Deborah Chavrit
email: deborahchavrit@gmail.com

Manuel A Moreira
email: moreira@ipgp.fr

David A Fike
email: dfike@levee.wustl.edu

Frédéric Moynier
email: moynier@ipgp.fr

Déborah Chavrit

Déborah Chavrit

Unusual neon isotopic composition in Neoproterozoic sedimentary rocks: Fluorine bearing mineral contribution or trace of an impact event?

Keywords: noble gas, Ediacaran sedimentary rocks, Ne-A, nucleogenic, fluorite, Acraman impact Highlights

Introduction

The accretion of extraterrestrial material falling onto Earth can be tracked using geochemical tracers, which are more abundant and isotopically distinct in the extraterrestrial materials compared to the Earth's surface. For example, Ir concentrations, He isotopes and Os isotopes (e.g. [START_REF] Peucker-Ehrenbrink | Iridium and osmium as tracers of extraterrestrial matter in marine sediments[END_REF]) and Cr isotopes [START_REF] Mougel | Chromium isotope evidence in ejecta deposits for the nature of Paleoproterozoic impactors[END_REF] are particularly well suited geochemical tracers. Amongst those elements, He has the largest abundance variations between extraterrestrial and terrestrial material, with 3 He abundances and 3 He/ 4 He ratios in extraterrestrial material respectively ∼10 9 and ∼10 5 times higher than that of the Earth's crustal rocks (see review in [START_REF] Mcgee | Extraterrestrial He in sediments: from recorder of asteroid collisions to timekeeper of global environmental change[END_REF]). Consequently, the measurements of He isotopes in marine sediments, or sedimentary rocks of different ages have been the subject of numerous studies for several decades, in order to track the accretion of extraterrestrial material onto Earth over geological timescales (e.g. [START_REF] Amari | Extra-terrestrial noble gases in deep-sea sediments[END_REF][START_REF] Farley | A late Miocene dust shower from the break-up of an asteroid in the main belt[END_REF][START_REF] Marcantonio | A comparative study of accumulation rates derived by He and Th isotope analysis of marine sediments[END_REF][START_REF] Murphy | An extraterrestrial 3He-based timescale for the Paleocene-Eocene thermal maximum (PETM) from Walvis Ridge, IODP Site 1266[END_REF][START_REF] Takayanagi | Temporal variation of 3 He/ 4 He ratio recorded in deep-sea sediment cores[END_REF])) However, since He is an element with a high diffusivity [START_REF] Hiyagon | Retention of Solar Helium and Neon in IDPs in Deep Sea Sediment[END_REF], it is likely to be partially or totally lost from the samples either during their entry into the atmosphere of after their deposition at the Earth's surface. For instance, among the extraterrestrial micrometeorites falling onto Earth, He can only be retained in the smallest ones (e.g. <10µm) because, due to their micrometric size, they experience less frictional heating during their entry in the Earth atmosphere than bigger objects [START_REF] Farley | Helium-3 evidence for a comet shower in the late Eocene[END_REF].

Although the oldest samples with a record of extraterrestrial 3 He are 480 Ma old carbonates [START_REF] Patterson | Preservation of extraterrestrial 3He in 480-Ma-old marine limestones[END_REF], due to its high diffusivity, He is likely to be partially or totally lost during its stay at the surface for millions of years. Thus, it is interesting to focus on Ne isotopes, because Ne is less sensitive to diffusion than He. According to the diffusion experiments of [START_REF] Hiyagon | Retention of Solar Helium and Neon in IDPs in Deep Sea Sediment[END_REF], Ne would be retained in IDP at typical seafloor temperatures during the transit times of the ocean crust on the seafloor, unlike He, which would be significantly released from the IDP at these temperatures over million-year timescales.

Moreover, Ne isotopes have also distinct signatures in extraterrestrial materials compared to the Earth's surface. The Ne IDP signature is close to that of the Ne-B value (12.52 ±0.18, (Black,4/24 1972), 12.73±0.02, [START_REF] Moreira | Noble gas constraints on the origin and evolution of earth's volatiles[END_REF][START_REF] Moreira | Origin of the neon in Earth and in chondrites[END_REF] and has been recently recorded in marine sediments together with IDP-derived He [START_REF] Chavrit | Estimation of the extraterrestrial 3He and 20Ne fluxes on Earth from He and Ne systematics in marine sediments[END_REF]. Ne-B results from irradiation and sputtering processes in space [START_REF] Moreira | Noble gas constraints on the origin and evolution of earth's volatiles[END_REF]. As such, it preferentially is found in the outer 100 nm of objects and thus recovered in objects with a high surface to volume ratio such as IDP. Bigger objects such as meteorites can also include the Ne-B component if they derive from a regolith, however, they can additionally contain the Ne-A component (e.g. [START_REF] Black | On the origins of trapped helium, neon and argon isotopic variations in meteorites-I. Gas-rich meteorites, lunar soil and breccia[END_REF][START_REF] Pepin | Trapped neon in meteorites[END_REF]), which has been recorded in pre-solar nano-diamonds [START_REF] Huss | Noble gases in presolar diamonds II : Component abundances reflect thermal processing[END_REF] and is particularly found in carbonaceous chondrites (see review in [START_REF] Moreira | Noble gas constraints on the origin and evolution of earth's volatiles[END_REF]).

Those two extraterrestrial signatures are distinct from the one that characterizes the material at the surface of the Earth. Combining He and Ne isotopes can allow to assess the amount of He lost from the samples [START_REF] Chavrit | Estimation of the extraterrestrial 3He and 20Ne fluxes on Earth from He and Ne systematics in marine sediments[END_REF]. Thus, this could be used to trace the accretion of different size ranges of the extraterrestrial material falling onto Earth. Neon being less sensitive to diffusion than He, it has the potential to trace different size ranges of extraterrestrial objects accreting onto Earth (from micrometeorites to meteorites).

In this study, we coupled He and Ne isotopic measurements in samples of ∼550 to ∼600 Ma old sedimentary rocks from the same location in Oman in order to trace the extraterrestrial material accretion during this time period.

Samples and geological context

The samples are 13 limestones, dolostones and calcareous shale from the Ediacaran-aged Huqf Supergroup in Oman. Samples come from MQR-1 well and span a depth range from 3228 -3936 m, corresponding to the Khufai, Shuram, and Buah formations of the Nafun group. The samples are believed to have been deposited in the interval between ∼600 Ma to ∼550 Ma (Figure 1) [START_REF] Bowring | Geochronological constraints on terminal Neoproterozoic events and the rise of Metazoan[END_REF][START_REF] Condon | U-Pb Ages from the Neoproterozoic Doushantuo Formation[END_REF][START_REF] Fike | Oxidation of the Ediacaran Ocean[END_REF][START_REF] Grotzinger | Biostratigraphic and geochronologic constraints on early animal evolution[END_REF][START_REF] Guerroué | 50 Myr recovery from the largest negative δ13C excursion in the Ediacaran ocean[END_REF], although the middle Ediacaran interval has notoriously poor chronostratigraphic resolution, frustrating attempts to link together records of environmental and ecological change over this critical interval of Earth history.

These samples have been previously characterized for their carbon and sulfur isotopic composition [START_REF] Burns | Carbon isotopic record of the latest Proterozoic from Oman[END_REF][START_REF] Fike | Oxidation of the Ediacaran Ocean[END_REF]. The data record the largest known 5/24 δ 13 Ccarbonate negative excursion in Earth history, called the Shuram excursion, which is characterized by δ 13 Ccarb values decreasing to -12‰ [START_REF] Fike | Oxidation of the Ediacaran Ocean[END_REF], Figure 1). The origin of this excursion is still subject to debate. The Shuram excursion was first attributed to diagenetic alteration [START_REF] Burns | Carbon isotopic record of the latest Proterozoic from Oman[END_REF]. However, methods to assess the diagenesis process have failed to validate this hypothesis (Le Guerroué, 2010). In addition, this excursion has been documented in multiple sites in the world, and it is more likely that the δ 13 Ccarbonate values reflect a primary signal encoded into the sediments during deposition [START_REF] Fike | Oxidation of the Ediacaran Ocean[END_REF][START_REF] Grotzinger | Enigmatic origin of the largest-known carbon isotope excursion in Earth's history[END_REF][START_REF] Le Guerroué | Veracity of Neoproterozoic negative C-isotope values: the termination of the Shuram negative excursion[END_REF].

Methods

He and Ne isotopes measurements

The sediment sample preparation is the same as in [START_REF] Chavrit | Estimation of the extraterrestrial 3He and 20Ne fluxes on Earth from He and Ne systematics in marine sediments[END_REF]. Fractions of powdered samples weighting between 0.2 and 2 g (Table 1) were decarbonated on a heating plate at 60°C using 10% acetic acid. For noble gas analysis, the carbonate fraction of the samples has to be removed to improve the gas purification. After three rinsing with milliQ water, each residue was dried at 60°C and packed into 1 or 2 Al foil packets, allowing for the measurement of duplicates.

Sample 3404 was prepared twice. The packets were loaded into a sample tree made of Pyrex connected to the mass spectrometer. The extraction line was baked out at ∼120°C and sample tree baked out at ∼60°C for ∼12 h during pumping prior to the measurements. The gases were extracted from the samples under high vacuum using a furnace at 1200°C. After purification, the 3 He, 4 He, 20 Ne, 21 Ne and 22 Ne were sequentially analyzed using the Noblesse mass spectrometer (Nu instruments) located at the Institut de Physique du Globe de Paris (IPGP), France, following the method described in [START_REF] Chavrit | Estimation of the extraterrestrial 3He and 20Ne fluxes on Earth from He and Ne systematics in marine sediments[END_REF]. Together with an air standard for Ne, the isotopic compositions of the homemade gas standard used to calibrate the data are the following:

3 He/ 4 He=7.73 RA, 20 Ne/ 22 Ne=9.94, 21 Ne/ 22 Ne=0.0309. On average, the furnace blank values, made at a temperature of 1200°C, were 6×10 -9 cm 3 STP for 4 He and 3.6×10 -12 cm 3 STP for 22 Ne.

Seven duplicates were analyzed in order to assess the reproducibility of the measurements.

Duplicates of each sample were all from the same residual fraction which was split into several

6/24

Al foil packets, except for sample 3404 which aliquots 2 and 3 were from a residual fraction different from that of aliquot 1 (Table 1).

In order to understand the origin of the Ne composition of the samples, we publish data of 3 Durango fluorapatites (Table 1) analyzed in 2006 by laser heating (Yb doped fiber laser) using the ARESIBO mass spectrometer at the IPGP [START_REF] Moreira | Rare gas systematics on Mid Atlantic Ridge (37°-40°)[END_REF].

Results

The results are reported in Table 1. The non-carbonate fractions (NCF) range from 11% for the dolostone at 3428 m depth to 73% for the calcareous shale at 3722 m depth. 3 He/ 4 He ratios range between 0.006±0.003 RA (3228#1) and 0.27±0.01 RA (3404#2). RA is the atmospheric ratio, with a value of 1.384×10 -6 (Clarke et al., 1976b). The three highest 3 He/ 4 He ratios are similar and correspond to the 3 duplicates of 3404, indicating a good reproducibility of the results. 3 He contents range between 0.6 and 31 ×10 -13 cm 3 STP.g -1 . They are in the same range as the data from the literature on samples that show evidence for an extraterrestrial 3 He contribution (Figure 2). In addition, our He data show a positive correlation between the 3 He content and the 3 He/ 4 He ratio, confirming the mixing of an extraterrestrial IDP component and a terrestrial component in the samples. 3 He/ 4 He positively correlates with 4 He and 3 He meaning that these two isotopes contribute to the variation of the ratio, which can be related both to the contribution of radiogenic 4 He from the terrestrial component and to the contribution of 3 He from the extraterrestrial component. 20 Ne/ 22 Ne ratios are close to or below the atmospheric ratio of 9.8 [START_REF] Eberhardt | A redetermination of the isotopic composition of atmospheric neon[END_REF], with the minimum values equal to 9.05±0.03 for sample 3636 (Figure 3). 21 Ne/ 22 Ne ratios show a high range of variation, going from 0.0345±0.0009 (3360#1) to up to three times the atmospheric ratio of 0.0290 [START_REF] Eberhardt | A redetermination of the isotopic composition of atmospheric neon[END_REF] with the data of sample 3636 ( 21 Ne/ 22 Ne=0.0935±0.0023).

In Figure 1, different trends are highlighted in the evolution of the NCF and the noble gases isotopic ratios over time and depth. From 3936 m to 3722 m, the NCF progressively increases from 16% to reach a maximum value of 73%. Then, the NCF follows a constant decrease down to 11% at 3428 m, followed by a relatively regular increase up to 30% at 3228 m. 3 He/ 4 He ratios 7/24 do not show a clear evolution, apart from two sharp positive peaks at 3826 m and 3404 m. 20 Ne/ 22 Ne ratios and 21 Ne/ 22 Ne ratios show an opposite evolution. Whereas 20 Ne/ 22 Ne ratios decrease progressively from 3936 m to reach a minimum value at 3636 m, the 21 Ne/ 22 Ne ratios increase. Between 3636 m and 3428 m, the 20 Ne/ 22 Ne ratios increases whereas the 21 Ne/ 22 Ne ratio decreases. At 3404 m, the 20 Ne/ 22 Ne ratios show a sharp negative spike, associated to a positive 3 He/ 4 He peak. Three duplicates of the sample at 3404 m show the same composition (Table 1) excluding a possible analytical problem during the measurement.

Discussion

In this part, we will highlight an unusual Ne isotopic signature. We will detail the two hypotheses that could explain such value.

The recording of an unusual Ne signature

In a 20 Ne/ 22 Ne vs 21 Ne/ 22 Ne diagram, our new data do not plot in the same area as previously analyzed terrestrial sediments (Figure 3), that showed evidence of IDP derived Ne [START_REF] Chavrit | Estimation of the extraterrestrial 3He and 20Ne fluxes on Earth from He and Ne systematics in marine sediments[END_REF]. It does not necessarily mean that IDP are absent from the samples. More preferentially, the Ne IDP signal may be overprinted, as it is also the case for helium, because the samples contain much higher NCF than the samples of [START_REF] Chavrit | Estimation of the extraterrestrial 3He and 20Ne fluxes on Earth from He and Ne systematics in marine sediments[END_REF] (Annex Figure A1 of the supplementary material).

Except for the sample at 3404 m, our samples fall into the field defined by crustal fluids [START_REF] Holland | Deep fracture fluids isolated in the crust since the Precambrian era[END_REF][START_REF] Kennedy | Crustal neon: a striking uniformity[END_REF][START_REF] Lippmann-Pipke | Neon identifies two billion year old fluid component in Kaapvaal Craton[END_REF] or by the continental crust from the KTB drill in Germany [START_REF] Drescher | The rare gas inventory of the continental crust, recovered by the KTB continental deep drilling project[END_REF]. Sample 3404 presents an unusual Ne isotopic composition, with a 20 Ne/ 22 Ne ratio lower than that of the other samples at similar 21 Ne/ 22 Ne ratio and falls outside this crustal fluid field. The three analyses of the Ne isotopic composition of this sample are in excellent agreement (Table 1, Figure 3) indicating that this signature is not an analytical artifact. In order to better understand this Ne isotopic composition, we have calculated the slope of the mixing line going through each sample and the air component in the Ne three isotopes diagram. In Figure 4, these slopes do not show a progressive evolution with depth of the samples while the 3404m sample shows a strong negative spike. Consequently, this 8/24 unusual Ne isotopic signature seems to be localized and to only affect the layer at 3404 m. It corresponds to the late stage of the Shuram excursion, at a slope break where the δ 13 C starts to recover more rapidly, at the beginning of the Buah formation (Figure 1). In the following we are going to explore the two hypotheses that could explain such composition.

A F-bearing mineral for the unusual Ne signature?

Neon nucleogenic production in Earth samples

In the continental crust, the radioactive decay of 235,238 U and 232 Th produce α particles, most of which stabilize in radiogenic 4 He. The other α particles react with surroundings atomic nuclei, such as 17 O, 18 O and 19 F isotopes to produce respectively nucleogenic 20 Ne, 21 Ne and 22 Ne (Figure 5). 20 Ne, 21 Ne and 22 Ne are also produced by neutron activation from 23 Na, 24 Mg, 25 Mg (Figure 5), however, these reactions account for less than 1% of the Ne production in the crust [START_REF] Yatsevich | Production of nucleogenic neon in the Earth from natural radioactive decay[END_REF]. Based on the previous works of [START_REF] Yatsevich | Production of nucleogenic neon in the Earth from natural radioactive decay[END_REF] and [START_REF] Hünemohr | Edelgase in U-und Th-reichen Mineralen und die Bestimmung der 21 Ne-Dicktarget-Ausbeute der 18 O(a[END_REF], production rates (in cm 3 STP g -1 .yr -1 ) have been empirically quantified by [START_REF] Ballentine | Production, Release and Transport of Noble Gases in the Continental Crust[END_REF] These equations show that the nucleogenic 20 Ne/ 22 Ne and 21 Ne/ 22 Ne production ratios are principally dependent on the O/F ratios of the samples as F in involved only on the production of 22 Ne. As Mg is a major element, its natural variations of concentration remain small compared to other trace elements such as F, U, Th. Variable O/F ratios will have the effect to change the slopes of the mixing lines between the atmospheric endmember and the nucleogenic endmember in a graph 20 Ne/ 22 Ne versus 21 Ne/ 22 Ne (Figure 3).

The continental crust nucleogenic trend predicted by [START_REF] Yatsevich | Production of nucleogenic neon in the Earth from natural radioactive decay[END_REF] corresponds to a O/F ratio of 752 (atomic ratio) [START_REF] Ballentine | Production, Release and Transport of Noble Gases in the Continental Crust[END_REF], according to the previous

9/24
equations (1), ( 2) and (3). Archean fluids also follow a similar trend [START_REF] Holland | Deep fracture fluids isolated in the crust since the Precambrian era[END_REF].

Crustal fluids data of [START_REF] Kennedy | Crustal neon: a striking uniformity[END_REF]) lie on a trend with O/F=113 [START_REF] Ballentine | Production, Release and Transport of Noble Gases in the Continental Crust[END_REF]. Almost all data from the literature, being crustal fluids of different ages, plot in the field with an O/F ratio ranging from ∼113 to ∼752 (Figure 3).

After equations ( 1), ( 2) and (3), O/F ratios lower than 113 can be obtained for minerals very rich in F and/or very poor in oxygen. It is the case for a full range of accessory minerals, for example bastnäsite, containing 7.35 wt%F (O/F=4), or fergusonite (F=1.42 wt%, O/F=34) [START_REF] Ballentine | Production, Release and Transport of Noble Gases in the Continental Crust[END_REF][START_REF] Eikenberg | U-Xe, U-Kr, and U-Pb systematics for dating uranium minerals and investigations of the production of nucleogenic neon and argon[END_REF][START_REF] Hünemohr | Edelgase in U-und Th-reichen Mineralen und die Bestimmung der 21 Ne-Dicktarget-Ausbeute der 18 O(a[END_REF]. It has the consequence of significantly shifting common nucleogenic trends towards trends with more negative slopes (Figure 3). Some major minerals can also present a low O/F ratio. For example, fluorite (CaF2), which does not contain any oxygen, is characterized by an O/F ratio of 0. Fluorapatite, the most common phosphate mineral in the environment is also characterized by a low O/F (O/F=12). Neon isotopic measurements of the Durango fluorapatite (Table 1, Figure 3) are in agreement with the nucleogenic trend defined using O/F=12, determined theoretically using the mineral formula (Ca5(PO4)3F), suggesting that the empirical equations are reliable to estimate the 20 Ne/ 22 Ne and 21 Ne/ 22 Ne production ratios and consequently the nucleogenic trends.

Unusual Ne signature: a particular terrestrial nucleogenic trend?

Except one sample, our sedimentary rock samples fall into the same field as the one of crustal fluids, corresponding to an O/F ratio ranging from ∼113 to ∼752 (Figure 3). For these samples, the extents of the 21 Ne/ 22 Ne ratios and 22 Ne contents are related to the non-carbonate fraction (Annex Figure A1 of the supplementary material), an expected correlation since Ne is produced by nuclear reactions of isotopes present in the detrital fraction of the samples, containing U and Th.

On the other hand, an O/F equal to ∼34 is required to explain the composition of the 3404 m deep sample (Figure 3). An O/F ratio of 34 corresponds to the fergusonite (O=40.4 wt%, F=1.42 wt%), an accessory mineral phase. However, this sample could not only be composed of this mineral, since it contains 85% carbonates (Table 1). Alternatively, it would be more probable that the singular Ne isotopic composition could be the result of a mixing between an endmember 10/24

with production ratios similar to the one fitting the literature data and all the samples from this study except at 3404 m, that is with 113≤O/F≤752 and another endmember characterized by a mineral phase with a O/F lower than 34 such as fluorapatite or fluorite characterized respectively by O/F=12 and O/F=0 (Figure 3). The Ne isotopic compositions of these two endmembers can be apprehended. The predicted crust production ratios are the following: 20 Ne/ 22 Ne=0.36, 21 Ne/ 22 Ne=3.67 [START_REF] Yatsevich | Production of nucleogenic neon in the Earth from natural radioactive decay[END_REF], corresponding to an O/F ratio of 752 (Figure 3).

For the other endmember, we use the isotopic neon production ratios of fluorapatite ( 20 Ne/ 22 Ne=0.0043; 21 Ne/ 22 Ne=0.0458) and fluorite ( 20 Ne/ 22 Ne=0; 21 Ne/ 22 Ne=0) that we calculated using the elemental proportion calculated after their mineral formulae and equations

(1), ( 2) and ( 3). The details of our calculations are provided in the Annex 2 of the supplementary material. After our calculations, the decrease of the O/F ratio from 752 down to 34 in order to fit the data points of sample 3404 would require the presence of 96% 22 Ne derived from fluorite or 98% 22 Ne derived from fluorapatite in the sample.

Using equations ( 1), ( 2) and (3), we estimate that for 600 Ma old fluorites and fluorapatites, the produced 22 Ne concentration is ∼2 orders of magnitude higher than the 22 Ne concentration of the samples from this study. It implies that ∼0.4 to 37% fluorite, depending on the U and Th content of the fluorine-bearing phase, would be sufficient to account for the Ne composition of the ∼600

Ma old sample 3404. If the signal came exclusively from fluorapatite, this mineral would need to have an abundance of 0.3-16%. To evaluate the presence of F in several of the samples, we performed SEM-EDS elemental mapping for some of the residual fractions of the samples (noncarbonate fractions) including sample 3404 (See Annex 3 of the supplementary material for the details on the SEM measurements). The maps show some spots with a F-enrichment (Figure 6).

It is not possible to get reliable quantification of the data because the samples were powders that were consequently not flat and Au-coated. Nevertheless, the decarbonated fraction of sample 3404 is the one which seems to visually contain the most abundant F-bearing phases amongst the 5 analyzed samples (Figure 6), particularly compared to sample 3428 which has a similar amount of non-carbonate fraction. From these observations, it is thus possible that sample 3404 has an O/F ratio lower than those of the other samples. However, a precise quantification is required to determine if its O/F ratio is low enough to explain its peculiar Ne composition. This can be assessed using theoretical calculations. The F-rich spots in sample 3404 are associated with Caenrichments (Figure 6, Annex Figure A2 of the supplementary material), suggesting that these 11/24 phases are fluorites. We did not see any P peak associated with F peak in the spectra that could witness the presence of fluorapatite. If we assume that all the F-rich phases in sample 3404 were fluorite, we can estimate that ∼1 wt.% of fluorite is present in the ∼15 wt.% of the noncarbonate fraction of sample 3404 (Table 1) analyzed with SEM-EDS (Figure 6), thus corresponding to 0.15 wt.% of fluorite in the whole-rock sample. This first order calculation gives a value too low to explain the composition of sample 3404. However, our calculations and SEM measurements are not precise enough to be definite, especially because fluorite can contain highly variable amounts of U and Th and that we did not consider any ejection factor of α particles, which is dependent on the crystal size [START_REF] Gautheron | U-Th)/Ne chronometry[END_REF]. Measurements of fluorite crystals of different ages and sizes are needed to better evaluate the nucleogenic Ne production in this mineral, in order to definitely test whether fluorite is responsible for the peculiar Ne composition of sample 3404.

A meteoritic origin for the unusual

Ne signature?

The Ne-A component in sedimentary rocks

In this study, the layer at 3404 m is characterized by an unusual Ne isotopic signature, with 20 Ne/ 22 Ne=9.13-9.25 and 21 Ne/ 22 =0.0368-0.0373 (Table 1). If this composition cannot be explained by a nucleogenic trend (see previous section, 5.2), alternatively, it could be the result of mixing three distinct components: air, "common" nucleogenic Ne and Ne-A (Figure 3). The Ne-A component is a component present in meteorites, mainly in carbonaceous chondrites (see review in [START_REF] Moreira | Noble gas constraints on the origin and evolution of earth's volatiles[END_REF], with a distinct signature ( 20 Ne/ 22 Ne=8.500±0.057, 21 Ne/ 22 Ne=0.036±0.001, [START_REF] Huss | Noble gases in presolar diamonds II : Component abundances reflect thermal processing[END_REF] 65 Ma old sediment from the LL-44 GPC-3 deep sea sediment core, with 20 Ne/ 22 Ne=8.85 and 21 Ne/ 22 Ne=0.025 (n=2), values very different from those characterizing their other samples [START_REF] Darrah | Evaluating the accretion of meteoritic debris and interplanetary dust particles in the GPC-3 sediment core using noble gas and mineralogical tracers[END_REF]. This composition is close to the Ne-A value. As evidenced from platinum group elements excesses and the presence of the large Chicxulub crater, the Earth underwent a large meteorite impact 65 Ma ago (e.g. [START_REF] Alvarez | Extraterrestrial cause for the Cretaceous-Tertiary extinction[END_REF][START_REF] Hildebrand | Chicxulub Crater: a possible Cretaceous/Tertiary boundary impact crater on the Yucatán Peninsula, Mexico[END_REF][START_REF] Sharpton | New links between the Chicxulub impact structure and the Cretaceous/Tertiary boundary[END_REF]. Therefore, the presence of a Ne-A component in the sediments may be a witness of a major meteoritic impact. This peculiar neon isotopic signature is also associated to the highest 3 He/ 4 He ratio (~0.27Ra), although helium alone could be used to trace an impactor [START_REF] Mukhopadhyay | A short duration of the Cretaceous-Tertiary boundary event: evidence from extraterrestrial Helium-3[END_REF].

It is extremely difficult to estimate the mass fraction of presolar nanodiamonds in the samples, as this deals with concentrations in the different endmembers, which are not well constrained.

However, Huss et al. [START_REF] Huss | Noble gases in presolar diamonds II : Component abundances reflect thermal processing[END_REF] provide a typical concentration of 22 Ne of ~8 10 -6 cm 3 STP.g -1 for the carrier of Ne-A (pre-solar diamonds). If we assume a simple binary mixing between a terrestrial and extraterrestrial neon, we can calculate the mass fraction of extraterrestrial material in the NCF of sample 3404 as follow: Where R is the 20 Ne/ 22 Ne ratio. The term 0.5 is roughly estimated from Figure 3. We can therefore estimate a mass fraction of presolar diamonds carrying the neon-A signature in the NCF to be ~10 ppm. This however assumes that no neon loss has occurred and that the neon concentrations in nanodiamonds are well preserved during the impact and after. Using the 3 He/ 22 Ne of ~1 measured in presolar nanodiamonds, by [START_REF] Huss | Noble gases in presolar diamonds II : Component abundances reflect thermal processing[END_REF] we can also estimate the theoretical extraterrestrial 3 He within sample 3404. The calculation above suggests that half of the 22 Ne (~10 -10 ccSTP/g) in the NCF should be of extraterrestrial origin and therefore the sample should contain ~10 -10 ccSTP/g of extraterrestrial 3 He in the NCF. The NCF of sample 3404 contains only ~5 10 -12 ccSTP/g of total 3 He, much lower than expected from neon isotope calculation, suggesting either helium was lost during the impact or after deposition.

+ = , -./

Evidence of an impact at the time sample 3404 m formed?

13/24

The presence of a Ne-A component would imply that an object impacted the Earth and that the sedimentary rock at 3404 m would have included some ejecta containing the Ne-A component during its formation. This could be related to the Acraman impact. The Acraman Lake in South Australia is part of a deeply eroded impact structure in South Australia [START_REF] Williams | The Acraman impact structure: source of ejecta in late Precambrian shales, South Australia[END_REF][START_REF] Williams | Acraman, South Australia: Australia's largest meteorite impact structure[END_REF] with an estimated original diameter of ∼40-90 km [START_REF] Haines | Impact cratering and distal ejecta: the Australian record[END_REF], making it one of the ten largest impact structures on Earth. The presence of ice-rafted material in layers directly below and above the ejecta layer related to the Acraman impact structure, suggests that the Acraman impact occurred during a phase of glaciation [START_REF] Gostin | Ediacaran ice-rafting and coeval asteroid impact, South Australia: insights into the terminal Proterozoic environment[END_REF]. This could possibly correspond to the Gaskiers glaciation dated at ∼580 Ma [START_REF] Bowring | Geochronological constraints on terminal Neoproterozoic events and the rise of Metazoan[END_REF][START_REF] Pu | Dodging snowballs: geochronology of the Gaskiers glaciation and the first appearance of the Ediacaran biota[END_REF] using U-Pb zircon geochronology on tuffs bracketing glacial diamictites. Similarly, [START_REF] Walter | Dating the 840-544 Ma Neoproterozoic interval by isotopes of strontium, carbon, and sulfur in seawater, and some interpretative models[END_REF] suggested an age of 578 Ma for the Acraman impact using chemostratigraphic correlation. However, the Acraman impact lacks a robust isotopic age for the ejecta layers due to difficulties of dating [START_REF] Schmieder | Zircons from the Acraman impact melt rock (South Australia): shock metamorphism, U-Pb and 40 Ar/ 39 Ar systematics, and implications for the isotopic dating of impact events[END_REF]. Thus, the age of the Acraman impact can only be loosely constrained in the Ediacaran period between ∼541 and ∼635 Ma by stratigraphic bracketing [START_REF] Schmieder | Zircons from the Acraman impact melt rock (South Australia): shock metamorphism, U-Pb and 40 Ar/ 39 Ar systematics, and implications for the isotopic dating of impact events[END_REF].

The layer at 3404 m, possibly containing the signature of Ne-A component, is not directly dated.

Nevertheless, the age range for the Nafun group layers has been assessed through correlations to other sections [START_REF] Fike | Oxidation of the Ediacaran Ocean[END_REF] with U-Pb zircon ages obtained on ash beds [START_REF] Bowring | Geochronological constraints on terminal Neoproterozoic events and the rise of Metazoan[END_REF][START_REF] Pu | Dodging snowballs: geochronology of the Gaskiers glaciation and the first appearance of the Ediacaran biota[END_REF]. The unconformity below the 3836 m layer, at the base of the Shuram formation, probably includes the time period of the ∼580 Ma Gaskiers glaciation, based on stratigraphic correlation [START_REF] Bowring | Geochronological constraints on terminal Neoproterozoic events and the rise of Metazoan[END_REF][START_REF] Fike | Oxidation of the Ediacaran Ocean[END_REF]. The recovery of the Shuram excursion (3300 m) is thought to have been completed by ∼550 Ma [START_REF] Condon | U-Pb Ages from the Neoproterozoic Doushantuo Formation[END_REF]. Thus, the sample at 3404 m should have an age between ∼580 and ∼550 Ma (Figure 1). However, an alternative view based on thermal subsidence curves to transform stratigraphic thicknesses of the Nafun Group into time suggests that the basal Shuram is ∼600 Myr (Le [START_REF] Guerroué | 50 Myr recovery from the largest negative δ13C excursion in the Ediacaran ocean[END_REF]. In this scenario, the base on the Buah formation, where the layer at 3404 is located, might have an age of ∼575-570 Ma (Le [START_REF] Guerroué | 50 Myr recovery from the largest negative δ13C excursion in the Ediacaran ocean[END_REF], close to the Acraman impact age of 578 Ma determined by [START_REF] Walter | Dating the 840-544 Ma Neoproterozoic interval by isotopes of strontium, carbon, and sulfur in seawater, and some interpretative models[END_REF].

The estimated age range for the Acraman impact is thus consistent with the estimated time range for the formation of the sample at 3404 m. Thus, the presence of an impact-derived Ne-A component present in this sample that would witness an impact event is possible.

14/24

However, more precise age constraints for both the Acraman impact and for the deposition of the Nafun group layers are required to confirm the validity of this hypothesis.

Conclusion

We have analyzed He and Ne isotopes in Ediacaran sedimentary rocks from the Nafun Group strata, Sultanate of Oman that are believed to have been deposited between ∼600-580 and ∼550

Ma. Extraterrestrial He is still present in these samples and is in similar amounts to that found in 3 to 480 Myr old samples. However, we were unable to detect Ne isotopic composition of interplanetary dust, unlike other data [START_REF] Chavrit | Estimation of the extraterrestrial 3He and 20Ne fluxes on Earth from He and Ne systematics in marine sediments[END_REF]. This may be related to the higher non-carbonate fraction in these samples as compared to previously published samples. Except for one sample, the Ne isotopic composition follows a common nucleogenic trend, similar to those defined by crustal fluids from the literature and the one predicted for the continental crust.

However, the other sample, from basal strata of the Buah formation, shows an unusual Ne isotopic composition that can be explained by two hypotheses.

First, a singular nucleogenic trend deviating from the common nucleogenic trend could explain the data. This would be the case if the sample contains a fluorine bearing mineral, such as fluorite or fluorapatite, in amounts ranging respectively from 0.4 to 37% and 0.3 to 16% by mass, depending on the U and Th contents of these minerals. Using SEM, we measured that the sample contains ∼0.15% fluorine and calcium rich phases. However, our estimates indicate that this abundance is lower than the minimum fluorite proportion required to explain its singular Ne composition. Nevertheless, our calculations are based on theoretical predictions and greatly rely on the amount on U and Th used, which can lead to very different yields of 20 Ne produced after 600 Ma, and thus lead to a wide range for the required proportion of fluorine-bearing mineral contained in this singular sample.

The other alternative is that this unusual Ne isotopic composition is related to the assimilation of an extraterrestrial Ne-A component in this sample. This could be the result of a significant meteoritic impact and, if so, would indicate that this highly volatile element was not lost to the atmosphere upon the impact. This would be consistent with the Neoproterozoic Acraman impact 15/24 in Australia. However, the ages of both the impact event and the deposition of the basal Buah Formation that contains the singular Ne isotopic composition are loosely constrained and more precise ages are required to validate the impact hypothesis for the origin of the Ne signal observed here.

For the time being, it is not possible to definitely rule out or validate one of the two hypotheses. Future work will be to investigate the Ne isotopic composition of fluorine bearing minerals of different ages to better assess their nucleogenic production over time. Sedimentary rocks containing impact ejecta should also be analyzed.

Due to its signature in chondrites, Ne can potentially be an indicator of meteoritic impact in sedimentary rocks. However, this study shows that in samples that are hundreds of million years old, small amounts of F-bearing minerals can significantly affect the resulting Ne isotopic composition. Thus, it is essential to consider the nucleogenic Ne production, in deciphering the Ne isotopic composition in sedimentary rocks, especially for very old ones.

Figure Captions

Figure 1: The noble gas composition as a function of depth. NCF = non-carbonate fraction.

Error bars for the isotopic ratios are smaller than or equal to the symbol size when non visible.

The δ 13 C carbonates data are from [START_REF] Fike | Oxidation of the Ediacaran Ocean[END_REF]. Geochronological constraints are from [START_REF] Bowring | Geochronological constraints on terminal Neoproterozoic events and the rise of Metazoan[END_REF][START_REF] Condon | U-Pb Ages from the Neoproterozoic Doushantuo Formation[END_REF][START_REF] Fike | Oxidation of the Ediacaran Ocean[END_REF][START_REF] Grotzinger | Biostratigraphic and geochronologic constraints on early animal evolution[END_REF][START_REF] Guerroué | 50 Myr recovery from the largest negative δ13C excursion in the Ediacaran ocean[END_REF]. The Khufai and Shuram formations are separated by an unconformity (wavy curve) that probably includes the Gaskiers glaciation [START_REF] Bowring | Geochronological constraints on terminal Neoproterozoic events and the rise of Metazoan[END_REF]. The dotted lines are the limits of the Shuram carbon isotope negative excursion [START_REF] Fike | Oxidation of the Ediacaran Ocean[END_REF].

Figure 2: 3 He/ 4 He ratio versus 3 He contents. The lines represent mixing lines between an IDP component, rich in 3 He, with 3 He/ 4 He=2.4×10 -4 and two hypothetical terrestrial endmembers, rich in radiogenic 4 He and poor in 3 He considering 3 He/ 4 He=10 -9 -10 -7 . The 2 mixing lines are derived from [START_REF] Mcgee | Extraterrestrial He in sediments: from recorder of asteroid collisions to timekeeper of global environmental change[END_REF]. Examples of literature data (small grey dots) are from [START_REF] Chavrit | Estimation of the extraterrestrial 3He and 20Ne fluxes on Earth from He and Ne systematics in marine sediments[END_REF][START_REF] Farley | A late Miocene dust shower from the break-up of an asteroid in the main belt[END_REF]Murphy et al., 2010) , and Chavrit et al. (unpublished data). [START_REF] Yatsevich | Production of nucleogenic neon in the Earth from natural radioactive decay[END_REF]. The measured crustal fluids trend with a O/F=113 [START_REF] Ballentine | Production, Release and Transport of Noble Gases in the Continental Crust[END_REF] is obtained from the nucleogenic trend defined by the crustal fluid data from [START_REF] Kennedy | Crustal neon: a striking uniformity[END_REF]. The line "KTB" represents the continental crust as sampled by the KTB drill in Germany [START_REF] Drescher | The rare gas inventory of the continental crust, recovered by the KTB continental deep drilling project[END_REF]. The slopes of the mixing lines between the air component and the nucleogenic components are a function of the O/F ratio (presented as atomic ratios here). The compositions of IDP are close to that of the Ne-B value ( 20 Ne/ 22 Ne=12.73±0.02, 21 Ne/ 22 Ne=0.0321±0.0001) [START_REF] Moreira | Noble gas constraints on the origin and evolution of earth's volatiles[END_REF], the Ne-A composition is from [START_REF] Huss | Noble gases in presolar diamonds II : Component abundances reflect thermal processing[END_REF], the air composition is from [START_REF] Eberhardt | A redetermination of the isotopic composition of atmospheric neon[END_REF].

22/24

23/24

Table 1: Helium and Ne concentrations and isotopic composition of the sedimentary rocks from the Huqf Supergroup and fluorapatites analyzed in this study using the Noblesse mass spectrometer sited at IPGP. Durango fluorapatites were analyzed in 2006 using the ARESIBO mass spectrometer sited at that time in IPGP. Helium and neon concentrations for these three samples cannot be compared to other Durango analyses since these samples were baked under vacuum before analysis.

The sample units, depths and lithologies are from [START_REF] Fike | Oxidation of the Ediacaran Ocean[END_REF]. In the text, the samples names used correspond to their depth of collection in the drill hole (2 nd column of this table).

Dup. = duplicate. NCF=non-carbonate fractions, corresponding to residual weight/initial weight ratio in percent. *For the sedimentary rocks, the loaded mass is estimated from the residual fraction after leaching. RA is the atmospheric 3 He/ 4 He ratio (1.384×10 -6 ; (Clarke et al., 1976a).

The concentrations are expressed in cm 3 STP.g -1 of initial mass. Uncertainties are ±5% on absolute abundances and ±2% on element ratios. # indicates the sample with the unusual Ne composition.

24/24

  compared to those of the Earth's surface, such as air and crustal fluids (Figure 3). Consequently, a 20 Ne/ 22 Ne ratio below the air value and a relatively low 21 Ne/ 22 Ne ratio in a sedimentary rock, such as in sample 3404, open the question of the presence of a meteorite component in this sample. Such a component would imply that an impact occurred on Earth at the time the sediment formed and that pre-solar nano-diamonds contained in the bolide survived the impact, allowing this volatile element and thus the Ne-A component to be trapped in the sediments. Darrah et al. measured a Ne isotopic composition in a 12/24

Figure 3 :

 3 Figure 3: 20 Ne/ 22 Ne ratios versus 21 Ne/ 22 Ne ratios. Sedim. stands for sediments or sedimentary rocks, mfl = mass fractionation line. The plain lines are nucleogenic trends for different O/F ratios (atomic ratios). Cont. crust (=continental crust predicted) is from (Yatsevich and Honda,

Figure 4 :

 4 Figure 4: Slope of the line passing through the air component and a given sample (reflecting the O/F value) in the three Ne isotopes diagram (Figure 3) as a function of the depth. The unusual Ne composition characterizes a negative spike at the depth 3404 m. The dotted lines indicate the limits of the Shuram excursion (Figure 1).

Figure 5 :

 5 Figure 5 : Nuclear reactions producing Ne isotopes in the crust.

Figure 6 :

 6 Figure 6: SEM-EDS mapping of the residual powders (=non-carbonate fractions, NCF) of several samples from this study. All the acquisitions were done with a resolution of 467 nm.px -1 .

  as a function of the U, Th and F contents (in ppm) and O, Mg,

	Na contents (in wt%), with the following equations:
		= 6.39	+ 0.770 ℎ 0.0226	+ 0.0022	× 10	(1)
	!	= " 1.48	+ 0.186 ℎ	+ 0.105	+ 0.0179 ℎ &' ( × 10	(2)
		= " 3.06	+ 0.417 ℎ ) + 4.20	+ 0.663 ℎ &' ( × 10 *	(3)

Acknowledgments

We thank the Oman Ministry of Oil and Gas for permission to publish this paper. We thank

Stephan Borensztajn for assistance during with the SEM measurements and Cécile Gautheron for providing the Durango fluorapatite. FM thanks the European Research Council under the European Community's H2020 framework program/ERC grant agreement # 637503 (Pristine), the Agence Nationale de la Recherche for a chaire d'Excellence Sorbonne Paris Cité (IDEX13C445) and the Plateform PARI. FM and MM thank the UnivEarthS Labex program (ANR-10-LABX-0023 and ANR-11-IDEX-0005-02). P. Sarda, J. Hopp and the two other anonymous reviewers are thanked for their constructive reviews. This is IPGP contribution number 4027.