Basaltic Melts Research Articles

Nature and sources of anomalous contamination of basalt magmas (Shufan plateau, Russian Far East)

When the primary mantle melts, the crustal contamination of basaltic magmas distorts the source composition of the melting substrate and the degree of its trace element and isotopic enrichments. Such contamination is also important in understanding the interactions of contrasting structures of melts with mafic and felsic compositions or solid-phase systems (xenoliths) and the capacity of basaltic magmas in relation to the volumes of assimilation of incoherent highly polymerized crustal matter. First, the mechanism of diffusive interaction with crustal matter in nonsuperheated magmas is of interest when elements coherent to the melt or structural groups near the emerging cluster groups of the melt are extracted from xenoliths. Second, the selectivity of the contamination is of interest. The features of interactions between contrasting magmatic melts and melt-solid phase systems (melt-xenolith) are considered by analyzing the reaction products of such interactions, i.e., basalts contaminated with acid aluminosilicate matter and buchites (crustal xenoliths subjected to chemical modification and melting). For the largest late Miocene Shufan basalt plateau in southern Primorye (Russia), a process that involves the contamination of tholeiitic basalts with crustal aluminosilicate matter, which is unique in scale, has been recognized. As a result, large volumes of andesite–trachyandesite formed, which compose the upper horizons of the volcanic plateau. They are represented by massive “nonvesicular” lavas with banded texture caused by oriented layers of silicic potassium granophyre immiscible felsic selective melts from xenoliths. Two associated processes are clearly traced: mingling and mixing. Contaminated varieties with SiO2 contents of 57–60 wt% abnormal are enriched with xenocrysts of quartz and sieve-like melted plagioclase evenly distributed throughout the volume, which suggests an unusual mechanism of convective homogenization of xenogenic crustal substances. Among the xenoliths, buсhites are found to be chemically modified and remelted rocks of primary pelitic composition. They are represented by glassy varieties with different and unusual chemical compositions (SiO2 = 37–45 wt%, Al2O3 = 29–36 wt%, and FeO + Fe2O3 = 17–23 wt%) and are depleted in Mg, Ca, Na, and K. The mineral associations of buchites are represented by ultraferrous sekaninaites (f = 85–95%), hercynites (f up to 100%), whose compositions have no analogs in metamorphogenic rocks, sillimanite, mullite, tridymite, bytownite, baddeleyite, zirconium and high‑aluminum armalcolite rich in pseudobrookite molecules, mono‑lanthanum monazite, scandium‑aluminum zircon and aluminosilicate‑barium-phosphate matrix. The melting of pelitic xenoliths and diffusion interactions with basaltic magma formed iron‑titanium rhyolitic liquids and anorthosite-like reaction coatings. The formation of buchites reflects a complementary relationship with contaminated basalt varieties due to the selective thermal and diffusive extraction of silicic-alkaline granitoid melts and a selective group of cations from pelitic xenoliths into the basaltic melt with the corresponding accumulation of refractory cations (Al, Fe, Ti, and Zr) in the restitic material. In the process of remelting xenoliths, the phenomenon of destruction and dispersion of the crystalline substance of xenoliths into fractal fragments and clusters (up to micro - to nanosizes), which are grouped into immiscible liquid-like suspensions of quartz, feldspar, and hercynite-ilmenite composition, is identified.

Petrogenesis and Tectonics of Eocene–Oligocene Phonolites of Mecejana, Ceará, NE Brazil: the Role of the Fernando de Noronha Fracture Zone, Equatorial Atlantic

Abstract Located on the western continental tip of the Fernando de Noronha transform fracture (FNTF), northeastern Brazil, the Eocene–Oligocene Mecejana Phonolitic Province (MPP) was subjected to a multi-methodological study, comprising field and microscopic characterization, mineral and whole-rock chemistry, and Sr-Nd isotopes. These rocks intrude the Precambrian crystalline basement, forming mainly plugs oriented in the NE–SW direction. They are composed of alkali basalt, phono-tephrite, tephri-phonolite, and phonolite with phenocrysts of nepheline and sanidine, and minor diopside, aegirine-augite, aegirine, kaersutite, biotite and titanite. Chemically, they are sodic (Na2O/K2O > 1) and display Light Rare Earth Elements (LREE) enrichment, concave upward or flat Heavy Rare Earth Elements (HREE) patterns, weak to strong Eu anomaly, and high contents of Nb, Ta and Zr. They delineate two chemical groups, the less-evolved one comprising basalt to tephri-phonolites and the more evolved constituted by phonolites. Oxide and trace element modeling points out to an evolution by fractional crystallization, leaving olivine-rich gabbro and foidolite/nepheline syenite cumulates, respectively. The analyzed samples have Sr-Nd isotopic ratios akin to ocean island basalt (OIB) rocks (positive εNd of +3.0 to +0.9 at 30 Ma) and low to high 87Sr/86Sri (at 30 Ma) of 0.70264–0.70955. The data suggest a three-stage evolution model for the MPP: (1) <5% partial melting of a metasomatized phlogopite-amphibole-garnet peridotite generates an alkali basalt melt, (2) polybaric differentiation of this melt at crustal level (~570–220 MPa, 1040–645°C) leads firstly to phono-tephrites and tephri-phonolites and then to (3) foidolite and nepheline syenite. Petrographic, chemical, and isotopic characteristics of the MPP and similar rocks of the Fernando de Noronha islands suggest the control by the FNTF and reactivation of NE–SW trending deep-rooted Precambrian faults allowing channeling and emplacement of the alkaline magmas.

Eu speciation in apatite at 1 bar: An experimental study of valence-state partitioning by XANES, lattice strain, and Eu/Eu* in basaltic systems

Abstract Partition coefficients for rare earth elements (REEs) between apatite and basaltic melt were determined as a function of oxygen fugacity (fO2; iron-wüstite to hematite-magnetite buffers) at 1 bar and between 1110 and 1175 °C. Apatite-melt partitioning data for REE3+ (La, Sm, Gd, Lu) show near constant values at all experimental conditions, while bulk Eu becomes more incompatible (with an increasing negative anomaly) with decreasing fO2. Experiments define three apatite calibrations that can theoretically be used as redox sensors. The first, a XANES calibration that directly measures Eu valence in apatite, requires saturation at similar temperature-composition conditions to experiments and is defined by: ( E u 3 + ∑ E u ) Apatite = 1 1 + 10 - 0.10 ± 0.01 × l o g ⁡ ( f o 2 ) - 1.63 ± 0.16 . The second technique involves analysis of Sm, Eu, and Gd in both apatite and coexisting basaltic melt (glass), and is defined by: ( Eu E u * ) D Sm × Gd = 1 1 + 10 - 0.15 ± 0.03 × l o g ⁡ ( f o 2 ) - 2.46 ± 0.41 . The third technique is based on the lattice strain model and also requires analysis of REE in both apatite and basalt. This calibration is defined by ( Eu E u * ) D lattice strain = 1 1 + 10 - 0.20 ± 0.03 × l o g ⁡ ( f o 2 ) - 3.03 ± 0.42 . The Eu valence-state partitioning techniques based on (Sm×Gd) and lattice strain are virtually indistinguishable, such that either methodology is valid. Application of any of these calibrations is best carried out in systems where both apatite and coexisting glass are present and in direct contact with one another. In holocrystalline rocks, whole rock analyses can be used as a guide to melt composition, but considerations and corrections must be made to either the lattice strain or Sm×Gd techniques to ensure that the effect of plagioclase crystallization either prior to or during apatite growth can be removed. Similarly, if the melt source has an inherited either a positive or negative Eu anomaly, appropriate corrections must also be made to lattice strain or Sm×Gd techniques that are based on whole rock analyses. This being the case, if apatite is primary and saturates from the parent melt early during the crystallization sequence, these corrections may be minimal. The partition coefficients for the REE between apatite and melt range from a maximum DEu3+ = 1.67 ± 0.25 (as determined by lattice strain) to DLu3+ = 0.69 ± 0.10. The REE partition coefficient pattern, as observed in the Onuma diagram, is in a fortuitous situation where the most compatible REE (Eu3+) is also the polyvalent element used to monitor fO2. These experiments provide a quantitative means of assessing Eu anomalies in apatite and how they be used to constrain the oxygen fugacity of silicate melts.

Implications of high-Mg# adakitic magmatism at Hunter Ridge for arc magmatism of the Fiji - Vanuatu region

The mostly submarine Hunter Ridge, located in the SW Pacific records a ∼12 Myr to present history of magmatism related to the opening of the North Fiji Basin and subduction of oceanic lithosphere of the South Fiji Basin. Although the Hunter Ridge is probably composed primarily of an older Vitiaz-related basement, young volcanic features are present from Matthew Island to Kadavu Island. Some dredged volcanic rocks from these features have low-FeO and high-Mg# affinities, ranging from picrites to high-Mg# andesites and dacites. Elevated Sr (500 – 3400 ppm) and Sr/Y (50 – 240) coupled to fractionated (adakitic) rare-earth element patterns (La/Yb = 5 – 40, Gd/Yb = 1.5 – 5.7) indicate a garnet-signature derived from the melting of eclogite-facies basalt. Pacific-type MORB Nd-Hf-Pb isotopic ratios of these rocks contrast with the Indian-type MORB nature of the underlying North Fiji mantle but match closely the subducted South Fiji ocean crust. Low values of Th/La (< 0.15), Ba/La (< 22), unradiogenic 87Sr/86Sr (0.7026 – 0.7032) and Pacific-MORB Nd-Hf-Pb isotopic ratios indicate that sediment is a minor contributor to the source. The isotopic data clearly connect Hunter Ridge arc rocks of all compositions (picrites, low- to medium K2O arc lavas, basalts, high-Mg# andesites and dacites) to source components predominantly within the subducting plate. Unradiogenic 87Sr/86Sr (0.7026 – 0.7029) at high Sr abundances (700 – 1400 ppm) are common in hot-slab localities and are interpreted to reflect flux-melting of MORB under eclogite-facies conditions driven by dehydration in the underlying mantle of the subducting plate. Such an adakitic slab-melt component can be detected in more common (non-adakitic) arc rocks along the Hunter Ridge and Vanuatu arc as well. Evidence of slab melting along the western Pacific indicates that melting of subducting oceanic lithosphere is likely a common occurrence at convergent margins.

Detailed petrogenesis of the unsampled Oceanus Procellarum: The case of the Chang'e-5 mare basalts

Lunar mare basalts provide a probe to study the magmatic and thermal evolution of the Moon. The Chang'e-5 (CE-5) mission returned samples from a young and hitherto unsampled mare terrain, providing fresh opportunities to understand lunar volcanic history. A detailed petrologic survey was conducted in this study on basalt fragments and glasses from the returned CE-5 soil samples. Relatively large-sized (100–400 μm) basaltic fragments were hand-picked and examined for texture, mineral assemblage and mineral chemistries. Basaltic fragments exhibit dominantly subophitic textures and are phenocryst-free, with low to intermediate-Ti (2.1–5.5 wt%) and low Mg# (Mg/(Mg + Fe) × 100, 19–47, with an average whole-rock Mg# of 33) consistent with olivine-melt equilibrium calculation (Mg# = 34). A range of highly evolved basaltic materials have been identified, in which abundant fayalitic olivine, symplectitic intergrowths, and Si + K-rich mesostasis co-exist were found resulting from late-stage silicate liquid immiscibility. Basaltic glass compositions largely overlap with basaltic fragment compositions suggesting they are locally derived. The CE-5 basalts have a relatively limited range of eruption temperatures of 1150–1230 °C. Based on their petrographic and geochemical characteristics, some CE-5 mare basalts are highly evolved and some of the resultant basaltic melt products underwent high crystallization. Thermodynamic modeling using MELTS suggests highly evolved basaltic magma was produced by a low-pressure and simple fractional crystallization under reduced conditions. This may have occurred at the surface in the inflated Em4/P58 flow with a thickness of ~50 m. The low degree of partial melting mantle source of the parental melts is the late-stage lunar magma ocean cumulates in a similar manner to some evolved low-Ti mare basalt meteorites, although the source of CE-5 basalts may have been slightly more Ti-rich.

MagmaFOAM-1.0: a modular framework for the simulation of magmatic systems

Abstract. Numerical simulations of volcanic processes play a fundamental role in understanding the dynamics of magma storage, ascent, and eruption. The recent extraordinary progress in computer performance and improvements in numerical modeling techniques allow simulating multiphase systems in mechanical and thermodynamical disequilibrium. Nonetheless, the growing complexity of these simulations requires the development of flexible computational tools that can easily switch between sub-models and solution techniques. In this work we present MagmaFOAM, a library based on the open-source computational fluid dynamics software OpenFOAM that incorporates models for solving the dynamics of multiphase, multicomponent magmatic systems. Retaining the modular structure of OpenFOAM, MagmaFOAM allows runtime selection of the solution technique depending on the physics of the specific process and sets a solid framework for in-house and community model development, testing, and comparison. MagmaFOAM models thermomechanical nonequilibrium phase coupling and phase change, and it implements state-of-the-art multiple volatile saturation models and constitutive equations with composition-dependent and space–time local computation of thermodynamic and transport properties. Code testing is performed using different multiphase modeling approaches for processes relevant to magmatic systems: Rayleigh–Taylor instability for buoyancy-driven magmatic processes, multiphase shock tube simulations propaedeutical to conduit dynamics studies, and bubble growth and breakage in basaltic melts. Benchmark simulations illustrate the capabilities and potential of MagmaFOAM to account for the variety of nonlinear physical and thermodynamical processes characterizing the dynamics of volcanic systems.

Cenozoic delamination of the southwestern Yangtze craton owing to densification during subduction and collision

Abstract It is widely thought that oceanic subduction can trigger cratonic keel delamination, but the southwestern Yangtze craton (SYC; southwestern China) lost its lower keel during Cenozoic continental collision. The upper mantle beneath the thinned SYC contains its incompletely delaminated keel, which has high-velocity seismic anomalies. Combining geophysical observations with the geochemistry of Eocene mafic potassic lavas derived from the SYC mantle at different depths, we suggest that the deep (~130 km) delaminated lithosphere was more fertile and dense, with low-forsterite (Fo; molar 100 × Mg/[Mg + Fe] = 91.3) and high-δ18O (5.9‰) olivine, than the shallow (~55 km) intact lithosphere (Fo = 94.2; δ18O = 5.2‰), although both were rehydrated and oxidized. The deep keel underwent strong refertilization and densification owing to the addition of Fe-rich basaltic melts during earlier oceanic subduction. Subduction-driven refertilization and subsequent collision-driven cooling caused cratonic keel delamination due to compositional and thermal densification rather than hydration- or oxidation-induced rheological weakening. Our study provides an example of Cenozoic cratonic keel delamination in a collisional orogen and highlights the key roles of compositional and thermal densification in delamination during subduction and collision.

Experimental Study of the Multicomponent Chemical Diffusion of Major Components (SiO2, Al2O3, Na2O, CaO, MgO, and FeO) and the $$\text{CO}_{3}^{2-}$$ Anion at Interaction between Basalt and Kimberlite Melts under a Moderate Pressure

Experimental Study of the Multicomponent Chemical Diffusion of Major Components (SiO2, Al2O3, Na2O, CaO, MgO, and FeO) and the $$\text{CO}_{3}^{2-}$$ Anion at Interaction between Basalt and Kimberlite Melts under a Moderate Pressure

Mineralogical and Isotope-Geochemical Characteristics of the Zharnichikha River Basanites (Southern Transbaikalia)

Abstract —Cenozoic alkali basaltoids are developed in southern Transbaikalia (Dauria–Khentei Ridge). They are represented by thin flows and sheets. Unlike the volcanic fields of the South Baikal Volcanic Region (SBVR), the investigated fields are not associated with rifting. Volcanics from the flow of the Zharnichikha River were studied. Compositionally, they belong to basanites. For the first time, the mineral compositions of the volcanics have been determined. In their geochemical parameters, the basanites correspond to basic volcanics of oceanic islands. The P–T parameters of the equilibrium crystallization of olivine and clinopyroxene phenocrysts have been determined. For the calculated parental melt, the temperature of formation was 1536 °C, and the pressure was 3.28 GPa. The obtained original data on Sr, Nd, and Pb isotope composition have shown that the isotope composition of the magmatic melt formed from a PREMA source with a minor contribution of the latter. The mantle source for the studied basanites is assumed to be olivine pyroxenites (Ol + Cpx + Grt). The generation of basaltic melts occurred under the influence of a mantle plume, which caused Cenozoic activation.

STUDY OF PHYSICAL AND MECHANICAL CHARACTERISTICS OF CERAMIC COMPOSITE BASED ON WASTE OF BASALT FIBER PRODUCTION

The use of accumulated waste from the production of basalt fibers from local raw materials in the production of composite materials contributes to the rational use of natural basalt and forms an additional raw material base. In this work, waste products from the production of basalt fibers, having a fractional composition with sizes less than 0,315 mm, were used as a composite filler. Using the plastic molding method, prototypes of ceramic composites based on waste products from the production of basalt fibers were obtained. Further, by the methods of physical and technical tests, an experimental study was carried out to determine the physical and mechanical properties of the resulting composite material. The method of least squares was used to process the experimental data obtained as a result of physical and technical tests of the samples. It has been established that the strength of the ceramic composite increases with the maximum amount of waste of the basalt melt fractions less 0,315mm and high firing temperature.

Partitioning of Ti, Al, P, and Cr between olivine and a tholeiitic basaltic melt: Insights on olivine zoning patterns and cation substitution reactions under variable cooling rate conditions

The mechanism governing the kinetic growth of olivine in dynamic volcanic settings has been the subject of considerable attention in recent years. Under variable cooling rate (CR) and undercooling (−ΔT) regimes, the textual maturation of olivine proceeds from skeletal/dendritic crystals to polyhedral morphologies by infilling of the crystal framework. Owing to the establishment of a diffusion-controlled growth regime, a sharp diffusive boundary layer develops in the melt next to the advancing olivine surface. In this context, we have quantified the apparent partitioning of Ti, Al, P, and Cr between olivine and a Hawaiian tholeiitic basaltic melt at P = 1 atm, fO2 = QFM-2 buffer, and CR = 4, 20, and 60 °C/h over a constant -ΔT = 85 °C. Differences in charge and/or size between the substituent minor cations and the major species in the olivine crystallographic site dominate the energetics of homovalent and heterovalent cation substitutions. While the entry of Ti in the olivine lattice site accounts for the simple exchange [TSi4+] ↔ [TTi4+], more complex charge-balancing coupled-substitution mechanisms have been determined for the incorporation of Al, P, and Cr, i.e., [MMg2+, TSi4+] ↔ [MAl3+, TAl3+], [2 TSi4+] ↔ [TP5+, TAl3+], and [MMg2+, TSi4+] ↔ [MCr3+, TAl3+], respectively. In order to maintain charge balance, the disequilibrium uptake of minor cations in rapidly growing crystals is controlled by the same substitution mechanisms observed under equilibrium crystallization. This finding is consistent with the achievement of a local interface equilibrium at the olivine-melt interface independently of the diffusive boundary in the melt. A statistical approach based on multivariate analysis of olivine/melt compositional parameters confirms that the control of melt structure on the partitioning of Ti, Al, P, and Cr is almost entirely embodied in the olivine structure and chemistry via charge compensation reactions. Therefore, the magnitude of minor element partition coefficients is weakly dependent on diffusion kinetics in the melt but rather strongly governed by olivine zoning patterns resulting from fast crystal growth rates.

Oxygen isotope evidence for progressively assimilating trans-crustal magma plumbing systems in Iceland

Abstract The oxygen isotope composition of mantle-derived melts can place important constraints on how magmas are processed as they traverse the crust. Assimilation of crustal material is a crucial aspect of basalt petrogenesis, as it affects the chemical and rheological characteristics of eruptive magmas at active volcanoes. We report oxygen isotope (δ18O) and trace element (TE) data from a suite of well-characterized basaltic melt inclusions and groundmass glasses from the Bárðarbunga volcanic system in Iceland to assess how and where in the plumbing system crustal rocks interact with ascending magmas. While both melt inclusions and groundmass glasses record a large range in δ18O values (+3.2‰ to +6.4‰ and +2.6‰ to +5.5‰, respectively) groundmass glasses record lower values on average. Relationships between incompatible trace element (e.g., Zr/Nb) and oxygen isotope ratios are best explained with three-component mixing, where primary melts derived from depleted and enriched mantle components with distinct δ18O values mix and acquire a low-δ18O character upon progressive contamination with altered Icelandic crust. The majority (60%) of melt inclusions require 10–30% exchange of oxygen with the Icelandic crust. In addition, for the first time, we link the extent of oxygen isotope exchange with melt equilibration depths, showing that most of the contamination occurs at 1–2 kbar (3–7 km depth). We propose that a progressively assimilating, multi-tiered plumbing system is a characteristic feature of the Bárðarbunga volcanic system, whereby chemical modifications resulting from interaction with the crust systematically increase as melts migrate through higher crustal levels. We show that similar processes may also occur across the active rift zone in Iceland.

Experimental determination of N2 solubility in silicate melts and implications for N2–Ar–CO2 fractionation in magmas

The solubility of nitrogen in magmatic melts is key for understanding the degassing of Earth’s interior and formation of Earth’s N-rich atmosphere. We performed high-pressure experiments to determine the solubility of nitrogen at N2-saturation in basaltic to granitic melts at pressures of 0.3–8 GPa, temperatures of 1200–1600 °C, and oxygen fugacities between the Fe–FeO and Ni–NiO oxygen buffers. The solubility and speciation of nitrogen in the quench silicate melts were quantified by Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), electron probe microanalyzer (EPMA), and Nano-scale secondary ion mass spectrometry (NanoSIMS). The results show that the predominant N-species in our silicate melts is N2, which dissolves physically by filling the interstitial sites of the silicate melt network. The measured N-solubilities in silicate melts (SN2melt) by using different approaches agree with each other, and they vary from 200 to 17,000 ppm by weight. The SN2melt increases with increasing pressure and/or decreasing silicate melt NBO/T (the ratio of non-bridging oxygens to tetrahedrally coordinated cations), but it is temperature-independent. Using our newly obtained SN2melt and previous SN2melt and SArmelt data, we develop an empirical model that describes the physical dissolution of N2 and Ar in silicate melts as a function of gas partial pressure and silicate melt composition, and this model can be used to predict SN2melt and SArmelt in magmatic melts of Earth’s mantle and crust. The comparison of our SN2melt data with previous SArmelt and SCO2melt data shows that the solubility in granitic melts is in the order of SArmelt >SCO2melt >SN2melt, but in basaltic melts it is in the order of SCO2melt >SArmelt > SN2melt. The lower SN2melt than SArmelt at Earth’s mantle and crustal conditions suggests N2–Ar fractionation during degassing of mid-ocean ridge basalts and arc magmas. Therefore, the N2/Ar ratios in degassed magmas may not fully represent those in their mantle source regions. The solubility-controlled, preferential degassing of N2 relative to CO2 in Earth’s oxidized shallow magma ocean, in conjunction with collision-induced atmospheric loss, can explain the superchondritic C/N ratio in the bulk silicate Earth. However, the two orders of magnitude higher N2/36Ar ratio in Earth’s mantle than in atmosphere could have resulted from preferential retention of reduced N-species in Earth’s reduced silicate magma ocean or preferential deep subduction of nitrogen.

Sulfide and sulfate saturation of dacitic melts as a function of oxygen fugacity

Sulfur is a key element in terrestrial magmatic processes yet its geochemical behavior remains one of the most difficult to model due to its heterovalent chemistry. The maximum amount of sulfur a silicate melt can dissolve before saturating with sulfide (e.g., pyrrhotite) or sulfate (e.g., anhydrite) changes with the redox state of the system and has important implications for the sulfur budget of a magmatic system. Several empirical models have been developed to predict the sulfur content of a silicate melt at either sulfide (under reducing conditions) or sulfate (under oxidizing conditions) saturation, but only one model existed that systematically assessed how the sulfur content of a basaltic melt changes as a function of oxygen fugacity (fO2) across the transition from sulfide- to sulfate-dominated conditions. The applicability of that model to intermediate and felsic melts rests on the assumption that changes in melt composition do not affect how sulfide or sulfate dissolves in the melt. Here, we report new experimental data that constrain the sulfur concentration at sulfide saturation (SCSS) and the sulfur concentration at anhydrite saturation (SCAS) in a dacitic melt as a function of fO2. The experiments were conducted using a H2O-saturated natural dacitic melt at 1000 °C, 300 MPa, and at log fO2 varying over four orders of magnitude encompassing the sulfide-sulfate transition (log fO2 = ΔFMQ−0.7, ΔFMQ+0, ΔFMQ+0.5, ΔFMQ+1, ΔFMQ+1.48, ΔFMQ+1.54, ΔFMQ+1.75, ΔFMQ+2.08 and ΔFMQ+3.3). New SCSS and SCAS data and modeling for dacitic melts reveals that the sulfide-sulfate transition occurs at ΔFMQ+1.81 ± 0.56, defined by the following equations to predict the sulfur content of intermediate to evolved silicate melts as a function of fO2:SCSSdacitic = [S2−] (1 + 10(2.00ΔFMQ – 3.05))SCASdacitic = [S6+] (1 + e(1.26 – 2.00ΔFMQ))The results presented here demonstrate that the basaltic-derived SCSS-SCAS model is not appropriate for dacitic melts and that the sulfide-sulfate transition is shifted to higher fO2 in more evolved silicate melts. Implications include the stability of sulfides to higher fO2 in more evolved silicate melts and the potential for a narrower transition from a sulfide- to a sulfate-dominated melt than that predicted by thermodynamics.

Cast Stone Matrices Based on Basaltic Melt with Uranium-Bearing Silica Gel

Cast Stone Matrices Based on Basaltic Melt with Uranium-Bearing Silica Gel

Crustal and uppermost mantle structure of the Porcupine Basin west of Ireland from seismic and gravity methods

The Porcupine basin is a failed rift that formed during the opening of the North Atlantic Ocean. Here, we provide additional insights into the crustal structure of the northern Porcupine Basin and assess the degree of crustal extension from long streamer seismic data and free-air gravity modelling. Recent re-processing of the 2013/2014 streamer data involved de-multiple and de-noise processing with pre-stack-depth migration using velocity models derived from reflection travel time tomography and full waveform inversion. Forward modelling of satellite gravity data was used to constrain the velocity and mass density of the basement and lower crust, where the seismic constraints are limited. Prior knowledge, from previous geophysical and geological studies, was included to guide the modelling and reconcile the gravity data with the seismic sections. It also served to minimize the non-uniqueness inherent in the forward problem. Crustal thinning occurs across the basin axis and increases southwards. We infer maximum stretching factor (β) at the basin axis, increasing from ∼4 to 6 in the North Porcupine at 52.5°N to >10 at ∼51°N in the Central Porcupine. The gravity modelling supports the presence of mantle serpentinisation, and the high β implies that crustal break up and mantle exhumation may have locally occurred in the Central Porcupine Basin, in agreement with previous seismic tomographic studies. Basaltic melt generation in the centre of the basin where the maximum bulk stretching factor exceeds10 in the hyper-extended crust may have led to localized sea-floor-spreading.

Refertilization of depleted mantle peridotite in the Nagaland–Manipur ophiolite, north‐east India: Constraints from PGE, mineral, and whole‐rock geochemistry

This paper discusses whole‐rock geochemistry, mineral chemistry, and platinum group element (PGE) systematics of depleted mantle rocks (harzburgite and dunite) from the northern part of Nagaland–Manipur Ophiolite (NMO), north‐east India, to comprehend their source features, fractionation behaviour of PGE during magmatic evolution, and its tectonic origin. The studied ultramafic rocks are characterized by a low concentration of CaO (0.57–0.71 wt%), Al2O3 (0.18–0.92 wt%) with ∑REE of 1.135–2.702 ppm and high concentrations of MgO (38.70–44.21 wt%), Cr (1,843–4,572 ppm), and Ni (894–4,138 ppm). They show U‐shaped REE patterns [LREE and HREE enrichment (La/Sm)N = 1.85–4.11, (Dy/Yb)N = 0.51–0.85]. Olivine ranges Fo 88.18 to Fo92.23, whereas Cpx and Opx range En44.84 to En47.89 and En86.37 to En93.37 respectively. The chrome spinel Cr# [Cr/(Cr + Al)] and Mg# [Mg/(Mg + Fe2+)] are 0.47–0.83 and 0.31–0.60, respectively, which indicates recrystallization from a boninitic magma in a Supra‐Subduction Zone setting. Conventional thermometry indicates the equilibration temperatures of the dunite sample yielded high temperatures of ~850°C, suggesting their formation due to later interaction with high‐temperature percolating melts. The PGE contents in harzburgite are low (125.6–142.8 ppb) as compared to the dunite (248–360 ppb). They have high PPGE/IPGE and negative Pt* (Pt/Pt* = 0.73) anomaly, which is characteristic of re‐entry of PPGE into the system via reaction with percolating basaltic melt in the mantle wedge. Significantly higher concentration of PPGEs than IPGEs in the samples, indicating recrystallization of PPGEs with early sulphide fractionation. The presence of significant Rh and Pd enhancements relative to Pt in all samples suggests that Pt was removed during PGE fractionation. This could be one of the reasons for both harzburgite and dunite's sulphide undersaturation. PGE distribution in NMO ultramafic rocks was therefore validated as being governed by sulphide saturation in parental magma and altered not only by partial melting but also by fractionation during their production in the Supra‐Subduction Zone environment.

Melting of hydrous pyroxenites with alkali amphiboles in the continental mantle: 1. Melting relations and major element compositions of melts

Melting experiments on ultramafic rocks rich in the hydrous minerals phlogopite or phlogopite + K-richterite, some including 5% of accessory phases, have been conducted at 15 and 50 kbar. The assemblages represent probable source components that contribute to melts in cratonic regions, but whose melt compositions are poorly known. A main series of starting compositions based on MARID xenoliths consisted of a third each of clinopyroxene (CPX), phlogopite (PHL) and K-richterite (KR) with or without 5% ilmenite, rutile or apatite. Additional experiments were run without KR and with higher proportions of accessory phases. Melt traps were used at near-solidus temperatures to facilitate accurate analysis of well-quenched melts, for which reversal experiments demonstrate equilibrium.Results show that KR melts rapidly and completely within 50 °C of the solidus, so that melts reflect the composition of the amphibole and its melting reaction. Melts have high SiO2 and especially K2O but low CaO and Al2O3 relative to basaltic melts produced from peridotites at similar pressures. They have no counterparts amongst natural rocks, but most closely resemble leucite lamproites at 15 kbar. KR and PHL melt incongruently to form olivine (OL) and CPX at 15 kbar, promoting SiO2 contents of the melt, whereas orthopyroxene OPX is increasingly stable at lower lithosphere pressures, leading to an increase in MgO and decrease in SiO2 in melts, which resemble olivine lamproites. Melts of mica pyroxenites without KR are richer in CaO and Al2O3 and do not resemble lamproites. These experiments show that low CaO and Al2O3 in igneous rocks is not necessarily a sign of a depleted peridotite source. Accessory phases produce melts exceptionally rich in P2O5 or TiO2 depending on the phases present and are unlike any melts seen at the Earth’s surface, but may be important agents of metasomatism seen in xenoliths. The addition of the 5% accessory phases ilmenite, rutile or apatite result in melting temperatures a few ten of degrees lower; at least two of these appear essential to explain the compositions of many alkaline igneous rocks on cratons.Melting temperatures for CPX + PHL + KR mixtures are close to cratonic geotherms at depths > 130 km: minor perturbations of the stable geotherm at >150 km will rapidly lead to 20% melting. Melts of hydrous pyroxenites with a variety of accessory phases will be common initial melts at depth, but will change if reaction with wall-rocks occurs, leading to volcanism that contains chemical components of peridotite even though the temperature in the source region remains well below the melting point of peridotite. At higher temperatures, extensive melting of peridotite will dilute the initial alkaline melts: this is recognizable as alkaline components in basalts and, in extreme cases, alkali picrites. Hydrous pyroxenites are, therefore, components of most mantle-derived igneous rocks: basaltic rocks should not be oversimplified as being purely melts of peridotite or of mixtures of peridotite and dry pyroxenite without hydrous phases.

The nature of the sub-continental lithospheric mantle beneath Thailand: evidence from xenocrysts in Cenozoic basalts

ABSTRACT Shortly after the cessation of the Cenozoic seafloor spreading in the South China Sea, a period of basaltic magmatism occurred in the Indochina Peninsula. The various types of xenoliths/xenocrysts present in these igneous rocks can provide information about the lithospheric mantle under the Indochina Peninsula. In this study, we present new ages and in situ major and trace element compositions for the xenoliths and olivine xenocrysts from the Wichian Buri and Mae Tha basalt flows in Thailand. The new ages obtained in this study for the Wichian Buri and Mae Tha basalt flows are 9.24 and 2.35 Ma, respectively. Under Thailand, the lithology of the sub-continental mantle lithosphere (SCLM) could be spinel-lherzolite, with a mantle temperature range of 936°C–1096°C. The mantle source of the Cenozoic basalts from Thailand and its adjacent regions could be affect by recycled oceanic crust and carbonates. The origin of spongy textures in clinopyroxene should be resulted from partial melting induced by decompression. SCLM under Thailand were transformed by peridotite-melt interaction, and basaltic melt trapped the mantle xenocryst and xenolith quickly ascended to the surface.

Chromitite layers indicate the existence of large, long-lived, and entirely molten magma chambers

The classical paradigm of the ‘big magma tank’ chambers in which the melt differentiates, is replenished, and occasionally feeds the overlying volcanos has recently been challenged on various grounds. An alternative school of thought is that such large, long-lived and largely molten magma chambers are transient to non-existent in Earth’s history. Our study of stratiform chromitites in the Bushveld Complex—the largest magmatic body in the Earth’s continental crust—tells, however, a different story. Several chromitites in this complex occur as layers up to 2 m in thickness and more than 400 kms in lateral extent, implying that chromitite-forming events were chamber-wide phenomena. Field relations and microtextural data, specifically the relationship of 3D coordination number, porosity and grain size, indicate that the chromitites grew as a 3D framework of touching chromite grains directly at the chamber floor from a basaltic melt saturated in chromite only. Mass-balance estimates imply that a few km thick column of this melt is required to form each of these chromitite layers. Therefore, an enormous volume of melt appears to have been involved in the generation of all the Bushveld chromitite layers, with half of this melt being expelled from the magma chamber. We suggest that the existence of thick and laterally extensive chromitite layers in the Bushveld and other layered intrusions supports the classical paradigm of big, albeit rare, ‘magma tank’ chambers.