Formation process of a compact Type A Ca-Al-rich inclusion from Northwest Africa 7865 reduced CV3 chondrite : the condensation process after the igneous process

概要

Formation process of a compact Type A Ca-Al-rich inclusion from Northwest Africa
　7865 reduced CV3 chondrite : the condensation process after the igneous process
　　　　　　　　　　　　　　　　　　　　　　　　　　　　　Akimasa Suzumura
Ca-Al-rich inclusions (CAIs) in chondrites are the oldest objects in the Solar System and are
commonly used as benchmarks to constrain the evolution of solid objects in the early Solar System.
CAIs have experienced thermal events such as the condensation process and the igneous process.
According to Al-Mg isotope measurements for condensate CAIs and igneous CAIs, it is suggested that
the condensation process and the igneous process occurred simultaneously if the

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Al was

homogeneously distributed. On the other hand, it is not well understood the relationship between the
condensation process and the igneous process in the CAI-forming region regarding the oxygen isotopic
environment, temperature, formation order, and time interval. One CAI which recorded the condensation
and the igneous processes would constrain the relationship between the condensation process and the
igneous process from a perspective of material science regardless of the possibility of heterogeneous
distributions of 26Al. However, it has not yet been studied for one CAI which recorded the condensation
process that occurred after the igneous process. In this thesis, I conducted the integrated study of the
petrographic observation, oxygen isotope measurement, and Al-Mg systematics for the KU-N-02
compact Type A (CTA) CAI from Northwest Africa 7865 reduced CV3 chondrite. This CAI is composed
of a core part and a mantle part, and has experienced the condensation process and the igneous process.
In Chapter 2, the oxygen isotope measurement combined with the petrographic-mineralogical
observations was conducted for the core part of this CAI. The core part has an igneous texture and
mainly consists of 16O-rich spinel (Δ17O ~ −23‰), 16O-poor melilite (Δ17O ~ −2‰), and fassaite. Spinel
crystals are poikilitically enclosed by melilite and fassaite, and their occurrences can be explained by
the crystallization from their melt. The distinct oxygen isotopic compositions of spinel and melilite were
caused by the partial melting process with the oxygen isotope exchange between 16O-rich melt and 16O-

poor gas at a temperature between approximately 1820 K and 1720 K. According to the line profile of
Ti contents and oxygen isotopic compositions, the oxygen isotopic compositions of blocky and
intergranular fassaite changed from

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O-poor (Δ17O ~ −6‰) to 16O-rich (Δ17O ~ −23‰) with crystal

growth. These oxygen isotopic variations and petrography indicate that the oxygen isotopic
compositions of the core part change from 16O-rich (Δ17O ~ −23‰) to 16O-poor (Δ17O ~ −2‰), and then
toward 16O-rich again (Δ17O ~ −23‰) during the formation.
In Chapter 3, the integrated study of petrographic observations and oxygen isotope analysis
was carried out for the mantle part of this CAI. The mantle part surrounds the whole core part, and
consists of spinel, melilite, and minor perovskite. The melilite shows the concentric reverse zoning from
core to rim and exhibits variable oxygen isotopic compositions (Δ17O ~ −2‰ to −9‰). It is difficult to
find any correlations between their oxygen isotopic compositions and not only their chemical
composition but also the distances from the margin of KU-N-02. On the other hand, there are grain-tograin variations of oxygen isotopic compositions of them. These petrographic textures and oxygen
isotopic features indicate that the mantle melilite was formed by condensation from the solar nebular
gas with various oxygen isotopic compositions at a temperature approximately of 1450 K to 1400 K,
indicating that the formation of the mantle part occurred after those of the core part.
In Chapter 4, high-precision Al-Mg isotope analysis was carried out for the core and mantle of
this CAI. The minerals in the mantle part show a significant depleted δ25Mg variation compared to the
constant δ25Mg values of the minerals in the core part. This signature is consistent with that the core part
is the igneous origin and the mantle part is the condensation origin. Al-Mg isotopic compositions of
minerals in the core part and in the mantle part yield well-defined mineral isochrons with inferred initial
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Al/27Al, (26Al/27Al)0 of (4.68 ± 0.15) × 10−5 and initial δ26Mg*, (δ26Mg*)0 of 0.041 ± 0.036‰, and

(26Al/27Al)0 of (4.74 ± 0.14) × 10−5 and (δ26Mg*)0 of 0.041 ± 0.066‰, respectively. These inferred
(26Al/27Al)0 values are indistinguishable from each other, suggesting that the condensation process would

have occurred after the igneous process within about 0.03 Myr even if it is long.
The summary of this thesis is that (1) the core part was formed by the igneous process with
multiple heating events and the oxygen isotopic environment surrounding the core part would evolve
from

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O-rich to

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O-poor and then toward

16

O-rich during their formation, (2) the mantle part was

formed by condensation from the solar nebular gas with various oxygen isotopic compositions after the
formation of the core part, and (3) the condensation process occurred after the igneous process in a short
period within about 0.03 Myr if the 26Al was homogeneously distributed in the CAI-forming region. The
main points of this thesis are that it is revealed that the condensation process occurred after the igneous
process and that the genetic relationship between the core part and the mantle part regarding the oxygen
isotopic environment, temperature, formation sequence, and time interval. These results suggest that the
condensate CAI-forming region and the igneous CAI-forming region are identical to each other. It would
be the first step to connecting the igneous and condensation processes in order to understand the thermal
events in the CAI-forming region. ...