Natural amphibole and biotite crystals as potential reference materials for in-situ Fe isotope analysis

The in-situ analysis of stable isotope ratios, e.g. ⁷Li/⁶Li, ⁵⁶Fe/⁵⁴Fe, in natural and experimental samples has become an indispensable tool for investigating inter-mineral, intra-mineral, and mineral–melt isotope fractionation processes, such as diffusion, equilibrium isotope exchange, or crystal growth / dissolution. A pre-requisite for reliable high-precision measurements of those isotope ratios, however, is the availability of suitable solid reference materials that serve as bracketing standards during the in-situ analyses. Here, we have investigated a series of mm-sized natural amphibole and biotite crystals using both, solution-based MC-ICP-MS and fs-LA-MC-ICP-MS, in order to assess their suitability to serve as reference materials for in-situ δ⁵⁶Fe measurements. These crystals have previously been employed to establish Fe²⁺/ΣFe analyses by electron microprobe using the flank method [1]. The amphibole candidates are classified as hastingsite (Amp-11), pargasite (Amp-12, Amp-40), or tremolite (Amp-15), showing FeO_T contents between 4.2 and 12.7 wt%. The four biotite candidates (Bt-18, Bt-25, Bt-26, Bt-41) display FeO_T concentrations between 8.2 and 28.6 wt% [1]. The results of our in-situ δ⁵⁶Fe analyses by fs-LA-MC-ICP-MS indicate that the investigated biotite crystals are sufficiently homogeneous, and the in-situ data are in good agreement with the Fe isotopic compositions obtained by solution-based MC-ICP-MS within analytical uncertainty. Regarding the amphibole crystals, the in-situ and solution-based Fe isotope measurements reveal that only Amp-11 and Amp-40 represent promising candidates for serving as reference materials. In contrast to this, Amp-15 shows significant internal inhomogeneity and Amp-12 displays large crystal-to-crystal variations in δ⁵⁶Fe.

References:

[1] Li et al. (2019): Chem. Geol., 509, 152-162.