Some of the synthetic and natural apatite grains used for this project. Photo: Heejin Jeon.

Cracking the code: HR-SIMS techniques and data corrections for Sr isotopes in apatite

Early Earth – the first billion years after Earth was created. With significant improvements in analytical techniques, such as SIMS (secondary ion mass spectrometry), small inclusions (typically <50 µm) trapped inside zircon (the oldest mineral ever reported; 4.4 Ga) have become an attractive subject for studying ancient Earth. Apatite inclusion for Sr isotopes (⁸⁷Sr/⁸⁶Sr) is an intriguing target to study crustal evolution in Early Earth.

Project overview

Project period: 2019 – present

Participating departments from the museum: Department of Geosciences, GEO

Apatite provides a powerful means to determine Sr isotopic composition (⁸⁷Sr/⁸⁶Sr) of ancient crust as it contains enough Sr to be measured but typically little Rb. Despite of its importance, it is technically challenging to measure Sr isotopes in apatite with good accuracy and precision mainly due to the analytical difficulties:

not enough Sr signal to collect with a small (< inclusion size) in-situ beam; and
strong molecular interferences to Sr isotopes.

This project is aiming to build a reliable method to determine the apatite Sr isotopic composition with good accuracy and precision (~0.5-0.7 permil 1SD, which is approximately equivalent to the fourth decimal place in 2SD for the typical range of Sr isotopic ratio of crust).

Project description

We faced several challenges in our quest to measure Sr isotopes in apatite from the beginning, mostly caused by lots of molecular interferences. This study evaluated all theoretically possible molecules that interfere Sr isotopes. The most significant are related to Ca dimers and 40Ca31P16O. We dealt with them by using a moderate mass resolution, employing greatly controlled instrument conditions and applying a correction that is carefully calculated based on measurements.

Despite this comprehensive approach, the Ca2 correction remained inadequate, possibly due to as yet unidentified doubly charged molecules interfering at the reference mass 82 peak (40Ca42Ca). We made a mathematically modelled correction for the residual mass 82 interference. A slightly positive offset (ca. +0.0007) of unknown origin in ⁸⁷Sr/⁸⁶Sr was also consistently observed across sessions with different analytical conditions, which may also be empirically corrected.

It is thus necessary to make several stages of corrections for the proven interferences and residual bias, which works well for the reference apatite materials across a wide range of Sr concentration, with good repeatability obtained for Sr from < 100 to 1500 ppm. It is a principle protocol that can be modified.

The reference materials tested earlier yet have low levels of trace elements (TEs). However, natural apatite commonly contains abundant TEs, producing molecules that can seriously interfere Sr isotopes depending on the content of TEs. We further need to demonstrate the contribution and corrections of other interferences formed from TEs. The project is continuing using some natural samples having high TEs contents and synthesized apatite.

Funding

Vetenskapsrådet (VR, 2019 – 2022)

Selected publications

Gillespie, J., Nemchin, A.A., Kinny, P.D., Martin, L., Aleshin, M., Roberts, M.P., Ireland, T.R., Whitehouse, M.J., Jeon, H., Cavosie, A.J., Kirkland, C.L. (2021). Strontium isotope analysis of apatite via SIMS. Chemical Geology 559, 119979. https://doi.org/10.1016/j.chemgeo.2020.11997 External link.
Jeon, H. & Whitehouse, M.J. (2021). A Robust LG‐SIMS Method for Sr Isotope Determination in Apatite Across a Wide Sr Concentration Range. Geostandards and Geoanalytical Research 45, 325-340. https://doi.org/10.1111/ggr.12377 External link.