Trace element behaviour during interaction between basalt and crustal rocks at 0.5–0.8 GPa: an experimental approach

We experimentally investigate the major and trace elements behavior during the interaction between two partially molten crustal rocks (meta-anorthosite and metapelite) and a basaltic melt at 0.5–0.8 GPa. Results show that a hybrid melt is formed at the basalt-crust contact, where plagioclase crystallizes. This contact layer is enriched in trace elements which are incompatible with plagioclase crystals. Under these conditions, the trace element diffusion coefficients are one order of magnitude larger than those expected. Moreover, the HFSE diffusivity in the hybrid melt is surprisingly higher than the REE one. Such a feature is related to the plagioclase crystallization that changes the trace elements liquid-liquid partitioning (i.e. diffusivity) over a transient equilibrium that will persist as long as the crystal growth proceeds. These experiments suggests that the behaviour of the trace elements is strongly dependent on the crystallization at the magma-crust interface. Diffusive processes like those investigated can be invoked to explain some unusual chemical features of contaminated magmatic suites.     

Temporal and genetic relationships between the Kidston gold-bearing Breccia Pipe and the Lochaber Ring Dyke Complex, North Queensland, Australia: insights from in situ U–Pb and Hf-isotope analysis of zircon

An existing model for the temporal and genetic relationships between the Kidston gold-bearing Breccia Pipe and the nearby Lochaber Ring Dyke Complex has been evaluated using in situ U–Pb and Hf-isotope analyses of zircon grains. The Oak River Granodiorite, the host rock to the Kidston Breccia Pipe, has 1,551?±?6 Ma old zircon cores overgrown by 417.7?±?2.2 Ma rims. The Black Cap Diorite and Lochaber Granite within the Lochaber Ring Dyke Complex have crystallisation ages of 350.7?±?1.3 and 337.9?±?2.6 Ma respectively. The gold-rich Median Dyke within the Kidston Breccia Pipe has a crystallisation age of 335.7?±?4.2 Ma, and thus is temporally related to the Lochaber Granite. However, zircon grains from the Median Dyke have less radiogenic Hf-isotope compositions (? _Hf from ?7.8 to ?15.8) than those from the Black Cap Diorite ?_Hf?=?0.4 to ?7.2) and the Lochaber Granite (? _Hf?=??1.0 to ?7.5), but within the range defined by zircons from the Oak River Granodiorite ? _Hf?=??8.0 to ?29.2). The Hf-isotope data thus rule out the proposed fractional crystallisation relationship between the Kidston gold-bearing rocks and the Lochaber Ring Dyke Complex. The Kidston Median Dyke may have been produced by mixing between Lochaber Granite magmas and magmas derived by remelting of the Oak River Granodiorite, which was itself derived from Proterozoic crust. There is no evidence for a juvenile component in the Lochaber Ring Dyke Complex or the Median Dyke. The gold enrichment in the Kidston rocks thus may reflect the multi-stage reworking of the Proterozoic crust, which ultimately produced the Carboniferous felsic magmas.     

Volcanic tremor at Mount Etna (January 1984–March 1985): its relationship to the eruptive activity and modelling of the summit feeding system

Spectral analyses of volcanic tremor at Etna during January 1984–March 1985, have been performed and the relationship between tremor energy and observed volcanic phenomena have been examined. The highest energy levels have been observed during the paroxysmal phases of eruptions, whereas a gradual decrease was linked to the lowering of eruptive activity. Amplitude variations with time of some spectral frequency peaks (0.95, 1.20, 1.45, 1.65, 1.80 and 2.40 Hz) have been compared with volcanic activity at the summit craters, and on the basis of these results a new schematic diagram for the feeding system of the summit vents is proposed.     

Time-space variation of volcano-seismic events at La Fossa (Vulcano, Aeolian Islands, Italy): new insights into seismic sources in a hydrothermal system

We investigated the relationship between volcano-seismic events, recorded at La Fossa crater of Vulcano (Aeolian Islands, Italy) during 2004-2006, and the dynamics of the hydrothermal system. During the period of study, three episodes of increasing numbers of volcano-seismic events took place at the same time as geothermal and geochemical anomalies were observed. These geothermal and geochemical anomalies have been interpreted as resulting from an increasing deep magmatic component of the hydrothermal fluids. Three classes of seismic events (long period, high frequency and monochromatic events), characterised by different spectral content and various similarity of the waveforms, have been recognised. These events, clustered mainly below La Fossa crater area at depths of 0.5–1.1 km b.s.l., were space-distributed according to the classes. Based on their features, we can infer that such events at Vulcano are related to two different source mechanisms: (1) fracturing processes of rocks and (2) resonance of cracks (or conduits) filled with hydrothermal fluid. In the light of these source mechanisms, the increase in the number of events, at the same time as geochemical and geothermal anomalies were observed, was interpreted as the result of an increasing magmatic component of the hydrothermal fluids, implying an increase of their flux. Indeed, such variation caused an increase of both the pore pressure within the rocks of the volcanic system and the amount of ascending fluids. Increased pore pressures gave rise to fracturing processes, while the increased fluid flux favoured resonance and vibration processes in cracks and conduits. Finally, a gradual temporal variation of the waveform of the hybrid events (one of the subclasses of long period events) was observed, likely caused by heating and drying of the hydrothermal system.     

Temporal correlation of magmatic-tectonic events in the lower and upper crust in north-east Australia

Hf model ages yielded by rutiles in a garnet-rich lower-crustal granulite xenolith from the McBride Province and whole-rock Nd and Hf model ages for the plagioclase- and garnet-rich Chudleigh and McBride granulites overlap with the well-defined U–Pb ages for detrital zircons and zircons in granitoids (Murgulov et al. Chem Geol 245:198–218, 2007; Mineral Petrol 95:17–45, 2009), suggesting temporal correlation of magmatic-tectonic events in the lower and upper crust in the north-east Australian craton. Intrusion of basaltic magmas into and below the lower crust beneath the Chudleigh Province around 1.4, 1.7 and 2.3?Ga and beneath the McBride Province around 1.5, 1.7, 2.3 and 2.5?Ga provided heat for remelting and supplied magmas with juvenile mantle isotope signatures to the upper crust. Similar magmas provided enough heat to cause melting in the lower crust beneath the Chudleigh Province around 0.12 and 0.8–0.9?Ga and beneath the McBride Province around 0.42, 0.8–0.9?Ga, 1.1 and 1.3?Ga but were not sufficient to cause significant melting in the upper crust. A wide range in initial ¹⁷⁶Hf/¹⁷⁷Hf values and a ~1.55?Ga Hf model age yielded by rutiles in the McBride granulite provide a link to the genesis of ~420?Ma granitoids. The data for a plagioclase-rich granulite from the Atherton Province are similar to those for the Chudleigh and McBride granulites. However, additional samples are required to test whether the lower crust of the Tasman orogenic belt is lithologically and isotopically similar to the lower crust of the craton.     

An energy budget for the subtidal bivalve Modiolus barbatus (Mollusca) at different temperatures

Clearance rates, respiration rates and food absorption efficiencies of the commercially interesting subtidal bivalve Modiolus barbatus were measured at different temperatures under laboratory conditions and scope for growth calculated. Clearance rates were highest at temperatures from 20 °C to 28 °C, whereas respiration rate was maximal at 9 °C and minimal at 26 °C. Highest mean values of absorbed energy occurred at 20 °C and 26 °C. Scope for growth trend had negative values at 9 °C, 15 °C and 28 °C and positive values at temperatures 20 °C and 26 °C. The profitable thermal window for M. barbatus to have energy sufficient for growth and reproduction corresponded to <5 months per year. Seawater temperature increases will potentially impact the eco-physiological responses of subtidal M. barbatus causing life history traits to change with important repercussions for subtidal biodiversity in the Mediterranean.     

Field observations of SPM using ADV, ADP, and OBS in a shallow estuarine system with low SPM concentration—Vitória Bay, SE Brazil

The present study investigated direct and indirect methods using optical and acoustic instruments for the acquisition of information required to estimate the concentration of suspended particulate matter (SPM) in Vitória Bay, a shallow estuarine system in SE Brazil. The aim was to calibrate and compare the use of different instruments (OBS, ADP, and ADV) to estimate SPM concentration in the water column and near the bed. Concentration was determined by correlating filtered water samples with the optical and acoustic measurements. In general, the methodology proved tenable for the chosen shallow estuarine environment with low SPM concentration (<60?mg/L). Pearson’s coefficient ranged from 0.75 to 0.85, when correlating measurements taken at three sampled depths. Differences in the correlation coefficient values showed that calibration at three depths (near the bed, mid-water column, and near the surface) was more effective than for surface samples alone, even in shallow (???m deep) water. When calibration was attempted for concentration in the entire water column with samples at just one elevation, the correlation value was very low, thus increasing the error in estimating the SPM concentration.     

Seismic assessment of phreatic-explosion hazard at ‘La Fossa’ Volcano (Island of Vulcano,Italy)

Active volcanic areas are sometimes affected by phreatic eruptions, which are explosions due to the outbreak of a confined pocket of steam and gas without the direct involvement of molten magma. Eruptive activity at La Fossa Volcano, Island of Vulcano (Italy), typically starts with a phreatic explosion, continues as phreatomagmatic and turns into a late magmatic stage, depending on the reduction in the efficiency of magma-water interaction. The present risk is mainly related to a village located at the very foot of the active volcanic cone, which in summer seasons is inhabited by more than 10,000 people. Because the last eruptive episode occurred in 1888–1890, when no local instrumental control was performed, the scientific community does not have any information about the seismic phenomena which could precede the initial phreatic blast. Inferences from similar monitored volcanoes, which awakened in the last few years after more or less long quiescence, are therefore made to tentatively depict possible pre-eruptive seismic scenarios and also to evaluate the most appropriate surveillance and alert criteria to adopt. Appreciable signs of volcanic unrest have been detected in recent years, mainly related to modifications in the physical and chemical features of fumarolic fluids discharged at the active crater. Following the classification of local seismic shocks, the recording oflong-period events (peak frequency 2 Hz) should be taken into account as a realistic seismic warning of volcanic hazard enhancement. A likely short-term seismic precursor may be finally given by the appearance of the so-calledbanded tremor, a seismic signal likely produced by intermittent hydrothermal boiling accompanying the hydraulic fracturing of rocks at the top of a pressurized aquifer. Real-time recognition of tremor can be therefore very useful to avoid, at least, injuries to volcanologists and visitors working or staying on the most dangerous sectors of the volcano.     

Platform margin collapses and sequence stratigraphic organization of carbonate slopes: Cretaceous–Eocene,Gargano Promontory,southern Italy

We describe the sequence stratigraphic organization and the associated sedimentological characteristics of Cretaceous to Eocene slope and base-of-slope carbonate successions. The study area is located in the Gargano Promontory which belongs to the stable foreland of southern Italy. The succession consists of three superimposed depositional sequences separated by major unconformities. The upper two sequences are clear examples of sequence stratigraphic organization; in fact, they both start with huge megabreccia wedges (LST) followed upward by thin pelagic units (TST) and a thick package of calciturbidites and debrites that alternate with pelagic mudstone (HST). The Cretaceous highstand systems tract is clearly arranged in a number of coarsening-upward cycles while the Eocene one which also comprises a toplap shallow water unit, is not. The Gargano stratigraphic palimpsest and the entire margin of the Apulia Platform show remarkable similarities with present-day carbonate platform margins and slopes where irregular, convex-bankward embayments suggest large-scale failures. It is clear that classic sequence stratigraphic organization can result from simple platform dismantling, having no or little time relation with global sea-level fluctuations. In fact, as the margin failure (LST) interrupts the carbonate production, a period of starvation (TST) along the entire slope and base-of-slope follows necessarily. Finally, when the margin once again becomes active and productive, sediment exportation starts again and the system begins to prograde (HST).