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Volume 27 Issue 4
Jul.  2016
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Tao Chen, Zhenmin Jin, Andy H. Shen, Wei Li. Altered spinel as a petrotectonic indicator in abyssal peridotite from the easternmost part of Southwest Indian Ridge. Journal of Earth Science, 2016, 27(4): 611-622. doi: 10.1007/s12583-016-0707-3
Citation: Tao Chen, Zhenmin Jin, Andy H. Shen, Wei Li. Altered spinel as a petrotectonic indicator in abyssal peridotite from the easternmost part of Southwest Indian Ridge. Journal of Earth Science, 2016, 27(4): 611-622. doi: 10.1007/s12583-016-0707-3

Altered spinel as a petrotectonic indicator in abyssal peridotite from the easternmost part of Southwest Indian Ridge

doi: 10.1007/s12583-016-0707-3
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  • Corresponding author: Tao Chen, summerjewelry@163.com
  • Received Date: 2015-01-16
  • Accepted Date: 2015-12-16
  • Publish Date: 2016-07-12
  • The easternmost part of Southwest Indian Ridge (SWIR) has special crustal structure, magmatic and tectonic processes. Abyssal peridotite from the easternmost part of Southwest Indian Ridge (63.5ºE/28ºS) is serpentinized spinel lherzolite. The accessory spinel has zoned texture, which was studied by petrography, electron probe micro-analysis (EPMA), and backscattered electron (BSE) imaging to reconstruct the petrotectonic and hydrothermal metamorphic history of the host abyssal peridotite. The fresh core is magmatic Al-spinel with low Cr#. The average extent of melting of the abyssal peridotite is about 5.9%. The composition of fresh magmatic spinel core indicates the studied area to be an anomalously thin crust with a melt-poor system. Hydrothermal reaction modifies the chemical composition of magmatic spinel. Ferritchromit is the first product forming the inner rim during pre-serpentinization. The abyssal ferritchromit crystalized as micro- to nano-sized particle with no triple grain boundary, indicating they crystalized in a rapid cooling process during hydrothermal alteration. Chemical compositions of ferritchromit indicate a hydrothermal metamorphism in amphibolite facies. Magnetite in the outer rim was formed by replacement of ferritchromit during syn- or post-serpentinization. Authigenic chlorites crystallized in two events: (1) after formation of ferritchromit crystallized as vein in fracture-zone near the core of spinel and (2) after formation of magnetite crystallized at outermost rim. They are different in compositions, indicating their formation temperature was about 289 ºC and declined to 214 ºC. These results show that the abyssal peridotite had undergone amphibolite to lower-greenschist facies hydrothermal events during pre- to syn-serpentinization or post-serpentinization.
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Altered spinel as a petrotectonic indicator in abyssal peridotite from the easternmost part of Southwest Indian Ridge

doi: 10.1007/s12583-016-0707-3

Abstract: The easternmost part of Southwest Indian Ridge (SWIR) has special crustal structure, magmatic and tectonic processes. Abyssal peridotite from the easternmost part of Southwest Indian Ridge (63.5ºE/28ºS) is serpentinized spinel lherzolite. The accessory spinel has zoned texture, which was studied by petrography, electron probe micro-analysis (EPMA), and backscattered electron (BSE) imaging to reconstruct the petrotectonic and hydrothermal metamorphic history of the host abyssal peridotite. The fresh core is magmatic Al-spinel with low Cr#. The average extent of melting of the abyssal peridotite is about 5.9%. The composition of fresh magmatic spinel core indicates the studied area to be an anomalously thin crust with a melt-poor system. Hydrothermal reaction modifies the chemical composition of magmatic spinel. Ferritchromit is the first product forming the inner rim during pre-serpentinization. The abyssal ferritchromit crystalized as micro- to nano-sized particle with no triple grain boundary, indicating they crystalized in a rapid cooling process during hydrothermal alteration. Chemical compositions of ferritchromit indicate a hydrothermal metamorphism in amphibolite facies. Magnetite in the outer rim was formed by replacement of ferritchromit during syn- or post-serpentinization. Authigenic chlorites crystallized in two events: (1) after formation of ferritchromit crystallized as vein in fracture-zone near the core of spinel and (2) after formation of magnetite crystallized at outermost rim. They are different in compositions, indicating their formation temperature was about 289 ºC and declined to 214 ºC. These results show that the abyssal peridotite had undergone amphibolite to lower-greenschist facies hydrothermal events during pre- to syn-serpentinization or post-serpentinization.

Tao Chen, Zhenmin Jin, Andy H. Shen, Wei Li. Altered spinel as a petrotectonic indicator in abyssal peridotite from the easternmost part of Southwest Indian Ridge. Journal of Earth Science, 2016, 27(4): 611-622. doi: 10.1007/s12583-016-0707-3
Citation: Tao Chen, Zhenmin Jin, Andy H. Shen, Wei Li. Altered spinel as a petrotectonic indicator in abyssal peridotite from the easternmost part of Southwest Indian Ridge. Journal of Earth Science, 2016, 27(4): 611-622. doi: 10.1007/s12583-016-0707-3
  • The Southwest Indian Ridge (SWIR) is one of the two ultraslow spreading ridges in the world, with the slowest rate of ocean ridge magma supply (Dick, 1989). Crustal structure, spreading history, and volcanic, magmatic and tectonic processes of SWIR have been studied by various groups of geophysicists, geochemists, and petrologists (e.g., Cannat et al., 2008, 1999; German, 2003; Seyler et al., 2003; Mendel et al., 1997). However, because of its remote location, SWIR has remained extremely difficult to conduct field work (Tao et al., 2011). Recently, the easternmost part of the SWIR (east of Melville fracture zone) has been suggested to be an anomalously thin crust and a magma-poor, melt-poor end-member ridge according to geophysical data (Sauter and Cannat, 2010).

    Hydrothermal systems at mid-ocean ridges (MOR) are known to play a major role in elemental exchange between ocean and crust through the interactions of circulating seawater with oceanic crust at various temperatures (Nakamura et al., 2007). The first indirect evidence for the presence of hydrothermal venting was obtained in 1997 from water-column anomalies overlying the eastern SWIR (German et al., 1998). Later, geochemical and mineralogical characteristics of the easternmost part of the SWIR hydrothermal field have been reported (Münch et al., 2001), and a different type of MOR hydrothermal activity in magma-starved ridge segments has been discovered (Bach et al., 2002). However, studies on hydrothermal alteration of the Indian Ocean MOR basalt and peridotite are limited (Nakamura et al., 2007).

    Abyssal peridotites are generated at mid-ocean ridges, which belong to oceanic mantle. They are considered as residues of mantle partial fusion and magma genesis (Dick et al., 2003; Dick and Bullen, 1984). Lherzolite is one of the two main rock types of peridotites in the uppermost mantle, which is less depleted and less common than harzburgite appearing in ophiolites at slow-spreading ridges. Along the Earth's mid-ocean ridges, abyssal peridotites undergo hydration reactions and become serpentinized—especially in slow to ultraslow spreading mid-ocean ridges (Lee, 1999). Abyssal peridotites are characteristically highly altered, with 20%-100% serpentine replacing olivine and pyroxene (Dick, 1989).

    Spinel is commonly present as an accessory mineral in small quantities in ultramafic and mafic rocks. The spinels have a large span in compositions, reflecting their primary magmatic origin and secondary alteration (Khalil and El-Makky, 2009; Lee, 1999; Kimball, 1990). The chemical formula of the spinel group minerals can be expressed by AB2O4, in which A represents divalent cations and B represents trivalent cations. These group minerals have been divided into three series according to their trivalent cations, which are spinel series (A1), chromite series (Cr) and magnetite series (Fe3+; Khalil, 2007; Lee, 1999).

    The chemical composition of the fresh primary spinel is a useful tracer of melting in residual peridotites (Hellebrand et al., 2001; Dick and Bullen, 1984). It has been used fairly extensively as a petrogenetic indicator for magmatic environment geotectonic settings, because it is relatively refractory and more resistant to alteration than associated silicate minerals during hydration reactions or serpentinized reactions (Arai et al., 2011, Voigt and von der Handt, 2011; Hellebrand et al., 2002; Sack and Ghiorso, 1991). However, when the composition of magmatic spinel is modified by serpentinization or metamorphism, it will be changed into chromite, ferritchromit, or magnetite (Kimball, 1990). Such a zoned spinel with different compositions has been used to study the metamorphic history of hydrothermal alteration or regional metamorphism of serpentinized orogenic peridotites and ophiolites (e.g., Sansone et al., 2012; Aswad et al., 2011; Hamdy and Lebda, 2011; Khalil and El-Makky, 2009; Mellini et al., 2005). Morishita et al. (2007) first reported local concentration of chromian spinel in a dunite from ultraslow spreading SWIR in details. However, metamorphic spinel in lherzolite from SWIR has not been reported.

    The abyssal peridotite sample studied in this paper is from the easternmost part of SWIR. The sample dredge point is near a hydrothermal field (Münch et al., 2001; German et al., 1998). In this paper, we use a detailed composition analysis of spinel and its alteration minerals to reconstruct the petrotectonic and hydrothermal history of the host abyssal peridotite in the easternmost part of SWIR.

  • The studied abyssal peridotite samples were dredged from the easternmost part of SWIR (63.5ºE/28ºS, water depth 3 660 m) during DY115-21 cruise on R/V Dayang Yihao. The dredge point is near the SWIR axis and away from main fraction zones (Fig. 1).

    Figure 1.  Location (black star) of the study sample on the easternmost part of Southwest Indian Ridge (modified from Sauter and Cannat, 2010). RTJ. Rodrigues triple junction; FZ. fracture zone.

    The SWIR forms at the boundary of the Africa and Antarctica plates in the Indian Ocean, which is among the world's slowest spreading ridges. Its full spreading rate is ~14 mm/year (at 64ºE/28ºS) varying only slightly along the 7 700-km ridge axis (Sauter and Cannat, 2010). The physiography of the SWIR shows no transform faults between Melville transform and the Rodriguez triple junction (Searle and Bralee, 2007). Compared with the West Melville, the East Melville has an unusual segmentation pattern with a few very high relief segments, a very low magma supply, and a cold subaxial mantle (Cannat et al., 1999). Seismic studies between 61ºE and 63ºE show evidence of presence of an anomalously thin crust, 4-5 km thick, compared to the uniform thickness of oceanic crust, 7 km, generated away from mantle plumes and fracture zones (Minshull and White, 1996). These results suggest that magma production is restricted under easternmost part of the SWIR (Mendel et al., 1997), and reveal a new type of seafloor, which corresponds to thin crust with little to no volcanism (Sauter and Cannat, 2010). However, very large volcanic centers with thick crust were considered to occur between these nonvolcanic sections (Sauter and Cannat, 2010; Cannat et al., 2008). Based on side scan and bathymetry superimposed on bathymetry image, the dredge point of the studied samples is on volcanic terrains near detachment faults (Searle et al., 2003).

    On the other hand, the easternmost part of the SWIR is among the deepest part of the oceanic ridge system. The average axial depth between the Gallieni fracture zone and the Rodrigues triple junction (RTJ) is 4 630 m. It is inferred to represent a melt-poor end-member for this system (Sauter and Cannat, 2010; Seyler et al., 2003).

    Hydrothermal activity on the eastern SWIR had been reported to indicate the presence of hydrothermal plumes by geophysical and geochemical studies (e.g., Tao et al., 2011; German, 2003; German et al., 1998). The reported hydrothermal plumes at some locations indicate a higher frequency of venting than expected. One of large hydrothermal plumes (63º33′E/27º58′S, depth 3 300 m) found by German et al. (1998) is near the dredge point of the studied samples. The newly discovered hydrothermal field is situated at the summit of an axial volcanic ridge (63º56′E/27º51′S). Within the field, inactive chimneys as well as hydrothermal mounds are observed on the ocean floor (Münch et al., 2001).

  • Quantitative chemical analysis of minerals was carried out by JEOL-JXA-8100 electron microprobe using wavelength dispersive X-ray spectrometer (WDS) with 15 kV accelerating voltage and 20 nA beam current at the Second Institute of Oceanography (SIO) and the State Key Laboratory of Geological Processes and Mineral Resources (GPMR), China University of Geosciences, Wuhan. The Fe2+-Fe3+ redistribution from electron microprobe analyses was calculated using the general equation of Droop (1987) to estimate Fe3+. The backscattered electron (BSE) images were obtained by a Quanta 450 FEG environment scanning electron microscope (ESEM). The analytical condition was 20 kV accelerating voltage and 35 nA beam current.

  • The mineral assemblages of the SWIR serpentinized peridotite was determined by petrography. The peridotite is composed of serpentine, olivine, orthopyroxene, clinopyroxene, spinel, magnetite, and chlorite (Figs. 2 and 3). These mineralogical assemblages indicate that the original ultramafic protolith is spinel lherzolite.

    Figure 2.  Photomicrographs of serpentinized spinel lherzolite. (a) Relict olivine are porphyroclasts. Some of orthopyroxenes occur as deformed crystals showing wavy extinction. Clinopyroxene occurs as subhedral crystal. Serpentine veins cut relict olivine, orthopyroxene and clinopyroxene. (b) Anhedral spinels and fibrous serpentine aggregates. Opx. Orthopyroxene; Ol. olivine; Serp. serpentine; Sp. spinel.

    Figure 3.  The SEM/BSE images of zoned spinel. (a) Anhedral spinel (Sp) surrounded by spongy textural magnetite (Mag) rim. (b) Fractured spinel surrounded by discontinuous narrow magnetite rim. (c) Spinel replaced by ferritchromit (Ftc) to form inner rim and further replaced by magnetite to form outer rim. Chlorites (Chl) fill in fractures or form as outermost rims beside magnetite. (d) Micro- to nano-sized ferritchromits surround spinel core. Chlorites surround ferritchromit rims. Ol. Olivine; Serp. serpentinite; Sp. spinel; Ftc. ferritchromit; Mag. magnetite.

    The preserved textures of the studied abyssal peridotite are pseudomorphic type (mesh and hourglass) after olivine and pyroxene, preserving the preserpentinization textures of the ultramafic precursors, and of non-pseudomorphic (interpenetrating and interlocking) type. As shown in Fig. 2a, relict olivine occurs as porphyroclast. As serpentinization proceeding, olivine has been replaced by serpentine. Relict orthopyroxenes also occur as porphyroclast, and some of them occur as deformed crystals. Clinopyroxene occurs as porphyroclast and subhedral crystals in peridotite indicating that the protolith is lherzolite. Relict spinels scatter throughout the peridotite (Fig. 2b), and occur as fractured crystals with lobate contours. Cores of spinels are amber to deep-red, often surrounded by opaque rims that resemble the highly reflective ferritchromit rims (e.g., Mellini et al., 2005; Kimball, 1990).

    The BSE images are employed to distinguish different zones of spinel by using the contrast function, which essentially depends upon differences in average atomic number. Atom of higher atomic number shows a brighter contrast. The location chosen for electron microprobe analysis is usually guided by differences in BSE images. Figure 3 was obtained by ESEM for higher imaging quality. Detailed BSE images of the spinel specify four different textures from core to rim including three spinel subtypes.

    (1) The primary spinel is anhedral contour with different grain size (60-2 500 m), which suffered brittle deformation forming fractured crystals (Figs. 3a and 3b). The unaltered part is the core (grey) of the particle.

    (2) Ferritchromit (inner rim, brighter) starts along fracture and progressively replaces the rim of the spinel grain. The spinel-ferritchromit reaction boundary is irregular. The extent of replacement of ferritchromit is variable from grain to grain (e.g., Figs. 3a and 3c). Ferritchromit is micro-crystal aggregation with micro- to nano-sized anhedral particle (Figs. 3c and 3d). There is no triple grain boundary among ferritchromit grains or among ferritchromit and spinel grains (Fig. 3d).

    (3) The outer reaction rim is composed of magnetite (brightest), showing different morphology around spinel. A spongy texture of magnetite rim is shown in Fig. 3a. Serpentinized olivine and pyroxene supply extra Fe to form a wide magnetite rim. However, most magnetites show discontinuous and narrow rims with linear to curvilinear grain boundaries, as shown in Figs. 3b and 3c. Such rims are formed by diffusive replacement of Mg2+ and Al3+ by Fe2+ and Fe3+ in the spinel structure (Aswad et al., 2001).

    (4) Chlorite (dark gray) occurs in two positions in the studied areas. Most of them occur as veins in the fracture-zone in spinel to surround the ferritchromit rims (called vein chlorite, hereafter, e.g., Fig. 3d). Some of them occur as outermost rim beside magnetite (called rim chlorite, hereafter, e.g., left up part in Fig. 3c).

  • Our sample contains unaltered spinel cores surrounded by altered products. The chemical compositions of spinels with unaltered cores and altertered rims have been tested by electron microprobe.

    Because many ferritchromit particles are smaller than the electron spot size and disperse at the rim, the chemical composition of ferritchromit can only be obtained from relatively larger particles. The chemical formula of spinel (core, average: (Mg0.77Fe0.222+Ni0.01)∑=1.00(Al1.60Cr0.32Fe0.073+)∑=1.99O4), ferritchromit (inner rim, average: (Fe0.872+Mg0.14)∑=1.01(Cr0.81Fe0.753+Al0.31 Si0.06Mn0.05Ti0.01)∑=1.99O4), and magnetite (outer rim, average: (Fe0.852+Mg0.09)∑=0.94(Fe1.933+Cr0.03Mn0.08Si0.02)∑=2.06O4) was determined based on 4 oxygen atoms (Table 1).

    Spinel (core)
    Oxides (wt.%) 01-01 01-02 02-01 02-02 03-01 03-02 04-01 04-02 05-01 06-01 07-01 08-01 09-01 10-01 11-01 Ave
    Al2O3 51.02 50.83 51.1 51.35 50.64 50.98 50.15 49.77 50.17 50.33 50.59 53.02 50.37 52.02 52.39 50.98
    Na2O 0.02 0.02 0.01 0.01 0.00 0.00 0.02 0.00 0.00 0.00 0.02 0.00 0.04 0.02 0.01 0.01
    K2O 0.00 0.00 0.00 0.01 0.01 0.00 0.01 0.02 0.00 0.00 0.00 0.00 0.02 0.00 0.00 0.00
    Cr2O3 14.91 14.86 14.8 14.82 15.16 14.84 15.96 15.94 15.55 15.56 16.34 12.22 16.01 14.21 13.13 14.95
    SiO2 0.06 0.02 0.02 0.04 0.03 0.06 0.27 0.37 0.01 0.06 0.00 0.01 0.02 0.02 0.02 0.07
    MgO 19.38 19.16 19.42 19.39 19.2 19.31 18.79 18.91 19.12 19.82 19.92 19.2 19.75 19.31 19.05 19.32
    CaO 0.00 0.01 0.00 0.00 0.00 0.01 0.16 0.37 0.00 0.03 0.01 0.00 0.01 0.01 0.00 0.04
    MnO 0.11 0.1 0.15 0.17 0.12 0.12 0.16 0.17 0.14 0.09 0.07 0.14 0.15 0.05 0.11 0.12
    TiO2 0.16 0.13 0.12 0.13 0.11 0.12 0.18 0.15 0.14 0.13 0.08 0.04 0.16 0.04 0.03 0.11
    FeO 13.04 13.11 13.1 12.95 13.25 13.32 13.1 13.37 13.39 12.6 12.55 13.77 13.52 12.93 15.39 13.29
    NiO 0.38 0.38 0.36 0.36 0.37 0.33 0.38 0.35 0.34 0.37 0.37 0.33 0.28 0.32 0.33 0.35
    Total 99.08 98.61 99.09 99.22 98.89 99.07 99.19 99.42 98.86 99 99.95 98.73 100.34 98.93 100.48 99.26
    Cations
    Si 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
    Al 1.61 1.61 1.61 1.61 1.6 1.61 1.58 1.57 1.59 1.59 1.58 1.66 1.57 1.63 1.63 1.60
    Ti 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
    Cr 0.31 0.32 0.31 0.31 0.32 0.31 0.34 0.34 0.33 0.33 0.34 0.26 0.34 0.3 0.27 0.32
    Fe3+ 0.07 0.07 0.07 0.07 0.07 0.07 0.05 0.07 0.07 0.08 0.08 0.08 0.09 0.06 0.1 0.07
    Fe2+ 0.22 0.22 0.22 0.22 0.22 0.22 0.24 0.24 0.23 0.2 0.2 0.23 0.2 0.23 0.23 0.22
    Mn 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
    Mg 0.77 0.77 0.77 0.77 0.77 0.77 0.75 0.75 0.77 0.79 0.79 0.76 0.78 0.77 0.75 0.77
    Ni 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01
    ΣCation 2.99 3.00 2.99 2.99 2.99 2.99 2.98 2.99 3.00 3.00 3.00 3.00 2.99 3.00 2.99 2.99
    aCr# 0.16 0.16 0.16 0.16 0.17 0.16 0.18 0.18 0.17 0.17 0.18 0.13 0.18 0.15 0.14 0.16
    bMg# 0.78 0.77 0.78 0.78 0.78 0.78 0.75 0.76 0.77 0.80 0.80 0.77 0.79 0.77 0.76 0.78
    cF 5.91 5.92 5.84 5.81 6.12 5.88 6.62 6.68 6.4 6.38 6.74 3.89 6.61 5.35 4.62 5.92
    Ferritchromit (inner rim) Magnetite (outer rim)
    Oxides (wt.%) 01-04 01-05 01-06 03-03 04-04 04-05 04-06 05-03 06-02 06-04 09-04 Ave. 02-03 03-05 04-08 09-09 Ave.
    Al2O3 6.19 6.92 16.21 6.35 8.98 4.2 7.68 6.31 6.81 8.52 3.61 7.43 0.02 0.05 0.06 0.03 0.04
    Na2O 0.00 0.00 0.03 0.03 0.01 0.01 0.01 0.01 0.03 0.01 0.00 0.01 0.00 0.01 0.01 0.00 0.01
    K2O 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.02 0.00 0.00 0.00 0.00 0.00 0.00 0.02 0.01
    Cr2O3 26.87 27.76 22.29 22.39 28.38 34.54 34.94 28 30.1 36.28 25.9 28.86 0.72 0.95 1.07 0.74 0.87
    SiO2 1.82 0.18 0.31 3.12 0.16 0.58 0.46 4.65 5.49 0.20 0.79 1.61 0.41 0.65 0.50 0.52 0.52
    MgO 2.14 1.39 2.85 3.64 1.31 0.78 1.33 6.69 6.23 1.48 1.47 2.66 1.68 1.19 1.76 1.58 1.55
    CaO 0.00 0.00 0.00 0.00 0.00 0.04 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
    MnO 2.27 2.08 2.15 1.81 1.73 1.2 0.96 1.19 0.92 0.87 1.71 1.54 2.44 1.87 2.39 2.77 2.37
    TiO2 0.28 0.28 0.19 0.20 0.31 0.29 0.38 0.24 0.24 0.29 0.20 0.26 0.04 0.04 0.02 0.04 0.04
    FeO 56.19 58.8 53.67 58.09 55.48 54.4 51.8 47.81 46.31 50.68 64.21 54.31 88.96 88.5 86.92 88.74 88.28
    NiO 0.40 0.16 0.15 0.40 0.15 0.27 0.07 0.50 0.27 0.15 0.08 0.24 0.09 0.36 0.10 0.10 0.16
    Total 96.16 97.58 97.84 96.03 96.51 96.32 97.62 95.42 96.42 98.48 97.95 96.94 94.36 93.63 92.82 94.54 93.84
    Cations
    Si 0.07 0.01 0.01 0.11 0.01 0.02 0.02 0.16 0.19 0.01 0.03 0.06 0.02 0.02 0.02 0.02 0.02
    Al 0.26 0.29 0.65 0.27 0.38 0.18 0.32 0.26 0.28 0.36 0.15 0.31 0.00 0.00 0.00 0.00 0.00
    Ti 0.01 0.01 0.00 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.00 0.00 0.00 0.00 0.00
    Cr 0.77 0.79 0.60 0.63 0.81 1.01 0.99 0.77 0.82 1.02 0.74 0.81 0.02 0.03 0.03 0.02 0.03
    Fe3+ 0.82 0.89 0.72 0.87 0.78 0.75 0.63 0.63 0.51 0.6 1.04 0.75 1.94 1.92 1.93 1.94 1.93
    Fe2+ 0.88 0.87 0.80 0.85 0.88 0.94 0.92 0.77 0.83 0.91 0.90 0.87 0.84 0.89 0.84 0.83 0.85
    Mn 0.07 0.06 0.06 0.05 0.05 0.04 0.03 0.04 0.03 0.03 0.05 0.05 0.08 0.06 0.08 0.09 0.08
    Mg 0.12 0.07 0.14 0.19 0.07 0.04 0.07 0.35 0.32 0.08 0.08 0.14 0.10 0.07 0.10 0.09 0.09
    Ni 0.01 0.00 0.00 0.01 0.00 0.01 0.00 0.01 0.01 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.00
    ΣCation 3.01 2.99 2.98 2.99 2.99 3.00 2.99 3.00 3.00 3.02 3.00 3.00 3.00 3.00 3.00 2.99 3.00
    aCr# 0.74 0.73 0.48 0.7 0.68 0.85 0.75 0.75 0.75 0.74 0.83 0.73
    bMg# 0.12 0.08 0.15 0.18 0.07 0.04 0.07 0.31 0.28 0.08 0.08 0.13
    aCr#=Cr/(Cr + Al); bMg#=Mg/(Mg+Fe2+); cF is the extent of melting; Ave. average.

    Table 1.  Microprobe analysis of zoned spinels in easternmost SWIR serpentinized peridotite

    Fresh spinel cores have high Al content (average Al2O3% is ~51%), indicating they are magmatic Al-spinels (Mellini et al., 2005). Their Cr# [molar ratio Cr/(Cr+Al)] is between 0.13 and 0.18, and Mg# is between 0.75 and 0.80. Most abyssal spinel-peridotites have spinel with low or negligible TiO2 content (< 0.25%, Hellebrand et al., 2001). Here, titanium content in spinel core is very low (average TiO2% is 0.11%).

    The most significant chemical variation of unaltered and altered spinel in abyssal peridotites is a large reciprocal variation of Cr and Al and a strong correlation of increasing Cr# and decreasing Mg# (Lee, 1999). The data shown in Table 1 have such characteristics. The average Cr# value increases from 0.16 to 0.73 when spinel replaced by ferritchromit. Average Mg# value decreases from 0.78 to 0.13 in the alteration.

    As shown in Figs. 4 and 5, chemical data of our samples sharply plot into three groups that mark the discontinuous changes from magmatic Al spinels to altered rims. From core to altered zones, Al is rapidly removed from the spinel lattice during ferritchromit replacement, and subsequently, slowly removed from following magnetite replacement. The chemical composition of spinel and magnetite are homogeneous, but the chemical composition of ferritchromit changes relatively larger. For example, Cr2O3% ranges from 22.29 wt.% to 36.28 wt.% (Fig. 4). On the other hand, titanium content increases to more than 0.2 wt.% in ferritchromit replacement but decreased in magnetite replacement (Fig. 5). Iron (Fe) is continuously increased in inner and outer rims, so the altered material consists mostly of iron oxides.

    Figure 4.  Cr2O3 vs. Al2O3 plot of zoned spinel compositions from easternmost SWIR.

    Figure 5.  Al2O3 vs. TiO2 plot of zoned spinel compositions from easternmost SWIR.

    Hellebrand et al. (2001) used spinel as a quantitative melting indicator of mantle residues. The chromium number of spinel (Cr#) is an important geochemical parameter for the estimation of the degree of partial melting (Lee, 1999; Dick and Bullen, 1984). The equation [F=10ln(Cr#)+24] has been widely used to describe the extent of melting (F) as a function of spinel Cr#, which is applicable when spinel Cr# values is between 0.10 and 0.60 (Hellebrand et al., 2001). In this study, the fresh core of spinel can be used to estimate melting degree of dredge abyssal peridotite (Table 1). The average extent of melting (F) calculated by Cr# is about 5.9%.

  • Serpentinized orogenic peridotites and ophiolites with subsequent formation of ferritchromit and chlorite have been reported (e.g., Aswad et al., 2011; Hamdy and Lebda, 2011; Mellini et al., 2005). It has been suggested that diffusion of A1 and Mg out of the spinel or chromite during metamorphism results in the formation of chlorite (Kimball, 1990).

    The chemical composition of chlorite in the veins of fracture-zones and at the rims beside magnetites had been analyzed and was calculated on the basis of 14 oxygens (shown in Table 2). The volume of rim chlorite is much less than that of vein chlorite. The major silicate mineral situated beside magnetite is serpentine. The data of serpentine (beside magnetite) and coexisting chlorite (at rim) are also listed in Table 2. In general, it has much lower Al2O3 content than chlorite.

    Chlorite (vein)
    Oxides (wt.%) 3-01 3-02 3-04 3-05 4-01 4-02 4-03 4-04 4-05 4-06 13-01 13-02 13-03 13-04 Ave
    Al2O3 19.70 17.64 17.97 16.69 18.46 17.90 18.42 19.21 19.20 18.53 18.99 19.16 18.31 18.54 18.48
    Na2O 0.17 0.07 0.04 0.05 0.01 0.02 0.03 0.02 0.01 0.05 0.01 0.01 0.03 0.03 0.04
    K2O 0.00 0.02 0.01 0.00 0.00 0.01 0.00 0.02 0.02 0.01 0.01 0.00 0.00 0.00 0.01
    Cr2O3 0.37 0.47 1.23 1.35 0.83 0.74 0.85 0.45 0.45 1.19 0.45 0.43 0.48 0.61 0.71
    SiO2 31.69 32.36 30.65 31.69 31.36 30.19 31.10 30.16 30.45 28.81 29.14 28.97 29.98 29.63 30.44
    MgO 32.34 32.33 30.87 31.46 31.06 30.99 31.03 30.60 30.61 31.01 30.30 30.60 30.80 30.81 31.06
    CaO 0.03 0.01 0.01 0.00 0.00 0.01 0.06 0.01 0.00 0.04 0.01 0.01 0.00 0.04 0.02
    MnO 0.01 0.02 0.05 0.06 0.05 0.04 0.01 0.00 0.01 0.01 0.03 0.00 0.05 0.03 0.03
    TiO2 0.01 0.03 0.03 0.02 0.04 0.00 0.02 0.04 0.01 0.01 0.01 0.04 0.03 0.01 0.02
    FeO 4.83 5.98 7.08 6.52 6.22 5.79 6.00 6.10 5.89 6.06 6.99 6.86 7.06 6.83 6.30
    NiO 0.07 0.06 0.07 0.09 0.09 0.09 0.06 0.05 0.08 0.06 0.08 0.04 0.11 0.09 0.07
    Total 89.23 89.00 87.99 87.92 88.12 85.78 87.59 86.65 86.73 85.77 86.03 86.10 86.86 86.60 87.17
    Structure formula (half-cell)
    Tetrahedron
    Si 2.93 3.02 2.93 3.02 2.97 2.93 2.96 2.90 2.92 2.82 2.77 2.82 2.90 2.87 2.91
    Al 1.07 0.98 1.07 0.98 1.03 1.07 1.04 1.10 1.08 1.18 1.23 1.18 1.10 1.13 1.09
    Octahedron
    Al 1.08 0.96 0.95 0.89 1.02 0.98 1.02 1.07 1.09 0.95 0.99 1.02 0.98 0.99 1.00
    Fe2+ 0.37 0.47 0.56 0.52 0.49 0.47 0.48 0.49 0.47 0.5 0.5 0.56 0.57 0.55 0.50
    Mg 4.46 4.5 4.39 4.46 4.38 4.49 4.4 4.38 4.37 4.52 4.52 4.44 4.44 4.45 4.44
    Cr 0.03 0.03 0.09 0.10 0.06 0.06 0.06 0.03 0.03 0.09 0.06 0.03 0.04 0.05 0.05
    Ti 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
    Mn 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
    Ni 0.00 0.00 0.01 0.01 0.01 0.01 0.00 0.00 0.01 0.00 0.01 0.00 0.01 0.01 0.01
    aFe# 0.08 0.09 0.11 0.10 0.10 0.09 0.10 0.10 0.10 0.10 0.10 0.11 0.11 0.11 0.10
    T (℃) 281 252 283 254 271 281 273 292 286 319 333 317 293 302 288
    Chlorite (outmost rim) Serpentine
    Oxides (wt.%) 2-08 2-09 5-02 10-05 Ave 2-11 5-03 5-04 5-05 5-06 10-01 10-02 10-03 Ave
    Al2O3 16.16 17.36 14.39 14.45 15.59 4.26 6.26 10.75 10.45 10.4 10.03 10.6 6.10 8.61
    Na2O 0.00 0.01 0.01 0.00 0.01 0.06 0.01 0.01 0.02 0.01 0.01 0.02 0.00 0.02
    K2O 0.01 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.01 0.02 0.03 0.00 0.01 0.01
    Cr2O3 0.39 0.41 1.70 0.42 0.73 0.42 1.52 0.74 0.97 0.84 0.09 0.10 0.26 0.62
    SiO2 33.89 32.37 33.85 34.01 33.53 37.39 39.58 35.33 36.62 36.65 38.24 38.02 39.89 37.72
    MgO 34.84 32.27 31.31 33.42 32.96 35.16 36.2 30.14 31.2 31.71 36.5 36.62 37.16 34.34
    CaO 0.02 0.01 0.01 0.00 0.01 0.06 0.02 0.01 0.01 0.04 0.01 0.00 0.03 0.02
    MnO 0.02 0.07 0.11 0.04 0.06 0.13 0.09 0.13 0.11 0.11 0.00 0.00 0.05 0.08
    TiO2 0.02 0.04 0.02 0.02 0.03 0.04 0.02 0.02 0.01 0.02 0.01 0.00 0.01 0.02
    FeO 2.81 6.47 7.58 4.87 5.43 7.76 6.47 11.83 9.85 9.65 3.8 2.93 4.52 7.10
    NiO 0.10 0.19 0.16 0.19 0.16 0.20 0.10 0.06 0.11 0.11 0.09 0.13 0.21 0.13
    Total 86.46 89.19 89.15 87.42 88.06 85.47 90.27 89.00 89.36 89.55 88.80 88.42 88.24 88.64
    Chlorite (outmost rim) Serpentine
    Structure formula (half-cell)
    Si 3.14 3.03 3.19 3.21 3.14 1.84 1.83 1.70 1.73 1.73 1.76 1.75 1.86 1.78
    Al 0.86 0.97 0.81 0.79 0.86 0.16 0.17 0.30 0.27 0.27 0.24 0.25 0.14 0.23
    Octahedron
    Al 0.90 0.94 0.79 0.82 0.86 0.09 0.17 0.31 0.32 0.31 0.31 0.33 0.19 0.25
    Fe2+ 0.22 0.51 0.60 0.38 0.43 0.32 0.25 0.48 0.39 0.38 0.15 0.11 0.18 0.28
    Mg 4.81 4.50 4.40 4.71 4.61 2.58 2.49 2.16 2.20 2.23 2.51 2.51 2.58 2.41
    Cr 0.03 0.03 0.13 0.03 0.06 0.02 0.06 0.03 0.04 0.03 0.00 0.00 0.01 0.02
    Ti 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
    Mn 0.00 0.01 0.01 0.00 0.01 0.01 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00
    Ni 0.01 0.01 0.01 0.01 0.01 0.01 0.00 0.00 0.00 0.01 0.00 0.01 0.01 0.01
    aFe# 0.04 0.10 0.12 0.08 0.09
    T (℃) 216 251 198 191 214
    aFe#=Fe2+/(Fe2++Mg); Ave. average.

    Table 2.  Microprobe analysis of chlorite and serpentine in easternmost SWIR serpentinized peridotite

    Vein chlorite shows SiO2 content ranging from 28.81 wt.% to 32.36 wt.%, and Al2O3 content ranging from 16.69 wt.% to 19.70 wt.%. The Fe# ratio is constant at about 0.1. The average chemical formula of vein chlorite is (Mg4.44Al1.00 Fe0.502+Cr0.06Ni0.01)∑=6.01(Si2.91Al1.09)∑=4.00O10(OH)8. Rim chlorite shows higher SiO2 content and lower Al2O3 content. The Fe# ratio ranges from 0.04 to 0.12. The average chemical formula of rim chlorite is (Mg4.61Al0.86Fe0.43Cr0.06Ni0.01)∑=5.97 (Si3.14Al0.86)∑=4.00 O10(OH)8. Chlorite in our samples is characterized by low Cr content both in veins and at rims. The average chemical formula of serpentine around magnetite is (Mg2.41Al0.25Fe0.28 Cr0.02Ni0.01)∑=2.97(Si1.78Al0.23)∑=2.01O5(OH)4.

    Caritat et al. (1993) divided chlorite into three end-members as Al-chlorite [Al12Si2O10(OH)8], Mg-chlorite [Mg6Si4O10(OH)8], and Fe-chlorite [Fe6Si4O10(OH)8], and two commom chlorites as clinochlore [Mg5AlSi3O10(OH)8] and daphnite [Fe5AlSi3O10(OH)8]. According to the characteristic of chemical composition of the studied chlorite, it belongs to clinochlore.

    The Al in chlorite is used to estimate the formation temperature (Caritat et al., 1993). Cathelineau and Izquierdo (1988) suggested an equation (T= -61.92+321.98Al) used as a chlorite geothermometer of general applicability in diagenetic, hydrothermal, and metamorphic settings. The formation temperatures of vein and rim chlorite are listed in Table 2, which is calculated by this equation. The average formation temperature of vein chlorite is 289 ℃, and 214 ℃ for the rim.

  • In this study, the cores of spinels are homogeneous. Accordingly, the fresh cores can represent the primary phase and the magmatic chemical composition of the disseminated spinel grains, as well as primary mantle values despite advanced serpentinization and oxidation of the rock (Khalil, 2007; Hellebrand et al., 2001).

    The Cr# value of spinel provides specific information about the types of their source rocks in different tectonic settings. The spinels of the abyssal peridotites are limited by higher Cr# value of 0.6. The Cr# value of abyssal peridotites of fast spreading ridge is about 0.3-0.6, while that of abyssal peridotites of slow spreading ridge is about 0.1-0.3 (Lee, 1999).

    From the above analysis, it is known that the Cr# values of the primary (first-stage) spinels are very low, ranging from 0.13 to 0.18. Those abyssal peridotites with spinel compositions at the low Cr-end of the abyssal range are mostly lherzolites (Hellebrand et al., 2001). Figure 6 shows Cr# values of the studied sample, which are almost the lowest ones of abyssal peridotites, it agrees with the studied spinel in lherzolite from ocean ridge (Hamdy and Lebda, 2011; Lee, 1999).

    Figure 6.  Diagram showing the chemical composition of unaltered core spinels (black squares) which are restricted to lherzolite and abyssal peridotite fields. The range of spinel from different rocks was concluded by Dick and Bullen (1984).

    Chromian spinel composition is mainly controlled by the composition and oxygen fugacity of the melt, and is only weakly dependent on temperature and pressure (Hamdy and Lebda, 2011). The Cr# value of spinel in mantle-derived harzburgite and lherzolite is an indicator of degree of magma extraction (Hellebrand et al., 2001). The average extent of melting of the studied abyssal lherzolite calculated using Cr# is about 5.9%. Previous studies concluded that the extent of mantle melting is about 10% at the end of slow-spreading rate and about 22% at the end of fast-spreading rate of the entire spreading rate variation range (Niu and Hékinian, 1997). Hellebrand et al. (2001) demonstrated that the range of melting for abyssal peridotites extends from ~2% to ~18%. For the SWIR, the additional degree of melting related to the vicinity of the Bouvet hotspot is 8% (Hellebrand et al., 2001). For the East Melville, mean extents of melting are 6% to 6.4%. This resulting melting thickness is about 2.7 km for the east of Melville (Cannat et al., 1999). Very low degrees of melting of abyssal peridotites were inferred from the Cr# in spinels along the 61ºE-64ºE section of the SWIR. The values of melting degree range from ~5% to ~8% in this section, which is lower than those in 53ºE-68ºE section (Seyler et al., 2003).

    The easternmost part of SWIR is typical of the slowest spreeding, melt-poor end-member ridge system. Typically, it has the deepest seafloor, anomalously thin crust, and cold mantle (Sauter and Cannat, 2010). Our data of melting match well with the previous studies. Such a low degree of melting indicates 63ºE in the easternmost part of SWIR is located in anomalously thin crust in an ultraslow spreading ridge with a melt-poor system (Sauter and Cannat, 2010; Meyzen et al., 2003; Niu and Hékinian, 1997).

  • Hydrothermal interaction between peridotites or serpentinites and seawater is gaining increasing interest because of its importance in the rheology of the oceanic lithosphere (Nakamura et al., 2007). Recently, hydrothermally reset magmatic spinels in retrograde serpentinites have been used to reconstruct metamorphic history (e.g., Sansone et al., 2012; Aswad et al., 2011; Mellini et al., 2005).

    The studied accessory spinels in serpentinized peridotite are found as zoned crystals. BSE images (Fig. 3) show four different textures and mineral compositions from core to rim. The products of Al-spinel are ferritchromit, magnetite, and chlorite. Alteration and zonation of spinel group minerals have been concluded to relate to regional metamorphism, magmatic alteration or hydrothermal alteration with serpentinization (Khalil, 2007; Kimball, 1990). The sample dredge point is in a newly found active hydrothermal vent field (Tao et al., 2011; German et al., 1998). One of the large hydrothermal plumes (63º33′E/27º58′S, depth 3 300 m) found by German et al. (1998) is near the dredge point. Therefore, the alteration mechanism should relate to hydrothermal alteration with pre-, syn- and post-serpentinization.

    The formation of ferritchromit is caused by replacement of Al-rich chromite or spinel through dissolution crystallization processes (Sansone et al., 2012). The alteration rims of the studied ferritchromit have a chemical composition that is characterized by both FeO and Fe2O3 enrichment and Cr# increase (caused by loss of Al2O3). Their chemical compositions are different from those of chromite altered under greenschist facies (Hamdy and Lebda, 2011; de Freitas Suita and Streider, 1996). The chemical compositions of studied ferritchromits are plotted on the triangular Fe3+-Cr-Al diagram (Fig. 7), which shows the spinel compositional fields mainly lie on lower-amphibolite metamorphic facies. However, other composition points plot up to upper-amphibolite metamorphic facies.

    Figure 7.  Triangular Fe3+-Cr-Al diagram showing chemical compositions of ferritchromit (black spots) are plotted mainly in lower-amphibolite facies. Fields of different metamorphic facies for altered spinel phases were concluded by Hamdy and Lebda (2011).

    Figures 3c and 3d show that the crystalized ferritchromit is micro- to nano-size and there are no triple grain boundaries among them. These ferritchromit particles are different from those altered spinel rims in orogenic peridotites. This phenomenon suggests that the replaced ferritchromits have not reached an equilibrium state, and they suffered a rapid cooling process during hydrothermal alteration (Dick and Bullen, 1984). A large detachment fault is very near our sample's dredge point. The study of this detachment fault indicates that denuded oceanic crust cooled rapidly (< 1 Ma), yielding cooling rates > 1 200 ℃/Ma, consistent with existing models for the cooling of oceanic crust (Searle et al., 2003). This rapid cooling process may be the reason why well crystallized amphibolite has not been observed in petrographic and XRD studies. On the other hand, antigorite can form in amphibolite facies (Khalil, 2007); but there is no antigorite found in this sample indicating that the ferritchromit was formed during pre-serpentinization.

    When Al diffuses out of magmatic spinel and forms (Fe, Cr)-rich spinel, the reaction promotes the formation of chlorite. In this study, vein chlorite surrounds ferritchromit rims, indicating it formed after ferritchromit. The forming temperature of vein chlorite calculated by the Al is 289 ℃. However, the formation temperature of rim chlorite (outermost rim) beside magnetite is 214 ℃, which is lower than the formation temperature of the vein chlorite. The alteration of spinel group minerals may end at this temperature. The formation temperature of authigenic vein and rim chlorite indicates the hydrothermal metamorphic reaction is under greenschist facies to lower greenschist facies metamorphism (Hamdy and Lebda, 2011).

    In the hydrothermal metamorphic processes, temperatures decrease progressively from the cores to the rims for spinel in the most metamorphosed ultramafics (Sack and Ghiorso, 1991). Lee (1999) considered metamorphism and alteration of ultramafic rocks transformed spinel into ferritchromit at higher temperature (amphibolite to greenschist facies) or into Cr-bearing magnetite at lower temperature. Magnetite is frequently associated with serpentine polymorphs, and considered to be formed during syn- and post-serpentinization (Aswad et al., 2011; Mellini et al., 2005). Under lower temperature in serpentinization metamorphic processes, magnetite rim shows directly alteration of magmatic spinel (Fig. 3b) and mixture reaction with serpentinized olivine and pyroxene (Fig. 3a).

    It was reported that the serpentinite samples recovered from SWIR at 62ºE, east of the Melville fracture zone, suggesting pressure and temperature conditions at the beginning of deformation-mylonitization to have been less than 800 ℃ and 7 kb, and during cooling to decrease through the low amphibolite facies until nearly 300 ℃ and 2 kb (Bagsias and Triboulet, 1992). In this study, the alteration products of magmatic spinel indicate the abyssal peridotite from the easternmost part of SWIR (63ºE) has undergone amphibolite to lower-greenschist facies hydrothermal metamorphism, although the amphibolite facies metamorphic period was short according to the rapid crystallization of ferritchromit.

  • The Southwest Indian Ridge (SWIR) is one of the two ultraslow spreading ridges in the world. Due to its remote location, mineralogical studies and hydrothermal alteration of peridotite focused on SWIR are still rare. The geochemical characteristics of abyssal peridotite samples from one dredge station (63.5ºE/28ºS, water depth 3 660 m) on the ultraslow spreading SWIR were investigated. The studied abyssal peridotite is spinel lherzolite. During hydrothermal metamorphic reactions the magmatic Al-spinel changed into zoned texture with newly formed ferritchromit, magnetite, and chlorite. These mineral zones represent the events of partial melting of primary spinel, pre-serpentinization spinel metamorphism, and syn- and post-serpentinization spinel metamorphism, respectively. The detailed composition analysis of spinel and its alteration minerals indicate the easternmost part of SWIR is in an anomalously thin crust with a melt-poor system. The abyssal peridotite from dredge station may be affected by hydrothermal vents nearby, so it had undergone amphibolite to lower-greenschist facies hydrothermal metamorphism but with a rapid cooling process caused by sea water.

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