Phase Equilibrium Constraints Research Articles

Phase equilibrium constraints on the pre-eruptive conditions of alkaline basalts of the Main Ethiopian Rift and their bearing on the production of peralkaline rhyolites

Abstract Bimodal magmatism is characteristic of the geodynamic evolution of the Main Ethiopian Rift (MER), which is a reference area for the study of the processes leading to continental break-up before seafloor spreading. There are abundant emissions of basalts and rhyolites, which are in possible parent-daugther relationships. However, the P-T-H2O conditions of production and storage of the basaltic end member remain unclear. Crystallization experiments have been conducted on an alkali basalt from the MER to define its pre-eruptive conditions and shed light on the compositional evolution of derivative liquids and source conditions. The experiments were performed at 100-200 MPa, 975-1080°C, varying H2O/CO2 ratios, corresponding to melt water contents of 1-5 wt%, at fO2 slightly lower than the Fayalite-Magnetite-Quartz (FMQ) solid buffer. Comparison between the petrological attributes of the starting rock and the experiments shows that the basaltic magma was stored at 150-200 MPa, 1050 ± 10°C, with 1-2 wt% H2O in melt, with fO2 near FMQ prior to eruption. Geochemical modelling shows that the corresponding mantle source contained about 0.1 wt% H2O, reflecting a metasomatized source. Extensive crystallization of such basalts produces SiO2-rich liquids, which are not yet peralkaline, however. This underscores that extreme fractionation (>90 wt%) is required in order to produce peralkaline derivatives from mildly alkaline basalts. This extreme fractionation and the water-rich nature of the starting basalt readily explain the H2O-rich condition of peralkaline rhyolites that have fueled caldera forming eruptions in the Rift.

An Algorithm for Thermodynamic Parameter Optimization: Application to the Martian Mantle

AbstractThe compilation of thermodynamic models for geophysical applications is such a tedious and complex process that it is generally impractical for researchers to refit parameters in existing models in light of new constraints. To mitigate this difficulty, we develop a Bayesian algorithm that permits the modification of a thermodynamic model to account for additional observational constraints. This algorithm can be applied to any thermodynamic dataset and can utilize a wide variety of experimental constraints. To demonstrate the applicability of the algorithm it is used to revise the Stixrude and Lithgow‐Bertelloni (2011, https://doi.org/10.1111/j.1365‐246x.2010.04890.x), whole‐mantle terrestrial thermodynamic model, using phase equilibrium constraints provided by Bertka and Fei (1997, https://doi.org/10.1029/96jb03270), for the more iron‐rich compositions that are thought to be relevant to the Martian mantle. The revised thermodynamic model provides a more reliable prediction of phase equilibria in the Martian mantle. Seismic properties are calculated in an internally self‐consistent manner along hot and cold areotherms to constrain the upper and lower bounds of these properties for different bulk silicate Mars compositional models.

Paired metamorphism in the Neoarchean: A record of accretionary-to-collisional orogenesis in the North China Craton

Paired metamorphism is the hallmark of asymmetric subduction at Phanerozoic convergent plate margins, yet spatially and temporally linked paired metamorphic belts have rarely been documented in the Archean rock record. Here, we investigate the Neoarchean Dengfeng Complex, a typical granite–greenstone belt in the southern part of the North China Craton (NCC). Using petrography, geochronology, and phase equilibrium constraints on rocks from different lithostructural units, we suggest the ca. 2.54–2.50-Ga Dengfeng Complex represents a remnant of a spatially and temporally linked Neoarchean paired metamorphic belt. In the western part of the complex, a garnet amphibolite from a dominantly metabasic unit (MBU) records near peak P–T conditions of 6.3–10 kbar and ∼675–750°C, corresponding to a high thermobaric ratio (T/P) of ∼720–1200°C/GPa. Tonalite–trondhjemite–granodiorite (TTG) gneisses preserve evidence for partial melting, also consistent with relatively high T/P (∼875–1400°C/GPa). In the eastern part of the Dengfeng Complex, two garnet amphibolites and three garnet quartz–mica schists from a dominantly metasedimentary unit (MSU) record peak P–T conditions of >9.8 kbar and ∼525–655°C, corresponding to intermediate thermobaric ratios of ∼425–600°C/GPa. Zircon and titanite U–Pb dating, coupled with existing ages of post-kinematic intrusions, constrain the metamorphic age to ca. 2.52–2.50 Ga. The metamorphic P–T data indicate that the MSU was buried to >30 km then exhumed to the near-surface by the early Paleoproterozoic. Our metamorphic results, when combined with recent structural and geochemical data, suggest the Dengfeng Complex records Neoarchean paired metamorphism, in which the higher thermal gradients reflect the arc–forearc region and the lower thermal gradients correspond to the accretionary complex. Our data indicate that the southern NCC experienced accretionary-to-collisional orogenesis with intra-oceanic subduction, forearc accretion (ca. 2.54–2.50 Ga), and subsequent arc–continent collision (ca. 2.50–2.47 Ga). The recognition of paired metamorphism in the NCC is consistent with widespread subduction and the operation of a plate tectonic regime in the late Archean.

Phase equilibria constraints on crystallization differentiation: insights into the petrogenesis of the normally zoned Buddusò Pluton in north-central Sardinia

Abstract The Buddusò Pluton in NE Sardinia (Italy) is a normally zoned intrusion composed of three units with chemical composition ranging from hornblende-bearing tonalites (SiO 2 ∼ 65 wt%) to leucocratic monzogranites (SiO 2 ∼ 76 wt%). Zircon crystals in the pluton are dated at 292.2 ± 0.7 Ma and have ε Hf values ranging from −4 to −8, with no systematic differences observed between the units. The pluton, which is isotopically homogeneous at the whole-rock scale in terms of Sr and Nd isotopes, shows textural evidence indicating local crystal–melt segregation. In this paper, we have implemented a novel approach based on path-dependent phase-equilibria modelling to test the hypothesis that the internal chemical variability of the pluton was generated by crystallization differentiation of a homogeneous parental magma. Our modelling indicates that this hypothesis is valid if the mechanism by which this occurs is compaction in a rheologically locked crystal-rich magma and if the separation occurs at 0.3 GPa from a tonalitic magma with water content >2 wt%. Finally, a subset of the magmatic enclaves in the pluton are considered to be autoliths, formed by the disruption of the compacted crystal mush and interaction between these cumulates and the felsic melt.

Effect of water on the phase relations of primitive K-basalts: Implications for high-pressure differentiation in the Phlegraean Volcanic District magmatic system

Phase relations, compositions, and modes of two natural primitive alkaline basalts were determined through experiments performed at 0.8 GPa, temperatures between 1000 and 1310 °C, and nominal water (H2Oi) contents varying from <1 (natural water rock content) to 6 wt%. The two natural samples used as starting material are high-Mg, moderate-K, alkaline basalt scoria clasts (Mg# = 0.68–0.66, Σalkalis = 4.4 wt%) dispersed within the hydromagmatic tuff of the Solchiaro eruption (Procida Island, southern Italy). These samples are considered representative of parental magmas feeding the Phlegraean Volcanic District, which includes the Campi Flegrei caldera and the Procida and Ischia islands. The composition of the experimental phases has been compared with that of natural rocks to constrain the early stages of crystallization of poorly differentiated magmas from the Phlegraean Volcanic District. Our study shows that the liquidus temperature at H2Oi < 1 wt% is ~1300 °C whereas with water contents between 1.5 and 3 wt%, and between 4 and 6 wt%, the apparent liquidus temperature decreases of about 50–100 °C, respectively. Under natural water conditions (<1 wt%), the liquidus phase is clinopyroxene, followed by olivine and Cr-spinel and, then, by plagioclase. The increase of H2O content in the system enhances olivine stability along with clinopyroxene at the liquidus temperature. The 4–6 wt% water bearing experiments show evidence of H2O saturation at 1100 °C, with olivine being replaced by orthopyroxene later joined by pargasitic amphibole and minor amounts of oxide starting from 1080 °C. The composition of glasses in the experiments with low H2Oi content is consistent with the trachybasaltic products of Procida; glasses obtained in the runs with 1.5–3 wt% water resemble the trend formed by the shoshonitic products of Campi Flegrei. The water-saturated experiments produced variably alkali-depleted residual glasses whose composition converges toward the subalkaline field. The compositions of the experimental clinopyroxenes match well those found in the Campi Flegrei products for which we have estimated crystallization conditions from 1140 to 1200 °C of temperature and from 0.6 to 0.9 GPa of pressure (22–33 km). Phase equilibria constraints and clinopyroxene compositions suggest that the parental magmas of Procida likely contained <2 wt% of H2O and were affected by an early stage of fractionation at a depth corresponding to the local Moho (~ 25 km corresponding to ~ 0.8 GPa assuming an average crustal density of 2.8 g/cm3). Based on our results, an early magmatic storage at this level could have caused a deep degassing process able to remove variable proportions of Na and K from the melt.

Syn-collisional felsic magmatism and continental crust growth: A case study from the North Qilian Orogenic Belt at the northern margin of the Tibetan Plateau

The abundant syn-collisional granitoids produced and preserved at the northern Tibetan Plateau margin provide a prime case for studying the felsic magmatism as well as continental crust growth in response to continental collision. Here we present the results from a systematic study of the syn-collisional granitoids and their mafic magmatic enclaves (MMEs) in the Laohushan (LHS) and Machangshan (MCS) plutons from the North Qilian Orogenic Belt (NQOB). Two types of MMEs from the LHS pluton exhibit identical crystallization age (~430 Ma) and bulk-rock isotopic compositions to their host granitoids, indicating their genetic link. The phase equilibrium constraints and pressure estimates for amphiboles from the LHS pluton together with the whole rock data suggest that the two types of MMEs represent two evolution products of the same hydrous andesitic magmas. In combination with the data on NQOB syn-collisional granitoids elsewhere, we suggest that the syn-collisional granitoids in the NQOB are material evidence of melting of ocean crust and sediment. The remarkable compositional similarity between the LHS granitoids and the model bulk continental crust in terms of major elements, trace elements, and some key element ratios indicates that the syn-collisional magmatism in the NQOB contributes to net continental crust growth, and that the way of continental crust growth in the Phanerozoic through syn-collisional felsic magmatism (production and preservation) is a straightforward process without the need of petrologically and physically complex processes.

Ti 3+ – and Ti 4+ – rich fassaites at the birth of the solar system: Thermodynamics and applications

A thermodynamic model is developed for pyroxenes in the chemical (mineralogical) system CaMgSi2O6 (diopside, Di, 1) – CaTiAlSiO6 (grossmanite, Gr, 2) –CaMg1/2 Ti1/2AlSiO6 (alumino – buffonite, AlBf, 3) – CaAl2SiO6 (Calcium Aluminum Tschermaks, CATS, 4), aka fassaites. It is formulated for the assumptions of molecular mixing between AlBf and Di, Gr and CATS components, and that the reduction in configurational entropy resulting from short – range Al – Si ordering on tetrahedral sites may be described using, as a proxy, a long–range, convergent ordering parameter for ordering of Al and Si between A and B tetrahedral sites on the AlBf – free tetrahedral sublattice. Nine out of sixteen independent thermodynamic parameters are calibrated to be consistent with calorimetric measurements of Benisek and others (2007), spectroscopic and statistical mechanical constraints on tetrahedral Al – Si ordering in pyroxenes on the Di – CATS binary (Cohen, 1986; Vinograd, 2001; Warren and others, 2001), and phase equilibrium constraints on Di – AlBf miscibility gaps in the Di – AlBf – CATS ternary subsystem (Yang 1975, 1976). The remaining seven independent parameters (for Gr – bearing fassaites) are constrained to be consistent with the distribution of fassaite compositions in calcium – aluminum – rich inclusions in carbonaceous chondrites (CAIs), experimental determinations of fassaite single – phase regions produced at the extremely low oxygen fugacities prevailing during their primary condensation/crystallization (for example, Beckett, ms, 1986), and the assumption that secondary fassaites in Wark – Lovering rims enclosing CAIs were in local equilibrium with the outer surface layers of the earlier (canonical) AlBf – rich fassaites at their layer contacts at temperatures of around 1473 K (Wark and Lovering, 1982; Meeker and others, 1983; Murrell and Burnett, 1987; Ruzicka, 1997; Dyl and others, 2011). In this model there is a miscibility gap between Di–rich and AlBf–rich fassaites on the Di – AlBf join which displays only very limited extension into the fassaite tetrahedron. On the Di – AlBf – CATS face of this tetrahedron there is a second miscibility gap (upper buffonite gap, UBG) with 0.08 ≤ X4fas Within the fassaite tetrahedron at 1473 K the UBG is rapidly destabilized with addition of the Gr component, diminishing in dimension and collapsing to a critical endpoint at X2fas The model for fassaite thermochemistry is used to develop a Ti3+ – Ti4+ fassaite oxygen cosmobarometer based on experimental constraints (for example, Beckett, ms, 1986). Values of oxygen fugacity calculated with this cosmobarometer for fassaites and melilites, judged by Grossman and others (2008) to be coeval in refractory CAIs from the Allende carbonaceous chondrite, are more than three orders of magnitude below that of a gas of solar composition.

Mineral chemistry and phase equilibrium constraints on the origin of accretions formed during copper flash smelting

This paper delves into the constraints on the nature, origin and thermal evolution of the accretions formed in the uptake shaft of the flash smelting furnace operated by Atlantic Copper in Huelva, Spain, outlining recommended practices for preventing accretion buildup. The accretions were investigated using quantitative electron probe microanalysis, X-ray diffraction and digital imaging techniques, and the experimental data on mineral composition, crystal chemistry and textural relationships were interpreted in terms of thermodynamic phase equilibrium in the SiO2-Fe-O-S system. The results suggest that two distinct types of accretions were formed by the fractional crystallization of two coexisting immiscible melts, under changing conditions of oxygen partial pressure (pO2). The type I accretion of magnetite + delafossite ± cuprite ± tridymite ± metallic copper crystallized from a fractionating copper-rich melt at pO2 above about 10−5 atm, while the type II accretion of magnetite + fayalite + metallic copper + chalcocite derived from a melt with lower copper concentration when pO2 levels dropped below that critical level. Phase compositions and textures were consistent with a cooling history of both compositionally contrasting liquids from about 1,250 °C, the liquidus temperature of magnetite, to eutectic or near-eutectic temperatures of around 1,100 °C. The maintenance of appropriate temperatures — above the liquidus temperature of magnetite — and oxygen partial pressure levels may be critical for the prevention of accretion buildup.

Genesis of the Mandongshan gold deposit (Xinjiang, NW China): T-P-ƒS2 and phase equilibria constraints from the Au-As-Fe-S system

The Mandongshan gold deposit, located in the west Junggar (Xinjiang, China), is hosted in Early Carboniferous basalt and tuff. Ore bodies mainly consist of gold-bearing hydrothermal veins, mineralized basalt and tuff with disseminated sulfides. Hydrothermal process could be identified based on ore petrographic studies: quartz-albite-sulfide-native gold (stage I), arsenopyrite-calcite-native gold (II), and pyrite-calcite (III). Microthermometric and laser Raman spectroscopic analysis of fluid inclusions in quartz demonstrate the ore-forming fluid at stage I was moderate temperature (360–398°C), low-salinity (0.5–5.1wt% NaCl equiv), and H2O-CH4-dominant, which evolved to low-salinity (0.2–1.2wt% NaCl equiv) with insignificant content of CH4 at 219–267°C (stage III). Contents of ore elements (As, Au, Cu, Sb, Ba, Cs, Rb) in mineralized basalt are higher than those in fresh basalt. Phase analysis for the Au-As-Fe-S system using SUPCRT92 software package with the updated database of slop98.dat indicates that a decrease of H2S activity and oxygen fugacity, caused by phase separation and fluid-rock reactions, induced native gold precipitation at stage I. The decrease of H2S activity triggered by arsenopyrite crystallization caused native gold deposition at stage II.

The evolution of calcite-bearing kimberlites by melt-rock reaction: evidence from polymineralic inclusions within clinopyroxene and garnet megacrysts from Lac de Gras kimberlites, Canada

Megacrystic (>1 cm) clinopyroxene (Cr-diopside) and garnet (Cr-pyrope) xenocrysts within kimberlites from Lac de Gras (Northwest Territories, Canada) contain fully crystallized melt inclusions. These ‘polymineralic inclusions’ have previously been interpreted to form by necking down of melts at mantle depths. We present a detailed petrographical and geochemical investigation of polymineralic inclusions and their host crystals to better understand how they form and what they reveal about the evolution of kimberlite melt. Genetically, the megacrysts are mantle xenocrysts with peridotitic chemical signatures indicating an origin within the lithospheric mantle (for the Cr-diopsides studied here ~4.6 GPa, 1015 °C). Textural evidence for disequilibrium between the host crystals and their polymineralic inclusions (spongy rims in Cr-diopside, kelyphite in Cr-pyrope) is consistent with measured Sr isotopic disequilibrium. The preservation of disequilibrium establishes a temporal link to kimberlite eruption. In Cr-diopsides, polymineralic inclusions contain phlogopite, olivine, chromite, serpentine, and calcite. Abundant fluid inclusion trails surround the inclusions. In Cr-pyropes, the inclusions additionally contain Al-spinel, clinopyroxene, and dolomite. The major and trace element compositions of the inclusion phases are generally consistent with the early stages of kimberlite differentiation trends. Extensive chemical exchange between the host phases and the inclusions is indicated by enrichment of the inclusions in major components of the host crystals, such as Cr2O3 and Al2O3. This chemical evidence, along with phase equilibria constraints, supports the proposal that the inclusions within Cr-diopside record the decarbonation reaction: dolomitic melt + diopside → forsterite + calcite + CO2, yielding the observed inclusion mineralogy and producing associated (CO2-rich) fluid inclusions. Our study of polymineralic inclusions in megacrysts provides clear mineralogical and chemical evidence for an origin of kimberlite that involves the reaction of high-pressure dolomitic melt with diopside-bearing mantle assemblages producing a lower-pressure melt that crystallizes a calcite-dominated assemblage in the crust.

Low-salinity Liquid-rich Or Vapor-like Fluids in a Porphyry-type Mo Deposit, South Korea

AbstractSmall porphyry-type molybdenum (Mo) mineralization, the Geumeum deposit in the Gyeongsang Basin, South Korea, is associated with the crystallization of a Cretaceous granodiorite, exsolution of magmatic hydrothermal fluids, and related hydrofracturing. Quartz and molybdenite occur with minor amounts of uneconomic chalcopyrite, pyrite, sphalerite, and galena that precipitated from exsolved magmatic fluids and formed hydrothermal fissure-filling vein ores. Three distinct fluid inclusion assemblages responsible for the precipitation of molybdenite are present in the vein quartz. The earliest fluid is represented by low-salinity liquid-rich Type I fluid inclusions, which displayed homogenization temperatures ranging from 298 to 352 °C, and salinities from ∼0 to 9 wt.% NaCl equiv. The Type I inclusions were trapped in a liquid-stable, vapor-absent, one-phase field. The intermediate fluid is represented by CO2-bearing vapor-rich Type IV inclusions, generally showing no visible CO2 liquid phase. These CO2-bearing inclusions totally homogenized by vapor disappearance at 327∼340 °C or vapor bubble expansion at 327∼369 °C, exhibiting near-critical behaviors. The textural relationships and phase equilibria constraints indicate that the Type IV inclusions were derived from a single population, which formed as pressure and temperature fluctuated at near-critical conditions. The latest fluid can be represented by vapor-rich Type II fluid inclusions. No microthermometric data were obtained for these latest assemblages as no visible amounts of liquid phases are evident in small inclusions. Type III inclusions, which commonly refer to halite-bearing inclusions, are absent in the quartz veins studied. The oxygen and hydrogen isotopic fluid compositions of the vein quartz (δ18OSMOW = 4.3 to 6.9‰ and δDSMOW = −65 to −84‰ at 400 °C) is consistent with a magmatic origin with a possible slight influence from meteoric water. Molybdenum mineralization at Geumeum is a product of hypogene hydrothermal processes that were strongly fracture-controlled, highlighting the importance of low-salinity liquid-rich to vapor-like supercritical fluids for the mineralization. It seems likely that the magmas responsible for the formation of the deposit at Geumeum were emplaced at greater depths than those reported for economic porphyry copper deposits in the world. The deposit could thus have survived long periods of erosion, and represent the weakly mineralized “base” of porphyry systems in the Gyeongsang Basin, South Korea.

High-MgO lavas associated to CFB as indicators of plume-related thermochemical effects: The case of ultra-titaniferous picrite–basalt from the Northern Ethiopian–Yemeni Plateau

A comprehensive petrological and geochemical dataset is reported in order to define the thermo-compositional characteristics of Ti (Fe)-enriched picrite–basalt lavas (HT2, TiO2 3–7 wt%), erupted close to the axial zone of the inferred Afar mantle plume, at the centre of the originally continuous Ethiopian–Yemeni CFB plateau (ca. 30Ma) which is zonally arranged with progressively lower Ti basalts (HT1, TiO2 2–4 wt%; LT, TiO2 1–3 wt%) toward the periphery. Integrated petrogenetic modelling based on major and trace element analyses of bulk rocks, minerals, and melt inclusions in olivines, as well as Sr–Nd–Pb–He–O isotope compositional variations enables us to make several conclusions. 1) The phase equilibria constraints indicate that HT2 primary picrites were generated at ca. 1570°C mantle potential temperatures (Tp) in the pressure range 4–5GPa whereas the HT1 and LT primary melts formed at shallower level (<2–3GPa, Tp 1530°C for HT1 and 1430°C for LT). Thus, the Afar plume head was a thermally and compositionally zoned melting region with maximum excess temperatures of 300–350°C with respect to the ambient mantle. 2) The HT2 primary melts upwelled nearly adiabatically to the base of the continental crust (ca. 1GPa) where fractionation of olivine, followed by clinopyroxene, led to variably differentiated picritic and basaltic magmas. 3) Trace element modelling requires that the primary HT2 melts were generated—either by fractional or batch melting (F 9–10%)—from a mixed garnet peridotite source (85%) with 15% eclogite (derived from transitional MORB protoliths included in Panafrican terranes) that has to be considered a specific Ti–Fe and incompatible element enriched component entrained by the Afar plume. 4) The LT, HT1, and HT2 lavas have 143Nd/144Nd=0.5131–0.5128, whereas Sr–Pb isotopes are positively correlated with TiO2, varying from 87 Sr/86Sr 0.7032 and 206Pb/204Pb 18.2 in LT basalts to 87Sr/86Sr 0.7044 and 206Pb/204Pb 19.4 in HT2 picrite–basalts. High 3He/4He (15–20RA) ratios are exclusively observed in HT2 lavas, confirming earlier evidence that these magmas require a component of deep mantle in addition to eclogite, while the LT basalts may more effectively reflect the signature of the pre-existing mantle domains. The comparison between high-MgO (13–22%) lavas from several Phanerozoic CFB provinces (Karoo, Paranà–Etendeka, Emeishan, Siberia, Deccan, North Atlantic Province) shows that they share extremely high mantle potential temperatures (Tp 1550–1700°C) supporting the view that hot mantle plumes are favoured candidates for triggering many LIPs. However, the high incompatible element and isotopic variability of these high-MgO lavas (and associated CFB) suggest that plume thermal anomalies are not necessarily accompanied by significant and specific chemical effects, which depend on the nature of mantle materials recycled during the plume rise, as well as by the extent of related mantle enrichments (if any) on the pre-existing lithospheric section.

Origin of the Powai ankaramite, and the composition, P–T conditions of equilibration and evolution of the primary magmas of the Deccan tholeiites

The Powai ankaramite flow from Mumbai, western Deccan Traps, contains abundant crystals of augite (En59–47Fs10–14Wo27–42, 22–40 modal %, 3–5 mm) and olivine (Fo84–74, 11–16 modal %, 1–2 mm), and minor plagioclase (An71, ~0.5 mm) embedded in a fine-grained matrix. Minor orthopyroxene (En79–77Fs16–19Wo5–4) with irregular and embayed margins is mantled by thick augite overgrowth rims. The Fe–Mg distribution between the large crystals and bulk rock shows disequilibrium, indicating that the ankaramite is a cumulate rock. The ankaramite probably formed by the intrusion of tholeiitic melt into a cumulate pile with olivine, augite, and orthopyroxene left by the crystallization of earlier magmas, resulting in orthopyroxene dissolution and subsequent precipitation of augite. Olivine-hosted melt inclusions and melts represented by the ankaramite groundmass and some associated tholeiitic dikes are multiply saturated with olivine + plagioclase + clinopyroxene at ≤6 kb according to phase equilibrium constraints. Calculations involving incremental addition of equilibrium phases to these melts and to the common aphyric tholeiites of the voluminous Ambenali and Mahabaleshwar Formations show that their primary magmas (wt% SiO2: 48–50, MgO: 9.8–11.8, and FeO: 7.2–7.9, and Mg# 70–74) last equilibrated with spinel lherzolite at ~8–13 kb (~30–49 km depths) and ~1268–1332 °C, and trace element considerations indicate ≤15 % batch melting of mantle. These tholeiitic primary magmas then underwent olivine gabbro fractionation in upper crustal magma chambers at depths ≤23 km. The minimum depth of equilibration of the primary magmas is shallower than the present-day Moho in the Mumbai area based on seismological data, indicating localized mantle upwelling and lower crustal interactions previously inferred from the Nd–Sr isotopic ratios and other geochemical characteristics of the ankaramite and associated tholeiites.

Phase equilibria constraints on the melt fertility of crustal rocks: the effect of subsolidus water loss

AbstractDuring regional prograde metamorphism, H2O generated by ongoing dehydration reactions is likely to be continuously lost from a rock by compaction. Classical melting experiments cannot easily simulate this phenomenon, because ideally, all run products are conserved within the experimental charge, although significant equilibration and H2O generation may occur during heating. Phase equilibria modelling is used to consider the effect of subsolidus water loss (SWL) on subsequent melting relationships of felsic lithologies (including metapelite, metagreywacke and metatonalite) in the suprasolidus. SWL drives the bulk composition towards the minimum saturation point on the boundary of the wet‐melting field and results in significantly reduced subsequent melt generation when compared to melting experiments involving conservation of subsolidus H2O. This effect is most significant at P–T conditions just above the solidus. For initially hydrated rocks, the reduction in melt production causes rheologically critical thresholds (e.g. melt connectivity threshold, melt escape threshold and the solid‐to‐liquid transition) to be intersected at temperatures generally more than 100 °C, higher than predicted by idealized experimental melting curves.

Crystal-poor, multiply saturated rhyolites (obsidians) from the Cascade and Mexican arcs: evidence of degassing-induced crystallization of phenocrysts

A detailed petrological study is presented for six phenocryst-poor obsidian samples (73–75 wt% SiO2) erupted as small volume, monogenetic domes in the Mexican and Cascade arcs. Despite low phenocryst (+microphenocryst) abundances (2–6 %), these rhyolites are each multiply saturated with five to eight mineral phases (plagioclase + orthopyroxene + titanomagnetite + ilmenite + apatite ± zircon ± hornblende ± clinopyroxene ± sanidine ± pyrrhotite). Plagioclase and orthopyroxene phenocrysts (identified using phase-equilibrium constraints) span ≤30 mol % An and ≤15 % Mg#, respectively. Eruptive temperatures (±1σ), on the basis of Fe–Ti two oxide thermometry, range from 779 (±25) to 940 (±18) °C. Oxygen fugacities (±1σ) range from −0.4 to 1.4 (±0.1) log units relative to those along the Ni–NiO buffer. With temperature known, the plagioclase-liquid hygrometer was applied; maximum water concentrations calculated for the most calcic plagioclase phenocryst in each sample range from 2.6 to 6.5 wt%. This requires that the rhyolites were fluid-saturated at depths ≥2–7 km. It is proposed that the wide compositional range in plagioclase and orthopyroxene phenocrysts, despite their low abundance, can be attributed to changing melt water concentrations owing to degassing during magma ascent. Phase-equilibrium experiments from the literature show that higher dissolved water concentrations lead to more Fe-rich orthopyroxene, as well as more calcic plagioclase. Loss of dissolved water leads to a progressive increase in melt viscosity, and phenocrysts often display diffusion-limited growth textures (e.g., dendritic and vermiform), consistent with large undercoolings caused by degassing. A kinetic barrier to microlite crystallization occurred at viscosities from 4.5 to 5.0 log10 Pa s for these rhyolites, presumably because the rate at which melt viscosity changed was high owing to rapid loss of dissolved water during magma ascent.

Phase equilibria constraints on melting of stromatic migmatites from Ronda (S. Spain): insights on the formation of peritectic garnet

AbstractStromatic metatexites occurring structurally below the contact with the Ronda peridotite (Ojén nappe, Betic Cordillera, S Spain) are characterized by the mineral assemblage Qtz+Pl+Kfs+Bt+Sil+Grt+Ap+Gr+Ilm. Garnet occurs in low modal amount (2–5 vol.%). Very rare muscovite is present as armoured inclusions, indicating prograde exhaustion. Microstructural evidence of melting in the migmatites includes pseudomorphs after melt films and nanogranite and glassy inclusions hosted in garnet cores. The latter microstructure demonstrates that garnet crystallized in the presence of melt. Re‐melted nanogranites and preserved glassy inclusions show leucogranitic compositions. Phase equilibria modelling of the stromatic migmatite in the MnO–Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2–O2–C (MnNCaKFMASHOC) system with graphite‐saturated fluid shows P–T conditions of equilibration of 4.5–5 kbar, 660–700 °C. These results are consistent with the complete experimental re‐melting of nanogranites at 700 °C and indicate that nanogranites represent the anatectic melt generated immediately after entering supersolidus conditions. The P–T estimate for garnet and melt development does not, however, overlap with the low‐temperature tip of the pure melt field in the phase diagram calculated for the composition of preserved glassy inclusions in garnet in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O (NCKFMASH) system. A comparison of measured melt compositions formed immediately beyond the solidus with results of phase equilibria modelling points to the systematic underestimation of FeO, MgO and CaO in the calculated melt. These discrepancies are present also when calculated melts are compared with low‐T natural and experimental melts from the literature. Under such conditions, the available melt model does not perform well. Given the presence of melt inclusions in garnet cores and the P–T estimates for their formation, we argue that small amounts (&lt;5 vol.%) of peritectic garnet may grow at low temperatures (≤700 °C), as a result of continuous melting reactions consuming biotite.

Phase equilibria constraints on pre-eruptive magma storage conditions for the 1956 eruption of Bezymianny Volcano, Kamchatka, Russia

Phase equilibria experiments were performed on andesites from the catastrophic 1956 eruption of Bezymianny Volcano, Kamchatka, Russia, to determine pre-eruptive magma storage conditions. Fifteen experiments were conducted under water-saturated conditions, with oxygen fugacity equal to the Ni–NiO oxygen buffer, at temperatures between 775 and 1100°C and pressures between 50 and 200MPa. Simultaneous amphibole and plagioclase crystallization is reproduced at ≤850°C and ≥200MPa. The simultaneous crystallization temperature range of the plagioclase–clinopyroxene–orthopyroxene–Fe–Ti oxide assemblage increases with decreasing pressure, from 840 to 940°C at 150MPa to 940–1020°C at 50MPa. Melt inclusion compositions in plagioclase phenocrysts and matrix glass match experimental melt compositions reproduced at 50–100MPa and ≤50MPa, respectively. Presence of the silica phase in groundmass and mature amphibole breakdown rims suggests that magma has been stored at ca. 3km depth prior to the final ascent for at least 40days. Syn-eruptive ascent led to decompression-driven crystallization, which caused a temperature increase from 850–900°C to 950–1000°C.

Whole-Rock and Mineral Composition Constraints on the Genesis of the Giant Hongge Fe-Ti-V Oxide Deposit in the Emeishan Large Igneous Province, Southwest China

The Hongge giant Fe-Ti-V oxide ore deposit is hosted by a layered intrusion located in the central part of the Emeishan large igneous province, SW China. The intrusion is relatively small in comparison with other typical oxide-bearing intrusions worldwide; it consists of a lower olivine clinopyroxenite zone, a middle clinopyroxenite zone, and an upper gabbro zone (herein referred to as the lower, middle, and upper zones). Most of the economic Fe-Ti-V oxide ore layers occur within the middle zone. The Hongge oxide ores are depleted in REE and enriched in Zr, Hf, Nb, and Ta as compared to the associated clinopyroxenites. This enrichment of elements that are compatible in titanomagnetite is consistent with the interpretation that the ores formed by accumulation of magnetite and ilmenite. As in the nearby coeval Panzhihua Fe-Ti-V oxide deposit described previously by others, mafic silicates in the Hongge deposit have much higher MgO contents than those in other oxide deposits associated with large layered intrusions in the world. This highlights the importance of relatively primitive parental magma becoming saturated in titanomagnetite at an early stage in the genesis of the giant Fe-Ti-V oxide deposits in the Emeishan large igneous province. Phase equilibrium constraints suggest that the parental magma of the Hongge deposit is similar to that of some of the most primitive high Ti basalts in the Emeishan large igneous province. The ferrobasaltic parental magma and the ferropicritic primary magma of the Hongge intrusion are similar in major and trace element composition to the ferropicritic-ferrobasaltic magma in the Pechenga belt, Kola peninsula, Russia. Depletion of incompatible trace elements in the oxide ores and associated rocks in the Hongge intrusion as compared to the coeval high-Ti basalts suggest that not all the magma involved in the development of the Hongge intrusion has been retained in the intrusion. The occurrence of multiple Fe-Ti oxide layers alternating with Fe-Ti oxide-bearing silicate layers within a single zone and the repetitive appearance of sulfides, olivine, and Cr-rich layers suggest that multiple pulses of magma were involved in the formation of the Hongge intrusion and related Fe-Ti-V oxide deposit. We propose that the middle zone of the Hongge intrusion was a magma stepwise flow-through system and that some of the liquid was lost to the peripheral lavas to form the basalt.

Phase equilibrium constraints on a deep crustal metamorphic field gradient: metapelitic rocks from the Ivrea Zone (NW Italy)

AbstractThe metamorphic rocks of the Ivrea Zone in NW Italy preserve a deep crustal metamorphic field gradient. Application of quantitative phase equilibria methods to metapelitic rocks provides new constraints on the P–T conditions recorded in Val Strona di Omegna, Val Sesia and Val Strona di Postua. In Val Strona di Omegna, the metapelitic rocks show a structural and mineralogical change from mica‐schists with the common assemblage bi–mu–sill–pl–q–ilm ± liq at the lowest grades, through metatexitic migmatites (g–sill–bi–ksp–pl–q–ilm–liq) at intermediate grades, to complex diatexitic migmatites (g–sill–ru–bi–ksp–pl–q–ilm–liq) at the highest grades. Partial melting in the metapelitic rocks is consistent with melting via the breakdown of first muscovite then biotite. The metamorphic field gradient in Val Strona di Omegna is constrained to range from conditions of ∼3.5–6.5 kbar at ≈650 °C to ∼10–12 kbar at &gt;900 °C. The peak P–T estimates, particularly for granulite facies conditions, are significantly higher than those of most earlier studies. In Val Sesia and Val Strona di Postua, cordierite‐bearing rocks record the effects of contact metamorphism associated with the intrusion of a large mafic body (the Mafic Complex). The contact metamorphism occurred at lower pressures than the regional metamorphic peak and overprints the regional metamorphic assemblages. These relationships are consistent with the intrusion of the Mafic Complex having post dated the regional metamorphism and are inconsistent with a model of magmatic underplating as the cause of granulite facies metamorphism in the region.

Sr2+–SAPO-34 Prepared via Coupled Partial Detemplation and Solid State Ion Exchange: Effect on Textural Properties and Carbon Dioxide Adsorption

Sr2+–SAPO-34 materials were prepared via solid-state ion exchange (SSIE) to improve their CO2 adsorptive properties, particularly at low partial pressure, and study the effect of the ion exchange treatments on the structural and textural properties of the materials. In the past, these materials have been prepared with traditional liquid-state ion exchange (LSIE) methods yielding a strontium(II) content of about one cation per unit cell, well below the theoretical maximum and probably due to aqueous phase equilibrium constraints. Characterization of the SSIE materials included coupled thermal gravimetric analyses/Fourier transform infrared spectroscopy (TGA/FT-IR), X-ray diffraction (XRD), energy-dispersive analysis by X-rays (EDAX), surface area, and pure component CO2 equilibrium adsorption. Coupled TGA/FT-IR studies were used for the selection of the SSIE temperature for both NH4+–SAPO-34 and as-synthesized Na+–SAPO-34 starting materials. In general, the results indicated that temperatures well above th...