Low H2O Contents Research Articles

Reconstructing the Crustal Section of the Intra‐Oceanic Caribbean Island Arc: Constraints From the Cumulate Layered Gabbronorites and Pyroxenites of the Rio Boba Plutonic Sequence, Northern Dominican Republic

AbstractLocated in northern Dominican Republic, the Early Cretaceous Rio Boba mafic‐ultramafic plutonic sequence constitutes a lower crust section of the Caribbean island arc, made up by gabbroic rocks and subordinate pyroxenite. Modal compositions, mineral chemistry, whole‐rock compositions and thermobarometric calculations indicate that pyroxenites and gabbronorites represent a cumulate sequence formed by fractionation of tholeiitic magmas with initially very low H2O content in the lower crust of the arc (0.6–0.8 GPa). Melts evolved along a simplified crystallization sequence of olivine → pyroxenes → plagioclase → Fe‐Ti oxides. The magmatic evolution of the Rio Boba sequence and associated supra‐crustal Puerca Gorda metavolcanic rocks is multi‐stage and involves the generation of magmas from melting of different sources in a supra‐subduction zone setting. The first stage included the formation of a highly depleted substrate as result of decompressional melting of a refractory mantle source, represented by a cumulate sequence of LREE‐depleted island arc tholeiitic (IAT) and boninitic gabbronorites and pyroxenites. The second stage involved volumetrically subordinate cumulate troctolites and gabbros, which are not penetratively deformed. The mantle source was refractory and enriched by a LILE‐rich hydrous fluid derived from a subducting slab and/or overlying sediments, and possibly by a LREE‐rich melt. The third stage is recorded in the upper crust of the arc by the Puerca Gorda “normal” IAT protoliths, which are derived from an N‐MORB mantle source enriched with a strong subduction component. This magmatic evolution has implications for unraveling the processes responsible for subduction initiation and subsequent building of the Caribbean island arc.

Microwave spectra, molecular geometries, and internal rotation of CH3 in N-methylimidazole⋯H2O and 2-methylimidazole⋯H2O Complexes.

Broadband microwave spectra have been recorded between 7.0 and 18.5 GHz for N-methylimidazole⋯H2O and 2-methylimidazole⋯H2O complexes. Each complex was generated by co-expansion of low concentrations of methylimidazole and H2O in argon buffer gas. The rotational spectra of five isotopologues of each complex have been assigned and analysed to determine rotational constants (A0, B0, C0), centrifugal distortion constants (DJ, DJK) and parameters that describe the internal rotation of the CH3 group. The results allow the determination of parameters in the (r0) molecular geometry of each complex. H2O is the hydrogen bond donor and the pyridinic nitrogen of imidazole is the hydrogen bond acceptor in each case. The ∠(O–Hb⋯N3) angles are 177(5)° and 166.3(28)° for N-methylimidazole⋯H2O and 2-methylimidazole⋯H2O respectively. These results are consistent with the presence of a weak electrostatic interaction between the oxygen atom of H2O and the hydrogen atom (or CH3 group) attached to the C2 carbon atom of imidazole, and with the results of density functional theory calculations. The (V3) barrier to internal rotation of the CH3 group within N-methylimidazole⋯H2O is essentially unchanged from the value of this parameter for the N-methylimidazole monomer. The same parameter is significantly higher for the 2-methylimidazole⋯H2O complex than for the 2-methylimidazole monomer as a consequence of the weak electrostatic interaction between the O atom and the CH3 group of 2-methylimidazole.

Characterizing Hydration of the Ocean Crust Using Shortwave Infrared Microimaging Spectroscopy of ICDP Oman Drilling Project Cores

AbstractAlthough ocean crust covers over 60% of Earth's surface, the processes that form, cool, and alter the ocean crust are not completely understood. We utilize shortwave infrared micro‐imaging spectroscopy of ∼1.2 km of rock cored by the International Continental Scientific Drilling Program's Oman Drilling Project to quantify hydration of basaltic dikes and gabbros from the Samail ophiolite as a function of depth, mineralogy and deformation. We develop a regression (R2 = 0.66) between area of the ∼1,350–1,650 nm OH/H2O absorption and measurements of loss on ignition of samples and apply this relationship to generate quantitative ∼250 μm/pixel hydration maps for all cores. The lowest mean hydration is observed in the most pervasively altered dike‐gabbro boundary (GT3A, H2Omean = 2.1 ± 1.6 wt%), consistent with the low H2O content of the dominant alteration minerals, amphibole and epidote. The highest H2O content occurs in deeper foliated and layered gabbros (GT2A, H2Omean = 3.2 ± 3.0 wt%) and layered gabbros (GT1A, H2Omean = 2.8 ± 3.1 wt%). The greater prevalence with depth of zeolite alteration as opposed to lower wt% H2O amphibole at shallow stratigraphic depths, as well as the occurrence of zones of intensive hydration associated with fault zones (H2Omean = 5.7 ± 4.0 wt%) lead to greater hydration of the lower ocean crust. This new approach provides an objective quantification of hydration in these cores, enabling an improved understanding of quantities and characteristics of ocean crust hydration. It highlights the importance of specific phases and faulting in controlling hydration, which has implications for ocean crust cooling, rheological properties, and the role of alteration in global biogeochemical cycling.

Selectivity of vacuum ammonia stripping using porous gas-permeable and dense pervaporation membranes under various hydraulic conditions and feed water compositions

Recovery of ammonia (NH3) from residual waters offers various reuse opportunities, such as the production of fertilisers and the generation of electricity and heat. However, simultaneous evaporation of water (H2O) during NH3 stripping under vacuum results in diluted recovered NH3 gas with high H2O contents. Whereas porous gas-permeable membranes are already used for vacuum NH3 stripping, the use of non-porous silica-based pervaporation (PV) membranes showed promising results in recent literature, with respect to more selective transfer of NH3 compared to H2O. In this study, we assessed the selectivity of NH3 over H2O transfer (SNH3/H2O) for different types of membranes, under various hydraulic conditions and feed water compositions. The three following membranes were tested: a porous gas-permeable polytetrafluoroethylene (PTFE) membrane, a hydrophilic (Hybrid Silica PV) membrane and a hydrophobic polydimethylsiloxane PV (PDMS PV) membrane.For the PTFE and the Hybrid Silica PV membrane, SNH3/H2O ranged between 0.1 and 0.4, indicating that the transfer of NH3 was consistently less preferred compared to the transfer of H2O. The preference for H2O over NH3 transfer through the membranes at various hydraulic conditions and feed water compositions can be assigned to the similarity in polarity and kinetic diameter of NH3 and H2O and the low relative concentration of NH3 in the used feed waters (approximately 0.1–1.0 wt%). The PDMS PV membrane showed negligible NH3 transfer and deteriorated rapidly during the NH3 stripping experiments. The SNH3/H2O of both gas-permeable and PV membranes was higher for unsteady than for steady hydraulic conditions. Furthermore, the SNH3/H2O of the both PTFE and the Hybrid Silica decreased when the ionic strength of the feed water increased from 0.0 to 0.8 mol∙L−1 and when the NH3 feed water concentration increased from 1 to 10 g∙L−1. According to the results, the used PV membranes did not show selectivity of NH3 over H2O transfer. In fact, the used PV membranes consistently had a lower SNH3/H2O than the PTFE membrane. Hence, the dense silica-based PV membranes did not allow for the recovery of gaseous NH3 from water, with lower H2O content in the recovered gas, compared to porous PTFE membranes.

(Keynote) Towards Robust Li-Mediated Electrosynthesis of Ammonia at High Rate and Faradaic Efficiency

Synthesis of ammonia will most likely rely on the Haber-Bosch technologies for a reasonable period of time, but will now use increasing amounts of H2 derived from water electrolysis to substantially decrease the CO2 footprint. The next stage of the technological evolution of NH3 production will build upon electrochemical systems combining the nitrogen reduction and hydrogen oxidation half-reactions. The preference of H2 as a source of electrons and protons over water is dictated by the very high sensitivity to even very low concentrations of H2O of the current most efficient (and arguably the only) approach to convert N2 to NH3 electrochemically, viz. via a lithium redox mediated process.The presentation will discuss possible strategies towards further improvements in the sustainable NH3 synthesis from N2 and H2 in aprotic electrolyte solutions and will present latest developments in the field from our group. Notwithstanding the hydrogen oxidation reaction is the most studied and best understood electrocatalytic process in aqueous electrolytes, it presents a broad host of challenges when attempted in organic media. This will be demonstrated through the comparison of the H2 oxidation kinetics in various tetrahydrofuran-based electrolytes for different nanoparticulate metal catalysts. In the domain of the nitrogen reduction reaction, the development of stable high-performance Li-mediated systems through the design of a genuinely stable proton shuttle and optimised electrolytes will be presented. Implementation of these strategies enables robust ammonia electrosynthesis on a timescale of days with the yield rates of ca 200 nmol s-1 cm-2 (per cathode surface area) and with a faradaic efficiency for the N2 reduction to NH3 of 99 ± 2 %. In conclusion, a perspective on possible future directions that are likely to further advance the presented technologies and the overall concept of the Ammonia Economy for the future energy will be discussed.

Boundaries of High-Temperature Co-Electrolysis Towards Direct CO2-Electrolysis

High-temperature co-electrolysis of CO2 and H2O at elevated temperatures between 700 °C and 900 °C valorises CO2 to produce a mixture of carbon monoxide (CO) and hydrogen (H2), called syngas. Co-electrolysis has the great advantage over conventional processes, that the desired syngas ratios of downstream processes can be realized by varying process parameters such as temperature and feed gas composition accordingly in a one step process. Co-electrolysis can also play a vital role in counteracting power fluctuations of renewable energy sources by storing temporarily unused electricity through conversion to other energy resources like chemicals or heat for later use.The underlying processes in co-electrolysis for CO production are direct electrochemical CO2 reduction and reverse water gas shift equilibrium (RWGS). Their specific significance has not been clarified in detail yet and was controversially discussed in literature up to this day [1,2].The impact of the equilibrium partial pressure of H2O on the physical processes in the transition boundary of co-electrolysis towards direct CO2-electrolysis was investigated by AC and DC measurements for various gas compositions. The analysis led to identifying the role of the underlying electrochemical processes during co-electrolysis, in particular the electrochemical CO2 reduction compared to the conversion of CO2 in the reverse water-gas shift reaction and the electrochemical H2O reduction. The area specific resistance (ASR) was, amongst others, taken as an indicator to determine, which of the reduction reactions (H2O or CO2 reduction) is dominant depending on the gas composition. The experiments were conducted using commercially available cathode-supported full cells (Elcogen) made of Ni-8YSZ/8YSZ/CGO/LSC.Results as seen in Figure 1 show that the ASR for an equilibrium concentration of 5 % H2O is considerably larger than for higher H2O contents. Above 15 % H2O, the ASR shows no dependency on the gas composition and is comparable to pure H2O-electrolysis [3]. These observations underline the hypothesis that CO2-electrolysis becomes pre-dominant compared to H2O-electrolysis for low H2O content during co-electrolysis. With increasing H2O content, CO2-electrolysis becomes less significant and carbon dioxide is converted in the reverse water gas shift equilibrium. The origin of the discrepancy in literature was found to be the different operating H2O concentrations. A threshold has been established for the perception of CO2-electrolysis during co-electrolysis experiments.figure caption: Arrhenius plot of ASROCV for different steam concentrations at 6 l·h-1.[1] C. Stoots, J. O'Brien, J. Hartvigsen, Int. J. Hydrogen Energy 2009, 34, 4208.[2] S. D. Ebbesen, R. Knibbe, M. Mogensen, J. Electrochem. Soc. 2012, 159, F482-F489.[3] L. Dittrich, M. Nohl, E. E. Jaekel, S. Foit, L.G.J. (Bert) de Haart, R.-A. Eichel J. Electrochem. Soc. 2019, 166, F971-F975. Figure 1

Magma mixing, degassing and late sulfide saturation: Insights into the 1976–2000 eruptive sequence at White Island, New Zealand

This paper reports new petrological and chemical data of ejecta from the 1976–2000 eruptive sequence of White Island, New Zealand. We provide evidence of mixing between shallow stored (0.7–1 km) andesitic to dacitic melts and deeper primitive mafic magma carrying high-Fo olivine, repeatedly injected throughout the sequence. Melt inclusions hosted in plagioclase, pyroxene and olivine were analyzed for major, trace and volatile elements. The low H2O contents in inclusions suggest it exsolves at depth while SO2 degassing is recorded during the rise and crystallization of the mafic magma. The higher solubility of Cl (as HCl and NaCl) delays its exsolution until shallow depth. Convection degassing is suggested to explain the large imbalance between elevated gas emissions and low amount of lava erupted. Frequent inputs of gas-rich, deeply derived magma are essential drivers for the counterflow of viscous degassed melts. Metal contents (Cu, Zn) of the residual mafic magma increases during olivine crystallization. This behavior changes during shallow differentiation of the andesitic magma, with metals leaving the silicate melt. While some metals may be lost to degassing, Cu, Ni, Ag and Au strongly partition into an immiscible sulfide melt. Saturation of such a sulfide melt phase occurs in the shallow environment, likely related to changes in redox state associated with magnetite crystallization and/or SO2 degassing.

Catalytic upcycling paper sludge for the recovery of minerals and production of renewable high-grade biofuels and bio-based chemicals

Paper sludge is a solid waste by-product abundantly produced in the paper industry and contains fine minerals and lignocellulosic biomass. In this contribution, the ex-situ catalytic fast pyrolysis of paper sludge with a high mineral content of ca. 71 wt% is reported for the first time and demonstrated in a pilot-scale unit (feeding rate of 11.3 kg h−1) using a granular Na2CO3/γ-Al2O3 catalyst (loading of 650 g) to produce a high-grade bio-oil at a fast pyrolysis temperature of 475 ± 5 °C (pre-screened using a pyroprobe) and a catalytic upgrading temperature of 500 °C. Besides, > 99 wt% minerals in the paper sludge could be recovered, including paper fillers such as CaCO3 (major) and talc. The bio-oil obtained at a carbon yield of 21 C.% has a low oxygen content of 3.2 wt%, a low total acidity number of 5.2 mg KOH g−1, low H2O content of 0.7 wt%, and a high higher heating value of 40.9 MJ kg−1. It consists of value-added bio-based chemicals such as paraffins, olefins, and low molecular weight aromatics. The results demonstrate that the use of paper sludge to recover minerals and to obtain fuels and chemicals using ex-situ catalytic pyrolysis is technically feasible at a pilot plant scale, which will be of importance for the development of future bio-based economy.

Rippite, K2(Nb,Ti)2(Si4O12)O(O,F), a New K-Nb-Cyclosilicate from Chuktukon Carbonatite Massif, Chadobets Upland, Krasnoyarsk Territory, Russia

Rippite K2(Nb,Ti)2(Si4O12)(O,F)2, a new K-Nb-cyclosilicate, has been discovered in calciocarbonatites from the Chuktukon massif (Chadobets upland, SW Siberian Platform, Krasnoyarsk Territory, Russia). It was found in a primary mineral assemblage, which also includes calcite, fluorcalciopyrochlore, tainiolite, fluorapatite, fluorite, Nb-rich rutile, olekminskite, K-feldspar, Fe-Mn–dolomite and quartz. Goethite, francolite (Sr-rich carbonate–fluorapatite) and psilomelane (romanèchite ± hollandite) aggregates as well as barite, monazite-(Ce), parisite-(Ce), synchysite-(Ce) and Sr-Ba-Pb-rich keno-/hydropyrochlore are related to a stage of metasomatic (hydrothermal) alteration of carbonatites. The calcite–dolomite coexistence assumes crystallization temperature near 837 °C for the primary carbonatite paragenesis. Rippite is tetragonal: P4bm, a = 8.73885(16), c = 8.1277(2) Å, V = 620.69(2) Å3, Z = 2. It is closely identical in the structure and cell parameters to synthetic K2Nb2(Si4O12)O2 (or KNbSi2O7). Similar to synthetic phase, the mineral has nonlinear properties. Some optical and physical properties for rippite are: colorless; Mohs’ hardness—4–5; cleavage—(001) very perfect, (100) perfect to distinct; density (meas.)—3.17(2) g/cm3; density (calc.)—3.198 g/cm3; optically uniaxial (+); ω = 1.737-1.739; ε = 1.747 (589 nm). The empirical formula of the holotype rippite (mean of 120 analyses) is K2(Nb1.90Ti0.09Zr0.01)[Si4O12](O1.78OH0.12F0.10). Majority of rippite prismatic crystals are weakly zoned and show Ti-poor composition K2(Nb1.93Ti0.05Zr0.02)[Si4O12](O1.93F0.07). Raman and IR spectroscopy, and SIMS data indicate very low H2O content (0.09–0.23 wt %). Some grains may contain an outermost zone, which is enriched in Ti (+Zr) and F, up to K2(Nb1.67Ti0.32Zr0.01)[Si4O12](O1.67F0.33). It strongly suggests the incorporation of (Ti,Zr) and F in the structure of rippite via the isomorphism Nb5+ + O2− → (Ti,Zr)4+ + F1−. The content of a hypothetical end-member K2Ti2[Si4O12]F2 may be up to 17 mol. %. Rippite represents a new structural type among [Si4O12]-cyclosilicates because of specific type of connection of the octahedral chains and [Si4O12]8− rings. In structural and chemical aspects it seems to be in close with the labuntsovite-supergroup minerals, namely with vuoriyarvite-(K), K2(Nb,Ti)2(Si4O12)(O,OH)2∙4H2O.

Experimental Studies on Thermal Effect of H2O on the Combustion of Pulverized Coal Char

ABSTRACT The differences in the gas thermophysical properties (including heat capacity, thermal conductivity and diffusivity) between char combustion in wet and dry environments have remarkable influences on the combustion process. We conducted a detailed experimental study on the thermal effects of H2O on the coal char combustion at 1773 K in a high-temperature drop tube furnace (HTDTF). An appropriate portion of Ar was introduced into the bulk gas (O2/H2O/N2) during combustion to eliminate the opposed thermal effect of H2O. Results indicate that the H2O thermal effect initially increased with higher H2O concentrations from 0 to 10 vol.%, but it declined with a further increase in H2O concentrations in the bulk gas. Meanwhile, its effect was dominant in the preliminary and middle stages of char burning at low H2O concentrations. The increased H2O concentration and residence time promoted the gasification reaction and the water-gas shift reaction so as to weaken the negative thermal effect of H2O and improve the carbon conversion. In the 15%O2/5%H2O/80%N2 environment, the relative contribution of thermal effects to carbon conversion ratios ranked up to −36% at the residence time of 0.3 s, but it decreased to −0.3% at 21%O2/30%H2O/49%N2 atmosphere and the residence time of 0.4 s.

Hydration of Na-saturated synthetic stevensite, a peculiar trioctahedral smectite

AbstractSmectite interlayer water plays a key role in the mobility of elements and molecules and affects a variety of geological processes. In trioctahedral smectites, in contrast to saponite and hectorite, the layer charge of which originates from isomorphic substitutions, the stevensite layer charge originates from the presence of octahedral vacancies. Despite its common occurrence in lacustrine environments, stevensite hydration has received little attention compared to saponite and hectorite. Early reports mention a specific hydration behaviour, however, with the systematic presence of a low-angle reflection attributed to the regular interstratification of various hydration states. The present study aims to revisit this specific hydration behaviour in more depth. Within this scope, the hydration behaviour of the three smectite varieties above are compared using synthetic trioctahedral smectites of similar layer charge and various compositions of their octahedral sheets. The chemical composition of the octahedral sheet does not appear to influence significantly smectite hydration for saponite and hectorite. Compared to its saponite and hectorite equivalents, H2O content in stevensite is lower by ~2.0 mmol H2O per g of dry clay. Consistent with this lower H2O content, Zn-stevensite lacks a stable monohydrated state, with dehydrated layers prevailing from 60% to 0% relative humidity. The presence of the regular interstratification of 0W and 1W layers is responsible for the low-angle reflection commonly observed for stevensite under air-dried conditions. Finally, the stevensite identification method based on X-ray diffraction of heated and ethylene glycol-solvated samples is challenged by the possible influence of the octahedral sheet chemical composition (Zn or Mg in the present study) on hectorite swelling behaviour in synthetic Zn-smectites. The origin of this effect remains undetermined and further work is needed to propose a more general identification method.

Slow Ascent of Unusually Hot Intermediate Magmas Triggering Strombolian to Sub-Plinian Eruptions

AbstractTo assess whether magma ascent rates control the style of volcanic eruption, we have studied the petrography, geochemistry and size distribution of microlites of plagioclase and pyroxene from historical eruptions from Tongariro, Ruapehu and Ngauruhoe volcanoes located in the southern Taupo Volcanic Zone, New Zealand. The studied deposits represent glassy andesitic and dacitic tephra shards from the Mangamate, Mangatawai, Tufa Trig and Ngauruhoe tephra formations, ranging in age from 11 000 years bp to ad 1996. Covering a range in eruption styles and sizes from Strombolian to Plinian, these samples provide an excellent opportunity to explore fundamental volcanic processes such as pre-eruptive magma ascent processes. Our quantitative petrographic analysis shows that larger microlites (>30 µm) display complex growth zoning, and only the smallest crystals (<30 µm) have formed during magma ascent in the conduit. Using a combination of orthopyroxene geothermometry, plagioclase hygrometry, and MELTS modelling, we show that these microlites nucleated at maximum pressures of 550 MPa (c. 16·5 km) from hot andesitic magmas (1010–1130 °C) with low H2O content (0–1·5 wt%). Size distributions of a total of >60 000 microlites, involving 22 tephras and 99 glass shards, yield concave-up curves, and the slopes of the pyroxene microlite size distributions, in combination with well-constrained orthopyroxene crystal growth rates from one studied tephra, indicate microlite population growth times of ∼3 ± 1 days, irrespective of eruption style. These data imply that microlites form in response to cooling of melts ascending at velocities of <5 cm s–1 prior to H2O exsolution, which occurs only at <33 MPa. Maximum magma ascent rates in the upper conduit, calculated using the exsolution of water during final decompression, range between 3 and 12 m s–1; that is, at least an order of magnitude lower than the hypersonic vent velocities typical of Vulcanian or sub-Plinian eruptions (up to 400 m s–1). This implies that magma ascent from depths of an average of 4 km occurs in dykes, and that vent velocities at the surface are controlled by a reduction of conduit cross-section towards the surface (e.g. dyke changing to cylindrical conduit).

Role of Adsorbate Coverage on the Oxygen Dissociation Rate on Sr-Doped LaMnO3 Surfaces in the Presence of H2O and CO2

Sr-doped LaMnO3 (LSM) is a promising oxygen reduction reaction electrocatalyst in solid oxide fuel cells and other electrochemical devices. The presence of CO2 and H2O has been reported to promote the oxygen dissociation reaction on LSM surfaces. Here, we investigate the coadsorption mechanism of O2 with H2O or CO2 by combining first-principles calculations of the (0 0 1) surface containing 25–100% Sr with thermodynamic adsorption models. The molecules were found to chemisorb by formation of charged oxygen, hydroxide, and carbonate species, and the adsorption energies were exothermic up to monolayer coverage. Low concentrations of H2O or CO2 do not compete with O2 for adsorption sites under relevant conditions. However, their presence contributes to the total amount of oxygen-containing species. The increased coverage of oxygen species provides a quantitative explanation for the reported enhancement in oxygen dissociation kinetics in the presence of H2O/CO2. This study thereby provides insights into oxygen exchange mechanisms on LSM surfaces.

Venus: A phase equilibria approach to model surface alteration as a function of rock composition, oxygen- and sulfur fugacities

Rock alteration processes on Venus are still poorly understood due to the limited geochemical data on surface rocks and uncertainties in atmospheric compositions. We use phase equilibria calculations to constrain mineral stabilities at Venus surface conditions for different rock and gas compositions resulting in the chemical system SiO2-TiO2-Al2O3-FeO-MgO-CaO-Na2O-K2O with C-O-H-S gas at varying O2 and S2 fugacities. While the low concentrations of H2O in the present-day atmosphere result in conditions, under which anhydrous mineral assemblages dominate, higher amounts of water, possibly during an earlier stage in Venus' history, could have resulted in the formation of amphibole and biotite. Even in a sulfur-free atmosphere, carbonates would be stable only in alkali-rich basalts. The presence of SO2 in the atmosphere, however, causes the formation of anhydrite. The stabilities of iron oxides and sulfides are highly sensitive to gas fugacities (i.e., the composition of the atmosphere), as well as temperature. While the modeled magnetite-hematite transition is located close to conditions relevant for planetary radius, the assemblage of anhydrite + hematite ± pyrite may be stable at higher elevations if a similar range of fO2 as at the lowlands is assumed. Therefore, our model agrees with pyrite as the proposed cause of the high radar backscatter observed at high elevations in the northern highlands.

Magma evolution at La Fossa volcano (Vulcano Island, Italy) in the last 1000 years: evidence from eruptive products and temperature gradient experiments

The intense explosive and effusive volcanic activity of the last 1000 years at La Fossa volcano (Vulcano Island, Italy) was characterized by the eruption of magmas ranging in composition from latites to trachytes and rhyolites, as well as K-rich trachytes. Evidence of syn-eruptive mixing among these magmas is frequently observed in the form of magmatic enclaves and bands in lava flows and pyroclastic products. The petrological and volcanological diversity of the erupted materials suggests that complex differentiation processes occurred in the shallow part of the plumbing system. With the aim to reconstruct the magmatic feeding system and to identify the differentiation processes behind such a petrologic complexity, we analysed lavas and pyroclastic products representative of the recent eruptive sequences at La Fossa and combined the petro-chemical features with thermo-barometric calculations, geochemical modelling and temperature gradient experiments. Thermo-barometric calculations indicate that the K-rich trachytic magma crystallized at lower pressure (160 ± 54 MPa) compared to the latitic (307 ± 47 MPa) and trachytic (208 ± 30 MPa) magmas. Differentiation modelling suggests that both trachytic and rhyolitic compositions can be obtained through differentiation of a common latitic magma, essentially by varying the plagioclase/sanidine ratio. Temperature gradient experiments, performed at the conditions inferred for the shallow plumbing system of La Fossa volcano (150 MPa and 1050–900 °C), indicate different paths of melt differentiation that overall produce an increase of the SiO2/K2O ratio with the increasing H2O in the system (from 0 to 4 wt.%). This is consistent with the origin of K-rich trachytes at lower pressure and lower H2O content. In turn, the formation of crystal-poor rhyolites is explained by the segregation of the interstitial melt formed in a latitic–trachytic crystal mush, favoured by the second boiling of the melt and consequent exsolution of a fluid phase.

Metasomatism and Hydration of the Oceanic Lithosphere: a Case Study of Peridotite Xenoliths from Samoa

AbstractWater influences geodynamic processes such as melting, deformation and rheology, yet its distribution in the oceanic upper mantle is primarily known indirectly from melt inclusions and glasses of erupted mantle melts (i.e. mid-ocean ridge and ocean island basalts). To better constrain the mechanisms influencing the distribution of H2O in the mantle, particularly regarding the role of metasomatism, we analyzed 15 peridotite xenoliths from Savai‘i and two dunite xenoliths from Ta‘ū (Samoa) for structural H2O (by polarized Fourier transform infrared spectroscopy), and major and trace element concentrations. Clinopyroxenes from the Ta‘ū dunites show trace element concentrations consistent with equilibration with their host lavas, but lower H2O contents than expected. Savai‘i peridotites are highly depleted harzburgites (melt depletion ≥17 %). They show strong evidence of transient metasomatism by both carbonatite and silicate melts, with highly variable Ti and Zr depletions and light rare earth element enrichments. However, despite metasomatism the H2O concentrations in olivines (0 − 4 ppm H2O) and orthopyroxenes (17 − 89 ppm H2O) are among the lowest reported in oceanic xenoliths, but higher than expected for the estimated degree of depletion. In general, H2O concentrations vary less than those of other incompatible trace elements in these samples. Transects across mineral grains show generally homogeneous distributions of H2O, indicating no significant H2O loss or gain during ascent. Raman spectroscopy on inclusions in minerals shows the presence of CO2 but an absence of molecular H2O. This agrees with the absence of H2O concentration variations between inclusion-rich and -poor domains in minerals. The above data can be explained by transient metasomatism along grain boundaries, now recorded as planes of inclusions within annealed grains. Fast diffusion of hydrogen (but not lithophile elements) from the inclusions into the host mineral phase will simultaneously enrich H2O contents across the grain and lower them in the inclusion-rich domains. The result is highly variable metasomatism recorded in lithophile elements, with smaller magnitude H2O variations that are decoupled from lithophile element metasomatism. Comparison with xenoliths from Hawai‘i shows that evidence for metasomatism from lithophile elements alone does not imply rehydration of the oceanic lithosphere. Instead, H2O concentrations depend on the overall amount of H2O added to the lithosphere through metasomatism, and the proximity of sampled material to areas of melt infiltration in the lithosphere.

Pre- and syn-eruptive conditions of a basaltic Plinian eruption at Masaya Volcano, Nicaragua: The Masaya Triple Layer (2.1 ka)

The Masaya Triple Layer tephra was deposited ~2100 years ago during a basaltic Plinian eruption of Masaya caldera, Nicaragua, and is one of few known examples of this extreme endmember of basaltic explosive volcanism. Masaya caldera is located approximately 25 km from Managua, the capital of Nicaragua, and a Plinian eruption presents a high potential risk to its population of 1 million people. Here we use geochemical and petrological tools to constrain the pre- and syn-eruptive physico-chemical magmatic conditions, in order to understand how low-viscosity basaltic magmas can erupt with Plinian style.By combining thermometric models with Rhyolite-MELTS simulations, we find that the Masaya Triple Layer magma was last stored between 1080 and 1100 °C and 21–42 MPa, with a pre-eruptive volatile content of ~2 wt% H2O. A small phenocryst volume fraction of 0.1 crystallised under water-saturated conditions within a shallow magma reservoir located at 0.8–1.6 km depth. During ascent, rapid microlite crystallisation occurred and lateral velocity gradients across the conduit produced heterogeneous inter-mingling regions of varying crystallinity, between 20 and 50 vol%. Crystal size distribution analysis shows substantial microlite crystallisation over a timescale of 1–5 min, which induced significant changes in viscosity and magma rheology during ascent, increasing the effective magma viscosity from ~10 Pa s to 106 Pa s and approaching the brittle fragmentation threshold. Comparable pre- and syn-eruptive conditions have been deduced for other examples of basaltic Plinian activity, indicating a common eruption mechanism. We find that the common requirements for a basaltic Plinian eruption are a low magmatic H2O concentration of ~2 wt%, storage at moderate temperatures of 1080–1100 °C, and rapid crystallisation during magma ascent. Ultimately, these conditions contributed to the production of this highly hazardous Plinian eruption of Masaya.

Particular H2O dissolution mechanism in iron‐rich melt: Application to martian basaltic melt genesis

AbstractMartian basalts are different from Earth by their iron‐rich abundance with 18 wt% FeOtot in average for Mars upper crust. The H2O dissolution mechanism in this atypical melt composition is not well understood. We have synthesized H2O‐bearing martian basaltic glass analogs (10 < FeOtot < 16 wt%) under high pressures (0.5–1.5 GPa) and temperatures (>1500°C) conditions. We used Raman and Fourier transform infrared spectroscopies to investigate the effect of H2O as well as the high FeOtot content on the molecular structure of Fe‐rich glasses. Increasing Fe content appears to inhibit the dissolution of H2O in the melt. In fact, it appears that Free OH groups are formed at relatively low H2O content in Fe‐rich melts (1 wt%), whereas they only appeared at high H2O content (~6.5 wt%) for Fe‐poor glasses. We suggest that the Free OH are bonded to Fe2+ cations in the melt forming isolating clusters of Fe (OH)2. Such configurations are suspected to induce an increase in the melt polymerization. The presence of free hydroxyls dissolved in the melt is likely to have a major impact on the genesis of aqueous fluid phase at the surface of Mars.

Degassing-induced chemical heterogeneity at the 2011-2012 Cordón Caulle eruption

Low H2O concentrations preserved in the erupted products from the 2011-2012 eruption at Cordón Caulle, Chile, have limited its use as a recorder of the shallow degassing processes responsible for the observed hybrid activity. However, volatile trace element heterogeneity (e.g. Li, Pb, Tl) is observed in different textural domains (e.g. breccias, tuffisites), which each represent the competing processes of fracturing, gas fluxing and melt relaxation occurring at different locations within the upper conduit. Elemental depletions in tuffisite veins record scavenging of metals by the fluxing gas phase. Preserved concentration gradients indicate deep-conduit degassing events are short-lived (~minutes), whilst late-stage activity involves hours of gas venting from a single location. Later-erupted material is more degassed than earlier-erupted bombs indicating a progressive loss of volatiles with eruption duration. Combining in situ textural and volatile trace element analyses can constrain evolving degassing processes in systems that are depleted in H2O and CO2.

Experimental constraints on the differentiation of low-alkali magmas beneath the Tonga arc: Implications for the origin of arc tholeiites

Hydrous melting experiments on a basaltic andesite from Late Volcano in the Tonga Arc were conducted at 900–1250 °C and 1 atm to 2.5 GPa, with a range of added H2O concentrations (0–9 wt%). These were used to constrain conditions of phenocryst precipitation in the basaltic andesite and to better understand the processes of magmatic differentiation beneath Tongan volcanoes. Comparison between the products of experiments and Tongan lavas indicates that basaltic andesites from Late crystallised plagioclase + augite + pigeonite ± orthopyroxene while water-saturated at comparatively low pressures (≤0.2 GPa) but high temperatures (≥1000 °C). Glasses produced at 950 °C and 0.2 GPa are similar in composition to dacites from neighbouring volcanoes, including Fonualei. This confirms previous interpretations of dacite magma genesis by crystal fractionation of plagioclase and clinopyroxene from basaltic andesite parent magmas, rather than by partial melting of lower crust. This produces decreases in Dy/Yb with increasing SiO2 without a role for amphibole, probably due to the high temperatures and low alkali concentrations involved. Tongan magmas are primarily tholeiitic, and their high water contents bring into question previous hypotheses that attribute the tholeiitic trend to low concentrations of magmatic H2O. Attributes inherited from the Tongan parent magmas, including low alkali concentrations, appear to drive tholeiitic differentiation, regardless of water content.