The Depleted Mantle Component in Kerguelen Archipelago Basalts: Petrogenesis of Tholeiitic-Transitional Basalts From the Loranchet Peninsula

A geochemical study of 28 Ma tholeiitic to transitional basaltsfrom the Kerguelen Archipelago (Mont des Ruches and Mont Fontaine)indicates that three distinct magma types erupted within     

The Montecristo monzogranite (Northern Tyrrhenian Sea,Italy): a collisional pluton in an extensional setting

The Montecristo monzogranite (MM) is a near-circular peraluminous monzogranite pluton occupying the entire 10 km² of Montecristo Island. Outcrops of country rock are scarce, and are mainly roof pendants of metagabbros and calcsilicate hornfels of the Apenninic ophiolite sequence. Emplacement of the pluton (Rb–Sr age=7·1±0·2 Ma), following the early Miocene onset of continental collision, occurred during an extensional phase which migrated eastward via a combined process of subduction–delamination. The MM rocks are strongly porphyritic, the assemblage being composed of alkali-feldspar, quartz, plagioclase (all occurring as mega- or phenocrysts), biotite and minor cordierite. Accessory minerals include tourmaline, apatite, zircon, ilmenite, allanite, monazite, rutile and hellandite. Reconstructed crystallization histories for the mineral phases reveal a polybaric crystallization starting at about 5 kb. Textural variations of MM occur in sharp contact with each other; darker types often form globular masses containing fewer megacrysts and more abundant mafic microgranular enclaves. Geochemical, isotopic and petrographic data indicate that the MM magma was produced by anatectic melting of an intermediate to deep pelitic crustal source. On the basis of the geochemical and mineralogical characteristics of the enclaves, modification of their parent magma occurred by crystal fractionation coupled with mixing and mingling of components from the MM magma. The limited geochemical variation in MM is interpreted as due to crystal fractionation processes during the magma's ascent. Younger porphyritic dykes with more potassic and alkaline affinities cut the pluton; these dykes are concentrated in a major fracture zone and are associated with contemporaneous pseudotachylites. © 1997 John Wiley & Sons, Ltd.     

Detrital carbonate grains as provenance indicators in the Upper Cretaceous Pietraforte Formation (northern Apennines)

The Upper Cretaceous Pietraforte Formation, an allochthonous unit of the Ligurian domain in the northern Apennines, provides a case study of the importance of detrital carbonate grains for provenance determination in sandstones. The Pietraforte Formation is composed of turbidite sandstones with subordinate conglomerate, deposited in an external sector of the Ligurian ocean, close to the Adriatic margin. The sandstones have a lithic composition, characterized by abundant sedimentary and metasedimentary rock fragments (35–56% of the terrigenous framework), little feldspar (<7%) that is almost exclusively plagioclase, and a high ratio of fine- to coarse-grained polycrystalline quartzose grains to total quartzose grains (average Q_p/Q_t=0.37). Carbonate rock fragments dominate the lithic association of both sandstones and conglomerates and provide the most detailed information for provenance determination. They are composed primarily of dolostones and a wide variety of limestones containing identifiable age-diagnostic microfossils. Fossils and rock textures of carbonate clasts document the erosion of Upper Triassic to Lower Cretaceous shelf and pelagic carbonate units which can be matched with Mesozoic rock types present in the Tuscan domain of the northern Apennines. Compositional results constrain the source of the Pietraforte Formation sandstones to the western margin of the Adriatic plate, from uplifted sedimentary and metasedimentary rocks of the Tuscan domain and its low-grade metamorphic basement. Coeval intrabasinal sources provided additional supplies to the depositional basin of the Pietraforte Formation; this intrabasinal supply consists of shelf carbonate allochems, planktonic foraminifera and argillaceous rip-up clasts. The presence of carbonate grains from shallow-water environments may indicate the existence during deposition of marginal shelf areas favourable for carbonate allochem production.     

Syros Metasomatic Tourmaline: Evidence for Very High-{delta}11B Fluids in Subduction Zones

High-pressure (HP) metamorphic blocks enclosed in a mafic toultramafic matrix from a mélange on the island of Syrosare rimmed by tourmaline-bearing reaction zones (blackwalls).The B isotopic composition of dravitic tourmaline within theseblackwalls was investigated in situ by secondary ion mass spectrometry.Boron in these tourmalines is unusually heavy, with ¹¹B valuesexceeding +18 in all investigated samples and reaching an extremevalue of +28·4 in one sample. Blackwalls formed duringexhumation of the HP mélange at a depth of 20–25km at temperatures of 400–430°C, by influx of externalhydrous fluids. The compositions of the fluids are estimatedto be in the range of 100–300 µg/g B with ¹¹B valuesof +18 to +28. The high ¹¹B values cannot be explained by tourmalineformation from unmodified slab-derived fluids. However, suchfluids could interact with the material in the exhumation channelon their way from the dehydrating slab to the site of tourmalineformation in the blackwalls. This could produce exceptionallyhigh ¹¹B values in the fluids, a case that is modelled in thisstudy. The model demonstrates that subduction fluids may beeffectively modified in both trace element and isotopic compositionduring their migration through the material overlying the subductingslab. Blackwall tourmaline from Syros has a large grain size(several centimetres), high abundance, and an exceptionallyhigh ¹¹B value. The formation of tourmaline at the contact betweenmafic or felsic HP blocks and their ultramafic matrix involvedfluids released during dehydration reactions in the subductingslab. It forms a heavy-boron reservoir in hybrid rocks overlyingthe subducting slab, and may, thus, have a significant impacton the geochemical cycle of B and its isotopes in subductionzones. KEY WORDS: boron isotopes; tourmaline; subduction zone; fluid, high pressure     

COMMERCIAL AGRICULTURE AND AGROCHEMICALS IN ALMOLONGA,GUATEMALA*

ABSTRACT. Large amounts of synthetic pesticides are used in Guatemala to increase the production of nontraditional crops. Almolonga, a township in the western highlands, produces vegetables for markets in Mexico and Central America. Almolongueños make intensive use of limited cultivable land and abundant water resources, as well as sizable amounts of synthetic pesticides. Among Almolongueños, congenital malformations, cancer, anemia, and respiratory infections await substantiated links to exposure to pesticides. The prevalence of infectious and parasitic diseases and nutritional deficiencies—the dominant health concerns—may be heightened by excessive use of pesticides. Public-health initiatives involve the rational use of pesticides or their elimination.     

Ice nuclei characteristics from M-PACE and their relation to ice formation in clouds

This paper presents airborne measurements of ice nuclei (IN) number concentration and elemental composition from the mixed-phase Arctic cloud experiment (M-PACE) in northern Alaska during October 2004. Although the project average IN concentration was low, less than 1 L⁻¹ STP, there was significant spatial and temporal variability, with local maximum concentrations of nearly 60 L⁻¹ STP. Immersion and/or condensation freezing appear to be the dominant freezing mechanisms, whereas mechanisms that occur below water saturation played a smaller role. The dominant particle types identified as IN were metal oxides/dust (39%), carbonaceous particles (35%) and mixtures of metal oxides/dust with either carbonaceous components or salts/sulphates (25%), although there was significant variability in elemental composition. Trajectory analysis suggests both local and remote sources, including biomass burning and volcanic ash. Seasonal variability of IN number concentrations based on this study and data from SHEBA/FIRE-ACE indicates that fall concentrations are depleted relative to spring by about a factor of five. Average IN number concentrations from both studies compare favorably with cloud ice number concentrations of cloud particles larger than 125 μm, for temperatures less than −10 °C. Cloud ice number concentrations also were enhanced in spring, by a factor of ∼2, but only over a limited temperature range.     

Authigenic seep-carbonates cementing coarse-grained deposits in a fan-delta depositional system (middle Miocene, Marnoso-arenacea Formation, central Italy)

Seep‐carbonates (¹³C‐depleted) are present at different levels within the Miocene terrigenous succession of Deruta (Marnoso‐arenacea Formation, central Italy); they are associated with pebbly sandstones and conglomerates in a tectonically active fan‐delta slope depositional system. Most of these seep‐carbonates are included in slide/slump horizons as scattered blocks. The occurrence of seep‐carbonates is clear evidence of the flow of methane‐rich fluids pervading the sediments. Fluids, probably of biogenic origin, may have reached the sea‐bottom through thrust faults and selectively infiltrated the more permeable coarse‐grained horizons deposited along the slope. Different stages of fluid emissions are documented: slow flux stage, corresponding to the development of large carbonate bodies and dense chemosynthetic communities; and fast fluid flow associated with intense carbonate brecciation, pipes and veins. Large amounts of authigenic carbonates are reworked by slope failures triggered by tectonics and fluids reducing sediment strength; in situ cementation of slide blocks may also have occurred due to remobilization of methane‐rich fluids by mass‐wasting processes.