Signature Of Melting Research Articles

Experimental evidence for a shallow cumulate remelting origin of lunar high-titanium mare basalts

High-titanium mare basalts were among the earliest rocks to be returned from the Moon, but key aspects of their formation remain enigmatic. Here we show twenty partial remelting experiments conducted on three bulk compositions based on shallow ilmenite-bearing cumulate compositions from experimental studies of lunar magma ocean crystallization, with varying amounts of plagioclase. Melts produced at about 1200–1250 °C and 0.4 gigapascal with residual iron metal, representing approximately 40 percent partial melting, are close to high-Ti mare basalt compositions in terms of both TiO2 content and magnesium number. These results, compared with Monte-Carlo simulations and modelled rare earth element abundances and samarium–neodymium-lutetium–hafnium isotopic signatures of the experimental melts, remove the requirement of a deep origin of the lunar high-Ti mare basalts. Instead, they provide new evidence supporting the hypothesis that high-degree impact-induced partial melting at shallow depth, with minor post-formation modification, played a key role.

Deconvolution of high and low-temperature alteration processes along the contact zones of basaltic-dike intrusions in basaltic host rocks of different permeabilities – implications for geothermal exploration

Magmatic intrusions serve as crucial heat sources for geothermal systems, facilitating mass transfer, mineral transformations, and elemental exchange, which result in the formation of contact aureoles. While these processes have been extensively studied in large intrusive complexes in ancient geological formations, understanding of their occurrence in active geothermal systems remains limited. The extreme conditions present in active volcanic systems often obscure the geochemical processes occurring within host rock/intrusion zones. Uncertainties persist regarding whether relatively small dike intrusions (∼0.5 m thick) possess sufficient heat content to induce textural or geochemical changes in the surrounding wall rock, and what implications this may hold for future exploratory drilling projects. The analyses in this study were conducted on two distinct outcrops, each featuring 50-cm thick basaltic intrusions within both high- and low-permeability basaltic host rocks. The low permeability host rock hosts high-temperature mineral phases (>800 °C), such as sanidine + hedenbergite + albite-rich plagioclase in the contact zone, which we interpret to have formed during partial melting. Immobile, incompatible trace elements (such as Zr, Nb and La) retain the signatures of partial melting in both outcrops. We demonstrate that low degree partial melting (F ≈ 3–10 %) results in a compositional shift in the host rock from basalt to dacite and/or trachyandesite. Thermal modelling suggests that these small dikes, along with their partial melts in the contact zone, form over a very short period of time (< 1 day), but can elevate the ambient temperature. This type of events play an important role in the development of active geothermal systems. In theory, these small dikes do not pose a significant risk during geothermal drilling, unless they are too small to be detected during geophysical exploration.

Garnet stability during crustal melting: Implications for chemical mohometry and secular change in arc magmatism and continent formation

Understanding how new felsic crust is formed and subsequently evolves through time is critical to identifying the geodynamic regimes that have dominated various parts of Earth history, and have important implications for feedbacks between the lithosphere and biosphere, such as controlling the influx of continental detritus into the oceans. In recent years, several trace element-based geochemical proxies have been proposed to allow determination of paleo-crustal thicknesses, which have been calibrated primarily using data collected from modern-day arcs. The application of these proxies through deep time has revealed surprising results, including the suggestion that the mid-Proterozoic continents were atypically thin compared to those in the Archean and the Phanerozoic; however, a range of factors may influence commonly cited trace element ratios (e.g. Sr/Y) rather than just crustal depth, leading to additional and unexpected magnitudes of uncertainty. Here we perform geochemical modelling to deduce the effect of variable bulk-rock composition and geothermal gradient on the trace element signature of felsic melts generated in arc systems. Using a range of protoliths representative of deep arc crust, the results show that considerable care must be taken when analysing simple trace element ratios of granitoid melts and making direct interpretations of the pressure of crystallisation. In particular, changes in geothermal gradients and differences in arc basalt composition impart strong controls on the relative stability of garnet and plagioclase during metamorphism and partial melting, and wide ranges of Sr/Y and La/Yb may be produced in derivative felsic melts produced at the same crustal depth. The interpretation of mid-Proterozoic continental arcs being atypically thin may instead be an artefact of underestimation of the active geothermal gradient at the time of magma formation, which acts to reduce Sr/Y and La/Yb ratios, even in normal thickness (∼35–40 km) crust. Furthermore, we argue that the potentially garnet-free residua during the formation of mid-Proterozoic felsic magmas points to crust formation without lower crustal foundering, and thus, that this commonly invoked paradigm for formation of the continental crust may only be applicable to certain periods of Earth history.

Eocene mafic alkaline volcanism in north Central Iran; Signatures of development and subsequent melting of a pyroxenite mantle source

Volcanic rocks from the Miami-Davarzan area are basalts to trachyandesites interlayered with fossiliferous beds and bear significant implications on the Eocene geodynamic evolution of northern central Iran. The rocks constitute two distinct groups, a low-Nb and a high-Nb suite. The least evolved low-Nb suite includes clinopyroxene-olivine phyric samples and is characterized by higher CaO and lower Al2O3 as compared to peridotite partial melts. The high-Nb suite includes more evolved plagioclase-clinopyroxene phyric samples. Fractional crystallization modeling using least-squares mass balance calculations for major elements suggests that the high-Nb suite was formed from a low-Nb parental melt by differentiation of a clinopyroxene-dominated mineral assemblage. The Miami-Davarzan volcanic rocks are characterized by significant large ion lithophile elements (LILE) and light rare earth elements (LREE) enrichment and high field strength elements (HFSE) depletion, which are typical signatures of subduction-related settings. Immobile, highly to moderately incompatible elements such as NbTa, LaCe, PrNd are higher in the Miami-Davarzan volcanic rocks as compared to the typical magmatic arcs, which is an indication of slab melt involvement and metasomatism of the mantle source. The Sr-Nd-Pb isotopic data also confirm a common mantle source involving the addition of subducted sediments into a depleted mantle source for both the low-Nb and high-Nb suites. The geochemical characteristics such as high CaO, low Al2O3, and the isotopic data can be best explained by derivation of the low-Nb suite parental melt from a pyroxenite mantle source. The pyroxenite mantle source was probably developed as a result of interaction between shallow depleted mantle and slab plus sediment-derived melts. It is suggested that the extensional phase that dominated the northern central Iran during the Eocene prompted partial melting of a pyroxenite mantle source.

Calcium isotopic compositions of eclogite melts and negligible modification during reaction with lithospheric mantle

Subduction-driven recycling of crustal materials plays a substantial role in the generation of geochemical and lithological heterogeneities in the Earth’s mantle. Because the distinct compositions and/or mineral-specific equilibrium isotopic fractionation effects are present in eclogitized crustal materials, stable isotopes of major elements that exert significant impacts on geochemical and petrologic properties of the mantle may serve as powerful tracers for linking the geochemical anomaly to lithological heterogeneity within the Earth’s interior. Here we present high-precision Ca isotope data, combined with previously reported Mg-Fe isotopes, for a suite of intraplate basalts with a large contribution from recycled crustal material in their mantle sources. The δ44/40Ca in these potassic basalts are consistently lower (0.66–0.78 ‰) than those of mid-ocean ridge basalt (MORBs; 0.85 ± 0.09 ‰), with an average of 0.72 ± 0.09 ‰. The co-variations of major elements with radiogenic isotopes (e.g., MgO-ɛNd) reflect the mixing of two distinct endmembers, one of which is the low-MgO primitive melts and the other is the lithospheric mantle. The low-MgO, SiO2-rich primitive melts are characterized by low CaO/Al2O3 (<0.4) and high Dy/Yb (>5). Remarkably, the primitive melts exhibit low δ44/40Ca (ca. 0.70 ‰)-δ26Mg (ca. –0.60 ‰), and elevated δ57Fe (ca. 0.30 ‰) relative to MORBs. These features are consistent with their origin as partial melts of eclogitic crustal materials. During ascending to the surface, these melts reacted with the surrounding lithospheric peridotite. This process resulted in a noticeable shift of the Mg-Fe isotopes towards the typical mantle values but very limited δ44/40Ca modification. These results suggest that Ca isotopes largely hold the signature of primary melts and are a promising tracer of eclogite in the mantle source of basalts. We further show that the combined isotopes of Ca-Mg-Fe, which are stoichiometrically incorporated into mantle minerals, offer substantial potential to establish the links between lithological and geochemical heterogeneity in the mantle.

Osmium Isotope Heterogeneity of the Upper Mantle: Evidence From the Bay of Islands Ophiolite Complex, Newfoundland

AbstractUnderstanding and determining the composition and evolution of the upper mantle is valuable to unravel Earth's evolutionary geodynamics. The compositions of oceanic basalts are inevitably biased by the preferential melting of fusible mantle components and the pooling of melts from multiple sources. Here we present whole rock and mineral chemistry as well as Re‐Os isotopic systematics of the Table Mountain ophiolitic mantle massif from the Bay of Islands ophiolite complex, complemented by thermodynamic models of melt extraction. The Table Mountain lherzolites were formed by moderate degrees (5%–15%) of shallow polybaric melting. We divide the harzburgites into two types. Type I harzburgites represent residues of high‐pressure melting, and Type II harzburgites appear to have been further modified by second‐stage flux melting above a subduction zone induced by slab sediment‐derived aqueous fluids. The Table Mountain peridotites display highly variable “initial” 187Os/188Os485 Ma isotope compositions, including 0.1252–0.1253 for the Type I harzburgites, 0.1217–0.1236 and 0.1192–0.1198 for both lherzolites and Type II harzburgites. These Os isotopic variations cannot be generated by different degrees of partial melting of a homogenous fertile mantle immediately prior to obduction, and hence must have been inherited from ancient melting events. The Table Mountain lherzolites with inherited ancient melting signals are of particular significance because they demonstrate that the record of earlier melting events is not limited to ultra‐refractory harzburgites but is also present even in relatively fertile lherzolites. Our observations underscore the ubiquity of ancient melting signatures in the entire compositional spectrum of the depleted upper mantle.

Into the High to Ultrahigh Temperature Melting of Earth’s Crust: Investigation of Melt and Fluid Inclusions within Mg-Rich Metapelitic Granulites from the Mather Peninsula, East Antarctica

Abstract Precise constraints on the compositions of melts generated by anatexis under ultrahigh temperature (UHT) conditions are critical for understanding processes of partial melting and differentiation of the Earth’s crust. Here we reveal geochemical and physical signatures of anatectic melts preserved as nanogranitoids (i.e. crystalized melt inclusions) within sapphirine-bearing UHT metapelitic granulites from the Mather Peninsula, East Antarctica. Their coexistence with high−Al orthopyroxene as inclusions in garnets strongly suggests that the investigated melts were at least partially UHT in origin. The nanogranitoids are enriched in SiO2 (69.9–75.6 wt.%), strongly peraluminous (ASI values = 1.2–1.6) and potassic to ultrapotassic (Na2O + K2O = 7.1–9.5 wt.%, K/Na = 2.2–9.3). When compared to the granulitic restite, the melts are enriched in Li, Cs, Rb, Ta, Sm, Nd, Zr, U and Pb, and depleted in Ce, Th, Ba, Sr and Nb. Their geochemical characteristics are consistent with biotite−dehydration melting in the absence of plagioclase. Our calculation results indicate that these hot crustal melts have low densities of 2.47 ± 0.07 g/cm3, low viscosities of 104.9 ± 1.2 Pa·s and high heat production values of ∼2.8 μW/m3. Therefore, such melts are mobile and susceptible to be extracted from the source, and consequently their flow and removal from the deep crust may greatly affect the chemical and thermal structure of the continental crust. Secondary C − O − H fluid inclusions within garnet and orthopyroxene have also been detected. These inclusions contain magnesite, pyrophyllite, corundum, with or without residual CO2. The minerals within the fluid inclusions are interpreted as stepdaughter minerals, which were produced by the reaction of the fluid with its host. The metamorphic timing of the investigated rocks is still a matter of debate. Zircon U–Pb dating results obtained in this study suggest that the metapelitic granulites may have undergone two separated thermal events at ∼1000 and ∼530 Ma, respectively. The presence of fluid inclusions indicates that fluid infiltration and Pan–African reworking may have played an important role in obscuring chronological information of the early thermal scenario in poly-metamorphic terranes.

Crystallizability of Free and Tethered Chains in Nanometer‐Sized Droplets

AbstractConfining polymers to nanometric sizes often interferes with the characteristic length scales of structure formation and thus significantly alters crystallization or nucleation behavior at the nanoscale. To investigate such effects in nanometric samples, past studies relied either on heavily constraining environments like block copolymer mesophases or on deposited droplets that exhibit significant variation in size, thus requiring extensive analysis to assign size classes. This studypresents a novel method to create nanometer‐sized polymer droplets of narrow volume distribution and to study their dynamic and thermodynamic behavior using the change in orientational polarization. A combination of gold‐nanoparticle deposition with a grafting‐to reaction and a nanostructured electrode setup enabled the measurement of poly‐ε‐caprolactone (PCL) droplets consisting of ten chains on average for a relatively small molecular weight. Distinct crystallization and melting signatures in spin‐cast droplets demonstrate the sensitivity of this approach. In contrast, the grafted aggregates exhibit no signs of crystallization, which is attributed to considerable constraints in this configuration, like limited chain motion or too wide chain spacing. Nevertheless, the presented setup is capable of studying ensembles of individualized (macro‐) molecules.

Origin of nitrogen on Mars: First in situ N isotope analyses of martian meteorites

Martian meteorites are key for assessing the isotopic characteristics of nitrogen in different martian reservoirs (i.e., mantle, crust, and atmosphere), and, ultimately, for constraining the source(s) of nitrogen trapped during the earliest stages of planetary accretion in the terrestrial planet-forming region. In this study, we analysed, for the first time, the nitrogen content and isotopic composition of glassy melt inclusions of Chassigny and of the mesostasis of five nakhlites (MIL 03346, Nakhla, NWA 6148, NWA 998, and Y 000593) by in situ secondary ion mass spectrometry. The nitrogen content of Chassigny melt inclusions, corrected for olivine overgrowth on the inclusion walls, varies from 4 ± 1 to 860 ± 45 ppm N, and the majority of δ15N values range from –35 ± 41 to +73 ± 36‰. The estimated nitrogen isotopic signature of the primitive melt, prior to degassing of N2 or NH3, is 0 ± 32‰. The mesostasis of nakhlites contains 2.7 ± 0.2 to 943 ± 156 ppm N, with δ15N values from –30 ± 37 to +348 ± 43‰. Whereas degassing of N2 or NH3 can explain the lowest nitrogen isotopic ratios measured in the nakhlite mesostasis, the 15N-enriched isotopic composition (δ15N > 150‰) of four nakhlites (MIL 03346, Nakhla, NWA 6148, and Y 000593) likely results from interaction of the mesostasis melt with the martian atmosphere during ejection. The δ15N values (+25 ± 42 and +77 ± 19‰) of two melt inclusions in Y 000593 are comparable to those of Chassigny, further confirming that these meteorites likely sample a common volatile reservoir in the martian interior. Overall, the new results indicate that the chassignite-nakhlite reservoir did not inherit nitrogen from the solar nebula but, instead, from chondritic-like materials. These findings further confirm that planetary bodies in the inner solar system accreted (isotopically) chondritic nitrogen during the first few million years of solar system history.

Simulated Signatures of Greenland Melting in the North Atlantic: A Model Comparison With Argo Floats, Satellite Observations, and Ocean Reanalysis

AbstractIncreased Greenland ice sheet melting has an impact on global mean and regional sea level rise and the ocean circulation. In this study, we explore whether Greenland melting signatures found in ocean model simulations are visible in observations from radar altimetry, satellite gravimetry and Argo floats. We have included Greenland freshwater flux (GF) in the global Finite‐Element‐Sea ice‐Ocean Model (FESOM) for the years 1993–2016. The reference run is computed by excluding Greenland freshwater input. These experiments are performed on a low resolution (ca. 24 km) and a high resolution (ca. 6 km) eddy‐permitting mesh. For comparison with the model experiments, we use different observational data, such as Argo floats, satellite observations, and reanalyses. We find that surface GF maps into signatures in temperature and salinity down to about 100 m in the surroundings of Greenland. The simulated melting signatures are particularly visible in steric heights in Baffin Bay and Davis Strait. Here, we find an improvement of the mean square error of up to 30% when including GF. For the Nordic part of the Nordic Seas, however, we find no improvement when including GF. We compare steric heights with reanalysis data and a new setup of the inversion method from gravimetric and altimetric satellite data. We cannot confirm that the GF signatures on variables such as temperature and salinity are visible in the observations on the time scales considered. However, we find that increased model resolution often causes larger improvements than occur due to including the simulated melting effect.

Zircon U-Pb ages and Hf isotope characteristics of granitic rocks from the Shapinggou molybdenum deposit, Dabie Shan, China

The giant Shapinggou porphyry molybdenum deposit in Jinzhai county, Anhui province is located in eastern Dabie Shan, but its genesis, genetic type and tectonic setting are still debated. New zircon U − Pb age data presented in this contribution show that the granitic rocks at Shapinggou area have been episodically formed at ∼ 135 Ma, ∼127 Ma, ∼117 Ma and ∼ 113 Ma in Early Cretaceous, and can be classified into the early-stage (135–127 Ma) granodiorite and monzogranite plutons, and the late-stage (117–113 Ma) quartz-orthophyre stock and cryptoexplosive breccia pipes. Mo mineralization is closely associated with the late-stage shallow-seated granitic stock or pipes. Granitic rocks of both stages have high contents of SiO2, K2O + Na2O and Al2O3, low contents of TiO2, CaO and MgO, showing a metaluminous to peraluminous, high-K calc-alkaline to shoshonite affinity. They are generally featured by enrichments of LILE (Rb, U, Th) and LREE, and depletions of HFSE (Nb, Ta, Ti) and HREE, with high (La/Yb)N ratios. The early-stage granitic plutons show trace element geochemical signatures of partial melting of over-thickened continental crust (>50 km), whereas element geochemistry of the late-stage small-sized granitic intrusions suggests a shallow source depth (ca. 35 km) under post-collisional extension setting. The Hf isotope data indicate that the Shapinggou granitic rocks are originated from a crustal source predominated by the Kuanping Group mixed with other components such as the mantle. The southward subduction and partial melting of the southern North China Block (including Kuanping Group) beneath the Dabie Shan lead to the generation of the Shapinggou porphyry Mo system in the post-collisional extension setting. Hence the Shapinggou Mo system distinguishes for its crustal source and post-collisional tectonic setting from the porphyry Mo deposits of Climax- or Endako-types, and should belong to collision-type or Dabie-type porphyry Mo deposit.

Ground Penetrating Radar (GPR) survey over a melting blue ice area (BIA) at the south of Schirmacher Oasis, Dronning Maud Land, East Antarctica

A blue ice area (BIA) is located in Dronning Maud Land, East Antarctica between Schiermacher Oasis (SO) and Ulthat mountain ranges. High-resolution ground-penetrating radar (GPR) survey has been carried out in the northern boundary of the BIA surrounded by Schirmacher Oasis (SO), Shivalinga and Veteheia nunataks to examine if the BIA in the area is wind-induced or melt-induced in nature. GPR study reveals that the glacier in this part of BIA is warm and showing prominent signatures of subsurface melting in the vicinity of the nunataks and other exposed rock masses. The signatures of bottom warming also have been detected in few profiles which are in agreement with the drilling results in nearby areas. Melting signatures vary over the area depending on the distances from the exposed rocks and proximity of subglacial bedrock topography. The results indicate the possible higher heat supply by the exposed rock mass to the BIA. Folding of the internal layers and the presence of two firn pockets also have been observed near the nunatak Veteheia. The present GPR study concludes that the survey area is predominantly melt-induced BIA. Further investigations are recommended to study the firn pockets and mass balance of the area.

Destabilization of Long‐Lived Hadean Protocrust and the Onset of Pervasive Hydrous Melting at 3.8 Ga

AbstractThe nature of Earth's earliest crust and crustal processes remain unresolved questions in Precambrian geology. While some hypotheses suggest that plate tectonics began in the Hadean, others suggest that the Hadean was characterized by long‐lived protocrust and an absence of significant plate tectonic processes. Recently proposed trace‐element proxies for the tectono‐magmatic settings in which zircons formed are a relatively novel tool to understand crustal processes in the past. Here, we present high‐spatial resolution zircon trace and rare earth element geochemical data along with Hf and O isotope data of a new location with Hadean materials, 4.1–3.3 Ga detrital zircons from the 3.31 Ga Green Sandstone Bed, Barberton Greenstone Belt. Together, the hafnium isotope and trace element geochemistry of the detrital zircons record a major transition in crustal processes. Zircons older than 3.8 Ga show evidence for isolated, long‐lived protocrust derived by reworking of relatively undepleted mantle sources with limited remelting of surface‐altered material. After 3.8 Ga, Hf isotopic evidence for this protocrust is muted while relatively juvenile source components for the zircon's parental magmas and flux‐like melting signatures become more prominent. This shift mirrors changes in Hf isotopes and trace element geochemistry in other Archean terranes between ∼3.8 and 3.6 Ga and supports the notion that the global onset of pervasive crustal instability and recycling—A possible sign for mobile‐lid tectonics—Occurred in that time period.

Geochemical Characterization of the Oman Crust‐Mantle Transition Zone, OmanDP Holes CM1A and CM2B

AbstractThe transition from the gabbroic oceanic crust to the residual mantle harzburgites of the Oman ophiolite has been drilled at Holes CM1A and CM2B (Wadi Tayin massif) during Phase 2 of the International Continental Scientific Drilling Program Oman Drilling Project (November 2017–January 2018). In order to unravel the formation processes of ultramafic rocks in the Wadi Tayin massif crust‐mantle transition zone and deeper in the mantle sections beneath oceanic spreading centers, our study focuses on the whole rock major and trace element compositions (together with CO2 and H2O concentrations) of these ultramafic rocks (56 dunites and 49 harzburgites). Despite extensive serpentinization and some carbonation, most of the trace element contents (REE, HFSE, Ti, Th, U) record high temperature, magmatic process‐related signatures. Two major trends are observed, with good correlations between (a) Th and U, Nb and LREE on one hand, and between (b) heavy REE, Ti and Hf on the other hand. We interpret the first trend as the signature of late melt/peridotite interactions as LREE are known to be mobilized by such processes (‘‘lithospheric process’’) and the second trend as the signature of the initial mantle partial melting (‘‘asthenospheric process’’), with little or no overprint from melt/rock reaction events.

Oxygen fugacity range of subducting crust inferred from fractionation of trace elements during fluid-present slab melting in the presence of anhydrite versus sulfide

Subducted slabs, which include altered-oceanic crust and overlying sediments, impart geochemical and redox fingerprints to arc magmas and affect long-term geochemical evolution of the deep mantle. Yet, the oxygen fugacity of the subducting slab remains poorly constrained. Light Rare Earth Element (LREE) to chalcophile element (ChE) ratios of arc magmas may serve as redox proxies for downgoing slabs because of the difference in the compatibility of these groups of elements between sulfide (e.g., pyrrhotite: Po) and sulfate (e.g., anhydrite: Anh) whose presence in the subducting crustal lithologies depend on the oxygen fugacity of the subducting slab. However, evaluating LREE/ChE of the arc magma require a complete understanding of element partitioning between residual phases in the subducting slab and the slab derived partial melts or fluids. Although previous studies have explored trace element partitioning between sulfide and silicate melts, similar data are not available for anhydrite-melt systems at the conditions of fluid-present slab melting. Here we performed laboratory experiments at 2 GPa and 900–1000 °C to investigate the partitioning of 26 lithophile and chalcophile elements between anhydrite and a hydrous silicic slab melt. Phases were analyzed using EPMA and LA-ICPMS. Sr, Y, Ba, and the REEs were found to be compatible in anhydrite while the other lithophile and chalcophile elements behave oppositely. Since Ce is compatible in anhydrite and Mo and Cu are not, we can use the Ce/Cu and Ce/Mo ratios of the near-primary arc magmas to try to fingerprint processes involved in modifying the sub-arc mantle by reduced or oxidized slab melt components. The bulk D (D¯) for Ce, Cu, and Mo were calculated using the mineralogical modes from partial melting experiments carried out on subducting sediments and Altered-Oceanic Crust (AOC). The differences in D¯Ce/D¯Mo and D¯Ce/D¯Cu values for anhydrite-saturated and sulfide-saturated subducting lithologies indicate preferential partitioning of Ce in sulfide-saturated sediment melts, whereas Mo and Cu are partitioned into anhydrite-saturated melts. Arc localities featuring slab melt signature like Marianas, L. Antilles, Kurile, Cascades, and Mexico show that the Ce/Mo and Ce/Cu ratios can be attributed to the mixing of depleted mantle-derived melts and sulfide-saturated slab melts for some arcs (e.g., Mexico and Cascades), whereas for lavas from Marianas and Kurile, most of the Ce/Mo and Ce/Cu values needs the mixing of depleted mantle-derived melts and anhydrite-saturated slab melts. Our experimental data and the resulting geochemical modeling, therefore, suggest that oxygen fugacity of subducting crusts is likely variable from one subduction zone to another.

Effect of annealing on structure and morphology of copper sulfide nanoparticles prepared by green methodology

Biosynthesis of copper sulfide nanoparticles (CuS NPs) is reported in this study. Leaf extract of Urtica dioica has been used as an alternative to chemicals, as a reducing/stabilizing agent. The synthesized CuS NPs have been annealed at temperatures; 100, 180, 250, 320 and 390 °C to study the changes in structure, morphology and composition of the formed CuS NPs with varying temperature. The as-synthesized CuS NPs were found to be amorphous in nature and their crystallinity was observed to improve on annealing from 100 to 320 °C. The XRD pattern revealed formation of Cu2S (monoclinic structure) at 320 °C. SEM images displayed morphological changes and some structural growth on annealing, which was most pronounced at 320 °C, however signature of melting and re-solidification of CuS NPs was observed afterwards at 390 °C. The FTIR study illustrated the presence of different functional groups namely C-O, C = O and O-H in synthesized nanoparticles due to the use of Urtica dioica leaves extract, which played the role of reducing and capping agent. Agglomeration of particles was observed by TEM measurements. The absorption peak of CuS NPs annealed at 320 °C was found to be around 425 nm corresponding to an enhanced optical band gap of about 2.17 eV.

Sound velocity and compressibility of melts along the hedenbergite (CaFeSi2O6)-diopside (CaMgSi2O6) join at high pressure: Implications for stability and seismic signature of Fe-rich melts in the mantle

Iron-rich silicate melts play an important role in the magmatic history of the Earth and the Moon. However, their elastic properties at high pressures, especially the sound velocities, are poorly understood. Here we determined the ultrasonic sound velocities of two silicate melts along the hedenbergite (Hd, CaFeSi2O6) – diopside (Di, CaMgSi2O6) join at high pressure and temperature conditions up to 6 GPa and 2329 K, using the high-pressure ultrasonic technique combined with synchrotron radiation in a multi-anvil apparatus. Our results show that Fe can significantly reduce the sound velocity while increasing the density of silicate melts. Comparing the melts of Di, Hd, and a mixture of 50 mol% hedenbergite + 50 mol% diopside (Hd50Di50), we find that although densities of the Hd-Di melts can be well-described by a linear mixing law empirically at high pressures, sound velocities do not vary with composition linearly. Assuming that the low-velocity regions in Earth's upper mantle are mainly due to the presence of partial melt, we applied our results to study the gravitational stability and seismic signature of Fe-rich silicate melts in the mantle, combined with melt geometry and compaction models. For the low-velocity zone (LVZ) in mantle asthenosphere, although the degree of seismic velocity reduction can be explained by the presence of a small amount of partial melt distributed in film/band geometry along grain boundaries, silicate melts formed in this depth range are too light to be gravitationally stable, but may be completely entrained in the convecting mantle due to the small melt fraction and low melt-matrix separation velocity. For the low-velocity layer (LVL) above the mantle transition zone, the presence of Fe-rich melts (with FeO>∼10 wt%) distributed in textural equilibrium with the ambient mantle is a plausible explanation.

Apollo 15 regolith breccia provides first natural evidence for olivine incongruent melting

Abstract The Apollo 15 mission returned various samples of regolith breccias, typical lunar rocks lithified by impact events on the Moon’s surface. Here we report our observations on shock features recorded in a section of the Apollo Sample 15299. We observe the presence of ferropericlase crystals confined in a shock-melt pocket and conclude that their formation is related to a shock-induced incongruent melting of olivine. While predicted by experiments, this phenomenon has never been observed in a natural sample. The incongruent melting of olivine provides an important signature of melting under high-pressure conditions and allows for estimating the pressure-temperature (P-T) experienced by the studied sample during the impact event. We infer that the fracture porosity that likely characterized the studied sample prior to the shock event critically affected the P-T path during the shock compression and allowed the studied sample to be subjected to elevated temperature during relatively low shock pressures.

Origin and timing of volatile delivery (N, H) to the angrite parent body: Constraints from in situ analyses of melt inclusions

Angrites are derived from the earliest generation of differentiated planetesimals that accreted sunward of Jupiter’s orbit, and are, thus, key to constraining the timing and source(s) of volatile delivery to planetary bodies in the inner solar system. Here we investigate the nitrogen and hydrogen isotopic signatures of angrite melts by in situ secondary ion mass spectrometry (SIMS) analyses of mineral-hosted melt inclusions and interstitial glass in two of the oldest volcanic angrites: D’Orbigny and Sahara 99555. The most primitive melt trapped in Mg-rich olivines in D'Orbigny is characterized by δ15N values ranging from 0 ± 25 to +56 ± 29‰ and δD values between −348 ± 53 and −118 ± 31‰. This shows that the angrite mantle source sampled by D'Orbigny has a N-H isotopic composition that is similar to that of CM carbonaceous chondrites, whose parent bodies are thought to have accreted in the outer solar system. The low nitrogen and water contents measured in Sahara 99555 possibly indicate that its parental melt underwent a higher degree of degassing compared to D’Orbigny or, alternatively, that the two angrites do not sample the same volatile reservoir within the angrite parent body. Given the very old crystallisation age of D’Orbigny, our findings imply that nitrogen- and water-rich objects, presumably formed beyond the orbit of Jupiter, must have been present in the terrestrial planet-forming region within the first ~4 Ma after the formation of Ca-Al-rich inclusions (CAIs, the oldest materials in the solar system).

Origin of Antecrysts in Igneous Rocks from the Salavat Range (NW Iran): an Explanation for the Geochemical Signature of Potassic Alkaline Rocks

AbstractAbundant silica-undersaturated potassic lavas are found in the centre of the Turkish–Iranian plateau (NW Iran) as flows, pillows and dykes. They display abundant zoned clinopyroxene macrocrysts and xenoliths of igneous cumulates. We determined four types of zoned crystals (Type-I, -II, -III and -IV) on the basis of their composition and zoning patterns. Use of in situ compositional data, together with whole-rock major and trace elements and the isotopic signatures of the host lavas provided evidence for the derivation of the different types of zoned clinopyroxenes from at least two contrasting parental melts. Our findings are consistent with an origin of the ultrapotassic and sodic alkaline melts from the deep-seated compaction pockets inferred from our previous studies of the alkaline magmatism throughout the Turkish–Iranian plateau. The ultrapotassic melt, which accumulated at the top of the compaction pockets, eventually ponded close to the spinel–garnet mantle transition and generated colourless antecrysts (Type-I and Type-II) and clinopyroxenite cumulates. When the compaction pocket impinged on the continental lithosphere, interstitial melts segregated and flowed inside dykes where grass green antecrysts (Type-III) and zoned phenocrysts (Type-IVa) crystallized from a melt having a geochemical signature of sodic alkaline melt. Later, at the crustal level, melt crystallization processes produced Type-IVb zoned phenocrysts. Our results are at odds with the paradigm of potassic magmas in NW Iran being derived strictly from a single mantle source.