Nitrogen speciation in upper mantle fluids and the origin ......origin of Earth’s N2-‐rich atmosphere Sami Mikhail & Dimitri Sverjensky S1. Supplementary discussion S1.1 The selection

Supporting Online Material for

Prediction of nitrogen speciation in upper mantle fluids explain the origin of Earth’s N2-‐rich

atmosphere

Sami Mikhail & Dimitri Sverjensky

S1. Supplementary discussion

S1.1 The selection of N2, 20Ne, 36Ar, 84Kr, and 130Xe

In comparing the chemistry of telluric planetary atmospheres, several factors must be considered to

enable accurate comparisons between different planets. The relative abundances of atmospheric N2 and

primordial noble gases can be used to investigate the role of mantle petrology and geochemistry on

volcanic degassing, and to place constraints on the similarities and differences during the evolution of the

terrestrial planets (Table S1).

Here we use the noble gases and nitrogen because Earth’s atmosphere has been terraformed by life 1,

resulting in a CO2-‐poor and O2-‐rich atmosphere, whereas the atmosphere of the lifeless terrestrial planets

(Venus and Mars) contain >95% CO2 and only trace quantities of O2 2. To enable comparative

investigations of these planetary atmospheres the data must be selective to avoid effects of biological

processes and geology/weathering (i.e. the decay of 40K to 40Ar). Most Ar in the atmospheres of Earth and

Mars is not primordial, but instead produced through the decay of radiogenic 40K (with 40Ar/36Ar ratios of

298 and 1900 ± 300 respectively) 1-‐2, whereas the ratio of primordial to radiogenic Ar in the Venusian

atmosphere is anomalously low, with a 40Ar/36Ar ratio of 1.03 ± 0.04 3. Therefore, the abundance of total

Ar (combined 40Ar, 38Ar and 36Ar) in planetary atmospheres appears to be more a function of the

abundance of exposed K to the atmosphere, and not the amount of degassed Ar released through

volcanism.

Nitrogen speciation in upper mantle fluids and the origin of Earth’s nitrogen-rich atmosphere

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Our approach is to use total atmospheric N2 relative to the abundance of primordial noble gases (20Ne,

36Ar, 84Kr, and 130Xe). This approach is based on the following factors. Firstly, the ratio of surficial nitrogen

(biosphere, sediments and oceans) to atmospheric nitrogen is only ~0.0001 4, therefore Earth’s

atmospheric nitrogen abundance is not controlled by the biosphere (contrary to CO2, CH4, O2 and H2O).

Secondly, the primordial noble gas abundances in the atmosphere are not fractionated in the biosphere.

S1.2 The reason for ruling out the role of bridgmanite to explain the data in Figure 1

Historically, geochemists have assumed that they should group nitrogen with the noble gases (Ne, Ar,

Kr, Xe). This is because molecular nitrogen (N2) and noble gases are inert gases within planetary

atmospheres and are highly incompatible elements in common mantle minerals 5-‐6. However, recent

experimental data show this assumption to be incorrect regarding mantle differentiation. Molecular

nitrogen is indeed incompatible in silicate minerals, but ammonic nitrogen can be a moderately

compatible element in K-‐Ca-‐Na bearing silicates phases, such as phlogopite and clinopyroxene 5-‐6.

Likewise, Ne, Ar and Kr have been show to be soluble in bridgmanite (a recently named mineral, formerly

known as MgSiO3-‐Perovskite), whereas Xe remains incompatible 7. This means nitrogen can be

fractionated from the noble gases, depending upon speciation, and theoretically, the noble gases can be

fractionated from nitrogen during bridgmanite crystallization. If bridgmanite was involved in the origin of

Earth’s atmospheric N2/noble gas enrichment one would expect the Martian and Venusian atmospheres to

show very different results for ratios such as Ne/Xe or Ar/Xe because of the large differences in

bridgmanite modal abundances within the mantles of these planets. However, this is certainly not the case

(Figs.S1 and 1 respectively).

S1.3 The reason for ruling out the role of Earths magnetic field to explain the data in Figure 1

It is theoretically possible that Earths magnetic field has enabled more atmospheric nitrogen retention

relative to the Martian and Venusian atmospheres. This shielding effect has certainly occurred, but cannot

explain the data in Fig. 1 for three reasons. Firstly, the Venusian atmosphere contains more nitrogen by

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mass relative to Earth (factor of 3) 3. Secondly, one would expect mass-‐dependent fractionation reflected

from N to Xe abundances, and also between light and heavy primordial noble gases during loss to space,

which would be recorded within the given planetary atmosphere and is not seen here. Instead, the

20Ne/84Kr ratio of Earth’s atmosphere falls alongside the Venusian and Martian values (Fig.S1). Finally, the

high 40Ar/36Ar ratio of the Martian atmosphere (1900±300) relative to Earth’s (298) 1-‐2 has been proposed

to be a function of early loss of more primordial 36Ar from a weak Martian atmosphere relative to Earth,

which was followed by volcanic degassing of 40Ar produced by 40K decay on both planets. Interestingly, the

40Ar/36Ar ratio of the Venusian atmosphere is ~1 3, and shows comparable 36Ar/primordial noble gas and

20Ne/primordial noble gas abundances to Earth and Mars (Fig.S1 and 1 respectively). Therefore the

shielding effect of Earth’s magnetic field cannot explain the differences between Earth’s N2-‐enrichment

and the relative, and comparable, N2-‐depletions for Mars and Venus.

S1.4 The reason for ruling out N-‐rich cores for Mars and Venus relative to Earth to explain the data in

Figure 1

Another hypothetical possibility is that Nitrogen should partition into a metallic phase during metal-‐

silicate differentiation under equilibrium conditions 8. However, because of their similarity in size (within

5%), Earth and Venus would likely have had similar P-‐T-‐fO2 conditions of core formation, provided that

the giant moon-‐forming impact and core formation occurred under similar conditions. Ergo, Mars should

be an outlier to the Earth-‐Venusian system, which is not the case (assuming comparable chemical

composition for the starting materials during accretion). Overall then, the partitioning of N during core

formation is not adequate to explain the differences for the atmospheric N2/primordial noble gas ratios

for the atmospheres of Earth and Venus

S1.5 The reason for ruling out preferential primordial noble gas loss from Earth during the Moon-‐

forming impact to explain the data in Figure 1

It could be hypothesized that loss of Earth’s early atmospheric N2 and the primordial noble gases during





a large impact (i.e. the proposed Moon-‐forming impact) would be manifested as a distinct geochemical

signature in Earths atmosphere and not in the atmospheres of Venus and Mars. In other words, we would

predict highly fractionated primordial noble gas ratios relative to the Martian and Venusian atmospheres

9. In fact, Earth’s atmosphere does show lower total concentrations of noble gases and molecular nitrogen

relative to Venus, but there is no indication of fractionation of the heavy/light primordial noble gases (i.e.

20Ne/36Ar and 20Ne/83Kr ratios; Fig.S1). This provides evidence that the relative noble gas patterns are

preserved through secondary loss processes (i.e. the Moon-‐forming Giant Impact). Therefore, major loss

of atmosphere appears to be possible without significantly fractionating the relative proportions of the

primordial noble gases and molecular nitrogen 9.

There is also the question of how much of the telluric planetary volatiles were delivered by the late

veneer, which is required to explain the moderate to volatile elements (such as H, C, S, and Se) 10, and the

HSE 11-‐12. The late veneer was widespread throughout the solar system 12-‐13 and certainly post-‐dates the

formation of the moon 11-‐12. However, it is unlikely that Earth received its N2/primordial noble gas

enrichment from the late veneer, because the Venusian and Martian atmospheres show comparable

N2/primordial noble gas abundances. The ‘late veneer’ cannot explain these data in Figs 1a+b and S1. This

is because the late veneer should/would have affected all planets in the inner solar system as a function of

size (surface area). Ergo, the atmospheres of Earth and Venus should be comparable, and not different. In

addition, the late veneer would have affected the atmospheres of Mars and Venus differently, however,

they display comparable N2/primordial noble gas ratios. In fact, as shown in Fig.1a, and S1, the three

planets exhibit comparable primordial noble gas/ primordial noble gas ratios (e.g. 20N/36Ar), something

that would not be predicted if the late veneer was the explanation for the Earth’s N2/primordial noble gas

enrichment.

S1.6 The redox state of the interiors of the terrestrial planets

The Earth’s upper mantle redox state (expressed as fO2 in log units) can be determined by studying

basaltic glasses and mantle xenoliths. These data show the LOGfO2 of Earth’s volcanic sampling field to be 4 NATURE GEOSCIENCE | www.nature.com/naturegeoscience




around the QFM redox buffer (average LOGfO2 = ΔQFM 0 ± 2) 13. However, if the data for these mantle

xenoliths are subdivided into two groups, a clearer picture emerges. Mantle xenoliths from arc settings

above the mantle wedge are shown to be more oxidizing (LOGfO2 = ΔQFM 0 to +2) relative to primitive

basalts and kimberlitic/oceanic xenoliths (LOGfO2 = ΔQFM 0 to -‐3) 13-‐15 , which thermodynamic data show

should become even more reducing with depth (i.e. below ca. 250 km the mantle is buffered around IW,

not QFM) 13. The LOGfO2 of the Martian mantle has been determined from using the most primitive SNC

meteorites to be around ΔQFM -‐1 to -‐3 16. Due to the scarcity of data, any comments on the redox state of

the Venusian mantle are highly speculative. Data from landers Veneras 13 and 14 show FeO contents of

basaltic rocks on the Venusian surface to be between ca. 8-‐10 wt.% 17, these compositions are similar to

mid-‐ocean ridge basalts on Earth.17. Because Venus and Earth are of a comparable size and bulk

composition, their respective mantles should be dominated by bridgmanite, which during core-‐mantle

differentiation forces the disproportionation of ferrous iron into ferric iron plus metal 18. This process has

previously been described as an ‘oxygen pump’, which would have injected ferric iron into the Earth’s

upper mantle during mantle differentiation 18 thus raising the ambient redox state from near IW towards

QFM. This process would not have occurred within the Martian interior because of the limited stability of

bridgmanite in the smaller planet 18. This explains the upper mantle redox discrepancy between Earth and

Mars, and would imply the Venusian mantle should have a redox state akin to the Earth’s ambient mantle.

Therefore, we assume the fO2 of the Venusian mantle is similar to that of Earth’s primitive mantle (LOGfO2

= ΔQFM 0 to -‐3).





S1.7 References cited

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S7. Shcheka, S.S., & Keppler, H. The origin of the terrestrial noble-‐gas signature, Nature, 490, 531-‐536

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S10. Wang, Z, & Becker, H. Ratios of S, Se and Te in the silicate Earth require a volatile-‐rich late veneer, Nature,

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S2. Supplementary data

Table S1: Data used in this study for the atmospheres of Earth 1, Mars 1-‐2 and Venus 1,3, 17.





Figure S1: The abundances of atmospheric 20Ne relative to the abundance of molecular nitrogen and the

primordial noble gases of Earth 1, Mars 1-‐2 and Venus 1,3. Molecular nitrogen and the primordial noble

gases are listed in order of their relative abundances ref.1.





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Nitrogen speciation in upper mantle fluids and the origin ......origin of Earth’s N2-‐rich atmosphere Sami Mikhail & Dimitri Sverjensky S1. Supplementary discussion S1.1 The selection