bluuebunny/crossref_metadata_2025_split huggingface dataset:
Trace element geochemical effects of imperfect crystal-liquid separation
Authors: O’Hara M. J. 
Published: April 2008

Abstract
The strongly contrasted effects of melting or crystallization under perfect equilibrium conditions; of perfect fractional partial melting; and of perfect fractional crystallization upon the relative concentrations of the highly incompatible and highly compatible elements are well known and are surveyed briefly at the start. A major role for either of the two latter processes in the genesis of natural basaltic magmas and peridotitic residues is apparently excluded provided that the liquids or crystals are removed in infinitesimally small batches, the crystals and melts are perfectly separated and there is no flux of source material or residue in or out of the greater system during the process (either liquid or crystals must be removed from the immediate, or lesser, system if it is undergoing perfect fractional melting or perfect fractional crystallization).
The consequences of small departures from (i) infinitesimal size of the batches of liquid or crystals removed, (ii) perfect separation of the crystal and liquid phases, or (iii) perfect closure of the greater system to fluxes of the source material and the residue are explored. These departures can very substantially reduce the differences between the trace element concentrations generated in the products and those expected in a perfect equilibrium process.
These effects are illustrated here by means of 3D plots of the logarithm of relative concentration in the liquid phase as a function of the logarithm of the distribution coefficient and the logarithm of the mass fraction of liquid involved in each process. There are also significant complementary effects on the compositions of the residues or precipitates from these processes which are not illustrated in this paper, although the necessary equations and some worked examples are provided. With the values of parameters chosen in these examples, the fluxing of liquid or solid through a ‘box’ in which fractional crystallization or fractional melting is taking place proves to be by far the most important factor in modifying the results towards those of the equilibrium processes.
Although much trace element behaviour in basalts can be quite successfully modelled using the perfect equilibrium melting relationships, it is unsafe to exclude on these grounds a major role for physical processes of fractional melting and fractional crystallization in the genesis of these magmas or to conclude that equilibrium partial melting is the principal physical process involved in their genesis.
Development of concepts concerning the Troodos ophiolite and adjacent units in Cyprus
Authors: Robertson Alastair , Xenophontos Costas 
Published: April 2008

Abstract
The foundations for study of the Troodos ophiolite and adjacent units in Cyprus were laid down by systematic mapping by the Cyprus Geological Survey Department (1952–1970). Prior to the late 1950s, Cyprus geology was interpreted in classic geosynclinal terms. The first, integrated description of the Troodos was given by Gass & Masson Smith in 1963. Publication of the Vine & Matthew’s (1963) sea-floor spreading hypothesis was followed by Ian Gass’s key interpretation in 1968 of the Troodos as oceanic lithosphere formed by sea-floor spreading. During the early 1970s, the dominant view of the Troodos was of a mid-ocean ridge, formed by spreading at a narrow, linear spreading zone, fed by a single, axial magma chamber. However, early geochemical studies suggested that genesis could have taken place above a subduction zone. During the 1980s, drilling related to the International Cyprus Crustal Study Project stimulated much new field work. By 1990 a popular view of the Troodos was that it had formed above a northward-dipping subduction zone during the earlier stages of convergence within an oceanic basin, similar in some respects to the origin of SW Pacific forearcs. Others favoured an origin as a marginal basin formed by spreading above a southward-dipping subduction zone, either in the vicinity of Cyprus, or located in central Turkey. In whatever scenario, it is generally accepted that the spreading fabric was unstable and was fed by small, multiple magma chambers. The extension outpaced magma supply and this favoured extensional detachment faulting, mainly near the base of the sheeted dykes. The Troodos lithosphere was also bounded by an oceanic fracture zone to the present south. Studies of the Late Cretaceous-Recent sediment cover have documented deep-sea sedimentation and later uplift, related to Africa-Eurasia convergence. In W Cyprus, the adjacent Mamonia Complex was pieced together as a Mesozoic passive margin bordering a small ocean basin, although whether this was a Red Sea-type basin, or a backarc marginal basin is still debated. The Mamonia lavas and sediments were juxtaposed with the Troodos ophiolite by processes, variously interpreted as subduction/accretion, oceanic transform faulting, strike-slip, collision and/or palaeorotation of a Troodos microplate. The Kyrenia Range in N Cyprus also includes remnants of Mesozoic passive margin; it was emplaced in stages apparently by a combination of strike-slip and thrusting. On a regional scale, the Troodos ophiolite preserves part of one of a number of inferred small oceanic basins within the Mesozoic Tethyan area of the Eastern Mediterranean.
Structure and tectonic evolution of the Southern Troodos Transform Fault Zone, Cyprus
Authors: MacLeod C. J. , Murton B. J. 
Published: April 2008

Abstract
A major strike-slip fault zone, at least 5 km in width, trends E-W along the southern margin of the Troodos ophiolite. It is an ocean-floor feature, overlain by undeformed pelagic sediments, and is orientated perpendicular to the mean strike of the sheeted dyke complex of the main ophiolite massif. It has long been recognized as probably the foremost example of a fossil transform fault within an ophiolite.

We present here detailed description of the structure and tectonic evolution of this so-called ‘Southern Troodos Transform Fault Zone’ (STTFZ), based upon 1:5000 scale mapping of the whole of the southern margin of the Troodos ophiolite. The styles of strike-slip deformation at all levels of the oceanic crust and upper mantle, and the variations in deformation style with time, are documented and the role of vertical-axis rotations within and adjacent to the transform zone is discussed. Evidence for the kinematics of deformation is reviewed; previous studies have been unable to agree even on such fundamentals as the sense of slip of the STTFZ. In an attempt to resolve the controversy we have made a systematic re-examination of the evidence for the direction of motion along the transform, supplemented by new field observations where necessary. We conclude that, although there is incontrovertible evidence for both sinistral
and
dextral shear along the STTFZ whilst it was in an oceanic environment, the overwhelming indications are for dextral slip. We show that sinistral indicators are restricted to a few small mylonite shears that are probably related to local geometrical complexities associated with the intrusion of gabbroic plutons into the transform; there is no need to invoke wholesale reversal of slip sense along the STTFZ as a whole.
Petrogenesis of ophiolitic chromitite
Authors: Roberts Stephen , Neary Christopher 
Published: April 2008

Abstract
Chromite deposits comprise an integral part of the mantle sequences observed within many ophiolite complexes; in particular, those where a harzburgite is the dominant mantle sequence lithology. The chromite segregations are invariably contained within a dunite envelope and show a wide variety of ore textures, some of which are directly analogous to those observed within major layered intrusions. The majority of chromite segregations contain chromite and olivine of variable modal proportions although in certain instances a wide variety of interstitial silicates may be preserved including plagioclase, orthopyroxene, clinopyroxene and amphibole. Evidence of deformation linked to increasingly lower temperatures is invariably present which can be linked either to high temperature ‘flow fabrics’ away from the spreading axis, the obduction of the ophiolite or post-obduction regional deformation events.

Analysis of chromite grains from a variety of ophiolitic chromite segregations shows them to be chrome-spinels with variable Cr* values (Cr/(Cr+Al)), typically between 0.4 and 0.85, with Mg* values (Mg/(Mg+Fe
2+
)) between (0.5 and 0.8) and invariably restricted Fe
3+
contents. By way of a simple contrast to major layered intrusions the ophiolitic chromite segregations tend to range to lower Cr* values and show higher and more restricted Mg* ratios. The variation of the Cr* ratio from chromite segregations within an individual ophiolite can be related to the position of the segregation within the mantle sequence. The more aluminous deposits being located towards the petrological moho, whereas the more chrome-rich horizons tend to occur deeper within the sequence, in particular toward the harzburgite lherzolite transition.

A model is described which considers the chromite deposits to represent the early fractionates of ascending magmas within highly depleted mantle, likely to have developed above subduction zones, or at fast spreading centres. Transport of the mantle away from the spreading zone results in deformation of the chromite deposits within a falling P-T regime involving both ridge-axis and post-obduction deformation.
An overview of the PGE concentrations in the Shetland ophiolite complex
Authors: Prichard H. M. , Lord R. A. 
Published: April 2008

Abstract
Extremely anomalous platinum-group element (PGE) concentrations are known at one site, Cliff, in the Shetland ophiolite complex. Systematic PGE analysis of all the mafic and ultramafic igneous and alteration lithologies in the ophiolite has revealed many lower grade anomalous PGE occurrences throughout the cumulate ultramafic sequence and their distribution is described here. More sites with PGE enrichments as anomalous as those at Cliff have not been found.
At Cliff the anomalous values of over 60 ppm Pt + Pd are restricted to a chromite-rich sulphide-bearing dunite forming a small part of a 200 m dunite lens in mantle harzburgite close to the basal thrust of the ophiolite. Lower values of up to 6.4 ppm Pt + Pd occur in chromite-poor, sulphide-rich dunites also within this dunite lens. The only other chromite-rich dunite lens in the Cliff area also has detected Pt and Pd but with low Pt + Pd values of 200 ppb. In contrast, harzburgites, metasediments underlying the basal thrust, serpentinites from the basal thrust and internal faults are all barren of Pt and Pd (detection limit = 20 ppb). In the Cliff area, therefore, detected PGE values are restricted to magmatic chromite-rich dunite lenses. PGE analyses of drill core from beneath the disused chromite quarries at Cliff are low (227 ppb Pt + Pd) indicating that the extremely anomalous PGE values are very restricted. This supports the idea that PGE, concentrated magmatically within the dunite lens at Cliff, may have been remobilized locally to produce extremely anomalous PGE values in an alteration zone only a few metres in diameter.
Elsewhere in the ophiolite, anomalous Pt + Pd values of up to 4 ppm occur in chromite-rich, sulphide-bearing dunite within the dunite unit, which stratigraphically overlies the harzburgite. Pt + Pd concentrations of up to 1 ppm are present in the pyroxenites and wehrlites of the upper part of the ultramafic sequences. High-level wehrlites and pyroxenites within the gabbro unit, representing late ultramafic differentiates, contain lower levels of PGE of up to 310 ppb Pt + Pd. The gabbros all have less than 20 ppb Pt and Pd. This distribution indicates that the PGE concentrations occur in specific primary magmatic lithologies and pathfinder analysis shows that they are associated with Ni, Cu, Au and Cr. PGE have not been found above detection limits in fault zones away from primary PGE mineralization sites and it is concluded that PGE are not significantly hydrothermally reconcentrated. At Cliff, remobilization may have occurred over short distances of only a few metres at most.
Geochemical and thermal fluxes, high-temperature venting and diffuse flow from mid-ocean ridge hydrothermal systems: the TAG hydrothermal field, Mid-Atlantic Ridge 26°N
Authors: Elderfield H. , Mills R. A. , Rudnicki M. D. 
Published: April 2008

Abstract

Submarine hydrothermal activity provides a source of many elements to sea water at rates of a similar order to those of input by rivers. Estimates of hydrothermal fluxes from geophysical models which attribute only about 200f the advection to the axial region are up to an order of magnitude lower than those based upon geochemical budgets (from
3
He, Mg and
87
Sr/
86
Sr). Evidence for ridge-flank and on-axis low-temperature hydrothermalism is reviewed. Diffuse flow appears to account for about ¾ to

3/4 to 6/7
of the total heat flux of 500–900 MW for the TAG vent field, but is poorly characterized chemically. Processes in hydrothermal plumes moderate the primary hydrothermal fluxes of trace elements to the oceans. Recent studies of the plume above the TAG hydrothermal field are discussed. Oxyanions and particle-reactive trace metals are co-precipitated with iron oxyhydroxides that form during mixing in the buoyant plume. Particle-reactive metals are further scavenged during lateral transport of the plume whereas chalcophile elements are removed from the plume. The overall effect of plume reactions is that hydrothermal activity acts as an oceanic sink for several trace elements.
Orogenic uplift and collapse, crustal thickness, fabrics and metamorphic phase changes: the role of eclogites
Authors: Dewey J. F. , Ryan P. D. , Andersen T. B. 
Published: April 2008

Abstract
Coesite-bearing eclogites in several deep crustal metamorphic assemblages now exposed in extensionally-collapsed orogens indicate the tectonic denudation of more than 90 km of crustal rocks and pre-collapsed crustal thicknesses of at least 120 km. For mountain ranges and orogenic plateaux up to 5 km in elevation and average crustal densities of about 2.8, crustal thickness cannot exceed about 80 km unless pre-shortening crustal/lithosphere thickness ratios were less than 0.135 or some way can be found to preferentially thicken the lithospheric mantle. This problem can be avoided and very thick orogenic crusts built up if granulite facies rocks transform to denser eclogite facies during shortening, where the petrographic Moho is continuously depressed below a density/seismic velocity Moho buffered at about 70 km and mountains at about 3 km. Advective thinning of the lithosphere combined with the resultant heating and eclogite to sillimanite-granulite/amphibolite transformation causes surface uplift of about 2 km, a rapid change in isostatic compensation level, and a switch from a shortening to an extensional/collapse regime. We have developed a simple numerical model based upon field observations in southwestern Norway in which coherent regional-scale transformation of lower crustal rocks to eclogite facies during lithospheric shortening is followed by heating, transformation of eclogite to amphibolite and granulite, extension, and crustal thinning by coaxial then non-coaxial mechanisms. The model also explains strong lower crustal layering (eclogite and other lenses in horizontally-extended amphibolites), regionally horizontal gneissic fabrics, rapid return from orogenic to ‘normal’ crustal thickness with minor erosion, the lateral and vertical juxtaposition of low-grade and high-grade rocks and rapid marine transgression shortly after orogeny.
Amphibolite dehydration-melting: sorting out the solidus
Authors: Wyllie Peter J. , Wolf Michael B. 
Published: April 2008

Abstract

Amphibolite melts under vapour-absent conditions by dehydration-melting, during which H
2
O released from hornblende is transferred directly into H
2
O-undersaturated silicate liquid. Five independent 1991 studies on amphibolite dehydration-melting differ from each other and from earlier conceptual treatments. Five solidi between 7 and 10 kb vary from 975°C to 740°C. Two solidi above 10 kb are near-vertical, near 975°C and 850°C. Some differences are due to different amphibolite compositions and mineralogy. We use the results of earlier experiments involving garnet, supported by our own new results, to construct a closed-system phase diagram for a simple, fully-hydrated amphibolite (hornblende + plagioclase) showing the solidus and the melting interval for (hornblende + garnet + other minerals + liquid). The amphibolite vapour-absent solidus is defined by the beginnings of two multivariant reactions: (1) a near-vertical curve (large positive dP/dT) where the formation of augite dominates; and (2) a near-horizontal curve at higher pressures where the formation of garnet dominates. The solidus curve bends back (estimated near 900°C, 9 kb) with slope changing to low positive dP/dT. The new phase diagram expands the field for liquid generation with garnet-amphibolite residues to much lower temperatures and pressures than the other recent experimental results, increasing the range from which small amounts of hydrous granitoid melts may be segregated by compaction. The reversed, near-horizontal slope of the solidus near 10 kb has interesting petrological consequences.
The Proterozoic Gardar rift zone, south Greenland: comparisons with the East African Rift System
Authors: Macdonald Ray , Upton Brian G. J. 
Published: April 2008

Abstract
The late Tertiary to Recent Kenya rift valley and the deeply dissected, Proterozoic, Gardar province of south Greenland are both parts of continental rift systems thousands of kilometres long. Both were generated within stabilized orogenic belts skirting cratonic forelands, in regions in which repeated reactivation of earlier structures has occurred. Both may represent failed attempts at continental separation.
Mafic magmatism in Kenya has ranged from strongly silica-undersaturated, melanephelinitic types through basanites to mildly alkaline and transitional basalts. Gardar activity was dominated by mildly alkaline and transitional basalts; basanites and nephelinites are rare or absent, the most undersaturated representatives being ultramafic lamprophyres. Very primitive (possibly primary) magmas are scarce in both rifts. Trace element chemistry suggests that the majority of the mafic rocks have lithospheric signatures, and that asthenospheric magmas had no direct access to the crust. The primary magmas were generated as small-degree melts in mantle sources which had experienced earlier depletion events and which were subsequently metasomatized. An important, mantle-derived feature of the Gardar basalts is their strongly aluminous nature, which appears to have generated an abundance of anorthositic bodies in the crust; this feature is not shown in the Kenya rift. Considerable amounts of underplating are inferred to characterize both rifts, limiting crustal thinning in regions of lithospheric extension.
Both rift zones have extensive axial dyke swarms, which are particularly focused on central complexes/caldera volcanoes. Kenya apparently lacks, however, the massive dykes, up to several hundred metres wide, of the Gardar province.
Alkali basalts from Shuqra, Yemen: magmas generated in the crust-mantle transition zone?
Authors: Cox K. G. , Charnley N. , Gill R. C. O. , Parish K. A. 
Published: April 2008

Abstract
The alkali basalts from Shuqra, Yemen, contain a megacryst suite consisting of olivine, aluminous diopside, kaersutite, pleonaste, plagioclase, and apatite. Deformational kink bands in the olivines, and the existence of multi-phase inclusions (e.g. olivine + pleonaste, aluminous diopside + pleonaste, kaersutite + pleonaste, aluminous diposide + plagioclase) suggest that the megacrysts are derived from disaggregated plutonic rocks. Whole-rock compositional variation cannot have been generated by the fractionation of the phenocrysts present (olivine, augite, labradorite), but is readily explicable as a mixing line between alkali basalt melt and restite, represented by the megacryst suite. It is suggested that the source rock was a deep-crustal, or uppermost mantle, hydrous, and very alkalic, picritic, gabbroic, or syenogabbroic intrusion, which was remelted, perhaps by the injection of further picritic magma. Although the Shuqra rocks themselves do not appear to contain mantle xenoliths, the proposed mechanism of magma generation, if operating on an interleaved crust/mantle transition zone, could provide a general explanation for the occurrence of occasional alkali basalt flows which are extremely rich in spinel lherzolite inclusions.
Coal and coal-bearing strata as oil-prone source rocks: an overview
Authors: Fleet Andrew J. , Scott Andrew C. 
Published: April 2008

Abstract
Despite many advances in our knowledge over the last decade, understanding of why some coals and coal-bearing strata are oil prone, and our ability to predict such sequences ahead of drilling in petroleum exploration, is relatively poor. Here we review the current status of the knowledge, highlighting contributions made in this volume. Oil-prone coals are hydrogen rich. Those which are generally acknowledged to have given rise to significant oil accumulations occur either as low latitude Tertiary deposits or within late Jurassic-Palaeogene sequences of the Australian region. Oils derived solely from coals and other terrigenous kerogens can be recognized using geochemical criteria. Recognition of oil-prone coals and associated mudrocks visually or geochemically, however, is problematical. What controls the expulsion of petroleum in the liquid phase from coal-bearing sequences is probably the critical factor. Our knowledge of the botanical, depositional and diagenetic controls which determine the formation of oil-prone coaly sequences, and hence our ability to predict their presence, is currently empirical and lacks understanding of the inputs and processes involved.
Geochemical characteristics of oils derived predominantly from terrigenous source materials
Authors: Philp R. P. 
Published: April 2008

Abstract

It is well established that there are many major accumulations of crude oils that are derived predominantly from higher plant source materials. All of these oils have a number of geochemical characteristics in common. These features include
n
-alkane distributions, presence of a variety of sesqui-, di-, tri-, tetra-, and pentacyclic terpanes and isoprenoids. Variations in the distribution of these compounds reflect variations in the nature of the source materials and the original depositional environment. The purpose of this paper is to review these characteristic features so that they can be used to determine whether or not new oil samples are derived predominantly from higher plant source materials. The geochemical approach has been used successfully in the past to characterize samples derived from hypersaline and carbonate environments.