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Thesis etd-10142013-112428

Thesis type
Tesi di dottorato di ricerca
Thesis title
Detecting small-scale geochemical and petrological variations within historical and modern lavas from Mt Etna
Academic discipline
Course of study
commissario Prof. Teresa, Trua
commissario Prof. Michele, Lustrino
commissario Prof. Rocchi, Sergio
tutor Prof. D'Orazio, Massimo
  • Beryllium
  • clinopyroxene
  • geochemistry
  • historical and modern lavas
  • iron oxidation ratio
  • lava rising paths
  • Mt Etna
  • rare earth elements
  • Thallium
Graduation session start date
Although Mt Etna is one of the most studied volcanoes all over the world, it has yet some poorly understood aspects related both to the geodynamical frame and to the evolution of the volcanic activity. The significant volcanological and geochemical variations recorded since XVIIth century, particularly during the last 50 years puzzled the scientific community, but their causes are not yet clarified (Di Geronimo et al., 1978; Frazzetta and Villari, 1981; Borgia et al., 1992; Continisio et al., 1997; Hirn et al., 1997; McGuire et al., 1997; Tanguy et al., 1997; Clocchiatti et al., 1998; Gvirtzman and Nur, 1999; Doglioni et al., 2001; Schiano et al., 2001; Patanè et al., 2003; Monaco et al., 2005; Cadoux et al., 2007; Catalano et al., 2008a). The most intriguing variations in etnean activity since XVIIth century and strikingly since 1970’s are:
an increase of days of activity/ number of eruptive events and total volume of emitted products (Andronico and Lodato, 2005; Smethurst et al., 2009; Harris et al., 2011);
a significant variation in porphyritic index (P.I.: vol.%, area % phenocrysts/ total volume, area of rock/thin section) shifting from almost totally aphyric to high porphyritic lavas (P.I.= 45)
marked K, Rb, Cs and radiogenic Sr enrichments in emitted products distinctly evident since 1970’s when, from then on, etnean volcanics show a potassic affinity (Cristofolini et al., 1984; Clocchiatti et al., 1988; Armienti et al., 1989).
The aim of this work is to provide further information to decipher the complex processes related to Mt Etna magmatism by deeply investigating historical and modern lavas with contemporary techniques. The bulk of petrographical, geochemical, crystallochemical, crystallographical and historical data on sampled products make up one of the most comprehensive and homogeneous databases for etnean historical activity.
Due to the relative short interval of time investigated and the volcano nature itself the variations are usually very small, often too small to be spotted by routine analyses. To best describe them we improved the traditional analytical procedures and methods by meticulously and accurately calibrating all the instruments more than we usually did.
This work was based on an exhaustive sampling of historical and modern eruptions of Mt Etna careful choosing lavas with certain age/year of eruption. To achieve this goal, we critically revised all the available literature on Mt Etna eruption starting from the most ancient reports (aged 1st century b.C.) until nowadays. We then revised eruption ages and, with the aid of the most updated maps of Mt Etna, we planned our sampling survey. Due to the fact that Mt Etna is an highly anthropized volcano, most of the more ancient lavas are covered by buildings and cultivated areas dramatically reducing the possibility to find fresh outcrops. To overcome this we used Google Street View™ that literally permits us to remotely walk over the flows to spot the right outcrop to be sampled. This trick helps to minimize the ‘wasted’ time spent on the field searching for the good outcrop.
The 59 collected samples cover a time span from A.D. 300 to 2011. Unfortunately several eruptions (particularly the most ancient and the recent ones) were not sampled because of their uncertain age and/or because they are covered by more recent flows.
The next step was focused on finely calibrating all the instrumental apparatus (XRF, volumetry, ICP-MS, EMPA and XRD) to obtain the best precision and accuracy. This was achieved by:
analyzing our samples in a single batch,
repeatedly analyzing international/ in house standards along with the unknown samples,
improving the whole analytical procedures and methods.

We recognize three main groups of lavas on the basis of porphyritic index, crystal sizes and felsic/mafic minerals ratio (vol.):
High-porphyritic: characterized by very large phenocrysts (0.5 to 2 cm), high porphyritic index (P.I.35-40) and a high felsic/mafic minerals ratio;
Low-porphyritic: showing an aphyric/ sub-aphyric texture (P.I.<10) with millimetric/sub-millimetric phenocrysts and low felsic/mafic minerals ratio;
Intermediate group: represented by lavas showing petrographical features intermediate between Highly Porphyritic and Low-porphyritic groups (P.I. between 10 and 35) with variable felsic/mafic mineral ratio (generally high). This is the most populated group of the last 2000 years of activity.

No major changes in mineral chemistry are visible between the sampled lava; the paragenesis is composed by phenocrysts of andesitic to bytonitic plagioclase (usually the most abundant phase), augitic to diopsidic clinopyroxene, olivine (Fo82-69) and Ti-magnetite in a holocrystalline/hypocrystalline (max 5% vol. glass) matrix. More interestingly very few samples are characterized by the occurrence of sedimentary, metamorphic or igneous xenoliths and/or by the presence of biotite and/or amphibole xenocrysts.
By a volcanological and geochemical point of view, etnean historical products are commonly grouped into three time intervals (Branca et al., 2011b):

post 122 b.C. Plinian eruption – 1669 A.D. eruption  in this work: i3;
post 1669 A.D. eruption – pre 1971 A.D. eruption  i4;
1971 A.D. eruption – present  i5 (and i5bis for the A.D. 2000 – 2012 period).
As stated before, significant changes in the geochemistry of erupted products occurred during the investigated period. These changes are mainly characterized by the emission of more mafic products (shifting from basaltic trachy-andesite to trachy-basalt) together with an enrichment of K, Rb, Cs and radiogenic Sr. Thanks to our precise and accurate analyses, we documented for the first time, similar enrichment in Tl, Be, MREEs and HREEs. The entity of this anomalous enrichment are referred to the expected values extrapolated by the main evolutionary trend of etnean lavas and is in the order of +10% + 40% for K, Rb, Cs, Be, MREEs and HREEs while Tl reaches values up to +300%. These enrichments are called ‘anomalous’ because of all of these elements show major affinities with evolved magmas whereas we found them concentrated in the more recent and primitive etnean products.

Anomalously high fO2 values for etnean magmas and gases were spottily reported in literature (Sato et al., 1973) but unfortunately no further investigations were made. To best describe this feature we determined Fe2O3/FeO ratio of 39 lavas using and improving the volumetric method (titration) developed by Yokoyama and Nakamura (2002) which permits to achieve analytical error better than 0.3% (1σ). We found that Fe2O3/FeO ratios for etnean samples markedly differ from the World average value of 0.3 for fresh trachybasaltic rocks (Middlemost, 1989) reaching an average value of 0.60± 0.13. This higher value gives important information about the oxidation state and the water content of etnean magmas setting, moreover, tight constraints for thermodynamic models (e.g. melt-phase equilibria recalculations, phase reactions, etc.).
Clinopyroxene is one of the most important phases in etnean lavas carrying lots of information about the source and the magmatic processes affecting Mt Etna. Clinopyroxene chemistry is thoroughly investigated by over than 900 EMPA analyses on over 450 phenocrysts separated from 38 selected eruptions. Despite the geochemical variations observed in the whole rocks, only minor time-related variation were detected in clinopyroxenes, such as slight increase of Al2O3 and CaO and decrease in Na2O.
Intra-sample clinopyroxene compositional variations are very useful to decipher the magma ascent paths within Mt Etna feeding system. We used the Putirka et al. (1996; 2003) clinopyroxene-liquid geothermobarometers to estimate P and T conditions during magma ascent.

As all the exchange geothermobarometers, Putirka’s ones need that clinopyroxene and liquid have to be in equilibrium but, since clinopyroxenes in equilibrium with whole rock composition (taken as liquid composition) in natural samples usually are very few, lots of samples, geochemical data and time are necessary to find a statistically relevant number of suitable clinopyroxenes. To overcome this drawback we used the Armienti et al. (2007) algorithms that, given a bulk rock and a clinopyroxene analysis, it allows to calculate the equilibrium liquid compositions on the basis of mineral-melt distribution coefficients (K_Ds). For each of the 38 investigated eruptions we defined the P-T paths. We found that studied eruptions can be broadly classified into two distinct groups on the basis of their paths:
Linear-direct paths, characterized by a linear distribution of P and T points along a straight line (where dP/dT=k). These trends are though belonging to those eruptions where magma rises fast throughout the feeding system.
Saw tooth paths, characterized by ‘ponding’ zones where clinopyroxene crystallization mainly occurs (portions where dP/dT=0).
Resulting data markedly differs from those published by Armienti et al. (2007, 2009, 2012) where clinopyroxenes crystallizing depth are usually deeper, commonly reaching the crust-mantle boundary. Our estimated pressures/depths are markedly shallower (0 to 7 km) with respect to the Armienti’s ones; this is probably due to the fact that they used poor-quality SEM clinopyroxene analyses, in particular wrong Na2O and Fe2O3/FeO values.
Looking in detail saw tooth paths we found that magma ‘ponding’ zones are located around the limit between the Hyblean carbonates and their sedimentary covers (density contrast) (Corsaro and Pompilio, 2004a). Furthermore our depths perfectly match with a ‘seismically anomalous zone’ (hereafter SAZ) revealed by 3D- topographies extending beneath Mt Etna up to about 8- 9 km below sea level. Recent studies suggest that SAZ, or at least some portions, could represent regions of magma storage where fractionation and differentiation processes occur (Villaseñor et al., 1998; Chiarabba et al., 2000; Nicolich et al., 2000; Patanè et al., 2003; Spilliaert et al., 2006a; Schiavone and Loddo, 2007; Corsaro et al., 2013).
For 9 of these eruptions we performed single-crystal XRD analyses to describe the crystal structure of clinopyroxenes and to obtain ‘independent’ P estimates using the Nimis’ geobarometer. Crystallographic data shows an increase over the time of Vcell and VT and a decrease in VM1 that could be accounted to changes in the depth of crystallization (Nimis and Ulmer, 1998).
The discovery of Be, MREEs and HREEs anomalies (Tl strictly correlate with K) give important hints on the cause of the anomalous enrichment affecting etnean magmas. The whole geochemical observations strengthen the hypothesis that these enrichments were inherited from the magma source and not by shallow crustal contamination processes (Clocchiatti et al., 1988). The source of etnean magmas may have suffered metasomatism by H2O-rich slab-released fluids carrying lots of elements to the mantle wedge (Schiano et al., 2001; Tonarini et al., 2001).
In fact, altered oceanic crust is usually enriched in Fe, K, H2O, Mn, Cl, B, Li, Rb, Cs, (Tl, REEs, etc.) (Pearce and Cann, 1973; Thompson, 1973; Hart et al., 1974; Humphris and Thompson, 1978; Staudigel and Hart, 1983; Jochum and Verma, 1996; Kelley et al., 2003; John et al., 2004; Spandler et al., 2004; Kessel et al., 2005; Kelley and Cottrell, 2009) while hydrothermally metasomatized sediments are normally enriched in phengitic-mica (and other phases such as allanite, monazite, garnet, etc.) that is considered a chief-carrier of K, Li, Rb, Cs, Sr, Ba, Cr, V, Tl, Be, B, 87Sr, etc. from the sedimentary/metamorphic environment to the igneous one (Hanson, 1980; Hart and Reid, 1991; Fleet et al., 2003; Schmidt and Poli, 2003; Hermann et al., 2006; Bebout, 2007; Hermann and Rubatto, 2009; Bebout, 2011).

Although quantitative data supporting this hypothesis are not yet available (e.g. the volumes of fluids interacting with the mantle wedge, the volumes of magma produced, etc.) and the whole geodynamic frame is not widely accepted (e.g. the presence of a slab window in the Ionian slab, etc.), we present a qualitative model trying to explain all the anomalies we found within the investigated products:

Subduction- related fluids, rich in H2O, K, Rb, Cs, Tl, Cl, MREEs, HREEs, were released from the Ionian altered crust and its sedimentary covers;

Released fluids rise through the slab window in the Ionian slab and interact with the overlying mantle wedge creating a veined-mantle;

Partial melting of the veined mantle;

Uprising/ ponding of newly produced magma batch within the crust.