11:00 〜 13:00
[PPS08-P07] Secondary Processes in the Crustal Material caused by Chondrite Clasts
キーワード：隕石、HED、炭素質コンドライト
Secondary Processes in the Crustal Material caused by Chondrite Clasts
Small carbonaceous chondritic materials, meteorites, and micrometeorites are considered to be one of the major contributors to the formation of the early atmospheres and oceans on rocky planetary bodies, including the Earth, and could have contributed to subsequent events, including the emergence of life. By avoiding the destructive impact events, large flux of carbonaceous chondritic materials could effectively bring the volatile-rich materials to the volatile-poor surface of the rocky planets after extensive silicate melting events that can lead to the production of a magma ocean. Volatiles in carbonaceous chondrites are locked mostly in organic compounds and phyllosilicates. However, after accretion of those materials, subsequent chemical reactions could have occurred such as degassing, production of volatile species, crustal materials interactions, and pre-biotic organic chemical reactions, etc. Such events cannot be studied by analyzing terrestrial rocks. However, one can utilize carbonaceous chondrite-bearing HED breccia to investigate crustal and volatile material interactions unlocked from organics found in hydrous silicates. The sample evidence from HED meteorites confirms that a large flux of carbonaceous chondrite materials impacted small differentiated rocky bodies at sufficiently low speed (<1000 m/s or even lower than 300 m/s) to avoid major heating or total disintegration. The chondritic material appears to have been gently incorporated into regolith before the lithification of the rocks. Carbonaceous chondrite (CM and CR) clasts and micro-clasts are located in HED polymict breccia. Among the clasts, the most common type of exotic clast in HEDs are comprised of CM chondritic material that contains on average 10 wt.% water, complex organicsand also could have delivered volatile gases (i.e., H2O, H2, CO, CO2 and H2S) if they are heated to high temperature. Indeed, the presence of H2O fluid mineral interactions hsve been proposed, and the volatile source of these fluids would likely be CM chondrite clasts. However, significantly heated CM chondrite clasts are absent from howardites so that the interpretation that secondary phases represent textual evidence of water-bearing fluids is still controversial. In order to better understand the contribution of such volatile rock interactions, we have investigated NWA 6659, a howardite carbonaceous chondrite clast.
The carbonaceous chondrite clast forms sharp boundaries with the eucrite host. The original carbonaceous chondrite texture is intact and obvious secondary thermal metamorphism features such as reaction rims (PV3 clast in Plainview (1917) H5) are absent from the clast. The texture is consistent with CM2 carbonaceous chondrite. The clast contains an abundant matrix, tochilinite-cronstedtite clumps, sulfide, chondrules, and CAI. Metal grains are also present in a chondrule. We did not observe dehydration TCI texture.
The surrounding texture is mostly equilibrated eucrite, and chemical zoning was not apparent. Recrystallized shock melt-like texture: opaque minerals, plagioclase, and pyroxene intergrowth, is present at the boundary of the clast and eucrite. Pyroxene vein-lets are cutting through eucrite plagioclase yet not through the carbonaceous chondrite clast.
The Raman spectra were obtained as spots analyses. We analysed about 20 randomly selected spots on the matrix in the clast and in artificially heated Murchison samples (600 ℃ and 900 ℃) to estimate the peak temperature. For organics in the clast, the peak position and full-width half-maximums (FWHM) of D1- and GL-bands were determined. We did not see any significant variations in the FWHM and the peak position from spot to spot within the clast, and secondary heating signatures are absent. We compared peak position and FWHM against the Murchison standard. The data fell in between the Murchison standards indicating that the clast experienced temperatures in the range from 600 to 900 ℃. Such temperatures are significantly higher than those previously reported for CI and CM clasts in Howardite (from 50 to 70 ℃, Visser et al., 2017). This discrepancy suggests secondary metamorphism occurred on the HED parent body. If so, the surrounding minerals may have experienced chemical exchange with the gas produced by the carbonaceous chondrite clast.
Small carbonaceous chondritic materials, meteorites, and micrometeorites are considered to be one of the major contributors to the formation of the early atmospheres and oceans on rocky planetary bodies, including the Earth, and could have contributed to subsequent events, including the emergence of life. By avoiding the destructive impact events, large flux of carbonaceous chondritic materials could effectively bring the volatile-rich materials to the volatile-poor surface of the rocky planets after extensive silicate melting events that can lead to the production of a magma ocean. Volatiles in carbonaceous chondrites are locked mostly in organic compounds and phyllosilicates. However, after accretion of those materials, subsequent chemical reactions could have occurred such as degassing, production of volatile species, crustal materials interactions, and pre-biotic organic chemical reactions, etc. Such events cannot be studied by analyzing terrestrial rocks. However, one can utilize carbonaceous chondrite-bearing HED breccia to investigate crustal and volatile material interactions unlocked from organics found in hydrous silicates. The sample evidence from HED meteorites confirms that a large flux of carbonaceous chondrite materials impacted small differentiated rocky bodies at sufficiently low speed (<1000 m/s or even lower than 300 m/s) to avoid major heating or total disintegration. The chondritic material appears to have been gently incorporated into regolith before the lithification of the rocks. Carbonaceous chondrite (CM and CR) clasts and micro-clasts are located in HED polymict breccia. Among the clasts, the most common type of exotic clast in HEDs are comprised of CM chondritic material that contains on average 10 wt.% water, complex organicsand also could have delivered volatile gases (i.e., H2O, H2, CO, CO2 and H2S) if they are heated to high temperature. Indeed, the presence of H2O fluid mineral interactions hsve been proposed, and the volatile source of these fluids would likely be CM chondrite clasts. However, significantly heated CM chondrite clasts are absent from howardites so that the interpretation that secondary phases represent textual evidence of water-bearing fluids is still controversial. In order to better understand the contribution of such volatile rock interactions, we have investigated NWA 6659, a howardite carbonaceous chondrite clast.
The carbonaceous chondrite clast forms sharp boundaries with the eucrite host. The original carbonaceous chondrite texture is intact and obvious secondary thermal metamorphism features such as reaction rims (PV3 clast in Plainview (1917) H5) are absent from the clast. The texture is consistent with CM2 carbonaceous chondrite. The clast contains an abundant matrix, tochilinite-cronstedtite clumps, sulfide, chondrules, and CAI. Metal grains are also present in a chondrule. We did not observe dehydration TCI texture.
The surrounding texture is mostly equilibrated eucrite, and chemical zoning was not apparent. Recrystallized shock melt-like texture: opaque minerals, plagioclase, and pyroxene intergrowth, is present at the boundary of the clast and eucrite. Pyroxene vein-lets are cutting through eucrite plagioclase yet not through the carbonaceous chondrite clast.
The Raman spectra were obtained as spots analyses. We analysed about 20 randomly selected spots on the matrix in the clast and in artificially heated Murchison samples (600 ℃ and 900 ℃) to estimate the peak temperature. For organics in the clast, the peak position and full-width half-maximums (FWHM) of D1- and GL-bands were determined. We did not see any significant variations in the FWHM and the peak position from spot to spot within the clast, and secondary heating signatures are absent. We compared peak position and FWHM against the Murchison standard. The data fell in between the Murchison standards indicating that the clast experienced temperatures in the range from 600 to 900 ℃. Such temperatures are significantly higher than those previously reported for CI and CM clasts in Howardite (from 50 to 70 ℃, Visser et al., 2017). This discrepancy suggests secondary metamorphism occurred on the HED parent body. If so, the surrounding minerals may have experienced chemical exchange with the gas produced by the carbonaceous chondrite clast.