Delivery of carbon, nitrogen, and sulfur to the silicate Earth by a giant impact

Abstract

Earth's status as the only life-sustaining planet is a result of the timing and delivery mechanism of carbon (C), nitrogen (N), sulfur (S), and hydrogen (H). On the basis of their isotopic signatures, terrestrial volatiles are thought to have derived from carbonaceous chondrites, while the isotopic compositions of nonvolatile major and trace elements suggest that enstatite chondrite-like materials are the primary building blocks of Earth. However, the C/N ratio of the bulk silicate Earth (BSE) is superchondritic, which rules out volatile delivery by a chondritic late veneer. In addition, if delivered during the main phase of Earth's accretion, then, owing to the greater siderophile (metal loving) nature of C relative to N, core formation should have left behind a subchondritic C/N ratio in the BSE. Here, we present high pressure-temperature experiments to constrain the fate of mixed C-N-S volatiles during core-mantle segregation in the planetary embryo magma oceans and show that C becomes much less siderophile in N-bearing and S-rich alloys, while the siderophile character of N remains largely unaffected in the presence of S. Using the new data and inverse Monte Carlo simulations, we show that the impact of a Mars-sized planet, having minimal contributions from carbonaceous chondrite-like material and coinciding with the Moon-forming event, can be the source of major volatiles in the BSE.

Fig. 1. Experimentally determined N contents and C solubilities as a function of S contents in Fe-Ni alloy melts.

(A) The N content in the alloy melt is not significantly affected by the presence of S. The N content does not show a significant change between experiments with S-free and low S-bearing (~12 to 15 wt %) alloy melts for a given P-T condition, while it drops by 50% for high S-bearing (~21 to 28 wt %) alloys. The N content in a C-free system (29) is higher than that in a C-present system in this study and previous studies (12, 63). (B) The C solubility in the alloy melt decreases steadily with an increasing S content in the alloy melt with an order of magnitude difference between S-free and high S-containing alloys. The C solubility in a N-free system (3, 4, 14, 24, 25, 56) is higher than that in a N-present system in this study and previous studies (12, 63). Error bars in both cases are 1−σ and, if absent, are smaller than the symbol size.

Fig. 2. Partition coefficients of N and C between the Fe-Ni alloy melt and the silicate melt as a function of S content in the alloy melt.

(A) For a given P-T condition, the ${\mathit{D}}_{\mathrm{N}}^{\text{alloy}/\text{silicate}}$ does not vary notably with an increase in S in the alloy melt. (B) In the presence of N in the alloy melt, the ${\mathit{D}}_{\mathrm{C}}^{\text{alloy}/\text{silicate}}$ drops by an order of magnitude from S-free to high S-bearing alloys with values as low as ~10 for N-bearing, S-rich alloys. The ${\mathit{D}}_{\mathrm{C}}^{\text{alloy}/\text{silicate}}$ in a N-free system (3, 4, 14, 24, 25, 56) is higher than that in a N-present system in this study and a previous study (12). Error bars in both cases are 1−σ and, if absent, are smaller than the symbol size.

Fig. 3. Ratios of the measured partition coefficients of C and N and C and S between the alloy melt and the silicate melt as a function of S content in the alloy melt.

(A) The ${\mathit{D}}_{\mathrm{C}}^{\text{alloy}/\text{silicate}}$/${\mathit{D}}_{\mathrm{N}}^{\text{alloy}/\text{silicate}}$ ratios show that with S-free and intermediate S contents in the alloy melt, C is more siderophile than N, while for high S contents in the alloy, C is less siderophile than N. (B) The ${\mathit{D}}_{\mathrm{C}}^{\text{alloy}/\text{silicate}}$/${\mathit{D}}_{\mathrm{S}}^{\text{alloy}/\text{silicate}}$ ratios show that under S-poor conditions, C is more siderophile than S (for N-free systems), while at intermediate S (for N-bearing systems) and high S contents (for both N-free and N-bearing systems) in the alloy melt, C is less siderophile than S. Dashed horizontal lines in both panels delineate data fields where the (C/N)_mantle and (C/S)_mantle increase (↑) or decrease (↓) from the initial values due to equilibrium alloy-silicate fractionation. Error bars in both cases are 1−σ and, if absent, are smaller than the symbol size.

Fig. 4. C-N-S delivery via a giant planetary impact with proto-Earth and the results of inverse Monte Carlo simulations used to obtain the composition and the mass of the impactor.

(A) Illustration showing the merger scenario of a volatile-bearing planetary embryo with a C-saturated, S-rich core to volatile-depleted proto-Earth. (B) The alloy/silicate ratio and the bulk S content are mutually dependent on the S content in the alloy of the impactor (the controlling variable from our experiments and calculations). (C) The bulk C content of the impactor as a function of the bulk S content as well as the alloy/silicate ratio of the impactor that yields possible volatile delivery solutions. Various chondritic meteorites are also plotted for comparison. (D) The mass of the impactor is constrained in a narrow range similar to that of Mars.

Fig. 5. Effect of a nonzero volatile content in the bulk silicate portion of proto-Earth on the composition and the mass of the impactor, which establishes the C-N-S budget of the present-day BSE.

Increasing the C-N-S abundance in chondritic proportions in the silicate portion of proto-Earth (represented by circles) results in a decreasing bulk C content and mass of the impactor and an increasing S content in the impactor’s core. A decrease in the mass of the impactor is due to the delivery of a smaller fraction of the C-N-S budget of the present-day BSE by the impactor, while a superchondritic C/N ratio greater than that of the present-day BSE in the impactor necessitates a higher C enrichment in the impactor’s mantle via C exsolution from the core, which results in a higher S content of the core and lower bulk C content of the impactor. However, if the C-N-S inventory in the silicate portion of proto-Earth was affected by terrestrial core formation and atmospheric loss(es) (represented by diamonds), then, owing to the highly siderophile character of C, the bulk silicate portion of proto-Earth would be essentially C free but could retain N and S. Increasing the N-S abundance in the silicate portion of proto-Earth by up to 20% of the N-S budget of the present-day BSE results in a decreasing mass and bulk C content of the impactor, while the S content of the core almost remains constant. However, with greater than 20% of the N-S budget of the present-day BSE in silicate proto-Earth, the S content of the impactor’s core increases, delivering C/N and C/S ratios that are much greater than the C/N and C/S ratios of the BSE, and the bulk C of the impactor also increases, delivering the present-day C budget of the BSE via a proportionally smaller impactor. The star represents a C-N-S–free bulk silicate proto-Earth.

Fig. 6. Results of inverse Monte Carlo simulations showing the effect of the degree of equilibration of the impactor’s core with the post-merger MO on the estimated bulk C content and the mass of the impactor.

(A) The most probable bulk C content (wt %) of the impactor as shown by the peak increases with an increasing degree of equilibration of the impactor’s core with the post-merger MO. (B) The most probable mass of the impactor with respect to the present-day Earth’s mass decreases with an increasing degree of equilibration of the impactor’s core with the post-merger MO (see Materials and Methods for details). These calculations assume the bulk silicate portion of the proto-Earth to be C-N-S free (star in Fig. 5).