273 research outputs found
Experimental constraints on pre-eruption conditions of the 1631 Vesuvius eruption
No full textWe established the phase equilibria of a representative tephriphonolitic sample of the 1631 eruption of Vesuvius (Italy). Experiments were conducted at 100 MPa, in the temperature range 950–1050 °C for melt water content ranging from 1.3 to 3.2 wt%, and at an oxygen fugacity (fO2) of NNO+1 to NNO+3 (one to three log unit above the fO2 of the Ni-NiO solid redox buffer). Results show that clinopyroxene, biotite and leucite dominate the crystallizing phase assemblage, with minor proportions of plagioclase and amphibole, in agreement with the petrological attributes of the tephra. Comparison between the phase proportions and compositions obtained in experiments and those observed in the rock indicates a pre-eruptive temperature of 950 ± 30° and a melt water content H2Omelt = 2.3 ± 0.3 wt%, for an oxygen fugacity around NNO+1. These T-H2O estimates are confirmed by empirical geothermometers based on experimental clinopyroxene and melt compositional trends. As for other Vesuvius eruptions, the most felsic part of the 1631 reservoir appears to have reached pre-eruptive leucite saturation, although a large amount of leucite microcrystals in the studied samples likely grew syn-eruptively. Our results confirm that magma storage conditions beneath Vesuvius became hotter, shallower, and more CO2-rich after the AD 79 Pompeii Plinian event
Geochemical and petrological characterization of two large silicic ignimbrites from the Main Ethiopian Rift (Ethiopia)
Get PDFRhyolites as the most abundant and scarce or absent intermediate terms (i.e., Daly gap). The mafic products are associated with cinder cones and lava flows while the felsic magmas produced large explosive eruptions often resulting in the formation of large calderas. This work focuses on these highly evolved compositions, the genesis of which still represents a matter of debate. We conducted analyses on two Central MER large silicic units with similar age and petrological characteristics: the 1.16 Ma Golja ignimbrite (GI) and the 1.3 Ma Kencherra ignimbrite (KI). The two units are remarkably crystal-poor with less than 10% of K-feld, qtz, pl, cpx, aen and Fe-Ti oxides and contain different types of juvenile material including white and banded pumices and fiamme as well as dark scoria.
In GI detailed analysis of matrix glass and melt inclusions from all juvenile types reveal a broad compositional spectrum ranging from basalts (found only in the pl-hosted melt inclusions) to rhyolites, with intermediate compositions (basaltic trachyandesites to trachydacites) present in the mingled pumices and dark scoria. Trace element plots show distinct evolutionary trends, not visible in major element compositions, while in-situ 87Sr/86Sr analyses of feldspars display wide isotopic variation ranging from mantle-like to crustal values, akin to the Pan African crust.
In the KI welded pyroclastic sequence, matrix glass and fiamme have rhyolitic composition while scoriaceous clasts, increasing in size and abundance up-sequence, range between trachybasalts and trachytes. Two different groups of rhyolites can be distinguished based on incompatible trace elements and REE concentrations. Overall, geochemical and isotopic data suggest involvement of fractional crystallization, assimilation of old crustal material and magma mixing in petrogenesis of the intermediate and evolved composition from KI and GI. Our data represent the first geochemical characterization of these large ignimbrites and contribute to a better understanding of silicic magmatism in the MER
Volcanological and petrological characterization of the Golja Ignimbrite (Main Ethiopian rift)
Get PDFThe Main Ethiopian Rift (MER) is an ideal natural laboratory to study magmatism related to rifting. The MER is an active magmatic rift that records variations in rift evolution from mature rifting in the north to less evolved rifting southwards. The Plio-Pleistocene volcanism in the MER is mainly characterized by eruptions of mafic products (i.e., transitional basalts) associated with cinder cones and lava flows, alternating with ignimbritic eruptions emplacing large volume of felsic products (i.e., peralkaline rhyolitic and trachytic pyroclastics) with the formation of large calderas. After widespread Miocene-Pliocene volcanism, the Quaternary magmatic activity became mostly localized in the MER axis, with products showing a typical bimodal composition (i.e., dominantly basaltic and rhyolitic composition) with a notable compositional gap, also known as the Daly gap, which remains a poorly understood aspect of the rift-related magmatism.
In this context, we present the first volcanological and petrological characterization of the Golja Ignimbrite, a crystal-poor (<5% crystals of qtz+K-feld+pl+cpx+aen), low aspect ratio ignimbrite sourced from the MER floor, that crops out over ~400 km2 and has an estimated bulk-tephra volume of ~100 km3. The Golja Ignimbrite pyroclastic sequence is characterized from bottom to top by: 1) a coarsening upward basal fallout layer; 2) an obsidian vitrophyre with rare, scattered fiammae; 3) a weakly to partially welded, lithic-rich PDC deposit and 4) a thick, unwelded PDC deposit containing white and banded pumices together with black scorias.
Both density and welding degree are relatively low at the transition between the basal fallout and the vitrophyre (1.32 ± 0.03 g /cm3) and reach their maximum in the vitrophyre (2.38 ± 0.02 g/cm3), where most of the glass shards are not distinguishable from the groundmass glass. Subsequently, density tends to decrease up-sequence (down to 1.20 ± 0.01 g/cm3 at the top), as indicated by the increasing occurrence of glass shards without a preferential orientation, glass spherules and vesicular juvenile clasts.
40Ar/39Ar dating of single K-feldspars from a white pumice and a black scoria yielded and age of 1.159 ± 0.006 Ma relative to the Fish Canyon Tuff (FCs) sanidine age 28.02 ± 0.28 Ma (Renne et al., 1998).
The ignimbrite composition plots in a transitional field between the subalkaline and peralkaline series, which is typical of magmas associated with continental rifts. Compositional differences have been observed between the various juvenile clasts with rhyolitic white pumices and, trachy-basaltic to trachy-andesitic black scorias. Mingled, streaky pumices range between trachytes and rhyolites, while truly basaltic compositions were only found in melt inclusions contained in bytownitic plagioclase crystals. The presence of intermediate compositions is an uncommon feature for MER magmas and our data support that they are likely due to two different processes: fractional crystallization and mingling between magmas with strong compositional differences (i.e, basalts and rhyolites) occurring before eruption. Such evidence places important constraints on the evolution of large magma reservoirs in this peculiar geodynamic setting
Low-P hydrous phase equilibria of a pantellerite melt: constraints on pre-eruptive conditions of recent felsic explosive volcanism at Pantelleria.
No full text
Experimental investigations on the role of sulfur in magmas
No full textSulfur (S) is the third volatile element in importance in Earth's magmas, after water and carbon dioxide. Recent work has shown that sulfur abundance (melt+gas) in typical arc magmas commonly exceeds 0.1 wt%, with maxima in the range 0.5-1 wt% [Scaillet et al., 2003]. In this presentation we will review recent advances with respect to the solubility and partitioning of S in silicate melts and give a general outline of on going, as well as next, research priorities. The solubility of S in metaluminous rhyolitic and phonolitic melts has been extensively investigated by Clemente et al. [2003] and Moncrieff et al. [in prep], respectively. Experiments have been performed at 800- 1000°C, 1-4 kb, and fO2 ranging from NNO-2 up to NNO+4. Both studies have shown that S solubility is sensitive to T, fO2 and fS2 variations, pressure control being less important. At any given fO2, the higher the fS2, the higher the S solubility. Rhyolite and phonolite melts follow the same pattern. The modeling of S solubilities has been attempted using either empirical or thermodynamical approaches. Empirical equations are available for either a given melt composition (rhyolite) or are valide over a large compositional spectrum (rhyolite-basalt). The thermodynamic modelling of S solubilities can be done considering that the total dissolved S results from the addition of H2S and SO2 species dissolution reactions, whose relative abundances depend on the prevailing fO2. This model has been calibrated only on rhyolite compositions. The S2-/S6+ proportions predicted by this model are in good agreement with experimental observations. The partition coefficient of S between melt and gas has been determined mostly in silicic magmas [Scaillet et al., 1998; Keppler, 1999], for which there appears to be a strong control of fO2. Thermodynamic calculations predict that the partition coefficient between gas and melt should decrease with melt silica content, from ca 1000 in rhyolite, to 100 in andesite, to 10 in basaltic melts [Scaillet and Pichavant, 2003]. Recent experimental work has explored the S behaviour in peralkaline rhyolites, which appears to dissolve up to 20 times more sulfur than their metaluminous counterpart [Scaillet and Macdonald, in prep]. Despite its relative low concentrations, S affects the stability of phases such as pyroxenes, amphiboles and biotite. In moderately oxidized dacite magmas, the addition of a few thousands ppm of S enhances the thermal stability of biotite by as much as 60°C [Costa et al., submitted]. In strongly oxidized dacites, the incorporation of S leads to the breakdown of horblende at the expense of orthopyroxene [Scaillet and Evans, in prep]. These effects on phase relations suggest the existence of various sulfur complexes in the silicate melt. We are currently concentrating our efforts on (1) calibrating existing solubility models on mafic compositions, (2) determining the S partitioning between melt and gas in mafic melts. We suggest that additional investigations should be devoted to (1) investigating the effects of S on silicate melt transport properties (density, diffusivity and viscosity), (2) determining the nature and proportion of S species dissolved in silicate melts so as to build more realistic thermodynamic models. References Clemente, B., Scaillet, B. and Pichavant, B. (2003). The solubility of sulphur in rhyolitic melts. Journal of Petrology, in press. Keppler, H. (1999). Experimental evidence for the source of excess sulfur in explosive volcanic eruptions. Science, 284, 1652-1654. Scaillet, B., Clemente, B., Evans, B. and Pichavant, M. (1998). Redox control of sulfur degassing in silicic magmas. Journal of Geophysical Research, 103, 23937-23949. Scaillet, B. and Pichavant, M. (2003). Experimental constraints on volatile abundances in arc magmas and their implications for degassing processes. Geol. Soc. Spec. Pub., in press. Scaillet, B., Luhr, J. and Carroll, M. (2003). Petrological and volcanological constraints on volcanic sulfur emissions to the atmosphere. In Volcanoes and the Earth atmosphere, A. Robock and C. Oppenheimer (eds.), AGU, in press
Eruptive paroxysms at Stromboli: an experimental simulation of pre-eruptive conditions of "yellow pumice"
No full text- …
