[S-CG52] 岩石・鉱物・資源

[SCG52-01] Coordinated sample preparation with cryo-FIB-SEM and X-ray CT: Applications to
chemical analysis for fluid inclusions in minerals

*兒玉 優¹、富岡 尚敬²、川本 竜彦³、黒田 潤一郎⁴、村山 雅史⁵、伊藤 元雄² (1.株式会社マリン・ワーク・ジャパン、2.海洋研究開発機構 高知コア研究所、3.静岡大学 理学部 地球科学科、4.東京大学大気海洋研究所、5.高知大学農林海洋科学部)

キーワード：cryo-FIB-SEM-EDS、流体包有物、マイクロフォーカスX線CT

Hydrothermal fluids and seawater are often entrapped in minerals as fluid inclusions in
crustal/mantle materials, marine halites and even in rare meteorites [1, 2, 3, 4]. The
chemical compositions of fluid inclusions and mineral inclusion therein are keys to
decipher their origins and evolution of the geofulids. A laser-ablation inductively-
coupled-plasma mass-spectrometry (LA-ICP-MS), a Raman spectrometry and an
environmental SEM (cryo-SEM with EDS) are methods for chemical analysis of fluid
inclusions, entrapped tiny minerals and gasses. For example, Kawamoto and coworkers
recently determined Na/K ratio of fluid inclusions in olivine crystals from mantle
xenoliths from the Pinatubo volcano by LA-ICP-MS in 20-µm spatial resolution [1].
The attempt successfully provided new insight on the fate of seawater subducting into
the mantle in a plate convergent region. Lowenstein et al. investigated major chemistry
of fluid inclusions from marine halites by a cryo-SEM with EDS system, and found
systematic and oscillating changes in Phanerozoic seawater chemistry (Na⁺ vs. Cl^- and
Mg²⁺ vs. Cl^-) [3].
As unique sample preparation and analytical approach to a fluid inclusion, combined
instrument of focused ion beam system and scanning electron microscope with a cryo
sample stage (cryo-FIB-SEM) is recently utilized (e.g., Yoshida et al., 2018 [2]). Fluid
inclusions can be exposed as ices in the vacuum sample chamber of cryo-FIB-SEM, and
then the frozen fluids can be directly analyzed by EDS attached to SEM. Advantages of
this approach are a direct access to fluid inclusions by precise sample milling with FIB,
and simultaneous analysis for major elements with a relatively high spatial resolution
(~5 µm). The method has been first applied to fluid inclusions in quartz [2].
A cryo-FIB-SEM approach is, however, time-consuming: entire processing and analysis
with cryo-FIB-SEM requires several to few tens of hours for a several tens micrometer-
sized sample. Descriptions of exact locations and spatial distribution of each fluid
inclusion in host minerals will help to minimize a processing time with FIB. We,
therefore, used a laboratory-based micro-focus XCT apparatus (µXCT) at the Center for
Advanced Marine Core Research, Kochi University for acquiring an accurate three-
dimensional characterization of fluid inclusions in minerals with a spatial resolution of
0.5 to 2.5 µm depending on sample size. In the present study, we aim to improve the
throughput of the fluid inclusion analysis using cryo-FIB-SEM and establish a
reasonable analytical flow. We have obtained spatial distributions of fluids in olivine
from the Pinatubo xenoliths and those in halite in Sicilian rock by µXCT. Pretreatments
for the samples have been carried out by micro-sampling system, and then
microfabrication and chemical analysis of frozen fluids in the samples have been carried
out by cryo-FIB-SEM.

In this talk, we will present the effectiveness of coordinated sample preparation utilizing
cryo-FIB-SEM and µXCT, and a comprehensive microanalysis of fluid inclusions in
addition to cryo-FIB-SEM. For instance, residues of fluid inclusions and associated
minute minerals in FIB-processed samples can be further characterized in a higher
spatial resolution by an analytical transmission electron microscope (TEM) and a
secondary ion mass spectrometer (SIMS).
[1] Kawamoto, T., Kimura, J., Chang, Q., Yoshikawa, M., Okuno, M., Kobayashi,
T., in prep.
[2] Yoshida, K., Orozbaev, R., Hirajima, T., Miyake, A., Tsuchiyama, A., Bakirov,
A., Takasu, A., Sakiev, K. (2018). Micro-excavation and direct chemical analysis
of individual fluid inclusion by cryo-FIB-SEM-EDS: Application to the UHP talc-
garnet-chloritoid schist from the Makbal Metamorphic Complex, Kyrgyz Tian-
Shan. Geochemical Journal, 52(1), 59-67.
[3] Lowenstein, T. K., Timofeeff, M. N., Brennan, S. T., Hardie, L. A., & Demicco,
R. V. (2001). Oscillations in Phanerozoic seawater chemistry: Evidence from
fluid inclusions. Science, 294(5544), 1086-1088.
[4] Zolensky, M. E., Bodnar, R. J., Gibson, E. K., Nyquist, L. E., Reese, Y., Shih,
C. Y., & Wiesmann, H. (1999). Asteroidal water within fluid inclusion-bearing
halite in an H5 chondrite, Monahans (1998). Science, 285(5432), 1377-1379.