Experimental constraints on amphibole stability in primitive alkaline and calc-alkaline magmas

Equilibrium crystallization experiments were carried out on two primitive basaltic rocks (APR16: Na2O+K2O=4.40 wt%; CM42: Na2O+K2O=2.59 wt%) with the aim to investigate the amphibole stability in the differentiation processes at deep crustal level, of primitive alkaline (APR16) and calc-alkaline (CM42) magmas. The experiments were performed with different initial H2O contents (0-5 wt%), at pressure of 800 MPa, in the temperature range of 975-1225 °C. For the explored conditions, amphibole crystallization occurs in both compositions at H2O in the melt >7wt% while the temperature of their occurrence is lower in the alkaline composition (<1050 °C in APR16 and ≥1050 °C in CM42). Moreover, amphibole crystallization seems to be influenced by the Na2O/K2O ratio rather than the absolute Na2O content in the melt. This is evident when experimental results on the APR16 and CM42 are compared with experimental data obtained from a primitive ultrapotassic composition (leucite-basanite: Na2O+K2O=4.58 wt%) and with thermodynamic modelling by the Rhyolite-MELTS algorithm. The comparison shows that amphibole never saturates the leucite-basanite at any of the investigated/modelled conditions, even when an extended crystallization increases the Na2O of melts up to contents like those of calc-alkaline experimental glasses. We conclude that, at pressure of 800 MPa and hydrous conditions, only primitive liquids with Na2O/K2O ratio ≥0.9 are more prone to crystallize amphibole

Are spherulitic lacustrine carbonates an expression of large-scale mineral carbonation? : A case study from the East Kirkton Limestone, Scotland

BP Exploration Co. is thanked for funding, and particularly the Carbonate Team for supporting this research and for fruitful discussions. West Lothian Council and Scottish Natural Heritage are thanked for allowing access and permission for sampling the site. The Core Store Team at BGS Keyworth is particularly acknowledged for their assistance. Mark Anderson, Tony Sinclair (University of Hull), and Bouk Lacet (VU University Amsterdam) are thanked for technical support. Anne Kelly (SUERC) for carrying out the Strontium Isotope analyses. Mark Tyrer is thanked for his advice on PHREEQC modelling.Peer reviewedPostprin

Subsurface biodegradation of crude oil in a fractured basement reservoir, Shropshire, UK

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Análise de proveniência das contas verdes dos Perdigões

A variscite é um mineral raro que oferece uma excelente opotunidade de estudo dos padrões de comércio e troca na Europa durante a Pré- História através da determinação da sua fonte de proveniência. No presente texto será discutida a proveniência das contas de colar verdes com base em análises de XRF e XRD, através das quais se criou uma identidade geoquímica passível de ser comparada com as das fontes de variscite conhecidasVariscite is a rare mineral that offers an excellent opportunity to study trade and exchange patterns in prehistoric Europe through proveniencing of source material. In this paper we discuss the provenance of Perdigões’ green beads by means of XRF and XRD analyses, thus creating a geochemical baseline that is compared with that of the known variscite source

Mingling and mixing features in basaltic Andesites of the Eastern Cordillera (Central Andes, 24°s): a petrographic and microanalytical study

The basaltic andesites of the Diego de Almagro volcanic complex (Central Andes, 24°S, 66°W) were erupted during Late Miocene at 300 km from the active arc, in the Eastern Cordillera, at the easternmost end of an important strike-slip structure, the Calama-Olocapato-El Toro. These volcanic rocks preserved traces of their pristine history in the form of xenocrysts and in the diverse populations of phenocrysts coexisting due to magma mixing. In Central Andes, crustal contamination commonly conditions the composition of magmas ascending through the thick continental crust, masking the characteristics of the mantle sources. Thus, we carried out detailed petrography and mineral chemistry studies on selected Diego de Almagro basaltic andesites, with the aim to identify the populations of crystals in equilibrium with mafic melts. The results suggested that mixing of basaltic and andesitic magmas occurred during the ascent and storage in the crust. The fraction of basaltic magma prevailed in the mixing process compared to the andesitic one. Crystal zoning and disequilibrium features indicate that magma mixing produced intimate hybridisation. The preservation of primitive crystal assemblages is probably due to the geological situation in which the Diego de Almagro complex emplaced, that favoured the formation of monogenetic volcanic centres rather than of a central volcanic edifice, thus limiting the residence of mafic magmas in the crust and their interaction with crustal partial melts.Le andesiti basaltiche appartenenti al complesso vulcanico Diego de Almagro (Ande Centrali, 24°S, 66°W) sono state emesse nel Miocene Superiore a ben 300 km verso E dall’arco attivo, nella Cordigliera Orientale, all’estremo orientale di una delle più importanti strutture trascorrenti trasversali andine, la Calama-Olocapato-El Toro. Le lave e le piroclastiti di questo complesso vulcanico conservano tracce della loro storia evolutiva sotto forma di xenocristalli e di popolazioni di cristalli in disequilibrio che coesistono grazie a processi di mixing di magmi a diversa composizione. Nelle Ande Centrali, la contaminazione crostale condiziona fortemente la composizione dei magmi che risalgono attraverso la spessa crosta continentale, mascherando le caratteristiche della sorgente mantellica. Per questo motivo, abbiamo ritenuto interessante effettuare uno studio di dettaglio della petrografia e della chimica dei minerali delle andesiti basaltiche di Diego de Almagro, allo scopo di identificare le popolazioni di cristalli in equilibrio con i magmi più primitivi. I risultati raccolti finora hanno mostrato che le andesiti basaltiche sono il prodotto di un processo di mescolamento di magma basaltico ed andesitico verificatosi durante la risalita del magma nella crosta, in condizioni pre-eruttive. Nel processo di mixing che ha prodotto le andesiti basaltiche la frazione di magma basaltico era prevalente in volume rispetto a quella di magma andesitico, e le proprietà fisicochimiche dei due magmi permisero la realizzazione di una avanzata ibridizzazione. L’associazione mineralogica primitiva si è potuta preservare grazie alla particolare situazione geologica del complesso Diego de Almagro, che favoriva la formazione di centri monogenetici piuttosto che lo sviluppo di vulcani centrali, limitando così lo stazionamento dei magmi basici nei serbatoi crostali e di conseguenza l’interazione con la crosta.Fil: Gioncada, Anna. Università degli Studi di Pisa; ItaliaFil: Hauster, Natalia. Universidad Nacional de Salta. Facultad de Ciencias Naturales; ArgentinaFil: Matteini, Massimo. Universidade do Brasília; BrasilFil: Mazzuoli, Roberto. Università degli Studi di Pisa; ItaliaFil: Omarini, Ricardo Hector. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Salta. Facultad de Ciencias Naturales; Argentin

Mineralogic and petrologic study of the low-temperature minerals in carbonaceous chondrites

Samples and petrographic thin sections of the Orgueil, Ivuna, and Alais chondrites were examined by optical, X-ray, and SEM techniques. Mineral species identified as primary vein constituents were epsomite, gypsum, and a calcium-magnesium carbonate. Relative abundances and textural relationships have suggested that fracture mineralization was a multi stage process, with individual mineralizations closely associated with impact brecciation events. Mass balance considerations of carbonaceous chondrite matrix support the prevailing view that the source of the fracture filling minerals was local. By inference they also suggest that the phyllosilicate matrix has been chemically altered and that there are probably very few primitive mineral phases in the primitive Cl chondrites

An investigation into the controls of granite plutonism in the Sierra da Freita region, Northern Portugal

The Serra da Freita region of north central Portugal was chosen for study as it displays the complex relationships between regional structure, plutonism, regional and contact metamorphism typical of this part of Iberia. The region was mapped on a scale of 1:10000. The Serra da Freita pluton, which intrudes the core of the Porto-Viseu metamorphic belt developed in the late Pre-Cambrian - Cambrian Beira Schists, is shown to lie in a sinistral transpressive shear zone, the Serra da Freita shear zone. Early structures are progressively modified over a protracted period by shear zone deformation, during which time metamorphism reached a peak and the granite was emplaced. Mapping of the intrusive contacts of the granite show that following initial intrusion of a steeply inclined sheet of magma into the zone of highest strain, magmas were injected into a region of progressively lower strain where the magma was acconunodated as a nearly flat sheet. The distal end of this mass ballooned upvards to form the small intrusion of Castanheira which has abundant biotite nodules which acted as near perfect strain markers. The main pluton is shown to intrude obliquely the core of a narrow metamorphic belt characterized by parageneses of biotite, andalusite/staurolite, sillimanite, which maps distinctly from a younger cordierite sillimanite contact aureole around an adjacent quartz diorite body. Several facies of granite within the pluton have been recognized; petrographical and structural studies allow the interpreted emplacemen~ mechanism of these units to be integrated within a more general model for the evolution of the shear zone. Geochemical analyses of major and trace elements show that certain compositional trends within these facies cannot be simply related as part of a fractionation sequence. A model is put forward in which repeated melting of a heterogeneous source is followed by sequential emplacement of discrete batches of magma as sheets and wedges wi thin the acti ve shear zone. A Rb-Sr whole rock isochron age of 324 Ma was obtained and this dates not only the emplacement age of the syn-tectonic granite, but also constrains the time of movement along the shear zone. Radiogenic and stable isotope data strongly point to the local high grade Beira Schists as being sui table source rocks for generation of magmas wi th marked S-type characteristics which now form the Serra da Freita pluton. 180 values for the granites of 10.64 ± 0.24 - 13.00 ± 0.12 overlap those of the schists which lie in the range 12.38 ± 0.24 - 14.15 ± 0.4. The whole rock Rb-Sr isochron for the granite has an initial ratio of 0.7136 ± 0.0008 (MSWD = 3.2). A regional and tectonothennal model is put forward in which end-Palaeozoic oblique strike slip collision took place in the Ibero-Armorican Arc. The resulting peturbation in continental heat flow, coupled with the possible effects of shear heating, fluid concentration and local high ductility contrasts in the heterogeneous metasediments, are invoked as being responsible for causing anatexis of the Beira Schists at a depth of 10-12 km, and the generation of granitic melts. Emplacement of these bodies gave rise to the Porto-Viseu metamorphic belt, into which later smaller higher-level melts were injected. It is argued that some of these later magmas which reached higher levels are now exposed as the constituent facies of the Serra da Freita pluton. The Serra cia Freita shear zone, active throughout metamorphism, anatexis and magma emplacement was a dominant feature of the geological history of the region