Original Garnet Research Articles

Ultrahigh-pressure peridotites record Neoarchean collisional tectonics

It is debated when plate tectonics first operated on Earth. One of the arguments against the Archean (> 2.5 Ga) operation of plate tectonics is the lack of rock records that can be best explained to be formed at convergent plate boundaries, such as continental lithosphere metamorphosed at ultrahigh-pressures (> 2.7 GPa or 80–100 km). Here we report Archean ultrahigh-pressure peridotites in Eastern Hebei, the North China Craton. Bulk-rock and mineral compositions suggest that these peridotites are likely cumulates or slivers of metasomatized continental lithospheric mantle. Garnet pseudomorphs and pyroxene exsolution textures are preserved in these Archean peridotites, indicating decompression-induced breakdown of the original garnet and pyroxene from high pressures. We reintegrate the original garnet and clinopyroxene based on mass proportions and compositions of decompression-induced breakdown products. The reconstructed garnet and clinopyroxene compositions of these Archean peridotites indicate that they were brought up from mantle depths of 110–130 km. We propose that these ultrahigh-pressure peridotites are tectonic slivers of a collisional complex, possibly subducted to mantle depths and then exhumed to crustal levels during Neoarchean subduction and subsequent arc/continental collision, similar to those from Phanerozoic continental collisional zones. The Archean ultrahigh-pressure peridotites in the North China Craton provide direct evidence for operation of continental collisional plate tectonics since at least 2.5 billion years ago.

Garnet major-element composition as an indicator of host-rock type: a machine learning approach using the random forest classifier

The major-element chemical composition of garnet provides valuable petrogenetic information, particularly in metamorphic rocks. When facing detrital garnet, information about the bulk-rock composition and mineral paragenesis of the initial garnet-bearing host-rock is absent. This prevents the application of chemical thermo-barometric techniques and calls for quantitative empirical approaches. Here we present a garnet host-rock discrimination scheme that is based on a random forest machine-learning algorithm trained on a large dataset of 13,615 chemical analyses of garnet that covers a wide variety of garnet-bearing lithologies. Considering the out-of-bag error, the scheme correctly predicts the original garnet host-rock in (i) > 95% concerning the setting, that is either mantle, metamorphic, igneous, or metasomatic; (ii) > 84% concerning the metamorphic facies, that is either blueschist/greenschist, amphibolite, granulite, or eclogite/ultrahigh-pressure; and (iii) > 93% concerning the host-rock bulk composition, that is either intermediate–felsic/metasedimentary, mafic, ultramafic, alkaline, or calc–silicate. The wide coverage of potential host rocks, the detailed prediction classes, the high discrimination rates, and the successfully tested real-case applications demonstrate that the introduced scheme overcomes many issues related to previous schemes. This highlights the potential of transferring the applied discrimination strategy to the broad range of detrital minerals beyond garnet. For easy and quick usage, a freely accessible web app is provided that guides the user in five steps from garnet composition to prediction results including data visualization.

Pronounced and rapid exhumation of the Connecticut Valley Trough revealed through quartz in garnet Raman barometry and diffusion modelling of garnet dissolution–reprecipitation reactions

AbstractMetamorphic rocks from the Connecticut Valley Trough (CVT), Vermont, and Massachusetts, have been examined using quartz‐in‐garnet (QuiG) and conventional thermobarometry, thermodynamic reaction modelling, diffusion modelling, and 40Ar/39Ar thermochronology to constrain their P–T–t paths during Acadian metamorphism and subsequent exhumation. Numerous samples, collected in the vicinity of the Acadian domes, contain garnet porphyroblasts that display cloudy zones characterized by numerous fluid inclusions and modified garnet compositions associated with the replacement of the original garnet by biotite±muscovite±plagioclase±quartz±low Xgrs/enriched Xsps. QuiG and conventional thermobarometry constrain both the conditions of garnet nucleation and peak P–T conditions to have occurred at ~0.85–1.05 GPa, ~550–600°C. Most notably, QuiG barometry was performed on inclusions adjacent to these reaction zones in conjunction with Gibbs method reaction modelling to reveal that these dissolution–reprecipitation reactions occurred during nearly isothermal decompression from the peak P–T conditions to around ~0.3 GPa, 550°C. Diffusion modelling reveals that the Mn zoning profiles created during garnet resorption that accompanied decompression formed in less than c. 3 Ma, which constrains the tectonic exhumation to have occurred at 8–10 mm/year. Subsequent cooling to 500°C occurred rapidly at a rate of 100°C/Ma, followed by slower cooling reaching 1.7°C /Ma by the mid Carboniferous. This is the first reported example of QuiG barometry revealing a multi‐stage metamorphic history and highlights the utility of this method for unravelling complex metamorphic terranes.

Formation of atoll garnets in the UHP eclogites of the Tso Morari Complex, Ladakh, Himalaya

The eclogites of the Tso Morari Complex, Ladakh, NW Himalayas preserve both garnets with spectacular atoll textures, as well as whole porphyroblastic garnets. Whole garnets are euhedral, idiomorphic and enclose inclusions of amphibole, phengite and zoisite within the cores, and omphacite and quartz/coesite towards the rims. Detailed electron microprobe analyses and back-scattered electron images show well-preserved prograde zoning in the whole garnets with an increase in Mg and decrease in Ca and Mn contents from the core to the rim. The atoll garnets commonly consist of euhedral ring over island/peninsular core containing inclusions of phengite, omphacite and rarely amphibole between the core and ring. Compositional profiles across the studied atoll grains show elemental variations with higher concentrations of Ca and Mn with low Mg at the peninsula/island cores; contrary to this low Ca, Mn and high Mg is observed at the outer rings. Temperature estimates yield higher values at the Mg-rich atoll garnet outer rings compared to the atoll cores. Atoll garnet formation was favoured by infiltration of fluid formed due to breakdown of hydrous phases, and/or the release of structurally bounded OH from nominally anhydrous minerals at the onset of exhumation. Infiltration of fluids along pre-existing fracture pathways and along mineral inclusion boundaries triggered breakdown of the original garnet cores and released elements which were subsequently incorporated into the newly-grown garnet rings. This breakdown of garnet cores and inward re-growth at the outer ring produced the atoll structure. Calibrated geo-thermobarometers and mineral equilibria reflect that the Tso Morari eclogites attain peak pressures prior to peak temperatures representing a clockwise path of evolution.

REE partition among zircon, orthopyroxene, amphibole and garnet in a high-grade metabasic system

AbstractA mafic amphibole-bearing granulite with porphyroblastic garnet was investigated to evaluate: (1) the rare earth element (REE) partition among garnet, zircon, orthopyroxene and amphibole during the metamorphic evolution; (2) the significance of the REE distribution along lobes and bights of reabsorbed garnet rim; and (3) REE distribution coefficient values (DREE) suggestive of chemical equilibrium, assuming garnet as a reference. The results have been compared with those deriving from an intermediate granulite containing porphyroblastic garnet, without amphibole. Porphyroblastic garnet from both samples is rimmed by a continuous corona formed during post-peak decompression characterized by REE-enriched lobes and REE-poor bights. The amphiboles from corona have various REE abundances, reflecting a different dissolution rate of original garnet rim. The initial slow rate of garnet dissolution caused high REE concentration in the new garnet rim due to intra-crystalline diffusion, leading to the formation of REE-poorer amphiboles in corona. Subsequently, under an increasing geothermal gradient and fluid-present conditions, the faster dissolution of garnet determined the formation of bights and the transfer of REEs towards the corona. The timing of garnet growth and its dissolution were checked by U–Pb zircon ages. The zircons dated from 339 Ma to 303 Ma in two rock types combined with the garnet domains (core, outer core, rim) show similar distribution of patterns relative to heavy rare earth elements for zircon and garnet (DHREEzrn/grt), suggesting chemical equilibrium. Zircons dated at c. 300 Ma do not appear in equilibrium with REE-rich garnet lobes, and younger zircons (278 Ma) show a new equilibrium with REE-poor garnet bights. On this basis, the DHREEamph/grt values obtained in specific textural sites might be interpreted as suggestive of equilibrium under granulite conditions.

Multi-stage evolution of the lithospheric mantle beneath the westernmost Mediterranean: Geochemical constraints from peridotite xenoliths in the eastern Betic Cordillera (SE Spain)

Spinel (±plagioclase) peridotite xenoliths from the Tallante and Los Perez volcanic centres in the eastern Betics (SE Spain) range from depleted (clinopyroxene-poor) harzburgites to fertile (clinopyroxene-rich) lherzolites and orthopyroxene-free wehrlites. Significantly, only one harzburgite, which is depleted in heavy rare earth elements (HREE), retains the imprint of ca. 20% ancient melting of an original garnet lherzolite source. In contrast, REE abundances of other harzburgites and lherzolites from the eastern Betics have been increased by melt-rock reaction. The whole-rock and mineral compositions of these mantle rocks are largely controlled by three types of modal metasomatism: 1) common clinopyroxene-orthopyroxene addition and olivine consumption which increased FeOt, SiO2 and Al2O3, and decreased MgO compared to the refractory melting products; 2) subordinate orthopyroxene dissolution and precipitation of clinopyroxene and olivine, which led to higher FeOt and MgO and lower SiO2 than in common (orthopyroxene-rich) lherzolites; and 3) rare orthopyroxene consumption and olivine addition that caused higher FeOt and lower SiO2 compared to the original melting residues. These mineral modal and major element variations have been produced mostly by interactions with relatively FeOt-rich/SiO2-poor melts, likely derived from a peridotite-pyroxenite lithospheric mantle with a highly heterogeneous isotopic composition. Melting of the lithospheric mantle in the western Mediterranean was triggered by upwelling of the asthenosphere induced by back-arc extension in the Late Oligocene-Early Miocene. Trapping of small fractions of exotic melts in whole-rocks — likely the parental magmas of Miocene back-arc dykes that intruded the Betic crust — caused local disequilibrium between the trace element signatures and Pb isotopic compositions of clinopyroxene and whole-rock. Subsequent interaction with SiO2-undersaturated magmas, similar to the parental melts of the Pliocene alkali basalts that host the xenoliths, promoted orthopyroxene consumption and clinopyroxene-olivine enrichment at locations close to magma conduits, and finally generated orthopyroxene-free wehrlites. This event constitutes the last episode of the Cenozoic magmatic evolution of the westernmost Mediterranean which is recorded in the mantle xenoliths from the eastern Betics.

Experimental synthesis of isochemical kelyphite — a preliminary report

We observed the breakdown of natural garnet in a high–pressure experiment (0.85 GPa and 1100 °C for 2 days) using a piston–cylinder apparatus. Two different types of kelyphites were formed: (1) isochemical and (2) metasomatic. The isochemical kelyphite was composed of orthopyroxene + spinel + plagioclase, whereas the metasomatic one, having a composition altered significantly from the original garnet, was composed of olivine + spinel + plagioclase. These synthetic kelyphites were studied and characterized using EPMA, SEM, EBSD, and TEM and were shown to have many microstructural features as well as their mineral assemblages in common with observed natural kelyphites. The kelyphitization was very heterogeneous and local and was apparently controlled by grain surfaces and intra–grain cracks in garnet. The metasomatic kelyphite apparently formed by a reaction between garnet and aqueous fluids that may have been introduced from outside the platinum capsule through holes accidentally made in the capsule.

Palaeoproterozoic metamorphism and cooling of the northern Nagssugtoqidian orogen, West Greenland

Lu–Hf and Sm–Nd garnet geochronology combined with geothermobarometric calculations allowed us to gain new insight into the metamorphic evolution of the Northern Nagssugtoqidian orogen (NNO), in West Greenland. Three samples were collected from the southern (SM 310 and SM 316) and northern (SM 339) parts of the NNO. They revealed amphibolite-grade Palaeoproterozoic metamorphism under peak temperature conditions decreasing from c. 810 to 840°C at 7.5kbar (SM 310, 316) to c. 690°C at 10kbar (SM 339) in the southern and northern part of the NNO, respectively. Our garnet dating defines two episodes: (1) at c. 1780Ma, determined by Lu–Hf in sample SM 316 and Sm–Nd in sample SM 339, most likely approximating the time of garnet growth, and (2) at c. 1750Ma defined by the remaining Sm–Nd dates, reflecting time of cooling below isotopic closure. Original garnet growth patterns preserved by HREE (SM 316) and clearly seen zonation of Sm and Nd (SM 339) suggest that the garnets may not have been completely reset and thus, the obtained age of 1780Ma could be fairly close to the time of garnet crystallisation.The growth of peak metamorphic mineral assemblages in the southern NNO took place after the end of deformation, as demonstrated by sample SM 316 that was collected from an undeformed mafic dyke cutting across the regional fabric. In contrast, sample SM 339 revealed synkinematic growth of peak metamorphic paragenesis that crystallised in a low angle top-to-SW shear zone overprinted on the pre-existing fabric. Our results are consistent with the interpretation of the NNO as an Archaean crustal block, the Aasiaat domain, that was affected by Palaeoproterozoic metamorphism and localised deformation at 1780Ma, i.e., 70Ma after the Nagssugtoqidian collision. This event can be compared to the formation of steep belts (major left-lateral strike-slip shear zones) in the central part of the Nagssugtoqidian orogen. The Aasiaat domain is considered to be an independent microplate separating the Nagssugtoqidian and Rinkian orogens which define respectively its southern and northern margins. The interpretation presented implies the existence of two discrete tectonic sutures to the north and south of the Aasiaat domain.

In Situ Genesis of Alumino-Ferruginous Nodules in a Soil Profile Developed on Garnet Rich Micaschist in the High Reliefs of South Cameroon Rainforest Zone (Central Africa)

The aim of the study is to highlight in detail using microscopic observations, coupled with mineralogical and geochemical analyses, the genesis of alumino-ferruginous nodules formed during weathering of a garnet rich micaschist in the high reliefs of the south Cameroon plateau. Morphologically, the studied profile is characterized by four horizons from bottom to top: (i) an isalteritic horizon in which there are numerous crystals of garnet with intragranular fracture coatings, which isolate either the original garnet grain fragments or alveolar voids with crystals of gibbsite; (ii) an alloteritic horizon with slightly indurated nodules surrounded by a discontinuous perinodular micaceous cortex which marks the original flow structures of the original bedrock, and where intragranular fracture coatings still isolate alveolus with gibbsite crystals; (iii) a nodular horizon characterized in addition to the properties noted below either by an orange-brown birefringent micromass or a darker and undifferentiated micromass; and (iv) a set of clayey and loose horizons with few nodules, which always show gibbsite crystallarias as in other horizons. Mineralogically, traces of kaolinite are recorded only in the weathering garnet in the isalteritic horizon. Other nodules are made of three secondary minerals gibbsite, hematite and goethite. Geochemically, there is a high expression of aluminium in nodules whose composition remains dominated by silica. Globally, those nodules are formed in situ from original garnet grains as well illustrated by the presence of discontinuous perinodular micaceous cortex and the compartmentalized structure. They are named alumino- ferruginous nodules due to their enrichment in gibbsite, hematite and goethite. This is a typical example of in situ formation of nodules from primary minerals which characterize the south Cameroon plateau high reliefs. This process may be extended to the high reliefs of the intertropical rainforest zone not capped in the past by iron duricrust, thus without any iron duricrust relicts.

Using calculated chemical potential relationships to account for coronas around kyanite: an example from the Bohemian Massif

AbstractCorona textures around kyanite, involving for example zoned plagioclase separating kyanite from the matrix, reflect the instability of kyanite with the matrix on changing P–T conditions, commonly related to decompression. The chemical potential gradients set up between the kyanite and the matrix as a consequence of slow Al diffusion drive corona development, with the zoning of the plagioclase reflecting the gradients. Calculated mineral equilibria are used to account for corona textures involving plagioclase ± garnet around kyanite, and replacement of kyanite by plagioclase + spinel symplectite, in quartz + plagioclase + K‐feldspar + garnet + kyanite granulite facies gneiss from the Blanský les massif in the Bohemian massif, Czech Republic. In the garnet‐bearing coronas, a commonly discontinuous garnet layer lies between the kyanite and the continuous plagioclase layer in the corona, with both the garnet and the plagioclase appearing mainly to replace matrix rather than kyanite. The garnet layer commonly extends around kyanite from original matrix garnet adjacent to the kyanite. Where garnet is missing in the corona, the kyanite itself may be replaced by a spinel–plagioclase corona. In a local equilibrium model, the mineral and mineral compositional spatial relationships are shown to correspond to paths in μ(Na2O)–μ(CaO)–μ(K2O)–μ(FeO)–μ(MgO)–μ(SiO2) in the model chemical system, Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2 (NCKFMAS). The discontinuous nature of the garnet layer in coronas is accounted for by the effect of the adjacent original garnet on the chemical potential relationships. The replacement of kyanite by spinel + plagioclase appears to be metastable with respect to replacement by corundum + plagioclase, possibly reflecting the difficulty of nucleating corundum.

Stress-induced redistribution of yttrium and heavy rare-earth elements (HREE) in garnet during high-grade polymetamorphism

Almandine garnet (Alm62.6Pyr11.4Sps8.4And6.5Grs4.0) exceptionally rich in Y2O3 (1.8–2.3 wt%), Sc2O3 (0.11–0.23 wt%), HREE2O3 (1.9–2.3 wt%), and Na2O (~0.3 wt%) occurs in Precambrian quartzofeld-spatic granulites ( T ~ 860 °C) on Hisaroya, Western Gneiss Region, Norway. The granulites were partially reequilibrated to eclogite-facies ( P = 14.9 ± 1.3 kbar, T = 649 ± 67 °C) and amphibolite-facies assemblages during the Caledonian Orogeny. The granulite-facies garnet is reequilibrated in an outer zone, typically ~4 μm thick, where both Y2O3 and HREE2O3 concentrations decrease to 1.2 wt%, and overgrown by Caledonian garnet with Y2O3 and HREE2O3 both below 0.1 wt%. The granulite-facies cores are also reequilibrated to lower Y2O3 and HREE2O3 (both ~1.8 wt%) along deformation structures including microfaults with horsetails, en-echelon bands, and splaying features around the tip of fractures. Locally, these reequilibrated zones, typically 10–20 μm across, have shoulders with higher Y and HREE (~2.9 wt%) than in the original garnet, suggesting closed-system behavior for these elements. In addition, the garnet locally displays a complex mesh-like pattern with high and low HREE and Y bands, possibly related to strain. Two charge-balancing mechanisms account for Y and HREE incorporation: (1) the yttrium aluminum garnet (Y2Al2O6, YAG) substitution involving incorporation of Al into the tetrahedral site is dominant in the granulite-facies garnet, and (2) coupled substitution with Na into the dodecahedral site is increasingly important during high- P reequilibration. The internal closed-system reequilibration of the granulite-facies garnet may be related to stress concentrations build up in garnet during the eclogite-facies event. Local stress concentrations during propagation of fractures may be the driving force behind the redistribution of elements seen within cores of granulite-facies garnet.

Characterization of an early metamorphic stage through inclusions in zircon of a diamondiferous quartzofeldspathic rock from the Erzgebirge, Germany

About 1000 zircon grains from a diamondiferous quartzofeldspathic rock of the Saxonian Erzgebirge were investigated for inclusions using optical microscopy and confocal laser-Raman spectroscopy. Cathodoluminescence imagery was applied to characterize the growth zone of zircon where the inclusions occurred. The most abundant inclusion minerals are microdiamonds. Coesite was not detected. However, garnet and jadeite occur as rare inclusions in zircon cores where diamonds are lacking. Jadeite was detected for the first time in quartzofeldspathic rocks from the crystalline complex of the Erzgebirge. The compositions of the pristine garnets in the zircons are similar to those of core areas of millimeter-sized garnets but the original garnet composition of the early metamorphic stage is only preserved in zircon. Intracrystalline diffusion at temperatures as high as 1000 °C resulted, for instance, in higher Ti concentrations in garnet cores compared with garnet enclosed in zircon. Rutile, quartz, and the compositions of jadeite and garnet inclusions in zircon and of phengite inclusions in cores of large garnets were applied for geothermobarometry. The results, related to an early metamorphic stage, are 650 °C and 18 kbar, which represent conditions at the base of a thickened continental crust before deeper subduction.

Ion irradiation-induced amorphization and nano-crystal formation in garnets

The radiation susceptibility of garnet structure types (A 3B 2(XO 4) 3, I a3d, Z=8) has been examined by 1.0 MeV Kr 2+ irradiation with in situ transmission electron microscopy over the temperature range of 50–1070 K. The target garnets included five natural garnets: almandine, andradite, pyrope, grossular and spessartine, and five synthetic garnets incorporating various contents of actinides. The synthetic garnets were silicates (N-series) and ferrate–aluminates (G-series). The critical amorphization temperature ( T c), above which amorphization does not occur, were determined to be 1050 K for N77, 1130 K for N56, 1100 K for G3, 890 K for G4 and 1030 K for andradite. T c of the synthetic garnets increased with increasing mass density of the target and as the electronic-to-nuclear stopping power ratio decreased. During ion irradiation of the G3 garnet at a temperature of 1023 K near the T c, nano-crystals were produced in which the unit-cell parameter of the original garnet was halved (∼0.64 nm) as a result of a radiation-induced recrystallization process, without evidence for ordering of the cations.

Constraints on Sm-Nd Diffusion Rates from Laser Ablation ICP-MS Analyses and Conventional Dating of Prograde Garnets

Metamorphic garnets are important phases for obtaining both P-T and age information on metamorphic processes. Establishing the rates of cation diffusivity and closure temperatures of the relevant isotopic systems in garnet are therefore crucial for any P-T-t interpretations in metamorphic terranes. While the generally slow diffusion rates of major and trace cations in garnets do not allow a precise experimental determination of their respective diffusion data, calculation of diffusion parameters from the composition of natural garnets is often hampered by the lack of information on their prograde evolution and independent P-T-t constraints. Here we report results of a major element, REE and Sm-Nd isotopic study of prograde garnets from the HP-HT felsic granulites from the Variscan Bohemian Massif. The garnets (Aim 0.53, Prp 0.27, Grs 0.19, Sps 0.01, up to 2 mm in diameter) are zoned in Mg, Fe, Ca and Mn with Fe/(Fe+Mg) decreasing from core to rim. The outer rim (c. 200 microns) shows retrograde zoning with increasing Fe/ (Fe+Mg), decreasing Ca and a large increase of Mn content (Fig. 1). As garnet is the only Mn-bearing phase in the rock, the Mn increase in the rim can only be attributed to garnet resorption during the retrograde decompression path. Composition maps suggest that the zoning in the rim is not related to proximity of any other phase (e.g. biotite or plagioclase) in the matrix. Laser probe ICP-MS analyses from the identical grain show zoning in REE on a comparable scale to that of the major elements, with Sm and Nd concentrations of 10 and 6 ppm in the core and 5 and 1 ppm in the rim, respectively. The Sm/Nd ratio of 1.5 is constant across the grain, suggesting an infinite REE reservoir (i.e. efficient REE mobility) during garnet growth. However, it increases to 5.5 in the outer 200 microns of the profile. This is also likely to have resulted from the preferential uptake of Sm to Nd during garnet resorption. Modelling the garnet core and plagioclase compositions using THERMOCALC suggests that they equilibrated at c. 20 kbar and 1000~ but approx. 20% of the original garnet volume has been resorbed during decompression and exhumation to upper crustal levels. However, such P-T conditions contrast with biotite inclusions which are present as a prograde phase in the garnet. Simple diffusion modelling using the available diffusion parameters for major cations suggests that in order to preserve the residual growth zoning, the studied garnet must have spent only a short time (<<1 Ma) at the peak conditions. In such a short time span the heat could not have been conducted to the crust but it was rather heated through convection of mantle melts. A minimum final equlibration temperature of the garnet rim and matrix biotite of 750~ is calculated from the Fe-Mg exchange. A whole-rock-garnet SmNd isochron yields an age of 351 + 4 Ma. Because of the presence of the high Sm/Nd rim in the garnets, this data can only represent a minimum age for prograde garnet growth. The end of prograde evolution is further constrained by a series of U-Pb zircon and monazite data as well as Sm-Nd dating of other garnets and accessory phases. These mark the drop in temperature below c. 700~ following isobaric decompression and mostly yield ages in the range of 345-338 Ma. In addition, hornblende, muscovite and biotite Ar-Ar ages, which cluster within the range of 330-295 Ma, indicate a cooling rate of c. 9-10~ between 340 and 295 Ma. The studied compositional profiles show that both prograde and retrograde zoning of major cations and REE in the garnet have been preserved on the same (1 ram) scale. In addition, it can be inferred from the geochronological data that the garnet must have stayed above c. 700~ for a minimum of 10 Ma without resetting its Sm-Nd isotopic system.

Rare Earth Element Geochemistry of Selected Mafic-Ultramafic Units from Singhbhum Craton: Implications to Source Heterogeneity

Abstract Modelling ofREE data indicates that while mafic rocks of Dhanjori Group are partial melting products of a spinel Iherzolite source, Dalma ultrabasics are the residue after the melt-extraction from an original garnet lherzolite rock. Ultramafics of Newer dolerite dykes indicate either derivation by mantle meta somatism or effect of post magmatic serpentinisation. REE data in conjunction with the available geochronological data suggest that significant mantle heterogeneity existed over relatively smaller distances (~100 km) even during Proterozoic and that the mantle evolved in a non-uniform manner with distinct localised processes.

Lower crustal xenoliths from Mongolia and their bearing on the nature of the deep crust beneath central Asia

Rare lower crustal xenoliths found in Cenozoic alkali basalts from the Tariat region in central Mongolia and the Dariganga Plateau in south-eastern Mongolia are the only direct samples of lower crustal material known so far from central and eastern Asia. They are two-pyroxene granulites, including some garnet granulites, as well as scarce amphibolite-facies rocks. The xenoliths are broadly basaltic to andesitic in bulk chemical composition. Their igneous protoliths appear to represent underplated fractionated liquids and cumulates from such liquids. The xenoliths yield equilibration temperatures of 840 ± 30 °C (Wells, 1977) and, for Tariat garnet granulites only, pressures of 14 ± 1.5 kbar. For central Mongolia, these estimates indicate unusually great depths of origin which, however, are in line with some geophysical models for that area.Extensive to complete kelyphitisation has affected the garnets where originally present in the Tariat suite; nevertheless, the kelyphite has largerly preserved the major element and REE compositions of the original garnet. Mineral and whole-rock Sm-Nd data obtained for three samples from Tariat and Dariganga indicate, within large errors, low or zero ages. These may either indicate that the rocks are young (Cenozoic) or that ambient temperatures in the lower crust were high enough to permit continuous isotopic equilibration on a mineral-to-mineral scale.

Crustal contamination in early Basin-and-Range hawaiites of the Los Encinos Volcanic Field, central México

The Los Encinos Volcanic Field (LEVF) consists of Miocene (10.6–13.6 Ma) hawaiitic volcanic necks and lava-capped mesas that crop out sparsely over an area of 11,500 km2 at the eastern margin of the Mexican Basin and Range Province (BRP). The LEVF rocks are similar to other early extensional hawaiites from the southern BRP, and provide numerous contrasts with younger basanites and alkali basalts that erupted during the Quaternary at the Ventura and Santo Domingo Volcanic Fields about 100 km to the south. A suite of 18 LEVF hawaiites was studied in thin section, and analyzed for mineral compositions, whole-rock major and trace element compositions, and Sr, Nd, and Pb isotopic ratios. All samples contain the stable minerals plagioclase (An53–64), olivine (Fo61–88), clinopyroxene, titanomagnetite, and minor biotite. Most samples also contain a complex assemblage of resorbed and reacted xenocrysts and megacrysts. Some of these minerals appear to have crystallized slowly from related, but more differentiated magmas, but other xenocrysts were clearly derived from lower-crustal, high-grade orthogneisses and paragneisses that are found as large xenoliths in the nearby Quaternary volcanic fields. Quartz xenocrysts are especially common in many hawaiites (up to 3.9 vol.%) and show a wide range of reaction styles. One sample contains microxenoliths of sillimanite- and quartz-bearing paragneiss with fine-grained domains that are interpreted as coronal structures related to original garnet xenocrysts. The geochemical effects of crustal contamination in the LEVF hawaiites vary widely. Five samples appear to be essentially uncontaminated (type U). Aside from being somewhat differentiated from a more primitive parent, the type-U samples can be used to infer the geochemistry of the mantle that was melting during the early stages of basin-and-range rifting. Type-U samples range up to ɛNd and down to 87Sr/86Sr=0.70286 and 206Pb/204Pb=18.74, compositions that are more extreme than any of the nearby Quaternary volcanic rocks. The other 13 samples are divided into two contamination types, A and B. Both types show trends toward higher 87Sr/86Sr (to 0.7040) and 206Pb/204Pb (to 18.98), lower ɛNd (to +3.1), and elevated Yb, which appear to reflect bulk or AFC-style contamination by granulites, particularly garnet-bearing paragneisses. Type-A hawaiites also show selective enrichments in Cs, Rb, Th, Sb, U, Pb, K, and Si. These elements were probably transferred into the type-A hawaiitic magmas through mixing with low-degree partial melts from deep-crustal granulites. The enrichments of these elements in type-A hawaiites complement the depletions of many of these same elements in highgrade granulites worldwide and provide insight into the origin of those depletions. Mixing models between type-U hawaiites and paragneiss xenoliths indicate that up to 45% of the Pb found in type-A hawaiites is crustally derived. In comparison with more mafic Quaternary basanitic rocks from the volcanic fields to the south, which carried large peridotite and granulite xenoliths to the surface, the LEVF hawaiites are relatively differentiated and megacryst rich, but free of large xenoliths, and show a wide variety of petrographic and geochemical evidence for crustal contamination. These differences probably reflect the slow and interrupted ascent of the LEVF hawaiites during early stages of basin-and-range extension in the Miocene, when the crust had a somewhat lower density and the entire lithosphere was relatively thick and cool. We argue that Quaternary basanites were able to ascend significantly faster through the thinner, hotter, and more fractured and extended lithosphere, whose crust was made denser by mafic intrusions during the preceding magmatic episode. Consequently the Quaternary basanites rose without stagnating and interacting with crustal lithologies, and without losing their entrained peridotite xenoliths.

Aluminian orthopyroxene in pyrometamorphosed garnet megacrysts from Liaoning and Shandong provinces, northeast China

Aluminian subsilicic orthopyroxene (Al 2 O 3 = 16.8 wt% maximum) was found in pseudomorphs after garnet megacrysts in alkali basaltic rocks occurring at the Mt. Huangyishan, Liaoning province and at Mt. Chishan, Shandong province of northeast China. The orthopyroxene-bearing pseudomorphs are mainly composed of fine-grained (usually less than 2-3 μm in size) orthopyroxene, spinel and anorthite crystals. The pseudomorphs have many internal cracks, and aggregates of coarser-grained (20-300 μm in size) orthopyroxene, spinel and anorthite occur sporadically along the cracks. Olivine or clinopyroxene occurs rarely in the coarser-grained aggregates of a Huangyishan megacryst. In this sample, orthopyroxenes in the olivine- or clinopyroxene-bearing coarser-grained aggregates have fairly restricted Al 2 O 3 contents of 10.5 ± 1.2 and 10.7 ± 1.2 wt%, respectively, and that in the olivine- and clinopyroxene-free aggregates is richer in Al 2 O 3 (12.3 ± 1.2 wt%). The Al enrichment of orthopyroxene is strongly controlled by the Tschermaks substitution [4] Al [6] Al(Mg, Fe) −1 Si −1 . The equilibrium conditions of the orthopyroxene-bearing assemblages are estimated at T = 1180-1270°C and P = 9-15 kbar. The orthopyroxene-bearing pseudomorphs have been formed by solid-state and isochemical break-down of original garnet megacrysts during the transportation by the alkali basaltic magma. The transportation speed was high and is estimated at 3.5-7.0 km/h from a calculation of the velocities of the host magma ascent and gravitational sedimentation of the garnet megacryst. This suggests that the transportation time of the garnet megacryst from the garnet-stable depth to the surface was about 6-14 hours, which is probably sufficient for its partial decomposition

Distribution of trace elements during breakdown of mantle garnet: an example from Zabargad

Ion probe investigations on mineral phases forming the Al-Di pyroxenites from the Zabargad peridotite body indicate that porphyroclastic pyroxenes in composite mafic layers record an unusual HREE, Zr, Sc enrichment not registered by pyroxenes in spinel websterites. Orthopyroxene in the opx+sp clusters forming the inner, cpx-free zone of layered pyroxenites shows strongly fractionated REE patterns (HREEN/LREEN>1000; Yb>100xch) and very high Zr, Sc and Y abundances (up to 30,672 and 60ppm, respectively). In the outer, cpx-rich zone porphyroclastic clinopyroxene is strongly HREE enriched (HREEN/LREEN∼29; Yb∼ 269xch) and displays very high Sc and Zr abundances (up to 819 and 164 ppm, respectively). It is suggested that the unusual trace element abundances are inherited from a precursor garnet. Composite pyroxenite layers are interpreted as former garnet clinopyroxenites characterized by gnt/cpx modal zoning. The sp+opx(cpx-free) assemblage in the inner part is a product of the break-down reaction of garnet upon decompression, with Ca of the original garnet completely entering the enstatite solid solution. The temperature at which the breakdown reaction occurred is estimated to be higher than 1000°C (P in the range 20–30 kbar). In the outer part, decompression caused the garnet to form a sp+opx assemblage; however, the grossularite component participated in the formation of new clinopyroxene which reacted with the clinopyroxene present in the original mode before the decompression reaction, thus forming a cpx2+sp+opx assemblage. As a result of garnet breakdown, pyroxenes have peculiar HFSE anomalies. Progressive upwelling during the Red Sea rifting produced incomplete reaction under pl-facies conditions. The geochemical signatures of precursor garnet in pyroxenes were partially crased during the recrystallization from granular spincl-bearing to granoblastic plagioclase-bearing assemblages, being preserved only in a few porphyroclast relies. The finding of pyroxenes with trace element characteristics of precursor garnet has important geodynamic and geochemical implications. Al-Di pyroxenite layers had a long history within the mantle, before the continental lithosphere rifting and thinning took place in the region. It is suggested that Al-Di pyroxenites were formed by deep-seated tholeiitic magmatism unrelated to the Red Sea evolution, thus representing the earliest event in the Zabargad upper mantle. Garnet breakdown significantly preceded the metasomatism induced by hydrous fluids (crystallization of Ti-rich pargasite) and the later intrusion of hydrous (Cr-Di) pyroxenite dykes. During the stages of mantle evolution, the HFSE anomalies in pyroxenes varied significantly. We note that the study of HFSE anomalies in mineral phases reveals complex geochemical histories which are not recorded by the whole-rock system.

Magnetic transition temperatures of ion-implanted layers on bubble garnet films

By means of susceptibility measurements, it has been found that the magnetic ordering temperatures of ion-implanted layers are depressed relative to those of the original garnet films. Implantation of 1015 Ne+ ions/cm2 at 100 keV brings the ordering temperature of the layer down to ∼30 °C, in the film composition which was investigated. Such behavior explains why implanted films can become ’’nonmagnetic’’ near room temperature, as is reported in the literature. Thus it appears unnecessary to invoke an overdamage mechanism which is qualitatively different than that of implanted layers normally used for hard bubble suppression. Annealing raises a layer’s transition temperature, as expected, but high-temperature annealing results in an anomalous behavior which suggests that healing of the radiation damage does not restore a low-damage structure.