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Shoji Arai - One of the best experts on this subject based on the ideXlab platform.
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Origin and significance of spinel–pyroxene Symplectite in lherzolite xenoliths from Tallante, SE Spain
Mineralogy and Petrology, 2008Co-Authors: Yohei Shimizu, Tomoaki Morishita, Shoji Arai, Yoshito IshidaAbstract:We found spinel–pyroxene Symplectites in lherzolite xenoliths from Tallante, SE Spain, and investigated their petrographical and geochemical signatures. The spinel–pyroxene Symplectites are divided into two types, a spinel-type (= opx + cpx + sp) and a plagioclase-type (= opx + cpx + sp + pl) Symplectites. The Symplectites are always surrounded by lenticular aggregates of coarser-grained spinel–pyroxene. The petrography and major-element chemistry of bulk Symplectites indicate an origin through subsolidus reaction between olivine and garnet like at Horoman (Japan; Morishita and Arai, Contrib Mineral Petrol 144:509–522, 2003). The spinel–pyroxene Symplectites at Tallante were of garnet origin. However, the bulk Tallante spinel–pyroxene Symplectites show a relatively flat rare earth element (REE) distribution with slight light REE (LREE) enrichment, i.e. there was no trace-element signature typical of mantle garnet. They also differ from the Horoman Symplectites that occasionally preserve a garnet trace-element signature, i.e. depletion of LREE and enrichment of heavy REE. These conflicting results indicate that the Symplectites record slight enrichment in pyroxene compositions during or after depletion by melt extraction and breakdown of garnet by decompression, and all the minerals including Symplectite constituents have been homogenized in the stability field of spinel to plagioclase lherzolite, with the assistance of some melt (possibly an alkaline silicate melt; Downes, J Petrol 42:233–250, 2001). Moreover, some of the spinel-type Symplectites experienced heating by injection of Si-rich melt, and consequently have been transformed to the plagioclase-type Symplectite. The Tallante spinel–pyroxene Symplectites developed from garnet + olivine and were carried from the garnet–lherzolite stability field to the spinel– and to the plagioclase–lherzolite stability fields. Our data indicates mantle upwelling (mantle diapirism) beneath the Betic–Rif zone in southern Spain.
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origin and significance of spinel pyroxene Symplectite in lherzolite xenoliths from tallante se spain
Mineralogy and Petrology, 2008Co-Authors: Yohei Shimizu, Tomoaki Morishita, Shoji Arai, Yoshito IshidaAbstract:We found spinel–pyroxene Symplectites in lherzolite xenoliths from Tallante, SE Spain, and investigated their petrographical and geochemical signatures. The spinel–pyroxene Symplectites are divided into two types, a spinel-type (= opx + cpx + sp) and a plagioclase-type (= opx + cpx + sp + pl) Symplectites. The Symplectites are always surrounded by lenticular aggregates of coarser-grained spinel–pyroxene. The petrography and major-element chemistry of bulk Symplectites indicate an origin through subsolidus reaction between olivine and garnet like at Horoman (Japan; Morishita and Arai, Contrib Mineral Petrol 144:509–522, 2003). The spinel–pyroxene Symplectites at Tallante were of garnet origin. However, the bulk Tallante spinel–pyroxene Symplectites show a relatively flat rare earth element (REE) distribution with slight light REE (LREE) enrichment, i.e. there was no trace-element signature typical of mantle garnet. They also differ from the Horoman Symplectites that occasionally preserve a garnet trace-element signature, i.e. depletion of LREE and enrichment of heavy REE. These conflicting results indicate that the Symplectites record slight enrichment in pyroxene compositions during or after depletion by melt extraction and breakdown of garnet by decompression, and all the minerals including Symplectite constituents have been homogenized in the stability field of spinel to plagioclase lherzolite, with the assistance of some melt (possibly an alkaline silicate melt; Downes, J Petrol 42:233–250, 2001). Moreover, some of the spinel-type Symplectites experienced heating by injection of Si-rich melt, and consequently have been transformed to the plagioclase-type Symplectite. The Tallante spinel–pyroxene Symplectites developed from garnet + olivine and were carried from the garnet–lherzolite stability field to the spinel– and to the plagioclase–lherzolite stability fields. Our data indicates mantle upwelling (mantle diapirism) beneath the Betic–Rif zone in southern Spain.
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evolution of spinel pyroxene Symplectite in spinel lherzolites from the horoman complex japan
Contributions to Mineralogy and Petrology, 2003Co-Authors: Tomoaki Morishita, Shoji AraiAbstract:We examined the textural and geochemical characteristics of spinel–pyroxene Symplectites in spinel–lherzolites collected from the lowest, middle, and upper parts (LZ1, MZ, and UZ1, respectively) of the Horoman Peridotite Complex, Japan. The modal proportion of the minerals within Symplectite is almost the same, i.e., orthopyroxene:clinopyroxene:spinel = 2:1:1. The size of the Symplectite minerals increases from the lowest through to the middle to the upper parts in the complex. The reconstructed major element composition of the bulk Symplectites is intermediate between pyrope-rich garnet and olivine. The model garnet compositions of the LZ1satisfies garnet stoichiometry and those of the MZ and UZ1 are not consistent with garnet stoichiometry. The primitive mantle-normalized pattern in trace elements for the LZ1 Symplectite is similar to that of pyrope-rich garnet from fertile peridotites, particularly in its enrichment of HREE and a positive Zr anomaly. Thus, the LZ1 Symplectite has inherited both major and trace element signatures from pre-existing garnet whereas the compositions of the MZ and UZ1 Symplectites were modified during and/or after breakdown of pre-existing garnet. Geochemical and textural variations of Symplectites might basically correspond to temperature differences within the complex during upwelling of the Horoman Complex. The basal part of the complex (LZ1) experienced the lowest temperature decompression path in the complex, which resulted in less textural and chemical modification. On the other hand, the higher part of the complex (UZ1) experienced a relatively higher temperature decompression path than other parts of the complex, resulting in chemical equilibration among the constituent minerals and coarsening of the Symplectite minerals. Selective enrichment of Sr and LREE in the Symplectite may indicate that the metasomatism by a Sr- and LREE-rich melt/fluid occurred during and/or after the formation of Symplectite.
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Evolution of spinel–pyroxene Symplectite in spinel–lherzolites from the Horoman Complex, Japan
Contributions to Mineralogy and Petrology, 2003Co-Authors: Tomoaki Morishita, Shoji AraiAbstract:We examined the textural and geochemical characteristics of spinel–pyroxene Symplectites in spinel–lherzolites collected from the lowest, middle, and upper parts (LZ1, MZ, and UZ1, respectively) of the Horoman Peridotite Complex, Japan. The modal proportion of the minerals within Symplectite is almost the same, i.e., orthopyroxene:clinopyroxene:spinel = 2:1:1. The size of the Symplectite minerals increases from the lowest through to the middle to the upper parts in the complex. The reconstructed major element composition of the bulk Symplectites is intermediate between pyrope-rich garnet and olivine. The model garnet compositions of the LZ1satisfies garnet stoichiometry and those of the MZ and UZ1 are not consistent with garnet stoichiometry. The primitive mantle-normalized pattern in trace elements for the LZ1 Symplectite is similar to that of pyrope-rich garnet from fertile peridotites, particularly in its enrichment of HREE and a positive Zr anomaly. Thus, the LZ1 Symplectite has inherited both major and trace element signatures from pre-existing garnet whereas the compositions of the MZ and UZ1 Symplectites were modified during and/or after breakdown of pre-existing garnet. Geochemical and textural variations of Symplectites might basically correspond to temperature differences within the complex during upwelling of the Horoman Complex. The basal part of the complex (LZ1) experienced the lowest temperature decompression path in the complex, which resulted in less textural and chemical modification. On the other hand, the higher part of the complex (UZ1) experienced a relatively higher temperature decompression path than other parts of the complex, resulting in chemical equilibration among the constituent minerals and coarsening of the Symplectite minerals. Selective enrichment of Sr and LREE in the Symplectite may indicate that the metasomatism by a Sr- and LREE-rich melt/fluid occurred during and/or after the formation of Symplectite.
Tomoaki Morishita - One of the best experts on this subject based on the ideXlab platform.
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Origin and significance of spinel–pyroxene Symplectite in lherzolite xenoliths from Tallante, SE Spain
Mineralogy and Petrology, 2008Co-Authors: Yohei Shimizu, Tomoaki Morishita, Shoji Arai, Yoshito IshidaAbstract:We found spinel–pyroxene Symplectites in lherzolite xenoliths from Tallante, SE Spain, and investigated their petrographical and geochemical signatures. The spinel–pyroxene Symplectites are divided into two types, a spinel-type (= opx + cpx + sp) and a plagioclase-type (= opx + cpx + sp + pl) Symplectites. The Symplectites are always surrounded by lenticular aggregates of coarser-grained spinel–pyroxene. The petrography and major-element chemistry of bulk Symplectites indicate an origin through subsolidus reaction between olivine and garnet like at Horoman (Japan; Morishita and Arai, Contrib Mineral Petrol 144:509–522, 2003). The spinel–pyroxene Symplectites at Tallante were of garnet origin. However, the bulk Tallante spinel–pyroxene Symplectites show a relatively flat rare earth element (REE) distribution with slight light REE (LREE) enrichment, i.e. there was no trace-element signature typical of mantle garnet. They also differ from the Horoman Symplectites that occasionally preserve a garnet trace-element signature, i.e. depletion of LREE and enrichment of heavy REE. These conflicting results indicate that the Symplectites record slight enrichment in pyroxene compositions during or after depletion by melt extraction and breakdown of garnet by decompression, and all the minerals including Symplectite constituents have been homogenized in the stability field of spinel to plagioclase lherzolite, with the assistance of some melt (possibly an alkaline silicate melt; Downes, J Petrol 42:233–250, 2001). Moreover, some of the spinel-type Symplectites experienced heating by injection of Si-rich melt, and consequently have been transformed to the plagioclase-type Symplectite. The Tallante spinel–pyroxene Symplectites developed from garnet + olivine and were carried from the garnet–lherzolite stability field to the spinel– and to the plagioclase–lherzolite stability fields. Our data indicates mantle upwelling (mantle diapirism) beneath the Betic–Rif zone in southern Spain.
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origin and significance of spinel pyroxene Symplectite in lherzolite xenoliths from tallante se spain
Mineralogy and Petrology, 2008Co-Authors: Yohei Shimizu, Tomoaki Morishita, Shoji Arai, Yoshito IshidaAbstract:We found spinel–pyroxene Symplectites in lherzolite xenoliths from Tallante, SE Spain, and investigated their petrographical and geochemical signatures. The spinel–pyroxene Symplectites are divided into two types, a spinel-type (= opx + cpx + sp) and a plagioclase-type (= opx + cpx + sp + pl) Symplectites. The Symplectites are always surrounded by lenticular aggregates of coarser-grained spinel–pyroxene. The petrography and major-element chemistry of bulk Symplectites indicate an origin through subsolidus reaction between olivine and garnet like at Horoman (Japan; Morishita and Arai, Contrib Mineral Petrol 144:509–522, 2003). The spinel–pyroxene Symplectites at Tallante were of garnet origin. However, the bulk Tallante spinel–pyroxene Symplectites show a relatively flat rare earth element (REE) distribution with slight light REE (LREE) enrichment, i.e. there was no trace-element signature typical of mantle garnet. They also differ from the Horoman Symplectites that occasionally preserve a garnet trace-element signature, i.e. depletion of LREE and enrichment of heavy REE. These conflicting results indicate that the Symplectites record slight enrichment in pyroxene compositions during or after depletion by melt extraction and breakdown of garnet by decompression, and all the minerals including Symplectite constituents have been homogenized in the stability field of spinel to plagioclase lherzolite, with the assistance of some melt (possibly an alkaline silicate melt; Downes, J Petrol 42:233–250, 2001). Moreover, some of the spinel-type Symplectites experienced heating by injection of Si-rich melt, and consequently have been transformed to the plagioclase-type Symplectite. The Tallante spinel–pyroxene Symplectites developed from garnet + olivine and were carried from the garnet–lherzolite stability field to the spinel– and to the plagioclase–lherzolite stability fields. Our data indicates mantle upwelling (mantle diapirism) beneath the Betic–Rif zone in southern Spain.
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formation and deformation mechanisms of pyroxene spinel Symplectite in an ascending mantle the horoman peridotite complex japan an ebsd electron backscatter diffraction study
Journal of Mineralogical and Petrological Sciences, 2007Co-Authors: Norihiro Odashima, Tomoaki Morishita, Akira Tsuchiyama, Kazuhito Ozawa, Hiroko Nagahara, Ryoko NagashimaAbstract:Symplectites, fine vermicular intergrowth (< 10 μm) of orthopyroxene, clinopyroxene, and spinel, and the sur- rounding lenticular coarser-grained (100-200 μm) aggregate (seam) with the same mineral assemblage of the Symplectites define remarkable foliation and lineation in spinel lherzolites of the Horoman complex, northern Japan. They are inferred to be products of reaction between garnet and olivine during decompression of the host peridotite accompanying deformation. Microstructure of a Symplectite was investigated with automated electron backscattered diffraction (EBSD) analysis using a field -emission gun SEM (FE-SEM) in order to clarify reaction and deformation mechanisms and thereby better constraining mechanical interaction between the Earth's upper mantle and lower crust. The Symplectite is composed of two segments with a large misorien- tation angle of ~ 60° only for spinel, and the two spinel crystals are in mirror symmetry with the segment boundary approximately parallel to the mirror plane. The segment boundary is interpreted as spinel law twin formed during phase transition from garnet. Each segment is further subdivided into several sectors with grad- ual lattice distortion smaller than a few degrees/mm and intra-sector misorientation mostly smaller than 25° for all constituent minerals and with misorientation axes nearly perpendicular to the lineation and parallel to the foliation. The sector boundaries are inferred to be subgrain boundaries formed by dislocation creep of py- roxenes and spinel in the spinel stability field. The spinel twin suggests that a garnet was decomposed directly into entangled aggregate of pyroxenes and spinel, which grew from an embryo nucleated on the surface of the reactant garnet. The Symplectite minerals in each sector show systematic crystallographic orientations (topo- taxy) with each other. The topotaxial relationship in the fine intergrowth with subgrain structure demonstrates that the systematic crystallographic orientations were acquired when the crystals grew by decomposing the garnet and were later modified by deformation during the consecutive ascent of the complex.
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observations of three dimensional microstructures of Symplectite minerals in the horoman peridotite complex using a high resolution x ray ct system at spring 8
Journal of the Geological Society of Japan, 2003Co-Authors: Tomoaki Morishita, Akira Tsuchiyama, Tsukasa Nakano, Kentaro UesugiAbstract:X-ray computerized tomography (CT) has become a powerful tool for completely non-destructive examination of the interiors of geological objects. An X-ray CT image is a map of the spatial distribution of values related to the linear attenuation coefficient (LAC), which depends on mass density, chemical composition and incident X-ray energy (Nakano et al., 2000). In this report we show the application of this method to a Symplectite consisting of irregular fine-grained orthopyroxene, clinopyroxene and spinel from the Horoman peridotite, Japan. The Symplectite in the Horoman peridotites is interpreted to be of pyropic garnet origin. Textural characteristics of Symplectite from the Horoman complex will therefore provide opportunities to investigate the phase transition from garnet to spinel lherzolite. We imaged Symplectites using a highresolution X-ray CT system at SPring-8 (Uesugi et al., 2001) with a monochromatic X-ray beam of 15 keV (Fig.1). Brighter regions on the CT images correspond to higher values of LAC. Three-dimensional structures of the Symplectites were obtained from 1018 slices of the CT images with 1000 × 1000 matrix (Figs.2, 3). The voxel size is 0.5 μm × 0.5 μm × 0.5 μm, and the effective spatial resolution is about 1 μm. Clusters of orthopyroxene, clinopyroxene and spinel in the Symplectites were clearly identified using the contrast in the CT images, which coincided with the theoretical LAC values of these minerals (Fig.1).Acknowledgements: The authors thank to Greg Yaxley to improve the English of the manuscript. Constructive review by Susumu Ikeda improved the manuscript.
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evolution of spinel pyroxene Symplectite in spinel lherzolites from the horoman complex japan
Contributions to Mineralogy and Petrology, 2003Co-Authors: Tomoaki Morishita, Shoji AraiAbstract:We examined the textural and geochemical characteristics of spinel–pyroxene Symplectites in spinel–lherzolites collected from the lowest, middle, and upper parts (LZ1, MZ, and UZ1, respectively) of the Horoman Peridotite Complex, Japan. The modal proportion of the minerals within Symplectite is almost the same, i.e., orthopyroxene:clinopyroxene:spinel = 2:1:1. The size of the Symplectite minerals increases from the lowest through to the middle to the upper parts in the complex. The reconstructed major element composition of the bulk Symplectites is intermediate between pyrope-rich garnet and olivine. The model garnet compositions of the LZ1satisfies garnet stoichiometry and those of the MZ and UZ1 are not consistent with garnet stoichiometry. The primitive mantle-normalized pattern in trace elements for the LZ1 Symplectite is similar to that of pyrope-rich garnet from fertile peridotites, particularly in its enrichment of HREE and a positive Zr anomaly. Thus, the LZ1 Symplectite has inherited both major and trace element signatures from pre-existing garnet whereas the compositions of the MZ and UZ1 Symplectites were modified during and/or after breakdown of pre-existing garnet. Geochemical and textural variations of Symplectites might basically correspond to temperature differences within the complex during upwelling of the Horoman Complex. The basal part of the complex (LZ1) experienced the lowest temperature decompression path in the complex, which resulted in less textural and chemical modification. On the other hand, the higher part of the complex (UZ1) experienced a relatively higher temperature decompression path than other parts of the complex, resulting in chemical equilibration among the constituent minerals and coarsening of the Symplectite minerals. Selective enrichment of Sr and LREE in the Symplectite may indicate that the metasomatism by a Sr- and LREE-rich melt/fluid occurred during and/or after the formation of Symplectite.
Werner E Halter - One of the best experts on this subject based on the ideXlab platform.
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Symplectite in spinel lherzolite xenoliths from the little hungarian plain western hungary a key for understanding the complex history of the upper mantle of the pannonian basin
Lithos, 2007Co-Authors: Gyorgy Falus, Csaba Szabo, I Kovacs, Zoltan Zajacz, Werner E HalterAbstract:Two spinel lherzolite xenoliths from Hungary that contain pyroxene–spinel Symplectites have been studied using EPMA, Laser ablation ICP-MS and universal stage. Based on their geochemical and structural characteristics, the xenoliths represent two different domains of the shallow subcontinental lithospheric mantle beneath the Pannonian Basin. The occurrence of Symplectites is attributed to the former presence and subsequent breakdown of garnets due to significant pressure decrease related to lithospheric thinning. This implies that both mantle domains were once part of the garnet lherzolitic upper mantle and had a similar history during the major extension that formed the Pannonian Basin. Garnet breakdown resulted in distinct geochemical characteristics in the adjacent clinopyroxene crystals in both xenoliths. This is manifested by enrichment in HREE, Y, Zr and Hf towards the clinopyroxene porphyroclast rims and also in the neoblasts with respect to porphyroclast core compositions. This geochemical feature, together with the development and preservation of the texturally very sensitive Symplectites, enables us to determine the relative timing of mantle processes. Our results indicate that garnets had been metastable in the spinel lherzolite environment and their breakdown to pyroxene and spinel is one of the latest processes that took place within the upper mantle before the xenoliths were brought to the surface. © 2006 Published by Elsevier B.V.
Rainer Abart - One of the best experts on this subject based on the ideXlab platform.
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Ca-rich garnets and associated Symplectites in mafic peraluminous granulites from the Gföhl Nappe System, Austria
Solid Earth, 2018Co-Authors: Konstantin Petrakakis, Nathalie Schuster-bourgin, Gerlinde Habler, Rainer AbartAbstract:Abstract. Mafic peraluminous granulites associated with the mantle-derived peridotites of the Dunkelsteiner Wald provide evidence of the tectono-metamorphic evolution of the Gfohl Nappe System, Austria. They contain the primary assemblage garnet + Al-rich clinopyroxene + kyanite. Large Ca- and Mg-rich garnets are embedded in a granoblastic matrix of Al-rich clinopyroxene, Ca-rich plagioclase and minor hornblende. They were partially replaced by different generations of Symplectites: (a) corundum + sapphirine + spinel + plagioclase formed around kyanite inclusions, (b) orthopyroxene + spinel + plagioclase ± hornblende formed at their rims and (c) clinopyroxene + orthopyroxene + spinel + plagioclase ± hornblende formed within cracks. Large garnets show complex compositional structure comprising several repeatedly occurring garnet types, which are characterized by specific compositions. The areal extent and the cross-cutting relations observed in element distribution maps allowed for the derivation of the relative timing of the formation of the different garnet types. The compositional features of the garnets indicate post-formational modification by intra-crystalline diffusion and metasomatic agents. The garnet composition isopleths in equilibrium assemblage diagrams are in line with compositions modification as indicated by the element distribution maps. They confirm the deviation of composition from equilibrium for all garnet types. Furthermore, at least the youngest garnet types show evidence of metasomatic (Fe + Mg) loss affecting their Ca content. Pressure–temperature (P–T) estimates are based on equilibrium assemblage diagrams that reproduce satisfactorily the observed mineral assemblages and measured mineral compositions. Criteria for checking the existence of preserved equilibrium compositions are suggested. The results call into question the invariability of the assumption that the Ca content and/or zoning in garnet preserves primary P–T information from garnet growth in every case. Recrystallization and compositional readjustment of the reactive garnet volume during Symplectite formation led to the development of pronounced, secondary diffusion-induced zoning profiles overprinting the different garnet types and post-dating the complex garnet compositional structure. The primary assemblage is stable between 760 and 880 °C and pressures > 11 kbar. The bulk composition of the crack Symplectites is almost isochemical to the oldest, broken-down garnet type. These Symplectites were formed above 730 °C and pressures between 5 and 7.5 kbar. The rocks studied underwent more or less isothermal decompression from pressures above 11 to ∼ 6 kbar at temperatures of about 800 °C. Crack and rim Symplectites were formed after decompression during the early stage of approximately isobaric cooling under conditions of low differential stress. Due to limited availability of fluids promoting Symplectite formation, the timescale of Symplectite formation calculated from secondary diffusion profiles associated with crack Symplectites is shown to be geologically very short (
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Ca-rich garnets and associated Symplectites in mafic peraluminous granulites from the Gföhl Nappe System, Austria
2018Co-Authors: Konstantin Petrakakis, Nathalie Schuster-bourgin, Gerlinde Habler, Rainer AbartAbstract:Abstract. Mafic peraluminous granulites associated with the mantle-derived peridotites of the Dunkelsteiner Wald provide evidence of the tectono-metamortphic evolution of rocks in the Gföhl Nappe System, Austria. They contain the primary assemblage garnet + Al-rich-clinopyroxene + kyanite. Large Ca-and Mg-rich garnets are embedded in a granoblastic matrix of Al-rich-clinopyroxene, Ca-rich-plagioclase and minor hornblende. They have been partially replaced by different, locally controlled Symplectites of (a) corundum + sapphirine + spinel + plagioclase formed around kyanite inclusions, (b) orthopyroxene + spinel + plagioclase ± hornblende formed at their rims and (c) clinopyroxene + orthopyroxene + spinel + plagioclase ± hornblende formed within cracks. Garnets are built up from repeatedly occurring garnet types characterized by specific component distributions. Areal extend and cross-cutting relations allowed for the derivation of the relative timing of garnet types formation. Recrystallization and compositional readjustment of the reactive garnet volume during Symplectite formation has led to the development of pronounced, secondary diffusion-induced zoning profiles overprinting the different garnet types and post-dating the complex garnet compositional structure. Thermodynamic analysis showed that none of the garnet types represents a preserved equilibrium composition. Furthermore, the latest garnet types show evidence of metasomatic (Fe+Mg)-loss affecting their Ca-content. The primary assemblage is stable between 760 and 880 °C and pressures > 11 kbar. The crack-Symplectites are almost isochemical to the oldest garnet type and have been formed above 730 °C and pressures between 7.5 and 5 kbar. The studied rocks have undergone a more or less isothermal decompression from pressures above 11 kbar to ~ 6 kbar at temperatures about 800 °C. Crack- and rim-Symplectites have been formed after decompression during approximately isobaric cooling under conditions of low differential stress. Due to limited availability of fluids promoting Symplectite formation, the time-scale of Symplectite formation calculated from secondary diffusion profiles associated with crack-Symplectites is shown to be geologically very short (
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garnet breakdown Symplectite formation and melting in basanite hosted peridotite xenoliths from zinst bavaria bohemian massif
Journal of Petrology, 2013Co-Authors: Gerlinde Habler, Rainer Abart, Petr Spacek, Lukas Ackerman, Jaromir UlrychAbstract:Complex, Symplectite-bearing pseudomorphs after garnet recently found in unique basanite-hosted peridotite xenoliths from Zinst, Bavaria, allow the study of the interaction between garnet peridotite and melts or fluids both prior to entrainment of the xenoliths and during their ascent. Based on microstructures and crystallographic fabric, and major and trace element mineral chemistry, four distinct concentric zones were defined in various types of pseudomorph: Zone I, coarse-grained (1 mm) aggregate of orthopyroxene + clinopyroxene + spinel with a granular structure; Zone II, fine- to medium-grained (order of 10-100 mu m) orthopyroxene + spinel Symplectite; Zone III, fine-grained (5-300 mu m), radially fibrous orthopyroxene + spinel Symplectite with interstitial anorthite; Zone IV, ultrafine-grained (1 mu m) orthopyroxene + spinel + anorthite Symplectite with an internal domain substructure. Zones III and IV have bulk compositions of pyrope-rich garnet. All zones exhibit perfect inter-sample correlation and document the discontinuous evolution of peridotite under changing conditions with successively increasing rates of garnet breakdown. Based on thermometry and microstructural relations, a sequence of three pre- and syn-volcanic events is discerned. The first traceable event corresponds to regional heating in the uppermost mantle probably related to the early stages of Tertiary rifting, which triggered the reaction between garnet and olivine (Zone I) leading to a partial re-equilibration of the rock at 1040-1080 degrees C within the spinel peridotite stability field. Subsequently a short period of heating by similar to 100-250 degrees C led to largely isochemical, fluid-mediated in situ melting of garnet and to the formation of kelyphite by crystallization from the melt (Zone III). The subsequent metasomatic alteration by external, Na-rich, K-poor, carbonate-bearing melts or fluids suggests that this phase of garnet breakdown occurred largely prior to formation of the xenolith, preceding the emplacement of the basanite magma. Finally, after xenolith formation, and associated with rapid, isochemical, decompression during exhumation, the garnet relics were transformed into microSymplectite (Zone IV). The positive volume change associated with this reaction caused fracturing, producing radial cracks that emanate from Zone IV and extend into the adjacent peridotite, allowing infiltration of basanite-derived melt components. The well-developed and clearly separated Symplectite zones indicating the isochemical breakdown of garnet are uncommon in garnet peridotites worldwide. Their existence at Zinst is explained by an extremely short time span between the formation of the kelyphite, metasomatism by Na- and carbonate-rich agents and the final garnet breakdown during the host basanite eruption, allowing for rapid quenching of the multiple advancing reaction fronts.
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Symplectite formation during decompression induced garnet breakdown in lower crustal mafic granulite xenoliths: mechanisms and rates
Contributions to Mineralogy and Petrology, 2010Co-Authors: Júlia Dégi, Rainer Abart, Kálmán Török, Enikő Bali, Richard Wirth, Dieter RhedeAbstract:The complex microstructure of kelyphitic rims around garnet in lower crustal garnet granulite xenoliths from the Bakony–Balaton Highland Volcanic Field, Central Pannonian Basin has been studied in order to identify controls on garnet breakdown. Symplectites comprised of a vermicular intergrowth of submicron sized anorthite, orthopyroxene and spinel replace garnet at a sharp reaction front. Based on element distribution maps the transformation of garnet to Symplectite is isochemical. Phase diagram calculations indicate that this reaction was induced by a pressure decrease and/or a temperature increase. In site-specific TEM foils prepared by focused ion beam technique and oriented parallel and perpendicular to the reaction front 200 nm wide rods of anorthite and 20 nm wide rods of spinel are identified. The rods are oriented approximately perpendicular to the replacement front and are embedded in an orthopyroxene matrix. The regular spacing of the Symplectite phases along the reaction front suggests that their growth is controlled by diffusion. The kinetics of Symplectite formation has been modelled based on irreversible thermodynamics. During interaction of the xenolith with the host basalt the microstructure and chemistry of the An–Opx–Spl Symplectite was significantly modified and it was partially replaced by an olivine bearing Symplectite. In contrast to primary Symplectite formation, these processes were metasomatic in nature including addition of sodium, titanium and some trace elements from the basaltic melt and can clearly be discerned from the garnet breakdown. Based on these observations it is inferred that Symplectite formation took place within the deep crust during the extension of the Pannonian Basin between 15 and 30 km depth at high temperature (850–1,050°C) prior to the volcanic transport to the surface.
Stephen E. Haggerty - One of the best experts on this subject based on the ideXlab platform.
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Symplectites in upper mantle peridotites: Development and implications for the growth of subsolidus garnet, pyroxene and spinel
Contributions to Mineralogy and Petrology, 1994Co-Authors: Stephen W. Field, Stephen E. HaggertyAbstract:Silicate-oxide Symplectites in complex mineral intergrowths are relatively common in upper mantle xenoliths and in xenoliths in the Jagersfontein Kimberlite, South Africa. Harzburgites of olivine and high-Al (1.9–3.6 wt%), Ca (0.6–0.9 wt%) and Cr (0.3–0.9 wt%) enstatite contain Symplectites of spinel and diopside, or spinel, diopside and lower-Al (0.8–2.2 wt%), Ca (0.1–0.4 wt%) and Cr (0.02–0.8 wt%) enstatite. From textures and mineral chemistries these Symplectites are interpreted to have formed by mineral unmixing and migration from Al−Ca−Cr discrete enstatite to adjoining mineral interfaces. Garnet harzburgites are composed of large (0.5–1 cm) olivine, equally large discrete low-Al (0.6–1.1 wt%), Ca (0.1–0.5 wt%), and Cr (0.1–0.3 wt%) enstatite and smaller interstitial garnet, diopside, and high-Cr and low-Al spinel. Symplectites are composed of either spinel+diopside+garnet, or garnet+spinel. Spinel diopside garnet Symplectites have cores of spinel+diopside, resembling Symplectites in harzburgites , but surrounded by rims of garnet or garnet+undigested globular spinel. From textures and chemistries we suggest that the spinel+diopside cores formed from Ca-Al-Cr-rich orthopyroxene initially as a nonstoichiometric homogeneous single phase clinopyroxene enriched in Fe, Cr and Al. This was followed by decomposition of the clinopyroxene to diopside+spinel, and subsequent garnet formation in a prograde reaction with olivine or enstatite. In both harzburgites and garnet harzburgites the metastable cellular structures may also have formed by the simultaneous precipitation of pyroxene and spinel. In all cases there is a strongly preferred embayment of Symplectite bodies into olivine. Olivine appears to have activated adjacent
