ALBERT

Description: Proficiency testing (PT) is one of the few ways for an analytical laboratory to assess data quality under routine operating conditions. Here we report the results of Round 1 of the G-Chron PT programme, which is sponsored by the International Association of Geoanalysts. G-Chron is the first PT scheme devoted to the U-Th-Pb dating of mineral phases, primarily zircon, in geological materials. In this first round of G-Chron a total of 72 geochronology laboratories received the test material “Rak-17”, which previously had been characterized by seven well-established isotope dilution TIMS laboratories. A total of 63 of the PT participating laboratories reported data by the 15 December 2019 deadline. Here we both report and assess the measurement results submitted to this round. Our analysis provides a means for participating laboratories to assess their individual performance in relation to the isotope ages assigned, the experimental fitness-for-purpose criteria proposed by the scheme’s organisers and the results of similar laboratories participating in this round.

Description: At the DREAMS (DREsden AMS) facility we are implementing a so-called Super-SIMS (SIMS = Secondary Ion Mass Spectrometry) device, which combines the micron-scale spatial resolution of a commercial SIMS (CAMECA IMS 7f-auto) with the very high selectivity through molecule suppression by AMS. We have demonstrated high transmission for major element ions including silicon, fluorine and iodine, however the lack of well characterized calibration materials makes a true quantification of trace and ultra-trace elements contents difficult. Measurements of P in Si show the linearity of the instrument’s relative sensitivity factor over more than three orders of magnitude, and measurements of the isotopic ratio of bromine in ZnS document the reliability of our approach. The goal of the DREAMS Super-SIMS project is to provide quantitative concentration data of ultra-trace elements in geological samples in the context of resource technology.

Description: Lithium and boron are trace components of magmas, released during exsolution of a gas phase during volcanic activity. In this study, we determine the diffusivity and isotopic fractionation of Li and B in hydrous silicate melts. Two glasses were synthesized with the same rhyolitic composition (4.2 wt% water), having different Li and B contents; these were studied in diffusion-couple experiments that were performed using an internally heated pressure vessel, operated at 300 MPa in the temperature range 700–1250 °C for durations from 0 s to 24 h. From this we determined activation energies for Li and B diffusion of 57 ± 4 kJ/mol and 152 ± 15 kJ/mol with pre-exponential factors of 1.53 × 10–7 m2/s and 3.80 × 10–8 m2/s, respectively. Lithium isotopic fractionation during diffusion gave β values between 0.15 and 0.20, whereas B showed no clear isotopic fractionation. Our Li diffusivities and isotopic fractionation results differ somewhat from earlier published values, but overall confirm that Li diffusivity increases with water content. Our results on B diffusion show that similarly to Li, B mobility increases in the presence of water. By applying the Eyring relation, we confirm that B diffusivity is limited by viscous flow in silicate melts. Our results on Li and B diffusion present a new tool for understanding degassing-related processes, offering a potential geospeedometer to measure volcanic ascent rates.

Description: The solubility of N2 in basaltic (MORB) and haplogranitic melts was studied at oxidizing conditions (oxygen fugacity about two log units above the Ni–NiO buffer). Under these conditions, N2 is expected to be the only significant nitrogen species present in the melt. Experiments were carried out from 0.1 to 2 GPa and 1000–1450 ˚C using either an externally heated TZM pressure vessel, an internally heated gas pressure vessel or a piston cylinder apparatus. Nitrogen contents in run product glasses were quantified by SIMS (secondary ion mass spectrometry). To discriminate against atmospheric contamination, 15N-enriched AgN3 was used as the nitrogen source in the experiments. According to infrared and Raman spectra, run product glasses contain N2 as their only dissolved nitrogen species. Due to interactions with the matrix, the N2 molecule becomes slightly infrared active. Nitrogen solubility in the melts increases linearly with pressure over the entire range studied; the effect of temperature on solubility is small. The data may, therefore, be described by simple Henry constants Khaplogranite = (1461 ± 26) ppm N2/GPa and KMORB = (449 ± 21) ppm N2/GPa, recalculated to ppm by weight (μg/g) of isotopically normal samples. These equations describe the solubility of nitrogen during MORB generation and during melting in the crust, as we show by thermodynamic analysis that N2 is the only significant nitrogen species in these environments. Nitrogen solubility in the haplogranitic melt is about three times larger than for the MORB melt, as is expected from ionic porosity considerations. If expressed on a molar basis, nitrogen solubility is significantly lower than argon solubility, in accordance with the larger size of the N2 molecule. Notably, N2 solubility in felsic melts is also much lower than CO2 solubility, even on a molar basis. This implies that the exsolution of nitrogen may drive vapor oversaturation in felsic melts derived from nitrogen-rich sediments. We also measured the partitioning of nitrogen between olivine, pyroxenes, plagioclase, garnet, and basaltic melt by slowly cooling MORB melts to sub-liquidus temperatures to grow large crystals. The mineral/melt partition coefficients of nitrogen range from 0.001 to 0.002, and are similar to argon partition coefficients. These new data, therefore, support the assumption that there is little fractionation between nitrogen and argon during mantle melting and that the N2/Ar ratio in basalts and xenoliths is, therefore, representative of the N2/Ar ratio in the mantle source. This assumption is essential for assessing the size of the nitrogen reservoir in the mantle. Our data also show that for an upper mantle oxidation state that is similar to the one observed today, nitrogen outgassing by partial melting is extremely efficient and even low melt fractions in the range of a few percent may extract nearly all nitrogen from the mantle source.

Description: This data publication consists of two parts: (1) the questionnaire for round 1 of the G-Chron proficiency test as provided to the participating laboratories via the on-line data submission portal, and (2) the complete data set of submitted results. The results of the survey are published as Scientific Technical Report - Data (STR 21/06, Webb et al., 2020). The questionnaire is structured into distinct segments. The first “metadata segment” allowed each laboratory to report key parameters describing their analytical technique. This part was structured as five independent tracks based on laboratory technique: isotope dilution thermal ionization mass spectrometry (ID-TIMS), secondary ion mass spectrometry (SIMS), laser ablation inductively coupled sector field mass spectrometry (LA-ICP-MS: SF), laser ablation inductively coupled quadrupole / time-of-flight mass spectrometry (LA-ICP-MS: Quad&ToF), and “other”. When reporting results a given laboratory was to select a single one of these options and then was provided a series of questions relevant to that specific method. The second segment of the questionnaire was provided to all laboratories for them to submit their determined age results. The on-line portal required submission of the determined age and uncertainty for both the 206Pb/238U and 207Pb/206Pb chronometers. Submission of results for the 208Pb/232Th chronometer was optional. Both the April 2020 report and the subsequent manuscript for publication are based on the table that forms the second part of this document. This contains both the reported age values and information about key aspects of each laboratory’s analytical method. In a small number of cases the input data were corrupted due to an apparent incompatibility between the data portal and the character set used by the submitting laboratory. Where the intent of the laboratory reporting was obvious these have been corrected. For two of the reporting laboratories there a software malfunction resulted in their submissions being classified as “other technique”. These data have now been reassigned to their correct technique categories, but the report has not been accordingly modified. All age values are in Ma. The software specified that uncertainty values should be reported as 1s or 1SE, depending on the nature of the value being addressed, though in a number of instances this instruction appears not to have been applied.