Chemie und Prozesstechnik
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In isotope ratio applications metrological principles such as measurement uncertainty and SI traceability often are not considered or realized. This is also well-documented by the traceability exception related to delta scale isotope ratio measurements CCQM requested from the CIPM. Generally, delta scale isotope ratio measurements can be performed on a precision level that is significantly lower than the uncertainty level of isotope amount ratio measurements. In the case of magnesium, we demonstrated for the first time that isotope amount ratios can be measured with uncertainties close to the typical precision of magnesium delta values, δ26/24Mg, which are at the 0.1 ‰ level.
In the past, δ26/24Mg measurements were referenced to NIST SRM 980, the initial zero of the δ26/24Mg scale. With the development of MC-ICPMS, the detection of small but measurable isotopic differences in different chips of NIST SRM 980 became apparent and NIST SRM 980 was replaced by an intermediate artefact, DSM3. To solve this problem a suite of magnesium isotope reference materials, ERM-AE143, -AE144, and -AE145, has been certified in a first study by applying an ab initio calibration for absolute Mg isotope ratios without any a priori assumptions, a procedure which fulfils all requirements of a primary method of measurement. Thus, these materials qualified to establish SI-traceability for magnesium delta measurements. In a second study five expert laboratories participated to cross-calibrate all available magnesium isotope standards, which are NIST SRM 980, IRMM-009, ERM-AE143, ERM-AE144, ERM-AE145 and the standards DSM3 and Cambridge-1. The mean δ26/24Mg values for the individual iRMs, calculated from the laboratory means show 2 SD reproducibilities varying between 0.025 and 0.093 ‰. Propagated measurement uncertainties suggest a standard uncertainty of about 0.1 ‰ for δ26/24Mg determinations. Thus, SI traceability for magnesium isotope amount ratios and delta values is demonstrated to be established.
Si isotope fractionation during BIF formation – inferences from a modern Archean ocean analogue
(2019)
Silica-rich sedimentary rocks like cherts and BIFS, typical for the Archean, have been used to reconstruct temperatures and other properties of the early oceans through the study of their Si isotope variations. Precambrian cherts and BIFS span a δ30Si range of ~7‰, with BIFs being about 2‰ lower in δ30Si than cherts. These lower δ30Si signatures have been attributed to represent contributions from different input sources such as hydrothermal fluids, variable continental weathering regimes or sorption onto Fe oxides/hydroxides [e.g. 2 and references therein]. In this study, fluids and BIF-like sediments have been investigated for their Si isotope compositions in Paulina Lake (PL), a hydrothermally-influenced crater lake in the Newberry Caldera, Oregon, USA. PL lake sediments are rich in silica (~65wt% SiO2) and are composed of up to 22.5wt% Fe2O3, which is comparable to Archean BIFs and thus serve as a modern Archean ocean analogue. We compared our analyses with East Lake (EL), the twin Newberry crater lake without hydrothermal input. Dissolved Si in EL has an average δ30Si signature of +1.55±0.16‰ (1sd) and sediments an average δ30Si signature of +0.18±0.28‰ (1sd). Dissolved Si in PL has an average δ30Si signature of +2.02±0.15‰ (1sd), whereas the sediments show a large range in δ30Si values between +0.59‰ and -1.24‰. PL sediments show a trend towards more negative δ30Si with increasing Fe2O3 contents. The magnitude of Si isotope fractionation thus appears to depend on the presence of Fe. This fractionation induced by interaction with Fe precipitation is defined here as the offset in δ30Si between PL and EL sediments at comparable depths (Δ30SiPL-EL). The resulting Δ30SiPL-EL values range between +0.69 and -1.42‰ and increase with increasing Fe2O3 content in the sediments. Our results are the first to quantify the magnitude of Fe-induced δ30Si fractionation observed in a natural analogue of the Archean ocean and can explain the lighter δ30Si signatures found in BIFs.