Experimental Evidence for a Reduced Metal-saturated Upper Mantle

The uppermost mantle as sampled by xenoliths, peridotite massifs and primitive basaltic melts appears to be relatively oxidized, with oxygen fugacities between the magnetite-wüstite and fayalite-ferrosilite-magnetite equilibria. Whether this range in oxygen fugacity is a shallow mantle signature or representative of the entire upper mantle still is unclear and a matter of debate because mantle regions deeper than 200 km are not well sampled. To constrain the redox state of the deeper upper mantle, we performed experiments from 1 to 14 GPa and 1220 to 1650°C on a model peridotite composition, encompassing the convecting asthenospheric mantle down to the Transition Zone at 410 km depth. The experiments were run in iron metal capsules to buffer fO2 close to an oxygen fugacity about 0·5 log units below the iron-wüstite equilibrium. Analysis of the experimental phases for ferric iron using electron energy loss spectroscopy reveals that at pressures higher than 7 GPa, subcalcic pyroxene and majoritic garnet incorporate appreciable amounts of ferric iron, even though at the experimental conditions they were in redox equilibrium with metallic iron. The major ferric iron carrier in the upper mantle is majoritic garnet, followed by subcalcic pyroxene. At around 8 ± 1 GPa, corresponding to ∼250 ± 30 km depth in the upper mantle, sufficient quantities of subcalcic pyroxene and majoritic garnet are stabilized that all the ferric iron thought to be present in fertile upper mantle (i.e. ∼2000 ppm) can be accommodated in solid solution in these phases, even though they were synthesized in redox equilibrium with metallic Fe. Based on the results of the experiments, it can be stated that, on a global scale, an oxidized upper mantle near the fayalite-ferrosilite-magnetite equilibrium is the exception rather than the rule. More than 75 vol. % of the Earth's present-day mantle is likely to be saturated with metallic iro

Zircons from Syros, Cyclades, Greece—Recrystallization and Mobilization of Zircon During High-Pressure Metamorphism

Zircons were studied from high-pressure/low-temperature metamorphosed meta-igneous lithologies from Syros. These rocks carry several zircon generations related to each other by dissolution-reprecipitation processes. One generation is pristine zircon that shows growth zoning, relatively elevated contents of trivalent cations and high Th/U ratios. The other end-member is a skeletal zircon generation with negligible trivalent cation contents and low Th/U ratios (≤0·1). Texturally between these two, there is a range of zircon crystals with complex inclusion populations of Y-HREE-Th phases and fluid inclusions, showing variable progress of replacement- recrystallization. Both end-members yield distinct sensitive high-resolution ion microprobe (SHRIMP) U-Pb ages. The pristine generation has an age of 80·2 ± 1·6 Ma from a metagabbro, and 76·4 ± 2·1 Ma from a meta-plagiogranite dyke. The skeletal, low-Th/U zircon generation yields an age of 52·4 ± 0·8 Ma. The older, Late Cretaceous, zircons are interpreted to date emplacement of the magmatic protoliths in a small segment of oceanic crust. The younger, Eocene, age, however, dates a zircon recrystallization event, which possibly coincides with high solubility and mobility of high field strength elements in a high-pressure aqueous fluid phase. Intergrowth relations between zircon and peak-metamorphic garnet, and excellent agreement of the U-Pb ages with white mica Ar-Ar ages for the same samples support the conclusion that Eocene is the true age of high-pressure metamorphism on Syro

Coeficientes de reparto de Elementos del Grupo del Platino entre fundidos sulfurados y arseniurados

Depto. de Mineralogía y PetrologíaFac. de Ciencias GeológicasTRUEpu

Rheological properties of calcite oozes: Implications for the fossilisation in the plattenkalks of the Solnhofen-Eichstatt lagoons in the Franconian Alb, Germany

We report on an experimental study to investigate the sedimentation behaviour and rheological properties of extremely fine-grained calcite oozes. The experiments are aimed at clarifying if thixotropic behaviour may have played a role in the preservation of marine biota in plattenkalks of the Solnhofen lagoons of the Franconian Alb. Calcite particles with grain sizes from 2.2 to 4.4 mu m were sedimented from water, seawater proxies, and hypersaline brines with up to 14 wt.% NaCl, for 170 days. High salinities as envisioned for the bottom waters of some Solnhofen lagoons slow down settling rates of calcite and may produce plattenkalks more porous and more friable than plattenkalks elsewhere in the Solnhofen archipelago. Rheological properties of calcite suspensions were measured with an oscillation rheometer. Calcite oozes with 40 vol.% calcite in suspension behave thixotropically regardless of the salinity of the pore solutions. Thixotropic behaviour may have the potential to promote the fossilisation of marine biota. Even if the sediment cover is thin, i.e. a few millimeters, a carcass covered by a thixotropic sediment would be largely isolated from the overlying water column because pore solutions in thixotropic media hardly communicate with the overlying water column. A fish carcass covered by a thixotropic sediment could impose local-scale physicochemical conditions on its direct sedimentary envelope favourable for preservation and the replacement of organic material by inorganic materials

Effect of boiling on the acidity of hydrothermal solutions

Natural seawater and H2O-NaCl solutions were equilibrated along the two-phase (liquid-vapor) curves between 150 and 390 degrees C to re-examine the effect of liquid-vapor phase separation on element fractionations between saline brines and vapor phases. The experimental setup allows vapor and brine to be sampled separately at in situ P-T conditions. Experimental vapor pressure is constrained by temperature and the electrolyte concentration of the brines. With increasing temperature, dissolved electrolytes react with increasing temperature to hydroxides and HCl, in both seawater and H2O-NaCl solutions. The extent of hydrolysis is more pronounced in seawater than in H2O-NaCl solutions because seawater contains, in addition to NaCl, a range of other electrolytes capable of hydrolysis. Associated HCl has a great affinity to fractionate to the vapor phase when phase separation occurs. At 365 degrees C, halite-saturated vapor phases have a pH(25) (pH after condensation to 25 degrees C) of 1.8 (seawater) and 2.8 (H2O-NaCl brines). Our data suggest that boiling of hydrothermal solutions followed by mixing of vapor condensates with seawater can impose pH(25) values as acidic as the most acidic natural hydrothermal solutions vented on the ocean floor

Experimental Evidence for a Reduced Metal-saturated Upper Mantle

ISSN:0022-3530ISSN:1460-241

Zircons from Syros, Cyclades, Greece-recrystallization and mobilization of zircon during high pressure metamorphism.

Zircons were studied from high-pressure/low-temperature metamorphosed meta-igneous lithologies from Syros. These rocks carry several zircon generations related to each other by dissolution---reprecipitation processes. One generation is pristine zircon that shows growth zoning, relatively elevated contents of trivalent cations and high Th/U ratios. The other end-member is a skeletal zircon generation with negligible trivalent cation contents and low Th/U ratios (01). Texturally between these two, there is a range of zircon crystals with complex inclusion populations of Y---HREE---Th phases and fluid inclusions, showing variable progress of replacement---recrystallization. Both end-members yield distinct sensitive high-resolution ion microprobe (SHRIMP) U---Pb ages. The pristine generation has an age of 802 16Ma from a metagabbro, and 764 21Ma from a meta-plagiogranite dyke. The skeletal, low-Th/U zircon generation yields an age of 524 08Ma. The older, Late Cretaceous, zircons are interpreted to date emplacement of the magmatic protoliths in a small segment of oceanic crust. The younger, Eocene, age, however, dates a zircon recrystallization event, which possibly coincides with high solubility and mobility of high field strength elements in a high-pressure aqueous fluid phase. Intergrowth relations between zircon and peak-metamorphic garnet, and excellent agreement of the U---Pb ages with white mica Ar---Ar ages for the same samples support the conclu-sion that Eocene is the true age of high-pressure metamorphism on Syros. KEY WORDS: zircon dissolution---reprecipitation; zircon U---Pb geochronology; paragonite 39Ar/40Ar dating; Syros meta-ophiolit

Concentrations of Pt, Pd, S, As, Se and Te in silicate melts at sulfide, arsenide, selenide and telluride saturation: evidence of PGE complexing in silicate melts?

Even though platinum group elements (PGE) solubilities are measured relative to pure metals, the PGE are assumed to dissolve as oxide complexes in silicate melts. PGE-oxide phases are, however, not known in magmatic rocks; in many cases PGE are associated with discrete magmatic phases (alloys, arsenides, bismuthotellurides, antimonides and sulfides). Here, we determine the concentrations of Pt, Pd, S, As, Se and Te in basaltic melts saturated with Fe, Pt or Pd sulfides, arsenides, selenides and tellurides and note that the solubilities of these elements are largely variable and depend on the metal-ligand reservoir in equilibrium. We equilibrated basaltic melts with immiscible Fe, Pt, and Pd sulfide, arsenide, selenide and telluride melts in a piston cylinder apparatus at 1250 degrees C, 0.5 GPa and relativefO(2)of similar to FMQ to FMQ-1.5. The concentrations of S, As, Se and Te in the basaltic melt vary considerably with the metal-ligand reservoir; the highest concentrations are recorded when the ferrous iron cation is the principal metal ligand. When instead Pt-(S/As/Se/Te) or Pd-(S/As/Se/Te) are used, the concentrations of S, As, Se and Te fall drastically. Platinum and Pd increase the activities of semimetals and chalcogenes in the silicate melt more than Fe does. Implications are that Pt and Pd can preferentially form stable associations (fundamental building blocks) with chalcogens and semimetals before the melt attains saturation in Fe-chalcogens or Fe-semimetals. Estimated concentrations of Pt-ligand and Pd-ligand required to saturate silicate melts in some Pt-ligand and Pd-ligand minerals are close to their abundances in the parent magmas of some layered intrusions