Trace element and Pb–B–Li isotope systematics of olivine-hosted melt inclusions: insights into source metasomatism beneath Stromboli (southern Italy)

We studied the elemental and isotopic (Pb, B and Li isotopes) composition of melt inclusions hosted in highly forsteritic (Fo_83–91) olivines that were collected from San Bartolo lava and pumice (ST79p, ST82p and ST531p) samples erupted by Stromboli in historical times. The studied melt inclusions have primitive calcalkaline to shoshonitic basaltic compositions. They cover a compositional range far wider than that exhibited by the whole-rocks and differ in key trace element ratios. San Bartolo melt inclusions are characterized by lower incompatible trace element abundances, higher ratios between fluid-mobile (B, Pb, U and LILE) and less fluid-mobile (REE, Th, HFSE) elements and lower La/Yb ratios relative to the pumice-hosted melt inclusions and pumiceous melts erupted during paroxysmal events. Trace elements, along with different Pb, B and Li isotopic signatures, attest to source heterogeneity on the small scale and provide new insights into subducted components beneath Stromboli. Results of a mixing model suggest that metasomatism of the mantle source of pumice-hosted melt inclusions was driven by solute-rich high-pressure fluids (<20%) expelled from the deep portion of the slab. Heterogeneous Pb isotopic composition together with light δ¹¹B (−8.6 to −13.7‰) and δ⁷Li (+2.3 to −1.7‰) indicates that high-pressure liquids were released in variable proportions from highly dehydrated metabasalts and metasediments. On the other hand, the elemental and isotopic (δ¹¹B ~ −1.9 to −5.9‰) composition of San Bartolo melt inclusions is better explained by the addition of a prevalent aqueous component (~2 to 4%) escaped at shallower depths from sediments and altered basaltic crust in almost equivalent proportions, with a smaller contribution by high-pressure fluids. Owing to the high-angle dip of the subducted cold Ionian slab, aqueous fluids and high-pressure fluids would rise through the mantle wedge and locally superimpose on each other, thus giving origin to variously metasomatized mantle domains.     

The Aspen–Amsterdam void finder comparison project

Despite a history that dates back at least a quarter of a century, studies of voids in the large-scale structure of the Universe are bedevilled by a major problem: there exist a large number of quite different void-finding algorithms, a fact that has so far got in the way of groups comparing their results without worrying about whether such a comparison in fact makes sense. Because of the recent increased interest in voids, both in very large galaxy surveys and in detailed simulations of cosmic structure formation, this situation is very unfortunate. We here present the first systematic comparison study of 13 different void finders constructed using particles, haloes, and semi-analytical model galaxies extracted from a subvolume of the Millennium simulation. This study includes many groups that have studied voids over the past decade. We show their results and discuss their differences and agreements. As it turns out, the basic results of the various methods agree very well with each other in that they all locate a major void near the centre of our volume. Voids have very underdense centres, reaching below 10 per cent of the mean cosmic density. In addition, those void finders that allow for void galaxies show that those galaxies follow similar trends. For example, the overdensity of void galaxies brighter than   m _B =−20  is found to be smaller than about −0.8 by all our void finding algorithms.     

Annual and spatial variations of chemical and physical properties in the Ross Sea surface waters (Antarctica)

Temperature, salinity, meltwater percentage, water column stability, dissolved oxygen and nutrients were measured in seawater samples collected at three fixed depths (0, 20, 100 m) in 104 stations located in three different areas of the Ross Sea (Antarctica), during four Italian Antarctic surveys carried out between 1998 and 2006. Nutrient data were used to quantify the nutrient removal, which appears particularly high in 2006, especially in polynya area. The N:P and Si:N disappearance ratios were studied to estimate the dominant phytoplanktonic community. No significant differences in the nutrient drawdown ratio were observed, in fact the N:P ratio was always below the Redfield standard ratio.     

Petrology,geochemistry and U–Pb zircon geochronology of lower crust pyroxenites from northern Apennine (Italy): insights into the post-collisional Variscan evolution

Spinel pyroxenites occur locally as clasts in polygenic breccias from the Late Cretaceous sedimentary mélanges of the Northern Apennine (Italy). They are of cumulus origin and formed in the deep crust by early precipitation of clinopyroxene and minor olivine and late crystallisation of orthopyroxene, spinel, Ti-pargasite and sulphides. Pyroxenites underwent high-temperature (~850°C) subsolidus re-equilibration and ductile deformation with development of mylonitic bands made of clinopyroxene, orthopyroxene, Ti-pargasite and spinel. U–Pb geochronology on zircons revealed the occurrence of inherited grains of Early Proterozoic to Late Devonian age. The inherited zircons are locally rimmed by recrystallised zircon domains. The oldest rims yield a mean concordia U–Pb age at 306 ± 8 Ma, which is considered to date the emplacement of the pyroxenites, in the framework of the post-Variscan lithospheric extension. The incompatible element compositions of calculated melts in equilibrium with clinopyroxenes from the pyroxenites are characterised by Ba, Nb, LREE and Sr enrichment relative to N-MORB. The depleted Nd isotopic signature of the pyroxenites (initial ε_Nd values of +5.3 to +6.1) may be thus linked to primary magmas produced by low degrees of melting of asthenospheric mantle. In addition, the pyroxenites locally record the infiltration of plagioclase-saturated hydrous melts, most likely evolved through fractional crystallisation and enriched in highly incompatible elements, within the clinopyroxene-dominated crystal mush. A thermal event in Late Permian–Middle Triassic caused the partial resetting of zircon U–Pb system.     

The Finero phlogopite-peridotite massif: an example of subduction-related metasomatism

The Finero peridotite massif is a harzburgite that suffered a dramatic metasomatic enrichment resulting in the pervasive presence of amphibole and phlogopite and in the sporadic occurrence of apatite and carbonate (dolomite)-bearing domains. Pyroxenite (websterite) dykes also contain phlogopite and amphibole, but are rare. Peridotite bulk-rock composition retained highly depleted major element characteristics, but was enriched in K, Rb, Ba, Sr, LREE (light rare earth elements) (La_N/Yb_N = 8–17) and depleted in Nb. It has high radiogenic Sr (⁸⁷Sr/⁸⁶Sr₍₂₇₀₎ = 0.7055–0.7093), low radiogenic Nd (ɛNd₍₂₇₀₎= −1 to −3) and EMII-like Pb isotopes. Two pyroxenite – peridotite sections examined in detail show the virtual absence of major and trace element gradients in the mineral phases. In both rock types, pyroxenes and olivines have the most unfertile major element composition observed in Ivrea peridotites, spinels are the richest in Cr, and amphibole is pargasite. Clinopyroxenes exhibit LREE-enriched patterns (La_N/Yb_N ∼16), negative Ti and Zr and generally positive Sr anomaly. Amphibole has similar characteristics, except a weak negative Sr anomaly, but incompatible element concentration ∼1.9 (Sr) to ∼7.9 (Ti) times higher than that of coexisting clinopyroxene. Marked geochemical gradients occur toward apatite and carbonate-bearing domains which are randomly distributed in both the sections examined. In these regions, pyroxenes and amphibole (edenite) are lower in mg## and higher in Na₂O, and spinels and phlogopite are richer in Cr₂O₃. Both the mineral assemblage and the incompatible trace element characteristics of the mineral phases recall the typical signatures of “carbonatite” metasomatism (HFSE depletion, Sr, LILE and LREE enrichment). Clinopyroxene has higher REE and Sr concentrations than amphibole (^amph/cpxD_REE,Sr = 0.7–0.9) and lower Ti and Zr concentrations. It is proposed that the petrographic and geochemical features observed at Finero are consistent with a subduction environment. The lack of chemical gradients between pyroxenite and peridotite is explained by a model where melts derived from an eclogite-facies slab infiltrate the overhanging harzburgitic mantle wedge and, because of the special thermal structure of subduction zones, become heated to the temperature of the peridotite. If the resulting temperature is above that of the incipient melting of the hydrous peridotite system, the slab-derived melt equilibrates with the harzburgite and a crystal mush consisting of harzburgite and a silica saturated, hydrous melt is formed. During cooling, the crystal mush crystallizes producing the observed sequence of mineral phases and their observed chemical characteristics. In this context pyroxenites are regions of higher concentration of the melt in equilibrium with the harzburgite and not passage-ways through which exotic melts percolated. Only negligible chemical gradients can appear as an effect of the crystallization process, which also accounts for the high amphibole/clinopyroxene incompatible trace element ratios. The major element refractory composition is explained by an initially high peridotite/melt ratio. The apatite, carbonate-bearing domains are the result of the presence of some CO₂ in the slab-derived melt. The CO₂/H₂O ratio in the peridotite mush increased by crystallization of hydrous phases (amphibole and phlogopite) locally resulting in the unmixing of a late carbonate fluid. The proposed scenario is consistent with subduction of probably Variscan age and with the occurrence of modal metasomatism before peridotite incorporation in the crust. Received: 20 July 1998 / Accepted: 28 October 1998     

Pacific interdecadal variability driven by tropical–extratropical interactions

Interactions between the tropical and subtropical northern Pacific at decadal time scales are examined using uncoupled oceanic and atmospheric simulations. An atmospheric model is forced with observed Pacific sea surface temperatures (SST) decadal anomalies, computed as the difference between the 2000–2009 and the 1990–1999 period. The resulting pattern has negative SST anomalies at the equator, with a global pattern reminiscent of the Pacific decadal oscillation. The tropical SST anomalies are responsible for driving a weakening of the Hadley cell and atmospheric meridional heat transport. The atmosphere is then shown to produce a significant response in the subtropics, with wind-stress-curl anomalies having the opposite sign from the climatological mean, consistent with a weakening of the oceanic subtropical gyre (STG). A global ocean model is then forced with the decadal anomalies from the atmospheric model. In the North Pacific, the shallow subtropical cell (STC) spins down and the meridional heat transport is reduced, resulting in positive tropical SST anomalies. The final tropical response is reached after the first 10 years of the experiment, consistent with the Rossby-wave adjustment time for both the STG and the STC. The STC provides the connection between subtropical wind stress anomalies and tropical SSTs. In fact, targeted simulations show the importance of off-equatorial wind stress anomalies in driving the oceanic response, whereas anomalous tropical winds have no role in the SST signal reversal. We further explore the connection between STG, STC and tropical SST with the help of an idealized model. We argue that, in our models, tropical SST decadal variability stems from the forcing of the Pacific subtropical gyre through the atmospheric response to ENSO. The resulting Ekman pumping anomaly alters the STC and oceanic heat transport, providing a negative feedback on the SST. We thus suggest that extratropical atmospheric responses to tropical forcing have feedbacks onto the ocean dynamics that lead to a time-delayed response of the tropical oceans, giving rise to a possible mechanism for multidecadal ocean-atmosphere coupled variability.     

Evaluating asbestos fibre concentration in metaophiolites: a case study from the Voltri Massif and Sestri–Voltaggio Zone (Liguria, NW Italy)

This work is part of the project study for a road tunnel bypassing the town of Genova and was aimed at evaluating the amount of asbestos fibres in the metaophiolites belonging to the Voltri Group and the Sestri–Voltaggio Zone (Liguria, Northern Italy). The 85 studied rock samples (mainly mafic and ultramafic rocks) derive from exposed outcrops and prospecting boreholes. The study of field relations and petrographic/microtextural investigations under the optical microscope allowed for the identification and characterisation of asbestos-bearing settings and lithotypes. Mineralogy and concentration of asbestos fibres in powdered specimens were determined by means of a scanning electron microscope equipped with an energy-dispersive X-ray spectroscopy device. These investigations were combined with petrography on thin-section, X-ray diffraction analysis and phase contrast optical microscopy on rock powders. Mafic and ultramafic rocks commonly contain asbestos in concentrations below 1,000 mg/kg (considered as the contamination threshold under Italian law). However, the fibre concentration rises abruptly within localised zones, where the metaophiolite sequences were involved into late ductile to brittle tectono-metamorphic events. Two groups of asbestos-bearing settings have been so far identified in the area: (a) fracture networks within serpentinites (dominated by fibrous chrysotile), and (b) boudins of chlorite-tremolite schists, likely deriving from dynamic recrystallisation of mafic rocks under greenschist facies conditions (dominated by fibrous amphibole). Even considering the low volumetric incidence of these settings (metres to few tens of metres), their high asbestos content locally controls the total fibre amount in the excavation products, thus requiring special prevention measures during excavation, management and final storage of the contaminated debris.     

Geophysical interpretation of a high-resolution seismic refraction profile in the Northern Apennines

A combined seismic and gravimetric interpretation in the Northern Apennines area (Italy) is presented. To the knowledge of the authors, this is one of the few attempts to apply tomographic methodology to a seismic refraction profile. This procedure, together with the classical interpretation for defining lower reflectors, led to the formulation of quite an accurate model of the upper crust. A gravity analysis was performed concurrently taking into account the seismic results at different depths which correspond to different frequency domains in the gravity signal. While the medium- and high-frequency patterns have been solved by trial-and-error, the regional trend has been modelled applying the collocation procedure to the gravity data.     

Distribution of trace elements during breakdown of mantle garnet: an example from Zabargad

Ion probe investigations on mineral phases forming the Al-Di pyroxenites from the Zabargad peridotite body indicate that porphyroclastic pyroxenes in composite mafic layers record an unusual HREE, Zr, Sc enrichment not registered by pyroxenes in spinel websterites. Orthopyroxene in the opx+sp clusters forming the inner, cpx-free zone of layered pyroxenites shows strongly fractionated REE patterns (HREE_N/LREE_N>1000; Yb>100xch) and very high Zr, Sc and Y abundances (up to 30,672 and 60ppm, respectively). In the outer, cpx-rich zone porphyroclastic clinopyroxene is strongly HREE enriched (HREE_N/LREE_N29; Yb 269xch) and displays very high Sc and Zr abundances (up to 819 and 164 ppm, respectively). It is suggested that the unusual trace element abundances are inherited from a precursor garnet. Composite pyroxenite layers are interpreted as former garnet clinopyroxenites characterized by gnt/cpx modal zoning. The sp+opx(cpx-free) assemblage in the inner part is a product of the break-down reaction of garnet upon decompression, with Ca of the original garnet completely entering the enstatite solid solution. The temperature at which the breakdown reaction occurred is estimated to be higher than 1000°C (P in the range 20–30 kbar). In the outer part, decompression caused the garnet to form a sp+opx assemblage; however, the grossularite component participated in the formation of new clinopyroxene which reacted with the clinopyroxene present in the original mode before the decompression reaction, thus forming a cpx₂+sp+opx assemblage. As a result of garnet breakdown, pyroxenes have peculiar HFSE anomalies. Progressive upwelling during the Red Sea rifting produced incomplete reaction under pl-facies conditions. The geochemical signatures of precursor garnet in pyroxenes were partially crased during the recrystallization from granular spincl-bearing to granoblastic plagioclase-bearing assemblages, being preserved only in a few porphyroclast relies. The finding of pyroxenes with trace element characteristics of precursor garnet has important geodynamic and geochemical implications. Al-Di pyroxenite layers had a long history within the mantle, before the continental lithosphere rifting and thinning took place in the region. It is suggested that Al-Di pyroxenites were formed by deep-seated tholeiitic magmatism unrelated to the Red Sea evolution, thus representing the earliest event in the Zabargad upper mantle. Garnet breakdown significantly preceded the metasomatism induced by hydrous fluids (crystallization of Ti-rich pargasite) and the later intrusion of hydrous (Cr-Di) pyroxenite dykes. During the stages of mantle evolution, the HFSE anomalies in pyroxenes varied significantly. We note that the study of HFSE anomalies in mineral phases reveals complex geochemical histories which are not recorded by the whole-rock system.     

Equivocal carbonatite markers in the mantle xenoliths of the Patagonia backarc: the Gobernador Gregores case (Santa Cruz Province,Argentina)

Amphibole ± phlogopite ± apatite-bearing mantle xenoliths at Gobernador Gregores display modal, bulk-rock and phase geochemical characteristics held as indicators of carbonatitic metasomatism. However, part of these xenoliths has high TiO₂/Al₂O₃ and those displaying the most pronounced carbonatitic geochemical markers modally trend towards harzburgite. Bulk-rock, clinopyroxene and amphibole show Zr, Hf and Ti negative anomalies, which increase at decreasing Na₂O and high field strength elements (HFSE) concentrations. Steady variation trends between xenoliths which have and do not have carbonatitic characteristics suggest a control by reactive porous flow of only one agent, inferred to be initially a ne-normative hydrous basalt (because of the presence of wehrlites) evolving towards silica saturation. Variation trends exhibit cusps when amphibole appears in the mode. Appearance of amphibole may explain the Ti anomaly variations, but not those of Zr and Hf. Numerical modelling [Plate Model (Vernières et al. in J Geophys Res 102:24771–24784, 1997)] gives results consistent with the observed geochemical features by assuming the presence of loveringite. Modest HFSE anomalies in the infiltrating melt may be acquired during percolation in the garnet-facies.In memory of Carlo Rivalenti