2O Content Research Articles

Hydrogen Bubble Templated Electrodeposited Cu As CO2 Reduction Catalyst: Substrate Effect

Selective conversion of CO2 into high quality chemical feedstock and fuels represents an emerging energy storage technology1. The electrochemical reduction of carbon dioxide using renewable sources of energy is a promising way of utilizing the only intermittently available renewable energies to produce chemical feedstocks and fuels for later use. Presently, copper is the one metal in focus because of its ability to catalyze CO2 reduction towards energy-dense hydrocarbon products. However, the electrochemical reduction of CO2 on copper results in many different products at a wide range of overpotentials. The selectivity and efficiency of the Cu-based catalysts towards specific products can be improved by fine tuning the structure and composition2-4. In this work, high surface area porous Cu films were electrodeposited either on Cu or Pt substrates at various deposition currents and durations using the dynamic hydrogen bubble template method5. During electrodeposition at high current values, the size and residence time of H2 bubbles vary based on the substrate and on the in-situ deposited Cu, leading to significantly different catalyst morphologies. Consequently, well-defined dendritic structures with large pores are observed (inset of Fig. 1) for Cu deposited on Cu substrate (CuCat/Cu) as compared to Cu deposited on Pt substrate (CuCat/Pt). Interestingly, product quantification by gas chromatography during electrochemical measurements showed that the product selectivity observed for all variations of the catalysts deposited on a particular substrate does not depend significantly on the differences in pore size and film thickness of the material. However there is a difference in product selectivity for the respective catalysts deposited on different substrates as represented in Fig. 1, where ethylene and carbon-monoxide selectivities are shown at two different potentials. The formation of methane is significantly suppressed (<1%) on both substrates in the potential range where the catalysts show high selectivity for ethylene. From X-ray absorption measurements it is observed that the catalysts are composed of metallic Cu and CuI 2O species with no evidence of CuIIO species. The morphological changes, atomic state and composition of the catalyst may have a dialectic role in deciding the selectivity. In the follow-up work, the selectivity of various Cu films electrodeposited on both the substrates will be correlated with respect to surface area and CuI 2O content after comprehensive quantification of the gaseous and liquid reaction products by gas chromatography, liquid gas chromatography and HPLC. These studies are expected to reveal strategies by which catalysts can be designed for higher selectivity towards a specific hydrocarbon product. References M. Gattrell, N. Gupta and A. Co, J. Electroanal. Chem., 594, 1 (2006). 2. A. Dutta, M. Rahaman, N. C. Luedi, M. Mohos and P. Broekmann, ACS Catal. 6, 3804 (2016).H. Mistry, A. S. Varela, C. S. Bonifacio, I. Zegkinoglou, I. Sinev, et. al, Nat. Commun., 7, 12123 (2016).A. S. Varela, C. Schlaup, Z. P. Jovanov, P. Malacrida, S. Horch, et. al, J. Phys. Chem. C, 117, 20500 (2013).H. -C. Shin, J. Dong and M. Liu, Adv. Mater ., 15, 1610 (2003). Figure 1

The geochemical and petrological characteristics of prenatal caldera volcano: a case of the\xa0newly formed small dacitic caldera, Hijiori, Northeast Japan

Evaluating the magma depth and its physical properties is critical to conduct a better geophysical assessment of magma chambers of caldera volcanoes that may potentially cause future volcanic hazards. To understand pre-eruptive conditions of a magma chamber before its first appearance at the surface, this paper describes the case of Hijiori caldera volcano in northeastern Japan, which emerged approximately 12,000 years ago at a place where no volcano ever existed. We estimated the depth, density, bulk modulus, vesicularity, crystal content, and bulk H_2O content of the magma chamber using petrographic interpretations, bulk and microchemical compositions, and thermodynamic calculations. The chemical mass balance calculations and thermodynamic modeling of the erupted magmas indicate that the upper portion of the Hijiori magmatic plumbing system was located at depths between 2 and 4 km, and had the following characteristics: (1) pre-eruptive temperature: about 780 ^{circ }C; (2) bulk magma composition: 66 ± 1.5 wt% SiO_{2}; (3) bulk magmatic H_2O: approximately 2.5 wt%, and variable characteristics that depend on depth; (4) crystal content: le57 vol%; (5) bulk modulus of magma: 0.1–0.8 GPa; (6) magma density: 1.8–2.3 g/cm3; and (7) amount of excess magmatic H_2O: 11–32 vol% or 48–81 mol%. The range of melt water contents found in quartz-hosted melt inclusions (2–9 wt%) suggests the range of depth phenocrysts growth to be wide (2sim13 km). Our data suggest the presence of a vertically elongated magma chamber whose top is nearly solidified but highly vesiculated; this chamber has probably grown and re-mobilized by repeated injections of a small amount of hot dacitic magma originated from the depth.

Phosphate minerals in the H group of ordinary chondrites, and fluid activity recorded by apatite heterogeneity in the Zag H3-6 regolith breccia

Phosphate minerals in ordinary chondrites provide a record of fluids that were present during metamorphic heating of the chondrite parent asteroids. We have carried out a petrographic study of the phosphate minerals, merrillite and apatite, in metamorphosed H group ordinary chondrites of petrologic type 4–6, to understand development of phosphate minerals and associated fluid evolution during metamorphism. In unbrecciated chondrites, apatite is Cl rich and shows textural evolution from fine-grained apatite-merrillite assemblages in type 4 toward larger, uniform grains in type 6. The Cl/F ratio in apatite shows a similar degree of heterogeneity in all petrologic types, and no systematic change in compositions with metamorphic grade, which suggests that compositions in each meteorite are dictated by localized conditions, possibly because of a limited fluid/rock ratio. The development of phosphate minerals in H chondrites is similar to that of L and LL chondrites, despite the fact that feldspar equilibration resulting from albitization is complete in H4 chondrites but not in L4 or LL4 chondrites. This suggests that albitization took place during an earlier period of the metamorphic history than that recorded by preserved apatite compositions, and chemical equilibrium was not achieved throughout the H chondrite parent body or bodies during the late stages of metamorphism. A relict igneous clast in the H5 chondrite, Oro Grande has apatite rims on relict phenocrysts of (possibly) diopside that have equilibrated with the host chondrite. Apatite in the Zag H3–6 regolith breccia records a complex fluid history, which is likely related to the presence of halite in this meteorite. The porous dark H4 matrix of Zag, where halite is observed, has a high apatite/merrillite ratio, and apatite is extremely Cl rich. One light H6 clast contains similarly Cl-rich apatite. In a second light H6 clast, apatite compositions are very heterogeneous and more F-rich. Apatites in both H4 matrix and H6 clasts have very low H_2O contents. Heterogeneous apatite compositions in Zag record multiple stages of regolith processing and shock at the surface of the H chondrite parent body, and apatite records either the passage of fluids of variable compositions resulting from different impact-related processes, or the passage of a single fluid whose composition evolved as it interacted with the chondrite regolith. Unraveling the history of apatite can potentially help to interpret the internal structure of chondrite parent bodies, with implications for physical and mechanical properties of chondritic asteroids. The behavior of halogens recorded by apatite is important for understanding the behavior of volatile elements in general: if impact-melt materials close to the surface of a chondritic asteroid are readily degassed, the volatile inventories of terrestrial planets could be considerably more depleted than the CI carbonaceous chondrite abundances that are commonly assumed.

Neoproterozoic nascent island arc volcanism from the Nubian Shield of Egypt: Magma genesis and generation of continental crust in intra-oceanic arcs

The Neoproterozoic Wadi Ranga metavolcanic rocks, South Eastern Desert of Egypt, constitute a slightly metamorphosed bimodal sequence of low-K submarine tholeiitic mafic and felsic volcanic rocks. The mafic volcanic rocks are represented by massive and pillow flows and agglomerates, composed of porphyritic and aphyric basalts and basaltic andesites that are mostly amygdaloidal. The felsic volcanic rocks embrace porphyritic dacites and rhyolites and tuffs, which overlie the mafic volcanic rocks. The geochemical characteristics of Wadi Ranga volcanic rocks, especially a strong Nb depletion, indicate that they were formed from subduction-related melts. The clinopyroxene phenocrysts of basalts are more akin to those crystallizing from island-arc tholeiitic magmas. The tholeiitic nature of the Wadi Ranga volcanics as well as their LREE-depleted or nearly flat REE patterns and their low K 2O contents suggest that they were developed in an immature island arc setting. The subchondritic Nb/Ta ratios (with the lowest ratio reported for any arc rocks) and low Nb/Yb ratios indicate that the mantle source of the Wadi Ranga mafic volcanic rocks was more depleted than N-MORB-source mantle. Subduction signature was dominated by aqueous fluids derived from slab dehydration, whereas the role of subducted sediments in mantle-wedge metasomatization was subordinate, implying that the subduction system was sediment-starved and far from continental clastic input. The amount of slab-derived fluids was enough to produce hydrous magmas that follow the tholeiitic but not the calc-alkaline differentiation trend. With Mg# > 64, few samples of Wadi Ranga mafic volcanic rocks are similar to primitive arc magmas, whereas the other samples have clearly experienced considerable fractional crystallization. The low abundances of trace elements, together with low K 2O contents of the felsic metavolcanic rocks indicate that they were erupted in a primitive island arc setting. The felsic volcanic rocks are characterized by lower K/Rb ratios compared to the mafic volcanic rocks, higher trace element abundances (~ 2 to ~ 9 times basalt) on primitive arc basalt-normalized pattern and nearly flat chondrite-normalized REE patterns, which display a negative Eu anomaly. These features are largely consistent with fractional crystallization model for the origin of the felsic volcanic rocks. Moreover, SiO 2-REE variations for the Wadi Ranga volcanic rocks display steadily increasing LREE over the entire mafic to felsic range and enriched La abundances in the felsic lavas relative to the most mafic lavas, features which are consistent with production of the felsic volcanic rocks through fractional crystallization of basaltic melts. The relatively large volume of Wadi Ranga silicic volcanic rocks implies that significant volume of silicic magmas can be generated in immature island arcs by fractional crystallization and indicates the significant role of intra-oceanic arcs in the production of Neoproterozoic continental crust. We emphasize that the geochemical characteristics of these rocks such as their low LILE and nearly flat REE patterns can successfully discriminate them from other Egyptian Neoproterozoic felsic volcanic rocks, which have higher LILE, Zr and Nb and fractionated REE patterns.

Effect of Na 2O on alumina leaching property and phase transformation of MgO-containing calcium aluminate slags

In order to remove or reduce the negative effect of MgO in calcium aluminate slags, the method of adding Na 2O into calcium aluminate slags was studied and its effect on leaching mechanism was also analyzed. The results show that the alumina leaching efficiency of the calcium aluminate slag increases from 68.73% to 80.86% with Na 2O content increasing from 0 to 4% when MgO content is 3%. The XRD results show that the quaternary compound C 20A 13M 3S 3 disappears when Na 2O content increases to 4%. The addition of Na 2O cannot remove the negative effect of MgO on leachability completely. XRD and EDS results indicate that Na 2O can come into the lattice of 12CaO·7Al 2O 3 and promote the formation of 12CaO·7Al 2O 3

U–Pb zircon age, geochemical and Sr–Nd isotopic data as constraints on the petrogenesis and emplacement time of andesites from Gerze, southern Qiangtang Block, northern Tibet

Mesozoic post-collisional andesites occur in the Gerze area within the Qiangtang Terrane of northern Tibet, China. Geochronological, geochemical, and whole-rock Sr–Nd isotopic analyses were performed on this suite of volcanic rocks to characterize their petrogenesis. Laser ablation-inductively coupled plasma-mass spectrometry U–Pb zircon analyses yielded consistent ages ranging from 122.4 ± 0.4 Ma to 124.4 ± 0.4 Ma for two andesite samples (JC01 and GZ01). The andesites belong to the alkaline and sub-alkaline magma series in terms of K 2O + Na 2O contents (4.7–10 wt.%), and belong to the high-K calc-alkaline and calc-alkaline series on the basis of their high K 2O contents (1.0–3.3 wt.%). The volcanic rocks are further characterized by the enrichment of light rare earth elements [(La/Yb) N = 7.4–11.5] and large-ion lithophile elements (e.g., Rb), slightly negative Eu anomalies (Eu/Eu * = 0.79–0.94), and negative anomalies in high field strength elements (e.g., Nb, Ta, and Ti) relative to primitive mantle. The samples show slightly elevated ( 87Sr/ 86Sr) i values that range from 0.7049 to 0.7057, and low ε Nd( t) values of −3.4 to 0.7. These results suggest that the rocks were derived from a compositionally heterogeneous mantle source and that their parent magmas are basalt magma. The parent mafic magmas may have undergone fractional crystallization of olivine, pyroxene, hornblende, and to a lesser extent, plagioclase during ascent through the thickened crustal pile, with minor or no crustal contamination, before the finally erupting as andesites.

The Corredoiras orthogneiss (NW Iberian Massif): Geochemistry and geochronology of the Paleozoic magmatic suite developed in a peri-Gondwanan arc

The Corredoiras orthogneiss belongs to the intermediate pressure upper units of the Órdenes Complex (Variscan belt, NW Spain), mainly composed by granodioritic orthogneisses, with small bodies of tonalitic orthogneisses, amphibole-rich orthogneisses and metagabbronorites. In this work we study their chemical and isotopic composition, to gain insight into the linkage between plate tectonics and magmatism and to improve the knowledge of the paleogeographic evolution of the European Variscan Belt. Granodioritic and tonalitic orthogneisses range from intermediate to felsic rocks, with K 2O/Na 2O ratios ≈ 1, typical of calc-alkaline rocks, and high Na 2O content, characteristic of I-type granites. Metagabbronorites are basic rocks, but some of them are contaminated by interaction with the felsic magmas, showing enrichment in SiO 2, Na 2O and K 2O. All Corredoiras metaigneous rocks are enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE) relative to high field strength elements (HFSE), resulting in a high LILE/HFSE ratio. These geochemical features are the most characteristic of magmas related to subduction zones; furthermore all orthogneisses display significant negative anomalies in Ta, Nb and Zr, which together with their low contents in Y and Yb match up with granitoids generated in volcanic arcs or subduction zones. SHRIMP U–Pb zircon dating provides a concordia age of 492 ± 3 Ma. Granodioritic orthogneiss has negative εNd (492 Ma) values (− 2.2 to − 3.6) and high ( 87Sr/ 86Sr) i ratios (0.707 to 0.708), on the other hand tonalitic orthogneisses and metagabbronorites have positive εNd (492 Ma) (1.0 to 2.4) and low ( 87Sr/ 86Sr) i (0.703 to 0.705), suggesting that granodioritic orthogneisses have a clear crustal influence in their generation, whereas tonalitic orthogneisses and metagabbros can be related to basic magmas extracted from the mantle or from a basic lower continental crust. The Corredoiras chemical characteristics permit us to interpret that this rocks were probably generated in an ensialic island arc and may represent a peri-Gondwanan fragment drifted away to open the Rheic Ocean.

H 2O and CO 2 in parental magmas of Kliuchevskoi volcano inferred from study of melt and fluid inclusions in olivine

This paper reports new FTIR data on the H 2O and CO 2 concentrations in glasses of 26 naturally quenched and experimentally partially homogenized melt inclusions in olivine (Fo 85–91) phenocrysts from rocks of the Kliuchevskoi volcano. Measured H 2O concentrations in the inclusions range from 0.02 to 4 wt.%. The wide variations in the H 2O content of the inclusions, which do not correlate with the host olivine composition and contents of major elements in the melts, are explained by the H 2O escape from inclusions via diffusion through the host olivine during the magma eruption and the following cooling. The largest H 2O loss is characteristic of inclusions from lava samples which cooled slowly after eruption. The minimal H 2O loss is observed for inclusions from rapidly quenched pyroclastic rocks. Parental magmas of the Kliuchevskoi volcano are estimated to contain 3.5 wt.% H 2O. The new data imply a 40 °C lower mantle temperatures than that estimated earlier for the Kliuchevskoi primary melts. The concentrations of CO 2 in glasses range from <0.01 to 0.13 wt.% and do not correlate with the type of studied inclusions and their composition. The calculated pressures of melt equilibria with H 2O–CO 2 fluid inside the inclusions are lower than 270 MPa. They are significantly lower than a pressure of 500 MPa calculated from the density (~0.8 g/cm 3) of cogenetic fluid inclusions in high-Fo olivine. The significant pressure drop inside the melt inclusions after their trapping in olivine might be due to the H 2O loss and redistribution of CO 2 from melt to daughter fluid phase. Compared with melt inclusions, cogenetic fluid inclusions provide independent information about the crystallization pressures of olivine and initial CO 2 content in the Kliuchevskoi magma, which were estimated to be at least 500 MPa and 0.35 wt.%, respectively. The maximum CO 2 concentrations in the primary Kliuchevskoi melts are estimated at 0.8–0.9 wt.%. The decompression crystallization of the Kliuchevskoi magmas starts at depths of 30–40 km and proceeds with a continuous decrease in CO 2 content and an increase (up to 6–7 wt.%) and then a decrease (at <300 MPa) in H 2O content in melts, which explains the origin of the whole spectrum of rocks and melt inclusions of the Kliuchevskoi volcano.

Silver effect on the structure of SiO 2-CaO-P 2O 5 ternary system

In this study are reported results obtained from the structural, morphological and textural investigations of 56SiO 2∙(40-x)CaO·4P 2O 5·xAg 2O system, where 0 ≤ x ≤ 10 mol%. The samples obtained by sol–gel method were annealed and then characterized by X-ray diffraction (XRD), differential thermal analysis (DTA), thermal gravimetric analysis (TGA), vibrational spectroscopy (Raman, FTIR), N 2-adsorption measurements, and transmission electron microscopy (TEM). XRD patterns of the investigated samples exhibit tricalcium phosphate (TCP) nanostructured phase and show the existence of metallic silver as dispersed phase. The presence of apatite-like phase is underlined by the recorded Raman and especially FT-IR spectra. TEM pictures indicate the presence of silver nanoparticles of almost spherical shapes and various sizes inside the matrix, depending on the Ag 2O content. Regarding the textural properties, it was observed both a decrease of the specific surface area as well as a progressive change of the mesoporous characteristics with the silver addition, the latter behavior recommending the potential use of these samples for applications, where the morphology control is required.

Processing of a Zimbabwean petalite to obtain lithium carbonate

Processing of petalite concentrate from the Bikita deposits in Zimbabwe for production of high-purity Li 2CO 3 has been studied. XRF and ICP-OES analysis showed that the concentrate consists of oxides of Li, Si, and Al as major components, with an average Li 2O content of 4.10%. XRD examination confirmed that the sample is a petalite. Processing of the petalite involves roasting the pre-heated concentrate with concentrated H 2SO 4 followed by water leaching of the resulting Li 2SO 4, solution purification and precipitation of Li 2CO 3. The effects of roasting temperature, stirring speed, solid to liquid ratio, leaching temperature and time on the lithium dissolution are reported. The dissolution rates are significantly influenced by roasting temperature and stirring speed. Water-washed lithium carbonate with a purity of 99.21% (metal basis) was produced. Synthesised and commercial Li 2CO 3 samples were characterised and compared using X-ray diffraction (XRD) and thermogravimetric analysis (TGA).

Insights into shallow magma storage and crystallization at Volcán Llaima (Andean Southern Volcanic Zone, Chile)

Scoriae produced by four mafic historic eruptions of Volcán Llaima are used to elucidate magma differentiation, recharge, mixing, and eruption triggering. Whole-rock, mineral, and olivine-hosted melt inclusion chemistry suggest that basaltic andesites (~ 52–53.5 wt.% SiO 2) stall at immediately sub-edifice depths (≤ 4 km beneath the base of the volcano), where they crystallize to large extents and form mush bodies. Melt inclusion trends overlap with the whole-rock data trends defined by the entire volcano up to 53–55% SiO 2, but more evolved compositions define divergent trends for some elements, with up to 2.2 wt.% TiO 2 (increasing Zr) and Al 2O 3 as low as 12.5 wt.% (decreasing Sr) at 57–58 wt.% SiO 2. These more evolved melt compositions are inferred to be the result of shallow evolution of interstitial melt during the formation of crystal mush bodies, as a consequence of degassing and crystallization with a reduced participation of Fe–Ti-oxides. The inferred suppression of Fe–Ti-oxide stability and the modal dominance of plagioclase crystallization are consistent with low fluid-saturation pressures of ~ 40–70 MPa inferred from average H 2O contents for mafic arc magmas (dominantly 1–4 wt.%) but low CO 2 contents (dominantly 0–300 ppm) in melt inclusions. The broad range in olivine core compositions (Fo 69–83) and the absence of correlated degassing and magma evolution trends in historic Llaima magmas suggest that they are stored as multiple dike-like bodies created by a high frequency of magma replenishment relative to the frequency of large eruptions. This temporal–spatial disconnection leads to isolated evolution and degassing of discrete magma batches, followed by remobilization and assembly just prior to eruption. Eruptions are probably triggered by recharge of relatively hot, mafic, and much less degassed magma, in accord with dominantly reversely-zoned olivine crystals, and higher olivine-melt temperatures recorded by the relatively primitive matrix glasses and coexisting olivine rims.

Spectroscopic investigations and physical properties of Mn 2+ doped mixed alkali zinc borate glasses

Spectroscopic investigations on Mn 2+ doped 20ZnO + xLi 2O + (30− x)Na 2O + 50B 2O 3 (5 ≤ x ≤ 25) (ZLNB) glasses reveal the non-linear behavior in their physical and structural properties. FT-IR spectra of ZLNB glasses reveal diborate units in the borate network. EPR spectra exhibit characteristic resonance signals of Mn 2+ ions. A well resolved six line hyperfine structure around g = 2.02 corresponds to the sites of Type II, and a weak broad shoulder around g = 2.7 followed by an unresolved intense signal at g = 4.25 corresponding to sites of Type I are observed for their respective transitions. A large value of zero-field splitting parameter ( D) is observed and it behaves non-linearly, reaches a maximum at x = 15 mol% with increase in Li 2O content. Δ g values reveal the ionic nature of glass systems. Optical absorption spectra suggest the distorted octahedral site symmetry of Mn 2+ ions in the host. Mixed alkali effect is evident in bandgap and Urbach energies.

A general viscosity model of Campi Flegrei (Italy) melts

Viscosities of shoshonitic and latitic melts, relevant to the Campi Flegrei caldera magmas, have been experimentally determined at atmospheric pressure and 0.5 GPa, temperatures between 840 K and 1870 K, and H 2O contents from 0.02 to 3.30 wt.%. The concentric cylinder technique was employed at atmospheric pressure to determine viscosity of nominally anhydrous melts in the viscosity range of 10 1.5 − 10 3 Pa s. The micropenetration technique was used to determine the viscosity of hydrous and anhydrous melts at atmospheric pressure in the high viscosity range (10 10 Pa s). Falling sphere experiments were performed at 0.5 GPa in the low viscosity range (from 10 0.35 to 10 2.79 Pa s) in order to obtain viscosity data of anhydrous and hydrous melts. The combination of data obtained from the three different techniques adopted permits a general description of viscosity as a function of temperature and water content using the following modified VFT equation: l o g η = − a + b T − c + d T − e ⋅ e x p g ⋅ w T where η is the viscosity in Pa·s, T the temperature in K, w the H 2O content in wt.%, and a, b, c, d, e, and g are the VFT parameters. This model reproduces the experimental data (95 measurements) with a 1σ standard deviation of 0.19 and 0.22 log units for shoshonite and latite, respectively. The proposed model has been applied also to a more evolved composition (trachyte) from the same area in order to create a general model applicable to the whole compositional range of Campi Flegrei products. Moreover, speed data have been used to constrain the ascent velocity of latitic, shoshonitic, and trachytic melts within dikes. Using petrological data and volcanological information (geometrical parameters of the eruptive fissure and depth of magma storage), we estimate a time scale for the ascent of melt from 9 km to 4 km depth (where deep and shallow reservoirs, respectively, are located) in the order of few minutes. Such a rapid ascent should be taken into account for the hazard assessment in the Campi Flegrei area.

Progress of hydration reactions in olivine–H 2O and orthopyroxenite–H 2O systems at 250 °C and vapor-saturated pressure

We conducted hydrothermal experiments in olivine (Ol; Fo 91)–H 2O and orthopyroxenite (Opx; composed of 95% of orthopyroxene, En 66)–H 2O systems under conditions of 250 °C and vapor-saturated pressure (P sat) to examine the temporal evolution of the solution chemistry and products in runs of up to 1008 h in duration. The maximal degree of hydration (i.e., H 2O content in the solid sample) in the Ol–H 2O experiments (3.6 wt.%) was much higher than that in the Opx–H 2O experiments (0.4 wt.%). In the Ol–H 2O experiments, Mg and Si in solution showed an initial increase (stage I) before decreasing (stage II) and finally attaining a steady state after 504 h (stage III). Following a drop in silica activity toward the level of brucite stability field, the products also changed from serpentine + magnetite (stages I and II) to serpentine + brucite + magnetite (stage III). Serpentine minerals also changed from lizardite (stages I and II) to lizardite + chrysotile (stage III). The zoning pattern of the products around olivine at 1008 h (brucite + serpentine at the olivine contact, and serpentine at the outermost rim) is consistent with the temporal changes in the mineralogy of the products, and is similar to the pattern observed in the mesh rims in partly serpentinized dunites. In the Opx–H 2O experiments, chlorite formed after orthopyroxene grains, which differs from the formation of talc and serpentine after orthopyroxene (bastite), as observed in natural hydrated harzburgites. The Opx–H 2O system maintained 10–10 3 times higher silica activity than Ol–H 2O system, suggesting that brucite does not form after olivine during hydration of peridotites when the Ol–H 2O system is linked to the Opx–H 2O system. The progress of hydration reactions is affected by mechanical properties of host rocks. The hydration reactions observed in this study produced hierarchical fractures in the reactants, which became filled with reaction products, similar to mesh textures after olivine in natural peridotites. This reaction-induced fracturing produced new reaction surfaces and fluid pathways that enhanced the hydration reactions. The overall reaction producing serpentine + brucite in the Ol–H 2O experiments showed the large volume expansion ( V/V 0 = 1.43 at stage III), whereas that producing only serpentine proceeded with near constant volume ( V/V 0 = 1.19 at stage I). The volume expansion is more difficult to occur in the oceanic lithosphere than in our experiments during serpentinization. Thus, in the case that volume expansion is prevented at reaction sites, one of the following outcomes occurs: (1) the hydration reaction stops until new fractures form, or (2) the reaction proceeds with low volume expansion (absence of brucite) by removing Mg from the system. These two outcomes would produce contrasting distributions or extent of hydration in oceanic lithosphere.

Garnet pyroxenite, eclogite and alkremite xenoliths from the off-craton Gibeon Kimberlite Field, Namibia: A window into the upper mantle of the Rehoboth Terrane

Different types of mantle xenoliths, including garnet peridotite, garnet pyroxenite, eclogite, and alkremite, occur in kimberlites of the Gibeon Kimberlite Field (GKF), Namibia. The off-craton GKF is situated within the Palaeoproterozoic Rehoboth Terrane, about 400–500 km west of the inferred margin of the Archaean Kaapvaal Craton. Petrological data are available only for garnet peridotite; the other three rock types are described here for the first time. Both types of eclogite, Type I and II, are present in the GKF. The criteria to distinguish between the two types, i.e. Na 2O in garnet and K 2O in clinopyroxene, based on cratonic samples are less suited for off-craton eclogite xenoliths. As demonstrated in this study, Na 2O contents and Cr 2O 3/K 2O ratios in clinopyroxene are useful proxies for classifying off-craton eclogites. All GKF eclogites are bi-mineralic garnet-clinopyroxene rocks with rare accessory spinel and phlogopite. Minor rutile, amphibole and K-feldspar occur only in Type I eclogites. Alkremite is dominated by primary garnet and corundum and contains minor spinel and clinopyroxene of probably secondary origin. Garnet pyroxenite is websteritic and contains clinopyroxene, orthopyroxene and garnet. The garnet pyroxenite protolith was a high-pressure clinopyroxene-rich cumulate, in which garnet exsolution from Al-rich clinopyroxene took place by cooling below 1350 °C. Genetically linked with the garnet pyroxenite is Type II eclogite, which also formed by crystallization from a melt in the mantle. In contrast, the nature of Type I eclogite remains ambiguous. It may be of similar origin as Type II eclogite, or it may represent fragments of subducted oceanic crust. The equilibrium temperature of garnet pyroxenite of ca. 770 °C is consistently lower than that of eclogite (965–1190 °C). The pressure estimate for garnet pyroxenite (21–23 kbar) implies that the rocks formed and re-equilibrated at some 65–75 km depth. The inferred eclogite depth range is 90–130 km. A similar difference in temperature, and by implication also in depth, between pyroxenite and eclogite is known from other xenolith suites in southern Africa. These localities are more than 1000 km apart and extend from the Namibian GKF in the west to the Rietfontein and Kaalvallei kimberlites in South Africa and beyond to the area around the Premier diamond mine close to Pretoria. This implies a common thermal and lithological depth pattern for large parts of southern Africa, with a layer rich in garnet pyroxenite being underlain by an eclogite-rich layer. This supports a recent proposition ( Bell et al., 2003; Janney et al., 2010) that the thermal state and thickness of Proterozoic off-craton and Archaean cratonic lithosphere were largely similar prior to Mesozoic Gondwana break-up, which caused thermal erosion of the off-craton lithosphere with respect to the Kaapvaal Craton. In many cases, Proterozoic lithospheric mantle may represent reworked Archaean rather than juvenile Proterozoic material (e.g. Begg et al., 2009). If this also applies to the GKF mantle lithosphere has yet to be shown. The correlation inferred from this study may also be explained in terms of similar processes that formed and modified both Archaean and Proterozoic mantle lithosphere.

The imprint of subduction fluids on subducted MORB-derived melts (Sierra del Convento Mélange, Cuba)

Major and trace element signatures and Sr–Nd–Pb isotope data for muscovite (Ms)-bearing amphibolite blocks and associated muscovite-bearing trondhjemite and quartz-muscovite rocks from the Sierra del Convento mélange (eastern Cuba) indicate that Proto-Caribbean oceanic crust underwent wet partial melting processes during Mesozoic subduction and after accretion to the upper plate. Trace element normalized patterns of Ms-bearing amphibolites are enriched in light rare earth elements (LREE) and large-ion lithophile elements (LILE) and evidence variable trace element transfer from the Proto-Caribbean subducting slab to the mantle wedge. Ms-bearing trondhjemites show LREE enrichment and HREE depletion and have geochemical features similar to adakites, including SiO 2 > 56 wt.%, high Na 2O contents (5.5–9.0 wt.%) and high Sr/Y (16–644). We consider that the trondhjemites represent primary and natural melts formed by deep partial melting of the subducting slab which did not significantly react with the mantle wedge before intrusion in the subduction channel. The Ms-bearing trondhjemites show different geochemical and petrological signatures compared with the Ms-free tonalites–trondhjemites from Sierra del Convento, that are interpreted as primary slab melts. These differences support the idea that partial melting processes in the Sierra del Convento subduction channel were triggered by the infiltration of fluids derived from three distinct subducted sources: sediments, altered mid-ocean ridge basalts (MORB), and serpentinites. In this scenario, the pegmatitic quartz-muscovite rocks, which are highly enriched in LILE, probably represent the crystallization products of fluids derived by differentiation of the trondhjemitic melts.

H 2O contents and their modification in the Cenozoic subcontinental lithospheric mantle beneath the Cathaysia block, SE China

The water contents of nominally anhydrous minerals (NAMs) in 35 peridotite xenoliths have been analyzed by Fourier transform infrared spectrometry (FTIR). The xenoliths are hosted by Cenozoic volcanic rocks from four localities (Mingxi, Anyuan, Niutoushan and Qilin) in the Cathaysia Block, SE China. Water contents of clinopyroxene (cpx), orthopyroxene (opx), and olivine (ol) range from 58 to 488 ppm, 38 to 213 ppm, and 0 to 41 ppm, respectively. Water contents of cpx are positively correlated with Na and Al, and negatively correlated with Mg and Ca. Considering (1) the homogeneity of water distribution within single pyroxene grains; (2) the partitioning of water between cpx and opx with an average D cpx/opx of 2.3; and (3) the correlations between the water contents and major element concentrations in cpx, it is suggested that the pyroxenes have largely preserved the water content of their mantle source. The whole-rock water contents calculated from mineral modes range from 12 to 94 ppm (average 60 ± 20 ppm). This is much higher than the previously-reported water contents of xenoliths from the North China Craton (NCC) (average 26 ± 17 ppm). However, it is still quite low compared to those of continental lithospheric mantle worldwide, as inferred from analyses of typical cratonic (122 ± 54 ppm) and off-craton (81 ± 40 ppm) peridotites. The SCLM beneath the Cathaysia block is a medium-poor water reservoir; this can be explained by the refertilization of old lithospheric mantle which has undergone multiple geological events through time. Water itself plays an important role during the modification of the subcontinental lithospheric mantle. The wet and fertile garnet-facies peridotites found in the Mingxi locality represent the accretion of young lithosphere near the lithosphere–asthenosphere boundary, with almost no melt extraction. This process marks the completion of the lithospheric thinning and the upwelling of asthenosphere. A negative correlation between pyroxene water contents and the oxygen fugacity has only been found in xenoliths from Niutoushan (Mg# < 90), which lies on the Changle–Nan'ao fault zone. The fault may have facilitated the infiltration of Niutoushan peridotites by oxidized fluids (or hydrous melts) rising from the subducting Pacific plate.

Effect of alumina on the structure and properties of Li 2O–B 2O 3–P 2O 5 glasses

The structure of glasses within the system Li 2O–Al 2O 3–B 2O 3–P 2O 5 has been studied through 31P, 11B and 27Al Nuclear Magnetic Resonance, and the effect of Al 2O 3 substitution by B 2O 3 and P 2O 5 network formers on the structure and properties investigated for a constant Li 2O content. Multinuclear NMR results reveal that substitution of Al 2O 3 for B 2O 3 and P 2O 5 network formers in a glass with composition 50Li 2O·15B 2O 3·35P 2O 5 produces a change in boron environment from four-fold to three-fold coordination. Meanwhile aluminum can be present in four-, five- and six-fold coordinations a higher amount of Al(IV) groups is found for increasing alumina contents. The behavior of the glass transition temperature and electrical conductivity of the glasses has been interpreted as a function of the structural changes induced in the glass network when alumina is substituted for B 2O 3, P 2O 5 or both. Small additions of alumina produce a drastic increase in glass transition temperature, while it does not change for [Al 2O 3] greater than 3 mol.%. However, the electrical conductivity shows very different behavior depending on the type of substitution; it can remain constant when B 2O 3 content decreases or sharply decrease when P 2O 5 is substituted by Al 2O 3, which is attributed to a higher amount of BO 3 and phase separation.

Polaron transport studies in La 2O–V 2O 5–P 2O 5 glasses

The glass transition temperature and electrical conductivity in the temperature range 325 K to 525 K have been investigated in a set of La 2O doped vanado-phosphate glasses. The measured variation of density and glass transition temperature with La 2O content revealed that glass network becomes continuously loose packed with increasing La 2O content. Conductivity variation with La 2O revealed the contribution of lanthanum ions to the total conductivity. Temperature behavior of conductivity has been explained using Mott’s small polaron and variable range hopping models. Various physical and polaron hopping parameters such as, activation energy, polaron hopping energy, polaron radius, effective dielectric constant, polaron bandwidth, electron-phonon coupling constant, hopping site energy and variable range hopping distance have been determined.

Physicochemical conditions for melting in the Earth’s mantle containing a C–O–H fluid (from experimental data)

Experimental data on phase transformations and melting in peridotite and eclogite systems with a C–O–H fluid at 6–30 GPa have been analyzed with special attention to the influence of redox conditions. It has been found that melting in systems with H 2O depends heavily on its total content and considerably on its solubility in nominally anhydrous rock-forming minerals. Partial melting occurs when the total H 2O content of the system exceeds the H 2O storage capacity in the rock under given physicochemical conditions. Melting in CO 2-containing systems is determined by carbonate stability and the chemical composition of the system, mainly its Na 2O and K2O contents, and, to a smaller extent, the content of CO 2 itself. Studies of peridotite and eclogite systems containing H 2O, CO 2, H 2O + CO 2, and a reduced C–O–H fluid show that most solidi flatten out at pressures above 6–8 GPa when intersecting the geotherms of subduction and average mantle. Mantle melting at constant pressure in the presence of a C–O–H fluid depends not only on temperature but also on redox conditions. The oxidation of the system causes redox melting. The stability boundary of a Fe–Ni alloy (it may coincide with the lithosphere–asthenosphere boundary under cratons, 200–250 km) and the 410-km discontinuity are paramount to redox and decarbonation–dehydration melting. Also, the paper provides evidence that subducted carbonates play the leading role in the “big” mantle wedge model for stagnant slabs. Volatile-containing eclogite systems melt at lower temperatures than peridotite ones (the difference is up to 100–200 °C). This suggests that eclogites are of global importance in mantle melting, which agrees with modern geochemical models.