Petrology of titanian clinohumite and olivine at the high-pressure breakdown of antigorite serpentinite to chlorite harzburgite (Almirez Massif,S. Spain)

Rocks of the Cerro del Almirez ultramafic massif (Sierra Nevada, Betic Cordillera, S. Spain) record the high-pressure dehydration of antigorite–olivine serpentinite to form chlorite harzburgite (ol + opx + chl). In the field, these two rock types are separated by a well-defined isograd. Titanian clinohumite (TiCl) and olivine show textural and compositional differences depending on the rock type. OH–TiCl occurs in the serpentinite as disseminated grains and in veins. F–OH–TiCl is observed exclusively in the chlorite harzburgite, where it occurs as porphyroblastic grains and within prograde olivine as irregular and lamellar, planar intergrowths at microscopic and submicroscopic scales. Petrological evidence of partial to complete breakdown of TiCl to olivine + ilmenite is preserved in both rock types. Chlorite harzburgite is characterized by a brown pleochroic olivine with abundantally oriented microscopic to submicroscopic oxide particles. The mean Ti-content of the brown olivine is 144 ppm. The brown olivine preserves TiCl lamellae that sometimes grade into ghost lamellae outlined by the oxide trails. This observation suggests that some of the oxide inclusions in the brown olivine are derived from the breakdown of TiCl intergrowths. Thermodynamic modelling of selected Almirez bulk rock compositions indicates a temperature increase from 635°C to 695°C, at pressures ranging from 1.7 GPa to 2.0 GPa, as the cause for the compositional adjustment of TiCl between the Almirez antigorite serpentinite and chlorite harzburgite. These P–T estimates are in good agreement with the sequence of phase relations observed in the field. The computed phase diagrams in conjunction with the geothermal conditions envisaged for different subduction settings indicate that TiCl is stable in the vicinity of the antigorite serpentinite/chlorite harzburgite phase boundary in some subduction settings. In these circumstances, clinohumite–olivine intergrowths in chlorite harzburgite may act as a sink for high field strength elements, and probably other elements, that are present in the mantle–wedge fluids.     

Evaluation of thermochemical data on Fe-Mg olivine,orthopyroxene, spinel and Ca-Fe-Mg-Al garnet

Thermochemical data on Fe-Mg olivine, orthopyroxene, spinel and Ca-Fe-Mg garnet have been tested and reevaluated in reproducing experimental equilibrium data. All data (except of spinel) adjusted in this process lie within the error limits of original calorimetric experiments. For spinel, an enthalpy of −2307.2 kJ/mol and an entropy of 81.5 J/mol-K has been recommended. Recommended interaction parameters for the spinel-hercynite and forsterite-fayalite solutions are as follows:Spinel: W_{spinel-hercynite} = 9124.0 J/mol. W_{hercynite-spinel} = 0.0 J/molOlivine: W = 4500.0 J/mol for 1 cation.Excess entropies (on 1 cation basis) necessary to reproduce phase equilibria for the pyrope-almandine and almandine-grossular solutions are as follows:Mg-Fegarnet: W^s_{pyrope-almandine} = 11.760 − 0.00167 J/mol-K. W^s_{almandine-pyrope} = −10.146 +0.0037T J/mol-K.Fe-Ca garnet: W^s = −16.07 + 0.0126T J/mol-K.     

Titanian hydroxyl-clinohumite: Formation and breakdown in antigorite rocks (Malenco,Italy)

Petrographic evidence is presented for the breakdown of titanian hydroxyl-clinohumite to olivine+magnesian ilmenite (or geikielite)±magnetite in the outermost zone of the Bergell aureole in the Malenco Serpentinite, Prov. Sondrio, Italy. The breakdown coincides in the field with the isograd reaction: antigorite+diopside=olivine+tremolite+H₂O. It is therefore concluded that this variety of clinohumite is unstable above approximately 520° C at a pressure of 3 kbars. Elsewhere in the Malenco Serpentinite, titanian hydroxyl-clinohumite may be found to have reacted with CO₂ to produce antigorite, magnesian ilmenite and magnesite. Titanian hydroxyl-chondrodite was detected in one sample. Under crustal pressures, the stability field of F-free clinohumite is entirely contained inside that of antigorite. The stable occurrence of titanian clinohumite in high-grade metamorphic ultrabasic rocks may be attributed to the substitution of F for OH.     

The behavior of Mg, Fe, and Ni during the replacement of olivine by orthopyroxene: experiments relevant to mantle metasomatism

Replacement of olivine by orthopyroxene is a frequently observed phenomenon in mantle metasomatism. In order to study element redistribution in SiO₂ metasomatism we synthesised orthopyroxene reaction rims at the contacts between forsterite-rich olivine and quartz. The orthopyroxene rims grew from the original quartz-olivine interface into both directions implying counterdiffusion of iron/magnesium and silicon. Following local equilibrium partitioning the X_Fe is lower in the orthopyroxene than in the reactant olivine at the olivine-orthopyroxene replacement front. The resulting local iron excess is compensated by formation of orthopyroxene with a higher X_Fe at the quartz-orthopyroxene interface, which is out of equilibrium with the reactant olivine. This is facilitated through short circuit diffusion along grain boundaries within the orthopyroxene rim. Due to the low capacity of orthopyroxene to accommodate Ni, this component is forced to diffuse back into the olivine producing a Ni enriched zone ahead of the replacement front. This leads to Ni contents in the orthopyroxene rim, which are higher than what is expected in equilibrium with the unaltered olivine. Taking quartz as a proxy for a silica rich fluid or liquid metasomatising agent, we conclude that the overall element fractionation between olivine and the silica rich phase may deviate from equilibrium partitioning so that the Fe and Ni concentrations in the orthopyroxene which is in contact with quartz are higher than in equilibrium with the reactant olivine. This indicates that kinetic fractionation is important for the chemical evolution of both the mantle rocks and the metasomatising agents.     

Experimental Study on the Solubility of Cr^2 in Olivine,Orthopyroxene and Spinel Solid Solutions

Experiments have been performed on the system MgO-SiO2-Cr-O at 0-2.88 GPa and 1100-1450℃,focusing on the stability of Cr^2 in olivine(O1),orthopyroxene(Opx) and spinel(Sp) and its partitioning between these phases.Analytical reagent grade chemicals,MgO,SiO2,Cr2O3.and Cr were used to make starting mixtures.Excess Cr(50%) was then added in these mixtures to ensure that the resultant phases were in equilibrium with the metal Cr.Flux of BaO B2O3(%) was added for facilitating experimental equilibrium and crystal growth.Cr was used as capsule material.All phases in the product were identified by X-ray and analyzed by electron microprobe,The contents of CrO in the different phases(O1,Opx and Sp)were calculated according to stoichiometry.The obtained results of calculation indicate that Cr^3 in Ol and Opx is negligible.The experimental results show;(a) with increasing temperature and decreasing pressure,Cr^2 solubility in Ol,Opx and Sp increases;(b) with in creasing temperature,the partitioning coefficient of Mg and Cr^2 between Ol and Opx decreases,that between Opx and Sp increases,and that between Ol and Sp remains almost unchanged;(c) the effect of pressure on all partitioning coefficients is negligible.     

Experimentally determined mineral-melt partition coefficients for Sc,Y and REE for olivine,orthopyroxene, pigeonite,magnetite and ilmenite

Our current lack of understanding of the partitioning behavior of Sc, Y and the REE (rare-earth elements) can be attributed directly to the lack of a sufficiently large or chemically diverse experimental data set. To address this problem, we conducted a series of experiments using several different natural composition lavas, doped with the elements of interest, as starting compositions. Microprobe analyses of orthopyroxene, pigeonite, olivine, magnetite, ilmenite and co-existing glasses in the experimental charges were used to calculate expressions that describe REE partitioning as a function of a variety of system parameters. Using expressions that represent mineral-melt reactions (versus element ratio distribution coefficients) it is possible to calculate terms that express low-Ca pyroxene-melt partitioning behavior and are independent of both pyroxene and melt composition. Compositional variations suggest that Sc substitution in olivine involves either a paired substitution with Al or, more commonly, with vacancies. The partitioning of Sc is dependent both on melt composition and temperature. Our experimentally determined olivine-melt REE Ds (partition coefficients) are similar to, but slightly higher than those reported by McKay (1986) and support their conclusions that olivines are strongly LREE depleted. Y and REE mineral/melt partition coefficients for magnetite range from 0.003 for La to 0.02 for Lu. Ilmenite partition coefficients range from 0.007 for La to 0.029 for Lu. These experimental values are two orders of magnitude lower than many of the published values determined by phenocryst/matrix separation techniques.     

An experimental study of Fe-Mg partitioning between olivine and orthopyroxene at 1173, 1273 and 1423 K and 1.6 GPa

The partitioning of Mg and Fe²⁺ between coexisting olivines and orthopyroxenes in the system MgO-FeO-SiO₂ has been investigated experimentally at 1173, 1273, 1423 K and 1.6 GPa over the whole range of Mg/Fe ratios. The use of barium borosilicate as a flux to promote grain growth, and the identification by back-scattered electron imaging of resulting growth rims suitable for analysis by electron microprobe, results in coexisting olivine and orthopyroxenene compositions determined to a precision of±0.003 to 0.004 in molar Fe/(Mg+Fe). Quasi-reversal experiments were performed starting with Mg-rich olivine and Fe-rich orthopyroxene (low K_D) and vice versa (high K_D), which produced indistinguishable results. The distribution coefficient, K_D, depends on composition and on temperature, but near Fe/(Mg+Fe)=0.1 (i.e. mantle compositions) these effects cancel out, and K_D is insensitive to temperature. The results agree well with previous experimental investigations, and constrain the thermodynamic mixing properties of Mg-Fe olivine solid solutions to show small near-symmetric deviations from ideality, with between 2000 and 8000 J/mol. Multiple non-linear least squares regression of all data gave a best fit with (implying 5450 J/mol at 1 bar) and , but the two W _G parameters are so highly correlated with each other that our data are almost equally well fit with , as obtained by Wiser and Wood. This value implies , apparently independent of temperature. Our experimental results are not compatible with the assessment of olivine-orthopyroxene equilibria of Sack and Ghiorso.     

An erupted migmatite from Cerro del Hoyazo,SE Spain

The almandine-bearing biotite-cordierite-labradorite dacite of the Cerro del Hoyazo is part of the Neogene volcanic range in SE Spain, extending roughly from Cabo de Gata to Cartagena. About 1 vol. % of the lava consists of rock inclusions, measuring over 1 cm, made up of almandine-biotite-sillimanite gneiss, quartz-cordierite gneiss and spinel-cordierite rock. On the ground of their abundance, chemical composition, mineral content and structure, the first and the second type are interpreted as restite inclusions and the third type as recrystallized restite. These restites and the dacitic magma were derived syngenetically from a (semi-) pelitic rock sequence by means of anatexis: the (semi-)pelitic rocks separated into a granitoid melt and Al-rich restites. Euhedral almandine crystals found in the glass base of the dacite have a pre-magmatic origin, and may be compared directly to those in the restites. Another type of inclusion is represented by basic igneous rocks of varying grain size, comprising mainly basaltoid rocks and quartz-rich gabbros. These inclusions commonly bear some restite fragments of the kind mentioned above, and therefore are interpreted as representing basic magma of deeper origin that has absorbed some anatectic material. In part, the composite basic melt thus formed crystallized under plutonic conditions and fragments of the resulting quartz-rich gabbro were incorporated in a later stage in the dacitic melt. Another portion of the composite basic magma was incorporated in the dacitic melt (probably shortly before the eruption of the dacitic magma) as magma blebs (Ø ?1–20 cm) which subsequently crystallized in part, some of them showing a comparatively fine-grained border zone. Two possible hypotheses are suggested for the time relation between the anatexis of the (semi-) pelitic complex and the appearance of the basic magma: (a) the anatexis was of a regional nature, and was in progress when the basic magma entered the stage (and assumedly triggered the eruption of the granitoid magma); (b) the regional anatexis took place considerably earlier and the basic magma intruded an essentially solid migmatite complex, which was then melted down (contact anatexis) and subsequently erupted.     

Petrogenesis of augite-bearing ureilites Hughes 009 and FRO 90054/93008 inferred from melt inclusions in olivine, augite and orthopyroxene

Melt inclusions in ureilites occur only in the small augite- and orthopyroxene-bearing subgroups. Previously [Goodrich C.A., Fioretti A.M., Tribaudino M. and Molin G. (2001) Primary trapped melt inclusions in olivine in the olivine-augite-orthopyroxene ureilite Hughes 009. Geochim. Cosmochim. Acta65, 621-652] we described melt inclusions in olivine in the olivine-augite-orthopyroxene ureilite Hughes 009 (Hughes). FRO 90054/93008 (FRO) is a near-twin of Hughes, and has abundant melt inclusions in all three primary silicates. We use these inclusions to reconstruct the major, minor and rare earth element composition of the Hughes/FRO parent magma and evaluate models for the petrogenesis of augite-bearing ureilites.Hughes and FRO consist of 23-47 vol % olivine (Fo 87.3 and 87.6, respectively), 7-52 vol % augite (mg 89.2, Wo 37.0 and mg 88.8, Wo 38.0, respectively), and 12-56 vol % orthopyroxene (mg 88.3, Wo 4.9 and mg 88.0, Wo 4.8, respectively). They have coarse-grained (?3 mm), highly-equilibrated textures, with poikilitic relationships indicating the crystallization sequence olivine → augite → orthopyroxene. FRO is more shocked than Hughes, experienced greater secondary reduction, and is more weathered. The two meteorites are probably derived from the same lithologic unit.Melt inclusions in olivine consist of glass ± daughter cpx ± metal-sulfide-phosphide spherules ± chromite, and have completely reequilibrated Fe/Mg with their hosts. We follow the method of Goodrich et al. (2001) for reconstructing the composition of the primary trapped liquid they represent (olPTL), but correct an error in our treatment of the effects of reequilibration. Inclusions in augite consist of glass, which shows only partial reequilibration of Fe/Mg. The composition of the primary trapped liquid they represent (augPTL) is reconstructed by reverse fractional crystallization of wall augite from the most ferroan glass. Inclusions in orthopyroxene consist of glass + 30-50 vol % daughter cpx. The cpx shows complete, but the glass only partial, reequilibration of Fe/Mg. A range of possible compositions for the primary trapped liquid they represent (opxPTL) is calculated by modal recombination of glass and cpx, followed by addition of wall orthopyroxene and adjustment of Fe/Mg for equilibrium with the primary orthopyroxene. Only a small subset of these compositions is plausible on the basis of being orthopyroxene-saturated.Results indicate that olPTL, assumed to represent the parent magma of these rocks, was saturated only with olivine and in equilibrium with Fo ∼ 83. AugPTL and opxPTL are very similar in composition; both are close to augite + orthopyroxene co-saturation and in equilibrium with Fo 87/8. We suggest that olPTL was reduced to Fo 87/8 due to smelting during ascent, and show that this produces a composition very similar to that of augPTL and opxPTL.REE data for each of the three primary silicates and the least evolved melt inclusions in olivine are used to calculate REE abundances in the Hughes/FRO parent magma. All four methods yield very similar results, indicating a REE pattern that is strongly LREE-depleted (Sm/La = 3.3-3.7), with a small negative Eu anomaly (Eu/Eu* = 0.82) and slight HREE-depletion (Gd/Lu = 1.4-1.6).The Hughes/FRO parent magma provides a robust constraint on models for the petrogenesis of augite-bearing ureilites. Its major, minor and rare earth element composition suggests derivation through mixing and/or assimilation processes, rather than as a primary melt on the ureilite parent body.     

High δ18O and possible pre-eruptional Rb-Sr isochrons in cordierite-bearing Neogene volcanics from SE Spain

O- and Sr-isotope data are reported for cordieritebearing dacites and andesites forming part of the Neogene volcanic province of SE Spain. The almandine-bearing biotite-cordierite-labradorite dacite from the Cerro del Hoyazo with its numerous inclusions of metamorphic and igneous rocks has been studied in some detail. A syngenetic derivation of the Hoyazo dacite magma and part of its inclusions (interpreted as restite) by means of anatexis of (semi-)pelitic rocks has previously been proposed. ¹⁸O values and ⁸⁷Sr/⁸⁶Sr ratios, thought to be closely representative of the original magmas, vary from +12.2 to +15.6 and 0.7095 to 0.7171, respectively. The metamorphic rock inclusions have ¹⁸O values (+13.0-+16.2) comparable to the range for the volcanics. These results support an anatectic origin for the entire suite of cordierite-bearing volcanics. The inclusions of basic igneous rocks in the Hoyazo dacite have, in comparison with the dacite, lower ¹⁸O values (+11.1-+13.1) and equal to lower ⁸⁷Sr/⁸⁶Sr ratios (0.7081–0.7112), confirming an independent origin.The six analysed samples of the Hoyazo dacite show a strong linear correlation of ⁸⁷Sr/⁸⁶Sr versus ⁸⁷Rb/⁸⁶Sr which, if interpreted as an isochron, yields an age of 210±17 Ma. Similar linear arrays for samples from the Mazarrón and Mar Menor areas yield nearly concordant ages. Samples for the Vera area define a 535±22 Ma line. These linear correlations may be interpreted either as (1) mixing lines, in which case the indicated ages have no geological meaning, or (2) true isochrons dating pre-eruptional events of the parent (meta-)sediments.     

The distribution of chromium among orthopyroxene,spinel and silicate liquid at atmospheric pressure

A series of experiments has been carried out in which a synthetic silicate melt, of composition equivalent to a “U-type” Bushveld Complex parent liquid, was equilibrated with bronzitic orthopyroxene and chromite spinel between 1334 and 1151°C over a range of oxygen fugacities between the nickel-nickel oxide and iron-wüstite buffers. The partition coefficient for Cr between bronzite and melt increases with falling temperature along a given oxygen buffer, and decreases with falling oxygen fugacity at a given temperature, showing an overall range from 1.1 to 11.7. The Cr content of the melt in equilibrium with spinel (Cr solubility) decreases with falling temperature and increases with lower oxygen fugacity. This variation may be quantified in terms of temperature-dependent solubility of Cr³⁺ combined with a changing ratio of Cr³⁺ to Cr²⁺ in the melt. Iso-oxidation curves for Cr are approximately parallel to Fe-Si-O buffer curves and to curves for equal Fe³⁺/Fe²⁺ determined by Hill and Roeder (1974), and agree within experimental error with the results of Schreiber and Haskin (1976). When the changing oxidation state of Cr is allowed for, the orthopyroxene partition coefficient may be expressed as the sum of the temperature-dependent coefficient for Cr³⁺ and a partition coefficient of about 1 for Cr²⁺.The experimental results are used to construct a series of model curves for liquid and bronzite compositional variations during fractional crystallization of a Bushveld parent liquid. Trends for Cr variation are shown to depend critically upon oxygen fugacity, and on whether the liquid is saturated with chromite. The position of the peritectic between chromite and orthopyroxene is shown to be very sensitive to oxygen fugacity within one and a half log units of the QFM buffer. This observation may explain the contrasting distribution of chromite seams in the Bushveld Complex, where chromite occurs within bronzitite-norite sequences, and in the Stillwater Complex in which chromite is restricted to olivine cumulate layers.     

Intracrystalline relationships in olivine,orthopyroxene, clinopyroxene and spinel from a suite of spinel lherzolite xenoliths from Mt. Noorat,Victoria, Australia

A detailed crystal chemical study of coexisting olivine, orthopyroxene, clinopyroxene and spinel from selected Victorian (Australia) lherzolite suites was carried out by means of single crystal x-ray diffraction and electron probe microanalysis to obtain actual site occupancies. The aim of this study was primarily to characterise the intracrystalline configurations and related cation ordering on sites in major mantle constituents. The results demonstrate that cation ordering on sites is subject to distinctive crystallographic controls which depend on the petrological evolution of the suite. Mg-Fe²⁺ ordering in M1–M2 pyroxene sites depends on variations of the smaller cations, mainly Al^vi, Ti⁴⁺, Fe³⁺, and related configurations of M 1. Pressuresensitive Al^vi is crucial to Fe²⁺, the more ordered clinopyroxene showing high Al^vi configurations which tend to exclude the larger bivalent cations and yield small polyhedral volumes for M 1, M 2, T sites and the unit cell. Conversely, the coexisting orthopyroxene, characterised by lower Al^vi configuration and higher M 1 and unit cell volumes, is relatively more disordered. Olivine is consistent with the coexisting clinopyroxene, the more disordered crystals coexisting with more disordered clinopyroxene, while Al-Mg order in the coexisting spinel shows the reverse relationship. Estimated temperatures of apparent equilibration based on current geothermometers are not considered realistic. Assumptions of ideal cation mixing on sites in pyroxene and spinel are not supported.     

Structures and fabrics in a crustal-scale shear zone,Betic Cordillera,SE Spain

A broad zone of dominantly ductile high-strain deformation lies beneath the Aguilón nappe in the Sierra Alhamilla, southern Spain. It forms part of a crustal-scale movement zone, traceable through much of the Betic Cordillera, which separates the Higher Betic Nappes from the underlying Nevado-Filabride Complex. The zone is characterized in outcrop by a distinctive platy foliation and a strong NNE-trending stretching lineation. Microstructural characteristics include quartz ribbons, mica fish, augen of feldspar and other minerals in a matrix of dynamically recrystallized quartz, and extensional crenulation cleavages. Narrow bands of ultramylonite and cataclasite occur within and on the margins of the movement zone. Deformation occurred under lower greenschist-facies conditions and was accompanied by retrogression of earlier higher-grade mineral assemblages.Structures in the movement zone developed in a temporal sequence, beginning with isoclinal folding and transposition of older foliations. This was followed by the formation of extensional crenulation cleavages, and the progressive localization of strain into the ultramylonite bands. Mylonitic foliation in these bands is deformed by syn-mylonite folds restricted to the bands. All these structures were then deformed by S- to SE-vergent small-scale folds restricted to the movement zone as a whole. Cataclasis, associated with alteration, is localized along the ultramylonite bands and indicates a transition to late-stage brittle deformation. The lower boundary of the movement zone is gradational: strain decreases, recrystallized grainsize and the degree of recrystallization of quartz increases, and pressure solution becomes the dominant deformation mechanism in mica-schist.Asymmetric quartz fabrics in the movement zone indicate a NNE sense of shear; but variations in the degree of asymmetry suggest that flow was partitioned, with the ultramylonite bands taking up much of the shear strain, and the intervening rocks deforming more slowly and with a lower degree of non-coaxiality. Diffuse fabrics in the fine-grained ultramylonite bands may indicate a switch to a grainsize-sensitive deformation mechanism, and an overall downward increase in the opening angle of crossed-girdle fabrics may reflect increased water activity at depth.     

Calcium zoning in olivine and its relationship to silica activity and pressure

Microprobe analyses of olivine phenocrysts from nephelinite and basanite lavas show strong zoning toward calcium enrichment. Tholeiitic and other more siliceous lavas show little or no such calcium enrichment, but more extensive magnesium to iron zoning. Consideration of reactions between clinopyroxene and olivine indicate that silica activity as well as pressure is an important variable. Zoning trends toward calcium enrichment in olivine could be interpreted as a response to pressure release during crystallization. A normal, magnesium to iron, trend with no increase in calcium would, then, reflect crystallization with stable pressure conditions.     

Evolving turbidite systems on a deforming basin floor, Tabernas, SE Spain

The Tabernas‐Sorbas basin was a narrow, east‐west trending, marine trough of Late Miocene age. Sediment gravity flow deposits dominate the basin fill and provide a record of changing bathymetry in response to tectonically induced sea bed deformation. A reanalysis of the western end of the basin in the vicinity of Tabernas establishes an upward evolution involving: (1) sand‐starved marls that were incised by axial channels recording a period of bypass, during which sand deposition took place in a depocentre further to the east; (2) punctuated infilling of the incisions, locally by high‐sinuosity embedded channels. Channel filling is related to a gradient reduction, which presaged collapse of the axial slope as the depocentre began to migrate westwards into the Tabernas area; (3) draping of the earlier incision fills by laterally extensive sheet turbidites, which were initially contained in structurally controlled depressions. These ‘deeps’ opened up as active faults propagated through the former axial slope. Flow containment is inferred on account of the unusual structure of the sheet sandstone beds, complex palaeoflow relationships and thick mudstone caps; (4) fault‐controlled topography was subsequently healed, and further sheet turbidites showing evidence of longer range containment and progressive slope onlap were emplaced. These record mixed supply from both seismically trigged ‘axial’ failures and a reactivated, fault‐controlled slope building out from the northern margin of the basin. Flows traversing the trough floor were strongly reflected off slopes marking the southern limit of the basin. The studied succession is capped by (5) the Gordo megabed event, a large, probably seismically triggered, failure which blanketed the basin floor, demonstrating an enlarged but still contained basin now devoid of significant intrabasinal fault topography. Tectonics played a key role in driving the evolution of the turbidite systems in this basin. Deformation of the basin floor had an important impact on gradients, slope stability, bathymetry and the ability of flows to bypass along the trough axis. Westward migration of the depocentre into the Tabernas area led to a change from incision and bypass to conduit backfilling to flow containment, as fault‐induced subsidence generated a ‘sump’, which trapped flows moving along the basin axis.     

Selenium content in soils from Murcia Region (SE,Spain)

The objective of this study was to determine the selenium content in soils from the Region of Murcia (SE, Spain) and to study the possible relationship between mineralogical composition, soil properties and selenium content. In addition, generic reference level (GRL) for selenium was determined according to Spanish legislation. For this study, 490 samples were selected in areas subjected to agricultural activity or abandoned agricultural soils with natural vegetation. Five different zones were established in order to study selenium variability. Four different mineralogical groups were established, based on significant relationship with the mineralogical composition, and each sample was included in one mineralogical group. The results showed that the concentration of selenium is correlated with the electrical conductivity, sulphate content and with the mineralogical composition, particularly positively with quartz and in a negative way with calcite percentage. In addition, selenium level was positively correlated with the phyllosilicate content. Established GRL for selenium varies with the mineralogical group, having a great transcendence from the legal and economic points of view.     

Reaction rims of orthopyroxene and plagioclase around chrome spinels in olivine from Skye and Rhum (NW Scotland)

Reaction between chrome spinel and its supernatant magma due to changes in magmatic composition or temperature is well known; its effects can be observed in these rocks. However, evidence of solid-solid reaction between chrome spinels and enclosing olivine is also present, in the form of relict diffusion profiles in some of the olivines, and an orthopyroxene reaction product rimming the spinel. In addition, the Rhum specimens contain rims of plagioclase resulting from solid-liquid reaction around chrome spinels within the margins of olivine. Detailed microprobe traverses and analysis of reaction products have been made in an attempt to determine the equations for these reactions.