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291.
Silvio Mollo Pierdomenico Del Gaudio Guido Ventura Gianluca Iezzi Piergiorgio Scarlato 《Lithos》2010,118(3-4):302-312
The compositional variation of clinopyroxene and the partitioning of major elements between clinopyroxene and melt are estimated as a function of the cooling rate. Clinopyroxenes were crystallized under variable cooling regimes (15, 9.4, 3, 2.1, and 0.5 °C/min from 1250 down to 1000 °C) and at isothermal conditions of 1000 °C from a basaltic composition at a pressure of 500 MPa under anhydrous and hydrous (H2O = 1.3 wt.%) conditions. The clinopyroxene chemistry shows that, as the cooling rate increases, crystals are progressively depleted in Ca, Mg, Fe2+ and Si and enriched in Na, Fe3+, Al (mainly AlIV), and Ti. Di and Hd versus CaTs and CaFeTs form a continuous binary solid solution characterized by higher amounts of tschermakitic components with increasing cooling rate. Two parameters (DH = Di + Hd and TE = CaTs + CaFeTs + En) are calculated to describe the effect of cooling rate on the clinopyroxene composition. The variation of DH/TE with increasing cooling rate evidences the kinetic process induced by rapid cooling in basic rocks under hydrous and anhydrous conditions.Dynamic crystallization conditions affect the partitioning of major elements between clinopyroxene and melt; with increasing cooling rate, the value of crystal–melt partition coefficient departs from that obtained at the isothermal condition. However, in spite of these variations, the values of cpx–meltKdFe–Mg remain almost constant. Therefore, the Fe2–Mg exchange between clinopyroxene and melt is not suitable to prove the (dis)equilibrium conditions in basaltic cooling magmas, giving rise to possible mismatches in the application of thermobarometers. The results of our study are consistent with that observed at the margin of dikes or in the exterior portions of lavas, where the cooling rate is maximized and disequilibrium compositions of clinopyroxene have been found. 相似文献
292.
The long prism/needle‐shaped polycrystalline quartz aggregates and square/parallelogram‐shaped singlephase quartz inclusions in omphacite and garnet of ultrahigh pressure eclogite were first discovered from the Jiangalesayi area, South Altyn UHP belt. Based on their morphology, these quartz inclusions are quartz paramorphs after stishovite. The minimum peak pressure of the eclogite is estimated to be >8–9 GPa at 800– 1000 °C based on the stability field of stishovite. This new evidence, together with previous stishovite exsolution microstructure in the gneiss from the same region, suggests an ultra‐deep subduction and exhumation of the South Altyn continental rocks to/from mantle depths in stishovite stability field. Evidence of ultra‐deep subduction of continental materials might be more common and diverse than previous thought. Exhumation of subducted continental rocks from≥300 km has been considered impossible because they are denser than mantle at these depths. How did the stishovite bearing continental rocks of the South Altyn exhumated? As we all know, the densities of stishovite (4.3 g/cm3) are much higher than coesite (2.9 g/cm3), and stishovite transforms into coesite with temperature increases. Density calculations were performed for subducted continental rocks along phase transition of stishovite to coesite, using the third‐order Birch‐Murnaghan equation of state based on mineral fractions obtained from experiments and Perple_X. The results show that the density of Siliceous rocks decrease remarkably, lower than the surrounding mantle in coesite stability field, whereas the density of Oligosiliceous and Silicon unsaturated rocks is higher than surrounding mantle. Thus, we propose that the thermal induced transformation could provide an initial driven force for the exhumation of ultra‐deep subducted silica‐enriched felsic continental rocks. Temperature increase could be derived from an increased geothermal gradient from convective mantle or mantle plume. Mafic to ultra‐mafic rocks and silica‐deficient rocks may be captured by the upwelling subducted continental rocks and exhumated together. 相似文献
293.
294.
Guido Baeten Jan Willem de Maag René‐Edouard Plessix Rini Klaassen Tahira Qureshi Maren Kleemeyer Fons ten Kroode Zhang Rujie 《Geophysical Prospecting》2013,61(4):701-711
Velocity model building and impedance inversion generally suffer from a lack of intermediate wavenumber content in seismic data. Intermediate wavenumbers may be retrieved directly from seismic data sets if enough low frequencies are recorded. Over the past years, improvements in acquisition have allowed us to obtain seismic data with a broader frequency spectrum. To illustrate the benefits of broadband acquisition, notably the recording of low frequencies, we discuss the inversion of land seismic data acquired in Inner Mongolia, China. This data set contains frequencies from 1.5–80 Hz. We show that the velocity estimate based on an acoustic full‐waveform inversion approach is superior to one obtained from reflection traveltime inversion because after full‐waveform inversion the background velocity conforms to geology. We also illustrate the added value of low frequencies in an impedance estimate. 相似文献
295.
The Calcium Isotope Composition of Modern Seawater Determined by Thermal Ionisation Mass Spectrometry 总被引:1,自引:0,他引:1
Anne-Désirée Schmitt Guido Bracke Peter Stille Bernard Kiefel 《Geostandards and Geoanalytical Research》2001,25(2-3):267-275
The Ca isotopic composition of modern seawater has been determined using a 43 Ca-48 Ca double spike, which was calibrated using a 42 Ca/44 Ca seawater ratio of 0.30587 ± 0.00026. This ratio was determined from a total evaporation experiment in which the ion beam was measured from the beginning to the end of the emission. With integration of the peak intensities, the fractionation effects can be minimised, since total evaporation of the reservoir cancels out the effect of vapour enrichment in the light isotopes. This experiment avoids the gravimetric uncertainty inherent in the double spike calibration. This calibration allows the precise redetermination of the seawater isotopic composition of Ca. A mean 40 Ca/44 Ca ratio for two Atlantic water samples of 45.143 (2smean = 0.003) was found. The good reproducibility of the Ca isotope ratios in present seawater and the very strong isotopic homogeneity of Ca in the oceans illustrate the advantage of using seawater as the common standard, with the advantage of decreasing interlaboratory bias. 相似文献
296.
Sebastián M. Jovic Diego M. Guido Isidoro B. Schalamuk Francisco J. Ríos Colombo C. G. Tassinari Clemente Recio 《Mineralium Deposita》2011,46(3):257-271
The Pingüino deposit, located in the low sulfidation epithermal metallogenetical province of the Deseado Massif, Patagonia,
Argentina, represents a distinct deposit type in the region. It evolved through two different mineralization events: an early
In-bearing polymetallic event that introduced In, Zn, Pb, Ag, Cd, Au, As, Cu, Sn, W and Bi represented by complex sulfide
mineralogy, and a late Ag–Au quartz-rich vein type that crosscut and overprints the early polymetallic mineralization. The
indium-bearing polymetallic mineralization developed in three stages: an early Cu–Au–In–As–Sn–W–Bi stage (Ps1), a Zn–Pb–Ag–In–Cd–Sb stage (Ps2) and a late Zn–In–Cd (Ps3). Indium concentrations in the polymetallic veins show a wide range (3.4 to 1,184 ppm In). The highest indium values (up
to 1,184 ppm) relate to the Ps2 mineralization stage, and are associated with Fe-rich sphalerites, although significant In enrichment (up to 159 ppm) is
also present in the Ps1 paragenesis associated with Sn-minerals (ferrokesterite and cassiterite). The hydrothermal alteration associated with the
polymetallic mineralization is characterized by advanced argillic alteration within the immediate vein zone, and sericitic
alteration enveloping the vein zone. Fluid inclusion studies indicate homogenisation temperatures of 308.2–327°C for Ps1 and 255–312.4°C for Ps2, and low to moderate salinities (2 to 5 eq.wt.% NaCl and 4 to 9 eq.wt.% NaCl, respectively). δ34S values of sulfide minerals (+0.76‰ to +3.61‰) indicate a possible magmatic source for the sulfur in the polymetallic mineralization
while Pb isotope ratios for the sulfides and magmatic rocks (206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios of 17.379 to 18.502; 15.588 to 15.730 and 38.234 to 38.756, respectively) are consistent with the possibility that
the Pb reservoirs for both had the same crustal source. Spatial relationships, hydrothermal alteration styles, S and Pb isotopic
data suggest a probable genetic relation between the polymetallic mineralization and dioritic intrusions that could have been
the source of metals and hydrothermal fluids. Mineralization paragenesis, alteration mineralogy, geochemical signatures, fluid
inclusion data and isotopic data, confirm that the In-bearing polymetallic mineralization from Pingüino deposit is a distinct
type, in comparison with the well-known epithermal low sulfidation mineralization from the Deseado Massif. 相似文献
297.
Simone Bernardini Fabio Bellatreccia Giancarlo Della Ventura Armida Sodo 《Geostandards and Geoanalytical Research》2021,45(1):223-244
Manganese oxides are important geomaterials, widespread in terrestrial and Martian environments. Characterisation of the oxidation state of Mn is a central issue in science; this task has been addressed up to the present by X‐ray spectroscopy or diffraction techniques. The former, however, requires access to synchrotron facilities, while the latter does not provide crystal‐chemical information at the local scale. In this work, we compare a large set of Raman data from well‐characterised samples, already published by the same authors of this paper or as found in the literature. We show a clear correlation between the oxidation state of Mn and the wavenumber of peculiar bands; octahedrally co‐ordinated Mn2+ is recognised by a band around 530 cm?1, Mn3+ by a band around 580 cm?1 and Mn4+ by a band around 630 cm?1, while tetrahedrally co‐ordinated Mn2+ is recognisable by a band around 650 cm?1. Strongly distorted Mn3+ octahedra are indicated by the appearance of Jahn–Teller modes. Our method allows a reliable, easily accessible tool to characterise the oxidation states of Mn in oxides, also suitable for microscale mapping. It provides a robust analytical basis for the use of these minerals as redox indicators in geology/geochemistry, in exoplanetary research or for monitoring technological processes. 相似文献