This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and

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1 This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier s archiving and manuscript policies are encouraged to visit:

2 Lithos 106 (2008) Contents lists available at ScienceDirect Lithos journal homepage: Chromian spinels in mafic ultramafic mantle dykes: Evidence for a two-stage melt production during the evolution of the Oman ophiolite Marie Python a,b,, Georges Ceuleneer a, Shoji Arai b a Université Paul Sabatier, Centre National de la Recherche Scientifique U.M.R Observatoire Midi-Pyrénées, 14, av. É. Belin Toulouse, France b Faculty of Science, Department of Earth sciences, Kanazawa University-Kakuma-machi, Kanazawa , Japan article info abstract Article history: Received 18 October 2007 Accepted 4 July 2008 Available online 19 July 2008 Keywords: Oman ophiolite Chromian spinel Gabbros Pyroxenites Dykes This paper describes a comprehensive study of the chromian spinels present in mafic ultramafic dykes cropping out along the mantle section of the Oman ophiolite. We studied about 1100 samples in thin section and with the electron microprobe. Chromian spinel is almost ubiquitous in primitive dykes (high-mg# troctolites and pyroxenites) and less common in more differentiated ones (olivine gabbros and gabbronorites). The Cr#, TiO 2 content, and other compositional parameters are well correlated to the nature and composition of the co-genetic silicate assemblage. Chromian spinel composition contributes to establish that the mantle dykes of Oman are more or less evolved cumulates that crystallised from two main types of primary melts: tholeitic melt similar to Mid-Ocean Ridge basalts (MORB; 0.45bCr# b 0.63; 0.3bMg# b0.6; TiO 2 up to2 wt.%), and more silicic melts issued from a highly depleted mantle source, similar to boninitic andesitic melts that preferentially form in subduction zone settings (0.35bCr# b0.80; 0.1bMg#b0.7; TiO 2 up to 0.2 wt.%). The chromian spinel composition presents a higher variability than the associated silicates and allows us to further unravel the petrological evolution and segmentation of the Oman ophiolite. The composition of chromian spinel in mantle dykes and in the spatially related residual harzbugites display well correlated variations at the scale of the Oman ophiolite. This shows that these two lithologies share a common magmatic history, even if, strictly speaking, they cannot be related through direct parent daughter relationships. The Cr# is on average higher, and the TiO 2 lower in the NW than in the SE, consistent with an increasing influence of marginal magmatic processes in the NW, while the southeastern area has petrological characteristics closer to those of an open ocean. In this southeastern part, compositional variations of the chromian spinel are correlated to structural characteristics related to the spreading history: the distribution of the Cr# around a former mantle diapir cropping out in the Maqsad area is concentric, with the highest values in the centre of this structure, consistent with higher degrees of partial melting of the mantle. On the other hand, unusually low Cr# diagnostic of a low melting degree in a ridge-parallel band of about 20 km to the northeast of the Maqsad area can be related either to temporal variations in the partial melting degree or to off-axis magmatic activity Elsevier B.V. All rights reserved. 1. Introduction Chromian spinel has a general formula ([Mg,Fe 2+ ][Cr,Al,Fe 3+ ] 2 O 4 ) where Cr concentration is appreciable. Mn, Zn, V and Ti are also usually present as minor components. This mineral is commonly found in mantle-derived peridotites where it is considered to be a powerful petrogenetic indicator, its composition being highly dependant on the degree and conditions of partial melting and of magma/ rock interactions (Irvine, 1965, 1967; Arai, 1992). The atomic ratio Corresponding author. Faculty of Science, Department of Earth sciences, Kanazawa University-Kakuma-machi, Kanazawa , Japan. Tel.: ; fax: addresses: marie@earth.s.kanazawa-u.ac.jp (M. Python), Georges.Ceuleneer@dtp.obs-mip.fr (G. Ceuleneer), ultrasa@kenroku.kanazawa-u.ac.jp (S. Arai). Mg MgþFe 2þ ð¼ Mg# Þ of spinel in equilibrium with olivine depends on the equilibrium temperatures (Irvine, 1967; Jackson, 1969; Evans and Frost, 1975). The chromium number Cr# ¼ Cr atomic ratio is CrþAl especially important for understanding the genesis of magmas as it indicates the degree of partial melting or the degree of depletion of the source (Dick and Bullen, 1984; Arai, 1994). Chromian spinels in peridotites are traditionally used as indicators of the degree of partial melting for tectonic discrimination between Mid-Ocean Ridge (MOR) and Supra-Subduction Zone (SSZ) settings (Dick and Bullen, 1984; Parkinson and Pearce, 1998; Pearce et al., 2000; Arai et al., 2006). Interpreting the composition of chromian spinel in plutonic rocks is more complicated: it depends on many parameters including the nature of the parent melts, the conditions of crystallisation (T, P, f O2 ), the composition of coexisting phases and subsolidus processes like diffusion and recrystallisation. In the present paper, we examine /$ see front matter 2008 Elsevier B.V. All rights reserved. doi: /j.lithos

3 138 M. Python et al. / Lithos 106 (2008) chromian spinel composition in dykes, sensu lato i.e. all kinds of former melt migration features cropping out in the mantle section of the Oman ophiolite. In a previous study (Python and Ceuleneer, 2003), we presented detailed description of these dykes at the scale of the entire Oman ophiolite except for the characteristics of their oxide minerals, including especially chromian spinels. Accordingly, the general philosophy of our approach in the present paper is quite simple: we use the constraints from this previous study (field relations, distribution at the scale of the entire ophiolite, nature and composition of the silicates) to understand which kind of petrogentic information can be deduced from the study of chromian spinel. A key question is to what extent can the variability in the composition of plutonic chromian spinel be used to distinguish different magma series, and hence different tectonic settings. In the case of Oman, this question has still not been resolved in spite of extensive studies. The ophiolite has been interpreted as a fragment of oceanic lithosphere generated along a Mid-Ocean Ridge by researchers who mapped the plutonic and mantle section of the ophiolite (Reinhardt, 1969; Coleman, 1981), or alternatively as a fragment of a marginal basin (some kind of immature island arc) by those who focused their investigations on the volcanic section (Pearce et al., 1981; Alabaster, 1982). The comprehensive dataset about chromian spinel we present in this paper will, hopefuly, help to progress in tectonic discrimination based on the petrological characteristics of plutonic rocks. 2. Geological settings and previous work on mantle dykes The Oman ophiolite is one of the largest ophiolites from the now disappeared Tethys Ocean. It was obducted on to thearabian margin in Maestrichtian times (see review in Nicolas, 1989). As it was not included in the Alpine collision belt, its internal structures are remarkably well preserved, although it was slightly dismembered during its final obduction and, more recently, during the Miocene uplift of the Oman mountains. The large-scale structures of the Oman ophiolite (mostly the attitude of the sheeted dyke complex) indicate that the strike of the spreading axis was roughly parallel to the longest axis of the Oman mountains, i.e rotating approximately from N S in the north to NW SE in the south (Fig. 1). This arcuate-like structure may be the result of the opening of NW SE basins in a lithosphere formerly accreted along an S axis, the largest one being centred on the Maqsad area, which extends for of about 80 km from the Nakhl area to the Ibra area in the south eastern part of the ophiolite (Ceuleneer, 1991; Python and Ceuleneer, 2003). Additional support for a two-stage accretion history is given by the petrological nature of the mantle dykes, which are predominantly MORB-derived troctolites and olivine gabbros in the Maqsad area and depleted andesite-derived pyroxenites and gabbronorites elsewhere (Benoit et al., 1999; Python and Ceuleneer, 2003). The study of the associated lavas suggets that the Oman ophiolite is possibly polygenic, i.e. formed in a Mid-Ocean Ridge context that migrated toward a subduction zone and was modified by arc magmas before being obducted (Pearce et al., 1981; Ishikawa et al., 2002). Nevertheless, a monogenetic history in a Supra-Subduction Zone tectonic environment can also explain the magmatic variations observed in the Oman ophiolite, a back-arc basin being a natural setting to simply account for bimodal magmatism (tholeiitic and andesitic Alabaster et al., 1982). Recently, in a study of the detrital spinels collected in wadi beds, Arai et al. (2006) showed that an appreciable proportion of the Oman peridotites are of sub-arc origin. In the mantle section, mafic and ultramafic dykes are ubiquitous and, until recently, it was inferred that they all resulted from the crystallisation of melts similar to MORB at various mantle pressures, pyroxenites crystallising at more than3 GPa and plagioclase bearing lithologies at lower pressure (Browning, 1982; Lippard et al., 1986; Ernewein, 1987; Boudier et al., 1983; Nicolas et al., 2000). Occurrences of gabbronorite dykes showing compositions depleted in incompatible elements compared with the olivine gabbro dykes were also reported but were considered very uncommon (Lippard et al., 1986). Fig. 1. Simplified geological map of the Oman ophiolite. The Oman ophiolite is traditionally divided into North and South with a middle part located around the Haylayn area. Strictly speaking, the Nakhl, Khawad and Musibit massifs are not located in the South of Haylayn but they belong to the so-called southern part. In this study, we divide the ophiolite into NW and SE, with a boundary between the Nakhl and the Haylayn zones. The Sumail massif itself is divided in three parts by two main shear zones: the Muqbaryiah shear zone (from Muqbaryiah to Sayma) in the South and the Wadi 'Uqq Shear zone in the North. Taking into account the petrographical and chemical affinities of the samples from some areas, the Miskin massif will be grouped with the Wuqbah area; the Nakhl, the Samad and the Maqsad (the centre of the Sumail massif, between the two shear zones) areas will be grouped into Sumail zone, centre; the Bahla, the South of Sumail (Southward to the Muqbaryiah shear zone) and the Ibra areas will be grouped into Sumail zone, South; and the Khawad, the Musibit, the Wadi Tayin areas and the North of Sumail (Northward to the Wadi 'Uqq shear zone) will be grouped into Sumail zone, North (see Fig. 11 and 9c, 9d).

4 M. Python et al. / Lithos 106 (2008) Table 1 Summary of the petrological characteristics of the major lithological facies of the mantle dykes of Oman Type Field Texture Mineralogy Distribution Depleted series Pyroxenites Clinopyroxenites Dyke Adcumulate/pegmatitic Cpx±Opx, Ol, Sp, Ox NW+South of Sumail Orthopyroxenites Dyke Adcumulate/mosaic Opx±Cpx, Ol, Sp, Ox Websterites Dyke Adcumulate/mosaic Opx+Cpx ±Ol, Sp, Ox Wehrlites Dyke Adcumulate Ol+Cpx ±Pl, Sp, Ox Gabbronorites Olivine gabbronorites Dyke Adcumulate Opx+Pl+Ol+Cpx±Sp, Ox, Amp Gabbronorites Dyke Pegmatitic/adcumulate Opx+Pl+Cpx ±Sp, Ox, Amp Norites Dyke Pegmatitic/adcumulate Opx+Pl ±Sp, Ox, Amp MORB series Ol gabbros Olivine gabbros Dyke Adcumulate Pl+Ol+Cpx±Opx, Sp, Ox, Amp SE+Hilti Opx-Olivine gabbros Dyke Adcumulate Pl+Ol+Cpx+Opx±Sp, Ox, Amp Troctolites Impregnations Diffuse segregation Mesocumulate (late Pl, Cpx) Pl+Ol+Cpx±Opx, Sp, Ox, Amp Troctolites Dyke/PFC Adcumulate/ mosaic Ol+Pl ±Cpx, Opx±Sp, Ox Cpx troctolites Dyke Mesocumulate (late Cpx) Pl+Ol+Cpx±Opx, Sp, Ox, Amp Exotic facies like diabases, plagiogranites or hydrothermal dykes are not included. The first column gives the magmatic series to, which is linked the lithological family provided in the second column. The last column gives the geographic distribution of these two series. PFC: Porus Flow Channel, see Python and Ceuleneer (2003) for a detailed explanation of field relationships, mineralogical, textural and chemical decriptions, and of the geographic distribution. Sp: chromian spinel; Ox: other oxide; Ol: olivine; Pl: plagioclase; Cpx: clinopyroxene; Opx: orthopyroxene; Amp: amphibole. This has been shown to be incorrect by Benoit et al. (1999) and Python and Ceuleneer (2003) who demonstrated that mantle dykes derived from depleted melts are the most common. The first detailed study of mantle dykes in the Oman ophiolite focused on the Maqsad area where a mantle diapir was mapped at Moho level (Rabinowicz et al., 1987; Ceuleneer et al., 1988, 1996; Benoit et al., 1996, 1999). Dykes and porous flow channels of troctolites and olivine gabbros derived from a MORB-type melt are dominant there. Gabbronorite and pyroxenite pegmatites are, conversely, very abundant at the periphery of the Maqsad area and their petrographical and chemical characteristics suggest that they crystallised from a liquid depleted in incompatible elements and enriched in silica relative to N-MORB. These dykes have the characteristics of cumulates from boninites or depleted high-mg andesites that were tentatively interpreted as the product of hydrated melting of the lithosphere surrounding the Maqsad diapir (Benoit et al., 1999). That this remelting process can occur at present-day ocean ridges was recently confirmed by Nonnotte et al. (2005). We extended the study of mantle dykes to the entire ophiolite, the results are detailed in Python and Ceuleneer (2003) and summarised in the following section. 3. Petrographic characteristics of the mantle dykes of the Oman ophiolite 3.1. Field relationships and general petrographic characteristics On the basis of petrographic observations (modal compositions and textures), thirty six petrographical types were distinguished among the 1200 samples we collected for this study. The mineral composition of each sample, determined with the electron microprobe, was used as independent evidence to relate these dykes to two different differentiation series (Python and Ceuleneer, 2003). Thirty of the thirty six petrographic types can be grouped into four main families corresponding to primitive and variably evolved cumulates that crystallised from two major magma types (see Table 1 and Python and Ceuleneer, 2003). These two magma suites crop out in distinct areas so that cross-cutting relationships and relative injection ages are impossible to establish. The six other lithological types are exotic facies (mainly granitic and hydrothermal lithologies) and only represent about 5% of the dykes. The first magmatic series is composed of troctolites, olivine gabbros and more evolved cumulates. It shows characteristics similar to that of the crustal section of the Oman ophiolite and of common oceanic gabbros drilled along present-day Mid-Ocean Ridges: they crystallised at low pressure ( 0.7 GPa, Villiger et al., 2007) from MORB-like melts. In terms of modal composition, a perfect continuum exists between troctolites and olivine gabbros, the intermediate lithologies being clinopyroxene-troctolites, i.e. a troctolitic cumulate assemblage with more or less abundant interstitial to poikilitic clinopyroxene (see Fig. 4 in Python and Ceuleneer, 2003). Olivine gabbros occur in brittle injection structures, i.e. dykes with sharp contacts with their host harzburgites and devoid of reactional dunitic margins. They display fine to medium grained adcumulate textures. Troctolites show more varied modes of occurrence compared to other lithologies. Half of them occur in classical dykes with more or less developed reactional dunitic margins, and the rest occur in more irregular structures ranging from impregnation pockets with irregular shapes(see below) to porous flow channels with gradational contacts with their host (see Fig. 4 in Python and Ceuleneer, 2003). Troctolites frequently show textures diagnostic of high temperature subsolidus reequilibration (triple grains junctions and pyroxenitic corona between olivine and plagioclase). The impregnations in harzburgites or dunites are a kind of melt migration structure that has no clear-cut boundary with its host peridotite (see Fig. 4a in Python and Ceuleneer, 2003). In the field, it seems that these features are related to minute crystallisation of plagioclase and/or clinopyroxene within the peridotite and the thin section observations show typical peridotite porphyroclastic and granoblastic textures with low amounts of interstitial plagioclase and/or clinopyroxene. Cumulates from the MORB-derived suite only crop out in a few areas, the main one being the 80 km long and up to 20 km wide NW SE corridor centred on the Maqsad mantle diapir (Python and Ceuleneer, 2003). The second magma suite principally consists of orthopyroxenerich gabbronorites and various kinds of pyroxenites, displaying a wide range of textures, from fine grained to pegmatitic. Some pegmatite Fig. 2. Histogram showing the frequency of ore minerals in each lithological family for each type of ore mineral. None refers to the absence of any ore minerals. Taking into account that our sampling was random in the various geographical areas and that the number of samples is quite high, the relative number of sample is likely representative of the actual proportions in the ophiolite.

5 140 M. Python et al. / Lithos 106 (2008) Table 2 Distribution of the opaque minerals contained in the main lithological facies of the dykes Lithology Chromian spinel Magnetite Sulphide Hematite Ilmenite Rutile None Number of samples Depleted series Pyroxenites Clinopyroxenites Orthopyroxenites Websterites Wehrlites Gabbronorites Olivine gabbronorites Gabbronorites Norites MORB series Ol gabbros Olivine gabbros Orthopyroxene-olivine gabbros Troctolites Impregnations Troctolites Cpx troctolites Each column gives the number of samples of each lithological facies bearing the opaque mineral indicated by its heading (column none: number of samples that do not contain any opaque of any sort). The last column Number of samples gives the total number of samples studied for each lithology. Note that a single sample may contain several types of opaque mineral, thus the number in the last column is not the sum of the other columns. dykes belonging to this suite reach several metres in thickness. The early and abundant crystallisation of orthopyroxene, the late crystallisation of plagioclase and the mineral composition are not compatible with a MORB origin but are likely derived from depleted andesite or boninite. Contrasting with early views, we have shown that these dykes are the most abundant in the mantle section of Oman, occupying about 75% of its surface and being absent or very rare in the areas where the MORB-derived troctolites and olivine gabbros are found, especially in the centre of the Maqsad area (Fig. 19 in Python and Ceuleneer, 2003) Chromian spinels Most of the samples in this study contain at least traces of various ore minerals, mainly chromian spinel, magnetite, sulphide, hematite, ilmenite, rutile (Fig. 2 and Table 2). Several of these ore minerals often Fig. 3. Photomicrographs of samples from the various families of dyke, showing examples of the textural relationships between the phases. Chromian spinels are euhedral to subeuhedral and may be included in either of the major phases. Sp: chromian spinel; Mt: magnetite; Olv: olivine; Plg: plagioclase; Cpx: clinopyroxene; Opx: orthopyroxene.

6 M. Python et al. / Lithos 106 (2008) Table 3 Selected representative microprobe analyses of spinels for various lithologies Sample TiO 2 Al 2 O 3 Cr 2 O 3 Fe 2 O 3 FeO MnO MgO NiO Total Cr# Mg# Cr/R 3+ Al/R 3+ Fe 3+ /R 3+ Impregnated dunites 05SDM05a HI53a SD12b KW6e Impregnated harzburgites 99HI KW16c WT9b Troctolites 01MB4b MB9a KW6d IB NA Troctolites (Maqsad centre) 92OG M23A M Clinopyroxene troctolites 01MB4a M30a NA SD Olivine gabbros 99KW16b KW NS WT M Gabbronorites 00BA FZ40c FZ40b HI71c Norites 00FZ30a Wehrlites 97HI33a WT14b Clinopyroxenites 97HI33c SA7b KW RU Olivine websterites 00FZ RU52a RU55a Websterites 00BA SA6b NA Opx-rich websterites 00FZ SA M Orthopyroxenites 00BA NA SS (continued on next page)

7 142 M. Python et al. / Lithos 106 (2008) Table 3 (continued) Sample TiO 2 Al 2 O 3 Cr 2 O 3 Fe 2 O 3 FeO MnO MgO NiO Total Cr# Mg# Cr/R 3+ Al/R 3+ Fe 3+ /R 3+ Orthopyroxenites 99NA RU Olivine orthopyroxenites 00WU26a OM11b OM21b The composition are given in weight percents, Cr#, Mg# and Cr/R 3+, Al/R 3+,Fe 3+ /R 3+ in atomic ratios. R 3+ =Cr+Al+Fe 3+. coexist in the same sample, and chromian spinel is present in each lithological type in variable quantities. It is the dominant opaque mineral in the primitive cumulates of both suites, being present in more than 70% of the samples of troctolites and pyroxenites. It can be also observed in about half of the samples from the olivine gabbro family and is less common in gabbronorites. Fig. 4. Trivalent cation ratio of chromian spinels. Data are plotted according to the lithological nature of the samples. The symbols will be the same in subsequent figures: black circles for olivine gabbros, white squares for clinopyroxene-troctolites, grey squares for troctolites, white triangles for impregnations (plagioclase & clinopyroxene) in harzburgites and dunites, black squares for websterites, grey triangles for clinopyroxenites, grey circles for orthopyroxenites, crosses for wehrlites and stars for gabbronorites. Various chemical fields are given for comparison: dotted field for the Mid-Ocean Ridge peridotites (MOR peridotites Prinz et al., 1976; Dick, 1989; Fujii, 1990; Komor et al., 1990; Allan and Dick, 1996; Arai and Matsukage, 1996; Dick and Natland, 1996; Cannat et al., 1997a,b; Niida, 1997; Tartarotti et al., 2002, and about 70 references from the PetDB ( light grey field for the Mid-Ocean Ridge crust (MOR gabbros, same references as for the MOR peridotites); dashed thick line for the arc peridotites (Bloomer and Hawkins, 1983; Ishii, 1985; Bloomer and Fisher,1987; Spadea et al.,1991; Ishii et al.,1992; Ohara and Ishii, 1998; Ishii et al., 2000; Ohara et al., 2002; Ishimaru, 2004); plain thick line for the Oman peridotites (far from any dykes, Takazawa et al., 2003; Suzuki, 2006; Monnier et al., 2006); squared surface for the gabbroic crust of the Oman ophiolite (Hilti block, Ernewein, 1987); light grey field delimited by a dashed line for the boninites (Barnes and Roeder, 2001, and references therein). The chemical domain of the troctolite from the centre of the Maqsad area (Maqsad diapir) is outlined in dark grey in the diagrams showing the troctolite family.

8 M. Python et al. / Lithos 106 (2008) The troctolite family The samples from the troctolite family usually contain a small proportion of chromian spinel (from 0.5 to 2 vol.%, but it reaches vol.% in two samples of impregnated dunite taken close to a chromitite pod). In the impregnated harzburgites and dunites, chromian spinel is anhedral, and is found at the olivine grain boundaries, and often co-exists with Fe Ni sulphides. In the other troctolites (dykes, porus flow channel), chromian spinel is often euhedral to subhedral and usually partially or totally included in olivine or plagioclase. Only a few samples of troctolite are devoid of ore minerals (a dozen samples out of a total of 124), and igneous magnetite is virtually absent (observed in only two samples of clinopyroxene-troctolites, see Table 2). In the troctolite family, chromian spinel is relatively coarse-grained compared to the other lithologies and their size may reach 0.5 mm when included in large poikilitic clinopyroxene, in which case they may be slightly eroded and appear as sub-rounded crystals (Fig. 3a) The olivine gabbro family The olivine gabbro family includes olivine gabbros but also differentiated facies like oxide gabbros, amphibole-bearing olivine gabbros, and gabbros and gabbronorites with orthopyroxene as a late crystallising phase. About one quarter of the samples from the olivine gabbro family are devoid of ore minerals (Table 2), and chromian spinels and nickel iron sulphides coexist in about 75% of the opaque bearing samples (Fig. 2). Ilmenite or rutile is sporadically present in the most differentiated samples (i.e. in the orthopyroxene-bearing olivine gabbros or oxide-rich olivine gabbros) where chromian spinel is less abundant than in the more primitive samples. In these differentiated samples, igneous magnetite is common. In this family, grains of chromian spinel are slightly smaller (several tens of microns) than in troctolites but, as in the latter, they occur as euhedral to subhedral, sometimes slightly eroded, crystals included in the other phases (Fig. 3b). Their modal proportion is usually less than 1 or 2 vol.% but exceed 5 10 vol.% in the case of oxide-rich olivine gabbros, which contain more than 10 vol.% of opaque mineral of various nature (ilmenite, rutile, magnetite and chromian spinel are coexisting in the oxide-rich gabbros) The pyroxenite family In the pyroxenite family, opaque minerals are on average slightly more abundant than in the other families (between 2 and 3 vol.%). Only few samples are devoid of any ore minerals and more than 75% of the pyroxenites contain chromian spinels (Table 2 and Fig. 2).They are subhedral and their relationships with the silicate phases suggest their co-crystallisation (Fig. 3d). Fe Ni sulphides may be relatively abundant (Table 2 and Fig. 2), commonly coexist with the chromian spinels in clinopyroxenites and websterites but are very rare in the orthopyroxenite group (with only two samples bearing this phase). Conversely, chromian spinels appear as abundant (up to 3 5 vol.%) small grains (few tens of micrometre) in this latter group (Fig. 3d). Fig. 5. TiO 2 of the chromian spinels plotted against Cr#. The Cr# of chromian spinels was calculated using atomic proportions as the ratio Cr CrþAl when Y Fe3þ 3þ Fe ¼ b0:2. The nonmagnitude spinels that exhibit a Y Fe CrþAlþFe 3þ 3þ over 0.2 are not represented. Symbols and fields areas in Fig. 4. The chemical field of chromian spinels of the crustal section of the Oman (Hiltiblock, Ernewein, 1987) mostly overlaps that of the Mid-Ocean Ridge gabbros (MOR crust) and is represented with the depleted series.

9 144 M. Python et al. / Lithos 106 (2008) The gabbronorite family In the gabbronorite family, 84 samples of 258 are devoid of ore minerals. When present, they are usually magnetites, sulphides, ilmenites and hematites. Chromian spinel bearing gabbronorites are scarce (found in only 25 out of 200 samples, Table 2 and Fig. 2). Three samples of norite and most olivine-bearing gabbronorites contain traces of chromian spinel but the oxide-gabbronorites (opaque modal proportion N15 vol.%) are totally devoid of it. When present, chromian spinels have a very low abundance (b0.5 vol.%) as small euhedral to subhedral crystals associated with magnetite. Magnetite grows at the rim of the grains or as an interstitial phase at the silicate grains boundaries (see Fig. 3c). The chromian spinel itself is totally or partially included in the other minerals. 4. Chemical characteristics of the spinels Mineral compositions of each dyke was analysed by electron microprobe (the Cameca SX-50 of Toulouse University for the large majority of the samples, the JEOL 8800 of the Kanazawa University was used for a few of them). With both instruments, a classical analysis program, with an acceleration voltage of 20 kv, a probe current of 20 na and a beam diameter of 3 µm was used. Counting times varied from s on the peak, and from 5 to 10 s on the background. The chemical characteristics of the silicates are detailed in Python and Ceuleneer (2003). Chromian spinels are usually quite small and only their cores were analysed and presented in Table 3, Figs.4,5,6,7and9, and in the table given in electronic supplement The troctolite and olivine gabbro families The nature, proportion, order of crystallization and composition of the silicate assemblage in the troctolites and olivine gabbros call for crystallisation at moderate pressure (around 0.2 GPa) from an olivine tholeiite similar to MORB (Python and Ceuleneer, 2003). The chemical compositions of their chromian spinels are also very similar to that of the gabbros drilled at Mid-Ocean Ridges and plot in chemical domains that widely or even completely overlap (Figs. 4a, 5a, 6a, 7a and 8). The Cr# globally ranges from 0.3 to0.72, but a large majority of samples show a Cr# between 0.45 and 0.63 (Table 3, Figs. 4a, 5a, 6a, 7a and 8). The Mg# Mg number ¼ Mg MgþFe 2þ in atomic number ranges from 0.1 for orthopyroxene-rich gabbros to 0.75 for some impregnated dunites, but most samples show a Mg# between 0.3 and 0.6 (Fig. 7a). One sample of olivine gabbro from the Wadi Tayin area (Fig.1) exceptionally contains a purely aluminous spinel associated with magnetite (Figs. 4a and 7a). The TiO 2 content reaches 6 wt.% in three samples containing ilmenite in addition to chromian spinels, but is below 2 wt.% in all the other samples and lower than 1 wt.% in most cases (Table 3, Figs. 5a and 6a). It increases slightly with increasing Y Fe 3þ or with decreasing h i Fig. 6. TiO 2 vs Y Fe 3þ ¼ Fe3þ atomic ratio of chromian spinels. Besides the various compositional fields given in the Fig. 4, chemical domains for Mid-Ocean Ridge Basalts (MORB, Fe 3þ þcrþal dashed surface) and arc magmas (Arai, 1992; Barnes and Roeder, 2001, and PetBD ( are given. FC: Fractional crystallisation trend (from Barnes and Roeder, 2001). Symbols are as in Fig. 4.

10 M. Python et al. / Lithos 106 (2008) h i h i Fig. 7. Relationship between Mg# ¼ Mg atomic ratio and Cr# ¼ Cr atomic ratio of chromian spinel. Chemical fields for Oman, Mid-Ocean Ridges and Arc peridotites, MgþFe 2þ CrþAl boninites and Mid-Ocean Ridge gabbros and troctolites, the approximate trends for peridotite partial melting and fractional crystallisation (Barnes and Roeder, 2001) are given for comparison. The chemical fields of chromian spinels of the plutonic crustal section of the Oman (Hilti block, Ernewein, 1987) and the boninite of Oman (Ishikawa et al., 2002) are shown on (b) with the depleted series. FC: fractional crystallisation trend; PM: partial melting trend (from Barnes and Roeder, 2001). forsterite content of olivine (Figs. 6a and 9a). At a given Fo in coexisting olivine, the Ti content in chromian spinel is on average higher in the troctolites than in the olivine gabbros (Fig. 9a). MnO ranges from 0.2 to 0.6 wt.% and tends to decrease with decreasing Cr and increasing Mg#. The samples from the Wadi Fayd and the region located to the North of the Wadi 'Uqq shear zone exhibit peculiarly low concentrations in Ti and Mn. The NiO content is always lower than 0.3 wt.% and shows no significant evolution with the Cr# or the Mg#. The troctolites and olivine gabbros are distributed in three subgroups based on the value of Cr#: the normal ones (0.45bCr#b0.63), the low Cr# ones (b0.43) and the high Cr# ones (N0.65). The Cr# is the only parameter that allows us to distinguish these three groups, the other parameters (Mn, Ni content, Mg#) are not discriminant. The TiO 2 content is peculiarly low (lower than 0.6 wt.%) in the low Cr# ( ) chromian spinels and high ( wt.%) in the high Cr# (N0.65) ones. But the MnO ( wt.%), NiO (b0.3) and Mg# ( ) are in the same range as the normal chromian spinels (see Table 3, Figs. 5a, 7a and 9a) The pyroxenite family The chemistry of the chromian spinel and the modal composition of the pyroxenites allow us to distinguish two groups in this family: an orthopyroxene-rich one composed of orthopyroxenites and websterites with more than 70 vol.% of orthopyroxene and a clinopyroxenite orthopyroxene-poor websterite suite containing less than 55 vol.% of orthopyroxene. The orthopyroxene-rich websterites have chromian spinels with chemical compositions close to those of the

11 146 M. Python et al. / Lithos 106 (2008) Fig. 8. Histogram showing the respective abundances of analysed chromian spinel in each lithological type for various intervals of Cr#. orthopyroxenites. On the other hand, in the other websterites, chromian spinels show a chemical composition similar to that of the clinopyroxenite group. The chromian spinels observed in these two groups (orthopyroxene-rich and orthopyroxene-poor) share common depleted chemical characteristics, e.g. very low TiO 2 (below 0.2 wt.%) and NiO (b0.3 wt.%) contents but their Cr#, Mg#, Y Fe 3þ, MnO outline the difference The orthopyroxenite group This group includes orthopyroxenites sensu stricto and websterites bearing more than 70 vol.% of orthopyroxene. The chromian spinels in orthopyroxenites are rather homogeneous in terms of Cr# ( ) and only four samples have a Cr# lower than 0.50 (Table 3; Figs. 4b, 7b and 8). The MnO content never exceeds 0.45 wt.% and the Y Fe 3þ is lower than 0.06 (Fig. 6b). Except for three samples (00FZ45, 99NA20 and 00RU29) which contain up to5 vol.% of magmatic hornblende and exhibit low Mg (b0.45), chromian spinels show relatively high Mg# ( ). This parameter tends to linearly decrease with the Cr# (Fig. 7b) The clinopyroxenite websterite group The Cr# of chromian spinels ranges from 0.35 to 0.80, i.e. the largest scatter among the different lithological families (Figs. 4b, 7b and 8). Low Cr# values ( ) are more frequent in the case of clinopyroxenites than in the case of websterites. The TiO 2 concentration is lower than 0.2 wt.%, except for the wehrlites and for a few samples of clinopyroxenite that are associated in the field with pegmatites of gabbronorite, whose TiO 2 content of spinel is higher than 0.3 wt.%. Some of the latter (e.g. sample 00FZ9 and 99RU62, see Table 3) are pyroxenitic inner walls of gabbronorite dykes and show a relatively high Fe 3+ content with a Y Fe 3þ N0:15 (Fig. 4b). The MnO content is variable (0.01 to 0.8 wt.%) and the highest values are observed for the clinopyroxenites. Except for a few isolated samples of clinopyroxenite, the proportion of Fe 3+ is low (Fe 2 O 3 b5 wt.%, with a Y Fe 3þ N0:1, see Figs. 4b and 6b) and magnetites are present, in association with chromian spinels, only in nine websterites and about ten clinopyroxenites. The Mg ranges globally from 0.1 for some clinopyroxenites to 0.65 in some websterites, with more abundant values in the interval 0.35 to The Mg# slightly decreases with increasing Cr#. The TiO 2 content of chromian spinels tends to increase with decreasing Mg# of orthopyroxene in wehrlites and in the clinopyroxenites associated with gabbronorites (Fig. 9b). No evolution is observed for the other samples. The compositional field of chromian spinel in clinopyroxenite hardly overlap with that of orthopyroxenite (Figs. 4b, 5b and 7b), the Cr# being lower at similar Mg# and Ti, Mn, Ni contents. Chromian spinels from the websterite group cover chemical fields intermediate between those of orthopyroxenites and of clinopyroxenites Gabbronorite family In the gabbronorite family, chromian spinels are scarce and the most commonly observed spinel is magnetite (Table 2, Fig. 4b). In these magnetites, the TiO 2, MnO and NiO contents are always below detection limits, and the MgO content is also rather low with a Mg# below 0.1. For the chromian spinels, the Cr# is mainly between 0.4 and 0.6 and low Cr# is common in this lithological family (Fig. 8).Two exceptional samples, from the Bahla and Fizh blocks (00BA10 and 00FZ27a) show high Cr#, respectively 0.65 and 0.7 (Figs. 5b and 7b). The NiO content reaches 0.25 wt.% in the olivine-bearing gabbronorites and is always lower than this value in the other gabbronorites and norites (devoid of olivine). The MnO content varies from 0.2 to 0.4 wt.%, reaching 1.4 wt.% only in sample from the Fizh block that exhibits an unusually high Cr# (00FZ27a). MnO and NiO do not show any significant evolution with the Cr# or the Mg#. The TiO 2 concentration is relatively variable but remains below 0.5 wt.% in most samples, is around 1 wt.% in a few samples and reaches higher values (up to 1.8) in two samples only (00BA10 and 00FZ27a; Figs. 5b and 6b). The samples with high Ti content in chromian spinels (N0.5) are very evolved: the Mg# of their orthopyroxene is below 0.7, so that they are not represented on the Fig. 9b. 5. Discussion Our previous study of Oman mantle dykes focused on field and petrographic characteristics, and on the chemical composition of silicate minerals. These data allowed us to distinguish four main petrographical families and to group them into two magmatic series (MORB and depleted, see Python and Ceuleneer, 2003, and Table 1). The chemical characteristics of the chromian spinels largely fits with this previous classification: chromian spinel has, on average, lower Cr# and higher TiO 2 concentrations in the troctolite and olivine gabbro families than in the pyroxenite and gabbronorite families, confirming the MORB affinity of the former and the depleted nature of the latter. In the present section we will show that some second order variations are observed in the composition of the chromian spinel, allowing an advanced analysis of the petrogenesis of the two magmatic suites. We will also see to what extent it is a reliable parameter to further constrain the tectonic setting of the Oman ophiolite.

12 M. Python et al. / Lithos 106 (2008) Fig. 9. TiO 2 in chromian spinel plotted against the Mg# of coexisting olivine or orthopyroxene. Approximate trend for the fractional crystallisation (FC) of a MORB-type melt and for the melt rock reaction (MRR) are shown. In the case of melt rock reaction (MORB-type melt), the forsterite content of olivine is buffered by the mantle and hardly varies while the TiO 2 content of the spinel increases. The chemical domains for the oceanic gabbros (as in Fig. 4) and the crustal section of the Oman ophiolite (Hilti block, Ernewein, 1987) are given for comparison. (a) and (b): representation by lithological types; (c) and (d): representation by geographical zone. In magmatic systems, the Cr/Al ratio of the chromian spinel depends on various parameters (P/T conditions, Cr/Al ratio of the melt, nature of the cotectic silicates, cooling rate). Nevertheless, in basaltic systems, this ratio is predominantly controlled by the Cr/Al ratio of the melt, which reflects the composition of the source (Cr/Al of the mantle), the degree of partial melting, and the degree of fractional crystallisation (e.g. Roeder and Reynolds, 1991). The crystallisation of plagioclase, modifies this ratio by removing large quantities of Al from the melt. The evolution of the chemical composition of chromian spinel during fractional crystallisation of silicates follows the Fe Ti trend described by Barnes and Roeder (2001), i.e. a trend showing increasing Fe 3+, TiO 2 and decreasing Mg# at relatively constant or slightly decreasing Cr# (see Figs. 4, 6 and 7), that may be enhanced by reaction with trapped melt in orthocumulates. Chromian spinel is usually the first mineral to crystallise from dry silicate melts (e.g. Roeder and Reynolds, 1991). As a consequence of the low solubility of Cr in silicate melts, the low concentration of Cr in common magmas allow the crystallisation of only small amounts of chromian spinel during the early stages of fractional crystallisation. Most of the chromian spinels contained in the dykes of Oman appear

13 148 M. Python et al. / Lithos 106 (2008) as small euhedral grains partially or totally included in the silicates, confirming their early crystallisation (Fig. 3). In addition, most of our samples are primitive facies that have not experienced a long crystallisation history and are thus only moderately influenced by fractional crystallisation (Python and Ceuleneer, 2003). Therefore, the effect of the crystallising silicates on our chromian spinels Cr# is considered to be negligible compared to the variability attributable to the mantle source. At subsolidus conditions, spinels tend to react with the host silicates. Fortunately, this process mostly influences the Mg#, which is affected by subsolidus Mg Fe 2+ exchange with mafic silicates during cooling (e.g. Evans and Frost, 1975). Nevertheless, elements like Ti and Cr are in relatively low abundance in associated silicates, and elements like Al (abundant in plagioclase only) have very sluggish diffusion rates (e.g. Morse, 1984). The Cr 2 O 3,Al 2 O 3 and TiO 2 contents of the chromian spinels are therefore little affected by this post-magmatic reequilibration and are mostly controlled by the concentration of these elements in the parental melt and by the partition coefficient between melt and spinel (e.g. Roeder and Campbell, 1985; Kamenetsky et al., 2001) The MORB suite The normal troctolites and olivine gabbros The Cr# of the chromian spinel in most troctolites and olivine gabbros vary between 0.45 and 0.63, i.e. in the MORB range (Figs. 5a and 7a). The other chemical parameters (Mg#, TiO 2, MnO, NiO) are also similar to what is observed for the Mid-Ocean Ridge gabbros. In a given tectonic environment, the chemical domain of the chromian spinel in plutonic rocks is rather similar to that of the volcanic rocks (Arai, 1992) with slightly lower Ti content and Y Fe 3þ, diagnostic of a lower degree of fractional crystallisation in the plutonic rock than in the volcanics (e.g. Fig. 6a). In the mantle dykes of Oman, the Ti content of chromian spinel is lower than in Mid-Ocean Ridge gabbros, which is compatible with the lower degree of differentiation (olivine Fo content and clinopyroxene Mg on average higher in the olivine gabbros and troctolites from the mantle of Oman than in the Mid- Ocean Ridge gabbros, see Python and Ceuleneer, 2003). However, about half of the olivine gabbros of the Oman mantle dykes and a few troctolites and impregnations (as well as most chromian spinel from the crustal section) show a Y Fe 3þ higher than in the Mid-Ocean Ridge gabbros (Fig. 6a).This suggests a higher f O2 during the crystallisation of the MORB-type dykes and the crust of Oman than during the genesis of the present-day oceanic gabbros. This higher oxidation level is probably the consequence of general hydrous conditions in the former Oman spreading centre, symptomatic of a slighty hydrated mantle that could result from the tectonic environment (marginal basin) or from local variations in the mantle source (this type of mantle being absent in the present-day East Pacific Rise) The Khawad trend : a low Cr# trend in the Khawad Wadi Tayin belt The MORB-type dykes (troctolites, olivine gabbros, impregnated hazburgites and dunites) that were sampled northward to the Wadi 'Uqq shear zone (in the zone that we called North Sumail, see Fig. 1), are unusual in terms of Cr# of chromian spinel. This Cr# is restricted to values between 0.35 and 0.43 suggesting that the Cr/Al ratio of the parent melt was lower than that of the troctolites and olivine gabbros from other areas (Figs. 7a, 10a and 11a). All the impregnations, troctolites and olivine gabbros from the whole ophiolite show similar silicates chemical characteristics, and the samples from the north of Sumail differ only by their low Cr# of chromian spinels. We can thus infer that the melts in equilibrium with the troctolites and olivine gabbros from this area were slightly depleted in Cr relative to Al compared to those from the other parts of the ophiolite. This lower Cr# melt could result from a low degree partial melting of a relatively primitive mantle, still containing low Cr# spinels and clinopyroxene. The peridotites from the Oman ophiolite were extensively studied (Takazawa et al., 2003; Suzuki, 2006; Monnier et al., 2006) but unfortunately, very few petrological and geochemical data are available for the north of the Sumail area, and no data have been published about the peridotite of the Khawad or the Musibit areas. Monnier et al. (2006) published a complete set of peridotite data which includes two harzburgites from the north of the Sumail zone and one from the Wadi Tayin area. Interestingly, these three samples exhibit low Cr# (b0.45) for their chromian spinel consistent with a relatively low melting degree of the mantle in these areas Origin of the impregnations in Oman harzburgites or dunites The impregnated dunites and harbzburgites (i.e. containing interstitial plagioclase and/or clinopyroxene, see Section 3.1) share a number of common characteristics with the mantle peridotites, among others porphyroclastic textures and anhedral chromian spinels. It is likely that, with these kinds of features, chromian spinel is residual rather than cumulative. However, only few samples of these impregnated peridotites exhibit a composition similar to that of the Oman harzburgite in terms of TiO 2, Mg# and Cr# of spinel and forsterite content of olivine (Figs. 4a, 5a, 6a, 7a and 9a). For most impregnated peridotites, the evolution of the TiO 2 of the chromian spinel with the forsterite content of olivine suggests an origin in a context of strong melt rock reaction (Fig. 9a) which is consistent with their field characteristics. Nevertheless, some samples of impregnated dunites, in spite of this impregnation-like appearance in the field, show a composition compatible with a fractional crystallisation from a MORB-type melt (Fig. 9a). The variability in the composition of the chromian spinel associated with the silicate phases in the impregnated peridotites show that the petrogenetical conditions during the genesis of these impregnations are variable and that, in the troctolite family, lithological facies have evolved in a continuous way between the impregnation stricto sensu (minute crystallisation of plagioclase within the peridotite) to the cumulative troctolite Very high Cr# in the Maqsad diapir: evidence for intensive melting? Five samples of troctolite are out of the MORB trend, showing higher Cr# ( ), relatively high Mg# and TiO 2 contents (Figs. 5a and 7a) and low NiO and MnO. These samples were collected near the centre of the Maqsad diapir (Fig.12a) and, then, are probably located in a region of high degree of melting and active magma circulation. This high Cr# can be interpreted in terms of magmatic processes: the large abundance of dunite in the Maqsad area is symptomatic of intense magma circulation. In the opposite way to the phenomenon that occurred in the north Sumail area, the extensive melting and magma/mantle interaction (dissolution of orthopyroxene) could, at its end, produce Cr-enriched melt that could precipitate high-cr# chromian spinels. Quatrevaux (1995) showed that the peridotites in the very centre of the Maqsad diapir contain chromian spinels with Cr# on average ranging from 0.55 to 0.6, while from 0.5 to 0.55 in the periphery of the diapir. The Cr# of chromian spinel is not as high as in those included in the troctolite dykes from the centre of the diapir. Nevertheless, a correlation of Cr# between peridotites and troctolites exists, suggesting that the high Cr# reflects high degree of partial melting of the mantle Petrogenesis of the andesitic suite Compared to the MORB series, the depleted andesitic suite is much more variable in terms of modal and chemical composition. Lithology ranges from olivine-bearing gabbronorite (orthopyroxene+ clinopyroxene+olivine+plagioclase) to the pure orthopyroxenite or clinopyroxenite with intermediate cumulates (norite, hornblende-rich norite, websterite). In spite of this lithological diversity, all these facies share common chemical characteristics like the enrichment in SiO 2

14 M. Python et al. / Lithos 106 (2008) Fig.10. Schematic maps of the Oman ophiolite showing the distribution of the Cr# of chromian spinels for the two magmatic series. The dashed line shows the inferred paleo-axis that was used for calculation of along and across axis distribution on the Fig. 12; this is the same axis that was used by Le Mée et al. (2004) and Monnier et al. (2006) for their distribution diagrams. and the depletion in all incompatible elements (Ti, Na, Cr) pointing to a common origin from hydrous melting of a depleted mantle source. A continuum is observed from clinopyroxenite to gabbronorite through websterite. On the other hand, a gap in the modal composition exists between the orthopyroxenite group and the clinopyroxenite websterite gabbronorite suite. The significance of this gap was not understood using the composition of clinopyroxene and orthopyroxene alone (Python and Ceuleneer, 2003). However, the present study on chromian spinel allows us to reconsider the problem Intermediate chemistry in the chromian spinels from a gabbronorite clinopyroxenite websterite suite The primitive and depleted nature of pyroxenites is outlined by the unusually high Cr# and low TiO 2 of their chromian spinels (Figs. 5b, 8 and 9b). We previously interpreted the pyroxenite and gabbronorite families as the product of the crystallisation of a very depleted melt derived from the remelting of a hydrothermally altered peridotite (Benoit et al., 1999; Python and Ceuleneer, 2003). The composition of the chromian spinels from the clinopyroxenites, gabbronorites and websterites is intermediate between those of MORB and of boninites and supports this hypothesis. Their depleted chemistry suggests that they crystallised from a depleted magma while the marked scatter in composition (see Figs. 4b and 7b) may reflect a very heterogeneous source or variable degree of mixing between a depleted source and a MORB-like one. The frequent occurrence of magnetites, especially (but not only) in the most differentiated gabbronorites, and high Y Fe 3þ in some clinopyroxenites (Fig. 6b) suggest a high f O2 consistent with melting in hydrous conditions. Our data do not allow us to choose