In situ Mg isotope measurements of biogenic carbonates using laser ablation multi‐collector inductively coupled plasma mass spectrometry: A new tool to understand biomineralisation

Abstract

Rationale Magnesium is one of the most abundant elements in the earth's crust and in seawater. Fractionation of its stable isotopes has been shown to be a useful indicator of many geological, chemical, and biological processes. For example, biogenic carbonates display an ~5‰ range of δ26Mg values, which is attributed to variable degrees of biological control on Mg ions during biomineralisation. Understanding this biological control is essential for developing proxies based on biogenic carbonates. Methods In this work, we present a new approach of measuring Mg isotopes in biogenic carbonates using Laser Ablation Multi‐Collector Inductively Coupled Plasma Mass Spectrometry (LA‐MC‐ICPMS). Results Our results show that this microanalytical approach provides relatively fast, high spatial resolution (<0.2 μm) measurements with high precision and accuracy down to 0.2‰ (2SE). To achieve high levels of precision and accuracy, baseline interferences need to be monitored and a carbonate standard with a relatively low trace metal composition similar to biogenic carbonates should be used. We also demonstrate that the matrix effect on Mg isotopes in carbonates with low Fe and Mn is limited to less than 0.2‰ fractionation under different laser parameters and low oxide condition (<0.3% ThO/Th). Conclusions Our newly developed LA‐MC‐ICPMS method and its applications to biogenic carbonates show significant advantages provided by the microanalytical approach in understanding complex processes of biomineralisation in marine calcifiers.

Non-matrix-matched calibration of Mg isotopic ratios in silicate samples by fs-LA-MC-ICP-MS with low mass resolution under wet plasma conditions

“…14–16 With the development of multiple collector inductively coupled plasma mass spectrometry (MC-ICP-MS), 17–25 it has become the method of choice for Mg isotopic bulk- and micro-analysis due to the low sample consumption, high sample throughput, and time-independent mass fractionation during the short analytical time. In particular, the development of laser ablation-MC-ICP-MS (LA-MC-ICP-MS) has widened the application of adopting an Mg isotope as a tracing tool in earth sciences, 26–34 as microanalysis by LA-MC-ICP-MS can identify and distinguish the inter- and intracrystalline isotopic variations of mineral grains on the scale of tens of micrometers compared with solution nebulization MC-ICP-MS (SN-MC-ICP-MS). However, compared with isotopic analysis by SN-MC-ICP-MS, laser analysis cannot benefit from matrix purification by ion exchange chromatography, and thus, the matrix effect and the interference related to the matrix elements are the two dominant concerns in Mg isotopic analysis by LA-MC-ICP-MS.…”

“…12,26,27,33 The metal oxide ( 9 Be 16 O, 10 B 16 O, 6 Li 18 O, and 7 Li 17 O), hydride ( 23 Na 1 H + , 24 Mg 1 H + , 25 Mg 1 H + , 12 C 12 C 1 H + , 12 C 13 C 1 H + , and 12 C 12 C 1 H 1 H + ) and 12 C 12 C + and 12 C 13 C + interferences have also been reported to be negligible, which was due to the low content of the interfering element, the low hydride yield 27,38 and leak-free gas fittings. 31 The 12 C 14 N + is one of the interferences that usually needs to be considered because the extraction lens is composed of graphite and N 2 is a major component of the atmosphere, and the medium mass resolution (the theoretically needed mass resolution power is 3808–6347) is needed if 12 C 14 N + is a major interference. For example, Xie et al (2011), 32 Norman et al (2006) 28 and Deng et al (2021) 26 stated that the medium mass resolution mode should be applied, and 26 Mg was measured at the off-center and left shoulder of the peaks to avoid potential interference of 12 C 14 N + .…”

High-precision magnesium isotope analysis of carbonates by laser ablation MC-ICP-MS using wet and dry conditions

“…11 Moreover, d 26 Mg of biological carbonates displays a relatively large variation ($5&), and the current proposed biomineralization models can hardly explain the range in Mg isotopic compositions. 12,13 High-resolution and precision Mg isotopes in carbonates are crucial for further solving these geochemical and biological questions.…”

Strategies to overcome interferences in elemental and isotopic geochemical analysis by quadrupole inductively coupled plasma mass spectrometry: A critical evaluation of the recent developments

“…MC‐ICP‐MS has revolutionized the precise determination of isotope ratios required in geoscience studies but it remains limited in some critical applications in geochemistry as a result of spectral interferences, mainly because of the limited resolution capability 146 . Some of the non‐traditional isotopic systems of metals such as Cu, Zn, Mo, Tl, and Ni are now being developed as tracers in geochemical studies due to their tendency to be fractionated by redox‐related and/or biological processes 147,148 . The isotopic compositions of non‐traditional stable isotopes such as V, Cu, Mo, Ba, K, Pt, Pd, Ag, Ce, Er, and Si also fractionate during the partitioning of isotopes between two substances or phases due to the isotope effect 149,150 .…”