Pub Date : 2024-05-02DOI: 10.1007/s00410-024-02135-y
Tong Liu, Chuan-Zhou Liu, Zhen-Yu Zhang, Wei-Qi Zhang, Wen-Bin Ji, Chang Zhang, Ross N. Mitchell
The dynamics and magma transport at the boundary between the upper and lower oceanic crusts (i.e., the dike–gabbro transition) are crucial for understanding the crustal accretion beneath mid-ocean ridges, which however have been studied at quite a few sites such as the East Pacific Rise and ophiolites like Troodos and Oman. Here we present detailed geological, petrological, and geochemical data for the dike–gabbro transition and associated basalts in the Yunzhug ophiolite, central Tibet, to constrain the complex magmatic processes in this specific horizon. The Yunzhug ophiolite contains a large (~ 20 km2) well-exposed sheeted dike complex, which is rooted in a dike–gabbro transition that consists of diverse lithologies, including diabase, gabbro, and minor porphyritic diabase. Petrographically, the Yunzhug gabbros could be grouped into the dominant Plg (plagioclase)-euhedral gabbros (euhedral–subhedral plagioclases enclosed in clinopyroxene oikocrysts) and a small amount of Cpx (clinopyroxene)-euhedral gabbros (with abundant euhedral clinopyroxenes). Plagioclases and their equilibrated melts of the two types of gabbros are similar, whereas clinopyroxenes and their equilibrated melts of the Cpx-euhedral gabbros are more primary and depleted than those of the Plg-euhedral gabbros. These petrographic and geochemical features suggest an earlier crystallization of clinopyroxene for the Cpx-euhedral gabbros, which is best explained by occasional water input in the magmatic system. Nevertheless, the modeled equilibrated melts of the two types of gabbros have compositions indistinguishable from the whole rock compositions of diabases and basalts, indicating a direct genetic linkage between these rocks. The unusual porphyritic diabases, on the other hand, provide evidence supporting for plagioclase accumulation and aggregation during magma upward migration, thus may have served as a unique way for magma to transport from the lower to upper crust. Studies of the Yunzhug ophiolite thus provide some key constraints on the complex magmatic processes in the oceanic dike–gabbro transition, regarding its dynamic accretion and magmatic plumbing mechanisms.
{"title":"Dynamics of the oceanic dike–gabbro transition revealed by petrology and geochemistry of the Yunzhug ophiolite, central Tibet","authors":"Tong Liu, Chuan-Zhou Liu, Zhen-Yu Zhang, Wei-Qi Zhang, Wen-Bin Ji, Chang Zhang, Ross N. Mitchell","doi":"10.1007/s00410-024-02135-y","DOIUrl":"https://doi.org/10.1007/s00410-024-02135-y","url":null,"abstract":"<p>The dynamics and magma transport at the boundary between the upper and lower oceanic crusts (i.e., the dike–gabbro transition) are crucial for understanding the crustal accretion beneath mid-ocean ridges, which however have been studied at quite a few sites such as the East Pacific Rise and ophiolites like Troodos and Oman. Here we present detailed geological, petrological, and geochemical data for the dike–gabbro transition and associated basalts in the Yunzhug ophiolite, central Tibet, to constrain the complex magmatic processes in this specific horizon. The Yunzhug ophiolite contains a large (~ 20 km<sup>2</sup>) well-exposed sheeted dike complex, which is rooted in a dike–gabbro transition that consists of diverse lithologies, including diabase, gabbro, and minor porphyritic diabase. Petrographically, the Yunzhug gabbros could be grouped into the dominant Plg (plagioclase)-euhedral gabbros (euhedral–subhedral plagioclases enclosed in clinopyroxene oikocrysts) and a small amount of Cpx (clinopyroxene)-euhedral gabbros (with abundant euhedral clinopyroxenes). Plagioclases and their equilibrated melts of the two types of gabbros are similar, whereas clinopyroxenes and their equilibrated melts of the Cpx-euhedral gabbros are more primary and depleted than those of the Plg-euhedral gabbros. These petrographic and geochemical features suggest an earlier crystallization of clinopyroxene for the Cpx-euhedral gabbros, which is best explained by occasional water input in the magmatic system. Nevertheless, the modeled equilibrated melts of the two types of gabbros have compositions indistinguishable from the whole rock compositions of diabases and basalts, indicating a direct genetic linkage between these rocks. The unusual porphyritic diabases, on the other hand, provide evidence supporting for plagioclase accumulation and aggregation during magma upward migration, thus may have served as a unique way for magma to transport from the lower to upper crust. Studies of the Yunzhug ophiolite thus provide some key constraints on the complex magmatic processes in the oceanic dike–gabbro transition, regarding its dynamic accretion and magmatic plumbing mechanisms.</p>","PeriodicalId":526,"journal":{"name":"Contributions to Mineralogy and Petrology","volume":null,"pages":null},"PeriodicalIF":3.5,"publicationDate":"2024-05-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140836403","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-04-27DOI: 10.1007/s00410-024-02129-w
Elena-Marie Rogmann, Eleanor S. Jennings, Jennifer Ross, Nobuyoshi Miyajima, Michael J. Walter, Simon C. Kohn, Oliver Thomas Lord
{"title":"The effect of potassium on aluminous phase stability in the lower mantle","authors":"Elena-Marie Rogmann, Eleanor S. Jennings, Jennifer Ross, Nobuyoshi Miyajima, Michael J. Walter, Simon C. Kohn, Oliver Thomas Lord","doi":"10.1007/s00410-024-02129-w","DOIUrl":"https://doi.org/10.1007/s00410-024-02129-w","url":null,"abstract":"","PeriodicalId":526,"journal":{"name":"Contributions to Mineralogy and Petrology","volume":null,"pages":null},"PeriodicalIF":3.5,"publicationDate":"2024-04-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140651948","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
{"title":"Compositional evolution of slab-derived fluids during ascent: implications from trace-element partition between hydrous melts and Cl-free or Cl-rich aqueous fluids","authors":"Hajime Taniuchi, Tatsuhiko Kawamoto, Takayuki Nakatani, Osamu Ishizuka, Toshihiro Suzuki, Akihiko Tomiya","doi":"10.1007/s00410-024-02122-3","DOIUrl":"https://doi.org/10.1007/s00410-024-02122-3","url":null,"abstract":"","PeriodicalId":526,"journal":{"name":"Contributions to Mineralogy and Petrology","volume":null,"pages":null},"PeriodicalIF":3.5,"publicationDate":"2024-04-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140652652","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-04-25DOI: 10.1007/s00410-024-02131-2
A. G. Sokol, O. A. Kozmenko, A. Kruk, S. Y. Skuzovatov, D. V. Kiseleva
{"title":"Trace-element mobility in pelite-derived supercritical fluid-melt at subduction-zone conditions","authors":"A. G. Sokol, O. A. Kozmenko, A. Kruk, S. Y. Skuzovatov, D. V. Kiseleva","doi":"10.1007/s00410-024-02131-2","DOIUrl":"https://doi.org/10.1007/s00410-024-02131-2","url":null,"abstract":"","PeriodicalId":526,"journal":{"name":"Contributions to Mineralogy and Petrology","volume":null,"pages":null},"PeriodicalIF":3.5,"publicationDate":"2024-04-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140657582","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-04-22DOI: 10.1007/s00410-024-02120-5
Yuxiang Zhu, Lianxun Wang, Yuanming Pan, Changqian Ma, Zhenbing She
{"title":"Remobilization and enrichment of Nb during magmatic and hydrothermal processes: insights from titanite in Nb-rich dyke swarms of South Qinling, China","authors":"Yuxiang Zhu, Lianxun Wang, Yuanming Pan, Changqian Ma, Zhenbing She","doi":"10.1007/s00410-024-02120-5","DOIUrl":"https://doi.org/10.1007/s00410-024-02120-5","url":null,"abstract":"","PeriodicalId":526,"journal":{"name":"Contributions to Mineralogy and Petrology","volume":null,"pages":null},"PeriodicalIF":3.5,"publicationDate":"2024-04-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140677061","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-04-22DOI: 10.1007/s00410-024-02121-4
Q. Shu, A. Beranoaguirre, R. Albert, L. Millonig, J. B. Walters, H. Marschall, A. Gerdes, H. E. Hoefer, D. Hezel, G. P. Brey
{"title":"Multi-stage ultrahigh temperature metamorphism in the lower crust of the Kaapvaal craton recorded by U–Pb ages of garnet","authors":"Q. Shu, A. Beranoaguirre, R. Albert, L. Millonig, J. B. Walters, H. Marschall, A. Gerdes, H. E. Hoefer, D. Hezel, G. P. Brey","doi":"10.1007/s00410-024-02121-4","DOIUrl":"https://doi.org/10.1007/s00410-024-02121-4","url":null,"abstract":"","PeriodicalId":526,"journal":{"name":"Contributions to Mineralogy and Petrology","volume":null,"pages":null},"PeriodicalIF":3.5,"publicationDate":"2024-04-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140673097","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-04-22DOI: 10.1007/s00410-024-02116-1
P. Giacomoni, M. Masotta, G. Delpech, G. Lanzafame, C. Ferlito, J. Villeneuve, M. Coltorti
{"title":"Geochemistry and volatile contents of olivine-hosted melt inclusions from Mt. Etna tholeiitic and alkaline magmatism","authors":"P. Giacomoni, M. Masotta, G. Delpech, G. Lanzafame, C. Ferlito, J. Villeneuve, M. Coltorti","doi":"10.1007/s00410-024-02116-1","DOIUrl":"https://doi.org/10.1007/s00410-024-02116-1","url":null,"abstract":"","PeriodicalId":526,"journal":{"name":"Contributions to Mineralogy and Petrology","volume":null,"pages":null},"PeriodicalIF":3.5,"publicationDate":"2024-04-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140675679","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-04-15DOI: 10.1007/s00410-024-02117-0
Dillon A. Brown, Anthony Reid, Elizabeth A. Jagodzinski, Megan Williams, Alex Simpson, Mark Pawley, Christopher L. Kirkland, Claire Wade, Alexander T. De Vries Van Leeuwen, Stijn Glorie
In mafic systems where primary mineral assemblages have witnessed moderate- to high-temperature hydrous overprinting and deformation, little is known about the retentivity of the Lu–Hf isotopic system in apatite. This study presents apatite laser-ablation Lu–Hf and U–Pb geochronology, zircon geochronology, and detailed petrological information from polymetamorphic mafic intrusions located in the central-western Gawler Craton in southern Australia, which records an extensive tectonometamorphic history spanning the Neoarchaean to the Mesoproterozoic. Zircon records magmatic crystallisation ages of c. 2479–2467 Ma, coinciding with the onset of the c. 2475–2410 Ma granulite-facies Sleafordian Orogeny. The amphibole-dominant hydrous assemblages which extensively overprint the primary magmatic assemblages are hypothesised to post-date the Sleafordian Orogeny. The Lu–Hf and U–Pb isotopic systems in apatite are used to test this hypothesis, with both isotopic systems recording significantly younger ages correlating with the c. 1730–1690 Ma Kimban Orogeny and the c. 1590–1575 Ma Hiltaba magmatic event, respectively. While the early Mesoproterozoic apatite U–Pb ages are attributed to thermal re-equilibration, the older Lu–Hf ages are interpreted to reflect re-equilibration facilitated primarily by dissolution-reprecipitation, but also thermally activated volume diffusion. The mechanisms of Lu–Hf isotopic resetting are distinguished based on microscale textures and trace element abundances in apatite and the integration of apatite-amphibole textural relationships and temperatures determined from the Ti content in amphibole. More broadly, the results indicate that at low to moderate temperatures, apatite hosted in mafic rocks is susceptible to complete recrystallisation in rocks that have weak to moderate foliations. In contrast, at higher temperatures in the absence of strain, the Lu–Hf system in apatite is comparatively robust. Ultimately, the findings from this study advance our understanding of the complex role that both metamorphism and deformation play on the ability of mafic-hosted apatite to retain primary Lu–Hf isotopic signatures.
{"title":"Testing in-situ apatite Lu–Hf dating in polymetamorphic mafic rocks: a case study from Palaeoproterozoic southern Australia","authors":"Dillon A. Brown, Anthony Reid, Elizabeth A. Jagodzinski, Megan Williams, Alex Simpson, Mark Pawley, Christopher L. Kirkland, Claire Wade, Alexander T. De Vries Van Leeuwen, Stijn Glorie","doi":"10.1007/s00410-024-02117-0","DOIUrl":"https://doi.org/10.1007/s00410-024-02117-0","url":null,"abstract":"<p>In mafic systems where primary mineral assemblages have witnessed moderate- to high-temperature hydrous overprinting and deformation, little is known about the retentivity of the Lu–Hf isotopic system in apatite. This study presents apatite laser-ablation Lu–Hf and U–Pb geochronology, zircon geochronology, and detailed petrological information from polymetamorphic mafic intrusions located in the central-western Gawler Craton in southern Australia, which records an extensive tectonometamorphic history spanning the Neoarchaean to the Mesoproterozoic. Zircon records magmatic crystallisation ages of c. 2479–2467 Ma, coinciding with the onset of the c. 2475–2410 Ma granulite-facies Sleafordian Orogeny. The amphibole-dominant hydrous assemblages which extensively overprint the primary magmatic assemblages are hypothesised to post-date the Sleafordian Orogeny. The Lu–Hf and U–Pb isotopic systems in apatite are used to test this hypothesis, with both isotopic systems recording significantly younger ages correlating with the c. 1730–1690 Ma Kimban Orogeny and the c. 1590–1575 Ma Hiltaba magmatic event, respectively. While the early Mesoproterozoic apatite U–Pb ages are attributed to thermal re-equilibration, the older Lu–Hf ages are interpreted to reflect re-equilibration facilitated primarily by dissolution-reprecipitation, but also thermally activated volume diffusion. The mechanisms of Lu–Hf isotopic resetting are distinguished based on microscale textures and trace element abundances in apatite and the integration of apatite-amphibole textural relationships and temperatures determined from the Ti content in amphibole. More broadly, the results indicate that at low to moderate temperatures, apatite hosted in mafic rocks is susceptible to complete recrystallisation in rocks that have weak to moderate foliations. In contrast, at higher temperatures in the absence of strain, the Lu–Hf system in apatite is comparatively robust. Ultimately, the findings from this study advance our understanding of the complex role that both metamorphism and deformation play on the ability of mafic-hosted apatite to retain primary Lu–Hf isotopic signatures.</p>","PeriodicalId":526,"journal":{"name":"Contributions to Mineralogy and Petrology","volume":null,"pages":null},"PeriodicalIF":3.5,"publicationDate":"2024-04-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140562015","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-04-13DOI: 10.1007/s00410-024-02118-z
Cordula P. Haupt, Christian J. Renggli, Arno Rohrbach, Jasper Berndt, Sabrina Schwinger, Maxime Maurice, Maximilian Schulze, Doris Breuer, Stephan Klemme
Modeling the behavior of trace elements during lunar magma ocean solidification is important to further our understanding of the chemical evolution of the Moon. Lunar magma ocean evolution models rely on consistent datasets on how trace elements partition between a lunar silicate melt and coexisting minerals at different pressures, temperatures, and redox conditions. Here we report new experimental trace element partition coefficients (D) between clinopyroxene (cpx), pigeonite, orthopyroxene, plagioclase, olivine (ol), and silicate melt at conditions relevant for the lunar magma ocean. The data include Dcpx−melt at ambient and high pressures (1.5 GPa and 1310 °C), and partition coefficients at ambient pressure for pig, opx, ol, and pl. Overall, clinopyroxene is a phase that may control the fractionation of key geochemical trace element ratios, such as Lu/Hf and Sm/Nd, during the evolution of the lunar magma ocean. We explore the impact of the new silicate Dmineral−melt on the trace element evolution of the lunar magma ocean and we find that accessory phosphate minerals, such as apatite or whitlockite are of critical importance to explain the observed trace element and isotopic signature of the KREEP reservoir on the Moon. The new partition coefficients were applied to calculate the trace element evolution of the residual melts of the crystallizing lunar magma ocean and we propose a new trace element composition for the urKREEP reservoir. The new data will be useful for future thermo-chemical models in order to adequately predict the duration of the lunar magma ocean and the age of the Moon.
{"title":"Trace element partitioning in the lunar magma ocean: an experimental study","authors":"Cordula P. Haupt, Christian J. Renggli, Arno Rohrbach, Jasper Berndt, Sabrina Schwinger, Maxime Maurice, Maximilian Schulze, Doris Breuer, Stephan Klemme","doi":"10.1007/s00410-024-02118-z","DOIUrl":"https://doi.org/10.1007/s00410-024-02118-z","url":null,"abstract":"<p>Modeling the behavior of trace elements during lunar magma ocean solidification is important to further our understanding of the chemical evolution of the Moon. Lunar magma ocean evolution models rely on consistent datasets on how trace elements partition between a lunar silicate melt and coexisting minerals at different pressures, temperatures, and redox conditions. Here we report new experimental trace element partition coefficients (D) between clinopyroxene (cpx), pigeonite, orthopyroxene, plagioclase, olivine (ol), and silicate melt at conditions relevant for the lunar magma ocean. The data include D<sup>cpx−melt</sup> at ambient and high pressures (1.5 GPa and 1310 °C), and partition coefficients at ambient pressure for pig, opx, ol, and pl. Overall, clinopyroxene is a phase that may control the fractionation of key geochemical trace element ratios, such as Lu/Hf and Sm/Nd, during the evolution of the lunar magma ocean. We explore the impact of the new silicate D<sup>mineral−melt</sup> on the trace element evolution of the lunar magma ocean and we find that accessory phosphate minerals, such as apatite or whitlockite are of critical importance to explain the observed trace element and isotopic signature of the KREEP reservoir on the Moon. The new partition coefficients were applied to calculate the trace element evolution of the residual melts of the crystallizing lunar magma ocean and we propose a new trace element composition for the urKREEP reservoir. The new data will be useful for future thermo-chemical models in order to adequately predict the duration of the lunar magma ocean and the age of the Moon.</p>","PeriodicalId":526,"journal":{"name":"Contributions to Mineralogy and Petrology","volume":null,"pages":null},"PeriodicalIF":3.5,"publicationDate":"2024-04-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140562011","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-04-12DOI: 10.1007/s00410-024-02125-0
Sascha Zertani, Luiz F. G. Morales, Luca Menegon
The breakdown of omphacite plays an important role in the exhumation and retrogression of eclogites. Additionally, metamorphic reactions associated with grain size reduction have the potential to significantly impact deformation mechanisms and the rheology of crustal rocks. We analyze the breakdown reaction omphacite → diopsidic clinopyroxene + plagioclase ± amphibole and associated microstructures by electron backscatter diffraction. The reaction results in the formation of (diopsidic) clinopyroxene-plagioclase symplectites. Samples were chosen from localities on Holsnøy (western Norway) and Lofoten (northern Norway), that are representative of vermicular symplectites, partly recrystallized symplectites, and deformed symplectites. Interphase misorientation analysis based on the electron backscatter diffraction results reveals that the nucleation of (diopsidic) clinopyroxene-plagioclase symplectites was crystallographically controlled, with the diopside copying the lattice orientation of the omphacite, and the plagioclase growing along diopside planes with favorable, i.e., similar, interplanar spacing. Deformation of the (diopsidic) clinopyroxene-plagioclase symplectites occurred by fracturing, transitioning into grain boundary sliding accommodated by diffusion creep. The results indicate that the formation of vermicular symplectites is not associated with enhanced permeability and fluid flow. Subsequent recrystallisation and grain-size sensitive deformation of the symplectites facilitates fluid redistribution and weakening of the retrogressed eclogites.
{"title":"Omphacite breakdown: nucleation and deformation of clinopyroxene-plagioclase symplectites","authors":"Sascha Zertani, Luiz F. G. Morales, Luca Menegon","doi":"10.1007/s00410-024-02125-0","DOIUrl":"https://doi.org/10.1007/s00410-024-02125-0","url":null,"abstract":"<p>The breakdown of omphacite plays an important role in the exhumation and retrogression of eclogites. Additionally, metamorphic reactions associated with grain size reduction have the potential to significantly impact deformation mechanisms and the rheology of crustal rocks. We analyze the breakdown reaction omphacite → diopsidic clinopyroxene + plagioclase ± amphibole and associated microstructures by electron backscatter diffraction. The reaction results in the formation of (diopsidic) clinopyroxene-plagioclase symplectites. Samples were chosen from localities on Holsnøy (western Norway) and Lofoten (northern Norway), that are representative of vermicular symplectites, partly recrystallized symplectites, and deformed symplectites. Interphase misorientation analysis based on the electron backscatter diffraction results reveals that the nucleation of (diopsidic) clinopyroxene-plagioclase symplectites was crystallographically controlled, with the diopside copying the lattice orientation of the omphacite, and the plagioclase growing along diopside planes with favorable, i.e., similar, interplanar spacing. Deformation of the (diopsidic) clinopyroxene-plagioclase symplectites occurred by fracturing, transitioning into grain boundary sliding accommodated by diffusion creep. The results indicate that the formation of vermicular symplectites is not associated with enhanced permeability and fluid flow. Subsequent recrystallisation and grain-size sensitive deformation of the symplectites facilitates fluid redistribution and weakening of the retrogressed eclogites.</p>","PeriodicalId":526,"journal":{"name":"Contributions to Mineralogy and Petrology","volume":null,"pages":null},"PeriodicalIF":3.5,"publicationDate":"2024-04-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140561995","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}