Partial Melting Research Articles

Mineral chemistry and thermobarometry of the pre-rift Upper Cretaceous to Paleocene melilite-bearing dykes from the northern part of the Bohemian Massif (Ploučnice River region): Implications for compositional variations of spinels from ultracalcic melts

The Ploučnice River region (polzenite group) is uniquely characterized by its melilite-bearing subvolcanic rocks located in the northern section of the Bohemian Massif. They are the crystallization products of ultracalcic melts during the pre-rift evolution of the Ohře/Eger Rift, which is the easternmost part of the European Cenozoic Rift System. The melt was produced by low-degree partial melting of carbonate-bearing garnet peridotite and pyroxenite at a depth of approximately 100 km (P ~ 3.0 Gpa). The rapid ascent of the ultracalcic melts through the lithosphere was accompanied by fractional crystallization of olivine + spinel ± clinopyroxene mainly within the upper to middle crustal storage zone at depths between 12 and 24 km (0.3–0.6 Gpa). Notably, olivine crystallized generally at higher temperatures (1257–1356 °C) compared to clinopyroxene (1156–1203 °C) and plagioclase (1099–1112 °C). The calculated oxygen fugacity during fractional crystallization (perovskite, −4.8 to +3.9 ΔNNO) decreases at the late-stage of crystallization due to residual magma exsolving oxidizing fluids and decreased fO2 (oxygen fugacity) of the magmas from which monticellite was crystallized (ΔNNO −6.0 to −3.9). The rounded shapes and chemical composition (Cr/(Cr + Al) 0.52–0.82) of partially resorbed chromite xenocrystic cores in subhedral to euhedral spinel grains indicate that they originated in the mantle. The first stage of magmatic evolution for the studied rocks is related to the Cr-spinel (Cr/(Cr + Al) 0.35–0.50) crystallization, which successively changed to a high-alumina composition (Cr/(Cr + Al) 0.25–0.30). Magnetite (magnetite–ulvöspinel solid solution) forms an atoll texture or small euhedral crystals in the groundmass. Both textural types of magnetite crystallized during the late-stage magmatic evolution of the ultracalcic melt. Carbonate or quartz-rich xenoliths were incorporated during magma emplacement under the upper crust. Sr-Nd isotopic data, mineral composition, and whole-rock chemical composition all verified that the assimilation of the xenoliths only affected the chemical composition of the host magma in the immediate neighborhood of the contract (up to a few millimeters around the xenolith).

Trace Element Partitioning Between Saline Aqueous Fluids and Subducted Metasediments, and the Origin of the “Sediment Fingerprint” in Arc Magmas

AbstractIsotopic and elemental signatures in arc magmas bear similarities to marine sediments from the forearc. Partial melts of subducted sediments were previously invoked as the predominant carrier of material between the slab and mantle wedge, whereas aqueous fluids were deemed too inefficient at mobilizing trace elements to account for arc magma compositions. However, trace element solubility in aqueous fluids may be significantly affected by the formation of chloride complexes. We introduced NaCl into the sediment‐H2O system, and re‐investigated trace element partitioning between aqueous fluids and metasediments in a series of piston cylinder experiments at 2.5–4.5 GPa and 600–700°C. The diamond trap method was employed to separate the fluids, which were analyzed in the frozen state. Trace element compositions of fluids and all solid phases were obtained by LA‐ICP‐MS. We combined our data with published literature to demonstrate that saline fluids and sediment melts have a comparable efficiency in extracting incompatible elements from subducted sediments, producing similar element fractionation during mobile phase expulsion. Accessory phases such as allanite may only be stable in particularly Rare Earth Elements‐enriched lithologies. The Sr, Pb and Nd isotopic signatures of arc magma suggest that the mantle wedge is infiltrated by mobile phases derived from both the sediment and basalt sections of the subducted oceanic crust. Up to several percent of either sediment‐derived saline fluid or melt can account for the isotopic and elemental compositions of most arc magma. Geochemical similarities between arc magma and forearc sediments are not necessarily evidence of slab melting.

Formation of the Tibetan Plateau during the India-Eurasia Convergence: Insight from 3-D Multi-Terrane Thermomechanical Modeling

Various models exist to explain the formation of the Tibetan Plateau, including “tectonic escape”, “pure shear thickening”, “convective removal of the lithospheric mantle”, and “lower crustal flow” model. The first two models are primarily constructed on pure mechanical models but are unable to reasonably explain the tension and shear phenomena inside the plateau. The latter two are rheological dynamic models based on deep geophysical observations. However, the spatial range of the lower crustal flow and its role in the plateau formation/uplift remain controversial. Five multi-terrane viscoplastic thermomechanical models were constructed to simulate the uplift and lithospheric structure change of the Tibetan Plateau during the post-collision stage (since 35 Ma) under the convergence of the Indian Plate. Results show that the plateau’s formation begins with crustal thickening, blocked by strong terranes at the northern plateau, and expanded laterally to the east. The lithosphere thickens gradually and experiences delamination at its base, elevating temperature within the crust and forming partial melting layers in the central plateau. As convergence persists on the southern side, the northern plateau’s lithosphere bends downward and undergoes delamination, further heating the crust and promoting the northward and eastward flow of partial melting layers, leading to secondary uplift around the plateau.

The Vaca Muerta mesosiderite: The path under which Fe‐Ni alloy ±C phases could have formed

AbstractThe combined SEM and TEM studies on the metal (Fe‐Ni alloy ±C) portion of the Vaca Muerta mesosiderite reveal structural and compositional evidence at micro to nanoscale, which sheds light on the solidification of taenite + graphite as a cement, and later solid‐state precipitation process for the kamacite formation as grain boundary allotriomorph. Besides, it is proposed that the graphite veinlets formed through a complex partial melting process followed by a solidification toward the final eutectic transient coupled with the solid‐state precipitation and later decomposition ordering of taenite. The presence of defects and taenite in graphite signal formation in a liquid environment. The δ13C values of graphite in the graphite‐rich areas (e.g., ranging from −0.8 ± 1.7‰ to +15.3 ± 2.5‰) suggest a short‐circuit diffusion path for C isotope fractionation.

Recognizing crystal accumulation in plutonic system: Evidence from mafic microgranular enclaves in Early Silurian Qingshan granodiorite, North Qilian Orogenic Belt, NW China

Granitoids are the primary constituents of the continental crust; recognizing and quantifying the effects of crystal accumulation in evolved magmas is a challenging task. Here we present whole-rock chemical, Sr-Nd isotopic, zircon U-Pb geochronological, and Hf isotopic data as well as EPMA and in-situ LA-ICP-MS analyses of amphiboles from Qingshan granodiorite and mafic microgranular enclaves (MMEs) from the North Qilian Orogenic Belt (NQOB). The MMEs and granodiorite have identical crystallisation ages (ca. 434 Ma), the same mineral assemblage, but different mineral modal proportions as the MMEs comprise more amphibole. The MMEs and granodiorite have indistinguishable and depleted zircon Hf (εHf (t) values of 6.1–8.3, and 6.4–9.1, respectively) and whole-rock Sr-Nd isotopic compositions, i.e. (87Sr/86Sr)i values of 0.70514–0.70913 and 0.70207–0.70552, respectively; and εNd(t) values of 0.27–2.23 and 0.21–2.39, respectively. The occurrence of idiomorphic amphibole grains, low MREEs and HREEs concentration, low Mg# values of equilibrium melts (AEMs), and decreasing Zr/Hf ratios indicate that Qingshan MMEs represent cumulates formed at a thermal boundary layer between the ascending conduits and the wall rocks. The Qingshan granodiorite was formed by partial melting of subducted oceanic crust with crustal materials during southward subduction of the North Qilian Ocean, followed by further amphibole-dominated fractionation by compaction without crystal deformation. We propose a model of southward subduction with the back-arc basin for the tectonic evolution of the western NQOB. Both the subduction- and collision-related granitoids of the NQOB have played a vital role in continental crust growth.

Silicon isotopes in an Archaean migmatite confirm seawater silicification of TTG sources

Unraveling ancient melting processes is key to understanding how the earliest, tonalite-trondhjemite-granodiorite (TTG)-dominated continental crust formed from partial melting of amphibolite. Application of silicon isotope analysis to ancient crust reveals that Archaean TTGs exhibit consistently high Si isotope values (δ30Si) compared to modern granitoids, attributed to seawater-derived silica introduced by either (a) partial melting of variably silicified basalts or (b) assimilation of authigenic silica-rich marine lithologies in the melt source. However, both mechanisms can introduce highly variable δ30Si, conflicting with the strikingly consistent δ30Si compositions of Archaean TTGs. This study investigates an alternative model, whereby the distinct mineralogy and chemistry of TTG melt sources impart a distinct silicon isotope composition to the melt, compared with “modern” granitoids. We measured δ30Si in component parts (melanosome and leucosome) of an Archaean (2.7 Ga) mafic migmatite and coeval amphibolites and mafic granulites from the Kapuskasing uplift, Canada, to explore how Si isotopes fractionate during incipient TTG melt formation. Our data reveal leucosome (i.e., melt) exhibits consistently high δ30Si values compared to a relatively isotopically lighter melanosome (i.e., residuum). We also derive inter-mineral silicon isotope fractionation factors for mineral separates that agree well with those of ab initio estimates for the same minerals and show that the magnitude of equilibrium fractionation between TTG source rock and melt replicates that in Phanerozoic granitoids. We conclude the effects of magmatic differentiation on δ30Si have remained consistent throughout Earth history, meaning that Archaean TTGs must require a source isotopically heavier than unaltered basalt, as reflected by our amphibolites and mafic migmatite components. The consistently heavy δ30Si of seawater through Earth history, and the high SiO2 content of amphibolites relative to coeval leucosome-free granulites in our study area, imply seawater silicification is the source of the observed high δ30Si. Thus, the consistently heavy Si isotope compositions measured in Archaean melt products define a unique aspect of ancient crust formation: that of the silicification of TTG source rock, implying the intrinsic involvement of a primeval hydrosphere.

Archaean continental crust formed from mafic cumulates

Large swaths of juvenile crust with tonalite-trondhjemite-granodiorite (TTG) composition were added to the continental crust from about 3.5 billion years ago. Although TTG magmatism marked a pivotal step in early crustal growth and cratonisation, the petrogenetic processes, tectonic setting and sources of TTGs are not well known. Here, we investigate the composition and petrogenesis of Archaean TTGs using high field-strength-element systematics. The Nb concentrations and Ti anomalies of TTGs show the overwhelming effects of amphibole and plagioclase fractionation and permit constraints on the composition of primary TTG melts. These melts are relatively incompatible element-poor and characterised by variably high La/Sm, Sm/Yb and Sr/Y, and positive Eu anomalies. Differences in these parameters are not indicative of melting depth, but instead track differences in the degree of melting and fractional crystallisation. Primary TTGs formed by the melting of rutile- and garnet-bearing plagioclase-cumulate rocks that resided in proto-continental roots. The partial melting of these rocks is part of a causal chain that links TTG magmatism to the formation of sanukitoids and K-rich granites. Together, these processes explain the growth and differentiation of the continental crust during the Archaean without requiring external forcing such as meteorite impact or the start of global plate tectonics.

Effects of salts on the exchanges through high-pressure ice layers of large ocean worlds

The habitability of large icy moons has long been disputed due to the presence of an high-pressure (HP) ice layer located between the core and the internal liquid ocean, preventing a direct contact and facilitated exchanges between both. In our previous study (Lebec et al. 2023), we have shown that under certain conditions and considering the phase change at the upper boundary of the HP ice shell, heat and mass transfer can occur and be efficient. Now, the aim of this work is to study the effects of impurities inside the HP ices on the flow dynamics and the mass transfer efficiency from the core to the ocean. These salts can enter the ice layer either by diffusion, which is a very inefficient process, or by partial melting at the bottom of the ice layer, if the temperature is high enough. In that case, the liquid water can interact with the core and get enriched in salts. This salty water, which can be lighter than high-pressure ices, could rise through the ice shell and refreeze. The resulting salts/ice mixture can be transported into the ocean by convection in the solid state but could also impede it due to its density higher than that of pure ice. The efficiency of convection depends on the buoyancy number Bsalts, which is the ratio of density increase associated to salts compared to the decrease associated to temperature. To explore the impact of salts, we model a flux of salts from the core and the advection of these salts using Lagrangian tracers in order to study the effects of the Rayleigh number, the buoyancy number and K, which is the partition coefficient of the salts between the ocean and the HP ices at the top interface, on the heat and salts transfer efficiency through the HP ice layer, as well as the implications for the evolution of salt concentration in the ocean of large icy moons and ocean worlds. We present a scaling of the bottom temperature and the top radial velocity wtop as function of the Rayleigh number Raq for various values of the buoyancy number and a scaling of the effective flux of salts across the top interface with the ocean as function of the buoyancy number for various values of the partition coefficient. We perform a numerical application for Ganymede and show that depending on the values of the system parameters, a significant percentage of the salts entering the HP ice layer from the core can efficiently flow into the ocean.

Linking beryllium enrichment to crystal-melt separation in granitic magmatic systems: Insights from high-silica granites in the southern Great Xing’an Range, NE China

High-silica granites are significant carriers of highly incompatible elements and are closely associated with mineralization of the rare metal beryllium. Thus, understanding their origin and evolution is of paramount importance for comprehending the evolution of the continental crust and enrichment processes of beryllium. This study presents zircon U-Pb ages, whole-rock major and trace element compositions, and Nd-Hf-O isotopic data for Early Cretaceous high-silica granite porphyries, monazite U-Pb ages and Nd isotopic data for beryllium-rich quartz veins, and whole-rock and apatite Nd isotopic data for Permian tuffs in the Dongshanwan W-Mo-Be deposit in the southern Great Xing’an Range, northeastern China. Our aim is to elucidate the link between mineralization and magmatism and to further our insight into the processes of crystal-melt separation and the mechanisms driving beryllium enrichment in granitic magmatic systems. Zircon U-Pb dating of the high-silica granite porphyry yields a weighted mean 206Pb/238U age of 141 ± 1 Ma, which is consistent with the age of hydrothermal monazite (ca. 140 Ma) from the beryllium-rich quartz veins. Hydrothermal monazite from the intra-granite porphyry quartz vein has positive εNd(t) values (+0.76 to +1.63) that overlap broadly with whole-rock Nd isotopic compositions of the host granite porphyry [εNd(t) = +1.24 to +1.61]. These characteristics indicate that the Dongshanwan beryllium mineralization was temporally and genetically associated with magmatic-hydrothermal activity of the Dongshanwan high-silica granite porphyries. Our systematic studies suggest that both the Dongshanwan high-silica granite porphyries and coeval Be-barren high-silica granites in the study area are high-K calc-alkaline I-type granites. They originated from a shared magmatic system and formed by the partial melting of a predominantly juvenile medium- to high-K basaltic crustal source, with a minor addition of old crustal components. Rayleigh fractionation modeling indicates that the granitic magma underwent a two-stage crystal-melt separation process. During the first stage, the fractional crystallization of a large proportion of plagioclase, in which beryllium is compatible, not only effectively inhibited beryllium enrichment in the differentiated melt, but also removed a large amount of Ca. During the second stage, minerals in which beryllium is incompatible dominated the fractionating assemblage owing to the low Ca contents in the magma, resulting in a surge in beryllium concentration in the differentiated melt. Our findings reveal that beryllium cannot be appreciably enriched in calc-alkaline granitic magmatic systems until plagioclase fractionation has substantially removed Ca from the system.

Petrogenesis of the late Cenozoic potassium-rich volcanism in the NW Tibetan Plateau: Constraints on the mantle evolution through post-orogenic events

The late Cenozoic Heishi Beihu volcanic rocks represent potassium-rich magmatic events at the northwesternmost margin of the Tibetan Plateau following the Indian–Asian continental collision. The petrogenesis of this juvenile igneous activity plays a crucial role in revealing the geochemical features of the deep mantle and contributes to our understanding of orogenic geodynamic evolution. This study, identified three eruption episodes at Heishi Beihu volcano (7.1–6.2, 4.3–1.4, and 1.3–1.0 Ma) using SHRIMP zircon U-Pb dating. Geochemically, the three episodes are characterized by shoshonitic affinities (K2O = 2.96–4.36 wt%) and crust-like trace-element signatures (negative Ta, Nb, and Ti anomalies), with enrichment in light rare earth (LREE) and large-ion lithophile elements (LILE), depletions in high-field-strength elements (HFSE), and small negative Eu anomalies (0.74–0.86). Their enriched whole-rock Sr ([87Sr/86Sr]i = 0.7072–0.7090), Nd (εNd(t) = −6.91 to −6.24), and zircon Hf (εHf(t) –5.64 to −9.13) isotopic compositions, and high zircon δ18O values (+7.4‰ to +10.6‰), are indicative of an enriched and heterogeneous mantle source (EMII-type) that contained recycled continental crust. The Heishi Beihu volcanic rocks were derived by low-degree (2%) partial melting of a mantle source with dominant pyroxenite and minor peridotite under conditions of the garnet-eclogite-facies. Results obtained from clinopyroxene–liquid geothermobarometry are indicative of diverse magma reservoirs, ranging from deep crustal levels (41–56 km) during Episode I to shallower depths for Episodes II (26–35 km) and III (17–29 km). In combination with the geological development of the area, we infer that magmatism at Heishi Beihu was genetically related to asthenospheric upwelling and induced by underthrusting of the Indian lithosphere beneath Western Kunlun.

Cretaceous magmatism in the northern Lhasa Terrane: Implications for the tectonic evolution and crustal growth tempos of central Tibet

The subduction and demise of an ocean plate are generally recognized as essential processes that result in the reworking and maturation of the continental crust. The northern Lhasa Terrane in central Tibet represents the forefront of the Lhasa-Qiangtang collision belt following the closure of the Bangong-Nujiang Ocean. Thus, it is a pivotal location to study the transition processes from oceanic lithosphere subduction to continental collision as well as pertinent crustal growth mechanisms. Here, we present zircon U-Pb dating, whole-rock major and trace element and Sr-Nd isotope, and zircon Hf isotope and trace element data of the Mendang igneous complex, Baingoin County, northern Lhasa Terrane. Geochronological results show that the granodiorites, trachydacites, and rhyolites in the Mendang igneous complex formed at ca. 122−116 Ma, 97 Ma, and 73 Ma, respectively. The Early Cretaceous granodiorite samples are peraluminous with high SiO2, Al2O3, and K2O contents, and moderate A/CNK (molar ratio of Al2O3/[CaO + Na2O + K2O]) values, which are similar to those of typical felsic peraluminous I-type granites. The granodiorites are characterized by enrichment in light rare earth elements and large ion lithophile elements (e.g., Rb) and depletion in high field strength elements (e.g., Nb, Ta). They also show the most enriched whole-rock Sr-Nd [(87Sr/86Sr)i = 0.7072−0.7078; εNd(t) = −7.60 to −5.08] and zircon Hf [εHf(t) = −4.46 to +1.02] isotope compositions, indicating that the Early Cretaceous granodiorites were likely derived from an ancient basement under a subduction setting. The trachydacites have uniform SiO2, high Al2O3, Sr contents, and Sr/Y values, and low Y and Yb contents, belonging to adakitic rocks. They show more depleted whole-rock Sr-Nd [(87Sr/86Sr)i = 0.7065−0.7066; εNd(t) = −0.56 to −0.22] and zircon Hf [εHf(t) = 4.36−7.84] isotopes than the granodiorites, suggesting that the trachydacites may have generated from partial melting of the juvenile thickened lower continental crust. The rhyolites have the highest SiO2 and K2O contents in the Mendang igneous complex, and significant depletion of Ba, Sr, Eu, P, and Ti. They have slighter more enriched whole-rock Nd [εNd(t) = −3.71 to −1.16] and zircon Hf [εHf(t) = 1.03−4.31] isotope compositions than the trachydacites. These features suggest that the rhyolites were highly fractionated products of the crustal melts. Whole-rock Sr-Nd and zircon Hf isotopes of the Mendang igneous complex show a kink trend from enrichment to depletion and then transfer to enrichment again, signifying an increased contribution of juvenile materials in the northern Lhasa Terrane toward progressively replacing the ancient lower crust and accumulating to newly formed crust. The estimated crustal thickness beneath the northern Lhasa Terrane shows a sharp increase from the Early to Late Cretaceous and peak at ca. 97 Ma, whereas it largely decreases in the Late Cretaceous. Integrated with previous studies, we propose that the formation of the Mendang igneous complex (122−73 Ma) elaborately documents the regional tectonic transition from oceanic lithosphere subduction to demise as well as continental crustal differentiation and maturation. The rollback and breakoff of the southward subducted Bangong-Nujiang oceanic slab in the Early Cretaceous initiated diverse magmatism in the northern Lhasa Terrane. During the early Late Cretaceous, the widespread adakitic and Mg-rich magmatism was attributed to the delamination of the thickened lithosphere following the final amalgamation of the Lhasa and Qiangtang Terranes. In the late Late Cretaceous, post-collisional extension induced the formation of the rhyolites in the northern Lhasa Terrane. The growth and destruction of the continental crust had certain tempos from the oceanic lithosphere subduction to continental post-collision stage.

New Insights on the Early Proto-Tethys Subduction History: Evidence from Ages and Petrogenesis of Volcanic Rocks in the Bulunkuole Complex, West Kunlun Orogen

Abstract The Proto-Tethys Ocean has played a significant role in the geological history of Earth. However, ongoing debates persist regarding the timing and polarity of its early subduction. Volcanic rocks associated with iron deposits in the Bulunkuole Complex, West Kunlun Orogen, offer insights into both the complex’s formation age and Proto-Tethys evolution. This study presents newly obtained zircon U–Pb age data (~536 Ma) along with comprehensive whole-rock major and trace element and Sr–Nd–Hf isotope analyses of these volcanic rocks. Our dataset implies that the Bulunkuole Complex partly formed in the early Paleozoic rather than entirely in the Paleoproterozoic, as previously suggested. Geochemically, the volcanic rocks exhibit enrichments in large ion lithophile elements and light rare earth elements, along with depletions in high-field strength elements. They also display elevated initial 87Sr/86Sr values (0.71093, 0.72025) and negative εNd(t) values (−5.13, −6.18), classifying them as continental arc volcanic rocks. These geochemical fingerprints, complemented by zircon εHf(t) values (−12.7 to −1.6), indicate that the parental magmas of the volcanic rocks were produced by partial melting of the lithospheric mantle wedge, which had been metasomatized by subducted sediment-derived melts. The available data, in conjunction with previously published findings, strongly suggest that the Proto-Tethys Ocean subducted southward prior to approximately 536 Ma due to the assembly of Gondwana. Subsequent slab rollback may have resulted in a crustal thinning of 9–25 km during 536–514 Ma. Further shifts in subduction dynamics led to the transition from high-angle subduction to either normal or low-angle subduction, facilitating the formation of a thicker crust ranging from 39 to 70 km between 514 and 448 Ma. This study, therefore, provides valuable insights into the early evolution of the Proto-Tethys Ocean and contributes significantly to our understanding of the tectonic history of the West Kunlun Orogen.

Titanite geochemistry traces extreme differentiation of granitic magma in the collisional orogen

The genesis of granitic magma is important for the growth and evolution of continental crust. However, whether and how granitic magma can undergo efficient crystal fractionation from partial melting at the source to magma emplacement in the shallow crust level are still controversial. To address this problem, a combined study of whole-rock and titanite geochemistry was carried out for syn-exhumation granites from the Sulu ultrahigh-pressure metamorphic belt in eastern China. These high-Si granites were produced by partial melting of the deeply subducted continental crust. Significant geochemical differentiation has been revealed in these granites. Titanite in the granites can be divided into anatectic and magmatic origins, according to the Fe/Al ratios and trace element compositions. Both types of titanite yield similar UPb ages ranging from 209 ± 23 Ma to 233 ± 26 Ma, corresponding to the late collisional stage of the Sulu orogen. These titanite domains generally have consistent εNd(t) values, suggesting that they formed in a nearly closed system without significant magma mixing of different sources. The highly variable Nb/Ta ratios of titanite show good correlations with FeO, Al2O3, Ta, Sr, LREE and (Lu/Gd)N. This interesting feature is ascribed to the extensive crystal fractionation of titanite, plagioclase and allanite/epidote during magma evolution. The results suggest that the highly silicic granitic melts can experience extensive crystal separation from the liquid during granitic magma differentiation. This study shows that accessory minerals such as titanite play a key role in constraining the geochemical differentiation of granitic melts. Furthermore, titanite geochemistry has the potential to decode the complex processes from partial melting at the source to magma differentiation at shallow crust level in collisional orogens.

The evolution of catastrophically evaporating rocky planets

ABSTRACT In this work, we develop a rocky planet interior model and use it to investigate the evolution of catastrophically evaporating rocky exoplanets. These planets, detected through the dust tails produced by evaporative outflows from their molten surfaces, can be entirely destroyed in a fraction of their host star’s lifetime. To allow for the major decrease in mass, our interior model can simultaneously calculate the evolution of the pressure and density structure of a planet alongside its thermal evolution, which includes the effects of conduction, convection and partial melting. We first use this model to show that the underlying planets are likely to be almost entirely solid. This means that the dusty tails are made up of material sampled only from a thin dayside lava pool. If one wishes to infer the bulk compositions of rocky exoplanets from their dust tails, it is important to take the localized origin of this material into account. Secondly, by considering how frequently one should be able to detect mass loss from these systems, we investigate the occurrence of sub-Earth mass exoplanets, which is difficult with conventional planet detection surveys. We predict that, depending on model assumptions, the number of progenitors of the catastrophically evaporating planets is either in line with, or higher than, the observed population of close-in (substellar temperatures around 2200 K) terrestrial exoplanets.

An Iapetus origin for a layered eclogite complex in the northern Western Gneiss Region, Scandinavian Caledonides

AbstractThe Western Gneiss Region (WGR) is a Precambrian basement domain in the Scandinavian Caledonides and one of the world's largest high‐ and ultrahigh‐pressure terranes. The south–central WGR underwent regional eclogite facies metamorphism 415–400 Ma ago when Baltica subducted beneath Laurentia, during the Scandian orogeny. Eclogites in the WGR group into two traditional types: (1) Precambrian mafic intrusions metamorphosed in situ during Scandian continental subduction and (2) eclogites, garnet peridotites and garnet pyroxenites within ultramafic complexes derived from the subcontinental mantle beneath Laurentia. We document, using field relations, petrography, whole‐rock geochemistry and secondary ion mass spectrometry (SIMS) zircon geochronology, a hitherto unrecognized third type of eclogite in the WGR that places new constraints on its tectonic architecture: an eclogitized fragment of oceanic crust from the Iapetus Ocean. The Kråkfjord eclogite complex is a km2‐sized body with an interior consisting of kyanite eclogite (meta‐troctolite) and subordinate layers and lenses of garnet peridotite, garnet websterite and kyanite–garnet leucotonalite. This interior is capped by Fe–Ti‐rich eclogite, which locally contains subordinate pockets of migmatitic aluminous gneiss. The elemental abundances and isotopic compositions of the Fe–Ti‐rich eclogites resemble those of mid‐ocean ridge basalt (MORB). In contrast, the interior kyanite eclogites, peridotites and pyroxenites have compositions similar to the gabbroic cumulates in the lower oceanic crust of slow‐spreading ridges. U–Pb SIMS dating of igneous zircon cores from a leucotonalite pod in the interior of the Kråkfjord complex yields Cambro‐Ordovician igneous ages of 500–440 Ma, with the ~500 Ma age interpreted as the isotopically undisturbed age. This age matches those of Iapetan oceanic rocks exposed elsewhere in the mountain belt. Metamorphic zircon from an Fe–Ti‐rich eclogite in the carapace of the Kråkfjord complex dates the eclogite facies metamorphism at 421.9 ± 2.2 Ma, synchronous with the continental collision. Zircon from a leucosome in Fe–Ti‐rich retro‐eclogite indicates an age of 408.5 ± 2 Ma for the crystallization of partial melt following the decompression. Detrital zircon core ages from a pocket of aluminous migmatitic gneiss in the carapace indicate derivation of sediment from the Baltic crust. Collectively, the data show that the eclogite complex (1) originated at an Iapetus spreading centre near the continent Baltica, (2) subducted to eclogite conditions during Scandian continental collision and (3) was tectonically intercalated with the Precambrian Baltica basement of the WGR.

The Effects of Partial Melting and Metasomatism on Peridotite Water Contents: Insights from Shuangliao Volcano Group, Northeast China

ABSTRACT Many minerals within the subcontinental lithospheric mantle (SCLM) contain structurally bonded hydrogen (commonly referred to as ‘water’), which significantly impacts their physical properties and associated geodynamic processes. Observations from different localities worldwide make understanding the behavior of hydrogen during partial melting and mantle metasomatism a contentious issue, as different localities reveal either melting or metasomatism as the controlling factor. To provide new insights, major elements, trace elements and water contents of peridotite xenoliths from three volcanoes of the Shuangliao Volcano Group in Northeast China were analyzed. Minerals display variations in major and trace elements, particularly in clinopyroxene. Most olivine contains no observable water, while orthopyroxene and clinopyroxene retained 14–157 and 46–351 μg/g of water, respectively. Samples were divided into three types according to clinopyroxene trace element patterns, namely, type 1: light rare earth elements (LREEs)-depleted samples; type 2: ‘spoon-shaped’ samples featuring slight enrichment of the most incompatible elements (La and Ce) and relative depletion of mid-rare earth element; and type 3: LREE-enriched samples. Combined with major element trends, the Shuangliao SCLM experienced varying degrees of partial melting and cryptic metasomatism. Different water contents in the Shuangliao SCLM are a combined result of melting and metasomatism: less metasomatized samples (LREE-depleted and ‘spoon-shaped’) preserved the control of water contents by partial melting, while strongly metasomatized samples (LREE-enriched), equilibrated at higher oxygen fugacity and temperature, display considerable post-melting modifications of water contents, possibly associated with Fe redox. These characteristics suggest that hydrous and oxidized melts/fluids likely released by the stagnant Pacific slab in the big mantle wedge have metasomatized the shallow SCLM beneath Shuangliao, which indicates the circulation of materials released by the stagnant slab throughout the upper mantle.

Evidence for partial melting and earthquake activity in the crust from a 2-D electrical resistivity model in the vicinity of the Chayu region, southeastern Jiali-Chayu fault, Tibetan Plateau

The southeastern segment of the Jiali-Chayu fault is a prominent strike-slip fault that separates the eastern Lhasa terrane. Its formation and activity are influenced by the subduction of the Indian plate towards the Lhasa terrane and clockwise rotation in the Himalayan Eastern syntaxis region. Notably, the Chayu 1950 Ms 8.6 earthquake, triggered by this fault, remains the largest recorded inland earthquake to date. However, due to a lack of high-precision geophysical evidence, the deep crustal structure of the hypocenter area remains challenging to reconstruct, limiting our understanding of earthquake activity in the region. In this paper, the Magnetotelluric data is consisted of 70 sites spaced 2 km apart. Through a joint inversion of the transverse electric and magnetic modes, we recovered the 2-D resistivity model that is approximately 140 km long and 50 km deep. The resistivity model provides compelling evidence that the presence of magmatic rocks and metamorphic basement is responsible for the high resistivity anomaly. Conversely, the low resistivity anomaly observed in the northern region can be attributed to crustal melting. These results suggest that the variation in earthquake activity between the Gongrigabu fault and Guyu fault can be attributed to the contrasting characteristics of the crustal material. The Gongrigabu fault represents a brittle plate with low water content and high crystallinity, resulting in a distinctive high resistivity anomaly. On the other hand, the Guyu fault corresponds to a molten plate, exhibiting a low resistivity anomaly. We believe that the Chayu earthquake is a typical intraplate earthquake in the Lhasa terrane and its seismogenic fault is a deeper hidden fault located below the Gongrigab fault, challenging the belief held by other scholars that the Gongrigabu fault is solely responsible. We think the Gongrigabu fault is a high-risk zone for severe earthquakes that demands continuous vigilance in the future.

Titanium-rich basaltic melts on the Moon modulated by reactive flow processes

The origin of titanium-rich basaltic magmatism on the Moon remains enigmatic. Ilmenite-bearing cumulates in the lunar mantle are often credited as the source, but their partial melts are not a compositional match and are too dense to enable eruption. Here we use petrological reaction experiments to show that partial melts of ilmenite-bearing cumulates react with olivine and orthopyroxene in the lunar mantle, shifting the melt composition to that of the high-Ti suite. New high-precision Mg isotope data confirm that high-Ti basalts have variable and isotopically light Mg isotope compositions that are inconsistent with equilibrium partial melting. We employ a diffusion model to demonstrate that kinetic isotope fractionation during reactive flow of partial melts derived from ilmenite-bearing cumulates can explain these anomalously light Mg isotope compositions, as well as the isotope composition of other elements such as Fe, Ca and Ti. Although this model does not fully replicate lunar melt–solid interaction, we suggest that titanium-rich magmas erupted on the surface of the Moon can be derived through partial melting of ilmenite-bearing cumulates, but melts undergo extensive modification of their elemental and isotopic composition through reactive flow in the lunar mantle. Reactive flow may therefore be the critical process that decreases melt density and allows high-Ti melts to erupt on the lunar surface.

Volatile-rich and oxidized ore-forming magmas of the Zijinshan porphyry Cu deposit, China: A mineral composition perspective

The Zijinshan orefield (Fujian Province) is a representative Cu-Au-Mo-Ag porphyry-epithermal mineralization system in eastern China. It contains both Cu ore-bearing and ore-barren granitoids, which provide an opportunity to study the unique properties of fertile magmas. This study focuses on chemistry of minerals such as zircon, apatite, amphibole, and plagioclase in ore-related (zircon U–Pb ages of 103–101 Ma) and ore-barren (zircon U–Pb ages of 157–156 Ma and ∼105 Ma) granitoids to constrain the characteristics of mineralized magmas. The results of whole-rock geochemistry and zircon Hf isotopes suggest that both ore-barren and ore-related granitoids were derived from partial melting of the lower crust, with the ore-related magma being influenced by mantle materials. The analysis of mineral chemistry reveals that the Zijinshan Cu ore-bearing porphyry magmas have high oxidation states (ΔFMQ +0.23–+2.25), high water contents (4.4–4.9 wt%), and shallow emplacement depths (1.9–4.0 km). These magmatic conditions are favorable for fluid exsolution and mineralization, similar to the global Cu-fertile porphyry magmas, but distinct from regional pre-mineralization ore-barren igneous rocks. Apatites from the ore-related granitoids have significantly lower Cl contents (0.03–0.83 wt%) and higher F/Cl ratios (1.52–377) compared to coeval ore-barren monzogranites (Cl = 0.14–2.42 wt%; F/Cl = 0.07–2.82). Additionally, the apatites’ volatile budgets, including XOH, XCl and XF values, and their covariation relationship, in the ore-related intrusions are consistent with apatite crystallization in H2O-saturated conditions. This suggests that extensive fluid exsolution from the parent magma in the chamber plays a crucial role in the formation of porphyry Cu deposits from a low-Cl magma. These low-Cl-apatite intrusions have the potential to form porphyry deposits. Together, we propose that the high oxygen fugacity, high water contents, and fluid exsolution of the parent magma may have triggered the Zijinshan porphyry mineralization.

Formation and modulation mechanisms of Fe-As phases in melting-processed Ba(Fe1-xCox)2As2 superconductor

The Fe-As phases play a crucial role in determining the superconducting properties of iron arsenic-based superconductors. In this study, we employed high-temperature confocal laser scanning microscopy (HT-CLSM) to investigate the formation mechanism of Fe-As phases in the melting processed Ba(Fe0.92Co0.08)2As2 superconductor through in situ observations. By combining these observations with microstructural and magnetic characterizations, we have confirmed that the high-temperature Fe-As phases primarily originate from the partial incongruent melting of the Ba122 superconducting phase. Moreover, we have elucidated the transition process of the Fe-As liquid and identified two dominant eutectic reactions in normal cooling sample: "L → Fe2As + FeAs" and "L → Fe2As + α-Fe". In case of extremely slow cooling rate, a further eutectic reaction between the Fe-As and superconducting phases ("L → Ba122 + Fe2As + FeAs") occurs, leading to the elimination of α-Fe and an increased volume of non-superconducting phases. We have also analyzed the effects of the Fe-As phases on the magnetic behavior, critical current density, and vortex pinning behavior. This study provides valuable insights for designing the microstructure of new type iron arsenic-based superconductors through the melting process.