Michael Wiedenbeck

Experimental resetting of the U–Th–Pb systems in monazite

Abraded fragments (200–400 µm) of a large, chemically homogeneous, and non-metamict Brazilian monazite crystal, characterised by a concordant U–Pb ages of 474 +/- 1 Ma (208Pb/206Pb = 19.5), were hydrothermally treated at varying temperatures with solutions of different compositions. Product monazites were analysed with Scanning Electron Microscope (SEM), Electron Microprobe (EMP), Secondary Ion Mass Spectrometer (SIMS) and Isotope Dilution–Thermal Ionisation Mass Spectrometer (ID-TIMS). Experiments with pure water over a temperature range of 800–1200°C, at 700 MPa and durations ranging from 5 to 60 days showed that even at 1200°C any dissolution and recrystallization of new monazite is confined to the outermost surface of the grain. Neither Pb diffusion at the EMP scale, nor significant discordancy were observed. We performed experiments at 800 and 1000°C for different durations using different fluid compositions at quartz saturation: a 10 wt.% CaCl2 fluid, a 10 wt.% SrCl2 fluid, a 10 wt.% NaCl fluid and a fluid containing NBS 982 Pb standard (208Pb/206Pb = 1). For all runs, EMP traverses revealed no Pb-diffusion profiles. Significant overgrowths of newly formed monazite are documented by SEM analyses. They occurred only in the 1000°C experiments when either CaCl2 or Pb-bearing fluids were present. In the CaCl2 experiment, two zones could be distinguished within the crystal: a core possessing the initial monazite composition and a rim consisting of newly formed monazite produced by dissolution/precipitation, which was enriched in Ca and Pb-free. ID-TIMS dating of single grains treated with SrCl2 and CaCl2 solutions at 1000°C are significantly discordant. Experiments employing the NBS Pb-standard produced sub-concordant monazite, for which the 207Pb/206Pb apparent age has become older than prior to the experiment (544 Ma at 800°C and 495 Ma at 1000°C). The newly grown monazite rim had obviously incorporated Pb from the fluid. None of our reaction products contained a detectable diffusion profile. The only resetting mechanism we detected involved dissolution/precipitation. The extent of the dissolution/precipitation process depends on fluid composition and is a more efficient mechanism than diffusion for controlling the resetting of monazite in natural rocks.

Chemical and phase composition of particles produced by laser ablation of silicate glass and zircon—implications for elemental fractionation during ICP-MS analysis

The chemical and phase compositions of particles produced by laser ablation (266 nm Nd:YAG) of silicate NIST glasses and zircon were studied by SIMS and HR-TEM techniques. The data suggest that the formation of phases of different mineralogy and/or chemical composition from the original sample at the ablation site can result in elemental fractionation (non-stoichiometric sampling) in material delivered to the ICP-MS for quantitative analysis. Evidence of the element fractionation is preserved in chemically zoned ejecta deposited around the ablation pit. The chemical composition and mineralogy of particles varies with particle size so that the efficiency of transport of particles also plays a role in elemental fractionation. During the first 250 pulses in a typical ablation experiment using a 266 nm laser, particle sizes are mainly <2.5 μm; thereafter they decrease to <0.3 μm. Pb and U are fractionated significantly during the ablation of both silicate glass and zircon. During the ablation of glass, both micron-sized, melt-derived, spherical particles, and nm-sized, condensate-derived particle clusters, are produced; the very smallest particles (<0.04 μm) have anomalously high Pb/U ratios. For zircon, both larger (0.2–0.5 μm) spherical particles and agglomerates of smaller (∼0.005 μm) particles produced by ablation are mixtures of amorphous and crystalline materials, probably zircon, baddeleyite (ZrO2) and SiO2. Evidence for thermal decomposition of zircon to baddeleyite and SiO2 is preserved in the wall of the ablation pit, and may lead to the commonly observed increase in Pb/U recorded during laser ablation ICP-MS analysis. It follows that a matrix-matched external calibration is essential for achieving highly precise and accurate laser (266 nm wavelength) ablation ICP-MS analysis of Pb and U in silicate samples.

Boralsilite (Al 16 B 6 Si 2 O 37 ); a new mineral related to sillimanite from pegmatites in granulite-facies rocks

Single crystals of MnSiO(SiO4) with the titanite structure together with MnSiO3 clinopyroxene were synthesized from a MnO-SiO2 oxide mixture at 1000 8C and 9.2 GPa in a multi-anvil press. The crystal structure of MnSi2O5 [space group C2/c, a 5 6.332(1) Å, b 5 8.161(1) Å, c 5 6.583(1) Å, b 5 114.459(3)8, and V 5 309.66 Å3] was refined at room temperature from single-crystal X-ray data to R1 5 2.23%. The monoclinic MnSi2O5 phase has the titanite aristotype structure and is similar to the monoclinic Ca-analogue CaSi2O5. Si occurs in compressed octahedral coordination, replacing Ti in titanite, and in tetrahedral coordination as an orthosilicate group. Mn has a distorted sevenfold coordination with MnO distances between 2.086 and 2.365 Å.

Periodic precipitation pattern formation in hydrothermally treated metamict zircon

Abstract For more than 100 years mineralogists, physicists, chemists, geologists, and biologists have discussed the formation of periodic Liesegang patterns observed in natural and experimental systems. Spectacular examples of minerals showing complex periodic patterns are agate, malachite, and sphalerite. Here we report the first observation of Liesegang-like patterns in hydrothermally treated metamict (i.e., amorphous) zircon. The structures observed show curved bands, radial sets of pocketlike wave fronts or irregular curved patterns in both cathodoluminescence and backscattered electron images. They are composed of alternating zones of crystallographically well-aligned, polycrystalline zircon along with remnant amorphous pockets and a phase assemblage of randomly oriented zircon crystallites, monoclinic ZrO2, and amorphous SiO2, as revealed by transmission electron microscopy. Analyses by secondary ion mass spectrometry and electron microprobe reveal that the latter zones are characterized by higher hydrogen concentrations and higher Zr-Si ratios. Both zones are also distinguishable by a distinctly different crystallite size. We propose a possible pattern-forming mechanism that is based on a feedback of hydrogen diffusion, zircon nucleation, and the displacement of hydrogen atoms from growing crystallites.

Fractionation of alkali elements during laser ablation ICP-MS analysis of silicate geological samples

Data on elemental fractionation of alkali elements during laser ablation ICP-MS analysis of silicate reference glasses (NIST-610, BCR-2G, alkali element-doped andesite glasses) and crystalline mineral albite (NaAlSi3O8) are reported. Laser ablation ICP-MS and SIMS were used to determine differences in sample composition before and after laser interaction with the samples. The fractionation trends of the alkali elements are different from those of other lithophile elements, such as Ca and the rare earth elements (REEs). The rate of fractionation varies for different sample matrices and for different alkali elements in the same matrix. Data from SIMS analyses of the ejecta blanket deposited adjacent to the laser ablation crater in silicate NIST-610 glass and crystalline albite suggest a matrix dependent fractionation of alkali elements for different particle size fractions in the ablated aerosol. The extent of fractionation varies for different alkali elements and is independent of their ionic radii. Ion probe depth profiling into the bottom of laser craters showed laser ablation-induced chemical changes in the sample that involved alkali elements and major matrix elements including Si and Ca. This suggests that a combination of thermally-driven diffusion and size-dependent particle fractionation are responsible for the observed fractionation of alkali elements during laser ablation of silicate samples.

Matrix effects during laser ablation MC ICP-MS analysis of boron isotopes in tourmaline

Laser ablation multi-collector inductively coupled plasma mass spectrometry (LA MC ICP-MS) and secondary ion mass spectrometry (SIMS) were used to determine the boron isotopic compositions of several natural tourmaline group minerals (elbaite, schorl and dravite). This study reports on the effects of instrument parameters of LA MC ICP-MS and the composition of sample matrix on data accuracy, reproducibility and repeatability. We demonstrate that the tourmaline matrix has a significant effect on the obtained δ11B values and impacts on data accuracy, with a bias of up to ca. 2.5‰ if a non matrix-matched reference material is used to calibrate the measurements. In the case of a matrix-matched calibration, the boron isotopic data obtained by LA MC ICP-MS are comparable to the reference TIMS values, with typical repeatability for δ11B results between 0.2 and 0.5‰ (1s). This is somewhat better compared to the repeatability of 0.8–1.3‰ (1s) for small format single collector SIMS.