bluuebunny/crossref_metadata_2025_split huggingface dataset:
Parent magmas of the Semail ophiolite, Oman
Authors: Pallister J. S. 
Published: April 2008

Summary

The plutonic rocks of the Semail ophiolite in the southeastern Oman Mountains formed in a spreading-ridge magma chamber that was repeatedly replenished by primitive magma. An average composition of the parent magma is calculated by mass balance of the igneous stratigraphy. The average parent magma was in equilibrium with most early-formed cumulus minerals from the ophiolite and is very similar to primitive mid-ocean-ridge basalt (MORB). Comparison with experimentally determined liquidus relations for primitive MORB also suggests that most of the cumulus minerals could have been produced by fractionation of the average parent magma. The magma is basaltic—in contrast to the picritic melts proposed as parents for some ophiolites. Although the
average
parent magma composition is basaltic, individual batches of magma that fed the magma chamber varied in composition because of mantle processes. The primary mantle melts were probably picritic, but due to reaction during ascent through the upper mantle and mixing or density entrapment within the crustal-level magma chamber, these picritic melts were probably not erupted on the sea floor. A model with primary mantle melting at 40–60 km depth followed by reaction during ascent and possibly second-stage melting at ≤25 km depth is favoured for the ophiolite.
Occurrence and significance of gneissic amphibolites in the Vema fracture zone, equatorial Mid-Atlantic Ridge
Authors: Honnorez J. , Mevel C. , Montigny R. 
Published: April 2008

Summary
Numerous gneissic amphibolites were recovered together with variably deformed and metamorphosed gabbros and serpentinites in the Vema fracture zone during two separate cruises. Petrological studies and K/Ar dating show that these rocks result from recrystallization of gabbros under stress, at temperatures consistent with amphibolite facies, probably in the vicinity of the ridge axis. We suggest that gneissic amphibolites were formed in a large subvertical shear zone related to transform fault movement which favoured penetration of seawater in oceanic layer 3. Less deformed metagabbros recrystallized away from the fault zone under static conditions from interaction with evolved seawater-derived fluids.
Sr-Nd isotope and chemical evidence that the Ballantrae ‘ophiolite’, SW Scotland, is polygenetic
Authors: Thirlwall M. J. , Bluck B. J. 
Published: April 2008

Summary

Sr-Nd isotope and major- and trace-element analyses, including REE, are presented for a suite of basaltic lavas from the Ballantrae complex, SW Scotland. The chemistry of the lavas is discussed with reference to ‘spider’ diagrams, using normalization of incompatible element abundances to those estimated in the primordial earth, rather than conventional element-element plots. These diagrams facilitate assessment of element mobility during alteration, and some samples are shown to have close to original igneous concentrations of the mobile elements Rb, K, Sr etc. Clinopyroxene-phyric basalts from Games Loup and Mains Hill have chemical affinities with modern intra-oceanic arc volcanics, and their acid-leached augite phenocrysts have more radiogenic Sr than the mantle array, defined using clinopyroxene separates from Ordovician oceanic basalts from the Southern Uplands. Basalts from Pinbain, Bennane Head and Downan Point are in contrast typical ocean-island tholeiites in both chemistry and Nd-Sr isotopes. At least four independent volcanic sequences can be recognized from the chemical data: these do not form a continuous 5 km thick sequence but rather represent tectonically juxtaposed units generated in very different environments. The ‘ophiolite’ can not therefore be regarded as a single entity, produced in a single tectonic environment. Sm-Nd mineral ages have been determined for samples from Games Loup, Mains Hill and Downan Point in an attempt to provide a chronology for the development of the different units, but ages obtained are indistinguishable from the zircon age reported by Bluck
et al.
(1980) for a Ballantrae trondhjemite. Evaluation of the significance of the complex to Caledonian plate models will only be possible with more precise age determinations on the different members of the complex.
Petrology and mineralization of the Sakhakot-Qila ophiolite, Pakistan
Authors: Ahmed Zulfiqar , Hall A. 
Published: April 2008

Summary

The Sakhakot-Qila complex is an incomplete, dominantly ultramafic ophiolite. The geology, rock and mineral compositions, and occurrences of chromite ore deposits are described. The predominant ultramafic rock types are harzburgite, wehrlite and dunite, and dykes of pyroxenite. The mafic components of the complex are a metamorphosed gabbro and dykes of metadolerite. Postmagmatic alteration is common, and several kinds of rodingite are abundant. They formed by addition of Ca, Al, Cl and Cu and removal of Si, Mg, Fe and Ni from pre-existing rocks, and contain corundum, clintonite and Al-depleted ferritchromite in addition to the more usual minerals such as grossular and vesuvianite. The rodingites appear to have developed independently of serpentinization, probably with the participation of seawater. There are numerous small chromite deposits, varying widely in texture and composition. There is a strong reciprocal Cr-Al variation and lesser Mg-Fe
2+
variation, partly related to the stratigraphic position of the chromite. Nickeliferous opaque mineral assemblages are well developed.
An ophiolite suite in Fiji?
Authors: Colley H. 
Published: April 2008

Summary
In SW Viti Levu, Fiji rocks formerly described as part of an island-arc succession are regarded as possibly representing the upper part of an ophiolite suite. Foraminiferal oozes, cherts, red clays, Fe-Mn metalliferous sediments, fine-grained volcanic turbidites, and reworked polymict lapillistones can be equated with Layer 1 of the oceanic lithosphere. Underlying these sediments are pillow basalt sequences and deeper structural levels in the suite may be represented by a tonalite body intruded by a dyke swarm. The chemistry of the igneous rock is ambiguous showing affinities with both arc tholeiites and oceanic tholeiites.
Emplacement of the ophiolite suite against arc rocks has occurred along arcuate thrusts with the ophiolitic rocks, principally Layer-1 sediments, being folded along the leading edge of the thrusted block.
The Oligocene-Lower Miocene age of the ophiolite suite suggests it is an obducted portion of the South Fiji Basin, however, emplacement from this direction requires a complex tectonic model involving up to 180° anticlockwise rotation of Viti Levu. This is supported to some degree by palaeomagnetic data.
Emplacement of ophiolite in Papua New Guinea
Authors: Davies H. L. , Jaques A. L. 
Published: April 2008

Summary
The major ophiolite complexes of Papua New Guinea lie on the northeastern margin of the Australian craton and are flanked, externally, by Palaeogene volcanic arcs. The ophiolites are segments of oceanic lithosphere which occupied the forearc zone prior to arc-continent collision. In the case of the easternmost ophiolite, the Papuan ultramafic belt, there is evidence that emplacement was preceded by development of a secondary rupture in the down-going plate, at some distance from the subduction zone, and was followed by extension, buoyant emergence, and reversed movement of the ophiolite. The westernmost ophiolite (April ultramafics) has more complex structure and probably developed as a series of thrust sheets in a subduction system; the thrust sheets were subjected to renewed deformation and thrusting by continued convergence after arc-continent collision.
The role of Landsat Multispectral Scanner (MSS) imagery in mapping the Oman ophiolite
Authors: Rothery D. A. 
Published: April 2008

Summary

The Oman ophiolite is well exposed, and all the major structural and lithological features are visible on suitably processed Landsat images. Visual interpretation of such images provides a synoptic view of the Oman mountains which is in many aspects as useful as inspection of published maps at scales of 1:500,000 to 1:100,000. This study is an investigation of image processing for semi-automated lithological mapping. The differences between the visible and near-infrared multispectral signatures of ophiolite rock types in the Oman mountains have been shown by
in-situ
measurements to be considerably less than for fresh, particulate samples of similar rocks, in the Landsat MSS spectral bands. The muting of their reflectance spectra is due to a combination of alteration and weathering, and this hinders discrimination between rock types on MSS images. The problem is further compounded by topographically induced variations in brightness (BV), and the presence of shadows containing no valid data. The first-order topographic effects can be partly overcome by the generation of intensity-normalized bands. By using training sets of less than 2 km
2
from within an area of less than 2000 km
2
; a thematic map can be produced which shows the approximate distribution of major rock types over virtually an entire MSS scene (30,000 km
2
). The problem of misclassification due to topography and other effects can be largely side-stepped by the use of a suitable post-classification filter.
Erosion, transport and deposition of fine-grained marine sediments
Authors: McCave I. N. 
Published: April 2008

Summary

Fine-grained marine sediments are cohesive but their degree of cohesion is not simply determined by grain size. Cohesion controls erodibility, and water content, mineralogy, cation exchange capacity, salinity of interstitial and eroding fluid, organic mucus content and Bingham yield strength are all parameters relating to cohesion that have been proposed. No unique relation to erodibility has emerged and for erosion of slowly deposited sediment, modified by biota, it seems that measures of surface properties such as aggregate strength should be more relevant than bulk properties such as yield strength. The latter may be more appropriate for rapidly deposited estuarine muds. Once eroded, suspended sediment is subject to flocculation and biological aggregation, which alters size and settling velocity distributions, thereby controlling distribution in the flow and rate of deposition. Disaggregation may also occur through turbulent straining of particles, but in most marine situations this will permit stable particles over 1 mm in diameter, though under higher shear close to the bed the upper limit may be 100–200 μm. For deposition where
τ
0
< 0.1 Pa the maximum stable aggregate size is probably ≥ 100 μm. Deposition of fine sediment on smooth beds probably occurs by entrapment in, and settling through, the viscous sublayer. On rough beds particles are trapped in the interstices between roughness elements. We have only a rough idea of critical deposition conditions but can give a fairly good estimate of deposition rate in given conditions. The longer term net deposition rate as measured by radiometric methods over weeks to months probably owes as much to frequency of erosion as to frequency and rate of deposition and is not yet well predicted from fluid and sediment parameters.
Methods and observations in the study of deep-sea suspended particulate matter
Authors: Eittreim S. L. 
Published: April 2008

Summary

Forward light scattering and light transmission are the two best techniques, coupled with direct sampling, to measure the spatial and temporal variations in suspended sediments of the ocean. It has been difficult to calibrate these methods with satisfactory precision in terms of suspended particulate matter (SPM) concentration; the best precision is approximately ± 10 μgL
−1
. The poor resolution is due to the many unknown factors in the relationship between optical effects and concentration of SPM, in addition to errors in direct sampling of SPM concentration. Because typical deep-sea concentrations are in the range of 5 to 50 μgL
−1
, extreme care must be taken to avoid contamination by airborne dust and salt filter residues. As a short-lived tracer of water motions, SPM is useful because its settling rates range from about 10
−1
to 10
2
m day
−1
, rates giving half-life time-scales roughly comparable to time-scales of deep-water eddy motions and boundary layer events. More needs to be learned, however, to define these settling rates with some confidence. Much progress has recently been made in studies with sediment traps which directly measure the downward vertical flux of particles. The work with sediment traps is prompted by the realization that large fast-settling aggregates and faecal pellets are important in the delivery of material to the bottom, and due to their short residence time and hence low concentrations in the water column, they are not sampled adequately by water bottles.
Grain-size characteristics of turbidites
Authors: Kranck Kate 
Published: April 2008

Summary
Detailed sampling using a very small sample size and grain-size analysis with a Coulter Counter of three fine-grained turbidites enabled a distinction to be made between the well-sorted single-grain Stokes’-deposited and unsorted ‘whole suspension’ floc-deposited grain-size populations. The results indicate that each turbidite is a continuous sequence deposited from the same source suspension. Particles settle to the bottom as flocculated masses. Initially flocs are broken up by near-bottom shear forces and only the coarsest silt and sand remains on the bed. The remaining mud forms a temporary mud suspension near the bottom which reflocculates and intermittently deposits at some critical concentration producing mud interlayers between silt laminae. Decrease in current velocity eventually allows simultaneous deposition of single grains and flocs with the latter becoming progressively more abundant resulting in formation of graded beds. Eventually, all deposition occurs in the form of mud flocs and a massive sediment results. The study essentially confirms the basic mechanism of the fine-grained turbidite deposition model proposed by Stow & Bowen (1978).
Sedimentary sequences on the north-west Mediterranean margin during the Late Quaternary: a dynamic interpretation
Authors: Monaco A. , Mear Y. 
Published: April 2008

Summary
The methods we have used in this study of pelitic sediments are based on the concept of fine-grained particle populations, and include: the counting of grains in the >40 μm fraction (lithoclastic, biogenic, authigenic and aggregates); grain-size analysis of the pelitic fraction expressed as a bi-logarithmic frequency curve giving the ‘evolution index’ of three granulometric classes—silt, clayey-silt and clay; mineralogical indices based on the ratios between particles with different hydrodynamic behaviour—quartz, mica-illite and smectite; and geochemistry of Mn, Cu and Fe to characterize the biogenic nature of the sediment.
A succession of turbiditic (heterogeneous population), hemipelagic and pelagic (homogeneous population) sediments were examined from the NW Mediterranean margin. We identify three main sequences in the Pleistocene to Recent succession, each of which may be interrupted by episodic events: (1) a palaeoclimatic sequence (∼ 5 m thick) of Pleistocene to Holocene age, marking the end of deep channelized lobe construction on the Rhone deep-sea fan; (2) a dynamic sequence (repeated intervals of 20–50 cm thick) of Würmian and Holocene age respectively, in the lower fan and in the central basin; and (3) a uniform sequence (without granulometric, mineralogical or geochemical grading) corresponding to a ponded unit filled with reworked sediments (contourites) and slumps.
Sedimentary processes on the west Hellenic Arc margin
Authors: Got H. 
Published: April 2008

Summary
High-resolution seismic surveys across the western Hellenic Arc show that the Plio-Quaternary cover is highly discontinuous along most of the margin. The slope displays a series of basins separated by ridges generally devoid of sediment. Sedimentological analyses of the recent sediment permit the origin and processes of sedimentation to be defined. The sediments provided by a specific area on land are successively trapped, released by the slope basins and transferred via channels and canyons towards the deeper trench basins. Several main provinces can be distinguished. Each of them includes one or more slope basins and trenches without either lateral connection or the mixing of sediment inputs. The diversity and complexity of sedimentary structures, the granulometric characters, and the variability in rates of sedimentation argue for the prevalence of mass gravity processes. The transfer of sediment from the upper slope basins to the outer trench is effected by an alternation of short periods of giant mudflows and regular but slight reworking between slope basins. This cascade-feeding process, characteristic of the Hellenic subduction margin, does not appear to prevent terrigenous material from reaching the deep trenches. We compare this briefly with sedimentation on passive margins in the western Mediterranean Basin.
Open-ocean, off-bank transport of fine-grained carbonate sediment in the Northern Bahamas
Authors: Heath K. C. , Mullins H. T. 
Published: April 2008

Summary

The contribution of fine-grained, bank-derived versus pelagic-derived carbonate sediment in periplatform oozes has been evaluated by means of a detailed, quantitative carbonate mineralogy study of 35 surface sediment samples from north of Little Bahama Bank. Our data indicate that large volumes of aragonite and magnesian calcite are being exported along the open-ocean margin of Little Bahama Bank during the present high-stand of sea-level. Percentages of mud-size aragonite + magnesian calcite display a strong negative correlation (r = −0.912) with distance from the edge of the bank, whereas percentages of calcite show a strong positive correlation (+0.914). Immediately adjacent to the northern, windward margin of Little Bahama Bank, 86% of the fine-grained carbonates are bank-derived. This value decreases in a linear fashion to 50% bank-derived muds at a distance of 50 km from Little Bahama Bank. Direct seaward projection of our data suggests that fine-grained, metastable, shallow-water carbonate sediments are transported laterally at least 120 km from open-ocean bank margins. This has important implications for the global budget of CO
2
, the diagenetic potential of periplatform oozes, and the growth potential of carbonate slopes.
Depositional model for calcilutites: Scaglia Rossa limestones, Umbro-Marchean Apennines
Authors: Stow D. A. V. , Rainey S. C. R. , Angell G. , Wezel F. C. , Savelli D. 
Published: April 2008

Summary
The calcilutites of the mid-Cretaceous to uppermost Eocene Scaglia Rossa Formation in Italy have previously been interpreted as a continuous sequence of slowly deposited pelagic limestones. However, regional considerations of thickness, sedimentation rates and facies distribution, together with detailed analysis of composition, texture and sedimentary structures suggest that turbidity currents have played an important part in the deposition of, at least, the middle (Maastrichtian) member. Turbiditic calcilutites are very fine-grained and massive, whereas the pelagites are coarser-grained, mottled and bioturbated. There is, however, a continuous gradation of facies and processes between these two end-members, that we believe is applicable to many other modern and ancient deep-water carbonate systems. The common overlap of turbiditic and pelagic processes in fine-grained ‘pure’ carbonates arises partly from their relative lack of flocculation compared with muddier sediments.
Basin model for hemipelagic sedimentation in a tectonically active continental margin: Santa Barbara Basin, California Continental Borderland
Authors: Thornton S. E. 
Published: April 2008

Summary
A variety of Holocene sedimentary processes and depositional environments in Santa Barbara Basin may serve as a model for terrigenously-dominated hemipelagic basins devoid of submarine canyons: (1) suspensate transport; (2) turbidity current transport; (3) mass movement on the lower slope; (4) the imprint of current patterns on both fine-grained suspended load and bedload transport; and (5) preservation of resulting stratigraphy on the deep basin floor by anoxic water conditions which result in annual varves plus turbidites and flood suspensate layers. Silt patterns define a river-derived suspensate pathway driven by surface currents, which coincides with the distribution of 1969 flood layer sediments. A ‘hemipelagic core’ is present on the western portion of the basin floor, where terrigenous and biogenous input are most balanced. Mass-movement covers 13% of the basin-slopes and is typified by mud-flows interpreted from seismic profiles and characteristic sedimentary structures in cores. A generalized vertical sequence of sedimentary structures derived from radiograph analysis may serve as a model for mud-flow deposits. Turbidites and flood suspensate layers have been traced from the middle slope to the basin floor on a canyonless slope and their distribution is controlled by surface current delivery of suspensates. Vertical sequences of turbidite sedimentary structures in grey-layer turbidites provide support for Piper’s (1978) model for fine-grained turbidites. Varve counting in the central basin yields a sedimentation rate of 173 cm/1000 years and frequencies of 59 years for all grey-layers (flood + turbidite), and 120 years for grey-layer turbidites, which probably reflect roughly 50 and 100 year frequency floods. Fine-grained sedimentary processes in hemipelagic settings with high sedimentation rates and low slope gradients may be typified by: (1) mud-flows on the lower slope; (2) unchannelized turbidity currents and flood layer deposition controlled by surface currents and proximity to source; (3) laminated sediments in the deep basin if oxygen levels are sufficiently low, or preservation of only flood suspensate layers and turbidites if bottom waters are not anoxic; (4) sediment pathways definable from texture and coarse fraction analysis.