# Northwest Africa 16273: 4Vesta's mantle sampled during the giant impact

Ratiba Kared, Bertrand Moine, Jean-Luc Devidal, Abdelmadjid Seddiki

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Ratiba Kared, Bertrand Moine, Jean-Luc Devidal, Abdelmadjid Seddiki. Northwest Africa 16273: 4Vesta's mantle sampled during the giant impact. Journal of African Earth Sciences, 2025, pp.105604. 10.1016/j.jafrearsci.2025.105604. hal-04979380

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Northwest Africa 16273: 4Vesta's mantle sampled during the giant impact

Ratiba Kared a, Bertrand N. Moine b, Jean-Luc Devidal b and Abdelmadjid Seddiki a, *

<sup>a</sup> Laboratoire Géoressources et Risques Naturels (GEOREN); University of Oran 2 Mohamed Ben Ahmed, BP: 1510, 31000 Oran, Algeria

<sup>b</sup> Laboratoire Magmas et Volcans, F-63000, CNRS, IRD, OPGC; Université Clermont Auvergne, Clermont-Ferrand, France

*Corresponding author: abdelmadjid.seddiki69@mail.com

**Abstract**

We report here the results of a comprehensive petrological and geochemical study of Northwest Africa (NWA) 16273. NWA 16273 is classified as a mesosiderite-B2/3 and contains a significant amount of metal and silicates phases. It is composed of pyroxene, olivine, plagioclase, iron-metal + sulfide phases, and minor amounts of phosphate, chromite, phosphide, rutile, ilmenite and quartz. Olivines appear as xenocrysts surrounded by reaction rims composed of pyroxene and chromite. Some pyroxenes occur as xenocrysts and as small grains in the matrix. Pyroxene shows significant variation in composition: Low-Ca pyroxene (Fs<sub>18-36</sub> Wo<sub>3-4</sub> ; n=22) and pigeonite Fs<sub>22-26</sub> Wo<sub>12-20</sub> (n=2). The Fe/Mn ratios of all pyroxenes show a large variation and have molar Fe/Mn ratios of 27.6 ± 3.1 (n=24). Whereas, olivines have molar Fe/Mn ratios of 45.5 ± 4.2 (n=20). These ratios are compatible with 4Vesta compositions. Plagioclase are bytownites and anorthite (An<sub>87-92</sub> Or<sub>0.2-0.6</sub>). Additionally, Olivine displays a bimodal distribution with two populations: olivine-Mg-rich (Fo<sub>72-81</sub> Fa<sub>19-28</sub>) and olivine less magnesian (Fo<sub>60-67</sub> Fa<sub>33-40</sub>). The combination of an harzburgitic composition and a high metal content in the NWA 16273 suggests a complex origin involving several parent bodies, which may have undergone a collision, melting and ejection to form the mesosiderite. Textural, mineral and geochemical characteristics indicate that a metallic impactor melted an eucritic crust of 4Vesta. The eucritic magma and molten metal were injected into the harzburite mantle prior to ejection. Nickel phosphides oxidized on contact with silicate magma to form secondary phosphates. Olivine underwent a more or less intense peritectic reaction to produce secondary orthoproxenes and chromites. The orthopyroxene xenocrysts are zoned and partially chemically
re-equilibrated. These features show that a fraction of 4Vesta mantle was sampled during the giant impact.

**Keywords**

Mesosiderite, NWA 16273, stony-irons, diogenite, eucrite, Vesta, impact

**1.Introduction**

Mesosiderites, which are part of the stony-iron meteorite group, consist of silicate phases and Fe-Ni metal that are intimately mixed. This mixture has been fused together by intense heat and pressure. These meteorites are breccia consisting of around equal proportions of silicate and Fe-Ni metals plus sulfide (Scott et al., 2001). The silicates are composed of mineral and lithic clasts embedded in a fine-grained fragmented and impact-melt matrix (Libourel et al., 2023). These silicate parts show chemical composition similar to that of HED (howardite, eucrite and diogenite) meteorites considered to the asteroid 4Vesta (McCord et al., 1970; Wang et al., 2010). However, the parent body of metals in mesosiderite may be distinct from that of silicates (Sugiura et al., 2022). The Fe-Ni metal assemblages are similar to that of IIIAB iron meteorites, which are believed to have originated from the core of an asteroidal body (Hassanzadeh et al., 1990; Wasson et al., 1974). Based on the modal proportions, mesosiderites are grouped into two mineralogical subgroups A and sB; A has more abundant plagioclase and B has more abundant pyroxene (Kong et al., 2008). On the other hand, and based on their silicate textures, mesosiderites are subdivided into four petrographic types (1 – 4), reflecting an increasing degree in metamorphism (Powell, 1971).

One of the most interesting aspects of mesosiderites is their composition, which includes the presence of silicate clasts that have been embedded within the metal matrix. These clasts represent a mixture of different types of rock, including basaltic, gabbroic and diogenitic materials (Floran, 1978; Mittlefehldt, 1979; Powell, 1971; Prior, 1918), which suggest that the parent bodies from which mesosiderites formed were composed of diverse range of materials.

Mesosiderites are considered one of the most enigmatic groups of differentiated meteorites (Mittlefehldt, 1994). They contain more abundant modal silica minerals and phosphates in their basaltic and gabbroic clasts compared to eucrites, and their pyroxenes trend have higher MgO content and lower Fe/Mn ratios (Tamaki et al., 2006).

The origin and formation of mesosiderites are unclear. In this study, we aim to improve understanding of their formation by examining the mesosiderite NWA 16273 and its various
textures, as well as carrying out a complete petrological and geochemical analysis. Our purpose
is to explain the petrographic diversity of this sample and the magmatic processes involved
followed the impact. This study focusses on the problem of the origin of olivine in mesosiderites
and the implications for the mesosiderite parent body (MPB).

**2. Materials and Methods**

In this work, the petrographic observation, mineral chemistry, X-ray elemental mapping, and trace element concentrations of minerals were performed at Magmas and Volcanoes laboratory (LMV) – Clermont-Ferrand – France.

Mineral chemistry was determined with an electron microprobe (CAMECA S×5). The acceleration voltage and the beam current were 15 kV and 15 nA, respectively. Naturel minerals and metals were used as standards for the quantitative analysis. For silicates phases, wollastonite was employed as standard for Ca and Si; orthoclase for K and Al; manganese and titanium oxide for Ti and Mn; albite for Na; forsterite for Mg; fayalite for Fe; chromium oxide for Cr. For phosphate phases, apatite was used for P and Ca. For sulphide phases, pyrite was employed for S and Fe. For the metallic phases metallic standard was used for Fe, Ni and Si and oxide standard for Mn, Al and Cr. In addition, data were reduced using the ZAF algorithm corrections.

X-ray elemental mapping of selected element was performed using a scanning electron microscope (Jeol JSM-5910 LV) equipped with a PGT Spirit microanalysis system. The acquired images were obtained under high vacuum with an accelerating voltage of 15 keV, a current of 80 µA, a working distance of 20 mm. Mosaic image of the entire view of NWA 16273 is the result of accumulation of 123 backscattered images obtained using the scanning electron microscope (SEM).

The trace element concentrations of NWA 16273 were determined by a laser inductively coupled plasma–mass spectrometry (LA-ICP-MS) using a RESONETICS RESOLUTION M-50 laser powered by an ultra-short pulse ATL ATLEXEXCIMER laser system. Spot sizes ranged from 20 to 107 µm in diameter and the laser repetition frequency was 2–5 Hz. NIST 612 and BCR2 were used as an external standard and analyzed four times before and after each analytical session. Data reduction was achieved using the software package GLITTER (Van Achterberg et al., 2001). The procedure has been described else-where (Kared et al., 2019).
**3. Results**

**3.1. *Sample description***

NWA 16273 is a 53g mesosiderite from Morocco, measuring approximately 4.5 x 3 x 3 cm³ (Fig. 1) and lacking a fusion crust. The meteorite's uneven surfaces bear a distinct desert patina implying that it is merely a shard of a larger fragmented specimen that entered the Earth's atmosphere.

**3.2. *Texture and mineralogy***

The polished surface of NWA 16273 shows a metal-silicate association and contains 9.09 % olivine, 31.03 % pyroxene, 19.46 % plagioclase, 40.05 % iron-metal, 0.37 % phosphate and sulfide phases. (Fig. 2). Phase proportions are calculated using an image analysis surface mode converted to a mass mode, taking into consideration the specific densities of the phases. NWA 16273 is classified as a B2/3 mesosiderite, based on its igneous textured matrix and plagioclase-poor mineralogy. The textural and mineralogical composition shows that this mesosiderite is formed of a mixture of eucritic and diogenitic composition associated metal grains. The eucritic composition is reflected by the presence of pyroxene and plagioclase crystals, as well as the diogenitic composition is marked by the presence of pyroxene and olivine crystals.

**3.2.1. Olivine**

In mesosiderite NWA 16273, olivine occurs as sub-rounded coarse-grained (up to 5mm in diameter) with fractures. It presents reaction rims formed of pyroxenes and chromites. Anhedral olivine grains (20–700 µm in length) are enclosed in pyroxene grains (Fig. 3A and B). Some olivines show a peritectic reaction (Fig. 3C and D) reflected by their partial or complete transformation into pyroxenes and chromite according to the following reaction:

Olivine + Liquide 1 → Pyroxene + chromite + Liquide 2.

Despite the low representativeness or few analysable grains on the section, we can still see that there is a bimodal distribution of olivine with two distinct populations (Fig. 4). On the other hand, there are two types of olivine in this mesosiderite, olivine-Mg-rich (Fo72-81 Fa19-28) and other less magnesian (Fo60-67 Fa33-40). Olivine compositions in mesosiderites (Fo55-90) (Delany et al., 1980). The Fe/Mn ratio varied from 36.8-54.4. It should be noted that there is a notable difference in concentration between the core and the rim. Ni and Co concentrations in olivines show large variations (Table 3), Ni (11.87 to 513.16 ppm) vs. Co (8.81 to 30.49 ppm).
Based on Ni and Co contents and as shown in (Fig. 5), olivines of NWA 16273 are rich in Ni and fall in the field of Mg-rich harzburgites from 4Vesta from (Hahn et al., 2018).

Chondrite CI-normalized rare earth elements patterns show that olivines are strongly fractionated between LREE and HREE ($La_N = 0.001-0.02$ x CI; $Lu_N = 0.3-1.26$ x CI; and $La_N/Yb_N = 0.006-0.02$) with negative Eu anomalies (Eu/Eu* = 0.34-0.5 where Eu/Eu* is calculated as $Eu_N/(Sm_N*Gd_N)^{1/2}$) (Fig. 6A).

### 3.2.2. Pyroxene

Pyroxenes constitute the majority of the matrix, along with some fractures, and include all the minerals of NWA 16273. They are mainly subhedral to anhedral and vary in size from a few microns to several hundred microns (Fig. 7A, B, C and D). Some grains (up to 50 µm) are occasionally found as inclusions in plagioclases. There are also some large anhedral grains (Fig. 7A). Xenocrysts of olivine, chromite and plagioclase (Fig. 7B) are enclosed in low-Ca pyroxene. They reveal a wide range of variation: Pigeonite has a composition of $Fs_{22-26} Wo_{12-20}$ (n=2) and Low-Ca pyroxene has a composition of $Fs_{18-36} Wo_{3-4}$ (n=22) (Fig. 8). The Fe/Mn ratio (calculated from molar abundances) of pyroxenes is $27.6 \pm 3.1$ (n=24).

The REE concentrations are variable between low-Ca pyroxene xenocrysts, low-Ca pyroxene in the matrix and pigeonites. Low-Ca pyroxene xenocrysts show a strong fractionation between LREE and HREE (Fig. 6B; $La_N = 0.03-0.13$ x CI; $Lu_N = 0.70-8$ x CI; and $La_N/Yb_N = 0.02-0.04$) with negative Eu anomalies (Eu/Eu* = 0.05-0.17). It should be noted that low-Ca pyroxene xenocrysts rims are enriched in REE than their cores.

Low-Ca pyroxenes of the matrix show REE enrichment compared to low-Ca pyroxene xenocrysts (Fig. 6C; LREE and HREE >0.01 x CI), with less fractionated REE patterns ($La_N = 0.11-0.19$ x CI; $Lu_N = 5.15-8.06$ x CI; and $La_N/Yb_N = 0.02-0.03$). All patterns show negative Eu anomalies (Eu/Eu* = 0.09-0.29).

However, pigeonites show more enriched REE patterns than those from low-Ca pyroxene xenocrysts and low-Ca pyroxene of the matrix, especially for LREEs and MREEs (Fig. 6D; $La_N/Sm_N = 0.12-0.15$; $La_N/Yb_N = 0.15-0.30$) with a strong negative Eu anomalies (Eu/Eu* = 0.04-0.09).

### 3.2.3. Plagioclase

Plagioclase grains are mainly subhedral to anhedral with fractures and rounded border. They show a large variation in size which ranges from several microns to 1.6 mm. Hence,
round-shaped pyroxene grains (10–50 µm) are also enclosed in plagioclase grains. These pyroxenes inclusions are occasionally arranged with sulfide in chains especially along coarse-grained plagioclase.

Plagioclase grains are mainly bytownites and anorthite. They show a composition ranging from (An$_{87-92}$ Or$_{0.2-0.6}$) (Table 1) without zoning. Mesosiderite plagioclase grains (An88-95) (Delany et al., 1980) Plagioclase from NWA 16273 shows fractionated LREE and HREE (2.68-5.4 x CI for LaN and 0.05-0.2 for LuN) with well-defined positive Eu anomalies (Eu/Eu*= 20.52-37.06) (Fig. 6E). However, there is particularly one pattern which shows REE depletion and without fractionation between LREE and HREE (LaN/YbN= 1.6). It has a stronger Eu anomaly (Eu/Eu*= 128.88) compared to the other patterns.

### 3.2.4. Iron-metal

Metal grains are anhedral and characterized by a lobed shape with irregular borders serrated and/or invaginated on various scales. They have a large variation of size (from a few microns to ~4 mm in diameter) and usually show a halo of infiltrations in cracks of surrounding silicates. Metal is mostly exhibited in contact with phosphates, plagioclases, chromites and sulfides. Moreover, a few grains contain phosphide inclusions and pyroxenes + plagioclases. It should be noted that metal from NWA 16273 shows stripes of taenite (up to 250 µm in wide) settle between or in contact with kamacite (up to 600 µm in wide) (Fig. 9).

Kamacite shows small variations in Fe (92.21-94.99 wt.%) and Ni (4.54-6.67 wt.%), whereas taenite shows large variations in Fe (48.46-66.48 wt.%) and Ni (33.20-50.18 wt.%) (Table 2). The Ni/Co ratio is low for kamacite which varies from 14.1 to 15.6 and is high for taenite (265 to 288).

### 3.2.5. Phosphate

Phosphate occurs essentially as fine-grained anhedral at the borders metal grains, but sometimes can be found enclosed in metal. Their size varies from a few microns to several hundred microns. Phosphates are homogeneous and show small variation in composition with Fe # (in molar) ranging from 53 to 59. Concentrations of MnO (1.39 to 1.50 wt.%), CaO (22.48-23.02 wt.%) and P$_2$O$_5$ (46.76-47.51 wt.%) exhibit small variation.

According to REE patterns, there are two types of phosphates present in NWA 16273 (Fig. 6F): one (type I) shows LREE enrichment compared to HREE (LaN/YbN ratio varies from 1.6 to 2.13) with a negative Eu anomaly (Eu/Eu*= 0.25-0.5). Whereas, the other (type II) shows
a flat-REE shape for REE, with a very low HREE enrichment compared to LREE (LaN/YbN ratio varies from 0.67 to 0.89). In addition, type I phosphate is clearly enriched compared to type II, especially for LREE. (Eu/Eu*= 1.05-1.06).

### 3.2.6. Chromite

Chromites are present as anhedral grains in the reaction crowns of the olivines, and in or at the borders of metal grains. They are about a few micrometers to 300 µm. Most of them occurs as tiny grains that surround the grains of olivine. The composition of these chromites is ferroan FeO (32.09-34.54 wt. %). These chromites show large variations in TiO₂ (0.80-3.84 wt.%) and Al₂O₃ (10.05-15.77 wt. %). The Cr* ratio shows a variation ranging from 66.7 to 77.9 where Cr* is calculated as $Cr/ (Cr + Al) \times 100$ % in molar percent.

### 3.2.7. Phosphide

The phosphides are present in contact with metal grains. They appear in different sizes (a few tens microns to about 100 µm), although some larger grains are also observed included in metal (400 µm) (Fig. 7A). Phosphides have a large variation in Fe and Ni. The grains around the metal contain Fe (30.99-31.90 wt.%) and Ni (51.99-52.94 wt.%). However, the grains included in metal have Fe and Ni contents 36.02 wt.% and 47.44 wt.%, respectively.

### 3.2.8. Sulfides

Sulfides are mostly fine-grained (<80 µm) in the pyroxene matrix or at the grain boundary of impact-related fractures. Some grains (Up to 150 µm in longest dimension) are included in pyroxenes (Fig. 10A). They are essentially troilite and generally homogeneous in composition Fe (62.44-63.54 wt.%) and S (35.79-36.27 wt.%).

### 3.2.9. Rutile, ilmenite and quartz

The rare rutile grains observed in NWA 16273 can be up to 40 µm in longest dimension (Fig. 11A). They are mostly composed of TiO₂, with some traces of FeO (0.29-0.54 wt.%) and Cr₂O₃ (0-0.66 wt.%). One ilmenite grain of a hundred micrometer is also observed in this sample (Fig. 11B). This ilmenite is ferroan FeO (46.54 wt.%) with traces of MnO (0.4 wt.%), Cr₂O₃ (0.22 wt.%) and MgO (0.13 wt.%). Finally, it should be noted that interstitial quartz (<90 µm) was also seen associated with metal grains (Fig. 10B and C).
## 4. Discussion

### 4.1. Significance of olivine compositions in NWA 16273 Mesosiderite: Implications for parent body mixing and impact processes

The composition of olivines in NWA 16273, as shown in (Fig. 12), suggests that the protolith of this mesosiderite could either be an olivine diogenite (ultrabasic cumulate of pyroxenes, olivines, and plagioclase) or an harzburgite (olivines ± orthopyroxenes). To investigate this further, we used the mass balance calculation to calculate the composition of the whole silicate rock and impact melt in which the xenocrysts of olivines are subtracted. We then plotted these compositions on a Forsterite-Anorthite-Quartz pseudoternary of Stopler, (1975) (Fig. 13). We observed that the two plotted points align with the Fo pole, indicating that the total silicate rock is a mixture resulting from the impact melt and olivine. The presence of interstitial quartz and the alignment of our calculated compositions with the ternary eutectic of the diagram suggest that the liquid underwent fractional crystallization and approached this eutectic, where the last drop of liquid would have crystallized. It is also possible that the initial liquid was located on the eutectic and assimilated a portion of the harzburgite until it reached the composition of the impact melt and then underwent fractional crystallization until it returned to its initial composition on the eutectic, where it finished crystallizing.

Calculation of impact melt Rare Earth Elements concentration from mineral modes and in-situ phase analyses yielding concave REE pattern with positive Eu anomaly, highlighting the cumulative nature of plagioclase. This pattern is very similar in shape and concentration to cumulate eucrites. Furthermore, the REE pattern of the impact melt (Fig. 14) indicates that it is the result of the melting of a crust essentially made up of cumulative eucrites (part of the basaltic crust of 4Vesta) as it shows values and trends similar to the cumulate eucrites Momoa and Serra de Mage (Hamet et al., 1978; Ma & Schmitt, 1979). Additionally, it can be observed from Fig. 8 that the pyroxene composition is within the ranges of both cumulate eucrites and diogenite fields. This provides further evidence that the silicate part of NWA 16273 is a combination of cumulate eucrites and diogenites.

### 4.2. Diogenitic composition of olivines from NWA 16273: A link to 4Vesta?

The study of NWA 16273 has provided intriguing insights into the formation of mesosiderites. The high concentration of metal in this sample as well as the diogenitic composition of olivines,
suggest that the metal and silicate phases did not originate from the same source. Specifically, the presence of metal and olivine from different sources suggests that NWA 16273 was produced by an impact fusion event, possibly involving the parent body of 4Vesta. The composition of olivine in this sample is consistent with that found in the HED (howardite-eucrite-diogenite) group of meteorites, which are believed to originate from 4Vesta (Binzel & Xu, 1993; Drake, 2001). The link between HED meteorites and mesosiderite silicates suggest that both could have originated from asteroid 4Vesta, or their parent bodies could have formed in adjacent regions (Haba et al., 2019). However, the high concentration of metal in the NWA 16273 indicates that the sample may have also originated from a metallic parent body. This is consistent with previous studies that have suggested that some mesosiderites may have been formed by impact and mixing of a metallic core from a differentiated body with silicates from a large body (Powell, 1971). The combination of a diogenitic composition and a high metal content in the NWA 16273 sample suggests a complex origin involving multiple parent bodies, which may have undergone a collision and fusion event to form the mesosiderite.

These findings suggest that the NWA 16273 mesosiderite is the result of an impact event on a body with a diogenitic mantle and basaltic crust, possibly the asteroid 4Vesta. The bimodal distribution of olivines in this sample could be explained by the impact of a metallic impactor, possibly a pallasite, with the olivine diogenite or the harzburgite protolith. The resulting impact melt underwent fractional crystallization, approaching the eutectic ternary of the diagram and eventually solidified. These are consistent with the petrology of mesosiderites, which are believed to be the result of the impact disruption of differentiated asteroids (Haba et al., 2019).

### 4.3. *Origin of olivine and phosphate from NWA 16273*

The origin of the bimodal distribution of olivines observed in mésosidérite NWA 16273 remains an important open question. We suppose that the magnesian olivines represent the primary olivines that were not or slightly affected by the metallic impact. However, for unequilibrated olivines, there are two possible hypotheses.

The first hypothesis is that the metallic impactor could be a pallasite, which is a mixture of metal and olivines and that the diogenitic olivines reacted strongly with this impactor by impact melting. The strong reaction between the diogenitic olivines and the impactor through impact fusion could have resulted in compositions closer to or even at the limit of the pallasite group (Fig. 12). However, this hypothesis is not consistent with the Ni vs. Co diagram (Fig. 5),
which shows that the unequilibrated olivines are extremely enriched in Ni, whereas if the
impactor were a pallasite, it would be rather the opposite : the Ni of the olivines would be
absorbed by the metal.

The second hypothesis is that the diogenitic olivines reacted with the harzburgite of 4Vesta's mantle during the impact fusion, resulting in an intermediate composition between our diogenitic olivines and the harzburgite domain (Fig. 12). This hypothesis is supported by previous studies (Greenwood, 2006; Haba et al., 2019) that suggest that the parent body of mesosiderites, from which NWA 16273 likely originated is 4Vesta, a differentiated asteroid with a mantle made of harzburgite. In addition, pyroxene xenocrysts are in equilibrium with olivine xenocrysts (Fig. 8). It is therefore more likely that these olivines are derived from an harzburgitic from 4 Vesta.

For phosphates from NWA 16273, two types of phosphates are present: type I (ranges from 50 to 60 X CI) and type II (ranges from 70 to 200 X CI) (Fig. 6F). The REE enrichments observed in phosphates from mesosiderites range from 70 to 200 × CI, representing a moderate level compared to the enrichments found in other achondrites (Ward et al., 2017).

Texturally, phosphate is associated with metal-silicate boundaries, which can be formed
during primary crystallization. However, fine-grained enclosed in metal may be due to the
redox reactions of nickel phosphides. These Nickel phosphides oxidized on contact with silicate
magma to form secondary phosphates. This has also been interpreted by Harlow et al., (1982)
that phosphorus in Emery mesosiderite should have been dissolved in the metallic portion and
that it largely reacted to form phosphate. The relatively low levels of REE found in calcium-
phosphates within mesosiderites are likely due to significant enrichment of metals over
silicates, which leads to the separation of phosphorus from the metal (and related phosphide
compounds) and its incorporation into the silicate fraction. This process occurs because of a
moderate level of oxygen fugacity (Delany et al., 1980; Harlow et al., 1982). Mesosiderite
silicates have phosphate concentrations that are 5-20 times higher than those in eucrites (Prinz
et al., 1980), this is probably due to the incorporation of phosphorus into silicate clasts from the
metal during oxidation (Harlow et al., 1982; Olsen & Fredriksson, 1966; Prinz et al., 1980;
Stewart et al., 1994).

**5.Conclusions**
NWA 16273 displays a significant presence of metal and a bimodal distribution of olivine xenocrysts: one population of diogenitic olivine and an intermediate population between diogenite and pallasite or harzburgite. These olivine xenocrysts are embedded in a pyroxene matrix that formed through impact fusion and contains a considerable amount of plagioclase crystals. There appear to be some olivines that have achieved a state of equilibrium with the matrix, while others do not exhibit such a relationship.

Based on petrographic and geochemical descriptions, we can conclude that this is the result of a mixture between the mantle of 4Vesta and a metallic impactor, which caused an impact fusion and formed a new liquid that introduced itself into the mantle to assimilate a portion of it. However, we can not determine the nature of the metallic impactor, as our data only contradicts our hypothesis of a pallasitic impactor. In this study, we focused on the major phases and more specifically on olivines. So investigating minor phases such as phosphates, for which we did not find any correspondence, phosphides, sulfides, or chromites could be revealing and provide more insight into this sample.

In conclusion the NWA 16273 mesosiderite represents a combination of Vesta's mantle and a metallic impactor. Petrographic and geochemical studies have revealed that the impact resulted in a fusion event, producing a new magma that mixed with the mantle to assimilate a portion of it. The impactor had melted a part of the eucritic Vesta and this was injected into the mantle to give the eucritic and diogenitic composition of NWA 16273. Further studies, including isotopic analyses, may provide more insights into the origin and evolution of NWA 16273 and mesosiderites in general.

**Declaration of competing interest**

There is no conflict of interest with the publication of the present manuscript.

**Data availability**

Data will be made available on request.

**Acknowledgments**

The authors would like to thank Bernard Mougenot the owner of the main mass of the Mesosiderite of this study. We are very grateful to him for passing on this rare meteorite. We would like to warmly thank Lenny Riguet for the preliminary petrographic and mineralogical work. This is contribution no. 689 of the ClerVolc program of the International Research Center for Disaster Sciences and Sustainable Development of the University of Clermont Auvergne.
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<table><caption>**Table 1.** Representative chemical compositions (wt%) of silicate minerals in NWA 16273.</caption><thead><tr><th></th><th>Uneq. Ol</th><th>Eq. Ol</th><th>Opx</th><th>Pgt</th><th>Pl</th><th>Chr</th><th>Rt</th><th>Ilm</th><th>Phos</th></tr></thead><tbody><tr><td>SiO<sub>2</sub></td><td>37.32</td><td>39.62</td><td>53.45</td><td>52.95</td><td>45.22</td><td>n.d.</td><td>0.02</td><td>0.04</td><td>0.02</td></tr><tr><td>TiO<sub>2</sub></td><td>0.09</td><td>n.d.</td><td>0.49</td><td>0.50</td><td>0.03</td><td>2.31</td><td>97.62</td><td>53.10</td><td>0.01</td></tr><tr><td>Al<sub>2</sub>O<sub>3</sub></td><td>n.d.</td><td>0.02</td><td>0.90</td><td>0.75</td><td>33.44</td><td>12.42</td><td>0.04</td><td>0.01</td><td>0.02</td></tr><tr><td>Cr<sub>2</sub>O<sub>3</sub></td><td>0.09</td><td>n.d.</td><td>0.44</td><td>0.59</td><td>-</td><td>47.62</td><td>0.66</td><td>0.22</td><td>-</td></tr><tr><td>FeO</td><td>30.56</td><td>18.52</td><td>18.40</td><td>13.76</td><td>0.79</td><td>32.67</td><td>0.50</td><td>46.54</td><td>17.69</td></tr><tr><td>MnO</td><td>0.61</td><td>0.39</td><td>0.63</td><td>0.6</td><td>n.d.</td><td>n.d.</td><td>0.01</td><td>0.40</td><td>1.41</td></tr><tr><td>MgO</td><td>31.94</td><td>42.02</td><td>23.52</td><td>20.35</td><td>0.36</td><td>1.82</td><td>n.d.</td><td>0.13</td><td>11.17</td></tr><tr><td>CaO</td><td>0.05</td><td>0.04</td><td>2.59</td><td>9.94</td><td>18.10</td><td>0.07</td><td>0.09</td><td>0.05</td><td>23.02</td></tr><tr><td>Na<sub>2</sub>O</td><td>n.d.</td><td>n.d.</td><td>0.01</td><td>0.06</td><td>0.82</td><td>n.d.</td><td>0.02</td><td>n.d.</td><td>n.d.</td></tr><tr><td>K<sub>2</sub>O</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>0.01</td><td>0.04</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td></tr><tr><td>P<sub>2</sub>O<sub>5</sub></td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>47.21</td></tr><tr><td>Total</td><td>100.66</td><td>100.61</td><td>100.42</td><td>99.51</td><td>98.8</td><td>96.92</td><td>98.98</td><td>100.48</td><td>100.54</td></tr><tr><td>Mg#</td><td>65.1</td><td>80.2</td><td>69.5</td><td>72.5</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Fa</td><td>34.9</td><td>19.8</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Wo</td><td>-</td><td>-</td><td>5.2</td><td>20.3</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>En</td><td>-</td><td>-</td><td>65.9</td><td>57.8</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Fs</td><td>-</td><td>-</td><td>28.9</td><td>21.9</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>An</td><td>-</td><td>-</td><td>-</td><td>-</td><td>90.6</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Ab</td><td>-</td><td>-</td><td>-</td><td>-</td><td>9</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Or</td><td>-</td><td>-</td><td>-</td><td>-</td><td>0.4</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Fe/Mn</td><td>49.9</td><td>46.5</td><td>29.0</td><td>22.6</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Cr#</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>72</td><td>91.1</td><td>96.4</td><td>-</td></tr></tbody></table>

Note: (1) n.d. not determined. (2) Mg# = Mg/(Mg + Fe) * 100 in molar percent. (3) Cr# = Cr/(Cr+Al) * 100 in molar percent. (4) Fe/Mn ratios are calculated from molar abundances. *Abbreviations used:* Uneq. Ol, unequilibrated olivine; Eq. Ol, equilibrated olivine; Opx, orthopyroxene; Pgt, pigeonite; Pl, plagioclase; Chr, chromite; Rt, rutile; Ilm, ilmenite; Phos, phosphate.
<table><caption>**Table 2.** Representative compositions (wt%) of kamacite, taenite, phosphide and sulfide in NWA 16273.</caption><thead><tr><th>wt. %</th><th>Kamacite</th><th>Taenite</th><th>Phosphide</th><th>Sulfide</th></tr></thead><tbody><tr><td>Ti</td><td>n.d.</td><td>0.02</td><td>n.d.</td><td>-</td></tr><tr><td>Si</td><td>0.01</td><td>0.02</td><td>n.d.</td><td>0.03</td></tr><tr><td>Fe</td><td>93.12</td><td>61.26</td><td>31.9</td><td>62.44</td></tr><tr><td>Mn</td><td>n.d.</td><td>0.02</td><td>n.d.</td><td>0.06</td></tr><tr><td>Al</td><td>n.d.</td><td>n.d.</td><td>0.01</td><td>0.01</td></tr><tr><td>Cr</td><td>n.d.</td><td>0.01</td><td>0.03</td><td>0.02</td></tr><tr><td>Ni</td><td>5.81</td><td>37.61</td><td>51.99</td><td>0.08</td></tr><tr><td>Co</td><td>0.44</td><td>0.08</td><td>0.03</td><td>0.03</td></tr><tr><td>S</td><td>-</td><td>-</td><td>-</td><td>36.09</td></tr><tr><td>P</td><td>-</td><td>-</td><td>13.41</td><td>-</td></tr><tr><td>Total</td><td>99.39</td><td>99.02</td><td>97.37</td><td>98.76</td></tr></tbody></table>

Note: n.d. not determined.
<table><caption>**Table 3.** Representative trace element concentrations for olivine, plagioclase, orthopyroxene, Pigeonite and phosphate in NWA 16273.</caption><thead><tr><th>n°</th><th>Olivine</th><th>Plagioclase</th><th>Opx</th><th>Pigeonite</th><th>Phosphate</th></tr></thead><tbody><tr><td>Li(µg/g)</td><td>0.83</td><td>13.8</td><td>2.64</td><td>2.14</td><td>10.99</td></tr><tr><td>Si %</td><td>17.8</td><td>26.47</td><td>14.75</td><td>8.45</td><td>1.77</td></tr><tr><td>P(µg/g)</td><td>23.89</td><td>67.23</td><td>27.91</td><td>9.69</td><td>2032.52</td></tr><tr><td>Ca %</td><td>0.04</td><td>12.86</td><td>1.52</td><td>5.63</td><td>15.26</td></tr><tr><td>Sc(µg/g)</td><td>6.21</td><td>0.76</td><td>18.7</td><td>38.65</td><td>36.78</td></tr><tr><td>Ti %</td><td>0.02</td><td>0.01</td><td>0.13</td><td>0.41</td><td>n.d.</td></tr><tr><td>V(µg/g)</td><td>2.38</td><td>1.22</td><td>72.44</td><td>93.38</td><td>26.98</td></tr><tr><td>Cr %</td><td>0.005</td><td>n.d.</td><td>0.25</td><td>0.24</td><td>0.01</td></tr><tr><td>Mn %</td><td>0.39</td><td>0.01</td><td>0.21</td><td>0.01</td><td>0.01</td></tr><tr><td>Co(µg/g)</td><td>12.07</td><td>5.13</td><td>8.35</td><td>3.64</td><td>3.4</td></tr><tr><td>Ni(µg/g)</td><td>19.63</td><td>67.15</td><td>41.97</td><td>6.59</td><td>149.11</td></tr><tr><td>Zn(µg/g)</td><td>15.64</td><td>0.604</td><td>5.06</td><td>2.83</td><td>0.597</td></tr><tr><td>Ga(µg/g)</td><td>0.825</td><td>14.64</td><td>0.725</td><td>0.72</td><td>0.522</td></tr><tr><td>Ge(µg/g)</td><td>0.042</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>0.443</td></tr><tr><td>Rb(µg/g)</td><td>0.007</td><td>0.105</td><td>0.005</td><td>n.d.</td><td>0.089</td></tr><tr><td>Sr(µg/g)</td><td>0.005</td><td>186.97</td><td>0.881</td><td>2.76</td><td>13.16</td></tr><tr><td>Y(µg/g)</td><td>0.108</td><td>0.364</td><td>3.41</td><td>16.63</td><td>104.55</td></tr><tr><td>Zr(µg/g)</td><td>0.603</td><td>0.097</td><td>8.8</td><td>20.75</td><td>19.45</td></tr><tr><td>Nb(µg/g)</td><td>0.074</td><td>0.02</td><td>0.471</td><td>0.064</td><td>n.d.</td></tr><tr><td>Cs(µg/g)</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td></tr><tr><td>Ba(µg/g)</td><td>n.d.</td><td>55.22</td><td>0.525</td><td>n.d.</td><td>2.004</td></tr><tr><td>La(µg/g)</td><td>n.d.</td><td>0.843</td><td>0.026</td><td>0.369</td><td>36.92</td></tr><tr><td>Ce(µg/g)</td><td>0.001</td><td>1.605</td><td>0.099</td><td>2.012</td><td>92.47</td></tr><tr><td>Pr(µg/g)</td><td>n.d.</td><td>0.173</td><td>0.022</td><td>0.459</td><td>12.69</td></tr><tr><td>Nd(µg/g)</td><td>0.002</td><td>0.729</td><td>0.164</td><td>3.54</td><td>62.27</td></tr><tr><td>Sm(µg/g)</td><td>n.d.</td><td>0.151</td><td>0.113</td><td>1.856</td><td>19.12</td></tr><tr><td>Eu(µg/g)</td><td>n.d.</td><td>1.394</td><td>0.006</td><td>0.036</td><td>3.18</td></tr><tr><td>Gd(µg/g)</td><td>0.003</td><td>0.133</td><td>0.255</td><td>2.91</td><td>19.22</td></tr><tr><td>Tb(µg/g)</td><td>0.001</td><td>0.017</td><td>0.061</td><td>0.486</td><td>3.41</td></tr><tr><td>Dy(µg/g)</td><td>0.009</td><td>0.092</td><td>0.540</td><td>3.46</td><td>23.2</td></tr><tr><td>Ho(µg/g)</td><td>0.003</td><td>0.013</td><td>0.133</td><td>0.666</td><td>4.39</td></tr><tr><td>Er(µg/g)</td><td>0.026</td><td>0.033</td><td>0.451</td><td>1.829</td><td>12.24</td></tr><tr><td>Tm(µg/g)</td><td>0.008</td><td>0.003</td><td>0.075</td><td>0.246</td><td>1.74</td></tr><tr><td>Yb(µg/g)</td><td>0.11</td><td>0.028</td><td>0.55</td><td>1.384</td><td>12.01</td></tr><tr><td>Lu(µg/g)</td><td>0.031</td><td>0.002</td><td>0.084</td><td>0.199</td><td>1,43</td></tr><tr><td>Hf(µg/g)</td><td>0.01</td><td>0.005</td><td>0.135</td><td>0.966</td><td>0.25</td></tr><tr><td>Ta(µg/g)</td><td>0.003</td><td>n.d.</td><td>0.013</td><td>n.d.</td><td>n.d.</td></tr><tr><td>Pb(µg/g)</td><td>0.001</td><td>0.026</td><td>0.006</td><td>n.d.</td><td>1.4</td></tr><tr><td>Th(µg/g)</td><td>n.d.</td><td>n.d.</td><td>0.003</td><td>0.005</td><td>1.74</td></tr><tr><td>U(µg/g)</td><td>n.d.</td><td>0.007</td><td>0.008</td><td>n.d.</td><td>1.08</td></tr><tr><td>(La/Yb)n</td><td>0.002</td><td>21.22</td><td>0.03</td><td>0.18</td><td>2.13</td></tr><tr><td>(La/Sm)n</td><td>-</td><td>3.51</td><td>0.14</td><td>0.12</td><td>1.21</td></tr><tr><td>(Sm/Yb)n</td><td>-</td><td>6.05</td><td>0.23</td><td>1.48</td><td>1.76</td></tr><tr><td>Eu/Eu*</td><td>-</td><td>29.89</td><td>0.11</td><td>0.05</td><td>0.5</td></tr><tr><td>Ce/Ce*</td><td>0.45</td><td>1.01</td><td>1</td><td>1.17</td><td>1.02</td></tr></tbody></table>

Note: (1) n.d. not determined. (2) Eu/Eu* = Eu<sub>n</sub>/(Sm<sub>n</sub>xGd<sub>n</sub>)<sup>1/2</sup>). (3) Ce/Ce* = Ce<sub>n</sub>/(La<sub>n</sub>xPr<sub>n</sub>)<sup>1/2</sup>). (4)

Values normalized to CI chondrite [17].
**Figure captions**

**Fig. 1.** Photograph of mesosiderite NWA 16273.

**Fig. 2.** Mosaic backscattered electron (BSE) image of NWA 16273 showing the entire views of the polished section studied.

**Fig. 3.** Backscattered electron images of NWA 16273 highlighting the different steps of dissolution of olivine to pyroxene and chromite. (A) An olivine phenocryst surrounded by a few small grains of chromite; (B) Several small grains of chromite surround a grain of olivine, which may be the result of peritectic reactions; (C) Olivines are partially transformed into pyroxene and chromite; (D) Olivines are completely transformed into pyroxenes and chromites.

**Fig. 4.** Frequency histogram for olivines Mg # [100 × Mg/(Mg + Fe) in molar] in NWA 16273.

**Fig. 5.** Ni versus Co diagram for olivines from NWA 16273 compared with literature data taken from (Hahn et al., 2018; Kared et al., 2019; Lunning et al., 2015; Shearer et al., 2010).

Fig. 6. REE patterns of olivine (A), low-Ca pyroxene xenocrysts (B), low-Ca pyroxene of the matrix (C), pigeonite (D), plagioclase (E) and phosphate (F) from NWA 16273. The reference chondrite is from (Anders & Grevesse, 1989).

Fig. 7. Backscattered electron images of NWA 16273. (A) Zoned pyroxene, phosphate assembled with Fe-metal and plagioclase grains; (B) pyroxene inclusions within plagioclase. Note there are fine-grained of plagioclase are associated with SiO2 and fer-metal aggregates within pyroxene; (C) pyroxene and plagioclase inclusions within the fer metal; (D) A representative olivine grain of BM 206. This olivine is entirely surrounded by fine-grained of chromite. Phase abbreviations: Fe-met = iron-metal; Pl = plagioclase; Px = pyroxene; Phos = phosphate; Chr = chromite.

Fig. 8. Major element compositions of pyroxene and olivine from NWA 16273. Literature data of pyroxenes and olivines are taken from (Beck et al., 2012; Hahn et al., 2018; Liao & Hsu, 2017; Z. Wang & Tian, 2023).

**Fig. 9.** X-ray elemental mapping of NWA 16273. (A) A typical region containing stripes of taenite settle within kamacite. Note that iron-metal is associated with phosphate and SiO2. (B)
A typical region showing a phosphide included within iron-metal. Fe-met = iron-metal; Kam = kamacite; Phos = phosphate.

**Fig. 10.** X-ray elemental mapping of NWA 16273. (A) A typical region showing sulfide grains included within pyroxene. (B) A typical region containing quartz grains included and/or associated with iron-metal. (C) Quartz grains associated with iron-metal and sulfides.

**Fig. 11.** (A) A representative view of rutile and phosphate from NWA 16273; (B) Ilmenite associated with rutile, phosphate, quartz and fer-metal (backscattered electron images).

**Fig. 12.** FeO/MnO vs. Mg# for olivines from NWA 16273. The Mesosiderites, diogenites and main-group pallasites are from (Greenwood et al., 2015) and the harzburgite of 4Vesta are from (Hahn et al., 2018).

**Fig. 13.** Calculated data for silicate whole-rock and impact melt from NWA 16273 plotted in olivine-anorthite-quartz pseudoternary of (Stopler, 1975). The composition of the last liquid drop is deduced from the observation of interstitial quartz, which would coincide with the eutectic that aligns with the silicate whole-rock and impact melt. Harzburgite data is taken from (Hahn et al., 2018).

**Fig. 14.** CI chondrite normalized REE patterns of impact melt from NWA 16273 compared to REE patterns from Momoa cumulative eucrites (Hamet et al., 1978) and Serra de Magé (Ma & Schmitt, 1979). CI chondrite normalization from (Anders & Grevesse, 1989).