11:45 〜 12:15
[SGC32-04] The story of Earth’s volatile accretion and evolution
★Invited Papers
キーワード：volatiles, accretion, atmosphere, giant impacts, early Earth, subduction
The geochemistry of mantle-derived lavas requires long-term differences to be preserved between different regions of the Earth’s mantle, and between the mantle and the atmosphere. Reading this history has required developing higher precision measurement techniques, which has provided a wealth of information regarding volatile accretion. Often this information has come from mantle noble gases that have recorded events during Earth’s earliest history. Being chemically inert, the evolution of noble gas isotopic and elemental ratios trace physical processes during accretion, such as magma ocean degassing, early mantle degassing and atmospheric loss. Furthermore, there are large differences in the primordial isotopic ratios of noble gases between different volatile sources, allowing one to infer the sources that contributed volatiles to Earth during accretion.
The primordial neon isotopic ratio (20Ne/22Ne) of mantle plumes, which sample material from near the core-mantle boundary and separated from the mid-ocean ridge basalt source within the first 90 Ma, is dramatically different from the atmospheric and chondritic values, but close to values of the nebular gas [1]. This observation requires early acquisition of solar neon and likely reveals the growth of the proto-Earth in the presence of a gas disk [1]. These same deep mantle materials, however, also carry the fingerprint of chondritic krypton [2,3]. This surprising observation requires concomitant acquisition of solar as well as chondritic volatiles during the early phase of Earth’s formation; chondritic volatiles, including water, were therefore added not just late in Earth’s formation.
A careful characterization of the solid Earth’s noble gas fingerprint reveals that the atmospheric noble gases cannot be related to the solid Earth noble gases through any combination of mantle outgassing and atmospheric escape. The distinct signature of the atmospheric noble gases requires their derivation from sources different from those that contributed to solid Earth after the last major interior-atmosphere equilibration – the Moon forming giant impact. While the nature of these volatile sources to the atmosphere is not entirely clear, they include comets [4.5]. Additionally, the distinct noble gas signatures of the atmosphere and mantle place constraints on the rate of volatile exchange between the interior and the surface over 4.5 Ga. In particular, the Xe isotopic composition of modern day lavas and the observation of increasing Xe isotopic mass fractionation in the Archean atmosphere indicates efficient Xe subduction into the mantle occurred after 3.0-2.5 Ga [6,7].
[1] Williams and Mukhopadhyay, Nature 565, 2018 [2] Broadley M. et al. PNAS 117, 2020. [3] Peron et al. in review. [4] Marty B. et al Science 356, 2017. [5] Bekaert D. et al., Scientific Reports 10, 2020. [6] Parai and Mukhopadhyay, Nature 560, 2018. [7] Peron and Moreira GPL 9, 2018.
The primordial neon isotopic ratio (20Ne/22Ne) of mantle plumes, which sample material from near the core-mantle boundary and separated from the mid-ocean ridge basalt source within the first 90 Ma, is dramatically different from the atmospheric and chondritic values, but close to values of the nebular gas [1]. This observation requires early acquisition of solar neon and likely reveals the growth of the proto-Earth in the presence of a gas disk [1]. These same deep mantle materials, however, also carry the fingerprint of chondritic krypton [2,3]. This surprising observation requires concomitant acquisition of solar as well as chondritic volatiles during the early phase of Earth’s formation; chondritic volatiles, including water, were therefore added not just late in Earth’s formation.
A careful characterization of the solid Earth’s noble gas fingerprint reveals that the atmospheric noble gases cannot be related to the solid Earth noble gases through any combination of mantle outgassing and atmospheric escape. The distinct signature of the atmospheric noble gases requires their derivation from sources different from those that contributed to solid Earth after the last major interior-atmosphere equilibration – the Moon forming giant impact. While the nature of these volatile sources to the atmosphere is not entirely clear, they include comets [4.5]. Additionally, the distinct noble gas signatures of the atmosphere and mantle place constraints on the rate of volatile exchange between the interior and the surface over 4.5 Ga. In particular, the Xe isotopic composition of modern day lavas and the observation of increasing Xe isotopic mass fractionation in the Archean atmosphere indicates efficient Xe subduction into the mantle occurred after 3.0-2.5 Ga [6,7].
[1] Williams and Mukhopadhyay, Nature 565, 2018 [2] Broadley M. et al. PNAS 117, 2020. [3] Peron et al. in review. [4] Marty B. et al Science 356, 2017. [5] Bekaert D. et al., Scientific Reports 10, 2020. [6] Parai and Mukhopadhyay, Nature 560, 2018. [7] Peron and Moreira GPL 9, 2018.