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# Identification of a meteoritic component using chromium isotopic composition of impact rocks from the Lonar impact structure, India

Bérengère Mougel, Frédéric Moynier, Christian Koeberl, Daniel Wielandt, Martin Bizzarro

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## To cite this version:

Bérengère Mougel, Frédéric Moynier, Christian Koeberl, Daniel Wielandt, Martin Bizzarro. Identification of a meteoritic component using chromium isotopic composition of impact rocks from the Lonar impact structure, India. Meteoritics and Planetary Science, 2019, 54 (10), pp.2592-2599. 10.1111/maps.13312. insu-02916850

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https://insu.hal.science/insu-02916850v1

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Meteoritics & Planetary Science 54, Nr 10, 2592–2599 (2019)

doi: 10.1111/maps.13312

# Identification of a meteoritic component using chromium isotopic composition of impact rocks from the Lonar impact structure, India

Berengere MOUGEL ID <fnref n="1" />,<fnref n="2" />*, Frederic MOYNIER<fnref n="1" />,<fnref n="3" />, Christian KOEBERL ID <fnref n="4" />,<fnref n="5" />, Daniel WIELANDT<fnref n="6" />, and Martin BIZZARRO<fnref n="6" />

*(Received 21 September 2018; revision accepted 15 May 2019)*

**Abstract**–The existence of mass-independent chromium isotope variability of nucleosynthetic origin in meteorites and their components provides a means to investigate potential genetic relationship between meteorites and planetary bodies. Moreover, chromium abundances are depleted in most surficial terrestrial rocks relative to chondrites such that Cr isotopes are a powerful tool to detect the contribution of various types of extra-terrestrial material in terrestrial impactites. This approach can thus be used to constrain the nature of the bolide resulting in breccia and melt rocks in terrestrial impact structures. Here, we report the Cr isotope composition of impact rocks from the ~0.57 Ma Lonar crater (India), which is the best-preserved impact structure excavated in basaltic target rocks. Results confirm the presence of a chondritic component in several bulk rock samples of up to 3%. The impactor that created the Lonar crater had a composition that was most likely similar to that of carbonaceous chondrites, possibly a CM-type chondrite.

## INTRODUCTION

The Lonar impact structure is a relatively small bowl-shaped and almost circular crater (diameter 1.83 km, depth 135 m) located in the Deccan traps (~65 Ma) in the Buldhana District of Maharashtra, India. It has an age of ~0.57 Ma (Jourdan et al. 2011) and represents a well-preserved impact structure excavated in basaltic target rocks, which are rare on Earth but common on other planetary bodies in the inner solar system. As for many small impact structures, it was originally considered to be a volcanic crater. However, its impact origin was later confirmed with the discovery of shock metamorphic features in breccias found in drill core samples (Fredriksson et al. 1973; Fudali et al. 1980), despite the lack at that time of geochemical evidence for a meteoritic component (Osae

et al. 2005; Son and Koeberl 2007) and of projectile fragments (which were expected to be present because the crater was thought to be very young, similar to Meteor Crater in Arizona, before the current age determination by Jourdan et al. [2011] revealed it is much older). Only very recently did different research groups provide geochemical evidence for the presence of extra-terrestrial material in Lonar impact rocks (Misra et al. 2009; Schulz et al. 2016; Das Gupta et al. 2017; Ray et al. 2017). While they all agree on the chondritic nature of this component, the type of chondrite involved in this impact event remains unclear. Schulz et al. (2016) characterized the Re-Os isotopic composition of impactite rocks from the crater and showed that it was distinct from the target rocks, and that the impactites exhibit up to two orders of magnitude higher Ir, Os, and Ru abundances, and

<footnote n="1">¹Institut de Physique du Globe de Paris, Université Sorbonne Paris Cité, CNRS UMR7154, 1 rue Jussieu, 75238 Paris Cedex 05, France</footnote>

<footnote n="2">²Centro de Geociencias, Universidad Nacional Autónoma de México, Blvd. Juriquilla No 3001, Querétaro 76230, Mexico</footnote>

<footnote n="3">³Institut Universitaire de France, 1 rue Descartes, 75231 Paris Cedex 05, France</footnote>

<footnote n="4">⁴Department of Lithospheric Research, University of Vienna, Althanstrasse 14, 1090 Vienna, Austria</footnote>

<footnote n="5">⁵Natural History Museum, Burgring 7, 1010 Vienna, Austria</footnote>

<footnote n="6">⁶Centre for Star and Planet Formation, Natural History Museum of Denmark, University of Copenhagen, Øster Voldgade 5-7 DK-1350 Copenhagen, Denmark</footnote>

<footnote>*Corresponding author. E-mail: mougel@geociencias.unam.mx</footnote>
near-chondritic interelement ratios, compared to the target rocks. This was interpreted to represent an addition of up to 0.03% of a chondritic component to the target basalt. The analyses of spherules contained in the continuous ejecta blanket of the crater also indicated the presence of a chondritic component based on Ni, Co, Cr (Misra et al. 2009; Das Gupta et al. 2017; Ray et al. 2017), and Ir contents, but at a higher abundance. Misra et al. (2009) postulated an admixture of up to 12–20% of a chondritic impactor (possibly EH-type) based on the Ni content of some submillimeter-sized spherules, whereas more recent studies published a slightly lower value of 6–8% chondritic contribution in spherules (Das Gupta et al. 2017; Ray et al. 2017). Ray et al. (2017) suggested that the major oxides and moderately siderophile element composition of the impact spherules were most compatible with the admixture of 6 wt% of an EH-type chondrite. This hypothesis is based on the evaluation of the lowest error sum of squares (SS²) of calculated mixtures involving various types of chondrites, and using major oxide, Ni, Co, and Cr abundances. SS² values were systematically low for a chondrite proportion between 5 and 10 wt%, but the lowest value was obtained for an EH-type chondrite when its proportion in the mixture was ~6 wt%. However, the involvement of other types of chondrites remains possible, and thus, the exact nature of the Lonar impactor remains to be confirmed.

In contrast to previous studies, we use a different approach that is based on the chromium isotopic characterization of some impact rocks from the Lonar crater to assess the nature of the impactor. Although Cr is not the most sensitive element to detect small amounts of meteoritic components in terrestrial material, it is the only one capable of characterizing the nature of a large variety of meteorite types (e.g., Moynier et al. 2009). Well-resolved Cr isotopic differences do exist between meteorites and Earth (e.g., Lugmair and Shukolyukov 1998; Trinquier et al. 2007, 2008a; Qin et al. 2010), and make the Cr isotopic composition of each chondrite group unique in a $\epsilon^{54}$Cr versus $\epsilon^{53}$Cr diagram (Fig. 1), with $\epsilon^{54}$Cr and $\epsilon^{53}$Cr the deviation in parts per 10,000 of $^{54}$Cr/$^{52}$Cr and $^{53}$Cr/$^{52}$Cr ratios normalized to a terrestrial standard. If at least 1% of the impactite Cr has an extra-terrestrial origin, it is possible to detect and identify the type of meteorite (cf. Koeberl et al. 2002). This approach has been successfully used for the identification of the impactors involved in the formation of the Morokweng, Bosumtwi, Clearwater East, Lappajärvi, El'gygytgyn, and Rochechouart (Koeberl et al. 2007; Foriel et al. 2013) impact structures as well as for ejecta layers (e.g., Lugmair and Shukolyukov 1998; Kyte et al. 2003, 2011;

Trinquier et al. 2006; Quitté et al. 2007; Simonson et al. 2009; Magna et al. 2017; Mougel et al. 2017). Therefore, we took advantage of this sensitive method to investigate the type of chondrite involved in the formation of the Lonar crater.

## SAMPLES AND METHODS

Impactites were collected by one of us (C.K.) from different locations, at or near the rim of Lonar crater. They exhibit a variety of shapes and textures from splash form to irregular shape, and from compact to vesicular textures, and sometimes flow-textured surface. The samples selected for chemical processing are small-sized ejecta including impact glass, impact melt, and impact melt breccia (see Table 1). Sample LO-15-1 is a frothy impact glass collected in the northwest side of the crater; it is somewhat irregular in shape, with abundant bubbles, and had an initial weight of 20.3 g. Sample LO-13-1-1 is a dense impact glass, with an initial mass of 14.5 g, irregular in shape, with few bubbles, and collected in the southern area of the crater. Sample LO-10-2-1 is a dense, irregularly shaped impact glass, 4.2 g original mass, and contains few bubbles. Sample LO-10-05 is a large and compact, irregularly shaped impact glass, with an original mass of 60.2 g, and containing very few and small bubbles. These two samples are from the northern area, close to the Lonar village. Sample LHN-05-65 is a large, dense impact glass with smooth, shiny surface, an original mass of 136.2 g, and containing few bubbles. Finally, samples LO-60D and LO-72 are blocky impact melt and impact melt breccia samples, respectively, of initial weight of several hundred grams, collected in 2000 just outside the southeastern part of the crater rim. They are similar to those shown in Osae et al. (2005). Their major and trace element signatures are relatively homogeneous and consistent with a basaltic composition, in the range of the Deccan basalt target rocks (Osae et al. 2005). Further information about the samples analyzed in this study (including detailed geochemical data, map, as well as location and petrography information) can be found in Osae et al. (2005), Son and Koeberl (2007), and Jourdan et al. (2011).

All chemical separations were performed at the Institut de Physique du Globe de Paris, France. Between 150 and 350 mg of bulk impactites were dissolved in a mixture of concentrated HF and $HNO_3$ (ratio 3:1) in Teflon vials at 140 °C (including multiple ultrasonication steps) until complete dissolution. After centrifugation and filtration, no residual gels and/or refractory phases were observed during the final inspection of the solution. The chemical procedure was
<figure><img src="image_5.png" /><figcaption>Figure 1. ε<sup>53</sup>Cr versus ε<sup>54</sup>Cr plot. Comparison between Lonar impactites (gray diamonds), the Deccan Trap basalt (white circle, at 0%), terrestrial Cr standard (white square = Earth), average Earth composition (striated square), chondrites (black circles), and other meteorites (white squares). White dashed circles represent the binary mixing model between Deccan trap basalt and Murchison chondrite and the amount of impactor component considered in the admixture (0%, 1%, 2%, 3%, and 100%), respectively. The dashed lines represent the 95% confidence envelop of the Lonar samples linear regression (R<sup>2</sup> = 0.75). (See Table 1 and manuscript for references.)</figcaption></figure>

adopted from Trinquier et al. (2008b) and includes three separation steps of Cr on cationic exchange resin AG50W-X8, allowing a ≥90% yield. Note that while the chromium isotopic method originally developed for the study of chondritic meteorites (Cr-rich) is complex and time-consuming to reach the highest precision, in this study, the chemistry was adapted to the analysis of Cr-poor terrestrial crustal rocks. The mass of sample material that has to be processed is higher (up to 10 times what is usually used for meteorite work), which considerably extends the purification step time. In order to avoid heavy matrix load on the chromatographic columns, which could generate some unwanted isotopic fractionation (Trinquier et al. 2008b), the size of the first column was doubled and the initial digestion of samples was split into fractions equivalent to ≤60 mg per column. They were recombined at the end of the elution. This procedure has been successfully tested on

BHVO-2 standard (95% of Cr recovered), which has a basaltic matrix similar to Lonar samples. The ⁵³Cr/⁵²Cr and ⁵⁴Cr/⁵²Cr isotope ratios (see Table 1) were measured by multicollection (nine cups) thermal ionization mass spectrometry (TIMS), using a Fisher Scientific Triton instrument, and normalized using an exponential law to ⁵²Cr/⁵⁰Cr = 19.28323 (Shields et al. 1966). A purified Cr fraction, corresponding to 1–2 µg (conventional method), was loaded in chloride form on W filament, together with an Al-silica gel-H₃BO₃. Each sample was loaded twice. Cr isotopic data are given in ε-units relative to NIST SRM 3112a Cr terrestrial standard. All the uncertainties reported for the samples are twice the standard errors (2 SE). The reproducibility of the standards was 7 and 15 ppm (2 SD) for ⁵³Cr/⁵²Cr and ⁵⁴Cr/⁵²Cr, respectively. The Cr isotopic composition of one half of the samples (see Table 1) was measured at the Institut de Physique du Globe de Paris following
<table><caption>Table 1. Chromium isotope compositions (ε⁵³Cr and ε⁵³Cr) and Cr, Ni, Co abundances (ppm) of bulk impact rocks from Lonar crater, Murchison meteorite, target Deccan trap basalt.</caption><thead><tr><th>Samples</th><th>ε<sup>53</sup>Cr</th><th>2 s.e.</th><th>ε<sup>54</sup>Cr</th><th>2 s.e.</th><th>Ref.</th><th>Location</th><th>Cr</th><th>Co</th><th>Ni</th><th>Ni/Cr</th><th>Ni/Co</th><th>Co/Cr</th></tr></thead><tbody><tr><td>**LO10-05 (impact glass)</td><td>0.09</td><td>0.05</td><td>0.08</td><td>0.07</td><td>1</td><td>N</td><td>109</td><td>41.4</td><td>46</td><td>0.42</td><td>1.11</td><td>0.38</td></tr><tr><td>**LO10-2-1 (impact glass)</td><td>0.12</td><td>0.05</td><td>0.26</td><td>0.10</td><td>1</td><td>N</td><td>98.1</td><td>41.3</td><td>136</td><td>1.39</td><td>3.29</td><td>0.42</td></tr><tr><td>**LO13-1-1 (impact glass)</td><td>0.09</td><td>0.05</td><td>0.20</td><td>0.07</td><td>1</td><td>S</td><td>63.9</td><td>41.2</td><td>55</td><td>0.86</td><td>1.33</td><td>0.64</td></tr><tr><td>*LO-15-1 (impact glass)</td><td>0.08</td><td>0.06</td><td>0.22</td><td>0.09</td><td>1</td><td>NW</td><td>81.1</td><td>48.9</td><td>206</td><td>2.54</td><td>4.21</td><td>0.60</td></tr><tr><td>**LHN05-65 (impact glass)</td><td>0.18</td><td>0.04</td><td>0.22</td><td>0.10</td><td>1</td><td>Other</td><td>77.5</td><td>41.8</td><td>68</td><td>0.88</td><td>1.63</td><td>0.54</td></tr><tr><td>*LO-60D (impact melt rock)</td><td>-0.03</td><td>0.05</td><td>-0.13</td><td>0.06</td><td></td><td>SE</td><td>n.a</td><td>n.a</td><td>n.a</td><td>n.a</td><td>n.a</td><td>n.a</td></tr><tr><td>*LO-72 (melt-bearing impact breccia)</td><td>0.19</td><td>0.03</td><td>0.48</td><td>0.06</td><td></td><td>SE</td><td>n.a</td><td>n.a</td><td>n.a</td><td>n.a</td><td>n.a</td><td>n.a</td></tr><tr><td>*BHVO-2 (basalt)</td><td>0.03</td><td>0.04</td><td>0.07</td><td>0.07</td><td></td><td>n.a</td><td>280</td><td>45</td><td>119</td><td>0.43</td><td>2.64</td><td>0.16</td></tr><tr><td>**BHVO-1 (basalt)</td><td>0.05</td><td>0.08</td><td>0.07</td><td>0.13</td><td></td><td>n.a</td><td>n.a</td><td>n.a</td><td>n.a</td><td>n.a</td><td>n.a</td><td>n.a</td></tr><tr><td>Deccan trap basalt (target rock)</td><td>0.03</td><td>0.08</td><td>0.08</td><td>0.13</td><td>2</td><td>n.a</td><td>117</td><td>48</td><td>90</td><td>0.77</td><td>1.88</td><td>0.41</td></tr><tr><td>Average submillimeter-sized spherules</td><td>n.a</td><td>n.a</td><td>n.a</td><td>n.a</td><td>3</td><td>n.a</td><td>321</td><td>103</td><td>1138</td><td>3.55</td><td>11.05</td><td>0.32</td></tr><tr><td>Murchison chondrite (CM)</td><td>0.27</td><td>0.06</td><td>1.01</td><td>0.05</td><td>4</td><td>n.a</td><td>3050</td><td>560</td><td>12,300</td><td>4.03</td><td>22.0</td><td>0.18</td></tr></tbody></table>

*Cr isotopic data measured by TIMS at the Institut de Physique du Globe de Paris (see Göpel et al. 2015).

**Cr isotopic data measured by TIMS at the Natural History Museum of Denmark of Copenhagen (see Van Kooten et al. 2016)

1: Compositions from Son and Koeberl (2007).

2: Cr isotopic composition from Trinquier et al. (2006) and Cr, Co, Ni concentrations from Osae et al. (2005).

3: Composition from Ray et al. (2017).

4: Composition from Trinquier et al. (2007, 2008b).

the method detailed in Göpel et al. (2015). The Cr isotopic composition of the other sample set was analyzed at the Natural History Museum of Denmark (Copenhagen), using a filament exhaustion sample standard bracketing approach (for details, see Van Kooten et al. 2016). One of the advantages of the total evaporation mode is that it ensures rapid and reproducible measurement of >90% of the transmitted sample, and thus does not require as large a quantity of Cr as for the conventional method.

## CHROMIUM ISOTOPIC SIGNATURE OF LONAR IMPACT ROCKS

The results are presented in Table 1, together with complementary data from the earlier literature. The Cr isotopic compositions of the terrestrial basalts BHVO-1 (measured in Copenhagen) and BHVO-2 (measured in Paris) are consistent and show typical terrestrial value (e.g., Mougel et al. 2018). Note also the fact that the consistency of independent data sets on two instruments supports the absence of instrumental bias. The Lonar impactites exhibit some variations in isotopic compositions ranging from -0.03 ± 0.05 to 0.19 ± 0.03 in ε⁵³Cr and from -0.13 ± 0.06 to 0.48 ± 0.06 in ε⁵⁴Cr. These isotopic compositions do form a trend, as shown in Fig. 1. According to the data available in Osae et al. (2005), and Son and Koeberl (2007) reported in Table 1, the samples do not show any particular Cr enrichment for these samples and fall in the range of the Lonar target basalt average composition of

117 ± 69 ppm (Osae et al. 2005). Among the seven samples that have been analyzed, five (LO15; LHN05-65; LO10-2-1; LO13-1-1; LO-72) have distinct nonterrestrial Cr isotopic compositions as indicated by resolved positive anomalies and two of them (LO-60D; LO10-05) fall in the range of terrestrial compositions.

## DISCUSSION

Traditionally, the Cr isotopic composition of all terrestrial rocks is considered as being constant and similar to the standard used for normalization. This was supported by the analyses of a few terrestrial samples, which are in agreement with the standard reference value (e.g., Trinquier et al. 2006; Koeberl et al. 2007). However, Mougel et al. (2018) have recently shown that the ε⁵³Cr and ε⁵⁴Cr compositions of a wide variety of terrestrial rocks exhibit small variations independently of age and location and were shifted toward slightly positive values compared to the Cr standard reference. While the reasons for such variations have not yet been determined, it is important to mention that this might have implications on the interpretation of small anomalies in impact rocks. Because these rocks represent the admixture of a minor meteoritic component to terrestrial material, the reported Cr isotopic anomalies can potentially be biased depending on the terrestrial reference chosen. One way to prevent this kind of misinterpretation could be a systematic evaluation of the Cr isotopic composition of target rocks involved in each impact structures.
As previously noted, the Lonar crater is located within the Deccan traps, whose basalt shows no discernable Cr isotopic anomalies ($ε^{53}$Cr = 0.03 ± 0.08 and $ε^{54}$Cr = 0.08 ± 0.15; Trinquier et al. 2006). This value falls within the average composition of terrestrial rocks (Mougel et al. 2018) (Fig. 1). Considering the Cr isotope composition of the Deccan basalt as the best representative of the target rock for the Lonar impact structure, five of the seven impactite samples have isotopic compositions that fall outside of the terrestrial range, and confirm the presence in these rocks of meteoritic Cr. Note that large isotopic anomalies were not expected in Lonar impact rocks because (1) the Cr isotopic variations between meteorites are small (the variations in $^{53}$Cr/$^{52}$Cr and $^{54}$Cr/$^{52}$Cr do not extend over more than 1 and 3 ε-units, respectively, and when compared to the terrestrial value, they do not exceed $ε^{53}$Cr = -1, i.e., ureilites, and $ε^{54}$Cr = 1.7; i.e., CI chondrites), (2) the meteoritic signal is largely diluted into the target rock during the impact (only few %), and (3) basalt contains higher Cr concentrations (up to several hundreds ppm, e.g., $[Cr]_{BHVO_2}$ = 280 ppm) than most sedimentary rocks (only dozens of ppm) more often involved in impact structures, such as the K-T impact event ($[Cr]_{target-rock}$ = 44 ppm; Trinquier et al. 2006) or the Zhamanshin impact event ($[Cr]_{target-rock}$ <20 ppm; Jonášová et al. 2016). Thus, the usual contrast in Cr abundances between target rock and impactor is reduced for a basaltic target.

More importantly, these data define a trend ($R^2$ = 0.75) in a $ε^{54}$Cr versus $ε^{53}$Cr diagram (Fig. 1), which can be interpreted as a mixing line between the impactor and the target rock, and can be used to discuss the type of impactor involved. Although this mixing line does not trend precisely toward a specific group of chondrite, it falls closer to the carbonaceous chondrites (more specifically the CM group) than any other class of meteorites.

The $ε^{54}$Cr values in the Lonar samples are by far too high to be due to enstatite or ordinary chondrites, or achondrites; and carbonaceous chondrites are the only group showing both positive $ε^{54}$Cr and $ε^{53}$Cr as is observed in these samples. Therefore, in contrast to previous speculation concerning the nature of the Lonar impactor, the Cr isotopes show that a carbonaceous chondrite object must be involved with the origin of the Lonar crater. The absence of Cr enrichments in bulk impactites, despite the input of chondritic material, is nonintuitive because the contribution of chondritic material would expect to be reflected in the Cr abundances as well, as shown by the correlation between $ε^{54}$Cr and 1/Cr in KT ejecta samples presented in Trinquier et al. (2006) or by coupled Cr enrichments and isotopic anomalies in impact glasses from the

Zhamanshin impact structure (Jonášová et al. 2016; Magna et al. 2017). This is, however, not systematic; other studies like Foriel et al. (2013) and Mougel et al. (2017) have reported Cr isotopic anomalies in impactites without being correlated with siderophile element enrichments. This decoupling was attributed to the large chemical abundance variability of these elements in the target rock as well as their nonuniform redistribution in the impactites. In fact, little is known about chemical processes during the formation of impact-derived products. The high-energy character of impact events is connected with extreme pressure and temperature conditions for which the behavior of many elements remains poorly understood. The development of hypervelocity impact experiments is recent; however, they show the existence of significant interelement fractionation occurring during mixing of projectile and target material, and cooling. For example, Ebert et al. (2014) tested the effect of petrophysical properties (i.e., porosity, water saturation) of distinct target material on the partitioning of Cr, V, Fe, Co, Mo, and W in impact-induced products from steel spheres in hypervelocity impacts. In their study, the authors show that less porous target material reaches higher impact pressures and higher impact (and postimpact) temperatures than more porous material. It also reacts longer at high temperatures, which results in enhancing interelement fractionation during mixing of projectile and target material. Therefore, the composition of impact melts formed can differ strongly from the respective element ratios of the projectile and initial target. The determination of the projectile type via trace element ratios or direct abundances in terrestrial impactites could lead to misleading interpretations, in particular if they are used for quantitative purposes.

In the case of Lonar, the target rock is basaltic; therefore, fractionation effects mentioned above are likely to happen and affect the distribution of Cr and other siderophile element content in the impactites. Then, because target rocks exhibit a large variability in Cr abundances (from 45 to 186 ppm), the addition of a few % (or less) of chondritic material in the heterogeneous target rock would not necessarily drive Cr concentrations out of the range of the Deccan basalt (Osae et al. 2005), although it would increase sufficiently the fraction of Cr coming from the impactor so that it can be detected in the isotopic composition. This could explain the absence of Cr enrichment at bulk-rock scale and correlation with isotope ratios. However, these enrichments in moderately siderophile elements do exist at the scale of spherules (Misra et al. 2009; Ray et al. 2017) although only in submillimeter-sized spherules.

In their studies, Ray et al. (2017) and Das Gupta et al. (2017) show that millimeter-sized spherules have Ni
and Cr abundances similar to those of target and impact rocks, whereas submillimeter-sized spherules can be 2.5–12.6 times richer in these elements. Misra et al. (2009) also noted, in some spherules, very high and variable Ni content compared to lower and more constant Cr and Co. Therefore, it appears that moderately siderophile elements were strongly partitioned inside the impactites itself, with Cr, Co, and Ni preferentially hosted in the smallest spherules. However, although there is no doubt that these enrichments represent evidence for the addition of impactor material, these abundances and interelement ratios cannot be confidently used as a proxy to calculate the amount of impactor because they may have been altered by selective and multiple redistribution steps during or subsequent to the impact.

Cr isotopes are not fractionated during impact processes, and the isotopic anomalies observed represent undeniable evidence for the presence of meteoritic chromium. They can be used to evaluate the type of impactor involved, and the contribution required to generate these anomalies. The result of admixture calculations between target rock and CM-type chondrite is illustrated in Fig. 1. It represents a simple binary mixing solution for various fractions of impactor from 0% to 100%, and the ε⁵³Cr and ε⁵⁴Cr increases related to the progressive addition in the mixture of chondritic component. The compositions chosen for the target rock and the impactor are those of the Deccan trap basalt and the Murchison chondrite, respectively (see Table 1). The goals of this first-order modeling were to (1) test if the mixing between a CM-type object and the target rock basalt could reproduce the composition of the Lonar impactites showing anomalies, (2) to determine at first order the amount of chondritic chromium contained at the bulk level in these rocks, and (3) to compare it to previous results from the literature that were based on other geochemical proxies. These calculations indicate that, within error, the ε⁵³Cr and ε⁵⁴Cr enrichments observed in the Lonar-selected impactites are compatible with a mixture containing 1–3% of chondritic Cr from a CM-type impactor. This is supported by the fact that most of the samples show nonterrestrial Cr isotopic compositions that plot very close to this mixing line (except LHN 05-65). This estimate of 1–3% of chondritic fraction in the Lonar target matrix is consistent with the amount of impactor material found in impact rocks from other craters (e.g., Zhamanshin, Magna et al. 2017; Morokweng, Koeberl and Reimold 2003) or impact layers (e.g., Wittenoom layer, Simonson et al. 2009; Grænsesø layer, Mougel et al. 2017). This value is slightly lower than the latest estimates (6–8%) from other studies of Lonar crater samples (Misra et al. 2009; Das Gupta et al. 2017; Ray et al. 2017), but higher than those obtained from Os

isotopic compositions (Schulz et al. 2016). The lower meteoritic component estimated here may derive from the fact that we analyzed bulk rocks, like Schulz et al. (2016), whereas estimates from Ray et al. (2017) and Das Gupta et al. (2017) are based on submillimeter-sized spherules, which show a clear enrichment in moderately siderophile elements, thus likely a higher impactor contribution despite possible fractionation. In this context, our estimate based on the Cr isotopic method is in agreement with previous works, as well as it confirms the chondritic nature of the impactor, although it indicates a carbonaceous chondrite origin.

## CONCLUSIONS

Chromium is a powerful tracer for the detection and identification of the nature of objects that collided with the Earth over its history, and for which we have limited evidence. Some Lonar crater impactites have been analyzed for their Cr isotopic compositions. Our results show the presence of chondritic Cr in the majority of the measured samples. In contrast to recent studies that speculated upon the contribution by an enstatite chondrite impactor, our new isotopic data, showing positive ε⁵³Cr and ε⁵⁴Cr, suggest a better affinity with 1–3% of a carbonaceous chondrite component, possibly of CM type.

*Acknowledgments*—We thank the ERC under the European Community's H2020 framework program/ERC grant agreement # 637503 (Pristine) and the ANR for a Chaire d'Excellence Sorbonne Paris Cité (IDEX13C445) and for the UnivEarthS Labex program (ANR-10-LABX-0023 and ANR-11-IDEX-0005-02). Parts of this work were supported by IPGP multidisciplinary program PARI and by Region Île-de-France SESAME (grant no. 12015908). Editor Jeff Plescia and reviewers A. Trinquier and S. Goderis are thanked for the time spent on the manuscript, and their constructive and helpful comments.

*Editorial Handling*—Dr. Jeffrey Plescia

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