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To investigate the effect of carbon-bearing phases on the release of fluids in subducted serpentinites, we performed high-pressure multi-anvil experiments on representative ophicarbonate assemblages over a pressure range from 2.5 GPa to 5 GPa and from 450 °C to 900 °C, across the antigorite-out reaction. Parallel experiments were performed on carbonate-free serpentinites. In all experiments, we monitored and/or controlled the oxygen fugacity. The addition of 20 wt. % CaCO.sub.3 to a serpentinite assemblage at 2.5 GPa is found to decrease the onset of the serpentine dehydration by over 100 °C, in comparison to carbonate-free assemblages. Similarly, the final disappearance of serpentine is also affected by the presence of CaCO.sub.3. For a bulk CaCO.sub.3 content of 20 wt. %, this causes a decrease in maximum stability of antigorite by 50 °C. For a bulk CaCO.sub.3 content exceeding 25 wt. %, this difference can be as high as 100 °C in warm and 150 °C in cold subduction zones, causing antigorite to be completely dehydrated at 500 °C. This results from the reaction of CaCO.sub.3 with serpentine to form clinopyroxene and Mg-rich carbonates. This reaction, however, causes no discernible decrease in the proportion of carbonate, indicating that the amount of released carbon is insignificant. Whilst CaCO.sub.3, therefore, influences serpentine stability, there is no significant effect of hydrous phases on the carbonate stability. On the other hand, a MgCO.sub.3-bearing system shows no significant effects on the serpentinite stability field. Further experiments and oxygen fugacity calculations indicate that graphite is not stable in typical magnetite-bearing serpentinites. The reduction of carbonates to graphite would require oxygen fugacities that are 1-2 log units below those of magnetite-bearing serpentinites. This confirms earlier studies and indicates that reduction of carbonates can only occur through the infiltration of external H.sub.2-rich fluids., Author(s): Lisa Eberhard [sup.1] [sup.2], Oliver Plümper [sup.2], Daniel J. Frost [sup.1] Author Affiliations: (1) grid.7384.8, 0000 0004 0467 6972, Bayerisches Geoinstitut, University of Bayreuth, , Universitätsstrasse 30, 95440, Bayreuth, [...]

Dichotomous high-Ti and low-Ti magmas are ubiquitous in large igneous provinces (LIPs). These magmas often form economically critical Fe-Ti oxide ores in layered intrusions via mechanisms that remain debated. To constrain the evolution of high-Ti and low-Ti basalts during fractionation, we performed stepwise equilibrium crystallization experiments at atmospheric pressure. We specifically aimed to quantify the influences of starting composition and oxygen fugacity (fO.sub.2) on phase stability, phase compositions, and the onset of silicate liquid immiscibility during cooling. Both types of magma crystallize similar phase assemblages at QFM (quartz-fayalite-magnetite thermodynamic equilibrium) and QFM + 2: olivine, clinopyroxene, plagioclase, Fe-Ti oxides, and whitlockite. Tridymite crystallizes late in experiments at QFM + 2. The starting composition exerts a strong influence on phase and melt compositions. High CaO and Al.sub.2O.sub.3 contents in the melt favor the early crystallization of plagioclase and enhance FeO enrichment before Fe-Ti oxide saturation. fO.sub.2 affects the composition and stability of Fe-Ti oxides, and high fO.sub.2 conditions may promote melt differentiation into the calc-alkaline field. Silicate liquid immiscibility occurs in both compositional trajectories, producing Fe-rich melt globules in equilibrium with Si-rich melts. Strong iron enrichment is not necessary for immiscibility to develop; unmixing also occurs in Fe depleted compositions. We propose a new parameterization to map the binodal surface in temperature-composition space that successfully fits the two-liquid field in experiments and natural immiscible compositions. Our results indicate that Fe-Ti oxide ores in layered intrusions associated with LIPs form by the segregation of Fe-rich melts and/or the accumulation of early crystallized Fe-Ti oxides during fractionation., Author(s): Yishen Zhang [sup.1], Olivier Namur [sup.1], Bernard Charlier [sup.2] Author Affiliations: (1) grid.5596.f, 0000 0001 0668 7884, Department of Earth and Environmental Sciences, KU Leuven, , 3000, Leuven, Belgium [...]

The Machangqing Cu-Mo (Au) deposit is located in the central part of the Jinshajiang - Red River belt in the Sanjiang orogen, which lies across the Qiangtang terrane and western Yangtze craton, southwestern China. Zircon U-Pb dating constrains that the granite porphyry and porphyritic granite emplacements occurred at 35.92 [+ or -] 0.31 Ma and 34.92 [+ or -] 0.31 Ma, respectively. The Re-Os model ages of molybdenite are 34.94 [+ or -] 0.38 Ma. The new ages presented here, along with previously published data in the region, define a short duration of potassic magmatism and mineralization from 37 Ma to 34 Ma in the Jinshajiang - Red River belt. Zircon [Ce.sup.4+]/[Ce.sup.3+] values of the porphyritic granite and granite porphyry vary from 50.32 to 1579.20 (averaging 481.01) and 33.18 to 1511.80 (averaging 452.98), respectively, and the log(f[o.sub.2]) values vary from -6.66 to -23.86 and -9.88 to -25.18, respectively, which plot within the range of the fayalite-magnetite-quartz buffer curve to the magnetite-hematite buffer curve, indicating an oxidized magma source, which may have facilitated the Cu-Au enrichment. Zircons from granitoids show [epsilon]Hf(t) values ranging from -0.75 to +2.33 and crustal model ages between 0.9 and 1.1 Ga. The features of Lu-Hf isotopes and wide range of Mg#, Cr, and Ni contents imply that the magmas of the Machangqing granitoids were probably derived from partial melting of juvenile lower crust and mixed with some mantle melts. Combined with the features of the Machangqing granitoids, the following evolution process are concluded. During the Cenozoic, the India-Asia continental collision triggered upwelling of hot asthenosphere and underplating of the thickened juvenile lower crust, which caused the formation of mafic and felsic magmas. Those magmas ascended, mixed, crystallized, and formed Machangqing ore-bearing granitoids in an intracontinental extension setting. Key words: LA-ICP-MS zircon U-Pb dating, molybdenite Re-Os dating, oxidation magma, Machangqing Cu-Mo (Au) deposit, Jinshajiang - Red River belt, eastern Tibetan plateau. Le gisement de Cu-Mo (Au) de Machangqing est situe dans la partie centrale de la ceinture de failles de Jinshajiang - riviere Rouge de l'orogene de Sanjiang, qui enjambe le terrane de Qiangtang et le craton du Yangtze (sud-ouest de la Chine). La mineralisation à Cu-Mo (Au) est genetiquement associee à des stocks de granitoïdes comprenant un granite porphyrique et un porphyre granitique. La datation U-Pb sur zircons par LA-ICP-MS permet d'etablir que le porphyre granitique et le granite porphyrique ont ete mis en place à 35,92 [+ or -] 0,31 Ma et 34,92 [+ or -] 0,31 Ma, respectivement. L'âge modele Re-Os sur la molybdenite est de 34,94 [+ or -] 0,38 Ma, ce qui indiquerait un episode de magmatisme et de mineralisation paleogene à Machangqing. Les nouveaux âges presentes ici, combines à des âges dejà publies pour la region definissent un court intervalle de magmatisme potassique et de mineralisation de 37 Ma à 34 Ma dans la ceinture de Jinshajiang - riviere Rouge. Les valeurs de [Ce.sup.4+]/[Ce.sup.3+] des zircons du granite porphyrique et du porphyre granitique vont de 50,32 à 1579,20 (moyenne de 481,01) et de 33,18 à 1511,80 (moyenne de 452,98), respectivement, et les valeurs de log(f[o.sub.2]) vont de -6,66 à -23,86 et de -9,88 à -25,18, respectivement, du champ tampon redox fayalite-magnetite-quartz au champ tampon redox magnetite-hematite, indiquant une source de magma oxydee qui pourrait avoir facilite l'enrichissement en Cu-Au. Les zircons des granitoïdes presentent des valeurs de [epsilon]Hf(t) allant de -0,75 à +2,33 et des âges modeles crustaux entre 0,9 Ga et 1,1 Ga. Les caracteristiques des isotopes de Lu-Hf et les grandes fourchettes definies par les valeurs de Mg#, Cr et Ni indiqueraient que les magmas des granitoïdes de Machangqing sont probablement derives de la fusion partielle d'une croûte inferieure juvenile, melanges avec des produits de fusion du manteau. Ces donnees, combinees à l'histoire geologique, la geochimie et les donnees isotopiques à l'echelle regionale, permettent de conclure au processus d'evolution suivant: durant le Cenozoïque, la collision continentale de l'Inde et de l'Asie a declenche la remontee d'asthenosphere chaude et son plaquage sous la croûte inferieure juvenile epaissie, ce qui a entraîne la formation de magmas mafiques et felsiques. Ces magmas ont remonte, se sont melanges, ont cristallise et ont forme les granitoïdes mineralises de Machangqing dans un milieu d'extension intracontinentale dans l'est du plateau tibetain. [Traduit par la Redaction] Mots-cles: datation U-Pb sur zircons par LA-ICP-MS, datation par Re-Os sur molybdenite, magma oxydant, gisement de Cu-Mo (Au) de Machangqing, ceinture de Jinshajiang - riviere Rouge, est du plateau tibetain., Introduction Porphyry Cu-(Mo-Au) deposits are among the most important types of economic deposits that account for a significant proportion of global Cu production (e.g., Cooke et al. 2005; Sillitoe 2010). [...]

Bare, uncoated magnetite nanoparticles, synthesized using an electrochemical surfactant-free synthesis, have highly oxidized surfaces that prevent aggregation. These particles have demonstrated highly intriguing biological activity showing extremely potent antibiotic activity against both gram-positive and gram-negative bacteria with little toxicity to rats. This difference in activity could be ascribed to the nature of the protein corona. The kinetics and thermodynamics of the binding of bovine serum albumin (BSA), used as a model serum protein, to these magnetite nanoparticles were analyzed. There is no significant change in particle diameter by dynamic light scattering following adsorption, indicating corona formation does not induce aggregation. The maximum adsorption capacity of the particles was determined to be 300 mg of BSA per gram of magnetite. The particles are able to adsorb 90% of the BSA at protein concentrations as high as 500 mg/L. The adsorption is best described using a pseudo second order model and a Langmuir Type III isotherm model. Thermodynamic analysis showed that the process is entropically driven and is spontaneous at all tested temperatures and conditions. However, it appears to be a weak to moderate physical adsorption. This moderate binding affinity could indicate the differential biological activity of these particles towards bacteria and mammalian cells and further support the contention that these are potentially useful new tools for targeting antibiotic-resistant bacteria. Key words: magnetite nanoparticles, bovine serum albumin, Langmuir isotherm, adsorption kinetics, adsorption thermodynamics. Des nanoparticules nues, synthétisées par une méthode électrochimique sans surfactant, présentent des surfaces hautement oxydées qui préviennent l'agrégation. Ces particules ont révélé une intrigante activité biologique : elles ont montré une activité antibiotique extrêmement puissante contre des bactéries à gram positif et à gram négatif tout en étant très peu toxiques pour les rats. Cette différence d'activité pourrait être attribuable à la nature de la couronne protéique. Dans la présente étude, nous avons analysé la cinétique et la thermodynamique de la liaison de ces nanoparticules de magnétite à l'albumine de sérum bovin (BSA), employée comme modèle de protéine sérique. Après l'adsorption, aucun changement significatif du diamètre des particules n'a été observé par diffusion dynamique de la lumière, ce qui indique que la formation de couronnes n'induit pas l'agrégation. Nous avons déterminé que la capacité maximale d'adsorption des particules était de 300 mg de BSA/g de magnétite. Les particules sont capables d'adsorber 90 % de la BSA à des concentrations de la protéine pouvant atteindre 500 mg/L. Les modèles décrivant le mieux cette adsorption sont le modèle de pseudo-second ordre et l'isotherme de Langmuir de type III. L'analyse thermodynamique a révélé que le processus est entropique et spontané à toutes les températures et conditions évaluées. Cependant, l'adsorption physique semble être faible à modérée. Cette affinité de liaison modérée pourrait être révélatrice de l'écart entre l'activité biologique de ces particules envers les bactéries et celle envers les cellules de mammifères. Ces résultats pourraient permettre de corroborer l'hypothèse selon laquelle ces nanoparticules pourraient constituer des moyens nouveaux de cibler les bactéries résistantes aux antibiotiques. [Traduit par la Rédaction] Mots-clés : nanoparticules de magnétite, albumine de sérum bovin, isotherme de Langmuir, cinétique d'adsorption, thermodynamique d'adsorption., Introduction Albumin is the most abundant protein in circulating mammalian blood and is the primary component responsible for serum protein adsorption of drugs, fatty acids, anions, metal ions, anti-oxidants, and [...]

Basalts and peridotites from mid-ocean ridges record f.sub.O2 near the quartz-fayalite-magnetite buffer (QFM), but peridotite partial melting experiments have mostly been performed in graphite capsules (~ QFM-3), precluding evaluation of ferric iron's behavior during basalt generation. We performed experiments at 1.5 GPa, 1350-1400 °C, and f.sub.O2 from about QFM-3 to QFM+3 to investigate the anhydrous partitioning behavior of Fe.sub.2O.sub.3 between silicate melts and coexisting peridotite mineral phases. We find spinel/melt partitioning of Fe.sub.2O.sub.3 ( [Formula omitted]) increases as spinel Fe.sub.2O.sub.3 concentrations increase, independent of increases in f.sub.O2, and decreases with temperature, which is consistent with new and previous experiments at 0.1 MPa. We find [Formula omitted] = 0.63 ± 0.10 and [Formula omitted] = 0.78 ± 0.30. MORB Fe.sub.2O.sub.3 and Na.sub.2O concentrations are consistent with a modeled MORB source with Fe.sub.2O.sub.3 = 0.48 ± 0.03 wt% (Fe.sup.3+/[Sigma]Fe = 0.053 ± 0.003) at potential temperatures (T.sub.P) from 1320 to 1440 °C. The temperature-dependence of the [Formula omitted] function alone allows ~ 40% of the variation in MORB compositions. If we allow [Formula omitted] and [Formula omitted] to also vary with temperature by tying them to spinel Fe.sub.2O.sub.3 through intermineral partitioning, then all the MORB data are within error of the model. Our model Fe.sub.2O.sub.3 concentration for the MORB source would require that the convecting mantle be more oxidized at a given depth than recorded by continental mantle xenoliths. Our result is supported by thermodynamic models of mantle with Fe.sup.3+/[Sigma]Fe = 0.03 that predict f.sub.O2 of ~ QFM-1 near the garnet-spinel transition, which is inconsistent with f.sub.O2 of MORB. Our results support previous suggestions that redox melting may occur between 200 and 250 km depth., Author(s): Fred A. Davis [sup.1] [sup.2], Elizabeth Cottrell [sup.2] Author Affiliations: (1) grid.266744.5, 0000 0000 9540 9781, Department of Earth and Environmental Sciences, University of Minnesota Duluth, , 229 Heller [...]

Fe.sup.3+ and X.sub.Fe3+, defined as Fe.sup.3+/(Fe.sup.2+ + Fe.sup.3+) on a molar basis, are now recognised as key parameters in phase equilibrium modelling. A hindrance is that it is only possible to routinely measure total Fe, and not Fe.sup.3+ and Fe.sup.2+, in minerals using the electron microprobe. Charge balance techniques can be used to estimate Fe.sup.3+ and Fe.sup.2+ for some minerals, but not for those that contain vacancies. Whilst other analytical techniques can determine X.sub.Fe3+ in minerals, these are not commonly applied by metamorphic petrologists. Therefore, researchers must rely on estimates. This study collates wet chemical, Mössbauer spectroscopy, and X-ray absorption near edge structure (XANES) spectroscopy analyses of X.sub.Fe3+ in metapelitic minerals and rocks from the literature. The resulting database of 77 studies contains 591 samples, of which 261 have X.sub.Fe3+ determined for the whole rock. There are X.sub.Fe3+ measurements for 483 biotites, 192 white micas, 78 chlorites, and 32 staurolites. Average (± 1[sigma]) X.sub.Fe3+ values in whole rock, biotite, white mica, chlorite, and staurolite are 0.23 ± 0.16, 0.11 ± 0.08, 0.55 ± 0.18, 0.08 ± 0.07, and 0.07 ± 0.06, respectively. The average (± 1[sigma]) number of Fe.sup.3+ cations in biotite, white mica, chlorite, and staurolite is 0.28 ± 0.19 (22 O + Ti cations per formula unit, pfu), 0.17 ± 0.13 (22 O pfu), 0.31 ± 0.27 (28 O pfu), and 0.20 ± 0.17 (46 O pfu), respectively. The mean whole rock X.sub.Fe3+ is similar for metapelites containing ilmenite and magnetite, as well as those that report no Fe-oxide, but is considerably higher for hematite-bearing rocks. Whilst there is little variation with pressure and temperature, there is an increase in the number of Fe.sup.3+ cations and X.sub.Fe3+ of both white mica and biotite with the type of Fe-oxide present. Our observations are compared with the predictions of phase equilibrium modelling using thermodynamic dataset 6.2 (Holland and Powell, J Metamorph Geol 29:333-383, 2011) and the solution models of White et al. (J Metamorph Geol 32:261-286, 2014a) for Fe.sup.3+ and X.sub.Fe3+ in these minerals. The predicted X.sub.Fe3+ and number of Fe.sup.3+ cations in biotite, chlorite, and staurolite broadly match natural observations, but for white mica the predicted mean X.sub.Fe3+ is underestimated by 0.2-0.4 and the number of Fe.sup.3+ cations by 0.05-0.2. Whilst modelling correctly predicted increases in the X.sub.Fe3+ of white mica and biotite with whole rock X.sub.Fe3+, it also predicted variations in mineral X.sub.Fe3+ as a function of pressure and temperature which are not observed in the natural samples., Author(s): Jacob B. Forshaw [sup.1], David R. M. Pattison [sup.1] Author Affiliations: (1) grid.22072.35, 0000 0004 1936 7697, Department of Geoscience, University of Calgary, , 2500 University Drive NW, T2N [...]

The oxidation state of Cr, Cr.sup.2+/ΣCr (where ΣCr = Cr.sup.2+ + Cr.sup.3+ = 0.35 wt%), in Fe-free silicate glasses quenched from melts equilibrated as a function of pressure to 3.5 GPa at 1500 °C, and as a function of temperature to 1500 °C at atmospheric pressure, and at oxygen fugacities (fO.sub.2, in log units relative to the quartz-fayalite-magnetite (QFM) buffer) between âQFM = - 1 and - 2, was determined by XANES spectroscopy. Increasing temperature stabilises Cr.sup.2+ and increasing pressure stabilises Cr.sup.3+. A general expression for Cr.sup.3+/Cr.sup.2+ in silicate melts was derived: log(Cr.sup.3+/Cr.sup.2+) = 1/4(âQFM + 8.58 - 25,050/T + 940P/T - 0.02P) + 9770/T - 7.69 + 6.22[LAMBDA] + (900P - 172P.sup.2)/T, where P is pressure in GPa, T is temperature in K, and [LAMBDA] is the optical basicity of the composition. This equation reproduces 213 Cr.sup.2+/ΣCr values reported here and in the literature with an average [DELTA]Cr.sup.2+/ΣCr of 0.02. A MORB melt at 1400 °C and QFM is predicted to have Cr.sup.2+/ΣCr ~ 0.35 at the surface but Cr.sup.2+/ΣCr ~ 0 at a depth of ~ 60 km. Although Cr.sup.2+ is an important oxidation state in silicate melts it is not preserved at low temperatures due to an electron exchange reaction with Fe.sup.3+., Author(s): Andrew J. Berry [sup.1], Hugh St. C. O'Neill [sup.1], Garry J. Foran [sup.2] [sup.3] Author Affiliations: (1) grid.1001.0, 0000 0001 2180 7477, Research School of Earth Sciences, Australian National [...]

Plagioclase hosted, oriented magnetite micro-inclusions are a frequently observed phenomenon in magmatic and metamorphic rocks. Understanding the orientation relationships between these inclusions and the plagioclase host is highly relevant for interpreting paleomagnetic measurements. The systematics of the shape and crystallographic orientation relationships between needle- and lath-shaped magnetite micro-inclusions and their plagioclase host from oceanic gabbro were investigated using optical microscopy including universal stage measurements, scanning electron microscopy, and crystal orientation analysis by electron backscatter diffraction. The magnetite inclusions show preferred shape orientations following six well-defined directions and with specific crystallographic orientation relationships to the plagioclase host. These relationships are rationalized based on angular and dimensional similarities between the crystal structures of magnetite and plagioclase, which favor the parallel alignment of oxygen layers with similar lattice spacing in both phases. The parallel alignment of oxygen layers in plagioclase and magnetite can be traced back to the oriented nucleation of magnetite, which occurs by the accommodation of FeO.sub.6 octahedra in six-membered rings of SiO.sub.4 and AlO.sub.4 tetrahedra of the plagioclase structure. The orientation systematics of the magnetite micro-inclusions is related to four orientation variants for placing the FeO.sub.6 octahedra into the plagioclase structure., Author(s): Olga Ageeva [sup.1] [sup.2], Ge Bian [sup.1], Gerlinde Habler [sup.1], Alexey Pertsev [sup.2], Rainer Abart [sup.1] Author Affiliations: (1) grid.10420.37, 0000 0001 2286 1424, Department of Lithospheric Research, University [...]

Mafic-ultramafic layered rocks of the Lower and Critical Zones of the Bushveld Complex are believed to have formed from a parental magma usually labelled B1 with SiO.sub.2 content at 55 wt.%, MgO at about 12 wt.% and Cr.sub.2O.sub.3 concentration at 0.15 wt.%. Previous experimental study at 300 MPa and nominally dry conditions showed that this composition had orthopyroxene on liquidus between 1250 and 1300 °C, and the mineral was joined by plagioclase and clinopyroxene at and below 1166 °C. The objective of this study was to investigate the effects of variable additions of H.sub.2O (2, 4 and 6 wt.% in starting compositions) on liquidus and sub-liquidus phase equilibria. Main attention was given to the solubility of Cr-spinel and Cr distribution between melt and crystalline phases. Six experiments were carried out in the internally heated pressure vessel (IHPV) at 300 MPa and in the temperature interval 1100-1300 °C. Redox conditions were not controlled and thermal dissociation of the H.sub.2O component followed by hydrogen diffusion through container walls resulted in a noticeable oxidation of run products. The effective values of fO.sub.2 in experiments were calculated using SPINMELT 2.0 software bases on the regression of experimental data on the spinel-melt equilibrium. According to the calculations, H.sub.2O additions in isothermal series of experiments increased fO.sub.2 by 0.8-1.6 log-units from the level in a nominally dry run. Overall, the experiments showed that H.sub.2O additions suppressed crystallization of silicates, shifted olivine-orthopyroxene peritectic equilibrium towards higher olivine stability (olivine first appears in H.sub.2O-bearing compositions at 1175 °C) and increased the proportion of magnetite component in spinel solid solution. The addition of 4 wt.% H.sub.2O to the starting composition at 1200 °C resulted in complete melting of orthopyroxene, which left Cr-spinel as the only liquidus phase. Equal amount of H.sub.2O at 1125 °C leads to complete melting of plagioclase, orthopyroxene and clinopyroxene, which leaves only olivine and spinel stable and increases the liquid mass fraction from 0.62 in the nominally dry compositions to 0.9. The results of this study appear to support the idea that chromitite layers in the Critical Zone of the Bushveld Complex were formed by episodes of hydration melting of silicate cumulates., Author(s): Ilya V. Veksler [sup.1] [sup.2], Tong Hou [sup.3] [sup.4] Author Affiliations: (1) grid.23731.34, 0000 0000 9195 2461, GFZ German Research Centre for Geosciences, , Telegrafenberg, 14473, Potsdam, Germany (2) [...]

Iron oxidation state and environment in magmas affect their phase diagram and their properties, including viscosity and density, which determine magma mobility and eruptive potential. In turn, magma composition, pressure, temperature and oxygen fugacity affect iron oxidation state and coordination, potentially leading to complex feedbacks associated with magma ascent, degassing and eruption. While equilibrium experiments and models have led to a deep understanding of the role of iron in melts, our knowledge of the effects of disequilibrium processes on iron oxidation state and its structural role in lavas and magmas remains limited. Accordingly, we performed a series of dynamic disequilibrium experiments on a natural melt composition (a phonolite lava from Erebus volcano, Antarctica) at atmospheric pressure, in which oxygen fugacity and temperature were controlled and varied. During the experiments, we continuously measured iron oxidation and coordination using Fe K-edge dispersive X-ray Absorption Spectroscopy (XAS). We found that iron oxidation state changes in the phonolite melt are reversible and well reproduced by existing models. Changes in iron oxidation state are driven by joint diffusion of alkali cations and oxygen anions at magmatic temperatures (~ 1000 °C for Erebus phonolite). However, redox diffusion timescales are too slow for any significant oxygen exchange with the atmosphere at the lava/air interface or via air entrainment. Turning to iron coordination, while Fe.sup.2+ and Fe.sup.3+ are present mostly in an average five-fold coordination, complex coordination variations decoupled from redox changes were detected. The data suggest transitions between Fe.sup.3+ in four-fold and six-fold coordination prior to reduction or as a consequence of oxidation. This questions the possible implication of Fe coordination changes in triggering crystallisation of magnetite nanolites upon magma ascent, and, through such crystallisation events, in promoting magma explosivity., Author(s): Charles Le Losq [sup.1] [sup.2], Roberto Moretti [sup.1] [sup.3], Clive Oppenheimer [sup.4], François Baudelet [sup.5], Daniel R. Neuville [sup.1] Author Affiliations: (1) grid.5842.b, 0000 0001 2171 2558, Institut de [...]

Although particles >2 mm are not considered soil material, gravels composed of resistant secondary minerals can be useful records of past soil environments. We investigated gravels (2-8 mm) from a Plinthic tiaplustox in central Brazil in order to assess their composition, fabric and genesis. Gravels were initially grouped into six macromorphological types, and investigated by mineralogical, micromorphological and microprobe analyses. The results suggest that gravels can be classified as pedogenic or lithopedogenic. Pedogenic gravels comprise indurated soil materials and include black magnetic gravels, black non-magnetic gravels and red earthy gravels. Magnetic gravels contained mostly quartz, hematite and magnetite--maghemite and were poor in kaolinite, gibbsite and goethite, whereas non-magnetic and red earthy gravels had the same minerals as in the surrounding soil. Lithopedogenic gravels are fragments of resistant rocks impregnated by iron (Fe), aluminium (Al) and manganese (Mn) oxides, including platy shales, quartzites and coarse quartz. The latter two showed oxide impregnations around internal pores, whereas the shale fragments included considerable mica, which contributed illite and potassium to the soil clays and, thus, the pedogenic gravels. These results point to multiple cycles of formation and incorporation of cemented materials into the soil, as well as their later impregnation by Fe, Mn and Al oxides. Thus, the conventional classification of all such gravels as 'ironstone' is questionable, and their potential role as indicators of past and present soil environments is demonstrated, which deserves further investigation. Additional keywords: geochemistry, iron oxides, lateritic soils, micromorphology, tropical soils. Received 15 May 2015, accepted 22 September 2015, published online 6 June 2016, Introduction There is growing acknowledgement that polygenesis is the rule rather than the exception in soil formation, and that soils retain a cumulative imprint of past soil-forming factors and processes [...]

The origin of the more oxidized nature of arc magmas as compared to that of mid-ocean ridge basalts (MORB) is debated, considered to be either a feature of their mantle source, or produced during crustal transit and eruption. Fe.sup.3+/Fe.sup.T ratios (Fe.sup.3+/[Fe.sup.3+ + Fe.sup.2+]) in arc volcanic rocks and glasses and thermodynamic oxybarometry on mantle xenoliths from arc lavas indicate elevated magmatic oxygen fugacity ( [Formula omitted]), whereas, redox-sensitive trace elements ratios and abundances in arc volcanic rocks have been used to suggest that arcs have source regions with [Formula omitted] similar to the MORB source. Here, we take an alternative approach by calculating the [Formula omitted] of the uppermost mantle and lowermost ultramafic cumulates from the accreted Jurassic Talkeetna arc (Alaska). This approach allows us to quantify the [Formula omitted] of the sub-arc mantle and of primary arc magmas crystallizing at the base of an island arc, which have not been affected by processes during crustal transit and eruption which could affect their [Formula omitted]. Implementing olivine-spinel oxybarometry, we find that the upper mantle (harzburgites and lherzolites) and ultramafic cumulates (clinopyroxenites and dunites) crystallized between + 0.4 and + 2.3 log units above the fayalite-magnetite-quartz buffer, consistent with previous studies suggesting that the sub-arc mantle is oxidized relative to that of MORB. In addition, the Talkeetna paleo-arc allows us to examine coeval lavas and their redox-sensitive trace element ratios (e.g., V/Sc). The average V/Sc ratios of high MgO (> 6 wt%) lavas are 6.7 ± 1.6 (2[sigma]), similar to that of MORB. However, V/Sc ratios must be interpretted in terms the degree of partial melting, as well as, the initial V/Sc ratio of the mantle source in order to derive information about [Formula omitted] of their mantle source. The V/Sc ratios of Talkeetna lavas are consistent with the elevated [Formula omitted] recorded in the sub-arc mantle and primitive cumulates (olivine Mg# [Mg/(Mg + Fe)] x 100 > 82) if a depleted mantle source underwent 15-20% melting. Our results suggest that the arc mantle is, on average, more oxidized than the MORB source and that V/Sc ratios must be interpreted in the context of a partial melting model where all model parameters are appropriate for arc magma genesis. This study reconciles V/Sc ratios in arc volcanic rocks with [Formula omitted] of primary arc basalts and the sub-arc mantle from the same locality., Author(s): Claire E. Bucholz [sup.1] , Peter B. Kelemen [sup.2] Author Affiliations: (Aff1) 0000000107068890, grid.20861.3d, Division of Geological and Planetary Sciences, California Institute of Technology, , 1200 E California Boulevard, [...]

The mutual affinity between bubbles and oxide crystals (especially magnetite) is well established and their tendency to remain in contact once they become connected (either by nucleation of one upon the other, or by attachment) has led to models of oxide transport via bubbles in natural melts. However, despite the widespread acceptance of bubble-oxide association, there is little direct textural evidence for these processes. We present results from a series of decompression experiments on andesitic melts, during which aggregates of bubbles and oxides formed because of hydrogen loss through the capsule walls causing oxidation of the melt. Experimental charges were imaged using 3D X-ray computed tomography that revealed complex bubble + oxide aggregates, with small oxide crystals coating part of the outer bubble surfaces in a shell-like morphology. These shells have smooth inner and rugose outer surfaces. Sometimes, additional concentric shells or partial shells can be found around bubbles, in the glass between the bubble wall and another shell. We quantified the volumes of bubbles and oxides and the oxides' compositions. We measured the surface area where the bubbles and oxides are in contact, thus quantifying their interface in 3D, and used these measurements to investigate the process of oxide shell formation. The complexity of the oxide textures when studied in 3D reveals a range of bubble-oxide interactions, from continuous generation, detachment and disintegration. These processes carry important implications on why such textures seem to have a low preservation potential in natural environments. Nevertheless, we have found natural samples that resemble our experimental results in a range of rock compositions from different geological environments that could have formed either due to rapid oxidation via the fluid phase or by bubbles harvesting different crystals., Author(s): P. Plese [sup.1] , M. D. Higgins [sup.1] , D. R. Baker [sup.2] , G. Lanzafame [sup.1] , M. Kudrna Prasek [sup.2] , L. Mancini [sup.3] , S. M. [...]

The 3.1 Ga Nuggihalli greenstone belt in the Western Dharwar craton is comprised of chromitite-bearing sill-like ultramafic-mafic rocks that are surrounded by metavolcanic schists (compositionally komatiitic to komatiitic basalts) and a suite of tonalite-trondhjemite-granodiorite gneissic rocks. The sill-like plutonic unit consists of a succession of serpentinite (after dunite)-peridotite-pyroxenite and gabbro with bands of titaniferous magnetite ore. The chromitite ore-bodies (length [almost equal to]30-500 m; width [almost equal to]2-15 m) are hosted by the serpentinite-peridotite unit. Unaltered chromites from massive chromitites (>80 % modal chromite) of the Byrapur and Bhaktarhalli chromite mines in the greenstone belt are characterized by high Cr# (100Cr/(Cr + Al)) of 78-86 and moderate Mg# (100 Mg/(Mg + Fe.sup.2+)) of 45-55. In situ trace-element analysis (LA-ICPMS) of unaltered chromites indicates that the parental magma of the chromitite ore-bodies was a komatiite lacking nickel-sulfide mineralization. In the Ga/Fe.sup.3+# versus Ti/Fe.sup.3+# diagram, the Byrapur chromites plot in the field of suprasubduction zone (SSZ) chromites while those from Bhaktarhalli lie in the MOR field. The above results corroborate our previous results based on major-element characteristics of the chromites, where the calculated parental melt of the Byrapur chromites was komatiitic to komatiitic basalt, and the Bhaktarhalli chromite was derived from Archean high-Mg basalt. The major-element chromite data hinted at the possibility of a SSZ environment existing in the Archean. Altered and compositionally zoned chromite grains in our study show a decrease in Ga, V, Co, Zn, Mn and enrichments of Ni and Ti in the ferritchromit rims. Trace-element heterogeneity in the altered chromites is attributed to serpentinization. The trace-element patterns of magnetite from the massive magnetite bands in the greenstone belt are similar to those from magmatic Fe-Ti-V-rich magnetite bands in layered intrusions, and magnetites from andesitic melts, suggesting that magnetite crystallized from an evolved gabbroic melt. Enrichments of Ni, Co, Te, As and Bi in disseminated millerite and niccolite occurring within chromitites, and in disseminated bravoite within magnetites, reflect element mobility during serpentinization. Monosulfide solid solution inclusions within pyroxenes (altered to actinolite) in pyroxenite, and interstitial pyrites and chalcopyrites in magnetite, retain primary characteristics except for Fe-enrichment in chalcopyrite, probably due to sub-solidus re-equilibration with magnetite. Disseminated sulfides are depleted in platinum-group elements (PGE) due to late sulfide saturation and the PGE-depleted nature of the mantle source of the sill-like ultramafic-mafic plutonic rocks in the Nuggihalli greenstone belt. Electronic supplementary material The online version of this article (doi:10.1007/s00410-015-1148-1) contains supplementary material, which is available to authorized users., Author(s): Ria Mukherjee[sup.1] [sup.2] , Sisir K. Mondal[sup.1] [sup.4] , Jose M. Gonzalez-Jimenez[sup.3] [sup.5] , William L. Griffin[sup.3] , Norman J. Pearson[sup.3] , Suzanne Y. O'Reilly[sup.3] Author Affiliations: (1) PGE [...]

We have reconstructed the compositional evolution of the silicate and carbonate melt, and various crystalline phases in the subvolcanic reservoir of Kerimasi Volcano in the East African Rift. Trace element concentrations of silicate and carbonate melt inclusions trapped in nepheline, apatite and magnetite from plutonic afrikandite (clinopyroxene-nepheline-perovskite-magnetite-melilite rock) and calciocarbonatite (calcite-apatite-magnetite-perovskite-monticellite-phlogopite rock) show that liquid immiscibility occurred during the generation of carbonatite magmas from a CO.sub.2-rich melilite-nephelinite magma formed at relatively high temperatures (1,100 °C). This carbonatite magma is notably more calcic and less alkaline than that occurring at Oldoinyo Lengai. The CaO-rich (32-41 wt%) nature and alkali-'poor' (at least 7-10 wt% Na.sub.2O + K.sub.2O) nature of these high-temperature (>1,000 °C) carbonate melts result from strong partitioning of Ca (relative to Mg, Fe and Mn) in the immiscible carbonate and the CaO-rich nature (12-17 wt%) of its silicate parent (e.g., melilite-nephelinite). Evolution of the Kerimasi carbonate magma can result in the formation of natrocarbonatite melts with similar composition to those of Oldoinyo Lengai, but with pronounced depletion in REE and HFSE elements. We suggest that this compositional difference results from the different initial parental magmas, e.g., melilite-nephelinite at Kerimasi and a nephelinite at Oldoinyo Lengai. The difference in parental magma composition led to a significant difference in the fractionating mineral phase assemblage and the element partitioning systematics upon silicate-carbonate melt immiscibility. LA-ICP-MS analysis of coeval silicate and carbonate melt inclusions provides an opportunity to infer carbonate melt/silicate melt partition coefficients for a wide range of elements. These data show that Li, Na, Pb, Ca, Sr, Ba, B, all REE (except Sc), U, V, Nb, Ta, P, Mo, W and S are partitioned into the carbonate melt, whereas Mg, Mn, Fe, Co, Cu, Zn, Al, Sc, Ti, Hf and Zr are partitioned into the silicate melt. Potassium and Rb show no preferential partitioning. Kerimasi melt inclusions show that the immiscible calcic carbonate melt is strongly enriched in Sr, Ba, Pb, LREE, P, W, Mo and S relative to other trace elements. Comparison of our data with experimental results indicates that preferential partitioning of oxidized sulfur (as SO.sub.4.sup.2-), Ca and P (as PO.sub.4.sup.3-) into the carbonate melt may promote the partitioning of Nb, Ta, Pb and all REE, excluding Sc, into this phase. Therefore, it is suggested that P and S enrichment in calcic carbonate magmas promotes the genesis of REE-rich carbonatites by liquid immiscibility. Our study shows that changes in the partition coefficients of elements between minerals and the coexisting melts along the liquid line of descent are rather significant at Kerimasi. This is why, in addition to the REE, Nb, Ta and Zr are also enriched in Kerimasi calciocarbonatites. We consider significant amounts of apatite and perovskite precipitated from melilite-nephelinite-derived carbonate melt as igneous minerals can have high LREE, Nb and Zr contents relative to other carbonatite minerals. Electronic supplementary material The online version of this article (doi:10.1007/s00410-014-1093-4) contains supplementary material, which is available to authorized users., Author(s): Tibor Guzmics[sup.1] , Zoltan Zajacz[sup.2] , Roger H. Mitchell[sup.3] , Csaba Szabo[sup.1] , Markus Wälle[sup.2] Author Affiliations: (1) Lithosphere Fluid Research Laboratory, Department of Petrology and Geochemistry, Eotvos University [...]