Wolf Volcano, Galapagos Archipelago: Melting and Magmatic Evolution at the Margins of a Mantle Plume

Wolf volcano, an active shield volcano on northern Isabela Islandin the Galápagos Archipelago, has undergone two majorstages of caldera collapse, with a phase of partial calderarefilling between. Wolf is a typical Galápagos shieldvolcano, with circumferential vents on the steep upper carapaceand radial vents distributed in diffuse rift zones on the shallower-slopinglower flanks. The radial fissures continue into the submarineenvironment, where they form more tightly focused rift zones.Wolf's magmas are strikingly monotonous: estimated eruptivetemperatures of the majority of lavas span a total of only 22°C.This homogeneity is attributed to buffering of magmas as theyascend through a thick column of olivine gabbroic mush thathas been deposited from a thin, shallow (<2 km deep) subcalderasill that is in a thermochemical steady state. Wolf's lavashave the most depleted isotopic compositions of any historicallyactive intraplate ocean island volcano on the planet and haveisotopic compositions (except for ³He/⁴He) indistinguishablefrom mid-ocean ridge basalt erupted from the GalápagosSpreading Center (GSC) 250–410 km away from the peak ofinfluence of the Galápagos plume. Wolf's lavas are enrichedin incompatible trace elements and have systematic major elementdifferences relative to GSC lavas, however. Wolf's magmas resultfrom lower extents of melting, deeper melt extraction, and agreater influence of garnet compared with GSC magmas, but Wolfand the GSC share the same sources. These melt generation conditionsare attributed to melting in a thermal and mechanical boundarylayer of depleted asthenosphere at the margins of the Galápagosplume. The lower degrees of melting and extraction from deeperlevels result from a thicker lithospheric cap at Wolf than existsat the GSC. KEY WORDS: caldera; Galápagos; mush; partial melting; plume     

An Anorthosite Suite in a Ring-Complex: Crystallization and Emplacement of an Anorogenic Type from Abontorok, A?r, Niger

Laminated anorthosite grading outwards into leucogabbro, gabbro,and monzogabbro occurs in a 2.6-km-diameter funnel-shaped intrusion,cut by a quartz alkali syenite plug and concentric syenite andgranite ring-dykes. The anorthosite-gabbro series is laminatedbut not modally or otherwise texturally layered. The lamination,defined by large tabular plagioclase crystals, forms a set ofinwarddipping cones, the dips of which decrease from 60–45?in the central anorthosite to < 25? in the outer gabbros.Rocks close to the outer contact are medium-grained isotropicgabbros. Plagioclase, forming >80% of the series, generallyhas homogeneous labradorite cores (An_62–58 in the wholeseries) and thin strongly zoned rims, which follow progressivelylonger solidus paths from the anorthosites to the gabbros. Allrocks contain a late-magmatic alkali feldspar. Plagioclase isthe main or only cumulus phase, the anorthosites being ad- tomesocumulates and the gabbros orthocumulates. Olivine (FO_49–41)is more abundant than clinopyroxene in most of the series. Dependingon quartz content, the syenites and granites are hypersolvusor subsolvus and the depth of crystallization was calculatedto be 5 ? 2 km. A Rb/Sr isochron for the syenites and granites gave an age of399 ? 10 Ma with an initial strontium isotopic ratio of 0.7084? 0.0005. Ten samples from the anorthosite-gabbro scries havean average calculated initial ratio of 0.70582 ? 0-00004 at– 400 Ma, showing that the two series are not comagmatic.The anorthosite-gabbro series has parallel REE trends (La_N/Yb_N{small tilde} 7–10) with decreasing positive Eu anomaliesand increasing total REE contents from anorthosite to gabbro;two monzogabbros have almost no Eu anomaly. The liquid calculatedto be in equilibrium with the lowest anorthosite has almostno Eu anomaly and its normalized REE pattern lies just abovethose for the monzogabbros. The syenites and granites have complementaryREE patterns with negative Eu anomalies. The inferred parental magma was alkalic and leucotroctoliticwith high TiO₂ P₂O₅, Sr and K/Rb and with low MgO, very similarto parental magmas in the Gardar province, South Greenland.It was probably produced at depth by settling of olivine andclinopyroxene but not of plagioclase, which accumulated by flotation.It is suggested that plagioclase crystals from this lower chamberwere progressively entrained (from 0% in the gabbros to 30–40%in the anorthosites), giving rise to the flow lamination inthe upper chamber. The magma in the lower chamber may have beenlayered, because the plagioclase cores in the anorthosite areconsiderably richer in Or than those in the leucogabbros orgabbros. Overall convection did not occur in the upper chamber,whereas compositional convection occurred in the more slowlycooled central anorthositic adcumulates.     

Rapid Holocene climate changes in the North Atlantic: evidence from lake sediments from the Faroe Islands

Holocene records from two lakes on the Faroe Islands were investigated to determine regional climatic variability: the fairly wind-exposed Lake Starvatn on Streymoy and the more sheltered Lake Lykkjuvötn on Sandoy. Sediment cores were analysed for content of biogenic silica, organic carbon and clastic material, and magnetic susceptibility. In addition, a new qualitative proxy for past lake ice cover and wind activity was developed using the flux of clastic grains that are larger than 255 μm. Both long-term and short-term climatic developments were similar between the two lakes, suggesting a response to a regional climate signal. The long-term climate development is characterized by early Holocene rapid warming followed by Holocene climatic optimum conditions ending around 8300 cal. yr BP. A more open landscape as evidenced from increased sand grain influx in the period 8300–7200 cal. yr BP could reflect the aftermath of the 8200 cal. yr BP event, although the event itself is not recognized in either of the two lake records. From around 7200 cal. yr BP the mid-Holocene climate deterioration is observed and from 4200 cal. yr BP the climate deteriorated further with increased amplitude of centennial cooling episodes.     

Dolomite formation in the dynamic deep biosphere: results from the Peru Margin

Early diagenetic dolomite beds were sampled during the Ocean Drilling Programme (ODP) Leg 201 at four reoccupied ODP Leg 112 sites on the Peru continental margin (Sites 1227/684, 1228/680, 1229/681 and 1230/685) and analysed for petrography, mineralogy, δ¹³C, δ¹⁸O and ⁸⁷Sr/⁸⁶Sr values. The results are compared with the chemistry, and δ¹³C and ⁸⁷Sr/⁸⁶Sr values of the associated porewater. Petrographic relationships indicate that dolomite forms as a primary precipitate in porous diatom ooze and siliciclastic sediment and is not replacing the small amounts of precursor carbonate. Dolomite precipitation often pre‐dates the formation of framboidal pyrite. Most dolomite layers show ⁸⁷Sr/⁸⁶Sr‐ratios similar to the composition of Quaternary seawater and do not indicate a contribution from the hypersaline brine, which is present at a greater burial depth. Also, the δ¹³C values of the dolomite are not in equilibrium with the δ¹³C values of the dissolved inorganic carbon in the associated modern porewater. Both petrography and ⁸⁷Sr/⁸⁶Sr ratios suggest a shallow depth of dolomite formation in the uppermost sediment (<30 m below the seafloor). A significant depletion in the dissolved Mg and Ca in the porewater constrains the present site of dolomite precipitation, which co‐occurs with a sharp increase in alkalinity and microbial cell concentration at the sulphate–methane interface. It has been hypothesized that microbial ‘hot‐spots’, such as the sulphate–methane interface, may act as focused sites of dolomite precipitation. Varying δ¹³C values from −15‰ to +15‰ for the dolomite are consistent with precipitation at a dynamic sulphate–methane interface, where δ¹³C of the dissolved inorganic carbon would likewise be variable. A dynamic deep biosphere with upward and downward migration of the sulphate–methane interface can be simulated using a simple numerical diffusion model for sulphate concentration in a sedimentary sequence with variable input of organic matter. Thus, the study of dolomite layers in ancient organic carbon‐rich sedimentary sequences can provide a useful window into the palaeo‐dynamics of the deep biosphere.     

Ensemble covariances adaptively localized with ECO-RAP. Part 1: tests on simple error models

In atmospheric data assimilation (DA), observations over a 6–12-h time window are used to estimate the state. Non-adaptive moderation or localization functions are widely used in ensemble DA to reduce the amplitude of spurious ensemble correlations. These functions are inappropriate (1) if true error correlation functions move a comparable distance to the localization length scale over the time window and/or (2) if the widths of true error correlation functions are highly flow dependent. A method for generating localization functions that move with the true error correlation functions and that also adapt to the width of the true error correlation function is given. The method uses ensemble correlations raised to a power (ECO-RAP). A gallery of periodic one-dimensional error models is used to show how the method uses error propagation information and error correlation width information retained by powers of raw ensemble correlations to propagate and adaptively adjust the width of the localization function. It is found that ECO-RAP localization outperforms non-adaptive localization when the true errors are propagating or the error correlation length scale is varying and is as good as non-adaptive localization when such variations in error covariance structure are absent.     

Monazite-Xenotime Thermochronometry and Al2SiO5 Reaction Textures in the Picuris Range, Northern New Mexico, USA: New Evidence for a 1450-1400 Ma Orogenic Event

Al₂SiO₅ reaction textures in aluminous schist and quartziteof the northern Picuris range, north-central New Mexico, recorda paragenetic sequence of kyanite to sillimanite to andalusite,consistent with a clockwise P–T loop, with minor decompressionnear the Al₂SiO₅ triple-point. Peak metamorphic temperaturesare estimated at 510–525°C, at 4·0–4·2kbar. Kyanite and fibrolite are strongly deformed; some prismaticsillimanite, and all andalusite are relatively undeformed. Monaziteoccurs as inclusions within kyanite, mats of sillimanite andcentimetre-scale porphyroblasts of andalusite, and is typicallyaligned subparallel to the dominant regional foliation (S₀/S₁or S₂) and extension lineation (L₁). Back-scatter electron imagesand X-ray maps of monazite reveal distinct core, intermediateand rim compositional domains. Monazite–xenotime thermometryfrom the intermediate and rim domains yields temperatures of405–470°C (±50°C) and 500–520°C(±50°C), respectively, consistent with the progradeto peak metamorphic growth of monazite. In situ, ion microprobeanalyses from five monazites yield an upper intercept age of1417 ± 9 Ma. Near-concordant to concordant analyses yield²⁰⁷Pb–²⁰⁶Pb ages from 1434 ± 12 Ma (core) to 1390± 20 Ma (rim). We find no evidence of older regionalmetamorphism related to the 1650 Ma Mazatzal Orogeny. KEY WORDS: Al₂SiO₅; metamorphism; monazite; thermochronometry; triple-point     

Grain-size information from the statistical properties of digital images of sediment

The autocorrelation technique for estimating grain-size from digital images of sand beds has been extended and validated for use on coarse sand (0·7 mm) and gravel (up to ∼20 mm). A number of aspects of the technique have been explored and some potential improvements suggested. Autocorrelation is just one suitable statistical method sensitive to the grain-size of sediment in digital images; four additional techniques are presented and their relative merits discussed. A collective suite of techniques applicable to the general problem of grain-size estimation from digital images of sediment might broaden the applicability to more sedimentary environments, as well as improve its accuracy. These techniques are compared using a large data set from a gravel barrier beach in southern England. Based on over 180 samples, mean grain-size of sieved and imaged sediments correspond to within between 8% and 16%. Some theoretical aspects of the spatial arrangement of image intensity in digital images of natural sediments are addressed, including the fractal nature of sediments in images, which has potential implications for derivation of grain-size distributions from images of sand-sized material through segmentation and thresholding. The methods outlined in this contribution may also find application in further uncovering the geometric structure of sediment beds, as well as in the simulation of sedimentation processes.