Pub Date : 2024-08-03DOI: 10.1093/petrology/egae086
Anton Kutyrev, Vadim S Kamenetsky, Ivan F Chayka, Nikolai A Nekrylov, Lyudmila Kryuchkova, Vladimir V Shilovskikh, Alkiviadis Kontonikas-Charos, Stepan P Krasheninnikov, Anna Sapegina, Alexei L Perchuk
Platinum-group elements (PGE) are most effectively concentrated via sulfide-silicate melt immiscibility; however, under sulfide-undersaturated conditions, PGE may exhibit highly-siderophile behaviour and occur as native metals and alloys. In this case, they can form μm-size inclusions within Cr-spinel, but also large nuggets (up to several kilograms), found in chromitites and related placer deposits. The exact formation mechanism of such large nuggets and accumulations of PGE unrelated to sulfide melts remains controversial due to mass balance issues. In this study of multiphase inclusions in Os-Ir-Ru nuggets from the Adamsfield placer district (Tasmania, Australia), we constrain their crystallization environment. Multiphase inclusions comprise variable proportions of hornblende, enstatite, quartz, anthophyllite, anorthite, chlorite and native iridium and coexist with single-phase olivine and Cr-spinel inclusions. The heterogeneity in phase and chemical composition of the inclusions indicates a complex origin from an inhomogeneous source media. This is corroborated by grain scale disequilibrium mineral assemblages, where forsterite and quartz are both included within a single osmium grain. Our proposed multi-stage origin of Adamsfield Os-Ir-Ru nuggets involved magmatic olivine-Cr-spinel-Os-Ir-Ru cumulates in peridotite bodies that were subsequently overprinted by various hydrothermal and metamorphic processes, including serpentinization or growth of Os-Ir-Ru from supercritical fluids. The final metamorphic stage resulted in the obliteration of Os-Ir-Ru zonation, culminating in the current assemblage of inclusions.
{"title":"Metamorphic origin of large nuggets of platinum-group metals: evidence from multiphase inclusions in Os-Ir-Ru alloys from the adamsfield placer, tasmania","authors":"Anton Kutyrev, Vadim S Kamenetsky, Ivan F Chayka, Nikolai A Nekrylov, Lyudmila Kryuchkova, Vladimir V Shilovskikh, Alkiviadis Kontonikas-Charos, Stepan P Krasheninnikov, Anna Sapegina, Alexei L Perchuk","doi":"10.1093/petrology/egae086","DOIUrl":"https://doi.org/10.1093/petrology/egae086","url":null,"abstract":"Platinum-group elements (PGE) are most effectively concentrated via sulfide-silicate melt immiscibility; however, under sulfide-undersaturated conditions, PGE may exhibit highly-siderophile behaviour and occur as native metals and alloys. In this case, they can form μm-size inclusions within Cr-spinel, but also large nuggets (up to several kilograms), found in chromitites and related placer deposits. The exact formation mechanism of such large nuggets and accumulations of PGE unrelated to sulfide melts remains controversial due to mass balance issues. In this study of multiphase inclusions in Os-Ir-Ru nuggets from the Adamsfield placer district (Tasmania, Australia), we constrain their crystallization environment. Multiphase inclusions comprise variable proportions of hornblende, enstatite, quartz, anthophyllite, anorthite, chlorite and native iridium and coexist with single-phase olivine and Cr-spinel inclusions. The heterogeneity in phase and chemical composition of the inclusions indicates a complex origin from an inhomogeneous source media. This is corroborated by grain scale disequilibrium mineral assemblages, where forsterite and quartz are both included within a single osmium grain. Our proposed multi-stage origin of Adamsfield Os-Ir-Ru nuggets involved magmatic olivine-Cr-spinel-Os-Ir-Ru cumulates in peridotite bodies that were subsequently overprinted by various hydrothermal and metamorphic processes, including serpentinization or growth of Os-Ir-Ru from supercritical fluids. The final metamorphic stage resulted in the obliteration of Os-Ir-Ru zonation, culminating in the current assemblage of inclusions.","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":null,"pages":null},"PeriodicalIF":3.9,"publicationDate":"2024-08-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141937137","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 : 2024-07-30DOI: 10.1093/petrology/egae084
Manuel Pimenta Silva, Andrea Giuliani, Urs Schaltegger, Massimo Chiaradia, Alexandra Nowak, Blair Schoene, Peter Ulmer, Othmar Müntener
The incremental construction of plutons characterises magmatic activity in arc settings, where new continental crust is produced. This polyphasic growth entails interactions with one or more crustal components, which modulate the geochemical and isotopic compositions of the newly formed crust. However, the early stages of magmatism are not always preserved due to obliteration by later magmatic pulses. Spatial migration of magmatism during the construction of the Adamello batholith (Northern Italy) enables the examination of the early pulses of pluton formation, thus allowing a time-integrated study of the relative importance of crystallisation-differentiation and contamination in a continental arc setting. We conducted a detailed textural, major and trace element, and Sr isotopic study of plagioclase from the first intrusive pulses of the Adamello batholith, combined with new major, trace element and Sr-Nd isotopic analyses of bulk rock samples across the entire Adamello batholith. We selected well-characterised samples with published CA-ID-TIMS 206Pb-238U ages and Hf isotopic composition for zircons. Strontium isotopes in plagioclase from the same samples were determined by laser ablation multi-collector ICP-MS. The tonalitic samples in the early magmatic stages show elevated but constant Sr isotopic compositions despite large variations in anorthite contents (An90 to An13), indicating that crustal contamination occurred before significant differentiation. Invariant bulk-rock 87Sr/86Sr with variable SiO2 in all superunits of the Adamello batholith further supports contamination preceding significant melt differentiation. Contamination by lower crustal basement lithologies is due to the increasing thermal anomaly triggered by consecutive magmatic injections coupled with the heterogeneous and less restitic nature of the basement in the early stage of the magmatic system (i.e. before consumption of fusible components). In addition, we observe significant variability in crustal contamination proxies (e.g. 87Sr/86Srplag, 87Sr/86Srbulk, εNdbulk, εHfzircon) during the initial phases of magmatism. This variability likely reflects the uneven distribution of positive thermal anomalies in the lower crust during early magmatic stages as well as the diverse lithological and isotopic makeup of the lower crust. The processes identified in our case study are pertinent to continental arc magmatism, particularly where magmas interact with a metapelitic lower crust.
{"title":"Tracing Lower Crustal Contamination in Continental Arc Magmas Using Sr-Nd-Hf Isotopes: a Combined in-Situ and Bulk Rock Approach Applied to the Adamello Batholith","authors":"Manuel Pimenta Silva, Andrea Giuliani, Urs Schaltegger, Massimo Chiaradia, Alexandra Nowak, Blair Schoene, Peter Ulmer, Othmar Müntener","doi":"10.1093/petrology/egae084","DOIUrl":"https://doi.org/10.1093/petrology/egae084","url":null,"abstract":"The incremental construction of plutons characterises magmatic activity in arc settings, where new continental crust is produced. This polyphasic growth entails interactions with one or more crustal components, which modulate the geochemical and isotopic compositions of the newly formed crust. However, the early stages of magmatism are not always preserved due to obliteration by later magmatic pulses. Spatial migration of magmatism during the construction of the Adamello batholith (Northern Italy) enables the examination of the early pulses of pluton formation, thus allowing a time-integrated study of the relative importance of crystallisation-differentiation and contamination in a continental arc setting. We conducted a detailed textural, major and trace element, and Sr isotopic study of plagioclase from the first intrusive pulses of the Adamello batholith, combined with new major, trace element and Sr-Nd isotopic analyses of bulk rock samples across the entire Adamello batholith. We selected well-characterised samples with published CA-ID-TIMS 206Pb-238U ages and Hf isotopic composition for zircons. Strontium isotopes in plagioclase from the same samples were determined by laser ablation multi-collector ICP-MS. The tonalitic samples in the early magmatic stages show elevated but constant Sr isotopic compositions despite large variations in anorthite contents (An90 to An13), indicating that crustal contamination occurred before significant differentiation. Invariant bulk-rock 87Sr/86Sr with variable SiO2 in all superunits of the Adamello batholith further supports contamination preceding significant melt differentiation. Contamination by lower crustal basement lithologies is due to the increasing thermal anomaly triggered by consecutive magmatic injections coupled with the heterogeneous and less restitic nature of the basement in the early stage of the magmatic system (i.e. before consumption of fusible components). In addition, we observe significant variability in crustal contamination proxies (e.g. 87Sr/86Srplag, 87Sr/86Srbulk, εNdbulk, εHfzircon) during the initial phases of magmatism. This variability likely reflects the uneven distribution of positive thermal anomalies in the lower crust during early magmatic stages as well as the diverse lithological and isotopic makeup of the lower crust. The processes identified in our case study are pertinent to continental arc magmatism, particularly where magmas interact with a metapelitic lower crust.","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":null,"pages":null},"PeriodicalIF":3.9,"publicationDate":"2024-07-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141885302","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 : 2024-07-30DOI: 10.1093/petrology/egae083
Omar Bartoli, Leo J Millonig, Bruna B Carvalho, Horst R Marschall, Axel Gerdes
Rates of melt production, extraction and crystallization, as well as scales of melt transfer and interaction with their residuum change continuously in migmatite and granulite, affecting the behavior of monazite and zircon as time capsules. Therefore, accessory mineral chronometers may be ambiguous and incomplete in providing an overview of the temperature–time evolution of high-grade metamorphic rocks. In this study, we applied the novel technique of in-situ U–Pb dating of garnet to the archetypal lower continental crust of the Ivrea-Verbano Zone (IVZ), NW Italy. In the IVZ, the temporal relationship between granulite-facies metamorphism and mafic underplating has long been debated, because of the interplay between tectonic, magmatic, metamorphic and metasomatic processes over a period of more than a hundred million years. Garnet from mafic and pelitic granulites yielded U–Pb ages between 287.4 ± 4.9 Ma and 280.1 ± 12.4 Ma, overlapping within uncertainty the time proposed for the emplacement of the Mafic Complex (286–282 Ma). These results indicate that the thermal climax in granulitic rocks was caused by mafic underplating and concomitant asthenospheric upwelling, rather than being inherited from the post-Variscan Carboniferous evolution. Providing robust dating of garnet with as low as 4 ng/g U, this study demonstrates the strength of garnet petrochronology in resolving complex tectono-metamorphic histories of high-grade terranes. It also represents a further step forward towards establishing garnet as part of the in-situ U–Pb geochronology repertoire.
在辉绿岩和花岗岩中，熔体产生、萃取和结晶的速率，以及熔体转移和与其残余物相互作用的规模都在不断变化，影响着作为时间胶囊的独居石和锆石的行为。因此，附属矿物计时仪在概述高品位变质岩的温度-时间演化方面可能是模糊和不完整的。在这项研究中，我们将石榴石原位 U-Pb 定年的新技术应用于意大利西北部伊夫雷亚-韦尔巴诺区（IVZ）的典型下大陆地壳。在伊夫雷亚-维尔巴诺区（IVZ），由于构造、岩浆、变质和变质过程在一亿多年的时间里相互作用，花岗岩成因变质作用与黑云母底侵之间的时间关系一直存在争议。从黑云母和辉绿岩花岗岩中提取的石榴石的 U-Pb 年龄介于 287.4 ± 4.9 Ma 和 280.1 ± 12.4 Ma 之间，与所提出的黑云母复合体的形成时间（286-282 Ma）在不确定的范围内重叠。这些结果表明，花岗岩的热高潮是由黑云母下伏和随之而来的星体层上涌造成的，而不是从后石炭纪演化中继承而来。这项研究提供了低至 4 ng/g U 的石榴石可靠年代测定，证明了石榴石岩石年代学在解析高品位陆相复杂构造-变质历史方面的优势。它还标志着在将石榴石确定为原位 U-Pb 地质年代的一部分方面又向前迈进了一步。
{"title":"The Age of Granulite-Facies Metamorphism in the Ivrea-Verbano Zone (NW Italy) Determined through in-Situ U–Pb Dating of Garnet","authors":"Omar Bartoli, Leo J Millonig, Bruna B Carvalho, Horst R Marschall, Axel Gerdes","doi":"10.1093/petrology/egae083","DOIUrl":"https://doi.org/10.1093/petrology/egae083","url":null,"abstract":"Rates of melt production, extraction and crystallization, as well as scales of melt transfer and interaction with their residuum change continuously in migmatite and granulite, affecting the behavior of monazite and zircon as time capsules. Therefore, accessory mineral chronometers may be ambiguous and incomplete in providing an overview of the temperature–time evolution of high-grade metamorphic rocks. In this study, we applied the novel technique of in-situ U–Pb dating of garnet to the archetypal lower continental crust of the Ivrea-Verbano Zone (IVZ), NW Italy. In the IVZ, the temporal relationship between granulite-facies metamorphism and mafic underplating has long been debated, because of the interplay between tectonic, magmatic, metamorphic and metasomatic processes over a period of more than a hundred million years. Garnet from mafic and pelitic granulites yielded U–Pb ages between 287.4 ± 4.9 Ma and 280.1 ± 12.4 Ma, overlapping within uncertainty the time proposed for the emplacement of the Mafic Complex (286–282 Ma). These results indicate that the thermal climax in granulitic rocks was caused by mafic underplating and concomitant asthenospheric upwelling, rather than being inherited from the post-Variscan Carboniferous evolution. Providing robust dating of garnet with as low as 4 ng/g U, this study demonstrates the strength of garnet petrochronology in resolving complex tectono-metamorphic histories of high-grade terranes. It also represents a further step forward towards establishing garnet as part of the in-situ U–Pb geochronology repertoire.","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":null,"pages":null},"PeriodicalIF":3.9,"publicationDate":"2024-07-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141885303","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 : 2024-07-24DOI: 10.1093/petrology/egae081
M Chardelin, A Tommasi, J A Padrón-Navarta
This article documents the evolution of pressure and temperature conditions and the successive influence of hydrous melts and aqueous fluids on the operation of extensional shear zones, which exhumed mantle slivers from deep lithospheric or asthenospheric depths, in a rift-to-drift setting. These results are based on a re-analysis of 40 samples from three peridotite massifs of Zabargad island in the northern Red Sea. By integrating high-resolution mapping of the microstructure by electron backscattered diffraction with recent developments in barometry for plagioclase-bearing peridotites and thermodynamic modelling of peridotitic compositions, this study: (1) constrains the temporal and spatial evolution of petrological and tectonic processes in the shallow mantle during rifting and (2) documents the presence of melts or aqueous fluids throughout the activity of the shear zones, unravelling substantial feedback between petrological and tectonic processes. Thermobarometry and thermodynamic modelling, constrained by the microstructural observations, document progressive strain localization associated with shearing under decreasing pressure and temperature, from near solidus conditions at >1 GPa (in the north and central peridotite massifs) or ~0.7 GPa (in the southern massif) to < 600°C and <0.3 GPa in all three massifs. The data substantiate local aqueous fluid saturation in the shear zones. This together with higher contents of hydrous minerals in ultramylonites indicate fluid focusing in the shear zones, with seawater ingress extending to >10 km depth. The presence of melts or fluids enabled concurrent dislocation and dissolution-precipitation creep, resulting in weakening of the shear zones. However, fluid supply was spatially heterogeneous and likely intermittent, with equilibrium achieved only locally in the ultramylonites. The present study documents therefore how the feedback between progressive strain localization and fluid-focusing in extensional shear zones contributes to thinning and exhumation of the mantle during continental rifting and the rift-to-drift transition.
{"title":"PROGRESSIVE STRAIN LOCALIZATION AND FLUID FOCUSING IN MANTLE SHEAR ZONES DURING RIFTING: PETROSTRUCTURAL CONSTRAINTS FROM THE ZABARGAD PERIDOTITES, RED SEA","authors":"M Chardelin, A Tommasi, J A Padrón-Navarta","doi":"10.1093/petrology/egae081","DOIUrl":"https://doi.org/10.1093/petrology/egae081","url":null,"abstract":"This article documents the evolution of pressure and temperature conditions and the successive influence of hydrous melts and aqueous fluids on the operation of extensional shear zones, which exhumed mantle slivers from deep lithospheric or asthenospheric depths, in a rift-to-drift setting. These results are based on a re-analysis of 40 samples from three peridotite massifs of Zabargad island in the northern Red Sea. By integrating high-resolution mapping of the microstructure by electron backscattered diffraction with recent developments in barometry for plagioclase-bearing peridotites and thermodynamic modelling of peridotitic compositions, this study: (1) constrains the temporal and spatial evolution of petrological and tectonic processes in the shallow mantle during rifting and (2) documents the presence of melts or aqueous fluids throughout the activity of the shear zones, unravelling substantial feedback between petrological and tectonic processes. Thermobarometry and thermodynamic modelling, constrained by the microstructural observations, document progressive strain localization associated with shearing under decreasing pressure and temperature, from near solidus conditions at &gt;1 GPa (in the north and central peridotite massifs) or ~0.7 GPa (in the southern massif) to &lt; 600°C and &lt;0.3 GPa in all three massifs. The data substantiate local aqueous fluid saturation in the shear zones. This together with higher contents of hydrous minerals in ultramylonites indicate fluid focusing in the shear zones, with seawater ingress extending to &gt;10 km depth. The presence of melts or fluids enabled concurrent dislocation and dissolution-precipitation creep, resulting in weakening of the shear zones. However, fluid supply was spatially heterogeneous and likely intermittent, with equilibrium achieved only locally in the ultramylonites. The present study documents therefore how the feedback between progressive strain localization and fluid-focusing in extensional shear zones contributes to thinning and exhumation of the mantle during continental rifting and the rift-to-drift transition.","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":null,"pages":null},"PeriodicalIF":3.9,"publicationDate":"2024-07-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141785691","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 : 2024-07-19DOI: 10.1093/petrology/egae080
Antoine Godet, Marine Jouvent, Christiaan Laureijs, Carl Guilmette, Kyle Larson, Mark Coleman, Jérémie Darveau, Myriam Côté-Roberge
Recent advances in geochronological techniques now allow the ability to efficiently decipher the timing and duration of geological processes in complex high-grade polymetamorphosed orogenic terranes. This is the case of the Grenville Front Tectonic Zone, which truncates the Superior Craton to the southeast. The zone exposes parautochtonous Archean rocks that underwent mid- to high-pressure granulite facies metamorphism of uncertain age. The metamorphic assemblages have been either interpreted as Archean and associated with the final stages of the Superior craton assembly, or as the result of Mesoproterozoic Grenvillian metamorphism, based on cross-cutting relationships and traditional geochronology methods such as U-Pb zircon and 40Ar-39Ar mica dating. Herein, we revisit the extent of the Grenvillian metamorphic overprint in the parautochtonous domain and provide new age constraints for granulite-facies metamorphic assemblages through in-situ garnet dating within migmatitic paragneiss, migmatitic orthogneiss, and mafic granulites, combined with in-situ trace element mapping. Six samples, which show bell-shaped and occasionally sharp and oscillatory lutetium growth zoning in garnet, yield garnet Lu-Hf isochrons with identical Archean dates of c. 2.6 Ga. Sparce analyses of material trend toward Grenvillian ages (c. 1 Ga) in one sample from which garnet shows lutetium zoning consistent with post-growth fluid-assisted disturbance. Overall, our results indicate that the widespread granulite-facies metamorphism within the Grenville Front Tectonic Zone is dominantly late Neoarchean in age, unveiling a rare exposure of Archean lower crust in the southeastern Superior Craton. Our results also point towards a limited Grenvillian metamorphic overprint though the spatial extent and precise thermal conditions of this metamorphism are still unknown. The results presented herein demonstrate the potential of in-situ isotopic geochronology on rock-forming minerals like garnet in polymetamorphic terranes.
{"title":"In-situ Lu-Hf garnet dating of Archean deep crust granulites from the polymetamorphic Grenville Front Tectonic Zone","authors":"Antoine Godet, Marine Jouvent, Christiaan Laureijs, Carl Guilmette, Kyle Larson, Mark Coleman, Jérémie Darveau, Myriam Côté-Roberge","doi":"10.1093/petrology/egae080","DOIUrl":"https://doi.org/10.1093/petrology/egae080","url":null,"abstract":"Recent advances in geochronological techniques now allow the ability to efficiently decipher the timing and duration of geological processes in complex high-grade polymetamorphosed orogenic terranes. This is the case of the Grenville Front Tectonic Zone, which truncates the Superior Craton to the southeast. The zone exposes parautochtonous Archean rocks that underwent mid- to high-pressure granulite facies metamorphism of uncertain age. The metamorphic assemblages have been either interpreted as Archean and associated with the final stages of the Superior craton assembly, or as the result of Mesoproterozoic Grenvillian metamorphism, based on cross-cutting relationships and traditional geochronology methods such as U-Pb zircon and 40Ar-39Ar mica dating. Herein, we revisit the extent of the Grenvillian metamorphic overprint in the parautochtonous domain and provide new age constraints for granulite-facies metamorphic assemblages through in-situ garnet dating within migmatitic paragneiss, migmatitic orthogneiss, and mafic granulites, combined with in-situ trace element mapping. Six samples, which show bell-shaped and occasionally sharp and oscillatory lutetium growth zoning in garnet, yield garnet Lu-Hf isochrons with identical Archean dates of c. 2.6 Ga. Sparce analyses of material trend toward Grenvillian ages (c. 1 Ga) in one sample from which garnet shows lutetium zoning consistent with post-growth fluid-assisted disturbance. Overall, our results indicate that the widespread granulite-facies metamorphism within the Grenville Front Tectonic Zone is dominantly late Neoarchean in age, unveiling a rare exposure of Archean lower crust in the southeastern Superior Craton. Our results also point towards a limited Grenvillian metamorphic overprint though the spatial extent and precise thermal conditions of this metamorphism are still unknown. The results presented herein demonstrate the potential of in-situ isotopic geochronology on rock-forming minerals like garnet in polymetamorphic terranes.","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":null,"pages":null},"PeriodicalIF":3.9,"publicationDate":"2024-07-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141742541","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 : 2024-07-18DOI: 10.1093/petrology/egae082
Aristle Monteiro, Tushar Mittal, Raymond Duraiswami, Stephen Self
Continental flood basalts provinces are the product of the largest known volumetric eruptions on Earth (~104 km3), with individual flow fields commonly covering well over 10 000 km2 with a mean lava thickness of over 5 m. Studies focusing on the emplacement style of such lava flows have relied extensively on morphological observations and comparisons with modern lava flows and experimental analogs. In the present study, we compare the textures of flood basalt lavas with those from different eruption settings all over the world using data collected from pre-existing literature to gain detailed insights into the style of eruption. Comparison of crystal size distribution (CSD) data indicates that the eruption style of CFBs is similar to those of modern-day fissure eruptions (e.g. Iceland). This matches inferences based on observations of morphology. We also use a 1D thermal model to estimate the depth dependent cooling rates within a single lava lobe and test the validity of assumptions built into the formulation of these models. The results reveal that, on average, flood basalt lavas need to conductively cool much faster than we would expect (up to order of ~102 times faster) to match the textural observations. The model is also frequently unable to replicate the observed depth-wise relative variations in length with depth for CFB lavas. Furthermore, the calculated cooling rates from crystal shapes also do not match those calculated from crystal lengths, indicating the assumptions in cooling flow models need to be modified for large CFB flow fields. Given the large areas of CFB flow-fields and the relatively long eruption times inferred for the emplacement of individual flow fields, we hypothesize that inflation of lobes and formation of new lobes via breakouts combined with variable eruption rates are key processes that are missing when modeling the cooling of these flow fields. Accounting for these processes is essential to derive accurate cooling rates, which is important to better understand the environmental impact CFBs have at the time of emplacement.
{"title":"The crystallization of continental flood basalt (CFB) lavas: insights from textural studies","authors":"Aristle Monteiro, Tushar Mittal, Raymond Duraiswami, Stephen Self","doi":"10.1093/petrology/egae082","DOIUrl":"https://doi.org/10.1093/petrology/egae082","url":null,"abstract":"Continental flood basalts provinces are the product of the largest known volumetric eruptions on Earth (~104 km3), with individual flow fields commonly covering well over 10 000 km2 with a mean lava thickness of over 5 m. Studies focusing on the emplacement style of such lava flows have relied extensively on morphological observations and comparisons with modern lava flows and experimental analogs. In the present study, we compare the textures of flood basalt lavas with those from different eruption settings all over the world using data collected from pre-existing literature to gain detailed insights into the style of eruption. Comparison of crystal size distribution (CSD) data indicates that the eruption style of CFBs is similar to those of modern-day fissure eruptions (e.g. Iceland). This matches inferences based on observations of morphology. We also use a 1D thermal model to estimate the depth dependent cooling rates within a single lava lobe and test the validity of assumptions built into the formulation of these models. The results reveal that, on average, flood basalt lavas need to conductively cool much faster than we would expect (up to order of ~102 times faster) to match the textural observations. The model is also frequently unable to replicate the observed depth-wise relative variations in length with depth for CFB lavas. Furthermore, the calculated cooling rates from crystal shapes also do not match those calculated from crystal lengths, indicating the assumptions in cooling flow models need to be modified for large CFB flow fields. Given the large areas of CFB flow-fields and the relatively long eruption times inferred for the emplacement of individual flow fields, we hypothesize that inflation of lobes and formation of new lobes via breakouts combined with variable eruption rates are key processes that are missing when modeling the cooling of these flow fields. Accounting for these processes is essential to derive accurate cooling rates, which is important to better understand the environmental impact CFBs have at the time of emplacement.","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":null,"pages":null},"PeriodicalIF":3.9,"publicationDate":"2024-07-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141778826","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 : 2024-07-15DOI: 10.1093/petrology/egae075
Tereza Zelinková, M. Racek, V. Janoušek, P. Štípská, Rainer Abart, Rene Asenbaum
The St. Leonhard granulite massif in Lower Austria, dominantly formed by kyanite-bearing felsic granulite, encloses countless up to 5 cm sized mantle xenoliths of garnet clinopyroxenite and peridotite. The mineralogical, textural and chemical consequences of a mutual metasomatic interaction at the contact between these xenoliths and the host orthopyroxene-bearing felsic granulite are described. Movement of Mg, Al and, especially, Ca from the garnet clinopyroxenite to the granulite and migration of K and Na in the opposite direction, caused the breakdown of clinopyroxene and formation of orthopyroxene–plagioclase symplectite coronae at the expense of the garnet clinopyroxenite xenoliths. Around the peridotite xenoliths, monomineralic orthopyroxene coronae have developed due to the supply of Si from the host granulite. The P–T conditions of this interaction were established to 900–1000 °C and 1.0–1.2 GPa by thermodynamic modelling. The duration of coronae growth was constrained to 13–532 ka based on modelling of Fe–Mg interdiffusion underlying secondary compositional zoning of garnet from the garnet clinopyroxenite xenolith extending to the coronae. The most significant change in the host granulite was caused by the supply of Ca from the garnet clinopyroxenite xenolith, which led to the breakdown of the Al2SiO5 phase – probably kyanite – and stabilization of orthopyroxene. K-feldspar-poor haloes surrounding mantle xenoliths formed due to the depletion of K in the granulite adjacent to the garnet clinopyroxenite. The observed origin of felsic–intermediate orthopyroxene-bearing granulite by transformation of felsic kyanite-bearing granulite through the metasomatic interaction with mantle xenoliths implies that the deep crustal chemical exchange between mantle- and crust-derived lithologies may have an important consequences on composition, thermal structure and geodynamic evolution of orogenic lower crust especially in hot collisional orogens, such as the European Variscides.
{"title":"Metasomatic interaction of ultramafic mantle xenoliths with their felsic HP–UHT granulite host (Moldanubian Domain, Bohemian Massif in Lower Austria)","authors":"Tereza Zelinková, M. Racek, V. Janoušek, P. Štípská, Rainer Abart, Rene Asenbaum","doi":"10.1093/petrology/egae075","DOIUrl":"https://doi.org/10.1093/petrology/egae075","url":null,"abstract":"\u0000 The St. Leonhard granulite massif in Lower Austria, dominantly formed by kyanite-bearing felsic granulite, encloses countless up to 5 cm sized mantle xenoliths of garnet clinopyroxenite and peridotite. The mineralogical, textural and chemical consequences of a mutual metasomatic interaction at the contact between these xenoliths and the host orthopyroxene-bearing felsic granulite are described. Movement of Mg, Al and, especially, Ca from the garnet clinopyroxenite to the granulite and migration of K and Na in the opposite direction, caused the breakdown of clinopyroxene and formation of orthopyroxene–plagioclase symplectite coronae at the expense of the garnet clinopyroxenite xenoliths. Around the peridotite xenoliths, monomineralic orthopyroxene coronae have developed due to the supply of Si from the host granulite. The P–T conditions of this interaction were established to 900–1000 °C and 1.0–1.2 GPa by thermodynamic modelling. The duration of coronae growth was constrained to 13–532 ka based on modelling of Fe–Mg interdiffusion underlying secondary compositional zoning of garnet from the garnet clinopyroxenite xenolith extending to the coronae. The most significant change in the host granulite was caused by the supply of Ca from the garnet clinopyroxenite xenolith, which led to the breakdown of the Al2SiO5 phase – probably kyanite – and stabilization of orthopyroxene. K-feldspar-poor haloes surrounding mantle xenoliths formed due to the depletion of K in the granulite adjacent to the garnet clinopyroxenite. The observed origin of felsic–intermediate orthopyroxene-bearing granulite by transformation of felsic kyanite-bearing granulite through the metasomatic interaction with mantle xenoliths implies that the deep crustal chemical exchange between mantle- and crust-derived lithologies may have an important consequences on composition, thermal structure and geodynamic evolution of orogenic lower crust especially in hot collisional orogens, such as the European Variscides.","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":null,"pages":null},"PeriodicalIF":3.5,"publicationDate":"2024-07-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141645403","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 : 2024-07-10DOI: 10.1093/petrology/egae078
Dachuan Wang, Tong Hou, Roman Botcharnikov, Sarah Haselbach, Florian Pohl, Renat R Almeev, Andreas Klügel, Meng Wang, Jingyi Qin, Zhaochong Zhang, Francois Holtz
High-Ti basalts are commonly believed to represent parental magmas leading to the formation of mafic-ultramafic layered intrusions, such as Late Permian Panzhihua and Hongge in the Emeishan Large Igneous Province (SW China). Consequently, elucidation of the crystallisation and crustal differentiation of high-Ti basalts is critical for our understanding of the petrogenesis of these layered intrusions and the associated oxide ore mineralisation. Here, we present the results of crystallisation experiments carried out in internally heated pressure vessels using a primitive high-Ti basaltic composition. The experiments were conducted at 100 and 300 MPa, in the temperature interval of 950-1200 °C and with water activities (aH2O) from 0 to 1. The oxygen fugacity (fO2) was controlled and varied from FMQ - 1 to FMQ + 3.3 log units (FMQ corresponds to fayalite-quartz-magnetite buffer). The main mineral phases are olivine, clinopyroxene and plagioclase, accompanied by Cr-Fe-Ti-oxides, orthopyroxene, apatite and amphibole, depending on the conditions. Redox conditions primarily influence the stability fields of Cr-Fe-Ti oxides. Clinopyroxene, orthopyroxene and amphibole are pressure-dependent and have larger stability fields under high pressure conditions. The olivine→orthopyroxene and olivine→amphibole peritectic reactions are observed. Comparisons of phase equilibria between this study and experiments conducted with parental magma of Skaergaard layered intrusion demonstrate the effect of bulk system composition. For instance, ilmenite crystallisation is determined by intrinsic parameters such as fO2 but also by additional compositional parameters (e.g., melt Ti, Fe, Al and Mg content). Although COMAGMAT and MELTS modelling results generally reproduce the crystallisation sequence, only the stability field of clinopyroxene and its composition are perfectly modelled. The comparison of experimental results with the rocks from the lowest units of Panzhihua and Hongge layered intrusions are applied to constrain storage conditions in the magma reservoirs. Compared to Hongge, we conclude that the Panzhihua magma chamber was probably located at a shallower depth (~3-6 km), that magma crystallisation started at lower temperatures (~1125-1100 °C), higher fO2 (~FMQ + 1 to FMQ + 2), and that its initial melt H2O content was lower (~0.5-1 wt.%).
{"title":"Experimental Constraints on the Storage Conditions and Differentiation of High-Ti Basalts from the Panzhihua and Hongge Layered Intrusions, SW China","authors":"Dachuan Wang, Tong Hou, Roman Botcharnikov, Sarah Haselbach, Florian Pohl, Renat R Almeev, Andreas Klügel, Meng Wang, Jingyi Qin, Zhaochong Zhang, Francois Holtz","doi":"10.1093/petrology/egae078","DOIUrl":"https://doi.org/10.1093/petrology/egae078","url":null,"abstract":"High-Ti basalts are commonly believed to represent parental magmas leading to the formation of mafic-ultramafic layered intrusions, such as Late Permian Panzhihua and Hongge in the Emeishan Large Igneous Province (SW China). Consequently, elucidation of the crystallisation and crustal differentiation of high-Ti basalts is critical for our understanding of the petrogenesis of these layered intrusions and the associated oxide ore mineralisation. Here, we present the results of crystallisation experiments carried out in internally heated pressure vessels using a primitive high-Ti basaltic composition. The experiments were conducted at 100 and 300 MPa, in the temperature interval of 950-1200 °C and with water activities (aH2O) from 0 to 1. The oxygen fugacity (fO2) was controlled and varied from FMQ - 1 to FMQ + 3.3 log units (FMQ corresponds to fayalite-quartz-magnetite buffer). The main mineral phases are olivine, clinopyroxene and plagioclase, accompanied by Cr-Fe-Ti-oxides, orthopyroxene, apatite and amphibole, depending on the conditions. Redox conditions primarily influence the stability fields of Cr-Fe-Ti oxides. Clinopyroxene, orthopyroxene and amphibole are pressure-dependent and have larger stability fields under high pressure conditions. The olivine→orthopyroxene and olivine→amphibole peritectic reactions are observed. Comparisons of phase equilibria between this study and experiments conducted with parental magma of Skaergaard layered intrusion demonstrate the effect of bulk system composition. For instance, ilmenite crystallisation is determined by intrinsic parameters such as fO2 but also by additional compositional parameters (e.g., melt Ti, Fe, Al and Mg content). Although COMAGMAT and MELTS modelling results generally reproduce the crystallisation sequence, only the stability field of clinopyroxene and its composition are perfectly modelled. The comparison of experimental results with the rocks from the lowest units of Panzhihua and Hongge layered intrusions are applied to constrain storage conditions in the magma reservoirs. Compared to Hongge, we conclude that the Panzhihua magma chamber was probably located at a shallower depth (~3-6 km), that magma crystallisation started at lower temperatures (~1125-1100 °C), higher fO2 (~FMQ + 1 to FMQ + 2), and that its initial melt H2O content was lower (~0.5-1 wt.%).","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":null,"pages":null},"PeriodicalIF":3.9,"publicationDate":"2024-07-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141588020","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 : 2024-07-01DOI: 10.1093/petrology/egae077
{"title":"Correction to: Garnet–Quartz Inclusion Thermobarometry and Lu–Hf Chronology Detail the Pre-Ultra-High Pressure Metamorphic History of the Grapesvare Nappe, Scandinavian Caledonides","authors":"","doi":"10.1093/petrology/egae077","DOIUrl":"https://doi.org/10.1093/petrology/egae077","url":null,"abstract":"","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":null,"pages":null},"PeriodicalIF":3.5,"publicationDate":"2024-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141711696","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 : 2024-07-01DOI: 10.1093/petrology/egae076
{"title":"Correction to: Petrogenesis and Geodynamic Evolution of the Archean Shawmere Anorthosite Complex and Associated Gneisses, Kapuskasing Uplift, Superior Province, Canada","authors":"","doi":"10.1093/petrology/egae076","DOIUrl":"https://doi.org/10.1093/petrology/egae076","url":null,"abstract":"","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":null,"pages":null},"PeriodicalIF":3.5,"publicationDate":"2024-07-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141709226","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}
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