The origin of the neon isotopes in chondrites and on Earth

doi:10.1016/j.epsl.2015.11.002

Earth and Planetary Science Letters

Volume 433, 1 January 2016, Pages 249-256

https://doi.org/10.1016/j.epsl.2015.11.002 Get rights and content

Highlights

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The neon isotope signature on Earth reflects solar wind irradiation on dust during the first stage of the solar system formation.
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Irradiation of grains during a few kyr is able to explain the Neon B composition.
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A new value for the Neon B component is estimated at ∼12.7 instead of the ancient estimate of 12.5.
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Chondrites reflect the mixture between these irradiated grains and pre-solar grain having the neon A signature.
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There is no need for a dense primordial solar atmosphere to explain the neon composition of the Earth's mantle.

Abstract

We discuss the origin of the neon isotopic signatures in chondrites and in the terrestrial mantle. There are two primary possible origins for neon in the Earth's mantle. One origin is the dissolution of a dense primordial atmosphere with a solar composition of ²⁰Ne/²²Ne >13.4 into the mantle in a possible magma ocean stage during Earth's accretion. The second origin, developed in this study, is that mantle neon was already in Earth's parent bodies because of refractory grain irradiation by solar wind. We propose that solar wind implantation occurred early on dust within the accretion disk to allow such irradiation. Because solar wind implantation fractionates neon isotopes, the heavier isotopes are implanted deeper than the lighter ones because of different kinetic energies, and the process of implantation, if coupled with sputtering, leads to a steady state neon isotopic ratio (²⁰Ne/²²Ne ∼12.7) that is similar to what is observed in mantle-derived rocks (12.5–12.9), lunar soil grains (∼12.9) and certain gas-rich chondrites from all classes (enstatite, ordinary, rumuruti). Using a dust transport model in a turbulent and irradiated solar nebula, we estimated the equivalent irradiation age of a population of dust particles at three different distances from the sun (0.8, 1, 1.2 AU) and converted these ages into neon concentrations and isotopic ratios. The dust subsequently coagulated to form Earth's parent bodies, which have the mean neon isotopic composition of the irradiated dust (non-irradiated dust is assumed to be free of neon). If this scenario of solar wind implantation coupled with sputtering in the precursors of Earth's parent bodies is correct, it offers a simple alternative to the model of solar nebula gas incorporation by dissolution in a magma ocean.

Introduction

The origin of volatiles on terrestrial planets is a long-standing debate because it has major consequences in many subfields of the Earth and Planetary Science. Terrestrial planets are accreted with material that is depleted of moderately volatile elements, such as K, Rb or Pb (e.g., Albarède, 2009); therefore, these planets are expected to be depleted in highly volatile elements, such as C, H, N and the noble gases, and thus the question of these elements origins arises. Noble gases are the perfect tools to constrain possible origins of volatiles on terrestrial planets. These gases form a family of chemically inert elements, highly volatile and with a large mass spectrum from He to Xe. Chondrites and terrestrial planets, relative to the sun, are highly depleted in noble gases by several orders of magnitude compared to other elements, including other volatiles (e.g., C, N, and others). Noble gases have both radiogenic and “stable” isotopes, allowing them to be used to trace physical processes during planetary formation and evolution and to constrain these processes in time. Additionally, because they are strongly depleted in refractory material, noble gases are very sensitive to secondary processes involving solar gases, such as adsorption or solar wind implantation. Therefore, noble gases are ideal tools to trace physical processes involving dust/gas interactions in the accretion disk during solar system formation.

The neon isotopic signature allows us to constrain the process of noble gas incorporation into the parent bodies of terrestrial planets. Indeed, the “stable” ²⁰Ne/²²Ne ratio varies greatly in different solar system reservoirs (Fig. 1). Fig. 1 shows the isotopic compositions of solar wind, the estimated ratios of the Sun, different types of chondrites, the Earth's atmosphere, the mantle and the three main carriers of neon in chondrites (A, B and E). The cosmogenic end-member represents the production of three neon isotopes by spallation during the transfer of the meteorite from the asteroid belt to the Earth, which is a secondary process that is irrelevant to understanding the physical processes during the formation of the parent bodies of these objects. Neon A has been proposed to represent the composition of pre-solar grains, formed in red giant envelopes (e.g., Huss and Lewis, 1994). In pre-solar grains, there is a second end-member, neon E. Neon E composition is pure ²²Ne derived from the decay of ²²Na. This is, however, a trace component in chondrites (e.g., Huss and Lewis, 1994) and therefore will not be considered in this study as an important neon contributor. The solar wind neon isotopic composition has been recently precisely determined by Genesis probe targets that confirm previous estimates obtained on Apollo foils or lunar grains that were exposed to solar wind (Grimberg et al., 2006, Heber et al., 2009, Heber et al., 2012; Pepin et al., 2012). There is also the question of whether the solar wind composition reflects the solar composition because mass fractionation is induced during solar wind formation, and the recent estimate of the ²⁰Ne/²²Ne ratio of the sun was acquired using a model of fractionation in the solar wind (Heber et al., 2012). The ²⁰Ne/²²Ne ratio that was estimated for the sun is $13.4 \pm 0.2 (2 σ)$ , lower than generally assumed (<13.8).

It has been suggested that mantle neon could be of solar origin by the dissolution of a primordial dense solar-like atmosphere into a magma ocean (Ballentine et al., 2005, Mukhopadhyay, 2012, Yokochi and Marty, 2004). Such a scenario may explain the neon isotopic signature in the mantle if the subduction of atmospheric neon occurs in the mantle; however, there are issues regarding the elemental pattern of the noble gases observed in the mantle that are incompatible with the dissolution of solar gases here (e.g., Moreira, 2013) and the Ar and Kr isotopic compositions (Holland et al., 2009, Raquin and Moreira, 2009), which are better explained by a chondritic origin of noble gases, rather than by the dissolution of solar noble gases into a magma ocean. In this study, we discuss the origin of neon on Earth as reflecting the neon “B” signature (e.g., Black, 1971, Black, 1972a, Black, 1972b), which is observed in gas-rich meteorites and on lunar soil. However, the neon B isotopic signature is estimated to be 12.5 (Black, 1971, Black, 1972a, Black, 1972b), lower than the highest ²⁰Ne/²²Ne measured on oceanic basalts (∼12.9) (e.g. Mukhopadhyay, 2012, Yokochi and Marty, 2004), which therefore question the neon B as a possible primordial neon composition of the Earth.

Using a model of solar wind implantation coupled with sputtering, we show that the best ²⁰Ne/²²Ne ratio for the Neon-B component is ∼12.7, consistent with ratios measured from lunar soil grains, gas-rich meteorites and mantle-derived samples and is higher than a previous estimate based only on relatively few samples (Black, 1971, Black, 1972a, Black, 1972b). We also discuss the chronology of irradiation during the first stage of solar system formation using a dust transport model in the solar nebula. We propose that solar wind implanted neon into the dust in the inner solar system (0–1.2 AU) within a short period of time (a few Ky), before macroscopic grains and planetary embryos were formed.

Section snippets

The neon isotopes in the Earth's mantle

Since the study by Poreda and Radicati di Brozolo (1984), there have been many studies of neon isotopes in mantle-derived samples, producing a comprehensive data set that allows us to determine the neon isotopic composition of the mantle. Fig. 2 shows the neon isotope ratios of a large suite of Mid Ocean Ridge Basalt (MORB) samples, excluding two on-ridge hotspots called Shona and Discovery in the south Atlantic that have low ⁴He/³He (high ³He/⁴He) (Moreira et al., 1995, Sarda et al., 2000),

First-order model of implantation-sputtering

We present a progression of the Raquin and Moreira model (Raquin and Moreira, 2009). Their model assumed that the profile of ion implantation in grains follows an exponential law $\sim \exp (- x / ν)$ in which ν is the attenuation of the implantation depth, and x is the distance to a grain's surface. The implantation depth differs for the different isotopes, primarily because the isotopes from solar wind have the same speed (the escape velocity from the sun) and therefore different kinetic energies;

Conclusions

Neon can have two possible origins in the Earth's mantle. An origin by dissolution of solar neon into a magma ocean requires a dense primordial atmosphere captured from the solar nebula (and then its loss because no evidence of it exists in the present day atmosphere; e.g., Mizuno et al., 1980). Although the elemental ratios of the noble gases (Moreira et al., 1998) and the Kr isotopic composition in the mantle (Holland et al., 2009) do not favor such a scenario, neon data from mantle-derived

Acknowledgments

M.M. and S.C. acknowledge the financial support from the UnivEarthS Labex program of Sorbonne Paris Cité (ANR-10-LABX-0023 and ANR-11-IDEX-0005-02).

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This component, labelled Ne-B (Black and Pepin, 1969) is found in the different classes of meteorites including CCs, OCs, and ECs, as well as in some achondrites (Kapoeta, Pesyanoe), some ureilites and in the lunar regolith. Its end-member 20Ne/22Ne ratio of 12.50–12.75 (Black and Pepin, 1969; Moreira and Charnoz, 2016) is attributed to fractionation of solar wind ions (20Ne/22Ne = 13.74 ± 0.02, Heber et al., 2012) during implantation and regolith grain processing. From the analysis of solar wind implanted in the Genesis mission target material, Grimberg et al. (2006) proposed that this component is not a discrete end-member but is rather implanted solar wind fractionated to variable extent during implantation and subsequent space erosion (hence labelled f-SW where f stands for “fractionated”).
The elemental and isotopic compositions of noble gases trapped in primitive meteorites have the potential to yield stringent constraints on the origin of matter in the solar system. The isotopic compositions of key elements like O, Ti, Ru, Mo suggest that the Earth accreted from material having similarities with two classes of meteorites, carbonaceous chondrites (CC) and non‑carbonaceous chondrites (NC), in particular enstatite chondrites (EC). In this contribution, I examine published noble gas (neon and argon) data for CI-CM as representative of CCs, and ECs as representative of NC terrestrial building blocks. Data were corrected for contributions of cosmic ray-produced isotopes in order to identify the trapped component compositions. For both CCs and ECs, corrected noble gas data indicate that high temperature objects such as chondrules were evolving in a dusty environment. The dust consisted of refractory phases including nanodiamonds, impacts-related debris, medium to low temperature phases mainly made of organics and, in the case of CC, hydrated minerals and icy grains. Remnants of such a dust are found as rims around chondrules and as a matrix between high temperature assemblages. The dust was probably the main source of volatiles on Earth.
In terrestrial reservoirs, covariations of ²⁰Ne/²²Ne ratios with ³⁶Ar/²²Ne ratios are consistent with mixing between a solar-like neon component trapped in the mantle and a chondritic Ne–Ar component mainly present in the atmosphere and hydrosphere. The chondritic end-member is clearly of the CC type and excludes EC-like material as the source of atmospheric volatiles. In addition to CC-like material, the isotopic composition of heavy noble gases (Kr and Xe) in the atmosphere points to a ~ 20% contribution of cometary material akin of the composition of comet 67P/Churyumov-Gerasimenko. In contrast, comets might have contributed less than 1% terrestrial water, C and N. Solar-like neon in the terrestrial mantle might have originated from solar irradiation of free-floating dust before parent body compaction, but this would require a cleared, dust-free environment. Trapping of nebular gas into forming solids during the gas epoch of the nascent solar system appears a more promising possibility. For other mantle volatiles, the stable isotopes of H, N, Ar, Kr and Xe point to a chondritic origin. The hydrogen and nitrogen isotopic signatures of mantle rocks and minerals are consistent with an EC-like contribution whereas those of heavy noble gases are still too imprecise to conclude. Further progress in the field will require high precision analysis of noble gases (in particular, Kr and Xe) trapped in the terrestrial (and martian) mantle(s), as well as documenting the composition of the Venusian atmosphere.
The origin of nitrogen in Earth's mantle: Constraints from basalts 15N/14N and N<inf>2</inf>/3He ratios
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The recycling of surface-derived neon into the MORB mantle would have lowered its 20Ne/22Ne ratio by a limited but measurable amount, from ~13.4 to ~12.5 (Williams and Mukhopadhyay, 2018). Note that alternative hypotheses offer explanations for the MORB 20Ne/22Ne ratio without resorting to neon subduction (Moreira and Charnoz, 2016; Péron et al., 2016). The carbon budget also shows somewhat limited regassing signatures.
Plate tectonics is thought to be a major driver of volatile redistribution on Earth. The budget of nitrogen in Earth's mantle has been suggested to be almost entirely surface-derived. Recycling would contribute nitrogen with relatively heavy ¹⁵N/¹⁴N isotope ratios to Earth's mantle. This could explain why the Earth's mantle ¹⁵N/¹⁴N isotope ratio is substantially higher than both solar gases and chondritic parent bodies akin to enstatite chondrites. Here, published nitrogen isotope data of mid-ocean ridge and ocean island basalts are compiled and used to evaluate the nitrogen subduction hypothesis. Nitrogen isotope ratios are used in conjunction with published N₂/³He and K₂O/TiO₂ ratios on the same basalts. Assuming that ³He is not recycled, N₂/³He ratios are argued to trace nitrogen addition to mantle sources via subduction. Various mantle source enrichments for basalts are tracked with K₂O/TiO₂ ratios: elevated K₂O/TiO₂ ratios are assumed to primarily reflect the contributions of recycled components in the basalts mantle sources. The main result of our data compilation is that for most basalts, δ¹⁵N and N₂/³He remain constant across a vast range of K₂O/TiO₂ ratios. Mid-ocean ridge basalts have δ¹⁵N signatures that are lower than air by ~4‰ and an average N₂/³He ratio of 3.7 (±1.2) x10⁶ (95% confidence, n = 30). Published δ¹⁵N and N₂/³He are invariant across K₂O/TiO₂ ratios that vary over a factor of ~20. Using estimates of slab K₂O/TiO₂ and [TiO₂], the observed invariant δ¹⁵N and N₂/³He may be fit with slabs containing ~0.1 ppm N. A mass balance shows that adding ~10% recycled slabs to the convective mantle only raises the N₂/³He by <5%. Lavas from Iceland, Galapagos and Hawaii have high ³He/⁴He and ¹⁵N/¹⁴N ratios relative to the convective mantle. Only seven samples show nitrogen isotopic signatures that are unaffected by air contamination, although those samples are poorly characterized for N₂/³He. The seven basalts show δ¹⁵N between −2 and 0‰ that do not vary systematically with K₂O/TiO₂ ratios that vary over a factor of ~5. The N₂/³He ratios of these seven basalts is unknown, but the high ³He/⁴He mantle may be estimated by combining published N₂/³⁶Ar to ³He/³⁶Ar ratios. This yields a N₂/³He of 2.3 (±1.2) x 10⁶ (1σ uncertainty). This is indistinguishable from the MORB estimate of 3.7 (±1.2) x 10⁶. Invariant δ¹⁵N across variable degrees of mantle enrichments and MORB-like N₂/³He for the high ³He/⁴He mantle are not consistent with nitrogen addition to plume sources with elevated ³He/⁴He ratios. A δ¹⁵N between −2 and 0‰ for plume sources, only marginally higher than MORB, could be a primordial feature of undegassed mantle reservoirs. Nonetheless, nitrogen subduction may have contributed to a specific array of mantle sources, as revealed by the few published data on basalts with low ³He/⁴He ratios. Lavas from the Society plume with low ³He/⁴He ratios show an enriched mantle source, and they have elevated δ¹⁵N ≥ +0.5‰ and N₂/³He > 10⁷. For those, the addition of slabs with concentrations of ~0.1 ppm N to a mantle source can account for the integrated dataset. To summarize, the published data suggest that nitrogen subduction may explain a sub-set of published N isotope data on basalts, but that N recycling has an overall more limited impact on mantle nitrogen than previously thought.
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Noble gas isotope systematics, particular those of He, have been fundamental in showing that some ocean island basalts (OIB) were sourced from deep mantle plumes. Relationships between He, W, Os, Sr, Nd and Pb isotopes in Hawaiian, Samoan, Galapagos, and Icelandic lavas have been suggested to reflect contributions from less degassed lower mantle sources, and perhaps even materials advected from the core-mantle boundary. This study reviews the noble gas (He-Ne-Ar-Xe) isotope systematics of major OIB suites. Important in this evaluation are considerations of spatial and temporal variations, including the sample media (glass, minerals, hydrothermal gases and fluids) used for analyzing noble gases, as well as the degree of partial melting experienced to produce OIB. Limited availability of gas-rich samples means patchy coverage in definition of OIB Ne, Ar, Xe isotope compositions. Additionally, low-degree partial melting will lead to preferential sampling of more fusible, generally more enriched components which could affect noble gas isotope systematics. OIB with low- (<8R_A) to MORB-like ³He/⁴He (8 ± 2R_A) dominantly sample convecting mantle domains and can also contain some relatively undegassed (solar) components. Their range in Sr-Nd-Os-Pb isotope compositions reflect a strongly recycled crustal and/or lithospheric heritage. Intermediate (>10R_A) to high-³He/⁴He (>25R_A) OIB (Loihi, Hawaii; Iceland; Fernandina, Galapagos; Ofu, Samoa) sample a reservoir that has been relatively isolated since ~4.5 billion years as shown by noble gases (He, Ne, Ar, Xe), and by W isotopes. However, this reservoir is not pristine and these OIB show evidence for containing depleted and enriched recycled components from Xe isotopes as well as lithophile-siderophile radiogenic isotope systematics. Linking the highest-³He/⁴He OIB to a putative Focus Zone (FOZO) reservoir is also problematic; FOZO is not a reservoir least affected by recycling of crust, rather a mantle reservoir that contains recycled components, including depleted lithosphere, and that has been oxidized. This distinction in definition means that, while the FOZO reservoir is almost certainly sampled by the deepest, hottest mantle plumes, its status as a primitive reservoir is unsubstantiated. Models of mantle convection can satisfy seismic constraints on Earth's deep mantle and reveal extensive mixing throughout with enriched and depleted lithologies formed by partial melting processes at Earth's surface. Ancient deep isolated reservoirs are likely to be relatively minor present-day mantle features. More remarkable is the predicted abundance of strongly refractory mantle material. Focus on the noble gas, radiogenic and stable isotope attributes of such refractory reservoirs are required for a fuller understanding of mantle processes.
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Citation Excerpt :
This interpretation, however, is still debated. Other researchers argued that these ratios could be reproduced through implantation of solar wind onto accreted meteoritic material (Raquin and Moreira, 2009; Trieloff et al., 2000; Moreira and Kurz, 2013; Moreira and Charnoz, 2016; Péron et al., 2016; Jaupart et al., 2017; Péron et al., 2018). A more recent study measured values up to 13.03±0.04 from deep mantle plumes and determined a 20Ne/22Ne ratio for the primordial plume mantle of 13.23±0.22 (Williams and Mukhopadhyay, 2019) which is indistinguishable from the nebular ratio.
The atmospheric noble gas isotope and elemental bulk ratios on Venus and Earth provide important information on their origin and evolution. If the protoplanets grew to a certain mass (i.e. > 0.5 M_Earth), they could have captured H₂-dominated primordial atmospheres by accreting gas from the circumstellar disk during the formation of the Solar System, which were then quickly lost by hydrodynamic escape after the disk dissipated. In such a case, the EUV-driven hydrodynamic flow of H atoms dragged heavier elements with it at different rates, leading to changes in their initial isotope ratios. For reproducing Earth and Venus present atmospheric ³⁶Ar/³⁸Ar, ²⁰Ne/²²Ne, ³⁶Ar/²²Ne, isotope and bulk K/U ratios we applied hydrodynamic upper atmosphere escape and Smooth Particle Hydrodynamics (SPH) impact models for the loss calculations of captured H₂-dominated primordial atmospheres for various protoplanetary masses. We investigated a wide range of possible EUV evolution tracks of the young Sun and initial atmospheric compositions based on mixtures of captured nebula gas, outgassed and delivered material from ureilite, enstatite and carbonaceaous chondrites. Depending on the disk lifetime of ≈ 3–5 Myr (Bollard et al., 2017; Wang et al., 2017) and the composition of accreted material after disk dispersal, we find from the reproduction of the present atmospheric Ar, Ne, and bulk K/U ratios, that early Earth's evolution can be explained if proto-Earth had accreted masses between ≈ 0.53 − 0.58 M_Earth by the time the nebula gas dissipated. If proto-Earth would have accreted a higher mass during the disk lifetime the present atmospheric Ar and Ne isotope ratios can not be reproduced with our model approach. For masses > 0.75 M_Earth, Earth would have had a problem to get get rid of its primordial atmosphere. If proto-Earth accreted ≈ 0.53 − 0.58M_Earth of enstatite-dominated material as suggested by Dauphas (2017) during the disk lifetime, it would have captured a tiny primordial atmosphere that was lost ≈ 3 Myr after the disk dissipated. In such a case we find that the present-day atmospheric Ar and Ne isotope ratios can be best reproduced if the post-nebula impactors contained ≈ 5% weakly depleted carbonaceous chondritic material and ≈ 95% enstatite chondrites that are strongly depleted in Ar, Ne and moderately volatile elements like potassium. If higher amounts of carbonaceous chondrites were involved in early Earth's accretion as recently suggested by Schiller et al. (2018), then the Earth's present atmospheric Ar and Ne ratios can only be reproduced if the involved carbonaceous chondritic post-nebula material was also highly depleted in these noble gases and/or had to be partially be delivered as long as the primordial atmosphere was yet escaping. As long as primordial atmospheres surround the growing protoplanets the abundance of their volatile elements is overwritten by their respective captured solar-like atmospheric abundances. Therefore the initial composition of the protoplanets at the disk dispersal time can not be identified by our method. For masses < 0.5 M_Earth atmospheric escape cannot explain the present-day ratios, i.e. if Earth grew slower then these ratios have to be explained differently (Marty, 2012). If proto-Venus captured a primordial atmosphere it should have grown to masses of ≈ 0.85− 1.0 M_Venus during the time until the disk dissipated and if early Venus accreted its main mass during the disk lifetime than the present atmospheric Ar and Ne isotope ratios and the observed K/U ratios on Venus surface can also be reproduced by the escape of a captured primordial atmosphere that is lost within ≤ 100 Myr, if the Sun was born as a weakly to moderately active young G star. New precise re-measurements of atmospheric noble gases are necessary by future Venus missions to better constrain the material that was involved in the planet's accretion history and possibly also the EUV activity evolution of the young Sun. In addition, measurements of other moderately volatile element and isotope ratios on the surface such as Rb/U, ⁶⁴Zn/⁶⁶Zn, and ³⁹K/⁴¹K can give an insight on whether Venus accreted slow or fast, i.e. almost to its final mass within the disk lifetime.

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