Pub Date : 2024-05-30DOI: 10.1093/petrology/egae055
I N Bindeman
Taupō volcanic zone, the site of the 26ka Oruanui supereruption, produced ~70km3 of new rhyolites since 11ka, culminating in 50 km3 Taupō eruption 1.8 ka. Major phenocrysts decrease from 4 to 1 vol%, and Oruanui and post-Oruanui ignimbrites all have identical high-d18Omelt values of 7.39±0.1‰ and lack low-d18O values despite overlapping calderas. The D’17O values are -0.07‰, lower than the mantle and indicate source contamination of high-d18O, low-D’17O metasediments, and limited interaction with high-D’17O hydrothermally altered crust. Previously published U-Th-Pb zircon ages demonstrate their diversity spanning 104-105 years for each unit. Zircon crystal size distribution shows a decrease in abundance and the mean size, and some units lack small (<~10 um) zircons suggesting that zircons were both growing and dissolving in the coexisting magma generation areas. Isotope thermometry indicates heating of the system from ~812±35°C to 874±36°C past zircon saturation in 1.8ka eruption. We advocate that a deep vertically continuous and laterally discontinuous silicic magma system at the base of the Taupō rift, rather than a shallow batholith or an evolving mush, drives volcanism at Taupō. To explain the post-Oruanui magma production, rift-base silicic magma origin and moderate (~2 km3/1000years) rhyodacitic magma flux from a growing and heating liquid magma body there creates a sufficient solution for the most recent magmatism.
{"title":"Reinterpretation of the post 26ka Taupō rhyolitic magmatic system (New Zealand) as deep and vertically extensive based on Isotope thermometry and measured and modeled zircon destinies","authors":"I N Bindeman","doi":"10.1093/petrology/egae055","DOIUrl":"https://doi.org/10.1093/petrology/egae055","url":null,"abstract":"Taupō volcanic zone, the site of the 26ka Oruanui supereruption, produced ~70km3 of new rhyolites since 11ka, culminating in 50 km3 Taupō eruption 1.8 ka. Major phenocrysts decrease from 4 to 1 vol%, and Oruanui and post-Oruanui ignimbrites all have identical high-d18Omelt values of 7.39±0.1‰ and lack low-d18O values despite overlapping calderas. The D’17O values are -0.07‰, lower than the mantle and indicate source contamination of high-d18O, low-D’17O metasediments, and limited interaction with high-D’17O hydrothermally altered crust. Previously published U-Th-Pb zircon ages demonstrate their diversity spanning 104-105 years for each unit. Zircon crystal size distribution shows a decrease in abundance and the mean size, and some units lack small (&lt;~10 um) zircons suggesting that zircons were both growing and dissolving in the coexisting magma generation areas. Isotope thermometry indicates heating of the system from ~812±35°C to 874±36°C past zircon saturation in 1.8ka eruption. We advocate that a deep vertically continuous and laterally discontinuous silicic magma system at the base of the Taupō rift, rather than a shallow batholith or an evolving mush, drives volcanism at Taupō. To explain the post-Oruanui magma production, rift-base silicic magma origin and moderate (~2 km3/1000years) rhyodacitic magma flux from a growing and heating liquid magma body there creates a sufficient solution for the most recent magmatism.","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":"38 1","pages":""},"PeriodicalIF":3.9,"publicationDate":"2024-05-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141197195","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 : 2024-05-13DOI: 10.1093/petrology/egae048
Valentin Basch, Alessio Sanfilippo, Jonathan E Snow, Matthew Loocke, Alberto Zanetti
At mid-ocean ridges, melts that formed during adiabatic melting of a heterogeneous mantle migrate upwards and ultimately crystallize the oceanic crust. The lower crustal gabbros represent the first crystallization products of these melts and the processes involved in the accretion of the lowermost crust drive the chemical evolution of the magmas forming two thirds of Earth’s surface. At fast-spreading ridges, elevated melt supply leads to the formation of a ⁓6 km-thick layered oceanic crust. Here, we provide a detailed petrochemical characterization of the lower portion of the fast-spread oceanic crust drilled during IODP Expedition 345 at the East Pacific Rise (IODP Holes U1415), together with the processes involved in crustal accretion. The recovered gabbroic rocks are primitive in composition and range from troctolites to olivine gabbros, olivine gabbronorites and gabbros. Although textural evidence of dissolution-precipitation processes is widespread within this gabbroic section, only the most interstitial phases record chemical compositions driven by melt-mush interaction processes during closure of the magmatic system. Comparing mineral compositions from this lower crustal section with its slow-spreading counterparts, we propose that the impact of reactive processes on the chemical evolution of the parental melts is dampened in the lower gabbros from magmatically productive spreading centres. Oceanic accretion thereby seems driven by fractional crystallization in the lower gabbroic layers, followed by upward reactive percolation of melts towards shallower sections. Using the composition of clinopyroxene from these primitive, nearly unmodified gabbros, we estimate the parental melt trace element compositions of Hess Deep, showing that the primary melts of the East Pacific Rise are more depleted in incompatible trace elements compared to those formed at slower spreading rates, as a result of higher melting degrees of the underlying mantle.
{"title":"Accretion of the lower oceanic crust at fast-spreading ridges: Insights from Hess Deep (East Pacific Rise, IODP Expedition 345)","authors":"Valentin Basch, Alessio Sanfilippo, Jonathan E Snow, Matthew Loocke, Alberto Zanetti","doi":"10.1093/petrology/egae048","DOIUrl":"https://doi.org/10.1093/petrology/egae048","url":null,"abstract":"At mid-ocean ridges, melts that formed during adiabatic melting of a heterogeneous mantle migrate upwards and ultimately crystallize the oceanic crust. The lower crustal gabbros represent the first crystallization products of these melts and the processes involved in the accretion of the lowermost crust drive the chemical evolution of the magmas forming two thirds of Earth’s surface. At fast-spreading ridges, elevated melt supply leads to the formation of a ⁓6 km-thick layered oceanic crust. Here, we provide a detailed petrochemical characterization of the lower portion of the fast-spread oceanic crust drilled during IODP Expedition 345 at the East Pacific Rise (IODP Holes U1415), together with the processes involved in crustal accretion. The recovered gabbroic rocks are primitive in composition and range from troctolites to olivine gabbros, olivine gabbronorites and gabbros. Although textural evidence of dissolution-precipitation processes is widespread within this gabbroic section, only the most interstitial phases record chemical compositions driven by melt-mush interaction processes during closure of the magmatic system. Comparing mineral compositions from this lower crustal section with its slow-spreading counterparts, we propose that the impact of reactive processes on the chemical evolution of the parental melts is dampened in the lower gabbros from magmatically productive spreading centres. Oceanic accretion thereby seems driven by fractional crystallization in the lower gabbroic layers, followed by upward reactive percolation of melts towards shallower sections. Using the composition of clinopyroxene from these primitive, nearly unmodified gabbros, we estimate the parental melt trace element compositions of Hess Deep, showing that the primary melts of the East Pacific Rise are more depleted in incompatible trace elements compared to those formed at slower spreading rates, as a result of higher melting degrees of the underlying mantle.","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":"16 1","pages":""},"PeriodicalIF":3.9,"publicationDate":"2024-05-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140931849","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 : 2024-05-13DOI: 10.1093/petrology/egae049
José Francisco Molina, Fernando Bea, Pilar Montero, Faouziya Haissen, Leticia Barcos, Aitor Cambeses, Irene Morales, Maria Monika Repczynska, Othman Sadki, Antonio García-Casco
Underplated mafic intrusions ponded at the base of the lower continental crust in extensional settings can experience ultra-high temperature (UHT) granulite-facies metamorphism during tens of My due to slow cooling rates, being also the source of heat and carbonic fluids for regional high-temperature (HT) granulite-facies metamorphism in the continental crust. This work analyses the fluid-melt-rock interaction processes that occurred during the magmatic to HT-UHT-granulite- and amphibolite-facies metamorphic evolution of high-grade mafic rocks from the Eastern Ediacaran Adrar-Suttuf Metamafic Complex (EASMC) of the Oulad Dlim Massif (West African Craton Margin, Southern Morocco). P-T conditions were determined using Ti-in-amphibole thermometry, two-pyroxene and amphibole-plagioclase thermobarometry, and phase diagrams calculations. The thermobarometric study reveals the presence of tectonically juxtaposed lower- and mid-crustal blocks in EASMC that experienced decompression-cooling paths from, respectively UHT and HT granulite-facies conditions at ca. 1.2 ±0.28 GPa and 975 ±50 °C, and ca. 0.82 ±0.15 GPa and 894 ±50 °C, to amphibole-facies conditions at ca. 0.28 ±0.28 GPa and 787 ±45 °C (precision reported for the calibrations at 1s level). An age for the magmatic to UHT granulite-facies metamorphic transition of 604 Ma was constrained from published SHRIMP Th-U-Pb zircon ages of the igneous protoliths. An amphibole 40Ar-39Ar cooling age of 499 ±8 Ma (precision at 2s level) was obtained for the lower-crustal blocks. Amphibole 40Ar-39Ar closure temperatures of 520-555 °C were obtained for an age range of 600-499 Ma and an average constant cooling rate of 4.2 °C/My, suggesting that the lower-crustal blocks cooled down to the greenschist-amphibolite facies transition in ca. 100 My. During the high-temperature stage, interstitial hydrous melts caused incongruent dissolution melting of olivine and pyroxenes, and, probably, the development of An-rich spikes at the grain rims of plagioclase, and assisted textural maturation of the rock matrix and local segregation of pargasite into veins. Subsequent local infiltration of reactive hydrous metamorphic fluids along mineral grain boundaries during cooling down to amphibolite-facies conditions promoted mineral replacements by coupled dissolution-precipitation mechanisms and metasomatism. Ubiquitous dolomite grains, with, in some cases, evidence for significant textural maturation, appear in the granoblastic aggregates of the high-grade mafic rocks. However, calculated phase relationships reveal that dolomite could not coexist with H2O-CO2 fluids at HT-UHT granulite- and low-medium P amphibolite-facies conditions. Therefore, it is proposed that it may have been generated from another CO2-bearing phase, such as an immiscible carbonatitic melt exsolved from the parental mafic magma, and preserved during cooling due to the prevalence of fluid-absent conditions in the granoblastic matrix containing dolomite. The l
{"title":"Fluid-melt-rock interaction during the transition from magmatism to HT-UHT-granulite- and amphibolite-facies metamorphism in the Ediacaran Adrar-Suttuf Metamafic Complex, NW Margin of the West African Craton (Southern Morocco)","authors":"José Francisco Molina, Fernando Bea, Pilar Montero, Faouziya Haissen, Leticia Barcos, Aitor Cambeses, Irene Morales, Maria Monika Repczynska, Othman Sadki, Antonio García-Casco","doi":"10.1093/petrology/egae049","DOIUrl":"https://doi.org/10.1093/petrology/egae049","url":null,"abstract":"Underplated mafic intrusions ponded at the base of the lower continental crust in extensional settings can experience ultra-high temperature (UHT) granulite-facies metamorphism during tens of My due to slow cooling rates, being also the source of heat and carbonic fluids for regional high-temperature (HT) granulite-facies metamorphism in the continental crust. This work analyses the fluid-melt-rock interaction processes that occurred during the magmatic to HT-UHT-granulite- and amphibolite-facies metamorphic evolution of high-grade mafic rocks from the Eastern Ediacaran Adrar-Suttuf Metamafic Complex (EASMC) of the Oulad Dlim Massif (West African Craton Margin, Southern Morocco). P-T conditions were determined using Ti-in-amphibole thermometry, two-pyroxene and amphibole-plagioclase thermobarometry, and phase diagrams calculations. The thermobarometric study reveals the presence of tectonically juxtaposed lower- and mid-crustal blocks in EASMC that experienced decompression-cooling paths from, respectively UHT and HT granulite-facies conditions at ca. 1.2 ±0.28 GPa and 975 ±50 °C, and ca. 0.82 ±0.15 GPa and 894 ±50 °C, to amphibole-facies conditions at ca. 0.28 ±0.28 GPa and 787 ±45 °C (precision reported for the calibrations at 1s level). An age for the magmatic to UHT granulite-facies metamorphic transition of 604 Ma was constrained from published SHRIMP Th-U-Pb zircon ages of the igneous protoliths. An amphibole 40Ar-39Ar cooling age of 499 ±8 Ma (precision at 2s level) was obtained for the lower-crustal blocks. Amphibole 40Ar-39Ar closure temperatures of 520-555 °C were obtained for an age range of 600-499 Ma and an average constant cooling rate of 4.2 °C/My, suggesting that the lower-crustal blocks cooled down to the greenschist-amphibolite facies transition in ca. 100 My. During the high-temperature stage, interstitial hydrous melts caused incongruent dissolution melting of olivine and pyroxenes, and, probably, the development of An-rich spikes at the grain rims of plagioclase, and assisted textural maturation of the rock matrix and local segregation of pargasite into veins. Subsequent local infiltration of reactive hydrous metamorphic fluids along mineral grain boundaries during cooling down to amphibolite-facies conditions promoted mineral replacements by coupled dissolution-precipitation mechanisms and metasomatism. Ubiquitous dolomite grains, with, in some cases, evidence for significant textural maturation, appear in the granoblastic aggregates of the high-grade mafic rocks. However, calculated phase relationships reveal that dolomite could not coexist with H2O-CO2 fluids at HT-UHT granulite- and low-medium P amphibolite-facies conditions. Therefore, it is proposed that it may have been generated from another CO2-bearing phase, such as an immiscible carbonatitic melt exsolved from the parental mafic magma, and preserved during cooling due to the prevalence of fluid-absent conditions in the granoblastic matrix containing dolomite. The l","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":"1 1","pages":""},"PeriodicalIF":3.9,"publicationDate":"2024-05-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140931980","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 : 2024-05-07DOI: 10.1093/petrology/egae047
Zhe Chi, Jun-Yi Pan, Pei Ni
A-type granites typically exhibit enrichment and mineralization of critical metals such as molybdenum and tin, essential for emerging technologies. However, the key factors influencing their mineralization potential remain elusive. The scarcity of studies on barren systems impedes the understanding of this question. Here, a detailed melt and fluid inclusion study was conducted on the barren Wushan pluton to reconstruct its magmatic evolution and magmatic-hydrothermal transition and to explore the factors controlling the metallogenic potential of Mo and Sn in A-type granites. The Wushan pluton displays apparent lithological zoning consisting of two major phases, i.e., medium-grained seriate to porphyritic alkali feldspar granite and fine-grained porphyritic granite. Miarolitic cavities are widely developed in each lithofacies. The silicate melt inclusions from two granitic phases are rhyolitic, with moderate F contents (0.06–0.53 wt %) and depleted H2O contents (2.0–3.5 wt %). Melt inclusions show a wide range of incompatible element contents, such as Cs (9–1977 μg/g) and Rb (268–2601 μg/g), suggesting that Wushan has undergone a high degree of magma evolution. Mo behaves incompatibly in the magmatic evolution, and its content is enriched with the increasing degree of fractional crystallization, but remains constant after the Cs content exceeds 50 μg/g. Rayleigh fractionation model suggests a large amount of Mo is extracted from fluid exsolution, which restrains Mo from further enrichment. In contrast, Sn behaves as a mildly incompatible element during the entire magmatic evolution history. The contents of Sn increase slowly compared to the trend of Mo, and the maximum contents reach ~30 μg/g in the highly evolved melts. The separation and crystallization of Sn-bearing minerals such as biotite, magnetite, and titanite inhibit the enrichment of Sn. Intermediate-density (ID-type) fluid inclusions hosted in the miarolitic quartz, representing the initial fluid exsolving from magma, display high Mo but low Sn concentrations. Constrained from two assemblages of coexisting ID-type fluid and melt inclusions, the fluid/melt partition coefficients of metals are obtained, with DMo, fluid/melt at 16–19, while DSn, fluid/melt is only about 1. The comparison between Mo-mineralized and barren intrusions worldwide shows that the metal contents in melts and fluids are not fundamentally different. The mineralized intrusions are characterized by the lower melt viscosity and the development of apophyses, both of which facilitate the extraction of metals and fluids from large magma chambers, followed by their concentration into a small rock volume. Consequently, it appears that physical and structural conditions rather than chemical compositions play a crucial role in the Mo mineralization process. Enrichment of Sn in melts is necessary but not decisive for Sn mineralization, whereas Sn enrichment in the initial exsolving fluid determines the Sn mineralization potent
{"title":"Magmatic-Hydrothermal Evolution at the Barren Wushan Pluton, Southeast China: Insight into Controls on Mineralization Potential","authors":"Zhe Chi, Jun-Yi Pan, Pei Ni","doi":"10.1093/petrology/egae047","DOIUrl":"https://doi.org/10.1093/petrology/egae047","url":null,"abstract":"A-type granites typically exhibit enrichment and mineralization of critical metals such as molybdenum and tin, essential for emerging technologies. However, the key factors influencing their mineralization potential remain elusive. The scarcity of studies on barren systems impedes the understanding of this question. Here, a detailed melt and fluid inclusion study was conducted on the barren Wushan pluton to reconstruct its magmatic evolution and magmatic-hydrothermal transition and to explore the factors controlling the metallogenic potential of Mo and Sn in A-type granites. The Wushan pluton displays apparent lithological zoning consisting of two major phases, i.e., medium-grained seriate to porphyritic alkali feldspar granite and fine-grained porphyritic granite. Miarolitic cavities are widely developed in each lithofacies. The silicate melt inclusions from two granitic phases are rhyolitic, with moderate F contents (0.06–0.53 wt %) and depleted H2O contents (2.0–3.5 wt %). Melt inclusions show a wide range of incompatible element contents, such as Cs (9–1977 μg/g) and Rb (268–2601 μg/g), suggesting that Wushan has undergone a high degree of magma evolution. Mo behaves incompatibly in the magmatic evolution, and its content is enriched with the increasing degree of fractional crystallization, but remains constant after the Cs content exceeds 50 μg/g. Rayleigh fractionation model suggests a large amount of Mo is extracted from fluid exsolution, which restrains Mo from further enrichment. In contrast, Sn behaves as a mildly incompatible element during the entire magmatic evolution history. The contents of Sn increase slowly compared to the trend of Mo, and the maximum contents reach ~30 μg/g in the highly evolved melts. The separation and crystallization of Sn-bearing minerals such as biotite, magnetite, and titanite inhibit the enrichment of Sn. Intermediate-density (ID-type) fluid inclusions hosted in the miarolitic quartz, representing the initial fluid exsolving from magma, display high Mo but low Sn concentrations. Constrained from two assemblages of coexisting ID-type fluid and melt inclusions, the fluid/melt partition coefficients of metals are obtained, with DMo, fluid/melt at 16–19, while DSn, fluid/melt is only about 1. The comparison between Mo-mineralized and barren intrusions worldwide shows that the metal contents in melts and fluids are not fundamentally different. The mineralized intrusions are characterized by the lower melt viscosity and the development of apophyses, both of which facilitate the extraction of metals and fluids from large magma chambers, followed by their concentration into a small rock volume. Consequently, it appears that physical and structural conditions rather than chemical compositions play a crucial role in the Mo mineralization process. Enrichment of Sn in melts is necessary but not decisive for Sn mineralization, whereas Sn enrichment in the initial exsolving fluid determines the Sn mineralization potent","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":"62 1","pages":""},"PeriodicalIF":3.9,"publicationDate":"2024-05-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140931853","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 : 2024-05-06DOI: 10.1093/petrology/egae046
Bin Wang, Chunjing Wei
Identifying ultrahigh-temperature (UHT) metamorphism in basic granulites is challenged by the uncertainty of peak temperatures. Consequently, available temperature indicators are desiderated all along. In this study, we investigated low-pressure basic granulites from Daqingshan, North China Craton, and determined a UHT peak condition around 1000–1050 °C and 0.8–0.9 GPa. This peak condition was constrained by combining the AlT of 0.10–0.12 in clinopyroxene (Cpx) cores, maximum Ti of 0.36–0.38 in amphibole (Amp) cores and anorthite proportion (XAn) of 0.67–0.70 and 0.61–0.63 in plagioclase (Pl) cores/mantles analyzed in two selected samples. The P–T records obtained from several thermobarometers related to amphibole, plagioclase and/or clinopyroxene were also verified to be in agreement with the phase equilibria modelling results. Notably, AlT-in-Cpx is assessed to be a reliable temperature indicator in basic granulites with the Opx–Cpx–Pl–Amp assemblage, albeit could be influenced by bulk-rock compositions in some extent and has a temperature standard error of ~70 °C. Besides, the post-peak evolution was dominated by near isobaric cooling at 1.94–1.93 Ga, to a fluid-absent solidus as high as ~940 °C. During the cooling process, plagioclase and amphibole grew together at the expense of Al-rich clinopyroxene and unsegregated melts, accompanied by the (rimward) decrease of AlT-in-Cpx, Ti-in-Amp and XAn-in-Pl. A pre-peak process is inferred to be decompressional heating. The UHT metamorphism in Daqingshan is interpreted to result from asthenosphere upwelling and thermal advection from regional mantle-derived magmatism during post-orogenic extension, in a newly developed back-arc-related environment.
{"title":"Identifying Ultrahigh-Temperature Metamorphism in Basic Granulites","authors":"Bin Wang, Chunjing Wei","doi":"10.1093/petrology/egae046","DOIUrl":"https://doi.org/10.1093/petrology/egae046","url":null,"abstract":"Identifying ultrahigh-temperature (UHT) metamorphism in basic granulites is challenged by the uncertainty of peak temperatures. Consequently, available temperature indicators are desiderated all along. In this study, we investigated low-pressure basic granulites from Daqingshan, North China Craton, and determined a UHT peak condition around 1000–1050 °C and 0.8–0.9 GPa. This peak condition was constrained by combining the AlT of 0.10–0.12 in clinopyroxene (Cpx) cores, maximum Ti of 0.36–0.38 in amphibole (Amp) cores and anorthite proportion (XAn) of 0.67–0.70 and 0.61–0.63 in plagioclase (Pl) cores/mantles analyzed in two selected samples. The P–T records obtained from several thermobarometers related to amphibole, plagioclase and/or clinopyroxene were also verified to be in agreement with the phase equilibria modelling results. Notably, AlT-in-Cpx is assessed to be a reliable temperature indicator in basic granulites with the Opx–Cpx–Pl–Amp assemblage, albeit could be influenced by bulk-rock compositions in some extent and has a temperature standard error of ~70 °C. Besides, the post-peak evolution was dominated by near isobaric cooling at 1.94–1.93 Ga, to a fluid-absent solidus as high as ~940 °C. During the cooling process, plagioclase and amphibole grew together at the expense of Al-rich clinopyroxene and unsegregated melts, accompanied by the (rimward) decrease of AlT-in-Cpx, Ti-in-Amp and XAn-in-Pl. A pre-peak process is inferred to be decompressional heating. The UHT metamorphism in Daqingshan is interpreted to result from asthenosphere upwelling and thermal advection from regional mantle-derived magmatism during post-orogenic extension, in a newly developed back-arc-related environment.","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":"32 1","pages":""},"PeriodicalIF":3.9,"publicationDate":"2024-05-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140883148","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 : 2024-05-06DOI: 10.1093/petrology/egae043
Oliver Higgins, Michael J Stock
Melt-based thermobarometers are essential tools to recover pre-eruptive magma storage conditions through their application to bulk rock and liquid chemistry. In active volcanic systems, thermobarometric results can be combined with independent geophysical data during or after an eruption to validate conceptual models. In this contribution, we revisit the thermobarometer for melts equilibrated with the mineral assemblage of olivine + plagioclase + augitic clinopyroxene (OPAM). We first demonstrate that the most widely applied OPAM thermobarometer suffers from both random and systematic uncertainty even for anhydrous melts, and that the uncertainty increases proportionally with melt H2O. To address this issue, we use a modern compilation of anhydrous and hydrous OPAM-saturated experiments to regress a new empirical melt-based OPAM thermometer and barometer. Our new equations recover a validation dataset with a standard error estimate (SEE) of ±1.14 kbar and ±36 °C for pressure and temperature respectively, as well as a low systematic uncertainty that does not depend on melt H2O. Additionally, we present a novel statistical approach to determine the probability that a given melt is OPAM-saturated, which can be used alongside rigorous petrographic and geochemical observations. Our thermobarometer and saturation test are presented as a user-friendly R script which reads from an input csv file to be populated with natural data. We benchmark the new calibrations on the products of the 2015 eruption of Wolf Volcano (Isabela Island, Galápagos archipelago) and the 2014-2015 Holuhraun eruption (Iceland), both of which have independent geophysical estimates of magma storage that agree well with our thermobarometric results.
{"title":"A New Calibration of the OPAM Thermobarometer for Anhydrous and Hydrous Mafic Systems","authors":"Oliver Higgins, Michael J Stock","doi":"10.1093/petrology/egae043","DOIUrl":"https://doi.org/10.1093/petrology/egae043","url":null,"abstract":"Melt-based thermobarometers are essential tools to recover pre-eruptive magma storage conditions through their application to bulk rock and liquid chemistry. In active volcanic systems, thermobarometric results can be combined with independent geophysical data during or after an eruption to validate conceptual models. In this contribution, we revisit the thermobarometer for melts equilibrated with the mineral assemblage of olivine + plagioclase + augitic clinopyroxene (OPAM). We first demonstrate that the most widely applied OPAM thermobarometer suffers from both random and systematic uncertainty even for anhydrous melts, and that the uncertainty increases proportionally with melt H2O. To address this issue, we use a modern compilation of anhydrous and hydrous OPAM-saturated experiments to regress a new empirical melt-based OPAM thermometer and barometer. Our new equations recover a validation dataset with a standard error estimate (SEE) of ±1.14 kbar and ±36 °C for pressure and temperature respectively, as well as a low systematic uncertainty that does not depend on melt H2O. Additionally, we present a novel statistical approach to determine the probability that a given melt is OPAM-saturated, which can be used alongside rigorous petrographic and geochemical observations. Our thermobarometer and saturation test are presented as a user-friendly R script which reads from an input csv file to be populated with natural data. We benchmark the new calibrations on the products of the 2015 eruption of Wolf Volcano (Isabela Island, Galápagos archipelago) and the 2014-2015 Holuhraun eruption (Iceland), both of which have independent geophysical estimates of magma storage that agree well with our thermobarometric results.","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":"46 1","pages":""},"PeriodicalIF":3.9,"publicationDate":"2024-05-06","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140883098","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 : 2024-04-30DOI: 10.1093/petrology/egae045
Clara M Waelkens, John Stix, Fraser Goff, Dominique Weis
Repeated intrusions of mafic magma have long been known to be a driver of long-lived magmatic systems. Although the importance of mafic recharge of silicic magma systems is well-documented in igneous petrology, the origin of this recharge magma is sometimes obscure. By examining the pre-caldera intermediate dome complexes of the Tschicoma Formation and their relationship to a dacitic recharge event into the Tshirege Member of the Bandelier Tuff, we aim to better understand the origin of mafic recharge events into the Bandelier magma chamber of Valles caldera, and the relationship between different stages of volcanic activity within the broader Jemez Mountains volcanic field (JMVF). Based on major, trace element and radiogenic isotopic data, we divide the Tschicoma Formation into three geochemical groups with similar petrologic evolutionary paths. The Cerro Grande, Cerro Rubio and Pajarito Mountain volcanic dome complexes form group A and have assimilated various amounts of a granitoid crustal component with low εNd, εHf and radiogenic Pb. Group B consists of the Sawyer Dome, Rendija Canyon and Caballo Mountain dome complexes, which have principally evolved through different degrees of fractional crystallisation of the same parent magma, itself a result of complex interactions of a mafic mantle-derived magma with the crust. The dacite domes and flows around Tschicoma Peak and the newly-described Cañada Bonita dacite form group C and are the result of mixing of Rendija Canyon magma with mafic recharge magma which is preserved as distinct mafic enclaves. At a later stage of the JMVF, during the eruption of the Tshirege Member, distinctive hornblende-dacite pumices formed as a result of the influx of more mafic recharge magma into the system, which mobilised a pre-existing dacite intrusion and injected it into the Tshirege rhyolite (Stimac, 1996; Boro et al., 2020). Based on trace element and isotopic compositions, we propose that dacite which was injected into the Tshirege magma chamber was related to the earlier-erupted Tschicoma Formation and itself represents a mixing product of Tshirege rhyolite and a precursor to the Tschicoma dacites. This implies that the Tschicoma magmatic system was long-lived yet dormant during the eruption of the Otowi Member of the Bandelier Tuff, then was reactivated shortly before the Tshirege eruption, temporarily co-existing and interacting with the Bandelier system as it erupted.
{"title":"Trace element and isotope geochemistry of Tschicoma Formation intermediate composition dome complexes, Jemez Mountains volcanic field, New Mexico, USA","authors":"Clara M Waelkens, John Stix, Fraser Goff, Dominique Weis","doi":"10.1093/petrology/egae045","DOIUrl":"https://doi.org/10.1093/petrology/egae045","url":null,"abstract":"Repeated intrusions of mafic magma have long been known to be a driver of long-lived magmatic systems. Although the importance of mafic recharge of silicic magma systems is well-documented in igneous petrology, the origin of this recharge magma is sometimes obscure. By examining the pre-caldera intermediate dome complexes of the Tschicoma Formation and their relationship to a dacitic recharge event into the Tshirege Member of the Bandelier Tuff, we aim to better understand the origin of mafic recharge events into the Bandelier magma chamber of Valles caldera, and the relationship between different stages of volcanic activity within the broader Jemez Mountains volcanic field (JMVF). Based on major, trace element and radiogenic isotopic data, we divide the Tschicoma Formation into three geochemical groups with similar petrologic evolutionary paths. The Cerro Grande, Cerro Rubio and Pajarito Mountain volcanic dome complexes form group A and have assimilated various amounts of a granitoid crustal component with low εNd, εHf and radiogenic Pb. Group B consists of the Sawyer Dome, Rendija Canyon and Caballo Mountain dome complexes, which have principally evolved through different degrees of fractional crystallisation of the same parent magma, itself a result of complex interactions of a mafic mantle-derived magma with the crust. The dacite domes and flows around Tschicoma Peak and the newly-described Cañada Bonita dacite form group C and are the result of mixing of Rendija Canyon magma with mafic recharge magma which is preserved as distinct mafic enclaves. At a later stage of the JMVF, during the eruption of the Tshirege Member, distinctive hornblende-dacite pumices formed as a result of the influx of more mafic recharge magma into the system, which mobilised a pre-existing dacite intrusion and injected it into the Tshirege rhyolite (Stimac, 1996; Boro et al., 2020). Based on trace element and isotopic compositions, we propose that dacite which was injected into the Tshirege magma chamber was related to the earlier-erupted Tschicoma Formation and itself represents a mixing product of Tshirege rhyolite and a precursor to the Tschicoma dacites. This implies that the Tschicoma magmatic system was long-lived yet dormant during the eruption of the Otowi Member of the Bandelier Tuff, then was reactivated shortly before the Tshirege eruption, temporarily co-existing and interacting with the Bandelier system as it erupted.","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":"161 1","pages":""},"PeriodicalIF":3.9,"publicationDate":"2024-04-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140841977","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 : 2024-04-17DOI: 10.1093/petrology/egae042
Zhiguo Cheng, Zhaochong Zhang, Mingde Lang, M Santosh, Lijuan Xu, Jingao Liu
Plume-derived high-MgO lavas provide important information on the lithological, thermal and chemical variations of Earth’s deep mantle. Here we present results from detailed field, mineralogical and geochemical studies of Late Permian-Late Triassic high-MgO lavas near the Chalukou area in the Western Kunlun (WK) orogenic belt, NW China. The major element compositions of the lavas show extremely high MgO contents (26.6-33.8 wt.%) in accordance with olivine accumulation. The parental magma is inferred to be picritic in composition with MgO of 17.2±0.9 wt.%. Olivine Zn/Fe and Mn/Zn ratios suggest a peridotite dominated source with a minor fraction of pyroxenite. The temperature and oxygen fugacity estimates based on multi-methods including olivine-melt Mg-Fe equilibria, Al-in-olivine and olivine-spinel thermometry and oxybarometer yield a mantle potential temperature of 1522-1556 °C and high oxygen fugacity of FMQ+0.93. The H2O contents in the picrite flows are estimated as 3.67±1.0 wt.%, indicating the volatile-rich nature of parental magma and its mantle source. The immobile trace element features show that the WK picrites are OIB-like, with the enrichment in light rare earth elements and positive Nb, Ta, Zr and Hf anomalies. Furthermore, the Nd-O-Os isotopes display typical mantle values without involvement of recycled materials. Our results suggest the high-MgO volcanism in the WK orogenic belt originated from a volatile-rich plume source.
{"title":"Geochemical evidence of plume sources for high-MgO lavas in the WK Orogenic Belt","authors":"Zhiguo Cheng, Zhaochong Zhang, Mingde Lang, M Santosh, Lijuan Xu, Jingao Liu","doi":"10.1093/petrology/egae042","DOIUrl":"https://doi.org/10.1093/petrology/egae042","url":null,"abstract":"Plume-derived high-MgO lavas provide important information on the lithological, thermal and chemical variations of Earth’s deep mantle. Here we present results from detailed field, mineralogical and geochemical studies of Late Permian-Late Triassic high-MgO lavas near the Chalukou area in the Western Kunlun (WK) orogenic belt, NW China. The major element compositions of the lavas show extremely high MgO contents (26.6-33.8 wt.%) in accordance with olivine accumulation. The parental magma is inferred to be picritic in composition with MgO of 17.2±0.9 wt.%. Olivine Zn/Fe and Mn/Zn ratios suggest a peridotite dominated source with a minor fraction of pyroxenite. The temperature and oxygen fugacity estimates based on multi-methods including olivine-melt Mg-Fe equilibria, Al-in-olivine and olivine-spinel thermometry and oxybarometer yield a mantle potential temperature of 1522-1556 °C and high oxygen fugacity of FMQ+0.93. The H2O contents in the picrite flows are estimated as 3.67±1.0 wt.%, indicating the volatile-rich nature of parental magma and its mantle source. The immobile trace element features show that the WK picrites are OIB-like, with the enrichment in light rare earth elements and positive Nb, Ta, Zr and Hf anomalies. Furthermore, the Nd-O-Os isotopes display typical mantle values without involvement of recycled materials. Our results suggest the high-MgO volcanism in the WK orogenic belt originated from a volatile-rich plume source.","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":"28 1","pages":""},"PeriodicalIF":3.9,"publicationDate":"2024-04-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140615459","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 : 2024-04-16DOI: 10.1093/petrology/egae041
Denis S Mikhailenko, Sonja Aulbach, Aleksandr S Stepanov, Andrey V Korsakov, Le Zhang, Yi-Gang Xu
Rare-Earth Elements (REE) are key geochemical tracers of crust-mantle differentiation, but there are few direct data on REE-rich minerals in mantle rocks. Here, we report the combined petrography and comprehensive chemical and isotopic characterization of three coesite- and kyanite-bearing eclogite xenoliths from the Udachnaya kimberlite pipe (Siberian craton), which are unusual in that two xenoliths (one with diamond and graphite) contain discrete, idiomorphic crystals of allanite at the grain boundaries of garnet and omphacite. Another xenolith contains allanite as part of a complex aggregate of calcite, apatite, barite, and celestine hosted by serpentine, which is a low-temperature secondary minerals likely result from metasomatic reaction at shallower depths during the transport of eclogite by the erupting kimberlite melt. The bulk rock composition reconstructed from the trace element composition of garnet and omphacite show marked depletion in LREE, precluding equilibration with kimberlite melt, whereas the measured bulk compositions show chondrite-normalized REE patterns with conspicuous depletions of Ce-Pr-Nd relative to La and Sm. The presence of 0.005 – 0.008 wt. % of allanite, texturally and chemically out of equilibrium with the rock-forming minerals, allows balancing the LREE and Sm-Nd budget of the rock, whereas Th and U require additional hosts. This not only highlights the utility of measuring bulk eclogite xenoliths in bringing this unusual component to light, but also demonstrates that the long-known incompatible element enrichment in bulk eclogites reflects the deposition of discrete phases rather than merely bulk kimberlite melt addition. Although allanite is stable in metabasalts at the pressure-temperature conditions of 1025 – 1080 °C and 3.6 – 4.8 GPa recorded by the eclogite xenoliths, its association with Ba-Sr minerals suggests its formation via reaction of the host eclogites with kimberlite melt. This is supported by the similarity in 143Nd/144Nd ratios between bulk eclogite (0.51227 – 0.51249) and the host kimberlite at eruption, whereas clinopyroxene in part retains unradiogenic Sr (87Sr/86Sr = 0.70205 ± 0.00011) related to ancient depletion. The discovery of allanite in the Udachnaya eclogites demonstrates that this REE mineral can form when omphacite and grossular-rich garnet in eclogite breakdown in contact with REE- and alkali-rich carbonatite/kimberlite melt, and may be more common than hitherto recognized. Crystallization of allanite in the cratonic mantle eclogite reservoir may also help explain the difference in LREE abundances between the more strongly enriched carbonatite/kimberlite at depth and the final erupted product. It is likely that allanite is overlooked at eclogites xenoliths, while it is common accessory mineral, hosting REE in orogenic UHP/HP eclogites. Further studies are required to deciphered the peculiarities in metamorphic history recorded in eclogites xenoliths and orogenic eclogites as well
{"title":"Allanite in mantle eclogite xenoliths","authors":"Denis S Mikhailenko, Sonja Aulbach, Aleksandr S Stepanov, Andrey V Korsakov, Le Zhang, Yi-Gang Xu","doi":"10.1093/petrology/egae041","DOIUrl":"https://doi.org/10.1093/petrology/egae041","url":null,"abstract":"Rare-Earth Elements (REE) are key geochemical tracers of crust-mantle differentiation, but there are few direct data on REE-rich minerals in mantle rocks. Here, we report the combined petrography and comprehensive chemical and isotopic characterization of three coesite- and kyanite-bearing eclogite xenoliths from the Udachnaya kimberlite pipe (Siberian craton), which are unusual in that two xenoliths (one with diamond and graphite) contain discrete, idiomorphic crystals of allanite at the grain boundaries of garnet and omphacite. Another xenolith contains allanite as part of a complex aggregate of calcite, apatite, barite, and celestine hosted by serpentine, which is a low-temperature secondary minerals likely result from metasomatic reaction at shallower depths during the transport of eclogite by the erupting kimberlite melt. The bulk rock composition reconstructed from the trace element composition of garnet and omphacite show marked depletion in LREE, precluding equilibration with kimberlite melt, whereas the measured bulk compositions show chondrite-normalized REE patterns with conspicuous depletions of Ce-Pr-Nd relative to La and Sm. The presence of 0.005 – 0.008 wt. % of allanite, texturally and chemically out of equilibrium with the rock-forming minerals, allows balancing the LREE and Sm-Nd budget of the rock, whereas Th and U require additional hosts. This not only highlights the utility of measuring bulk eclogite xenoliths in bringing this unusual component to light, but also demonstrates that the long-known incompatible element enrichment in bulk eclogites reflects the deposition of discrete phases rather than merely bulk kimberlite melt addition. Although allanite is stable in metabasalts at the pressure-temperature conditions of 1025 – 1080 °C and 3.6 – 4.8 GPa recorded by the eclogite xenoliths, its association with Ba-Sr minerals suggests its formation via reaction of the host eclogites with kimberlite melt. This is supported by the similarity in 143Nd/144Nd ratios between bulk eclogite (0.51227 – 0.51249) and the host kimberlite at eruption, whereas clinopyroxene in part retains unradiogenic Sr (87Sr/86Sr = 0.70205 ± 0.00011) related to ancient depletion. The discovery of allanite in the Udachnaya eclogites demonstrates that this REE mineral can form when omphacite and grossular-rich garnet in eclogite breakdown in contact with REE- and alkali-rich carbonatite/kimberlite melt, and may be more common than hitherto recognized. Crystallization of allanite in the cratonic mantle eclogite reservoir may also help explain the difference in LREE abundances between the more strongly enriched carbonatite/kimberlite at depth and the final erupted product. It is likely that allanite is overlooked at eclogites xenoliths, while it is common accessory mineral, hosting REE in orogenic UHP/HP eclogites. Further studies are required to deciphered the peculiarities in metamorphic history recorded in eclogites xenoliths and orogenic eclogites as well ","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":"6 1","pages":""},"PeriodicalIF":3.9,"publicationDate":"2024-04-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140615403","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 : 2024-04-16DOI: 10.1093/petrology/egae040
A L Jaques
The 17.44 Ma Walgidee Hills lamproite in the West Kimberley province of Western Australia is the type locality for several K-, Ba- and Ti-rich minerals characteristic of lamproites and, at 490 ha, the largest known lamproite. The Walgidee Hills pipe comprises a thin sequence of tuffs and breccia formed by explosive eruptions that excavated a large shallow crater that was infilled by lamproite magma which cooled and crystallised in situ. The lamproite is zoned in grain size, mineralogy, and mineral and rock composition from porphyritic olivine lamproite at the margin through medium grained lamproite comprised of olivine (altered), titanian phlogopite, diopside, leucite (altered) and titanian potassic richterite to coarse gained lamproite rich in potassic richterite, priderite, jeppeite, perovskite, apatite, wadeite and noonkanbahite at the centre of the body. Compositional zoning is evident across the lamproite in phlogopite (to lower Mg and Al, higher Fe), potassic richterite (to higher Fe and Na, lower Ti), priderite (to lower Cr) and perovskite (to lower Cr and Fe, higher Na, Sr, Y, Nb, U, REE). The Walgidee Hills lamproite is ultrapotassic and ranges from olivine lamproite (up to ~21 wt % MgO, ~800 μg/g Ni, ~4 wt % K2O) to sanidine/leucite-rich lamproite (~7 wt % MgO, ≤100 μg/g Ni, ~7 wt % K2O) at the centre of the pipe. The lamproite has low Al2O3, total Fe, Na2O and CaO (except for intensely carbonate-veined rocks at the centre) and is highly enriched in TiO2 (3−6.5 wt %), Ba, Rb, Sr, Zr and LREE (LaN = 150−520 x primitive mantle). A transect and geochemical contours show MgO, Ni and Cr contents decrease and P, K, Ti, Fe, Rb, Sr, Y, Zr, Nb, REE, Hf, Pb, Th and U abundances increase inwards to the most evolved rocks at the centre of the pipe, consistent with fractionation by inwards in situ crystallisation. The parent magma is estimated from the composition of the porphyritic olivine lamproite at the margins of the pipe to have ~16 ± 1 wt % MgO, ~600 μg/g Ni, ~6 wt % K2O with La/Yb ~ 150. Modelling suggests that the most MgO-rich lamproites result from entrainment of ~15–20 wt % mantle olivine in the parent magma. Cooling of the magma resulted in fractional crystallisation of olivine and in situ crystallisation with the evolved coarse-grained lamproites at the centre of the pipe crystallised from residual magma enriched in the more incompatible elements. Mantle xenocrysts include abundant Cr-Al spinel, chrome diopside, chrome pyrope, and rare diamond. Thermobarometry on the Cr diopside xenocrysts defines a cold paleogeotherm of ~38 mW/m2 and a thick lithosphere (~235 km) extending from the Kimberley craton. Many of the Cr diopsides from the deeper lithospheric mantle are enriched in K, Ba and LREE and these, and the Ti-rich spinel xenocrysts, are inferred to be derived from metasomatised mantle peridotite. The enriched trace element and Sr-Nd-Pb isotopic signatures of the Walgidee Hills lamproite suggest derivation from or extensive incorpora
{"title":"Petrology, Geochemistry and Mantle Minerals of the Walgidee Hills Lamproite, West Kimberley, Western Australia","authors":"A L Jaques","doi":"10.1093/petrology/egae040","DOIUrl":"https://doi.org/10.1093/petrology/egae040","url":null,"abstract":"The 17.44 Ma Walgidee Hills lamproite in the West Kimberley province of Western Australia is the type locality for several K-, Ba- and Ti-rich minerals characteristic of lamproites and, at 490 ha, the largest known lamproite. The Walgidee Hills pipe comprises a thin sequence of tuffs and breccia formed by explosive eruptions that excavated a large shallow crater that was infilled by lamproite magma which cooled and crystallised in situ. The lamproite is zoned in grain size, mineralogy, and mineral and rock composition from porphyritic olivine lamproite at the margin through medium grained lamproite comprised of olivine (altered), titanian phlogopite, diopside, leucite (altered) and titanian potassic richterite to coarse gained lamproite rich in potassic richterite, priderite, jeppeite, perovskite, apatite, wadeite and noonkanbahite at the centre of the body. Compositional zoning is evident across the lamproite in phlogopite (to lower Mg and Al, higher Fe), potassic richterite (to higher Fe and Na, lower Ti), priderite (to lower Cr) and perovskite (to lower Cr and Fe, higher Na, Sr, Y, Nb, U, REE). The Walgidee Hills lamproite is ultrapotassic and ranges from olivine lamproite (up to ~21 wt % MgO, ~800 μg/g Ni, ~4 wt % K2O) to sanidine/leucite-rich lamproite (~7 wt % MgO, ≤100 μg/g Ni, ~7 wt % K2O) at the centre of the pipe. The lamproite has low Al2O3, total Fe, Na2O and CaO (except for intensely carbonate-veined rocks at the centre) and is highly enriched in TiO2 (3−6.5 wt %), Ba, Rb, Sr, Zr and LREE (LaN = 150−520 x primitive mantle). A transect and geochemical contours show MgO, Ni and Cr contents decrease and P, K, Ti, Fe, Rb, Sr, Y, Zr, Nb, REE, Hf, Pb, Th and U abundances increase inwards to the most evolved rocks at the centre of the pipe, consistent with fractionation by inwards in situ crystallisation. The parent magma is estimated from the composition of the porphyritic olivine lamproite at the margins of the pipe to have ~16 ± 1 wt % MgO, ~600 μg/g Ni, ~6 wt % K2O with La/Yb ~ 150. Modelling suggests that the most MgO-rich lamproites result from entrainment of ~15–20 wt % mantle olivine in the parent magma. Cooling of the magma resulted in fractional crystallisation of olivine and in situ crystallisation with the evolved coarse-grained lamproites at the centre of the pipe crystallised from residual magma enriched in the more incompatible elements. Mantle xenocrysts include abundant Cr-Al spinel, chrome diopside, chrome pyrope, and rare diamond. Thermobarometry on the Cr diopside xenocrysts defines a cold paleogeotherm of ~38 mW/m2 and a thick lithosphere (~235 km) extending from the Kimberley craton. Many of the Cr diopsides from the deeper lithospheric mantle are enriched in K, Ba and LREE and these, and the Ti-rich spinel xenocrysts, are inferred to be derived from metasomatised mantle peridotite. The enriched trace element and Sr-Nd-Pb isotopic signatures of the Walgidee Hills lamproite suggest derivation from or extensive incorpora","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":"20 1","pages":""},"PeriodicalIF":3.9,"publicationDate":"2024-04-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140617922","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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