Sulfur and selenium systematics of the subcontinental lithospheric mantle: Inferences from the Massif Central xenolith suite (France)     

Jean-Pierre Lorand  Olivier Alard  Ambre Luguet 《Geochimica et cosmochimica acta》2003,67(21):4137-4151

Selenium has been analyzed in addition to S in 58 spinel peridotite xenoliths collected in Cenozoic alkali basalts from the Massif Central (France). The S concentration range now available for this suite, calculated from 123 samples, is the largest ever reported for alkali basalt-hosted xenoliths (<3-592 ppm). Likewise, the Se concentrations range between 0.2 and 67 ppb. No partial melting signature can be identified from the S and Se systematic. Half of the analyzed xenoliths have lost S during supergene weathering. By contrast, neither surficial alteration, nor loss of chalcophile elements during eruption can explain the regional-scale variations of S and Se concentrations. A first group of lherzolite xenoliths sampled in Southern Massif Central, from volcanic centers older and spatially unrelated to the Massif Central plume that triggered the Cenozoic volcanism, contains between 20 and 250 ppm S (with occasional S concentrations up to 592 ppm) and 12-67 ppb Se. It is clear that the highest S values, originally interpreted as representing S abundances in the primitive mantle, were in fact enriched by metasomatism. Highly variable S and Se contents (<5-360 ppm; 9-52 ppb) have also been observed in peridotite xenoliths collected in the Northern Massif Central, from volcanic centers mostly older than the plume. Like Group I xenoliths, these Group II xenoliths were strongly metasomatized by volatile-rich carbonated/silicated melts which precipitated Cu-rich sulfides. A third group of xenoliths from Plio-Quaternary basalts spatially related to the Massif Central Plume are uniformly poor in S (10-60 ppm) and Se (9-29 ppb). In this Group III, poikiloblastic textured xenoliths have lost most of their S and Se budget by peridotite-melt interactions at high melt/rock ratios. Taken as a whole, the Massif Central xenolith suite provides further evidence for strong heterogeneities in the S and Se budget of the sub-continental lithospheric mantle. However, the few LREE-depleted fertile lherzolites that escaped strong metasomatic alterations suggest a S- and Se-depleted primitive mantle reservoir compared to currently accepted primitive mantle estimates.  相似文献   

Platinum-group element systematics in Mid-Oceanic Ridge basaltic glasses from the Pacific, Atlantic, and Indian Oceans     

A. Bézos  J.-P. Lorand  M. Gros 《Geochimica et cosmochimica acta》2005,69(10):2613-2627

The concentrations of Ir, Ru, Pt and Pd have been determined in 29 Mid-Oceanic Ridge basaltic (MORB) glasses from the Pacific (N = 7), the Atlantic (N = 10) and the Indian (N = 11) oceanic ridges and the Red Sea (N = 1) spreading centers. The effect of sulfide segregation during magmatic differentiation has been discussed with sample suites deriving from parental melts produced by high (16%) and low (6%) degrees of partial melting, respectively. Both sample suites define positive and distinct covariation trends in platinum-group elements (PGE) vs. Ni binary plots. The high-degree melting suite displays, for a given Ni content, systematically higher PGE contents relative to the low-degree melting suite. The mass fraction of sulfide segregated during crystallization (X_sulf), the achievement of equilibrium between sulfide melt and silicate melts (R_eff), and the respective proportions between fractional and batch crystallization processes (S_b) are key parameters for modeling the PGE partitioning behavior during S-saturated MORB differentiation. Regardless of the model chosen, similar sulfide melt/silicate melt partition coefficients for Ir, Ru, Pt and Pd are needed to model the sulfide segregation process, in agreement with experimental data. When corrected for the effect of magmatic differentiation, the PGE data display coherent variations with partial melting degrees. Iridium, Ru and Pt are found to be compatible in nonsulfide minerals whereas the Pd behaves as a purely chalcophile element. The calculated partition coefficients between mantle sulfides and silicate melts (assuming a PGE concentration in the oceanic mantle at ∼0.007 × CI-chondritic abundances) increase from Pd (∼10³) to Ir (∼10⁵). This contrasting behavior of PGE during S-saturated magmatic differentiation and mantle melting processes can be accounted for by assuming that Monosufide Solid Solution (Mss) controls the PGE budget in MORB melting residues whereas MORB differentiation processes involve Cu-Ni-rich sulfide melt segregation.  相似文献   

The behaviour of the upper mantle sulfide component during the incipient alteration of “Alpine”-type peridotites as illustrated by the Beni Bousera (northern Morocco) and Ronda (southern Spain) ultramafic bodies     

Dr. J. P. Lorand 《Mineralogy and Petrology》1985,34(3-4):183-209

Summary A volumetrically minor Cu–Fe–Ni–S component derived from the uppermost mantle is found within the Beni Bousera (northern Morocco) and Ronda (southern Spain) Alpine-type peridotites; it occurs today as inclusions within primary silicates or as assemblages disseminated in the intergranular sites of the host rocks. Detailed microtextural and microprobe data indicate that inclusions and intergranular assemblages behaved as two contrasting systems during the low temperature, incipient serpentinization of the host rocks. The former were equilibrated in closed systems whereas the latter behaved as open systems with respect to hydrothermal fluids; as a result, intergranular assemblages were controlled by redox conditions generated by serpentinization. An early stage of alteration is characterized by a slight decrease of the sulfur content and would be due to the first influx of water inside peridotites; a further transformation was produced by anomalous highly reducing conditions responsible for the crystallization of native iron-bearing alloy assemblages as well as for a preferential partioning of Fe from silicate into pentlandite. The production of anomalous, reducing conditions during incipient serpentinization is ascribed to a low permeability of the host rocks with respect to the diffusion of hydrogen out of the sites of serpentinization. Because of its low temperature behaviour, the intergranular sulfide component would not be of use reconstructing the initial composition of the upper mantle sulfide component; thus, it is concluded that only the sulfide inclusions would provide such informations.

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Le comportement du composant sulfuré du manteau supérieur pendant les premiers stades de serpentinisation des péridotites de type alpin; une discussion a partir de l'exemple des massifs ultramafiques de Beni Bousera (Maroc) et de Ronda (Espagne)

Résumé Des traces d'un composant sulfuré du système Cu–Fe–Ni–S d'origine mantellique sont présentes dans les péridotites alpines de Beni Bousera (Maroc) et Ronda (Espagne); il forme actuellement des inclusions dans les silicates primaires ou des plages intergranulaires disséminées dans les interstices des roches. Une analyse minéralogique et chimique détaillée démontre qu'inclusions et plages intergranulaires sulfurées ont eu un comportement opposé dès les premiers stades de serpentinisation des roches encaissantes. Les premières ont été reéquilibrées en système chimique ouvert vis-à-vis des fluides de serpentinisation; en conséquence les paragenèses intergranulaires résultent d'une suite de transformations du composant sulfuré mantellique, controlée par le degré d'oxydo réduction du fluide de serpentinisation. Dans un premier stade, l'entrée de l'eau dans les péridotites a provoqué une légère baisse de la fugacité du soufre et la cristallisation d'assemblages sulfurés riches en mackinawite. L'augmentation du degré de serpentinisation a ensuite engendré des conditions inhabituellement réductrices responsables d'un important fractionnement du fer libéré par la serpentinisation de l'olivine dans la pentlandite ainsi que de la cristallisation d'une paragenèse complexe de phases métalliques incluant localement le fer natif; ces conditions anormalement reductrices sont imputées à une faible perméabilité des roches encaissantes vi-à-vis de la diffusion de l'hydrogène en dehors dessites de serpentinisation. En raison de leur comportement à basse température, les plages sulfurées intergranulaires sont inutilisables pour reconstituer le composant sulfuré du manteau supérieur; sur ce plan, on conclut que seules les inclusions blindées dans les silicates peuvent apporter des informations.

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With 9 Figures  相似文献   

The influence of nonlinear material properties and resistance to bending on the development of internal structures     

J-P. Latham 《Journal of Structural Geology》1985,7(2):225-236

A reexamination of Biot's internal instability analysis, including the effects of bending resistance and nonlinear material properties, shows that internal buckling and oblique localized shearing into bands are the two end-member processes associated with internal instability. The first is a consequence of a high anisotropy which is an intrinsic property of the original unstressed layering or foliation. The second is a consequence of a highly anisotropic material response to perturbations which is induced during the initial uniform compression if material properties are nonlinear. The incremental anisotropy generating the instability is associated with planes of least resistance to shearing in directions parallel to the initial compression in the first case and at 45° to the initial compression in the second case.  相似文献   

Pétrologie de l'intrusion alcaline mésozoïque de la région d'Anemzi, Haut Atlas Central, Maroc     

A. Lhachmi  J.-P. Lorand  J. Fabries   《Journal of African Earth Sciences》2001,32(4):58

The Mesozoic Anemzi intrusion belongs to the peri-Atlantic Mesozoic alkali magmatism. It is composed of gabbros (with or without olivine), monzodiorites and quartz-bearing syenites. Dolerite dykes of alcaline affinity occur nearby. The alkaline rocks are intrusive into mid-Jurassic limestones. Liquidus phases crystallised following the characteristic order of alkali magmas, i.e. olivine + spinel, plagioclase, clinopyroxene and amphibole. Clinopyroxene and amphibole compositions are governed by Ti-tschermakite substitutions. Clinopyroxene and amphibole in syenites are enriched in aegirine and richterite components, respectively. The mineralogical and chemical evolution in the Anemzi intrusion is consistent with low-pressure fractional crystallisation from an alkali lava under low to moderate fO₂ (MW + [1 − 2 log units]). However, the parental liquid shows some geochemical characteristics of transitional magma, such as a high SiO₂ content (48 wt%) and low La/Yb_N ratio (5). Silica activity decreased from the parental magma (0.56) to 0.08 after fractionation of the gabbros and then increased to 1.0 at the end of the magmatic differentiation. Compared with rocks of similar ages, belonging to the peri-Atlantic Cretaceous alkaline magmatism, the Anemzi intrusion is distinguished by low REE contents ([La/Yb]_N = 6) and transitional basalt(s) as parental magma. This magma was emplaced through reactivation of Palæozoic fault systems.  相似文献   

Pervasive melt percolation reactions in ultra-depleted refractory harzburgites at the Mid-Atlantic Ridge, 15° 20′N: ODP Hole 1274A     

Monique Seyler  J. -P. Lorand  H. J. B. Dick  M. Drouin 《Contributions to Mineralogy and Petrology》2007,153(3):303-319

ODP Leg 209 Site 1274 mantle peridotites are highly refractory in terms of lack of residual clinopyroxene, olivine Mg# (up to 0.92) and spinel Cr# (∼0.5), suggesting high degree of partial melting (>20%). Detailed studies of their microstructures show that they have extensively reacted with a pervading intergranular melt prior to cooling in the lithosphere, leading to crystallization of olivine, clinopyroxene and spinel at the expense of orthopyroxene. The least reacted harzburgites are too rich in orthopyroxene to be simple residues of low-pressure (spinel field) partial melting. Cu-rich sulfides that precipitated with the clinopyroxenes indicate that the intergranular melt was generated by no more than 12% melting of a MORB mantle or by more extensive melting of a clinopyroxene-rich lithology. Rare olivine-rich lherzolitic domains, characterized by relics of coarse clinopyroxenes intergrown with magmatic sulfides, support the second interpretation. Further, coarse and intergranular clinopyroxenes are highly depleted in REE, Zr and Ti. A two-stage partial melting/melt–rock reaction history is proposed, in which initial mantle underwent depletion and refertilization after an earlier high pressure (garnet field) melting event before upwelling and remelting beneath the present-day ridge. The ultra-depleted compositions were acquired through melt re-equilibration with residual harzburgites. Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users.  相似文献   

Petrogenesis of the amphibole-rich veins from the Lherz orogenic lherzolite massif (Eastern Pyrenees, France): a case study for the origin of orthopyroxene-bearing amphibole pyroxenites in the lithospheric mantle     

J. Fabriès  J.-P. Lorand  M. Guiraud 《Contributions to Mineralogy and Petrology》2001,140(4):383-403

The Lherz orogenic lherzolite massif (Eastern French Pyrenees) displays one of the best exposures of subcontinental lithospheric mantle containing veins of amphibole pyroxenites and hornblendites. A reappraisal of the petrogenesis of these rocks has been attempted from a comprehensive study of their mutual structural relationships, their petrography and their mineral compositions. Amphibole pyroxenites comprise clinopyroxene, orthopyroxene and spinel as early cumulus phases, with garnet and late-magmatic K₂O-poor pargasite replacing clinopyroxene, and subsolidus exsolution products (olivine, spinel II, garnet II, plagioclase). The original magmatic mineralogy and rock compositions were partly obscured by late-intrusive hornblendites and over a few centimetres by vein–wallrock exchange reactions which continued down to subsolidus temperatures for Mg–Fe. Thermobarometric data and liquidus parageneses indicate that amphibole pyroxenites started to crystallize at P ≥ 13 kbar and recrystallized at P < 12 kbar. The high Al^VI/Al^IV ratio (>1) of clinopyroxenes, the early precipitation of orthopyroxene and the late-magmatic amphibole are arguments for parental melts richer in silica but poorer in water than alkali basalts. Their modelled major element compositions are similar to transitional alkali basalt with about 1–3 wt% H₂O. In contrast to amphibole pyroxenites, hornblendites only show kaersutite as liquidus phase, and phlogopite as intercumulus phase. They are interpreted as crystalline segregates from primary basanitic magmas (mg=0.6; 4–6 wt% H₂O). These latter cannot be related to the parental liquids of amphibole pyroxenites by a fractional crystallization process. Rather, basanitic liquids mostly reused pre-existing pyroxenite vein conduits at a higher structural level (P ≤ 10 kbar). A continuous process of redox melting and/or alkali melt/peridotite interaction in a veined lithospheric mantle is proposed to account for the origin of the Lherz hydrous veins. The transitional basalt composition is interpreted in terms of extensive dissolution of olivine and orthopyroxene from wallrock peridotite by alkaline melts produced at the mechanical boundary layer/thermal boundary layer transition (about 45–50 km deep). Continuous fluid ingress allowed remelting of the deeper veined mantle to produce the basanitic, strongly volatiles enriched, melts that precipitated hornblendites. A similar model could be valid for the few orthopyroxene-rich hydrous pyroxenites described in basalt-hosted mantle xenoliths. Received: 15 September 1999 / Accepted: 31 January 2000  相似文献   

The effects of stock size and environmental variability on Cape hake recruitment in Namibia: an unsolved puzzle (a comment on Kainge et al. 2013)     

J-P Roux  MR Wilhelm 《African Journal of Marine Science》2015,37(3):431-433

Measuring and forecasting recruitment are central to the understanding and management of fish stocks. Kainge et al. (2013) studied the effect of spawning stock size and environmental fluctuations on the recruitment levels of the Cape hake Merluccius capensis in Namibia. However, their study contains some flaws that undermine the conclusion that Cape hake recruitment is under the influence of upwelling in summer. Until those flaws are properly addressed, this conclusion, in our view, should be treated with caution.  相似文献   

Exsolution and shock microstructures of igneous pyroxene clasts in the Northwest Africa 7533 Martian meteorite          下载免费PDF全文

Hugues Leroux  Damien Jacob  Maya Marinova  Roger H. Hewins  Brigitte Zanda  Sylvain Pont  Jean‐Pierre Lorand  Munir Humayun 《Meteoritics & planetary science》2016,51(5):932-945

Northwest Africa (NWA) 7533 is a Martian regolith breccia. This meteorite (and its pairings) offers a good opportunity to study (near‐) surface processes that occurred on early Mars. Here, we have conducted a transmission electron microscope study of medium‐ and coarse‐grained (a few tens to hundreds of micrometers) Ca‐rich pyroxene clasts in order to define their thermal and shock histories. The pyroxene grains have a high‐temperature (magmatic) origin as revealed by the well‐developed pigeonite–augite exsolution microstructure. Exsolution lamella characteristics (composition, thickness, and spacing) indicate a moderately slow cooling. Some of the pyroxene clasts display evidence for local decomposition into magnetite and silica at the submicron scale. This phase decomposition may have occurred at high temperature and occurred at high oxygen fugacity at least 2–3 log units above the QFM buffer, after the formation of the exsolution lamellae. This corresponds to oxidizing conditions well above typical Martian magmatic conditions. These oxidizing conditions seem to have prevailed early and throughout most of the history of NWA 7533. The shock microstructure consists of (100) mechanical twins which have accommodated plastic deformation. Other pyroxene shock indicators are absent. Compared with SNC meteorites that all suffered significant shock metamorphism, NWA 7533 appears only mildly shocked. The twin microstructure is similar from one clast to another, suggesting that the impact which generated the (100) twins involved the compacted breccia and that the pyroxene clasts were unshocked when they were incorporated into the NWA 7533 breccia.  相似文献   

Platinum-group element signature of the primitive mantle rejuvenated by melt-rock reactions: evidence from Sumail peridotites (Oman Ophiolite)     

J.-P. Lorand  O. Alard  M. Godard 《地学学报》2009,21(1):35-40

Platinum-group element (PGE) abundances in mantle rocks are generally considered to result from a late meteorite addition to the early Earth, post-dating the core separation event. As such, PGEs are key tracers for the Earth accretion history. For decades, the PGEs systematics of undepleted mantle peridotites has been used to constrain the composition of meteorite impactors involved in the late veneer material. Despite multiple evidence of considerable modifications by partial melting, harzburgites from the Sumail ophiolite (Oman) display a mean PGE composition very akin to recently refined estimates for the Primitive Upper Mantle (PUM) of the Earth. These rocks document a resetting of the PUM signature by percolating basaltic melts, which precipitated Pd-enriched Cu–Ni sulphides within a strongly Pd-depleted residual harzburgitic protolith. Such a resetting casts doubt on both the reliability of any PUM estimates and relevance of the PUM concept itself, at least for PGEs.  相似文献