Neil R. Banerjee

Microbes and volcanoes: A tale from the oceans, ophiolites, and greenstone belts

Submarine volcanic glass alteration displays two easily discernable types of textures, one that is best interpreted as the result of an abiotic diffusive exchange process and another that involves microbial activity. Glass bioalteration textures dominate in the upper 300 m of the oceanic crust and have been found in nearly all ocean basins and in many ophiolites and greenstone belts back to 3.5 Ga. Bioalteration may involve a globally significant biomass and may influence geochemical fluxes from seafloor alteration. Glass bioalteration creates an entirely new discipline of research that involves microbiologists and volcanologists working in active volcanic systems and in the geologic record. Submarine volcanoes exposed on the ocean floor are studied along with ophiolites and greenstone belts to understand Earth not only as a physical and chemical heat engine but also as a bioreactor.

Oceanic Pillow Lavas and Hyaloclastites as Habitats for Microbial Life Through Time – A Review

This chapter summarizes research undertaken over the past 15 years upon the microbial alteration of originally glassy basaltic rocks from submarine environments. We report textural, chemical and isotopic results from the youngest to the oldest in-situ oceanic crust and compare these to data obtained from ophiolite and greenstone belts dating back to c. 3.8 Ga. Petrographic descriptions of the granular and tubular microbial alteration textures found in (meta)-volcanic glasses from pillow lavas and volcanic breccias are provided and contrasted with textures produced by abiotic alteration (palagonitization). The geological setting in particular the degree of deformation and metamorphism experienced by each study site is documented in outcrop photographs, geological maps and stratigraphic columns (where possible). In addition, X-ray mapping evidence and carbon isotopic data that are consistent with a biogenic origin for these alteration textures is explained and a model for their formation is presented. Lastly, the petrographic observations and direct radiometric dating techniques that have been used to establish the antiquity and syngenicity of these microbial alteration textures are reviewed.

Hydrothermal alteration patterns in supra-subduction zone ophiolites

The geologic and alteration characteristics of 11 supra-subduction zone (SSZ) ophiolites have been compiled in order to assess the variability of hydrothermal alteration patterns. All SSZ ophiolites show evidence for high-temperature hydrothermal discharge in the form of polymetallic sulfide deposits and/or metalliferous sedimentary rocks. Metamorphic zona-tions are commonly subparallel to the igneous stratigraphy, and metamorphic grade increases with depth, but varies laterally in response to local magmatic and tectonic activity. The degree of alteration is more pervasive in sheeted-dike complexes than in volcanic or plutonic sequences. The greatest variability in the style of alteration is in volcanic sequences, where alteration occurred at low-temperature to greenschist-facies conditions. The prevalence of high porosity and permeability within volcanic sequences in modern oceanic crust, which maintains low temperatures, indicates that special conditions are required to develop zeolite- or greenschist-facies zones away from sites of hydrothermal discharge. Recent study of sediment-covered ridges show that early burial of oceanic crust, at or near a spreading center, does not lead to the widespread development of high-temperature alteration zones in the uppermost volcanic rocks. Alteration patterns in the Izu-Bonin and Tonga forearcs show many similarities to SSZ ophiolites interpreted to have formed in a forearc, such as the presence of epidosites and the nature of alteration within plutonic sequences. However, modern forearc volcanic sequences only record low-temperature alteration. Much more detailed documentation of most ophiolites is required in order to fully understand the effects of geologic setting on the evolution of hydrothermal systems.

Discovery of epidosites in a modern oceanic setting, the Tonga forearc

The first suite of oceanic epidosites has been recovered from the Tonga forearc. Epidosites are metal-depleted rocks composed of subequal proportions of epidote and quartz that lack any original igneous texture. Epidosites have been interpreted to form in upflow zones at the base of ore-forming oceanic hydrothermal systems that vent as black smokers on the sea floor. Tongan epidosites metasomatically replaced basaltic and plagiogranite protoliths and formed under similar conditions to epidosites hosted in many ophiolites interpreted to have formed in a suprasubduction-zone setting. The similarities shared by the Tongan and suprasubduction-zone ophiolite epidosites, along with the absence of epidosites in collections from modern mid-ocean ridges, suggest that tectonic setting plays a role in their formation.

Hydrothermal alteration in a modern suprasubduction zone: The Tonga forearc crust

An extensive suite of hydrothermally altered basalts, gabbros, and plagiogranites was recovered from the trench-facing slope of the Tonga forearc. The tectonic setting, lithology, and geochemistry of these samples make them a unique collection for comparison with suprasubduction zone (SSZ) ophiolites. Petrography, mineral chemistry, and geothermometry are used to constrain the metamorphic evolution of ocean crust formed in a modern SSZ setting. Seawater-derived hydrothermal fluids first penetrated the lower crust along grain boundaries and microscopic fracture networks at temperatures >800°C. As the plutonic sequence cooled, amphibole progressively replaced the mafic phases, followed by chlorite and epidote below ∼550°C. Basalts record peak alteration temperatures up to 773°C; however, most were altered at lower temperatures typical of mid-ocean ridge (MOR) volcanic sequences [Kelman, 1998]. Epidosites formed by pervasive alteration of basalt and plagiogranite at greenschist facies conditions and at high water-rock ratios. Pervasively altered gabbros and basalts display evidence of late cataclastic deformation and/or contain veins that is likely related to a later tectonic event such as trench rollback. The range of alteration temperatures and mineral assemblages in basalts and gabbros are similar to those described from both SSZ ophiolites and MORs. However, the degree of alteration in basalts and the presence of epidosites in the Tonga collection are most similar to alteration characteristics in SSZ ophiolites. The initiation of high-temperature brittle deformation in the absence of ductile deformation suggests that the Tonga forearc crust was constructed in a magma-rich environment, similar to fast spreading MORs.

Low-temperature alteration of submarine basalts from the Ontong Java Plateau

Abstract We present a detailed mineralogical and petrological description of the low-temperature alteration patterns in basalts from four new sites drilled during ODP Leg 192 on the Early Cretaceous Ontong Java Plateau. Three main alteration types have been identified: pervasively altered dark grey basalt; black or dusky green halos; and brown halos. Dark grey basalts are the most common and represent the least intensive, but most pervasive, alteration phase. Early interaction of the basalts with low-temperature sea-water-derived hydrothermal fluids lead to the development of black and dusky green halos characterized by the replacement of groundmass and olivine phenocrysts by celadonitic phyllosilicates and smectite. Later interaction of basalts with cold oxidizing sea water produced brown halos characterized by replacement of primary phases and mesostasis by smectite and iron oxyhydroxides. Secondary minerals in order of decreasing abundance include phyllosilicates, calcite, iron oxyhydroxides, pyrite, chalcedony, quartz and zeolites. Veins, resulting from symmetrical infilling of open cracks, commonly contain phyllosilicates, iron oxyhydroxide or pyrite, and late calcite. Carbonate veins cross-cut all other alteration features and stable isotope analyses of vein carbonates indicate formation from marine bicarbonate below about 40°C. A positive correlation between vein density and overall degree of alteration is observed resulting in pervasive development of brown alteration halos in highly fractured rocks. Overall, alteration of basalts from the Ontong Java Plateau is similar to that observed from other DSDP/ODP sites throughout the oceans.