Fe-Ni-P-S Melt Pockets in Elga IIE Iron Meteorite: Evidence for the Origin at High-Pressures Up to 20 GPa †

: Here we report new data on high-pressure microstructures in Elga group IIE iron meteorites, made of solidiﬁed Fe-Ni-P-S melt pockets and microcrystalline aggregates, which could be formed only at high pressures and temperatures according to the experimental data. The bulk composition of the melt pockets and crystals correspond to the Fe 3 P-Fe 3 S solid solution with the closure of an immiscibility gap at pressures near 20 GPa in static experiments. Some other melt pockets ﬁt with the Fe 2 S-Fe 2 P compositions, which could also correspond to high pressures and temperatures. The results suggest a late shock episode during the formation of the IIE iron parent body, which may be prior or due to the ﬁnal disruption that caused the meteorite arrival to Earth. It also has an important implication to the shock features in other meteorites, such as ureilite.

Here for the first time, we report new evidence for high-pressure microstructures in Elga, made of Fe-Ni-P-S aggregates, which could be formed only at high pressures and temperatures according to the experimental phase diagrams [19,20].

Materials and Methods
The Elga meteorite was found in 1959 in Yakutia (Russia) [11,17]. The main mass of 23 kg is in the Central Siberian Geological Museum, Novosibirsk. We studied six polished slices of the meteorite from the working collection, 5-7 g each. Polished sample sections ( Figure 1) were characterized using a Tescan MYRA 3 LMU scanning electron microscope coupled with an INCA energy-dispersive X-ray microanalysis system 450 equipped with the liquid nitrogen-free Large area EDS X-Max-80 Silicon Drift Detector (Oxford Instruments) at the Sobolev Institute of Geology and Mineralogy, Novosibirsk. The energy-dispersive X-ray spectra (EDS) were collected by sharp 1-2 µm or scattered over a sample area electron beam at 20 kV accelerating voltage and 1 nA beam current. Counting times for spectra collection were 30-60 s. No beam damage or change in measured composition with time was observed when using the current setting. The EDS spectra were optimized for the quantification using the standard XPP (exponential model of Pouchou and Pichoir matrix correction) procedure included in the INCA Energy 450 software. Calibrated EDS spectra from the used device were reported to be of the same or even better quality as the wavelength dispersive analysis in many recent work [21][22][23]. In addition to standard minerals, oxides, and metals, mentioned in these papers, we used synthetic Fe 3 P, Ni 3 P, and FeS as internal standards for EDS analyses.
The crystal structure of the single crystal and nanocrystalline (Fe,Ni) 3 (P,S) was confirmed using the Electron Backscatter Diffraction (EBSD) of the chemo-mechanically polished plates. Colloidal silica was used for the final sample polishing. The EBSD data were collected on a Hitachi S-3400 N scanning electron microscope equipped with an Oxford Instruments HKL detector with an accuracy of misorientations of 0.5-1.0 • at 20 kV accelerating voltage, 8 nA beam current, 15.5 mm working distance, and 70 • tilt angle. The Kikuchi patterns of individual points were automatically indexed by the Oxford data collection software. The spatial resolution of EBSD was less than 1 µm.

Results
Schreibersite and troilite in Elga often form rim zones around silicate inclusions and located in fractures along the metal sub-grains ( Figure 1). Representative compositions of the primary phases are shown in Table 1. In the fractures, schreibersite and troilite are severely crumpled and re-melted  Various Fe-Ni-P and Fe-Ni-P-S melt pockets can be distinguished in the fractures and near silicate inclusions (Figures 2-7). In this paper, we consider only Fe-Ni-P-S aggregates. They can be subdivided into the following types according to their bulk compositions: (a) FN3-corresponding to stoichiometric (Fe,Ni) 3 (P,S); (b) FN2-corresponding to stoichiometric (Fe,Ni) 2 (P,S); (c) FN3-Ox-partially oxidized (Fe,Ni) 3 (P,S); (d) FN2-Ox-partially oxidized (Fe,Ni) 2 (P,S); (e) FNX-other compositions with the P and S contents deviating from stoichiometric proportions; and (f) FNX-Ox -partially oxidized (Fe,Ni) n (P,S) ( Table 2 and Table S1 in Supplementary Materials).    Table S1.
FN3 appears as a micro-or nanocrystalline mixture of two or three phases at the boundary between schreibersite and troilite, where melt pockets could form zoned patterns with FN2 near troilite, intermediate compositions between FN2 and FN3 ( Figure 2 and Table S2), and FN3 aggregates partially contaminated by metal. In the latter, FN3 is represented by cryptocrystalline zones surrounded by FNX (Figure 3 and Table S1).
FN3 can also form crystal-like aggregates surrounded by a partially oxidized quenched zone with the same composition ( Figure 4). Preliminary transmission electron microscope data indicate the nanocrystalline nature of this crystal-like aggregate. It consists of 10-20 nm crystals representing a mixture of S-containing schreibersite and S-bearing area with unknown composition.  Table S1). Another appearance of FN3 is connected with quenched textures, which may have crystallized from the melt (Figure 5a). Finally, it can form interstitial zones in non-stoichiometric mixtures with (Fe,Ni)-metal (Figure 5b,c). The composition of FN3 varies from (Fe,Ni) 3 P 0.8 S 0.2 to (Fe,Ni) 3 P 0.4 S 0.6 ( Figure 6, Table 2). The EBSD analyses on nanocrystalline aggregates of FN3 and single crystal-like areas of FN3 indicate the presence of a phase with the I4 schreibersite-type structure [24] and mean angular deviation (MAD) of measurements between 0.1 • and 0.3 • (Figure 7). No troilite or kamacite could be identified in such aggregates suggesting the presence of crystalline FN3 or a nanocrystalline mixture with the FN3 composition. In Figures 2 and 4, we could recognize two phases in the BSE images of Fe-Ni-P-S aggregates. It is obvious that one needs to add 20 to 60 mol.% (Fe,Ni)-metal to fit the compositions of FN3 (Figure 6), which is not the case. The FN3 compositions were also observed in some microcrystalline Fe-Ni-P-S aggregates with a grain size of 0.1-2.0 µm, where we could identify schreibersite, troilite, and kamacite (e.g., Figure 5a). In this case, it is difficult to suggest that this is a former high-pressure FN3 phase; however, a close resemblance to the stoichiometric compositions may not be accidental.  Table S1 for chemical compositions.
FN2 form micro-and nanocrystalline mixtures of dendritic crystals and appears near the boundary with troilite along with FN3 aggregates (Figure 2) or as interstitial pockets in the non-stoichiometric FNX zones (Figure 5b). Sometimes FN2 looks like a mechanical mixture of troilite and schreibersite crystals (Figure 8). Its composition varies from (Fe,Ni) 2 P 0.5 S 0.5 to (Fe,Ni) 2 P 0.2 S 0.8 ( Figure 6, Table 2), and it is easy to identify troilite microcrystals by EBSD measurements.
The oxidized regions of FN3 and FN2 appear due to partial oxidation of the metal or alloy with the formation of magnetite or trevorite (Figure 9, Table S3). In this case, the stoichiometry could remain the same or slightly deviate from the original FN3 or FN2 ( Figure 6). The composition of the various FNX and FNX-Ox aggregates deviates from stoichiometric FN3 and FN2 due to the admixture of the metallic phase ( Figure 6). Some of them correspond to the composition of the miscible Fe-Ni-P-S melts (Figures 5 and 6) and could be formed at low to zero pressures [25,26]. shows the compositional variation of Fe-Ni-P-S alloy from schreibersite to troilite (see Figure 2 and Table S2). Pressure values in GPa correspond to maximum S solubility in Fe 3 P 1-x S x alloys [20]. Black dashed line shows compositions of mechanical mixture between schreibersite and troilite. Grey dashed lines are same with an addition of 20, 40, and 60 mol.% of (Fe,Ni)-metal. Dotted lines show the limit of liquid immiscibility in the Fe-P-S system after Raghavan [25] (R-88) and Chabot and Drake [26] (Ch-00).   Table S1 for chemical compositions (Site 19).  Table S3 for chemical compositions.

High-Pressure Origin of (Fe,Ni) 3 (P,S)
Fe 3 P and Fe 3 S are isostructural at high pressure and form a complete solid solution at P > 20 GPa [19]. This may indicate partial solubility of sulfur in Fe 3 P at lower pressures. Gu et al. [20] calibrated the pressure dependence of sulfur solubility in tetragonal Fe 3 P and argued that it could be used as a pressure marker for natural Fe-Ni alloys containing P and S. They predicted that this solid solution could be found in meteorites, as it is the case in the studied Elga sample. We plotted calibration of Gu et al. [20] in Figure 6 and argue that most FN3 crystals and nanocrystalline aggregates correspond to pressures of 10-20 GPa according to the static high-pressure experiments. This calibration may be slightly distorted by the addition of Ni to the system since Ni 3 S, isostructural with schreibersite, appears at pressures about 5 GPa [27]. There is no experimental data to evaluate the pressure effect of Ni substitution for Fe in Fe 3 (P,S). However, assuming a linear relationship between the stability pressures of Ni 3 S and Fe 3 S, we can suggest that 20 mol. % of Ni in (Fe,Ni) 3 (P,S) would reduce the estimated pressures for 2-3 GPa [28]. Thus, the pressure estimations for the FN3 aggregates would be at least 7-17 GPa. Previous findings of tuite, which is stable at pressures above 12 GPa [29,30] in the shocked zones of silicate inclusions in Elga are consistent with the data for the FN3 aggregates.
The origin of FN2 is presently not clear. Gu et al. [20] reported that Fe 2 P dissolves S at high pressures. However, quantitative data on the S solubility in Fe 2 P as a function of pressure are not yet available. The close textural relations between FN3 and FN2 in the Elga meteorite and the lack of stable sulfides with the Fe 2 S stoichiometry in the studied pressure range [31,32] indicate the possibility of a high-pressure origin for FN2. In this regard, the stoichiometric composition of these aggregates is unlikely to be accidental ( Figure 6).

The Implication of the Results for Putative Impact Events during the Formation of IIE Iron Meteorites and Ureilites
Silicate-bearing iron meteorites are important for understanding the early evolutionary processes in the solar system that affected planetesimals and small bodies. Group IIE iron meteorites often contain 5-20% of silicates as inclusions, varying in composition from mafic to felsic and ultrafelsic. These inclusions show genetic relations to H-chondrites, as revealed by their isotopic and geochemical characteristics [33,34]. The origin of IIE irons with fractionated (non-chondritic) silicate inclusions, such as Elga meteorite, is best explained by a model of disruption and reassembly of partially molten asteroids with complex impact history, when some impact events are related to the formation of silicate inclusions and some to deformation and recrystallization along narrow zones in meteorites [34]. The formation of high-pressure minerals on a microscopic scale could be related to the late impacts. However, presumable extremely high-pressures exceeding 15 GPa recorded by Fe-Ni-P-S aggregates in the Elga indicate that the impact events were almost as intense as during the formation of high-pressure minerals in chondrites, where pressures exceeding 25 GPa were recorded [2].
S-bearing inclusions (S/(S + P) ≈ 0.12) with a bulk composition close to the (Fe,Ni) 3 (S,P) stoichiometry were found inside the diamond grains in Almahata Sitta ureilite [28]. These inclusions are interpreted to have been recrystallized to nanocrystalline aggregates of troilite, kamacite, and schreibersite by decompression. The authors relate the origin of these inclusions to pressures above 20 GPa inside the ureilite parent body. We show that these phases could be formed by the shock processes and do not support the idea about the origin of ureilites and their diamonds under the static high-pressure and high-temperature conditions. In our opinion, the FN3 aggregates in ureilitic diamonds, as well as diamonds themselves, were formed dynamically during a strong disruption impact event. This strong impact could cause the formation of enlarged diamond crystals (up to 100 µm) that exceed the normal size of impact diamond crystals (<5 µm) and is considered to be important evidence for the formation of ureilite diamonds in the deep interior of the ureilite parent body [35].
Author Contributions: K.D.L. and S.N.T. designed the study, K.D.L. and A.S. prepared and analyzed samples, K.E.K. and K.D.L. performed EBSD measurements. All the authors contributed to the text and figures.
Funding: This research was funded by the Russian Science Foundation, grant number 17-17-01177.