DEVELOPMENT OF SAMPLE PREPARATION TECHNIQUE
FOR ISOTOPE ANALYSIS OF FLUID INCLUSIONS BY SIMS
1 Isono Y., 2 Sakamoto N., 1 Ishibashi A., 1 Song J. and 1,3 Yurimoto H.
1Department of Natural History Sciences, Hokkaido University, 2Isotope Imaging Laboratory, Creative Research Institution, Hokkaido University,
3Institute of Space and Astronautical Science/Japan Aerospace Exploration Agency (isono@ep.sci.hokudai.ac.jp)

Fluid inclusions in halite crystals from meteorites provide direct information of extraterrestrial liquid water (Zolensky et al., 1999). In-situ isotope analysis of the fluid inclusions was realized by SIMS equipped with acold-sample-stage (Yurimoto et al., 2014). However, the analytical precision was limited to 14 ‰ (1s) for d18O because of severe electrostatic charging on the excavated area by ion beam to expose the fluid inclusion
embedded in the halite crystal before SIMS analysis. In order to improve the analytical precision, we developed a cryogenic sample preparation technique to expose fluid inclusions on the sample surface including acryogenic polishing, a cryogenic coating and cryogenic sample transfer systems. Here we demonstrate SIMS analysis of H2O ice prepared by our cryogenic sample preparation technique developed in this study.

Cryogenic sample preparation system

Procedure

1

1. Start up
The atmosphere of glove box is replaced by N2 gas generated from LN2. The dew point in the glove box decrease atabout -2.0 C/min at N2 gas flow to the LN2 supplier of 200 ccm. After the N2 gas replacement, the LN2 from the LN2
supplier was provided to the cryo-polisher to cooled down the polishing table. The cooling rate of the polishing table wasabout -2.0 C/min at N2 gas flow to the LN2 supplier of 200 ccm. Since the LN2 capacity of the LN2 supplier was 7 kg
and the consumption of LN2 was about 2 kg/hr, LN2 continues to be supplied for 3.5 hours.

248752. Cryo-polishing

63The frozen sample is polished to expose the fluid inclusions bypolishing sheets of 40, 12, 9 um on the polishing table cooled down to-30 C with checking the surface condition by the integrated optical
microscope. The sample is finished by 3 um polish sheet.
Sample3583. Au coating & mount1

The polished sample is cooled by LN2 in thecryo-work station to prevent sublimation ofSample
H2O in the coating chamber vacuumed at
LN2
several pascals during the gold coating. Afterthe cooling, the sample on the coating stage istransported to the coater. Since the interior of
the cryo-coating stage is filled with pure N2 gas evaporated from LN2, the formation of frost on
the sample surface during transport is prevented. The gold-coated sample is mounted on thesample holder for cryo-SIMS in order to introduce the sample to the cryo-sample-stage of SIMS.
The sample set on the holder is stored in the transfer vessel.

LN2

N2 gas72LN2

1 Glove box
The glove box is purged by the N2 gas evaporated from liquid nitrogen (LN2) to prevent frost
formation during sample preparation processes. The internal pressure of the glove box is kept about
0.5 kPa higher than the outside during operation to prevent invasion of water vapor.

2 LN2 supplier
LN2 is suppled to the inside of the glove box to keep low dew point and to cool cryo-polisher. TheLN2 is pushed out by the N2 gas controlled by the mass flow controller in the range from 0 to 500
ccm.

4. Transportation
The holder in the transfer vessel is transported to SIMS. Thesample is picked up from the vessel by the cryo-loadlock
chamber connected with the transfer vessel. After picking up theholder, a valve attached to the loadlock chamber is closed to shut
off the outside air. The inside of the cryo-loadlock chamber is fullof the N2 gas evaporated from LN2 in the vessel, frost formationon the sample is prevented. The loadlock chamber is quickly
connected with SIMS. The attached valve the loadlock chamberis opened simultaneously with the start of vacuum by a roots
pump. The holder is introduced to the cold-sample-stage ofSIMS cooled at -190 C. It takes 1 − 2 minutes for theinstallation of the holder to SIMS.

3 Cryo-polisher
This cryo-polisher can polish the sample under low temperature. The polisher is cooled throughthe trench under the polishing table with LN2 provided from the supplier. The temperature of
the polishing table is controlled by LN2 flow.

4 Optical microscopeThis optical microscope (VK-X200, KEYENCE) is used to identify target fluidinclusions embedded in the host crystal and observe the sample surface. The microscopeis incorporated into the cryo-polisher to reduce vibrations during observation.

5 Cryo-work station
Cryogenic samples cooled below the dew point in the glove box is treated in this work station in order toprevent frost formation. This cryo-work station contains LN2 and is filled with pure N2 gas evaporated fromthe LN2.

6 Au ion coaterGold coating is performed by an sputter ion coater (SC-701AT, SANYU DENSHI) to makeconductive sample surface for SIMS analysis. The vacuum of the coating chamber is about 7 Pa for
40 seconds from the start and 10 nm thickness of gold is deposited for 1 min.

5. SIMS analysis
Left figure shows the optical image of the ice on SIMS. The yellow green area spreadthroughout the image is the surface of the ice after gold coating. Crystal boundaries of theice are visible and the target position is clearly identified. The SIMS analysis of H2O ice isperformed by ims-1270 (CAMECA) using a Cs+ beam as primary beam and a normal
incident electron gun to compensate positive charging of the sputtered region due to primarybeam irradiation.

7 Cryo-holder
This sample holder for SIMS is made by machining from one metal block in order to prevent
deformation at cryogenic temperature due to differences in thermal shrinkage between different
materials.

8 Transfer vesselThis transfer vessel is designed to transport the sample to the SIMS with keeping sample at LN2 temperature.
The vessel can hold a SIMS holder in the center of the vessel and pick up the holder by the cryo-loadlock
using a connector compatible to the airlock system of SIMS instrument.

Ice on SIMS camera

Oxygen isotope analysis method for H2O ice
16O

17O

16OH

16OH

18O
17O

17OH
MRP=9000

MRP=9000

16O
17OH

18O

16OD
17O

17OH
16OD

16O
16OH

16O

17O

Mass spectra of oxygen isotopes from H2O ice were obtained by cryoSIMS analysis. Upper figures shows mass spectra of 16O–, 17O–,
16OH–, 18O–, 17OH– and 16OD– with mass resolving power (MRP) =5000. The intensity of 16O– was 1.0×107 cps, a tail was observed
on both side of the peak. The intensity of 16OH– was similar to that of16O– and three orders of magnitude higher than 17O–.
The interference of 16OH– tail to 17O– was estimated to be morethan 10‰ by overlapping 16O– peak (red) on 16OH–. Yurimoto et al.
(2014) corrected the contribution of the interference for 17O– by
16O–-tail/16O– ratio.

Upper figures show mass spectra of oxygen isotopes from H2O ice using higher MRP and primary ion
intensity. The contribution of interference from 16OH– to 17O– was similar to that of lower MRP
condition (upper left). On the other hand, the tail effect of 16OD– to 17OH– was much smaller than the caseof 16OH– because the intensities of 17OH– and 16OD– were similar (upper right). The contribution of from
16OD– to 17OH– was estimated to be less than 1‰. Isotope analysis using 16OH– and 17,18OH– signals would beeffective for further high precision analysis of H2O ice.

➢ A cryogenic polishing system was developed to expose fluid inclusions on the sample surface for the cryo-SIMS analysis.
➢ A cryogenic coating systems was developed to make sample surface conducive under cryogenic condition.
➢ A sample transfer system was developed to transport and introduce the cryogenic sample to SIMS.
➢ Oxygen isotope analysis using 16,17,18OH signals instead of 16,17,18O signals would be effective for H2O ice because of the severe tail effect to 17O for further high precision analysis.