Isotopic record of the atmosphere and hydrosphere

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1 Isotopic record of the atmosphere and hydrosphere W. F. McDonough 1 1 Department of Earth Sciences and Research Center for Neutrino Science, Tohoku University, Sendai , Japan (Dated: March 7, 2018) The atmosphere and hydrosphere document the Earth s accretion and degassing history of the highly volatile elements (those that condense from a nebular disk at half-mass temperatures of <500 K). Key radiogenic isotopic systems include 4 He, 21 Ne, 40 Ar, 129 Xe, and Xe and they provide critical constraints on the formation, evolution and present-day composition of the atmosphere. The δ 18 O, δ 2 D and δ 15 N composition of the atmosphere and hydrosphere differs from that of nebular gas and comets, but is consistent with values seen in primitive meteorites. Consequently, the Earth s inventory of O, H, and N originates from primitive meteorites, while the noble gases appear to represent a mixture of solar and chondritic compositions. However, the recent Rosetta spacecraft mission, which orbited the comet 67P/Churyumov-Gerasimenko (coming within kilometers of its surface), has provided data showing that 1/5 of the atmospheric xenon is of cometary origin. The 129 I and δ 37 Cl isotope systems and relative abundances of the halides (e.g., F/Cl, F/Br, F/I) all place limits on the existence and timing of the early hydrosphere. By far, the most abundant atoms in the Earth are oxygen (50%), with Mg, Si and Fe atoms adding sub-equal proportions at 15% each, and the remaining 5% of the atoms in the Earth are made up of all of the other elements combined. The Earth, the largest of the terrestrial planets, as well as the rest of the terrestrial bodies so far sampled, share a restricted range of oxygen isotopic compositional space that is distinctly removed from the oxygen isotopic of the Sun, implying compositional fractionation processes in the nebular disk. I. Summary Three groups of isotope systems give insights into the nature and evolution of the Earth s inventory of highly volatile elements: Noble gas isotopes: These elements have completely filled electron orbits and thus are inert to to chemical processes. Several of these systems 4 He, 21 Ne, 40 Ar, 129 Xe, and Xe are the products of nuclear reactions (isotope decay chain products). δ 18 O, δ 2 D, and δ 15 N isotope systems : The light elements, which form gases and ices (i.e., H, C, N, O), control many important properties of a planet, including initiating and nurturing life, the generation of atmosphere and hydrosphere, and controls on crust and mantle viscosity, which in turn influences the ease of convection and heat dissipation. These elements and their isotopes are readily fractionated by low temperature processes. Isotopic variations in these gases document a wide range of geological, biological, and cosmochemical processes. 129 I and δ 37 Cl isotope systems : The halides, which are strongly partitioned into the hydrosphere, record processes associated with the Earth s ocean and atmosphere. 129 I is an extinct nuclide that decayed to 129 Xe. This system documents the earliest evolution of our atmosphere - hydrosphere system. The halide abundances and the δ 37 Cl isotope system provides further constraints and insights into the origin and evolution of the hydrosphere; release of these elements from the silicate Earth may have lead to their fractionation from the lithophile elements. Their depletion in the silicate Earth may be explained by the loss of an early ocean and atmosphere, as well as providing an insight into the Moon s formation. A. β and α decay schemes in these isotope systems Noble gas systems [1] 238 U, 235 U, and 232 Th produce 8α, 7α, and 6α, respectively, which mostly become 4 He atoms [2] α particles from the above system can interact with atoms and occasionally produce other products: For example, 18 O(α, n) 21 Ne

2 2 [3] 40 K + e 40 Ar + ν e + Q (1.505 MeV) (t 1/2 = Ga), this branched decay system accounts for 10.9% of the decays from 40 K [4] 129 I 129 Xe + e + ν e + Q (0.189 MeV) (t 1/2 = 15.7 Ma) [5] 244 Pu fissions into various proportions of Xe (t 1/2 = 81.1 Ma) [6] 238 U fissions into various proportions of Xe (t 1/2 = 4.47 Ga) [7] 235 U fissions into various proportions of Xe (t 1/2 = Ga) Figure 1: Tables from The History of Planetary Degassing as Recorded by Noble Gases by D.Porcelli and K. K. Turekian, Volume 6.16, and The Origin of Noble Gases and Major Volatiles in the Terrestrial Planets by D. Porcelli and R. O. Pepin, Volume 6.17, in Treatise of Geochemistry (Turekian and Holland, eds) Elsevier (2015) Initial inventory of noble gases Noble gases in the Earth s atmosphere have a chondritic affinity and they appear not to be derived from the nebular. In contrast, the atmospheres of Venus and Mars appear, in part, to preserve a nebular composition. It is possible that the Earth s noble gases are derived from late-accreting planetesimals. The literature on He isotope ratios is simply very confusing. The literature reports both 4 He/ 3 He and 3 He/ 4 He values. Often this ratio is expressed relative to the atmospheric ratio (R A )and so the MORB source region is generally stated to have an R/R A value of 8 ± 1 and the OIB source region records R/R A values as low as 4 and as high as 50. Today, because of thermal (Jeans) escape mechanisms, the concentration of He in the atmosphere is only 5 (µg/g). He atoms do not have sufficient mass to be gravitationally bound to the Earth and are ultimately lost to space. Thus, He atoms have a mean atmospheric residence time of 1x10 6 year. ( 4 He/ 3 He) atmosphere 1.4 x 10 6 (R A ), ( 3 He) initial 10 8 atoms kg 1 ( 4 He/ 3 He) initial is estimated to be between 120 R A and 330 R A, with preference towards the lower estimates. The mantle produces about 2 x10 32 atoms of 4 He per second. The Ne isotopic composition of the atmosphere, crust, and mantle changes with time due to 18 O(α, n) 21 Ne interaction, the mantle produces about 1 x10 25 atoms of 21 Ne per second and thus, a 4 He/ 21 Ne production ratio of 2.5x10 7. ( 20 Ne/ 22 Ne) atmosphere 10, ( 21 Ne/ 22 Ne) atmosphere 0.02 ( 20 Ne/ 22 Ne) Solar wind 13.7, ( 21 Ne/ 22 Ne) Solar wind The Argon isotope system provides a direct measure of the decay of 40 K, but it lacks specific information about the absolute amount of Ar in the mantle. The atmosphere has 1% by mass Ar and its ( 40 Ar/ 36 Ar) atmosphere = 295.5, whereas the initial composition at Ga was 0.01.

3 3 Figure 2: LEFT: Tables from The History of Planetary Degassing as Recorded by Noble Gases by D.Porcelli and K. K. Turekian, Volume 6.16, and The Origin of Noble Gases and Major Volatiles in the Terrestrial Planets by D. Porcelli and R. O. Pepin, Volume 6.17, in Treatise of Geochemistry (Turekian and Holland, eds) Elsevier (2015). RIGHT: Allegre CJ, Hofmann AW, and O Nions RK (1996) The argon constraints on mantle structures. Geophysical Research Letters 23: Many people have estimated the amount of mantle degassing of the noble gases. Much of these estimates have depended on the Ar isotope system. A good illustration is given in the figure below. The assumed values in the figure above are best guesses for our state of knowledge at the end of the 1990s. Today, seismic tomography shows the mantle to be mixed and thus simple, 2 layer mantle models (upper and lower mantle, as illustrated here) can no longer be justified. Moreover, we do not know the absolute concentration of K in the upper or lower mantle, or bulk silicate Earth (but we have good estimates for the latter). We know best the atmospheric Ar isotopic composition and its absolute concentration; we also know the amount of K in the continental crust, but are less certain about its Ar isotopic composition. A more recent attempt at this scenario is given in the figure below, where the depth to the boundary layer (i.e,. separation between the MORB and OIB sources is estimated at 2000 km. Figure 3 illustrates the rate of 40 K decay (1.3 Ga half life) in the Earth over its history. By the Archean -Proterozoic boundary 2.5 Ga, which is 2 halflives, <50% of the Earth s radiogenic heat was produced from 40 K decay. We now have 15% of our original inventory of 40 K. Thus, in the early Earth, this isotope played an important role in heating the Earth. Note, however, that 40 K is only 0.015% of all potassium. The right panel illustrates the accumulation of 40 Ar in the planet over the last billion years. This is a straightforward calculation just using the standard decay equation and an estimate of the original amount of 40 Ar in the Earth at the end of accretion, which is essentially zero atoms on this scale. The initial 40 Ar/ 36 Ar of the Earth is estimated from iron meteorites and it is taken to be What can be seen here is the amount of Ar in the atmosphere is equivalent to 66 exa grams or 66 x kg of 40 Ar, with a 40 Ar/ 36 Ar =

4 Figure 3: Arevalo, Jr., R., McDonough W. F. and Luong M. (2009) The K/U ratio of the silicate earth: insights into mantle composition, structure and thermal evolution. Earth Planet. Sci. Lett.

In Figure 4 are illustrations of a mantle model of it Ar isotopic composition (left panel) and the evolution of the atmospheric Ar isotopic composition. Figure 4: LEFT: Arevalo, Jr., R., McDonough W.

4 4 Figure 3: Arevalo, Jr., R., McDonough W. F. and Luong M. (2009) The K/U ratio of the silicate earth: insights into mantle composition, structure and thermal evolution. Earth Planet. Sci. Lett. 278, Thus, radiogenic argon dominate the entire Ar budget of the Earth. In Figure 4 are illustrations of a mantle model of it Ar isotopic composition (left panel) and the evolution of the atmospheric Ar isotopic composition. Figure 4: LEFT: Arevalo, Jr., R., McDonough W. F. and Luong M. (2009) The K/U ratio of the silicate earth: insights into mantle composition, structure and thermal evolution. Earth Planet. Sci. Lett. 278, , RIGHT: Pujol, M. et al (2013) Argon isotopic composition of Archaean atmosphere probes early Earth geodynamics, Nature 498: B. The H, O, N isotope systems It is likely that water, nitrogen, sulfur and other volatiles in the Earth were derived from chondrites, perhaps dominated by carbonaceous chondrites, that accreted post mantle and Moon formation (e.g., after 4.50 Ga) on to the planet as a late veneer, contributing a mass fraction on the order of 1%. As an aside, this same mechanism is invoked to account for the addition of the highly siderophile elements (HSE) to explain their chondritic relative abundances and their anomalously high absolute abundances in the bulk silicate Earth. [Metal-silicate partition coefficients for the HSE predict that core formation would have

5 5 greatly depleted their absolute abundances to negligible levels and would never have maintained chondritic proportions.] However, in the case of the HSE, their Os isotopic composition appears to be a closer match to enstatite or ordinary chondrites than carbonaceous chondrites. Overall, models invoking late, small mass additions to the planet appears to have support from multiple sources of observations. Models of planetary dynamics during the first billion years of solar system evolution envisage continued interactions between the giant planets and clustered masses at Kuiper belt distances that lead to mass instabilities potentially from the Jovian Trojan belt. [The Kuiper belt, like the asteroid belt, is a circumstellar disk of material extending from Neptune to about 50 AU from the Sun. Importantly, Asteroids are made up of rocks and metal, whereas Kuiper belt objects are composed mostly of ices, the highly volatile elements.] Early in the solar system s evolution material transport, via orbit crossing, may have been more common, including transfer of outer solar system materials inwards. Such planetary dynamics might provide the impetus to bring primordial material from Jupiter and move outer solar system material to Trojan orbits and explain the source of late mass additions (late veneer) to the Earth (and Moon). Additions of a late veneer to the Earth likely brought with it volatiles and HSE. This mass addition stands in contrast with what is referred to as the Late Heavy Bombardment (LHB). This solar system event is well recorded on the Moon between 4.0 and 3.8 Ga and is responsible for the largest of the lunar craters. The true nature of this LHB event is not known and competing hypotheses are considered; the LHB is either (1) the final trailing stage marking the end of planetary accretion (i.e., a continuously decreasing mass versus time curve for the solar system) or (2) a punctuated, cataclysmic event, triggered by the breakup of a large asteroid or the migration of asteroidal material forced by resonance orbits of the Sun with the giant planets. Figure 5 shows the volatile element abundance pattern for the Earth relative to chondrites and the non-solar Oxygen isotopic composition of the Earth and terrestrial bodies. These terrestrial bodies likely accreted in the 0.5 to 5 AU annulus surrounding the Sun and nebula conditions this annulus probably experiences higher temperatures and a high dust to gas ratio than other portions of the disk. Consequently, these conditions will lead to silicates and water ices with enrichments in heavy oxygen relative to the nebula gas. The higher dust fraction heated more readily and these solids absorbed the heavy water from the disk. The Earth s elemental pattern of the abundant volatiles and of the noble gases is approximately chondritic, with depletions in nitrogen and xenon. Both nitrogen and xenon are depleted by about a factor of 10 relative to the other volatiles. It is likely that nitrogen was retained in the mantle and/or core, while xenon depletion might have taken place during the giant impact formation of the Moon accompanying the loss of the ancient atmosphere and accompanying hydrosphere (see next section). C. 129 I and δ 37 Cl isotope systems The δ 37 Cl value of carbonaceous, ordinary, and enstatite chondrites is -0.3 ± 0.3 (relative to standard mean ocean water), consistent with a compositionally homogenous inner solar nebula. The abundances of Cl, Br, and I in the Earth are depleted by 5-10 times that predicted from the Earth s volatility curve (Figure 6). In contrast, fluorine is not depleted. Chlorine, Br, and I are considerably more hydrophilic than fluorine and it is likely that these heavy halogens were outgassed from the mantle to a surface ocean in the earliest days of the Earth s history. Subsequently, these elements, along with much of the Earth s hydrosphere and atmosphere, were likely lost during the late stages of planetary accretion and/or during giant impact event that formed the Moon. Finally, the 129 I 129 Xe (t 1/2 = 15.7 Ma) is a sensitive monitor of early Earth degassing and potential loss of an atmosphere (and ocean) and recycling of gas into the mantle. It appears from xenon isotopic studies that the volatiles in the atmosphere and mantle originated from distinct cosmochemical sources. It appears that an increasing amount of volatile rich materials was being accreted on to the Earth during the first tens of millions of years of its history. The xenon isotopic composition of the present-day mantle shows a strong deficits in the heavy xenon isotopes and matches that of a primordial atmospheric component, while

6 Terrestrial compositions (planets, meteorites) Figure 5: LEFT: Marty B. (2012) The origins and concentrations of water, carbon, nitrogen and noble gases on Earth. Earth Planet. Sci. Lett.

Lithophile elements Heavy halides Highly Siderophile Chalcophile Loss of halides and early ocean Figure 6: The Earth s volatility curve.

6 6 Terrestrial compositions (planets, meteorites) Figure 5: LEFT: Marty B. (2012) The origins and concentrations of water, carbon, nitrogen and noble gases on Earth. Earth Planet. Sci. Lett , RIGHT: K. D. McKeegan et al (2011) The Oxygen Isotopic Composition of the Sun Inferred from Captured Solar Wind, Science, 332: Lithophile elements Heavy halides Highly Siderophile Chalcophile Loss of halides and early ocean Figure 6: The Earth s volatility curve. Modified from McDonough and Sun (1995) and McDonough (2014) that of the present-day Earth atmosphere contains 22 ± 5% cometary xenon plus chondritic (or solar) xenon. The mantle is made up of two distinctive components, depleted and enriched mantle, and their xenon isotopic compositions imply that these two reservoirs separated within the first 100 million years of Earth?s history and have had limited mixing since. The MORB source region (i.e., the depleted mantle) appears to be significantly degassed and has recycled atmospheric xenon.

Composition and the Early History of the Earth

Composition and the Early History of the Earth Sujoy Mukhopadhyay CIDER 2006 What we will cover in this lecture Composition of Earth Short lived nuclides and differentiation of the Earth Atmosphere and