Harald Furnes

Ophiolite genesis and global tectonics: Geochemical and tectonic fingerprinting of ancient oceanic lithosphere

Ophiolites, and discussions on their origin and significance in Earths history, have been instrumental in the formulation, testing, and establishment of hypotheses and theories in earth sciences. The definition, tectonic origin, and emplacement mechanisms of ophiolites have been the subject of a dynamic and continually evolving concept since the nineteenth century. Here, we present a review of these ideas as well as a new classification of ophiolites, incorporating the diversity in their structural architecture and geochemical signatures that results from variations in petrological, geochemical, and tectonic processes during formation in different geodynamic settings. We define ophiolites as suites of temporally and spatially associated ultramafic to felsic rocks related to separate melting episodes and processes of magmatic differentiation in particular tectonic environments. Their geochemical characteristics, internal structure, and thickness vary with spreading rate, proximity to plumes or trenches, mantle temperature, mantle fertility, and the availability of fluids. Subduction-related ophiolites include suprasubduction-zone and volcanic-arc types, the evolution of which is governed by slab dehydration and accompanying metasomatism of the mantle, melting of the subducting sediments, and repeated episodes of partial melting of metasomatized peridotites. Subduction-unrelated ophiolites include continental-margin, mid-ocean-ridge (plume-proximal, plume-distal, and trench-distal), and plume-type (plume-proximal ridge and oceanic plateau) ophiolites that generally have mid-ocean-ridge basalt (MORB) compositions. Subduction-related lithosphere and ophiolites develop during the closure of ocean basins, whereas subduction-unrelated types evolve during rift drift and seafloor spreading. The peak times of ophiolite genesis and emplacement in Earth history coincided with collisional events leading to the construction of supercontinents, continental breakup, and plume-related supermagmatic events. Geochemical and tectonic fingerprinting of Phanerozoic ophiolites within the framework of this new ophiolite classification is an effective tool for identification of the geodynamic settings of oceanic crust formation in Earth history, and it can be extended into Precambrian greenstone belts in order to investigate the ways in which oceanic crust formed in the Archean.

A Younger Dryas Ash Bed in western Norway, and its possible correlations with tephra in cores from the Norwegian Sea and the North Atlantic

Abstract A bed of volcanic ash up to 23 cm thick is found in lacustrine and marine sediments in western Norway. It is formally mamed the Vedde Ash Bed, and its age is approximately 10,600 yr B.P., i.e., mid-Younger Dryas. The bed consits of pure glass having a bimodal basaltic and rhyolitic somposition. The geochemistry of the glass shards suggests an Icelandic source. By means of stratigraphic position and geochemistry, the ash is correlated with ash zones found in cores from the continental shelf, the Norwegian Sea, and the North Atlatic.

The importance of microbiological activity in the alteration of natural basaltic glass

Abstract The textural development of palagonite may differ profoundly depending on whether alteration occurred in the outermost 6–7 mm thick light-exposed surface zone of deposits, or elsewhere. In the former case, a pit-textured development of the parent basaltic glass develops as a consequence of local establishment of a highly alkaline micro-environment (pH > 9) for which the light-dependent cryptoendolithic cyanobacteria are considered most likely to be responsible. A highly porous, sponge-textured variety of palagonite, frequently defining zoned layers, contains abundant examples of bacteria. The shape and size of the pores combined with the geochemical development strongly suggest that bacteria have played an important role in the development of this texture.

Microbes play an important role in the alteration of oceanic crust

Microbes have been identified within altered parts of the glass rims of pillow lavas in the upper oceanic crust, 237 m below the top of the volcanic basement of ODP Hole 896A at the Costa Rica Rift. Their presence is verified by spherical and vermicular microbodies containing DNA. The elemental composition of the microbially processed areas differ from the parent glass. Further, the microbially processed parts, showing different morphological forms also show different elemental composition. Extreme K-enrichment indicate that microbes are presently active in the alteration process. The carbon isotopic composition of disseminated carbonates within the basaltic section of the Hole 504B (1 km distance from Hole 896A) also give evidence for microbial activity during rock alteration. The σ13C values of most of these trace carbonates are very low, reflecting metabolic control of the carbon cycle in these rocks. Microbial alteration of basaltic glass, comprising a substantial volume and surface area of the upper oceanic crust, may thus play an important role in the element exchange between oceanic crust and seawater.

Textural and chemical effects of bacterial activity on basaltic glass: an experimental approach

Abstract Naturally altered basaltic glass may show features such as pitted textures and variable degree of element mobilization relative to the fresh parent. The alteration process has generally been considered from only a chemical/physical point of view, but recent observations of bacteria in altered glass have, however, led to questions about the importance of microbial activity. In order to examine this, an experiment has been performed in which basaltic glass samples were immersed in growth media at room temperature for up to 394 days, inoculated with bacteria derived from a naturally altered pyroclastic deposit (Surtsey tuff). During the experiment it was observed that bacteria had a great affinity for attachment to the glass surface, which is in most cases connected to the production of extracellular polymers. Further, different species of bacteria were dominant at different time intervals. The bacteria activity caused a general decrease in pH from 8.0 to 5.8 during the time of the experiment. After 46 days of incubation, SEM studies of samples show rare examples of clear etching marks on the surface corresponding in size and shape of a minor group of bacteria. A local corrosion in a more irregular manner was observed after 181 days. Chemical analyses of the glass surface show no difference in composition compared to the fresh glass at this stage, i.e. any dissolution is congruent. Bacteria and biofilms attached to the glass surface show accumulation of elements, of which Al and Si could only have been derived by dissolution of the glass. However, the extent of accumulation of various elements may differ pronouncedly within and between the runs at 44, 77 and 181 days. This scatter probably reflects the diversity of the community and the ability of the different species of bacteria to accumulate elements. After 394 days the outermost glass rim, 1 μm in thickness, is highly depleted in all cations, except Si, which is relatively enriched. This incongruent dissolution of the glass, has only been active during the last 7 months of the experiment. The alteration rate is increased, at least, by a factor of 10 compared to that of the first 6 months. This is thought to be caused by the activity of a new, dominant bacterium group during this period. Microanalyses of the bacteria attached to the residual, leached glass rim, show more frequent accumulation of Si, and generally their chemistries are more homogenous than that observed in the other, shorter-termed runs. Bacterial activity may hence have a great influence on the textural and chemical developments commonly observed in naturally altered basaltic glass deposits.

A Vestige of Earth's Oldest Ophiolite

A sheeted-dike complex within the ∼3.8-billion-year-old Isua supracrustal belt (ISB) in southwest Greenland provides the oldest evidence of oceanic crustal accretion by spreading. The geochemistry of the dikes and associated pillow lavas demonstrates an intraoceanic island arc and mid-ocean ridge–like setting, and their oxygen isotopes suggest a hydrothermal ocean-floor–type metamorphism. The pillows and dikes are associated with gabbroic and ultramafic rocks that together make up an ophiolitic association: the Paleoarchean Isua ophiolite complex. These sheeted dikes offer evidence for remnants of oceanic crust formed by sea-floor spreading of the earliest intact rocks on Earth.

Evidence for microbial activity at the glass-alteration interface in oceanic basalts

A detailed microbiological and geochemical study related to the alteration of basaltic glass of pillow lavas from the oceanic crust recovered from Hole 896A on the Costa Rica Rift (penetrating 290 m into the volcanic basement) has been carried out. A number of independent observations, pointing to the influence of microbes, may be summarized as follows: (1) Alteration textures are reminiscent of microbes in terms of form and shape. (2) Altered material contains appreciable amounts of C, N and K, and the N=C ratios are comparable to those of nitrogen-starved bacteria. (3) Samples stained with a dye (DAPI) that binds specifically to nucleic acids show the presence of DNA in the altered glass. Further, staining with fluorescent labeled oligonucleotide probes that hybridize specifically to 16S-ribosomal RNA of bacteria and archaea demonstrate their presence in the altered part of the glass. (4) Disseminated carbonate in the glassy margin of the majority of pillows shows d 13 C values, significantly lower than that of fresh basalt, also suggests biological activity. The majority of the samples have d 18 O values indicating temperatures of 20‐100oC, which is in the range of mesophilic and thermophilic micro-organisms.

Biological mediation in ocean crust alteration: how deep is the deep biosphere?

Abstract Biological mediation has been suggested as a control of the chemical exchange between the oceanic crust and seawater, but very little is known about its distribution within the oceanic crust and the relative importance of biotic and abiotic processes. Alteration textures in glassy pillow lava margins record the proportions of biotic and abiotic alteration, and the fraction of biotic alteration may be determined by point counting methods. We used this method at DSDP/ODP Sites 417D and 418A (110 Ma crust south of Bermuda Rise) and Holes 504B and 896A (5.9 Ma Costa Rica Rift). Biotic alteration dominates glass alteration in the upper 250 m of the oceanic crust (60–85% of the total glass alteration) and steadily declines in importance down to 10–20% at 500 m. The consistency of data between two crustal sections of very different age and tectonic setting suggest that microbially mediated glass alteration may be largely confined to the upper oceanic crust. However, both sites studied are sealed by thick sedimentary layers and, thus, are typical for ocean crust underlying the oceanic basins, rather than crust at mid-ocean ridges with possibly deep and rapid hydrothermal circulation. Down-hole temperature measurements at the Costa Rica Rift suggest that glass-altering microbes are hyperthermophilic and thrive at least up to temperatures of about 90°C. Microbial activity does occur at higher temperatures (up to about 110°C) but with reduced apparent abundance.

A 9000-Year-old Ash Bed on the Faroe Islands

Abstract Radiocarbon datings of the Saksunarvatn ash bed on the Faroe Islands indicate an age of 9000–9100 yr B.P. The Saksunarvatn ash bed differs geochemically from both ash zone 1 in the North Atlantic and the Vedde Ash Bed of Norway and the Norwegian Sea. All mentioned ashes are assumed to originate from Iceland. The Vedde Ash has been dated at 10,600 ± 60 yr B.P. Consequently, the Saksunarvatn and Vedde ash beds provide an opportunity for precise dating of events around the Pleistocene/Holocene boundary in marine cores, especially from the region where the two plumes overlap.

A textural and chemical study of Icelandic palagonite of varied composition and its bearing on the mechanism of the glass-palagonite transformation

Palagonite of basalt and basaltic andesite parentages from hyaloclastite deposits in Iceland has been investigated. SEM studies indicate a sharp to diffuse alteration front which may propagate along microfractures in the glass, resulting in a progressive partial dissolution yielding palagonite of variable, but generally increasing porosity towards grain surfaces. The palagonite has a granular texture. Incipient alteration is indicated by the development of individual globules (ca. 0.01 μm in diameter), whereas at an advanced stage chains of globules, defining a sponge-like texture, characterize the palagonite. In the basaltic andesite, the precursor to brown Ti- and Fe-rich palagonite is a white variety, for which an evolutionary model is presented. EDS line-scans across the fresh glass-palagonite boundary show the existence of a 2–4 μm thick zone of glass-like material in which all elements have been depleted, except Si, and in some cases Al, which have been relatively enriched. The white palagonite is characterized by strong depletion of Ti, Fe, Na, Mg, and to lesser extent, Ca, Al (in order of decreasing loss). Regardless of the degree of porosity development (ca. 1–43 vol%), the extent of element depletion relative to SiO2 is constant. This gives evidence for selective element mobility prior to a variable degree of congruent network dissolution of the Si-rich residue, yielding zoned palagonite with a different porosity. In order for Fe, Ti, and Al to dissolve, a pH 3, Fe, Ti, and Al will precipitate in the pores of the white palagonite as oxides/hydroxides, thus creating the brown variety, which is characterized by highly variable contents of the above-mentioned elements. The applicability of this model to palagonite derived from basalt parentages at different pH conditions is discussed.