Anette Meixner

MPI‐DING reference glasses for in situ microanalysis: New reference values for element concentrations and isotope ratios

We present new analytical data of major and trace elements for the geological MPI-DING glasses KL2-G, ML3B-G, StHs6/80-G, GOR128-G, GOR132-G, BM90/21-G, T1-G, and ATHO-G. Different analytical methods were used to obtain a large spectrum of major and trace element data, in particular, EPMA, SIMS, LA-ICPMS, and isotope dilution by TIMS and ICPMS. Altogether, more than 60 qualified geochemical laboratories worldwide contributed to the analyses, allowing us to present new reference and information values and their uncertainties (at 95% confidence level) for up to 74 elements. We complied with the recommendations for the certification of geological reference materials by the International Association of Geoanalysts (IAG). The reference values were derived from the results of 16 independent techniques, including definitive (isotope dilution) and comparative bulk (e.g., INAA, ICPMS, SSMS) and microanalytical (e.g., LA-ICPMS, SIMS, EPMA) methods. Agreement between two or more independent methods and the use of definitive methods provided traceability to the fullest extent possible. We also present new and recently published data for the isotopic compositions of H, B, Li, O, Ca, Sr, Nd, Hf, and Pb. The results were mainly obtained by high-precision bulk techniques, such as TIMS and MC-ICPMS. In addition, LA-ICPMS and SIMS isotope data of B, Li, and Pb are presented.

Ocean acidification and the Permo-Triassic mass extinction

Ocean acidification and mass extinction The largest mass extinction in Earths history occurred at the Permian-Triassic boundary 252 million years ago. Several ideas have been proposed for what devastated marine life, but scant direct evidence exists. Clarkson et al. measured boron isotopes across this period as a highly sensitive proxy for seawater pH. It appears that, although the oceans buffered the acidifiying effects of carbon release from contemporary pulses of volcanism, buffering failed when volcanism increased during the formation of the Siberian Traps. The result was a widespread drop in ocean pH and the elimination of shell-forming organisms. Science, this issue p. 229 A rapid injection of massive amounts of carbon into the atmosphere acidified the oceans, causing mass extinction. Ocean acidification triggered by Siberian Trap volcanism was a possible kill mechanism for the Permo-Triassic Boundary mass extinction, but direct evidence for an acidification event is lacking. We present a high-resolution seawater pH record across this interval, using boron isotope data combined with a quantitative modeling approach. In the latest Permian, increased ocean alkalinity primed the Earth system with a low level of atmospheric CO2 and a high ocean buffering capacity. The first phase of extinction was coincident with a slow injection of carbon into the atmosphere, and ocean pH remained stable. During the second extinction pulse, however, a rapid and large injection of carbon caused an abrupt acidification event that drove the preferential loss of heavily calcified marine biota.

Boron in central Andean ignimbrites: implications for crustal boron cycles in an active continental margin

Abstract Calc-alkaline pumices and lavas from the Neogene–Pleistocene central Andean Altiplano–Puna Volcanic Complex (APVC) contain quartz phenocrysts with abundant primary melt inclusions. Melt inclusions represent the melt composition of dacitic to rhyodacitic magmas prior to eruption and they are widely protected from eruptive degassing, low-temperature vapor-phase alteration and post-depositional contamination with boron-rich dusts and aerosols. The average boron isotopic compositions of melt inclusion and matrix glasses are uniform in the units studied (overall average δ 11 B=−3.8±2.8‰, 1 SD) and overlap with the range of local basement rocks ( δ 11 B=−11‰ to −5‰). B is enriched in glasses (55±14 ppm, 1 SD), whereas whole-rock contents range between 10 and 40 ppm. At similar B contents, incompatible trace element ratios like B/Nb are generally higher in the ignimbrites and lavas (B/Nb=2.4 and 4.0) than in crustally derived Paleozoic to Cretaceous granitoids from NW-Argentina (B/Nb=1.0–2.7). This suggests a minor contribution of high-B/Nb arc andesites and is consistent with petrogenetic models in which APVC dacites originated from hybridization of dominantly crustally derived melts with andesitic magmas. Boron-rich hydrothermal brines and salt-lake water from within the APVC have δ 11 B=−4.1 to +5.7‰. These values are close to the APVC ignimbrite values and contrast with hydrothermal brines from the eastern Puna and eastern Cordillera, which have lower δ 11 B=−10‰ to −17‰ and occur in an area of little volcanic cover. This regional pattern suggests that borate enrichments resulted from the leaching of local country rocks: ignimbrites in the W Altiplano–Puna and Paleozoic basement in the eastern Puna and eastern Cordillera.

Li-isotope fractionation between silicates and fluids: Pressure dependence and influence of the bonding environment

Isotope fractionation experiments and molecular simulations were performed to determine the relation of Li-isotope fractionation between silicates and fluids and the corresponding cation coordination environments. The effect of pressure-induced changes of Li hydration in aqueous fluids on solid-fluid Li-isotope fractionation was studied by performing experiments in the system spodumene-fluid at three different pressures of 1, 4 and 8 GPa at temperatures ranging from 500 to 625 °C. 7 Li preferentially partitioned into the fluid in all three experiments. The Li-isotope fractionation of experiments at 1 and 4 GPa does not show a significant P dependence in comparison to previously published data at 2 GPa. At 8 GPa the Li-isotope fractionation is slightly decreased compared to the low-pressure data. In addition, the fractionation of lithium isotopes between Li-bearing amphibole and fluid was determined experimentally at 700 °C and 2 GPa, which resulted in a Δ 7 Li (Li-amph-fluid) of −1.7 ‰. Our experiments are complemented by ab initio molecular dynamics simulations of Li-bearing aqueous fluids aimed to determine structural properties at high P and T . Despite the increase in Li coordination from 4.0 to 5.4 with pressure at isothermal conditions, the mean Li-O distance of the fluid is almost unchanged between 1 and 8 GPa at 727 °C. This might explain the insignificant effect of pressure over a large P range observed experimentally. The new experimental results indicate a partial inapplicability of the coordination-principle on isotope fractionation. Therefore, we additionally analyzed the relation of isotope fractionation and Li-O bond length and applied the bond valence model. Using the available structural data of solids and fluids, in a first approximation, the bond valence model seems to be more appropriate to relate the local atomic structure to isotope fractionation than the simple coordination-dependent principle.

Interlaboratory comparison of magnesium isotopic compositions of 12 felsic to ultramafic igneous rock standards analyzed by MC-ICPMS

To evaluate the interlaboratory mass bias for high-precision stable Mg isotopic analysis of natural materials, a suite of silicate standards ranging in composition from felsic to ultramafic were analyzed in five laboratories by using three types of multicollector inductively coupled plasma mass spectrometer (MC-ICPMS). Magnesium isotopic compositions from all labs are in agreement for most rocks within quoted uncertainties but are significantly (up to 0.3‰ in 26Mg/24Mg, >4 times of uncertainties) different for some mafic samples. The interlaboratory mass bias does not correlate with matrix element/Mg ratios, and the mechanism for producing it is uncertain but very likely arises from column chemistry. Our results suggest that standards with different matrices are needed to calibrate the efficiency of column chemistry and caution should be taken when dealing with samples with complicated matrices. Well-calibrated standards with matrix elements matching samples should be used to reduce the interlaboratory mass bias.

An experimental approach to quantify the effect of tetrahedral boron in tourmaline on the boron isotope fractionation between tourmaline and fluid

Abstract This study investigates the effect of tetrahedral B ([4]B) in synthetic tourmaline on the B-isotope fractionation between tourmaline and fluid. This is important for the correct interpretation of B-isotope variations in natural tourmalines containing “excess” B (greater than three atoms per formula unit), which substitutes for Si at tetrahedral sites. Such tourmalines commonly occur in Li, Al-rich pegmatites and have been reported from glaucophane schists that formed at high pressures during subduction. Tourmaline synthesis experiments were performed in a piston-cylinder apparatus in the system SiO2-Al2O3-B2O3-NaCl-H2O at 4 GPa and 700 °C using different run durations, starting from quartz-γ-Al2O3-H3BO3 solid mixtures and NaCl-solutions. We were able to produce “olenitic” tourmaline with excess B between 1.2 and 2.5 [4]B per formula unit. The B-isotope compositions of the olenitic tourmaline and coexisting fluids were determined by secondary ion mass spectrometry and multi-collector plasma source mass spectrometry to derive isotope fractionation coefficients. The results indicate that for every 10 mol% of total B in tourmaline in tetrahedral coordination, the value of Δ11Btur-fluid is shifted to more negative values by about 1‰ at 700 °C. This is in good agreement with published ab initio calculations and corresponds to an intracrystalline fractionation of B-isotopes between the trigonal B and tetrahedral T sites of tourmaline on the order of 8 ± 5‰, whereby 10B partitions to the T site.