Primary Magmas Research Articles

Estimating Primary Magmas From Mars With PRIMARSMELT: Implications for the Petrogenesis of Some Martian Rocks and the Thermal Evolution of Mars

AbstractPrimary magmas form by partial melting in the mantle of a terrestrial planet and represent the starting material for building its crust. The compositions of primary magmas are critical for understanding the thermal history of planetary interiors, as they can be used to estimate mantle potential temperatures (TP) and track changes in the conditions of mantle partial melting over time. Here, we introduce PRIMARSMELT, a new member of the PRIMELT software family, calibrated to estimate the composition of Martian primary magmas and their formation conditions. We applied PRIMARSMELT to a comprehensive database of basaltic compositions from Mars. Our results are consistent with their petrology, requiring olivine addition to restore fractionated compositions to their primary parents and olivine subtraction from cumulate rocks. Individual primary magma solutions provide insights into the petrogenesis of specific Martian meteorites, with implications for the near‐primary nature of some primitive meteorites and the relationship between lithologies A and B in meteorite EETA 79001. Taken together, our results suggest nearly constant or potentially increasing mantle potential temperatures throughout the geological history of Mars. The average TP for young shergottite meteorites is ∼1,442 ± 40°C, similar to ambient mantle temperatures inferred from geophysical models. In contrast, older basaltic rocks record potential temperatures as low as ∼1,320 ± 48°C for igneous clasts in meteorites NWA 7034/7533. We suggest that, rather than plume‐related magmatism, shergottite meteorites record ambient mantle temperatures, with the thermal evolution trend possibly resulting from inefficient heat loss, as expected for a planet in stagnant‐lid mode.

Parental magma and mantle source compositions of chromitites in the Mesoarchean Nuggihalli greenstone belt, India: Evidence for Archaean subduction zone magmatism

We present comprehensive petrological and geochemical analyses of chromitites from the 3.3-3.1 Ga Nuggihalli greenstone belt in Western Dharwar Craton, India. Our findings provide insights into the primary magma and mantle source compositions associated with these Archean oceanic lithosphere chromitites. Major and trace element data from chromites and clinopyroxenes in the chromitites, combined with thermobarometry and hygrometry calculations, and numerical modeling, indicate that the Nuggihalli chromites are characterized by high-Cr chromites (Cr# = 68.1-78.6) with low TiO 2 (0.19-0.29 wt. %), Al 2 O 3 (9.10-13.13 wt. %) and Ga (4.24-10.37 ppm). These chemical signatures are comparable to those of chromites from high-Cr podiform chromitites and boninites. Clinopyroxenes are mainly augites with high Mg# (93-95). Equilibrated melts with the clinopyroxenes from the Nuggihalli chromitites have high Mg# values (79-86), similar to ancient komatiitic or picritic magmas (Mg# = 75-90). Thermobarometry and hygrometry calculations indicate that the clinopyroxenes likely formed at temperatures of 1194-1221℃ and pressures of 0.7-5.8 kbar with high water contents (∼2.4-2.6 wt. %), exceeding water contents of mid-ocean basalts (MORB) and oceanic island basalts (OIB). Numerical modeling of rare earth elements indicates moderate degrees of partial melting (1% to ∼10%) of a hydrated spinel lherzolite mantle source (water content: ∼478-5408 ppm). The parental magmas of the 3.3-3.1 Ga Nuggihalli chromitites are characterized by enrichment of water contents (∼2.4-2.6 wt. %) and depletion of high field strength elements (HFSEs, e.g., Nb and Hf), which are similar to boninites and arc magmas. Furthermore, the rock associations, including ultramafic rocks with chromitites, high-Mg basalts, and sheets of gabbro-anorthosites in the Nuggihalli greenstone belt, are similar to the rock assemblages of ophiolites, which could represent a relic of a Mesoarchaean oceanic lithosphere formed in a forearc setting.

Petrological Characteristics and Physico-Mechanical Properties of Dokhan Volcanics for Decorative Stones and Building Material Applications

Wide varieties of igneous rocks are extensively utilized as stones for decoration purposes and as a potential source for building. With the use of petrological (mineralogical and chemical) and physico-mechanical analyses, the current work accurately mapped the Dokhan Volcanics (DV) and utilized them as decorative stones and their prospective in building materials using Frattini’s test. Field observations indicate that metavolcanics, DV, and monzogranites are the principal rock units exposed in the studied area. The DV rocks are characterized by a dense series of stratified, rhyolitic to andesitic lava interspersed with a few pyroclastics. Andesite, andesite porphyry, dacite, and rhyolite are the primary representatives of the selected DV. The lack of infrequent appearance of mafic units in the current volcanic eruptions indicates that the primary magma is not mantle-derived. This is supported by their Mg# (17.86–33.57). Additionally, the examined DV rocks have Y/Nb ratios above 1.2, suggesting a crustal source. The role of fractionation is interpreted by their variation from andesite passing through dacite to rhyolite, which is indicated by gradual negative distribution groups between silica and TiO2, Fe2O3, CaO, MgO, Co, and Cu from andesite to rhyolitic lava. Additionally, a wide range of widely used DV rocks like Y/Nb, Rb/Zr, and Ba/Nb point to crustal contamination in the rhyolitic rocks. The partial melting of the lower crust can produce andesitic magma, which ascend to higher crustal levels and form lava of calc-alkaline. A portion of this lava may split, settle at shallow crustal depths, and undergo differentiation to create the DV rocks. Based on the results of physico-mechanical properties, the studied samples met the requirements for natural stone to be used as decorative stones, whether as interior or exterior installations. The pozzolanic assessment of the studied rocks revealed their usability as supplementary cementitious materials in the building sector.

Origin of the Oligocene–Miocene Sailipu ultrapotassic volcanic rocks in southern Tibet: Melting of Asian mantle pyroxenites triggered by eastward tearing of the subducting Indian continental slab

Widely distributed Oligocene−Miocene ultrapotassic volcanic rocks in the Lhasa terrane of southern Tibet have been associated with the melting of the lithospheric mantle, plateau uplift, and porphyry Cu-Au mineralization. This study presents the mineral chemistry of olivine and clinopyroxene phenocrysts, whole-rock major and trace element data, and zircon U-Pb geochronological and Hf isotopic data for the Sailipu primitive ultrapotassic volcanic rocks. The Sailipu volcanic rocks exhibit high MgO (5.6−11.4 wt%), Cr (386−981 ppm), Co (22−43 ppm), and Ni (95−423 ppm) concentrations and have highly fractionated rare earth elements [REEs; (La/Yb)N = 23−73] and high-Fo (89.1−90.8) olivine phenocrysts containing elevated NiO (up to 0.59 wt%), which suggests a pyroxenitic mantle source that partially melted in the garnet stability field. Their high K2O contents (4.8−8.0 wt%) and global subduction sediment-like trace element patterns suggest that the metasomatic agents, which reacted with mantle peridotites to form phlogopite-bearing pyroxenites, were dominantly derived from the melting of subducted continental sediments. Their high whole-rock Ba/La and Th/Nd ratios are consistent with this hypothesis. The Sailipu ultrapotassic volcanic rocks also exhibit low initial 176Hf/177Hf ratios that resemble those of Himalayan leucogranites, and high Ca contents in olivine phenocrysts, which is consistent with contributions from the subducted carbonate-rich sedimentary strata on top of the thinned Greater Indian continental crust. The zircon U-Pb chronological data yielded concordant ages of 24.33 ± 0.19 Ma, 21.20 ± 0.62 Ma, and 17.05 ± 0.31 Ma for different exposures of the Sailipu volcanic rocks, which establishes a maximum age of ca. 24 Ma for these rocks. The northwest−southeast spatial distribution and the southeastward decrease in age (80°E−90°E) suggest west-to-east tearing of the thinned Greater Indian slab, which caused asthenospheric upwelling and melting of the Tibetan lithospheric mantle. Geothermometric calculations show relatively high primary magma temperatures (∼1250 °C) that are consistent with asthenospheric upwelling. We propose a mechanism that could genetically link the coeval Cu-Au ore-forming granitoids with the ultrapotassic magmatism of the Gangdese belt. The ultrapotassic rocks supply a large-volume of external magmatic volatiles, particularly H2O, which could trigger melting of the Tibetan lower crust and lead to the generation of the ore-forming granitoids and the establishment of oxidizing conditions for porphyry deposits. The oxygen fugacity (log ƒO2 values of ΔFMQ) of the primitive Sailipu ultrapotassic volcanic rocks (ΔFMQ = 0.48 ± 0.51 based on the Dol/melt V oxybarometer and ΔFMQ = 0.33 ± 1.19 according to the magmatic zircon U-Ce-Ti oxybarometer) is slightly lower than that of porphyry Cu-Au ore-forming granitoids in the eastern Gangdese (ΔFMQ = +0.8 to +2.9), which suggests that the direct injection of ultrapotassic melts into ore-forming granitoids played a limited role in changing oxygen fugacity, but more oxidized fluids/volatiles exsolved from these ultrapotassic melts may have facilitated the remelting of sulfide-bearing lower crust and/or directly scavenged sulfides from the mush-state reservoirs of the ore-forming granitoids in the middle−upper crust.

Formation of the Lianhuashan Cu deposit in the southern Great Xing’an Range, NE China: Constraints from fluid inclusions, whole-rock geochemistry, zircon U–Pb geochronology, and H–O–S–Pb isotopes

The medium-sized Lianhuashan Cu sulfide deposit is located in the southern Xing’an Range of Inner Mongolia, NE China. The zoned massive sulfide vein ores are hosted mainly in the Permian Dashizhai Formation, and the ore veins are controlled by NW–NNW-trending structures. The ore-forming process comprises four stages: arsenopyrite–quartz (I); chalcopyrite–pyrite–quartz (II); pyrite–chalcopyrite–sphalerite–galena–quartz (III); and ore-barren sulfide–quartz–carbonate (IV). Three types of fluid inclusions (FIs), namely vapor-rich two-phase (LV-type), liquid-rich two-phase (VL-type), and daughter mineral-bearing three-phase (SL-type), are distinguished. Stage I, II and III contain all types of FIs (LV-, VL-, and SL-type), with homogenization temperatures (Th) of 268–462°C, 230–382°C and 180–340°C and salinities of 3.4–52.3, 3.4–44.5 and 3.7–39.9 wt% NaCl eqv., respectively, whereas stage IV has only VL-type FIs, with Th = 152–232°C and salinity = 3.4–7.9 wt%. Fluid geochemical data show that the Lianhuashan ore fluids were of medium–high temperature (236–382 °C), high-salinity (31.5–44.5 wt%), and relatively oxidizing conditions, typical of a NaCl-H2O system. The microthermometric and H–O isotope data (δ18OH2O = − 9.0 to 6.1 ‰; δD = − 149.0 to − 99.0 ‰) indicate that the ore fluids were initially magmatic with later meteoric water incursion. The sulfide S (δ34S = − 2.9–3.8 ‰) and Pb (206Pb/204Pb = 17.954 − 18.492, 207Pb/204Pb = 15.427 − 15.739, 208Pb/ 204Pb = 37.815 − 38.357) isotopes support that the metals were magmatic-derived. Fluid boiling, cooling, and meteoric water mixing were likely the main ore precipitation mechanism at Lianhuashan.We suggest that Cu minerals at Lianhuashan were precipitated with boiling at ∼ 1km depth. For the ore-forming granodiorite porphyry (zircon U-Pb age: 252.8 ± 1.8 Ma), geochemical data indicate that the primary magma was formed by partial melting of the thickened or delaminated lower crust. Integrating the available age, geological, and geochemical evidence, we suggest that mineralization at Lianhuashan is spatial–temporal and genetically associated with the granodiorite porphyry, and was formed in a volcanic arc setting after the Paleo-Asian Ocean closure.

Petrology of Granitoids of the Mayorka Massif (Gorny Altay): Contribution to the Problem of High-Silica Magma Formation

Abstract —The paper presents data from a comprehensive study of granitoids identified in the Mayorka intrusion that is located in the western part of the Altai Mountains. It is shown that the massif is composed of rocks of four intrusive phases, the age of these rocks ranges from 391 to 372 Ma, and the intrusion of the main volume of granitoids dates back to a relatively short interval of 386–384 Ma. The massif contains rocks of two geochemical types. The first type is differentiated calc-alkaline granite-leucogranites with near-clark contents of high-field-strength elements and rare earth elements: ɛNd(T) = + 4.3…+ 4.5 and σ18O V-SMOW = +10.7…+11.2 ‰. The second is alkaline and moderately alkaline A-type alaskites, strongly enriched in high-field-strength elements and rare earth elements, having ɛNd(T) + 5.3 and σ18O V-SMOW = +11.6 ‰. Granitoids of the first group are of crustal source, while the rocks of the second group contain a significant portion of mantle material. The near-simultaneous introduction of these melts to the level of formation of the intrusion caused their interaction and the formation of hybrid magmas. Low crystallization temperatures of granitoids (<700 °C) and the presence of syngenetic melt and fluid inclusions in most rock varieties indicate a high fluid saturation of the melts. The abundance of leucogranites, whose geochemical characteristics cannot be explained from the standpoint of shallow differentiation of primary magmas, indicates the leading role of fluid-magmatic interaction processes in the formation of high-silica magmas.

Petrogenesis and an Evaluation of the Melting Conditions of the Late Permian ELIP Picrites, SW China: Constraints Due to Primary Magma and Olivine Composition

The late Permian Emeishan large igneous province (ELIP) in SW China is a melting product of the Emeishan mantle plume. Recently, it has been debated whether peridotite or pyroxenite is the dominant lithology of the mantle source in the ELIP. To address this, systematic analyses of bulk-rock and coexisting spinel and olivine compositions were conducted on picrites from Lijiang–Yongsheng, Dali–Binchuan, Yumen, Muli, and Ertan. The ELIP picrites exhibit positive TiO2–CaO and negative MgO–CaO correlations, as well as low FC3MS values (−0.24–0.1), supporting a peridotite-dominated mantle source. This lithology of the mantle source is also supported by the high 100 × Mn–Fe (1.43–1.73) and Mn–Zn (13.6–18.4) values but low 10,000 × Zn–Fe (8.0–12.7) ratios of the olivine phenocrysts. The estimated mantle potential temperature for Lijiang, Yongsheng, Yumen–Ertan, Muli, and Dali–Binchuan picrites decreased away from Lijiang and Yongsheng, suggesting that the Lijiang and Yongsheng areas were the center of the ELIP. The Lijiang–Yongsheng primary magma shows similar SiO2 content but lower Al2O3 contents (average of 8.24 wt.%) and higher MgO contents (average of 21.42 wt.%) than those of Dali–Binchuan primary magma (Al2O3: 9.86 wt.%; MgO: 19.02 wt.%). Also considering the high Gd–Yb (average of 3.05) and La–Yb (average of 14.61) ratios and mantle potential temperature (average of 1599 °C), we proposed that Lijiang–Yongsheng lavas are produced via the melting of a garnet–peridotitic mantle. In contrast, the Dali–Binchuan lavas with low Gd–Yb (average of 1.91) and La–Yb (average of 5.88) ratios can be explained by their formation in the garnet–spinel transition zone of a peridotitic mantle. The Yumen–Ertan primary magma displays similar mantle potential temperature (average of 1600 °C), Al2O3 and FeO content, and Gd–Yb ratios to those of Lijiang–Yongsheng lavas, indicating that YumenvErtan primary magma may be attributed to the partial melting of garnet with minor peridotite. Therefore, heterogeneous plume-head mantle sources lead to the evaluation of melting conditions of the late Permian ELIP picrites.

Petrogenesis of the Deccan high-Mg basalts and picrites

Tholeiitic basalts and picrites from the Deccan Traps were used to constrain the pressure and temperature conditions of mantle melting for their origin. Clinopyroxene thermobarometry indicates that all Deccan tholeiites crystallized at low pressures in the upper crust (< 6 kbar/1047–1221 °C). In comparison, the Deccan alkalic rocks crystallized at pressures up to ~ 12.7 kbar. Rare samples of the tholeiites plot on their low-pressure olivine-plagioclase-clinopyroxene (Ol-Pl-Cpx) cotectic boundaries or olivine control lines in phase diagrams. These samples represent unmodified magmatic liquids. Primary magmas of the basalts that plot on their cotectic boundaries were modeled through reverse fractionation by incrementally adding equilibrium Ol + Pl + Cpx, Ol + Pl and Ol ± spinel, until the liquid was multiply saturated with lherzolite at a high pressure. The high-Mg basalts are contaminated with continental crust. Hence, a crustal partial melt was simultaneously subtracted according to energy constraints at each reverse fractionation step for these samples. The results show that the high-Mg basalts are 41–53% fractionated and 1–6% contaminated, and the low-Mg basalts are 63–67% fractionated. Their primary magmas were last equilibrated with spinel lherzolite at 10–13 kbar/1289–1333 °C. A picrite and two very high-Mg basalts plot on their olivine control lines. So, their primary magmas were calculated by adding only equilibrium olivine. These samples are 9–25% fractionated, and their primary magmas were last equilibrated with garnet lherzolite at 25–36 kbar/1452–1531 °C. The estimated mantle potential temperatures are 1400–1500 °C for the Deccan tholeiites, consistent with their origin from a mantle plume.

Trace element partitioning between minerals and melt: parameterization of experimental data for olivine, pyroxenes, and feldspars

Experimental data were analyzed on the partitioning of trace elements between minerals (olivine, orthopyroxene, clinopyroxene, and feldspars) and silicate melts. The data span over a wide range of conditions (from 1 atm to 10 GPa and ~1000–2000 °C) and compositions of melt (from ultrabasic to ultrasilicic) and minerals. The dependence of logarithms of partition coefficients (lnDi ) on conditions and compositions was approximated by linear functions of 1/T, P/T (P is pressure, and T is absolute temperature) and compositional parameters of mineral and melt. It was found that Di /Dj values for a number of element pairs are independent of experimental parameters and vary within narrow ranges. The coefficients of Di dependencies on conditions and compositions were estimated by minimization of quadratic deviations of model Di and Di /Dj values from experimental values. The obtained equations can be used to calculate Di values for a number of elements with an accuracy of approximately a factor of 1.2–2.0. The obtained values were applied as an example for a model of formation of mafic melts of mid-ocean ridges through melting of a peridotite source and crystallization of primary magmas under crustal conditions.

Unraveling the Magmatic-to-Carbothermal Processes in the Ba-Sr-REE Mineralization of the Sevattur Carbonatites, India

Abstract The Neoproterozoic Sevattur carbonatite complex (India) consists of calcite carbonatite, silicate-poor- and silicate-rich (banded and blue) dolomite carbonatite. The trace element composition of the calcite and ferroan dolomite shows that most of them have magmatic characteristics. The predominance of dolomite carbonatite over calcite carbonatite and the absence of a genetically related silicate suite at Sevattur suggest a dolomitic parental magma derived by the direct melting of the mantle. Regardless of the prevalent magmatic character of the Sevattur carbonatites they host a plethora of Ba-Sr-REE minerals formed throughout the orthomagmatic-to-carbothermal stages under varying activities of ${\mathrm{F}}^{-}$,${\mathrm{CO}}_3^{2-}$, ${\mathrm{PO}}_4^{3-}$and ${\mathrm{SO}}_4^{2-}$. The orthomagmatic stage begins with the crystallization of calcite carbonatite containing xenocrystic diopside, antecrystic Y-bearing calcite (Cal-I), and primary benstonite, apatite, and richterite in a Sr-Mg-rich calcite (Cal-II) matrix. Fractionation of apatite, carbocernaite, and cordylite-(Ce) together with the enrichment of Mg, Fe, Na, and F in late-magmatic stages, forms the dolomite carbonatite. The decomposition of benstonite to norsethite, Ba-Ca carbonate, and strontianite occurs within dolomite carbonatite at the same stage with minimal wall-rock assimilation. Interaction between the xenocrystal clinopyroxenite and the primary magma during late-magmatic stages has resulted in the formation of aegirine and phlogopite in calcite carbonatite. Similarly, the prolonged assimilation of clinopyroxenite by the evolving magma leads to the formation of magnesio-riebeckite and magnesio-arfvedsonite within the banded- and blue carbonatites. The increasing activities of ${\mathrm{PO}}_4^{3-},{\mathrm{SO}}_4^{2-}$and H2O during the carbothermal stage, destabilizes precursor carbocernaite and cordylite-(Ce), resulting in an assemblage consisting of synchysite-(Ce), ancylite-(Ce), daqingshanite, hydroxylbastnäsite-(Ce), ferriallanite-(Ce), monazite-(Ce), and baryte in dolomite carbonatite. In the banded carbonatite, the carbothermal stage is characterized by ferri-winchite, tremolite-actinolite, and phlogopite crystallization. Further contamination and differentiation of the carbothermal fluid have resulted in the crystallization of quartz and albite in blue carbonatite. Phlogopitization within the blue carbonatite and clinopyroxenite results from late K enrichment in the carbothermal fluids.

Crystals and melt inclusions record deep storage of superhydrous magma prior to the largest known eruption of Cerro Machín volcano, Colombia

Abstract Cerro Machín, a volcano located in the northern segment of the Andes, is considered one of the most dangerous volcanoes in Colombia with an explosive record that involves at least five plinian events. Prior studies focused on the last dome-building eruption have suggested the presence of a water-rich mid-crustal magma reservoir. However, no direct volatile measurements have been published and little work has been completed on the explosive products of the volcano. Here, we study the largest known eruption of Cerro Machín volcano which occurred 3600 yr BP producing dacitic pyroclastic fall deposits that can be traced up to 40 km from the vent. Lapilli pumice clasts have a mineral assemblage of plagioclase, amphibole, quartz, and biotite phenocrysts, with accessory olivine, Fe-Ti oxides, and apatite. The occurrence of Fo89-92 olivine rimmed by high Mg# amphibole and the established high-water contents in the magma imply the presence of magma near or at water saturation at pressures &amp;gt;~500 MPa. Measurements of up to 10.7 wt% H2O in melt inclusions hosted in plagioclase and quartz in the 3600 years BP eruption products support the idea that Cerro Machín is a remarkably water-rich volcanic system. Moreover, this is supported by measurements of ~103 – 161 ppm H2O in plagioclase phenocrysts. The application of two parameterizations of water partitioning between plagioclase and silicate melt allows us to use our water in plagioclase measurements to estimate equilibrium melt water contents of 5 ± 1 – 11 ± 2 wt% H2O, which are in good agreement with the water contents we measured in melt inclusions. Results of amphibole geobarometry are consistent with a magma reservoir stored in the mid-to-lower crust at a modal pressure of 700 ± 250 MPa, corresponding to a depth of ~25 km. Minor crystallization in the shallow crust is also recorded by amphibole barometry and calculated entrapment pressures in melt inclusions. Amphibole is present as unzoned and zoned crystals. Two populations of unzoned amphibole crystals are present, the most abundant indicate crystallization conditions of 853 ± 26 °C (1 se; standard error), and the less abundant crystallized at an average temperature of 944 ± 24 °C (1 se). Approximately 18% of the amphibole crystals are normally or reversely zoned, providing evidence for a minor recharge event that could have been the trigger mechanism for the explosive eruption. Plagioclase crystals also show normal and reverse zoning. The moderate Ni concentrations (&amp;lt;1600 μg/g) in the high-Fo olivine xenocrysts suggest that Cerro Machín primary magmas are generated by inefficient interaction of mantle peridotite with a high-silica melt produced by slab melting of basaltic material. Some sediment input is also suggested by the high Pb/Th (&amp;gt;2.2), Th/La (0.3 – 0.4), and low La/Th (&amp;lt;13; relative to mantle array) ratios. Whole rock chemistry reveals heavy rare earth element (HREE) depletion and Sr enrichment that likely formed during the crystallization of garnet and amphibole in the upper part of the mantle or lower portion of the crust, promoting the formation of water-rich dacitic magma that was then injected into the middle-to-lower crust. Textural and compositional differences in the crystal cargo that erupted during dome-building and plinian events support the idea that large volumes of magma recharge lead to effusive eruptions, while only small recharge events are needed to trigger plinian eruptions at Cerro Machín.

Plate tectonics through Earth’s history: constraints from the thermal evolution of Earth’s upper mantle

ABSTRACT Temporal changes in Earth’s tectonic style play a crucial role in understanding the planet’s evolution. Modern-style tectonics is characterized by the formation of basaltic crust at divergent plate boundaries and its subsequent recycling at subduction zones, accompanied by wedge mantle formation and arc magmatism. It is commonly believed that the secular cooling of the mantle modified the tectonic style from stagnant lid or heat pipe on an early hotter Earth to horizontal tectonics during Meso-Neoarchean. However, various field, petrographic, and geochemical studies suggest that the onset of plate tectonics ranges from Hadean to Neoproterozoic. In this study, we re-evaluate the primary magma temperature (mantle potential temperature, Tp) of the upper ambient mantle, spanning from Eoarchean to Neoproterozoic. We used basalts from several Archean and Proterozoic greenstone belts worldwide, along with Proterozoic ophiolites, to (re)calculate the Tp using the FRACTIONATE-PT method. We observed a Tp range of 1444–1639°C during the Archean and 1414–1611°C during the Proterozoic. These findings indicate a strong correlation with previously estimated Tp values obtained from PRIMELT3 method. This further highlights strong internal consistency among different methods and supports models of a hot ambient mantle during the Archean and Proterozoic. We further reviewed numerical models regarding the effect of mantle temperature on the viability of early Earth plate tectonics. Such model results, composition of Archean continental crust, recent crustal growth models, and field evidence are consistent with the operation of plate tectonics on an early hotter Earth. The transition from a hotter mantle to a colder one from Eo-Neoarchean resulted in thinner oceanic lithosphere and a less depleted lithospheric mantle. Subduction of this thinner oceanic lithosphere led to modern-style tectonics. The intensity and style of plate tectonics on the early Earth differed from modern tectonics, which emerged during the Neoarchean.

Diverse magmatism along the northern margin of Tarim during the Ediacaran: Transition from Rodinian dispersing to Gondwana assemblage

The Ediacaran igneous rocks and sedimentary sequences play a key role for our understanding the transition from dispersing of the Rodinian continents to the assemblage of the Gondwana. On the other hand, some continental massifs at the marginal area of the Rodinia and Gondwana could preserve solid evidences for deciphering the geodynamic background from the break-up of Rodinia to the assemblage of Gondwana. In this contribution, we report field observations, petrography, ages and systematic geochemistry of the diverse Ediacaran igneous activities in the northern margin of the Tarim Block, which was favored as a marginal continental massif in the Rodinia configuration by most geologists. The Ediacaran igneous rocks along the northern margin of Tarim include the Baicheng and Yangxia granites, Tiemenguan syenite, Kurle mafic dykes and the basalt layers in the Zamoketi Formation (northeastern Tarim) and the Sugetbrak Formation (northwestern Tarim). Precise zircon UPb dating revealed that these distinct rocks were broadly coeval with crystallization/eruption ages of 630–600 Ma. Bulk-rock elemental and SrNd isotope compositions as well as zircon LuHf isotope compositions demonstrate that the Baicheng and Yangxia granites were derived from Neoarchean mafic lower crust and underwent intensive fractionation before their emplacements. The Tiemenguan syenites have potassic andesitic compositions, in combination with their pronounced enriched NdHf isotopes and extremely high Zr saturation temperatures (∼900 °C), we argue that their possible shoshonitic primary magmas were derived from previously metasomatized subcontinental lithospheric mantle source (amphibole-bearing) and then, coupling with assimilation and fractional crystallization (AFC) effects in magma chamber and/or en route to the emplacement space. This can account for their intriguing geochemical features. The ca. 615 Ma Zamoketi and Sugetbrak basalts and the slightly younger porphyrite dykes (ca. 580 Ma) cutting the Zamoketi basalt layer, show typical OIB-like geochemical signatures, they were most likely derived from a depleted asthenospheric mantle source with variable fractionation of olivine and pyroxene and negligible crustal contamination before eruption. On the whole, the geochemistry of the diverse Ediacaran igneous rocks unambiguously demonstrates that they were genetically related to an extensional environment. In combination with the significant passive continental features of the Ediacaran-Cambrian sedimentary sequences, we consider that these Ediacaran igneous rocks were the latest phase of igneous activity related to the dispersing of the Tarim at the margin of the Rodinia supercontinent.The dispersing of the Rodinian supra-continent was diachronic with the assemblage of the Gondwana. Late Tonian to Ediacaran dispersing of the Tarim from northern margin of Australia induced the opening of the Proto-Tethys Ocean. Then the initiation of the southward subduction of the Proto-Tethys Ocean at ca. 530 Ma (Fortunian) led to the Tarim assemblage to Gondwana at ca. 440 Ma.

Complex structure of the center Oktyabr’sky deposit, Norilsk district, Russia

The PGE-Cu-Ni Oktyabr’sky deposit, related to the Kharaelakh intrusion, contains the largest sulfide ore bodies in the world. Various models have been proposed to explain its formation based on the mineralogical and textural characteristics of the sulfide ores. However, the underlying structure of the intrusion, critical to understanding the deposit’s formation, has not been well studied until now. For the first time a 3D model of the center deposit with C-3 and C-4 orebodies representing its main part has been created. The model reveals that the intrusion in this area is a thin plate, rather than a tubular or honolithic body, as previously assumed. This morphology does not support the idea of ore formation by in situ reaction between primary magma and host rocks. Instead, sulfides were transported by magma from a deeper zone in the chamber. Within this area, two pulses of magma were identified, forming the Northern and Southern intrusive branches. They are similar in inner structure; however, some differences exist. The rocks of these intrusions are identical in terms of their major components but differ in metals (Cu/Ni ratios, Co, Zn, and V contents), which is particularly reflected in the composition of the chalcopyrite group of minerals. The picritic gabbro-dolerites presented in both intrusive bodies crystallized under similar conditions, namely, T and fO2, with an accuracy of the applied methods (±15 °C and ± 0.4 lgfO2, respectively) from two portions of magma that differ from those of the taxitic gabbro-dolerites. Therefore, the Oktyabr’sky deposit has a more complex structure in its central part than has been previously reported. Considering earlier published data from the western and eastern parts, it can be concluded that the Karaelakh intrusion consists of several intrusive bodies rather than a single large massif that should be consider in future models.

Translithospheric magma plumbing system fossilized in the Emeishan large igneous province

Lighting up the magma plumbing system beneath a large igneous province (LIP) is challenging because the complex magma migration paths are often covered by flood basalts and sediments. Here, we present a three-dimensional seismic image of the Permian Emeishan LIP in Southwest China, constructed by joint inversion of Rayleigh wave dispersions and receiver functions. The results outline a cylindrical, high-velocity anomaly extending to ~135 km depth below the inner zone of this continental LIP. The geometry and magnitude of the high-velocity anomaly suggest that it represents culminated crystallized materials of primary magmas, thereby mirroring a magma plumbing system fossilized in the lithosphere. Furthermore, our geodynamic models illustrate that the nearly vertical plumbing system was controlled by slow plate motion during the magma emplacement. The plume head beneath a nearly static plate has higher thermal buoyancy and thus promotes more intensive magma emplacement. This phenomenon may apply to other LIPs throughout Earth’s history

Discovery of mafic dikes in northeastern Tibet plateau belonging to the Emeishan large igneous province: Implications for paleocontinental reconstruction and mineral exploration

Due to the lack of paleomagnetic data and paleoclimatic records, the paleocontinental belonging of the Yidun terrane in northeastern Tibet plateau is debated. In this paper, we use the distribution of the Emeishan large igneous province (ELIP) to put the issue to rest. The ELIP is the product of mantle plume activity, which took place at 260 ± 3 Ma in the western margin of the Yangtze craton. The Yangtze craton is the northern part of the South China paleocontinental block. Our new discovery of four mafic dikes in the Yidun terrane with ages and chemistry like the ELIP strongly supports a new interpretation that this terrane was an integral part of the Yangtze craton during the ELIP magmatic event. Baddeleyites from these mafic dikes yield the U–Pb ages of 257.1 ± 4.8, 259.8 ± 8.5, 260.2 ± 4.5 and 261.5 ± 6.8 Ma for each of these dikes, which are similar to the zircon U-Pb ages of the mafic–ultramafic intrusions of the ELIP within the Yangtze craton. The Yidun mafic dikes and some high-Ti basalts of the ELIP within the Yangtze craton have similar Sr-Nd isotope compositions and mantle-normalized incompatible trace element distribution patterns, indicating similar mantle source compositions. Mixing calculation using Sr-Nd isotope compositions shows that the parental magmas for the Yidun mafic dikes experienced only 2–3 % crustal contamination. Partial melting modelling shows that the primary magmas for the Yidun mafic dikes were produced by 5–6 % mantle partial melting at the depths > 85 km below surface, consistent with mantle plume-related magmatism in a continental setting. The Yidun terrane is a new exploration frontier for magmatic Fe-Ti-V oxide deposits that are present elsewhere in the ELIP.

Garnet Pyroxenite Cumulates from Cretaceous Alkaline Intraplate Magmas Underplate the Zealandia Mantle Lithosphere

Abstract The elemental and isotopic properties of garnet pyroxenites can yield information on lithospheric mantle composition, thermal state, and evolution. The 34Ma Kakanui Mineral Breccia in New Zealand contains spectacular but little-studied mantle peridotite and pyroxenite xenoliths that yield new insights into the evolution of a portion of the underlying mantle lithosphere of a former Gondwana margin. The moderately depleted and metasomatized spinel peridotites, as judged from spinel and olivine compositions and bulk rock major and platinum group element abundances, give mineral equilibration temperatures &amp;lt;1020°C and are derived from the middle to shallow (~35 to 50 km) lithospheric mantle when projected onto a 70 mW∙m−2 geotherm. These residues have low Re/Os and Re-depletion 187Os/188Os model ages that range from Eocene (0.05 Ga) to Paleoproterozoic (1.9 Ga), consistent with extraction from a lithospheric mantle comprising fragments with complex depletion histories. Although the peridotites have restricted δ18O (olivine +5.2 to 6.2), evidence for an isotopically heterogeneous mantle column in addition to the 187Os/188Os is seen in clinopyroxene 87Sr/86Sr (0.70244 to 0.70292), εNd (+4.1 to 18.8), 206Pb/204Pb (17.8 to 20.3), and εHf (+10 to +101). Higher metamorphic equilibrium temperatures of the garnet pyroxenites (Fe–Mg exchange of &amp;gt;1150°C) compared to the peridotites indicate their Eocene extraction was from towards the base of this isotopically heterogeneous mantle lithosphere. Pyroxenite bulk compositions point to cumulate origins, and the mineral isotope ratios of 87Sr/86Sr (0.70282 to 0.70294), εNd (+5.5 to 8.0) and 206Pb/204Pb (18.1 to 19.3) match many of the Zealandia metasomatized mantle peridotite xenoliths as well as the primitive intraplate basalts but not the garnet pyroxenite host magmas. In contrast to many global pyroxenite studies, the garnet pyroxenite 87Sr/86Sr and δ18O (+5.2 to 5.8) data provide no evidence for subducted crustal material in the primary magma source region, and Sm–Nd and Lu–Hf isotope data yield mid-Cenozoic ages that are probably related to isotope closure during eruption. An exception is one sample that yields a Lu–Hf isochron age of 111.9 ± 9.1 Ma, which corresponds to the convergence of the Lu–Hf isotope evolution curves of three other samples. Liquids calculated to have been in equilibrium with these cumulates have trace element compositions comparable to primitive alkaline intraplate basalts like those found at the surface of Zealandia. The new data, therefore, indicate that a pulse of intraplate magmatism occurred during or directly after the cessation of long-lived subduction on the former Zealandia Early Cretaceous forearc Gondwana margin, despite any volcanic surface exposure having been long eroded away. The lower lithospheric mantle emplacement of the garnet pyroxenites suggests that the source of the alkaline parent magmas was probably the convecting mantle, which supports conclusions that intraplate magmas in Zealandia have asthenospheric and lithospheric mantle sources.

Fast, furious, and gassy: Etna's explosive eruption from the mantle

The 3930 BP Fall Stratified (FS) eruption at Mt. Etna is a rare example of a highly explosive eruption of primitive (picritic) magma directly from the mantle. The eruption produced ash plumes up to an estimated 20 km height, leading to a volcanic explosivity index (VEI) 4 (subplinian). Given its volatile-rich and primitive nature, the FS magma may have ascended rapidly from great depths to avoid fractionation and mixing within the extensive plumbing system beneath Etna. To determine the pressures from which the FS magma derived, we perform rehomogenization experiments on melt inclusions hosted in Fo90–91 olivines to resorb shrinkage bubbles and determine the initial H2O and CO2 in the melt. With measured CO2 concentrations of up to 9600 ppm, volatile solubility models yield magma storage pressures of 630–800 MPa. These correspond to depths of 24–30 km, which are comparable to the seismologically estimated Moho. Therefore, the magma's high CO2 concentration must come from carbon in the mantle (likely from subducted carbonates), as opposed to assimilation of shallow (<10 km) crustal carbonates.Diffusion modeling of H2O and forsterite zonation profiles in clear, euhedral, and crystallographically oriented olivines indicates rapid ascent of magma directly from its source region to the surface. Forsterite profiles exhibit a narrow rim of growth zoning but no detectable diffusional zoning, reflecting maximum ascent times of 1–5 days. Eighteen measured H2O profiles result in remarkably uniform decompression rates of 0.47 MPa/s (95% confidence interval of 0.16–1.28 MPa/s), which is among the fastest measured for basaltic-intermediate magmas. These decompression rates indicate that the final stage of magma ascent over the region in which H2O degasses (between the surface and ∼ 15 km) occurred extremely fast at ∼ 17.5 m/s. This eruption may provide a link between primary magma composition and eruption intensity: we propose that the unusually explosive nature of this picritic eruption was driven by high H2O and CO2 concentrations, which led to continuously rapid ascent without stalling, all the way from the Moho.

Magmatic degassing and fluid metasomatism promote compositional variation from I-type to peralkaline A-type granite in the late Cretaceous Fuzhou felsic complex, SE China

Abstract A-type granites generally have much lower water, higher temperature, and incompatible element concentrations than I-type granitoids. Yet it remains unclear why I-A-type granitic complexes occur in convergent plate margins. Here we conduct geochemical analyses on apatite and mafic minerals from the late Cretaceous I-A-type granitic complex in Fuzhou area, SE China, aiming to decipher differentiation, fluid metasomatism, and degassing that primarily control the compositional diversity of felsic magmas. Apatites in both rock types are F-rich and show large H2O and δD variations, i.e., 341–3892 ppm H2O and –325 to +336‰ δD in I-type granitoids; 67–1366 ppm H2O and –251 to +1439‰ δD in A-type granites. H2O in apatite is negatively correlated with La/Sm and Sr/Y in the I-type granitoids, whereas it is positively correlated with Ce and total rare earth element (REE) concentrations in the A-type granites. Once H2O increases up to hundreds of ppm, both rock types show a rapid decrease of H2O/Ce, an increase of F/Cl, and extensive H isotope fractionation. Arfvedsonite occurs as a late crystallizing mineral in the A-type granite and has much higher contents of Na2O, K2O, F, and high field strength elements (HFSE) than hornblende in the I-type granitoids, indicating the addition of F-HFSE-rich alkaline fluids during its magmatic evolution. The consumption of arfvedsonite and formation of aegirine further indicate the role of fluid metasomatism and H2 degassing via a reaction of 3Na3Fe5Si8O22(OH)2 + 2H2O = 9NaFeSi2O6 + 2Fe3O4 + 6SiO2+5H2. The combined geochemical data demonstrate that the systematic differences in mineral assemblage, whole-rock composition, magma temperature, H2O content, and δD of apatite between the I- and A-type granites are likely attributed to varying degrees of differentiation, fluid metasomatism and magmatic degassing. The I-type granitoids experienced hornblende, biotite, plagioclase, K-feldspar, and apatite fractionation and close-system degassing. The A-type granite was likely formed from the I-type monzogranitic magma that was metasomatized by the mantle-derived F-HFSE-rich alkaline fluids to produce the peralkaline magma, which further experienced K-feldspar + plagioclase + biotite + apatite fractionation and open-system degassing. Further numerical estimation indicates that the primary magma of Fuzhou granitic complex contained ~3.0 wt% H2O, and the lower water content of A-type granite was likely attributed to strong degassing during its emplacement. Our results indicate that some peralkaline A-type granites can be generated from relatively water-poor I-type granitic magmas by fluid metasomatism and degassing.

Late Neoproterozoic tholeiitic plutonism in far-east Avalonia (the Dirgine Batholith, Istanbul Zone): Exposed mid-crustal section of an oceanic arc

Avalonia, a large composite terrane, extends from the northeastern regions of America and Canada to southern Britain and Belgium/Netherlands, continuing on to Poland. It was accreted to Laurentia and Baltica during early Paleozoic. The late Neoproterozoic basement of the Istanbul Zone (NW Turkey), which is considered the far-east extension of Avalonia, is poorly known. This study deals with the petrology and age of the Dirgine Batholith, the largest component of the late Neoproterozoic basement. The batholith, ∼70 km long and ∼12 km wide, was emplaced into Neoproterozoic low-grade mafic to felsic metavolcanic and metapyroclastic rocks. Tonalite and quartz diorite form the bulk of the batholith. Trondhjemite, granodiorite, and anorthositic gabbro occur in minor amounts. Overall, rock types resemble low-Al tonalite-trondhjemite-granodiorite series and trondhjemites in ophiolite sequences. The batholith formed over a narrow time interval (562–574 Ma, late Ediacaran), as inferred by U–Pb zircon geochronology. In-situ Lu-Hf analyses of igneous zircons reveal a limited range of significantly positive initial εHf values (9.47 ± 1.46, 2σ) and young Hf depleted mantle model ages (0.78 ± 0.06 Ga), indicating the relatively juvenile character of original magmas. All rock types have typical subduction-related trace-element signatures such as depletion in high-field-strength elements and enrichment in several fluid-mobile elements as well as nearly flat chondrite-normalized REE patterns. Geochemical characteristics indicate that crustal assimilation was insignificant, and the batholith formed by fractional crystallization of mantle-derived low-K tholeiitic magmas involving mainly clinopyroxene, hornblende and ± plagioclase. The Dirgine Batholith and metamorphic country rocks probably represent the mid-crustal section of a late Neoproterozoic oceanic arc. The interpretation of an oceanic rather than continental arc is based on the significantly lower concentrations of incompatible elements relative to continental arcs, nearly flat REE patterns of the rock types, and the juvenile nature of the primary magmas. These data, in concert with those in the literature, reveal that late Neoproterozoic oceanic arcs constitute an important component of far-east Avalonian terranes.