Earth’s accretion and the potential origins of its volatile elements

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Dynamical and elemental constraints on the origin of volatile elements on Earth

N-body simulations are currently the best numerical tools to understand how the terrestrial planets formed from a given starting distribution of planetesimals. State-of-the art numerical models comprise a significant step that is the inward and outward migration of the giant planets Jupiter and Saturn: the so-called « Grand Tack » event (Walsh et al., 2011). It caused a truncation of the disk of planetesimals and embryos with all the mass in the terrestrial planet region concentrated between 0.7 and 1.0 AU. This truncation around 1.0 AU appears necessary to form small Mars analogs. In this model the inward and then outward migration of the giant planets has also severe implications on the distribution of planetesimals. Figure 1.8 depicts the effect of this « Grand Tack » on planetesimals initially located between 0.3 and 3 AU (red dots), between giant planets orbits (light blue dots) and in more external regions of the solar system (dark blue dots). The inward migration of Jupiter scatters planetesimals originally inside the giant planets orbits toward outer regions of the solar system. The outward migration of Jupiter, captured by Saturn, repopulates the inner asteroid belt with these objects but also with objects that were originally beyond the orbits of giant planets. At the end, the inner asteroid belt is mainly constituted by planetesimals originally located inside giant planets orbits but it also contains some planetesimals originally formed in the outer regions of the solar system (blue dots in the last panel in Fig. 1.8). These objects formed in cold regions of the solar system and are thus potentially volatile-rich. Their presence in the inner asteroid belt at the end of the Grand Tack and their later disturbance during the Late Heavy Bombardment (Gomes et al., 2005) are probably keys to bring volatile-rich bodies in the Earth’s originally dry area.
A volatile-rich contribution to the Earth during the late stages of terrestrial accretion and inferred from modeling work is also proposed by geochemical studies (Marty, 2012; Halliday, 2013). For example, Marty (2012) pointed out that a contribution of 1-3 % of volatile-rich material (carbonaceous chondrites) to an originally dry proto-Earth well explains the abundances of terrestrial water, carbon and noble gases (except for Xe, see section 1.2.4).

Volatile delivery versus atmospheric loss during impacts

Whatever the provenance of impactors that may have supplied volatile elements to the originally « dry » proto-Earth, special attention must be paid to the mass balance of atmospheric escape versus volatile delivery when objects were impacting the growing Earth. The simplest assumption that impacts were conservative with no loss of volatile elements from the target or from the impactor during an impact event is probably wrong when one considers the high energy delivered by giant impacts (Genda and Abe, 2005). There are numerous parameters governing the amount of atmospheric escape during an impact such as the size of impacting bodies, their velocity, atmospheric entry angles, the presence of an ocean on the proto-Earth etc. Impacts do not always imply a total loss of atmospheric volatile elements. de Niem et al. (2012) already demonstrated that accumulation of volatile elements probably dominates over atmospheric erosion during a heavy bombardment. For example for a late heavy bombardment of 2 × 1023 g containing between 10
% and 90 % of comet-like objects, the initial pressure of the atmosphere would have been multiplied by 2.5 to 6 demonstrating that impacts can efficiently deliver volatile elements to the growing Earth (Fig.1.9). Similarly, Schlichting et al. (2015) demonstrated that the Earth’s atmosphere reflects the equilibrium between atmospheric erosion and volatile delivery from impactors.


Fig. 1.8: Evolution of the small-body populations (eccentricity vs. semi-major axis) during the growth and migration of the giant planets. Jupiter, Saturn, Uranus and Neptune are represented with black-filled circles. Planetary embryos are represented with large empty circles. The dashed curves represent the boundaries of the main belt. Red dots, light blue dots and dark blue dots are planetesimals originally inside 3 AU, between giant planets orbits and in the outer solar system, respectively. Figure from Walsh et al. (2011).
Fig. 1.9: Evolution of the terrestrial atmospheric pressure during heavy bombardment vs. the total impactor mass. Colors represent different mass contribution to the late heavy bombardment from Kuiper Belt (cometary) Objects (KBOs). The vertical grey line represents the mass of the terrestrial late heavy bombardment (2 × 1023 g) taken by Marty and Meibom (2007) to estimate the potential contribution of comets to the budget of terrestrial volatiles. Modified after de Niem et al. (2012).
The potential contribution from comets to the budget of volatile elements on Earth during final stages of Earth’s accretion
A potential contribution from comets to the budget of volatile elements and organics on terrestrial planets and specially on Earth has often been advocated mainly based on elemental ratios of volatile elements in the Earth’s atmosphere (Owen et al., 1992; Dauphas, 2003; Hartogh et al., 2011; Marty and Meibom, 2007). However, knowing the isotopic composition of noble gases, and specially Xe, in comets is of primordial importance here to precisely estimate their potential contribution to the budget of volatile elements on Earth. Analyses of samples from comet 81P/Wild 2 returned by the Stardust NASA mission already revealed the presence of chondritic gases (« Q » gases) in cometary grains sampled by the Stardust spacecraft (Marty et al., 2008). However there is, up to now, no in-situ measurement of the isotopic composition of heavier noble gases (Kr, Xe) in such objects.
Recently, Kramers et al. (2013) analyzed noble gases contained in an unusual diamond-rich pebble found in SW Egypt and called « Hypatia ». Among other results, Kramers et al. (2013) found that Ne, Kr and Xe isotopic ratios were tending toward the G component. This component is believed to carry nucleosynthetic anomalies and to be very primitive in the history of our solar system (Ott, 2014). The authors interpreted the presence of this component as an evidence that Hypatia was originating from a cometary object that exploded when penetrating in the Earth’s atmosphere, causing an outburst responsible for the formation of the Libyan Desert Glass. However, this interpretation was subsequently criticized (Reimold and Koeberl, 2014). During this study, an international collaboration has been conducted in order to measure abundances and isotopic compositions of nitrogen and noble gases in this unusual stone. Indeed, if Kramers et al. (2013) were right, Hypatia was an unique opportunity to measure the isotopic composition of N and noble gases (specially Xe) in cometary matter with very precise laboratory equipments. Results and interpretations are presented and discussed in a recent publication (Avice et al. (2015), see Annexe 5.1 for the full edited paper). Unfortunately, this study does not confirm the presence of the G component and thus does not confirm the cometary nature of the parent body of Hypatia. However the presence of Q gases (see section 1.2.3), of 129Xe excess due to the decay of extinct 129I (Fig. 1.10a) and of a major component released at high temperature with a δ15N value of -110 ‰(Fig. 1.10b) make this extraterrestrial material unique among other known groups of meteorites. Analyses of noble gases and nitrogen contained in a graphite nodule from the Canyon Diablo iron meteorite are planned in the near future in order to make a comparison with Hypatia. Furthermore this study demonstrates another time the ubiquitous presence of Q gases in primitive and differentiated meteorites even if its origin remains unknown.
All results presented in this section call for a contribution of volatile-rich bodies to the budget of volatile elements on Earth. However, the exact contribution of comets in this late addition remains unknown.

The early Earth: from the Hadean to the Archean

The Hadean is the first geological age of our planet and covers 600 Ma, from ≃4.5-4.6 to 4.0 Ga. It started after the last giant impact, probably the Moon-forming event, whose precise age is still debated and that probably melted a significant part of the Earth’s mantle. Most of the rocky remnants of the first 600 Ma of our planet are zircon crystals contained in younger sedimentary rocks of the Jack Hills (Australia) geological area. However, numerous studies permitted to put constraints on this epoch by applying diverse analytical techniques to these crystals (Harrison, 2009). One major implication of these studies is that the oxygen isotope systematics applied to these crystals demonstrates that the protolith of these grains contained 18O-rich clay minerals formed by interaction with liquid water (Mojzsis et al., 2001). This observation and other arguments reviewed by Harrison (2009) imply the presence of liquid water at the Earth’s surface around 4.3 Ga and that this water was already acting as a weathering agent. Another implication of the presence of Jack Hills zircons is that they come from granites that constitute the upper crust. Measurements of initial 176Hf/177Hf ratios in these crystals are compatible with an early extraction of the continental crust during the Hadean (e.g. Blichert-Toft and Albarède (2008)). However, the crustal volumes at this time remain debated (e.g. Kemp et al. (2015)).
The Hadean was also the time for large-scale differentiation of the Earth’s mantle. For example, Touboul et al. (2012) measured 182W anomalies in Kostomushka komatiites signing the persistence, at least 2.8 Ga ago, of a reservoir that isolated from the whole mantle during the first tens of Ma of Solar System history. Anomalies are also found in Sm-Nd isotope systematics with a Sm/Nd fractionation occurring no later than 100 Ma after Solar System formation (Caro et al., 2003). Finally, noble gas studies of mantle-derived samples demonstrated the existence of an undegassed reservoir in the mantle with high Xe/I, Ar/K and He/(U+Th) ratios which has been isolated from whole mantle convection within the first 100 Ma of Solar System history (e.g. Marty (1989); Mukhopadhyay (2012)).
Even if Jack Hills zircons sign the presence of water on the surface, only little is known on the composition of the atmosphere during the Hadean and up to the early Archean. CO2 and N2 were probably dominating the budget of atmospheric gases as CH4 released from meteoritical impacts would have been dissipated by photolysis within several tens of Ma (Kasting, 2014). The end of the Hadean (4.0 Ga) is often described as the epoch when the Earth looked much as it is today. However, the next section will demonstrate that the Earth had still very different geological and atmospheric features.
The Archean spans a long period of 1.5 Ga of Earth’s history (4.0 Ga to 2.5 Ga). This epoch is funda-mental in Earth’s geological history since life probably emerged at this epoch (Mojzsis et al., 1996). One remarkable feature of the Earth system during the Archean is that the Earth’s mantle was probably hotter than today. Several observations and considerations argue in favor of higher temperatures for the mantle in the past. First, the whole budget of heat production was higher due to very active radioactivities, ongoing core crystallization, residual heat released during accretion. For example, Korenaga (2008) computed an internal heat flux up to 4 times higher than the modern one (around 10 TW) for the Earth 3.5 Ga ago. The widespread presence of komatiites during the Archean is probably a direct evidence that the mantle was hotter than today (Arndt and Nisbet, 2012). A somehow rough conclusion would be that, at this time, the Earth was very active and that the mantle presented a high degassing rate of its volatile elements due to a very active convection regime. However, this not clear how the high internal heat flux translated in terms of tectonics mode and extent of mantle convection (Arndt and Nisbet, 2012).
The Archean atmosphere of Earth’s atmosphere during this period are scarce due to the lack of an extended and well preserved geological record. The transition between the Archean and the Proterozoic (2.5 Ga to 0.542 Ga) is marked by an evolution of the atmosphere from an anoxic to an oxic state. In this section, key points on the Archean atmosphere and on main causes and consequences of the oxygenation of this atmosphere are presented and discussed in order to build a coherent framework for the interpretation of the results obtained during this study.
There are several lines of evidence for low levels of 02 (< 10−5 PAL (Present Atmospheric Level)) in the
Archean atmosphere from 3.9 to 2.2 Ga (Holland, 2006). This subject has been recently critically reviewed by Ohmoto et al. (2014) and some of the arguments for an O2-poor Archean atmosphere are listed below:
• Absence of red beds usually formed by oxidation by O2-rich water of FeII originally in biotites or hornblendes;
• Presence of detrital grains of uraninite or pyrite very labile in O2-rich conditions;
• Presence of banded iron formations (BIFs) of Lake Superior type due to the abundance of FeII in deep oceans;
• Absence of Mo enrichment and isotopic fractionation in black shales (Anbar et al., 2007);
Mass-independent fractionation (MIF) of sulfur isotopes recorded in Archean sediments (Fig. 1.11) are one of the most clear evidence that oxygen atmospheric levels remained very low (< 10−5 PAL) during the Archean (Farquhar and Wing, 2003). Photochemistry in the atmosphere under UV photons is the only known process able to produce significant S-MIF during photolysis of SO2 and SO (Farquhar et al., 2001). Furthermore conservation of S-MIF signals is possible uniquely if products of UV-photolysis in the atmosphere, that are carrying Δ33S with opposite signs, are separated in two categories: one with polymerized elemental S and another with sulfates produced by oxidation of elemental sulfur by CO2 and H2O. These two products then rain out and remain segregated in the rock record. Even if the mechanisms behind UV photolysis and conservation of the S-MIF signature in rocks are still not fully understood, production of mass-independent isotopic fractionation signatures in the past required: i) the presence of methane (CH4), the reducing agent responsible for formation of polymers of elemental S; ii) very low levels of O2 to ensure the formation of both sulfates and elemental sulfur carrying different Δ33S signatures and to prevent any absorption of shortwave photons by tropospheric ozone (O3 ); iii) a sufficient input of sulfur-bearing species to the atmosphere by abiotic or biogenic processes (Catling (2014) and refs. therein).

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The faint young Sun paradox: how to warm the early Earth?

One major motivation for determining the molecular composition of the Archean atmosphere is based on one question: how to warm the early Earth sufficiently despite a faint young Sun? It has long been recognized that the Sun’s luminosity is not constant and evolved from the Sun’s formation to Present. Models of stellar evolution and measurements of solar analogs of different ages demonstrate that the solar energy input was around 25 % lower than today during the Archean (Sagan and Chyba, 1997). In these conditions, a modern-like composition for the Archean atmosphere would have led to a snowball Earth because modern abundances of greenhouse warming gases would not have been sufficient to compensate such a lower luminosity. However, there are several lines of evidence for the presence of liquid water at this time (Feulner, 2012; Mojzsis et al., 2001). The composition of the Archean atmosphere was thus probably different from Present times. Feulner (2012) reviewed ways to warm the early Earth with enhanced greenhouse warming driven by higher atmospheric concentrations of NH3, CH4 and CO2 and pointed out the need for geochemical constraints on the abundances of these molecules in the ancient atmosphere. Goldblatt et al. (2009) proposed another way to solve the faint young Sun paradox. N2 is not a greenhouse gas by itself but a higher partial pressure of this gas in the past would have amplified the greenhouse impact of other gases by broadening absorption lines. Doubling the partial pressure of nitrogen would thus have led to a global warming of 4.4 °C. However, Marty et al. (2013) demonstrated by using Ar-N2 correlations on fluids contained in Archean samples that the atmospheric end-member presented a modern-like N2/36Ar ratio corresponding to a modern-like partial pressure of nitrogen (pN2 ).
Studying fluids trapped in ancient samples has thus the potential to constrain the elemental composition of the ancient atmosphere and to follow its evolution with time.

Table of contents :

1 Introduction 
1.1 Context of the study
1.1.1 Earth’s accretion and the potential origins of its volatile elements
1.1.2 The early Earth: from the Hadean to the Archean
1.2 Introduction to Xenology: principles and applications
1.2.1 Isotopic structure of Xenon and physico-chemical caracteristics
1.2.2 I-Pu-Xe systematics and the dating of reservoirs closure
1.2.3 Relations between solar system components
1.2.4 Atmospheric Xenon: paradox, plausible explanations and implications
1.2.5 The case of Mars-Xe: a similar story?
1.3 Questions and research opportunities
2 Samples Characterization and Analytical Methods 
2.1 Geological setting of the samples
2.1.1 The Barberton greenstone belt (South Africa)
2.1.2 The Hamersley Basin and the Fortescue Group (Australia)
2.1.3 Other studied geological areas
2.2 Analytical methods
2.2.1 Noble gas mass spectrometry
2.2.2 Ar-Ar extended method: Ar-Ar ages and halogens (I, Cl, Br) abundances
2.2.3 Analyses of other noble gases (Ne, Ar) and nitrogen
3 Results and Implications 
3.1 Article Chondritic Xenon in the Earth’s Mantle
3.1.1 Additional Comments and Research Perspectives
3.2 Article Archean Xenon Reveals a Possible Cometary Origin for the Earth’s Atmosphere
3.3 Article Evolution of Atmospheric Xenon and other Noble Gases Inferred from the Study of Archean to Paleoproterozoic rocks
3.3.1 Additional Comments and Research Perspectives
3.4 Article The I-Pu-Xe age of the Moon-Earth system revisited
3.4.1 Additional Results and Research Perspectives
4 Conclusions and Perspectives 
4.1 Main Results of this Study
4.2 The Potential Contribution from Comets
4.3 An Emerging Picture for the Origin and Evolution of Terrestrial Xenon
4.4 Open Issues and Research Perspectives
5 Annexes 
5.1 Article A comprehensive study of noble gases and nitrogen in « Hypatia », a diamond-rich pebble from SW Egypt
5.2 Article Multiple carriers of Q noble gases in primitive chondrites
5.3 Article Tissint Martian Meteorite: A Fresh Look at the Interior, Surface, and Atmosphere of Mars
5.4 Article Origins of volatile elements (H, C, N, noble gases) on Earth and Mars in light of recent results from the ROSETTA cometary mission


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