Minerals as key markers of melt/fluid percolation in the lithospheric mantle from the French Massif Central

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Petrology and mineralogy of the angrite Northwest Africa 1670

International audienceNorthwest Africa (NWA) 1670, contains olivines of up to 5 mm in size representing about 30% of the studied section. With subordinate clinopyroxene and chrome-spinel microphenocrysts (0.2- 0.5 mm), they represent a xenocrystic association. Phenocrysts are surrounded by a groundmass, predominantly comprising bundles of plagioclase and clinopyroxene (typically 20 × 200 μm crystals). Olivine and kirschsteinite are present in the groundmass in lesser amounts. The olivine xenocrysts (Fo90) are significantly fractured and show mosaicism for their major part, the remaining showing faint undulatory extinction. They are surrounded with a rim of 100-200 μm zoned down to Fo80 and overgrown with serrated olivine, Fo80 to Fo60 (about 100 μm). Olivine in the groundmass is zoned from Mg# 0.55 to 0.15; its CaO content ranges 2.0 to 8.4%. Subcalcic kirschsteinite is zoned from Mg# 0.13 to 0.03, CaO increasing from 15.8 to 21.3%. Pyroxenes xenocrysts (Mg# = 0.77) are superseded in the groundmass by less magnesian pyroxenes, Mg# 0.61 to 0.17, with an average FeO/ MnO of 98. Their compositions range from En30Fs22Wo27Al-Ts28Ti-Ts2 to En2Fs37Wo22Al-Ts40Ti- Ts1. Anorthite microcrysts (An99-100) are restricted to the groundmass. Accessories are pyrrhotite, kamacite, Ca-phosphate, titanomagnetite, hercynite and Ca-carbonate. The bulk chemical composition confirms that NWA 1670 corresponds to a normal angrite melt that incorporated olivine. High Mg olivine xenocrysts and the associated mineralogy are typical of angrites. We suggest that it is an impact melt with relict phenocrysts. The strong silica undersaturation, the presence of Fo90 olivine xenocrysts and carbonate support their derivation as melilite-like melts in the presence of carbonate

Evolution des cortèges argileux de l' Albien au Santonien (113-84 Ma) au large des côtes sud-ouest australiennes au cours : Données préliminaires et origine(s) possible(s) des argiles

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Decoding multi-stage processes in mantle xenoliths from microtextural and chemical evidences

International audienceMantle-derived xenoliths rapidly brought to the surface by alkaline magmas commonly exhibit reaction features valuable for elucidating magmatic and metasomatic processes that occur in the mantle, or during the transport to the surface, or post-entrainment processes. In this contribution, we focus on a suite of fresh anhydrous and hydrous amphibole-bearing spinel lherzolites from Devès in the Southern Domain of the French Massif Central. We present three different cases: (1) reaction zones developed around primary pyroxenes, amphibole and spinel; (2) the presence of carbonate as veins and vesicles in composite carbonate-silicate pockets from well-developed reaction zones; (3) the presence of secondary orthopyroxene in veins cross-cutting or rimming primary olivine. Case 1 * The presence of low Ti (0.5 wt.% TiO 2), Cr-and Mg-rich (1.2 wt.% Cr2O 3 , mg# = 88) pargasite coronae around relict spinel testifies for mantle modal metasomatism by a melt of alkali basalt composition. * Reactional textures around amphibole consist of secondary clinopyroxene, olivine, spinel, ± plagioclase, formerly volatile-filled bubbles, and small (10 µm) patches of fresh colourless glass. A single decompression process is not sufficient for explaining the amphibole breakdown and the secondary phase composition, but additional components (alkalis) brought by a percolating agent are required. * In anhydrous samples, similar secondary assemblages are observed at the contact between primary spinel and orthopyroxene. The presence of former amphibole is ruled out by the Al-rich compostion of secondary spinels. The secondary assemblage is likely to result from reaction between the primary phases and the same percolating agent as above. Case 2 * Some carbonate veins cut spongy large clinopyroxene crystals resulting from a former metasomatic event by an infiltrating (alkali?) melt as in case (1). * The carbonate fills globular vesicles in reaction zones that contain secondary clinopyroxene, olivine, spinel, ± quenched plagioclase and small glass patches or rims. The secondary clinopyroxene and olivine indent or are included in the carbonate crystals. The carbonate is a low Mg (MgO <1 wt.%) calcite poor in alkalis and REE. Although apparent immiscibility features, the tie-line glass-carbonate does not intersect experimentally determined immiscibility and, thus, the carbonate represents crystal cumulates. The co-precipitation of carbonate and associated secondary minerals occurs near the base of the crust (as inferred from secondary silicate-liquid pairs estimates) and results from a magmatic fractionation of an alkali/carbonate mantle-derived melt. Case 3 * The secondary orthopyroxene is characterized by a lower content in Al2O 3 <2.5 wt.% compared to 2.9-4.4 wt.% in primary orthopyroxene, depending on the sample. It also contains less Cr2O 3 and TiO 2 (<0.2 wt.%) and has similar mg# (90-91). It includes small (<1µm) rounded Cl-apatite (2.5 wt.% Cl). Small (2 µm) pores are observed throughout the veins and at the contact with the primary phases. Secondary orthopyroxene formed in the mantle (1.5 GPa, 1150 • C) from a metasomatic volatile-rich hydrous silicate melt

Carbonate and silicate cementation of siliciclastic sediments of the New Jersey shelf (IODP Expedition 313): relation with organic matter diagenesis and submarine groundwater discharge

International audienceThe New Jersey continental shelf extends 150 km off the shoreline. During IODP Expedition 313, siliciclastic deposits of late Eocene to late Pleistocene age were drilled down to 631, 669 and 755 m below seafloor at sites 27A, 28A and 29A respectively in very shallow waters (33.5 to 36 m depth). Pore water salinities display multilayered brackish-salty-brine units 10 to 170 m thick, where low-salinity water is preferentially stored in fine-grained sediments. The sharp boundaries of these buried aquifers are often marked by cemented layers a few centimetres thick. The mineralogy and scanning electron microscope observations of these layers show two phases of cementation by authigenic minerals: (1) the early carbonate cement is frequently associated with pyrite, and (2) the late silicate cement infills the residual porosity. The isotopic compositions of the carbonate cements vary widely: −2.4 18O ‰ VPDB 13C ‰ VPDB 18O values indicate that the carbonate cements precipitated with pore waters comprising variable mixtures of seawater and 18O-depleted fresh water originating from submarine groundwater discharge. The δ13C values of the carbonate cements are related to organic matter diagenesis, providing 13C-depleted dissolved inorganic carbon during bacterial sulphate reduction and anaerobic oxidation of methane, and 13C-rich dissolved inorganic carbon during methanogenesis. The diagenetic cementation processes included chemical weathering of reactive silicate minerals by the CO2-rich pore waters issued from organic matter diagenesis that released bicarbonate, cations and dissolved silica, which were further precipitated as carbonate and silicate cements. The estimated range of temperature (18±4 °C) during carbonate precipitation is consistent with carbonate cementation at moderate burial depths; however, silicate cementation occurred later during diagenesis at deeper burial depths

Tirhert and Aouinet Legraa: Rare unbrecciated eucrite falls

International audienceTirhert and Aouinet Legraa are the only documented unbrecciated eucrite falls in Africa. Aouinet Legraa fell in Algeria on July 17, 2013. Tirhert's fall occurred about a year later in Morocco, on July 9, 2014. Both meteorites are covered by a black and glossy fusion crust as is typical of eucrites. Tirhert has a poikilitic texture with remnant subophitic pockets, and consists of millimeter-sized grains of plagioclase (An87-91), pigeonite (Mg# 42) with augite exsolution lamellae, and interstitial opaque minerals. Aouinet Legraa has a subophitic texture, and it is dominated by plagioclase laths (An82-89) enclosed by pigeonite (Mg# 37), with exsolution lamellae of augite. Remnant Ca zoning in pyroxene is observed in both rocks, although it is more abundant in Aouinet Legraa than Tirhert. The presence of exsolved pyroxenes suggests that these meteorites have undergone thermal metamorphism. Equilibration temperatures estimated from pigeonite and augite pairs using the QUILF program are ∼931 °C in Tirhert and ∼758 °C in Aouinet Legraa. This indicates that these rocks had distinct thermal histories. Aouinet Legraa has trace element abundances similar to the typical main group eucrite Juvinas, confirming its origin as a main group eucrite. The trace element abundances of Tirhert fall between those of cumulate and main group eucrites. Its rare earth element pattern is flat with a positive Eu anomaly. This likely suggests that Tirhert is a partial cumulate of plagioclase from a main group magma, or a flotation cumulate formed by flotation of plagioclase in a subvolcanic chamber or by scavenging crystals during eruption

Utilisation de l'analyses d'images multispectrales en Sciences de la terre traitements et applications

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Atmospheric element deposit on tree barks : the opposite effects of rain and transpiration

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Re-equilibration processes in magnetite from an Iranian BIF deposit.

International audienceMagnetite is common in various iron ore deposits including BIF, Fe-skarn, IOGC (iron oxide-copper-gold), IOA (iron oxide-apatite) and porphyry Cu-Au deposits. Magnetite incorporates a number of trace elements in tetrahedral and octahedral site in its structure. Its trace element composition has been used in numerous studies to fingerprint deposit type and ore genesis and to provide a guide for mineral exploration, based on a series of discriminant diagrams (e.g. [1]). However, the applicability of such diagrams must be carefully evaluated as the composition of magnetite can be modified by various processes. In this study we present textural and compositional data for magnetite from a late Ediacaran BIF in NW Iran [2] to illustrate complex re-equilibration processes. Three stages of magnetite (Mt) has been identified. Mt1 forms large (&#8804;1 mm) inhomogeneous dark grey grains surrounded and locally invaded by a light grey porous Mt2. The Mt1/Mt2 boundary is irregular and sharp, consistent with replacement textures. Bright Mt3 forms needle-like bands (10-80 &#181;m width) aligned along fracturing planes. Mt3 contains rare hematite relicts and is porous in close proximity to hematite. Mt1 shows a variable trace element composition and contains the highest Si (average 1.14 wt.%), Al and Ca (0.28 wt.%), Mg (0.13 wt.%) and the lowest Fe (68 wt.%) contents. Both Mt2 and Mt3 show a restricted range of composition. Mt2 has lower Si (0.68 wt.%), Al (0.14 wt.%), Ca (0.15 wt.%) and Mg (0.08 wt.%) contents, while Mt3 is characterized by the lowest Si (0.16 wt.%), Al and Ca (0.05 wt.%) and Mg (0.01 wt.%) and the highest Fe (71.1 wt%) contents. The three magnetite have low Mn (&#8804;0.03 wt.%), and Ti (&#8804;0.02 wt.%), and Ni and Cr are mostly below the detection limit. The silician dark Mt1 magnetite likely forms in a rather reduced Si-rich environment. The presence of structural silicon is supported by correlations / antithetic correlations with R2+/R3+ cations and the lack of inclusions. The incorporation of Si may cause lattice defects or deformation facilitating fluid alteration. A fluid-assisted coupled dissolution of Mt1 and precipitation of Mt2 (CDR process) is supported by close spatial relationship, sharp compositional boundaries, similar crystallographic structure of MT1 and Mt2 and abundant porosity in Mt2. The increase in porosity promotes the infiltration of hydrothermal fluids and further advances the CDR process. By removing trace elements from the early Mt1 this process increases the iron grade of Mt2. Micro-fracturing allows the penetration of a more oxidized fluid along cleavage planes and formation of needle-like bands of hematite. Then porous mushketovite Mt3 formed after hematite under a more reduced fluid composition by a redox transformation supported by the volume decrease. All these processes significantly modified the texture and composition of the magnetite and point to a predominant imprint of hydrothermal fluid, thus causing difficulties in using magnetite as a genetic indicator. &#160;[1] Dupuy and Beaudoin, 2011; [2] Honarmand et al, in press

The inclusion of atmospheric particles into the bark suber of ash trees

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