1,721,038 research outputs found
Mantle domains interlayered at crustal depth: evidence provided by peridotite xenoliths from Cofrentes, Iberian peninsula
No full textThe Iberian Peninsula is characterized by the outcrops of significant alpine-type pridotite massifs both in the Betic and the Pyrenean orogenic belts, ophiolitic peridote occurrences as well as by several volcanic centres entraining mantle xenoliths (Bodinier and Godard, 2014, Ancochea and Nixon, 1987), allowing a multiple outlook of deep mantle processes occurring in the region. Mantle xenoliths, such as those from the volcanic districts of Calatrava (central Spain), Olot (south-east Spain) and from the Betic locality of Tallante were extensively investigated by various petrological studies, whereas those of the volcano of Cofrentes have been reported, but so far never investigated in detail. They represent a mantle section located between the Betic and Pyrenean belts, in a sector faulted and deformed at least since the Mesozoic (Villaseñor et al., 2018). The study of these xenoliths is therefore very important to complete the lithospheric mapping of the Iberian region. Xenoliths from Cofrentes are extremely fresh, but rarely exceed
1-2 cm in size. Thin section observation, coupled with in situ-major and trace element analyses, show that they are spinel-bearing peridotite characterized by protogranular textures. Using an iterative method that combines the T-P results calculated with various geothermometers and geobarometers, calibrated for ultramafic systems, we obtained that all samples record equilibration temperatures in the range of 610 to 930 °C, and pressure from
13 to 7 Kbar. The xenoliths from Cofrentes, if compared with mantle xenoliths from other Iberian localities (and more in general with xenoliths from other volcanic districts of the circum-Mediterranean area studied by our research group) display the lowest equilibration temperatures, thus suggesting a provenance by a mantle domain equilibrated at relatively shallow conditions, as confirmed by the pressure values. Therefore, it appears that the
MOHO beneath Cofrentes is shallower than beneath other volcanic districts, and/or that significant slivers of mantle rocks penetrated at crustal level, a process that has to be explained within the geodynamic evolution of the Iberian margin that was affected by multiple extensional and compressional phases.
REFERENCES
Bodinier, J.L., Godard, M., (2014): Orogenic, Ophiolitic, and Abyssal Peridotites. In: The Mantle and Core. Treatise on Geochemistry. Carlson, R.W. (ed.), Elsevier, Amsterdam. 3 p. 103–167
Ancochea, E, Nixon, P., (1987). Xenoliths in the Iberian Peninsula. In: Nixon P.H, ed. Mantle Xenoliths. New York: John Wiley & Sons
Villaseñor, A., Chevrot S., Harnafi M., et al. (2015): Subduction and volcanism in the Iberia–North Africa collision zone from tomographic images of the upper mantle. Tectonophysics 663, 238-249
Aluminium distribution in an Earth's non–primitive lower mantle
No full textThe aluminium incorporation mechanism of perovskite was explored by means of quantum mechanics in combination with equilibrium/off-equilibrium thermodynamics under the pressure-temperature conditions of the Earth's lower mantle (from 24 to 80 GPa). Earth's lower mantle was modelled as a geochemically non-primitive object because of an enrichment by 3 wt% of recycled crustal material (MORB component). The compositional modelling takes into account both chondrite and pyrolite reference models. The capacity of perovskite to host Al was modelled through an Al2O3 exchange process in an unconstrained Mg-perovskite + Mg-Al-perovskite + free-Al2O3(corundum) system. Aluminium is globally incorporated principally via an increase in the amount of Al bearing perovskite [Mg-Al-pv(80 GPa)/Mg-Al-pv(24 GPa) ≈ 1.17], rather than by an increase in the Al2O3 content of the average chemical composition which changes little (0.11–0.13, mole fraction of Al2O3) and tends to decrease in Al. The Al2O3 distribution in the lower mantle was described through the probability of the occurrence of given compositions of Al bearing perovskite. The probability of finding Mg-Al-perovskite is comparable to Mg-perovskites. Perovskite with Al2O3 mole fraction up to 0.15 has an occurrence probability of ∼28% at 24 GPa, increasing up to ∼43% at 80 GPa; on the contrary, perovskite compositions in the range 0.19–0.30 Al2O3 mole fraction drop their occurrence probability from 9.8 to 2.0%, over the same P-range. In light of this, the distribution of Al in the lower mantle shows that, among the possible Al bearing perovskite phases, the (Mg0.89Al0.11)(Si0.89Al0.11)O3 composition is the likeliest, providing from 5 to 8% of the bulk perovskite in the pressure range from 24 to 80 GPa. The occurrence of the most Al rich composition, i.e. (Mg0.71Al0.29)(Si0.71Al0.29)O3, is a rare event (probability of occurrence < 1.7%). This study predicts that perovskite may globally host Al2O3 in terms of 4.3 and 4.8 wt% (with respect to the non-primitive lower mantle mass), thus accounting for ∼90% and 100% of the bulk Al2O3 estimated in the framework of pyrolite and chondrite reference models, respectively. A calcium-ferrite type phase (on the MgAl2O4-NaAlSiO4 join) seems to be the only candidate that can compensate for the 10% gap of the perovskite Al incorporation capacity, in the case of a pyrolite non-primitive lower mantle model
A TEACHING INTERVENTION WITH DYNAMIC INTERACTIVE MEDIATORS TO FOSTER AN ALGEBRAIC DISCOURSE
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Outdoor science experiments, hands-on learning in nature: the Nirano Mud Volcano (NMV, Fiorano Modenese, Italy) open-field laboratory
No full textMud volcanoes (MVs) are part of the “sedimentary volcanism” due to their morphological resemblance
to igneous volcanoes and are characterized by the uplift of sediments and fluids (Martinelli & Judd, 2004;
Mazzini & Etiope, 2017). They are widely diffused and, although not always as fascinating as “ordinary”
volcanoes, no less investigated by various disciplines, e.g. botanics, microbiology, geophysics, geomorphology,
geochemistry and structural geology. In particular, they are taken into account for hydrocarbon prospection,
mainly gas, since they are mostly located in petroliferous basins and constitute the second natural source of
CH4 (Sciarra et al., 2019) and as earthquake precursors (Martinelli & Judd, 2004).
In Italy, MVs occur in both Northern and Central Apennine and in Sicily (Martinelli & Judd, 2004; Sciarra
et al., 2019). Nirano mud volcano (NMV) of the Emilia Romagna region is one of the biggest in Italy, situated
close to a small anticline in the outcrop of the Plio-Pleistocene “Argille Azzurre” clays (Martinelli & Judd,
2004), and is widely investigated for the gas composition (Mazzini & Etiope, 2017). Thousands of people of all
ages and levels of education visit this magnificent area every year. Master geology students from the University
of Ferrara, the course “geochemical prospecting”, have been putting into practice the theoretical geochemical
skills of measuring in-situ temperature, pH, and electrical conductivity, as well as collecting water and mud
samples for analyses (X-Ray Fluorescence, XRF; Inductively Coupled Plasma Mass Spectrometry, ICP-MS;
Ion Chromatography, IC; Elemental Analyzer for coupling to Isotope Ratio Mass Spectrometers, EA-IRMS)
in the laboratory.
The results of their curiosity and abilities focused on the geochemical composition of NMV mud and
water, as shown during the second Italian Geochemistry Society congress. The aim of this work is also to
raise and increase awareness of peculiar geochemical threats in the national territory at many levels using the
NMV area as an open-field laboratory, which is suitable for everyone interested in improving their geological
knowledge and understanding and the processes occurring in the Earth crust (fault formation, fluid circulation,
and earthquake indicators). In particular, this outdoor experience fits very well with exploring teens, who can
make first-hand observations, mud, water and gas sampling (be prepared to get hands dirty!) and experience
simple analyses on the field, such as pH, EC, and water salinity measurement throughout specific probes.
Martinelli G. & Judd A. (2004) - Mud volcanoes of Italy. Geol. J., 39, 49-61, https://doi.org/10.1002/gj.943.
Mazzini A. & Etiope G. (2017) - Mud volcanism: An updated review. Earth-Sci. Rev., 168, 81-112, https://doi.
org/10.1016/j.earscirev.2017.03.001.
Sciarra A. et al. (2019) - Geochemical characterization of the Nirano mud volcano, Italy. Appl. Geochem., 102, 77-87,
https://doi.org/10.1016/j.apgeochem.2019.01.006
Mass balance vs Rietveld refinement to determine the modal composition of ultramafic rocks: The case study of mantle peridotites from Northern Victoria Land (Antarctica)
No full textThis paper aims at applying the Rietveld refinement method to X-ray powder diffraction (XRPD) spectra in order to determine the mineralogical assemblages of ultramafic rocks. The results are compared to those obtained by mass balance (MB) calculations, a least squares method that reduces major element chemical analyses to the whole rock composition. This methodological work was carried out in five hydrous and anhydrous mantle xenoliths from Northern Victoria Land, Antarctica.The intrinsic goodness of the XRPD and MB results, evaluated by with Rwp and r2 respectively, shows similar values of modal compositions. Independent of the mineralogy (amphibole-bearing and amphibole-free) and textures (fine to coarse grained), good agreement (<2%) is observed for modal contents lower than 10%, whereas a discrepancy of up to 10% is recorded for phase abundances higher than 70%. The comparison of the two methods has allowed us to demonstrate that the Rietveld method is applicable even when limited amount of material (<1g) is available, and the "classical" chemical approaches (XRF and EMPA) cannot be applied. Moreover it not only provides information on rock mineral concentrations (wt.%), but also gives supplementary information on crystallographic data (i.e., mineral crystallite size, mineral lattice parameters, density, site occupancies).On the other hand, the MB procedure provides direct chemical information of both bulk rock and minerals, but requires a large amount of material (i.e., for XRF at least >. 1. g of material is needed). Dealing with MB procedure, problems can occur if accessory minerals cannot be identified in thin sections, which are, however, part of the whole-rock analysis. Besides, the strength of XRPD, if compared with all the suitable methods for the quantitative mineralogical identification, is to provide direct information about the physical properties, and mineral site occupancies that could indirectly give a mineral chemical composition. © 2015 Elsevier B.V
Geochemical and petrological study of eastern limb of Rustenburg Layered Suit (Bushveld mafic complex)
No full textSouth Africa’s Bushveld Complex is the most significant and important example of layered mafic complex in the world. It is an iconic geological site, where many generation of geologists were ventured to understand the phenomenology of intrusive magma crystallization process. The interst in Bushveld Compelex expanded
since the discovery (Merensky, 1925) of the world’s large reserve of platinum (and platinum elements group). The Complex comprises four exposed sectors - the eastern limb, the western limb, the far western limb and the northern limb, with a fifth limb, the southeastern Bethal limb, obscured by younger sediments. These sectors are formed by mafic-ultramafic layered suite at the base, a granite suite and a sequence of heterogeneous predominantly felsic volcanic rocks of the Rooiberg Group. Both extrusive and intrusive Bushveld magmatism occurred with a time span of a few million years around 2057±3 (Mungall et al., 2016). The majority of the
ore deposits are, however, restricted to the intervening group of ultramafic-mafic rocks, or Rustenburg Layered Suite. The Rustenburg layered Suite comprises a package of rocks which range in composition from dunite to diorite. This layered suite is subdivided into marginal, lower (LZ), critical (CZ), main (MZ) and upper (UZ)
zones, although their exact boundaries have been the subject of much debate (e.g. Kruger, 2005). Despite the countless published papers since the Twenties of the previous century, there is no consensus yet on the details of its mode of formation. However, it is generally assumed that the layered rocks represent an upward-aggrading pile of crystals deposited on the floor of a vast, long-lived and repeatedly replenished magma chamber (e.g. Mungall et al., 2016). In 1998 the field excursion poposed by the programme of four-yearly International Vocanic Congress (IAVCEI), held in Cape Town (SA), was focused on the eastern limb of the Rustenburg Layered Suite. Petrologists of the Unviersity of Ferrara (Luigi Beccaluva and Franca Siena) attended the field excursion and collected 31 samples representing all the main rock types of each zone (dunites, orthpyroxenites, pyroxenites, anorthosites). This rock collection remained unworked for many years and just recentely were resumed by
the authors of this contribution. Here, the initial stage of a geochemical and petrological study of eastern limbof Rustenburg Layered Suite is presented. On the basis of preliminary bulk geochemistry (major and trace elements) and mineral modal distribution, the majority of the samples are cumulates: dunites, orthopyroxenites,
peridotites norite, anorthosites, gabbros; a few metasedimentary rocks of the Transvaal basement are also included. The general order of appearance (and disappearce) of cumulus minerals suggests a multiple a crystallization processes in an open melt-filled chamber.
Kruger F.J. (2005) - Filling the Bushveld Complex magma chamber: lateral expansion, roof and floor interaction, magmatic unconformities, and the formation of giant chromitite, PGE and Ti-V-magnetitite deposits. Miner. Deposita,
40, 451-472.
Merensky H. (1925) - How we discovered platinum. Mining Ind. Mag. South. Africa, 1, 265-266.
Mungall J.E. et al. (2016) - U-Pb geochronology documents out-of-sequence emplacement of ultramafic layers in the
Bushveld Igneous Complex of South Africa. Nat. Commun., 7, 13385
Can a simple lherzolitic mantle source explain the geochemical variation of Etnean magmas through time?
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