Mn/Cr relative sensitivity factors of SIMS determined for synthetic Mn-, Cr-, and Fe-bearing
dolomites

Shingo Sugawara1, Wataru Fujiya1, Akira Yamaguchi2, Ko Hashizume1
1
Ibaraki University
2
National Institute of Polar Reserach
Dolomite [CaMg(CO3)2] is a carbonate mineral commonly found in aqueously altered carbonaceous chondrites and contains
some amounts of Mn and Fe. Dolomite in meteorites formed in the early solar system and incorporated a short-lived
radionuclide 53Mn that decays to 53Cr with a half-life of 3.7 million years [1]. Thus, the timescale of aqueous alteration and theformation age of parent bodies can be constrained by 53Mn-53Cr dating of meteoritic dolomite.
The Mn-Cr dating of dolomite has commonly been performed using secondary ion mass spectrometry (SIMS). For SIMSanalyses, the relative sensitivity factor (RSF) defined as a Mn+/Cr+ ion intensity ratio divided by a Mn/Cr elemental ratio mustbe evaluated using a standard material with the same composition as the meteoritic carbonate to be analyzed to correct thematrix effect. However, because carbonates in the natural environment contain little Cr, it is challenging to utilize a proper Crbearing dolomite standard, and therefore, the previously reported Mn-Cr ages of meteoritic dolomite may have systematic
uncertainties. To overcome this problem, the synthesis of Mn- and Cr-bearing dolomite has been attempted [8,9], although
dolomite crystallization proceeds only under hydrothermal conditions, making it difficult to incorporate Cr. Alternatively, Mnand Cr-bearing calcite [CaCO3] [10], which can be synthesized in aqueous solution at room temperature, has been used [2-5],
but the RSFs of calcite and dolomite may be different. Recently, natural dolomite to which Cr ions were implanted wasproduced as a Cr-bearing dolomite standard [11,12]. However, the depth of the implantation was within 1 m from the sample
surface, and thus, the analytical conditions of this standard and meteoritic dolomite were much different, which may lead asystematic error on the obtained Mn-Cr ages.
In this work, we synthesized Mn-, Cr-, and Fe-bearing dolomites via amorphous carbonates. Amorphous calcium carbonate(ACC), which can incorporate large amounts of incompatible elements for crystal phases, crystalizes into calcite by heating[e.g., 13]. Applying this method, we synthesized Mn-, Cr-, and Fe-bearing dolomites via amorphous calcium magnesium
carbonate (ACMC) with FeO contents of (i) 0 wt%, (ii) 0.95 wt%, and (iii) 3.17 wt%. Also, we synthesized Mn- and Crbearing calcite from ACC for comparison with previous studies [10].
We produced ACMCs by mixing a 0.2 M Na2CO3 solution and a 0.2 M (Mg, Ca, Cr, Mn, Fe) Cl2 solution. The ACMCs
were dehydrated at 270-300 ℃ and crystallized to dolomite by heating at 310-420 ℃ under a 1-22 bar CO2 atmosphere. The
heated samples were washed with ultrapure water and dried in a vacuum desiccator. The run products were identified as singlephase dolomite by powder X-ray diffraction (XRD). We observed the dolomites with a field emission scanning electron
microscope (FE-SEM) and found that they were from several 10s nanometer- to submicron-sized particles. Also, a Mn-, andCr-bearing calcite was synthesized via ACC [13]. The Mn-, Cr-, and Fe-bearing dolomite and Mn- and Cr-bearing calcite
particles were separately put in a tungsten carbide piston cylinder and pressed using a hydraulic press to produce pellets. Thesepellets were embedded in resin, and their surfaces were polished and then coated with carbon. Electron probe micro analysis
confirmed that the 55Mn/52Cr ratios of the dolomites were (i) 1.32 ± 0.02 (2SE, n = 185), (ii) 5.46 ± 0.14 (2SE, n = 391), and(iii) 2.70 ± 0.06 (2SE, n = 220), and that of the calcite was 0.86 ± 0.01 (2SE, n = 200).
Then, we analyzed the synthesized carbonates and a natural olivine from San Carlos using SIMS. Secondary ions of 43,44Ca+,Cr+, 55Mn+, and 57Fe+ were produced with a ~1 nA O- primary ion beam focused to ~10 µm in diameter. The RSFs ofolivine and calcite were 0.93 and ~0.75, respectively, consistent with previous studies [2-5, 10] (Fig. 1). We found that theRSFs of Fe-bearing dolomites increase with their FeO contents from ~0.81 to ~1.00, which is also consistent with the previousstudy [12] (Fig. 2). The observed variation in the RSFs between Fe-bearing dolomites and calcite could result in a systematicerror on the ages of meteoritic dolomites by 1.6 Myr. We noted, however, that the reproducibility was worse when we used a
primary ion beam with a lower intensity of 150 pA, probably due to the influence of the surface topography. This indicates that
it is important to keep analytical conditions the same between the standard and meteoritic dolomites.
52,53

Fig.1. The relative sensitivity factors [RSF = (55Mn/52Cr)SIMS/(55Mn/52Cr)EPMA] of synthetic dolomites, synthetic calcite, and San CarlosOlivine. RSFs obtained by Jilly et al. [4] are shown by open symbols for comparison.

Fig. 2. The RSFs of synthetic dolomites as a function of Fe contents. RSFs obtained by McCain et al. [12] are shown by open symbols forcomparison.

References[1] Honda M. and Imamura M. 1971. Phys. Rev. C 4, 1182–1188. [2] Fujiya W. et al. 2012. Nat. Commun. 3, 627. [3] FujiyaW. et al. 2013. Earth Planet. Sci. Lett. 362, 130–142. [4] Jilly C. et al. 2014. Meteorit. Planet. Sci. 49, 2104–2117. [5] Jilly C.
et al. 2017. Geochim. Cosmochim. Acta 201, 224–244. [5] Yokoyama T. et al. 2022. Science eabn7850. [7] Nakamura E. et al.
2022. Proc. Jpn. Acad., Ser. B 98, 227-282. [8] Ichimura K. and Sugiura N. 2015. Abstract #1795 LPSC. [9] Donohue P. et al.
2019. Abstract #1949 LPSC. [10] Sugiura N. et al. 2010. Geochem. J. 44, 11–16. [11] Steele R. et al. 2017. Geochim.
Cosmochim. Acta 201, 245–259. [12] McCain K. et al. 2020. J. Vac. Sci. Technol. B38, 044005 [13] Miyajima Y. et al. 2020.
Geostand. Geoanal. Res. 45, 189–205.