17:15 〜 18:45
[MZZ45-P01] Using 54Cr to trace the original zone of the impactor collided with asteroid 4 Vesta
キーワード：Vesta, HED meteorite, Cr isotopes, Impact events
Asteroid 4 Vesta is the second largest body in the asteroid belt and has always been thought to be a differentiated, rocky protoplanet. The mineralogy and chemical composition of the Howardite, Eucrite, Diogenite (HED) clan of meteorites, which are thought to be originated from Vesta suggested that such an asteroid has experienced multiple impacts events. Especially in the case of howardite, it is the most thoroughly mixed polymict breccia consisting of fragments of eucrite and diogenite, as well as fragments from impactors. Despite many previous attempts to study howardites using various isotope systems, the understanding of the chronological sequence and characteristics of these impact events are still under debate. In general, crusts of differentiated planetary bodies are strongly depleted in siderophile elements, whereas chondrites are comparatively enriched in these elements [e.g., 1]. Thus, the materials produced by the collision at Vesta are often enriched in siderophile elements inherited from the impactor [2]. Using howardite as a research sample, siderophile elements have been excellent cosmochemical tracers for identifying exogenous materials. In particular, the 54Cr heterogeneity in the Solar System is thought to be inherited from its original regions, by using it, it is possible to distinguish the different types of materials present on impactites. In this study, we conducted a coordinated study of mineralogical and isotopic analyses of howardite samples to explore the clasts of the impactor and aimed to find out the effect caused by multiple impact events presented recorded in individual samples.
We investigated one howardite Northwest Africa (NWA) 11899, which is a polymict breccia consisting of approximately 60% diogenite and 40% eucrite clasts, with accessory Fe-Ni metal, Fe-sulfide, and silica [3]. A specimen of NWA 11899 has been subdivided into three sections and examined by an optical microscope and a field emission electron microprobe analyzer (FE-EPMA). Subsequently, we subdivided each section into four regions (25_mm×25_mm per region) based on surface mineral characteristics, each region from which powdered sample was collected by using a micromilling system (Geomill 326, Izumo, Japan). The collected 12 samples were dissolved in acids following the method described in [4]. The elemental abundances were determined by inductively coupled plasma-mass spectrometry (ICP-MS).
NWA 11899 is nearly homogeneous in major element composition. However, the data we obtained indicate significant variations in the Cr abundances between different regions, with some areas showing abundances even twice as high as others. These differences could be due to the influence of impactor materials or the inherent presence of relatively more chromite in certain regions of the sample. Compared to the previous data for eucrite and diogenite [e.g., 5], other siderophile elements in NWA 11899 exhibit relative enrichments. This is especially true for Ni abundances, which show a large variation ranging from 36.4 to 1081 µg g-1, suggesting varying degree of residual material from the impactor in individual samples.
Interestingly, in the CI-normalized rare earth element (REE) abundance patterns, three exhibit negative Eu anomalies, six show positive anomalies, while the remaining samples display flat CI-normalized REE abundance patterns. The observed mineralogical distribution, mineralogical differences can cause the variation of Eu anomalies in different regions within the same sample. Alternatively, the impact events induced extreme temperature and pressure conditions, resulting in the fragmentation and mixing of rocks. This could also lead to changes in the redox conditions of the surrounding environment, resulting in anomalous Eu occurrences. The observed elemental anomalies suggest that the distinct episodes of impact events may be recorded in different regions of the same sample. However, we cannot currently determine the specific cause, and further research is needed to provide support. In the future study, we will measure Cr and Ni isotopes as in the 12 regions collected. The impact events on Vesta’s evolution will be further investigated.
References:
[1] Dale C W., et al. Science, 2012, 336(6077): 72-75.
[2] Norman M D., et al. Earth Planet. Sci. Lett., 2002, 202(2): 217-228.
[3] Gattacceca J., et al. Meteoritics. Planet Sci, 2020, 55(2): 460-462.
[4] Yokoyama T., et al. Geostandard. Geoanal. Res., 2023, 47(2): 415-435.
[5] Warren P H., et al. Geochim. Cosmochim. Acta, 2009, 73(19): 5918-5943.
We investigated one howardite Northwest Africa (NWA) 11899, which is a polymict breccia consisting of approximately 60% diogenite and 40% eucrite clasts, with accessory Fe-Ni metal, Fe-sulfide, and silica [3]. A specimen of NWA 11899 has been subdivided into three sections and examined by an optical microscope and a field emission electron microprobe analyzer (FE-EPMA). Subsequently, we subdivided each section into four regions (25_mm×25_mm per region) based on surface mineral characteristics, each region from which powdered sample was collected by using a micromilling system (Geomill 326, Izumo, Japan). The collected 12 samples were dissolved in acids following the method described in [4]. The elemental abundances were determined by inductively coupled plasma-mass spectrometry (ICP-MS).
NWA 11899 is nearly homogeneous in major element composition. However, the data we obtained indicate significant variations in the Cr abundances between different regions, with some areas showing abundances even twice as high as others. These differences could be due to the influence of impactor materials or the inherent presence of relatively more chromite in certain regions of the sample. Compared to the previous data for eucrite and diogenite [e.g., 5], other siderophile elements in NWA 11899 exhibit relative enrichments. This is especially true for Ni abundances, which show a large variation ranging from 36.4 to 1081 µg g-1, suggesting varying degree of residual material from the impactor in individual samples.
Interestingly, in the CI-normalized rare earth element (REE) abundance patterns, three exhibit negative Eu anomalies, six show positive anomalies, while the remaining samples display flat CI-normalized REE abundance patterns. The observed mineralogical distribution, mineralogical differences can cause the variation of Eu anomalies in different regions within the same sample. Alternatively, the impact events induced extreme temperature and pressure conditions, resulting in the fragmentation and mixing of rocks. This could also lead to changes in the redox conditions of the surrounding environment, resulting in anomalous Eu occurrences. The observed elemental anomalies suggest that the distinct episodes of impact events may be recorded in different regions of the same sample. However, we cannot currently determine the specific cause, and further research is needed to provide support. In the future study, we will measure Cr and Ni isotopes as in the 12 regions collected. The impact events on Vesta’s evolution will be further investigated.
References:
[1] Dale C W., et al. Science, 2012, 336(6077): 72-75.
[2] Norman M D., et al. Earth Planet. Sci. Lett., 2002, 202(2): 217-228.
[3] Gattacceca J., et al. Meteoritics. Planet Sci, 2020, 55(2): 460-462.
[4] Yokoyama T., et al. Geostandard. Geoanal. Res., 2023, 47(2): 415-435.
[5] Warren P H., et al. Geochim. Cosmochim. Acta, 2009, 73(19): 5918-5943.