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Volume 28 Issue 3
Jun.  2017
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High Water Content in Primitive Mid-Ocean Ridge Basalt from Southwest Indian Ridge (51.56° E): Implications for Recycled Hydrous Component in the Mantle

  • The Southwest Indian Ridge (SWIR) is an ultraslow spreading end-member of mid-ocean ridge system and is characterized by weak or even an absence of magmatism. The segment between Indomed (ITF) and Gallieni (GTF) transform faults in the SWIR, however, displays extremely magmatic accretion with an unusual thick crust (up to 9.5 km). Although H2O is present in trace amounts in the mantle, it has a strong influence on mantle melting and magmatism in the shallow crust. The mid-ocean ridge basalts (MORB) worldwide show strong variation in H2O contents, but with a nearly uniform H2O/Ce ratio. Regionally distinctive H2O contents and H2O/Ce ratios are inferred to be related to the H2O variation in the source and can be used to constrain the mantle heterogenity. In this study, we measured the H2O and trace elements of clinopyroxene phenocrysts from one basalt dredged from the ITF-GTF segment, SWIR (51.56oE). The estimated H2O content (1.3 wt.%±0.3 wt.%) in the primitive ITF-GTF basaltic melt is much higher than that in typical MORB samples, but similar to oceanic island basalts (OIB) and back-arc basalts (BABB). In addition, the calculated H2O/Ce ratio (1 672–4 990) are extremely high, bearing "arc-like" signature. This study provides evidence that arc-related hydrous components are involved in the mantle source beneath the ITF-GTF ridge segment. It further lends support to the hypothesis that the mantle beneath the central SWIR may have experienced an ancient hydrous melting event in an arc terrain prior to or during the closure of the Mozambique Ocean in the Neoproterozoic.
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High Water Content in Primitive Mid-Ocean Ridge Basalt from Southwest Indian Ridge (51.56° E): Implications for Recycled Hydrous Component in the Mantle

    Corresponding author: Zhenmin Jin, zmjin@cug.edu.cn
  • 1. State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China
  • 2. Key Laboratory of Submarine Geosciences, The Second Institute of Oceanography, SOA, Hangzhou 310012, China

Abstract: The Southwest Indian Ridge (SWIR) is an ultraslow spreading end-member of mid-ocean ridge system and is characterized by weak or even an absence of magmatism. The segment between Indomed (ITF) and Gallieni (GTF) transform faults in the SWIR, however, displays extremely magmatic accretion with an unusual thick crust (up to 9.5 km). Although H2O is present in trace amounts in the mantle, it has a strong influence on mantle melting and magmatism in the shallow crust. The mid-ocean ridge basalts (MORB) worldwide show strong variation in H2O contents, but with a nearly uniform H2O/Ce ratio. Regionally distinctive H2O contents and H2O/Ce ratios are inferred to be related to the H2O variation in the source and can be used to constrain the mantle heterogenity. In this study, we measured the H2O and trace elements of clinopyroxene phenocrysts from one basalt dredged from the ITF-GTF segment, SWIR (51.56oE). The estimated H2O content (1.3 wt.%±0.3 wt.%) in the primitive ITF-GTF basaltic melt is much higher than that in typical MORB samples, but similar to oceanic island basalts (OIB) and back-arc basalts (BABB). In addition, the calculated H2O/Ce ratio (1 672–4 990) are extremely high, bearing "arc-like" signature. This study provides evidence that arc-related hydrous components are involved in the mantle source beneath the ITF-GTF ridge segment. It further lends support to the hypothesis that the mantle beneath the central SWIR may have experienced an ancient hydrous melting event in an arc terrain prior to or during the closure of the Mozambique Ocean in the Neoproterozoic.

0.   INTRODUCTION
  • As a main constituent of oceanic crust, the formation of MORB (mid-ocean ridge basalts) provides an opportunity to investigate the physical and chemical states of the Earth's upper mantle. The interpretation of MORB geochemistry, however, strongly depends on the understanding of partial melting process, as well as melts extraction and differentiation. In principle, the melting regime is affected by variables such as mantle potential temperature, mantle source compositions and surface conduction cooling (Robinson et al., 2001; White et al., 2001; McKenzie and Bickle, 1988; Klein and Langmuir, 1987). Recently, the presence of water in sub-oceanic basalts and mantle peridotites make water an unneglectable role in magmatic systems. Although water is only present in trace amounts in the sub-oceanic upper mantle, it is thought to play a signifycant rolein mantle melting, magma crystallization and the volcanic eruption at mid-ocean ridges (Asimow et al., 2004; Asimow and Langmuir, 2003; Gaetani and Grove, 1998). For example, water triggers mantle melting to initiate at deeper depth and enlarges the whole melting regime, which consequently increase the total melt production (Katz et al., 2003). Water addition will also suppress the crystallization of plagioclase (Danyushevsky, 2001). Furthermore, the temporal and spatial distribution of water bears significance on identifying mantle source heterogeneity (Asimow et al., 2004; Hauri et al., 1994), mantle convection (Jung and Karato, 2001; Hirth and Kohlstedt, 1996) and crust recycling (Dixon et al., 2002; Zhang and Zindler, 1993; Javoy et al., 1982).

    There were abundant studies to obtain H2O content in MORB glasses (Danyushevsky et al., 2000; Michael, 1995; Dixon et al., 1988). Previous study found that the H2O content of normal MORB (N-MORB) is below 0.2 wt.% (Danyushevsky et al., 2000). However, basalts from topographically swollen portions of mid-ocean ridges (i.e., ocean rise) have H2O concentrations higher than those of the N-MORB. For example, basalts from Galapagos Rise contain up to ~1.5 wt.% H2O, higher than those of the N-MORB, and basaltic lavas from Iceland and Azores Rise contain up to 1.02 wt.% (Nichols et al., 2002) and up to 1.8 wt.%-2.0 wt.% H2O (Métrich et al., 2014), respectively. These ocean rises are generally interpreted as being influenced by hot mantle plumes rooting from the mantle transition zone or from even deeper in the mantle (Dick and Zhou, 2015). Thus, the H2O contents of the mantle source of ocean rise basalts are probably higher than those of the N-MORB source. For example, the mantle source of the Icelandic and Azores Rise may contain 620 ppm-920 ppm and ~700 ppm H2O (Asimow et al., 2004; Nichols et al., 2002), respectively, which is several times higher than that of the N-MORB mantle source (50 ppm-200 ppm) (Hirschmann, 2006). Concerning off-ridge hotspots (expressed as oceanic island or seamount), a H2O content of 405 ppm±190 ppm has been estimated for the mantle source of Hawaiian basalts (Dixon and Stolper, 1995), which supports the hypothesis that plume-type mantle is H2O-rich relative to the N-MORB mantle. However, regional high water contents (up to 1.1 wt.%) in basaltic glasses were also found from a cold portion of mid-ocean ridges in the equatorial Atlantic Ocean (Ligi et al., 2005). These water-rich basalts are enriched in sodium and LREEs (i.e., E-MORB type) and were interpreted to be generated by low degrees of 'wet' melting (Ligi et al., 2005). Therefore, their observation links high H2O content to E-MORB, with no need of H2O-rich mantle plume sources. Until now, the global MORB data compilation shows that H2O contents are strongly variable. Over 90% MORB samples contain < 0.8 wt.% H2O (Fig. 1). The H2O contents in MORB glasses are positively correlated with the enrichment index such as (La/Sm)N (Fig. 1a), consistent with the interpretation that E-MORB may contain more H2O (Fig. 1b). It should be noted that some D-MORB (defined with (La/Sm)N < 0.8 according to Gale et al. (2013)) also contain high H2O (Fig. 1). Therefore, the genetic link between water-rich MORB source and mantle plume is still controversial.

    Figure 1.  (a) H2O plotted versus chondrite normalized La/Sm for global mid-ocean ridge basalt glasses. Note that two enrichment trends exist for global MORB glasses. (b) Histogram of H2O concentrations in different types of MORB glasses. D-MORB and E-MORB is defined with (La/Sm)N < 0.8 and (La/Sm)N > 1.5, respectively according to Gale et al. (2013). (c) H2O contents in melt inclusions from four mid-ocean ridges. All the melt inclusions are hosted in olivine. Orange squares are averages of each site (error bars are one standard deviation) and blue circles represent the maximum values. Grey vertical bar reflect average of all melt inclusions (i.e. 0.25 wt.%±0.13 wt.%). (d) Histogram of H2O contents in global MORB melts inclusions. All MORB glasses data are from the Pet DB database (http://www.petdb.org/). Melt inclusions from the East Pacific Rise, Mid-Atlantic Ridge, Juan de Fuca Ridge and Gakkel Ridge are from Kamenetsky et al. (1998), Saal et al. (2002), Shaw et al. (2010), Wanless and Shaw (2012), Wanless et al. (2014) and Wanless et al. (2015).

    Olivine-hosted melt inclusions have been increasingly used in several studies to determine the H2O content of primitive magmas at mid-ocean ridges. In contrast to erupted basaltic magmas, melt inclusions provide the following advantages: (1) because melt inclusions form prior to magma eruption, they are often trapped at pressures exceeding the pressure of eruption, and they are protected by their crystal host from eruptive degassing and contamination (e.g., McDonough and Ireland, 1993). (2) Melt inclusions trapped at high pressures can preserve original volatile contents (e.g., H2O, CO2, SO2 and Cl), rather than be degassed on eruption at lower pressures (Kent, 2008). Taking a look at the existing data on H2O contents in olivine-hosted melt inclusions from global MORBs (Figs. 1c and 1d), two important conclusions are obtained. First, the studied melt inclusions contain H2O contents (0.25 wt.%±0.13 wt.%) within the range of D-MORB type glasses. Second, the studied melt inclusions display a heterogeneous H2O distribution, independent of spreading rate (Fig. 1c). The heterogeneous H2O contents, therefore, probably reflect the mantle source heterogeneity at a local scale. For example, Wanless et al. (2014) found that the olivine-hosted melt inclusions from the eastern volcanic zone (EVZ) of the Gakkel Ridge show heterogeneous H2O distribution (0.16 wt.%-0.4 wt.%), with an increasing trend from the eastern to the western end of the EVZ. They attributed the elevated H2O contents to the hydrated metasomatized component in the mantle source. Furthermore, Koleszar et al. (2009) investigated the olivine-hosted melt inclusions from two lavas from the Galapagos Archipelago with contrasting compositions. Melt inclusions from the lava with high 3He/4He signature contain 0.64 wt.%-1.0 wt.% H2O, significantly higher than that in melt inclusions from the MORB-like lava (0.06 wt.%-0.12 wt.%). Again, it suggests that the water contents in primitive magma are related to the mantle sources.

    Although the existing data (MORB and/or melt inclusions) indicate that H2O heterogenity in MORB worldwide may exist at different sites and scales, it remains controversial to which extent they could reflect the initial H2O content in primary magma. In this study, we calculate the initial H2O content of a basalt from the ridge section between the Indomed (ITF, 46°E) and Gallieni (GTF, 52°20'E) transform faults in Southwest Indian Ridge (SWIR) using the H2O content of clinopyroxene (Cpx) phenocrysts crystallized from the primitive basaltic magma and the H2O partition coefficient between Cpx and basaltic melt. The inferred H2O content in the initial basaltic melt is significantly higher than that of N-MORB. Such high water is associated with an axial shallow region in SWIR. It provides the first evidence that H2O plays an important role in the generation of ITF-GTF basalts, with a hydrated mantle source being identified. The origin of such water-enriched component and potential influence of Crozet plume on the shallow mantle beneath the ITF-GTF segment will be discussed.

1.   GEOLOGICAL BACKGROUND AND SAMPLE DESCRIPTION
  • The Southwest Indian Ridge (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 which varies only slightly along the 7 700 km ridge axis (Sauter and Cannat, 2010). The ITF-GTF ridge section lies in the central shallow portion of the SWIR. Several nontransform faults were observed in this ridge section, resulting in an overall 15° spreading obliquity (Rommevaux-Jestin et al., 1997).Unlike other regions in SWIR (Chen et al., 2016; Seyler et al., 2003), ultramafic rocks (i.e., peridotites) were rarely exhumed on the seafloor between the ITF and GTF. Geophysical observations indicate the presence of thicker crust or hotter mantle between the ITF and GTF relative to other ridge sections in SWIR (Li et al., 2015; Niu et al., 2015; Zhao et al., 2013; Sauter et al., 2009; Cannat et al., 2008). Over the past 10 Ma, the ITF-GTF section experienced seafloor accretion, resulting in an axial shallower area (Sauter et al., 2009). This area was interpreted as a volcanic plateau due to a sudden increase of the magma supply that may be ascribed to a regional high mantle temperature provided by mantle outpouring from the Crozet hotspot toward the SWIR (Sauter et al., 2009).

    The studied basalt (20Ⅶ-L12) was collected at segment 25 (following the nomenclature of Cannat et al. (2009)) from SWIR during DY115-20 cruise on R/V Dayang Yihao (Fig. 2). This sample displays a variolitic texture varying from glassy quenched rims (~10 mm thick) to more devitrified textures towards the interior (Fig. 3a). Olivine (10 vol.%-15 vol.%) and plagioclase (5 vol.%-10 vol.%) are the dominant phenocrysts in this sample whereas Cpx occurs as rare (small) phenocrysts or macrocrysts (3-10 mm of grain size) in the groundmass (Figs. 3b and 3d). The Cpx phenocrysts are fresh and usually prismatic and subhedral to euhedral in shape (Fig. 3a). The intergranular texture of the matrix is composed of plagioclase, anhedral olivine, Cpx and ilmenite (Fig. 3c).

    Figure 2.  (a) Bathymetric map of the SWIR from Smith and Sandwell (1997). The inset indicates the global location of study area (red square): Southeast Indian Ridge (SEIR), Central Indian Ridge (CIR) and Rodrigues Triple Junction (RTJ). (b) Physiographic sketch of Indomed and Gallieni TF (after Sauter et al., 2009), showing sample locations (red filled circle). FZ. Fault zone.

    Figure 3.  (a) A polished section of sample 20Ⅶ-L12 collected from segment #25 between Indomed and Gallieni TF, SWIR. (b) Backscattered electron (BSE) image of a typical clinopyroxene phenocryst coexisting with olivine and plagioclase. Note that Cpx phenocryst shows compositional reverse zoning. (c) BSE image of matrix showing plagioclase, Cpx, ilmenite and olivine. (d) BSE image of a large Cpx phenocryst displaying compositional zoning.

2.   ANALYTICAL METHODS
  • The backscattered electron (BSE) images were firstly used to check the chemical homogeneity of each Cpx phenocryst. BSE images were obtained by a Quanta 450 FEG environment scanning electron microscope (ESEM) at the State Key Laboratory of Geological Processes and Mineral Resources (GPMR), China University of Geosciences (CUG), Wuhan. The major element compositions of Cpx phenocrysts were measured using a JEOL-JXA-8100 electron microprobe at the Second Institute of Oceanography (SIO). During quantitative analysis, the operating conditions were set as following: 15 kV accelerating voltage, 20 nA beam current and focused beam of 1 μm diameter. Natural minerals and synthetic oxides were used as standards and the data correction was obtained by a program based on the ZAF procedure. The reproducibility is < 1% for elements with concentration > 5% and < 3% for elements with concentration > 1%. The analyzed points were set within the Fourier Transform Infrared Spectroscopy (FTIR) analysis region.

    Trace element concentrations were determined by an Agilent 7500a laser-ablation ICP-MS spectrometer operating with a GeoLas 2005 laser at GPMR, China University of Geosciences (Wuhan). Detailed operating conditions for the laser ablation system and the ICP-MS instrument and data reduction are the same as described by Liu et al. (2008). Each analysis incorporated a background acquisition of approximately 20-30 s (gas blank) followed by 50 s data acquisition from the sample. The Agilent Chemstation was utilized for the acquisition of each individual analysis. The analyzed points were also set within the FTIR analysis region and the ablation diameter of the spot analyses was 44 µm. Element contents were calibrated against multiple-reference materials (BCR-2G, BIR-1G, NIST 610 and BHVO-2G) without applying internal standardization (Liu et al., 2008).

  • Cpx grains were handpicked from the crushed sample under a binocular microscope. Randomly oriented Cpx grains then were fixed with epoxy resin to make a double-polished section for FTIR analysis. The mineral thicknesses were measured by a double-pointed digital micrometer with ±1 μm precision, varying from 140 to 250 μm. Unpolarized spectra were obtained from 1 000 to 5 000 cm-1 on a Nicolet 6700 FTIR spectrometer coupled with a continuum microscope at GPMR, CUG (Wuhan), using a KBr beam splitter and a liquid-nitrogen cooled MCT-A detector. For each Cpx grain, a total of 128 scans were accumulated for each spectrum with a resolution of 4 cm-1. Several spots were set in the optically clean core areas (without crack and submicroscopic inclusions) and the aperture size was set 50×50 µm for measurement. Since they display almost same spectra, the average spectrum was used to calculate the H2O content of that grain. A modified Beer-Lambert law [c=A/(I×t)] was used to calculate the H2O content, where c is the water concentration in ppm H2O, A is the total integral absorption of OH bands (cm-2) which is 3 times of the integrated area (Kovács et al., 2008), I is the integral specific absorption coefficient (7.09 ppm-1·cm-2, Bell et al., 1995), t is thickness (cm). The total uncertainty of H2O content is estimated to be less than 30% (Xia et al., 2013).

    In order to examine the possibility of fine-scale water zoning, some large Cpx phenocrysts were analyzed using a Thermo Scientific Nicolet iN10 MX FTIR microscope at GPMA, CUG (Wuhan). The FTIR microscope is equipped with a single element MCT-A detector and an FPA (MCT-A) multichannel detector, and a KBr beam splitter. The FPA detector measurements were done in transmission mode by placing the samples on a BaF2 window on a high precision (1 mm accuracy) automated X-Y table, using a spectral resolution of 8 cm-1 with 128 scans on each spectrum in the wave number range of 4 000-600 cm-1.

3.   RESULTS
  • Some large Cpx phenocrysts display compositional zoning (Figs. 3b and 3d), indicating that they may have reacted with the host basaltic lava during ascent. Since hydrogen diffusion in Cpx during decompression may modify the initial H2O content and result in a heterogeneous distribution of water within a single grain, we only measured the central area of each Cpx grain in this study (see Fig. 4), in order to get real information of the initial and primitive melt. For each grain, 2-5 spots in the central area were firstly selected for FTIR and EPMA analysis. In individual grains, they show the same chemical compositions and IR spectra (Fig. 4). The FPA detector mapping of some large Cpx grains also revealed a remarkable homogeneity with respect to water content (Figs. 4c and 4d). Therefore, the average values of the analyzed spots of each grain were used to represent the major element and H2O contents of that grain. After that, LA-ICP-MS analyses were performed in the grain core to obtain trace element compositions.

    Figure 4.  (a), (b) Representative unpolarized FTIR spectra for two Cpx phenocrysts from basalt 20Ⅶ-L12. Insert shows three spots for FTIR analyses in each grain. Note that IR spectra of the three spots display similarities in OH band positions and heights. All spectra are normalized to a thickness of 1 mm. (c) Photomicrograph of Cpx grain (#37) displaying the area used for FPA mapping. (d) IR image of the indicated crystal area displaying the intensity of defect water peaks in the absorption range 3 700-3 200 cm-1. Note that water is homogeneously distributed. The apparent intensity increase is cause by melt inclusions, seen in (c).

    The Cpx grains are diopsidic with TiO2 and Na2O contents of 0.29 wt.%-0.46 wt.% and 0.26 wt.%-0.34 wt.%, respectively (Table S1). Cr2O3 contents are generally less than 1 wt.% and they are positively correlated with Al2O3 (2.33 wt.% to 3.89 wt.%) (Fig. 5a). In general, the major element compositions of Cpx grains are different from those in the SWIR abyssal peridotites (Fig. 5a). Therefore, it rules out the possibility of mantle-derived xenocrysts. CaO contents are negatively correlated with MgO (Fig. 5b). The Mg# (=100×Mg/(Mg+Fe)) of Cpx are 84.0 to 88.0, corresponding to a Mg# of 65-70 for the equilibrated basaltic melts using the experimental Mg-Fe partition coefficient (0.34±0.04, Kinzler, 1997). The trace element concentrations of Cpx phenocrysts are given in Table S1. All the Cpx phenocrysts display nearly consistent HREE patterns with depleted MORB mantle (DMM) concentrations but steeply plunging LREE (Fig. 6).

    Figure 5.  (a) Cr2O3 versus Al2O3 and (b) CaO versus MgO plot for Cpx in 20Ⅶ-L12 basalt from ITF-GTF segment, SWIR. Cpx in SWIR peridotites are plotted for comparison (Warren and Shimizu, 2010; Warren et al., 2009; Seyler et al., 2003). The compositions of Cpx in DMM is from Workman and Hart (2005).

    Figure 6.  Plots of REE variations of Cpx phenocrysts in 20Ⅶ-L12 basalt from ITF-GTF segment, SWIR. Concentrations are normalized to chondrite (Anders and Grevesse, 1989). The compositions of Cpx in DMM is from Workman and Hart (2005).

    The IR absorption spectra of the 20Ⅶ-L12 Cpx can be divided into four different groups: 3 630-3 620, 3 540-3 520, 3 470-3 450 and 3 350-3 360 cm-1 (Figs. 4a and 4b). The absorption band positions of the 20Ⅶ-L12 Cpx phenocrysts are similar to those reported in earlier studies, and are interpreted as resulting from the vibration of structural OH (Xia et al., 2016, 2013; Wang et al., 2013; Sundvall and Stalder, 2011; Green et al., 2010; Li et al., 2008; Grant et al., 2007; Peslier et al., 2002; Ingrin and Skogby, 2000; Bell and Rossman, 1992; Skogby et al., 1990; Skogby and Rossman, 1989). The H2O contents of 35 phenocrysts are 60 ppm-312 ppm (average 157 ppm±58 ppm) by weight.

4.   DISCUSSION
  • Among the 35 analyzed Cpx phenocrysts, 5 Cpx grains have Mg# > 86 (Table S1). These values are similar to those of the most forsteritic olivine phenocrysts (unpublished data) from ITF-GTF basalts. CaO negatively correlate with MgO (Fig. 5b), which indicates that these high Mg# Cpx phenocrysts were co-crystallized with the forsteritic olivine from a nearly primary basaltic magma. Therefore, the H2O and Ce contents in the 5 Cpx phenocrysts can be used to infer the H2O and Ce content of the primitive magma. Using the specific partition coefficients of H2O (see APPENDIX I) and H2O contents of Cpx, the calculated H2O contents in primitive melt is from 0.9 wt.% to 1.8 wt.%, at average H2O=1.3 wt.%±0.3 wt.% (Table S1). As shown in Fig. 7a, within the 40% uncertainty, the H2O content of the primitive basaltic magma is higher than typical values of N-MORB (~0.1 wt.%-0.3 wt.%) (Asimow et al., 2004; Saal et al., 2002; Simons et al., 2002; Danyushevsky et al., 2000; Sobolev and Chaussidon, 1996; Michael, 1995, 1988; Dixon et al., 1988), but significantly lower than island arc basalt (IAB) (2 wt.%-8 wt.%) (Plank et al., 2013; Wallace, 2005; Dobson et al., 1995; Sisson and Layne, 1993). Instead, these values fall in the range of BABB (0.2 wt.%-2 wt.%) (Stolper and Newman, 1994; Danyushevsky et al., 1993; Hochstaedter et al., 1990) and OIB (0.3 wt.%-1.9 wt.%) (Nichols et al., 2002; Simons et al., 2002; Dixon and Clague, 2001; Wallace, 1998; Dixon et al., 1997). The elevated H2O contents in the primitive ITF-GTF magma, therefore, indicate an addition of water either from a mantle plume or back arc related process. Applying the similar calculation (see APPENDIX I), the Ce contents in the primitive magma are 2.7 ppm to 6.7 ppm. In contrast to the typical MORB and OIB samples, the H2O/Ce ratios in primitive ITF-GTF melts are extremely high (1 672-4 990) and fall in the range of BABB and IAB (Fig. 7b). Although the ITF-GTF segment is far from any subduction related terrain in SWIR at present, in the Neoproterozoic, the central SWIR mantle experienced a melting event in an arc terrain prior to or during the closure of the Mozambique Ocean, and the subsequent assembly of Gondwana (Dick and Zhou, 2015; Gao et al., 2016). Therefore, the high H2O contents and H2O/Ce ratios most likely reflect the contribution of such arc-related hydrous components in the mantle source.

    Figure 7.  Comparison of the initial H2O content and H2O/Ce ratios of the ITF-GTF basalt with those of MORB, OIB, BABB and IAB. All MORB glasses data are from the PetDB database (http://www.petdb.org/).

  • Based on the experimentally determined partition coefficients between peridotite and basaltic melt (O'Leary et al., 2010; Hirschmann et al., 2009), the calculated H2O contents in primitive ITF-GTF magma can be further used to constrain the H2O contents in the peridotite source using a partial melting model with variable melting degrees (10%-20%). This calculation shows that the peridotite source should hold at least 900 ppm H2O (see APPENDIX I). Clearly, such a value is > 4 times higher than the H2O content of the MORB source (50 ppm-200 ppm) (Green et al., 2010; Asimow et al., 2004; Saal et al., 2002; Simons et al., 2002; Sobolev and Chaussidon, 1996; Michael, 1995, 1988; Dixon et al., 1988). It is also over 8 times higher than that of abyssal peridotites ( < 118 ppm, Hesse et al., 2015; Warren and Hauri, 2014; Peslier et al., 2007), which are regarded as the residue after MORB extraction (Dick et al., 1984). Moreover, this value is considerably higher than that (~190 ppm) reported in an experimental study of Green et al. (2010), which applied fully buffered fertile lherzolite compositions in equilibrium with hydrous melt. Thus, our calculation is not consistent with the traditional DMM source model.

    Recently, it was suggested that the oceanic upper mantle may contain a certain amount of fusible lithologies such as volatile-bearing peridotite and mafic eclogite or pyroxenite (Zhang et al., 2012; Sobolev et al., 2007; le Roux et al., 2002). Specifically, pyroxenite is thought to play a significant role in the petrogenesis of E-MORB and OIB. In contrast to peridotites, pyroxenite may contain high H2O contents (up to 467 ppm, Bizimis and Peslier, 2015), which is similar to OIB source estimates (like Hawaii). It is noted that our results also fall in the range (albeit at the high side) of estimates for E-MORB or OIB sources (300 ppm-1 000 ppm) (Tenner et al., 2012; Asimow et al., 2004; Dixon et al., 2002; Hauri et al., 1994, and references therein). Since the partition coefficient of H2O between pyroxenite and melt (DH2OPeridotite/melt=0.015) is only slightly higher than that for peridotite (DH2OPeridotite/melt=0.005-0.013), and small variations in partition coefficients will not significantly change our results (see APPENDIX I). Apparently, pyroxenite can be a potential lithology in the ITF-GTF mantle. However, we infer that the pyroxenite cannot fully explain the high H2O and H2O/Ce ratio of ITF-GTF primitive melt for the following two reasons. First, the young basaltic lavas from ITF-GTF ridge segment did not display any E-MORB or OIB-like geochemical signatures (e.g., enriched LREE). Elevated H2O/Ce ratios cannot be imported to depleted sources by mixing with enriched components without causing the MORB to appear enriched in terms of ratios of highly to moderately incompatible elements (e.g., La/Sm and Rb/K) (Michael, 1995). Second, pyroxenites always display low H2O/Ce ratios ( < 100, Bizimis and Peslier, 2015; Gibler et al., 2014; Warren and Hauri, 2014). During mantle melting along a typical oceanic geotherm, the pyroxenite-derived melts will inherit the low H2O/Ce signature of the pyroxenite source (Bizimis and Peslier, 2015). Therefore, the high H2O and H2O/Ce of ITF-GTF primitive melt cannot be fully explained by pyroxenite in the source.

    The source of H2O and other trace elements in the MORB could also be recycled materials, which is strongly related to subduction process. The arc-related hydrous melting events in the central SWIR were inferred in the Neproterozoic (Gao et al., 2016). In addition, the ITF-GTF basalts show arc-like geochemical signatures, such as strong LILE enrichments and HFSE depletion (Gale et al., 2013; Yang et al., 2013; Font et al., 2007; Bézos et al., 2005; Meyzen et al., 2003; Li et al., unpublished data). It then indicates that the elevated water in the source of ITF-GTF basalts was from the released fluids from the Neoproterozoic arcterranes, which is delaminated and stagnated in the normal SWIR asthenosphere.

  • It was inferred that the ITF-GTF ridge section experienced a suddenly enhanced magmatism about 10 Ma ago, which resulted in the off-axis shallow domains (i.e., volcanic plateau) in this ridge section (Sauter et al., 2009). Such magmatism is probably still active at segment #27 but weak or ended along the remaining part of the ITF-GTF section (e.g., segment #25, Fig. 2a) (Sauter et al., 2009). Furthermore, based on the physiographic characteristics of ITF-GTF section, Sauter et al. (2009) inferred that the melt supply anomaly is not related to the change of the ridge obliquity, but to the higher mantle temperatures caused by Crozet hotspot. Consider that the ITF-GTF ridge section lie on the rifted Marion Rise, which is supported by compositionally buoyant depleted mantle (Zhou and Dick, 2013). Along a normal oceanic geotherm and without external enhanced heat, extensive melting cannot occur for such refractory mantle to produce a normal to even unusual thick (~10 km) oceanic crust at ITF-GTF section (Li et al., 2015; Niu et al., 2015). The Crozet hotspot, therefore, provides a good explanation for the heat source of the enhanced magmatism since 10 Ma ago. The young on-axis magmatism is probably due to the continued influence of Crozet hotspot. An alternative explanation for such an enhanced melt production is low-melting components in the mantle source. Such low-melting components can be fusible lithologies such as pyroxenite or hydrousmetasomatized peridotite. In this study, we have identified the arc-related hydrous components in the mantle source based on the high H2O and H2O/Ce ratio of ITF-GTF primitive melt. It is likely that Crozet hotspot triggered the release of H2O frozen in hydrous minerals during melting of such hydrous component. Alternatively, Crozet hotspot may also feed fertile mantle laterally to the ITF-GTF ridge section. However, ITF-GTF MORB does not display any OIB like signature in trace elements, nor they show any direct Sr-Nd-Pb isotopic affinity to Crozet basalts (Breton et al., 2013). Although Breton et al. (2013) proposed that the shallow mantle beneath the ITF-GTF segment is contaminated by deep material from the Crozet plume based on Sr-Nd-Pb-He isotopic compositions of basalts from the Crozet Archipelago, it remains controversial on the chemical interaction between ITF-GTF and Crozet hotspot.

5.   CONCLUSIONS
  • The estimated H2O content in the primary ITF-GTF basalt is higher than those of normal MORB (N-MORB), but similar to that of BABB and OIB. In addition to the "arc-like" geochemistry of ITF-GTF basalts, the elevated H2O content and H2O/Ce ratios in primary ITF-GTF magma provide evidence that arc-related hydrous components are involved in the mantle source. It is likely that the Crozet hotspot provided the heat source and consequently triggered the enhanced magmatism since 10 Ma ago.

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