Petrology of serpentinized peridotites in the
Mariana forearc, western Pacific
著者
Murata Keiko
内容記述
学位授与大学: Osaka Prefecture University(大阪
府立大学), 学位の種類: 博士(理学), 学位記番号:
論理第81号, 学位授与年月日: 2009-09-30, 指導教
員: 前川 寛和
Petrology of serpentinized peridotites in the Mariana forearc,
western Pacific
Keiko Murata
Osaka Prefecture University
2009
Abstract
In the Mariana forearc, horst and graben structures are well developed in the outer forearc basement, which is composed of both island arc and oceanic crust-mantle rocks. A zone of dome-shaped diapiric seamounts, which are composed mainly of serpentinized peridotites, formed on the basement in the outer forearc regions. Serpentine minerals in peridotites from both diapiric seamounts and basement are mostly chrysotile and/or lizardite. Antigorite, however, is rarely found in peridotites recovered from Conical, Big Blue, Celestial, and South Chamorro Seamounts. Antigorite-bearing peridotites recovered from Conical, Big Blue, South Chamorro Seamounts contain secondary iron-rich olivine and metamorphic clinopyroxene (diopside), and antigorite seems to coexist stably with them. The stable association of antigorite, diopside and olivine in these peridotites indicates that peridotites underwent serpentinization at approximately 450-550 °C.
Iron-rich secondary olivine (Fo86-90) occurs as overgrowth on the rim or along the cleavage traces of
primary olivine (Fo90-92). Olivine with conspicuous iron-rich stripe patterns are found in
serpentinized peridotites from Conical and South Chamorro Seamounts. The iron-rich stripe patterns (Fo86-89) are formed in the olivine crystal (Fo90-92) as a parallel alignment of narrow straight parts of widths ranging from 0.5 to 2.0 μm. The iron-rich stripe patterns are well developed near the rim of the host olivine where fiber crystals of antigorite are pierced into olivine. These patterns are not found in the inside of olivine grain except in the periphery of cracks. Generally, olivine is highly deformed and has well-developed cleavages in (010), (100), and (001) directions, and the stripe is commonly parallel to (100). Modes of occurrence of iron-rich stripe patterns in olivine suggest that the infiltration of iron-rich fluid along the cleavage trace or the subgrain boundary formed by dislocations is probably responsible for the formation of the iron-rich stripe patterns. The iron-poor
parts intervened between the iron-rich parts are slightly lower in XMg [= Mg/(Mg+Fe2+)] than the
inside of olivine grain that is homogeneous in composition and lacks iron-rich stripe patterns, suggesting that metasomatic alteration also occurred in iron-poor parts.
Iron-rich olivine is only found in antigorite-bearing peridotites, and always occurs in intimate
association with antigorite. Antigorite formation causes extra iron component because XMg of
antigorite (= 0.94-0.97) is higher than that of host olivine. Therefore, the iron-rich fluid may have been produced by serpentinization and infiltrated through olivine crystal to form iron-rich stripe patterns. It, however, still remain a possibility that the iron-rich olivine had formed before
serpentinization by fluid that had initially invaded from outside under the high temperature mantle conditions and thereafter caused serpentinization.
Less altered peridotite samples well containing primary olivine and spinel in the Mariana forearc, Mid-Atlantic Ridge, Hess Deep and Tonga forearc were examined to clarify thermal history using the olivine-spinel geospeedometry. The cooling history of peridotites from Mariana forearc
was estimated to 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 results are different from
those of the Mariana peridotites in that both initial and latest temperatures of peridotites in these ridges are slightly higher than those in the Mariana forearc. It is sure that these cooling rates do not show the subduction process seen in Mariana peridotites. Therefore, these cooling rates may reflect the regression process from hot ridge axis or uplift process within upper mantle.
Contents
1. Introduction --- 1
2. Geologic outline of western Pacific region --- 3
3. Dome-shaped serpentinite seamounts --- 4
4. Petrographical characteristics of serpentinized peridotites --- 6
5. Constituent minerals of serpentinized peridotites --- 7
Analytical procedures --- 7 5-1. Olivine --- 8 5-2. Spinel --- 8 5-3. Pyroxene --- 9 5-3-1. Orthopyroxene --- 9 5-3-2. Clinopyroxene --- 10 5-4. Serpentine minerals --- 10 5-5. Amphibole --- 11 5-6. Brucite --- 11 5-7. Phlogopite --- 12 5-8. Chlorite --- 12 5-9. Other minerals --- 12 6. Discussions --- 13 6-1. Serpentinization --- 13
Origin of iron-rich olivine --- 13
Metamorphic conditions --- 15
6-2. Cooling history of peridotites --- 16
7. Conclusions --- 22
Figures --- 24
Tables --- 57
Acknowledgements --- 76
List of figures
Fig. 1. Regional map of the western Pacific, showing the location of Izu-Bonin-Mariana forearc. 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).
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).
Fig. 4. Bulk rock compositions of peridotites from South Chamorro Seamount on the CaO vs.
100Mg/(Mg+Fe2+) and CaO vs. Al
2O3 diagrams. Data source for degree of melting is
Ishiwatari (1985). The data from Conical Seamount is after Ishii et al. (1992).
Fig. 5. Relict olivine crystals in peridotite from Big Blue Seamount (783-4R-1). (A) Plane polarized light. (B) Crossed polars.
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.
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 of primary olivine in peridotite from South Chamorro Seamount (1200B-1W-1, 92-100 cm).
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.
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. 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.
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).
Fig. 13. A Backscattered electron image and line scan of spinel in peridotite from Twin Peaks Seamount (371-5R-1).
Fig. 14. Cr-Al-Fe3+ plots of analyzed spinels in the peridotites from dome-shaped seamounts (A)
and horst blocks (B) on the Mariana forearc. A number at the lower left is the number of the samples.
Fig. 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).
Fig. 16. Deformed orthopyroxene crystal in peridotite from South Chamorro Seamount (1200A-7R-2, 103-109 cm). (A) Plane polarized light. (B) Crossed polars.
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.
Fig. 19. CaO vs. Al2O3 diagram of clinopyroxenes. Secondary metamorphic clinopyroxene is
clearly poor in Al2O3 in comparison with primary one.
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.
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.
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.
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 calculated on the basis of 23 oxygen atoms. (B) Al vs. Mg/(Mg+Fe2+) in
amphiboles from peridotites.
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.
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).
Fig. 26. Chlorite around spinel in peridotite from South Chamorro Seamount (1200B-1W-1, 92-100 cm). Plane polarized light.
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).
by crystallization of feather-like antigorite crystals.
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.
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.
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.
Fig. 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.
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).
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.
for the peridotites from fault scarps around horst blocks in the Mariana forearc. (A) Staircase Plateau, (B) Eastern ridge of Big Blue Seamount.
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.
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.
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.
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.
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.
Fig. 41. A tectonic model explaining the cooling history of peridotites from dome-shaped seamounts and basement.
List of tables
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
Table 2A. Representative chemical compositions of olivine Table 2B. Representative chemical compositions of olivine Table 3. Representative chemical compositions of spinel
Table 4. Representative chemical compositions of orthopyroxene Table 5. Representative chemical compositions of clinopyroxene Table 6. Representative chemical compositions of serpentine minerals Table 7. Representative chemical compositions of amphiboles
Table 8. Representative chemical compositions of brucite Table 9. Representative chemical compositions of phlogopite Table 10. Representative chemical compositions of chlorite
1. Introduction
Serpentinized peridotites are often found along the trench-landward slope of the western Pacific margin. Fisher and Engel (1969) first reported serpentinized peridotites from the landward slope of Tonga Trench, and after the second half of the 1970s, it was shown clearly that serpentinized peridotites and related ophiolitic rocks are widely exposed on the trench landward slope of the Izu-Bonin and Mariana forearc regions (e.g., Honza and Kagami, 1977; IGCP Working Group, 1977). Bloomer (1983) reported that the landward slope of Mariana Trench is composed largely of igneous rocks, and explained that these rocks were probably the initial products of arc volcanism caused by the subduction of the Pacific Plate beneath the Philippine Sea Plate; their exposures on the trench slope today imply a significant amount of tectonic erosion of the landward slope since Eocene time. Fryer et al. (1985) showed that the zone of seamounts which mainly consist of serpentinized peridotites is distributed in parallel to the trench axis between the trench axis and the volcanic front (= forearc) in the Mariana region. Furthermore, based on the morphological features, they interpreted that each dome-shaped seamount was diapirically formed on the forearc basement where horst and graben structures dominate. They distinguished the dome-shaped seamounts from the horst blocks that are surrounded by normal or strike-slip faults. ODP Leg 125 (1989) drilled two dome-shaped seamounts, Conical Seamount in the Mariana forearc and Torishima Forearc Seamount in the Izu-Bonin forearc, and recovered serpentinized peridotites, metamorphosed basalts and dolerites. Ishii et al. (1992) studied serpentinized peridotites recovered from these seamounts, and concluded that the peridotites were residues of extensive partial melting (= 30 %), the last episode of which occurred in the mantle wedge, probably associated with the generation of incipient island arc magma, including boninite and/or arc-tholeiite sources. On the other hand, Parkinson and Pearce (1998) examined geochemical nature of the peridotites, and indicated that the forearc is underlain by two types of mantle lithosphere, one being trapped or accreted oceanic lithosphere, the other being lithosphere formed by subduction-related melting. The blueschist-facies rock clasts found from the cores recovered at the southern flank of Conical Seamount suggest a blueschist-facies metamorphism beneath the forearc (Maekawa et al., 1992, 1993). An analysis of the fluids seeping from the chimneys at the summit of Conical Seamount suggests that the fluids were derived from the dehydration processes of descending oceanic slab (Fryer et al., 1990; Mottl, 1992).
The summit of South Chamorro Seamount was drilled during ODP Leg 195 (Shipboard Scientific Party, 2002). D’Antonio and Kristensen (2004) studied the serpentinized peridotites recovered from Hole 1200A and demonstrated that serpentine + brucite paragenesis characterizes the serpentinization of peridotites at South Chamorro Seamount, and estimated the upper temperature limit for serpentinization of 200-300 °C. Sand-sized fragments of blue amphibole-rich schists and tremolite-chlorite schists were found from the recovered cores at South Chamorro Seamount during ODP Leg 195 (Shipboard Scientific Party, 2002). Maekawa et al. (2004) pointed out that these fragments are metasomatic products formed between wedge mantle serpentinites and pelagic sediments on top of the subducting plate along the subduction boundary. The analysis of inner structures of serpentinite seamounts using multichannel seismic and bathymetric data suggest that each of serpentinite seamounts formed by episodic eruptions of mudflows from a central edifice (Oakley et al., 2007).
Recent seismological researches have given evidence of widespread serpentinization of the mantle wedge. Based on the seismic reflection-refraction study, Takahashi et al. (1998, 2007) showed that the upper mantle velocity beneath the forearc gradually decreases toward the trench axis to become indiscernible from the velocity of the lower crust in the Izu-Bonin and Mariana subduction systems. Unusually low velocity (= 7.1 km/s) of the upper mantle beneath the east side of the forearc suggests that a large amount of water is carried down and released by subduction for serpentinization of the mantle peridotites. They demonstrated that the root of the serpentinite diapir on the inner trench wall is a low-velocity mantle wedge that was probably caused by a large amount of water released from the subducting Pacific Plate at depths shallower than 30 km. Shimamoto (1985) and Shimamoto et al. (1993) discussed the seismicity and deformation mechanisms in subduction zones and divided a subduction plate boundary into three zones: shallow, intermediate, and deep interfaces. He ascribed the aseismic and decoupled natures of the shallow interface to the existence of enormous amount of water. The shallow interface may well correspond to the low-velocity mantle wedge. The low velocity wedge at the western side of the trench may indicate the path of serpentinite diapir (Takahashi et al., 1998). The low electric resistance at the boundary between the arc and subducting plate is due to high water content of the region, which corresponds to the low-velocity mantle wedge (Toh, 1993).
mantle wedge due to water supplied from the sediments on top of the subducting Pacific Plate below the Izu-Bonin and Mariana forearcs, and that the resultant voluminous and gravitationally unstable serpentinized materials have diapirically risen up from wedge mantle, and have gone through forearc crust to form dome-shaped seamounts on the seafloor. Although the constituent rocks of dome-shaped seamounts are very important to understand geotectonic evidence and physicochemical conditions in mantle wedge, there are only few petrological and mineralogical studies for these rocks mainly due to the difficulty of getting rock samples.
The R/V Kairei KR06-15 cruise of Japan Agency for Marine-Earth Science and Technology (JAMSTEC), intensive petrological investigations of dome-shaped seamounts were carried out, and obtained abundant rock samples (Maekawa et al., 2007a). In this thesis, I present the results of the petrological and mineralogical study of serpentinized peridotites obtained from the Mariana forearc mainly during the R/V Kairei KR06-15 cruise (2006), and partly during ODP Leg 125 (1989), ODP Leg 195 (2001), and the R/V Yokosuka YK03-07 cruise (2003) of JAMSTEC. This study deals with peridotites from eleven sites, which cover almost whole area in the southern Mariana forearc. Furthermore, I discuss nature of serpentinization and thermal history of the Mariana forearc peridotites comprehensively.
2. Geologic outline of western Pacific region
In the western Pacific region, the major topographic constituents of the Mariana area from east to west are the Mariana Trench, the Mariana Ridge (volcanic arc), the Mariana Trough, the West Mariana Ridge (remnant arc), and the Parece Vela Basin (Fig. 1). The Mariana Trench marks the eastern boundary of the Philippine Sea Plate, which comprises a series of basins and ridges. The western half of the Philippine Sea Plate is occupied by the West Philippine Basin. The Kyushu-Palau Ridge and the West Mariana Ridge are remnant arcs which are separated by the Parece Vela Basin. The Mariana Trough has been spreading since 6 Ma to separate West Mariana Ridge (remnant arc) from Mariana Ridge which corresponds to still active volcanic front.
Since Eocene time, the Pacific Plate has been subducting along the Mariana Trench below the Philippine Sea Plate (Uyeda and Ben-Avraham, 1972). At present, the Mariana Trench is the deepest trench in the world, and has an average depth of 6,000 m. In the southern Mariana area, the trench greatly inflected to the west, and seems to cross Mariana Ridge, Mariana Trough and West
Mariana Ridge. The distance from the trench to the volcanic front (Mariana Ridge) is about 200 km in the middle of the Mariana area. The Mariana forearc occupies a region between the trench and the active volcanic arc (= Mariana Ridge).
In the Mariana forearc, horst and graben structures are well developed in the outer forearc basement, the western limit of which is 60-90 km from the trench. The outer forearc is composed of both island arc and oceanic crust-mantle rocks (Johnson et al., 1991). At the initiation of subduction of the Pacific Plate, a part of oceanic plate must have been trapped in the region from the trench to initial volcanic front. Bloomer (1983), however, found that island arc volcanics are exposed throughout the outer forearc, even just west of the trench, and explained the reason that the trapped oceanic crust has been tectonically eroded by the subducting slab and dragged down into depths. Johnson and Fryer (1990) and Johnson et al. (1991) reported that, in addition to the island arc tholeiite and boninite, the Cretaceous radioralian cherts and basalts of mid-oceanic ridge origin are recognized in the outer Mariana forearc. Johnson et al. (1991) showed the complex mixed occurrences of old oceanic crust and younger arc rocks, and suggested that the fragments of Cretaceous oceanic materials accreted to the Mariana forearc have been extensively faulted and intruded by island arc tholeiite and boninite magmas.
3. Dome-shaped serpentinite seamounts
In the outer Mariana forearc, there are many seamounts composed mainly of serpentinized peridotites. The seamounts have commonly smooth-sided dome-like shape, and are up to 30 km in diameter with up to 2 km of relief. Geological, geophysical, and geochemical features are best investigated in Conical Seamount, which was drilled during ODP Leg 125. Conical Seamount is one of the youngest forearc seamounts in the outer Mariana forearc, and covers an area of
approximately 700 km2. It shows long sinuous flow features on its flanks on side scan image (Fryer
et al., 1995). Alvin submersible dives on the flank of Conical Seamount revealed that serpentinite blocks of varying sizes are scattered in a fractured and crushed serpentinite mud matrix on the surface of the seamounts. Drilled samples of serpentinized peridotites from ODP Leg 125 are highly sheared, and show "block-in-matrix" fabrics, which are typical in on-land serpentinite melanges. Similar to the dredged samples from the Torishima Forearc Seamount in the Izu-Bonin forearc area described by Maekawa (1995), each block is characteristically surrounded by several slickensided
flat surfaces, their edges and corners being more or less rounded. Chimneys composed of carbonate and silicate minerals with up to 1.5 m high are formed on the southwest side of the summit (Fryer et al., 1990). The fluids actively seeping from the chimneys and pore fluids from the summit drill samples are characterized by high pH, more than one-half deficiency of Cl and Br relative to seawater, and contain methane, ethane, propane, acetate and organic acids (Haggerty and Fisher, 1992; Mottl, 1992). They have probably the highest pH value ever measured in deep-sea sediment, up to 12.6, and have lower chlorinity than at any other site drilled in a subduction zone, up to 57% lower than that in seawater (Mottl, 1992). The presence of aromatic compounds in the chimneys and of ethane and propane along with methane in the pore waters indicates a thermogenic origin for the organic compounds. The presence of acetate ions limits the temperature range in the source region of the organics to less than 150˚C. Taking account into the tectonic environment of the Mariana subduction system, the most likely source of sediments is that at the top of the subducting slab about 30 km below Conical Seamount (Mottl, 1992). The forearcs were intensively destroyed by many normal faults caused by the tensional stress in the west of the trenches in the Izu-Bonin and Mariana regions (Fryer et al., 1985). Conical Seamount lies at the apex of the large forearc graben along at least two major forearc fault zones (Fryer et al., 1995).The voluminous and low-density serpentinites generated just above the subducting slab have probably uplifted along the normal faults to form a huge chain of seamounts on the ocean floor. Sediments recovered during drilling at the flank of the seamount contain Pleistocene nannofossils (<1.6 Ma) (Ciampo, 1992), suggesting the recent uplift of serpentinite materials to the seafloor. As demonstrated before, the fluids (which converted peridotite to serpentinite) are still actively upwelling from the chimneys at its summit. According to Fryer et al. (1992), Newson (1992) carried out gravity study in his master
thesis and unusually low densities of 2.0 to 2.2 g/cm3 were obtained for Conical Seamount. It is
consistent with the idea that solid serpentinite fragments are supported by water-saturated low-density matrix. The seamount shows no coherent internal reflectors on seismic profiles (Fryer et al., 1990). Drilling records from Conical Seamount during ODP Leg 125 indicate that drilling rate and degree of serpentinization of peridotites lessen gradually with the drilling depth; that is, higher density and larger blocks tend to be accumulated at the lower portion (Maekawa et al., 2001, Fig. 2).
High-pressure blueschist-facies rocks were found from Conical Seamount (Maekawa et al., 1992, 1993, 1995), South Chamorro Seamount (Shipboard Scientific Party, 2001), and Twin Peaks Seamount (Maekawa et al., 2007b). This fact suggests that they were formed at depth after initial subduction and were then entrained by uprising fluidized serpentinite materials. The presence of high-pressure rocks in these seamounts confirms that blueschist-facies metamorphism took place beneath the Mariana forearc.
4. Petrographical characteristics of serpentinized peridotites
The Mariana forearc peridotites obtained during KR06-15, YK03-07, ODP Leg 125, and ODP Leg 195 were moderately to highly serpentinized; degrees of serpentinization are from 40 to 100 vol% (volume percent). The samples taken from sea bottom surfaces by ROV KAIKO 7000II and submersible vessel SHINKAI 6500 are weathered variously by seawater to commonly show brown to brownish-red color with the naked eye. There is a clear difference in constituent rock types between diapiric seamounts and fault scarps around horst blocks (Maekawa et al., 2007b). The diapiric seamounts tend to be rich in serpentinized peridotite in comparison with the horst blocks, that is, serpentinized peridotites occupy 60 % of the total number of recovered rocks, whereas in the fault scarps rock samples contain significant amounts of dolerites, basalts and gabbros, and serpentinized peridotites are less than 40 % (Fig. 2).
Some peridotites well preserve their primary mineralogy. Olivine is abundant in the majority of the samples. Primary olivine, orthopyroxene, and clinopyroxene have locally survived in the rock
samples. Primary chromian spinel is found in most rocks.Bastite after orthopyroxene is commonly
found. Modal analyses of peridotites indicate that harzburgite is predominant and dunite is subordinate. Constituent minerals of serpentinized peridotites are given in Table 1. Fig. 3 shows examples of modal analyses of constituent minerals of peridotites in South Chamorro Seamount and Conical Seamount. It is sometimes difficult to estimate the rock type from modal analyses in the case that the primary minerals were highly obliterated due to severe serpentinization. Primary spinel
chemistry, however, can help to estimate primary protolith (Ishiwatari, 1985). Al-rich features of
spinels in highly serpentinized peridotites from North Chamorro and Turquoise Seamounts suggests a possible lherzolite protoliths for these rocks.
melting experienced by ophiolitic residual peridotites. The degree of partial melting can be
estimated using bulk-rock chemistry in the Mg# [= 100Mg/(Mg+Fe2+)]-CaO wt% and Al
2O3-CaO
wt% diagrams (Fig. 4). Rocks type and whole rock compositions from Conical and South
Chamorro Seamounts indicate that these peridotites are the products of moderate degree of partial melting (about 30 %) (Fig. 4).
Serpentine minerals in peridotites from the Mariana forearc are commonly chrysotile and lizardite. Antigorite was also found in peridotites from Conical, Big Blue, Celestial, and South Chamorro Seamounts. Peridotites also contain magnetite, chlorite, amphiboles, brucite, calcite, secondary clinopyroxene, and phlogopite.
5. Constituent minerals of serpentinized peridotites
Analytical proceduresMinerals were analyzed with a JEOL JSM-840A scanning electron microscope equipped with an Oxford energy-dispersive analytical system (Link ISIS series L200I-S) at the Department of Physical Science, Osaka Prefecture University. Accelerating voltage and beam current were kept at 15.0 kV and 0.5 nA, respectively. Corrections were made using the ZAF method. About spinels, the trivalent iron was estimated from the stoichiometry. Mineral species identification for the serpentine mineral polymorphs was done by polarizing microscope and confirmed by an imaging-plate X-ray microdiffractometer with two-dimensional area detector using graphite-monochromatized Cu-Kα radiation, operating at 40 kV and 40 mA (IP-XRD, RINT RAPID, Rigaku Co.) at the Department of Physical Science, Osaka Prefecture University. Collimators of 0.1 and 0.3 mm diameters were used for analysis of selected area in thin section by CCD camera. Using the IP-XRD, serpentine polymorphs were easily identified by the diagnostic peaks at 2.525Å (2θ=35.6˚) for antigorite, 2.501Å (2θ = 35.9˚) for lizardite, and 2.519Å (2θ = 36.6˚) for chrysotile (Kohyama, 2007). Rigaku thermogravimetry-differential thermal analysis (TG-DTA) analyzer at Osaka Application Laboratory, Rigaku Co. Ltd., was also used for part of samples to identify the three serpentine polymorphs by the differences of dehydration temperatures, that is, in the case of 10 ˚C/hour increasing temperature ratio, they are about 600 ˚C for lizardite, 700 ˚C for chrysotile, and 750 ˚C for antigorite.
5-1. Olivine
Olivine is a major constituent mineral in peridotites. It occurs as 1.0-2.0 mm colorless grain (Fig. 5), and more or less deformed to show wavy extinction and kink bands. In most cases, olivine is hardly serpentinized, but some peridotites well retain primary olivines. An aggregate of fine-grained olivine of less than 30 μm is also found around porphyroclastic olivine and orthopyroxene (Fig. 6). It is thought to have been formed during mylonitization that promoted
serpentinization. Both types are chemically equivalent (Fo89-93) (Table 2), and are regarded as
primary ones.
In antigorite-bearing peridotites, olivines with perfect cleavages on the (100), (010), and (001) crystal planes (‘cleavable olivine’ of Hawkes, 1946) were recognized in peridotites from Conical,
Big Blue, Celestial, and South Chamorro Seamounts (Fig. 7). Iron-rich olivine (Fo86-90) is
sometimes found as an overgrowth or along cleavage trace of cleavable olivine from Conical, Big Blue and South Chamorro Seamounts (Murata et al., 2009a) (Fig. 8). As will be stated in detail in the later section, iron-rich olivines are thought to have formed secondarily during serpentinization. The iron-rich olivine is only found in the peridotites with cleavable olivine and is always accompanied with antigorite and acicular clinopyroxene (Figs. 9 and 10).
Olivine grains with conspicuous iron-rich stripe are found from the peridotites of 779A-19R-2, 105-108 cm, 779A-10R-1, 39-43 cm, and 779A-22R-2, 9-14 cm obtained from Conical Seamount during ODP Leg 125, and 1200B-1W-1, 92-100 cm from South Chamorro Seamount during ODP Leg 195 (Murata et al., 2009b) (Fig. 11). The modes of occurrence and origin will be given in the Chapter 6.
5-2. Spinel
Serpentinized peridotites from the Mariana forearc almost always contain chromian spinel, which is partly replaced by magnetite along the rim and cleavage trace during serpentinization (Fig. 12A). Spinels seem to be the most resistant to serpentinization and alteration. Euhedral to subhedral spinel is common in the dunite samples (Fig. 12B), suggesting that they may have crystallized from a melt or formed by melt-mantle interaction. In general, Cr, Al and Mg in spinel show different behaviors between fractional crystallization and partial melting. Spinels in peridotites from the horst blocks and dome-shaped seamounts have chemical zonings with Cr- and Fe-rich core and Al-
and Mg-rich rim (Fig. 13), but those with Cr-rich rim and Cr-poor core are sometimes found. Spinel chemistry is sensitive to temperature and bulk-rock composition, and can be used as a petrogenetic indicator in peridotites (Dick and Bullen, 1984). Dick and Bullen (1984) pointed out that the Cr# [= 100Cr/(Cr+Al)] of spinel reflects degrees of partial melting in the mantle, and divided peridotites and associated volcanics into three groups on the basis of Cr# of spinels. Type I and Type III contain spinels with Cr#<60 and Cr#>60, respectively. Type II is a transitional group and contains spinels spanning the full range of spinel compositions in Type I and Type III peridotites. The
compositions of spinel core in peridotites from the Mariana forearc are plotted in the Cr-Al-Fe3+
and Mg#-Cr# diagrams (Figs. 14 and 15). The spinels exhibit a negative correlation between Cr# and Mg#. In Fig. 15, spinels in peridotites from the Mariana forearc are plotted in the range of “Type II” of Dick and Bullen (1984), and seem to have been formed in the island arc that developed on oceanic crust.
Peridotites from Turquoise and North Chamorro Seamounts contain aluminous spinels, suggesting the existence of lherzolite although these rocks are severely serpentinized. Spinels in peridotites recovered from the horst blocks developed in the basement are plotted on the region with high Cr# ranging from 58 to 91, and those recovered from the dome-shaped seamounts have wider range of Cr# from 26 to 75 (Figs. 14 and 15, Table 3). The wide range of Cr# in the latter relative to the former may be explained that the dome-shaped seamounts had been formed by diapiric rise of serpentinized peridotites with various degrees of melting which were incorporated from the various depths and throughout the pathway of diapiric rise in the subduction zone.
5-3. Pyroxenes 5-3-1. Orthopyroxene
Orthopyroxene occurs as a porphyroclast of 1-5 mm in diameter. Some orthopyroxenes show micro structures indicating plastic deformation, such as kink band and wavy extinction (Fig. 16). Orthopyroxene crystals are sometimes surrounded by aggregates of fine-grained olivine crystals (Fig. 6). They are more or less replaced by chrysotile and/or lizardite, but orthopyroxene pseudomorphs are usually well preserved as bastite texture (Fig. 17). In antigorite-bearing peridotites, orthopyroxene and/or bastite are not found. They may have been completely obliterated during antigorite recrystallization even if they had once existed. Orthopyroxenes are enstatite with
compositions of En89.7-92.7. The XMg ratios of orthopyroxene are almost the same as that of olivine.
Al2O3 and CaO contents of orthopyroxene vary from 0.59 to 2.16 wt% and from 0.13 to 1.56 wt%,
respectively. Cr2O3 contents range from 0.06 to 0.84 wt%. Representative chemical compositions of
orthopyroxene are given in Table 4.
5-3-2. Clinopyroxene
Primary clinopyroxene occurs as a euhedral to subhedral crystals, and is sometimes in association with or in proximity to orthopyroxene grains. Thin exsolution lamellae of clinopyroxene are commonly found within orthopyroxene crystals (Fig. 18). In most cases, clinopyroxene is partly
or fully altered to serpentine and/or dusty clay minerals. The XMg ratios of clinopyroxene range
from 0.87 to 0.97 (Table 5). The Ca/(Ca+Mg+Fe2+) ratios range from 0.46 to 0.50. Al
2O3 and Cr2O3 contents of clinopyroxene vary widely from 0.56 to 2.19 wt% and from 0.11 to 1.24 wt%, respectively (Table 5).
Secondary clinopyroxenes are found as a fine-grained acicular crystal in the peridotites from Conical, South Chamorro, and Big Blue Seamounts (Fig. 10). It always associates with antigorite
and secondary olivine. Secondary clinopyroxenes have less Al2O3 contents and slightly higher CaO
contents than primary ones (Fig. 19). The XMg ratios of secondary clinopyroxene range from 0.94 to
0.98, slightly higher than those of primary one. The CaO vs. XMg diagram of primary clinopyroxene
shows that clinopyroxenes from dunite (372-1R-1) tend to have low CaO content and low XMg ratio
in comparison with those from harzburgite samples (Fig. 20). Both types of clinopyroxene are classified into diopside.
5-4. Serpentine minerals
There are three polymorphs of serpentine minerals. They are antigorite, chrysotile, and lizardite. Serpentine minerals in peridotites from all horst blocks and most of dome-shaped seamounts are chrysotile and lizardite, which occur as platy and fibrous crystals that replaced olivine, orthopyroxene and clinopyroxene, and as vein fillings. Hourglass and mesh textures after olivine and bastite textures after both orthopyroxene and clinopyroxene are commonly found (Figs. 21 and 17). Antigorite was found in peridotites from Conical, Celestial, Big Blue and South Chamorro Seamounts. Antigorite-bearing samples from Conical, Big Blue and South Chamorro
Seamounts characteristically have cleavable olivine and secondary iron-rich olivine. Feather-like crystals of antigorite often have interpenetrating texture and cut olivine in random directions. The mode of occurrence of antigorite is different from that of chrysotile/lizardite in that the latter is always associated with fine-grained magnetite, but the former accompanied with magnetite is rare. Antigorite often coexists with acicular clinopyroxene (Fig. 10B). In antigorite-bearing samples, chrysotile and lizardite also occur in a vein or matrix as early-stage and later-stage serpentine minerals. Chrysotile/lizardite veins cut by antigorite veins are often recognized, whereas antigorite
veins cut by chrysotile/lizardite veins are also common. Antigorite has high XMg ratio and tends to
be slightly rich in Si in comparison with chrysotile/lizardite (Fig. 22, Table 6).
5-5. Amphibole
Colorless to pale green amphiboles are found in samples from Conical, Big Blue, Pacman, Twin Peaks, and South Chamorro Seamounts. They commonly occur as euhedral to subhedral prismatic
crystals less than 0.3 mm in length. Most of them have less than 5 wt% Al2O3, and correspond to
tremolite to magnesiohornblende of Leake et al. (1997) (Table 7, Fig. 23). The sample Leg 195-1200A-6R-1, 70-76 cm from South Chamorro Seamount, contains amphiboles with 10-11 wt%
Al2O3, which are classified as edenite or pargasite (after Leake et al., 1997) (Fig. 23). The modes of
occurrence suggest that these amphiboles occur as primary phases formed in hydrous mantle, as suggested by Ohara and Ishii (1998). These amphiboles had lost Al components in varying degrees, probably during serpentinization.
5-6. Brucite
Brucite is found in peridotite from Pacman, Staircase Plateau, Big Blue, Conical, and South Chamorro Seamounts. Fine-grained brucite (5-50 μm) is found in veins and around magnetite or spinel of chrysotile and/or lizardite-bearing peridotites. Colorless brucite is found in veins of the antigorite-bearing peridotite from Conical Seamount (779A-22R-2, 9-14 cm) (Fig. 24). It occurs as 0.3-0.5 mm fibrous crystals, and is always accompanied with magnetite. This brucite may not coexist stably with antigorite, because it occurs only as a vein mineral. Pale-brown iron-rich brucite described by D’Antonio and Kristensen (2004) from South Chamorro Seamount is also found as a vein filling in peridotites from Conical and South Chamorro Seamounts. It does not
chemically coincide with that of ideal brucite with the Mg(OH)2 chemical formula, and contains
significant amounts of Fe(OH)2: FeO contents are 1.4-32.5 wt%. Pale-brown brucite does not
coexist with antigorite, but is probably in equilibrium with chrysotile and/or lizardite as suggested by D’Antonio and Kristensen (2004). Representative chemical compositions of brucite are given in Table 8.
5-7. Phlogopite
Fine-grained Mg-rich phlogopite (XMg = 0.95-0.96) was reported from one peridotite sample of
Conical Seamount (Sample 779A-11R-1, 94-96 cm) by Parkinson and Pearce (1998). In this study, phlogopite was newly found in the peridotite from South Chamorro Seamount (Sample 195-1200B-1W-1, 92-100 cm) as 1-5 μm inclusions of primary olivine and clinopyroxene (Fig. 25).
The phlogopite crystals often have smaller K2O contents than the idealized formula due to
later-stage serpentinization or weathering, but those from less altered ones contain significant
amounts of K2O, as much as 8.05 wt%. The XMg ratios of phlogopites in the peridotite from South
Chamorro Seamount range from 0.88 to 0.95, and are lower than those from Conical Seamount reported from Parkinson and Pearce (1998) (Table 9).
5-8. Chlorite
A pale-green chlorite is often found around spinel and in crack of spinel with serpentine minerals (Fig. 26). Chlorite is a solid solution between endmember serpentine ((Mg,
Fe)12Si8O20(OH)) 16) and amesite ((Mg, Fe)8 Al8Si4O20(OH)16). The Al2O3, MgO and FeO contents in
chlorite vary widely from 4.3 to 19.1 wt%, 25.9 to 40.5 wt%, and 3.3 to 18.5 wt%, respectively. The ratios of Si/Al and Mg/Fe vary from 1.2 to 7.8 and 2.6 to 21.0, respectively (Table 10).
5-9. Other minerals
Magnetite is precipitated during serpentinization of olivine and orthopyroxene, and it commonly forms bands or dusty clusters of tiny opaque crystals around altered primary phases or in veins. Magnetite commonly formed as a secondary mineral phase partly replacing spinel. Calcites are
6. Discussions
6-1. Serpentinization Origin of iron-rich olivine
Antigorite-bearing peridotites in the Mariana forearc commonly contain primary olivine with well-developed cleavage (= cleavable olivine) (Fig. 7). Cleavages of primary olivine are always filled with antigorite films (Figs. 8B, 8C, and 27C). Primary olivine is highly deformed and has conspicuous undulose extinction and subgrain boundaries (tilt boundaries) parallel to (100). It also has well-developed cleavages on (010), (100) and (001) directions. Similar olivines are known as ‘cleavable olivine’ (Hawkes, 1946).
Cleavable olivine often develops in the aureoles thermally metamorphosed by magma intrusion. Uda (1984) studied cleavable olivine developed in the thermally metamorphosed Oeyama peridotites, Japan. He found that cleavage represents antigorite films which are continuous to dislocation cell walls, and concluded that the dislocation cell structure has been formed through annealing recovery process and the cell walls have been successively invaded by the antigorite film, resulting in the formation of the cleavage. In cleavable olivines found from the Mariana forearc peridotites, cleavages are always filled with antigorite film, suggesting a strong correlation between the origin of cleavage and antigorite recrystallization. Ohara & Ishii (1998) also has reported cleavable olivine from peridotites containing talc, tremolite, and antigorite as secondary phases from the landward slope near the southern end of Mariana Trench, and indicated the similarity to the Oeyama peridotites in that cleavable olivine-bearing peridotites in both areas contain secondary hydrous phases. As any heat source is difficult to consider in the Mariana subduction system, cleavable olivine in the Mariana forearc may have formed under high-pressure environment as suggested by Kuroda & Shimoda (1967).
The secondary iron-rich olivine irregularly occurs along the rim or cleavages trace of cleavable olivine (Fig. 8). Iron-rich olivine seems to have formed secondarily as an overgrowth or
replacement of primary olivine (Fo90-92) (Fig. 27), and as irregular bands in olivine that are thought
to have been once-developed conduits of fluids (Fig. 8A).
In olivines of peridotites of 779A-19R-2, 105-108 cm, 779A-10R-1, 39-43 cm, and 779A-22R-2, 9-14 cm obtained from Conical Seamount during ODP Leg 125, and 1200B-1W-1,
develop from the crack formed by the crystallization of feather-like antigorite crystals (Fig. 11A). The orientation of the stripe is parallel to (100) subgrain boundary (Fig. 27C). This direction is the same as that of striped iron-zoning of olivine described by Ando et al. (2001) from deformed peridotites of the Hidaka metamorphic belt of central Hokkaido, Japan, where the iron concentration increases at subgrain boundaries due to edge dislocations. They considered that the zoning is formed by alignment of edge dislocations dragging a so-called Cottrell ‘atmosphere’ of solute atoms (Kitamura et al., 1986) into subgrain boundaries during deformation of the olivine by dislocation creep. However, the iron-rich stripe found in this study is thought to have different origin from the zoning reported by Ando et al. (2001) and Kitamura et al. (1986). The formation of iron-rich stripe in olivines found from the Mariana forearc peridotites seems to have been intimately related to antigorite formation. The iron-rich stripe is commonly found in olivines with distinct cleavages. It occurs along the cleavage traces (Fig. 27C), but also is found in part where cleavage is not visible. In the latter case, a cluster of thin line parallel to iron-rich stripe is recognized by polarizing microscopy . These thin lines were probably formed by dislocation creep. There is a clear deflection for the distribution of the stripe in olivine grain, that is, iron-rich stripe is intensively developed near the margin of olivine where fiber crystal of antigorite pierce olivine crystals, and is not recognized in the inside of olivine grain except for periphery of a crack (Figs. 11A and 28). An arrow in Fig. 11A shows a crack filled with antigorite from where a cluster of iron-rich stripe emanates. It suggests that infiltration of iron-rich fluids along the cleavage trace or the subgrain boundary formed by dislocations is probably responsible for the formation of the iron-rich stripe.
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 occurred also in iron-poor parts (Fig. 30).
Although antigorite has Mg-rich composition (XMg = 0.94-0.97) in comparison with primary
olivine (XMg = 0.90-0.92), antigorite crystals replaced with primary olivine commonly are not
associated with magnetite, suggesting that antigorite formation was unaccompanied with magnetite formation. Excess iron component resulted by antigorite formation after primary olivine is thought
to have dissolved in an H2O-rich fluid that was responsible for the crystallization of the secondary
Fe-rich olivine. Fine-grained crystals of magnetite, however, are locally abundant along the grain boundaries of fine-grained secondary olivines (Fig. 31A). These olivines have Mg-rich
compositions and are plotted in the area between primary olivine and secondary olivine that is free from magnetite association (Fig. 29C). It may reflect some physicochemical change during the formation of secondary olivine. Later-stage chrysotile/lizardite vein cuts both olivine and antigorite crystals. Fine- to medium-grained magnetite is locally abundant also in later stage chrysotile/lizardite pool or vein (Fig. 31B), suggesting that the excess iron component was precipitated to form magnetite after the formation of the Fe-rich olivine.
As mentioned above, Iron-rich olivine is only found in antigorite-bearing peridotite, and always occurs in intimate association with antigorite. Although some iron-rich olivines seem to have been cut by feather-like antigorite, the crystallization of iron-rich olivines has been formed by iron-rich fluid. It, however, still remains a possibility that the iron-rich olivine had formed before serpentinization by fluid that had initially invaded from outside under the higher temperature mantle conditions and thereafter caused serpentinization.
Metamorphic conditions
Chrysotile and lizardite are ubiquitous serpentine minerals in peridotites throughout the Mariana forearc, and were found from the investigated all sites. Mineral associations of chrysotile/lizardite ± brucite of serpentinized peridotites from the Mariana forearc indicate that the temperature conditions of serpentinization are 200-300 °C. Antigorite-bearing rocks are rare, but were found from Conical, Big Blue, Celestial, and South Chamorro Seamounts. The possibility that once-prevailed antigorite in serpentinized peridotites were completely obliterated to chrysotile/lizardite can probably be denied, because most of chrysotile/lizardite-bearing peridotites well retains mesh and bastite textures filled with chrysotile/lizardite. In most cases, these textures are wholly obliterated by feather-like antigorite crystals in antigorite-bearing peridotites. Antigorite is stable at higher temperatures than chrysotile (Iishi and Saito 1973; Evans et al., 1976), and lizardite is considered to have the same P-T stability field as chrysotile (Peacock, 1987). Antigorite commonly coexists with acicular clinopyroxene (= diopside) and secondary olivine, suggesting high temperature serpentinization at about 450-550 °C (Fig. 32). In antigorite-bearing samples, we recognized both early- and later-stage chrysotile veins; that is, the former was cut by antigorite vein and the latter cut antigorite vein (Figs. 31C and 31D). This suggests the chrysotile/lizardite formations occurred before and after the antigorite formation. The early-stage formation of
chrysotile/lizardite is considered to have occurred before subduction event or initial-stage of progressive serpentinization within subduction. On the other hand, the later-stage chrysotile and lizardite indicate the later low-temperature serpentinization probably during uplift stage. In the Mariana forearc, serpentinite diapirs are considered to have generated along the subduction boundary where serpentinization is 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. The stable association of antigorite + diopside + olivine is well known in blueschist- and eclogite-facies terrains (e.g. Scamberruri et al., 1991). The finding of the same assemblage in the Mariana forearc peridotites shows that the stable existence of the assemblage actually formed in a subduction zone environment. It is difficult to explain the reason why we did not find antigorite + brucite assemblage that is located at an intermediate region between chrysotile/lizardite-stable and ‘antigorite + olivine’-stable regions in Fig. 32. As only small amounts of peridotite samples have been examined in this study, more comprehensive study in the Mariana forearc is necessary to understand the precise nature of serpentinization in subduction zone.
6-2. Cooling history of peridotites
Some elements of solid solution minerals in rocks are strongly influenced with temperature, pressure, and bulk rock compositions. Therefore, the temperature and pressure conditions during mineral recrystallization can be inferred using geothermometers and geobarometers, respectively. The geothermometers of orthopyroxene-clinopyroxene pair (Wood & Banno, 1973; Wells, 1977, etc.) and olivine-spinel pair (Fabriès, 1979, etc.) are commonly used for peridotites. When we estimate temperature of mineral formation, it is required to examine carefully whether a pair of associated minerals were in equilibrium mutually. Minerals with compositional zoning show nonequilibrium recrystallization, and are sometimes useful to deduce thermal history during mineral recrystallization. Ozawa (1984) discussed about the cooling history of peridotites using the relation between estimated temperature obtained from the olivine-spinel geothermometer of Fabriès (1979)
and the grain size of spinel. Fabriès (1979)’s geothermometer is shortly introduced here as follows. Irvine (1965) showed the exchange reaction between olivine and spinel, and pointed out the
possibility as a geothermometer; that is, Mg-Fe2+ distribution between olivine and spinel in peridotite was sensitive to temperature.
1/2Fe2SiO4 + Y sp CrMgCr2O4 + Y sp AlMgAl2O4 + Y sp Fe3+MgFe2O4 = 1/2Mg2SiO4 + Y sp CrFeCr2O4 + Y sp AlFeAl2O4 + Y sp Fe3+Fe3O4
Based on the reaction, Fabriès (1979) found that temperature can be estimated by the following formula. T(°K)=(4250YCrsp+1343) / (lnK 0D+1.825 YspCr+0.571) KD = X ol Mg · X sp Fe / X ol Fe · X sp Mg lnK 0 D = lnKD - 4.0Y sp Fe3+
It is thought that temperatures obtained from this formula are always lower than those by other geothermometers (e.g. Wood & Banno, 1973; Lindsley, 1983). The reason is that the interdiffusion
coefficients of Mg-Fe2+ between olivine and spinel are larger than any diffusion coefficients
between elements in pyroxene; that is, during cooling process of peridotites, even after the reactions
among other minerals are completed, an exchange of Mg-Fe2+ between olivine and spinel still
continue effectively.
The cooling of peridotite causes increase of XMg of olivine and decrease of XMg of spinel near
the grain boundary between olivine and spinel (Ozawa, 1984). When temperature conditions are
changed, Mg-Fe2+ diffusion starts between adjoining spinel and olivine grains to attain equilibrium.
Thus, the diffusion occurs from rim toward core in both minerals. Since diffusion velocity of
Mg-Fe2+ in olivine is fast, olivine tends to become homogeneous in composition and does not have
chemical zoning. Olivine commonly occupies a large part of whole volume of peridotite, and thus
XMg of olivine is considered to be nearly equal to XMg of whole rock. Therefore, we can regard that
XMg of olivine is almost constant irrespective of temperature conditions except for the rim of olivine
spinel has remarkable zoning in many cases. When we use the geothermometer of Fabriès (1979), rim of spinel grain in contact with olivine shows equilibrium compositions corresponding to most recent tectonic event. However, if spinel has enough large grain size, the spinel core retains older nonequilibrium compositions of various stages. Therefore, the geothermometer using olivine and spinel core of various grain sizes gives temperatures of various stages. Ozawa (1984) assumed that the core part of spinel and the semi-infinite spherical shell of olivine which encloses spinel do
exchange reaction by diffusion rate controlling of Mg and Fe2+, and discussed the rates of
temperature cooling and rising of peridotite using the geothermometer of Fabriès (1979). Ozawa (1984) showed that peridotites from Miyamori ultramafic complex, dunite from the Iwanaidake
mass, and harzburgite from Biei area in the Kamuikotan belt have cooling rate of 10-5-10-2 °C/yr
from 800 °C to 600 °C, and harzburgite from Horoman has 10-4-10-3 °C/yr from 800 °C to 600 °C.
Peridotites obtained from the dome-shaped seamounts in the Mariana forearc consist mainly of olivine, so the method of Ozawa (1984) can be useful for the analysis of the thermal history of the Mariana peridotites. Ban (1991) first examined the thermal history of peridotites from Conical Seamount obtained during ODP Leg 125 using this method in her master thesis. She showed that
peridotites from Conical Seamount were cooled with the cooling rate of 10-4-10-2 °C /yr from
800 °C to 600 °C, that is similar to that of peridotites from Miyamori ultramafic complex and Kamuikotan belt.
In this study, less-altered peridotite samples well containing primary olivine and spinel in the Mariana forearc were carefully selected. Six dome-shaped seamounts (Conical, Pacman, Twin Peaks, Big Blue, Celestial and South Chamorro Seamounts) and two horst blocks (Eastern ridge of Big Blue Seamount and Staircase Plateau) were investigated to obtain thermal history of the Mariana peridotites. Furthermore, peridotites from the Mid-Atlantic Ridge recovered during ODP Leg 153 Site 920, Hess Deep recovered during ODP Leg 147 Site 895, and Tonga forearc during Boomerang Leg 8 were also investigated for comparison.
The geologic outlines and analyzed samples of Mid-Atlantic Ridge, Hess Deep, and Tonga forearc are given below.
Mid-Atlantic Ridge
been known along oceanic fracture zones, especially on slow-spreading ridges (Miyashiro et al., 1969; Bonatti, 1976; Bonatti and Hamlyn, 1981; Fox and Stroup, 1981; Fox and Gallo, 1986; Dick, 1989). Oceanic crust and upper mantle materials rise up tectonically along transform fault of slow-spreading ridge axis, and gabbros and ultramafic rocks were exposed there. The typical examples are the Atlantis bank in the southwest Indian Ocean and the MARK area near 22°N in Mid-Atlantic Ridge.
The median valley of Mid-Atlantic Ridge about 30 km wide is spreading at a full-rate of 2.7 cm/yr (Purdy et al., 1979; Schulz et al., 1988), and has formed in about 1 m.y.. Serpentinized peridotites crop out in a belt approximately 2 km wide and 20 km long along the western median valley wall of the Mid-Atlantic Ocean Ridge, just south of the Kane Transform in the MARK area. Three serpentinized peridotites recovered from the western wall of the Mid-Atlantic Ridge in the MARK area during ODP Leg 153 Site 920 (Karson and Lawrence, 1997) were analyzed in this study (920D-10R-3, 90-95 cm, 920D-21R-2, 45-50 cm, 920D-22R-2, 86-92 cm). Primary spinel chemistry of these peridotites shows lherzolite (Cr# = 29-32) (Fig. 33A).
Hess Deep
Hess Deep Rift Valley is a tectonically complex, westward propagating oceanic rift valley located near the junction of Pacific, Nazca, and Cocos plates west of the Galapagos Island. The deepest part (>5400 m below seafloor) of this rift valley is the result of deep faulting associated with the westward propagation of the east-west trending Cocos-Nazca spreading center that is opening up the eastern flank of young oceanic crust (0.5-1 Ma) created at the fast-spreading equatorial East Pacific Rise (Lonsdale, 1988; Francheteau et al., 1990). In Hess Deep, the upper mantle of East Pacific Rise, which is a fast-spreading ridge axis (~13.5 cm/yr), is exposed. It is characterized by lithofacies of harzburgite, troctolite and dunite (Arai and Matsukage, 1996; Dick and Natland, 1996).
ODP Leg 147 drilled two sites (Sites 894 and 895) at Hess Deep, and recovered continuous sections of 1 Ma lower crust and upper mantle that were created at the East Pacific Rise (Früh-Green et al., 1996). Peridotites analyzed in this study (895C-3R-1, 35-42 cm, 895D-4R-2, 105-109 cm) were recovered at Site 895, which is located along the southern slope of the intra-rift ridge where the Nautile recovered peridotites derived from the EPR shallow mantle. Primary spinel
chemistry of peridotites in this study shows harzburgite (Fig. 33B).
Tonga Trench
The Tonga Trench is the site of westward subduction of the Pacific Plate beneath the northeastern corner of the Australian Plate. Tonga is a typical example of extension-dominated, non-accretionary convergent margin (e.g. Tappin, 1994; Tappin et al., 1994; MacLeod, 1996), with active extensional tectonism throughout the forearc and trench landward slope. The estimated convergence rate is approximately 15 cm/yr (Lonsdale, 1986); however, recent GPS measurements indicate an instantaneous convergence of 24 cm/yr across the northern Tonga Trench, which is the fastest plate velocity yet recorded on the Earth (Bevis et al., 1995).
Peridotites were recovered from Tonga forearc during Boomerang Leg 8 in 1996 (Ishii and Sato, 2001). They exhibit wider chemistry including more fertile, intermediate and deplete peridotites (i.e., harzburgite, lherzolite and dunite). Three samples from the mid-slope (D93) and lower-slope (D98) of Tonga Trench were used for this study. Primary spinel chemistry of peridotites in this study shows harzburgite (Fig. 33C).
Cooling rate
Positive correlations are recognized between grain size of spinels and estimated temperature in peridotites obtained from the dome-shaped seamounts in the Mariana forearc (Fig. 34). These values are mostly in agreement with the values estimated by Ban (1991), that is, the cooling rate is
10-5-10-2 °C/yr from 800 °C to 600 °C (Figs. 34, 37, and 40A). The results shows that peridotites
from the Mariana cooled at the similar rate as the those from the Miyamori peridotite mass and the Kamuikotan belt estimated by Ozawa (1984). Similar correlation to the peridotites from dome-shaped seamounts is not recognized in those of the horst blocks in the Mariana forearc (Figs. 35 and 38). Any correlation is also not recognized in the peridotites from Tonga forearc (Figs 36C and 39C). It may be responsible to less variation of spinel grain size in the Tonga peridotites.
The highest temperature of peridotites obtained from the Mariana forearc is about 800 °C (Figs. 34, 35, 37, 38 and 40A), and show upper limit plateau temperature. Ban (1991) suggested a possibility that the plateau temperature shows a temperature once prevailed in the mantle before cooling.
After the Pacific Plate had started to subduct under the Philippine Sea Plate, mantle wedge peridotites had been cooling gradually, and thereafter within the mantle wedge the temperature attained steady-state. The cooling process from 800 °C to 600 °C of peridotites obtained from the Mariana forearc may reflect a change of thermal structure of wedge mantle from the initiation of subduction to the attainment of steady-state subduction (Fig. 41).
Positive correlations are recognized between grain size of spinels and estimated temperature in peridotites obtained from the Mid-Atlantic Ridge and Hess Deep (Figs. 36A and 36B). These
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 (Figs. 39A, 39B, and 40B). These
results are different from those of the Mariana peridotites in that both initial and latest temperatures of peridotites in these ridges are slightly higher than those in the Mariana forearc. It is sure that these cooling rates do not show the subduction process seen in Mariana peridotites. Therefore, these cooling rates may reflect the regression process from hot ridge axis or uplift process within upper mantle.
7. Conclusions
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.
