Garnet-in Reaction Research Articles

Nucleation and Initial Growth of Garnet in Low-Grade Metamorphic Rocks of the Sanbagawa Metamorphic Belt, Kanto Mountains, Japan

The beginning of the recrystallization of minerals within a subducting oceanic plate provides a valuable record of dehydration within the subduction zone. Pelitic schists of the Nagatoro area, Kanto Mountains, Japan, record the initial stages of garnet growth. Consequently, these rocks were studied to analyze garnet nucleation and growth during metamorphism of the Sanbagawa metamorphic belt, one of the world’s most comprehensively studied subduction complexes. The garnet grains are small, euhedral, and occur only within micaceous lamellae that define the schistosity. Crystal size distribution analyses revealed most of the garnet grains follow the log-normal size distribution, indicating that they formed in the same event. A few exceptionally large garnet grains exist that do not seem to follow the log-normal distribution. The latter garnet grains contain a rounded fragmental area with a different chemical composition inside the core. It is possible that detrital fragments of garnet contribute to the irregular crystal size distribution of garnet in the studied area. Many of the smaller (log-normal) garnet grains have relatively large, homogeneous Mn-rich cores. The lack of chemical zoning within the garnet cores suggests that they grew under constant pressure and temperature in response to overstepping of the garnet-in reaction. The chemical composition changes very sharply at the boundary between the core and the surrounding mantle. The size of the Mn-rich core is different from sample to sample, suggesting that the nucleation was controlled by the local chemical condition of each sample.

Evaluation of the effective bulk composition (EBC) during growth of garnet

The effective bulk composition (EBC) for the initial growth of garnet that has nucleated only after considerable overstepping of the equilibrium garnet-in reaction has been calculated for eight samples from central Vermont. The method developed assumes that the composition of garnet is determined by the maximum driving force (maximum change in Gibbs free energy) at the assumed conditions of nucleation. The method of intersecting isopleths is used to find the EBC where the isopleth intersection matches the P–T conditions of nucleation.The calculated EBC for all samples (four epidote-free and four epidote-bearing) contains less MnO and FeO (or lower FeO/MgO) than the whole rock bulk composition. The CaO content of the EBC is lower than the whole rock bulk composition in all epidote-free samples and in two of the four epidote-bearing samples. It is proposed that the differences between the EBC and whole rock bulk composition are caused by the sluggish kinetics of dissolution of reactants rather than diffusive transport of nutrients to the garnet.

Significance of “stretched” mineral inclusions for reconstructing P–T exhumation history

polymorph kyanite also has the capacity to retain tensile stresses if exhumed to Earth's surface after being trapped as an inclusion in plagioclase at P-T conditions within the kyanite stability field, with the stresses developed during exhumation sufficient to produce a transformation to anda - lusite. These results highlight the elastic environment that may arise during exhumation and provide a potential expla- nation of observed inclusions whose stability fields are well removed from P-T paths followed during exhumation. Abstract Analysis of mineral inclusions in chemically and physically resistant hosts has proven to be valuable for reconstructing the P-T exhumation history of high-grade metamorphic rocks. The occurrence of cristobalite-bear- ing inclusions in garnets from Gore Mountain, New York, is unexpected because the peak metamorphic conditions reached are well removed (>600 °C too cold) from the sta- bility field of this low-density silica polymorph that typi - cally forms in high temperature volcanic environments. A previous study of samples from this area interpreted pol- ymineralic inclusions consisting of cristobalite, albite and ilmenite as representing crystallized droplets of melt gen- erated during a garnet-in reaction, followed by water loss from the inclusion to explain the reduction in inclusion pressure that drove the transformation of quartz to cristo- balite. However, the recent discovery of monomineralic inclusions of cristobalite from the nearby Hooper Mine can- not be explained by this process. For these inclusions, we propose that the volume response to pressure and tempera- ture changes during exhumation to Earth's surface resulted in large tensile stresses within the silica phase that would be sufficient to cause transformation to the low-density (low- pressure) form. Elastic modeling of other common inclu- sion-host systems suggests that this quartz-to-cristobalite example may not be a unique case. The aluminosilicate

Garnet zonation in metapelitic schists from the Eclogite Zone, Tauern Window, Austria: comparison of observed and calculated profiles

Three quartzitic metapelites with garnet-chloritoid-kyanite assemblage from the Eclogite Zone, Tauern Window, Austria have been studied based on pseudosections in the KNCFMMnASTH and partly KNCFMMnASTHO system compared with measured and calculated garnet zonations along three P-T paths. Deduced garnet core growth is in the range 502–521 °C, 20.2–21.4 kbar, corresponding to overstepping the garnet-in reaction by ~25–100 °C for P-T paths 1, 2 and ~72–133 °C for P-T path 3. The peak assemblage garnet-chloritoid-kyanite-phengite-paragonite-rutile-quartz in sample A is stable in a narrow maximum ~10 °C band from ~598 °C/14 kbar to ~570 °C/24.9 kbar. In sample B and C this stability is much more restricted with ~ 2 °C width from ~581 °C/22.3 kbar to ~572 °C/24.5 kbar and ~4 °C/0.7 kbar width at ~570 °C/24.8 kbar. Temperature differences between measured and calculated pyrope and grossular isopleths are generally in the range from 0 to 20 °C, however larger values are sometimes obtained. The proposed P-T paths reach peak values around 570–575 °C/24.6 kbar. Comparison of measured and calculated garnet zonation suggests a better accordance for unfractionated than for fractionated models. Several factors could be responsible for this and the partly large overstepping values, e.g . insufficient thermodynamic data, or non-equilibrium reactions due to sluggish nucleation or breakdown reactions and later garnet re-equilibration.

An exhumation pressure–temperature path and fluid activities during metamorphism in the Tseel terrane, SW Mongolia: Constraints from aluminosilicate-bearing quartz veins and garnet zonings in metapelites

The Tseel terrane of the Central Asian Orogenic Belt, SW Mongolia, contains a record of amphibolite-facies (locally granulite-facies) metamorphism and multiple igneous intrusions. However, the metamorphic history of the terrane and the relation of metamorphism to the intrusion of granitoids remain uncertain. In this study, we investigated pelitic gneisses and aluminosilicate-bearing quartz veins located at various distances from a granitoid body in the central part of the Tseel terrane.The pelitic gneisses are composed mainly of garnet (Grt)+biotite (Bt)+plagioclase (Pl)+sillimanite (Sil)+quartz (Qtz). Garnet in the pelitic gneiss located far from the granitoid shows compositional zoning, characterized by decreases in Ca and Mn from core to rim, and increases in Fe and Mg, along with minor retrograde zoning at the outermost rim. The P–T path during garnet growth was estimated by garnet isopleth thermobarometry based on a P–T pseudosection and by conventional Grt–Bt geothermometry and Grt–Bt–Pl–Qtz geobarometry. The two approaches yield similar P–T paths, with the decompression P–T path extending from the kyanite (Ky) stability field (560±10°C and 6.5±0.5kbar) to the sillimanite stability field (600±5°C and 3.8±0.5kbar), accompanied by a slight increase in temperature (by 40–50°C). Garnet grains in pelitic gneisses located near the granitoid body lack a high-Ca core and yield P–T conditions of the sillimanite stability field, suggesting the local thermal effect of a granitoid intrusion at shallow crustal depths (<∼20km).Aluminosilicate-bearing quartz veins in the pelitic gneisses contain kyanite, sillimanite, and andalusite (And) at their margins. The textural relations among aluminosilicate polymorphs in the veins indicate their formation in the order of Ky→Sil→And. Microthermometric analyses of fluid inclusions reveal that these veins formed in the kyanite stability field, that corresponds to the growth of high-Ca garnet cores in the pelitic gneiss. The precipitation of coarse-grained kyanite in fractures was probably assisted by the focused flow of fluid produced by a dehydration reaction (i.e., the garnet-in reaction). After the main period of vein growth, the replacement of kyanite by sillimanite and andalusite occurred in association with the formation of muscovite in the veins. These reactions were probably facilitated by fluid supply from granitoids at shallow crustal depths.

The influence of Cr on the garnet–spinel transition in the Earth's mantle: experiments in the system MgO–Cr 2O 3–SiO 2 and thermodynamic modelling

The position of the transition from spinel peridotite to garnet peridotite in a simplified chemical composition has been determined experimentally at high pressures and high temperatures. The univariant reaction MgCr 2O 4+2Mg 2Si 2O 6=Mg 3Cr 2Si 3O 12+Mg 2SiO 4, has a negative slope in P– T space between 1200 °C and 1600 °C. The experimental results, combined with assessed thermodynamic data for MgCr 2O 4, MgSiO 3 and Mg 2SiO 4 give the entropy and enthalpy of formation of knorringite garnet (Mg 3Cr 2Si 3O 12). Thermodynamic calculations in simplified chemical compositions indicate that Cr shifts the garnet-in reaction to much higher pressures than previously anticipated. Moreover, in Cr-bearing systems a pressure–temperature field exists where garnet and spinel coexist. The width of this divariant field strongly depends on the Cr/(Cr+Al) of the system.

The influence of Cr on the garnet?spinel transition in the Earth's mantle: experiments in the system MgO?Cr2O3?SiO2 and thermodynamic modelling

The position of the transition from spinel peridotite to garnet peridotite in a simplified chemical composition has been determined experimentally at high pressures and high temperatures. The univariant reaction MgCr 2 O 4 +2Mg 2 Si 2 O 6 =Mg 3 Cr 2 Si 3 O 12 +Mg 2 SiO 4 , has a negative slope in P – T space between 1200 °C and 1600 °C. The experimental results, combined with assessed thermodynamic data for MgCr 2 O 4 , MgSiO 3 and Mg 2 SiO 4 give the entropy and enthalpy of formation of knorringite garnet (Mg 3 Cr 2 Si 3 O 12 ). Thermodynamic calculations in simplified chemical compositions indicate that Cr shifts the garnet-in reaction to much higher pressures than previously anticipated. Moreover, in Cr-bearing systems a pressure–temperature field exists where garnet and spinel coexist. The width of this divariant field strongly depends on the Cr/(Cr+Al) of the system.

The Effect of Reaction Overstep on Garnet Microtextures in Metapelitic Rocks of the Ilesha Schist Belt, SW Nigeria

Garnet-bearing assemblages of K-rich and K-poor metapelites from the Ilesha Schist belt, SW Nigeria, are investigated. K-rich samples contain the assemblages (A) garnet±staurolite±muscovite±chlorite±magnetite, (B) andalusite±garnet±staurolite±muscovite±chlorite±magnetite and (C) sillimanite±andalusite±garnet±muscovite±chlorite± magnetite. K-poor samples contain the assemblages (D) garnet±staurolite±cordierite±chlorite and (E) garnet±cordierite±chlorite staurolite. All assemblages contain quartz, plagioclase, biotite and ilmenite. P±T pseudosections calculated in the system CaO±Na2O±K2O±TiO2±MnO±FeO±MgO± Al2O3±SiO2±H2O O2 suggest peak metamorphism at 590 20 C at 5 0 5 kbar, followed by retrogression to 550 C at 3 0 kbar, in agreement with field evidence, domain assemblages, mineral compositions, modes and geothermobarometry. The absence of compositional zonation shows that garnet in all investigated rocks nucleated and grew at constant P±T±X in equilibrium with associated minerals on the thin-section scale. However, the garnet-in reaction did not begin until the establishment of a significant temperature overstep of 80 C, with consequent rapid dehydration of chloriteand muscovite-bearing assemblages resulting in interface-controlled garnet growth under locally hydrostatic pressure conditions. Garnet in rock types A and B formed with a characteristic pattern of biotite and quartz inclusionsÐa `house of cards' texture. Additionally, clusters and aggregates of unzoned garnet grains formed in rock types A and D. Garnet resorption in rock types A±C resulted from prograde stauroliteand retrograde andalusite-forming reactions.

Integration of phase equilibria modelling and garnet Sm-Nd chronology for construction of P-T-t paths: examples from the Cordilleran Coast Plutonic Complex, USA

Abstract Integration of petrographic observations, mineral chemistry, garnet Sm-Nd isochrons, and MnNaCaKFMASH pseudosection phase equilibria models constructed with THERMOCALC provides quantitative metamorphic pressure-temperature-time (P-T-t) paths which allow determination of assemblage/reaction history for pelites. Examples are presented for the Cretaceous to Tertiary magmatic arc of the North American Coast Plutonic Complex. Metamorphism in the western Coast Plutonic Complex of southeastern Alaska and in the North Cascades of Washington resulted from at least three widespread events from &gt; 100 Ma to c. 60 Ma, and in both areas partly resulted from crustal thickening, evidenced by local occurrences of kyanite after andalusite. Pressure-temperature pseudosections constructed from bulk rock compositions and the intersection of garnet core composition isopleths provide estimates for the pressure and temperature of garnet core growth. Intersections of these isopleths indicate garnet growth 18 to 85 °C above the predicted garnet-in reaction temperatures. Rim and near-rim garnet compositions and matrix mineral chemistry provide estimates for near-peak metamorphic conditions. Finite pressure-temperature-time paths of garnet zone metamorphism were determined from the combined core growth and pressure-temperature conditions determined from near-rim garnet and matrix mineral compositions. The western Coast Plutonic Complex near the Stikine River, southeastern Alaska, displays a complex pattern of regional metamorphism overprinted by contact metamorphic aureoles. Many of the c. 90 Ma aureoles contain andalusite, andalusite plus sillimanite, or andalusite plus kyanite with complex replacement textures. A pseudosection constructed for a contact metamorphic rock on Kadin Island (95GL11c), predicts that garnet grew c. 555 ± 10 °C and 4.8 ± 0.7 kbar, above the garnet-in line and the aluminium silicate triple-point pressure. These results suggest that andalusite in samples from this aureole likely grew prior to garnet and that the pressure may have increased by ≤ 1 kbar during metamorphism. The southern part of the North Cascades in Washington also contain complex aluminium silicate replacement textures with early andalusite and later kyanite and sillimanite. A sample (96NC67), collected near the andalusite-bearing aureole of the Mt Stuart batholith, contains sillimanite and c. 10 mm garnet crystals containing staurolite inclusions in their cores. Temperatures estimated from the garnet core of this sample are within the pseudosection staurolite stability field, compatible with initial garnet growth significantly above the garnetin line. The garnet rim thermometry estimate of c. 668 ± 59 °C for this sample is c. 85 °C higher than the core growth temperature. The calculated P-T-t path provides important information for interpreting regional and contact metamorphism. An extensive region NE of the Mt Stuart batholith in the North Cascades underwent a significant pressure increase; however, the timing and nature of medium- to high-pressure metamorphism is controversial. Quantitative P-T-t paths constructed for garnet growth along the NE margin of the batholith indicate that 87-85 Ma garnet growth was younger than the nearby Mt Stuart batholith (93.5 ± 1.4 Ma, U-Pb zircon). Garnet core and rim segments are isochronous indicating a short interval for garnet growth. P-T-t paths indicate that garnet growth occurred in the sillimanite stability field during a maximum pressure increase of 1 to 2 kbar, after rocks passed through the andalusite stability field (Mt Stuart contact metamorphism). Careful sampling, hand-picking, acid leaching, and isotopic analysis of garnet provide geologically consistent ages with uncertainties of ≤ 1.0 Ma. Thermodynamic modelling in the MnNaCaKFMASH system provide reasonable P-T predictions for pelite mineral stability that can be integrated with isotope ages to provide quantitative P-T-t paths. The P-T-t paths developed for both regional and contact metamorphic rocks allow critical evaluation of tectonic models and of interpretations for mineral textures.

Experimental study of the stability of cordierite and garnet in pelitic compositions at high pressures and temperatures

The stability of cordierite and garnet relative to their anhydrous breakdown products, i.e. hypersthene, sapphirine, olivine, spinel, sillimanite and quartz, has been studied experimentally in model pelitic compositions (system MgO-FeO-Al2O3-CaO-K2O-SiO2). Below 1000° C cordierite breaks down according to the divariant reaction cordierite⇄ garnet+sillimanite+quartz (1) for most values of the MgO/MgO + FeO ratio (X). At very high values of X (ca. X〉0.9) garnet in reaction (1) is replaced by hypersthene. The position and width of the divariant field (in terms of pressure and temperature) in which cordierite and garnet coexist, is a function of the MgO/MgO + FeO ratio. If this ratio is increased then the stability field of garnet is reduced and that of cordierite extended towards higher pressure. Compositions of coexisting cordierite and garnet in divariant equilibrium have been analysed by electron probe micro-analyser. These compositions are unique functions of pressure and temperature. Above ca. 1000° C the breakdown of cordierite involves the phases sapphirine and hercynite-rich spinel in Mg-rich and Fe-rich compositions respectively.

Experimental study of the stability of cordierite and garnet in pelitic compositions at high pressures and temperatures

The stability of cordierite and garnet relative to their anhydrous breakdown products, i.e. hypersthene, sapphirine, olivine, spinel, sillimanite and quartz, has been studied experimentally in model pelitic compositions (system MgO-FeO-Al2O3-CaO-K2O-SiO2). Below 1000° C cordierite breaks down according to the divariant reaction cordierite⇄ garnet+sillimanite+quartz (1) for most values of the MgO/MgO + FeO ratio (X). At very high values of X (ca. X〉0.9) garnet in reaction (1) is replaced by hypersthene. The position and width of the divariant field (in terms of pressure and temperature) in which cordierite and garnet coexist, is a function of the MgO/MgO + FeO ratio. If this ratio is increased then the stability field of garnet is reduced and that of cordierite extended towards higher pressure. Compositions of coexisting cordierite and garnet in divariant equilibrium have been analysed by electron probe micro-analyser. These compositions are unique functions of pressure and temperature. Above ca. 1000° C the breakdown of cordierite involves the phases sapphirine and hercynite-rich spinel in Mg-rich and Fe-rich compositions respectively.