The Lyngen Gabbro: the lower crust of an Ordovician Incipient Arc

We present evidence for the origin of the Lyngen Gabbro of the Ordovician Lyngen Magmatic Complex in Troms, Northern Norway. The two magmatic suites of the Lyngen Gabbro strike parallel NNE-SSW, and have distinct magmatic signatures. We define these signatures by using major and trace-element analyses together with selected major- and trace-element mineral analyses and ¹⁴³Nd/¹⁴⁴Nd-isotope whole-rock analyses of gabbroic to tonalitic plutonic rocks from seven detailed cross-sections from this large gabbro-complex. The Western suite of the Lyngen Gabbro precipitated from magma that may have been derived from the same system as the associated volcanic rocks. The gabbros have high An-content (An_>90) of their plagioclases relative to co-existing mafic minerals. Together with somewhat high _Nd(t) values (+6), this implies that the parental magmas were hydrous tholeiites similar to those found in back arc basins today. The Eastern suite, on the other hand, consist of cumulates that were precipitated from melts resembling those of ultra-depleted high-Ca boninitic magmas found in fore-arcs. Extremely high-An plagioclases (An_>95) co-exist with evolved mafic minerals and oxides, and the _Nd(t) values are lower (+4) than in the Western suite. The Eastern suite has no volcanic counterpart, but dikes intersecting the suites have compositions that possibly represent its parental magma. The oceanic Rypdalen Shear Zone generally separates the two suites in the north, but several non-tectonic transitions from boninitic to tholeiitic signatures southwards advocate that the magmatism happened concurrently. The magmatic proximity between the suites, the hydrous magmatism and the absence of a silicic or calc-alkaline mature arc section, suggests that the Lyngen Gabbro formed in the Iapetus Ocean under conditions presently found in incipient arcs later emplaced as outer arc highs.     

Timing of post-collisional H-type to A-type granitic magmatism: U–Pb titanite ages from the Alpine central Anatolian granitoids (Turkey)

The last stages of the continental collision during the closure of the Neotethyan ocean in central Anatolia are characterized by post-collisional H- and A-type granitoids intruding both the metamorphic country rocks and allochthonous ophiolitic rocks of the central Anatolian crystalline complex. Available Rb–Sr and K–Ar whole-rock and mineral age data on the H- and A-type granitoids in central Anatolia are inconsistent. To better constrain the geological relevance and the timing of the change in the chemical character of magmatism in the wake of the Alpine orogeny in Anatolia, we re-evaluated the geochemical characteristics and dated titanite from representative H- (Baranadag quartz-monzonite: BR) and A-type (Çamsari quartz-syenite: CS) granitoids by the U–Pb method. BR is a high-K calc-alkaline intrusion with mafic microgranular enclaves and shows enrichment of LILE relative to HFSE. The alkaline CS displays higher SiO₂, Na₂O+K₂O, Fe/Mg, Rb, Th and HFSE with corresponding depletion in CaO, MgO, Fe₂O₃^tot, P₂O₅, Ba, Sr, and Ti. Chondrite-normalized REE patterns of the BR and CS samples have LREE-enriched and flat HREE patterns, whereas CS differs from BR by higher LREE enrichment and lower MREE and HREE contents. Mineralogical and geochemical characteristics suggest that BR and CS were not products of the same magma source. BR was derived from a subduction-modified depleted hybrid-source and CS had an enriched mantle source with significant crustal contribution. The U–Pb titanite ages of the H-type central Anatolian granitoids (BR) and the A-type granitoids (CS) are 74.0±2.8 and 74.1±0.7 Ma, respectively. The coeval evolution of post-collisional/calc-alkaline H- to A-type magmatism was possibly associated with source heterogeneity and variable involvement of continental materials during the evolution of these granitoids. These new age data constrain the timing of the onset of a post-collision extensional period following the Alpine thickening within the passive margin of the Tauride–Anatolide platform, which occurred before the opening of the latest Cretaceous central Anatolian basins.An erratum to this article can be found at     

Photoreactivation of Escherichia coli and Yersinia enterolytica after Irradiation with a 222 nm Excimer Lamp Compared to a 254 nm Low‐pressure Mercury Lamp

Photoreactivation of Escherichia coli ATCC 11229 and Yersinia enterolytica ATCC 4780 after irradiation with a 222 nm krypton‐chloride excimer lamp compared to a 254 nm mercury lamp was investigated under laboratory conditions. The bacteria samples were irradiated each with different doses of both wavelengths. After irradiation one sample of the bacteria was illuminated with fluorescent light, the other sample was stored in darkness to prevent photoreactivation. The inactivation curves were determined. Without photoreactivation, an irradiation of 69 J/m² at 254 nm was sufficient for a 4 log reduction for E. coli, and only 59 J/m² for Y. enterolytica. To get a 4 log reduction with following photoreactivation, 182 J/m² were necessary for E. coli and 180 J/m² for Y. enterolytica. After irradiation with the 222 nm excimer lamp the ratios were different. Without photoreactivation, an irradiation of 106 J/m² at 222 nm was sufficient for a 4 log reduction for E. coli and 88 J/m² for Y. enterolytica. With photoreactivation 161 J/m² were necessary for E. coli to get a 4 log reduction and 117 J/m² for Y. enterolytica. When the photoreactivation after irradiation is excluded, the mercury lamp with 254 nm clearly shows better results regarding inactivation. Whereas, when included, the excimer lamp with 222 nm wavelength obviously shows better results.     

The geochemical cycle of boron: Constraints from boron isotope partitioning experiments between mica and fluid

The fractionation of boron isotopes between synthetic boromuscovite and fluid was experimentally determined at 3.0 GPa/500 °C and 3.0 GPa/700 °C. For near-neutral fluids Δ¹¹B_(mica-fluid) = δ¹¹B_(mica) − δ¹¹B_(fluid) is − 10.9 ± 1.3‰ at 500 °C, and − 6.5 ± 0.4‰ at 700 °C. This supports earlier assumptions that the main fractionation effect is due to the change from trigonal coordination of boron in neutral fluids to tetrahedrally coordinated boron in micas, clays and melts. The T-dependence of this effect is approximated by the equation Δ¹¹B_{(mica,clay,melt–neutral fluid)} = − 10.69 · (1000/T [K]) + 3.88; R² = 0.992, valid from 25 °C for fluid–clay up to about 1000 °C for fluid–silicate melt. Experiments at 0.4 GPa that used strongly basic fluids produced significantly lower fractionations with Δ¹¹B_{(mica–fluid)} of − 7.4 ± 1.0‰ at 400 °C, and − 4.8 ± 1.0‰ at 500 °C, showing the reduced fractionation effect when large amounts of boron in basic fluids are tetrahedrally coordinated. Field studies have shown that boron concentrations and ¹¹B/¹⁰B-ratios in volcanic arcs systematically decrease across the arc with increasing distance from the trench, thus reflecting the thermal structure of the subducting slab. Our experiments show that the boron isotopic signature in volcanic arcs probably results from continuous dehydration of micas along a distinct P–T range. Continuous slab dehydration and boron transport via fluid into the mantle wedge is responsible for the boron isotopic signature in volcanic arcs.     

Experimental calibration of lake-sediment spectral reflectance to chlorophyll a concentrations: methodology and paleolimnological validation

Chlorophyll a preserved in lake sediments reflects, in part, past primary production. This study assesses the spectral properties of sedimentary chlorophyll a using visible-near infrared reflectance (VNIR) spectroscopy, with the objective of establishing a new, non-destructive paleolimnological proxy. Reflectance spectra were determined from a dilution series (n = 10) involving incremental additions of pulverized modern algae to a lake sediment matrix of low organic content. This enabled an assessment of the development of sediment reflectance spectra in relation to different sediment chlorophyll a concentrations, and subsequent regression of spectral features against measured concentrations of chlorophyll a and derivatives obtained by high performance liquid chromatography (HPLC). The experiment demonstrates that ubiquitous troughs in sediment reflectance near 675 nm are attributable to chlorophyll a and derivative compounds. A significant correlation (r ² = 0.98, P < 0.01) was obtained between the area of the reflectance trough in the 650–700 nm interval and summed concentrations of chlorophyll a, all derivative isomers, and degradational pheopigments. A simple linear inference model derived from this experiment was applied to a down-core sequence of VNIR spectra from a productive prairie lake (Alberta, Canada), where it produced inferred sediment chlorophyll a concentrations in concordance with HPLC measurements. Although a larger training set is desirable to further refine the inference model, the analyses reported here demonstrate that reflectance spectroscopy provides a rapid, semi-quantitative method for assessing the chlorophyll a content of lake sediments.     

The Formation of Knots and Filaments in Shocks

We present a survey of different kinds of instabilities in the context of radiative colliding flows which greatly contribute to structure formation. In particular, this includes analytical results for different kinds of thin shell instabilities (DI, NDI, NTSI). New numerical results for the non-linear evolution of such instabilities in two dimensions, and their coupling with the thermal cooling instability are presented. The astrophysical implications are briefly outlined, in particular the formation of knots and filaments. This revised version was published online in July 2006 with corrections to the Cover Date.     

Ellenbergerite,a new high-pressure Mg-Al-(Ti,Zr)-silicate with a novel structure based on face-sharing octahedra

Ellenbergerite occurs as purple millimetre-size grains associated with talc, kyanite, clinochlore, rutile, and zircon in composite inclusions within decimetre-large pyrope crystals (90–98 mole percent end-member) in the quartzite layer of the Dora Maira massif, Western Alps, from which coesite has been recently reported (Chopin 1984). It is hexagonal, a=12.255(8), c=4.932(4) Å, Z=1, space group P6₃. Mohs hardness 6.5; D_mes 3.15, D_cal 3.10; no cleavage. Uniaxial negative and vividly pleochroic, colourless, colourless to deep lilac with colour zoning. The intensely coloured variety has 1.6789(5), 1.670(1); microprobe analysis yields SiO₂ 39.1, P₂O₅ 0.45, Al₂O₃ 25.1, TiO₂ 4.0, MgO 22.2, FeO 0.20, sum 99.05 wt.% including H₂O 8.0 (coulometrically). The formula calculated on a O₂₈(OH)₁₀ basis (implying 7.5 wt.% H₂O) is Mg_6.71 Fe_0.03 Ti_0.61 Al_6.00 Si_7.92 P_0.08 O₂₈(OH)₁₀ The colour zoning is due to nearly complete TiZr substitution. In addition ellenbergerite may contain more than 8 wt.% P₂O₅ with strictly correlated changes of Si, Mg, Al and Ti+Zr contents, over 80% of which represent the SiAlPMg substitution.The structure has been determined from 1049 observed independent reflections and refined to R=0.034, R_w=0.031, including six of ten protons. It consists of single chains of face-sharing octahedra with one third vacancies extending along the six-fold screw axes, and of pairs of fully occupied face-sharing octahedra linked by edge-sharing to form octahedral double chains parallel to the twofold screw axes, all interconnected by SiO₄ tetrahedra. It may be compared with the dumortierite polymorph with space group P6₃mc derived hypothetically by Moore and Araki (1978). The structural formula is (Mg,Ti,Zr,)₂ Mg₆(Al,Mg)₆ (Si,P)₂ Si₆ O₂₈(OH)₁₀ Face-sharing octahedra are an unusual feature in silicates which results in a dense structure and reflects, considering the common bulk composition, the uncommon high-pressure formation conditions (about 25–30 kbar, 700–800° C). Ti⁴⁺-Fe²⁺ charge transfer between face-sharing octahedra on the six-fold screw axes most likely accounts for the absorption scheme.