Search results for " petrology"
showing 10 items of 1353 documents
A Reappraisal of Redox Melting in the Earth’s Mantle as a Function of Tectonic Setting and Time
2010
Redox melting refers to any process by which melt is generated by the contact of a rock with a fluid or melt with a contrasting oxidation state. It was originally applied to melting owing to the oxidation of reduced CH4and H2-bearing fluids in contact with more oxidized blocks in the mantle, particularly recycled crustal blocks.This oxidation mechanism causes an increase in the activity of H2O by the reaction of CH4 with O2, and the increased aH2O causes a rapid drop in the solidus temperature, and is here termed hydrous redox melting (HRM). Recently, a second redox melting mechanism (carbonate redox melting; CRM) has been discovered that operates in more oxidized conditions, and may post-d…
The fate of subducted oceanic slabs in the shallow mantle: Insights from boron isotopes and light element composition of metasomatized blueschists fr…
2012
Abstract Serpentine muds from South Chamorro Seamount (SCS), drilled during ODP Leg 195 at Site 1200 contain metamafic clasts that experienced blueschist-facies metamorphism (including the critical mineral assemblage pumpellyite – Na-amphibole – epidote). These schists represent fragments from the actual slab–mantle interface at ~ 27 km depth. Their heterogeneous lithology with a metasomatic character indicates significant mobility of major elements in the Mariana forearc, a region of melange formation as it can also be observed in onland exposures such as the Catalina Schist. As the Mariana forearc blueschists show no late stage alteration they permit the direct study of material transfer …
Stable Isotope Transport and Contact Metamorphic Fluid Flow
2001
Stable isotopes are a powerful tool for deciphering the fluid histories of metamorphic terranes. The nature of fluid flow, fluid sources, and fluid fluxes can be delineated in well-constrained studies. Observed isotopic gradients in metamorphic rocks and minerals can thus shed light on many processes involved in mass-transport including diffusion, recrystallization, fluid infiltration, volatilization, metasomatism, and heat flow. Modeling of fluid flow and mineral exchange kinetics offers greatly enhanced understanding of metamorphic processes that can be tested and refined by application of new micro-analytical techniques. This review will concentrate on the principles of stable isotope fl…
Petrogenesis of tourmaline rocks associated with Fe-carbonate-graphite metapelite, metabasite and strata-bound polymetallic sulphide mineralisation, …
2007
Abstract Tourmalinite and tourmaline-rich rocks associated with Fe-carbonate–graphite phyllite, strata-bound polymetallic sulphide deposits, metabasite and marble were studied, for information on the mechanism of tourmaline formation in the pre-Hercynian low-grade metamorphic sequence of the Mandanici Unit in the Peloritani Mountains of Sicily, southern Italy. The major and trace element compositions of the tourmaline rocks suggest the existence of a sedimentary protolith with pre-metamorphic black shale and bedded chert. Boron was interpreted to be accumulated in a restricted sedimentary basin, between platform carbonate formations, with abundant organic matter and Fe–Al–Ti-rich laterite–b…
Bader's analysis of the electron density in the Pbca enstatite - Pbcn protoenstatite phase transition
2011
The Bader’s topological analysis of the electron density obtained via ab initio quantum mechanical simulation at Hartree–Fock and DFT level has been performed for experimentally in situ heated structures of enstatite–protoenstatite MgSiO 3 . The measurements have been performed in the temperature range 1200–1400 K. The work was aimed at characterizing both the evolution of the electron arrangement in the crystal and that of the crystal-structure at the enstatite–protoenstatite phase transition in terms of topology of the electron density, with particular care about the Mg–O and the O–O bonds. The observed breaking of some chemical bonds with increasing temperature and/or due to the phase tr…
Structural modulations and phase transitions in β-eucryptite: an in-situ TEM study
1999
Beta-eucryptite as grown by the flux method has been investigated by in-situ cold- and hot-stage transmission electron microscopy (TEM). Using electron diffraction and dark-field TEM imaging, we found no evidence for the merohedral twinning that accompanies structural collapse at the β-to-α-quartz transition, suggesting a true hexagonal symmetry for the aluminosilicate framework of β-eucryptite. Selected-area electron diffraction (SAED) patterns exhibited a variety of incommensurate structures along the three a axes with an average modulation period of about 6.5 a. These modulated structures arise from superperiodic stacking parallel to (100) of two structural units with different Li config…
Potassic dyke swarm in the Sapucai Graben, Eastern Paraguay: Petrographical, mineralogical and geochemical outlines.
1992
Abstract The western side of the Parana Basin of Brazil extends to central Paraguay, where repeated and widespread magmatic activity developed from Lower Cretaceous to Oligocene, associated with late Mesozoic crustal extension trending NE-SW. In central Paraguay this trend is characterized by a zone of NW-SE normal faults which formed the Asuncion-Sapucai graben, up to 45 km wide and 200 km long, where alkaline rocks occur as volcanic domes, complexes, lava-flows and dykes. These rocks, 128 Ma aged, are dominantly potassic and ne-normative. A swarm of at least 200, mainley NW-SE trending, dykes occurs in the Sapucai region and seems to be formed by two main lineages: tephrite to phonolite (…
Gradients in physical parameters in zoned felsic magma bodies: Implications for evolution and eruptive withdrawal
1990
Abstract Five diverse, well documented, chemically zoned magmas have been chosen from the literature to demonstrate the extent and patterns of density and viscosity gradients in zoned magma chambers. The patterns are used to assess implications for development of zonation, and withdrawal dynamics and preservation of systematic chemical variations in the final pyroclastic deposit. These examples are: Bishop Tuff, California (high-silica rhyolite); Los Humeros, Mexico (calc-alkaline rhyolite to andesite); Fogo A, Azores (trachyte); Laacher See, Eifel (phonolite) and Tenerife, Canary Islands (phonolite). It was necessary to make several simplifying assumptions in order to calculate viscosity a…
Luminescence study of defects in synthetic as-grown and HPHT diamonds compared to natural diamonds
2005
The optically active defects in as-grown, high-pressure high-temperature-treated (HPHT), boron-doped, and synthetic diamonds (SD) grown with a nitrogen-getter, as well as of natural diamonds (ND), were characterized by absorption and luminescence spectroscopies using different excitation sources. The laser-excited photoluminescence (PL) spectra of SDs show numerous sharp lines characteristic for nickel-related centers, whereas NDs yield mainly broad PL bands. The emission from the nickel-related defects in NIR range increases and the maxima of the bands shift to lower energies with increasing temperature. Under UV and electron beam excitation, the yellow synthetic diamonds display green lum…
Production altitude and time delays of the terrestrial gamma flashes: Revisiting the Burst and Transient Source Experiment spectra
2008
[1] On the basis of the RHESSI results it has been suggested that terrestrial gamma flashes (TGFs) are produced at very low altitudes. On the other hand some of the Burst and Transient Source Experiment (BATSE) spectra show unabsorbed fluxes of X rays in the 25–50 keV energy range, indicating a higher production altitude. To investigate this, we have developed a Monte Carlo code for X-ray propagation through the atmosphere. The most important features seen in the modeled spectra are (1) a low-energy cutoff which moves to lower energies as TGFs are produced at higher altitudes, (2) a high-energy cutoff which moves to lower energies as TGFs are observed at larger zenith angles, and (3) time d…