1 High-resolution *in situ* Fe isotope measurements of silicate minerals and glasses by femtosecond laser ablation MC-ICP-MS.

S. RABIN¹,², F.-X. d'Abzac¹, †, J. CHMELEFF¹, F. POITRASSON¹, M. GREGOIRE¹

¹Géosciences Environnement Toulouse, Université de Toulouse, CNRS, IRD, UPS, CNES,
14, av. E. Belin, 31400 Toulouse, France

² Vrije Universiteit Brussel, Analytical, Environmental, and Geo-Chemistry, Pleinlaan 2, 1050 Brussels, Belgium (segolene.rabin@vub.be)

† deceased

ABSTRACT

In this study, we present a high-precision and high-spatial resolution *in situ* Fe isotope protocol using femtosecond (fs) laser ablation multiple collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS). Long term (~ 3 years) external reproducibility obtained on the USGS glass BIR-1G against Putratonic reference standard is 0.17 ‰ (2SD) for δ⁵⁶Fe. Uncertainties achieved on individual analysis of glass and olivines are < 0.15 ‰ for δ⁵⁶Fe. This high precision is associated to high spatial resolution of about 170 x 25 µm. Our results display good consistency between LA-MC-ICP-MS and solution nebulized multiple collector inductively coupled plasma mass spectrometry (SN-MC-ICP-MS) data from the literature. Obtained δ⁵⁶Fe values on different USGS glass (BIR-1G, BHVO-2G and BCR-2G) show that these reference materials have homogenous Fe isotopes compositions and therefore can be used as bracketing standard during laser ablation sessions. On the other hand, the San Carlos Olivine displays high Fe isotope heterogeneity, and therefore cannot be considered as a good bracketing standard.

We also applied our fs-LA-MC-ICP-MS protocol on olivines and pyroxene from the Kerguelen Archipelago. This technique appears to be a relevant tool to resolve isotopic zoning in chemically zoned silicate phenocrysts, even small-sized ones. We demonstrated that within a single lava, olivine crystals display various zoning depending on their size, therefore related to their residence time in the magma. Both equilibrium and diffusive processes were observed in olivine crystals from Kerguelen, uncovering complex histories. Hence, Iron
isotope measurements by fs-LA-MC-ICP-MS open news possibilities for studying highly zoned silicates in magmatic rocks to better understand their formation.

Keywords: Fe stable isotopes, femtosecond, LA-MC-ICP-MS, silicate minerals

## INTRODUCTION

Iron is the fourth most abundant chemical element in the Earth's crust and the most abundant redox-active metal in the entire Solar System (Johnson et al. 2020). Iron is found in a large variety of minerals due to its aptitude to bind with both sulfur and oxygen and is therefore strongly involved in a lot of biological and geological processes on Earth. Iron has four natural stable isotopes $^{54}$Fe, $^{56}$Fe, $^{57}$Fe, $^{58}$Fe with respective mean abundances of 5.80, 91.72, 2.20 and 0.28 %. The most interesting characteristic of iron is its sensitivity to redox changes. Iron has indeed three oxidation states, metallic iron ($Fe^{0}$), ferrous iron ($Fe^{2+}$) and ferric iron ($Fe^{3+}$). Ferric iron, displaying stronger bonds, tends to favor heavy isotopes ($^{57}$Fe) compared to ferrous iron (e.g. Polyakov and Mineev, 2000; Schauble, 2004). Measurements of iron isotope ratio have been developed for various purposes in Earth and planetary sciences and essentially performed on bulk samples. However, to enhance our knowledge of geological processes, there is the need to develop methods to measure iron isotopic ratios at the mineral scale. To do so, in situ techniques were developed such as secondary ionization mass spectrometry (SIMS; see, e.g., Whitehouse and Fedo, 2007) and laser ablation multiple collectors inductively coupled plasma mass spectrometry (LA-MC-ICP-MS). Laser ablation systems coupled to ICP-MS became indeed a center of interest for the scientific community since few decades. In situ iron isotopes measurements using both nanosecond and femtosecond LA-MC-ICP-MS were performed on magnetite to characterize banded iron formations (Czaja et al. 2013; Li et al. 2013; Knipping et al. 2019), on various oxides and hydroxides to characterize hydrothermal ore formation (Horn et al. 2006), on sulfides and iron meteorites (Košler et al. 2005; Chernonozhkin et al. 2017), on micrometeorites (González De Vega et al. 2020), on microtektites to better understand impact events (Chernonozhkin et al. 2021), or on silicates minerals such as olivine in order to constrain kinetic isotope fractionation in magmatic settings (Oeser et al. 2014, 2015, 2018; Collinet et al. 2017). Laser ablation appears to have two major advantages over the conventional
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65 solution nebulization: i) It eliminates the preparation steps of the samples (dissolution, ion chromatography, dilution) that can be time consuming. Laser ablation analyses are fast and only need really small amount (from femtograms to micrograms) of samples with high spatial resolution compared to solution nebulization that need much more material (milligrams). The major limitations of laser ablation system coupled to MC-ICP-MS is the precision of the measurements that is poorer than the one obtained by solution nebulization due to the laser/matter interaction, the efficiency of aerosols transportation (Košler et al. 2005; d'Abzac et al. 2014), the ionization of the produced aerosols in the plasma and the potential matrix effects (e.g. Steinhoefel et al. 2009; Zheng et al. 2018). Much progress has been made in terms of uncertainties improvement that enforce LA-MC-ICP-MS as one the most powerful and advanced analysis technique in laboratories, especially after the emergence of femtosecond laser ablation systems (Poitrasson and d'Abzac, 2017).

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78 Recent studies (Oeser et al. 2014, 2015, 2018; Collinet et al. 2017; Xu et al. 2021) reported indeed in situ iron isotopes measurements using femtosecond laser coupled to MC-ICP-MS with a δ⁵⁶Fe measurement reproducibility < 0.12 ‰. High precision in situ Fe measurements is necessary to study high-temperature processes such as magmatic differentiation, as the Fe isotope fractionation at equilibrium is limited at high temperature (<1 ‰; Johnson et al. 2020). Silicates in magmatic rocks, such as olivine and pyroxene, can display complex histories illustrated by chemical and isotopic zoning. These zonings can be divided in two categories; i) crystal growth zoning, that corresponds to the evolution of the crystal chemistry in the evolving melt during magmatic differentiation. The crystal is then assumed to be in equilibrium with the melt. ii) diffusion zoning, that corresponds to kinetic driven processes (Sio et al. 2013, 2018; Oeser et al. 2014, 2015, 2018; Collinet et al. 2017). In situ Fe isotopes measurements enable the elucidation of genetic versus kinetic processes as diffusion generates strong Fe isotopic fractionation (up to 1.7 ‰) and specific inter-element concentrations and isotopic patterns (Oeser et al. 2015; 2018) whereas equilibrium processes produce limited Fe isotope fractionation (< 0.5 ‰; Polyakov and Mineev, 2000; Rabin et al. 2021) at magmatic temperature.

94 Here, we present a protocol to obtain accurate, precise, and reproducible *in situ* Fe measurements associated to higher spatial resolution than previously published, to be able to study small olivine and pyroxene phenocrysts. First, we discuss the Fe isotope
homogeneity of the USGS glasses as well as the San Carlos olivine, and therefore their potential as reference material for Fe isotope analysis. Then, the application of the laser ablation method on olivine and pyroxenes from Kerguelen Archipelago lavas enable us also to discuss the processes involved in the Fe isotopes variations within olivines of different sizes.

REFERENCE MATERIALS AND SAMPLES

The investigated reference materials include glasses and minerals (olivine) to determine a relevant protocol for iron isotope measurements by fs-LA-MC-ICP-MS in a magmatic context. The USGS reference glasses BIR-1G, BHVO-2G and BCR-2G were used in this study to optimize the analytical conditions. Additionally, we investigated the San Carlos olivine reference material. Puratronic, a metal rod from Alfa Aesar, was used as a bracketing standard as an alternative to IRMM-14 (Horn et al. 2006). Our Puratronic sample was found to be isotopically homogeneous through several lateral profiles and solution nebulization (SN) MC-ICP-MS measurements of an aliquot displaying a $\delta^{56}$Fe of 0.249 ±0.032 ‰ and a $\delta^{57}$Fe 0.389 ± 0.057 ‰ (2SE) relative to IRMM-14. Even though femtosecond laser ablation allows non-matrix matching calibration without analytical bias (Horn et al. 2006; Poitrasson and d'Abzac, 2017), in situ determination of Fe isotopes in most USGS glasses and olivines were carried out relative to the USGS reference material BIR-1G, to minimize laser setting changes between sample and standard since they have closer Fe concentrations than a metal.

In addition, we investigated olivine and pyroxene phenocrysts in two distinct oceanic island basalts (OIB) from Kerguelen Archipelago. The first lava, ARC 603, is an oceanite characterized by a large amount of millimeter-sized olivine crystals. Sample ARC 603 represents a lava erupted around 20 Ma ago and belongs to the Lower Miocene series characterized by a silica-saturated alkaline volcanism (e.g., Weis et al. 1993). This sample comes from an outcrop located near the Sourcils Noirs River in the Jeanne d'Arc Peninsula (49°36′55″S; 69°48′21″E). The second lava sample, RR08-88, is a sub-alkaline basalt
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3 129 belonging to the Mont Ross series (e.g., Weis et al. 1998) and comes from an outcrop
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5 130 located in the Gallieni Peninsula (49°35′00″ S; 69°37′58″ E). This RR08-88 lava erupted during
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7 131 the last basaltic volcanic event recorded in the Kerguelen Archipelago, around 1Ma ago. The
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9 132 olivine phenocrysts in both lavas display only limited chemical zoning with only a small drop
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11 133 of the Mg# in the last 50-100 µm of the rim (Fig 7 to 10).
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13 134
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15 135 METHODS
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18 137 *In situ* Fe isotope determinations by fs-LA-MC-ICP-MS
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20 138
21 139 In situ Fe isotopes measurements were performed using a high mass resolution MC-ICP-MS
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23 140 Thermo Scientific Neptune Plus combined to an ESI "UC Femto dual-wavelength"
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25 141 femtosecond laser ablation system at the Observatoire Midi-Pyrénées, France. The laser
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27 142 pulse amplification system delivers ~280 fs pulses at the fundamental wavelength of 1030
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29 143 nm and the unit can be operated as is. This wavelength can also be converted to a 257 nm
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31 144 beam, by fourth harmonic generation, with a gaussian shape (yield of 35% and a limited loss
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33 145 on the pulse width (< 1%). This last setup was used for this study (Table 1). The polished
34 146 samples and standards are placed in a two volumes ablation cell from ESI (Portland USA),
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36 147 permitting a high sensitivity on rather large samples (thin sections and plots). A beam
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38 148 imaging system controls the spot size (1 to 60 µm) delivered on the samples. The ablated
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40 149 material is transported with 250 ml.min⁻¹¹ +-50 % (depending on conditions) of Helium into
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42 150 a 2.5 m PTFE-inner-coated tubing and is mixed with argon using a Y-shape connector (fig. 1).
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44 151
45 152 The MC-ICP-MS measurements were performed in high resolution and static modes in order
to measure 5 masses of iron and Ni (54, 56, 57, 58 and 60) to correct for instrumental mass-bias with Ni and ⁵³Cr in order to correct for the isobaric interference of ⁵⁴Cr on ⁵⁴Fe. Average
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47 153
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49 154 signal intensities on olivine for ⁵⁴Fe, ⁵⁶Fe, ⁵⁷Fe were about 1 V, 18 V and 0.4 V respectively,
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51 155 using faraday cup amplifiers with 10¹¹ Ohm resistors (Table 1).
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53 156
54 157 The measurements were performed in wet plasma condition to improve the stability of the
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56 158 ICP signal and to diminish the mass-bias fractionation drift during the laser session.
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58 159 Furthermore, the concomitant injection of Ni-spiked solution to the aerosol, enables us to
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60 160 correct for instrumental mass-bias fractionation as demonstrated by Poitrasson and Freydier
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161 (2005) for solution nebulization and by Oeser et al. (2014) for laser ablation. Instrumental mass bias correction using ⁵⁸Ni/⁶⁰Ni following the routine of Oeser et al. (2014) as well as corrections for isobaric interferences (⁵⁸Fe on ⁵⁸Ni, ⁵⁴Cr on ⁵⁴Fe) are detailed in Supplementary Data – 1. In this study, the spiked Ni-solution is added to the mix via a 50 µl.min⁻¹ PTFE nebulizer and a cyclonic spray chamber and using an in-house glass T-connector (Verres Vagner, Toulouse, France) that is directly connected to the MC-ICP-MS torch injector (fig. 1). The shape of the in-house connector was investigated though different configuration (Supplementary Data – 1 – fig. S1) to obtain the most homogenous mix and optimized relative sensitivity between the Ni reference solution and the aerosol containing the ablated particles. Besides the coolant argon gas flow going through the MC-ICP-MS torch gas flow (14 L.min⁻¹), additional gases consist in the torch auxiliary Ar flow of ~0.59 L.min⁻¹, the sample Ar flow of ~0.62 L.min⁻¹ and the Ar flow of ~0.46 L.min⁻¹ added in order to nebulize the Ni-spiked solution (Table 1). Moreover, the ablation cell is swiped by ~250 ml/min.of He. The nickel reference solution used is an atomic absorption solution or NIST SRM 986 (depending on the session), at a concentration of 5 or 10 ppm depending on the the cyclonic spray chamber used (Glass Expansion Cinnabar and Glass Expansion Twinabar – 20mL) to obtain a signal of at least 10 V for ⁵⁸Ni and 4 V for ⁶⁰Ni respectively. The analysis of minerals and glasses containing Ni within their structures (10-3000 ppm) generated a maximum increase of the Ni signal by 2 %, which can be considered as negligible compared to the total Ni signal (Oeser et al. 2015). Also, the samples might have variable Ni isotope signatures but the uncertainties related to the potential presence of Ni in the samples is much smaller (< 0.02 ‰; Oeser et al. 2015) than the overall uncertainties of the measurements (0.1‰), considering the range of Ni isotopes fractionation observed in terrestrial rocks (< 2‰; Gueguen et al. 2013).

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186 Individual analyses consisted of one block of 60 cycles with a cycle integration time of 2.09s, including ~ 8 cycles of background (gas blank, dwell time) at the beginning of each analysis (laser off; Fig. 2).

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190 The δ⁵⁶Fe values relative to the reference material (BHVO-2G or Puratronic) were calculated as follow:
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$$ \delta^{56}\text{Fe} = \left( \frac{\frac{^{56}\text{Fe}}{^{54}\text{Fe}_{\text{sample}}} - 1}{\frac{^{56}\text{Fe}}{^{54}\text{Fe}_{\text{ref. mat.}}}} \right) \times 1000 \qquad (1) $$

Three samples were bracketed between two reference material analyses. Finally, the $\delta^{56}\text{Fe}$ were recalculated relative to IRMM-014, the international standard for iron isotopes using this approximation:

$$ \delta^{56}\text{Fe}_{\text{sample/IRMM-014}} = \delta^{56}\text{Fe}_{\text{sample/ref. mat.}} + \delta^{56}\text{Fe}_{\text{ref. mat./IRMM-014}} \qquad (2) $$

where $\delta^{56}\text{Fe}_{\text{sample/ref. mat.}}$ correspond to $\delta^{56}\text{Fe}_{\text{sample/BHVO-2G.}}$ or $\delta^{56}\text{Fe}_{\text{sample/Puratronic}}$ that represent the $\delta$ values measured relative to BHVO-2 or Puratronic. The value of $\delta^{56}\text{Fe}_{\text{BHVO_2G/IRMM-014}}$ were taken from Craddock and Dauphas (2011) and the value of $\delta^{56}\text{Fe}_{\text{Puratronic/IRMM-014}}$ from our solution nebulization MC-ICP-MS analyses (See Reference materials and samples part above).

The overall uncertainty (2SE) for the $\delta^{56}\text{Fe}$ values relative to the reference material is determined by propagating the analytical errors of the bracketing standard with those of the samples (Horn et al. 2006).

For both standards and samples analysis, the laser parameters were set up in order to obtain a fluence below 3 J.cm$^{-2}$. A laser rate of 10-15 Hz and a spot diameter of 13 µm for the Puratronic reference material resulted in signal intensities of 12 V on mass 56. However, silicate glasses and minerals have much lower iron concentration compared with metallic iron. Therefore, the laser rate was increased up to 20 Hz as well as the spot diameter (between 24 and 27 µm) to match the Fe signal intensity between the USGS reference glasses/silicate minerals and the metallic reference material. Variations of repetition rates during fs-LA-MS-ICP-MS session does not affect the isotope ratio measurements and no matrix effect is observed during femtosecond laser analysis (e.g. Steinhoefel et al. 2009; Oeser et al. 2014; Poitrasson and d'Abzac, 2017). However, such differences in ablation parameters increase the time between reference materials and samples analysis as the laser system needs to reset the parameters. To increase the productivity of data acquisition and
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223 given the isotopic homogeneity of USGS glass reference material, BIR-1G was used as a reference material for standard bracketing during olivine and pyroxene analysis towards the end of this study.

226 All the samples and references materials were analyzed in the "line-mode" of the laser ablation system, except for one Kerguelen olivine (RR08-88-017) that was also analyzed in the "spot-mode" for comparison. The USGS glasses measurements were conducted in "line-mode" with a frequency of 12-20 Hz, a spot size of 24-27 µm and a laser progression speed (sweeping rate) of 8 µm/s. The line-mode in olivine was conducted with a frequency of 15-20 Hz, a spot size of 12-30 µm and a laser progression speed (sweeping rate) of 8 µm/s. The resulting line lengths ranged between 129 and 265 µm during olivine analysis, whereas they were about 170 µm for USGS reference material analysis. Typically, 3-4 laser paths were performed during each analysis. The signal intensity appears to be stable during the "line-mode" ablation. However, each new path is characterized by a small decrease of intensity as the line grove gets slightly deeper with increasing number of passes (Fig. 2). This incremental decrease has no effect on the isotope ratios which remain fairly stable during the analysis (Fig. 2). The spot-mode in Kerguelen olivines were conducted with a frequency of 10 Hz and a spot size of 23-30 µm. The "spot-mode" allows a better lateral spatial resolution. However, the signal drastically decreases as the pit is getting deeper. In the "spot-mode", the ⁵⁶Fe/⁵⁴Fe appears less stable than in the "line-mode".

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Overall, the laser parameters selected for the Kerguelen olivine and reference material allowed us to obtain the highest and more stable signal according to the MC-ICP-MS sensitivity while achieving high spatial resolution.

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Chemical Fe-Mg zoning determination in Kerguelen olivines and pyroxenes from Kerguelen Archipelago

249
The FeO and MgO profiles of the silicate minerals were determined with the electron probe micro-analyzer (EPMA) SXFive - CAMECA at the Centre de microcaractérisation Raimond Castaing, Toulouse. The EPMA was operated at a beam current of 20 nA and an accelerating voltage of 15 kV. The counting time was 10 s on the peak for each element and 5 s on each side of the peak for the background. The Fe and Mg concentration profiles were performed
from the center to the rim of the minerals with associated analytical uncertainties of ± 0.15 and ± 0.17% respectively, next to the fs-LA measurements.

# RESULTS

## Reference material

The measured *in situ* δ⁵⁶Fe values for the USGS reference glasses BIR-1G, BCR-2G and BHVO-2G are given in Table 2 and illustrated in Fig 3. Using the protocol described in the method section, internal precision during a single analysis (~ 52 cycles) was typically lower than 0.15‰ (2SE) for δ⁵⁶Fe/δ⁵⁴Fe. The measurements reproducibility, based on repeated analyses of the reference material Puratronic and BIR-1G (n= 124) over a period of three years, was better than 0.17 ‰ (2SD) for δ⁵⁶Fe (Table 2). The Fe isotopes variations observed in “line-mode” across the USGS glasses was smaller than 0.1 ‰ for an area of about 170 x 24-27 µm. To access any heterogeneity within the glasses and the San Carlos olivine, Fe measurements were evenly distributed over the individual glass and mineral fragments. All glasses were at least measured 22 times. The *in situ* Fe measurements of the USGS glasses all fall in the terrestrial mass-dependent fractionation line (MFL) with a slope of 1.49 (Fig. 4, e.g., Beard and Johnson, 2004; Steinhoefel et al. 2009).

*In situ* iron isotopes data of the four USGS reference glasses BHVO-2G, BIR-1G and BCR-2G yielded δ⁵⁶Fe values of 0.01 ± 0.09 ‰, 0.15 ± 0.18 ‰ and 0.02 ± 0.10 ‰, respectively (Table 2). The value of BHVO-2G, BIR-1G and BCR-2G agreed with the values determined by SN-MC-ICP-MS and fs-LA-MC-ICP-MS from the literature (Fig. 2; e.g., Dideriksen et al. 2006; Weyer and Ionov, 2007; Schuessler et al. 2009; Wombacher et al. 2009; Craddock and Dauphas, 2011; Millet et al. 2012; M Oeser et al. 2014; Williams and Bizimis, 2014; Nebel et al. 2015; Peters et al. 2019; González De Vega et al. 2020; Xu et al. 2021). This confirms that *in situ* Fe isotopes compositions determined using this fs-LA-MC-ICP-MS protocol are accurate. The precision and reproducibility of the data were monitored using BIR-1G on a period of 3 years (fig. 5). A total of 124 analysis were performed across multiple fragments of BIR-1G glasses. The δ⁵⁶Fe measured on BIR-1G (fig. 5b) shows a normal distribution given the statistical Shapiro-Wilk test (p-value of 0.69). The normal distribution of the data associated to the
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287 small variability of the δ⁵⁶Fe measured on BIR-1G on two different fragments over a period of 3 years suggest that this USGS glasses can be used as bracketing standard for Fe isotopes analysis. The long-term reproducibility of BIR-1G is 0.17 ‰ (2SD), over a period of three years, with an internal precision of individual analysis that vary between 0.09 and 0.2 ‰ (2SD). This shows that the Fe isotopes determination by fs-LA-MC-ICP-MS is precise enough for high temperature geological studies (see also Oeser et al. 2014, 2015).

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The San Carlos olivine (SCO) was also investigated as a potential reference bracketing standard for silicate minerals analysis and displayed a δ⁵⁶Fe values of 0.11 ± 0.30 ‰. This value is consistent with literature δ⁵⁶Fe values of San Carlos Olivine 0.034 ± 0.038 ‰ (Fo₈₈) and 0.015 ± 0.038 ‰ (Fo₉₀; Sio et al. 2013). However, the δ⁵⁶Fe measured on SCO (fig. 6) does not show a unimodal, normal distribution given the statistical Shapiro-Wilk test (p-value of 0.3) underlining the fact that the δ⁵⁶Fe values within a single crystal of SCO are widespread. The 102 analysis of SCO show indeed a significant δ⁵⁶Fe variations from -0.18 ± 0.13 ‰ to 0.541 ± 0.12 ‰ (fig. 6). This shows that San Carlos Olivines does not have homogenous Fe isotope composition and thus cannot be used as bracketing standard in a straightforward way.

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305 Kerguelen minerals
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The measured *in situ* δ⁵⁶Fe variations within olivine and pyroxene crystals are illustrated in figure 7, 8, 9 and 10. The Fe isotope variations observed in line modes across silicate minerals was smaller than 0.15 ‰ for areas ranging between 129 x 12 to 265 x 29 µm. In spot mode, the ⁵⁶Fe/⁵⁴Fe variations across olivine is about 0.2 ‰ for a spot size of 23-30 µm. This higher Fe isotope variations is caused by the decrease of intensity during the analysis as the laser pit gets deeper as observed for Si isotopes (Guitreau *et al.* 2020). Nonetheless, both line and spot modes give similar δ⁵⁶Fe values across olivine 7 in the RR08-88 basalt (fig. 7). This millimeter sized olivine displays a mean δ⁵⁶Fe value of 0.12 ± 0.23 ‰ in the core, from 600 to 350 µm, followed by a steady decrease of the δ⁵⁶Fe value down to negative values -0.22 ± 0.14 ‰ in the last 100 µm of the rim (fig. 7). Olivine 2 in the RR08_88 basalt displays uniform δ⁵⁶Fe value from the core to the rim of about -0.39 ± 0.12 ‰ (fig. 8). On the other hand, the Mg number (Mg#) profile on Olivine 2 displays a chemical diffusion pattern
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3 319 with the rim displaying lower Mg# (0.55) than the core (0.6). This confirms Oeser et al.
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5 320 (2015) observation that isotopic and chemical zoning are not systematically related.
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7 321 The large olivine phenocryst, olivine 1, from ARC 603, displays heavy δ⁵⁶Fe values in the core
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9 322 up to 0.64 ± 0.09 ‰ and sharp decrease down to 0.19 ± 0.09 or, on the other side of the
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11 323 crystal, 0.34 ± 0.08 ‰, in the last 300 µm (fig. 9a). The millimeter-sized olivine, olivine 2 and
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13 324 olivine 3, in the ARC 603 picrite, display similar δ⁵⁶Fe variations but with a lighter Fe isotope
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15 325 core of about 0.1 ‰ associated to a sharp decrease of the δ⁵⁶Fe in the last 300 µm of the rim
down to -0.1 and -0.3 ‰ (fig. 9b and 9c). The Mg# profiles of these three olivine mirrors well
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18 327 the Fe isotopic variations with lower Mg# in the rim than in the core.
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20 328
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22 329 The Fe isotopic profile performed in the ARC 603 pyroxene shows a reverse isotopic zoning
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24 330 compared to the olivine. The pyroxene core displays a δ⁵⁶Fe value of -0.52 ± 0.12 ‰ while
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26 331 the last 100 µm forming the rim displays heavier δ⁵⁶Fe values of about -0.14 ± 0.16 ‰ and -
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28 332 0.31 ± 0.08 ‰ (fig. 10). These variations are mirrored by the Mg# evolution with a core of
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30 about 0.75 and a rim as low as 0.6.
31 334
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33 335 Overall, the Fe isotope variations within individual Kerguelen olivine phenocrysts are small
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35 336 but may show significant variations given the improved precision of individual
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37 337 measurements in line mode, (See Figs. 7-10), associated with high spatial resolution.
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39 338 Although Fe isotope compositions of the olivine from Kerguelen lavas vary from one to the
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41 339 other, the range and variations of δ⁵⁶Fe measured are consistent with olivine δ⁵⁶Fe value
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43 340 from other localities (e.g. Teng et al. 2011; Oeser et al. 2015, 2018; Collinet et al. 2017; Sio
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45 and Dauphas, 2017).
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48 342
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52 345
53 346 *In situ* Fe compositions of USGS glasses obtained by fs-LA-MC-ICP-MS exhibit good
consistency with SN- and LA-MC-ICP-MS literature data. This indicates that Fe isotope
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57 347 analysis of silicates by fs-LA-MC-ICP-MS are accurate and robust. The isotopic patterns found
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59 348 in olivines are also consistent with previous works. The good agreement between
60 349 femtosecond laser ablation and solution nebulization also demonstrated that there is no

DISCUSSION

*In situ* Fe compositions of USGS glasses obtained by fs-LA-MC-ICP-MS exhibit good consistency with SN- and LA-MC-ICP-MS literature data. This indicates that Fe isotope analysis of silicates by fs-LA-MC-ICP-MS are accurate and robust. The isotopic patterns found in olivines are also consistent with previous works. The good agreement between femtosecond laser ablation and solution nebulization also demonstrated that there is no
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351 resolvable composition-dependent matrix effect, considering our analytical precision, as demonstrated by previous studies (e.g. Horn and von Blanckenburg, 2007; Steinhoefel et al. 2009; Oeser et al. 2014). The *in situ* data appears to be less precise than SN-MC-ICP-MS measurements, but the spatial resolution is much higher than what can be achieved by micro-drill (300 µm; Sio et al. 2013). In RR08-88-O17, the spatial resolution is 129 x 12 µm in line-mode and 23-30 µm in spot-mode, with measurement uncertainties of ~ 0.15 ‰ and ~ 0.20 ‰, respectively. Moreover, during USGS reference material analysis, the spatial resolution is 170 x 24-27 µm associated to long term uncertainties down to ~0.1‰ (Table 2). These high reproducibility and high spatial resolution allow the analysis of zoned Fe-bearing minerals and relatively small crystals (> 200 µm). Line-mode should be favored, when the size of the samples allows it, as it produces better precision measurements. Therefore, small Fe isotopes variations can be better determined. Spot-mode should be reserved to small samples that display Fe isotopes variations higher than 0.2‰, and if the sample thickness allows it. Compared to MC-SIMS (Marin-Carbonne et al. 2011; Sio et al. 2013) and nanosecond laser ablation MC-ICP-MS (e.g., Sio et al. 2013), fs-LA-MC-ICP-MS analyses provide smaller long-term uncertainties (see Table 2; ~0.1-0.2 ‰ against ~0.3-0.4‰ (2SD) for the two other techniques) and above all, it does not necessitate matrix-matching between samples and reference material. Moreover, the spatial resolution achieved in our study on silicate minerals is often higher than ns-LA-MC-ICP-MS (20-50 µm), MC-SIMS (50 µm) and previous fs-LA-MC-ICP-MS (200x50µm, Oeser et al. 2014; 200x40-50 µm, Oeser et al. 2015; 150-200 x 40 µm, Collinet et al. 2017) studies.

372
373 In situ Fe isotopes measurement of USGS reference materials BIR-1G, BHVO-2G and BCR-2G demonstrated that these glasses are homogenous. They can thus be used as bracketing standard for Fe measurements by fs-LA-MC-ICP-MS. However, investigation of San Carlos Olivine demonstrated intra-grain Fe isotope heterogeneities (fig. 6). The only feasible way to use the SCO as a reference standard would be to isolate the core of the crystals that appears to be homogenous. But this is risky and again, matrix-matched calibration is not required for fs LA-MC-ICP-MS Fe isotope analysis.
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380
381 Application to Kerguelen silicates minerals
<table>
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    <td>382</td>
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    <td>383</td>
    <td>In the past years, studies have demonstrated that chemical diffusion at magmatic temperature generates Fe isotopes fractionation in olivine that exceeds the expected equilibrium isotopes fractionation (e.g., Teng et al. 2011; Oeser et al. 2014, 2015, 2018; Collinet et al. 2017; Sio and Dauphas, 2017). The diffusion-driven fractionation of Fe isotopes observed in olivine is up to 1.7 ‰ (Oeser et al. 2015). In this study, we investigated Fe chemical and isotopic zoning in olivine phenocrysts from Kerguelen Archipelago. It appears that the Fe isotopes variations in olivine crystals seem to be correlated to the olivine phenocryst size, and thus to the crystallization stage events. Three olivine categories can be established based on size and Fe chemical and isotope compositions: i) The pluri-millimeter sized olivine, O1 in Arc 603, displays normal zoning with lighter Fe isotope composition and lower Mg# in the outer rim (0.73 Compared to 0.78 in the core; fig. 9a). However, the δ<sup>56</sup>Fe values of the core is really heavy (~ 0.5 ‰) and in the opposite direction expected for equilibrium Fe isotopes fractionation between olivine and melt. Indeed, forsterite is assumed to have slightly lighter Fe composition than the melt (Rabin et al. 2023). Iron diffusion out of the olivine can possibly explain heavy Fe isotope compositions of this olivine core. As previously suggested by Oeser et al. (2015), a magma mixing event with hotter and more primitive magma could generate such Fe isotope fractionation; ii) Millimeter sized olivines (O12, O13 in ARC 603 and O17 in RR08-88; fig. 7 and fig. 9b, c) exhibit normal zoning although the δ<sup>56</sup>Fe variations are barely resolvable outside analytical uncertainties. The cores exhibit δ<sup>56</sup>Fe values of ~0.10-0.12 ‰ (Mg# = 0.76-0.61) that corresponds to the expected δ<sup>56</sup>Fe value of olivine in equilibrium with the melt. The δ<sup>56</sup>Fe values are 0.15-0.36 ‰ lighter in the rims than in the cores, associated to lower Mg# (0.64-0.46). These small variations, considering the analytical uncertainties (~0.1-0.2‰), are also coherent with Fe equilibrium fractionation. Accordingly, Rabin et al. (2021) computed that olivines with a higher Fe content should display lighter isotopic compositions than Fe-poor olivine (Forsterite). A prolonged crystal growth in an evolving melt during magmatic differentiation, without diffusion exchange between the crystal and the melt, could indeed lead to the growth of a rim with higher Fe iron concentration and thus slightly lighter isotopic composition. Then, these olivines are most likely in Fe isotopic equilibrium. iii) The small olivine crystal (RRO08-88-O12; fig. 8), smaller than 500 µm, displays a homogenous δ<sup>56</sup>Fe value (-0.39 ± 0.12 ‰), indicating no resolvable Fe isotopic zoning. Therefore, the low δ<sup>56</sup>Fe value (~0.39 ± 0.12 ‰), indicating no resolvable Fe isotopic zoning. Therefore, the low δ<sup>56</sup>Fe</td>
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414 value of this olivine can be considered as generated by equilibrium processes as it is unlikely that diffusion would have completely overprinted the core signature of the olivine. The small size of this olivine phenocryst as well as its low Mg# (0.53-0.60) indicate that it rather represents a later stage of crystallization relative to the mm-sized olivine cores.

418
419 The ARC 603 pyroxene displays lower δ⁵⁶Fe in the core than mm-sized ARC 603 olivines (Compare Fig. 10 with Fig. 9b, c, respectively), which is coherent with recent theoretical calculation showing that Fe²⁺ bearing pyroxene displays lighter Fe isotopes signatures than forsterite (Rabin et al. 2021). This pyroxene displays, in contrast to the olivines, Fe isotope zoning with heavier δ⁵⁶Fe in the rim (-0.14 ± 0.16‰ and -0.31 ± 0.08‰) than in the core (-0.52 ± 0.12 ‰). This Fe isotope zonation is associated with an inversed Mg# zoning with lower Mg# (0.60) in the rim than in the core (0.77). These δ⁵⁶Fe and Mg# variations can be explained by equilibrium processes as the chemical evolution of pyroxenes during crystal growth tends to favor ferric iron in their structure as the magma evolves. The incorporation of ferric iron generates a strong enrichment in heavy Fe isotopes in the pyroxene rims compared to the core. The pyroxene Fe isotope variations, of about 0.2-0.3‰, can be then also interpreted as Fe isotopes equilibrium fractionation during magmatic differentiation.

431
432 Overall, the measurement of Fe isotope variations in the different sizes of olivines are reflecting multi-stage olivine crystallization during the formation of these basalts. In situ Fe isotope measurements of olivine can indeed highlight complex events related to the lava formation, such as magma mixing in ARC 603 basalt. On the other side, Fe isotope measurements of silicates (olivines, pyroxenes) can also underline equilibrium processes as in RR08-88 and ARC 603 basalts, showing that diffusion processes, even if common, may not be systematic in natural olivines and pyroxenes.

439
440 CONCLUSION

441
442
443 We report a high spatial resolution protocol for Fe isotope measurements in silicate minerals and glasses by femtosecond laser ablation MC-ICP-MS. This technique offers an additional approach to existing methods to measure Fe isotopes variations within silicates.
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446 Measurements in line and spot modes appears to both provide precise Fe isotope data.

447 Nonetheless, line-mode analyses are preferred as it achieves better precision. The precision

448 for USGS reference glasses is < 0.10 ‰, associated to a long-term reproducibly of 0.17 ‰.

449 Moreover, errors achieved on individual analysis of olivines are inferior to 0.15 ‰ (2SD). This precision is associated to high resolution, with line as small as 129 µm, and 12 µm.

450 Therefore, this method can resolve olivine profile with a resolution < 30 µm if the lines are

451 taken perpendicular to the profile. This technique will enable the study of wide range of

452 phenocryst size and highly zoned crystal, such as pyroxene, for a better understanding of

453 magma genesis and magmatic differentiation.

454

456 ACKNOWLEDGEMENTS

457

458 We are grateful to Ingo Horn and Martin Oeser for their precious help and advice during the

459 development of the *in situ* iron isotopes MC-ICP-MS methods at the GET laboratory. We

460 thank Bertand N. Moine and Hervé Leyrit for providing samples of the Kerguelen

461 Archipelago. We also thanks Phillipe de Parseval for his help with the EPMA analysis. SR

462 thanks Lisa Krämer-Ruggiu for the BSE imaging of the silicate minerals. This work was

463 supported by the Programme National de Planétologie (PNP) of CNRS/INSU, co-funded by

464 CNES, to FP.

465

466

467 BIBLIOGRAPHY

468

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DATA AVAILABILITY STATEMENT

604

605 The data that support the findings of this study are available from the corresponding author

606 upon reasonable request.

607
<figure><img src="image_1.png" /><figcaption>Figure 1 - Schematic drawing of the set up permitting the addition of a Ni standard solution to the plasma during laser ablation.</figcaption></figure>

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<figure><img src="image_2.png" /><figcaption>Figure 2 - Signal and isotope ratio evolutions throughout a typical analysis of the USGS geostandard BIR-2G in raster mode using the following analytical conditions: energy of 17%, spot size of 24 µm, frequency of 12 Hz, and sweeping rate of 8 µm/s.</figcaption></figure>
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<figure><img src="image_3.png" /><figcaption>Figure 3 : Iron isotope compositions of three USGS references glasses (BHVO-2, BIR-1 and BCR-2) determined by fs-La-MC-ICP-MS in this study (calibrated initially against puratronic and recalculated relative to IRMM-14) compared with data from fs-La-MC-ICP-MS (highlighted with an asterisk) of Oeser et al. (2014), Gonzales de Vega et al. (2020), Xu et al. (2021) and rock powders analyzed in other studies Craddock and Dauphas (2011), Millet et al. (2012), Nebel et al. (2015), Peters et al. (2019), Dideriksen et al. (2006), Weyer and Ionov (2007), Wombacher et al. (2009), Schuessler et al. (2009), Williams and Bizimis (2014).</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>Figure 4 - Three-isotopes plot for iron of the United States Geological Survey (USGS) Reference glasses (BIR-1G, BHVO-2G, BCR-2G) analyzed by fs-LA-MC-ICP-MS. The line represents the mass-dependent fractionation line (MFL) with a slope of 1.49 (Beard and Johnson 2004, Steinhoefel et al. 2009b).</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Figure 5 – Characteristics of BIR-2G USGS standards: a) Long-term (> 2 years) reproducibility of Fe isotope signatures in BIR-2G (recalculated against IRMM-14); b) Histogram displaying the frequency distribution of the δ<sup>56</sup>Fe data for BIR-2G.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>Figure 6 - Iron isotopes compositions (a) and frequency distribution (b) of the San Carlos Olivine from *in-situ* fs-LA-MC-ICP-MS measurements (n = 102).</figcaption></figure>

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<figure><img src="image_7.png" /><figcaption>Figure 7 - Iron isotopes profile across olivine O17 in an alkaline basalt from Kerguelen Archipelago (RR08-88) obtained with raster analysis (blue circle) and spot (green diamond) using the following analytical condition for the raster analysis: spot size of 20 µm, frequency of 20 Hz, sweeping rate of 8 µm/s, and for the spot analysis: spot size of 23 µm and frequency of 10 Hz. Imaging of the olivine with the spot and line analyses profiles is displayed in Supplementary Data 1 - fig S2.</figcaption></figure>
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<figure><img src="image_8.png" /></figure>

Figure 8 - Iron isotopes profile across olivine Ol-2 in an alkaline basalt from Kerguelen Archipelago (RR08-88) obtained with raster analysis using the following analytical condition; spot size of 17 µm in the core and 20 µm at the rim, frequency of 20 Hz, and sweeping rate of 8 µm/s. Imaging of the olivine with the line analyses profile is displayed in Supplementary Data 1 - fig S3.
<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /></figure>

Figure 9 - Iron isotopes profile across olivines O1 (a), O12 (b), profile performed from the right to the left) and O13 (c) in an alkaline basalt from Kerguelen Archipelago (Arc 603) obtained with raster analysis using the following analytical condition; energy of 23%, spot size of 25 µm, frequency of 15 Hz, and sweeping rate of 8 µm/s. Imaging of the olivines with the line analyses profile is displayed in Supplementary Data 1 - Fig S4, S5 and S6.
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<figure><img src="image_12.png" /><figcaption>Figure 10 - Iron isotopes profile across pyroxene Px1 in an alkaline basalt from Kerguelen Archipelago (Arc 603) obtained with raster analysis using the following analytical condition: spot size of 25 µm, frequency of 15 Hz, and sweeping rate of 8 µm/s. Imaging of this pyroxene with the line analyses profile is displayed in Supplementary Data 1 - fig S7.</figcaption></figure>
<table><thead><tr><th></th><th></th></tr></thead><tbody><tr><td><strong>Laser ablation system</strong></td><td><strong>ESI "Femto UC"</strong></td></tr><tr><td>Wavelength</td><td>257 nm</td></tr><tr><td>Ablation Cell</td><td>Two volumes (ESI)</td></tr><tr><td>Helium carrier +-50%</td><td>250ml.min-1</td></tr><tr><td>Argon (cool/sample/aux) +-10%</td><td>14/0.62/0.59 L.min<sup>-1</sup></td></tr><tr><td>Acquisition pattern</td><td>Single spot and line patterns, 60 cycles of 2s</td></tr><tr><td>Dwell time</td><td>8s</td></tr><tr><td>Wash time</td><td>15s</td></tr><tr><td><u>Raster parameter</u></td><td></td></tr><tr><td>Repetition rate</td><td>20 Hz</td></tr><tr><td>Speed</td><td>8 µm.s<sup>-1</sup></td></tr><tr><td>Fluence (ref. material/sample)</td><td>2 / 2.5 J.cm<sup>-2</sup></td></tr><tr><td><u>Spot parameter</u></td><td></td></tr><tr><td>Repetition rate</td><td>35 Hz</td></tr><tr><td>Fluence (ref. material/sample)</td><td>2/2.5 J.cm<sup>-2</sup></td></tr><tr><td>Nebulizer</td><td>Micromist 50µl.min<sup>-1</sup></td></tr><tr><td>Ni concentration (in HNO3 0,35N)</td><td>5-10 ppm</td></tr></tbody></table>

<table><caption>Table 1 - Analytical conditions for femtosecond laser ablation - MC-ICP-MS iron isotopes measurements.</caption><thead><tr><th></th><th></th></tr></thead><tbody><tr><td><strong>MC-ICP-MS</strong></td><td><strong>ThermoFinnigan Neptune Plus</strong></td></tr><tr><td>Cup configuration</td><td><sup>53</sup>Cr (L4), <sup>54</sup>Fe (L2), <sup>56</sup>Fe (L1), <sup>57</sup>Fe (H1), <sup>58</sup>Fe (H2), <sup>60</sup>Ni (H4)</td></tr><tr><td>Amplifiers</td><td>10e11 Ohms</td></tr><tr><td>Mass resolution</td><td>High, Resolving power > 6500</td></tr><tr><td>RF Power</td><td>1200 W</td></tr><tr><td>Types of cones</td><td>Sample : normal, Skimmer: X</td></tr><tr><td>Nebulizer</td><td>Micromist 50µl.min<sup>-1</sup></td></tr><tr><td>Spray chamber</td><td>Glass Expansion cinnabar or twinabar cyclonic 20ml</td></tr><tr><td>Argon +-10%</td><td>0.46 L.min<sup>-1</sup></td></tr><tr><td>Y connector</td><td>in house</td></tr></tbody></table>
<table><thead><tr><th>Sample</th><th>δ<sup>56</sup>Fe<br>(‰ LAS)</th><th>2 SD</th><th>δ<sup>57</sup>Fe<br>(‰ LAS)</th><th>2 SD</th><th>δ<sup>56</sup>Fe<br>(‰ IRMM-14)</th><th>2 SD</th><th>δ<sup>57</sup>Fe<br>(‰ IRMM-14)</th><th>2 SD</th><th>n</th><th>Primary reference<br>material used</th></tr></thead><tbody><tr><td>BHVO-2G</td><td>-0.24</td><td>0.09</td><td>-0.45</td><td>0.17</td><td>0.01</td><td>0.09</td><td>-0.08</td><td>0.19</td><td>22</td><td>Puratronic</td></tr><tr><td>BIR-1G</td><td>0.02</td><td>0.17</td><td>-0.02</td><td>0.30</td><td>0.15</td><td>0.18</td><td>0.16</td><td>0.30</td><td>126</td><td>BHVO-2G</td></tr><tr><td>BCR-2G</td><td>-0.09</td><td>0.10</td><td>-0.11</td><td>0.27</td><td>0.02</td><td>0.10</td><td>0.05</td><td>0.18</td><td>26</td><td>BHVO-2G</td></tr></tbody></table>

Table 2 - Iron isotope compositions of USGS references glasses. δ<sup>56</sup>Fe (‰ LAS) and δ<sup>57</sup>Fe (‰ LAS) represent the δ<sup>56</sup>Fe and δ<sup>57</sup>Fe relative to the primary reference material used during the LA-MC-ICP-MS analytical session. δ<sup>56</sup>Fe (‰ IRMM-14) and δ<sup>57</sup>Fe (‰ IRMM-14) are the recalculated value relative to IRMM-014 with a value of δ<sup>56</sup>Fe<sub>BHVO-2/IRMM-14</sub> = 0.114 ± 0.011 ‰, and δ<sup>57</sup>Fe<sub>BHVO-2/IRMM-14</sub> = 0.174 ± 0.016 ‰ for BHVO-2 (Craddock and Dauphas, 2011) and δ<sup>56</sup>Fe<sub>Puratronic/IRMM-14</sub> = 0.249 ± 0.032 ‰ and δ<sup>57</sup>Fe<sub>Puratronic/IRMM-14</sub> = 0.389 ± 0.057 ‰ for our Puratronic sample (this study), both obtained by solution mode MC-ICP-MS after chemical purification. Measurements uncertainties (2SD) of the δ-values represents the two standard deviations based on replicate analyses (n) within the analytical sessions (over 3 years). The uncertainties of the re-calculated δ<sup>56</sup>Fe (‰ IRMM-14) and δ<sup>57</sup>Fe (‰ IRMM-14) are propagated uncertainties related to the laser ablation (SD<sub>LAS</sub>) and of those related to the solution nebulization (SD<sub>sol</sub>) measurements of δ<sup>56</sup>Fe<sub>BHVO-2/IRMM-14</sub> and δ<sup>57</sup>Fe<sub>BHVO-2/IRMM-14</sub> where 2SD = 2 × √SD<sub>sol</sub><sup>2</sup> + SD<sub>LAS</sub><sup>2</sup>.