Philippe Boulvais

Large colonial organisms with coordinated growth in oxygenated environments 2.1 Gyr ago

The evidence for macroscopic life during the Palaeoproterozoic era (2.5–1.6 Gyr ago) is controversial. Except for the nearly 2-Gyr–old coil-shaped fossil Grypania spiralis, which may have been eukaryotic, evidence for morphological and taxonomic biodiversification of macroorganisms only occurs towards the beginning of the Mesoproterozoic era (1.6–1.0 Gyr). Here we report the discovery of centimetre-sized structures from the 2.1-Gyr-old black shales of the Palaeoproterozoic Francevillian B Formation in Gabon, which we interpret as highly organized and spatially discrete populations of colonial organisms. The structures are up to 12 cm in size and have characteristic shapes, with a simple but distinct ground pattern of flexible sheets and, usually, a permeating radial fabric. Geochemical analyses suggest that the sediments were deposited under an oxygenated water column. Carbon and sulphur isotopic data indicate that the structures were distinct biogenic objects, fossilized by pyritization early in the formation of the rock. The growth patterns deduced from the fossil morphologies suggest that the organisms showed cell-to-cell signalling and coordinated responses, as is commonly associated with multicellular organization. The Gabon fossils, occurring after the 2.45–2.32-Gyr increase in atmospheric oxygen concentration, may be seen as ancient representatives of multicellular life, which expanded so rapidly 1.5 Gyr later, in the Cambrian explosion.

The 2.1 Ga old Francevillian biota: biogenicity, taphonomy and biodiversity.

The Paleoproterozoic Era witnessed crucial steps in the evolution of Earths surface environments following the first appreciable rise of free atmospheric oxygen concentrations ∼2.3 to 2.1 Ga ago, and concomitant shallow ocean oxygenation. While most sedimentary successions deposited during this time interval have experienced thermal overprinting from burial diagenesis and metamorphism, the ca. 2.1 Ga black shales of the Francevillian B Formation (FB2) cropping out in southeastern Gabon have not. The Francevillian Formation contains centimeter-sized structures interpreted as organized and spatially discrete populations of colonial organisms living in an oxygenated marine ecosystem. Here, new material from the FB2 black shales is presented and analyzed to further explore its biogenicity and taphonomy. Our extended record comprises variably sized, shaped, and structured pyritized macrofossils of lobate, elongated, and rod-shaped morphologies as well as abundant non-pyritized disk-shaped macrofossils and organic-walled acritarchs. Combined microtomography, geochemistry, and sedimentary analysis suggest a biota fossilized during early diagenesis. The emergence of this biota follows a rise in atmospheric oxygen, which is consistent with the idea that surface oxygenation allowed the evolution and ecological expansion of complex megascopic life.

Ophicalcites from the northern Pyrenean belt: a field, petrographic and stable isotope study

Brecciated and fractured peridotites with a carbonate matrix, referred to as ophicalcites, are common features of mantle rocks exhumed in passive margins and mid-oceanic ridges. Ophicalcites have been found in close association with massive peridotites, which form the numerous ultramafic bodies scattered along the North Pyrenean Zone (NPZ), on the northern flank of the Pyrenean belt. We present the first field, textural and stable isotopic characterization of these rocks. Our observations show that Pyrenean ophicalcites belong to three main types: (1) a wide variety of breccias composed of sorted or unsorted millimeter- to meter-sized clasts of fresh or oxidized ultramafic material, in a fine-grained calcitic matrix; (2) calcitic veins penetrating into fractured serpentine and fresh peridotite; and (3) pervasive substitution of serpentine minerals by calcite. Stable isotopic analyses (O, C) have been conducted on the carbonate matrix, veins and clasts of samples from 12 Pyrenean ultramafic bodies. We show that the Pyrenean ophicalcites are the product of three distinct genetic processes: (1) pervasive ophicalcite resulting from relatively deep and hot hydrothermal activity; (2) ophicalcites in veins resulting from tectonic fracturing and cooler hydrothermal activity; and (3) polymictic breccias resulting from sedimentary processes occurring after the exposure of subcontinental mantle as portions of the floor of basins which opened during the mid-Cretaceous. We highlight a major difference between the eastern and western Pyrenean ophicalcites belonging, respectively, to the sedimentary and to the hydrothermal types. Our data set points to a possible origin of the sedimentary ophicalcites in continental endorheic basins, but a post-depositional evolution by circulation of metamorphic fluids or an origin from relatively warm marine waters cannot be ruled out. Finally, we discuss the significance of such discrepancy in the characteristics of the NPZ ophicalcites in the frame of the variable exhumation history of the peridotites all along the Pyrenean realm.

Giant quartz vein formation and high-elevation meteoric fluid infiltration into the South Armorican Shear Zone: geological, fluid inclusion and stable isotope evidence

Giant quartz veins associated with the South Armorican Shear Zone record important fluid circulation during the Hercynian period. Regional-scale mapping of veins allows two groups of veins to be identified, on the basis of their geometric relationship with the South Armorican Shear Zone. Veins in the first group are parallel to the shear zone, whereas those in the second group developed in a direction oblique to it. The former probably record infiltration of fluids along permeable pathways in highly deformed zones; the latter may represent crustal-scale tension gashes in the regional context. Most quartz veins have δ18O values between 10 and 16‰ indicating a mid-crustal origin for the fluids. Microthermometry on fluid inclusions from euhedral quartz indicates that late fluids were mostly aqueous with very low salinity (0–1.7 wt% eq.) and with homogenization temperatures ranging between 150 and 270 °C. Together with very low δ18O values of some euhedral quartz, down to −2‰, these features argue for a surface origin. Corresponding δ18Ofluid values estimated near −11‰ are probably related to the high palaeo-elevation of meteoric precipitation. Scarce, but significant, H2O–CO2 fluid inclusions in euhedral quartz indicate a metamorphic contribution. These were probably sourced from the exhumed metamorphic basement in the southern part of the region.

Dissolution–precipitation processes governing the carbonation and silicification of the serpentinite sole of the New Caledonia ophiolite

Abstract The weathering of mantle peridotite tectonically exposed to the atmosphere leads commonly to natural carbonation processes. Extensive cryptocrystalline magnesite veins and stock-work are widespread in the serpentinite sole of the New Caledonia ophiolite. Silica is systematically associated with magnesite. It is commonly admitted that Mg and Si are released during the laterization of overlying peridotites. Thus, the occurrence of these veins is generally attributed to a per descensum mechanism that involves the infiltration of meteoric waters enriched in dissolved atmospheric CO2. In this study, we investigate serpentinite carbonation processes, and related silicification, based on a detailed petrographic and crystal chemical study of serpentinites. The relationships between serpentine and alteration products are described using an original method for the analysis of micro-X-ray fluorescence images performed at the centimeter scale. Our investigations highlight a carbonation mechanism, together with precipitation of amorphous silica and sepiolite, based on a dissolution–precipitation process. In contrast with the per descensum Mg/Si-enrichment model that is mainly concentrated in rock fractures, dissolution–precipitation process is much more pervasive. Thus, although the texture of rocks remains relatively preserved, this process extends more widely into the rock and may represent a major part of total carbonation of the ophiolite.

Geochemical fluxes related to alteration of a subaerially exposed seamount: Nintoku seamount, ODP Leg 197, Site 1205

Hole 1205A was drilled on Nintoku Seamount, which lies in the midportion of the Emperor Seamount Chain. This seamount was emergent ∼56 Myr ago but was submerged by 54 Ma, so the lavas have endured weathering in both subaerial and submarine environments. We have studied the petrology, mineralogy, and geochemistry of intercalated altered basalts, breccias, and soil samples recovered at Hole 1205A to quantify the chemical exchanges between the seamount and seawater and/or meteoric fluids. The secondary mineralogy is relatively uniform throughout the section and comprises smectite, Fe-oxyhydroxides, iddingsite, and Ca-carbonates. Soils are composed of variably altered basaltic clasts in a matrix of kaolinite, smectite, and vermiculite with minor goethite, hematite, and magnetite. Throughout the basement section, altered basalts, breccias, and soils are depleted in Si, Mg, Ca, Na, Sr, Rb, and Ba and enriched in Fe. Fe3+/FeT (up to ∼1), δ18O (up to ∼+20‰), and 87Sr/86Sr ratios are strongly elevated relative to primary igneous values. Differences in the 87Sr/86Sr ratios define an Upper Alteration Zone with 87Sr/86Sr close to 56 Ma seawater (∼0.7077) from a Lower Alteration Zone where 87Sr/86Sr are less elevated (∼0.704). The Lower Alteration Zone likely reflects interaction with a subaerial oxidizing fluid at low temperature. This zone probably retained most of the original subaerial weathering signature. The Upper Alteration Zone was altered through circulation of large quantities of cold oxidizing seawater that partially overprinted the subaerial weathering chemical characteristics. Altered samples were compared to estimated protolith compositions to calculate chemical gains and losses. Global chemical fluxes are calculated for the entire basement section using different lithological proportions models and different rates of oceanic island emplacement. Although the global construction rate of ocean islands is small compared to igneous accretion at mid-ocean ridges, the magnitude of the chemical changes indicates that ocean islands and seamounts may be a significant contributor to the chemical budget of the oceans.

Stable isotopes (O, C) and fluid inclusion study of quartz-carbonate veins from the antimony line, Murchison Greenstone Belt

In the homogeneously deformed Murchison Greenstone Belt region, fluids circulations were focused along a localized deformation brittle-ductile zone, and the so-called Antimony Line. In order to trace the origin of the fluid, its composition, the temperature of the precipitation along the Antimony Line, we investigate the stable isotope (oxygen and carbon) and fluid inclusion compositions of the quartz-carbonate veins, as well as chemical analyses of the carbonates and 40Ar-39Ar dating on the fuchsite associated with the veins. Three types of fluid inclusions are recognized for the H2O-CO2-dominated (plus minor CH4-N2) fluid(s). Microthermometric measurements together with the paragenesis of the veins and host rocks indicate that the pressure-temperature of the precipitation is 350 to 450 °C, 200 to 300 MPa, which is an unusually high temperature for an antimony deposit. The carbonates are Fe-Mg-rich and are characterized by flat MREE and HREE patterns, with slight depletions in LREE. The oxygen isotope compositions of the quartz are rather homogeneous at the scale of the Antimony Line (δ18Oquartz = 10.9-14.3‰). In detail, subtle differences in the δ18O values for the quartz and carbonate exist depending on the sampling sites. These differences are likely due to minor temperature and/or host rock geochemistry variations along the Antimony Line. Data point to a metamorphic origin for the fluids responsible for the mineralization, which likely escaped from the nearby greenstone lithologies. Finally, we present an overview of the Murchison Greenstone Belt geological evolution by integrating the antimony metallogeny of the Antimony Line, which can be compared to that of an orogenic gold deposit.

2H/1H measurements of amphiboles and nominally anhydrous minerals (clinopyroxene, garnet and diamond) using high-temperature continuous flow elemental analyser/pyrolysis/isotope ratio mass spectrometry

HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. 2H/1H measurements of amphiboles and nominally anhydrous minerals (clinopyroxene, garnet and diamond) using high-temperature CF-EA-PY-IRMS François Fourel, Christophe Lécuyer, Attila Demeny, Philippe Boulvais, Lutz Lange, Dorrit E. Jacob, Istvan Kovacs