Evidence for multiple magma ocean outgassing and atmospheric loss episodes from mantle noble gases

Abstract

The energy associated with giant impacts is large enough to generate global magma oceans during Earth's accretion. However, geochemical evidence requiring a terrestrial magma ocean is scarce. Here we present evidence for at least two separate magma ocean outgassing episodes on Earth based on the ratio of primordial 3 He to 22 Ne in the present-day mantle. We demonstrate that the depleted mantle 3 He/ 22 Ne ratio is at least 10 while a more primitive mantle reservoir has a 3 He/ 22 Ne ratio of 2.3 to 3. The 3 He/ 22 Ne ratios of the mantle reservoirs are higher than possible sources of terrestrial volatiles, including the solar nebula ratio of 1.5. Therefore, a planetary process must have raised the mantle's 3 He/ 22 Ne ratio.We show that long-term plate tectonic cycling is incapable of raising the mantle 3 He/ 22 Ne ratio and may even lower it. However, ingassing of a gravitationally accreted nebular atmosphere into a magma ocean on the proto-Earth explains the 3 He/ 22 Ne and 20 Ne/ 22 Ne ratios of the primitive mantle reservoir. Increasing the mantle 3 He/ 22 Ne ratio to a value of 10 in the depleted mantle requires at least two episodes of atmospheric blow-off and magma ocean outgassing associated with giant impacts during subsequent terrestrial accretion. The preservation of a low 3 He/ 22 Ne ratio in a primitive reservoir sampled by plumes suggests that the later giant impacts, including the Moon-forming giant impact, did not generate a whole mantle magma ocean. Atmospheric loss episodes associated with giant impacts provide an explanation forEarth's subchondritic C/H, N/H, and Cl/F elemental ratios while preserving chondritic isotopic ratios. If so, a significant proportion of terrestrial water and potentially other major volatiles were accreted prior to the last giant impact, otherwise the fractionated elemental ratios would have been overprinted by the late veneer.

Tracing the ingredients for a habitable earth from interstellar space through planet formation

“…Thus, substantial loss of N to space seems to be required. This may be most consistent with scenarios that include (i) late delivery of volatiles, chiefly from comparatively oxidized and metal-poor bodies (48), thereby adding volatile-rich material to the mantle without loss of metal to the core; (ii) multiple magma oceans punctuated by large atmospheric loss events (11); and (iii) atmospheric ablation from many smaller impacts following magma ocean solidification (32). The high C/N ratio of the BSE therefore appears to be a sensitive indicator of the balance of volatile accretion and loss during the final stages of the Earth's assembly.…”

“…The organics can derive in part from the dense ISM before stellar birth, but also can be created inside the gas-rich disk. Based on models and observations, disk chemistry and transport can imprint variable C/N ratios (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12) in ices comparable to those of comets and primitive meteorites. During the stages of planetesimal formation when bodies grow to large sizes (greater than tens of kilometers), impact processing and thermal metamorphism can lead to greater disparity in the C/N content of preplanetary material (Fig.…”

“…This volatile poor state of terrestrial planets is partially imparted from the starting materials. However, further differential loss of C and N can occur due to parent body processes such as thermal metamorphism, core segregation, planetary outgassing, and atmospheric loss (5)(6)(7)(8)(9)(10)(11).…”

Reconciling magma‐ocean crystallization models with the present‐day structure of the Earth's mantle

“…Even though any evidence for a strongly-enriched (and "hidden") early reservoir as originally proposed by Boyet and Carlson (2005) has been recently challenged (Burkhardt et al, 2016), at least a moderately-enriched early reservoir is indeed required by the preservation of ancient isotopic mantle heterogeneity through the Archean (Rizo et al, 2016;Touboul et al, 2012) and up to the present-day (Mundl et al, 2017). Since the overturned MO cumulate layers have never communicated with the surface except in the presence of a dense early steam atmosphere, a "secondary" BMO and the related present-day LLSVPs may further host ancient volatiles such as primordial noble gases (Caracausi et al, 2016;Mukhopadhyay, 2012), particularly as multiple BMO episodes (see above) are considered (Tucker and Mukhopadhyay, 2014). Integration of geophysical and geochemical data in a self-consistent geodynamic framework will be needed to map out primordial geochemical reservoirs in the present-day mantle.…”

Subterranean production of neutrons, 39Ar and 21Ne: Rates and uncertainties

“…Accurate estimates of production rates of noble gas isotopes, such as 39 Ar, 40 Ar and 21 Ne, in rocks are necessary for dating groundwaters and in proper interpretation of isotopic signatures of gases originating in Earth's interior (e.g., Graham, 2002;Tucker and Mukhopadhyay, 2014).…”