17:15 〜 19:15
[PPS03-P05] The Li- and O-isotope composition of Sahara 97103 EH3 chondrules: Constraining
heterogeneity in the protosolar nebula.
Enstatite chondrites are considered important building blocks for terrestrial planets including Earth [1]. Previous studies demonstrated that chondrules in unequilibrated enstatite chondrites (UEC) likely formed from similar precursor components to other chondrite groups, based on oxygen-isotope and major-element data [2-7]. The Mg/Si ratio of enstatite chondrites is smaller than that of ordinary and carbonaceous chondrites [8]. If enstatite chondrite chondrules were originally olivine-rich, their Mg/Si ratio must have been reduced to produce enstatite. This may occur by the interaction of a Si-rich gas reservoir with melted or partially melted olivine [4,9]. To confirm the composition of the proto-Solar nebula (PSN), the Li- and O-isotope compositions of chondrules within the UEC Sahara 91703 (EH3) were determined.
The O-isotope compositions of the enstatite was close to the intersection of the terrestrial fractionation (TF) and primitive chondrule minerals (PCM) lines. Whereas, the O-isotope composition of the olivine varied along the PCM line, indicating the olivine is relict. Cristobalite was observed within the mesostasis and enclosed by enstatite, with no accompanying metal. This indicates it is the product of Si saturation, rather than reduction of an FeO-rich silicate. Furthermore, chondrules with relict olivine lacked MgS, indicating that enstatite was not produced by sulfidation. This is consistent with the Si-enrichment experienced by EH3 chondrites. The cristobalite was consistently enriched in 18O relative to the enstatite, with the O-isotope composition of the cristobalite being scattered along the TF line. This indicates that some cristobalite formed during chondrule formation, and some formed within the parent asteroid. The O-isotope composition of the Si-rich reservoir was estimated from the values obtained for enstatite and relict olivine. Assuming the added SiO2 and Si-rich reservoirs were in isotopic equilibrium, the O-isotope composition of the Si-reservoir was estimated to be (delta18O, delta17O, Delta17O)=(5.4, 3.7, 1.0 permil). This differs from the O-isotope composition of the Si-rich reservoir for CR and CV chondrules, estimated to be (delta18O, delta17O, Delta17O)=(3.6, 1.8, -1.0 permil) [9].
The Li-isotope composition and concentration of the enstatite was larger than that of the olivine. The Li-isotope composition of the Si-rich reservoir was estimated from the enstatite and relict olivine and was found to be 2 permil. This is consistent with whole rock values for EH3 chondrites. However, it differs from the Li-composition of the Si-rich reservoir determined for the carbonaceous chondrule forming region (-11 permil) [3]. Therefore, the Li-isotope composition of the Si-rich gas reservoirs were likely heterogenous throughout the Solar nebula, indicating heterogeneity between the nebula gas composition of chondrule forming regions throughout the Solar system. The Li-isotope heterogeneity is likely due to the production of light lithium by cosmic-rays in the interstellar medium and the incorporation of this signature into the Solar nebula. Therefore, it is likely that the Li-isotope composition of the outer Solar nebula, where carbonaceous chondrules formed, retained the interstellar medium contribution, while the inner Solar nebula, where enstatite chondrules formed, lost this signature due to homogenisation associated with vaporisation, due to proto-Sun formation.
[1] Kallemeyn and Wasson 1986, Geochim Cosmochim Acta, 50, 2153-2164.
[2] Kimura et al. 2003, Meteorit. Planet. Sci., 40, 855–868. [3] Kunihiro et al. 2019, Geochim Cosmochim Acta, 252, 107–125. [4] Tanaka and Nakamura, 2017, Nature Astronomy, 1. [5]
Weisberg et al. 1994, Meteoritics, 29, 362–373. [6] Weisberg et al. 2011, Geochim Cosmochim Acta, 75, 6556–6569. [7] Weisberg et al. 2021, Geochim Cosmochim Acta, 300, 279–295. [8] Dauphas et al. 2015, Earth Planet. Sci. Lett. 427, 236–248. [9] Chaussidon et al. 2008, Geochim Cosmochim Acta, 72, 1924–1938.
The O-isotope compositions of the enstatite was close to the intersection of the terrestrial fractionation (TF) and primitive chondrule minerals (PCM) lines. Whereas, the O-isotope composition of the olivine varied along the PCM line, indicating the olivine is relict. Cristobalite was observed within the mesostasis and enclosed by enstatite, with no accompanying metal. This indicates it is the product of Si saturation, rather than reduction of an FeO-rich silicate. Furthermore, chondrules with relict olivine lacked MgS, indicating that enstatite was not produced by sulfidation. This is consistent with the Si-enrichment experienced by EH3 chondrites. The cristobalite was consistently enriched in 18O relative to the enstatite, with the O-isotope composition of the cristobalite being scattered along the TF line. This indicates that some cristobalite formed during chondrule formation, and some formed within the parent asteroid. The O-isotope composition of the Si-rich reservoir was estimated from the values obtained for enstatite and relict olivine. Assuming the added SiO2 and Si-rich reservoirs were in isotopic equilibrium, the O-isotope composition of the Si-reservoir was estimated to be (delta18O, delta17O, Delta17O)=(5.4, 3.7, 1.0 permil). This differs from the O-isotope composition of the Si-rich reservoir for CR and CV chondrules, estimated to be (delta18O, delta17O, Delta17O)=(3.6, 1.8, -1.0 permil) [9].
The Li-isotope composition and concentration of the enstatite was larger than that of the olivine. The Li-isotope composition of the Si-rich reservoir was estimated from the enstatite and relict olivine and was found to be 2 permil. This is consistent with whole rock values for EH3 chondrites. However, it differs from the Li-composition of the Si-rich reservoir determined for the carbonaceous chondrule forming region (-11 permil) [3]. Therefore, the Li-isotope composition of the Si-rich gas reservoirs were likely heterogenous throughout the Solar nebula, indicating heterogeneity between the nebula gas composition of chondrule forming regions throughout the Solar system. The Li-isotope heterogeneity is likely due to the production of light lithium by cosmic-rays in the interstellar medium and the incorporation of this signature into the Solar nebula. Therefore, it is likely that the Li-isotope composition of the outer Solar nebula, where carbonaceous chondrules formed, retained the interstellar medium contribution, while the inner Solar nebula, where enstatite chondrules formed, lost this signature due to homogenisation associated with vaporisation, due to proto-Sun formation.
[1] Kallemeyn and Wasson 1986, Geochim Cosmochim Acta, 50, 2153-2164.
[2] Kimura et al. 2003, Meteorit. Planet. Sci., 40, 855–868. [3] Kunihiro et al. 2019, Geochim Cosmochim Acta, 252, 107–125. [4] Tanaka and Nakamura, 2017, Nature Astronomy, 1. [5]
Weisberg et al. 1994, Meteoritics, 29, 362–373. [6] Weisberg et al. 2011, Geochim Cosmochim Acta, 75, 6556–6569. [7] Weisberg et al. 2021, Geochim Cosmochim Acta, 300, 279–295. [8] Dauphas et al. 2015, Earth Planet. Sci. Lett. 427, 236–248. [9] Chaussidon et al. 2008, Geochim Cosmochim Acta, 72, 1924–1938.