16:15 〜 16:30
[SGC35-10] Noble gases and nitrogen in two Ryugu grains (JAXA Hayabusa2 mission) analyzed at CRPG Nancy France: Implications for Earth’s accretion and differentiation
★Invited Papers
キーワード：Hayabusa2, noble gases, nitrogen
The JAXA-led Hayabusa2 mission returned to Earth in December 2020, carrying 5.4g of material collected during two separate touchdown operations on the C-type asteroid (162173) Ryugu [1]. These samples represent the first substantial return of asteroid material to Earth, and provide unique opportunities to understand the volatile element composition of solar system bodies. Preliminary analysis of Ryugu materials suggest that it is most similar in composition to the volatile-rich CI chondrite meteorites, which may have provided a key source of volatile elements during planetary accretion [2].
At CRPG Nancy, we received 2 solid sample grains, one from each sampling site, which were analyzed for all noble gases and nitrogen [4] using two mass spectrometers. Noble gases, in particular xenon, show that the volatile inventory of Ryugu is dominated by the endmember phase Q. Phase Q is a well-characterized carrier phase of noble gases, likely carbonaceous in nature, which is typically the most significant source of primordial heavy noble gases in chondritic meteorites. There is a small excess in primordial 128Xe which may record a minor contribution from an exotic volatile endmember such as Xe-HL and/or Xe-P6. These components are both derived from presolar diamonds.
Overall, the heavy noble gases in the Ryugu grains indeed display some similarity to CI chondrites. Yet the strong elemental enrichment, particularly in Xe, suggests that Ryugu may represent an even more primitive, volatile-enriched reservoir that is distinct from anything observed in the meteorite record. However, nitrogen abundances are significantly depleted compared to typical CI values, and additionally δ15N is well below the CI range. This suggests the preferential loss of a N-rich phase with a high associated δ15N.
The N/36Ar ratio of planetary reservoirs bears important constraints on the origin of planetary building blocks, on the behavior of volatile elements during planet differentiation as well as on the cycle of life-forming elements during geological periods of time [5]. The terrestrial mantle has a N/36Ar value of 3.4±1.6 x 106 [5], very comparable to the mean of CI-CM values (2.9±1.4 x 106). The Ryugu grains display values of 1.0-1.2 x 106, slightly lower but still consistent with the range of chondritic and mantle values. The observation that the terrestrial mantle appears to have a chondritic-like N/36Ar ratio is somewhat puzzling as the Earth is a differentiated body with an elevated C/N ratio. This suggests that nitrogen and noble gases were added late to the mantle when most nitrogen, but not carbon, had already been lost or partitioned into the core. In contrast, the surface inventory of our planet (atmosphere, oceans, crust) has a N/36Ar ratio of only 0.08 x 106, suggesting preferential loss of nitrogen relative to Ar, or contributions of cosmochemical bodies different from those which supplied volatile elements to the mantle.
Study supported by CNES and by ERC.
[1] Tachibana et al. (2014) Geochem. J. 48, 571–587. [2] Yada et al., (2021) Nat. Aston. [3] Tsuda et al. (2020) Acta Astronautica, 171, 42–54. [4] Boulliung et al., (2020) GCA 285, 120-133. [5] Marty et al., (2020) EPSL 551, 116574.
At CRPG Nancy, we received 2 solid sample grains, one from each sampling site, which were analyzed for all noble gases and nitrogen [4] using two mass spectrometers. Noble gases, in particular xenon, show that the volatile inventory of Ryugu is dominated by the endmember phase Q. Phase Q is a well-characterized carrier phase of noble gases, likely carbonaceous in nature, which is typically the most significant source of primordial heavy noble gases in chondritic meteorites. There is a small excess in primordial 128Xe which may record a minor contribution from an exotic volatile endmember such as Xe-HL and/or Xe-P6. These components are both derived from presolar diamonds.
Overall, the heavy noble gases in the Ryugu grains indeed display some similarity to CI chondrites. Yet the strong elemental enrichment, particularly in Xe, suggests that Ryugu may represent an even more primitive, volatile-enriched reservoir that is distinct from anything observed in the meteorite record. However, nitrogen abundances are significantly depleted compared to typical CI values, and additionally δ15N is well below the CI range. This suggests the preferential loss of a N-rich phase with a high associated δ15N.
The N/36Ar ratio of planetary reservoirs bears important constraints on the origin of planetary building blocks, on the behavior of volatile elements during planet differentiation as well as on the cycle of life-forming elements during geological periods of time [5]. The terrestrial mantle has a N/36Ar value of 3.4±1.6 x 106 [5], very comparable to the mean of CI-CM values (2.9±1.4 x 106). The Ryugu grains display values of 1.0-1.2 x 106, slightly lower but still consistent with the range of chondritic and mantle values. The observation that the terrestrial mantle appears to have a chondritic-like N/36Ar ratio is somewhat puzzling as the Earth is a differentiated body with an elevated C/N ratio. This suggests that nitrogen and noble gases were added late to the mantle when most nitrogen, but not carbon, had already been lost or partitioned into the core. In contrast, the surface inventory of our planet (atmosphere, oceans, crust) has a N/36Ar ratio of only 0.08 x 106, suggesting preferential loss of nitrogen relative to Ar, or contributions of cosmochemical bodies different from those which supplied volatile elements to the mantle.
Study supported by CNES and by ERC.
[1] Tachibana et al. (2014) Geochem. J. 48, 571–587. [2] Yada et al., (2021) Nat. Aston. [3] Tsuda et al. (2020) Acta Astronautica, 171, 42–54. [4] Boulliung et al., (2020) GCA 285, 120-133. [5] Marty et al., (2020) EPSL 551, 116574.