Jasper Berndt

A combined rapid-quench and H2-membrane setup for internally heated pressure vessels: Description and application for water solubility in basaltic melts

Abstract This study presents improvements of internally heated pressure vessels to realize high-pressure experiments at controlled fO₂ in low-viscosity systems such as basaltic ones. The new design is a combination of two experimental techniques: a hydrogen sensor membrane made of platinum to measure fH₂, and therefore fO₂, and a rapid-quench system to avoid crystallization of low-viscosity melts during quench. The experimental setup has been tested successfully at temperatures up to 1250 °C and pressures up to 500 MPa. Basaltic melts containing up to 9.38 wt% water can be quenched as bubble-free and crystal-free glasses. The improvements allow synthesis of hydrated glass or partly crystallized samples with a large volume (for further studies) and to perform routine phase-equilibrium studies in basaltic systems at geologically relevant conditions. We used the new technique to determine the effect of fO₂ on water solubility in a melt with MORB composition. The results show that there is a small but significant decrease of water solubility with decreasing fO₂ from MnO-Mn3O4 to QFM buffer conditions in the pressure range 50-200 MPa. Kinetic problems in crystallization experiments in basaltic systems and the duration necessary to attain equilibrium Fe2+/Fe3+ ratio in the charge are discussed.

Experimental constraints on storage conditions in the chemically zoned phonolitic magma chamber of the Laacher See volcano

Abstract Phase relations of three samples of the Laacher See Tephra (LST) have been determined experimentally as a function of temperature (760 to 880 °C), pressure (200, 300 and 400 MPa), water content of the melt and oxygen fugacity (ƒO2). The crystallization experiments were carried out at ƒO2=NNO buffer and at NNO=+ 2.3 log units. The melt water contents varied between 6 and more than 8 wt% H2O, corresponding to water-undersaturated and water saturated conditions respectively. The synthetic products are compared to the natural phases to constrain pre-eruptive conditions in the Laacher See magma chamber. The major phases occurring in the LST have been reproduced. The stability of hauyne is favoured at high ƒO2 (≈NNO + 2.3). The CaO contents in melt and plagioclase synthesized under water-saturated conditions are significantly higher than in the natural phases, implying that most of the differentiation of the phonolites took place under water-undersaturated conditions. However, this does not exclude the presence of a S-, Cl- and CO2-rich fluid phase in the upper parts of the magma chamber. The phase relationships and the TiO2 contents of melts show that the temperature was lower than 760 °C in the upper part of the magma column (probably down to 720 °C in the most differentiated levels) and that temperatures above 840–860 °C prevailed in the lower part. The variation of the XMg of ferromagnesian minerals observed in both natural and experimental phases reflects the strong variations in ƒO2 in the lower magma chamber just prior to eruption (probably variation of about 2 log units). The most probable explanation for these ƒO2 variations is the injection of an oxidized alkali-rich magma, containing Mg-rich phenocrysts, at the base of a chemically zoned and more reduced magma column prior to eruption. Although the amount of injected magma may not have been very important, it was sufficient to change the ƒO2 conditions locally, explaining the heterogeneous XMg of ferromagnesian minerals and the formation of hauyne at the base of the chamber.

U-Pb dating of zircon by laser ablation ICP-MS: recent improvements and new insights

A robust method for U-Pb dating of zircon by laser ablation ICP-MS was established using a Thermo-Finnigan Element2 sector field ICP-MS coupled to a New Wave UP193HE ArF Excimer laser system. This included the construction of a low-volume sample cell and the establishment of a data reduction routine. A detailed systematic approach for determining optimal ablation settings and ICP-MS parameters is presented, which is also applicable to other instrument set-ups. Laser induced time-dependent elemental fractionation is corrected using the intercept method, whereas instrument drift, mass bias and elemental fractionation caused by ionisation differences are corrected by applying external standardisation using the matrix-matched reference zircon GJ-1. The isobaric interference of 204Hg on 204Pb is reduced by using a gold trap enabling the application of an accurate common Pb correction. Typical laser craters are 12, 25 or 35 μm in diameter and ca . 20 μm deep. A single analysis takes 65 s and the automated analysis of a measurement sequence ensures optimal efficiency. The performance of the established procedure was assessed by analysing zircon ranging in age from Mesoproterozoic to Miocene. Concordia ages obtained from multiple analyses on the various well-characterised zircons are all within uncertainty of the accepted values, demonstrating the high accuracy of the method. The uncertainty on the individual ages ranges from 1 to 4 % (2σ), depending on the absolute age, Pb content and homogeneity of the analysed zircon. These results indicate that the precision and accuracy obtained using the technique presented in this study are similar to those of other LA-ICP-MS laboratories and SIMS analyses.

The Earth’s missing lead may not be in the core

Relative to the CI chondrite class of meteorites (widely thought to be the ‘building blocks’ of the terrestrial planets), the Earth is depleted in volatile elements. For most elements this depletion is thought to be a solar nebular signature, as chondrites show depletions qualitatively similar to that of the Earth. On the other hand, as lead is a volatile element, some Pb may also have been lost after accretion. The unique 206Pb/204Pb and 207Pb/204Pb ratios of the Earth’s mantle suggest that some lead was lost about 50 to 130 Myr after Solar System formation. This has commonly been explained by lead lost via the segregation of a sulphide melt to the Earth’s core, which assumes that lead has an affinity towards sulphide. Some models, however, have reconciled the Earth’s lead deficit with volatilization. Whichever model is preferred, the broad coincidence of U–Pb model ages with the age of the Moon suggests that lead loss may be related to the Moon-forming impact. Here we report partitioning experiments in metal–sulphide–silicate systems. We show that lead is neither siderophile nor chalcophile enough to explain the high U/Pb ratio of the Earth’s mantle as being a result of lead pumping to the core. The Earth may have accreted from initially volatile-depleted material, some lead may have been lost to degassing following the Moon-forming giant impact, or a hidden reservoir exists in the deep mantle with lead isotope compositions complementary to upper-mantle values; it is unlikely though that the missing lead resides in the core.

Zircon geochronology and trace element characteristics of eclogites and granulites from the Orlica-Śnieżnik complex, Bohemian Massif

U–Pb zircon geochronology and trace element analysis was applied to eclogites and (ultra)high-pressure granulites that occur as volumetrically subordinate rock bodies within orthogneisses of the Orlica-Śnieznik complex, Bohemian Massif. Under favourable circumstances such data may help to unravel protolith ages and yet-undetermined aspects of the metamorphic evolution, for example, the time span over which eclogite-facies conditions were attained. By means of ion-probe and laser ablation techniques, a comprehensive database was compiled for samples collected from prominent eclogite and granulite occurrences. The 206 Pb/ 238 U dates for zircons of all samples show a large variability, and no single age can be calculated. The protolith ages remain unresolved due to the lack of coherent age groups at the upper end of the zircon age spectra. The spread in apparent ages is interpreted to be mainly caused by variable and possibly multi-stage Pb-loss. Further complexities are added by metamorphic zircon growth and re-equilibration processes, the unknown relevance of inherited components and possible mixing of different aged domains during analysis. A reliable interpretation of igneous crystallization ages is not yet possible. Previous studies and the new data document the importance of a Carboniferous metamorphic event at c. 340 Ma. The geological significance of this age group is controversial. Such ages have previously either been related to peak (U)HP conditions, the waning stages of eclogite-facies metamorphism or the amphibolite-facies overprint. This study provides new arguments for this discussion because, in both rock types, metamorphic zircon is characterized by very low total REE abundances, flat HREE patterns and the absence of an Eu anomaly. These features strongly suggest contemporaneous crystallization of zircon and garnet and strengthen interpretations proposing that the Carboniferous ages document late-stage eclogite-facies metamorphism, and not amphibolite-facies overprinting.

Sulfide oxidation as a process for the formation of copper rich magmatic sulfides

Typical magmatic sulfides are dominated by pyrrhotite and pentlandite with minor chalcopyrite, and the bulk atomic Cu/Fe ratio of these sulfides is typically less than unity. However, there are rare magmatic sulfide occurrences that are dominated by Cu-rich sulfides (e.g., bornite, digenite, and chalcopyrite, sometimes coexisting with metallic Cu) with atomic Cu/Fe as high as 5. Typically, these types of sulfide assemblages occur in the upper parts of moderately to highly fractionated layered mafic–ultramafic intrusions, a well-known example being the Pd/Au reef in the Upper Middle Zone of the Skaergaard intrusion. Processes proposed to explain why these sulfides are so unusually rich in Cu include fractional crystallization of Fe/(Ni) monosulfide and infiltration of postmagmatic Cu-rich fluids. In this contribution, we explore and experimentally evaluate a third possibility: that Cu-rich magmatic sulfides may be the result of magmatic oxidation. FeS-dominated Ni/Cu-bearing sulfides were equilibrated at variable oxygen fugacities in both open and closed system. Our results show that the Cu/Fe ratio of the sulfide melt increases as a function of oxygen fugacity due to the preferential conversion of FeS into FeO and FeO1.5, and the resistance of Cu2S to being converted into an oxide component even at oxygen fugacities characteristic of the sulfide/sulfate transition (above FMQ + 1). This phenomenon will lead to an increase in the metal/S ratio of a sulfide liquid and will also depress its liquidus temperature. As such, any modeling of the sulfide liquid line of descent in magmatic sulfide complexes needs to address this issue.

Low-temperature aqueous alteration of crystalline pyrochlore: correspondence between nature and experiment

Abstract Pyrochlore has been considered as a waste form to immobilize high-level nuclear waste such as excess weapons-grade plutonium. In order to study the aqueous stability of pyrochlore, we have carried out hydrothermal experiments with a natural microlite (Nb + Ta > 2Ti; Ta ≥ Nb) at 175°C in a neutral and acidic solution for 14 days. The starting material and the experimental products were studied by electron microprobe, backscattered electron (BSE) imaging, powder infrared (IR) and micro-Raman spectroscopy, and powder X-ray diffractrometry (XRD). The microlite has small U (~200 ppm) and Th (~ 800 ppm) contents and is crystalline. The hydrothermal treatment in a solution containing 1 mol/l HCl and 1 mol/l CaCl2 causes the partial replacement of the microlite (up to ~10 mm) by a new pyrochlore phase. This new phase is characterized by a larger unit-cell volume and contains a large number of vacancies at the A site (A = Ca, Na) as well as anion vacancies, molecular water, and possibly OH groups. Analyses of the experimental fluid further revealed that U was also selectively lost to the solution. Treatment in pure water did not produce reaction zones detectable by BSE imaging or powder XRD. However, significant spectral changes in powder IR spectra of the reaction product and the detection of Na and Ca in the solution indicate that the microlite has also reacted in pure water. The experimental alteration features bear a remarkable resemblance to those seen in naturally altered microlite samples, suggesting that short-term experimental results can be applied to natural systems and vice versa in order to evaluate the long-term stability of a pyrochlore waste form.

Zircon saturation in silicate melts: a new and improved model for aluminous and alkaline melts

The importance of zircon in geochemical and geochronological studies, and its presence not only in aluminous but also in alkaline rocks, prompted us to think about a new zircon saturation model that can be applied in a wide range of compositions. Therefore, we performed zircon crystallization experiments in a range of compositions and at high temperatures, extending the original zircon saturation model proposed by Watson and Harrison (Earth Planet Sci Lett 64:295–304, 1983) and Boehnke et al. (Chem Geol 351:324–334, 2013). We used our new data and the data from previous studies in peraluminous melts, to describe the solubility of zircon in alkaline and aluminous melts. To this effect, we devised a new compositional parameter called G

An experimental study on the shallow-level migmatization of ferrogabbros from the Fuerteventura Basal Complex, Canary Islands

\left[ {\left( {3 \cdot {\text{Al}}_{2} {\text{O}}_{3} + {\text{SiO}}_{2} )/({\text{Na}}_{2} {\text{O}} + {\text{K}}_{2} {\text{O}} + {\text{CaO}} + {\text{MgO}} + {\text{FeO}}} \right)} \right]