14:00 〜 14:15
[SIT16-02] 火星の小さい核とマントル最下部の溶融層の化学組成
キーワード：組成、火星核、底部マントル溶融層、Fe-S-O系、W-Hfアイソトープ、短命同位体
Recent analyses of the Insight seismological data (Samuel et al., 2023; Khan et al., 2023) indicated a small core of Mars with the radius of 1650-1675 km with a surrounding basal molten mantle layer of the thickness around 150 km. The Vp and density of the Martian core are larger than those estimated previously. Both authors estimated the Martian core has high sulfur content exceeding 20 wt% with a low content of oxygen and some additional light elements, which is inconsistent with the cosmochemical constraint with about 8 wt% sulfur of the Martian core (Yoshizaki and McDonough, 2020).
Here, we present a compositional model for the Martian core consistent with cosmochemical constraint for Mars’ bulk composition. Our model core contains 8 % sulfur and 10 wt% oxygen. The W-Hf isotopic signature of Martian meteorites (Dauphas and Pourmand, 2011) indicated that Martian core was formed in the early stage at the time within 2 My after CAI formation. An early core separation with significant amounts of short-lived isotopes (e.g., 26Al and 60Fe) that produce significant amounts of radiogenic energy points to high temperatures of core formation under super-liquidus temperatures for the Martian mantle. Partitioning of O between the silicate mantle and metallic iron with a limited amount of S at super-liquidus temperature suggests a high O content in the Martian metallic core. According to the melting relation of Fe-S-O system (e.g., Tsuno and Ohtani, 2009), exsolution of ionic FeO liquid is inevitable during cooling of the core. A molten layer on the mantle side of the core-mantle boundary might be composed of the liquid FeO separated from the molten O-rich core during cooling or a mixture of the molten mantle and liquid FeO, which were reacted during cooling.
References:
Dauphas, N and Pourmand, A., Nature, 473, 489-492 (2011)
Khan et al., Nature, 622, 718 (2023)
Samuel et al., Nature 622, 712 (2023)
Tsuno and Ohtani, Phys Chem Minerals, 36:9–17 (2009)
Yoshizaki and McDonough, Geochim Cosmochim. Acta 273, 137-162 (2020)
Here, we present a compositional model for the Martian core consistent with cosmochemical constraint for Mars’ bulk composition. Our model core contains 8 % sulfur and 10 wt% oxygen. The W-Hf isotopic signature of Martian meteorites (Dauphas and Pourmand, 2011) indicated that Martian core was formed in the early stage at the time within 2 My after CAI formation. An early core separation with significant amounts of short-lived isotopes (e.g., 26Al and 60Fe) that produce significant amounts of radiogenic energy points to high temperatures of core formation under super-liquidus temperatures for the Martian mantle. Partitioning of O between the silicate mantle and metallic iron with a limited amount of S at super-liquidus temperature suggests a high O content in the Martian metallic core. According to the melting relation of Fe-S-O system (e.g., Tsuno and Ohtani, 2009), exsolution of ionic FeO liquid is inevitable during cooling of the core. A molten layer on the mantle side of the core-mantle boundary might be composed of the liquid FeO separated from the molten O-rich core during cooling or a mixture of the molten mantle and liquid FeO, which were reacted during cooling.
References:
Dauphas, N and Pourmand, A., Nature, 473, 489-492 (2011)
Khan et al., Nature, 622, 718 (2023)
Samuel et al., Nature 622, 712 (2023)
Tsuno and Ohtani, Phys Chem Minerals, 36:9–17 (2009)
Yoshizaki and McDonough, Geochim Cosmochim. Acta 273, 137-162 (2020)