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Project Progress

Oxygen Isotopes of Apatites in Precambrian banded iron formations

Dominic Papineau from the Carnegie team, received an NAI research scholarship to come work with team member Gary Huss to use the ion microprobe to look at apatite grains in a suite of Precambrian banded iron formations in order to document the ranges of δ¹⁸O values dot see if this technique could be used for biosignature studies in banded iron formations.

Dominic Papineau (Geophysical Laboratory) successfully executed the oxygen isotopic analyses of 41 apatite grains in 6 different samples of Precambrian banded iron formations (BIFs) with the multicollector 1280 Secondary Ion Mass Spectrometer (SIMS) at the University of Hawaii in Manoa. Analyses by SIMS were performed with a 1 nA primary Cs⁺ beam rastered over a 10×10 microns area, which provided the targeted spatial resolution to analyze the small apatite crystals in the BIFs (typically varying between about 15 and 60 microns in size). Analyses by SIMS were preceded by systematic determination of the chemical composition of all target apatite grains by wavelength dispersive spectroscopy in the electron microprobe at the Carnegie Institution of Washington. Because SIMS analyses require standardization with apatite crystals of known and homogeneous isotope and chemical compositions, we have developed several apatite crystals as potential standards that will be useful in future work. This contribution is significant because such SIMS analyses have not been systematically performed on apatite and thus, there currently exist very few apatite crystals that can be used as reliable oxygen-isotope standards. Oxygen isotope analyses of apatite standards were performed by thermal conversion elemental analyzer – isotope ratio mass spectrometry at the Geophysical Laboratory in order to correct SIMS data for instrumental mass fractionation. Over the course of the four-days session, Huss and Papineau performed over 130 oxygen isotope analyses of apatite standards and unknowns, which will be used as preliminary data to build a project that will aim at distinguishing possible biological fractionations of oxygen-isotopes in (meta-) sedimentary apatites compared to igneous apatites, which have yet to be analyzed with this technique.