Journal of Petrology Advance Access originally published online on January 9, 2007
Journal of Petrology 2007 48(4):647-660; doi:10.1093/petrology/egl076
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Pyroxenites from the Southwest Indian Ridge, 9–16°E: Cumulates from Incremental Melt Fractions Produced at the Top of a Cold Melting Regime
1CNRS-UMR5562, Dynamique Terrestre et Planétaire, Observatoire Midi Pyrénées, Université Paul Sabatier, 14, Av. E. Belin, 31400 Toulouse, France
2CNRS-UMR5563, Laboratoire D’etudes des Mécaniques de Transferts en Géologie, Observatoire Midi Pyrénées, Université Paul Sabatier, 14, Av. E. Belin, 31400 Toulouse, France
3Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA
RECEIVED JUNE 23, 2006; ACCEPTED NOVEMBER 24, 2006
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONGEOLOGICAL SETTINGANALYTICAL METHODSPETROGRAPHYMINERAL COMPOSITIONSDISCUSSIONCONCLUSIONREFERENCES |
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The Southwest Indian Ridge (SWIR) at 9–16°E and 52–53°S is characterized by ultra-slow, oblique spreading and contains one of the few documented occurrences of pyroxenite veins associated with abyssal peridotites. The origin of these uncommon lithologies is still debated. We present a detailed study (including electron microprobe and laser ablation inductively coupled plasma mass spectrometry) of spinel websterites collected during Cruise 162, Leg 9, of the R.V. Knorr. Rare earth element patterns in clinopyroxenes (Cpx) lead us to discard a possible origin of the pyroxenites as residues from partial melting of garnet pyroxenites (i.e. relics of a layered mantle protolith). Their composition and cumulate texture (when not obscured by mylonitization related to emplacement on the seafloor) are better interpreted in terms of fractional crystallization from a basaltic melt at relatively high pressure. Evidence for a high pressure of crystallization includes the lack of plagioclase in the cumulate assemblage and the high Al2O3 contents of the pyroxenes: up to 5 wt % in orthopyroxene (Opx) and up to 7 wt % in Cpx. These values are among the highest reported for pyroxenes in a mid-ocean ridge setting. Sub-solidus breakdown of spinel to plagioclase (now altered) is observed in one sample, providing a rough estimate of the final equilibration pressure of these cumulates, around 0· 6–0· 7 GPa (plagioclase–spinel transition for a bulk pyroxenite composition). The inferred pyroxenite parent melts were close to equilibrium with the associated residual peridotites; some samples have a slightly evolved composition in terms of the Mg-number [Mg/(Mg + total Fe)]. These parental melts had major and trace element compositions consistent with a mid-ocean ridge basalt (MORB) affinity, although they were not rigorously identical to MORB. Among other characteristics, these melts were relatively depleted in highly incompatible elements. We propose that they correspond to the latest, shallowest, incremental melt fractions produced during fractional decompression melting of a normal MORB (N-MORB) mantle source. These melts experienced fractional crystallization as soon as they segregated from the peridotite matrix, moved upward, and crossed the lithosphere–asthenosphere boundary (defined here as the base of the conductive lid). As a consequence, these shallow melt fractions produced beneath mid-ocean ridges did not fully mix with melt fractions produced and extracted at greater depths. Our study provides concrete evidence for the actuality of pyroxene crystallization in melt channels beneath mid-ocean ridges at relatively high pressures, a process frequently invoked to account for the ‘pyroxene paradox’ in MORB petrogenesis.
KEY WORDS: abyssal pyroxenites; cumulates; lithospheric mantle; melt migration; Southwest Indian Ridge
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGANALYTICAL METHODSPETROGRAPHYMINERAL COMPOSITIONSDISCUSSIONCONCLUSIONREFERENCES |
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Deep-seated processes related to melt production, migration and crystallization in the shallow mantle beneath oceanic spreading centers are poorly constrained by direct observations. The petrological and geochemical characteristics of mid-ocean ridge basalts (MORBs) call for a heterogeneous source and complex crystallization and/or melt–rock interaction processes at mantle pressures; however, these are indirect forms of evidence whose interpretation remains rather ambiguous. The influence of pyroxene on the differentiation trend of MORBs, which are virtually devoid of pyroxene phenocrysts (the ‘pyroxene paradox’; Francis, 1986
; Grove et al., 1992), is one of a number of problems that remain to be resolved. A precise knowledge of the physical nature and geometry of the melt plumbing system and of the petrological processes active during melt extraction is necessary to better understand MORB systematics. The study of mantle sections of ophiolites helps to fill this observational gap (e.g. Ceuleneer & Rabinowicz, 1992; Kelemen et al., 1995; Benoit et al., 1996, 1999; Ceuleneer et al., 1996; Python & Ceuleneer, 2003); however, the relevance of most ophiolite studies to mid-ocean ridge settings is questionable.
The origin of clinopyroxene (Cpx) in mantle peridotites is an important issue, as the behaviour of many major and trace elements used in petrological modelling is largely controlled by Cpx. A residual vs cumulate origin of the Cpx scattered in abyssal peridotites has been considered by Seyler et al. (2001). Those workers concluded, on the basis of textural and geochemical evidence, that a large proportion of these Cpx are actually crystallization products from interstitial melts that percolated through the mantle and not a residual phase after partial melting of a more fertile mantle source.
Analysis of the geochemical composition of pyroxenite layers and veins in abyssal peridotites can provide a new kind of observational constraint on the general problem of genesis of MORBs and their differentiation at high pressure. In contrast to the Cpx distributed in residual peridotites, pyroxenes found in veins provide unambiguous evidence that they are the crystallization products of former melt segregations. A major unknown remains, however, concerning the age of their formation: are they ancient or recent melt segregations? In the latter case, they can be simply interpreted as crystallization products from melts produced during the present-day spreading event. In the former case, they can be viewed as components of a layered, ‘marble cake’ mantle—in which case their petrological and geochemical characteristics need to be interpreted in terms of a polygenetic history. If they formed initially as cumulates or trapped melts equilibrated at high pressure but that experienced more recent episodes of decompression melting, they could be considered as residues of partial melting of lithologies such as garnet pyroxenite or eclogite, which are less refractory than their host peridotite (Dick & Sinton, 1979; Hirschmann & Stolper, 1996). Should this be the case, inversion of MORB compositions in terms of partial melting degree and mantle temperature assuming a mineralogically homogeneous source (e.g. Langmuir et al., 1992) would be flawed.
Pyroxenites are particularly uncommon among samples drilled or dredged along mid-ocean ridges. Only a few occurrences have been reported (Dick et al., 1984; Fujii, 1990; Juteau et al., 1990). Accordingly, their actual abundance at depth, their petrogenesis and their role in MORB formation remain largely unknown. Ultra-slow spreading of the SWIR from 9 to 16°E is particularly favourable to the exhumation of mantle lithologies. During R.V. Knorr Cruise 162, Leg 9, seven samples of pyroxenite veins in mantle peridotite were dredged. This study presents the results of a petrographic and geochemical study of these pyroxenites. The use of in situ analysis of primary mineral phases, by electron microprobe and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), allows us to circumvent the effects of hydrothermal alteration, which is a general problem with seafloor samples.
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GEOLOGICAL SETTING |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGANALYTICAL METHODSPETROGRAPHYMINERAL COMPOSITIONSDISCUSSIONCONCLUSIONREFERENCES |
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The SWIR is
8000 km long and extends from the Bouvet Triple Junction in the SW to the Rodriguez Triple Junction in the NE (Fig. 1). The spreading rate varies between slow and ultra-slow (i.e. full spreading rates ranging from <50 mm/year to <20 mm/year). From 9 to 16°E, the angle of spreading is highly oblique to the spreading direction by (up to 60°; Dick et al., 2003). As with other slow and ultra-slow spreading mid-ocean ridges (Dick et al., 2003), the SWIR is highly segmented and characterized by episodic melt delivery. This results in marked variations in the thickness of the basaltic crust from one segment to another and within a single segment (Cannat et al., 1999; Dick et al., 2003). Deep-seated mantle-derived peridotites and gabbroic cumulates are frequently exposed on the seafloor.
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During R.V. Knorr Cruise 162, Legs 7–9 (2000–2001), the 9–16°E Oblique Supersegment of the SWIR was mapped and extensively dredged for the first time. A few pyroxenite samples, among hundreds of abyssal peridotites and basalts, were collected in Dredges 47 and 55, the location of which is shown in Fig. 1. These dredges are from the 11°30'E–14°24'E section, where the ridge is at an angle of 32° to the spreading direction, resulting in an effective full spreading rate of 7· 8 mm/year (Dick et al., 2003
). As there is no evidence of recent volcanism, this segment is considered as ‘amagmatic’. In Dredge 47, pyroxenites make up 7% of the samples, with most other samples being residual harzburgites and lherzolites. In Dredge 55, pyroxenites constitute only 1% of the collected samples, the rest of which are basalts, diabases, gabbros, and peridotites.
The combined effect of ultra-slow spreading and obliquity leads to particularly low degrees of melting in the upwelling mantle, estimated to be <5% (Dick et al., 2003). This should prevent efficient mixing of individual melt fractions and thus preserve the geochemical signatures of mantle source heterogeneity (Python, 2001). Basalts from the Oblique Supersegment include alkaline lavas and enriched MORB (E-MORB) (Le Roex et al., 1992; Standish et al., 2005), which are uncommon in an oceanic spreading setting. Incompatible element enrichment could be attributed to very low degrees of partial melting of a source deeper than the average MORB source. This source could have been contaminated by the Bouvet mantle plume, currently located 700 km to the west of the Oblique Supersegment, but estimated to have been under this portion of the ridge at 20 Ma (Georgen et al., 2003). Eruption of alkaline lavas probably occurred during a gap in the production of tholeiites, possibly related to a readjustment in the spreading direction experienced by the SWIR (Le Roex et al., 1992).
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ANALYTICAL METHODS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGANALYTICAL METHODSPETROGRAPHYMINERAL COMPOSITIONSDISCUSSIONCONCLUSIONREFERENCES |
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Major and minor element concentrations in individual mineral phases (Ol, Cpx, Opx, and Sp) were determined with a CAMECA SX50 electron microprobe with SAMx automation at the Paul Sabatier University, Toulouse, France, using wavelength-dispersive spectrometry (WDS). Analyses were performed with an accelerating voltage of 15 kV, beam current of 20 nA and a spot size of 4 µm2. The following synthetic and natural minerals standards were used: albite (Na), periclase (Mg), corundum (Al), sanidine (K), wollastonite (Si, Ca), pyrophanite (Mn, Ti), hematite (Fe), chromium oxides (Cr), and nickel oxides (Ni). Nominal concentrations were corrected by the PAP data reduction method (Pouchou & Pichoir, 1985
). The detection limits range from 0· 07% for Al2O3 and TiO2 to 0· 09% for Cr2O3 and Na2O, for all mineral phases analysed. Concentrations of trace elements (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu, Rb, Ba, Th, Sr, Zr, Ti, Y, Ni, V and Sc) were determined by LA-ICP-MS, at the Paul Sabatier University (Toulouse). This allowed in situ analysis of Cpx in >120 µm thick polished sections. The Perkin Elmer 6000 ICP-MS instrument was coupled to a Cetac LSX-200 laser ablation module with a 266 nm frequency-quadrupled Nd–YAG laser. The NIST 610 glass standard was used to calibrate relative element sensitivities. Each analysis was normalized using CaO values determined by electron microprobe. Clinopyroxenes in five websterites from Dredge 47 were analyzed. The inter-cleavage area in the cores of the freshest cpx grains were ablated to obtain homogeneous results unaffected by alteration or exsolution processes. A beam diameter of 100–150 µm and a scanning rate of 20 µm/s were used. Typical theoretical detection limits are 10–20 ppb for rare earth elements (REE), Ba, Rb, Th, Sr, Zr and Y; 100 ppb for Sc and V; and 2 ppm for Ti and Ni. The relative precision and accuracy for a laser analysis ranges from 1 to 10%.
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PETROGRAPHY |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGANALYTICAL METHODSPETROGRAPHYMINERAL COMPOSITIONSDISCUSSIONCONCLUSIONREFERENCES |
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Seven pyroxenite samples were selected for detailed study (Table 1). Most of these (six samples out of seven) are websterites and olivine websterites from Dredge 47, with a seventh websterite coming from Dredge 55, <100 km away. The primary mineral assemblage in the websterites consists of orthopyroxene (Opx), clinopyroxene (Cpx), olivine (Ol) and chrome spinel (Cr-Sp). Excluding the low-temperature alteration phases, the modal composition of the websterites is 35–50% Opx, 40–60% Cpx, and minor amounts of Cr-Sp (1–10%) and Ol (0–15%). Samples D47-12, D47-25 and D47-62 are devoid of olivine. This estimate is not very accurate because of the combined effects of alteration, small sample sizes and sample heterogeneity (coarse and heterogeneous grain sizes). All the websterites are deformed to some degree. Two samples, D47-10 and D47-12, have undergone greater degrees of plastic deformation, resulting in mylonitic textures, as shown in Fig. 2d. In these samples the websterites occur as layers parallel to the host peridotite mylonitic foliation plane, as shown in Fig. 2a. Mylonitized peridotites are diagnostic of high strain rates, stress and lower temperature (<1000°C) conditions (Mercier & Nicolas, 1975
; Jaroslow et al., 1996). Kink bands are evident in most of the pyroxene porphyroclasts. The matrix is fine-grained (10–100 µm). All boundaries between porphyroclasts are highlighted by micro-mylonitic shear zones. Even when highly deformed, relics of coarse (up to 10 mm) grains are preserved in all the samples, allowing us to determine their pristine igneous geochemical characters. The layer boundaries are slightly boudinaged, indicating the importance of plastic deformation during exhumation of these veined peridotites. The layer thickness varies from 1 to 3 cm in the thickest parts of the boudins. Contact relationships with the host peridotites are not preserved in the other five samples, but they do not have similar degrees of deformation.
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Five samples are weakly to moderately deformed and are derived from coarse-grained (>8 mm) lithologies. Deformation is indicated by undulose extinction and bending of cleavage and exsolution lamellae, as shown in Fig. 2c. Grain-size reduction at the boundaries of coarser grains has led to the development of medium-grained (2–3 mm) mosaic textures with well-developed 120° triple junctions, which are diagnostic of textural equilibrium and static recovery at high temperature (>1000°C; Mercier & Nicolas, 1975
). These textures are particularly well developed in samples D47-62 and D47-63. Olivines occur as small relict grains surrounded by mesh-textured serpentine. Pyroxenes have well-developed exsolution lamellae. Cpx grains are anhedral and interstitial relative to Ol and Opx grains. Where the igneous texture is well preserved, inclusions of Ol and Opx in Cpx are frequently observed. Spinel is generally dark brown except in the Dredge 55 sample, where it is greenish brown. Sample D55-71 contains peculiarly coarse-grained Cr-spinels (up to 4 mm diameter). In this sample, spinel is anhedral and sparsely distributed in a fine-grained, deformed matrix of Ol, Cpx and Opx. The spinels have coronae of fine-grained chlorite and actinolite needles, which are probably alteration products of primary plagioclase (Fig. 2b). Sulfides have been observed in the mylonitized samples.
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MINERAL COMPOSITIONS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGANALYTICAL METHODSPETROGRAPHYMINERAL COMPOSITIONSDISCUSSIONCONCLUSIONREFERENCES |
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Major elements
Average compositions of the four primary igneous mineral phases in the websterites are presented in Tables 2–5
. All mineral phases are unzoned in major elements. The olivine that occurs in four of the websterites is highly magnesian, ranging from Fo88· 9 to Fo90· 5. NiO contents range from 0· 33 to 0· 53 wt %, with the lower value corresponding to sample D47-10. Orthopyroxenes are enstatites, with Mg-number ranging from 89 to 90. They have a low TiO2 content (<0· 12 wt %) and high Al2O3 (3· 6–5· 3 wt %) and Cr2O3 (0· 4–0· 8 wt %) contents.
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The clinopyroxenes are diopsides with Mg-number [100 x Mg/(Mg + Fetot)] ranging from 90 to 91. Figure 3a indicates that they are highly aluminous, with Al2O3 contents ranging from 5· 3 (D55-71) to 6· 9 wt % (D47-22). Their Na2O content is also high (0· 38–0· 63 wt %) relative to abyssal pyroxenite Cpx from other localities, as shown in Fig. 3cs. Cpx compositions in pyroxenites from the Mid-Atlantic Ridge at the Kane Fracture Zone (MARK area) have been documented in Ocean Drilling Program (ODP) Hole 670 (Fujii, 1990
; Juteau et al., 1990) and ODP Hole 920D (Kempton & Stephens, 1997). These pyroxenites have significantly lower Al2O3 contents (5· 5 wt % and 5 wt %, respectively) and are Na2O poor (0· 15 wt % and 0· 05 wt %, respectively). Figure 3a and c also indicates that interstitial clinopyroxenes disseminated in SWIR and MARK abyssal peridotites have similar Na2O and Al2O3 contents to the websterite Cpx from this study (Seyler et al., 2001). Moreover, their TiO2 content ranges from 0· 2 to 0· 5 wt %, values that are higher than those for Cpx from ODP sites 670 and 920 (<0· 15 wt %) but similar to those of peridotites from Dredge 47 (Fig. 3c). Figure 3b demonstrates that the Oblique Supersegment pyroxenite Cpx Cr2O3 contents (1–1· 3 wt %) are similar to those of abyssal peridotites. Among abyssal peridotites, the most ‘fertile’ sample (AT 196AE, Romanche Fracture Zone) ever found contains a Cpx characterized by high Al2O3 (7· 8 wt %) and Na2O (>1· 3 wt %) but a less magnesian olivine (Fo89) and a spinel characterized by a very low Cr-number (10; Seyler & Bonatti, 1994).
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, websterites from this study; 
, SWIR peridotites (Johnson et al., 1990
, MARK websterites (Fujii, 1990
, MARK peridotites (Fujii, 1990Websterite spinels have Mg-number ranging from 64· 9 to 74· 7 (Fig. 4). Their Cr-number [100 x Cr/(Cr + Al)] varies between dredges: samples from Dredge 47 have a Cr-number ranging from 13· 4 and 17· 2 whereas the Dredge 55 sample has a Cr-number of
28. The latter is therefore a Mg–Al chromite whereas the former (Site 47) are Cr-pleonastes (Haggerty, 1981). The Dredge 47 Cr-pleonastes are also TiO2 poor (0· 02–0· 08 wt %) whereas the Dredge 55 Mg–Al chromites have higher TiO2 contents (0· 2 wt %).
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Clinopyroxene trace element contents
Cpx from the websterite veins analyzed in this study have REE patterns similar to abyssal peridotite Cpx; that is, depleted in light REE (LREE) relative to the middle REE (MREE) and heavy REE (HREE), but with higher absolute REE concentrations in the Cpx of the websterites (Fig. 5). All analysed Cpx have MREE and HREE contents four to seven times chondrite. Dredge 47 websterite Cpx have very similar and homogeneous REE concentrations, with LREE depletion, as shown in Fig. 5. The CeN/YbN ratio is close to 0· 035 in Cpx from samples D47-62 and D47-25, and close to 0· 012 for the other samples. The CeN/SmN ratio ranges from 0· 02 to 0· 05. REE patterns of Cpx from SWIR abyssal peridotites (Johnson et al., 1990
; Johnson & Dick, 1992), including Dredge 47 peridotites (J. Warren, unpublished data), are shown for comparison in Fig. 5.
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The extended normalized trace element patterns—including Ba, Sr, Th, Ti and Zr—largely mimic the REE patterns, with incompatible element depletion. Cpx from the websterites in this study are homogeneous in their high field strength elements (HFSE) and other incompatible trace element contents (Fig. 6). Their trace element patterns are characterized by a marked depletion in the most incompatible elements and a higher concentration of moderately incompatible elements relative to SWIR abyssal peridotite Cpx. Superimposed on this general pattern is a negative anomaly in HFSE (Zr and Ti) and a slight positive Th anomaly (Fig. 6).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGANALYTICAL METHODSPETROGRAPHYMINERAL COMPOSITIONSDISCUSSIONCONCLUSIONREFERENCES |
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The petrographic and geochemical data presented in the previous sections allow us to infer the processes involved during the genesis of the SWIR Oblique Supersegment websterite veins.
Where did the pyroxenites form and crystallize?
SWIR pyroxenites occur as relatively thin veins (< 3 cm thick); their contact relationships with the host peridotites (although somewhat obscured by alteration) are relatively clear-cut, pointing to an efficient melt segregation and channellization process in the mantle before crystallization of the pyroxenitic assemblage. A textural continuum exists between samples that preserve magmatic textures and samples that show evidence for high degrees of plastic deformation. The least deformed pyroxenites are coarse-grained rocks that look like adcumulates with evidence for slightly later crystallization of clinopyroxene relative to orthopyroxene and olivine. In the samples that underwent plastic deformation, evidence for textural equilibrium and preservation of coarse grain size suggest that crystallization and re-crystallization of pyroxenites occurred while the host peridotite was still very hot. These textural arguments do not demonstrate but do support the view that the crystallization of the pyroxenites, and possibly the melt segregation event itself, took place close to the asthenosphere–lithosphere boundary, defined here as the limit between a hot (close to solidus temperature) mantle deformed by penetrative high-temperature, low-stress plastic flow (the asthenosphere) and a slightly colder, more rigid mantle, the lithosphere. This transition is classically estimated to occur around about 1200°C (see Ceuleneer & Rabinowicz, 1992, and reference therein).
One sample has preserved evidence of transformation of spinel into plagioclase. For a pyroxenitic bulk composition, the Spinel + Opx + Cpx 
Plag + Ol reaction occurs at P–T conditions estimated to be around 0· 6–0· 7 GPa and 1000–1250°C (Gasparik, 1984). The breakdown of spinel into plagioclase requires diffusion, and thus high temperatures, to be effective. Accordingly, it is likely that it occurred soon after pyroxenite crystallization, while the mantle had not yet undergone a significant amount of conductive cooling. This allows us to propose a rough estimate for the depth of crystallization of the SWIR websterites of c. 20 km. The peculiarly high Al2O3 content of the pyroxenes is a further argument for a relatively high pressure of crystallization and/or re-equilibration for these pyroxenites (e.g. Seyler et al., 2001). The formation of websterites, indicating crystallization of melts at depths greater than the stability field of plagioclase, is consistent with the existence of a thick conductive lid beneath the Oblique Supersegment of the SWIR inferred by Dick et al. (2003).
Discontinuous mylonitic–ultramylonitic shear zones affect some of the studied samples. Mylonitization, well developed in some samples, is more tenuous in other samples, evidenced by incipient grain-size reduction along grain boundaries. The mylonitization is probably related to normal faulting affecting the entire lithosphere during ‘amagmatic’ spreading, and it is likely that it eventually led to the exposure of mantle rocks on the seafloor (Jaroslow et al., 1996; Ceuleneer & Cannat, 1997).
Are SWIR pyroxenites trapped melts or cumulates?
The spinel-bearing pyroxenites might be former trapped basaltic melts that crystallized in the mantle, at depths below the stability field of plagioclase. We did not measure the bulk-rock compositions of the SWIR pyroxenite veins because the small, heterogeneous and partly weathered samples were inappropriate for such an exercise. Other arguments, however, allow us to discard the trapped melt hypothesis in favor of a cumulate origin for the SWIR pyroxenites. As spinel is never abundant, the bulk-rock composition of the pyroxenites would be too Al2O3 poor to correspond to a basaltic melt composition. In addition, the high Mg-number (0· 90) of the pyroxenes and olivine is beyond the range of any known melt erupted along a mid-ocean ridge, even among the most primitive MORBs. Conversely, these values are consistent with the Mg-numbers of pyroxenes and olivine from primitive mafic cumulates sampled along mid-ocean ridges. Similarly, clinopyroxene Cr contents and olivine Ni contents are on the fractional crystallization trend of oceanic cumulates, as shown in Fig. 3c. Therefore, we conclude that the pyroxenites formed by fractional crystallization from a passing melt, not by bulk crystallization of a stationary melt. Consequently, the residual liquids left after websterite crystallization could have segregated from the mantle, mixed with other melt fraction migrating to the surface and erupted as a basalt. The ‘footprint’ of pyroxene crystallization should be preserved in the geochemical characteristics of these basalts. Accordingly, our study provides concrete evidence for pyroxene crystallization in melt channels beneath mid-ocean ridges at relatively high pressures, a process frequently invoked to account for the ‘pyroxene paradox’ in MORB petrogenesis (Francis, 1986; Grove et al., 1992).
Can we infer the parent melt composition of the SWIR pyroxenites?
If the SWIR pyroxenites are cumulates, then the composition of the parent melt needs to be estimated to constrain the ridge partial melting processes. Given the number of unknowns, the precise calculation of equilibrium liquid compositions using partition coefficients is underconstrained. Therefore, our conclusions remain primarily qualitative.
The first-order homogeneity in the major and trace element concentrations of the SWIR pyroxenite veins is a major observational constraint, suggesting a similar mantle source and melting processes for the formation of the parent melts. The Mg-number of Cpx ranges from 0· 89 to 0· 91, which is typical of Cpx in equilibrium with mantle partial melts ranging from primitive types (Mg-number 0· 72) to those slighty differentiated by fractional crystallization. Consequently, the SWIR websterites can be considered to be a cogenetic suite of cumulates from parent melts generated by roughly the same degree of melting of a homogeneous mantle source. The lack, or low modal abundance, of an aluminous mineral phase such as spinel does not mean that these parent melts were Al poor, but that the cotectic proportion of spinel was low at the pressure and temperature of crystallization of the pyroxenites. The high Al content of the pyroxenes and the presence of aluminous spinel point to crystallization from melts with a high Al content. Even compared with high-pressure cumulates (Fig. 3), these are among the highest Al contents documented in pyroxenes from abyssal pyroxenites and peridotites (e.g. Dick et al., 1984; Fujii, 1990; Juteau et al., 1990). If these parent melts had crystallized at lower pressure, plagioclase would have been among the cotectic phases.
The normalized REE and extended multi-element patterns preclude the possibility that the parent melts of the SWIR pyroxenites were identical to MORB although they are derived from a MORB source. The hypothetical Cpx in equilibrium with E-MORB, normal MORB (N-MORB) (Sun & McDonough, 1989) and basalts erupted along the SWIR from 11°53' to 14°38'E (Le Roex et al., 1992) are plotted in Fig. 7. The Kdcpx/basalt values used to calculate the composition of these Cpx are from Hart & Dunn (1993: La, Ce, Nd, Sm, Dy, Er, Yb, Lu, Ba, Sr, Zr, Ti and Y), and from McKenzie & O'Nions (1991: Th, Pr, Eu, Gd and Ho). The HREE and MREE Cpx contents of SWIR websterites encompass the field of hypothetical Cpx in equilibrium with E-MORB, N-MORB and SWIR basalts, but their LREE abundances are about one order of magnitude too low to represent MORB cumulates. Because SWIR pyroxenites are primitive cumulates (i.e. formed by moderate degrees of fractional crystallization of a melt in equilibrium with the mantle), their REE compositions are close to those of residual pyroxenes in equilibrium with these melts. This makes it possible to compare the REE composition of SWIR Cpx with the calculated compositions of the residual Cpx of Johnson et al. (1990). We deduce that the REE patterns of the Cpx are consistent with crystallization from parent melts that are incremental melt fractions produced from a peridotite that is residual after a low degree of partial melting. This scenario is consistent with fractional decompression melting: the parent melts of the SWIR pyroxenites can be viewed as late-stage, non-aggregated melt fractions that have not mixed with earlier melt fractions from the same mantle source. The previous melting of the mantle source must have been of low to moderate degree; this is required by the relatively high concentrations of HREE but also of other incompatible elements such as Na and Ti in the Cpx. Although partly attributable to pure crystal chemical effects (high Na and Ti abundance in Cpx being a consequence of the high Al content of these cpx), it also requires a relatively high concentration of Na and Ti in the parent melts.
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, Sun & McDonough, 1989Could the SWIR pyroxenites be residues of melting of more fertile pyroxenites?
The melting of a ‘marble cake mantle’ has often been invoked to account for the isotopic heterogenity of MORB (Polvé & Allègre, 1980
; Allègre & Turcotte, 1986; Prinzhofer et al., 1989). In the specific case of the SWIR, Salters & Dick (2002) have shown that abyssal peridotites alone cannot account for the more enriched 143Nd/144Nd characteristics of the spatially associated basalts. They proposed that isotopic variations observed in the basalts are attributable to the melting of pyroxenite layers that have a more radiogenic Nd signature than the associated peridotites. Here we investigate the hypothesis that the SWIR pyroxenite veins are residues from melting of more fertile pyroxenite layers (garnet and/or spinel pyroxenites). As we have seen above, the fact that coarse-grained cumulate textures are preserved in some samples provides evidence for relatively recent crystallization of the websterites. Pyroxenitic layers in a marble cake mantle would have reached textural equilibrium during their long residence time at high temperature and pressure in the convective mantle, and would have similar textures to their host peridotite. The question addressed in this section is to check if this inference is consistent with the geochemical composition of the pyroxenites.
The idea that the pyroxenites have undergone a significant degree (more than a few per cent) of partial melting in the stability field of garnet can be easily discounted because they are not depleted in HREE relative to LREE (e.g. Johnson et al., 1990; Hirschmann & Stolper, 1996). However, from the REE patterns, melting of previously formed pyroxenite veins in the spinel stability field cannot be excluded. To test such a hypothesis we compared the REE composition of the SWIR pyroxenite Cpx with those of residual pyroxenes resulting from incremental melting of a spinel peridotite (Fig. 7; model of Johnson et al., 1990). In the incremental melting model described by Johnson et al. (1990), small but finite increments of melting and melt segregation occur, with a new starting composition after each segregation event. The starting composition corresponds to a cpx from a ‘primitive’ spinel peridotite collected along the SWIR, a composition particularly relevant to our case. By comparing compositions of our samples with the calculated compositions of Johnson et al. (1990), we deduce that the REE patterns of the cpx in our samples are consistent with parent melts that are the products of low to moderate incremental melt fractions (5–10%) of such a primitive spinel peridotite. Fractional melting models of the REE concentrations in the SWIR pyroxenite Cpx are consistent with a low degree of partial melting of more fertile spinel pyroxenites (5–10%; Johnson et al., 1990). High abundances of other incompatible trace elements also support this view. Similarly, the high Al–Na–Ti concentrations in Cpx and the low Cr-numbers of Sp are not inconsistent with formation of the SWIR pyroxenites by low degrees of melting of older pyroxenites, if the initial incompatible element content of the older pyroxenites is close to the inferred composition of Cpx in the MORB source region. The main problem with this hypothesis is that pyroxenites are assumed to be much less refractory than lherzolites. Accordingly, the lherzolites that host the pyroxenite veins would have to have remained virtually unmelted. Although, among the peridotites dredged during the Knorr Cruise 162, Leg 9, a significant proportion of samples are rather fertile peridotites, there are also, in this area, many more depleted peridotites, including harzburgites (40% of the samples collected in drege haul 47) that have experienced a high degree of melting. If the pyroxenites are residual lithologies, this would suggest that the degree of melting recorded by the peridotites and pyroxenites associated in the same dredge is extremely variable. Although this remains a possible explanation, the simpler explanation is to view the pyroxenites as cumulates formed from crystallization of melts from an already depleted mantle source.
Could the SWIR pyroxenites be produced by melt–rock reaction?
The pyroxenites could also be viewed as the products of melt–rock reaction. Reactions between infiltrating melts and host peridotites—and their geochemical consequences—are well documented in ophiolites, in the case of dunite channels (e.g. Kelemen et al., 1995; Suhr et al., 2003). If the pyroxenites are the results of a similar type of melt–rock reaction, the pyroxenite mineral compositions should largely reflect the compositions of the percolating melts. Here again, this hypothesis cannot be definitely excluded. However, the reacting melt would have to be both depleted in LREE relative to HREE and SiO2 rich. The pyroxene-rich composition of the veins and the lack of reaction zones around the veins (as far as can be documented) would require a melt oversaturated in silica relative to the surrounding peridotites, to limit the dissolution of peridotitic Opx. A SiO2-oversaturated melt with increasing fractionation of LREE relative to HREE is not a natural evolution of melt composition during progressive decompression melting. The formation of silica-rich melts from a mantle source markedly depleted in the most incompatible elements has been proposed to account for the formation of gabbronoritic cumulates in the Oman ophiolite and along the Mid-Atlantic Ridge (Benoit et al., 1999; Nonnotte et al., 2005). However, the formation of these melts is a shallow-level process, probably related to melting of depleted peridotite previously altered by hydrothermal fluids. This process is difficult to transpose to the relatively deep (
20 km) context of formation of the SWIR pyroxenites.
Do the SWIR pyroxenites differ from other mid-ocean ridge pyroxenites?
Pyroxenites are extremely uncommon lithologies among abyssal ultramafic samples: only a few occurrences have been reported, all from slow and ultra-slow spreading environments (Dick et al., 1984; Fujii, 1990; Juteau et al., 1990; Kempton & Stephens, 1997). We can only compare the major–minor element compositions from the Oblique Supersegment pyroxenites to data for four samples from the MARK area (Fujii, 1990; Juteau et al., 1990; Kempton & Stephens, 1997). Dick et al. (1984) interpreted pyroxenites as products of in situ crystallization of partial melts trapped in a peridotite matrix. Similarly, Juteau et al. (1990) concluded that websterites in their study represented an early crystallization product of a trapped liquid produced during a partial melting event. Alternatively, Fujii (1990) considered pyroxenites to be small-scale heterogeneities in the upper mantle beneath a mid-ocean ridge. Kempton & Stephens (1997) proposed two models of formation for what they called pyroxenite ‘nodules’: either remnants of former veins deformed during plastic flow or in situ crystallization from a partial melt trapped beneath the ridge axis.
Pyroxenite petrographic characteristics and mode of occurrence are identical between the MARK area and the SWIR Oblique Supersegment. The pyroxenites are all spinel websterites – some of them olivine-bearing – and occur as small-scale features. Despite similar modal proportions, the chemical composition of Cpx differs markedly between the two areas. The Oblique Supersegment websterite Cpx have higher TiO2 and Na2O concentrations, and significantly lower Mg-numbers, as shown by the squares in Fig. 3a and c. The concentrations of other major elements are similar between the two sites (Fig. 3a–c). Cpx from the MARK area pyroxenites are not distinct from the Cpx in the host peridotites, whereas SWIR pyroxenite Cpx are different. The Oblique Supersegment peridotite Cpx have lower Mg-numbers and less variable Ti, Al and Na concentrations with respect to those in the pyroxenites, which have slightly higher incompatible element concentrations. These differences may be related to slight differences in the compositions of their parent melts. The parent melts of the Oblique Supersegment websterites had a higher Ti and Na content than the parent melts of the MARK websterites. This can be accounted for by a lower degree of melting in the ultra-slow spreading environment of the SWIR Oblique Supersegment. This could be also related, in the case of the SWIR, to a hotspot influence, although we need to perform a detailed isotopic study to investigate this possibility further.
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CONCLUSION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGANALYTICAL METHODSPETROGRAPHYMINERAL COMPOSITIONSDISCUSSIONCONCLUSIONREFERENCES |
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The SWIR Oblique Supersegment from 9 to 16°E is one of the few areas along the global mid-ocean ridge system where a significant (although still small) amount of pyroxenite veins have been sampled in association with abyssal peridotites. Cpx REE patterns allow us to exclude the possibility that they are residues left behind after partial melting of garnet pyroxenites. On the other hand, they are probably not residual after partial melting of pre-existing spinel pyroxenites, as this would require an extremely low degree of partial melting (<5%) to produce the observed REE compositions of the Cpx. Such a low degree of partial melting would be difficult to reconcile with the presence of more depleted peridotites dredged in the same area.
Our data support the view that the pyroxenite veins are cumulates that crystallized at relatively high pressure (0· 6 GPa), above but close to, the spinel–plagioclase transition for a pyroxenite bulk composition. Their trace element concentrations are consistent with formation from parent melts produced by incremental decompression melting of a N-MORB mantle source that had already been depleted deeper in the melting column. These melt fractions were produced just beneath the base of the thermal lithosphere, which has a significant on-axis thickness as a result of the combined effects of ultra-slow spreading and oblique spreading (Dick et al., 2003). These melts underwent fractional crystallization soon after segregation from the peridotitic matrix, as a result of the proximity of the conductive cooling boundary layer (Rabinowicz & Ceuleneer, 2005). The resulting cumulates are in equilibrium with primitive (high Mg-number) to slightly evolved mantle partial melts.
Seyler et al. (2001) proposed a similar scenario to account for the texture and geochemistry of disseminated cpx in abyssal peridotites. We show here that these melts do not necessarily remain scattered and trapped in the peridotitic matrix but can segregate into veins and undergo fractional crystallization. As these melts preferentially crystallize pyroxenes, because of the relatively high-pressure conditions, this will affect the concentration of the trace elements fractionated by Cpx in the residual liquids migrating to the surface (REE, Ti, Na, etc.). If such fractionated melts eventually mix with other melt fractions produced in the melting regime, this will affect the calculation of the degree of source melting based on the interpretation of the major and trace element signatures of MORB (Langmuir et al., 1992).
On the other hand, our study provides new and concrete evidence that the pyroxene paradox could actually be attributable to fractionation of pyroxenes at depth, as proposed by previous workers (e.g. Grove et al., 1992). Our study allows us to clarify the scenario: we show that the pyroxene paradox is possibly related to fractional crystallization of not yet aggregated melt fractions produced and segregated at the interface between the melting regime and the lithosphere.
We tentatively suggest that the crystallization of pyroxenite veins is not specific to the slow spreading context but can occur in all kind of melting regimes (hot and cold). In the fast spreading context, relatively low-T–high-P conditions leading to the crystallization of pyroxenites are not realized below the spreading axis itself but are certainly realized somewhere at the edges of the melting regime; accordingly, part of the melt fractions contributing to the petrogenesis of MORB may record in their chemistry the fractional crystallization of pyroxenes, regardless of the spreading rate. Ultra-slow spreading conditions—with a relatively thick axial lithosphere and long intervals of amagmatic spreading—are required, however, if there is to be a reasonable probability of pyroxenites being exposed on the seafloor.
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ACKNOWLEDGEMENTS |
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The crew of the R.V. Knorr are thanked for their hard work, especially under some very adverse conditions. Thin sections, electron microprobe analyses and ICP-MS analyses were performed using the facilities of the Observatoire Midi-Pyrénées, Paul Sabatier University, Toulouse. We are particularly indebted to Fabienne de Parseval and Jean-François Mena for thin section preparation, and to Philippe de Parseval for his help during microprobe data acquisition. Nobu Shimizu, at WHOI, provided assistance with ion microprobe analyses of Knorr peridotites. This work was financially supported by the Fond Social Européen (FSE) and by the French Centre National de la Recherche Scientifique (CNRS). The manuscript benefited from constructive reviews and editing by Marjorie Wilson and Shoji Arai.
*Corresponding author. Tel: (+33)5 6133 3014, Fax: (+33)5 6133 2900. E-mail: dantas{at}dtp.obs-mip.fr
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