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Magnetite and spinel thermochronological (U-Th)/He dates obtained in different geological contexts often present significantly dispersed values, that could be related to the low concentration of U and Th isotopes, the lack of standard samples or other parameters. For this purpose, this study focuses on the analysis of U and Th content variability. U and Th in magnetite (natural and synthetic) and in natural Al-spinel samples containing different amounts of U and Th, from 0.02 to 116 µg/g, are analyzed using both wet chemistry and in-situ laser ablation extraction methods. To increase the number of reference samples, two U-Th doped nanomagnetite powders were synthesized and the U and Th concentration were firstly determined using wet chemistry extraction (U and Th of NMA is ~40 µg/g and NMB ~0.1 µg/g). We show that for both U and Th analyses, the obtained reproducibility of the wet chemistry protocol depends on their concentration and is below 11 % for U-Th values higher than 0.4 µg/g and reaches 22 % for U-Th content lower than 0.1 µg/g. It implies that (U-Th)/He thermochronological dates cannot be more reproducible than 24 % for magnetite containing less than 0.1 µg/g of U and Th, explaining part of the natural date variability. Secondly, U and Th concentration extracted by laser ablation on natural magnetite and Al-spinel samples were calibrated using both silicate glass standards and synthetic magnetite samples. The determined U and Th content using NMA sample give similar values than the one obtained by wet chemistry extraction but is 30 % overestimated using the glass standard samples. These results highlight the impact of matrix effect on the determination of the U-Th content and we recommend to use a well-characterized magnetite sample for calibrating the U-Th signals by laser ablation. In addition, the scatter on the (U-Th)/He magnetite dates can be expected to be ~20 % if the U and Th contents are determined by laser ablation. Such a precision level is not that different to the one obtained using wet chemistry extraction, opening the use the use of laser ablation extraction method for determining (U-Th)/He dates. In the absence of spinel reference for U and Th, the silicate glasses and NMA samples was used for laser ablation calibration and U and Th content and are of ~30 % lower compared to values obtained using wet chemistry extraction. This discrepancy underlines the importance of using a standard with a composition close to that of the mineral of interest. Although magnetite and Al-spinel have related crystal-structures, the magnetite standard is not appropriate for U and Th analysis in Al-spinel. [ABSTRACT FROM AUTHOR]

Magnetite and spinel thermochronological (U–Th) / He dates often display significantly dispersed values. In the present study, we investigated the contribution of analytical (and standardization) errors to this dispersion. U and Th content of magnetite (natural and synthetic) and natural Al spinel samples with U and Th concentrations between 0.02 and 116 µg g -1 were analyzed using both wet chemistry and in situ laser ablation sampling methods. New magnetite reference samples (NMA and NMB) were synthesized, consisting of U- and Th-doped nano-magnetite powders, whose U and Th concentrations were determined using a wet chemistry method (U and Th of NMA and NMB are ∼40 µg g -1 and ∼0.1 µg g -1 , respectively). We show that, for both U and Th analyses, the reproducibility obtained with the wet chemistry protocol depends on the U and Th concentration. It is below 11 % for U–Th values higher than 0.4 µg g -1 and reaches 22 % for U–Th content lower than 0.1 µg g -1. This result implies that (U–Th) / He thermochronological ages cannot be more reproducible than 24 % for magnetite containing less than 0.1 µg g -1 of U and Th, thus explaining part of the natural ages variability. U and Th data obtained by laser ablation ICP-MS on natural magnetite and Al spinel samples were calibrated using both silicate glass standards and synthetic magnetite samples. The U and Th contents determined using NMA are consistent with those obtained by means of the wet chemistry method, but they are overestimated by 30 % when using the glass standard samples only. These results highlight the impact of the matrix effect on the determination of the U–Th content in magnetite. We thus recommend the use of a well-characterized magnetite reference for the calibration of the U–Th signals obtained by laser ablation. The scatter in the (U–Th) / He magnetite ages can be expected to be ∼20 % if the U and Th contents are determined by laser ablation. This level of precision is actually not significantly different from that obtained using the wet chemistry method, which paves the way for the use of laser ablation for determining (U–Th) / He ages. In the absence of a spinel reference for U and Th calibration using LA-ICP-MS, silicate glass references, along with NMA, were used. U and Th contents were found to be ∼30 % lower than the values obtained using wet chemistry. This discrepancy underlines the importance of using a standard with a composition close to that of the mineral of interest. Although magnetite and Al spinel have related crystal structures, the magnetite standard is not appropriate for U and Th analysis in Al spinel using LA-ICP-MS. [ABSTRACT FROM AUTHOR]

Minor-element concentrations in marine ferromanganese nodules are primarily controlled by the mineralogy, which itself depends on redox conditions at the sediment-water interface. Results are presented for the first in-depth X-ray microstructural and microspectroscopic investigation of a mixed hydrogenetic-diagenetic nodule, which is representative of ferromanganese deposits on abyssal plains. The measurements were conducted by micro-X-ray diffraction and X-ray absorption spectroscopy (both XANES and EXAFS) on hydrogeneous and diagenetic regions of the nodule. The hydrogeneticdiagenetic interface was imaged by X-ray microfluorescence, after which regions of interest were chosen to represent mineralogical and chemical transformations that occurred at the early stage of suboxic diagenesis. In the hydrogenetic nodule (oxic environment), Mn is speciated as Fe-vernadite, a nanocomposite material composed of intergrown feroxyhite (δ-FeOOH) and monodispersed phyllomanganate layers having no interlayer Mn (vernadite). In the diagenetic nodule (suboxic environment), Mn is speciated dominantly as Mg-rich 10 Å vernadite, which consists of a random intergrowth of vernadite and its transformation product todorokite. The authigenic 10 Å vernadite precipitated from the components of vernadite in Fe-vernadite that were dissolved in suboxic microenvironments of the sediment. Direct evidence supporting a redox-driven dissolution reaction is provided by the valence composition of Mn, as measured by micro-XANES, which is 0.69Mn4+ + 0.24Mn3+ + 0.07Mn2+ (average = 3.62 ± 0.04 v.u.) for Fe-vernadite and 0.61Mn4+ + 0.23Mn3+ + 0.16Mn2+ (average 3.28 ± 0.04 v.u.) for 10 Å vernadite. Ni and Cu, derived mainly from dissolved vernadite and oxidized organic matter, replace structural Mn3+/4+ in both the MnO2 layer and todorokite domains of 10 Å vernadite. Pure todorokite in highly diagenetic regions of the nodule has an average formula of Mg2+ 0.167(Mn4+ 0.783 Mn3+ 0.099Co3+ 0.002Ni2+ 0.076Cu2+ 0.040)O2·nH2O, with an atomic ratio of (Cu+Ni+Co)/Mn = 0.13, which is slightly lower than 0.167 (1/6), the maximum metal uptake capacity reported for marine nodules. By analogy with synthetic todorokites we infer that Mg2+, which has a hydrated diameter close to that of the [3 × 3] tunnel size of todorokite, and Mn3+ and Cu2+, which prefer Jahn-Teller distorted octahedra, play a crucial role in templating the topotactic transformation of 10 Å vernadite to todorokite and stabilizing todorokite in suboxic marine sediments. [ABSTRACT FROM AUTHOR]

An iron-based powder material composed of zerovalent iron (8 wt.%) and magnetite (92 wt.%), has been synthesized hydrothermally at 200 °C from zero-valent iron. Its effect on the reduction of aqueous Hg 2+ into gaseous Hg 0 has been investigated at ambient conditions for pH comprised between 4 and 8.5. The production of Hg 0 was monitored with an online mercury vapor analyzer at the picogram level for concentrations of iron-based composite of a few tenths of mg L -1 . Starting from a solution having an Hg 2+ concentration of 25 ng L -1 at pH = 4, a succession of two Hg 0 production events was recorded. The first event is related to the Hg 2+ reduction by ZVI which fully dissolved within the first hours. Upon ZVI consumption, pH drifted towards the pH window where magnetite can efficiently reduce Hg 2+ at the hour timescale, resulting in a second Hg 0 production peak. The combined use of ZVI and magnetite to remove aqueous Hg 2+ by formation of Hg 0 (volatile) under mild acidic pH allows (1) to maximize the Hg 2+ reduction rate and (2) to take benefit of the longer lifetime of magnetite compared to ZVI.