Transcript スライド 1 - 2015 AGU Fall Meeting

V11C-2776
Hydrogen concentration in plagioclase as a hygrometer of magmas:
Approaches from melt inclusion analyses and hydrous melting experiments
1
Hamada *,
M.
M.
1
Ushioda ,
T.
2,3
Fujii
and E.
1
Takahashi
1Department
of Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo, Japan
2Earthquake Research Institute, University of Tokyo, Tokyo, Japan
(*E-mail:
3Crisis & Environmental Management Policy Institute, Tokyo, Japan
Experimental procedures
130ºE
Crushing
volcanic rock
135ºE 140ºE 145ºE
Separation of
An95 plagioclase
1 mg of An95 plagioclase
Hydrous melting
of crushed rock
Izu-Oshima volcano
Miyakejima volcano
Au80Pd20 capsule
Starting material of melting experiments (MTL rock,
collected from Miyakejima volcano in Izu arc)
Analyses of plagioclase-melt inclusion pairs from MORB
Studied sample: Sample# KH93-3 DR9 from
Rodriguez Triple Junction in the Indian Ocean
Whole-rock composition
Figure is from Nakamura et al. (2007 Marine Geol.)
SiO2
TiO2
Al2O3
FeO*
MnO
MgO
CaO
Na2O
K2O
P2O5
Total
50.5
1.52
15.1
11.1
0.19
7.44
11.1
2.80
0.10
0.15
100 (wt.%)
16
2
0
0
0 (vol.%))
Internally-heated pressure vessel
installed at Magma Factory, Tokyo Tech.
1300
plagioclase-in
fO2~NNO+3
augite-in
magnetite-in
Partitioning experiment of hydrogen
between An95 plagioclase and melt
1250
An95 plagioclase
80
85
An content of plagioclase
90
Temperature (℃)
Hydrogen concentration in
plagioclase (wt. ppm water)
Hydrogen concentration in
plagioclase (wt. ppm water)
75
plag
C H 2O
(wt. ppm) ≈94.3×
melt
CH2O (wt. %)
0
0.015
0
1
2
3
OH in glass (wt.%)
0.01±0.005
0.010
☞ Linear correlation
between H concentration
in plagioclase and OH in
melt suggests that H is
accommodated in
plagioclase as OH.
0.008±0.002
0.005
1
2
3
4
5
H2O in glass (wt.%)
☞ Hydrogen partition
coefficient DH slightly
decreases with increasing
H2O in coexisting melt.
6
－ Representative absorption spectra of plagioclase －
#RTJ-pl20
(11 wt. ppm water, 0.3 wt.% H2O in melt inclusion)
#RTJ-pl24
(38 wt. ppm water, 0.3 wt.% H2O in melt inclusion)
#MTL17
(68 wt. ppm water, 0.8 wt.% H2O in melt)
15
#MTL05
(153 wt. ppm water, 2.3 wt.% H2O in melt)
#MTL39
(157 wt. ppm water, 3.5 wt.% H2O in melt)
10
#MTL37
(210 wt. ppm water, 4.5 wt.% H2O in melt)
0
6000
#MTL41
(225 wt. ppm water, 5.6 wt.% H2O in melt)
5500
5000
4500
4000
3500
3000
2500
Natural plagioclase in MTL lava
(5 wt. ppm water)
Shorter hydrogen bond length d(O⋯O)
Obtained phase diagram of MTL rock at 0.35 GPa
80
70
50
Wavenumber (cm-1)
50
0
100
Longer hydrogen bond length d(O⋯O)
Analytical result using FT-IR
20
1050 ℃
5
Backscattered-electron image of
plagioclase and melt inclusion.
Plagioclase (An74-89) 5
Olivine (Fo90)
1 (vol.%))
40
1160 ℃
20
50 μm
Modal composition
60
150
25
51.2
0.96
17.5
11.2
0.21
4.9
11.5
2.2
0.25
100 (wt.%))
Modal composition
Plagioclase (An95)
Olivine (Fo78-81)
Clinopyroxene
Orthopyroxene
Magnetite
500 μm
Cross-Nicol image of studied MORB.
Quenched glass contains ~0.3 wt.% H2O.
1000 ℃
1130 ℃
Absorption per cm
SiO2
TiO2
Al2O3
FeO*
MnO
MgO
CaO
Na2O
K2O
Total
1050 ℃
200
0
0
Melting at 0.35 GPa
for 24-48 hours using
internally-heated
pressure vessel
☞ H concentration in
plagioclase can be
approximated as a square
root of H2O in melt.
Decreasing temperature
lowers H concentration in
plagioclase. However, the
effect of temperature is
minor.
Hydrous melting
experiments
Melt inclusions
volcanic front
Part 1)
KH93-3 DR9
250
10 mg of hydrous glass
Hydrogen in plagioclase (wt. ppm water) ≈ 94.3×√(H2O in melt, wt.%).
This empirical formulation can be used as a practical hygrometer of
magmas. In this poster, we apply this formulation to understand the 19861987 summit eruption of Izu-Oshima volcano, a frontal-arc volcano in
Izu arc, and discuss eruption process of H2O-saturated magma.
300
residual resin?
Plagioclase is one of the nominally anhydrous minerals
(NAMs) which accommodates hydrogen, and plagioclase in volcanic
rocks essentially contains structural OH in it. Hydrogen concentration in
NAMs can be a useful proxy for dissolved H2O in silicate melts if the
partitioning of hydrogen between plagioclase and melts is known. Here,
we performed two parts of studies in order to determine hydrogen
partitioning between plagioclase and hydrous basaltic melt: (i) analyses
of low-H2O melt inclusions (H2O ≈ 0.3 wt.%) hosted by plagioclase in
mid-ocean ridge basalt (MORB) and (ii) hydrogen partitioning
experiments between Ca-rich plagioclase (An95) and hydrous arc basaltic
melt (0.8 ~ 5.5 wt.% H2O) at 0.35 GPa. Obtained formulation is
Hydrous melting experiments to determine
H partitioning between plagioclase and melt
DH(plag/melt)
Purpose and quick summary
Integration of Part 1 and Part 2
H in plagioclase (wt.ppm water)
Part 2)
[email protected])
40
30
20
1200
1150
200 μm
magnetite-in
1100
10
0
0
0.1
0.2
0.3
0.4
0.5
H2O in plagioclase-hosted melt
inclusions (wt.%)
≈ 0.01
1050
1000
0
1
2
3
4
H2O in melt (wt.%)
5
Backscattered electron image
of the recovered sample
(1130℃, 2.6 wt.% in melt)
☞ Hydrogen concentration
in plagioclase and H2O
concentration in glass was
6 measured using FT-IR.
☞
Peak of infrared absorption spectra of plagioclase shifts from lower
wavenumbers (peak position: 3200-3400 cm-1) under H2O-poor conditions to higher
wavenumbers (peak position: 3600 cm-1) under H2O-rich conditions, meaning
expansion of O-H⋯O bond length with increasing H2O. These observations suggest
that hydrogen site in plagioclase slightly changes with increasing H2O, which also
changes hydrogen partitioning between plagioclase and melt as shown above.
Application
☞ High hydrogen concentration in plagioclase
from Izu-Oshima volcano, a frontal-arc volcano
in Izu arc (≥200 wt. ppm water, Hamada et al.,
2011 EPSL), suggests crystallization of
plagioclase from H2O-rich (≥4 wt.%) melt.
An80
An85
An90
An95
Anorthite content of plagioclase
Hamada et al. (2011, EPSL)