Internal structure and occurrence of accretionary lapilli — a case study at Laacher See Volcano

Accretionary lapilli are common in fine-grained pyroclastic flow and surge deposits and related co-ignimbrite/co-surge ash layers of Laacher See volcano. Two morphologically different types are distin-guished: (1) Rim-type lapilli are composed of a coarse-grained core surrounded by a fine-grained rim. Rims are internally graded or made up of several layers of alternating fine and very-fine grained ash. (2) Core-type lapilli lack fine-grained rims. Field relationships, internal, and grain-size characteristics are specific to accretionary lapilli from different types of tephra deposits. Accretionary lapilli may therefore be a helpful tool to infer the origin of tephra of different origin. In co-ignimbrite ashfall, accretionary lapilli are generally concentrated at the base, whereas pyroclastic flow and surge deposits contain lapilli in the upper parts of individual, thin-bedded layers. Rim-type lapilli are found in pyroclastic flow and surge deposits up to 4 km from the source. Core-type lapilli occur at greater distances or are associated with vesiculated tuffs where they are within 1 km from the vent. Accretionary lapilli from co-ignimbrite/co-surge ash show open framework textures and edge-to-face contacts of individual ash particles. Vesicularity is generally low but the overall porosity of 40% to 50% results in an average density of 1200 kg/m³. Accretionary lapilli in pyroclastic flow and surge deposits are more densely packed and platy particles are often in face-to-face contacts. Vesicularity of those from pyroclastic flow deposits is significantly higher; the overall porosity is about 30% to 40% and the average density 1600 kg/m³. Grain-size analyses show that the accretionary lapilli in co-ignimbrite/co-surge ashfall deposits are the most fine-grained with a median (Md) of 20 to 30 m and a maximum grain size of 250 to 350 m. Accretionary lapilli from pyroclastic flow deposits have intermediate Md-values of 30 to 50 m and a maximum grain size of 350 to 500 m. Those of surge deposits are the coarsest grained with Md-values of 30 to >63 m and a maximum grain size up to 2 mm.     

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.     

Direct dating of hydrothermal W mineralization: U-Pb age for hübnerite (MnWO4), Sweet Home Mine, Colorado

We have investigated the potential of hübnerite for U-Pb dating. Hübnerite forms typically at medium to low-temperatures in a wide range of pneumatolytic-hydrothermal mineral deposits, particularly porphyry molybdenum and Sn-specialized granites. Hübnerite from the Sweet Home Mine (Alma, Colorado) formed in a Pb-rich, U-poor environment, but still developed relatively radiogenic Pb isotopic compositions. The low Pb_common contents in hübnerite (0.075 to 0.155 ppm) demonstrate that Pb is efficiently excluded from the crystal lattice. In contrast, U may substitute for Mn. The U-Pb data of hübnerite scatter. Most of the scatter originates from samples with ²⁰⁶Pb/²⁰⁴Pb values below 50, where Pb_blank contributes up to 30% to Pb_total. Using the least radiogenic galena Pb, samples with ²⁰⁶Pb/²⁰⁴Pb values above 70 have overlapping ²⁰⁶Pb∗/²³⁸U and ²⁰⁷Pb∗/²³⁵U values and yield a ²⁰⁶Pb/²³⁸U age of 25.7 ± 0.3 Ma (2σ). Late stage apatite from the Sweet Home Mine yields a ²⁰⁶Pb/²⁰⁴Pb-²³⁸U/²⁰⁴Pb isochron corresponding to an age of 24.8 ± 0.5 Ma (2σ). A comparison of the U-Pb hübnerite ages with literature ⁴⁰Ar/³⁹Ar ages on earlier sericite and the U-Pb age on later apatite suggests that (i) hübnerite yields accurate U-Pb ages and (ii) the evolution of the Sweet Home mineralization from greisen-type mineralization to medium-temperature hydrothermal vein mineralization took place in a few hundred thousand years at most. Aqueous low-N₂-bearing and aqueous inclusions in the dated hübnerite have homogenization temperatures between 325 and 356 °C and moderate salinity (up to 6.7 wt% NaCl equiv.). Thus, hübnerite represents one of the rare examples of a mineral that can be dated accurately and carries petrological information.     

A skeleton design theory for spatial data infrastructure

In this paper, we look into the theory of designing geoservice systems, i.e., SDI networks and their constituent SDI nodes. As the field of SDI is strongly about bridging between geoservice systems, interoperability and harmonisation, it is not surprising that standardisation efforts are of crucial importance in it. These efforts have historically addressed abstract and concrete content models for data and metadata exchange, as well as abstract and concrete behavioural models for computational processes. The list of standards that are in use in the SDI field continues to expand, and reaches out to neighbouring fields such as sensor nets. We argue that given these trends, the resulting levels of standardisation in actual systems, and the complexity of geoservice systems in general, it appears only natural to look into the possibility to define a standardised design theory for SDI and its nodes, which addresses the function base and the communication base. Specifically, we provide an overview of those components that need to be designed, and what are their relationships. We do so in an abstract way, focussing on the concern of information content in this paper, and only hinting at an appropriate theory of realisation based on our skeleton theory.     

New findings on the tectono-metamorphic history of the western Rhenish Massif (Germany) by K–Ar dating of metasedimentary illite

Metapelites, clay-rich sandstones and volcanics from Cambrian, Ordovician and Lower Devonian strata of the western Rhenish Massif underwent a complex regional Variscan tectono-thermal evolution, as shown by mineralogical and K–Ar isotopic analyses of the illite to mica components from three NW-SE transects. The metamorphic degree extended from an anchimetamorphic to an epimetamorphic intensity during two major episodes of illite crystallization at 328 ± 6 and 282 ± 12 Ma. A further late orogenic or post-orogenic extensional activity could also be detected, but not precisely, around 270 Ma, probably recorded by the precipitation of illite in new or reactivated extensional faults with upward moving heat flows.