid solution markedly departed from the ideal, as was shown by concentration and pressure-temperature (PT) dependencies of Gibbs's molar excess energies and by mixing-volume concentration dependence.

High-pressure phase equilibria in a garnet lherzolite, with special reference to Mg 2+Fe 2+ partitioning among constituent minerals

Phase equilibria in a natural garnet lherzolite nodule (PHN 1611) from Lesotho kimberlite and its chemical analogue have been studied in the pressure range 45–205 kbar and in the temperature range 1050–1200°C. Partition of elements, particularly Mg 2+Fe 2+, among coexisting minerals at varying pressures has also been examined. High-pressure transformations of olivine(α) to spinel(γ) through modified spinel(β) were confirmed in the garnet lherzolite. The transformation behavior is quite consistent with the information previously accumulated for the simple system Mg 2SiO 4Fe 2SiO 4. At pressures of 50–150 kbar, a continuous increase in the solid solubility of the pyroxene component in garnet was demonstrated in the lherzolite system by means of microprobe analyses. At 45–75 kbar and 1200°C, the Fe 2+/(Mg + Fe 2+) value becomes greater in the ascending order orthopyroxene, Ca-rich clinopyroxene, olivine and garnet. At 144–146 kbar and 1200°C, garnet exhibits the highest Fe 2+/(Mg + Fe 2+) value; modified spinel(β) and Ca-poor clinopyroxene follow it. When the modified spinel(β)-spinel(γ) transformation occurred, a higher concentration of Fe 2+ was found in spinel(γ) rather than in garnet. As a result of the change in the Mg 2+Fe 2+ partition relation among coexisting minerals, an increase of about 1% in the Fe 2SiO 4 component in (Mg,Fe) 2SiO 4 modified spinel and spinel was observed compared with olivine. These experimental results strongly suggest that the olivine(α)-modified spinel(β) transformation is responsible for the seismic discontinuity at depths of 380–410 km in the mantle. They also support the idea that the minor seismic discontinuity around 520 km is due to the superposition effect of two types of phase transformation, i.e. the modified spinel(β)-spinel(γ) transformation and the pyroxene-garnet transformation. Mineral assemblages in the upper mantle and the upper half of the transition zone are given as a function of depth for the following regions: 100–150, 150–380, 380–410, 410–500, 500–600 and 600–650 km.

A solubility model for rare-earth iron garnets in a PbO/ B2O3 solution

It is shown that the solubility of non-substituted rare-earth iron garnets in a PbO/B 2O 3 solution can be well described if it is assumed that the different garnet components are dissolved separately. Two descriptions for the liquid are used, a molecular fluid consisting of molecules of PbO, B 2 O 3, Y 2 O 3 and Fe 2 O 3 and an ionic fluid consisting of Pb 2+, B 3+, Y 3+, F e 3+ and O 2- (Temkin model). The latter description gives good agreement between the measured heat of solution and the enthaply of solution obtained from solubility data. If it is assumed that both the liquid and the solid are ideal mixtures of the end-member components, the solubility of substituted garnets like La xY 3-xGa yFe 5-yO 12 can also be described. From experimental data it may be concluded that the degree of La substitution x is influenced by the degree of Ga substitution y, and vice versa. In addition a relation is obtained for the equilibrium distribution of Ga on tetrahedral and octahedral sites.

The state of mantle minerals

The static pressure-volume, high-pressure X-ray, shock, and ultrasonic equation of state data are used to describe the seismic properties in terms of the mineralogy of the upper mantle, the spinel-zone, and the post-spinel or lower mantle. For the upper mantle, elastic properties have been measured for olivines, pyroxenes, and garnets. The effect of pressure, temperature and the Fe/Mg ratio on elasticity is less well known for pyroxene and garnet than for olivine. In the spinel zone, ~ 400- ~ 650 km depth, the major minerals include β- and γ-(Mg,Fe) 2SiO 4 and a complex garnet, largely composed of aluminum-silicate garnet and pyroxene components. The zero-pressure bulk moduli of these phases lie in the range 1.8–2.1 Mbar. Although the high-pressure X-ray measurements of the bulk moduli of the Fe-rich γ- or spinel phase (ringwoodite) are reported, the shear properties of the silicate spinels are unmeasured, as are all the properties of the distorted spinel, β-phase. The bulk modulus of the complex-garnet phase is known approximately from shock-wave data for pyroxene, but since standard specific volumes are unknown, reduction of these data to adiabats is uncertain. The major minerals of the lower, or post-spinel, mantle are thought to be (Mg,Fe) 2SiO 4 in the close-packed Sr 2PbO 4-structure, and the pyroxene and garnet component, initially in the close-packed ilmenite structure, and at greater depths probably transforming to the perovskite structure. Only shock-wave data and results inferred from the elasticity of the close-packed oxides are available in this regime. Except for garnet, the zero-pressure densities of the shock-induced high-pressure phases are not known; this difficulty, and the lack of data or a theory for the Grüneisen parameter make reduction of these data difficult.

Compositions of magmas formed by partial zone melting of the Earth's upper mantle

The upper mantle of the earth is probably composed of peridotites consisting mainly of olivine, orthopyroxene, clinopyroxene, and garnet or spinel similar to those found as inclusions in kimberlite and basaltic rocks. The compositions of the magmas produced by the partial melting of the mantle peridotites are closely dependent on the position of the isobaric invariant point where olivine and orthopyroxene are involved in systems including components of minerals of the mantle peridotites. The positions of such invariant points have been investigated in the systems Mg2SiO4-SiO2-X, where X represents CaMgSiO4, CaAl2O4, NaAlSiO4, and MgAl2O4, in the pressure range 7 to 40 kb. It was found that the liquidus boundary between forsterite and enstatite solid solution and the invariant point where forsterite and enstatite solid solution are involved shift from silica-rich compositions toward silica-poor compositions with increasing pressure up to at least 40 kb in all the systems examined. The experimental results suggest that the compositions of the magmas produced by the partial melting of the mantle peridotites would become more silica-deficient with increasing pressure. It is also suggested that magmas consisting essentially of pyroxene and garnet components are formed by partial melting of the mantle peridotites at pressures less than 30 kb. Magmas formed in the upper mantle by partial melting would ascend by the mechanism of ‘partial zone melting’ in the levels where temperature is close to the beginning of melting of the mantle peridotites. Compositional change in the magmas that ascend by the partial zone melting is dependent on the isobaric invariant point where olivine and orthopyroxene are involved and can be estimated on the basis of the present experiments.

Effect of jadeite component on the paragenesis of eclogitic rocks

On the basis of an analysis of the paragenetic relationships in a simple hypothetical system, a general relationship between the jadeite component in omphacite and the grossular component in garnet in eclogitic rocks, is presented. It is concluded that in eclogitic rocks an increasing amount of jadeite in omphacite is accompanied by a decrease in the grossular content of garnet, whereas in grosspydite the relationship is reversed. This relationship is applied to the interpretation of the paragenesis of natural eclogitic rocks.

Der petrogenetische Werdegang der Klinopyroxene in den tertiären Vulkaniten der Hocheifel

The course of crystallization of basalt forming clinopyroxenes in the olivine-basalt-hawaiite-mugearite-trachyte-and basanitoid (= basanite)-series from the Hocheifel area (Western Germany) has been presented by the author in two earlier papers (Hucken-holz, 1965 a, 1965 b). The present paper deals with the evolution of clinopyroxenes from the ankaramites in the same area. The ankaramites are richer in olivines and clinopyroxenes than the normal olivine basalts and may be a product of accumulation of olivines and clinopyroxenes in an olivine basalt magma. The first clinopyroxene formed in the ankaramites is a greenish chromian salite Ca47.1 Mg40.9 Fe12.0 which is preserved in the cores of the phenocrysts. Strongly zoned brown titansalite Ca45 Mg42Fe13 surrounds the chromian salite (HF 5) or appears in independent microphenocrysts (HF53). The groundmass clinopyroxene is titansalite Ca45Mg37Fe18 with a small optic axial angle. Phenocrysts of nickel-rich olivine Fe12–14 and chrome spinell were formed together with chromium clinopyroxene. The same minerals together with orthopyroxenes have been observed in the olivine basalts, hawaiites and basanitoids (= basanites). This paragenesis indicates high pressure and high temperature in the alkali basalts of the Hocheifel during an initial stage of magmatic evolution. With the decrease of pressure, mineral reaction occurred with the alkali basalt melt. The high pressure clinopyroxenes of the ankaramites and basanitoids (= basanites) were changed to a larger extent than the clinopyroxenes of the olivine basalts and hawaiites or were completely destroyed (HF 53) because they are in contact with the hot magma for a longer period of time. This favours the reaction between the solids and the melt, and the high pressure garnet and/or enstatite components in the clinopyroxenes are substituted by Ca-Tschermak’s and titanaugite molecules.