Michele Lustrino

The role of lower continental crust and lithospheric mantle in the genesis of Plio-Pleistocene volcanic rocks from Sardinia (Italy)

Abstract The first comprehensive chemical and Sr–Nd–Pb isotopic data set of Plio–Pleistocene tholeiitic and alkaline volcanic rocks cropping out in Sardinia (Italy) is presented here. These rocks are alkali basalts, hawaiites, basanites, tholeiitic basalts and basaltic andesites, and were divided into two groups with distinct isotopic compositions. The vast majority of lavas have relatively high 87Sr/86Sr (0.7043–0.7051), low 143Nd/144Nd (0.5124–0.5126), and are characterised by the least radiogenic Pb isotopic composition so far recorded in Italian (and European) Neogene-to-Recent mafic volcanic rocks (206Pb/204Pb=17.55–18.01) (unradiogenic Pb volcanic rocks, UPV); these rocks crop out in central and northern Sardinia. Lavas of more limited areal extent have chemical and Sr–Nd–Pb isotopic ratios indicative of a markedly different source (87Sr/86Sr=0.7031–0.7040; 143Nd/144Nd=0.5127–0.5129; 206Pb/204Pb=18.8–19.4) (radiogenic Pb volcanic rocks, RPV), and crop out only in the southern part of the island. The isotopic ratios of these latter rocks match the values found in the roughly coeval anorogenic (i.e. not related to recent subduction events in space and time) mafic volcanic rocks of Italy (i.e. Mt. Etna, Hyblean Mts., Pantelleria, Linosa), and Cenozoic European volcanic rocks. The mafic rocks of the two Sardinian rock groups also show distinct trace element contents and ratios (e.g. Ba/Nb>14, Ce/Pb=8–25 and Nb/U=29–38 for the UPV; Ba/Nb

Ancient Fungal Life in North Pacific Eocene Oceanic Crust

We present evidence that eukaryotic life has existed in an extreme environment, inside the oceanic crust. Up to now only prokaryotes have been discovered within deep marine sediments and glass-rims of pillow basalts, no higher life forms are described as yet. This study demonstrates unique filamentous fossil structures observed within carbonate-filled vesicles of a massive lava flow unit from the upper oceanic crust in the North Pacific (ODP Site 1224). Based on morphological traits including branching, septa and central pores, the filaments are interpreted as fungi. The chemical composition of the fungal structures differs from the surrounding crystalline carbonate matrix in the deep basaltic rocks. Small open space between the fungi and the carbonate cement and undisturbed filamentous growth through different calcite crystals indicate endolithic fungal growth after the calcium carbonate filling. The presence of euhedral pyrite crystals within the carbonate cements points out anaerobic conditions in this habitat. Our results provide for the first time evidence for eukaryotic, fungal life in deep ocean basaltic rocks.

On the origin of EM-I end-member

On the basis of geochemical data available for oceanic basalts distinct mantle reservoirs have been identified during the last decades. Among these reservoirs, the EM-I end-member (Enriched Mantle type I) shows almost unique geochemical characteristics. The definition of the EM-I end member and the petrogenetic processes related to its origin are still debated: the geochemical composition of EM-I basalts has been interpreted in terms of crust-mantle interaction during 1) mantle recycling of subducted crustal material; 2) detachment of lithospheric keels in over-thickened regions; 3) thermal erosion by anomalously hot mantle. Key features of the EM-I end-member are: low uranogenic Pb ratios ( 2 0 6 Pb/ 2 0 4 Pb 0.85), and 2 0 8 Pb/ 2 0 6 Pb (>2.08). EM-I basalts show apparent or inexistent Ba, Pb, Eu or Sr anomalies in primitive mantle-normalized diagrams and have Ba/ Nb ratios ranging from 3.5 to 47.4, Ce/Pb from 1.2 to 24.6, Nb/U from 10.5 to 71.8, Sr/ Nd from 6.2 to 36.4, and Eu/Eu* from 0.83 to 1.25. On these basis, the origin and evolution of the EM-I end-member cannot be explained by a single petrological model, and even the definition of elemental and isotopic parameters is controversial.

Weathering and Relative Durability of Detrital Minerals in Equatorial Climate: Sand Petrology and Geochemistry in the East African Rift

This article investigates how, where, and to what extent the mineralogical and chemical composition of sand-sized sediments is modified by extreme weathering in modern equatorial settings, with the ultimate goal of learning to read climate from the sedimentary record. To single out the weathering effect, we studied the compositional trends of fluvial sands along the western branch of the East African Rift between 5°S and 5°N. The relative durability of different detrital components, as well as potential hydraulic-sorting and grain-size effects, were assessed by comparing samples with similar provenances in different climatic and environmental conditions or of different size classes within the same sample. Sands of equatorial central Africa at the headwaters of the Congo and Nile basins display the full spectrum of petrologic suites characterizing rift-shoulder and volcanic rift provenances. Unlike in arid Arabia, quartzose sands are not restricted to areas where detritus is recycled from prerift sedimentary covers. In a hot humid climate, weathering can effectively obliterate the fingerprint of parent rock lithology and produce a nearly pure quartz residue even where midcrustal basement rocks are being actively uplifted and widely unroofed. In such settings garnet is destroyed faster than hornblende, and zircon faster than quartz. Weathering control on detrital modes is minor only in the rain shadow of the highest mountains or volcanoes, where amphibole-dominated quartzofelicdspathic metamorphiclastic sands (Rwenzori Province) or clinopyroxene-dominated feldspatholithic volcaniclastic sands (Virunga Province) are generated. Our detailed study of the Kagera basin emphasizes the importance of weathering in soils at the source rather than of progressive maturation in temporary storage sites during stepwise transport and shows that the transformation of diverse parent rocks into a quartzose “white sand” may be completed in one sedimentary cycle in hydromorphic soils of subequatorial lowlands. Micas and heavy minerals, which are less effectively diluted by recycling than main framework components, offer the best key to identify the original source-rock imprint. The different behavior of chemical indexes such as the CIA (a truer indicator of weathering) and the WIP (markedly affected by quartz dilution) helps us to distinguish strongly weathered first-cycle versus polycyclic quartz sands.

The transition from alkaline to tholeiitic magmas: a case study from the Orosei-Dorgali Pliocene volcanic district (NE Sardinia, Italy)

Abstract During the Pliocene, simultaneously with the opening of the Tyrrhenian Sea, mafic magmas were erupted in NE Sardinia (Orosei-Dorgali area). These range from mildly alkaline with sodic affinity (about 80% of exposure) to tholeiitic (about 20%). The tholeiitic rocks (basaltic andesite) are slightly more evolved than the alkaline ones and show geochemical features (e.g., Mg# 20). Similar incompatible element ratios for both alkaline and tholeiitic rocks suggest different degrees of melting of a single mantle source. Mathematical modeling indicates ∼4–6% and ∼10–15% partial melting for alkaline and tholeiitic lavas, respectively. Trace element abundances of the Orosei-Dorgali volcanic rocks are typical of Plio-Pleistocene volcanic rocks of Sardinia but differ strongly from other Cenozoic anorogenic volcanic rocks of Europe. Similarly, Sr ( 87 Sr/ 86 Sr=0.70442–0.70455), Nd ( 143 Nd/ 144 Nd=0.512465–0.512558) and Pb ( 206 Pb/ 204 Pb=17.74–17.86; 207 Pb/ 204 Pb=15.53–15.60; 208 Pb/ 204 Pb=37.89–38.02) isotopic ratios are very unusual when compared with other Cenozoic European volcanic rocks. Trace element abundances and isotopic composition of the Orosei-Dorgali volcanic rocks suggest a lithospheric mantle origin.

Early activity of the largest Cenozoic shield volcano in the circum-Mediterranean area: Mt. Karacadag, SE Turkey

Volcanic activity at Mt. Karacadag, SE Turkey, developed between ~11 and ~0.01 Ma. In this paper we investigate the oldest products (older than 2.6 Ma) that created a large volcanic plateau and a N-S aligned volcanic edifice in the form of a shield volcano. These igneous rocks are mildly alkaline to transitional olivine-clinopyroxene phyric basalts with minor hawaiites, basanites and very rare differentiated lithologies (mugearites and benmoreites). The poor correlation of major elements with MgO in these lavas is qualitatively consistent with polybaric depths of magma production, variable degrees of partial melting (from ~2 to ~10 %), heterogeneous mantle sources and differences in the fractionating crystal assemblage. Primitive mantle-normalized patterns resemble typical anorogenic magma compositions, with peaks at HFSE (Nb, Ta, Hf, Zr) and high HFSE/LILE ratios. REE contents are compatible with derivation of the basanites from a mixed garnet-spinel facies peridotite after ~2 % partial melting. Alkali basalts are compatible with higher degrees of melting (between 5 and 10 %) from the same type of source. Initial 87 Sr/ 86 Sr ratios range from 0.70349 to 0.70522 while those of 143 Nd/ 144 Nd range from 0.512853 to 0.512659. Early-stage lavas show higher 87 Sr/ 86 Sr and lower 143 Nd/ 144 Nd compared to plateau-stage lavas. The Sr-Nd isotopic variations and their relation with major and trace elements cannot be explained by AFC-like (Assimilation and Fractional Crystallization) processes involving average crustal lithologies. More likely, the Sr-Nd isotopic ratios are related to the existence of heterogeneous mantle sources with only minor involvement of AFC-like processes. The Cenozoic lavas in a 200 x 800 km area between the Karasu Valley and the Syria-Iraq-Turkey border in south-eastern Anatolia form a distinct igneous province which can be characterised on the basis of Sr-isotope signatures. The lithospheric mantle beneath this area is characterized by anomalously enriched 87 Sr/ 86 Sr compositions (up to 0.7055) as well as more isotopically depleted compositi o ns ( 87 Sr/ 86 Sr down to 0.7030).

Phanerozoic geodynamic evolution of the Circum-Italian realm

The Phanerozoic geodynamic evolution of Europe is reviewed for the purpose of identifying its bearing on the petrogenesis of the Cenozoic European Volcanic Province. Several events capable of modifying the chemistry and mineralogy of the mantle, such as subduction of oceanic crust, continent-continent collision, and ocean formation are emphasized. The area now occupied by the Mediterranean Sea and, in general, all of Europe, underwent a complex geodynamic evolution, involving large relative crustal movements. The Paleozoic to Recent evolution of the circum-Mediterranean Sea area can be summarized as follows: (1) extension during the Precambrian (presence of ∼3000 to 4000 km wide oceanic crust between Laurussia (consisting of the Laurentian and Baltica-Fenno-scandian cratons) and Gondwana (South America, Africa, Australia, India, and Antarctica); (2) collisional movements with the formation of “Andean-type” margins during the Late Precambrian to Middle Paleozoic, followed by “Himalayan-type” margins during the Carboniferous (Hercynian orogeny sensu stricto); (3) change of plate movements and development of tensional (transtensive) stresses at the end of the Paleozoic, as indicated by the formation of the North Atlantic-Tethys rift system, with the Cretaceous formation of the Ligurian-Piedmontese and the Mesogean Ocean; (4) the Alpine orogeny, with a two-stage compressive cycle-(a) Eoalpine (Paleogene closure of the Ligurian-Piedmontese Ocean; formation of the Betic Cordillera, western-northern Alps, and Carpatho-Balkan Arc), with Europe-verging thrusts; and (b) Neoalpine (Neogene-Pleistocene formation of the Apennine, Maghrebide, Dinaride, and Hellenide chains, plus the backthrusted southern Alps, all with African vergence; opening of two diachronous backarc basins-the Ligurian-Provencal Basin and the Tyrrhenian Sea-in the western Mediterranean). Hercynian-age modifications (the most important of which are subduction-related) led to almost unique isotopic ratios, such as low 143Nd/144Nd, 206Pb/204Pb, 3He/4He, and slightly radiogenic 87Sr/86Sr ratios. During the Cenozoic and Quaternary, widespread magmatic activity developed throughout Europe. These products, mainly represented by mildly to strongly alkaline rocks with sodic affinity and tholeiitic mafic rocks (basanite, alkali basalts, tholeiitic basalts), show quite uniform geochemical and isotopic compositions typical of a within-plate tectonic setting. Moreover, subduction-related magmatism (mainly represented by low-to high-K calc-alkaline and shoshonitic series + ultrapotassic rocks such as lamproites) developed in response to the subduction systems of the Alpine orogeny. With respect to the circum-Italian realm, the igneous rocks emplaced during the last 30 Ma are essentially related to the Alpine orogeny. This activity is represented by rocks of extremely variable composition (alkaline-both sodic and potassic to ultrapotassic-and subalkaline [tholeiitic and calc-alkaline]) and probably carbonatitic.

Trace-element partitioning between plagioclase, alkali feldspar, Ti-magnetite, biotite, apatite, and evolved potassic liquids from Campi Flegrei (Southern Italy)

Abstract Partition coefficients (Min/LD) for a series of geochemically relevant elements have been calculated from combined EMP/LA-ICP-HRMS analyses of plagioclase, alkali feldspar, Ti-magnetite, biotite, apatite, and trachytic/trachyphonolitic melt pairs in selected Campi Flegrei rocks. Pl/LD and Kfs/LD values are generally very low for most of the trace-elements but Sr, Ba, and Eu. Kfs/LD for the latter elements record a systematic increase as the melt composition changes from trachyte to trachyphonolite, likely due to increasing structural compliance of the sanidine in the trachyphonolites related to larger Na/K values. Conversely, Kfs/LD values for transitional, highly charged incompatible elements (e.g., LREE) decrease from trachyte to trachyphonolite, possibly in response to the decrease of melt polymerization. Min/LD values for titanomagnetite generally decrease with the increasing melt evolution, the highest values being those measured for Ti, V, and Sc. Ti, Ba, Sc, Rb, Nb, Ta, and V are compatible in biotite in equilibrium with trachytic melt, whereas Cs, Sr, and Pb are incompatible and REE are strongly incompatible, as supported by the extremely low Bt/LDY (0.003-0.008). Partition coefficients for apatite and trachyphonolitic glass pairs are high for Sr, REE (particularly MREE), and Y, large for Th, U, and V, generally low for HFSE, and variable for other LILE. The comparison of measured Min/LD values for Campi Flegrei trachytes/trachyphonolites with other sets of partition coefficients reported in literature for evolved systems suggests that a reliable data set for magma evolution modeling requires: (1) a thorough preliminary selection of natural samples; (2) the adoption of accurate microanalytical techniques; (3) the direct measurement of Min/LD values for each specific melt composition.

What 'anorogenic' igneous rocks can tell us about the chemical composition of the upper mantle: Case studies from the circum-Mediterranean area

The composition of the upper mantle bounded by the Canaries, Eastern Anatolia, Libya and Poland is indirectly investigated by means of the chemical composition of igneous rocks with ‘anorogenic’ geochemical characteristics emplaced during the Cenozoic. The relatively homogeneous composition of these products in terms of incompatible trace-element content and Sr–Nd–Pb isotopic composition is unexpected, considering the variable lithospheric structure of this large area and the different tectono-thermal histories of the various districts. In order to reconcile the geochemical characteristics with a statistical sampling model, it would be necessary to propose volumes of the enriched regions much lower than the sampling volumes for each volcano (that is, less than 10 cubic metres), or alternatively, efficient magma blending from larger areas. The data are consistent with a relatively well-stirred and mixed sub-lithospheric upper mantle, in the solid state, which is also hard to understand. This contrasts with the situation under oceans where magma blending from diverse sources and sampling theory can explain the compositional statistics.

Petrological, geochemical and isotopic characteristics of the lithospheric mantle beneath Sardinia (Italy) as indicated by ultramafic xenoliths enclosed in alkaline lavas

Mantle xenoliths hosted in Miocene-Quaternary mafic alkaline volcanic rocks from Sardinia have been investigated with electron microprobe, laser ablation microprobe-inductively coupled plasma-mass spectrometry and thermal ionization mass spectrometry techniques. The xenoliths are anhydrous clinopyroxene-poor lherzolites and harzburgites, plus very rare websterites and olivine-websterites. Glassy pods having thin subhedral to euhedral microlites of olivine, clinopyroxene and spinel have been found in harzburgites and websterites. Clinopyroxene shows trace element variability, with values of (La/Yb)N ranging from sub-chondritic (0.01) to supra-chondritic (8.6). The Sr–Nd isotopic ratios of the clinopyroxenes fall mostly in the field of the European lithospheric mantle xenoliths (87Sr/86Sr from 0.70385 to 0.70568 and 143Nd/144Nd ranging from 0.512557 to 0.512953). The geochemical characteristics of the Sardinian xenoliths testify to the variable degrees of earlier partial melt extraction, followed by metasomatic modification by alkaline melts or fluids. Websterites are considered to represent small lenses or veins of cumulitic (i.e. magmatic) origin within the mantle peridotite.