Ancient hydrothermal seafloor deposits in Eridania basin on Mars

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Sorosite (eta-Cu6Sn5)-bearing native tin and lead assemblage from the Mir zone (Mid-Atlantic Ridge, 26 degrees N)

A number of small, irregular-shaped and spherical shiny metallic particles have been found in the sediments from the Mir zone, Trans-Atlantic Geotraverse hydrothermal held (Mid-Atlantic Ridge, 26 degreesN). The phase variety of the particles examined is represented by metallic tin: tin-rich lead, and tin-copper phases. A detailed mineralogical study of these particles was carried out using optical microscopy, nuclear microscopy, scanning electron microscopy, electron microprobe, proton microprobe and X-ray diffraction analysis. Tin-lead grains have the typical eutectic microtexture of the metal components. Tin-copper grains are formed from single crystals of sorosite, eta -Cu6Sn5. The Sn-Pb-Cu complex grains are composed of fine stannoan lead (Pb0.74Sn0.26) and tin crystals as well as fine (or occasionally larger) sorosite (Cu6.1Sn4.9) idiomorphic crystals, in a tin-lead matrix forming eutectic microtexture. On the basis of data obtained, a natural origin is proposed for the examined Sn-Pb-Cu association, and its parent relations with the tectono-magmatic events in the rift zone. This association has probably been formed (1) during the hydrothermal seepings through the Trans-Atlantic Geotraverse sediment cover, or (2) during the evolution of ridge crest magmatic systems. Crystallisation sequence from an initial melt with falling temperature is: firstly sorosite (Cu6Sn5) (T less than or equal to 380 degreesC) --> crystallisation of tin crystals (T less than or equal to 227 degreesC) --> crystallisation of Sn-Pb eutectic mixture, composed of tin + stannoan lead (T less than or equal to 183 degreesC) --> limited tin exsolutions in stannoan lead (T < < 183 degreesC). Sn-Pb-Cu grains might be present as accessory minerals in the basic rocks of the east rift wall, which have undergone degradation, permitting their deposition into rift valley sediments.De petites particules métalliques brillantes de forme irrégulière et sphériques ont été trouvées dans les dépôts de la zone hydrothermale de Mir (dorsale, médio-Atlantique, 26 °N). Les particules examinées représentent lˈétain métallique, lˈétain riche en plomb et lˈétain riche en cuivre. Une étude minéralogique détaillée de ces particules a été effectuée en utilisant la microscopie optique, la microscopie nucléaire, le microscope électronique à balayage, la micro-sonde dˈélectrons, la micro-sonde de protons et la diffraction de rayons X. Les grains étain–plomb présentent une microtexture typique des composants métalliques. Des grains d’étain–cuivre sont formés de cristaux simples de sorosite η-Cu6Sn5. Les grains complexes de Sn–Pb–Cu se composent de cristaux fins d’étain–plomb (Pb0.74Sn0.26) et d’étain ; les cristaux idiomorphes du sorosite (Cu6.1Sn4.9) fin dans une matrice d’étain–plomb formant la microtexture eutectique. Une origine « normale » est proposée pour lˈassociation examinée de Sn–Pb–Cu, et ses relations de parenté avec les événements tectono–magmatiques dans la zone de rift. Cette association a été probablement formée (1) pendant une infiltration hydrothermale par la couverture de dépôt géotraverse trans-Atlantique, ou (2) pendant lˈévolution des systèmes magmatiques des dorsales. La succession de cristallisations à partir d’une fusion initiale avec la température en baisse est la suivante : le sorosite (Cu6Sn5) (T ≤ 380°C) → cristallisation des cristaux d’étain (T ≤ 227°C) → cristallisation du mélange eutectique de Sn–Pb, composé d’étain + étain–plomb (T ≤ 183°C) → exsolutions limités d’étain dans étain–plomb (T << 183°C). Les grains de Sn–Pb–Cu pourraient être présents comme des minéraux accessoires des roches basiques de la paroi est du rift qui ont subi la dégradation, permettant leur dépôt dans les sédiments de la vallée de rift

Native Sn–Pb droplets in a zeolitic amygdale (Isle of Mull, Inner Hebrides)

Despite the particular scientific interest in the elements with high affinity to S and O2, but found in zero-valence state in nature, the origin of these native minerals has been little explored and remains obscure. Here we describe unique Sn–Pb droplets found in a closed analcime–calcite amygdale collected from a basaltic unit cropping out at Carsaig Bay (Isle of Mull, Inner Hebrides). The droplets consist of intimate intergrowths of nearly pure Sn0 and Pb0 domains in proportion 88:12 and are enveloped in a thin, brownish film of organic composition. The occurrence of the Sn–Pb droplets in a closed amygdale, their relationship with the host analcime + calcite and their Pb isotope composition (which does not match any known anthropogenic Pb source) rule out the possibility of anthropogenic contamination and support the natural origin of the Sn–Pb alloy. The variable isotope (Pb, Sr, Nd) compositions in different members of the host basaltic sequence suggest that a parent basaltic magma was modified by crustal assimilation and post-emplacement alteration processes. Considering all possible explanations, it appears that the most likely source of Pb for the Sn–Pb alloy is a discrete basaltic unit with an isotopic composition comparable to the Antrim basalts (Northern Ireland). The amygdale phases, on the other hand, show isotopic evidence for incorporation of elements from both local basaltic and sedimentary units. The apparent isotopic disequilibrium between Sn–Pb droplets and amygdale phases indicates a complex, multi-stage fluid evolution. The occurrence of Sn–Pb droplets in organic capsules suggests that the droplets and the enveloping organic substances are co-precipitates. This implies that the transportation and deposition of Sn and Pb might have occurred through organometallic compounds. We assume interaction of seawater fluids carrying metals leached from basaltic rocks with hydrocarbons from sedimentary units as a prerequisite for the formation of the organometallic complexes. The zeolites lining the basaltic vesicles might have destabilized the migrating organo-Sn and Pb compounds causing their breakdown and precipitation of Sn–Pb alloy