5. Geochemistry and Environmental Science

概要

CO5-1

Track observation in muscovite irradiated by 241Am sources and its thermal stability

N. Hasebe, T. Nakashima1, K. Miura1, U. Uyangaa1,
G.Shuukhaaz1, K. Oohashi2, S.Akutsu2, Y. Iinuma3, and
K. Takamiya3
Institute of Nature and Environmental Technology, Kanazawa University
1Graduate School of Natural Science and Technology,
Kanazawa University
2Graduate School of Science and Technology for Innovation, Yamaguchi University
3 Institute for Integrated Radiation and Nuclear Science,
Kyoto University
INTRODUCTION: 238U, 235U, and 232Th decay through
emission of alpha particles to stable lead. Alpha Recoil
Track (ART) is the damage created when daughter nuclides move back in reaction to alpha decay (Fleisher,
2003). Under the known decay constants of uranium and
thorium, the age since the start of ART accumulation can
be calculated by measuring the number of ARTs and uranium and thorium concentrations.
The purpose of this research is to establish a method to
artificially form ARTs on mineral surfaces using muscovite ART detectors to help the understanding of ART behavior in various minerals. An experiment on the annealing behavior of ARTs in muscovite was also performed.

by irradiation.
Based on the annealing experiment, It was concluded that
natural ARTs were annealed at ambient temperature for a long
period of time because the size of ART tends to decrease with
annealing time during the isothermal annealing experiment
(Figure 2). The establishment of the artificial ART formation
method will make it possible to observe the shape and
characteristics of ARTs in various minerals other than mica
and will contribute to further development of geochronological
use of ART, especially including zircon.
REFERENCES:
[1] Fleischer R., Geochim. cosmochim. acta, 67, (2003)
4769-4774.

EXPERIMENTS: A 300 Bq americium source was
tested to form artificial ART at Research Institute for
Complex Nuclear Science, Kyoto University. Irradiation
was performed under a vacuum at various time intervals
(1 hour, 3 hours, 6 hours, 12 hours, 2 days, 4 days and 1
week). The ARTs were enlarged by etching with 47 % HF
for two hours and observed with a phase contrast optical
microscope.
Several samples of muscovite that have been irradiated
for 3 hours were prepared and annealed at different temperatures (100℃, 150℃, 200℃) and times (30 min, 1
hour, 3 hours, 5 hours, 10 hours, 20 hours, 100 hours and
352 hours) to see the stability of ARTs under the geological condition.
RESULTS: The areal density of ARTs increased linearly
against the irradiation time (Figure 1). One could identify
each ART one by one in samples irradiated from 1 hour
to 12 hours, but the number of ARTs formed on the muscovite surface was too many to identify for samples irradiated from 2 days to 1 week. Size of artificially formed
ARTs (single alpha decay) was investigated and they
were larger than natural ARTs (multiple alpha decay). In
general, the artificial ART is formed by a single alpha
decay from 241Am to 237Np. In contrast, muscovite without annealing should record the ARTs by multiple decay
from the parent nuclide 238U, 235U, and 232Th to the stable
206Pb, 207Pb, and 208Pb, respectively, so that one can expect larger natural tracks than the artificial tracks formed

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Fig. 1. Areal density of ARTs is plotted against
irradiation time.

Fig. 2.The Arrhenius plot based on the annealing
temperatures and times (sec).

CO5-2

Mechanisms of high-pressure transitions in (Mg,Fe)2SiO4 under differential stress

N. Tomioka1, T. Okuchi2, M. Miyahara3
Kochi Institute for Core Sample Research, Japan Agency
for Marine-Earth Science and Technology
2
Institute for Integrated Radiation and Nuclear Science,
Kyoto University
3
Graduate School of Advanced Science and Engineering,
Hiroshima University

1

INTRODUCTION:
Phase equilibria studies have demonstrated that olivine [(Mg, Fe)2SiO4: a-phase] transforms into a spinelloid structure (wadsleyite:b-phase) and then into a spinel
structure (ringwoodite: g-phase) with increasing pressure.
Natural examples of wadsleyite and ringwoodite were
first discovered in heavily shocked meteorites. Based on
the characterizations of planer defects in these natural
phases, shear-promoted "diffusionless" mechanisms were
proposed in transformations among the olivine polymorphs [1]. The transformation models also predicted the
possible occurrence of an intermediate phase, which is
exhibiting the smallest unit cell among all spinel/spinelloid structures. We recently discovered the
phase as a mineral poirierite (e-phase) in shocked meteorites [2, 3]. In the present study, we have carried out a
transformation experiment on olivine to understand the
conditions and mechanism of the poirierite formation.
EXPERIMENTS:
Natural olivine with an Fe/(Mg+Fe) ratio of 0.09 was
used in the transformation experiments. The olivine single crystal was crushed into powder with heterogeneous
grain size less than 100 µm. The powder was kept at 16
GPa and 900 °C for 2 hours by using a Kawai-type
high-pressure apparatus. The recovered samples were
measured using an X-ray diffractometer (RIGAKU
SmartLab 9 kW) at the Institute for Integrated Radiation
and Nuclear Science, Kyoto University. The X-ray beam
is focused to a diameter of ~100 µm. The portions at the
reaction boundaries were extracted and to be ultrathin
sections of ~150 nm in thickness by using a focused ion
beam apparatus (Hitachi SMI-4050), and then examined
by a transmission electron microscope (TEM: JEOL
JEM-ARM200F).
RESULTS AND DISCUSSION:
The olivine grains are partially transformed at their
grain boundaries. Micro-area powder X-ray diffraction
patterns show that the product phase is exclusively ringwoodite (Fig. 1). The ringwoodite occurs as euhedral and
subhedral crystals with a grain size of 490 ± 270 µm under TEM (Fig. 2). Most of the ringwoodite grains exhibit
pervasive stacking faults on {110} planes. These defects
have also been reported in ringwoodite in shocked meteorites [e.g. 2,3]. The selected-area electron diffraction
(SAED) patterns of many ringwoodite grains with a high
density of planar defects show weak extra diffraction
spots corresponding to poirierite. In addition, the SAED

patterns of the ringwoodite-poirierite intergrowth show
that both phases have a topotaxial relationship:
(001)ε//{001}γ and (100)ε//{110}γ. The relict olivine has a
high dislocation density of 1.4 x 109 /cm2, corresponding
to a differential stress of ~0.6 GPa according to an olivine
piezometer [4].
Microstructural and crystallographic features of the
sample described above suggest that polycrystalline
ringwoodite grains were formed by a nucleation and
growth mechanism at grain boundaries of olivine, while
poirierite lamellae were metastably formed within ringwoodite grains by a shear mechanism. The latter transformation would be favored by relatively low temperature conditions, where atomic diffusion is kinetically
hindered, and by high differential stress, which causes
shearing of the oxygen sublattices of the spinel/spinelloid
structures. The transformation experiments at different
pressure conditions are currently ongoing.

Fig. 1. Micro-area X-ray diffraction pattern taken from
olivine kept at 16 GPa and 900 °C. Peaks with notations
are from ringwoodite (g), and all the other peaks are
from olivine.

Fig. 2. Transmission electron micrograph (Left) and selected-area electron diffraction patten (Right) of a ringwoodite (g) grain with intergrown poirierite (e).
REFERENCES:
[1] M. Madon and J. P., Poirier, Phys. Earth Planet Inter.,
33 (1983) 31–44.
[2] N. Tomioka and T. Okuchi, Sci. Rep., 7 (2017) 17351.
[3] N. Tomioka et al., Commun. Earth Environ., 2,
(2021) 16.
[4] Kohlstedt et al. (1976). in Physics and Chemistry of
Minerals and Rocks, R. G. J. Strens (ed.), (1976) 35–49.

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CO5-3

Experimental evaluation of impact-induced structure transformation of planetary minerals

T. Okuchi, N. Tomioka1, Y. Seto2, Y. Umeda and T. Sekine3
Institute for Integrated Radiation and Nuclear Science,
Kyoto University
1
Kochi Institute for Core Sample Research,
Japan Agency for Marine-Earth Science and Technology
2
Graduate School of Science,
Osaka Metropolitan University
3
Center for High Pressure Science & Technology
Advanced Research
INTRODUCTION: 4.6 billion years ago in a nebula
surrounding the primordial sun, numerous small bodies
were growing into planets and satellites through their
mutual collisions. After the collisions of small bodies,
some evidences had been recorded within themselves to
indicate their impact histories, where materials were
strongly and temporally compressed during the impact
events. A typical example of such evidence is frozen
dense structures occurring after such compression, which
emerged through structural transformations of the mineral
crystals that originally consisted the asteroids.
There are numbers of previous reports on such dense
structures occurring in primitive meteorites [1]. We have
been reporting some of these structures including newly
discovered one, which had been very possibly recording
hypervelocity impact events of the ancient asteroids [2,3].
In these meteorites, we observed that low-density olivine
crystals [α-(Mg,Fe)2SiO4] were transformed into one or
more of its three dense high-pressure polymorphs. These
are of particular interest because they could have recorded the timescale and the pressure scale of the ancient impact processes in quantitative manner [2,3].
The purpose of the current research is to reveal the
formation mechanisms of these dense mineral structures
as unique evidences of evolution history of the early solar
system, with particular attention to their nanoscale morphological structures and textures.

solid-state structure transformation mechanism of silicate
minerals. The mechanism proceeded even during shortlived shocks equivalent to those induced by impacts of
relatively-small (sub-kilometer-scaled) asteroids. ...