Report for the Joint Use/Research of the Institute for Planetary Materials,
Okayama University
2022 fiscal year first term / second term / others
30/4/2023
Category:

International Joint Research/ General Joint Research/ Joint Use of Facility/ Workshop

Name of the research project: Influence of oxygen fugacity on depth variation ofolivine-wadsleyite phase transformation: Implication for 410-km discontinuity depth anomaly
Principal applicant: Chengcheng Zhao
Affiliated institution and department: Zhejiang University
Collaborator
Name: Professor Takashi Yoshino
Affiliated institution and department: Institute for Planetary Materials
Research report:
1) Please write the research report with free format, but include followings: research purpose, actuallyconducted research, and research outcomes. If necessary, you can add another pages.
2) For the workshop, please write the report for the workshop. Also, attach the program, abstracts, and listof the participants etc.
3) Please add Collaborator's Name, Affiliated institution and department as needed.

To determine the exact depth (pressure) of olivine to wadsleyite transformation, synchrotron
experiment is required. So my research project changed to determining redox kinetics of hydrous
olivine at high pressure. Below is the detailed research report.
Research project: Redox kinetics of hydrous olivine at high pressure and its implication on redox
exchange between subducting slab and the surrounding upper mantle
Research purpose:
1. Determine redox kinetics of hydrous olivine at high pressure from 4 to 12 GPa;
2. Figure out the redox mechanism of hydrous olivine at high pressure;
3. Calculate the possible influence on redox budget of subducting slab with pressure.
Experimental procedures:
1. Preparation of starting materials
Reactant grade MgO, SiO2, Fe2O3 powder of stoichiometric forsterite composition were mixed,with 3 wt% additional SiO2 added as silica buffer. The well-mixed powder was made into pellets of

~ 0.5 mm thickness, which were then baked in a gas mixture furnace at 1300 oC and either
FMQ/RRO or IW buffered conditions for 4 hours. Then the olivine pellets were ground into powder
and 5 wt% Pt powder was added as oxygen sensor to monitor oxygen fugacity in-situ. After fully
mixing in alcohol, the Pt-doped olivine powder was made into 0.5 mm thick pellets again and bakedin the gas mixture furnace at the same temperature and oxygen fugacity conditions as before foranother 4 hours. Finally, Pt-doped FMQ/RRO and IW buffered olivine powders were obtained as
the starting materials.
2. Synthesis of olivine aggregates and sample characterization
FMQ/RRO and IW buffered olivine powders were loaded into Ni and Mo capsules with ~ 3 mmdiameter and ~ 4 mm length respectively, which were inserted into MgO sleeve, graphite/LaCrO3
heater, ZrO2 insulator and 14 mm octahedron in a sequence (Figs. 1a and 1b). WRe3 and WRe25
thermocouple was set in the middle of the sample capsule to monitor temperature. Before
compression, the whole cell assembly was kept in 200 oC vacuum furnace overnight to minimize
absorbed water. 32 mm anvils with 8 mm truncations was later adopted as the second stage in a
multi-anvil apparatus. Sintering experiments were conducted from 4 to 12 GPa. After reached thetarget pressure, temperature was increased to 1200 oC at a rate of ~ 100 oC/min. 1-2 hours was
adopted to get well sintered olivine aggregate. The detailed experimental conditions are
summarized in Table 1.
(a)

(c)

(b)

(d)

Fig. 1 14/8 cell assembly for synthesis and diffusion experiment. (a) (b) Sintering experimentconfiguration at 4-8 and 12 GPa respectively. (c) (d) Diffusion experiment configuration
After recovery, micro-texture was observed by scanning electron microscope (SEM). Averagegrain size of olivine was measured to be 3–9 μm, using intercept method (Mendelson, 1969) withthe relation d = cL where d is grain size, L is length of intercept, and c is a constant of 1.56.
Chemical composition of ~ 20 points of Pt alloys and neighboring olivine grain was measured byelectron microprobe (EPMA) from near Ni/Mo capsule to the center on the cross section of sample.
The almost constant oxygen fugacity distribution for samples at 4 and 8 GPa (Fig. 2 and Table 1)
calculated from method described in Faul et al. (2017) indicates that a new redox state has been

achieved across the sample. However, sample at 12 GPa shows a little bit higher standard deviationfrom constant oxygen fugacity.
Table 1 Synthesis experiment summary using multi-anvil apparatus
RunNo.
5K37695K37705K3780

1K3583

5K3787

1K3589Starting material
Ol+3wt%SiO2+5wt%Pt(FMQ)
Ol+3wt%SiO2+5wt%Pt(IW)
Ol+3wt%SiO2+5wt%Pt(RRO)
Ol+3wt%SiO2+5wt%Pt(IW)
Ol+3wt%SiO2+5wt%Pt(RRO)
Ol+3wt%SiO2+5wt%Pt
(IW)

fO2

fO2

Grain

H2O*

(ΔFMQ)(ΔIW)

size (μm)(ppm)-7.4(0.1) -1.5(0.1)

3.0(0.1)

6.3(3.5)140(20)

Mo foil-9.5(0.1)-3.7(0.1)

0.9(0.1)

3.4(1.8)80(10)

Ni foil-3.5(0.2)-0.2(0.2)

5.4(0.2)

8.3(4.5)/-7.5(0.1)-4.1(0.1)

1.5(0.1)

5.9(3.0)/1-1.1(0.4)-0.4(0.4)

6.3(0.4)

8.4(4.1)/

2-5.8(0.3)-5.0(0.3)

1.7(0.3)

6.1(2.9)/

CapsuleP

T

Duration

(GPa)

(oC)

(h)

Ni foil
4

8

1200

1200

1

1

Mo foilNi foil
12

fO2

1200

Mo foilNumber in parenthesis is the standard deviation error.
* “/” in water content column means water content was not measured.

(a)

(b)

(c)

(d)

(e)

(f)

Fig. 2 Oxygen fugacity distribution of the starting olivine aggregate at different pressures
3. Redox experiment and sample characterization
The recovered olivine aggregate was cut into two pieces of 1-1.5 mm thickness from the middle
perpendicular to the longitudinal direction. One FMQ/RRO buffered piece was stacked on one IW
buffered piece sintered at the same pressure and temperature condition to form a redox couple, withcontact interfaces of the two redox parts carefully polished to ensure good contact. The original Ni
and Mo capsules for both parts were retained. The redox couple was inserted into a MgO sleeve,graphite/LaCrO3 heater, ZrO2 insulator and 14 mm octahedron in a sequence (Figs. 1c and 1d).
WRe3 and WRe25 thermocouple was set in the bottom of the redox capsule to monitor
temperature.32 mm anvil with 8 mm truncation was used.
Redox experiment was performed at the same pressure and temperature with that for sinteringexperiment to minimize possible interference from re-equilibrating of oxygen fugacity. When target
pressure was achieved, temperature was increased to 1200 oC at a rate of ~ 100 oC/min. Different
durations were adopted at different pressures in order to get oxygen fugacity profile. Detailed
experimental conditions are summarized in Table 2.
Table 2 Experimental summary for redox experiment using multi-anvil apparatus
Run
No.

Starting sample

P

T

Duration

Grain size

Increase

H 2O

log D

(GPa)

(oC)

(h)

(μm)

(%)

(ppm)

(m2/s)

O: 9.1(5.0)

4450(10)-11.78R: 5.3(2.6)

56

30(10)-11.86

O: 11.7(6.5)

41

140(10)-11.24#

R: 8.0(3.9)

3670(10)-11.67

O: 9.5(5.2)

14180(10)-11.79#R: 6.3(3.1)

750(10)-11.79

O: /

/

/

/

R: /

/

/

/

5K3778

5K3769+5K3770

4

1200

3

5K3783

5K3780+1K3583

8

1200

4

5K3788

5K3780+1K3583

8

1200

2

5K3791

5K3787+1K3589

12

1200

1

# indicate the value is not robust.

Comment

diffuse out

fO2 dropped

After recovery, redox couple was cut into two halves at center, perpendicular to the initial contact
interface. One half was double polished to ~ 100 μm thickness for infrared (IR) spectroscopy
measurement. A JASCO IRT5200IMPY Fourier-transform IR (FTIR) spectrometer with an aligned
transmission geometry was used with an aperture size of 100 × 100 and accumulation scans of 256.
5–10 different points were measured for each redox part. After corrected baseline and normalized
thickness, water content was obtained by calibrating IR spectra according to Paterson’s method(Paterson, 1982) with an integration range of 3,000–3,730 cm−1 and an orientation factor of 1/2.
Water content before redox experiment is also acquired for samples synthesized at 4 GPa, while thatfor sample at 8 GPa is absent. Water content calibrated by other methods are also summarized inTable 3 for comparison. Typical IR spectra are presented in Fig. 3.
The other half of the redox couple was used for SEM and EPMA measurement. A homogeneous
texture with average grain size of 5–9 μm was observed (Table 2), with grain growth within 60%
for both FMQ/RRO and IW parts. Usually the initial interface was identified as parallel to thejuncture between Ni and Mo capsule (Fig. 3). In one couple whose initial interface is tilted, the backscattered electron under SEM provided clear boundary between the two parts (Fig. 4). Chemicalcomposition of Pt particle and neighboring olivine grain were analyzed in the center part of crosssection from one to the other end across the redox couple. fO2 profile as a function of distance fromthe initial contact interface is plotted (Fig. 5). Redox profile at 4 GPa forms a complete diffusion
profile, while those at 8 GPa either diffuse out or has significant drop of oxygen fugacity at theoxidized part.
Table 3 Water content using various calibration methods
Run No.

Redox
state

Paterson

Bell/3AubaudWithers

5K3769FMQ

140(20)230(40)190(30)140(20)5K3770IW80(10)

140(20)110(20)90(10)

FMQ

50(10)

80(20)

60(10)

50(10)

IW

30(10)

50(20)

40(10)

30(10)

RRO

140(5)

200(10)160(5)

120(5)

IW70(10)

100(10)90(10)

70(10)

RRO

180(10)260(10)210(10)160(10)IW50(5)80(10)

65(5)

50(5)

RRO

-

-

-

-

IW

-

-

-

-

5K3778

5K3783

5K3788

5K3791** “-” means water content is not measured.

(a)

(b)

(c)

Fig. 3 Typical IR spectra of redox couple before and after redox experiment. “b.r.” means watercontent before redox experiment. Water content before redox experiment at 8 GPa is not measured.

(a) 5K3778 4 GPa(b) 5K3778 FMQ part(c) 5K3778 IW part(d) 5K3783 8 GPa(e) 5K3783 RRO part(f) 5K3783 IW part(g) 5K3788 8 GPa(h) 5K3788 RRO part(i) 5K3783 IW partFig. 4 Sample texture after redox experiment at 4 and 8 GPa. Red dashed lines are the initial contact
interfaces of redox couples. Yellowish rectangles are EPMA measurement region, from where
oxygen fugacity profiles are obtained. The grey matrix is olivine, while the dark grey grains are
pyroxene. Holes without grains appears as black spots.
(a)

(b)

(c)

Fig. 5 Oxygen fugacity profile as a function of distance at different pressures and durations. Greendots are raw data in the original oxidized part of redox couple, while yellow ones are raw data in theoriginal reduced part. Horizontal dashed line with yellow shaded region indicates the initial oxygen
fugacity before redox experiment within standard deviation. Vertical dashed line is the initial
contact interface.
4. Determination of diffusion coefficient
Along oxygen fugacity profile, fO2 gradient leads to local re-equilibrium between Pt particle andsurrounding olivine grains, which results in changing iron content in Pt alloy (XFe). As the amountof surrounding olivine grains is more than Pt alloy and Fe content in olivine is rather insensitive tofO2, XFe is chosen to be the indicator of redox process. To determine diffusion coefficient, redox
process in olivine aggregate is assumed to be one-dimension diffusion in semi-infinite space. Since
XFe as a function of distance shows asymmetric feature (Fig. 6), Matano method was used to obtain
diffusion coefficient (Matano, 1933). To get the robust fitting, largely deviated data were firstremoved to minimize possible interference. And the remaining raw data were fitted to theBiDoseResp function as below,,(1)

where x is the distance of data points from the initial interface, C(x) is the mole fraction of Fe in Pt
alloy (XFe), others are fitting parameters. Except run 5K3783 which shows an obvious kink at theinterface, other two runs demonstrate a smoothly fitted lines. Then position of Matano interface x
was determined where the integration of x over the minimum (C1) and maximum concentration (C2)equals to each other, i.e.,.

(2)

Pseudo-Matano interface was used for runs diffuse out at 8 GPa, which is meaningless physically
except for getting diffusion coefficient in the next step. Iron-content dependent diffusion coefficient
D was calculated according to the equation below,,(3)

where C is the iron content calculated from fitting line at position x. Although diffusion coefficient
in redox couples at 8 GPa suffers from diffusion out and significant oxygen fugacity drop whichmakes it false, diffusion coefficient in the reduced part is still robust and reliable since it isindependent from the oxidized one. In this case, only diffusion coefficient at the reduced part isadopted.

Fig. 6 Iron content in Pt alloy as a function of distance at different pressures and durations. The
dashed vertical line is the initial contact interface, while the solid line is the (pseudo-) Matanointerface. Diffusion profile at 4 GPa is complete, while profiles at 8 GPa diffuse out.

Results
Water content of redox couples from 4 to 8 GPa is within 30–180 wt. ppm (Fig. 3 and Table 3).
Redox couples at 4 GPa suffers from water loss during redox experiment, while water content at 8
GPa before redox experiment is unknown. Within one redox couple, water content for the
FMQ/RRO buffered part (hereafter named the oxidized part) is usually higher than that of the IW
buffered part (hereafter named the reduced part).
Grain size of olivine in oxidized part before redox experiment is around 5–6.5 μm, and that in
reduced part is around 1.5–3.5 μm (Table 1). After redox experiment, 10–60 % grain growth occurs.
For the oxidized part, the grain size is around 9 – 12 μm, and for the reduced part, around 5-8 μm(Table 2). Since redox couple at 12 GPa is loosely sintered after redox experiment, its grain contrast

as well as oxygen fugacity profile is absent due to technical difficulty. Grain boundary for olivine
grain at higher pressures is more obvious than that at 4 GPa, especially in the oxidized part (Fig. 4).
Oxygen fugacity profile as a function of distance from the initial contact interface at 4 GPa
demonstrates asymmetric diffusion feature, while that at 8 GPa shows diffusing out feature orsystematical drop of oxygen fugacity in the oxidized part (Fig. 5). In XFe profile, superfluous
scattered data points were screened to ensure better fitting and thus more robust diffusion
coefficient. At 4 GPa, XFe first increases gently over a long distance in the oxidized part, thenincreases rapidly until it levels out in the reduced part at the right side (Fig. 6a). At 8 GPa, XFe
deviates from the initial XFe a little bit in the end of the left side which may result from too long run
duration or too much water for run 5K3783 and 5K3788 respectively (Figs. 6b and 6c). However,both of them demonstrate a well-sustained diffusion profile in the reduced part, with XFe levels out
in the end of the right side. The Matano interface shifted to the reduced part at 4 GPa, indicating
relatively slower diffusion coefficient there. Since the initial XFe in the oxidized part is not
maintained at 8 GPa, the Matano interface is pseudo one which is meaningless physically.

Fig. 7 Diffusion coefficient as a function of iron content in Pt alloy (XFe). Yellow and green points
are diffusion coefficient at 4 and 8 GPa respectively. Light and dark green points are diffusion
coefficient at 8 GPa for different durations. The grey shaded region is diffusion coefficient of dry
reduced olivine at 1 GPa and 1200 oC for different durations. Oxygen fugacity of all the data iswithin IW and FMQ buffered condition.
Diffusion coefficient obtained versus XFe shows that diffusion coefficient increases withincreasing pressure and water content (Fig. 7). Diffusion coefficient at 12 GPa is not available due
to loosely sintered redox couple after recovery. In general, diffusion coefficient at 4 GPa issignificantly higher than that at 1 GPa, which maybe due to the combined effect of pressure andwater. The water influence on diffusion coefficient is negligible small at 4 GPa, i.e. 20 wt. ppm

water content different results in 0.1 log unit difference in diffusion coefficient (Table 2). Diffusion
coefficient at 8 GPa seems to be a little higher than that at 4 GPa at the first glance (Fig. 7).
However, when water content is the same (50 wt. ppm), they give the same diffusion coefficient(Table 2). At 8 GPa, the redox couple with slightly higher water contents by 20 wt. ppm exhibits alittle bit faster diffusion coefficient (Fig. 8 and Table 2). Therefore, it is obvious that pressure has noinfluence on diffusion coefficient and thus redox kinetics, while water does have some, though may
be not significant.
Extrapolating high pressure data to low pressure gives diffusion coefficient on the order of 10-11.9
m2/s at 1 GPa (Fig. 8). Combined with diffusion coefficient of dry olivine at 1 GPa, it indicates that
small amount of water can facilitate redox process in olivine by 0.5 log unit when oxygen fugacity
is below FMQ buffer. The relationship between diffusion coefficient and water content is depicted
in Fig. 9. Diffusion coefficient is enhanced by increasing water content, but the accelerate rate isdiminishing. It shows that about 100 wt. ppm water can increase diffusion coefficient by about 1 log
unit, and 2000 wt. ppm by about 2 log units when water content in olivine reaches the watersolubility at the deep upper mantle according to the fitting equation. For the oxidized part in run
5K3788 whose oxygen fugacity dropped by about 1.7 log unit (Fig. 5c), its diffusion coefficient is
expected to be 10-11.2 m2/s according to the water content (Fig. 9 and Table 2), which is about 0.5
log unit faster compared with the apparent diffusion coefficient deduced from the final diffusion
profile. Therefore, it is probable that the redox process first take place at the contact interface, when
the initial oxygen reservoir runs out, the distant end far from the contact interface serves as oxygen
source to provide redox component until the new oxygen fugacity level approaches the diffusion
profile, which results in shortened apparent diffusion length and slowed apparent diffusion
coefficient.

Fig. 8 Diffusion coefficient as a function of pressure and water content at 1200 oC. Grey filled circle
is from dry reduced part in redox couple at 1 GPa (Zhao et al., 2022). Yellow filled circles are from

4 GPa with different water contents, of which dark yellow represents the initially FMQ bufferedpart (ΔFMQ-1.5) and light one represents the initially IW buffered part (ΔIW+0.9) in the same run.
Green filled circles are from 8 GPa with different water contents and durations, both of which
represent the IW buffered part (ΔIW+1.5).

Fig. 9 Relationship between diffusion coefficient and water content from 1 to 8 GPa. Grey filled
circle represents diffusion coefficient of dry olivine at 1 GPa whose water content is assumed to be
10 wt. ppm (Zhao et al., 2022). Yellow filled circles are diffusion coefficients of hydrous olivine at
4 GPa, and green ones are those of hydrous olivine at 8 GPa. Note that sample with the highest
water content in run 5K3783 should have higher diffusion coefficient than demonstrated here since
this value is derived from the profile diffusing out. Fitting line is shown below the data points. Grey
filled cubes are diffusion coefficient calculated using H-D exchange data from Sun et al. (2019).
Discussion
Mechanism of redox kinetics for wet olivine
Diffusion coefficient of wet olivine at high pressure is 0.5–0.7 log unit faster than that of dryolivine at low pressure. Pressure is shown to have no influence on diffusion coefficient, therefore,
the increased redox kinetics could be caused by mechanism whose diffuses faster than oxygen grainboundary diffusion. Since there is no cracks during redox experiment, high oxygen grain boundary
caused by cracks and other short circuits could not be the dominant mechanism. Hydrogen is themost probable candidate. Diffusion coefficients obtained in this study lie on extended line ofhydrogen diffusion associated with Mg site (Fig. 10). Furthermore, diffusion coefficient calculatedusing H-D exchange experiment matches diffusion coefficient in this study well, whose value is
within 5% of ours (Fig. 9).
When slab subducts into the upper mantle, oxygen budget within slab evolved according to theevolution of its water content. Regions with no or small amount of water cannot release oxygen
budget fast, but to preserve until it reaches deep mantle. However, regions with high water content
can reach new oxygen equilibrium quickly and release oxidizing component. When water content in
the surrounding reduced mantle is lower than the slab, there will be redundant oxidizer wanderaround. Otherwise, the surrounding mantle will be oxidized more quickly. However, even assuming
the maximum water content in olivine (2000 wt. ppm), the slab can only be altered by ~30%, in theabsence of fluid.

Fig. 10 Diffusion coefficient in wet olivine at high pressure compared with diffusivities of otherelements in olivine.