Garnet Growth Research Articles

Reinterpretation of a major terrane boundary in the northern Svalbard Caledonides based on metamorphic fingerprinting of rocks in northern Spitsbergen

Tectonic models for development of the Svalbard Caledonides depend on reliable assessment of the metamorphic evolution of the various basement provinces involved. The Mosselhalvøya Group (MG) and the Atomfjella Complex (AC) have previously been assigned to the Nordaustlandet and West Ny-Friesland terranes, respectively. New analytical data and petrographic observations indicate that both units experienced two-stage metamorphism under similar pressure–temperature (P–T) conditions. Two stages of amphibolite facies metamorphism (M1 and M2) are clearly recorded by garnet and staurolite porphyroblast textures. The results of thermodynamic phase equilibrium modeling indicate that peak M2 metamorphism occurred at ∼7–7.5 kbar and 590–600 °C in both units. Zirconium-in-rutile trace element thermometry confirms the temperature estimates for M1 and M2 stages of metamorphism. Monazite chemical Th-U-Pb dates from the MG resolve a two-stage garnet growth at 444 ± 7 Ma (M1) and 423 ± 6 Ma (M2). In contrast, monazite dated in the AC defines a single age of 420 ± 4 Ma interpreted as M2 growth. We suggest M2 was coeval with early strike-slip motion along the Billefjorden Fault Zone, whereas M1 reflects initial tectonic burial of the studied units. The similarity in metamorphic history between the both units suggests that the boundary between them is a subordinate thrust fault within the Atomfjella thrust stack rather than a major boundary separating the Nordaustlandet and West Ny-Friesland terranes. The MG should be included within the West Ny-Friesland terrane and the tectonic boundary with the Nordaustlandet terrane is likely the Eolussletta Shear Zone.

Lu–Hf Geochronology on Single Garnets in a Micaschist from the North Qilian Orogenic Belt

The acquisition of the timing and duration of metamorphic mineral growth is the key to understanding the evolution of metamorphic belts. Garnet Lu–Hf geochronology is becoming increasingly powerful in this aspect. It is believed that garnet Lu–Hf radiometric systems are preserved during low-temperature metamorphism. However, this hypothesis has not been systematically tested. To examine the Lu–Hf systematics of garnets during low-temperature metamorphism, we conducted radiometric dating on individual garnet crystals of different sizes from a single micaschist in the North Qilian orogenic belt. The garnet Lu–Hf dates correlate well with the grain sizes and their core Mn concentrations. The positive correlation between the ages and grain sizes suggests that grain size is a proxy for preserving garnet nucleation and growth history. Small grains nucleated and grew later than large crystals. The Lu–Hf date of the individual garnet crystals faithfully recorded the total growth time span. The date difference of ~16 Myr is the minimum duration of the total garnet growth. The age discrepancy between the micaschist and the eclogite indicates they may not have experienced the same subduction and exhumation in the North Qilian orogenic belt.

Bulk Single-Crystal Growth of Ce/Gd3(Al,Ga)5O12 from Melt without a Precious Metal Crucible by Pulling from a Cold Container

We report the growth of cerium-doped multicomponent garnet (Ce/GAGG) single crystals based on crystal pulling from a melt using a cold container without employing a precious metal crucible. This novel method is described in detail. We measured the optical, luminescence, and scintillation characteristics of several samples cut from three crystals and compared them with a commercial Ce/GAGG crystal. The best samples from the set are fully comparable in all characteristics with the commercial reference crystal, demonstrating the practical potential of this method. Avoidance of an expensive precious metal crucible in this method makes crystal growth much more economical.

Deciphering western Mediterranean kinematics using metamorphic porphyroblasts from the Alpujárride Complex (Betic Cordillera)

3D microstructural analysis of porphyroblast inclusion trails using X-ray Computed Tomography is integrated with analysis of field structures to unravel the Alpine deformation history of the Alpujárride Complex, which constitutes the partially submerged metamorphic core of the Gibraltar Arc. Prograde metamorphism in the complex has been traditionally linked to a 'D1' event witnessed by inclusion trails in garnet porphyroblasts. Orientation data for these microstructures reveal three age groups with differently oriented axes of inclusion-trail curvature (known as FIA). The successive development of FIAs trending WNW-ESE, ENE-WSW and NNW-SSE is shown and correlated with the Paleogene-Neogene relative plate-motion paths of Africa, Iberia and the Alborán Domain as known from paleomagnetic data. During the late-metamorphic evolution of the Alpujárride Complex, after garnet growth had ceased, two steeply dipping crenulation cleavages and associated folds with roughly suborthogonal N–S and E-W trends developed, in addition to two subhorizontal ones. Inclusion trails are also found to exhibit a general preference for subvertical and subhorizontal orientations, suggesting a protracted orogenic evolution characterized by multiple stress permutations causing alternations of crustal shortening and gravitational collapse.

Lithium, Phosphorus, and rare earth elements in magmatic garnets from granitic Pegmatites: Coupled substitution and petrogenetic implications

The general chemical formula of garnet can be expressed as X3Y2(ZO4)3 where X, Y, and Z represent dodecahedral, octahedral, and tetrahedral sites, respectively. Lithium, phosphorus, and rare earth elements (including yttrium) in garnets from the Luanzhuang-Guanpo and Ziyugou (ZYG) pegmatite fields in the East Qinling orogen and the Kalu’an (KLA) Li pegmatites in the Chinese Altay orogen have been investigated both at a mineral scale and in the context of pegmatite types to identify the substitution mechanisms for these elements. Our results show that Y + REE or P can enter the garnet lattice through coupled substitution with Li (+Na) that can compensate the charge differences due to Y + REE in the X-site or P in the Z-site of the garnet. Two substitutions, namely [X(Li,Na)+1X(Y,REE)3+1(R2+)–2] and [X(Li,Na)1ZP1XR2+-1ZSi-1], are likely the most important and can account for incorporation of the majority of Y + REE, P, and Li + Na in pegmatite garnets. Whichever substitution becomes predominant in garnets is strongly dependent on the bulk chemistry of pegmatite-forming melts. Consequently, the first substitution dominates in garnets from NYF (niobium-yttrium-fluorine) pegmatites and mixed-type pegmatites with NYF affinity whereas the other one dominates in garnets from relatively P-rich LCT (lithium-cesium-tantalum) and barren pegmatites. Incorporation of Sc and Ti in garnet may also need Li + Na, although Sc and Ti are not as much as Y and REE in pegmatite garnet. The zonings of Y, REE, and Sc in garnet are mainly controlled by the composition of pegmatite-forming melt and consumption of these elements due to the growth of garnet itself and other minerals enriched in Y + REE (e.g., xenotime, fergusonite, monazite, etc.) or Sc (e.g., biotite). Additionally, our results, combined with the data in the literature, show that garnets from NYF pegmatites and mixed-type pegmatites with NYF affinity commonly contain elevated Y (up to ∼ 20000 ppm), REE (up to ∼ 20000 ppm), and Sc (up to ∼ 2400 ppm) but low P (<100 ppm) compared to those from LCT and barren pegmatites. Nevertheless, individual garnets with low concentrations of Y, REE, Sc, and P can occur in any types of pegmatites. Therefore, the coupled substitution between Li + Na and Y + REE or P along with zonings and concentrations of Y, REE, and Sc in garnet should be investigated on a regular basis to interpret pegmatite petrogenesis.

Discrimination of thermodynamic and kinetic contributions to the heavy rare earth element patterns in metamorphic garnet

AbstractVariations of rare earth element (REE) concentrations in metamorphic garnet are an important source of information of geodynamic and geochemical processes in the deeper Earth. In order to extract this information, the thermodynamic equilibrium and kinetic contributions of the REE uptake in garnet must be distinguished and quantified. Utilizing high‐resolution trace element and μ‐Raman mapping together with combined thermodynamic–geochemical–diffusion models, we demonstrate that the equilibrium and kinetic aspects of the REE uptake in metamorphic garnet can be discriminated by interpreting 2D trace element mapping in a single sample. The heavy (H) REE (Tb to Lu) zoning in the investigated garnet from a high‐pressure blueschist comprises an inner part with an overall decrease from core to inner rim, followed by a concentric zone of HREE enrichment and a drastic HREE decrease towards the outermost rim. The central peak in the garnet core decreases in intensity with decreasing atomic number of the REE. The broad overall shape of this pattern resembles those often observed in metamorphic garnet from different rock types and tectonic settings. Superimposed on this trend is a concentric pattern of minor recurring fluctuations in the HREE concentrations with at least six regularly spaced sets of peaks and troughs along the entire garnet radius. Comparison of the observed inclusion suite, the trace element maps and thermodynamic–geochemical models show that the inner part with decreasing HREE concentrations results from fractional garnet growth in an unchanged mineral assemblage, whereas the REE enrichment zone is caused by the breakdown of titanite. We suggest that the width of the central peak is controlled by the bulk permeability of the interconnected transport matrix and the fraction of matrix minerals that the garnet equilibrates with. The superimposed REE fluctuations result from changing element transport properties of the host rock and mark recurring changes from equilibrium REE uptake to transport‐limited REE uptake in garnet. Such fluctuating element transport properties can be best explained by pulse‐like fluid fluxes that rhythmically change the interconnectivity of the intercrystalline transport matrix. Increasing numbers of published spatially highly resolved REE analyses show that such trace element fluctuations are common in metamorphic garnet indicating that recurring changes in rock permeabilities due to pulsed fluid fluxes are a common phenomenon during metamorphism.

Garnets in the Yinachang Fe-Cu deposit, Kangdian region, China: Mineralogical and geochemical insights into fluid-rock dominated iron oxide-copper–gold (IOCG) mineralization

Garnet is an important metasomatic mineral in iron oxide-copper–gold (IOCG) ore systems, but its crystallization mechanisms and relationship to fluid evolution in IOCG are poorly understood. In this study, major and trace element analyses were conducted on garnets from the Yinachang IOCG deposit in the Kangdian region, southwest China, in order to reveal their characteristics and forming process.Garnet commonly occurs in the K-Fe ± REE mineralization stage and Cu ± Au mineralization stage of the Yinachang deposit. Three types of garnets are identified from this deposit, which generally have high almandine but low grossular and spessartine contents (Alm65-78Gro5-17Sp5-17). The type 1 garnet (Grt 1) in the ore bodies and the type 2 garnet (Grt 2) in the proximal silicate alteration zones are pink to purple, euhedral to subhedral grains. Grt 1 exhibit zoning pattern, and Grt 2 were altered into chlorite and was mainly crosscut by ore-bearing veins. They are both extremely depleted in LILEs and enriched in HFSEs, with low total rare earth element ∑REE contents but variable LREE vs HREE ratios. The type 3 garnets (Grt 3) in the intermediate garnet-biotite-quartz zones are purple-brown, anhedral grains with high LILE, HFSE, and ∑REE contents, and exhibit LREE-depleted and HREE-enriched patterns, with marked positive Eu anomalies. All three types garnets that formed during alteration and mineralization are hydrothermal, but their composition is controlled by different mechanisms due to different environments. Compositions of Grt 1 and Grt 2 are mainly related to substitution mechanism controlled by crystal chemistry, physicochemical conditions of the closed hydrothermal system, and buffering effects of carbonate wall rocks during slow infiltration metasomatism, whereas the composition of Grt 3 is mainly controlled by surface adsorption and related to the composition of external fluid during later rapid garnet growth. Decreases in temperature, pressure, and fO2 triggered by mixing of fluids could be responsible for the distinct mineralogical and geochemical characteristics of the three types of garnets.Garnets from IOCG deposits show various textual and geochemical characteristics worldwide. Generally, almandine and andradite are dominating end members for IOCGs. Andradite is typical for metasomatic origin. Although almandine garnet is generally considered to form during metamorphism, the occurrence of hydrothermal almandine in the alteration zones of IOCG deposits reveal a metasomatism between Fe-bearing fluid and Al-enriched pelite protolith in a sufficiently reduced environment. Also, it is possible that the metasomatism during mineralization has enriched the protolith with Fe, and the post-ore metamorphism formed almandine by providing high temperature and pressure.

Deciphering the tectonometamorphic history of subducted metapelites using quartz‐in‐garnet and Ti‐in‐quartz (QuiG–TiQ) geothermobarometry—A key for understanding burial in the Scandinavian Caledonides

AbstractThe Seve Nappe Complex is a subduction‐related high‐grade metamorphic unit that was emplaced onto the margin of Baltica during Caledonian orogenesis. In this paper, the tectonometamorphic evolution of the Lower Seve Nappe in the Scandinavian Caledonides was characterized with the help of the continuous Collisional Orogeny in the Scandinavian Caledonides (COSC‐1) drill core, using a combination of various P–T estimation techniques based on garnet–quartz mineral pairs (quartz‐in‐garnet and Ti‐in‐quartz [QuiG–TiQ]), conventional thermobarometry and thermodynamic modelling of phase equilibria. This multi‐method approach yields complementary results and delivers critical data to constrain a comprehensive pressure–temperature–deformation–time (P–T–D–t) evolutionary path for the metasedimentary rocks of the Lower Seve Nappe. In the garnetiferous metasedimentary rocks, quartz inclusions in garnet preserve the P–T conditions of three distinct garnet growth stages corresponding to three metamorphic stages Ms1 to Ms3, including prograde and peak metamorphic conditions. Ms1 and Ms2 stages were constrained via quartz inclusions in garnet core and mantle. They are relatively close in the P–T space and could be considered as one single continuous prograde event occurring at epidote–amphibolite facies conditions of 460–520°C and 0.6–0.85 GPa. The growth of the garnet outermost rim defines the Ms3 stage at amphibolite facies conditions of 590–610°C and 1.13–1.18 GPa and corresponds to the peak metamorphic conditions. The microstructural analysis shows that the finite ductile strain pattern of the Lower Seve Nappe results from the superposition of four deformation phases. The initial phase D1 is defined by the S1 foliation that is still preserved as a curved inclusion trail in the garnet core. The D2 phase initiated contemporaneously with garnet core growth and the development of muscovite–biotite–plagioclase S2 foliation. Garnet outermost rim growth marks the end of the prograde path and peak metamorphic conditions. This stage is overprinted by the D3 phase and Ms4 stage associated with the development of the main regional metamorphic and mylonitic fabric S3 associated with C′‐type shear bands along the retrograde path. Ms4 stage, which was constrained using traditional thermobarometric techniques, corresponds to the chemical re‐equilibration of the metasedimentary minerals and occurred under amphibolite facies conditions at ~570–610°C and 0.78–1.00 GPa. The D3 phase is then generally weakly to strongly overprinted by later lower grade deformation D4 phase at greenschist facies conditions (Ms5). 40Ar/39Ar ages of syn‐kinematic white mica and biotite indicate that the final stage of the thrusting of the Lower Seve Nappe and thus the timing of its emplacement onto the Offerdal Nappe occurred at c. 423 Ma. Collectively, these results are consistent with previous estimates of the timing and conditions of metamorphism derived from the Lower Seve Nappe especially in west‐central Jämtland. However, application of QuiG–TiQ thermobarometry demonstrated that quartz inclusions in garnet can preserve different aspects of garnet growth, which are not accessible by traditional methods especially in complex terranes, and therefore provided new significant insights into the Lower Seve prograde evolution.

Multistage growth of garnet fingerprints the behavior and property of metamorphic fluids in a Paleotethyan oceanic subduction zone

Metamorphic fluids released from the subducting crust are of great importance to understand the recycling of volatile materials at convergent plate margins. Garnet in metamorphic rocks can retain chemical zonation and crystal inclusions produced during crustal subduction, providing the record of geochemical recycling in subduction zones. This paper presents a combined study of mineralogy and geochemistry for metamorphic garnets in oceanic-type eclogite from the Paleotethyan subduction zone in the Hong'an orogen, east-central China. The results show distinct zonation of major and trace elements, crystal inclusions, water contents, and oxygen fugacity in garnet. Five stages of garnet growth were recognized based on the garnet composition and phase equilibrium modelling. Growth of Stage I to IV garnets occurs during prograde metamorphism of the Paleotethyan oceanic crust from blueschist to eclogite facies along low geothermal gradients of 7.8–9.7 °C/km. Stage V garnet was formed through dissolution and reprecipitation during mineral-fluid reaction in the early exhumation stage. Garnet grains exhibit a decrease of water content from core to rim, indicating a gradual decrease of bulk water content in host eclogite during the garnet growth. Combined with the theoretical calculation based on the mineral modes and mineral water contents, it is suggested that the subducting oceanic crust would release reduced amounts of fluids during prograde metamorphism from greenschist to eclogite facies. The different garnet zones have f O 2 values varying from ΔFMQ+1.77 to ΔFMQ+3.26, suggesting that the fluids released from the subducting oceanic crust were oxidizing during subduction to peak eclogite facies and its early exhumation stage. These garnet f O2 values are correlated with the water contents of water-saturated garnet, implying that the water content of metamorphic garnet is a potential means to recover the redox state of its host eclogite. The increase of f O2 values during prograde metamorphism of the eclogite indicates the breakdown of chlorite and possible fluid influx from serpentinite, whereas the significant drop of f O2 values during the early exhumation stage indicates the breakdown of lawsonite to release the oxidized fluid. This study demonstrates that a combined study of water content, major and trace elements, and mineral inclusions in metamorphic garnet can potentially provide a new perspective into the behavior and property of fluids released from the subducting oceanic crust. • Garnet exhibits zoning in inclusions, major-trace elements, water content and oxygen fugacity. • Multiple stages of garnet growth were recognized along the P-T path of Hong'an eclogite. • Garnet water contents reflect decrease of fluid release from subducting oceanic crust. • Garnet water contents are correlated with redox conditions of its host eclogite. • Oxidized fluid is released during the dehydration of the subducting oceanic crust.

Garnet: The archive of prograde metamorphic records of granulite-facies terranes

<p indent="0mm">Whether granulite terranes can preserve prograde metamorphic records is a perturbing topic in metamorphic petrology, and related research is rare. This is mainly because the metamorphic records of the granulite terranes have information asymmetry. Generally, the mineral assemblages and compositions of granulite-facies rocks are considered to reflect peak and retrograde metamorphism, rather than prograde metamorphism. However, this assumption must be made with caution. Case studies of granulites from the junction area of Shanxi and Hebei province in the North China Craton suggest that garnet could serve as “temperature and pressure resistant containers” under granulite-facies conditions, and retain the evidence of pre-peak metamorphism. For pelitic granulites, their protoliths are supracrustal rocks, and must have undergone prograde metamorphic processes from the Earth’s surface to the deep crust. Therefore, a confirmatory study seeking prograde metamorphic records for pelitic granulites from the Huai’an-Datong area was conducted. In the studied pelitic granulites, inclusions in garnet display well-preserved microstructural zoning, with radical zonal distribution patterns: Crowded quartz grains in the core, a few K-feldspar-kyanite-bearing assemblages in the mantle, and some sillimanite flakes in the rim. The major element zoning (here, major endmember proportions X_Sps, X_Alm, X_Pyr, and X_Grs are used) of garnet is relatively homogeneous. However, the high rare earth elements (HREEs) exhibit bell-shaped profiles decreasing from the core to the rim. This suggests that even if the major elements of garnet are homogenous due to high-temperature diffusion, microstructural zoning and some trace elements with slow diffusion rates could be protected from peak- and retro-grade high-temperature modification. For garnet mafic granulites, their protoliths are considered as gabbro dykes, and there is no robust evidence of prograde metamorphism from the upper to lower crust; therefore, their tectonic significance is unclear. Here, an exploratory study of prograde metamorphism for garnet mafic granulites from the Huai’an-Hengshan area was conducted based on the study experience of pelitic garnet. In the investigated mafic granulites, garnet also displays a zoned distribution of various mineral assemblages from its center to rim, including quartz- and titanite/ilmenite-bearing assemblages in the core, rutile-bearing assemblages in the outer mantle to the inner rim, and a resorbed rim. Fortunately, garnet of the mafic granulites displays well-preserved compositional growth zoning patterns, with bell-shaped X_Sps, increasing X_Pyr from core to mantle, and a resorbed rim (X_Sps). The HREEs (for example, Yb) show bell-shaped patterns similar to those of the X_Sps. Both textural and compositional zoning patterns in the studied mafic granulites documented multiple generations of garnet growth history from the prograde to peak metamorphic stages. These findings provide robust metamorphic evidence for the Paleoproterozoic subduction and assembly of the Jin-Yu Mobile Belt in the North China Craton. The above two case studies suggest that high-temperature granulite-facies metamorphism may not completely reset the multi-generation growth information retained in the garnet. In contrast, minerals in the matrix only record the history of intensive re-equilibration at the peak or rehydration retrogression metamorphism. Thus, garnet is an important archive of prograde metamorphic information. This study further suggests that caution should be exercised when determining the peak<italic> P-T</italic> conditions of granulites using garnet core compositions.

Metamorphic P-T paths and zircon U-Pb ages of the intermediate to felsic granulites from the Jianping Complex, the North China Craton:Implications for the Neoarchean tectonic regime

The Jianping Complex (JPC), located at the northwestern margin of the Eastern Block in the North China Craton (NCC), is one of the high-grade complexes that subjected to granulite facies metamorphism. It has been suggested that the JPC at least experienced two phases of Precambrian metamorphism. This study focuses on the late Neoarchean to early Paleoproterozoic metamorphic evolution of the felsic and intermediate granulites with ‘red-eye socket’ texture. Typical peak stage minerals including plagioclase, orthopyroxene, clinopyroxene and quartz, as well as post-peak growth of garnet, clinopyroxene, plagioclase, amphibole and quartz were identified. Phase equilibria modelling for two selected samples was performed using THERMOCALC 3.45. Based on the petrographic observations and the modelling results, anticlockwise P-T paths are recovered for the granulites. Peak stage metamorphic P-T conditions are constrained to be 940–1000 °C/8.0–8.5 kbar, and the post-peak final solidus assemblages were stable at the conditions of 880–900 °C/9.5–10.5 kbar. U-Pb dating of three granulites yields magmatic ages of ~2.50–2.53 Ga and metamorphic ages of ~2.49 Ga, suggesting the short interval between the emplacement and the granulite facies metamorphism. Anticlockwise P-T paths can result from intrusion and underplating of mantle-derived magmas in several potential tectonic settings, among which scenarios of magmatic arc and mantle plume are both possible for the NCC at ~2.5 Ga. Comprehensively considering the lithological, metamorphic, geochronological, and structural characteristics of the Neoarchean basement rocks in the NCC, we favor the mantle plume model in this study. • The intermediate to felsic granulites from the JPC preserved ‘red-eye socket’ texture. • Anticlockwise P-T paths with peak conditions of 940–1000 °C/8.0–8.5 kbar were retrieved. • The timing of the magmatism and metamorphism are ~2.50–2.53 Ga and ~ 2.49 Ga.

Zircon and monazite reveal late Cambrian/early Ordovician partial melting of the Central Seve Nappe Complex, Scandinavian Caledonides

The Seve Nappe Complex (SNC) comprises continental rocks of Baltica that were subducted and exhumed during the Caledonian orogeny prior to collision with Laurentia. The tectonic history of the central SNC is investigated by applying in-situ zircon and monazite (Th-)U–Pb geochronology and trace element analysis to (ultra-)high pressure (UHP) paragneisses in the Avardo and Marsfjället gneisses. Zircons in the Avardo Gneiss exposed at Sippmikk creek exhibit xenocrystic cores with metamorphic rims. Cores show typical igneous REE profiles and were affected by partial Pb-loss. The rims have flat HREE profiles and are interpreted to have crystallized at 482.5 ± 3.7 Ma during biotite-dehydration melting and peritectic garnet growth. Monazites in the paragneiss are chemically homogeneous and record metamorphism at 420.6 ± 2.0 Ma. In the Marsfjället Gneiss exposed near Kittelfjäll, monazites exhibit complex zoning with cores enveloped by mantles and rims. The cores are interpreted to have crystallized at 481.6 ± 2.1 Ma, possibly during garnet resorption. The mantles and rims provide a dispersion of dates and are interpreted to have formed by melt-driven dissolution-reprecipitation of pre-existing monazites until 463.1 ± 1.8 Ma. Depletion of Y, HREE, and U in the mantles and rims compared to the cores record peritectic garnet and zircon growth. Altogether, the Avardo and Marsfjället gneisses show evidence of late Cambrian/early Ordovician partial melting (possibly in (U)HP conditions), Middle Ordovician (U)HP metamorphism, and late Silurian tectonism. These results indicate that the SNC underwent south-to-north oblique subduction in late Cambrian time, followed by progressive north-to-south exhumation to crustal levels prior to late Silurian continental collision.

High-Density Upper Amphibolite/Granulite Facies Fluid Inclusions in Magmatic Garnet from the Koralpe Mountains (Eastern Alps, Austria)

Fluid and solid inclusions in magmatic garnet from Permian pegmatites of the Koralpe Mountains were investigated. On the basis of MnO/(MnO + FeO) ratios, different degrees of melt fractionation during garnet growth were linked with fluid inclusion densities and chemistries. It is shown that garnet indicating low-melt fractionation trends contained primary CO2 ± N2-rich fluid inclusions of the highest densities, up to 1.15 g/cm3, compared to garnet samples of increased fractionation trends comprising CO2-N2-rich fluid inclusions with lower densities up to 0.85 g/cm3. This fluid composition is interpreted as a part of an unmixed CO2 ± N2-H2O-rich fluid that was present during garnet crystallization. Variabilities in the nitrogen composition up to 40.83 mol% resulted from different degrees of partial melting of mica and plagioclase from the metapelitic host rock. Densities, fluid chemistries, and mineral chemical data enabled a continuous upward trend for garnet crystallization during anatexis from lower (ca. 25 km) up to middle crustal levels (12–15 km). Resulting amphibolite/granulite facies conditions of 7.6 kbar/700 °C for garnet crystallization in spodumene-free pegmatites were significantly higher than previously suggested for pegmatite formation in the Koralpe Mountains.

Research on valorisation of spent garnets as addition in cementitious materials – preliminary experimental evaluation

Abrasive waterjet (AWJ) material processing represents a relatively new and extremely efficient method in the specific industries, implying various applicability areas, attributed to different materials, with different properties and domains of use. Besides numerous advantages of the AWJ cutting and generally of the material processing techniques, they also involve the waste generation: the abrasive sands are converted into sludge material, collected into recipients and further on, after natural drying, they become waste dumps randomly deserted. The enlargement of the AWJ techniques in the latest years determines the corresponding increase of the associated Garnet wastes (Spent Garnets, SG), thus leading towards the clear need of identifying opportunities for their recycling and valorisation. Aggregates are basic raw materials in the production of concrete, mortar and plasters, composite cementitious and/or cement-free materials (geopolymer concrete) in the building materials and generally, in the construction industry. The rapid growth of the population, recorded mainly in urban areas, determines an increasing demand on housing facilities and consequently, on concrete production and aggregates consumption. Aggregates and sand, mainly exploited from riverbeds or quarries, represent exhaustible natural resource for which substitution solutions need to be found, in order to control and reduce their extraction from the natural landscape. Considering the superposition of this independent cause and effect situations, the rapid growth of Spent Garnets (SG) landfills, generated by AWJ processes, and, on the other hand, the need to substitute the natural aggregate in construction industry, a reliable solution could emerge from the potential valorisation of SG wastes as partial or even complete substitution of sands / aggregates in the composition of construction materials. The present paper offers a preliminary overview regarding the possibility of incorporating SG wastes of local production in usual cement-based materials, as partial substitute of the aggregate, for the double purpose of waste management implementation and natural resources protection, on the transition path towards the implementing the Circular Economy (CE) concept in the Romanian industry.

Anatomy of Garnet from the Nanminghe Skarn Iron Deposit, China: Implications for Ore Genesis

Garnet is a common constituent of skarn type iron deposits and can be used to derive potential information on the genesis of skarn type deposits. Here, we investigate the petrologic, spectroscopic, and geochemical characteristics of garnet from the Nanminghe skarn iron deposit in China to elucidate the formation process, growth environment, and genesis. We employ a combination of multiple techniques including petrography, Infrared spectroscopy (IR), X-ray powder diffraction (XRD), Raman spectrum, electron microprobe, and LA-ICP-MS. The primary mineral assemblage in the skarn is garnet–diopside–magnetite–quartz–calcite–pyrite. The garnet occurs as granular aggregates or veins, and generally shows a combination form bounded by dodecahedral faces {110} and trapezohedron faces {211}. Oscillatory zoning and abnormal extinction of garnet are also noted. We identify at least three stages of garnet growth, with a gradual decrease in the iron content from early to late stage, accompanied by the precipitation of magnetite. Regarding the rare earth distribution model, the Nanminghe garnet is generally in the right-dipping mode enriched in LREE and depleted in HREE, which may be mainly controlled by adsorption. Major and trace elements of different generations of garnet suggest that the garnet in the iron skarn crystallized under high oxygen fugacity and is of hydrothermal origin.

Garnet crystallization mechanisms and localized polymetamorphism in the southwestern Meguma Terrane, Nova Scotia, Canada

AbstractWe present data on the pressure and temperature (P–T) conditions experienced by metamorphic rocks of the Meguma Terrane, Nova Scotia, Canada, also utilizing three‐dimensional microstructural data on one sample to better constrain the mechanisms that controlled garnet crystallization. Inverse and forward thermodynamic modelling place peak P–T conditions in the southwestern Meguma Terrane at ~650°C and 4.5 kbar. Interpretation of these results with petrographic observations and previous P–T constraints across the terrane suggests that amphibolite facies metamorphism occurred during the Devonian Neoacadian orogeny (406–388 Ma). Integration of quantitative 3D textural data with an estimated metamorphic heating rate of &lt;5°C/Myr is consistent with amphibolite facies metamorphism resulting from tectonic loading during the Neoacadian orogeny, though the exact nature of the orogeny is still not well understood. Further, the intrusion of granitic plutons into the Meguma metasediments at 373 Ma likely locally drove metamorphic recrystallization (polymetamorphism). The 3D size, shape, and location of garnet crystals in one sample reveal that the rate‐limiting step for garnet crystallization was likely the diffusion of aluminium through the intergranular matrix at length scales less than the mean nearest neighbour distance between garnet crystals. Nucleation was aided by epitaxial overgrowth onto a muscovite substrate, though it appears there may have been a decoupling between minerals providing a substrate and those providing nutrients during garnet growth.

Rapid, paced metamorphism of blueschists (Syros, Greece) from laser-based zoned Lu-Hf garnet chronology and LA-ICPMS trace element mapping

Unravelling the timing and rate of subduction-zone metamorphism and H2O release requires linking time to the compositional changes of petrogenetic indicator minerals in blueschists and eclogites. Garnet is a key mineral in this regard, as it forms by, and records, H2O-releasing mineral reactions that can be linked to pressure and temperature conditions. These can be placed in time using chronology. Bulk-or multi-grain ages are the norm in garnet chronology. While these ages provide important and precise time constraints on reactions across both facies, the duration, rates and style of growth—crucial parameters in the petrological and geochemical evolution of subducting oceanic crust—remain unconstrained. Domain dating, i.e., dating of individual growth zones, is necessary to investigate these aspects, but is generally impeded in “common”-sized garnet grains due to sample size requirements. To overcome these limitations, we use a low-loss micro-sampling by laser cutting combined with Lu-Hf chronology. We used this approach to precisely date multiple growth zones in a 1-cm garnet grain from glaucophane-bearing micaceous schist of the Cycladic Blueschist Unit (CBU), Syros Island, Greece. The analysis was combined with major- and trace-element mapping (EPMA, LA-ICPMS) to investigate garnet petrogenesis. Garnet core and mantle zones are chemically comparable and identical in age within 0.1 Myr precision (2σ), indicating an initial growth pulse at 51.8 ± 0.1 Ma (MSWD = 1.13). The two rim zones, which are chemically distinct, were resolvably younger at 51.3 ± 0.2 Ma (MSWD = 0.67). Integrated with compositional data, these age constraints provide a time-resolved record of extremely fast, pulsed garnet growth and fluid-release reactions, such as chloritoid breakdown. These reactions can be considered near-instantaneous on subduction time scales, indicating that re-equilibration far outpaces changes in pressure and temperature conditions. The analysed sample is associated with the upper-subunit of the CBU with oceanic affinities, which does not share the last part of the prograde history with the rest of the CBU, i.e., the margin sub-unit. We interpret our sample as a sliver that became attached to the base of the ophiolitic sub-unit and was thrusted onto the lower margin unit at high-pressure. Beyond providing insights into the timing of subduction-zone processes, our new protocol for zoned garnet Lu-Hf geochronology of “common-sized” garnet opens new possibilities for constraining the causes and rates of garnet growth and the pace of tectonic processes in general.

Constraining the P–T path of (U)HP rocks with reaction overstepping during subduction; example from the Western Gneiss Region (Norway)

AbstractThis paper presents the results of petrological observations and diffusion modelling on garnet from high‐pressure to ultrahigh‐pressure ((U)HP) metamorphic rocks of the Western Gneiss Region in the Nordfjord. Garnet from kyanite‐bearing micaschist preserves two generations of garnet growth that are related to the Pre‐Caledonian granulite facies and Caledonian eclogite facies metamorphic events. Mafic eclogite, forming lenses in the micaschist, contains only eclogite facies assemblages with partial recrystallization under amphibolite facies conditions. Caledonian garnet in both the micaschist and hosting eclogite indicates reaction overstepping and nucleation near or above 550°C/2.0 GPa. Maximum pressure and temperature, calculated using pseudosection modelling for the eclogite facies event, were ~2.6 GPa and 650°C. The interface between the Pre‐Caledonian and Caledonian garnet in the micaschist shows a strong compositional gradient or possibly a compositional jump. The preservation of such a gradient together with the hummocky‐shaped composition profiles in the Caledonian garnet from the eclogite indicates either no relaxation or a short‐time of relaxation of the rocks at their peak temperature conditions, as well as their exhumation by cooling. Possibly, heating or exhuming of the rocks by isothermal decompression could have easily modified such compositional irregularities along the garnet profiles. A cooling rate of ~187°C/Ma and exhumation rate in the vertical direction of ~2.5 cm/year for the HP rocks were obtained by considering that the temperature and transport distance changes from their maximum depth and peak temperature to the surface were proportional to the time (3.5 Ma) calculated by modelling for the garnet.

Timing and significance of pre-Olarian metamorphism in the Willyama Supergroup, Broken Hill, Australia

In the Willyama Supergroup of the Curnamona Province, the intensity of granulite facies Olarian metamorphism at c. 1600 Ma has hampered efforts to elucidate the timing and nature of preceding lower-grade metamorphic events. Previous dating of pre-peak monazite inclusions in aluminous metapelites has been complicated by potentially incomplete isotopic ‘shielding’ by their host porphyroblasts, owing to pervasive deformation and recrystallisation of these rocks during the c. 1600 Ma event. Near the Broken Hill Pb-Zn orebody, quartz-feldspar-biotite-garnet (‘Potosi-type’) gneiss proved more resistant to recrystallization during granulite facies metamorphism: new Sensitive High Resolution Ion MicroProbe (SHRIMP) U-Pb analyses of monazite from this rock delineated a population of relatively Y-rich (median 0.96 wt%) cores at 1636.8 ± 4.4 Ma (95% confidence), overprinted by Y-poor (median 0.10 wt%) rims and domains at 1597.3 ± 3.3 Ma. Garnet is a sink for Y in metamorphic rocks, which suggests that Y-rich c. 1637 Ma monazite growth predated garnet formation (and possibly occurred at relatively low metamorphic grade), whereas the Y-poor nature of the c. 1597 Ma rims and domains implies that dissolution and reprecipitation during Olarian metamorphism postdated porphyroblastic garnet growth. Coexisting low-Th/U zircon rims are dominated by a 1600.7 ± 3.6 Ma population, emphasising that the event responsible for c. 1637 Ma monazite growth was largely invisible to zircon. Interlayered mafic granulite 70 m away yielded partially recrystallised zircons with dark-CL internal domains which establish a minimum age of 1639 ± 8 Ma for the igneous precursor. Recrystallisation was accompanied by actinide depletion, with bright-CL, U-poor and low-Th/U domains dated at 1596 ± 8 Ma. Our new 1640–1635 Ma dates constitute robust isotopic evidence for pre-Olarian mafic magmatism and metamorphism within the lower Willyama Supergroup. We infer an extensional environment in the upper mid-crust for this tectonothermal activity, coeval with ongoing sedimentation and deposition of the upper Paragon Group at the surface. Extension of the eastern Curnamona Province at c. 1640 Ma is coeval with sediment-hosted base-metal mineralisation in the Mount Isa Inlier and the McArthur Basin, and is consistent with tectonic regimes prevailing throughout Proterozoic southern, central and eastern Australia at that time.

The Subducting Slab as a Chromatographic Column: Regimes of Sub‐Solidus Mass Transport as a Function of Lithospheric Hydration State, With Special Reference to the Fate of Carbonate

AbstractWe investigate the chemical budget of subduction zones at sub‐solidus conditions using a thermodynamic‐numerical simulation in which all major rock components are treated as soluble and potentially mobile in aqueous fluids. This new strategy significantly improves the accuracy of predicted fluid‐rock equilibrium compositions in open petrological systems. We show that all slabs release volatiles and nonvolatiles to the mantle wedge, contributing to its refertilization. But some mobile constituents, such as alkali and alumina, may be trapped along layer boundaries or traverse without interaction depending on chemical contrasts between adjacent lithologies. The accumulation of igneous alumina and silica in the limestones of the central‐eastern Pacific slabs drives their decarbonation and is marked by metasomatic garnet growth. Those slabs are also predicted to lose much of their alkalis before sub‐arc depth. Even when they are produced in the altered mafic and ultramafic layers, fluids reach the slab/mantle wedge interface with distinct compositional signatures that are typical of the sedimentary cover. We distinguished supply and transport limited regimes of element subduction by testing the sensitivity of our mass balance to changes in slab hydration state (HS). Transport limited slabs sensitive to HS include notably a hotspot of carbon release to the mantle wedge (e.g., Costa Rica). Finally, we show that the quantitative budgets do depend on the geometry of fluid flows, and on assuming that slabs are mechanically continuous structures, which is questionable. Taken together, these insights will help better constrain the long‐term chemical evolution of the shallow planetary interior, and the thermomechanical behavior of the subduction interface.