1. Serpentinized peridotites obtained from dome-shaped seamounts and horst blocks in the Mariana forearc, during R/V Kairei KR06-15 cruise (2006), ODP Leg 125 (1989), ODP Leg 195 (2001), and R/V Yokosuka YK03-07 cruise (2003) were petrologically examined. Up to now the studies on Mariana forearc peridotites had dealt with relatively small areas. However, this study dealt with peridotites from 11 sites, which cover almost the entire area of the southern Mariana forearc, and enables us to discuss the areal differences among peridotites in the forearc area.
2. Primary olivine, orthopyroxene, and clinopyroxene have survived in some peridotite samples.
Primary spinel is more or less altered to magnetite, but almost always retains its primary chemistry in the core. Serpentine minerals are commonly chrysotile and lizardite. Antigorite was also found in peridotites from Conical, Big Blue, Celestial, and South Chamorro Seamounts.
Antigorite-bearing peridotites in the Mariana forearc commonly contain primary olivine with well-developed cleavage (i.e., cleavable olivine).
3. Secondary iron-rich olivines (Fo86-90) formed as an overgrowth or a replacement of primary olivines (Fo90-92) were found in cleavable olivine-bearing peridotites from Conical, Big Blue, and South Chamorro Seamounts. They occur as irregular bands randomly running within olivine grain that are thought to have been once-developed conduits of fluids. In cleavable olivine-bearing peridotites from Conical and South Chamorro Seamounts, the iron-rich stripes (Fo86-89) develop from the crack formed by growth of feather-like antigorite crystal. Iron-rich stripes are only developed near the margin of olivine grain where fiber crystal of antigorite pierces into olivine crystals, and are not recognized in the inside of olivine grain except for periphery of crack. Iron-poor parts intervened between iron-rich parts are slightly lower in XMg
than the homogeneous core olivines which lack iron-rich stripe, suggesting that hydrothermal alteration by iron-rich fluids affect also in iron-poor part compositions.
4. The iron-rich olivines have been formed by iron-rich fluid. It, however, still remains a possibility that the iron-rich olivine had begun to crystallize before serpentinization by pre-existed iron-rich fluids that had initially invaded from outside under the high-temperature mantle conditions and
thereafter caused serpentinization.
5. Widespread occurrence of chrysotile and lizardite in serpentinized peridotites throughout the southern Mariana forearc indicates that the temperature conditions of serpentinization are about 200-300 °C. Newly found mineral associations of antigorite + secondary clinopyroxene + iron-rich olivine suggests higher temperature serpentinization at about 450-550 °C. In the Mariana forearc, serpentinite diapirs are thought to have generated along the subduction boundary where serpentinization proceeded due to the water supplied from the hydrated sediments on top of the subducting Pacific Plate. As antigorite-bearing assemblages favor the deep high-temperature portion in the subduction zone, antigorite-stable region lies farther from the trench axis along the subduction boundary than chrysotile/lizardite-stable region.
Antigorite-bearing peridotites are found in the dome-shaped seamounts regardless of distance from the trench. This may reflect a complex process of tectonic migration of mantle wedge serpentinized peridotites from depth to shallow region along the subduction boundary.
6. The cooling history of peridotites from Mariana forearc was estimated at 10-5-10-2 °C/yr from 800 °C to 600 °C. There is highly probable that the cooling history reflects the temperature change from hot wedge mantle environment before the initiation of subduction to steady-state lower temperature environment caused by subduction of cold slab. The peridotites from Mid-Atlantic Ridge and Hess Deep were estimated at 10-3-10-1 °C/yr from 900 °C to 700 °C and 10-3-10-2 °C /yr from 830 °C to 660 °C, respectively. These cooling rates may reflect the regression process from hot ridge axis or uplift process within upper mantle.
7. The occurrences of cleavable olivine and associated iron-rich stripe found in this study could be a key to solve kinematic environment within the subduction boundary in the near future.
Japan
Ridge Shikoku
Basin Nankai Trough
Oki Daito Ridge
West Philippine Basin
Ridge Parece
Mariana Trench Mariana Mariana
Mariana Ridge
West Trough
Dutton Ridge
Philippine Sea Plate
Pacific Plate
Ogasawara Plateau
Izu-
Izu-ni Bo Trn
ench
Kyushu
Palau
Palau Trench
Trench Yap Korean
Peninsula
30°N 40°N
20°N
10°N
150°E 140°E
130°E
forearc
Mariana
Fig. 1. Regional map of the western Pacific, showing the location of Izu-Bonin-Mariana forearc.
Vela Basin
Japan Trench
-8000
-8000
-8000
-6000 -6000
-6000
-6000 -6000
-4000
-4000
-4000
-4000
-4000
-4000
-4000
-2000
-2000 2000
-2000 -2000
-2000
-10000 -8000 -6000 -4000 -2000 0
0 50
km
2006 Dec 10 01:10:45 contour: 200 m scale 1:3,500,000
146° 147° 148°
13°
14°
15°
16°
17°
18°
19°
20°
#786 Coni-pack Triangle Leg 125 Site 779 & #369
Conical Seamount
#371 Twin Peaks Seamount
#372 Eastern ridge of Big Blue Seamount
#783 Big Blue Seamount
#373 Staircase Plateau
#374 Turquoise Seamount
#782 Celestial Seamount
#375 North Chamorro Seamount Leg 195 Site 1200 & #778-780
South Chamorro Seamount
7
#786
13
#370
23
#372
29
#373
19
#782 3
#369
26
#371
14
#375 7
#783
28
#374
10
#778-780
Fig. 2. Topographic map in the Mariana forearc, showing the locations of studied serpentinite seamounts and horst blocks. The ratios of constituent rock types from each dive site are also shown in this figure. The ratios of each diagram correspond to the number of recovered samples. The numbers enclosed by squares in the right bottom of each diagram indicate the numbers of total rock samples.
Data is from Maekawa et al. (2007b).
#370 & #784 Pacman Seamount
serpentinite basalt & dolerite gabbro
amphibolite chert mudstone
Conical Seamount
South Chamorro Seamount 90
40
Opx Cpx
Ol
lherzolite
harzburgite wehrlite
olivine websterite
90
olivine
orthopyroxenite olivine
clinopyroxenite dunite
websterite
orthopyroxenite clinopyroxenite
Fig. 3. Modal compositions of peridotites from Conical and South Chamorro Seamounts in the olivine (Ol) - orthopyroxene (Opx) - clinopyroxene (Cpx) triangular diagram. The data from Conical Seamount are after Ban (1991).
88
90
92
0 1 2 3
≃ ≃
100Mg/(Mg+Fe
2+ ) rock
0 2 4
1 2 3
melting
Al2O3 wt% rock
≃
CaO wt% rock CaO wt% rock
melting ≃
15%
30% 30% 15%
Conical Seamount
South Chamorro Seamount Harzburgite
Dunite Harzburgite
1 2 3
Fig. 4. Bulk rock compositions of peridotites from South Chamorro Seamount on the CaO vs. 100Mg/(Mg+Fe2+) and CaO vs. Al2O3 diagrams. Data source for degree of melting is Ishiwatari (1985).
The data from Conical Seamount is after Ishii et al. (1992).
0.5 mm
1 mm
Ol Ol OpxOpx
Ol Ol
Fig. 5. Relict olivine crystals in peridotite from Big Blue Seamount (783-4R-1). (A) Plane polarized light. (B) Crossed polars.
B
B A
A
Fig. 6. An aggregate of fine-grained olivine around orthopyroxene in peridotite from Conical Seamount (779A-17R-2, 20-25 cm). (A) Plane polarized light. (B) Crossed polars.
Fig. 7. Cleavable olivine in peridotite from South Chamorro Seamount (1200B-1W-1, 92–100 cm). (A) Plane polarized light. (B) Crossed polars.
A B
0.2 mm
Ol Ol
Atg
100 μm Atg
Atg
Spl
Ol
Ol
m-Ol m-Ol
A
20 μm Atg
Ol Ol
m-Ol m-Ol
B C
Atg m-Ol
Ol
m-Ol Atg
Atg
Ol
10 μm Fig. 8. Backscattered electron images of olivine. The olivine crystal partly has well-developed cleavages.
(A) Mode of occurrence of olivine in peridotite from Conical Seamount (779A-10R-1, 39-43 cm). Dark part is primary olivine and bright part is iron-rich secondary olivine (m-Ol). Secondary olivine
irregularly replaced primary olivine. It seems to occur along the once-existed conduits of fluid. (B) Iron-rich secondary olivine along the cleavage traces of primary olivine in peridotite from Conical Seamount (779A-10R-1, 39-43 cm). (C) Iron-rich secondary olivine formed along the cleavage traces
Ol
Fig. 9. Cleavable olivine in peridotite from Conical Seamount (779A-19R-2, 105-108 cm).
Feather-like crystals of antigorite were often developed in olivine crystals. (A) Plane polarized light. (B) Crossed polars.
Ol Ol
0.2 mm
Atg Atg
A B
0.2 mm
m-Cpx m-Cpx
A
25 μm Ol
Ol
m-Cpx m-Cpx
0.1 mm m-Cpx
m-Cpx
Atg Atg
Atg Atg
I
m-Ol m-Ol Spl
Spl
Fig. 10. Acicular crystals of metamorphic
clinopyroxene in peridotites. (A) Acicular crystals of metamorphic clinopyroxene (m-Cpx) in peridotite from South Chamorro Seamount (1200B-1W-1, 92-100 cm). (B) Backscattered electron images of metamorphic clinopyroxene in peridotite from South Chamorro Seamount (1200B-1W-1, 92-100 cm). Secondary acicular clinopyroxenes scattered in matrix of antigorite.
(C) Backscattered electron images of
metamorphic clinopyroxene in peridotite from Conical Seamount (779A-10R-1, 39-43 cm).
Secondary acicular clinopyroxenes scattered in olivine crystals.
A
A C
B
Fig. 11. Backscattered electron images and element maps of olivine crystals. (A) A backscattered electron image of olivine crystal. Iron-rich stripe pattern is well developed near the rim of olivine where fiber crystals of antigorite intensely pierce it.
(B) Close up of the square shown in (A).
(C) Fe element map of (B). (D) Mg element map of (B). Sample No. 779A-10R-1, 39-43 cm.
A
100 μm
100 μm Ol
Ol
Atg Atg
C
Ol Ol
B
Atg Atg 25 μm
D
0.2 mm
Spl
A B
0.2 mm Fig. 12. Modes of occurrence of spinels in peridotites. (A) A spinel crystal partly replaced by magnetite along the rim and cleavage in peridotite from Turquoise Seamount (374-2R-5). (B) Euhedral to subhedral spinel is commonly found in the dunite sample from eastern ridge of Big Blue Seamount (372-1R-1).
20 μm
Fig. 13. A Backscattered electron image and line scan of spinel in peridotite from Twin Peaks Seamount (371-5R-1).
5
Leg 125 & #369 Conica l
#786 Conipac k
2
(A) (B )
Cr Al Al
C r
0. 5 0. 5 Fig. 14. Cr-Al-Fe
3+plots of analyzed spinel s
in the peridotites from dome-shaped s
eamounts (A) and horst blocks (B) on the M ariana forearc . A number at the low er left is the number of the samples.
#370 & #784 Pacman
4 3
#783 Big Blu e
C r Al Fe
3+#372 Eastern ridge of Big Blu e
4
#373 Staircase Plateau
5 Al
C r 0. 5
#374 Turquois e
4
#782 Celestia l
3
#375 North Chamorr o
4 11
Leg 195 South Chamorr o
#371 Twin Peaks
3 Fe
3+00 10080 604020 100 90 80 70 60 50 40 30 20 10 100 80 60 40 20 0 Mg# (100Mg/(Mg+Fe2+ ))Mg# (100Mg/(Mg+Fe2+ ))
Cr# (100Cr/(Cr+Al)) 0
100 90 80 70 60 50 40 30 20 10
(A) (B)
ODP Leg 195 South Chamorro SeamountODP Leg 125 & #369 Conical Seamount #370 & #784 Pacman Seamount #371 Twin Peaks Seamount #783 Big Blue Seamount #374 Turquoise Seamount #782 Celestial Seamount #375 North Chamorro Seamount#372 Eastern ridge of Big Blue Seamount #373 Staircase Plateau#786 Coni-Pack TriangleFig. 15. Cr# vs.Mg# diagrams of spinels. (A) Peridotites from the dome-shaped seamounts. (B) Peridotites from horst blocks. Broken lines indicate the field for abyssal spinel peridotites of Dick and Bullen (1984).
Cr# (100Cr/(Cr+Al))
Ol
Fig. 16. Deformed orthopyroxene crystal in peridotite from South Chamorro Seamount (1200A-7R-2, 103-109 cm). (A) Plane polarized light. (B) Crossed polars.
1 mm
Opx
A
OpxB
1 mm
Opx Opx
A B
Fig. 17. Bastite texture filled with chrysotile and/or lizardite after orthopyroxene in peridotite from South Chamorro Seamount (1200A-16R-1, 83–89 cm). Orthopyroxene was partly survived at the left side crystal in the figure. (A) Plane polarized light. (B) Crossed polars.
Fig. 18. Thin exsolution lamellae of clinopyroxene within the orthopyroxene crystal in peridotite from South Chamorro Seamount (1200A-17G-2, 8-14 cm). (A) Plane polarized light. (B) Crossed polars.
0.5mm
Opx Opx
Cpx Cpx
secondary clinopyroxene primary clinopyroxene lA2O3%tw
CaO wt%
2.5
2.0
1.5
1.0
0.5
0.021.0 22.0 23.0 24.0 25.0 26.0 27.0
Fig. 19. CaO vs. Al2O3 diagram of clinopyroxenes. Secondary metamorphic clinopyroxene is clearly poor in Al2O3 in comparison with primary one.
A B
0.5 mm
Opx Opx
Cpx lamellae Cpx lamellae
20.0 21.0 22.0 23.0 24.0 25.0 26.0 27.0
0.85 0.90 0.95
XMg
CaO wt%
Fig. 20. XMg vs. CaO diagram of clinopyroxenes. Clinopyroxenes from dunite tend to have low CaO content and low XMg ratio in comparison with those from harzburgite samples.
harzburgite dunite
Fig. 21. Mesh texture of chrysotile and/or lizardite in peridotite from South Chamorro Seamount (1200A-17G-2, 8-14 cm). (A) Plane polarized light. (B) Crossed polars.
0.5 mm
Ol Ol
A B
3.8 3.9 4.0 4.1 4.2 4.3
0.88 0.89 0.90 0.91 0.92 0.93 0.94 0.95 0.96 0.97 0.98
Si
antigorite (779A-19R-2, 105-108 cm) chrysotile/lizardite (779A-19R-2, 105-108 cm) chrysotile/lizardite (779A-26R-2, 15-18 cm)
● ●
●
●
●
●
●
Mg/(Mg+Fe2+)
Fig. 22. Mg/(Mg+Fe2+) vs. Si diagram of serpentine minerals of peridotites from Conical Seamount.
Open square: antigorite from the sample 779A-19R-2, 105-108 cm. Closed square: chrysotile and/or lizardite from the sample 779A-19R-2, 105-108 cm. Closed circle: chrysotile and/or lizardite from the sample 779A-26R-2, 15-18 cm.
Eedenite
Pargasite Tschermakite
6.0 7.0 8.0
2.0 2.5 3.0
Si
Richterite
South Chamorro Seamount 1200A-6R- 1, 70-76 cm
Pacman Seamount 784-4R-2
Eastern ridge of Big Blue Seamount 372-1R-1 Twin Peaks Seamount 371-7R-3
(A)
Ca + Na + K
South Chamorro Seamount 1200B-1W-1, 49-55 cm
Conical Seamount 779A-17R-2, 20-25 cm
Eastern ridge of Big Blue Seamount 372-2R-3 Conical Seamount 779A-26R-2, 15-18 cm 0.90
0.91 0.92 0.93 0.94 0.95 0.96 0.97
0.0 1.0 2.0
Al Mg/(Mg+Fe2+)
6.0 7.0
2.0 2.5 3.0
Ca+Na+K Si
Tremolite8.0
Actinolite Richterite
Eedenite
Pargasite Tschermakite
Conical Seamount 369-1R-1
Fig. 23. Chemical compositions of calcic amphiboles. (A) Ca+Na+K vs. Si in amphiboles from peridotites from seamounts on Mariana forearc. Elements are expressed in structural formula
Fig. 24. Brucites accompanied with magnetites in vein in peridotite from Conical Seamount (779A-22R-2, 9-14 cm). (A) Plane polarized light. (B) Crossed polars.
0.5 mm
Brc Brc
MagMag
Atg Atg
A B
Phl Ol
5 μm 5 μm
Fig. 25. Backscattered electron images of phlogopite. (A) Phlogopite as inclusions of primary olivine in peridotite from South Chamorro Seamount (1200B-1W-1, 92-100 cm). (B) Phlogopite as inclusions of olivine in peridotite from South Chamorro Seamount (1200B-1W-1, 92-100 cm).
A B
Ol Phl
Fig. 26. Chlorite around spinel in peridotite from South Chamorro Seamount (1200B-1W-1, 92-100 cm). Plane polarized light.
0.5 mm
Spl
Chl
Fig. 27. Backscattered electron images. (A) Mode of occurrence of secondary olivine (bright part at the bottom of the figure).
Antigorite fibers penetrate olivine crystal from the top of the photograph, and iron-rich stripes run in the direction normal to the antigorite fibers. (B) Close up of the square in (A). (C) Iron-rich stripe patterns formed parallel to the cleavage (100) of olivine (779A-10R-1, 39-43 cm).
A B
200 μm Atg
Atg AtgAtg
Ol Ol
Atg Atg
100 μm
C
100 μm Ol
Ol
Atg Atg
Fig. 28. A backscattered electron image showing iron-rich stripe patterns developed from the crack by crystallization of feather-like antigorite crystals.
200 μm
92 91 90 89 88 87 86 15
10 5 0
Frequency
Fo
92 91 90 89 88 87 86
Frequency
0 5 10 15
Fo
South Chamorro Seamount Leg 195 1200B-1W-1, 92-100 cm Conical Seamount
Leg 125 779A-10R-1, 39-43 cm
92 91 90 89 88 87 86
15
10 5 0
Frequency
Fo
Conical Seamount
Leg 125 779A-19R-1, 105-108 cm
(A)
(B)
(C)
Primary olivine, Secondary olivine, Secondary olivine with abundant magnetite
Fig. 29. Frequency distributions of Fo contents of primary and secondary olivines. (A, B)
Secondary olivines without magnetite have low forsterite contents (Fo86–88). (C) Secondary
olivines coexisting with magnetite have Fo-rich compositions (Fo88–90). Representative
chemical compositions of olivine are given in Table 2A.
92 91 90 89 88 87 86
ycneuqerF
Conical Seamount (779A-10R-1, 39-43 cm)
92 91 90 89 88 87 86
85
85
ycneuqerF
South Chamorro Seamount (1200B1W-1, 92-100 cm)
Fo Fo
Fig. 30. Frequency distributions of Fo contents of olivines. White square represents olivines without iron-rich stripe patterns; gray square, iron-poor parts intervened between iron-rich stripe parts; black square, iron-rich parts of striped olivine.
1 2 3 4 5 6
SiO2 40.97 40.99 40.44 39.88 41.02 40.71
TiO2 0.00 0.18 0.00 0.06 0.03 0.00
Al2O3 0.00 0.10 0.00 0.00 0.10 0.02
Fe2O3 0.00 0.00 0.00 0.00 0.00 0.00
FeO 8.72 8.49 11.96 12.15 9.41 9.32
MnO 0.14 0.11 0.13 0.37 0.19 0.21
MgO 49.54 49.45 46.43 46.44 48.82 49.31
CaO 0.00 0.00 0.02 0.01 0.03 0.00
Cr2O3 0.02 0.00 0.02 0.00 0.00 0.00
NiO 0.18 0.43 0.41 0.63 0.27 0.48
Total 99.57 99.75 99.41 99.55 99.87 100.06
O= 4 4 4 4 4 4
Si 1.003 1.002 1.007 0.997 1.005 0.997
Ti 0.000 0.003 0.000 0.001 0.001 0.000
Al 0.000 0.003 0.000 0.000 0.003 0.001
Fe3+ 0.000 0.000 0.000 0.000 0.000 0.000
Fe2+ 0.179 0.173 0.249 0.254 0.193 0.191
Mn 0.003 0.002 0.003 0.008 0.004 0.004
Mg 1.808 1.801 1.724 1.730 1.782 1.800
Ca 0.000 0.000 0.001 0.000 0.001 0.000
Cr 0.000 0.000 0.000 0.000 0.000 0.000
Ni 0.004 0.008 0.008 0.013 0.005 0.009
Total 2.996 2.993 2.992 3.002 2.993 3.003
Fo 91.01 91.21 87.37 87.20 90.24 90.41
Conical Seamount (779A-10R-1, 39-43 cm) Table. 1: Representative chemical composition of olivines
1 and 2, olivines without iron-rich stripe patterns.
3 and 4, iron-rich parts of striped olivine.
5 and 6, iron-poor parts intervened between iron-rich stripe parts.
Fig.31. (A) Secondary iron-rich olivine coexisting with magnetite in peridotite from Conical Seamount (779A-19R-2, 105-108 cm).
Magnetite occurs along the grain boundaries of fine-grained secondary olivines. (B) Mesh texture of chrysotile and/or lizardite around olivines in peridotite from Conical Seamount (779A-26R-2, 15-18 cm). Chrysotile and lizardite are characteristically accompanied by fine-grained magnetite (bright-colored mineral). (C) Antigorite veins cut by a chrysotile and/or lizardite vein in peridotite from Conical Seamount (779A-10R-1, 39-43 cm). (D) A chrysotile and/or lizardite vein cut by antigorite veins in peridotite from Conical Seamount (779A-10R-1, 39-43 cm). Fine-grained bright minerals in chrysotile and/or lizardite vein are magnetite.
Mag Ol
Mag Ol
A
25 μm
20 μm Ctl/Lz
Ctl/Lz
Atg Atg Ol
Ol
Mag Mag
20 μm Ctl/Lz
Ctl/Lz
Atg Atg Ol
C D
OlFig. 32. Pressure-temperature diagram. Ctl: chrysotile, Lz: lizardite, Atg: antigorite, Brc: brucite, Fo: forsterite, Di:
diopside, Tr: tremolite, En: enstatite, Ath: anthophylite, Chl: chlorite, Tlc: talc, W: water. Data is from Bucher and Frey (2002), Berman et al. (1986), and Evans (2004). The approximate metamorphic condition of antigorite + diopside + secondary olivine of the Mariana peridotites is shown in shaded region.
50 μm Ol
Ol Ctl/Lz
Ctl/Lz
B
Chl Atg + Brc Fo + W
Tlc + Fo En + W
Tlc + FoAth + W Atg + Di
Fo + Tr + W
Atg
Fo + Tlc + W
2 4 6 8 10 12 14
200 300 400 500 600 700
Ctl/Lz Atg + Brc
Temperature (˚C)
Pressure (kb)
F + Ath
o
0100 90 80 70 60 50 40 30 20 10 100 80 60 40 20 0 Mg# (100Mg/(Mg+Fe2+ )) Cr# (100Cr/(Cr+Al))
(B) Hess Deep 0
100 90 80 70 60 50 40 30 20 10 100 80 60 40 20 0 Mg# (100Mg/(Mg+Fe2+ )) Cr# (100Cr/(Cr+Al))
(A) Mid-Atlantic Ridge 0
100 90 80 70 60 50 40 30 20 10 100 80 60 40 20 0 Mg# (100xMg/(Mg+Fe2+ )) Cr# (100Cr/(Cr+Al))
(C) Tonga forearc
Fig. 33. Cr# vs. Mg# diagrams of spinels. (A) Mid-Atlantic Ridge, (B) Hess Deep, (C) Tonga forearc. Broken lines indicate the field for abyssal spinel peridotites of Dick and Bullen (1984).
#782 Celestial Seamount
600 700 800 900
0.01 0.1 1 10
Grain size of spinel (mm)
Temperature (℃)
Pacman Seamount
600 700 800 900
temperature (℃)
0.01 0.1 1 10
grain size of spinel (mm)
700 800 900
temperature (℃)
Leg 195 South Chamorro Seamount
600 700 800 900
Temperature (℃)
0.01 0.1 1 10
Grain size of spinel (mm) Conical Seamount
Twin Peaks Seamount
600 700 800 900
Temperature (℃)
0.01 0.1 1 10
Grain size of spinel (mm)
600 700 800 900
temperature (℃)
0.01 0.1 1 10
#783 Big Blue Seamount
grain size of spinel (mm) (A)
(B)
(C)
Fig. 34. Measured and calculated relations between grain size of spinel and estimated temperature for the peridotites from dome-shaped seamounts in the Mariana forearc. (A) Conical
Seamount, (B) Pacman Seamount, (C) Twin Peaks Seamount, (D) Big Blue Seamount, (E) Celestial Seamount, (F) South Chamorro Seamount.
779A-19R-2, 105-108 cm 779A-10R-1, 39-43cm 779A-26R-2, 15-18 cm 779A-17R-2, 20-25 cm
370-6R-1 370-7R-1 784-4R-2
371-8R-2 371-5R-1
47
Celestial Seamount
600 700 800 900
0.01 0.1 1 10
grain size of spinel (mm)
temperature (℃)
#370 & #784 Pacman Seamount
600 700 800 900
Temperature (℃)
0.01 0.1 1 10
Grain size of spinel (mm)
600 700 800 900
0.01 0.1 1 10
Grain size of spinel (mm)
Temperature (℃)
South Chamorro Seamount
600 700 800 900
temperature (℃)
0.01 0.1 1 10
grain size of spinel (mm) Leg 125 Conical Seamount
#371 Twin Peaks Seamount
600 700 800 900
temperature (℃)
0.01 0.1 1 10
grain size of spinel (mm)
600 700 800 900
Temperature (℃)
0.01 0.1 1 10
Big Blue Seamount
Grain size of spinel (mm)
(A)
(B)
(C)
Fig. 34. Continued.
(D)
(E)
(F) (A)
(A)
783-4R-1
782-2R-2 782-2R-3 782-2R-4
1200A-6R-1, 70-76 cm 1200A-7R-1, 10-17 cm 1200A-7R-2, 103-109 cm 1200A-13R-1, 24-28 cm 1200A-16R-1, 56-60 cm 1200A-16R-2, 5-10 cm 1200A-17G-2, 8-14 cm 1200B-1W-1, 92-100 cm 1200B-2W-1, 42-48 cm 1200B-1W-1, 49-55 cm
1200B-2W-1, 110-115 cm
Fig. 35. Measured and calculated relations between grain size of spinel and estimated temperature for the peridotites from fault scarps around horst blocks in the Mariana forearc. (A) Staircase Plateau, (B) Eastern ridge of Big Blue Seamount.
600 700 800 900
Temperature(℃)
(A) Staircase Plateau
600 700 800 900
Temperature (℃)
(B)Eastern ridge of Big Blue Seamount Grain size of spinel (mm)
Grain size of spinel (mm)
0.01 0.1 1 10
0.01 0.1 1 10
373-4R-2
372-1R-1 372-2R-2
#782 Celestial Seamount
600 700 800 900
temperature (℃)
0.01 0.1 1 10
grain size of spinel (mm)
600 700 800 900
0.01 0.1 1 10
grain size of spinel (mm)
temperature (℃)
Leg 195 South Chamorro Seamount
600 700 800 900
temperature (℃)
0.01 0.1 1 10
#783 Big Blue Seamount
grain size of spinel (mm)
Fig. 36. Measured and calculated relations between grain size of spinel and estimated temperature for the peridotites. (A) Mid-Atlantic Ridge, (B) Hess Deep, (C) Tonga forearc.
(D)
(E)
(F) (C)Tonga forearc
600 700 800 900
0.01 0.1 1 10
Temperature(℃)
600 700 800 900
Temperature (℃)
0.01 0.1 1 10
Grain size of spinel (mm)
(B) Hess Deep
(A) Mid-Atlantic Ridge
600 700 800 900
Temperature (℃)
Grain size of spinel (mm)
0.01 0.1 1 10
Grain size of spinel (mm)
920D-10R-3, 90-95 cm 920D-21R-2, 45-50 cm 920D-22R-2, 86-92 cm
D93-1-9 D98-2-7 D98-2-10
895D-4R-2, 105-109 cm 895C-3R-1, 35-42 cm
1300 1200 1100 1000 900 800 700 600 500
cooling rate 1.0×10
/yr ℃
1.0
4
1.0×10
3
1.0×10
2
10.0 0.1 -2 10 -3 10 -4 10 -5 10
1300 ℃ 1200 ℃
initial temperature
0.010.0010.11.010.0
Temperature ( ) ℃
却速度
×1 1.0 0
℃/
yr
0 1.
4
1.0
0 ×1
3
×1 1.0 0
2
0 10. 1 0.
-2 10 -3 10 -4 10 -5 10
1300℃ 1200℃
初期温度 0.0110.11.010.0
) スピネルの粒径 (mm)
Grain size of spinel (mm)
(A) Conical Seamount(B) Pacman Seamount
(D) Big Blue Seamount
(A)
伴（1991）1300 1200 1100 1000 900 800 700 600 500
cooling rate 1.0×10
℃
/yr
1.0
4
1.0×10
3
1.0×10
2
10.0 0.1 -2 10 -3 10 -4 10 -5 10
1300 ℃ 1200 ℃
initial temperature
0.010.0010.11.010.0 Grain size of spinel (mm)
Temperature ( ) ℃
1300 1200 1100 1000 900 800 700 600 500
cooling rate 1.0×10
℃
/yr
1.0
4
1.0×10
3
1.0×10
2
10.0 0.1 -2 10 -3 10 -4 10 -5 10
1300 ℃ 1200 ℃
initial temperature
0.010.0010.11.010.0 Grain size of spinel (mm)
Temperature ( ) ℃
(C) Twin Peaks Seamount
Iwanaidake(Kamuikotan) Biei（Kamuikotan）
1300 1200 1100 1000 900 800 700 600 500
cooling rate 1.0×10
℃
/yr
1.0
4
1.0×10
3
1.0×10
2
10.0 0.1 -2 10 -3 10 -4 10 -5 10
1300℃ 1200℃
initial temperature
0.010.0010.11.010.0 grain size of spinel (mm)
temperature ( ) ℃
HoromanMiyamori Fig. 37. Measured and calculated relations between grain size of spinel and estimated temperature for peridotites from dome-shaped seamounts in the Mariana forearc proposed by Ozawa (1984). (A) Conical Seamount, (B) Pacman Seamount, (C) Twin Peaks Seamount, (D) Big Blue Seamount, (E) Celestial Seamount, (F) South Chamorro Seamount.
51
却速度
×1 1.0 0
℃/
yr
0 1.
4
1.0
0 ×1
3
×1 1.0 0
2
0 10. 1 0.
-2 10 -3 10 -4 10 -5 10
1300℃ 1200℃
初期温度 0.0110.11.010.0
) スピネルの粒径 (mm)
(D) Big Blue Seamount1300 1200 1100 1000 900 800 700 600 500
cooling rate 1.0×10
℃
/yr
1.0
4
1.0×10
3
1.0×10
2
10.0 0.1 -2 10 -3 10 -4 10 -5 10
1300 ℃ 1200 ℃
initial temperature
0.010.0010.11.010.0 Grain size of spinel (mm)
Temperature ( ) ℃
(F)South Chamorro Seamount1300 1200 1100 1000 900 800 700 600 500
cooling rate 1.0×10
℃
/yr
1.0
4
1.0×10
3
1.0×10
2
10.0 0.1 -2 10 -3 10 -4 10 -5 10
1300 ℃ 1200 ℃
initial temperature
0.010.0010.11.010.0 Grain size of spinel (mm)
Temperature ( ) ℃
(E)Celestial Seamount
(A)
伴（1991） Fig. 37. Continued.1300 1200 1100 1000 900 800 700 600 500
cooling rate 1.0×10
℃
/yr
1.0
4
1.0×10
3
1.0×10
2
10.0 0.1 -2 10 -3 10 -4 10 -5 10
1300 ℃ 1200 ℃
initial temperature
0.010.0010.11.010.0 Grain size of spinel (mm)
Temperature ( ) ℃
52
却速度
×1 1.0 0
℃/
yr
0 1.
4
1.0
0 ×1
3
×1 1.0 0
2
0 10. 1 0.
-2 10 -3 10 -4 10 -5 10
1300℃ 1200℃
初期温度 0.0110.11.010.0
) スピネルの粒径 (mm)
(A)
伴（1991） Fig. 38. Measured and calculated relations between grain size of spinel and estimated temperature for peridotites from fault scarps around horst blocks in the Mariana forearc proposed by Ozawa (1984). (A) Staircase Plateau, (B) Eastern ridge of Big Blue Seamount.1300 1200 1100 1000 900 800 700 600 500
cooling rate 1.0×10
℃
/yr
1.0
4
1.0×10
3
1.0×10
2
10.0 0.1 -2 10 -3 10 -4 10 -5 10
1300 ℃ 1200 ℃
initial temperature
0.010.0010.11.010.0 Grain size of spinel (mm)
Temperature ( ) ℃
(A) Staircase Plateau1300 1200 1100 1000 900 800 700 600 500
cooling rate 1.0×10
℃
/yr
1.0
4
1.0×10
3
1.0×10
2
10.0 0.1 -2 10 -3 10 -4 10 -5 10
1300 ℃ 1200 ℃
initial temperature
0.010.0010.11.010.0 Grain size of spinel (mm)
Temperature ( ) ℃
(B) Eastern ridge of Big Blue Seamount
53
却速度
×1 1.0 0
℃/
yr
0 1.
4
1.0
0 ×1
3
×1 1.0 0
2
0 10. 1 0.
-2 10 -3 10 -4 10 -5 10
1300℃ 1200℃
初期温度 0.0110.11.010.0
) スピネルの粒径 (mm)
(A)
伴（1991） Fig. 39. Measured and calculated relations between grain size of spinel and estimated temperature for peridotites proposed by Ozawa (1984). (A) Mid-Atlantic Ridge, (B) Hess Deep, (C) Tonga forearc.1300 1200 1100 1000 900 800 700 600 500
cooling rate 1.0×10
℃
/yr
1.0
4
1.0×10
3
1.0×10
2
10.0 0.1 -2 10 -3 10 -4 10 -5 10
1300 ℃ 1200 ℃
initial temperature
0.010.0010.11.010.0 Grain size of spinel (mm)
Temperature ( ) ℃
(A) Mid-Atlantic Ridge 1300 1200 1100 1000 900 800 700 600 500
cooling rate 1.0×10
℃
/yr
4 1.0
1.0×10
3
1.0×10
2
10.0 0.1 -2 10 -3 10 -4 10 -5 10
1300 ℃ 1200 ℃
initial temperature
0.010.0010.11.010.0 Grain size of spinel (mm)
Temperature ( ) ℃
1300 1200 1100 1000 900 800 700 600 500
cooling rate 1.0×10
℃
/yr
1.0
4
1.0×10
3
1.0×10
2
10.0 0.1 -2 10 -3 10 -4 10 -5 10
1300 ℃ 1200 ℃
initial temperature
0.010.0010.11.010.0 Grain size of spinel (mm)
Temperature ( ) ℃
(B) Hess Deep(C) Tonga forearc
54
Iwanaidake Biei
1300 1200 1100 1000 900 800 700 600 500
cooling rate 1.0×10
℃
/yr
1.0
4
1.0×10
3
1.0×10
2
10.0 0.1 -2 10 -3 10 -4 10 -5 10
1300 ℃ 1200 ℃
initial temperature
0.010.0010.11.010.0
Temperature ( ) ℃
却速度
×1 1.0 0
℃/
yr
0 1.
4
1.0
0 ×1
3
×1 1.0 0
2
0 10. 1 0.
-2 10 -3 10 -4 10 -5 10
1300℃ 1200℃
初期温度 0.0110.11.010.0
) スピネルの粒径 (mm)
Grain size of spinel (mm)
(A)
(D) Big Blue Seamount Fig. 40. Measured and calculated relations between grain size of spinel and estimated temperature for peridotites proposed by Ozawa (1984). (A) Dome-shaped seamounts, (B) Mid-Atlantic Ridge and Hess Deep. The colors are the same as those in Figs. 37-39.
Iwanaidake Biei
1300 1200 1100 1000 900 800 700 600 500
cooling rate 1.0×10
℃
/yr
1.0
4
1.0×10
3
1.0×10
2
10.0 0.1 -2 10 -3 10 -4 10 -5 10
1300 ℃ 1200 ℃
initial temperature
0.010.0010.11.010.0 Grain size of spinel (mm)
Temperature ( ) ℃
HoromanMiyamori
(B)
(A)
伴（1991）HoromanMiyamori
Iwanaidake(Kamuikotan) Biei（Kamuikotan）
1300 1200 1100 1000 900 800 700 600 500
cooling rate 1.0×10
℃
/yr
1.0
4
1.0×10
3
1.0×10
2
10.0 0.1 -2 10 -3 10 -4 10 -5 10
1300℃ 1200℃
initial temperature
0.010.0010.11.010.0 grain size of spinel (mm)
temperature ( ) ℃
HoromanMiyamori
55
50 Ma
800°C 700°C
800°C 700°C
800°C 700°C east
west
Plate
Trench
Serpentinite Diapir Trench
Lateral fault
Fig. 41. A tectonic model explaining the cooling history of peridotites from dome-shaped seamounts and basement.
Location Sample No. Rock type Ol Spl Opx Cpx m-Ol m-Cpx Amp Phl Chl Brc Cal Ctl/Lz Atg Mag
KR06-15 #369 Conical Seamount 369-1R-1 H ○ ○ ○ ○ ○ ○ ○
#370 Pacman Seamount 370-4R-3 H ○ ○ ○ ○
370-6R-1 H ○ ○ ○ ○ ○ ○
370-7R-1 H ○ ○ ○ ○ ○ ○
#371 Twin Peaks Seamount 371-5R-1 H ○ ○ ○ ○ ○ ○
371-7R-3 H ○ ○ ○ ○ ○ ○
371-8R-2 H ○ ○ ○ ○ ○ ○ ○
372-1R-1 D ○ ○ ○ ○ ○ ○ ○
372-2R-1 D ○ ○ ○ ○ ○
372-2R-2 H ○ ○ ○ ○ ○ ○
372-2R-3 H ○ ○ ○ ○ ○ ○
#373 Staircase Plateau 373-2R-2 H or D ○ ○ ○ ○
373-4R-2 H ○ ○ ○ ○
373-8R-1 H or D ○ ○ ○
373-12R-2 H or D ○ ○ ○ ○ ○
373-13R-1 H ○ ○ ○ ○
#374 Turquoise Seamount 374-2R-2 L? ○ ○ ○
374-2R-5 L? ○ ○ ○
374-3R-4 L? ○ ○ ○
374-4R-2 L? ○ ○ ○
#375 North Chamorro Seamount 375-3R-3 L? ○ ○ ○ ○
375-7R-3 H ○ ○ ○
375-7R-4 H ○ ○ ○
375-8R-1 H ○ ○ ○
YK03-07 #782 Celestial Seamount 782-2R-2 H ○ ○ ○ ○ ○ ○ ○ ○ ○
782-2R-3 H ○ ○ ○ ○ ○
782-2R-4 H ○ ○ ○ ○ ○ ○
#783 Big Blue Seamount 783-1R-1 H ○ ○ ○
783-1R-2 H ○ ○ ○
783-4R-1 H ○ ○ ○ ○ ○ ○
783-4R-2 H or D ○ ○ ○ ○ ○ ○ ○ ○ ○
#784 Pacman Seamount 784-4R-2 H ○ ○ ○ ○ ○ ○ ○ ○
#786 Conipack Triangle 786-6R-1 D ○ ○ ○
786-6R-2 D ○ ○ ○
Leg 125 Conical Seamount 779A-26R-2, 15-18 cm H ○ ○ ○ ○ ○ ○ ○ ○
779A-17R-2, 20-25 cm H ○ ○ ○ ○ ○ ○ ○ ○
779A-19R-2, 105-108 cm H or D ○ ○ ○ ○ ○ ○ ○ ○ ○
779A-22R-2, 9-14 cm H or D ○ ○ ○ ○ ○ ○ ○ ○ ○
779A-10R-1, 39-43 cm H or D ○ ○ ○ ○ ○ ○ ○ ○ ○ ○
Leg 195 South Chamorro Seamount 1200A-6R-1, 70-76 cm H ○ ○ ○ ○ ○ ○ ○
1200A-7R-1, 10-17 cm H ○ ○ ○ ○ ○ ○
1200A-7R-2, 103-109cm H ○ ○ ○ ○ ○ ○ ○
1200A-13R-1, 24-28 cm H ○ ○ ○ ○ ○ ○
1200A-16R-1, 56-60 cm H ○ ○ ○ ○ ○
1200A-16R-1, 83-89 cm H ○ ○ ○ ○ ○ ○
1200A-16R-2, 5-10 cm H ○ ○ ○ ○ ○ ○ ○
1200A-17G-2, 8-14 cm H ○ ○ ○ ○ ○ ○ ○
1200B-1W-1, 49-55 cm H ○ ○ ○ ○ ○ ○ ○ ○
1200B-1W-1, 92-100 cm H or D ○ ○ ○ ○ ○ ○ ○ ○ ○
1200B-2W-1, 42-48 cm H ○ ○ ○ ○ ○ ○
1200B-2W-1, 110-115 cm H ○ ○ ○ ○ ○ ○ ○
#372 Eastern ridge of Big Blue Seamount
Table 1. Constituent minerals of representative samples of peridotite in the Mariana forearc
H; harzburgite, L; lherzolite, D; dunite, Ol; olivine, Spl; spinel, Opx; orthopyroxene; Cpx; clinopyroxene, m-Ol;
secondary olivine, m-Cpx; secondary clinopyroxene, Amp; amphibole, Phl; phlogopite, Chl; chlorite, Brc; brucite, Cal;
calcite, Ctl/Lz; chrysotile and/or lizardite, Atg; antigorite, Mag; magnetite
Mineral Ol Ol Ol Ol Ol Ol Ol Ol Ol Ol
SiO2 42.16 41.80 41.31 41.39 41.62 41.14 42.01 40.92 40.86 41.12
TiO2 0.08 0.00 0.05 0.10 0.19 0.00 0.06 0.00 0.00 0.00
Al2O3 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
FeO 8.36 8.39 8.32 8.44 8.43 8.07 8.35 8.48 8.58 8.36
MnO 0.07 0.16 0.14 0.22 0.16 0.23 0.03 0.21 0.08 0.11
MgO 51.09 50.81 49.55 50.40 50.51 49.75 50.41 49.82 49.14 49.61
CaO 0.03 0.00 0.00 0.00 0.01 0.05 0.01 0.05 0.00 0.02
Na2O 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.02 0.11
Cr2O3 0.00 0.03 0.02 0.02 0.00 0.00 0.00 0.00 0.00 0.08
NiO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
Total 101.79 101.19 99.39 100.57 100.92 99.24 100.87 99.48 98.68 99.41
O= 4 4 4 4 4 4 4 4 4 4
Si 1.006 1.004 1.010 1.001 1.003 1.006 1.011 1.001 1.008 1.006
Ti 0.001 0.000 0.001 0.002 0.003 0.000 0.001 0.000 0.000 0.000
Al 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000
Fe2+ 0.167 0.169 0.170 0.171 0.170 0.165 0.168 0.174 0.177 0.171
Mn 0.001 0.003 0.003 0.005 0.003 0.005 0.001 0.004 0.002 0.002
Mg 1.817 1.819 1.805 1.818 1.814 1.813 1.808 1.817 1.806 1.809
Ca 0.001 0.000 0.000 0.000 0.000 0.001 0.000 0.001 0.000 0.001
Na 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.001 0.005
Cr 0.000 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.002
V 0.000 0.000 0.000 0.000 0.000 0.002 0.000 0.000 0.000 0.000
Ni 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000
Total 2.993 2.996 2.989 2.997 2.994 2.991 2.988 2.998 2.993 2.996
Fo 91.6 91.5 91.4 91.4 91.4 91.7 91.5 91.3 91.1 91.4
1200B-2W-1, 110-115 cm 1200A-17G-2,
8-14 cm 1200A-13R-1, 24-28 cm Table 2A: Representative chemical compositions of olivine
South Chamorro Seamount 1200A16R-2,
5-10 cm
Mineral Ol Ol Ol Ol Ol Ol m-Ol m-Ol m-Ol m-Ol
SiO2 41.34 41.90 42.01 41.42 40.67 41.01 40.07 40.38 40.85 40.24
TiO2 0.00 0.03 0.00 0.00 0.08 0.04 0.12 0.00 0.09 0.01
Al2O3 0.00 0.00 0.00 0.00 0.00 0.06 0.03 0.00 0.00 0.01
FeO 7.37 7.30 7.87 8.01 8.86 9.04 12.64 11.23 11.12 12.36
MnO 0.00 0.17 0.01 0.16 0.13 0.31 0.08 0.15 0.22 0.43
MgO 50.81 51.57 51.04 50.97 49.40 49.18 46.21 46.97 47.02 46.48
CaO 0.00 0.01 0.00 0.03 0.00 0.00 0.10 0.00 0.00 0.00
Na2O 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
Cr2O3 0.00 0.00 0.00 0.00 0.00 0.00 0.10 0.28 0.00 0.00
NiO 0.00 0.00 0.33 0.51 0.23 0.12 0.11 0.41 0.25 0.07
Total 99.52 100.98 101.26 101.10 99.37 99.76 99.46 99.42 99.55 99.60
O= 4 4 4 4 4 4 4 4 4 4
Si 1.005 1.004 1.007 0.998 0.999 1.004 1.001 1.004 1.011 1.003
Ti 0.000 0.001 0.000 0.000 0.001 0.001 0.002 0.000 0.002 0.000
Al 0.000 0.000 0.000 0.000 0.000 0.002 0.001 0.000 0.000 0.000
Fe2+ 0.150 0.146 0.158 0.161 0.182 0.185 0.264 0.233 0.230 0.258
Mn 0.000 0.003 0.000 0.003 0.003 0.006 0.002 0.003 0.005 0.009
Mg 1.841 1.841 1.823 1.830 1.809 1.794 1.720 1.740 1.735 1.726
Ca 0.000 0.000 0.000 0.001 0.000 0.000 0.003 0.000 0.000 0.000
Na 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000
Cr 0.000 0.000 0.000 0.000 0.000 0.000 0.002 0.006 0.000 0.000
V 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000
Ni 0.000 0.000 0.006 0.010 0.005 0.002 0.002 0.008 0.005 0.001
Total 2.995 2.996 2.993 3.002 2.999 2.995 2.996 2.994 2.987 2.997
Fo 92.5 92.6 92.0 91.9 90.9 90.7 86.7 88.2 88.3 87.0
South Chamorro Seamount 1200A-7R-1,
10-17 cm 1200B-2W-1,
42-48 cm 1200B-1W-1, 92-100 cm
Mineral Ol Ol Ol m-Ol m-Ol m-Ol Ol Ol Ol Ol
SiO2 41.16 40.64 41.13 40.06 40.50 40.15 41.22 41.17 41.10 40.84
TiO2 0.00 0.04 0.00 0.00 0.00 0.11 0.14 0.01 0.00 0.00
Al2O3 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.03
FeO 8.53 9.13 9.14 12.79 11.39 12.21 7.81 8.23 8.57 8.15
MnO 0.23 0.39 0.08 0.13 0.19 0.43 0.11 0.04 0.05 0.12
MgO 50.28 48.77 49.53 46.58 46.95 46.29 50.11 50.54 49.31 50.20
CaO 0.07 0.13 0.01 0.07 0.13 0.11 0.00 0.00 0.00 0.08
Na2O 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
Cr2O3 0.06 0.00 0.13 0.15 0.07 0.07 0.00 0.00 0.00 0.00
NiO 0.57 0.53 0.31 0.00 0.07 0.13 0.42 0.53 0.35 0.55
Total 100.90 99.63 100.34 99.78 99.30 99.50 99.81 100.52 99.38 99.97
O= 4 4 4 4 4 4 4 4 4 4
Si 0.997 1.000 1.002 0.998 1.007 1.002 1.004 0.998 1.007 0.996
Ti 0.000 0.001 0.000 0.000 0.000 0.002 0.003 0.000 0.000 0.000
Al 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.001
Fe2+ 0.173 0.188 0.186 0.266 0.237 0.255 0.159 0.167 0.176 0.166
Mn 0.005 0.008 0.002 0.003 0.004 0.009 0.002 0.001 0.001 0.002
Mg 1.815 1.789 1.798 1.729 1.739 1.721 1.818 1.826 1.802 1.825
Ca 0.002 0.003 0.000 0.002 0.003 0.003 0.000 0.000 0.000 0.002
Na 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000
Cr 0.001 0.000 0.003 0.003 0.001 0.001 0.000 0.000 0.000 0.000
V 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000
Ni 0.011 0.010 0.006 0.000 0.001 0.003 0.008 0.010 0.007 0.011
Total 3.003 2.999 2.997 3.001 2.993 2.996 2.994 3.002 2.993 3.003
Fo 91.3 90.5 90.6 86.7 88.0 87.1 92.0 91.6 91.1 91.7
Pacman Seamount
779A-10R-1, 39-43 cm 369-1R-1 370-6R-1
Conical Seamount
372-1R-1 372-2R-2
Mineral Ol Ol Ol Ol Ol Ol Ol Ol Ol
SiO2 41.05 41.24 41.48 40.92 41.21 41.08 40.91 40.54 40.18
TiO2 0.03 0.02 0.00 0.02 0.00 0.08 0.00 0.00 0.00
Al2O3 0.05 0.00 0.01 0.00 0.00 0.02 0.07 0.00 0.00
FeO 9.24 8.92 7.81 7.75 7.84 8.35 8.06 10.12 8.12
MnO 0.05 0.00 0.00 0.15 0.17 0.10 0.05 0.11 0.23
MgO 50.15 50.86 50.51 50.37 50.11 50.31 49.58 48.17 49.53
CaO 0.00 0.02 0.00 0.09 0.07 0.00 0.02 0.06 0.00
Na2O 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
Cr2O3 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
NiO 0.91 0.53 0.44 0.54 0.42 0.00 0.59 0.16 0.52
Total 101.48 101.59 100.25 99.84 99.82 99.94 99.28 99.16 98.58
O= 4 4 4 4 4 4 4 4 4
Si 0.992 0.992 1.005 0.998 1.004 1.000 1.003 1.003 0.995
Ti 0.001 0.000 0.000 0.000 0.000 0.001 0.000 0.000 0.000
Al 0.001 0.000 0.000 0.000 0.000 0.001 0.002 0.000 0.000
Fe2+ 0.187 0.179 0.158 0.158 0.160 0.170 0.165 0.209 0.168
Mn 0.001 0.000 0.000 0.003 0.004 0.002 0.001 0.002 0.005
Mg 1.807 1.824 1.823 1.830 1.819 1.825 1.812 1.777 1.827
Ca 0.000 0.001 0.000 0.002 0.002 0.000 0.001 0.002 0.000
Na 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000
Cr 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000
V 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000
Ni 0.018 0.010 0.009 0.011 0.008 0.000 0.012 0.003 0.010
Total 3.006 3.007 2.995 3.002 2.996 2.999 2.996 2.997 3.005
Fo 90.6 91.0 92.0 92.1 91.9 91.5 91.6 89.5 91.6
Pacman Seamount Eastern ridge of
Big Blue Seamount
370-7R-1 371-5R-1 371-8R-2
Twin Pekas Seamount
Eastern ridge of Big Blue Seamount
372-2R-2 782-2R-2
Mineral Ol Ol Ol Ol Ol Ol Ol Ol Ol
SiO2 40.68 40.42 40.65 40.58 40.82 40.43 40.92 41.20 40.66
TiO2 0.02 0.00 0.06 0.00 0.18 0.00 0.05 0.00 0.00
Al2O3 0.00 0.04 0.00 0.00 0.00 0.00 0.05 0.00 0.00
FeO 8.37 8.45 8.17 8.31 7.62 8.17 8.68 8.36 8.79
MnO 0.24 0.00 0.08 0.13 0.02 0.03 0.07 0.12 0.10
MgO 49.60 49.89 49.22 49.32 50.66 49.76 50.26 50.08 49.99
CaO 0.04 0.02 0.00 0.04 0.00 0.00 0.03 0.00 0.03
Na2O 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
Cr2O3 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
NiO 0.49 0.64 0.33 0.56 0.66 0.67 0.39 0.55 0.75
Total 99.44 99.46 98.51 98.94 99.96 99.06 100.45 100.31 100.32
O= 4 4 4 4 4 4 4 4 4
Si 0.999 0.993 1.004 1.001 0.994 0.996 0.995 1.001 0.992
Ti 0.000 0.000 0.001 0.000 0.003 0.000 0.001 0.000 0.000
Al 0.000 0.001 0.000 0.000 0.000 0.000 0.001 0.000 0.000
Fe2+ 0.172 0.174 0.169 0.171 0.155 0.168 0.176 0.170 0.179
Mn 0.005 0.000 0.002 0.003 0.000 0.001 0.001 0.002 0.002
Mg 1.815 1.826 1.812 1.813 1.838 1.827 1.821 1.814 1.818
Ca 0.001 0.001 0.000 0.001 0.000 0.000 0.001 0.000 0.001
Na 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000
Cr 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000
V 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000
Ni 0.010 0.013 0.007 0.011 0.013 0.013 0.008 0.011 0.015
Total 3.001 3.007 2.995 2.999 3.003 3.004 3.004 2.999 3.008
Fo 91.4 91.3 91.5 91.4 92.2 91.6 91.2 91.4 91.0
373-4R-2
Staircase Plateau Celestial Seamount
782-2R-4 782-2R-3
Mineral Ol Ol Ol Ol m-Ol m-Ol Ol Ol
SiO2 40.47 41.07 41.13 40.74 40.52 40.14 40.82 40.46
TiO2 0.02 0.04 0.09 0.00 0.00 0.10 0.19 0.07
Al2O3 0.04 0.00 0.00 0.00 0.00 0.01 0.00 0.00
FeO 8.95 9.04 9.93 9.37 10.54 10.97 7.78 8.31
MnO 0.06 0.17 0.11 0.05 0.29 0.20 0.15 0.14
MgO 49.94 50.74 49.25 48.70 47.41 47.61 50.00 49.48
CaO 0.00 0.00 0.02 0.00 0.00 0.06 0.00 0.02
Na2O 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
Cr2O3 0.00 0.00 0.00 0.00 0.07 0.00 0.00 0.00
NiO 0.25 0.46 0.41 0.68 0.67 0.34 0.61 0.30
Total 99.73 101.52 100.94 99.54 99.50 99.43 99.55 98.78
O= 4 4 4 4 4 4 4 4
Si 0.992 0.990 1.000 1.003 1.004 0.997 0.998 0.998
Ti 0.000 0.001 0.002 0.000 0.000 0.002 0.004 0.001
Al 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.000
Fe2+ 0.183 0.182 0.202 0.193 0.218 0.228 0.159 0.171
Mn 0.001 0.003 0.002 0.001 0.006 0.004 0.003 0.003
Mg 1.824 1.823 1.784 1.787 1.751 1.762 1.822 1.820
Ca 0.000 0.000 0.001 0.000 0.000 0.002 0.000 0.001
Na 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000
Cr 0.000 0.000 0.000 0.000 0.001 0.000 0.000 0.000
V 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000
Ni 0.005 0.009 0.008 0.013 0.013 0.007 0.012 0.006
Total 3.007 3.009 2.999 2.997 2.995 3.001 2.998 3.000
Fo 90.9 90.9 89.8 90.3 88.9 88.6 92.0 91.4
784-4R-2 Paman Seamount
783-4R-1 783-4R-2
Big Blue Seamount
1 2 3 4 5 6
SiO2 40.97 40.99 40.44 39.88 41.02 40.71
TiO2 0.00 0.18 0.00 0.06 0.03 0.00
Al2O3 0.00 0.10 0.00 0.00 0.10 0.02
Fe2O3 0.00 0.00 0.00 0.00 0.00 0.00
FeO 8.72 8.49 11.96 12.15 9.41 9.32
MnO 0.14 0.11 0.13 0.37 0.19 0.21
MgO 49.54 49.45 46.43 46.44 48.82 49.31
CaO 0.00 0.00 0.02 0.01 0.03 0.00
Cr2O3 0.02 0.00 0.02 0.00 0.00 0.00
NiO 0.18 0.43 0.41 0.63 0.27 0.48
Total 99.57 99.75 99.41 99.55 99.87 100.06
O= 4 4 4 4 4 4
Si 1.003 1.002 1.007 0.997 1.005 0.997
Ti 0.000 0.003 0.000 0.001 0.001 0.000
Al 0.000 0.003 0.000 0.000 0.003 0.001
Fe3+ 0.000 0.000 0.000 0.000 0.000 0.000
Fe2+ 0.179 0.173 0.249 0.254 0.193 0.191
Mn 0.003 0.002 0.003 0.008 0.004 0.004
Mg 1.808 1.801 1.724 1.730 1.782 1.800
Ca 0.000 0.000 0.001 0.000 0.001 0.000
Cr 0.000 0.000 0.000 0.000 0.000 0.000
Ni 0.004 0.008 0.008 0.013 0.005 0.009
Total 2.996 2.993 2.992 3.002 2.993 3.003
Fo 91.01 91.21 87.37 87.20 90.24 90.41
Conical Seamount (779A-10R-1, 39-43 cm) Table 2B: Representative chemical compositions of olivine
1 and 2, olivines without iron-rich stripe patterns.
3 and 4, iron-rich parts of striped olivine.
5 and 6, iron-poor parts intervened between iron-rich stripe parts.
Mineral Spl Spl Spl Spl Spl Spl Spl Spl Spl Spl
SiO2 0.15 0.10 0.16 0.00 0.14 0.16 0.12 0.13 0.17 0.17
TiO2 0.02 0.07 0.00 0.00 0.10 0.04 0.00 0.15 0.16 0.14
Al2O3 20.95 20.18 24.03 21.47 23.06 15.75 16.19 22.90 21.80 22.92
Fe2O3 2.94 3.24 3.04 2.63 2.26 0.48 1.17 0.11 0.00 0.00
FeO 17.23 18.16 18.13 17.69 17.94 18.90 19.71 18.81 16.78 17.19
MnO 0.41 0.47 0.46 0.23 0.40 0.72 0.72 0.33 0.59 0.34
MgO 11.67 10.88 11.25 11.70 11.23 10.09 9.68 11.37 12.11 11.63
CaO 0.02 0.08 0.01 0.06 0.05 0.06 0.00 0.07 0.04 0.06
Na2O 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
Cr2O3 46.52 46.72 42.27 46.51 43.32 53.54 52.37 46.10 47.95 46.82
NiO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
Total 99.90 99.89 99.35 100.28 98.50 99.74 99.97 99.97 99.60 99.27
O= 4 4 4 4 4 4 4 4 4 4
Si 0.005 0.003 0.005 0.000 0.004 0.005 0.004 0.004 0.005 0.005
Ti 0.000 0.002 0.000 0.000 0.002 0.001 0.000 0.003 0.004 0.003
Al 0.771 0.750 0.880 0.785 0.854 0.598 0.614 0.835 0.796 0.839
Fe3+ 0.069 0.077 0.071 0.061 0.053 0.012 0.028 0.003 0.000 0.000
Fe2+ 0.450 0.478 0.471 0.459 0.471 0.509 0.531 0.486 0.435 0.446
Mn 0.011 0.013 0.012 0.006 0.011 0.020 0.020 0.009 0.015 0.009
Mg 0.544 0.511 0.521 0.541 0.526 0.485 0.465 0.524 0.559 0.538
Ca 0.001 0.003 0.000 0.002 0.002 0.002 0.000 0.002 0.001 0.002
Na 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000
Cr 1.149 1.164 1.039 1.141 1.076 1.364 1.333 1.127 1.175 1.149
Ni 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000
Total 3.000 3.000 3.000 2.996 2.999 2.995 2.994 2.993 2.992 2.991
Mg# 54.7 51.6 52.5 54.1 52.7 48.8 46.7 51.9 56.3 54.7
Cr# 59.8 60.8 54.1 59.2 55.8 69.5 68.5 57.5 59.6 57.8
1200A-17G-2, 8-14 cm
1200A16R-2, 5-10 cm 1200B-1W-1,
49-55 cm South Chamorro Seamount
Table 3: Representative chemical compositions of spinel
1200B-2W-1, 110-115 cm
Twin Peaks Seamount
371-5R-1
Mineral Spl Spl Spl Spl Spl Spl Spl Spl Spl
SiO2 0.06 0.13 0.10 0.00 0.14 0.05 0.06 0.10 0.10
TiO2 0.00 0.00 0.00 0.05 0.11 0.14 0.21 0.07 0.19
Al2O3 28.99 22.59 28.60 27.55 14.71 15.59 15.62 14.90 29.17
Fe2O3 1.89 1.03 0.58 1.46 1.95 0.11 1.32 0.87 1.33
FeO 18.15 17.71 16.53 16.11 17.63 20.11 18.22 19.34 16.40
MnO 0.32 0.51 1.20 0.70 0.14 0.29 0.53 0.52 0.75
MgO 12.20 11.95 12.34 12.38 9.28 9.22 10.10 9.16 12.61
CaO 0.07 0.04 0.04 0.04 0.00 0.00 0.07 0.09 0.01
Na2O 0.00 0.00 0.00 0.00 0.58 0.03 0.00 0.00 0.00
Cr2O3 39.03 46.77 40.48 40.19 54.10 52.89 52.32 53.13 38.04
NiO 0.00 0.00 0.00 0.23 0.30 0.00 0.00 0.07 0.07
Total 100.71 100.73 99.87 98.71 98.94 98.43 98.45 98.25 98.67
O= 4 4 4 4 4 4 4 4 4
Si 0.002 0.004 0.003 0.000 0.005 0.002 0.002 0.003 0.003
Ti 0.000 0.000 0.000 0.001 0.003 0.003 0.005 0.002 0.004
Al 1.022 0.817 1.016 0.993 0.565 0.603 0.602 0.580 1.043
Fe3+ 0.042 0.024 0.013 0.033 0.048 0.003 0.032 0.022 0.030
Fe2+ 0.454 0.454 0.417 0.412 0.481 0.552 0.498 0.534 0.416
Mn 0.008 0.013 0.031 0.018 0.004 0.008 0.015 0.015 0.019
Mg 0.544 0.546 0.554 0.564 0.451 0.451 0.492 0.451 0.570
Ca 0.002 0.001 0.001 0.001 0.000 0.000 0.002 0.003 0.000
Na 0.000 0.000 0.000 0.000 0.037 0.002 0.000 0.000 0.000
Cr 0.923 1.134 0.965 0.971 1.395 1.372 1.352 1.388 0.912
Ni 0.000 0.000 0.000 0.006 0.008 0.000 0.000 0.002 0.002
Total 2.997 2.994 3.000 3.000 2.995 2.995 3.000 3.000 3.000
Mg# 54.5 54.6 57.1 57.8 48.4 45.0 49.7 45.8 57.8
Cr# 47.5 58.1 48.7 49.5 71.2 69.5 69.2 70.5 46.7
Conical Seamount 1200A-7R-2,
103-109 cm
1200B-1W-1,
92-100 cm 779A-19R-2, 370-4R-3
105-108 cm
Pacman Seamount South Chamorro Seamount
Turquoise Seamount
371-5R-1 371-8R-2 374-3R-4
Mineral Spl Spl Spl Spl Spl Spl Spl Spl Spl
SiO2 0.01 0.00 0.12 0.21 0.14 0.11 0.13 0.00 0.00
TiO2 0.04 0.01 0.00 0.00 0.00 0.01 0.07 0.04 0.00
Al2O3 23.97 20.49 4.59 4.31 18.17 18.13 16.77 17.74 41.35
Fe2O3 1.30 3.53 0.00 0.00 0.00 0.35 0.00 0.00 2.78
FeO 15.55 18.26 22.45 20.77 18.33 18.20 16.83 17.15 12.58
MnO 0.58 0.77 0.76 0.53 0.53 0.84 0.71 0.63 0.20
MgO 12.43 10.25 5.72 6.84 10.08 10.16 11.05 10.74 16.52
CaO 0.14 0.01 0.02 0.02 0.00 0.02 0.17 0.03 0.01
Na2O 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
Cr2O3 44.21 45.17 64.78 65.60 51.24 50.71 53.87 52.63 25.30
NiO 0.00 0.08 0.17 0.32 0.00 0.00 0.00 0.11 0.11
Total 98.23 98.57 98.61 98.60 98.49 98.54 99.60 99.07 98.85
O= 4 4 4 4 4 4 4 4 4
Si 0.000 0.000 0.004 0.007 0.005 0.004 0.004 0.000 0.000
Ti 0.001 0.000 0.000 0.000 0.000 0.000 0.002 0.001 0.000
Al 0.879 0.772 0.191 0.178 0.691 0.690 0.631 0.670 1.376
Fe3+ 0.030 0.085 0.000 0.000 0.000 0.009 0.000 0.000 0.059
Fe2+ 0.405 0.488 0.663 0.609 0.495 0.491 0.450 0.460 0.297
Mn 0.015 0.021 0.023 0.016 0.014 0.023 0.019 0.017 0.005
Mg 0.577 0.489 0.301 0.357 0.485 0.489 0.526 0.513 0.695
Ca 0.005 0.000 0.001 0.001 0.000 0.001 0.006 0.001 0.000
Na 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000
Cr 1.088 1.142 1.808 1.818 1.307 1.294 1.360 1.333 0.565
Ni 0.000 0.002 0.005 0.009 0.000 0.000 0.000 0.003 0.002
Total 3.000 3.000 2.996 2.995 2.996 3.000 2.998 2.997 3.000
Mg# 58.8 50.0 31.2 37.0 49.5 49.9 53.9 52.7 70.1
Cr# 55.3 59.7 90.4 91.1 65.4 65.2 68.3 66.6 29.1
Twin Peaks Seamount
373-12R-2_1
Stairacase Plateau 372-2R-1
Eastern ridge of Big Blue Seamount
373-13R-1
Big Blue Seamount
783-1R-1
Mineral Spl Spl Spl Spl Spl Spl Spl Spl Spl
SiO2 0.00 0.08 0.00 0.03 0.10 0.04 0.05 0.02 0.08
TiO2 0.08 0.12 0.02 0.16 0.09 0.00 0.00 0.00 0.05
Al2O3 42.01 40.73 40.55 44.74 42.42 22.21 20.72 20.62 25.58
Fe2O3 2.75 2.75 2.22 2.21 2.48 1.00 0.43 1.59 0.83
FeO 12.22 12.73 12.01 12.58 12.79 17.30 15.94 14.51 16.13
MnO 0.27 0.22 0.53 0.27 0.28 0.58 0.63 0.94 0.41
MgO 16.63 16.50 16.50 17.27 16.80 10.85 12.08 12.37 12.60
CaO 0.08 0.17 0.10 0.00 0.08 0.16 0.04 0.08 0.17
Na2O 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
Cr2O3 24.01 25.97 26.66 23.45 24.78 45.44 49.42 47.44 43.84
NiO 0.10 0.04 0.13 0.35 0.00 0.26 0.12 0.11 0.19
Total 98.16 99.32 98.72 101.06 99.83 97.84 99.42 97.68 99.88
O= 4 4 4 4 4 4 4 4 4
Si 0.000 0.002 0.000 0.001 0.003 0.001 0.002 0.001 0.002
Ti 0.002 0.003 0.000 0.003 0.002 0.000 0.000 0.000 0.001
Al 1.401 1.353 1.354 1.440 1.393 0.832 0.764 0.771 0.918
Fe3+ 0.059 0.058 0.047 0.046 0.052 0.024 0.010 0.038 0.019
Fe2+ 0.289 0.300 0.285 0.287 0.298 0.460 0.417 0.385 0.411
Mn 0.006 0.005 0.013 0.006 0.007 0.016 0.017 0.025 0.011
Mg 0.701 0.693 0.697 0.703 0.698 0.514 0.563 0.585 0.572
Ca 0.002 0.005 0.003 0.000 0.002 0.005 0.001 0.003 0.006
Na 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000
Cr 0.537 0.579 0.597 0.506 0.546 1.142 1.223 1.190 1.056
Ni 0.002 0.001 0.003 0.008 0.000 0.007 0.003 0.003 0.005
Total 3.000 3.000 3.000 3.000 3.000 3.000 3.000 3.000 3.000
Mg# 70.8 69.8 71.0 71.0 70.1 52.8 57.5 60.3 58.2
Cr# 27.7 30.0 30.6 26.0 28.2 57.8 61.5 60.7 53.5
374-3R-4
Turquoise Seamount North Chamorro Seamount Celestial Seamount
375-3R-3 782-2R-4