High‐Precision Fe and Mg Isotope Ratios of Silicate Reference Glasses Determined In Situ by Femtosecond LA‐MC‐ICP‐MS and by Solution Nebulisation MC‐ICP‐MS

Abstract

In this study, a technique for high precision in situ Fe and Mg isotope determinations by femtosecond‐laser ablation‐multi collector‐ICP‐MS (fs‐LA‐MC‐ICP‐MS) was developed. This technique was employed to determine reference values for a series of common reference glasses that may be used for external standardisation of in situ Fe and Mg isotope determinations in silicates. The analysed glasses are part of the MPI‐DING and United States Geological Survey (USGS) reference glass series, consisting of basaltic (BIR‐1G, BCR‐2G, BHVO‐2G, KL2‐G, ML3B‐G) and komatiitic (GOR128‐G and GOR132‐G) compositions. Their Fe and Mg isotope compositions were determined by in situ fs‐LA‐MC‐ICP‐MS and by conventional solution nebulisation multi‐collector ICP‐MS. We determined δ56Fe values for these glasses ranging between ‐0.04‰ and 0.10‰ (relative to IRMM‐014) and δ26Mg values ranging between ‐0.40‰ and ‐0.15‰ (relative to DSM‐3). Our fs‐LA‐MC‐ICP‐MS results for both Fe and Mg isotope compositions agreed with solution nebulisation analyses within analytical uncertainties. Furthermore, the results of three USGS reference glasses (BIR‐1G, BHVO‐2G and BCR‐2G) agreed with previous results for powdered and dissolved aliquots of the same reference materials. Measurement reproducibilities of the in situ determinations of δ56Fe and δ26Mg values were usually better than 0.12‰ and 0.13‰ (2s), respectively. We further demonstrate that our technique is a suitable tool to resolve isotopic zoning in chemically‐zoned olivine crystals. It may be used for a variety of different applications on isotopically‐zoned minerals, e.g., in magmatic or metamorphic rocks or meteorites, to unravel their formation or cooling rates.

Crystallization history of enriched shergottites from Fe and Mg isotope fractionation in olivine megacrysts

“…Fe and Mg isotope ratios were measured in situ using the same laser ablation system but connected to a ThermoFinnigan Neptune multicollector ICP-MS (MC-ICP-MS) operated in a high mass resolution mode (Weyer and Schwieters, 2003) with sample-standard bracketing following the exact procedure described in Oeser et al (2014Oeser et al ( , 2015. Analyzing simultaneously the 60 Ni/ 58 Ni ratio of a Ni standard solution (NIST SRM 986) with the ablation aerosol allowed to monitor the instrumental mass bias for Fe isotopes (Poitrasson and Freydier, 2005).…”

“…Additional trace elements (V, Co, Sc, Ni, Cu, Zn and Cr) were measured by femtosecond-laser ablation (fs-LA) of the sample with a Spectra-Physics Solstice system (see Oeser et al, 2014;Lazarov and Horn, 2015 for details) connected to a fast scanning sector field ICP-MS (ThermoScientific Element XR Ò ) at the Institute of Mineralogy, LUH. Sample ablation was performed by spot analyses with a beam diameter of 40 mm and a step width of 50 mm on average (Fig.…”

“…The within-session reproducibility, which is more appropriate for defining the uncertainties of the measured d-values for a single isotope profile, is better than 0.10‰ (2 SD), based on replicate analyses of silicate reference glasses (Oeser et al, 2014). Ablation of olivine crystals was performed along lines of 150-200 mm in length parallel to the crystal rim with a laser spot size of $40 mm (Fig.…”

Quantifying nutrient uptake as driver of rock weathering in forest ecosystems by magnesium stable isotopes

“…Mass bias correction was performed using the komatiite glass GOR132-G as bracketing standard. Using a δ 26 Mg value for GOR132-G of −0.17 ‰ relative to DSM-3 (Oeser et al, 2014), we converted results to δ values relative to DSM-3. Based on our current experience, we conservatively estimate the uncertainty of the fsLA-MC-ICP-MS method for Mg isotope ratios to be better than ±0.25 ‰ (2SD) for δ 26 Mg. Repeat measurements on reference material BHVO-2G (basaltic glass) (average δ 26 Mg = −0.07 ± 0.18 ‰, 2SD, n = 18) agree within uncertainties to published values ( Fig.…”

“…Instrumentation, data acquisition and evaluation procedures are described in detail in Schuessler and von Blanckenburg (2014 (Dai et al, 2016;Oeser et al, 2014). Mass bias correction was performed using the komatiite glass GOR132-G as bracketing standard.…”