Volatile Speciation Research Articles

Copper isotopes track the Neoproterozoic oxidation of cratonic mantle roots

The oxygen fugacity (fO2) of the lower cratonic lithosphere influences diamond formation, melting mechanisms, and lithospheric evolution, but its redox evolution over time is unclear. We apply Cu isotopes (δ65Cu) of ~ 1.4 Ga lamproites and < 0.59 Ga silica-undersaturated alkaline rocks from the lithosphere-asthenosphere boundary (LAB) of the North Atlantic Craton to characterize fO2 and volatile speciation in their sources. The lamproites’ low δ65Cu (−0.19 to −0.12‰) show that the LAB was metal-saturated with CH4 + H2O as the dominant volatiles during the Mesoproterozoic. The mantle-like δ65Cu of the < 0.59 Ga alkaline rocks (0.03 to 0.15‰) indicate that the LAB was more oxidized, stabilizing CO2 + H2O and destabilizing metals. The Neoproterozoic oxidation resulted in an increase of at least 2.5 log units in fO2 at the LAB. Combined with previously reported high fO2 in peridotites from the Slave, Kaapvaal, and Siberia cratonic roots, this oxidation might occur in cratonic roots globally.

Constraints on the formation of basaltic magmas. Comment on “Lithosphere thickness controls the extent of mantle melting, depth of melt extraction and basalt compositions in all tectonic settings on Earth – a review and new perspectives” – by Niu Y. (2001)

Niu (2021) reviews fundamental topics of basaltic magma generation in the oceans and continents, including passive and active margins as well as intraplate regions. In his approach, Niu (2021) focuses on the role exerted by lithospheric (“LID”) thickness in controlling basalt compositions and volumes, proposing only a minor role, if any, for the thermal state of the mantle. A consequence of this is rejection of mantle plumes in controlling the compositions of MORB and OIB, albeit not excluding completely their existence to explain basalt petrogenesis.Although the fundamental role of the lithospheric mantle on continental breakup, deformation and volcanism was recognized over 50 years ago, this region has only rarely been considered a candidate source for basaltic rocks, being thought to be too cold, stiff and sterile to produce significant amounts of partial melt. However, the stability of C-H-bearing mineral phases such as carbonates, phlogopite and pargasitic amphibole at lithospheric mantle depths indicates relatively low solidus temperature, and the possibility to produce partial melts even under relatively cold thermal regimes.We contribute to this debate by proposing that a wide range of variables explain igneous activity on Earth and, likely, the other rocky planets. The “lid effect” is certainly important but there is evidence that other factors and variables also influence the compositions and volumes of melts produced in the upper mantle. Among these, bulk- and trace-element chemical heterogeneities, the distribution and speciation of volatiles, and large-wavelength variations in temperature caused by radiogenic decay, are the most relevant.

Degassing of volcanic extrusives on Mercury: Potential contributions to transient atmospheres and buried polar deposits

The surface of Mercury is dominated by extensive, widespread lava plains that formed early in its history. The emplacement of these lavas was accompanied by the release of magmatic volatiles, the bulk of which were lost to space via thermal escape and/or photodissociation. Here we consider the fate of these erupted volatiles by quantifying the volumes of erupted volcanic plains and estimating the associated masses of erupted volatiles. The concentrations and speciation of volatiles in Mercury's magmas are not known with certainty at this time, so we model a wide range of cases, based on existing experimental data and speciation models, at 3–7 log fO2 units below conditions determined by the iron-wüstite buffer. Cases range from relatively low gas content scenarios (total exsolved gas mass of 9×1015 kg) to high gas content scenarios (total exsolved gas = 5 × 1019 kg). We estimate that the average duration of a transient volcanic atmosphere resulting from a single eruption would be between ∼250 and ∼210,000 years, depending on the volume, degassed volatile content, and eruption rate of an individual eruption, as well as the fO2 conditions of the planet's interior. If a dense transient atmosphere was ever surface-bound long enough for the released volatiles to be transported to and cold-trapped at Mercury's polar regions, those trapped volatiles are predicted to be well-mixed with the regolith, and at least 16 m beneath the surface given regolith gardening rates. These volatiles would have a composition and age distinctly different from those of the H2O-ice deposits observed at the poles of Mercury today.

Formation and dispersal of pyroclasts on the Moon: Indicators of lunar magma volatile contents

We use new estimates of the total content and speciation of volatiles released during the ascent and eruption of lunar mare basalt magma to model the generation and behavior of gas bubbles, the disruption of magma at shallow depth by bubble expansion, and the acceleration and dispersal of the resulting pyroclasts. Lunar eruptions in near-vacuum differ significantly from those on bodies with an atmosphere: 1) exposure to near-zero external pressure maximizes volatile release to form gas bubbles; 2) the infinite potential expansion of the gas bubbles both ensures and maximizes magma fragmentation into pyroclastic liquid droplets with sizes linked to the bubble size distribution; 3) the speeds to which gas and entrained pyroclasts can be accelerated by gas expansion are also maximized. Generation of CO gas bubbles at much greater depths and pressures than bubbles of other volatiles produces bimodal (~120 and 650 μm) total pyroclast size distributions. In the near-vacuum, gas expands to pressures so low that gas-particle interactions enter the Knudsen regime, resulting counter-intuitively in the median grainsize in pyroclastic deposits first increasing, then decreasing, and finally increasing again with increasing distance from the vent, instead of decreasing monotonically as when an atmosphere is present. These complex gas-particle interactions cause clast size distributions to vary in a complex way with distance from the vent and the maximum thickness of the deposit to occur at about 75% of the maximum pyroclast range. Lunar eruptions typically evolve through four stages, which significantly influence gas release patterns. Most volatiles are released during the second, hawaiian-style eruption stage. However, elevated gas concentration can occur both in the short first stage (due to gas accumulation in the dike tip during ascent from the mantle) and in the third and fourth stages (due to reduced volume flux, increased time for gas bubble formation, growth, rise and coalescence, and strombolian activity replacing the hawaiian eruption style). Such gas concentration mechanisms can increase pyroclast ranges by a factor of ~5, but result in very much thinner deposits than if no concentration occurs. Maximum pyroclast range scales essentially linearly with total mass fraction of released volatiles; thus determination of the deposit radius around specific vents can provide data on lunar magma volatile contents. If the volatile inventory of the Apollo 17 orange glass bead picritic magma (~3400 ppm maximum) is typical, maximum ranges of the majority of pyroclasts would have been ~20 km. Such eruptions could explain 79% of the currently recognized pyroclastic deposits on the Moon. A few larger deposits and vents, such as the Aristarchus Plateau Dark Mantle and Cobra Head, suggest higher magma volatile contents. Numerous lunar vents show little evidence of associated pyroclastic deposits. Together, these observations suggest a wide range of volatile contents in lunar basaltic magma mantle source regions.

Mixed incorporation of carbon and hydrogen in silicate melts under varying pressure and redox conditions

Volatiles including carbon and hydrogen are generally considered to be more soluble in silicate melts than in mantle rocks. How these melts contribute to the storage and distribution of key volatiles in Earth's interior today and during its early evolution, however, remains largely unknown. It is essential to improve our knowledge about volatiles-bearing silicate magmas over the entire mantle pressure regime. Here we investigate molten Mg1−xFexSiO3 (x=0, 0.25) containing both carbon and hydrogen using first-principles molecular dynamics simulations. Our results show that the dissolution mechanism of the binary volatiles in melts varies considerably under different conditions of pressure and redox. When incorporated as CO2 and H2O components (corresponding to oxidizing conditions) almost all carbon and hydrogen form bonds with oxygen. Their speciation at low pressure consists of predominantly isolated molecular CO2, carbonates, and hydroxyls. More oxygenated species, including tetrahedrally coordinated carbons, hydrogen (O-H-O) bridges, various oxygen-joined complexes appear as melt is further compressed. When two volatiles are incorporated as hydrocarbons CH4 and C2H6 (corresponding to reducing conditions), hydroxyls are prevalent with notable presence of molecular hydrogen. Carbon-oxygen bonding is almost completely suppressed. Instead carbon is directly correlated with itself, hydrogen, and silicon. Both volatiles also show strong affinity to iron. Reduced volatile speciation thus involves polymerized complexes comprising of carbon, hydrogen, silicon, and iron, which can be mostly represented by two forms: C1−4H1−5Si0−5O0−2 (iron-free) and C5−8H1−8Si0−6Fe5−8O0−2. The calculated partial molar volumes of binary volatiles in their oxidized and reduced incorporation decrease rapidly initially with pressure and then gradually at higher pressures, thereby systematically lowering silicate melt density. Our assessment of the calculated opposite effects of the volatile components and iron on melt density indicates that melt-crystal density crossovers are possible in the present-day mantle and also could have occurred in early magma ocean environments. Melts at upper mantle and transition zone conditions likely dissolve carbon and hydrogen in a wide variety of oxidized and non-oxygenated forms. Deep-seated partial melts and magma ocean remnants at lower mantle conditions may exsolve carbon as complex reduced species possibly to the core during core-mantle differentiation while retaining a majority of hydrogen as hydroxyls-associated species.

Oxidation State of the Lithospheric Mantle Beneath Komsomolskaya–Magnitnaya Kimberlite Pipe, Upper Muna Field, Siberian Craton

The oxidation state of the mantle plays an important role in many chemical and physical processes, including magma genesis, the speciation of volatiles, metasomatism and the evolution of the Earth’s atmosphere. We report the first data on the redox state of the subcontinental lithospheric mantle (SCLM) beneath the Komsomolskaya–Magnitnaya kimberlite pipe (KM), Upper Muna field, central Siberian craton. The oxygen fugacity of the KM peridotites ranges from −2.6 to 0.3 logarithmic units relative to the fayalite–magnetite–quartz buffer (∆logfO2 (FMQ)) at depths of 120–220 km. The enriched KM peridotites are more oxidized (−1.0–0.3 ∆logfO2 (FMQ)) than the depleted ones (from −1.4 to −2.6 ∆logfO2 (FMQ)). The oxygen fugacity of some enriched samples may reflect equilibrium with carbonate or carbonate-bearing melts at depths &gt;170 km. A comparison of well-studied coeval Udachnaya and KM peridotites revealed similar redox conditions in the SCLM of the Siberian craton beneath these pipes. Nevertheless, Udachnaya peridotites show wider variations in oxygen fugacity (−4.95–0.23 ∆logfO2 (FMQ)). This indicates the presence of more reduced mantle domains in the Udachnaya SCLM. In turn, the established difference in the redox conditions is a good explanation for the lower amounts of resorbed diamonds in the Udachnaya pipe (12%) in comparison with the KM kimberlites (33%). The obtained results advocate a lateral heterogeneity in the oxidation state of the Siberian SCLM.

Mantle redox state drives outgassing chemistry and atmospheric composition of rocky planets

Volcanic degassing of planetary interiors has important implications for their corresponding atmospheres. The oxidation state of rocky interiors affects the volatile partitioning during mantle melting and subsequent volatile speciation near the surface. Here we show that the mantle redox state is central to the chemical composition of atmospheres while factors such as planetary mass, thermal state, and age mainly affect the degassing rate. We further demonstrate that mantle oxygen fugacity has an effect on atmospheric thickness and that volcanic degassing is most efficient for planets between 2 and 4 Earth masses. We show that outgassing of reduced systems is dominated by strongly reduced gases such as text {H}_{2}, with only smaller fractions of moderately reduced/oxidised gases (text {CO}, text {H}_{2}text {O}). Overall, a reducing scenario leads to a lower atmospheric pressure at the surface and to a larger atmospheric thickness compared to an oxidised system. Atmosphere predictions based on interior redox scenarios can be compared to observations of atmospheres of rocky exoplanets, potentially broadening our knowledge on the diversity of exoplanetary redox states.

Role of the global water ocean on the evolution of Titan’s primitive atmosphere

During the accretion of Titan, impact heating may have been sufficient to allow the global melting of water ice and the release of volatile compounds, mainly constituted of CO2, CH4 and NH3. The duration and efficiency of exchange between the primitive massive atmosphere and the global impact-induced water ocean likely play a key role in the chemical evolution of the early Titan’s atmosphere. To investigate the atmospheric composition of early Titan for a wide range of global (atmosphere + ocean) composition in volatils, we first developed a gas–liquid equilibrium model of the NH3–CO2–H2O system, where the non-ideal behavior of both gas and liquid phases, and the speciation of volatiles dissolved in the aqueous phase are taken into account. We show that the relative abundance of CO2 and NH3 determine the composition of Titan’s atmosphere. For CO2/NH3 ≤ 1, CO2 is massively dissolved in the ocean. On the contrary, for CO2/NH3 > 1, CO2 is the main constituent of Titan’s primitive atmosphere while the NH3 atmospheric content is dramatically decreased. We then investigate the conditions for the formation of CH4-rich clathrates hydrates at Titan’s surface that could be the main reservoir of methane for the present-day atmosphere. In absence of reliable experimental data in the CH4–CO2–NH3–H2O system, the dissolution of methane in water is included using a simplified Henry’s law approach. We find that if the concentration of CH4 in Titan’s building block was higher than ∼ 0.1 mol kg−1 and CO2/NH3 < 3, a large fraction of methane may be stored in the primordial crust, which would form at temperature below ∼ 10 °C.

Tectonic significance and redox state of Paleoproterozoic eclogite and pyroxenite components in the Slave cratonic mantle lithosphere, Voyageur kimberlite, Arctic Canada

Mantle-derived eclogite and pyroxenite xenoliths from the Jurassic Voyageur kimberlite on the northern Slave craton in Arctic Canada were studied for garnet and clinopyroxene major and trace element compositions, clinopyroxene Pb and garnet O isotopic compositions, and garnet Fe3+/ΣFe contents. The Voyageur xenoliths record a wide range of pressures, but are cooler compared to mantle xenoliths derived from the nearby, coeval Jericho kimberlite. The CaO, TiO2 and Zr contents of Voyageur eclogites increase with depth, which is also observed in northern Slave peridotite xenoliths, demonstrating ‘bottom-up’ metasomatic processes within cratonic mantle lithosphere.The Voyageur eclogites have positive Eu anomalies, flat HREEN patterns, and major element compositions that are consistent with ultimate origins from basaltic and gabbroic protoliths within oceanic lithosphere. Clinopyroxene Pb isotope ratios intercept the Stacey-Kramers two-stage terrestrial Pb evolution curve at ca. 2.1Ga, and form an array towards the host kimberlite, indicating isotopic mixing. The 2.1Ga eclogite formation age broadly overlaps with known Paleoproterozoic subduction and collision events that occurred along the western margin of the Slave craton. Unlike the eclogites, the Voyageur pyroxenites contain garnet with distinctive fractionated HREEN, sinusoidal REE patterns of calculated bulk rocks, and clinopyroxene with 206Pb/204Pb ratios that intercept the Stacey-Kramers curve at 1.8Ga. This suggests a distinct origin as Paleoproterozoic high-pressure mantle cumulates. However, the pyroxenite Pb isotope ratios fall within the eclogite array and could also be explained by protoliths formation at ca. 2.1Ga followed by minor isotopic mixing during mantle metasomatism. Thus, an alternative scenario involves pyroxenite formation within the mantle section of Paleoproterozoic oceanic lithosphere followed by variable metasomatism after incorporation into cratonic mantle lithosphere. This model allows for a linked petrogenesis of the Voyageur eclogites (crust) and pyroxenites (mantle) as part of the same subducting oceanic slab.Oxygen fugacity determinations for one pyroxenite and ten eclogite xenoliths show a range of 3 log units, from −4.6 to −1.6 ΔFMQ, similar to the range observed for nearby Jericho and Muskox eclogites (ΔFMQ −4.2 to −1.5). Importantly, the northern Slave eclogite and pyroxenite mantle components are highly heterogeneous in terms of redox state provided that they range from reduced to oxidized relative to Slave peridotite xenoliths.Moreover, the Voyageur eclogites do not exhibit any trend between oxidation state and equilibration depth, which contrasts with the downward decrease in fO2 shown by Slave and worldwide cratonic peridotite xenoliths. Our investigation of mantle eclogite and pyroxenite fO2 reinforces the important influence of recycled mafic components in upper mantle processes, because their high and variable redox buffering capacity strongly controls volatile speciation and melting relations under upper mantle conditions.

Eclogite xenoliths from the Lace kimberlite, Kaapvaal craton: From convecting mantle source to palaeo-ocean floor and back

Major- and trace-element compositions of eclogite and pyroxenite xenoliths of ≥2.5 Ga age (in situ Pb–Pb data on clinopyroxene) from the Lace kimberlite on the Kaapvaal craton were investigated in order to constrain: (1) the nature and evolution of their protoliths; (2) the extent to which they preserve information on the state of the asthenospheric mantle source that gave rise to their low-pressure protoliths; and (3) the effect of their deep recycling on the radiogenic isotope evolution of the convecting mantle. Their elemental relationships are consistent with low-pressure fractionation of olivine ± plagioclase and clinopyroxene during oceanic crust formation, whereby the residual melt was enriched in rare-earth elements (REE), high field-strength elements and Y, producing inverse correlations of ΣREE with the size of Eu- and Sr-anomalies. LREE-depletion may indicate loss of on average 20% of a partial melt upon subduction and metamorphism (eclogitisation) of oceanic crust, which did not, however, contribute to juvenile growth of continental crust. The eclogites have median Sm/Nd (0.40) and Lu/Hf (0.27) similar to Depleted Mantle, and lower U/Pb (0.02) and Th/Pb (0.02). If deeply subducted, these rocks cannot explain unradiogenic Nd and Hf, and radiogenic Pb isotope compositions in the sources of some modern ocean island basalts.Low incompatible trace-element contents similar to picrites, and Yb concentrations at a given TiO2 content similar to modern MORB, indicate derivation of the protoliths by average melt fractions of ∼0.20–0.25 that left a spinel peridotite residue at pressures ≤2.5 to 3.0 GPa. This shallow intersection of the peridotite solidus suggests moderate Archaean ambient mantle potential temperatures of ≤1420 to 1470 °C. Samples filtered for clinopyroxene fractionation and metasomatism have V/Sc (4.7±1.2; n=11) indicating lower fO2 (−1.9 relative to the fayalite–magnetite–quartz buffer=ΔFMQ) than modern MORB. This is in part due to the higher average melt extraction pressure (∼1.5 GPa) during formation of their crustal protoliths. Extrapolation to 1 GPa, similar to the average pressure of present-day MORB generation, yields ΔFMQ of −1.7±1.1, corresponding to Fe3+# of 0.07±0.04. If these results are correct, they suggest an Archaean ambient mantle more reducing than at present, with implications for the speciation of volatiles, which affects the mantle solidus and the composition of volcanic gases. This has implications for the Archaean atmospheric redox evolution and the recycling of carbon and other volatiles.

Advances in Fourier transform infrared spectroscopy of natural glasses: From sample preparation to data analysis

Fourier transform infrared spectroscopy (FTIR) is an analytical technique utilized to measure the concentrations of H and C species in volcanic glasses. Water and CO2 are the most abundant volatile species in volcanic systems. Water is present in magmas in higher concentrations than CO2 and is also more soluble at lower pressures, and, therefore it is the dominant volatile forming bubbles during volcanic eruptions. Dissolved water affects both phase equilibria and melt physical properties such as density and viscosity, therefore, water is important for understanding magmatic processes. Additionally, quantitative measurements of different volatile species using FTIR can be achieved at high spatial resolution. Recent developments in analytical equipment such as synchrotron light sources and the development of focal plane array (FPA) detectors allow higher resolution measurements and the acquisition of concentration maps. These new capabilities are being used to characterize spatial gradients (or lack thereof) around bubbles and other textural features, which in turn lead to new insights into the behavior of volcanic feeder systems. Here, practical insights about sample preparation and analysis of the distribution and speciation of volatiles in volcanic glasses using FTIR spectroscopy are discussed. New advances in the field of FTIR analysis produce reliable data at high spatial resolution that can be used to produce datasets on the distribution, dissolution and diffusion of volatiles in volcanic materials.

The effect of CO2 and H2O on Etna and Fondo Riccio (Phlegrean Fields) liquid viscosity, glass transition temperature and heat capacity

Viscosity (η), glass transition (Tg) and heat capacity (Cp) of Etna trachybasalt and Fondo Riccio latite (Phlegrean Fields, Italy) were determined at low and high temperatures for dry, hydrous and CO2 bearing samples. High temperature experiments have been performed in the range of 1499 to 1700K by concentric cylinder measurements, while low temperature experiments were carried out in the interval between 633 and 1093K using the micropenetration technique. Glass transition temperature and glassy and liquid heat capacities were investigated using differential scanning calorimetry (DSC) up to 955K.The H2O content in the melts ranged from nominally dry to 6.32wt.%, while CO2 ranged from 229 to 1907ppm. We combined low- and high-temperature viscosities and parameterized them by the use of a modified Vogel–Fulcher–Tamman equation, which accommodates the non-Arrhenian temperature dependence of melt viscosity. Experimental measurements show that melt viscosity decreases with increasing temperature and water and CO2 contents. For latitic samples at 893K, the introduction of CO2 (up to 732ppm) decreases the liquid viscosity up to one order of magnitude with respect to the measured viscosity for H2O-bearing liquid. Moreover, the results of calorimetric measurements indicate that the glass transition temperature decreases with increasing volatile content (H2O+CO2). The glass transition temperature decreases by about 25K by adding up to 1907ppm of CO2 in the trachybasaltic samples. No appreciable effect on glassy [Cpg (Tg)] and liquid (Cpliq) heat capacities was observed with the addition of water and CO2. Structural and volcanological implications (i.e. volatile speciation and melt fragility) for water and CO2 dissolution in silicate melts are discussed in light of the presented results.

A micro-reflectance IR spectroscopy method for analyzing volatile species in basaltic, andesitic, phonolitic, and rhyolitic glasses

Volatile contents of geologic glasses are used to model magma chamber and degassing processes, thus, there is considerable interest in small-scale analytical techniques for analyzing volatiles in glasses. Infrared (IR) spectroscopy has the advantage of determining volatile speciation in glasses (e.g., OH − , molecular H 2 O, molecular CO 2 , and CO 3 2− ). However, sample preparation for the most common IR method used, micro-transmission IR spectroscopy, is complicated because glasses must be prepared as thin, parallel-sided wafers. Raman analysis, while valuable for Fe-poor samples, can be difficult to use for Fe-rich glasses. We have calibrated a micro-reflectance infrared method for determining volatile species using calculated Kramers-Kronig absorbance (KK-Abs.) spectra that requires that only one side of a glass be polished. The method is easier to use than other reflectance methods where it is difficult to determine the baseline for the IR bands. Total H 2 O wt% = m ·(3600 cm −1 KK-Abs.), where m , is the slope of the calibration line that is obtained from a fit to the data. The m value is related to the calculated refractive index, n , for a range of aluminosilicate glass compositions allowing the technique to be applied to samples with unknown calibration slopes. For calc-alkaline andesite glasses we determined calibration slopes for micro-reflectance IR measurements of molecular H 2 O, molecular CO 2 , and CO 3 2− . The method has been calibrated for glasses with up to 6.76 wt% total H 2 O (but is useful for glasses with more than 20 wt% total H 2 O) and has been calibrated for glasses with up to 0.575 wt% total CO 2 . This technique provides a means to analyze volatile abundances in samples that are not possible to analyze or prepare for analysis with transmission micro-IR techniques. We have determined volatile contents in fragile samples such as cracked, vesicular, or crystal-bearing glasses formed by volcanic or impact processes or in high-pressure bubble nucleation experiments and H diffusion experiments. We have monitored H uptake during weathering of basaltic glasses that cannot be polished and determined volatiles in melt inclusions and pumice.

Profiling food volatiles by comprehensive two-dimensional ga schromatography coupled with mass spectrometry: Advanced fingerprinting approaches for comparative analysis of the volatile fraction of roasted hazelnuts ( Corylus avellana L.) from different origins

This study examined how advanced fingerprinting methods (i.e., non-targeted methods) provide reliable and specific information about groups of samples based on their component distribution on the GC × GC chromatographic plane. The volatile fractions of roasted hazelnuts ( Corylus avellana L.) from nine different geographical origins, comparably roasted for desirable flavor and texture, were sampled by headspace-solid phase micro extraction (HS-SPME) and then analyzed by GC × GC-qMS. The resulting patterns were processed by: (a) “chromatographic fingerprinting”, i.e., a pattern recognition procedure based on retention-time criteria, where peaks correspondences were established through a comprehensive peak pattern covering the chromatographic plane; and (b) “comprehensive template matching” with reliable peak matching, where peak correspondences were constrained by retention time and MS fragmentation pattern similarity criteria. Fingerprinting results showed how the discrimination potential of GC × GC can be increased by including in sample comparisons and correlations all the detected components and, in addition, provide reliable results in a comparative analysis by locating compounds with a significant role. Results were completed by a chemical speciation of volatiles and sample profiling was extended to known markers whose distribution can be correlated to sensory properties, geographical origin, or the effect of thermal treatment on different classes of compounds. The comprehensive approach for data interpretation here proposed may be useful to assess product specificity and quality, through measurable parameters strictly and consistently correlated to sensory properties and origin.

Ilvaite stability in skarns from the northern contact of the Maladeta batholith, Central Pyrenees (Spain)

Two well developed hedenbergitic, ilvaite-bearing skarns formed along the northern contact of the Maladeta batholith (Central Pyrenees, Spain) have been studied in order to determine their physico-chemical conditions of formation. The skarns formed at ~3 kbar total pressure following a classical two-stage (prograde/retrograde) evolution. Oxygen fugacity during skarn formation was buffered close to the NNO oxygen buffer while it is likely that sulfur fugacity remained nearly constant (10−8–10−10) throughout the second, retrograde stage. Fluid inclusions indicate that the hydrothermal fluid was a hypersaline brine (25–30 % NaCl eq.), sometimes halite-saturated at room temperature. Based on fluid inclusions and volatile speciation calculations, H2O was the dominating volatile while CO2 was present in low levels (X CO2 < 0.02). Thermodynamic data for ilvaite have been retrieved from published works and available experiments. These data have made possible the construction of several phase diagrams for the Ca-Fe-Si-O-H ± C system with which the stability of ilvaite with respect to a number of parameters is illustrated. The stability relationships render unlikely that ilvaite can form under pressures below 2 kbar and show that this mineral can only develop from fluids with low or very low CO2 content. With the exception of Mn, the presence of significant amounts of Al, Mg and other elements drastically reduces the stability field of ilvaite. Hence, this mineral should not be expected to occur in environments departing significantly from the 6-component system studied here.

Iron isotope fractionation and the oxygen fugacity of the mantle.

The oxygen fugacity of the mantle exerts a fundamental influence on mantle melting, volatile speciation, and the development of the atmosphere. However, its evolution through time is poorly understood. Changes in mantle oxidation state should be reflected in the Fe3+/Fe2+ of mantle minerals, and hence in stable iron isotope fractionation. Here it is shown that there are substantial (1.7 per mil) systematic variations in the iron isotope compositions (delta57/54Fe) of mantle spinels. Spinel delta57/54Fe values correlate with relative oxygen fugacity, Fe3+/sigmaFe, and chromium number, and provide a proxy of changes in mantle oxidation state, melting, and volatile recycling.

Tectonomagmatic evolution of the Kenya rift valley: some geological perspectives

The Cenozoic Kenya rift valley has developed in an area of great lithospheric complexity, partly related to late Proterozoic orogenic events, which has strongly influenced the regional patterns of faulting, subsidence, uplift and magmatism. The mantle sources of the magmas are poorly understood, but seem to be heterogeneous as regards trace elements and isotopes, and to have undergone one or more depletion-enrichment events. Mafic magmas erupted in the rift zone may have both asthenospheric and lithospheric components, but the evidence is equivocal. Uncertainties about geothermal gradients and the abundances and speciation of volatiles in the melting zone make it difficult to constrain the depths and degrees of partial melting. Partial fusion may, however, have taken place over the depth range 100+ to 23 km. The largest volumes of basalt last equilibrated at pressures &lt;l5 kb and have trace element characteristics consistent with generation in the spinel-garnet peridotite transition zone. The thickness and internal structure of the lithospheric mantle and attempts to interpret rift evolution in terms of lithosphere-asthenosphere interactions are poorly constrained. Evidence in favour of active (plume-driven) rifting includes the long wavelength gravity and topographic anomalies, the volume of eruptive rocks, the relative timing of magmatism and rifting, the distribution of geophysical anomalies, and the pattern of the tectonomagmatic evolution of the rift. A passive role for the asthenosphere is suggested by the continent-scale correlation between episodes of rifting and magmatism and by fault geometry. Active and passive mechanisms may have acted together in rift development.