The Southwest Indian Ridge (SWIR) at 9^168E and 52^538S is characterized by ultra-slow, oblique s... more The Southwest Indian Ridge (SWIR) at 9^168E and 52^538S 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 Al 2 O 3 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 midocean 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 0Á6^0Á7 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 þ 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.
Mafic-ultramafic dykes scattered in the mantle section of ophiolites are generally crystallisatio... more Mafic-ultramafic dykes scattered in the mantle section of ophiolites are generally crystallisation products from common silicate melts. In the frame of a global survey of these melt migration relics in the Oman mantle harzburgites, we discovered a peculiar lithology made essentially of pure diopside (0:95b Mg MgþFetotal b1) whose characteristics do not match a magmatic or mantle origin. Arguments against a magmatic or mantle origin for these diopsidites combine compositional and textural evidence. In spite of their refractory composition, they are strongly depleted in Cr (Cr 2 O 3 b 0.2 wt.%). By the same way other minor elements (Al, Ti, Na…) and rare earth elements have peculiarly low abundances and plot away from magmatic differentiation trends. In a few samples, the paragenetic association includes pure anorthite (An% up to 0.99), minor amounts of forsterite (Fo N 0.95) or traces of andradite. When not deformed, the diopsides are automorphic and their texture points to metamorphic growth in a matrix composed of antigorite and/or carbonate. Diopsidites have textures reminiscent to that of skarns developing in contact metamorphic halos or that of rodingite frequently present in the serpentine bodies of the ophiolitic crust. They frequently appear as dykes (former cracks), with a few mm to few tens of cm wide transitional zones, which contain high amounts of hydrous minerals, between the diopsidite facies and its host rock. The diopsidites are not randomly distributed in the Oman ophiolite, being more abundant near former asthenospheric diapirs emplaced at shallow depth in the lithosphere. We interpret the diopsidites as the footprint of very high temperature circulation of seawater and carbonated fluids (N 800°C), which may have leached plagioclase rich lithologies before penetrating the mantle (as shown by a well developed positive Eu anomaly). Our data confirm the prediction of McCollom and Shock [T.M. McCollom, E.L. Shock, Fluid-rock interactions in the lower oceanic crust: Thermodynamic models of hydrothermal alteration, J. Geophys. Res. 103 (B1) (1998) 547-575.] who proposed that common anhydrous minerals like pyroxene, plagioclase and olivine may crystallise from high temperature fluids intermediate between silicate melts and supercritical water. This confirms that there is no clear-cut thermal and chemical boundary between the fields of magmatic and hydrothermal crystallisations.
This paper describes a comprehensive study of the chromian spinels present in mafic-ultramafic dy... more 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; 0.45 b Cr# b 0.63; 0.3 b Mg# b 0.6; TiO 2 up to2 wt.%), and more silicic melts issued from a highly depleted mantle source, similar to boniniticandesitic melts that preferentially form in subduction zone settings (0.35 b Cr# b 0.80; 0.1 b Mg# b 0.7; 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 degreein 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.
1] We conducted a comprehensive field, petrographic, and microprobe study of the dykes and porous... more 1] We conducted a comprehensive field, petrographic, and microprobe study of the dykes and porous flow channels cropping out in the Oman harzburgites. The 36 rock types we recognized among of about 1000 samples can be grouped in two main magma suites contrasted in terms of structural and textural characteristics, modal composition, order of crystallization, and phase chemistry. One suite (troctolites, olivine gabbros, opx-poor gabbronorites, and rare oxyde gabbros) derives from MORB-like melts. The other suite (pyroxenites, opx-rich gabbronorites, diorites, and tonalite-trondhjemites) derives from melts richer in silica and water than MORBs and ultradepleted in incompatible elements. Dykes and porous flow channels from the MORB suite are restricted to a few areas, covering only 25% of the mantle section. This is an unexpected result as the deep Oman crust is made essentially of cumulates from MORB-like melts. Their composition, texture, and relations with the host harzburgites point to high mantle temperatures at the time of crystallization (likely above 1100°C, up to 1200°C for part of them), i.e., conditions close to the ''asthenosphere/lithosphere'' boundary. The largest outcrop of mantle harzburgites enclosing MORB like dykes is a 80 km long and 10 km wide corridor, parallel to the strike of the sheeted dyke complex and centered on an area where a former mantle upwelling has been unambiguously defined (the Maqsad ''diapir''). A few other occurrences of mantle cumulates from the MORB suite are smaller than the Maqsad area and have a lesser abundance of troctolites (i.e., of high-temperature cumulates). We interpret the troctolite zones of Oman as the witnesses of former diapirs frozen at various stages of their development. Dykes belonging to the depleted suite are the most common in Oman harzburgites. Their structural and textural characteristics show that they crystallized in a mantle colder than the melt (likely in the range 600°C to 1100°C). A possible origin for the parent melts of this suite is in situ partial melting of the shallow and partly hydrated lithosphere residual after MORB extraction. Our data support the view that feeding magma chambers with MORBs is a focused (and likely episodic) process involving the rise of hot mantle to the base of the crust through a lithospheric lid accreted during a previous diapiric event. They suggest also that the shallow mantle beneath spreading centers is a place of important petrologic processes, some of them predicted on the basis of MORB composition (e.g., fractionation inside melt conduits) and other ones unexpected (e.g., remelting of the depleted lithosphere).
We discovered a new kind of dyke in the mantle section of the Oman ophiolite composed mainly of d... more We discovered a new kind of dyke in the mantle section of the Oman ophiolite composed mainly of diopside that we called diopsidite, although traces of forsterite, anorthite, titanite or andradite may be also observed. The texture and chemical composition, and the relationship between antigorite and diopside or rare forsterite, suggest that they formed by high T metamorphism of serpentine (antigorite) and are the result of interaction between hydrothermal fluid and percolating magma in the shallow mantle. Chemical data, obtained by electron microprobe and in situ laser ablation coupled to ICP MS, show large heterogeneities from one sample to another and between diopside crystals in each sample. The trace element patterns are characterised by strong positive anomalies in Sr and Eu and a negative anomaly in Ti. Small scale heterogeneities reflect the disequilibrium conditions in the dyke formation and show that a relatively Ti rich and Al Eu Sr LREE poor phase may be included in the diopside. The exact nature of this phase is difficult to determine but the Eu negative anomaly, and the low Sr and LREE concentration suggest that it is probably some partially altered, relict protolith mineral.
The Southwest Indian Ridge (SWIR) at 9^168E and 52^538S is characterized by ultra-slow, oblique s... more The Southwest Indian Ridge (SWIR) at 9^168E and 52^538S 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 Al 2 O 3 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 midocean 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 0Á6^0Á7 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 þ 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.
This paper details a new discovery of anorthitebearing diopsidites (= anorthite diopsidites) from... more This paper details a new discovery of anorthitebearing diopsidites (= anorthite diopsidites) from the lowermost crust along Wadi Fizh in the northern Oman ophiolite. The anorthite diopsidites occur as networks within layered gabbros 50 m above the gabbro/peridotite boundary, and are mainly composed of high -Mg diopsidic clinopyroxenes and anorthites with various amounts of uvarovite. They are intermediate in mineral chemical characteristics between the crustal gabbros and the diopsidites, and interpreted as an interaction product between hightemperature Carich hydrothermal fluids and peridotites within the mantle section. The anorthite diopsidites were probably formed by interaction between the fluids involved in diopsidization within the mantle section and the layered gabbro. The fluids responsible for the formation of the anorthite diopsidite were also Carich and carried Cr to form uvarovite from the underlying mantle section. Hightemperature hydrothermal circulation plays an important role in the transportation of elements, e.g., Ca and Cr, from the upper mantle to the lower crust across the Moho in the ocean floor.
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