17:15 〜 18:45
[PPS08-P14] Micrometeorites recovered from Proterozoic Ocean floor
キーワード：Micrometeorites, Cohenite, cosmic spherules, wüstite
Micrometeorites have been collected from sedimentary rocks spanning most of Earth history and thus record the past flux of extraterrestrial dust accreted by our planet (Onoue et al. 2011; Genge et al. 2020; Suttle et al. 2023). The recovery of micrometeorites from ancient sedimentary rocks is a natural consequence of the accumulation of these particles in surficial environments on Earth, which can become preserved during burial and lithification of sediments into rock. The collection and nature of micrometeorites suggest their sources dominated by comets and asteroids. Micrometeorites can be subdivided into melted or unmelted particles. Particles that have extensively melted and formed sub-spherical droplets owing to surface tension are known as cosmic spherules. Silicate-dominated particles are known as S-types, whilst those dominated by iron oxides and metal are known as I-types. The I-type micrometeorites are formed by the oxidation of chondritic metal dust during atmospheric entry.
Here I report the I-type micrometeorites collected from the Proterozoic Ocean floor in north China. This contribution shows major characteristics of these I-type micrometeorites. Most I-type spherules have smooth spherical shapes which implies that their surfaces are not weathered texturally. Their sizes range from 20 micro to 40 micro. The major minerals in these I-type spherules are cohenite, edscottite, and FeO rim. The core–shell structure of the cohenite-edscottite core with FeO rim is observed in the I-type micrometeorites recovered from the Proterozoic Ocean in north China. The FeO rim is not always continuous and some micrometeorites do not have any rims. As far as can be determined by TEM, most carbide grains have such core–rim structure. The widths of the FeO rims are between 0 micro and 30 micro. Such cohenite-edscottite core with FeO rim demonstrates that the cosmic spherules experienced a later oxidation event, and the observed characteristics suggest that there is a fundamental control that limits the extent to which carbide grains can undergo oxidation. The carbide cohenite is most abundant in iron meteorites. The heterogeneous distribution of cohenite in irons meteorites probably reflects the rapid diffusion of carbon and the rapid growth rate of cohenite (Goldstein et al. 2017). The cohenite-edscottite observed in the I-type spherules are suggested to form by a process of initial metal segregation due to pyrolysis of carbon in a carbonaceous chondritic parent body during atmospheric entry that results in the formation of a Fe-C metallic bead. Rapid melting results in the formation of spherical particles while subsequent oxidation progressively converts metal into metal-oxide. This leads to the development of a core-rim internal structure, with a metal bead encapsulated by a FeO shell.
This work is financially supported by the National Natural Science Foundation of China (Grand No. 42072077).
References: Genge MJ et al (2020) Planetary Space Sci 187, 104900; Goldstein JI et al (2017) GCA 200, 367–407; Onoue T et al (2011) Geology 39, 567–570; Suttle MD et al (2023) GCA 355, 75–88.
Here I report the I-type micrometeorites collected from the Proterozoic Ocean floor in north China. This contribution shows major characteristics of these I-type micrometeorites. Most I-type spherules have smooth spherical shapes which implies that their surfaces are not weathered texturally. Their sizes range from 20 micro to 40 micro. The major minerals in these I-type spherules are cohenite, edscottite, and FeO rim. The core–shell structure of the cohenite-edscottite core with FeO rim is observed in the I-type micrometeorites recovered from the Proterozoic Ocean in north China. The FeO rim is not always continuous and some micrometeorites do not have any rims. As far as can be determined by TEM, most carbide grains have such core–rim structure. The widths of the FeO rims are between 0 micro and 30 micro. Such cohenite-edscottite core with FeO rim demonstrates that the cosmic spherules experienced a later oxidation event, and the observed characteristics suggest that there is a fundamental control that limits the extent to which carbide grains can undergo oxidation. The carbide cohenite is most abundant in iron meteorites. The heterogeneous distribution of cohenite in irons meteorites probably reflects the rapid diffusion of carbon and the rapid growth rate of cohenite (Goldstein et al. 2017). The cohenite-edscottite observed in the I-type spherules are suggested to form by a process of initial metal segregation due to pyrolysis of carbon in a carbonaceous chondritic parent body during atmospheric entry that results in the formation of a Fe-C metallic bead. Rapid melting results in the formation of spherical particles while subsequent oxidation progressively converts metal into metal-oxide. This leads to the development of a core-rim internal structure, with a metal bead encapsulated by a FeO shell.
This work is financially supported by the National Natural Science Foundation of China (Grand No. 42072077).
References: Genge MJ et al (2020) Planetary Space Sci 187, 104900; Goldstein JI et al (2017) GCA 200, 367–407; Onoue T et al (2011) Geology 39, 567–570; Suttle MD et al (2023) GCA 355, 75–88.