Garnet Content Research Articles

Ni in chrome pyrope garnets: a new geothermometer

Proton microprobe analyses of the minerals in garnet-peridotite xenoliths from kimberlites show that the partitioning of Ni between chrome pyrope garnet and olivine is strongly temperature(T)-dependent. The range of Ni contents in olivines is small relative to that in the analyzed garnets; a geothermometer therefore can be derived, based only on the Ni content of garnet. This allows estimation of T for single Cr-pyrope grains, such as the inclusions in diamonds, if these can be assumed to have equilibrated with olivine.

The cancrinite-syenite/carbonatite complex of Lueshe, Kivu/NE-Zaire: petrographic and geochemical studies and its economic significance

The Luesche cancrinite-syenite/carbonatite complex is located in NE-Zaire about 50 km SSW of Lake Edward. The alkaline rocks have intruded metamorphic rocks of Burundian age (1.100 m.y.). The country rocks are quartzites and mica schists bearing chlorite, garnet, Kyanite and staurolite, which indicates P-T conditions of some 4 kbar and 430–500°C, maximum 6 kbar and 620°C. The K/Ar age of the biotites from the carbonatite is found to be 516 m.y. The core of the alkaline complex is composed of cancrinite-syenites which vary from place to place with respect to the contents of garnet, amphibole, aegirine, sodalite and cancrinite. Significant variations of the pyrochlore content are also observed. The core is surrounded by fine grained calcitic carbonatite (alvikite) which often shows a banded texture and grades into coarse grained calcitic carbonatite (søvite). In the SE of the søvite ring there is a separate dolomitic carbonatite body (beforsite) while at the contacts between the alkaline rocks and the country rocks a wide range of low to high grade fenites enriched in Na or K occurs. Within the alkaline complex the highest niobium values are found associated with metasomatic rocks of the contact zones such as carbosyenites, pyroxentic rocks and fenites. The main niobium mineral is a Ca, Na-pyrochlore enriched in Ti and/or Sr. The geochemical distribution patterns of the major elements as well as of the trace elements, especially the REE, suggests that the carbonatites are of primary magmatic origin derived from mantle material which was affected by later remobilizations, most likely as a result of postmagmatic hydrothermal solutions. Regarding the cancrinite-syenites, there is no evidence for a comagmatic formation with the carbonatites. They are rather of subcrustal origin; formed in the course of various metasomatic processes where the alkalic carbonate melts or volatile carbonate phases interacted with the metamorphic country rocks to form rheomorphic syenites which subsequently intruded. From an economic point of view only the lateritic zone derived from weathering of the carbo-syenites, pyroxenitic rocks and fenites is considered as ore due to the concentration of pyrochlore in the course of weathering. Ore grades vary from 1 % to 7 % Nb 2O 5. The ore is composed of clay minerals, iron oxides and phosphates. The weathering profile may be up to 150 m deep, whereby a gradual change with depth from crandallitic ore to crandallitic-apatitic ore and to apatitic ore in the bottom section occurs. Ore reserves amount to some 8 million tons with an average of 2.5 % Nb 2O 5. By these figures Lueshe is one of the few high grade niobium deposits in the world.

Perpotassic granulites from southern Bohemia

Garnetiferous, perpotassic granulites associated with voluminous, felsic calc-alkaline garnet-kyanite granulites in the Blanský les massif are described as a new rock type. A K2O content near 13 wt%, Zr content ranging from 17 to 5877 ppm, primary Ca-Mg-Fe garnet (pyrope up to 33 mol %), and the absence of primary plagioclase and quartz characterize the new type of granulite. Calcium is incorporated only in garnet and apatite. Owing to the content of garnet the bulk syenitic compositions are metaaluminous to peraluminous. The perpotassic granulites are accompanied by subordinated melanocratic perpotassic granulite and alkali-feldspar melanocratic granulite containing up to 20 vol % quartz. These rock types also contain highly variable amounts of zircon. High-pressure, noneutectic partial melting of crustal rocks and crystallization under upper mantle conditions is suggested as the main process in generation of perpotassic granulites. A polyphase, ductile to brittle deformation and recrystallization in pyroxene granulite and later in amphibolite facies accompanied obduction of the granulite massif which carries numerous enclaves of pyrope peridotite. Euhedral equant zircon crystals up to 4 mm long enclosed in large garnet crystals and in the alkali feldspar matrix of perpotassic granulite represent a uniform, high-temperature population with a nearly concordant U-Pb system (Aftalion et al. 1989). The Hercynian age of this zircon indicates the role of the Hercynian anatexis of crustal rocks under upper mantle conditions in generation of granulites in the Moldanubian zone of the Bohemian massif.

Magnetite exsolution in almandine garnet

AbstractThree almandine-rich metamorphic garnets have been studied by analytical electron microscopy and electron microprobe analysis. Electron microprobe analyses with total Fe calculated as Fe2+ show that there are no significant departures from stoichiometry due to the presence of Fe3+ in any of the garnets studied. However, in the transmission electron microscope (TEM) all the garnets were found to contain myriad spherical, iron-rich particles up to 400 Å in diameter. Microdiffraction techniques have revealed that the particles are a cubic spinel phase, consistent with magnetite. There is no crystallographic relationship between the host garnet and the particles, a rare situation for exsolution processes. The presence of such particles is interpreted in terms of the exsolution of magnetite from almandine garnet during cooling. This can apparently occur at temperatures below 55°C. The size of the particles is a qualitative indicator of the cooling rate of the rock, but is also dependent on the original Fe3+ content of the host garnet.

Phosphoran pyroxene and olivine in silicate inclusions in natural iron-carbon alloy, Disko Island, Greenland

A survey of literature data on the P contents of olivine, pyroxene, and garnet in mantle xenoliths and the bulk P contents of mantle xenoliths shows that available data is ambiguous as to whether mantle P is primarily contained in silicates or in phosphates, even though phosphates are rarely observed in mantle xenoliths. Garnet probably contains more of the mantle's P than olivine or pyroxene. Bulk P contents of mantle xenoliths vary significantly. Estimation of the primitive mantle P content from xenoliths requires interpretation of the partial melting and contamination histories of the xenoliths.

Chemical weathering of Mn-garnets under lateritic conditions in northwest Ivory Coast (West Africa)

Lateritic profiles developed on Mn 2+-bearing metamorphic rocks in the Ziemougoula area (northwest Ivory Coast) permit the observation of chemical weathering of garnets, essentially spessartites. Two main stages are found in the process of weathering. In the lower part of profiles, spessartites alter principally to birnessite and the Al content of the parent garnet is leached out of the zone. Higher in the profile, garnets alter directly to lithiophorite; in this geochemical environment Al is no longer mobile. In such a layer, the earlier formed birnessite is transformed into nsutite along with minor cryptomelane. These latter minerals can locally evolve into ramsdellite or pyrolusite. The associated nsutite and lithiophorite are the principal phases of the hard manganese crust capping the hillrocks of the lateritic landscape of the Ziemougoula area.

Garnet and associated minerals in the southern margin of the Menderes Massif, southwest Turkey

AbstractAssemblages with muscovite + quartz show a regular increase in grade from the Chlorite Zone at the base of the Lycian Nappe Complex to the Garnet Zone within the structurally underlying Menderes Massif. Biotite enters before garnet, which precedes oligoclase. Garnet-bearing assemblages in pelites are compared with those in re-equilibrated quartzofeldspathic gneisses, where garnet is unusually calcic (in one case approaching Gross50Alm50). Local retrograde effects are noted but no evidence is found for a polymetamorphic record in the mineral compositions. Garnet zoning, with Mn decreasing outwards, is interpreted as growth zoning; Ca decreases outwards in pelite garnets but shows the reverse effect in the gneisses. Chloritoid is common but rarely coexists with biotite, and garnet + chlorite + paragonite is found rather than chloritoid + albite. Garnet-biotite geothermometry, corrected for the effect of Ca in garnets with up to 29 mole % grossular, indicates temperatures of 530±5O°C near the garnet isograd. As in other areas, an attempt at muscovite-paragonite geothermometry gives an anomalous result. Metamorphic pressure isconsidered in the light of (i) Mn/Fe partition between garnet and biotite, (ii) Ca content of garnet coexisting with plagioclase + muscovite + biotite, (iii) Na in actinolite coexisting with albite + chlorite + magnetite, and (iv) celadonite content of muscovite which, however, shows variation due to disequilibrium within a specimen and does not provide an accurate geobarometer. Comparisons with published studies indicate a strong similarity to the Barrovian Dalradian of Scotland and lead to a tentative pressure estimate of approximately 5 kbar.

Elasticity of single‐crystal orthoferrosilite

The elastic moduli of a synthetic single crystal of pure FeSiO3, orthoferrosilite, have been measured at 1 bar and 20°C by Brillouin spectroscopy. The moduli are (in megabars) c11 = 1.98, c22 = 1.36, c33 = 1.75, c44 = 0.59, c55 = 0.58, c66 = 0.49, c12 = 0.84, c13 = 0.72, c23 = 0.55. A comparison of the pyroxene crystal structure with elasticity data shows that distortion of the tetrahedral chains parallel to c is related to c33 Straight or extended chains increase c33, and highly kinked chains weaken the structure. However, the ferrosilite data show that beyond a certain degree of distortion the tetrahedral chains are essentially passive. Previous models of pyroxene elasticity have indicated that c11 should be most sensitive to composition. This is supported by the current study and is reflected in the elasticity data via a positive linear correlation of c11 with the size of the M1 octahedron, as measured by the average M1‐0 distance. The average longitudinal modulus 11 ((c11 + c22 + c33)/3) varies linearly with composition in the system orthoenstatite‐orthoferrosilite, whereas the average shear moduli ¯44 and ¯12 are constant for Fe contents of at least 20%. By comparison, data for the solid solution forsterite‐fayalite are consistent with a linear variation of modulus versus composition between the end‐members for all three average moduli. We suggest that this difference between orthopyroxene and olivine is due to a strong preference of Fe2+ for the M2 octahedral site in pyroxene. The isotropic shear modulus μ is 30% lower for ferrosilite than enstatite. Despite a similar contrast in end‐member values of μ, the olivine data are consistent with a linear variation of μ between Fo100 and Fa100. At high temperatures, Fe and Mg in orthopyroxene will be randomly distributed, and a linear variation of μ with molar content of Fe is expected. A comparison of laboratory elasticity data with observed mantle properties shows that Vp, Vs, and ρ of the upper mantle are satisfied by pure olivine. However, the data strongly suggest that without the coexistence of a garnet phase, no more than 10% orthopyroxene may be present in a model mantle assemblage. Ca‐clinopyroxene increases the upper limit on mantle garnet content but does not greatly affect constraints on the quantity of orthopyroxene in the upper mantle.

Petrogenesis of Eclogite inclusions in the Moses Rock dyke, Utah, U.S.A.

A detailed electron microprobe study of the mineralogy of fifteen eclogites from the Moses Rock kimberlitic dyke, Utah, has demonstrated complexity in compositional zoning of minerals. Most of the eclogites examined are of the almandine-jadeite type and zonal and irregular variation in grossular content of garnet and acmite, jadeite and diopside-hedenbergite content of pyroxene produce large uncertainties in temperature estimates based on Fe/Mg partitioning between garnet and clinopyroxene. Zoning pattems of increasingXMg in both clinopyroxene and garnet, and increasingXJd in clinopyroxene, suggest the introduction of Mg and Na throughout the evolution of these essentially bimineralic assemblages. Averaged data yield temperatures from 340°C to 500°C at 10 kbar for compositions of rims of coexisting garnet and clinopyroxene. Two samples contain pyrope-rich gamets but coexisting pyroxenes are extremely magnesian and temperatures of equilibration of both primary omphacite-pyrope and secondary omphacite-almandine/pyrope-chlorite are only slightly higher (500–650° at 10 kbar) than those for almandine jadeite eclogites and estimates overlap with those of some examples of the latter type. Unlike the majority of almandine-jadeite eclogites, two examples contain garnets with almandine-cores and pyrope-rich rims without accompanying variation in grossular content. The simple interpretation of these samples as evidence of garnet growth during prograde metamorphism is precluded by complex zoning in coexisting clinopyroxene. The eclogites provide evidence for the presence of a metamorphic terrane including rocktypes resembling those of blueschist terranes beneath the Colorado Plateau but do not permit deduction of theP,T path by which such rocktypes reached theirP,TT-conditions (∼10 kbar, 400–600°C) of metamorphism.

Experimental rare earth element partition coefficients for garnet, clinopyroxene and amphibole coexisting with andesitic and basaltic liquids

The partitioning of La, Sm, Dy, Ho and Yb between garnet, calcic clinopyroxene, calcic amphibole and andesitic and basaltic liquids has been studied experimentally. Glasses containing one or more REE in concentrations of 500–2000 ppm were crystallized at pressures of 10–35 kbar, and temperatures of 900–1520°C. Water was added to stabilize amphibole and to allow study of partition coefficients over wide temperature ranges. Major element and REE contents of crystal rims and adjacent glass were determined by EPMA, with limits of detection for individual REE of 100–180 ppm. Measured partition coefficients, D REE Cryst-liq, are independent of REE concentration over the concentration ranges used. D-values show an inverse dependence on temperature, best illustrated for garnet. At a given temperature, they are almost always higher for equilibria involving andesitic liquid. Garnet shows by far the greatest range of D-values, with e.g. D La < 0.05 and D Yb ~ 44 for andesitic liquid at 940°C. D Yb falls to ~ 12 at 1420°C. D Sm Ga-liq also correlates negatively with temperature and positively with the grossular content of garnet. Patterns of D ree Cryst-Liq for calcic clinopyroxenes and amphiboles are sub-parallel, with D-values for amphibole generally higher. Both individual D-values and patterns for the crystalline phases studied are comparable with those determined for phenocryst-matrix pairs in natural dacites, andesites and basalts. D-values and patterns are interpreted in terms of the entry of REE 3+ cations into mineral structures and liquids of contrasted major element compositions. The significance of the partition coefficients for models of the genesis of andesitic and Hy-normative basaltic magmas is assessed. Most magmas of these types in island arcs are unlikely to be produced by melting of garnet-bearing sources such as eclogite or garnet lherzolite.

The deep structure of continents

The Lehmann discontinuity at 220‐km depth is an important global feature which occurs under both oceans and continents. It is a barrier to penetration by young lithosphere and marks the base of seismicity in regions of continent‐continent collision. The strong lateral variation in upper mantle velocities occurs mainly above this depth. Continental roots extend no deeper than about 150–200 km. The basalteclogite transformation and eclogite‐harzburgite separation may be responsible for the geometry of intermediate depth earthquakes. Oceanic and continental geotherms converge above about 200 km and become less steep than the melting gradient at greater depth. This implies a low viscosity channel near 250 km. This would give a decoupling zone of maximum shear beneath continental shields. The Lehmann discontinuity may be the interface between two distinct geochemical reservoirs. The velocity jump, and the inferred density jump, at 220 km are consistent with an increase in garnet content. The mantle may be garnet lherzolite above and eclogite immediately below the Lehmann discontinuity. The transition region may be mainly eclogite and be the source region for oceanic tholeiites.

Distribution of Fe and Mg between garnet and biotite in Scottish Barrovian metamorphic zones

SummaryThe distribution coefficient KD = (Mg/ Fe)garnet/(Mg/Fe)biotite has been calculated for eight garnet-biotite pairs from rocks ranging from the garnet isograd to the staurolite zone along the Stonehaven coast of NE Scotland. This distribution coefficient depends upon metamorphic temperature and is not dependent upon the Mn and Ca content of garnet. Garnet-biotite geothermometer yields temperatures of about 500, 550, and 575–620°C for the garnet, chloritoid + staurolite, and staurolite zones respectively.

Elastic properties of eclogite rocks from the Bohemian massif

Compressional velocity anistropy has been studied in detail at atmospheric pressure for 78 specimens of 23 types of eclogite rocks from the Bohemian massif. For nine of these rock types, compressional and shear velocities were measured as a function of pressure to750 MPa at room temperature. The velocity anisotropy for both compressional and shear waves is less than4% at high pressure. The velocities increase with increasing garnet content and decrease with increasing symplectitization. The Moldanubian eclogites have significantly higher velocities, on the average, than the eclogites from the Krusne hory crystalline complex, although the densities of both groups are comparable.

A model to explain the Mn enrichment in the rim of zoned garnet.

Zonal structure of garnet grown in metamorphism maintaining surface equilibrium was examined on the model system Mn3Al2Si3O12-Fe3Al2Si3O12-H2O. The model assumes that the diffusion of Mn and Fe in both garnet and chlorite is limited, or it is practically negligible in garnet. It is shown that if the rate of temperature change during metamorphism is large in comparison with diffusion rate, a zonal structure, in which the MnO content of garnet decreases from the center of the crystal towards rim and increases again at the rim, giving rise to the minimum MnO content at an intermediate zone, can be formed without accompanying resorption. Two natural examples which support the conclusion on the model system are discussed.

Garnet in silicic liquids and its possible use as a P-T indicator

Melting experiments on a model pelitic composition yield low-spessartine garnet as an important residual phase at pressures above 7 kb. The K D values for distribution of iron and magnesium between coexisting garnet and liquid in the pelitic composition are mainly sensitive to temperature, but also have a small pressure dependence. At temperatures above 950 ° C garnet has a higher $$\frac{{Mg}}{{Mg + Fe}}$$ value than coexisting liquid, but below 950 ° C the garnet $$\frac{{Mg}}{{Mg + Fe}}$$ value is lower than that of the coexisting liquid. Thus at temperatures below 950 ° C silicic magmas may fractionate garnet and produce more magnesian derivative liquids. Reconnaissance experiments with added MnO content in the model pelite demonstrate that spessartine-rich garnets are stable in silicic liquids to pressures as low as 3 kb. The MnO and CaO contents of the experimentally crystallized garnets show an antipathetic relation. Also, the grossular content of near-liquidus garnets crystallizing from a range of compositions increases with increasing pressure. The spessartine and grossular contents of most natural garnets in eastern Australian granitic rocks suggest that these garnets formed at pressures greater than 5 kb. Increased spessartine content allows crystallization of garnet in equilibrium with a silicic magma well within the pressure limit of stability of cordierite, provided the garnet contains ≧10 mol.% spessartine. Thus the depth range over which garnet and cordierite may coexist in a silicic melt is broadened, subject to the availability of MnO. The effect of increased Mn content on the low-pressure stability limit of garnet may also explain the lack of resorption of some garnets in granitic magmas, as these magmas rise to shallower levels. These euhedral garnets characteristically show zoning from an Mn-poor core (typically 4 % MnO) and may reflect continued growth of the garnet in a low pressure regime, stabilized by Mn concentrated in the residual liquid fractions of the crystallizing magma.

The stability of pyrope-grossular garnet with excess silica

In the system CaO-MgO-A12O3-SiO2 the tie lines connecting anorthite with other phases are sequentially broken down with increasing pressure according to the following univariant reactions: anorthite+ enstatitess+sillimanite⇌ pyrope-grossularss+quartz (3), anorthite+enstatitess ⇌ pyrope-grossularss+diopsidess+quartz (2), anorthite+pyrope-grossularss+ quartz ⇌ diopsidess+kyanite (4) and anorthite+diopsidess⇌ grossular-pyropess +kyanite+quartz (8). At 1,200 ° C these reactions occur at 14.5± 0.5, 15.5±0.5, 19.5±0.5 and 26.4±1 kilobar and have positive slopes (dP/dT) of 1±0.5, 2.8±0.5, 13.3±0.5 and 24±2bars/°C respectively. An invariant point involving kyanite rather than sillimanite, occurs at 850 °C±25 °C and 14.5±0.5kbar at the intersection of reactions (3), (2) and (4). Reaction(4) exhibits significant curvature with an increase in dP/dT from 13.3±0.5 to 18.5± 0.5 bars/°C between 1,050° and 850° C. The pressure at which the complete grossular-pyrope join is stable with quartz is estimated at 41 ± 1 kbar at 1,200 ° C. The pressure at which garnet appears according to reaction (2) is lowered by 5 kbar for a composition with anorthite and orthopyroxene (En0.5Fs0.5). Enstatite and plagioclase (An0.5Ab0.5) first produce garnet at 2 kbar higher pressure than enstatite and pure anorthite (reaction (2)). The calcium content of garnet in various divariant assemblages is relatively insensitive to temperature but very sensitive to pressure, it is therefore a useful geobarometer. At metamorphic temperatures of 700–850 °C pressures of 8–10 kbar are required for the formation of quartz-bearing garnet granulites containing calcic plagioclase and with (Mg/Mg+Fe) bulk = 0.5.

The petrogenesis of komatiites and related rocks as evidence for a layered upper mantle

Although the CaO/Al 2O 3 ratio of komatiites has been regarded as one of the distinguishing features of these rocks, a comparison of various komatiite and oceanic tholeiite analyses suggests that there is a continuum of ratios between the two. The extremely high MgO values of peridotitic komatiites suggest that they are the result of high degrees of partial melting of the mantle, leaving a harzburgitic residuum depleted in CaO and Al 2O 3, and hence preserving in the melt the original CaO/Al 2O 3 ratio of the parental material. Available chemical models of the mantle have CaO/Al 2O 3 ratios too low to explain the origin of komatiite by such a process. Shallow-level melting of a layered mantle in which clinopyroxene content decreases and garnet content increases with depth, may explain the chemistry of komatiites and related ultrabasic lavas.

Rare-earth element distribution in Archaean gneisses and anorthosites, Godthåb area, West Greenland

Rare-earth element (REE) distribution patterns have been determined for samples of Amîtsoq gneiss from the Godthåb and Isua areas. All have fractionated REE patterns with light REE enrichment, but some leucocratic components show extreme depletion in heavy REE. Various different crystal-liquid equilibria appear to have been involved in their petrogenesis. Fractional crystallization of garnet from a basic or intermediate magma, or partial melting of a basic or intermediate source with a high modal garnet content are considered as possible mechanisms for the generation of the leucocratic components. Anorthosites from Ameralik and Buksefjord show slight light REE enrichment with small or undetectable Eu anomalies, possibly due to crystallization under a high fO 2.

Origin of garnet in the basic granulites around Saltora, W. Bengal, India

Garnetiferous basic granulites occur, as parts of hornblende-pyroxene- and pyroxene granulites, in a Precambrian terrain around Saltora. The chemistry of the garnetiferous basic granulites is broadly similar to that of the hornblende-pyroxene granulites, their immediate precursors, but in detail they have distinctly higher Fe/Mg ratios. The compositions of the major mafic silicates of the garnetiferous varieties do not reflect higher pressures of formation: the Jd/Ts ratios in calcic pyroxenes are similar to those from the non-garnetiferous varieties, and the pyrope contents of garnets are low.

Progressive compositional change of garnet in metapelite

The MnO content of garnet is discussed on the basis of two models of crystalization representing complete fractionation and complete equilibrium. The progressive decrease of the MnO content or garnet in metapelite with increasing temperature is mainly due to the gradual increase in the amount of this mineral. The actually observed decrease of the MnO content, however, is probably enhanced by a statistical decrease in the MnO contents of garnet-bearing metapelites with increase in the temperature of metamorphism. Increasing rock pressure should have the same effect as increasing temperature on the MnO content of garnet. The progressive increase of the MgO content of garrnet can be explained in similar way.