Pub Date : 2023-08-01DOI: 10.1093/petrology/egad056
Jian Wang, Qiang Wang, Lin Ma, Wan-Long Hu, Jun Wang, Elena Belousova, Gong-Jian Tang
ABSTRACT Subduction recycling of sediments plays a key role in the geochemical evolution of Earth. The presence of recycled terrigenous sediments in upwelling plumes has been cited to explain the EM2 signature in ocean island volcanics, characterized by particularly high 87Sr/86Sr (>0.706). However, the origin of such isotopic anomalies in continental regions and the role of subducted sediments in the subcontinental lithospheric mantle (SCLM) remain unclear. The Himalaya–Tibet orogen is one of the world’s best places for deciphering continental subduction and the fate of subducted crustal materials in the mantle. Here we present a systematic study of the geochronology, mineral chemistry (especially clinopyroxene), whole-rock chemistry and Sr–Nd–Pb–Hf–O isotopic compositions of Cenozoic potassic–ultrapotassic lavas from the western Kunlun area of northwestern Tibet. New secondary ion mass spectrometry (SIMS) zircon U–Pb dating, coupled with published age results, constrain the timing of volcanism from ~8.3 Ma to the present. These lavas show geochemical characteristics that closely resemble the EM2 mantle end-member represented by the Samoan hotspot. Both whole rocks and individual magmatic clinopyroxenes display arc-like trace-element patterns and remarkably enriched Sr–Nd–Pb–Hf isotope compositions (87Sr/86Sr ≥ 0.7080; εNd ≤ −4.8; 206Pb/204Pb ≥ 18.704; εHf ≤ −2.6). Together with high zircon δ18O values (6.3–10.4‰), the data point to a mantle source enriched by recycled sedimentary materials. Geochemical modeling and geophysical evidence further indicate that the sediments were directly derived from the subducted Indian continental lithosphere during India–Eurasia collision. Partial melting models assuming a hybridized mantle source that contains ~5% Indian continental crust suggest that the primary melts of the potassic–ultrapotassic lavas could be formed by melting of a phlogopite-bearing garnet lherzolite at low melting degrees (1–5%). The magma geochemistry is consistent with the model of mélange melting, implying that the subducted sediments may detach from the downgoing Indian slab and rise up diapirically into the overlying mantle lithosphere. Unlike traditional models of subducted sediments entering the deep mantle, the western Kunlun EM2-like lavas reveal that subducted sediments can be rapidly recycled into the SCLM during continental subduction (probably <50 Myr). We suggest that the SCLM could be an important reservoir for subducted sediments. The findings are important to our understanding of mantle circulation rates and chemical heterogeneities.
{"title":"Rapid Recycling of Subducted Sediments in the Subcontinental Lithospheric Mantle","authors":"Jian Wang, Qiang Wang, Lin Ma, Wan-Long Hu, Jun Wang, Elena Belousova, Gong-Jian Tang","doi":"10.1093/petrology/egad056","DOIUrl":"https://doi.org/10.1093/petrology/egad056","url":null,"abstract":"ABSTRACT Subduction recycling of sediments plays a key role in the geochemical evolution of Earth. The presence of recycled terrigenous sediments in upwelling plumes has been cited to explain the EM2 signature in ocean island volcanics, characterized by particularly high 87Sr/86Sr (&gt;0.706). However, the origin of such isotopic anomalies in continental regions and the role of subducted sediments in the subcontinental lithospheric mantle (SCLM) remain unclear. The Himalaya–Tibet orogen is one of the world’s best places for deciphering continental subduction and the fate of subducted crustal materials in the mantle. Here we present a systematic study of the geochronology, mineral chemistry (especially clinopyroxene), whole-rock chemistry and Sr–Nd–Pb–Hf–O isotopic compositions of Cenozoic potassic–ultrapotassic lavas from the western Kunlun area of northwestern Tibet. New secondary ion mass spectrometry (SIMS) zircon U–Pb dating, coupled with published age results, constrain the timing of volcanism from ~8.3 Ma to the present. These lavas show geochemical characteristics that closely resemble the EM2 mantle end-member represented by the Samoan hotspot. Both whole rocks and individual magmatic clinopyroxenes display arc-like trace-element patterns and remarkably enriched Sr–Nd–Pb–Hf isotope compositions (87Sr/86Sr ≥ 0.7080; εNd ≤ −4.8; 206Pb/204Pb ≥ 18.704; εHf ≤ −2.6). Together with high zircon δ18O values (6.3–10.4‰), the data point to a mantle source enriched by recycled sedimentary materials. Geochemical modeling and geophysical evidence further indicate that the sediments were directly derived from the subducted Indian continental lithosphere during India–Eurasia collision. Partial melting models assuming a hybridized mantle source that contains ~5% Indian continental crust suggest that the primary melts of the potassic–ultrapotassic lavas could be formed by melting of a phlogopite-bearing garnet lherzolite at low melting degrees (1–5%). The magma geochemistry is consistent with the model of mélange melting, implying that the subducted sediments may detach from the downgoing Indian slab and rise up diapirically into the overlying mantle lithosphere. Unlike traditional models of subducted sediments entering the deep mantle, the western Kunlun EM2-like lavas reveal that subducted sediments can be rapidly recycled into the SCLM during continental subduction (probably &lt;50 Myr). We suggest that the SCLM could be an important reservoir for subducted sediments. The findings are important to our understanding of mantle circulation rates and chemical heterogeneities.","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":"40 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-08-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135930531","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 : 2023-08-01DOI: 10.1093/petrology/egad055
Benoît Welsch, F. Faure, E. First
We investigated the early stages of olivine crystal growth via in situ seeded experiments in a single plagioclase-hosted melt inclusion, using a heating stage microscope. Each experiment was subjected to a cooling ramp of 7800 °C/h followed by an isothermal dwell at 19, 38, 57, 77, 96 or 129 °C of undercooling. The seeds (6 – 16 μm in diameter Ø) grew into large crystals (Ø 80 – 169 μm) in 3 to 30 min through the symmetrical development of tabular, skeletal, and dendritic overgrowths as the undercooling of the system increased. Time-resolved image processing and incremental measurements of the overgrowth thicknesses indicate up to three stages of crystal growth: an acceleration stage, a linear (constant growth rate) stage, and a deceleration stage. At the isotherm, the growth velocities reach a stable maximum that in all experiments corresponds to the period of linear growth. The highest linear values are measured at the $left{101right}$ interfaces, from 2.1 x 10-8 m/s at 19 °C of undercooling to 4.8 x 10-7 m/s at 129 °C of undercooling. Crystal growth is slower at other interfaces, in the ranges 1.9 – 7.6 x 10-8 m/s and 4.5 x 10-9 – 7.6 x 10-8 m/s for the $left{100right}$ and $left{001right}$ forms, respectively. Growth in the $<010>$ dimension appears limited to less than 2.4 x 10-8 m/s at 129 °C of undercooling. We constrain the uncertainty on these growth velocities, which includes the environmental conditions (± 8.6 °C on the nominal undercooling) and the measurements of crystal lengths (underestimated by < 16 % at most fast interfaces). A systematic and comprehensive review of 19 pre-existing datasets indicates that our linear growth velocities are faster than most growth rates determined at comparable undercoolings. Growth rates determined as half crystal lengths divided by total time are intrinsically low estimates of the true maximum, linear growth velocities, because the total time includes periods of slower or non-growth, and measured crystal dimensions are subject to projection foreshortening or truncation. These errors can lead to values that are several times to several orders of magnitude lower than the true maximum growth rates. This study completes and refines previously published data on the crystallization kinetics of olivine, highlighting the sensitivity of growth rates to specific environmental conditions and measurement methods. We emphasize the importance of symmetrical growth and true maximum growth velocities for interpreting olivine growth histories.
{"title":"Reappraising Crystallization Kinetics with Overgrowth Chronometry: An In Situ Study of Olivine Growth Velocities","authors":"Benoît Welsch, F. Faure, E. First","doi":"10.1093/petrology/egad055","DOIUrl":"https://doi.org/10.1093/petrology/egad055","url":null,"abstract":"\u0000 We investigated the early stages of olivine crystal growth via in situ seeded experiments in a single plagioclase-hosted melt inclusion, using a heating stage microscope. Each experiment was subjected to a cooling ramp of 7800 °C/h followed by an isothermal dwell at 19, 38, 57, 77, 96 or 129 °C of undercooling. The seeds (6 – 16 μm in diameter Ø) grew into large crystals (Ø 80 – 169 μm) in 3 to 30 min through the symmetrical development of tabular, skeletal, and dendritic overgrowths as the undercooling of the system increased. Time-resolved image processing and incremental measurements of the overgrowth thicknesses indicate up to three stages of crystal growth: an acceleration stage, a linear (constant growth rate) stage, and a deceleration stage. At the isotherm, the growth velocities reach a stable maximum that in all experiments corresponds to the period of linear growth. The highest linear values are measured at the $left{101right}$ interfaces, from 2.1 x 10-8 m/s at 19 °C of undercooling to 4.8 x 10-7 m/s at 129 °C of undercooling. Crystal growth is slower at other interfaces, in the ranges 1.9 – 7.6 x 10-8 m/s and 4.5 x 10-9 – 7.6 x 10-8 m/s for the $left{100right}$ and $left{001right}$ forms, respectively. Growth in the $<010>$ dimension appears limited to less than 2.4 x 10-8 m/s at 129 °C of undercooling. We constrain the uncertainty on these growth velocities, which includes the environmental conditions (± 8.6 °C on the nominal undercooling) and the measurements of crystal lengths (underestimated by < 16 % at most fast interfaces).\u0000 A systematic and comprehensive review of 19 pre-existing datasets indicates that our linear growth velocities are faster than most growth rates determined at comparable undercoolings. Growth rates determined as half crystal lengths divided by total time are intrinsically low estimates of the true maximum, linear growth velocities, because the total time includes periods of slower or non-growth, and measured crystal dimensions are subject to projection foreshortening or truncation. These errors can lead to values that are several times to several orders of magnitude lower than the true maximum growth rates. This study completes and refines previously published data on the crystallization kinetics of olivine, highlighting the sensitivity of growth rates to specific environmental conditions and measurement methods. We emphasize the importance of symmetrical growth and true maximum growth velocities for interpreting olivine growth histories.","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":" ","pages":""},"PeriodicalIF":3.9,"publicationDate":"2023-08-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"48523741","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 : 2023-08-01DOI: 10.1093/petrology/egad061
Jian-Hui Su, Xin-Fu Zhao, Johannes Hammerli
Abstract Apatite can incorporate significant amounts of H2O and CO2, potentially recording volatile abundance and behavior during magma evolution. Here we conducted in situ measurements of CO2 and H2O concentrations in apatite, along with elemental compositions, from two contemporaneous alkaline volcanic suites (Tianbao and Tudiling) in the South Qinling belt in central China to better understand the CO2 and H2O contents and behavior in evolving alkaline melts. Apatite from alkali basalts in Tianbao contains variable CO2 contents ranging from <100 ppm to 2.7 wt.% and H2O contents ranging from 0.1 to 0.6 wt.%. Apatite from REE-enriched trachytes, which evolved from alkali basalt, shows significantly lower CO2 contents and a continuous decrease in H2O during magma fractionation. The observations suggest that CO2 loss commenced at the early stages of magma evolution, whereas significant H2O loss occurred during subsequent magma fractionation in tandem with REE-enrichment. In comparison, apatite grains from the Tudiling trachyte, which is genetically linked with carbonatite, contain higher CO2 contents (0.6 to 1.5 wt.%) but lower REE concentrations than the Tianbao trachytes. Apatite in the Tudiling trachyte is inferred to have crystallized from a carbonated alkaline magma prior to significant CO2 loss and the separation of Tudiling carbonatitic melts, where subsequent liquid immiscibility led to REE enrichment into the carbonatitic melts. The volatile characteristics of apatite from the two volcanic suites provide valuable insights into two different evolutionary processes of alkaline/carbonatitic magmas, the behavior of CO2 and H2O, and the enrichment of REE in alkaline systems.
{"title":"Apatite CO2 and H2O as Indicators of Differentiation and Degassing in Alkaline Magmas","authors":"Jian-Hui Su, Xin-Fu Zhao, Johannes Hammerli","doi":"10.1093/petrology/egad061","DOIUrl":"https://doi.org/10.1093/petrology/egad061","url":null,"abstract":"Abstract Apatite can incorporate significant amounts of H2O and CO2, potentially recording volatile abundance and behavior during magma evolution. Here we conducted in situ measurements of CO2 and H2O concentrations in apatite, along with elemental compositions, from two contemporaneous alkaline volcanic suites (Tianbao and Tudiling) in the South Qinling belt in central China to better understand the CO2 and H2O contents and behavior in evolving alkaline melts. Apatite from alkali basalts in Tianbao contains variable CO2 contents ranging from &lt;100 ppm to 2.7 wt.% and H2O contents ranging from 0.1 to 0.6 wt.%. Apatite from REE-enriched trachytes, which evolved from alkali basalt, shows significantly lower CO2 contents and a continuous decrease in H2O during magma fractionation. The observations suggest that CO2 loss commenced at the early stages of magma evolution, whereas significant H2O loss occurred during subsequent magma fractionation in tandem with REE-enrichment. In comparison, apatite grains from the Tudiling trachyte, which is genetically linked with carbonatite, contain higher CO2 contents (0.6 to 1.5 wt.%) but lower REE concentrations than the Tianbao trachytes. Apatite in the Tudiling trachyte is inferred to have crystallized from a carbonated alkaline magma prior to significant CO2 loss and the separation of Tudiling carbonatitic melts, where subsequent liquid immiscibility led to REE enrichment into the carbonatitic melts. The volatile characteristics of apatite from the two volcanic suites provide valuable insights into two different evolutionary processes of alkaline/carbonatitic magmas, the behavior of CO2 and H2O, and the enrichment of REE in alkaline systems.","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":"103 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-08-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"136161798","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 : 2023-07-22DOI: 10.1093/petrology/egad052
Jun Wang, Qiang Wang, Peng Sun, Wei Dan, Andrew C Kerr, Zhi-Ping Zhang, Le Zhang, Gangjian Wei, Han Dong, Wan-Long Hu, Zong-Yong Yang, Xiu-Zheng Zhang, Yue Qi
In recent studies of crustal growth using global zircon Hf–O isotopic datasets, high-δ18O zircons are typically attributed to intra-crustal reworking during which very little juvenile mantle-derived magmas were added to the crust. Although arc magmas may originate from a high-δ18O mantle wedge, it has been difficult to decipher the contribution of high-δ18O mantle to zircon-saturated felsic magma due to superimposed intra-crustal processes. We address this issue by combining the data from high-δ18O zircon-bearing ultramafic cumulates and coeval lavas from a Cretaceous magmatic arc in southern Tibet. The cumulates mainly consist of different proportions of cumulus olivine and intercumulus amphibole. Amphibole analyses show a transition from increasing to decreasing Zr with increasing SiO2 (50–74 wt.%) contents in the intercumulus melts, indicating zircon saturation in late-stage interstitial melts. The εNd(t) values (2.4 ± 1.4) of the apatite grains crystallised before and after zircon remain almost constant. Interstitial zircons have δ18O (6.1–7.2‰) values similar to the earliest crystallised olivine (δ18O = 6.3–7.1‰) in the cumulates. The coeval lavas may represent the intercumulus melts extracted from amphibole-rich cumulates at different depths. Both the lavas and cumulates were ultimately derived from high-δ18O arc mantle modified by small amounts (<12%) of subducted sediments, and crystallised zircon during intra-crustal magma evolution without involving crustal contamination or melting. These high-δ18O zircons therefore are not products of crustal reworking, but record crustal growth during their crystallisation (110 ± 2 Ma). Our study shows that the combination of zircon and olivine oxygen isotopes for ultramafic to felsic rocks is more effective than zircon data alone in evaluating the role of crustal growth vs. reworking in an arc system. The implication is that global zircon-based crustal evolution models that attribute all high-δ18O zircons to crustal reworking may conceal recent crustal growth.
{"title":"Crustal growth identified by high-δ18O zircon and olivine: A perspective from ultramafic arc cumulates in southern Tibet","authors":"Jun Wang, Qiang Wang, Peng Sun, Wei Dan, Andrew C Kerr, Zhi-Ping Zhang, Le Zhang, Gangjian Wei, Han Dong, Wan-Long Hu, Zong-Yong Yang, Xiu-Zheng Zhang, Yue Qi","doi":"10.1093/petrology/egad052","DOIUrl":"https://doi.org/10.1093/petrology/egad052","url":null,"abstract":"In recent studies of crustal growth using global zircon Hf–O isotopic datasets, high-δ18O zircons are typically attributed to intra-crustal reworking during which very little juvenile mantle-derived magmas were added to the crust. Although arc magmas may originate from a high-δ18O mantle wedge, it has been difficult to decipher the contribution of high-δ18O mantle to zircon-saturated felsic magma due to superimposed intra-crustal processes. We address this issue by combining the data from high-δ18O zircon-bearing ultramafic cumulates and coeval lavas from a Cretaceous magmatic arc in southern Tibet. The cumulates mainly consist of different proportions of cumulus olivine and intercumulus amphibole. Amphibole analyses show a transition from increasing to decreasing Zr with increasing SiO2 (50–74 wt.%) contents in the intercumulus melts, indicating zircon saturation in late-stage interstitial melts. The εNd(t) values (2.4 ± 1.4) of the apatite grains crystallised before and after zircon remain almost constant. Interstitial zircons have δ18O (6.1–7.2‰) values similar to the earliest crystallised olivine (δ18O = 6.3–7.1‰) in the cumulates. The coeval lavas may represent the intercumulus melts extracted from amphibole-rich cumulates at different depths. Both the lavas and cumulates were ultimately derived from high-δ18O arc mantle modified by small amounts (&lt;12%) of subducted sediments, and crystallised zircon during intra-crustal magma evolution without involving crustal contamination or melting. These high-δ18O zircons therefore are not products of crustal reworking, but record crustal growth during their crystallisation (110 ± 2 Ma). Our study shows that the combination of zircon and olivine oxygen isotopes for ultramafic to felsic rocks is more effective than zircon data alone in evaluating the role of crustal growth vs. reworking in an arc system. The implication is that global zircon-based crustal evolution models that attribute all high-δ18O zircons to crustal reworking may conceal recent crustal growth.","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":"60 2","pages":""},"PeriodicalIF":3.9,"publicationDate":"2023-07-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138503698","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 : 2023-07-21DOI: 10.1093/petrology/egad053
Charles E Lesher, Eric L Brown, Gry H Barfod, Justin Glessner, Niklas Stausberg, Peter Thy, Christian Tegner, Lars Peter Salmonsen, Troels F D Nielsen
The Skaergaard intrusion is one of the most thoroughly studied layered mafic intrusions on Earth and an exceptional example of (near) closed-system magmatic differentiation. We report new Fe isotope data for whole rocks, and magnetite and ilmenite separates through the layered series (LS) and upper border series (UBS) of the intrusion. δ56Fe values for gabbroic rocks range from 0.033 to 0.151 ‰ with an abrupt step increase at the base of Lower Zone c (LZc) within the LS with the appearance of cumulus magnetite and subsequent decline accompanying FeTi oxide fractionation. The lowest δ56Fe values are found near the Upper Zone b (UZb) – c (UZc) boundary followed by a sharp rise across UZc approaching the Sandwich Horizon. Magnetite - ilmenite separates straddle bulk rock compositions with fractionation factors (∆56Femt-ilm) of 0.081 to 0.239 ‰, consistent with subsolidus equilibration. Granophyric rocks occurring as pods, sheets and wispy layers from the upper zone and UBS equivalents, having unradiogenic Sr like gabbroic rocks of Skaergaard, are isotopically heavier than their host ferrodiorites (∆56Fegranophyre-ferrodiorite ≥ 0.1 ‰) reaching a maximum δ56Fe of 0.217 ‰ for UBS granophyre. A fused xenolith from UBS has δ56Fe = 0.372 ‰. This range in δ56Fe spans much of that reported for terrestrial igneous rocks, and like the global dataset, shows a pronounced increase in δ56Fe with inferred silica content of modelled Skaergaard liquids. Forward modelling of closed system fractional solidification was undertaken to account for Fe isotope systematics, first by testing published liquid lines of descent (LLD), and then by exploring improvements and considering the impacts of liquid immiscibility, crustal contamination, fluid exsolution and diffusional processes. Our modelling relies on published Fe+2 and Fe+3 force constants for magmatic minerals and silicate glasses, and the most reliable estimates of the average bulk composition and mass proportions of the well-defined subzones of the intrusion. We show that the increase in δ56Fe across the LZb – LZc boundary is readily explained by the increased incorporation of Fe+3 into the crystallizing solid including magnetite. We further demonstrate that the classic Fenner LLD, involving strong Fe-enrichment at nearly constant silica, does not lead to a rise in δ56Fe towards the end stages of evolution, while a Bowen-like LLD, with little Fe enrichment and strong Si-enrichment, also underestimates enrichment in heavy Fe isotopes in the ferrodiorites of UZc. A LLD following an intermediate path involving modest Fe and Si enrichment, followed by Fe depletion best explains the observations. We predict ~3.5% (by mass) residual liquid after crystallization of UZc having a composition similar to felsic segregations in pegmatitic bodies found in the intrusion. While liquid immiscibility may have been encountered within fractionating mush at the margins of the intrusion, the Fe isotope systematics do not support liqui
{"title":"Iron isotope systematics of the Skaergaard intrusion and implications for its liquid line of descent","authors":"Charles E Lesher, Eric L Brown, Gry H Barfod, Justin Glessner, Niklas Stausberg, Peter Thy, Christian Tegner, Lars Peter Salmonsen, Troels F D Nielsen","doi":"10.1093/petrology/egad053","DOIUrl":"https://doi.org/10.1093/petrology/egad053","url":null,"abstract":"The Skaergaard intrusion is one of the most thoroughly studied layered mafic intrusions on Earth and an exceptional example of (near) closed-system magmatic differentiation. We report new Fe isotope data for whole rocks, and magnetite and ilmenite separates through the layered series (LS) and upper border series (UBS) of the intrusion. δ56Fe values for gabbroic rocks range from 0.033 to 0.151 ‰ with an abrupt step increase at the base of Lower Zone c (LZc) within the LS with the appearance of cumulus magnetite and subsequent decline accompanying FeTi oxide fractionation. The lowest δ56Fe values are found near the Upper Zone b (UZb) – c (UZc) boundary followed by a sharp rise across UZc approaching the Sandwich Horizon. Magnetite - ilmenite separates straddle bulk rock compositions with fractionation factors (∆56Femt-ilm) of 0.081 to 0.239 ‰, consistent with subsolidus equilibration. Granophyric rocks occurring as pods, sheets and wispy layers from the upper zone and UBS equivalents, having unradiogenic Sr like gabbroic rocks of Skaergaard, are isotopically heavier than their host ferrodiorites (∆56Fegranophyre-ferrodiorite ≥ 0.1 ‰) reaching a maximum δ56Fe of 0.217 ‰ for UBS granophyre. A fused xenolith from UBS has δ56Fe = 0.372 ‰. This range in δ56Fe spans much of that reported for terrestrial igneous rocks, and like the global dataset, shows a pronounced increase in δ56Fe with inferred silica content of modelled Skaergaard liquids. Forward modelling of closed system fractional solidification was undertaken to account for Fe isotope systematics, first by testing published liquid lines of descent (LLD), and then by exploring improvements and considering the impacts of liquid immiscibility, crustal contamination, fluid exsolution and diffusional processes. Our modelling relies on published Fe+2 and Fe+3 force constants for magmatic minerals and silicate glasses, and the most reliable estimates of the average bulk composition and mass proportions of the well-defined subzones of the intrusion. We show that the increase in δ56Fe across the LZb – LZc boundary is readily explained by the increased incorporation of Fe+3 into the crystallizing solid including magnetite. We further demonstrate that the classic Fenner LLD, involving strong Fe-enrichment at nearly constant silica, does not lead to a rise in δ56Fe towards the end stages of evolution, while a Bowen-like LLD, with little Fe enrichment and strong Si-enrichment, also underestimates enrichment in heavy Fe isotopes in the ferrodiorites of UZc. A LLD following an intermediate path involving modest Fe and Si enrichment, followed by Fe depletion best explains the observations. We predict ~3.5% (by mass) residual liquid after crystallization of UZc having a composition similar to felsic segregations in pegmatitic bodies found in the intrusion. While liquid immiscibility may have been encountered within fractionating mush at the margins of the intrusion, the Fe isotope systematics do not support liqui","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":"60 3","pages":""},"PeriodicalIF":3.9,"publicationDate":"2023-07-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138503697","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 : 2023-07-05DOI: 10.1093/petrology/egad050
P. Wieser, A. Kent, C. Till
The chemistry of erupted clinopyroxene crystals (±equilibrium liquids) have been widely used to deduce the pressures and temperatures of magma storage in volcanic arcs. However, the large number of different equations parameterizing the relationship between mineral and melt compositions and intensive variables such as pressure and temperature yield vastly different results, with implications for our interpretation of magma storage conditions. We use a new test dataset composed of the average Clinopyroxene-Liquid (Cpx-Liq) compositions from N=543 variably-hydrous experiments at crustal conditions (1 bar to 17 kbar) to assess the performance of different thermobarometers, and identify the most accurate and precise expressions for application to subduction zone magmas. First, we assess different equilibrium tests, finding that comparing the measured and predicted EnFs and KD (using Fet in both phases) are the most useful tests in arc magmas, while CaTs, CaTi and Jd tests have limited utility. We then apply further quality filters based on cation sums (3.95-4.05), number of analyses (N>5), and the presence of reported H2O data in the quenched experimental glass (hereafter ‘liquid’) to obtain a filtered dataset (N=214). We use this filtered dataset to compare calculated versus experimental pressures and temperatures for different combinations of thermobarometers. A number of Cpx-Liq thermometers perform very well when liquid H2O contents are known, although the Cpx composition contributes little to the calculated temperature relative to the liquid composition. Most Cpx-only thermometers perform very badly, greatly overestimating temperatures for hydrous experiments. These two findings demonstrate that the Cpx chemistry alone holds very little temperature information in hydrous systems. The majority of Cpx-Liq and Cpx-only barometers show similar performance to one another (mostly yielding RMSEs of 2-3.5 kbar), although the best Cpx-only barometers currently outperform the best Cpx-Liq barometers. We also assess the sensitivity of different equations to melt H2O contents, which are poorly constrained in many natural systems. Overall, this work demonstrates that Cpx-based barometry on individual Cpx only provides sufficient resolution to distinguish broad storage regions (e.g., upper, mid, lower crust). Significant averaging of Cpx compositions from experiments reported at similar pressures can reduce RMSEs to ~1.3-1.9 kbar. We hope our findings motivate the substantial amount of experimental and analytical work that is required to obtain precise and accurate estimates of magma storage depths from Cpx±Liq equilibrium in volcanic arcs.
喷出斜辉石晶体(±平衡液体)的化学性质已被广泛用于推断火山弧中岩浆储存的压力和温度。然而，大量不同的方程参数化矿物和熔体成分之间的关系，以及压力和温度等密集变量产生了截然不同的结果，这对我们解释岩浆储存条件有影响。在地壳条件下(1 bar ~ 17 kbar)进行的N=543次变水实验中，我们使用了一个由斜斜成岩-液体(Cpx-Liq)平均成分组成的新测试数据集来评估不同温度气压计的性能，并确定了最准确和精确的表达式，用于俯冲带岩浆。首先，我们评估了不同的平衡测试，发现比较测量和预测的EnFs和KD(在两个阶段都使用Fet)是弧岩浆中最有用的测试，而CaTs、CaTi和Jd测试的实用性有限。然后，我们根据阳离子和(3.95-4.05)、分析次数(N>5)和在淬火实验玻璃(以下简称“液体”)中报告的H2O数据的存在进一步应用质量过滤器，以获得过滤后的数据集(N=214)。我们使用这个过滤的数据集来比较不同组合的温度计的计算压力和实验温度。当已知液态水的含量时，许多Cpx- liq温度计表现得很好，尽管相对于液体成分，Cpx成分对计算温度的贡献很小。大多数仅使用cpx的温度计性能很差，对含水实验的温度估计过高。这两项发现表明，在含水系统中，仅Cpx的化学性质就能提供很少的温度信息。大多数Cpx-Liq和Cpx-only气压计显示出彼此相似的性能(大多数产生的rmse为2-3.5 kbar)，尽管目前最好的Cpx-only气压计优于最好的Cpx-Liq气压计。我们还评估了不同方程对熔融水含量的敏感性，这在许多自然系统中受到很差的约束。总的来说，这项工作表明，基于Cpx的单个Cpx气压测量仅提供足够的分辨率来区分广泛的存储区域(例如，上、中、下地壳)。在类似压力下的实验中，Cpx成分的显著平均可以将rmse降低到1.3-1.9 kbar。我们希望我们的发现能够激发大量的实验和分析工作，这些工作需要从火山弧的Cpx±Liq平衡中获得精确和准确的岩浆储存深度估计。
{"title":"Barometers behaving badly II: A critical evaluation of Cpx-only and Cpx-Liq thermobarometry in variably-hydrous arc magmas","authors":"P. Wieser, A. Kent, C. Till","doi":"10.1093/petrology/egad050","DOIUrl":"https://doi.org/10.1093/petrology/egad050","url":null,"abstract":"\u0000 The chemistry of erupted clinopyroxene crystals (±equilibrium liquids) have been widely used to deduce the pressures and temperatures of magma storage in volcanic arcs. However, the large number of different equations parameterizing the relationship between mineral and melt compositions and intensive variables such as pressure and temperature yield vastly different results, with implications for our interpretation of magma storage conditions. We use a new test dataset composed of the average Clinopyroxene-Liquid (Cpx-Liq) compositions from N=543 variably-hydrous experiments at crustal conditions (1 bar to 17 kbar) to assess the performance of different thermobarometers, and identify the most accurate and precise expressions for application to subduction zone magmas. First, we assess different equilibrium tests, finding that comparing the measured and predicted EnFs and KD (using Fet in both phases) are the most useful tests in arc magmas, while CaTs, CaTi and Jd tests have limited utility. We then apply further quality filters based on cation sums (3.95-4.05), number of analyses (N>5), and the presence of reported H2O data in the quenched experimental glass (hereafter ‘liquid’) to obtain a filtered dataset (N=214). We use this filtered dataset to compare calculated versus experimental pressures and temperatures for different combinations of thermobarometers. A number of Cpx-Liq thermometers perform very well when liquid H2O contents are known, although the Cpx composition contributes little to the calculated temperature relative to the liquid composition. Most Cpx-only thermometers perform very badly, greatly overestimating temperatures for hydrous experiments. These two findings demonstrate that the Cpx chemistry alone holds very little temperature information in hydrous systems.\u0000 The majority of Cpx-Liq and Cpx-only barometers show similar performance to one another (mostly yielding RMSEs of 2-3.5 kbar), although the best Cpx-only barometers currently outperform the best Cpx-Liq barometers. We also assess the sensitivity of different equations to melt H2O contents, which are poorly constrained in many natural systems. Overall, this work demonstrates that Cpx-based barometry on individual Cpx only provides sufficient resolution to distinguish broad storage regions (e.g., upper, mid, lower crust). Significant averaging of Cpx compositions from experiments reported at similar pressures can reduce RMSEs to ~1.3-1.9 kbar. We hope our findings motivate the substantial amount of experimental and analytical work that is required to obtain precise and accurate estimates of magma storage depths from Cpx±Liq equilibrium in volcanic arcs.","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":" ","pages":""},"PeriodicalIF":3.9,"publicationDate":"2023-07-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43928037","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 : 2023-07-03DOI: 10.1093/petrology/egad051
Zhao Liu, B. B. Carvalho, Wan-Cai Li, L. Tong, O. Bartoli, Longyao Chen, Qinghe Yan, Hao Wu
Precise constraints on the compositions of melts generated by anatexis under ultrahigh temperature (UHT) conditions are critical for understanding processes of partial melting and differentiation of the Earth’s crust. Here we reveal geochemical and physical signatures of anatectic melts preserved as nanogranitoids (i.e. crystalized melt inclusions) within sapphirine–bearing UHT metapelitic granulites from the Mather Peninsula, East Antarctica. Their coexistence with high−Al orthopyroxene as inclusions in garnets strongly suggests that the investigated melts were at least partially UHT in origin. The nanogranitoids are enriched in SiO2 (69.9−75.6 wt.%), strongly peraluminous (ASI values = 1.2−1.6) and potassic to ultrapotassic (Na2O + K2O = 7.1−9.5 wt.%, K/Na = 2.2−9.3). When compared to the granulitic restite, the melts are enriched in Li, Cs, Rb, Ta, Sm, Nd, Zr, U and Pb, and depleted in Ce, Th, Ba, Sr and Nb. Their geochemical characteristics are consistent with biotite−dehydration melting in the absence of plagioclase. Our calculation results indicate that these hot crustal melts have low densities of 2.47 ± 0.07 g/cm3, low viscosities of 104.9±1.2 Pa·s and high heat production values of ∼2.8 μW/m3. Therefore, such melts are mobile and susceptible to be extracted from the source, and consequently their flow and removal from the deep crust may greatly affect the chemical and thermal structure of the continental crust. Secondary C−O−H fluid inclusions within garnet and orthopyroxene have also been detected. These inclusions contain magnesite, pyrophyllite, corundum, with or without residual CO2. The minerals within the fluid inclusions are interpreted as stepdaughter minerals, which were produced by the reaction of the fluid with their host. The metamorphic timing of the investigated rocks is still a matter of debate. Zircon U−Pb dating results obtained in this study suggest that the metapelitic granulites may have undergone two separated thermal events at ∼1000 and ∼530 Ma, respectively. The presence of fluid inclusions indicates that fluid infiltration and Pan–African reworking may have played an important role in obscuring chronological information of the early thermal scenario in poly–metamorphic terranes.
{"title":"Into the high to ultrahigh temperature melting of Earth’s crust: Investigation of melt and fluid inclusions within Mg–rich metapelitic granulites from the Mather Peninsula, East Antarctica","authors":"Zhao Liu, B. B. Carvalho, Wan-Cai Li, L. Tong, O. Bartoli, Longyao Chen, Qinghe Yan, Hao Wu","doi":"10.1093/petrology/egad051","DOIUrl":"https://doi.org/10.1093/petrology/egad051","url":null,"abstract":"\u0000 Precise constraints on the compositions of melts generated by anatexis under ultrahigh temperature (UHT) conditions are critical for understanding processes of partial melting and differentiation of the Earth’s crust. Here we reveal geochemical and physical signatures of anatectic melts preserved as nanogranitoids (i.e. crystalized melt inclusions) within sapphirine–bearing UHT metapelitic granulites from the Mather Peninsula, East Antarctica. Their coexistence with high−Al orthopyroxene as inclusions in garnets strongly suggests that the investigated melts were at least partially UHT in origin. The nanogranitoids are enriched in SiO2 (69.9−75.6 wt.%), strongly peraluminous (ASI values = 1.2−1.6) and potassic to ultrapotassic (Na2O + K2O = 7.1−9.5 wt.%, K/Na = 2.2−9.3). When compared to the granulitic restite, the melts are enriched in Li, Cs, Rb, Ta, Sm, Nd, Zr, U and Pb, and depleted in Ce, Th, Ba, Sr and Nb. Their geochemical characteristics are consistent with biotite−dehydration melting in the absence of plagioclase. Our calculation results indicate that these hot crustal melts have low densities of 2.47 ± 0.07 g/cm3, low viscosities of 104.9±1.2 Pa·s and high heat production values of ∼2.8 μW/m3. Therefore, such melts are mobile and susceptible to be extracted from the source, and consequently their flow and removal from the deep crust may greatly affect the chemical and thermal structure of the continental crust.\u0000 Secondary C−O−H fluid inclusions within garnet and orthopyroxene have also been detected. These inclusions contain magnesite, pyrophyllite, corundum, with or without residual CO2. The minerals within the fluid inclusions are interpreted as stepdaughter minerals, which were produced by the reaction of the fluid with their host. The metamorphic timing of the investigated rocks is still a matter of debate. Zircon U−Pb dating results obtained in this study suggest that the metapelitic granulites may have undergone two separated thermal events at ∼1000 and ∼530 Ma, respectively. The presence of fluid inclusions indicates that fluid infiltration and Pan–African reworking may have played an important role in obscuring chronological information of the early thermal scenario in poly–metamorphic terranes.","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":" ","pages":""},"PeriodicalIF":3.9,"publicationDate":"2023-07-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"41933014","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 : 2023-06-30DOI: 10.1093/petrology/egad049
J. Puziewicz, S. Aulbach, M. Kaczmarek, T. Ntaflos, A. Gerdes, H. Mazurek, A. Kukuła, M. Matusiak-Małek, Sylvin S. T. Tedonkenfack, Małgorzata Ziobro-Mikrut
The lithospheric mantle as sampled by peridotite xenoliths in some continental settings resembles the source of mid-ocean ridge basalts (MORB). Whether this resemblance is a primary feature or the result of post-formation secondary processes remains controversial. Here, the age, origin and thermochemical evolution of fertile continental mantle are constrained based on the chemical composition of minerals in spinel-facies lherzolite and websterite xenoliths from the Wum maar and Befang cinder cone of the Oku Volcanic Group (Cameroon Volcanic Line, West Africa), combined with in-situ Sr isotope compositions of clinopyroxene and fabric investigation by Electron Backscatter Diffraction (EBSD). The majority of lherzolites (here assigned to Group I) consist of minerals with fertile composition (olivine Fo89, Al-rich pyroxenes, spinel Cr# 0.08-0.10). Clinopyroxene is LREE-depleted and has depleted 87Sr/86Sr (0.7017-0.7020). Crystal-preferred orientation determined by EBSD reveals that clinopyroxene, and sporadically both clino- and orthopyroxene, post-date the olivine framework. Subordinate Group II lherzolites also contain secondary clinopyroxene which is LREE-enriched and has higher 87Sr/86Sr (0.7033). In contrast, the scarce lherzolites of Group III are more refractory: they contain 72-78 vol. % olivine, Al-poor pyroxenes, and spinel with Cr# 0.18. Clinopyroxene (87Sr/86Sr 0.7021) is texturally coeval with olivine and orthopyroxene. Few lherzolites contain amphibole (87Sr/86Sr 0.7031) which post-dates the nominally anhydrous minerals. Most of the websterites (Group A) are aluminous (spinel Cr# 0.04-0.06) with LREE-depleted clinopyroxene having depleted 87Sr/86Sr ratios (0.7017-0.7020) similar to Group I lherzolites. Chemical characteristics of minerals coupled with the crystal-preferred orientation data suggests that Group I lherzolites originated in the spinel stability field by reactive intergranular percolation of an incompatible element-depleted MORB-like melt. Group A websterites likely formed as cumulates from that melt. The Group II lherzolites supposedly occur close to lithosphere-asthenosphere boundary and record interaction with lavas of the Cameroon Volcanic Line, whereas Group III lherzolites occur in the shallow part of the mantle profile and represent the protolith from which the Group I lherzolites were formed. Local crystallization of amphibole and concomitant recrystallization of the host lherzolite were driven by supply of water in an event post-dating the formation of LREE-depleted rejuvenated rocks. Migration of alkaline melts of the CVL apparently did not significantly affect the mineral and chemical composition of the lithospheric mantle, which allowed Group I lherzolites and Group A websterites to retain very low 87Rb/86Sr (average 0.002) and depleted 87Sr/86Sr ratios in clinopyroxene. This not only indicates their formation in the Paleoproterozoic (~2.0-2.25 Ga), possibly during the Eburnean orogeny at the margin of the C
{"title":"The origin and evolution of DMM-like lithospheric mantle beneath continents: Mantle xenoliths from the Oku Volcanic Group in the Cameroon Volcanic Line, West Africa","authors":"J. Puziewicz, S. Aulbach, M. Kaczmarek, T. Ntaflos, A. Gerdes, H. Mazurek, A. Kukuła, M. Matusiak-Małek, Sylvin S. T. Tedonkenfack, Małgorzata Ziobro-Mikrut","doi":"10.1093/petrology/egad049","DOIUrl":"https://doi.org/10.1093/petrology/egad049","url":null,"abstract":"\u0000 The lithospheric mantle as sampled by peridotite xenoliths in some continental settings resembles the source of mid-ocean ridge basalts (MORB). Whether this resemblance is a primary feature or the result of post-formation secondary processes remains controversial. Here, the age, origin and thermochemical evolution of fertile continental mantle are constrained based on the chemical composition of minerals in spinel-facies lherzolite and websterite xenoliths from the Wum maar and Befang cinder cone of the Oku Volcanic Group (Cameroon Volcanic Line, West Africa), combined with in-situ Sr isotope compositions of clinopyroxene and fabric investigation by Electron Backscatter Diffraction (EBSD).\u0000 The majority of lherzolites (here assigned to Group I) consist of minerals with fertile composition (olivine Fo89, Al-rich pyroxenes, spinel Cr# 0.08-0.10). Clinopyroxene is LREE-depleted and has depleted 87Sr/86Sr (0.7017-0.7020). Crystal-preferred orientation determined by EBSD reveals that clinopyroxene, and sporadically both clino- and orthopyroxene, post-date the olivine framework. Subordinate Group II lherzolites also contain secondary clinopyroxene which is LREE-enriched and has higher 87Sr/86Sr (0.7033). In contrast, the scarce lherzolites of Group III are more refractory: they contain 72-78 vol. % olivine, Al-poor pyroxenes, and spinel with Cr# 0.18. Clinopyroxene (87Sr/86Sr 0.7021) is texturally coeval with olivine and orthopyroxene. Few lherzolites contain amphibole (87Sr/86Sr 0.7031) which post-dates the nominally anhydrous minerals. Most of the websterites (Group A) are aluminous (spinel Cr# 0.04-0.06) with LREE-depleted clinopyroxene having depleted 87Sr/86Sr ratios (0.7017-0.7020) similar to Group I lherzolites.\u0000 Chemical characteristics of minerals coupled with the crystal-preferred orientation data suggests that Group I lherzolites originated in the spinel stability field by reactive intergranular percolation of an incompatible element-depleted MORB-like melt. Group A websterites likely formed as cumulates from that melt. The Group II lherzolites supposedly occur close to lithosphere-asthenosphere boundary and record interaction with lavas of the Cameroon Volcanic Line, whereas Group III lherzolites occur in the shallow part of the mantle profile and represent the protolith from which the Group I lherzolites were formed. Local crystallization of amphibole and concomitant recrystallization of the host lherzolite were driven by supply of water in an event post-dating the formation of LREE-depleted rejuvenated rocks.\u0000 Migration of alkaline melts of the CVL apparently did not significantly affect the mineral and chemical composition of the lithospheric mantle, which allowed Group I lherzolites and Group A websterites to retain very low 87Rb/86Sr (average 0.002) and depleted 87Sr/86Sr ratios in clinopyroxene. This not only indicates their formation in the Paleoproterozoic (~2.0-2.25 Ga), possibly during the Eburnean orogeny at the margin of the C","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":" ","pages":""},"PeriodicalIF":3.9,"publicationDate":"2023-06-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45643694","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 : 2023-06-30DOI: 10.1093/petrology/egad048
Zisong Zhao, C. Wang, J. Dou, Bo Wei
Mafic magmatic enclaves (MMEs) hosted in granitic plutons are ideal to investigate the role of mushes on magma mixing processes in crustal magma chambers. However, the petrographic evidence for mixing of magmas through infiltration and percolation in coexisting mushes and magmas is desired. Here, we describe complex zoning patterns of plagioclase in the MMEs hosted in the monzogranite of the Early Cretaceous Sanguliu pluton in East China, to reveal a mush-facilitated magma mixing process. The MMEs appear as round to oval nodules about 10 to 20 cm in size and show diverse disequilibrium textures. Plagioclase in the MMEs can be identified as three populations (Plag1, Plag2 and Plag3) with distinct zoning patterns, anorthite contents (XAn) and initial Sr isotopic ratios (87Sr/86Sri). Plag1 is antecryst displaying normal zoning with An42-67 in the core and An20-36 in the mantle. The core of Plag1 shows coarse sieve texture with high-frequency oscillation in the margin, and the mantle displays resorption surface and patchy zoning. Plag2 is also antecryst with An23-66 in the core and An21-35 in the mantle. However, its core can be further recognized as Core I inside and Core II outside with distinctly different An23-43 and An44-66, respectively, showing reverse zoning. In addition, Core I contains aligned biotite inclusions and Core II shows sieve texture, resorption surface and patchy zoning. Amphibole inclusions are sporadically enclosed within Core I of Plag2 (Amp1) and mantles of Plag1 and Plag2 (Amp2), but rarely observed in Core II of Plag2. Plag3 is anhedral grain in the matrix and shows core-rim texture with An20-37 in the core. The three plagioclase populations all exhibit angular rims with resembling An9-22. Plag1 core and Plag2 Core II have (87Sr/86Sr)i (0.70920 to 0.71092) similar to the bulk (87Sr/86Sr)i of the mafic dykes intruding the Sanguliu pluton, and likely crystallized from basaltic andesitic magmas. In contrast, the rims of Plag1, Plag2 and Plag3 overall have (87Sr/86Sr)i (0.71391 to 0.71583) nearly identical to the (87Sr/86Sr)i of host monzogranite and the plagioclase in the monzogranite, likely crystallized from granitic magmas. The mantles of Plag1 and Plag2 and the core of Plag3 have (87Sr/86Sr)i (0.71141 to 0.71390) overlapping the (87Sr/86Sr)i of the MMEs, and may have crystallized from mixed melts. Calculation results based on amphibole thermobarometers show that Amp1 crystallized at ~775 °C and ~16 km depth, whereas Amp2 and the amphibole in the matrix of the MMEs and monzogranite crystallized at 730-744 °C and 8-9 km depth. We thus propose that the chemical and textural complexity of the three plagioclase populations in the MMEs can be attributed to that the MMEs may have come from a mushy hybrid layer that was developed through a molten granitic body being recharged by upwelling basaltic andesitic magma. Core I of Plag2 may have nucleated and grown from andesitic magma that was evolved from the basaltic andesitic magma
{"title":"A mush-facilitated magma mixing process revealed by complex zoning of plagioclase in mafic magmatic enclaves of the Early Cretaceous Sanguliu granitic pluton, East China","authors":"Zisong Zhao, C. Wang, J. Dou, Bo Wei","doi":"10.1093/petrology/egad048","DOIUrl":"https://doi.org/10.1093/petrology/egad048","url":null,"abstract":"\u0000 Mafic magmatic enclaves (MMEs) hosted in granitic plutons are ideal to investigate the role of mushes on magma mixing processes in crustal magma chambers. However, the petrographic evidence for mixing of magmas through infiltration and percolation in coexisting mushes and magmas is desired. Here, we describe complex zoning patterns of plagioclase in the MMEs hosted in the monzogranite of the Early Cretaceous Sanguliu pluton in East China, to reveal a mush-facilitated magma mixing process. The MMEs appear as round to oval nodules about 10 to 20 cm in size and show diverse disequilibrium textures. Plagioclase in the MMEs can be identified as three populations (Plag1, Plag2 and Plag3) with distinct zoning patterns, anorthite contents (XAn) and initial Sr isotopic ratios (87Sr/86Sri). Plag1 is antecryst displaying normal zoning with An42-67 in the core and An20-36 in the mantle. The core of Plag1 shows coarse sieve texture with high-frequency oscillation in the margin, and the mantle displays resorption surface and patchy zoning. Plag2 is also antecryst with An23-66 in the core and An21-35 in the mantle. However, its core can be further recognized as Core I inside and Core II outside with distinctly different An23-43 and An44-66, respectively, showing reverse zoning. In addition, Core I contains aligned biotite inclusions and Core II shows sieve texture, resorption surface and patchy zoning. Amphibole inclusions are sporadically enclosed within Core I of Plag2 (Amp1) and mantles of Plag1 and Plag2 (Amp2), but rarely observed in Core II of Plag2. Plag3 is anhedral grain in the matrix and shows core-rim texture with An20-37 in the core. The three plagioclase populations all exhibit angular rims with resembling An9-22. Plag1 core and Plag2 Core II have (87Sr/86Sr)i (0.70920 to 0.71092) similar to the bulk (87Sr/86Sr)i of the mafic dykes intruding the Sanguliu pluton, and likely crystallized from basaltic andesitic magmas. In contrast, the rims of Plag1, Plag2 and Plag3 overall have (87Sr/86Sr)i (0.71391 to 0.71583) nearly identical to the (87Sr/86Sr)i of host monzogranite and the plagioclase in the monzogranite, likely crystallized from granitic magmas. The mantles of Plag1 and Plag2 and the core of Plag3 have (87Sr/86Sr)i (0.71141 to 0.71390) overlapping the (87Sr/86Sr)i of the MMEs, and may have crystallized from mixed melts. Calculation results based on amphibole thermobarometers show that Amp1 crystallized at ~775 °C and ~16 km depth, whereas Amp2 and the amphibole in the matrix of the MMEs and monzogranite crystallized at 730-744 °C and 8-9 km depth. We thus propose that the chemical and textural complexity of the three plagioclase populations in the MMEs can be attributed to that the MMEs may have come from a mushy hybrid layer that was developed through a molten granitic body being recharged by upwelling basaltic andesitic magma. Core I of Plag2 may have nucleated and grown from andesitic magma that was evolved from the basaltic andesitic magma","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":" ","pages":""},"PeriodicalIF":3.9,"publicationDate":"2023-06-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45145423","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 : 2023-06-27DOI: 10.1093/petrology/egad047
M. Lara, R. Dasgupta
Many lines of evidence from high P-T experiments, thermodynamic models, and natural observations suggest that slab-derived aqueous fluids, which flux mantle wedges contain variable amounts of dissolved carbon. However, constraints on the effects of H2O-CO2 fluids on mantle melting, particularly at mantle wedge P-T conditions, are limited. Here we present new piston cylinder experiments on fertile and depleted peridotite compositions with 3.5 wt.% H2O and XCO2 [= molar CO2 / (CO2 + H2O)] of 0.04-0.17. Experiments were performed at 2-3 GPa and 1350 °C to assess how temperature, peridotite fertility, and XCO2 of slab-derived fluid affects partial melting in mantle wedges. All experiments produce olivine + orthopyroxene + 7 to 41 wt.% partial melt. Our new data, along with previous lower temperature data, show that as mantle wedge temperature increases, primary melts become richer in SiO2, FeO* and MgO and poorer CaO, Al2O3 and alkalis when influenced by H2O-CO2 fluids. At constant P-T and bulk H2O content, the extent of melting in the mantle wedge is largely controlled by peridotite fertility and XCO2 of slab-fluid. High XCO2 depleted compositions generate ~ 7 wt.% melt whereas, at identical P-T, low XCO2 fertile compositions generate ~ 30-40 wt.% melt. Additionally, peridotite fertility and XCO2 have significant effects on peridotite partial melt compositions. At a constant P-T-XCO2, fertile peridotites generate melts richer in CaO and Al2O3 and poorer in SiO2, MgO+FeO, and alkalis. Similar to previous experimental studies, at a constant P-T-fertility condition, as XCO2 increases, SiO2 and CaO of melts systematically decrease and increase, respectively. Such distinctive effects of oxidized form of dissolved carbon on peridotite partial melt compositions are not observed if the carbon-bearing fluid is reduced, such as CH4-bearing. Considering the large effect of XCO2 on melt SiO2 and CaO concentrations and the relatively oxidized nature of arc magmas, we compare the SiO2/CaO of our experimental melts and melts from previous peridotite + H2O ± CO2 studies to the SiO2/CaO systematics of primitive arc basalts and ultra-calcic, silica-undersaturated arc melt inclusions. From this comparison, we demonstrate that across most P-T-fertility conditions predicted for mantle wedges, partial melts from bulk compositions with XCO2 ≥ 0.11 have lower SiO2/CaO than all primitive arc melts found globally, even when correcting for olivine fractionation, whereas partial melts from bulk compositions with XCO2 = 0.04 overlap the lower end of the SiO2/CaO field defined by natural data. These results suggest that the upper XCO2 limit of slab-fluids influencing primary arc magma formation is 0.04 < XCO2 < 0.11, and this upper limit is likely to apply globally. Lastly, we show that the anomalous SiO2/CaO and CaO/Al2O3 signatures observed in ultra-calcic arc melt inclusions can be reproduced by partial melting of either CO2-bearing hydrous fertile and depleted peridotites
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