224 research outputs found
Evidence for serpentinite fluid in convergent margin systems: The example of El Salvador (Central America) arc lavas
Transantarctic Mountain microtektites: New petrographic data, water content, and Nd and Sr isotopic composition
The crystalline units of the High Himalayas in the Lahul-Zanskar region (northwest India): metamorphic-tectonic history and geochronology of the collided and imbricated Indian plate
In the High Himalayan belt of northwest India, crustal thickening linked to Palaeogene collision between India and Eurasia has led to the formation of two main crystalline tectonic units separated by the syn-metamorphic Miyar Thrust: the High Himalayan Crystallines sensu stricto (HHC) at the bottom, and the Kade Unit at the top. These units are structurally interposed between the underlying Lesser Himalaya and the very low-grade sediments of the Tibetan nappes. They consist of paragneisses, orthogneisses, minor metabasics and, chiefly in the HHC, leucogranites. The HHC registers: a polyphase metamorphism with two main stages designated as M1 and M2; a metamorphic zonation with high-temperature recrystallization and migmatization at middle structural levels and medium-temperature assemblages at upper and lower levels. In contrast, the Kade Unit underwent a low-temperature metamorphism. Rb-Sr and U-Th-Pb isotope data point to derivation of the orthogneisses from early Palaeozoic granitoids, while the leucogranites formed by anatexis of the HHC rocks and were probably emplaced during Miocene time. Most of the complicated metamorphic setting is related to polyphase tectonic stacking of the HHC with the âcooler' Kade Unit and Lesser Himalaya during the Himalayan history. However, a few inconsistencies exist for a purely Himalayan age of some Ml assemblages of the HHC. As regards the crustal-derived leucogranites, the formation of a first generation mixed with quartzo-feldspathic leucosomes was possibly linked to melt-lubricated shear zones which favoured rapid crustal displacements; at upper levels they intruded during stage M2 and the latest movements along the syn-metamorphic Miyar Thrust, but before juxtaposition of the Tibetan nappes along the late- metamorphic Zanskar Faul
Boron isotopic composition of olivine-hosted melt inclusions from Gorgona komatiites, Colombia : new evidence supporting wet komatiite origin
Author Posting. © The Author(s), 2011. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Earth and Planetary Science Letters 312 (2011): 201â212, doi:10.1016/j.epsl.2011.09.033.A fundamental question in the genesis of komatiites is whether 30 these rocks originate
from partial melting of dry and hot mantle, 400â500°C hotter than typical sources of MORB and
OIB magmas, or if they were produced by hydrous melting of the source at much lower
temperatures, similar or only moderately higher than those known today. Gorgona Island,
Colombia, is a unique place where Phanerozoic komatiites occur and whose origin is directly
connected to the formation of the Caribbean Large Igneous Province. The genesis of Gorgona
komatiites remains controversial, mostly because of the uncertain origin of volatile components
which they appear to contain. These volatiles could equally result from shallow level magma
contamination, melting of a âdampâ mantle or fluid-induced partial melting of the source due to
devolatilization of the ancient subducting plate. We have analyzed boron isotopes of olivine40
hosted melt inclusions from the Gorgona komatiites. These inclusions are characterized by
relatively high contents of volatile components and boron (0.2â1.0 wt.% H2O, 0.05â0.08 wt.%
S, 0.02â0.03 wt.% Cl, 0.6â2.0 ÎŒg/g B), displaying positive anomalies in the overall depleted,
primitive mantle (PM) normalized trace element and REE spectra ([La/Sm]n = 0.16â0.35;
[H2O/Nb]n = 8â44; [Cl/Nb]n = 27â68; [B/Nb]n = 9-30, assuming 300 ÎŒg/g H2O, 8 ÎŒg/g Cl and
0.1 ÎŒg/g B in PM; Kamenetsky et al., 2010. Composition and temperature of komatiite melts
from Gorgona Island constrained from olivine-hosted melt inclusions. Geology 38, 1003â1006).
The inclusions range in ÎŽ11B values from â11.5 to +15.6 ± 2.2â° (1 SE), forming two distinct
trends in a ÎŽ11B vs. B-concentration diagram. Direct assimilation of seawater, seawater-derived
components, altered oceanic crust or marine sediments by ascending komatiite magma cannot
readily account for the volatile contents and B isotope variations. Alternatively, injection of <3%
of a 11B enriched fluid to the mantle source could be a plausible explanation for the ÎŽ11B range
that also may explain the H2O, Cl and B excess.Financial support
to AAG during data acquisition and manuscript preparation was provided by Northeast National
Ion Microprobe Facility (Woods Hole Oceanographic Institution, USA) and the Centre de
Recherches PĂ©trographiqueset GĂ©ochimiques (France). This research was also supported by the
Australian Research Council (Research Fellowship and Discovery grants to VSK). We
acknowledge partial support of the Alexander von Humboldt Foundation, Germany (F.W. Bessel
Award to VSK and Wolfgang Paul Award to A.V. Sobolev who provided access to the electron
microprobe at the Max Planck Institute, Mainz, Germany
Geogenic and atmospheric sources for volatile organic compounds in fumarolic emissions from Mt. Etna and Vulcano Island (Sicily, Italy)
In this paper, fluid source(s) and processes controlling the chemical composition of volatile organic compounds (VOCs) in gas discharges from Mt. Etna and Vulcano Island(Sicily, Italy) were investigated. The main composition of the Etnean and Volcano gas emissions is produced by mixing, to various degrees, of magmatic and hydrothermal components. VOCs are dominated by alkanes, alkenes and aromatics, with minor, though significant, concentrations of O-, S- and Cl(F)-substituted compounds. The main mechanism for the production of alkanes is likely related to pyrolysis of organic-matterbearing sediments that interact with the ascending magmatic fluids. Alkanes are then converted to alkene and aromatic compounds via catalytic reactions (dehydrogenation and dehydroaromatization, respectively). Nevertheless, an abiogenic origin for the light hydrocarbons cannot be ruled out. Oxidative processes of hydrocarbons at relatively high temperatures and oxidizing conditions, typical of these volcanic-hydrothermal fluids, may explain the production of alcohols, esters, aldehydes, as well as O- and S-bearing heterocycles. By comparing the concentrations of hydrochlorofluorocarbons (HCFCs) in the fumarolic discharges with respect to those of background air, it is possible to highlight that they have a geogenic origin likely due to halogenation of both methane and alkenes. Finally, chlorofluorocarbon (CFC) abundances appear to be consistent with background air, although the strong air contamination that affects the Mt. Etna fumaroles may mask a possible geogenic contribution for these compounds. On the other hand, no CFCs were detected in the Vulcano gases, which are characterized by low air contribution. Nevertheless, a geogenic source for these compounds cannot be excluded on the basis of the present data
magma mixing history and dynamics of an eruption trigger
The most violent and catastrophic volcanic eruptions on Earth have been triggered by the refilling of a felsic volcanic magma chamber by a hotter more mafic magma. Examples include Vesuvius 79 AD, Krakatau 1883, Pinatubo 1991, and Eyjafjallajokull 2010. Since the first hypothesis, plenty of evidence of magma mixing processes, in all tectonic environments, has accumulated in the literature allowing this natural process to be defined as fundamental petrological processes playing a role in triggering volcanic eruptions, and in the generation of the compositional variability of igneous rocks. Combined with petrographic, mineral chemistry and geochemical investigations, isotopic analyses on volcanic rocks have revealed compositional variations at different length scales pointing to a complex interplay of fractional crystallization, mixing/mingling and crustal contamination during the evolution of several magmatic feeding systems. But to fully understand the dynamics of mixing and mingling processes, that are impossible to observe directly, at a realistically large scale, it is necessary to resort to numerical simulations of the complex interaction dynamics between chemically different magmas
Simultaneous determination of neodymium and samarium in silicate rocks and minerals by isotope dilution inductively coupled plasma mass spectrometry: results on twenty geochemical reference samples
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