Journal of Petrology最新文献

{"title":"Destruction of the lithosphere beneath the SW margin of the São Francisco Craton evidenced by refertilized and deformed mantle xenoliths","authors":"Luisa Gomes Braga, Tiago Jalowitzki, Fernanda Gervasoni, Rodrigo Antonio Freitas Rodrigues, Maurizio Mazzucchelli, Tommaso Giovanardi, Marina Marques Dalla Costa, Roberto Ventura Santos, Marcelo Peres Rocha, Reinhardt Adolfo Fuck, Georgina Rubiano Lorenzoni, Gustavo Walter Bertotto","doi":"10.1093/petrology/egae087","DOIUrl":"https://doi.org/10.1093/petrology/egae087","url":null,"abstract":"The destruction of the cratonic root has been documented for multiple cratons worldwide and is characterized by severe lithospheric thinning, extensive extensional deformation, and intense thermal activity. Here, we present detailed petrography accompanied by comprehensive geochemical and isotopic data for peridotites, pyroxenites, and eclogites from the SW margin of the São Francisco Craton that has also been severely thinned. The diamond-bearing Canastra-1 kimberlite represents a Cretaceous intrusion from the Alto Paranaíba Igneous Province and hosts garnet-bearing mantle xenoliths from different mantle sources, revealing a complex history of metasomatism/refertilization related to superimposed tectonic events since the cratonic consolidation. Eclogites (T = 978–982°C; P ~ 4.0 GPa) and pargasite-bearing websterites (T = 875–926°C; P = 2.0–3.0 GPa) represent the shallower and colder cratonic lithosphere. Mantle-derived high-MgO eclogitic clinopyroxenes show a light rare earth element (LREE)-enriched pattern (Ce/YbN = 60.90–93.63) while both clinopyroxene and garnet present high 87Sr/86Sr ratios (0.70842–0.70912) and negative εNd values (−5.6 to −7.3). These features, supported by the reconstructed whole-rock composition, suggest a mafic protolith probably metasomatized by fluid/melt derived from the overlying sedimentary rocks. The pargasite websterites are cumulates from an evolved (SiO2-rich) and hydrated basaltic andesitic magma. These xenoliths are characterized by high concentration of LREE relative to heavy-REE (HREE) in clinopyroxenes (Ce/YbN = 10.52–50.61) and pargasite (Ce/YbN = 10.26–57.06), and by the presence of Al-rich orthopyroxene. As observed in eclogites, clinopyroxene and garnet from pargasite websterites display high 87Sr/86Sr ratios (0.70894–0.71094) and strong negative εNd values (−7.2 to −13.3). Trace elements (i.e. Zr, Ti, and Y) in garnets of both rock types indicate the role of a depleted component affected by a metasomatic agent. Although we were unable to date the formation of these rocks or the metasomatic events, we suggest a possible relationship with the consolidation of Gondwana during the Neoproterozoic. Sheared lherzolites, clinopyroxenites, anhydrous websterites, and dunite are deeper fragments from the lithosphere-asthenosphere boundary (LAB: 1191–1290°C; 5.3–5.6 GPa). They have abundant kelyphitic rims around garnet grains, which indicate metasomatism promoted by the percolation of a high-temperature proto-kimberlite melt enriched in Ti, Zr, and Y. The presence of kelyphitic phlogopite with high-Ti-Cr contents reinforces this assumption. LREE-enriched clinopyroxenes (Ce/YbN = 12.06–48.02) confirm the enriched character of the silicate melt responsible for the refertilization process within the cratonic root. The proto-kimberlite metasomatism is further supported by the preferential enrichment of 87Sr/86Sr ratios (0.70560–0.70869) accompanied by positive εNd values (+1.8 to +10.10). The two-point clinopyroxene","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":"25 1","pages":""},"PeriodicalIF":3.9,"publicationDate":"2024-08-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142196744","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Metamorphic origin of large nuggets of platinum-group metals: evidence from multiphase inclusions in Os-Ir-Ru alloys from the adamsfield placer, tasmania","authors":"Anton Kutyrev, Vadim S Kamenetsky, Ivan F Chayka, Nikolai A Nekrylov, Lyudmila Kryuchkova, Vladimir V Shilovskikh, Alkiviadis Kontonikas-Charos, Stepan P Krasheninnikov, Anna Sapegina, Alexei L Perchuk","doi":"10.1093/petrology/egae086","DOIUrl":"https://doi.org/10.1093/petrology/egae086","url":null,"abstract":"Platinum-group elements (PGE) are most effectively concentrated via sulfide-silicate melt immiscibility; however, under sulfide-undersaturated conditions, PGE may exhibit highly-siderophile behaviour and occur as native metals and alloys. In this case, they can form μm-size inclusions within Cr-spinel, but also large nuggets (up to several kilograms), found in chromitites and related placer deposits. The exact formation mechanism of such large nuggets and accumulations of PGE unrelated to sulfide melts remains controversial due to mass balance issues. In this study of multiphase inclusions in Os-Ir-Ru nuggets from the Adamsfield placer district (Tasmania, Australia), we constrain their crystallization environment. Multiphase inclusions comprise variable proportions of hornblende, enstatite, quartz, anthophyllite, anorthite, chlorite and native iridium and coexist with single-phase olivine and Cr-spinel inclusions. The heterogeneity in phase and chemical composition of the inclusions indicates a complex origin from an inhomogeneous source media. This is corroborated by grain scale disequilibrium mineral assemblages, where forsterite and quartz are both included within a single osmium grain. Our proposed multi-stage origin of Adamsfield Os-Ir-Ru nuggets involved magmatic olivine-Cr-spinel-Os-Ir-Ru cumulates in peridotite bodies that were subsequently overprinted by various hydrothermal and metamorphic processes, including serpentinization or growth of Os-Ir-Ru from supercritical fluids. The final metamorphic stage resulted in the obliteration of Os-Ir-Ru zonation, culminating in the current assemblage of inclusions.","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":"30 1","pages":""},"PeriodicalIF":3.9,"publicationDate":"2024-08-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141937137","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Tracing Lower Crustal Contamination in Continental Arc Magmas Using Sr-Nd-Hf Isotopes: a Combined in-Situ and Bulk Rock Approach Applied to the Adamello Batholith","authors":"Manuel Pimenta Silva, Andrea Giuliani, Urs Schaltegger, Massimo Chiaradia, Alexandra Nowak, Blair Schoene, Peter Ulmer, Othmar Müntener","doi":"10.1093/petrology/egae084","DOIUrl":"https://doi.org/10.1093/petrology/egae084","url":null,"abstract":"The incremental construction of plutons characterises magmatic activity in arc settings, where new continental crust is produced. This polyphasic growth entails interactions with one or more crustal components, which modulate the geochemical and isotopic compositions of the newly formed crust. However, the early stages of magmatism are not always preserved due to obliteration by later magmatic pulses. Spatial migration of magmatism during the construction of the Adamello batholith (Northern Italy) enables the examination of the early pulses of pluton formation, thus allowing a time-integrated study of the relative importance of crystallisation-differentiation and contamination in a continental arc setting. We conducted a detailed textural, major and trace element, and Sr isotopic study of plagioclase from the first intrusive pulses of the Adamello batholith, combined with new major, trace element and Sr-Nd isotopic analyses of bulk rock samples across the entire Adamello batholith. We selected well-characterised samples with published CA-ID-TIMS 206Pb-238U ages and Hf isotopic composition for zircons. Strontium isotopes in plagioclase from the same samples were determined by laser ablation multi-collector ICP-MS. The tonalitic samples in the early magmatic stages show elevated but constant Sr isotopic compositions despite large variations in anorthite contents (An90 to An13), indicating that crustal contamination occurred before significant differentiation. Invariant bulk-rock 87Sr/86Sr with variable SiO2 in all superunits of the Adamello batholith further supports contamination preceding significant melt differentiation. Contamination by lower crustal basement lithologies is due to the increasing thermal anomaly triggered by consecutive magmatic injections coupled with the heterogeneous and less restitic nature of the basement in the early stage of the magmatic system (i.e. before consumption of fusible components). In addition, we observe significant variability in crustal contamination proxies (e.g. 87Sr/86Srplag, 87Sr/86Srbulk, εNdbulk, εHfzircon) during the initial phases of magmatism. This variability likely reflects the uneven distribution of positive thermal anomalies in the lower crust during early magmatic stages as well as the diverse lithological and isotopic makeup of the lower crust. The processes identified in our case study are pertinent to continental arc magmatism, particularly where magmas interact with a metapelitic lower crust.","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":"49 1","pages":""},"PeriodicalIF":3.9,"publicationDate":"2024-07-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141885302","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"The Age of Granulite-Facies Metamorphism in the Ivrea-Verbano Zone (NW Italy) Determined through in-Situ U–Pb Dating of Garnet","authors":"Omar Bartoli, Leo J Millonig, Bruna B Carvalho, Horst R Marschall, Axel Gerdes","doi":"10.1093/petrology/egae083","DOIUrl":"https://doi.org/10.1093/petrology/egae083","url":null,"abstract":"Rates of melt production, extraction and crystallization, as well as scales of melt transfer and interaction with their residuum change continuously in migmatite and granulite, affecting the behavior of monazite and zircon as time capsules. Therefore, accessory mineral chronometers may be ambiguous and incomplete in providing an overview of the temperature–time evolution of high-grade metamorphic rocks. In this study, we applied the novel technique of in-situ U–Pb dating of garnet to the archetypal lower continental crust of the Ivrea-Verbano Zone (IVZ), NW Italy. In the IVZ, the temporal relationship between granulite-facies metamorphism and mafic underplating has long been debated, because of the interplay between tectonic, magmatic, metamorphic and metasomatic processes over a period of more than a hundred million years. Garnet from mafic and pelitic granulites yielded U–Pb ages between 287.4 ± 4.9 Ma and 280.1 ± 12.4 Ma, overlapping within uncertainty the time proposed for the emplacement of the Mafic Complex (286–282 Ma). These results indicate that the thermal climax in granulitic rocks was caused by mafic underplating and concomitant asthenospheric upwelling, rather than being inherited from the post-Variscan Carboniferous evolution. Providing robust dating of garnet with as low as 4 ng/g U, this study demonstrates the strength of garnet petrochronology in resolving complex tectono-metamorphic histories of high-grade terranes. It also represents a further step forward towards establishing garnet as part of the in-situ U–Pb geochronology repertoire.","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":"21 1","pages":""},"PeriodicalIF":3.9,"publicationDate":"2024-07-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141885303","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"PROGRESSIVE STRAIN LOCALIZATION AND FLUID FOCUSING IN MANTLE SHEAR ZONES DURING RIFTING: PETROSTRUCTURAL CONSTRAINTS FROM THE ZABARGAD PERIDOTITES, RED SEA","authors":"M Chardelin, A Tommasi, J A Padrón-Navarta","doi":"10.1093/petrology/egae081","DOIUrl":"https://doi.org/10.1093/petrology/egae081","url":null,"abstract":"This article documents the evolution of pressure and temperature conditions and the successive influence of hydrous melts and aqueous fluids on the operation of extensional shear zones, which exhumed mantle slivers from deep lithospheric or asthenospheric depths, in a rift-to-drift setting. These results are based on a re-analysis of 40 samples from three peridotite massifs of Zabargad island in the northern Red Sea. By integrating high-resolution mapping of the microstructure by electron backscattered diffraction with recent developments in barometry for plagioclase-bearing peridotites and thermodynamic modelling of peridotitic compositions, this study: (1) constrains the temporal and spatial evolution of petrological and tectonic processes in the shallow mantle during rifting and (2) documents the presence of melts or aqueous fluids throughout the activity of the shear zones, unravelling substantial feedback between petrological and tectonic processes. Thermobarometry and thermodynamic modelling, constrained by the microstructural observations, document progressive strain localization associated with shearing under decreasing pressure and temperature, from near solidus conditions at >1 GPa (in the north and central peridotite massifs) or ~0.7 GPa (in the southern massif) to < 600°C and <0.3 GPa in all three massifs. The data substantiate local aqueous fluid saturation in the shear zones. This together with higher contents of hydrous minerals in ultramylonites indicate fluid focusing in the shear zones, with seawater ingress extending to >10 km depth. The presence of melts or fluids enabled concurrent dislocation and dissolution-precipitation creep, resulting in weakening of the shear zones. However, fluid supply was spatially heterogeneous and likely intermittent, with equilibrium achieved only locally in the ultramylonites. The present study documents therefore how the feedback between progressive strain localization and fluid-focusing in extensional shear zones contributes to thinning and exhumation of the mantle during continental rifting and the rift-to-drift transition.","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":"2 1","pages":""},"PeriodicalIF":3.9,"publicationDate":"2024-07-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141785691","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"In-situ Lu-Hf garnet dating of Archean deep crust granulites from the polymetamorphic Grenville Front Tectonic Zone","authors":"Antoine Godet, Marine Jouvent, Christiaan Laureijs, Carl Guilmette, Kyle Larson, Mark Coleman, Jérémie Darveau, Myriam Côté-Roberge","doi":"10.1093/petrology/egae080","DOIUrl":"https://doi.org/10.1093/petrology/egae080","url":null,"abstract":"Recent advances in geochronological techniques now allow the ability to efficiently decipher the timing and duration of geological processes in complex high-grade polymetamorphosed orogenic terranes. This is the case of the Grenville Front Tectonic Zone, which truncates the Superior Craton to the southeast. The zone exposes parautochtonous Archean rocks that underwent mid- to high-pressure granulite facies metamorphism of uncertain age. The metamorphic assemblages have been either interpreted as Archean and associated with the final stages of the Superior craton assembly, or as the result of Mesoproterozoic Grenvillian metamorphism, based on cross-cutting relationships and traditional geochronology methods such as U-Pb zircon and 40Ar-39Ar mica dating. Herein, we revisit the extent of the Grenvillian metamorphic overprint in the parautochtonous domain and provide new age constraints for granulite-facies metamorphic assemblages through in-situ garnet dating within migmatitic paragneiss, migmatitic orthogneiss, and mafic granulites, combined with in-situ trace element mapping. Six samples, which show bell-shaped and occasionally sharp and oscillatory lutetium growth zoning in garnet, yield garnet Lu-Hf isochrons with identical Archean dates of c. 2.6 Ga. Sparce analyses of material trend toward Grenvillian ages (c. 1 Ga) in one sample from which garnet shows lutetium zoning consistent with post-growth fluid-assisted disturbance. Overall, our results indicate that the widespread granulite-facies metamorphism within the Grenville Front Tectonic Zone is dominantly late Neoarchean in age, unveiling a rare exposure of Archean lower crust in the southeastern Superior Craton. Our results also point towards a limited Grenvillian metamorphic overprint though the spatial extent and precise thermal conditions of this metamorphism are still unknown. The results presented herein demonstrate the potential of in-situ isotopic geochronology on rock-forming minerals like garnet in polymetamorphic terranes.","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":"63 1","pages":""},"PeriodicalIF":3.9,"publicationDate":"2024-07-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141742541","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"The crystallization of continental flood basalt (CFB) lavas: insights from textural studies","authors":"Aristle Monteiro, Tushar Mittal, Raymond Duraiswami, Stephen Self","doi":"10.1093/petrology/egae082","DOIUrl":"https://doi.org/10.1093/petrology/egae082","url":null,"abstract":"Continental flood basalts provinces are the product of the largest known volumetric eruptions on Earth (~104 km3), with individual flow fields commonly covering well over 10 000 km2 with a mean lava thickness of over 5 m. Studies focusing on the emplacement style of such lava flows have relied extensively on morphological observations and comparisons with modern lava flows and experimental analogs. In the present study, we compare the textures of flood basalt lavas with those from different eruption settings all over the world using data collected from pre-existing literature to gain detailed insights into the style of eruption. Comparison of crystal size distribution (CSD) data indicates that the eruption style of CFBs is similar to those of modern-day fissure eruptions (e.g. Iceland). This matches inferences based on observations of morphology. We also use a 1D thermal model to estimate the depth dependent cooling rates within a single lava lobe and test the validity of assumptions built into the formulation of these models. The results reveal that, on average, flood basalt lavas need to conductively cool much faster than we would expect (up to order of ~102 times faster) to match the textural observations. The model is also frequently unable to replicate the observed depth-wise relative variations in length with depth for CFB lavas. Furthermore, the calculated cooling rates from crystal shapes also do not match those calculated from crystal lengths, indicating the assumptions in cooling flow models need to be modified for large CFB flow fields. Given the large areas of CFB flow-fields and the relatively long eruption times inferred for the emplacement of individual flow fields, we hypothesize that inflation of lobes and formation of new lobes via breakouts combined with variable eruption rates are key processes that are missing when modeling the cooling of these flow fields. Accounting for these processes is essential to derive accurate cooling rates, which is important to better understand the environmental impact CFBs have at the time of emplacement.","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":"166 1","pages":""},"PeriodicalIF":3.9,"publicationDate":"2024-07-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141778826","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Experimental Constraints on the Storage Conditions and Differentiation of High-Ti Basalts from the Panzhihua and Hongge Layered Intrusions, SW China","authors":"Dachuan Wang, Tong Hou, Roman Botcharnikov, Sarah Haselbach, Florian Pohl, Renat R Almeev, Andreas Klügel, Meng Wang, Jingyi Qin, Zhaochong Zhang, Francois Holtz","doi":"10.1093/petrology/egae078","DOIUrl":"https://doi.org/10.1093/petrology/egae078","url":null,"abstract":"High-Ti basalts are commonly believed to represent parental magmas leading to the formation of mafic-ultramafic layered intrusions, such as Late Permian Panzhihua and Hongge in the Emeishan Large Igneous Province (SW China). Consequently, elucidation of the crystallisation and crustal differentiation of high-Ti basalts is critical for our understanding of the petrogenesis of these layered intrusions and the associated oxide ore mineralisation. Here, we present the results of crystallisation experiments carried out in internally heated pressure vessels using a primitive high-Ti basaltic composition. The experiments were conducted at 100 and 300 MPa, in the temperature interval of 950-1200 °C and with water activities (aH2O) from 0 to 1. The oxygen fugacity (fO2) was controlled and varied from FMQ - 1 to FMQ + 3.3 log units (FMQ corresponds to fayalite-quartz-magnetite buffer). The main mineral phases are olivine, clinopyroxene and plagioclase, accompanied by Cr-Fe-Ti-oxides, orthopyroxene, apatite and amphibole, depending on the conditions. Redox conditions primarily influence the stability fields of Cr-Fe-Ti oxides. Clinopyroxene, orthopyroxene and amphibole are pressure-dependent and have larger stability fields under high pressure conditions. The olivine→orthopyroxene and olivine→amphibole peritectic reactions are observed. Comparisons of phase equilibria between this study and experiments conducted with parental magma of Skaergaard layered intrusion demonstrate the effect of bulk system composition. For instance, ilmenite crystallisation is determined by intrinsic parameters such as fO2 but also by additional compositional parameters (e.g., melt Ti, Fe, Al and Mg content). Although COMAGMAT and MELTS modelling results generally reproduce the crystallisation sequence, only the stability field of clinopyroxene and its composition are perfectly modelled. The comparison of experimental results with the rocks from the lowest units of Panzhihua and Hongge layered intrusions are applied to constrain storage conditions in the magma reservoirs. Compared to Hongge, we conclude that the Panzhihua magma chamber was probably located at a shallower depth (~3-6 km), that magma crystallisation started at lower temperatures (~1125-1100 °C), higher fO2 (~FMQ + 1 to FMQ + 2), and that its initial melt H2O content was lower (~0.5-1 wt.%).","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":"35 1","pages":""},"PeriodicalIF":3.9,"publicationDate":"2024-07-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141588020","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"The role of latent heat buffering in the generation of high-silica rhyolites","authors":"L Tavazzani, R C Economos, S Peres, G Demarchi, S Sinigoi, O Bachmann","doi":"10.1093/petrology/egae072","DOIUrl":"https://doi.org/10.1093/petrology/egae072","url":null,"abstract":"The physical process of crystal-melt separation is responsible for the accumulation of small to very large volumes (>100 km3) of eruptible rhyolitic melt in the shallow crust. Granitic intrusions, although providing a terminal, time-integrated image of melt segregation processes, host an unmatched record of the physical properties controlling mechanisms and rates of interstitial melt extraction from a crystal-rich source. We applied mass balance calculations and thermodynamic modeling simulations to an extensive bulk rock geochemistry dataset (>150 samples) collected in a Permian upper-crustal granitoid intrusion of the Italian Southern Alps. Textural and geochemical evidence indicate that this intrusion constituted a single, zoned magma body, with a crystal-rich base and a thick (~2 km), high-silica cap (75-77 wt% SiO₂). The large compositional variability of the crystal-rich materials (cumulate) suggests variable degrees of melt extraction efficiency and corresponding terminal porosities. Specifically, the loosely bimodal distribution of porosity values (φ) indicates that at least two distinct melt segregation mechanisms were operating in this system, which produced both high (0.65-0.45) and low terminal porosities (0.45-0.25) in the cumulate materials. Modeling of latent heat budget shows that coexistence of cumulate products with differing terminal porosity signature can be explained by melt segregation processes taking place at different depths across a thick, interconnected magmatic reservoir with an initial homogenous water content (~4 wt% H2O). Deep in the mush column, low water activities (aH₂O<0.5) promoted thermal buffering of cooling magma at high crystallinities, enabling residual melt extraction by percolation through a crystalline framework accompanied by compaction. Instead, at shallower depths, high water activities (aH₂O>0.5) ensured prolonged magma residence at porosities that promoted crystal melt separation via hindered settling. Distinct melt extraction processes, acting synchronously but at different depths in vertically extensive silicic mush columns, can account for the large volumes of residual, haplogranitic melt mobilized during the relatively short lifespan of upper crustal magma reservoirs (~105 years).","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":"157 1","pages":""},"PeriodicalIF":3.9,"publicationDate":"2024-06-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141507830","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Bulk rock and olivine chemistry and isotopes of 106-58 Ma basalts from Liaodong Peninsula and its adjacent areas: Implications for secular evolution of the big mantle wedge in eastern China","authors":"Chao Zhang, Qiang Ma, Lu-Bing Hong, Hong-Kun Dai, Liang Ma, Yi-Gang Xu, Jian-Ping Zheng, Hui-Ying Cao","doi":"10.1093/petrology/egae071","DOIUrl":"https://doi.org/10.1093/petrology/egae071","url":null,"abstract":"The subducting Pacific slab stagnates in the mantle transition zone and creates a big mantle wedge (BMW) system in East Asia. A similar BMW structure may have already existed since the Early Cretaceous (>120 Ma), but how such a structure evolved from Early Cretaceous to the present day remains unclear. We address this issue by comparing compositions and source heterogeneity of the 106-58 Ma basalts from Liaodong Peninsula and its adjacent areas (LPAA) in eastern China, with those formed in the modern BMW setting. The LPAA basalts display oceanic island basalts-like trace element patterns. Elemental and isotopic compositions of these basalts and their olivine phenocrysts point to peridotite and two recycled components in their source. One recycled component is altered lower oceanic crust given the low δ18Oolivine (2.8-5.2‰) of the ~99 Ma Liaoyuan alkali basalts. The second component consists of altered upper oceanic crust and pelagic sediments indicated by high δ18Oolivine (>6.0‰), represented by the ~58 Ma Luanshishanzi alkali basalts. The depleted mantle-like isotopes of these two components suggests derivation from a young HIMU source with characteristics of the Izanagi plate (e.g., Indian Ocean-type Sr-Nd-Pb-Hf isotopes), which may have resided in the mantle transition zone at that time. Our results reveal strong similarities between chemical and source characteristics of the mantle sampled by the 106-58 Ma LPAA basalts and those derived from the modern BMW. This implies that the BMW structure has been present since the Early Cretaceous, probably having lasted more than 120 Myr, and modulating the chemical properties of the upper mantle and influencing a variety of geological processes.","PeriodicalId":16751,"journal":{"name":"Journal of Petrology","volume":"88 1","pages":""},"PeriodicalIF":3.9,"publicationDate":"2024-06-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"141507831","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}