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Abstract: Over the last decade there has been renewed interest in determining the water contents of basaltic magmas. A commonly applied method is analysis of H2O from melt inclusions in olivine. However, it is also well known that these can rapidly lose (or gain) H2O by diffusion. An alternative is to measure the H2O contents of clinopyroxene phenocrysts and use a partition coefficient (D) to estimate the original H2O content of the host magma. This approach is not without complications and several recent studies have attempted to assess the effects of diffusive loss of H2O from magmatic clinopyroxenes. In the ideal case, these crystals should be taken from rapidly cooled tephra or lapilli but such materials are not always available. In order to further assess the potential of using 5–10 mm clinopyroxenes from lavas we undertook a detailed, multi-analytical investigation of clinopyroxenes from an ankaramite flow on Pico Island in the Azores. We conclude that these can be trusted to preserve (probably minimum) magmatic H2O contents if the H2O concentrations of multiple clinopyroxenes from a single sample form a linear correlation with the AlIV content that demonstrates a coupled substitution with little or no H2O loss. Conversely, if H2O contents decrease from core to rim whereas AlIV contents remain relatively constant then it is likely that those clinopyroxenes lost H2O during differentiation and/or cooling. We suspect that the olivine melt inclusions we analysed from Pico and São Miguel Islands also underwent diffusive loss of H2O. Using these criteria, we present clinopyroxene-derived magmatic H2O estimates for Corvo, Flores Faial, Pico and São Miguel Islands that range from 0.28 to 2.2 wt%. When combined with published data these show that H2O contents often extend to higher values on the islands than along the adjacent mid-Atlantic ridge. These localised, elevated H2O contents can explain why the islands are emergent despite being situated away from the ridge and perhaps also the asymmetric nature of the bathymetry of the archipelago. It is possible that this H2O was recycled from material subducted very early on in Earth's history.

Open Access

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Abstract: The desire to study biology in situ has been aided by many imaging techniques. Among these, X-ray fluorescence (XRF) mapping permits observation of elemental distributions in a multichannel manner. However, XRF imaging is underused, in part, because of the difficulty in interpreting maps without an underlying cellular ‘blueprint’; this could be supplied using contrast agents. Carbon nanotubes (CNTs) can be filled with a wide range of inorganic materials, and thus can be used as ‘contrast agents’ if biologically absent elements are encapsulated. Here we show that sealed single-walled CNTs filled with lead, barium and even krypton can be produced, and externally decorated with peptides to provide affinity for sub-cellular targets. The agents are able to highlight specific organelles in multiplexed XRF mapping, and are, in principle, a general and versatile tool for this, and other modes of biological imaging.

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Abstract: The spinel group minerals, found in a range of igneous rocks, are resistant to weathering and can incorporate several multivalent elements, meaning they have the potential to provide insight into redox conditions of parental magmas. Naturally occurring spinel can contain varying quantities of Mn, an element which occurs terrestrially and extraterrestrially as Mn2+, Mn3+, Mn4+ and Mn5+. However, a lack of information on the effects of oxygen fugacity (fO2) on: (1) Mn valence state and cation distribution, and (2) on spinel-melt partitioning means that the potential for a Mn-in-spinel oxy-barometer remains largely untested. Here, we use electron probe microanalysis, micro-focus X-ray Absorption Near Edge Structure (XANES) spectroscopy and single crystal X-ray diffraction (SC-XRD) to investigate cation distribution and valence state in spinels in the Al-Mn-O and Fe-Mn-O systems synthesized at ambient pressure under varying fO2 conditions. In contrast to previous studies, we find that the spectral resolution of the Mn K edge XANES spectra is insufficient to provide quantitative data on Mn valence state and site occupancy, although it does verify that Mn is incorporated as both Mn2+ and Mn3+, distributed over tetrahedral and octahedral sites. Combination of data from XANES and SC-XRD refinements can, however, be used to model Mn, Al and Fe valence and site occupancy. It would be expected that Mn-Fe spinels have the potential to record fO2 conditions in parental melts due to changes to the octahedral site under more reducing conditions. However, decoupling the effects of temperature and oxygen fugacity on the TFe3+-TMn2+ exchange in the Mn-Fe spinels remains challenging. In contrast, little variation is noted in Mn-Al spinels as a function of fO2, implying that crystal chemistry and cation site geometry may significantly influence cation distribution, and by inference, crystal-melt partitioning, in spinel group minerals.

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Abstract: The salt films formed on metal surfaces dissolving inside artificial corrosion pits formed in 1 M HCl have been probed with synchrotron X-ray diffraction. NiCl2 · 6H2O is the main phase in the salt film on nickel, whereas salt films on both iron and 316 L stainless steel are predominantly FeCl2 · 4H2O. However, the salt film on iron has a very fine homogeneous crystallite size whereas that on stainless steel is much coarser. The potential-dependence of the film formed on iron has been determined.