Fayalite Magnetite Quartz Research Articles

Comparison of Cretaceous granitoids of the Chaun tectonic zone to those of the Taigonos Peninsula, NE Asia: rock chemistry, composition of rock forming minerals, and conditions of formation

Abstract. The Cretaceous granitoid complexes of the Eastern Taigonos and the Prybrezhny Taigonos belts (southern part of the Taigonos Peninsula), Tanyurer pluton of the Okhotsk-Chukotka volcanic belt, and the Peekiney, Moltykan, and Telekay plutons of the Chaun tectonic zone are discussed in relation to their structural position, petrography, rock and mineral chemistry and physicochemical conditions of melt crystallization. These granitoid plutons were generated through melting of a compositionally heterogeneous crustal source, with direct contribution from mafic melts produced in the mantle wedge above active or extinct Benioff zones. Variations of the trace-element composition of granitoids are controlled to a greater extent by local compositional peculiarities of the source regions than by the geodynamic regime as such. The final crystallization of these plutons occurred at comparatively shallow depths, between 1–2 and 6–7 km, in a temperature interval of 700–770°C. The depth of emplacement of the bodies decreases with increasing distance from the areas with oceanic and transitional type crust, as does the degree of incompatible element enrichment of the mantle and crustal sources of melts. Variations in fo2 values at the late stages of crystallization of the plutons reach 3–4 orders of magnitude, exceeding the limits of the quartz-fayalite-magnetite (QFM) and nickel-nickel oxide (NNO) buffer equilibria, which likely results from local variations of the source composition.

Experimental calibration of oxygen isotope fractionation between quartz and zircon

We report the results of an experimental calibration of oxygen isotope fractionation between quartz and zircon. Data were collected from 700 to 1000 °C, 10–20 kbar, and in some experiments the oxygen fugacity was buffered at the fayalite–magnetite–quartz equilibrium. Oxygen isotope fractionation shows no clear dependence on oxygen fugacity or pressure. Unexpectedly, some high-temperature data (900–1000 °C) show evidence for disequilibrium oxygen isotope partitioning. This is based in part on ion microprobe data from these samples that indicate some high-temperature quartz grains may be isotopically zoned. Excluding data that probably represent non-equilibrium conditions, our preferred calibration for oxygen isotope fractionation between quartz and zircon can be described by: 1000 ln α qtz – zrc = ( 2.33 ± 0.24 ) × 10 6 / T 2 ( in K ) This relationship can be used to calculate fractionation factors between zircon and other minerals. In addition, results have been used to calculate WR/melt–zircon fractionations during magma differentiation. Modeling demonstrates that silicic magmas show relatively small changes in δ 18O values during differentiation, though late-stage mafic residuals capable of zircon saturation contain elevated δ 18O values. However, residuals also have larger predicted melt–zircon fractionations meaning zircons will not record enriched δ 18O values generally attributed to a granitic protolith. These results agree with data from natural samples if the zircon fractionation factor presented here or from natural studies is applied.

The Plinian Lower Pumice 2 eruption, Santorini, Greece: Magma evolution and volatile behaviour

The Plinian Lower Pumice 2 (LP2) eruption (172 ka) was one of the first major caldera-forming eruptions of the Santorini volcanic complex (Greece). The eruption shows some striking similarities to the caldera-forming Late Bronze Age (Minoan) eruption in terms of field, petrological and geochemical characteristics of its eruptive products, which are used to reveal the storage conditions of the LP2 magmas, pre-eruptive magmatic processes and the behaviour and degassing of volatiles prior to and during eruption. The LP2 eruption comprises four, predominantly rhyodacitic eruptive units (LP2-A, B, C, D). The lowermost unit of the Plinian LP2 deposits (LP2-A) consists of a basal phreatomagmatic bed (LP2-A1), which is overlain by three discrete pumice fall deposits (LP2-A2-1, A2-2, A3), the most prominent of which (LP2-A3) contains abundant, quench-textured scoriae that range in composition from basalt to basaltic andesite. The eruption proceeded with the deposition of pumice-rich pyroclastic flows (LP2-B) characterised by a lower, stratified and cross-bedded ignimbrite (LP2-B1) that may grade into a massive, non-welded ignimbrite (LP2-B2), a lithic-rich pumiceous breccia (LP2-C) and a co-ignimbrite lithic lag breccia (LP2-D). The main volume of rhyodacitic magma, which formed by fractionation of olivine, clinopyroxene, orthopyroxene, plagioclase, amphibole, Fe–Ti oxides, pyrrhotite and apatite from basaltic parental magmas and assimilation of crustal rocks, was held at mid-crustal levels (≤ 16 km depth), magmatic temperatures of 831 ± 12 °C and an oxygen fugacity slightly above the fayalite–magnetite–quartz (FMQ) oxygen buffer. Injection of ∼ 200 °C hotter mafic magma into the rhyodacitic reservoir and subsequent mingling and minor hybridisation with the resident magma helped to remobilise the rhyodacitic host magma and determined the final compositional range of the erupted products. Melt inclusion data show that sulphur concentrations were reduced to < 270 ppm in the rhyodacite, primarily due to partitioning of sulphur into pyrrhotite or, depending on temperature, a FeS-rich melt during magmatic differentiation at oxygen fugacities around the FMQ oxygen buffer. Sulphur concentrations in groundmass glasses of the LP2 pumices suggest that ∼ 43% of the remaining sulphur was released into the atmosphere during the LP2 eruption, the climatic effects of which are considered minor when compared to eruptions of more oxidised silicic arc magmas. Chlorine remained dissolved in the melt during magmatic differentiation prior to and during the LP2 eruption, indicating that chlorine emissions to the atmosphere were negligible.

Mid-ocean ridge and supra-subduction geochemical signatures in spinel–peridotites from the Neotethyan ophiolites in SW Turkey: Implications for upper mantle melting processes

The Lycian and Antalya ophiolite complexes in SW Turkey represent fragments of oceanic lithosphere emplaced following the closure of the Neotethys Ocean during the Late Cretaceous. The peridotites from both of these ophiolites have compositions ranging from relatively undepleted lherzolites to highly depleted harzburgites and display a diverse suite of geochemical signatures indicative of both anhydrous, mid-ocean ridge (MOR)-type and hydrous, supra-subduction zone (SSZ)-type melting regimes. Whole-rock major and trace element systematics and mineral chemistry indicate that the MOR- and SSZ-type peridotites represent the residues from 5–9% and 13–25% of mantle melting, respectively, and display evidence for a multi-stage evolution of an oceanic lithosphere. Olivine–orthopyroxene–spinel equilibria indicate that the clinopyroxene-bearing, MOR-type peridotites are moderately reduced with their oxygen fugacity (fO2) ranging from −2.22 to −1.44 log units relative to the FMQ (fayalite–magnetite–quartz) buffer and are similar to abyssal peridotites, while more refractory, SSZ-type harzburgites and dunites (<4% modal clinopyroxene) are more oxidized with their higher oxygen fugacity (FMQ −0.72 to +1.02). The latter can readily be explained by interaction with oxidizing melts/fluids originating from a subducted slab in a SSZ environment. Precise determination of trace element compositions of residual clinopyroxenes by LA-ICPMS indicates that the SSZ peridotites are strongly depleted in Ti and HREE and enriched in Zr and LMREE, compared to the most depleted MOR-type peridotites. The results of quantitative model calculations show that moderate degrees of anhydrous mantle melting accounts for the composition of MOR peridotites, while SSZ peridotites are likely to have originated from hydrous, orthopyroxene-dominated remelting of previously depleted mantle. For the case of hydrous melting, it is inferred that flux of a slab-derived fluid component sustained further melting of MORB-depleted peridotite, resulting in higher degrees of depletions in the SSZ mantle than the anhydrous melting in the MOR setting. Interaction of SSZ-type melts with depleted peridotites enriched the source mantle in more incompatible trace elements, resulting in elevated Zr/Ti and LMREE/HREE ratios in peridotites. The occurrence of both MOR and SSZ styles of melting regimes indicates that these ophiolites contain mantle residues from discrete stages of oceanic lithosphere generation.

Estimation of pre-eruptive magmatic water fugacity in the Phlegrean Fields, Naples, Italy

We estimated the water fugacity (fH(2)O) in the trachytic magma of Phlegrean Fields using the biotite-magnetite-sanidine equilibrium. We confirm that the partly ionic model is the most appropriate to estimate annite activity (a(annite)) for the experimental biotite. Crystallization experiments were carried out on a representative sample of trachytic Breccia Museo eruption, Naples, Italy. Experiments were performed in the temperature and pressure range or 725-870 degrees C and 50-200 MPa, and redox conditions ranging from NNO (nickel- nickel oxide) + 1 (+/- 0.2) to FMQ (fayalite + magnetite + quartz). Most experiments were done under conditions of H2O saturation (P-water = P-total). Few experiments were done using a mixed H2O-CO2 fluid phase. The pre-eruptive fH(2)O of the Phlegrean Fields magma is estimated at 735 bars, which compares well with available melt inclusion constraints. Our results confirm that this geohygrometer can be used in any magmatic systems bearing the worked assemblage.

Breakdown of hydrous ringwoodite to pyroxene and spinelloid at high P and T and oxidizing conditions

To get deeper insight into the phase relations in the end-member system Fe2SiO4 and in the system (Fe, Mg)2SiO4 experiments were performed in a multi-anvil apparatus at 7 and 13 GPa and 1,000–1,200°C as a function of oxygen fugacity. The oxygen fugacity was varied using the solid oxygen buffer systems Fe/FeO, quartz–fayalite–magnetite, MtW and Ni/NiO. The run products were characterized by electron microprobe, Raman- and FTIR-spectroscopy, X-ray powder diffraction and transmission electron microscopy. At fO2 corresponding to Ni/NiO Fe-ringwoodite transforms to ferrosilite and spinelloid according to the reaction: 9 Fe2SiO4 + O2 = 6 FeSiO3 + 5 Fe2.40Si0.60O4. Refinement of site occupancies in combination with stoichiometric Fe3+ calculations show that 32% of the total Fe is incorporated as Fe3+ according to \( \left ( {{\text{Fe}}_{1.60}^{2 + } {\text{Fe}}_{0.40}^{3 + } } \right)^{\text{VI}} \left ( {{\text{Si}}_{0.60} {\text{Fe}}_{0.40}^{3 + } } \right)^{\text{IV}} {\text{O}}_{4} . \) From the Rietveld refinement we identified spl as spinelloid III (isostructural with wadsleyite) and/or spinelloid V. As we used water in excess in the experiments the run products were also analyzed for structural water incorporation. Adding Mg to the system increases the stability field of ringwoodite to higher oxygen fugacity and the spinel structure seems to accept higher Fe3+ but also water concentrations that may be linked. At oxygen fugacity corresponding to MtW conditions similar phase relations in respect to the breakdown reaction in the Fe-end-member system were observed but with a strong fractionation of Fe into spl and Mg into coexisting cpx. Thus, through this strong fractionation it is possible to stabilize very Fe-rich wadsleyite with considerable Fe3+ concentrations even at an intermediate Fe–Mg bulk composition: assuming constant K D independent on composition and a bulk composition of x Fe = 0.44 this fractionation would stabilize spl with x Fe = 0.72. Thus, spl could be a potential Fe3+ bearing phase at P–T conditions of the transition zone but because of the oxidizing conditions and the Fe-rich bulk composition needed one would expect it more in subduction zone environments than in the transition zone in senso stricto.

Sulphur solubility in andesitic to basaltic melts: implications for Hekla volcano

The solubility of sulphur in sulphide-saturated, H2O-bearing basaltic–andesitic and basaltic melts from Hekla volcano (Iceland) has been determined experimentally at 1,050°C, 300 and 200 MPa, and redox conditions with oxygen fugacity (logfO2) between QFM−1.2 and QFM+1.1 (QFM is a quartz–fayalite–magnetite oxygen buffer) in the systems containing various amounts of S and H2O. The S content of the H2O-rich glasses saturated with pyrrhotite decreases from 2,500 ppm in basalt to 1,500 ppm in basaltic andesite at the investigated conditions. Furthermore, the reduction of water content in the melt at pyrrhotite saturation and fixed T, P and redox conditions leads to a decrease in S concentration from 2,500 to 1,400 ppm for basaltic experiments (for H2O decrease from 7.8 to 1.4 wt%) and from 1,500 to 900 ppm (for H2O decrease from 6.7 to 1.7 wt%) for basaltic andesitic experiments. Our experimental data, combined with silicate melt inclusion investigations and the available models on sulphide saturation in mafic magmas, indicate that the parental basaltic melts of Hekla were not saturated with respect to sulphide. During magmatic differentiation, the S content in the residual melts increased and might have reached sulphide saturation with 2,500 ppm dissolved S. With further magma crystallization, the S concentration in the melt was controlled by the sulphide saturation of the magma, decreasing from ~2,500 to 900 ppm S.

The origin of trachyte and pantellerite from Pantelleria, Italy: Insights from major element, trace element, and thermodynamic modelling

Trachyte and peralkaline rhyolite (pantellerite and comendite) frequently comprise the felsic end-member in bimodal suites in continental rift and oceanic island settings. In these settings, the relationship between the mafic (mildly alkaline, or transitional, basalt) and felsic lavas is ambiguous; major- and trace-element models and isotopic data are often consistent with an origin for felsic lavas from either fractional crystallization of transitional basalt or partial melting of alkali gabbro followed by fractional crystallization. In this paper, we present representative mineral analyses and whole-rock analyses from forty samples of a basalt–trachyte–pantellerite suite collected at Pantelleria, Italy, in the Strait of Sicily Rift Zone, and compare the results of major- and trace-element modelling with the results of thermodynamic (MELTS) modelling. From these results we conclude that metaluminous trachyte formed as a result of 70 to 75% low-pressure (0.1 GPa) fractional crystallization of an assemblage of plagioclase, clinopyroxene, olivine, magnetite, and apatite from a hydrous (1.0–1.5 wt.% H 2O) transitional basalt magma at relative oxygen fugacities approximately one log unit below the fayalite–magnetite–quartz buffer (FMQ−1). The “Daly gap”–a lack of intermediate (~ 49–62 wt.% SiO 2) volcanic rocks–at Pantelleria is concluded to be primarily the result of rapid differentiation through that interval. Relatively rapid crystallization at low pressure may have effected the partial degassing of water-saturated (~ 4 wt.% H 2O) metaluminous trachyte magma. Some metaluminous trachyte lavas have positive Eu anomalies, high K/Rb ratios, high concentrations of Ba, and low concentrations of incompatible trace elements; these are interpreted to be the result of up to 40% accumulation of alkali feldspar. Comenditic trachyte, pantelleritic trachyte, and pantellerite formed after an additional 20 to 80% fractional crystallization of an assemblage dominated by alkali feldspar from metaluminous trachyte magma. The most evolved pantellerite lavas and tuffs are the result of a total of ~ 95% fractional crystallization of transitional basalt, with phenocrysts that equilibrated at low temperatures (< 700 °C), low oxygen fugacities, and high (> 4 wt.%) water contents.

The Magmatic and Fluid Evolution of the Motzfeldt Intrusion in South Greenland: Insights into the Formation of Agpaitic and Miaskitic Rocks

The 1� 275 Ga Motzfeldt intrusive complex in the Gardar failed-rift Province in South Greenland formed from six successively intruding melt batches (SM1^SM6) interpreted to be derived from a common magma source at depth. Five units (SM1^SM5) crystallized an alkaline to peralkaline, miaskitic mineral assemblage of amphibole, clinopyroxene, feldspar, nepheline, Fe^Ti oxides, zircon, apatite, fluorite and rarely olivine.The last magmatic batch (SM6) is characterized by an agpaitic mineral assemblage of aegirine, nepheline, alkali-feldspar, eudialyte and rare fluorite or sodalite. Coexisting mafic minerals constrain the crystallization conditions of the miaskitic rocks to about 850^6008C, whereas solidus temperatures below 5008C are indicated by coexisting alkali feldspars in the agpaitic rocks. Oxygen fugacities during the orthomagmatic stage are below the FMQ (fayalite^ magnetite^quartz) buffer (FMQ ^0 � 5t o ^2� 0) whereas late hematite provides evidence of a higher relative oxygen fugacity during late-stage alteration. The Nd and oxygen isotope compositions of amphiboles and pyroxenes are homogeneous throughout the complex and suggest a common, mantle-derived magma source for all six units which is comparable with other Gardar intrusions.The hydrogen isotopic composition of amphiboles (dD^99 to ^132%) indicates lowtemperature fluid^rock interaction with low fluid^rock ratios. Fluid inclusion studies indicate that H2O^NaCl fluids present during the magmatic stages in the miaskitic units had salinities of510 wt % NaCl eq. Calcite crystals in fluid inclusions within these rocks suggest that CO2 or HCO3 ^ was an important component of the original fluid phase. In contrast, the agpaitic unit is characterized by a CH4^H2O^ NaClfluid.The C^O^H isotope compositionsofthefluid inclusions in all units are consistent with mixing between a small volume of magmatic fluid and a large volume of meteoric water. The chemical evolution of the Motzfeldt complex is a type example of the connection of the transition from miaskitic to agpaitic mineral assemblages with redox-dependent fluid^solid equilibria.Thetransitionfromarelatively oxidized to a relatively reduced fluid is correlated with a change from a more reduced, Fe 2þ -bearing miaskitic (taramite^arfvedsonite, zircon, SM1^SM5) to a more oxidized, Fe 3þ -bearing agpaitic assemblage (aegirine, eudialyte, SM6).We suggest that coupled fluid^solid redox equilibria involving Fe 2þ ,F e 3þ ,C O2 and CH4 in this case were simply driven by temperature decrease and an overall increase in Na (þ K) in the melt.This observation sheds light on the heavily debated miaskite^ agpaite transition.The combined temperature and compositional effect stabilized Fe 3þ ,C H 4 and enhanced the solubility of Zr (possibly as Na^Zr^Si^O complexes) in the latest stage melt (SM6) resulting in the crystallization ofan agpaitic mineral assemblage. In terms of redox conditions during crystallization, the Motzfeldt rocks represent an intermediate case in the Gardar Province between more oxidized, CO2dominated intrusions such as the syenite^carbonatite complex of

Interaction of magma with sedimentary wall rock and magnetite ore genesis in the Panzhihua mafic intrusion, SW China

In southwestern China, several large magmatic Fe–Ti–V oxide ore deposits are hosted by gabbroic intrusions associated with the Emeishan flood basalts. The Panzhihua gabbroic intrusion, a little deformed sill that contains a large titanomagnetite deposit at its base, concordantly intrudes late-Proterozoic dolostones. Mineralogical and chemical studies of the contact aureole in the footwall dolostones demonstrate that the metamorphism was largely isochemical but released large quantities of CO2 as the rocks were converted to marble and skarns during intrusion of the gabbroic magma. Petrological modelling of the crystallization of the intrusion, using H2O-poor Emeishan basalt as parent magma, shows that under normal conditions, Fe–Ti oxides crystallize at a late stage, after the crystallization of abundant olivine, clinopyroxene and plagioclase. In order for titanomagnetite to separate efficiently to form the ore deposit, this mineral must have crystallized earlier and close to the liquidus. We propose that CO2-rich fluids released during decarbonatization of sedimentary floor rocks passed up through the magma. Redox equilibria calculations show that when magma with the composition of Emeishan basalt is fluxed by a CO2-rich gas phase, its equilibrium oxygen fugacity (fO2) increases from the fayalite–magnetite–quartz buffer (FMQ) to FMQ + 1.5. From experimental constraints on magnetite saturation in basaltic magma under controlled fO2, such an oxidizing event would allow magnetite to crystallize near to the liquidus, leading to the formation of the deposit.

The Redox State of Earth's Mantle

Oxygen thermobarometry measurements on spinel peridotite rocks indicate that the oxygen fugacity at the top of the upper mantle falls within ±2 log units of the fayalite-magnetite-quartz (FMQ) oxygen buffer. Measurements on garnet peridotites from cratonic lithosphere reveal a general decrease in fo2 with depth, which appears to result principally from the effect of pressure on the controlling Fe3+/Fe2+ equilibria. Modeling of experimental data indicates that at approximately 8 GPa, mantle fo2 will be 5 log units below FMQ and at a level where Ni-Fe metal becomes stable. Fe-Ni alloy and an Fe2O3-garnet component will be formed as a result of the disproportionation of FeO, which is experimentally demonstrated through observations of high Fe3+/ΣFe ratios in minerals in equilibrium with metallic Fe. In the lower mantle, the favorable coupled substitution of Al and Fe3+ into (Fe,Mg)SiO3 perovskite results in very high perovskite Fe3+/ΣFe ratios in equilibrium with metallic Fe. As a result, the lower mantle should contain approximately 1 weight% metallic Fe formed through FeO disproportionation, if the bulk oxygen content is the same as the upper mantle. Loss of disproportionated metallic Fe from the lower mantle during core formation could explain the higher Fe3+/ΣFe ratio of the present-day upper mantle when compared to that expected during core formation. The influence of pressure on mantle fo2 has important implications for the speciation of C-O-H-S volatile phases in Earth today and during its early evolution.

Solubility of H 2O- and chlorine-bearing fluids in basaltic melt of Mount Etna at T = 1050–1250 °C and P = 200 MPa

An experimental study on the solubility of H–O–Cl-bearing fluids in basaltic melts has been conducted at total pressure of 200 MPa, in the temperature range from 1050 to 1250 °C and at redox conditions corresponding to lof fO 2 ∼ QFM + 1 (QFM = quartz–fayalite–magnetite oxygen buffer). A natural trachybasaltic sample of Mount Etna's July–August 2001 eruption has been used as starting material. Water concentrations in the melt (H 2O melt) increase from 4.6 up to 5.4 wt.% with an increase in Cl concentration in the melt (Cl melt) from 0 to 2.5 wt.%. The H 2O melt dramatically decreases to ∼ 2–3 wt.% at Cl melt > 2.5 wt.%. The saturation of the melt with Cl is reached at 4 wt.% Cl melt which is significantly higher than predicted by the existing models. Although increasing total Cl concentration in the system (bulk Cl in fluid and melt) leads to significant extraction of cations like Na, K and Fe from the melt into the fluid(s), the observed systematic increase in H 2O melt cannot be directly attributed to the change in melt composition, indicating that the presence of Cl may decrease the activity coefficient of dissolved H 2O in the melt. The complex non-linear relationships between H 2O and Cl concentrations in basaltic melt and fluid phase point to a strong non-ideality of mixing between fluid components, presumably resulting in an increase in the activity coefficient of Cl in the fluid and in the formation of two fluid phases (vapor and liquid) at the studied conditions. The effect of temperature on the solubility of H–O–Cl-bearing fluids in trachybasaltic melt and, hence, on the mixing properties of the fluid phase is not resolved from the obtained experimental data. The experimentally determined partition coefficient of Cl between fluid(s) and melt ( K d′Cl, based on a molar ratio) exponentially increases from 0.04 up to 14 with increasing Cl melt and shows almost linear dependence on bulk Cl content of the fluid(s). The K d′Cl values < 1 are observed at Cl melt lower than 1.0 wt.%, explaining the almost constant Cl concentrations (∼ 0.2 wt.%) in melt inclusions and variations of H 2O/Cl and S/Cl ratios in melt inclusions and volcanic gasses with ongoing magma degassing and changes in eruptive activity of Mt. Etna. Thus, the obtained data represent an experimental database necessary for the accurate modeling of natural processes of magma degassing at basaltic volcanoes.

The Alkaline-Peralkaline Tamazeght Complex, High Atlas Mountains, Morocco: Mineral Chemistry and Petrological Constraints for Derivation from a Compositionally Heterogeneous Mantle Source

The Eocene Tamazeght complex, High Atlas Mountains, Morocco is a multiphase alkaline to peralkaline intrusive complex. A large variety of rock types (including pyroxenites, glimmerites, gabbroic to monzonitic rocks, feldspathoidal syenites, carbonatites and various dyke rocks) documents a progression from ultramafic to felsic magmatism. This study focuses on the silicate plutonic members and the genetic relationships between the various lithologies. Based on detailed petrographic and mineral chemical data we show that the various units crystallized under markedly different oxygen fugacity and silica activity conditions and demonstrate how these parameters influence both the phase assemblage and the detailed chemical evolution of the fractionating phases. Nepheline, olivine–clinopyroxene and hornblende–plagioclase thermometry indicate equilibration temperatures ≥800°C for all major rock types. Highly oxidized conditions (close to the hematite–magnetite buffer) are characteristic of the garnet-rich pyroxenites, ultrapotassic glimmerites and associated olivine-shonkinites. The parental magmas to these rocks evolved from low initial a SiO2 values of 0·1 to values of 0·5–0·8 during nepheline and alkali feldspar saturation. In contrast, the monzonitic rocks evolved from initially high a SiO2 values (up to 0·75) down to about 0·1 at intermediate values of oxygen fugacity (ΔFMQ = +2–5 to −1, where FMQ is the fayalite–magnetite–quartz buffer). For nepheline syenites and malignites, more reduced conditions (ΔFMQ = −2) and intermediate a SiO2 values (between 0·25 and 0·5) dominate. We conclude that fractional crystallization is not a likely mechanism to explain the large variety of lithologies present in the Tamazeght complex. It is more probable that successive melting of a compositionally heterogeneous mantle source region gave rise to several melt batches with distinct chemical and physico-chemical characteristics. Low-degree melts from a K-phase-bearing mantle domain resulted in the formation of ultrapotassic glimmerites, whereas garnet-rich pyroxenites and olivine-shonkinites may have originated from hybrid melts and partly from a pyroxene-dominated source. Less alkaline lithologies such as monzonites potentially reflect larger degrees of melting and the increased importance of a basaltic component, whereas nepheline syenites and malignites may be explained by lower degrees of melting and a more alkaline character for the parental melt of these rocks.

The oxidation state of hydrothermal systems on early Enceladus

Abstract The discovery of CO 2 , CH 4 , and N 2 in a plume at Enceladus provides useful clues about the chemistry and evolution of this moon of Saturn. Here, we use chemical equilibrium and kinetic calculations to estimate the oxidation state of hydrothermal systems on early Enceladus, with the assumption that the plume's composition was inherited from early hydrothermal fluids. Chemical equilibrium calculations are performed using the CO 2 /CH 4 ratio in the plume, and kinetic calculations are conducted using equations from fluid dynamics and chemical kinetics. Our results suggest that chemical equilibrium between CO 2 and CH 4 would have been reachable at temperatures above ∼200 °C in hydrothermal systems. The oxidation state of the hydrothermal systems would have been close to the pyrrhotite–pyrite–magnetite (PPM) or fayalite–magnetite–quartz (FMQ) redox buffer (i.e., terrestrial-like) if the plume's CO 2 and CH 4 equilibrated in hydrothermal systems long ago. As for minerals, we suggest that iron metal would have been oxidized to magnetite by the escape of H 2 from the early satellite. Our calculations also indicate that, assuming CO 2 and CH 4 reached chemical equilibrium, magnetite would not have been oxidized to hematite in hydrothermal systems, perhaps due to insufficient H 2 escape. It is shown that, if Enceladus accreted as much NH 3 as comets contain, the presence of N 2 and deficiency of NH 3 in the plume can be understood in the context of chemical equilibrium in the C–N–O–H system. We conclude by proposing an evolutionary hypothesis in which the fairly oxidized nature of the plume can be explained by a brief episode of oxidation caused by short-lived radioactivity. These suggestions can be rigorously tested by acquiring gravity and isotopic data in the future.

Oxygen solubility and speciation in sulphide-rich mattes

Sulphide-rich liquids are common in magmatic environments forming over a wide range of temperature, pressure, fO 2 and fS 2. They are economically important because they sequester valuable metals such as Cu, Ni, Au and Pt from silicate melts. The presence of accessory amounts of primary oxides associated with sulphide mineralisations is often ignored or unexplained. Experimental work has shown that large amounts of oxygen can dissolve into mattes at fO 2 typical of terrestrial environments. At the quartz–fayalite–magnetite fO 2 buffer, the molar fraction of O in the matte exceeds that of S, placing the composition of the matte to the magnetite side of the mss (monosulphide solid solution)–magnetite join in the Fe–S–O system. However, sulphides crystallise before magnetite in most sulphide mineralisations and are much more abundant. Moreover, the speciation of O in a matte is not well known. Here we report the results of an experimental study of the solubility of O in mattes as a function of fS 2, fO 2, temperature, and composition. We confirm previous observations that Ni and Cu have a negative effect on the solubility of O in mattes. We show evidence for the existence of FeSO as a structural constituent of mattes in the Fe–S–O system. We present a simple parameterisation of the amount of O dissolved in mattes under relevant geological conditions, and use this parameterisation to discuss mechanisms for the crystallisation of primary spinels associated with sulphides in the Kambalda massive sulphide deposit (Western Australia) and the Sudbury Igneous Complex (Ontario, Canada).

Oxidation state of Paleozoic subcontinental lithospheric mantle below the Pali Aike volcanic field in southernmost Patagonia

Mantle xenoliths in the Quaternary Pali Aike alkaline basalts of southernmost Patagonia include lherzolites and harzburgites with and without garnet. The values of fO 2 for all xenoliths range from 0.33 logarithmic unit below the fayalite–magnetite–quartz buffer (FMQ − 0.33) to FMQ + 0.75, which overlap those for abyssal peridotites. The fO 2 data, together with the bulk rock and mineral compositions, suggest that the subcontinental lithospheric mantle (SCLM) below Pali Aike formed through the accretion of oceanic lithosphere. The oceanic accretion likely occurred in mid to late Paleozoic time when the southern Patagonian terrane formed along the southwest margin of Gondwana. Relict spinel inclusions in garnet suggest that garnet-facies peridotites formed from spinel-facies peridotites most likely in response to pressure increase during the accretion. Comparable fO 2 for garnet- and spinel-facies peridotites suggests that this spinel-garnet transformation was not accompanied by changes in fO 2. Metasomatism by asthenosphere-derived melt through a slab window resulted in the formation of Ti-bearing minerals, and lowering of Mg and enrichment of Ti in bulk rocks and minerals. The replacement of olivine by orthopyroxene formed orthopyroxenites in extreme cases. No significant change in fO 2 is associated with this metasomatism since similar oxidation state is observed between un-metasomatized (FMQ − 0.23 to + 0.48) and metasomatized (FMQ − 0.33 to + 0.75) samples. This is explained by the fO 2 of the asthenospheric melt (~ FMQ − 0.50) similar to the SCLM prior to the metasomatism.

Ferrospinels and petromagnetism of Siberian traps: A case study of traps drilled by the Norilsk borehole

Magnetic characteristics and compositions of ferromagnetic species of trap samples from the Norilsk borehole are studied; the borehole penetrated the complete section of a trappean rock sequence (nearly 3 km) represented by eight magmatic formations. The lowest (Ivakinskian) formation consists of rocks of the subalkalic type, and the remaining formations generally belong to the tholeiitic series. Primary ferromagnetic phases are represented by titanomagnetites with various ulvospinel concentrations in a solid solution. Three modes characterized by TiO2 concentrations of 14.6, 18, and 23.9 wt % are revealed. The Ivakinskian rocks are most magnetized, and magnetization is low in core samples from depths of 1700–2500 m. Various magnetic characteristics have polymodal distributions of their values. Plateau basalt samples of the Ivakinskian Formation from other regions of Siberia, the Lory Plateau (Armenia), and Kamchatka are additionally studied. On the whole, titanomagnetites of plateau basalts were crystallized under the quartz-fayalite-magnetite (QFM) buffer conditions.

How partial melts of mafic lower crust affect ascending magmas at oceanic ridges

We present experiments showing that the lower oceanic crust should melt efficiently and quickly when heated by hot ascending magmas. Average plagioclase–olivine and plagioclase–augite pairs from the lower crust at the Southwest Indian Ridge have melt–mineral saturation boundaries at 1,190 and 1,154°C, respectively, and melt rapidly (>0.01 mm/h) at 50°C or more above these temperatures. Melting experiments performed on olivine–plagioclase and augite–plagioclase mineral pairs from actual oceanic lower crustal rock samples and under conditions applicable to a MOR setting (1,220–1,330°C, 1 atm, quartz–fayalite–magnetite oxygen buffer, 0.25–24 h) indicate that the resulting disequilibrium melts are linear mixes of the mineral compositions. The rates of melting are slower than the rate of heat-diffusion into a sample and are approximated as: ξ=2.43×10(−26)×e0.004109×T(degC) (m/s1/2) for augite melted with plagioclase; ξ=3.47×10(−15)×e0.002041×T(degC)for plagioclase melted with augite; and ξ=1.79×10(−21)×e0.0032×T(degC) for plagioclase melted with olivine. Our results indicate that great care must be taken in backward models using basalt chemistry alone to explore mantle-melting processes, assuming only crystallization and fractionation during ascent, as partial melts may mix with intruded hot magma.

Ca-rich Ilvaite-Epidote-Hydrogarnet Endoskarns: a Record of Late-Magmatic Fluid Influx into the Persodic Ilimaussaq Complex, South Greenland

Endoskarn assemblages involving the Ca-silicates ilvaite, epidote and Ca-rich garnet occur along fracture zones in the persodic Il|¤ maussaq intrusion, South Greenland. The 1 16 Ga intrusion solidified at a depth of about 3^4 km, below a cover of sandstones and pillow-basalts of the Eriksfjord Formation. In contrast to typical skarn assemblages, the Il|¤ maussaq endoskarns contain albite as a main phase and they did not form in metacarbonate rocks, as these are completely lacking in the vicinity of Il|¤ maussaq. Instead, they record lateto post-magmatic interaction of possibly external Ca-rich fluids with the alkaline to agpaitic magmatic rocks. Accordingly, endoskarn textures clearly reflect the magmatic textures of the precursor rocks. Phase relations in two endoskarn varieties with epidoteþ albiteþ andradite-rich garnet ilvaite retrograde prehnite suggest their formation at about 5008C at high oxygen fugacities slightly above the hematite^magnetite oxygen buffer [FMQ (fayalite^magnetite^quartz)þ 5 to FMQþ 7] with later small modifications as a result of fluid influx or cooling of the original fluid at about 300^3508C (formation of prehnite) and at about 200^2508C (oxygen isotopic re-equilibration of the albite). One model for the formation of the observed assemblages is the decomposition of Ca-bearingminerals, such as primary eudialyte, clinopyroxene or ternary feldspar, and redistribution of the Ca by ametasomatizing late-magmatic fluid. Stable isotope (O, H) investigations, however, favour a model in which seawater was the metasomatizing fluid, which entered the Eriksfjord basalts above the intrusion, reacted with them (spilitization) and brought about 10 3 mol/l Ca along fractures into the metasomatized rocks. Fluid^rock interaction in the Eriksfjord basalts is documented by abundant chlorite^epidote^ quartz assemblages; high fluid/rock ratios allowed the fluid to retain its seawateroxygen isotope composition.

Metasomatism of the upper mantle beneath the Hyblean Plateau (Sicily): evidence from pyroxenes and glass in peridotite xenoliths

Abstract Coarse-grained, protogranular spinel peridotite xenoliths in the Upper Miocene Valle Guffari diatreme display a complex history for the shallow mantle underneath the Hyblean Plateau (SE Sicily). The mineral assemblage and composition (olivine Fo 89–91 , orthopyroxene En 88–91 , Cr-diopside En 48–49 Fs 4–6 Wo 45–48 , Cr-rich spinel with cr-number=25–39) record at least one depletion event caused by melt extraction, followed by metasomatic enrichment. One of these samples (HYB40) hosts a fresh glass vein. Rare earth elements (REE) in clinopyroxenes from these peridotites show three patterns: (1) light REE-enriched (La n /Yb n =7–17); (2) spoon-shaped (La n /Yb n =18–20; La n /Sm n =21–34; Sm n /Yb n &lt;1); (3) nearly flat (La n /Yb n ∼3). Whole-rock and clinopyroxene trace elements indicate that these patterns are associated with more or less complete equilibration with at least two distinct metasomatic melts: an alkaline silicate melt resembling the host basalt and a hawaiitic melt (for peridotite HYB40). P – T estimates yield 0.9–1.2 GPa and 870–1050 °C, suggesting that refertilization by metasomatizing melts occurred at the Crust–Mantle boundary or just below. In addition, the P – T data coincide with the palaeogeotherm reported by an earlier worker that is consistent with a high geothermal gradient. However, this thermal regime does not fit with the occurrence of an active mantle plume beneath the Hyblean area because of the deduced mantle potential temperatures, which are almost 200°C lower than those typical for a mantle plume. f O 2 calculation gives a redox state above the fayalite–magnetite–quartz buffer FMQ (up to +1.7 Δlog units) related to melt-driven metasomatism.