Silicic Bodies Research Articles

Harmonic and statistical analyses of the gravity and topography of Vesta

We examine the gravity and topography of the asteroid 4 Vesta, as recently revealed by the Dawn mission. The observed gravity is highly correlated with the observed topography, and suggests little lateral variation in density. The variance spectra of both gravity and topography follow power laws which are very similar to those seen for the Moon, Mars, Venus, and Earth. A significant way in which Vesta differs from these larger silicate bodies is that both gravity and topography are significantly anisotropic, with more north–south variation than east–west variation. Rapid rotation plausibly contributes to this anisotropy, but only at harmonic degree two. The remainder of the anisotropy appears related to the large impacts which formed the Rheasilvia and Veneneia basins. We note that, as usual, gravitational inverse problems are non-unique. While the observed gravity and topography of Vesta do not preclude existence of a metallic core, they certainly do not require it.

Stress transfer between magma bodies: Influence of intrusions prior to 2010 eruptions at Eyjafjallajökull volcano, Iceland

Stress transfer between separate magma bodies is evaluated by considering how pressure changes related to magma accumulation/propagation influence the stability of a separate nearby magma body. Three‐dimensional numerical models are used to evaluate the stability evolution of a magma body through the calculation of two variables: (i) the variation of the threshold pressure needed to cause failure around the magma body and (ii) the magma pressure change. A parametric study indicates that stress interactions are strongly dependent on the distance between magma bodies as well as the body's shape. Such models are then applied to evaluate stress influence of intrusive activity in 1994, 1999, and 2010 at Eyjafjallajökull volcano, which preceded two eruptions there in 2010. Two cases are considered: influence of these intrusions on (i) a magma reservoir at 20 km distance under the Katla volcano and (ii) a silicic magma body under Eyjafjallajökull. The distance between the Eyjafjallajökull intrusions and the Katla reservoir is sufficiently long to reduce the stress interaction to insignificant levels, with an amplitude of the same order as Earth tides (a few kilopascals). However, cumulative stress transfer due to the intrusions to a remnant silicic shallow body situated below the Eyjafjallajökull is much larger (0.5–2.5 MPa). This mechanical transfer could have contributed to the failure of the silicic body and promoted the chemical mixing/mingling between different magma types, which is commonly interpreted as the main cause of the 2010 explosive eruption of Eyjafjallajökull.

New insights on the occurrence of peperites and sedimentary deposits within the silicic volcanic sequences of the Paraná Magmatic Province, Brazil

Abstract. The PMP (Paraná Magmatic Province) is characterized by lava flows of the Early Cretaceous Serra Geral Formation which covers about 75% of the Paraná Basin (southern and southeastern Brazil), composed of a thick (up to 1600 m) volcanic sequence formed by a succession of petrographically and geochemically distinct units of basic and silicic composition. The whole package must have been emplaced during approximately 3 million years of nearly uninterrupted activity. A few aeolian sandstone layers, indicating arid environmental conditions (the Botucatu Formation), are interlayered in the lower basalts. Above the basalts, the Palmas and Chapecó Members are composed of silicic volcanic rocks (quartz latites, dacites, rhyodacites and rhyolites) and basalts. This paper presents new evidence of sedimentation episodes separating silicic volcanic events, expressed by the occurrence of sedimentary deposits. Interaction between the volcanic bodies and the coeval unconsolidated sediments formed peperites. The sediments were observed between basaltic lava flows and silicic rocks or interlayered in the Palmas-type rocks, between the Chapecó-type rocks and overlying basaltic flows, between silicic bodies of the Palmas and Chapecó types, and interlayered within Palmas-type units. The observed structures indicate that the sediments were still wet and unconsolidated, or weakly consolidated, at the time of volcanism, which, coupled with the sediment features, reflect environmental conditions that are different from those characterizing the Botucatu arid conditions.

Investigation of Glaze Layer Formation on Green Ceramic Bodies

Using a high-speed and high-resolution-camera system of droplets in glaze sprays and their impact on the surface of a green ceramic body based on silicate tiles have been characterized. The influence of the surface tension of the glaze slips on the droplet size distribution and their wetting behavior on contact with the green silicate body surface have been investigated. The unfired glaze layers have been studied by means of scanning electron microscopy and computer tomography. The impact angle of the droplets as well as the size of the droplets is of great importance for a high glaze surface quality.

Geophysical implications of the long-wavelength topography of the Saturnian satellites

[1] We use limb profiles to quantify the long‐wavelength topography of the Saturnian satellites. The degree 2 shapes of Mimas, Enceladus, and Tethys are not consistent with hydrostatic equilibrium. We derive 2‐D topographic maps out to spherical harmonic degree 8. There is a good correlation with topography derived from stereo techniques. If uncompensated, topography at degree 3 and higher is large enough to be detectable during close spacecraft flybys. If not properly accounted for, this topography may bias estimates of a satellite’s degree 2 gravity coefficients (which are used to determine the moment of inertia). We also derive a one‐dimensional variance spectrum (a measure of how roughness varies with wavelength) for each body. The short‐wavelength spectral slope is − 2t o−2.5, similar to silicate bodies. However, unlike the terrestrial planets, each satellite spectrum shows a reduction in slope at longer wavelengths. If this break in slope is due to a transition from flexural to isostatic support, the globally averaged elastic thickness Te of each satellite may be derived. We obtain Te values of ≥5 km, 1.5–5 km, ≈5 km, and ≥5 km for Tethys, Dione, Rhea, and Iapetus, respectively. For Europa, we obtain Te ≈ 1.5 km. These estimates are generally consistent with estimates made using other techniques. For Enceladus, intermediate wavelengths imply Te ≥ 0.5 km, but the variance spectrum at wavelengths greater than 150 km is probably influenced by long‐ wavelength processes such as convection or shell thickness variations. Impact cratering may also play a role in determining the variance spectra of some bodies. Citation: Nimmo, F., B. G. Bills, and P. C. Thomas (2011), Geophysical implications of the long‐wavelength topography of the Saturnian satellites, J. Geophys. Res., 116, E11001, doi:10.1029/2011JE003835.

Ganymede crater dimensions – Implications for central peak and central pit formation and development

The morphology of impact craters on the icy Galilean satellites differs from craters on rocky bodies. The differences are thought due to the relative weakness of ice and the possible presence of sub-surface water layers. Digital elevation models constructed from Galileo images were used to measure a range of dimensions of craters on the dark and bright terrains of Ganymede. Measurements were made from multiple profiles across each crater, so that natural variation in crater dimensions could be assessed and averaged scaling trends constructed. The additional depth, slope and volume information reported in this work has enabled study of central peak formation and development, and allowed a quantitative assessment of the various theories for central pit formation. We note a possible difference in the size-morphology progression between small craters on icy and silicate bodies, where central peaks occur in small craters before there is any slumping of the crater rim, which is the opposite to the observed sequence on the Moon. Conversely, our crater dimension analyses suggest that the size-morphology progression of large lunar craters from central peak to peak-ring is mirrored on Ganymede, but that the peak-ring is subsequently modified to a central pit morphology. Pit formation may occur via the collapse of surface material into a void left by the gradual release of impact-induced volatiles or the drainage of impact melt into sub-crater fractures.

HIGHLY METAMORPHOSED IRON-FORMATION ON ARCEDECKNE ISLAND, BOOTHIA PENINSULA, ARCTIC CANADA, AND THE PARAGENESIS OF HARRISONITE, Ca(Fe,Mg)6(SiO4)2(PO4)2

Summary A layered quartz-iron silicate body, ca. 700 m x 200 m, bordered concordantly by Paleoproterozoic, granulite facies, supracrustal gneisses, outcrops on Arcedeckne Island, off northern Boothia Peninsula, Nunavut and is the type locality of the rare silico-phosphate mineral harrisonite (Roberts et al. 1993; Grice and Roberts 1993). Layers and lenses, up to 0.7 m thick, of massive garnetite are intercalated with microscopically layered quartz-orthopyroxene (or, rarely, olivine)-garnet granofels and subordinate, graphitic biotite-orthopyroxene-garnet metapelite. Contacts among the three rock types are commonly gradational. Apatite is a near-ubiqitous constituent of the rocks, which are anomalously rich in phosphorus. Garnet has mol % almandine of 84-92 in garnetite and granofels and 76-80 in metapelite. Orthopyroxene composition is Fs71-83 in garnetite and granofels, Fs59-63 in metapelite. In the two most iron-rich rocks analyzed, olivine Fo5-7 has crystallized in place of orthopyroxene. Although variable in composition, biotite is consistently rich in iron and titanium. Ilmenite is the sole oxide phase identified throughout the Arcedeckne body; pyrrhotite is a sparse accessory mineral in garnetite.

Magma–tectonic interaction and the eruption of silicic batholiths

Due to its unfavorable rheology, magma with crystallinity exceeding about 50 vol.% and effective viscosity > 10 6 Pa s is generally perceived to stall in the Earth's crust rather than to erupt. There is, however, irrefutable evidence for colossal eruption of batholithic magma bodies and here we analyze four examples from Spain, Mexico, USA and the Central Andes. These silicic caldera-forming eruptions generated deposits characterized by i) ignimbrites containing crystal-rich pumice, ii) co-ignimbritic lag breccias and iii) the absence of initial fall-out. The field evidence is inconsistent with most caldera-forming deposits, which are underlain by initial fall-out indicating deposition from a sustained eruption column before the actual collapse sequence. In contrast, the documented examples suggest early deep-level fragmentation at the onset of eruption and repeated column collapse generating eruption volumes on the order of hundreds of cubic kilometers almost exclusively in the form of ignimbrites. These examples challenge our understanding of magma eruptability and eruption initiation processes. In this paper, we present an analysis of eruption promoters from geologic, theoretical and experimental considerations. Assessing relevant dynamics and timescales for failure of crystal-melt mush we propose a framework to explain eruption of batholithic magma bodies that primarily involves an external trigger by near-field seismicity and crustal failure. Strain rate analysis for dynamic and static stressing, chamber roof collapse and rapid decompression indicates that large “solid-like” silicic reservoirs may undergo catastrophic failure leading to deep-level fragmentation of batholithic magma at approximately 2 orders of magnitude lower strain rates than those characteristic for failure of crystal-poor magmas or pure melt. Eruption triggers can thus include either amplified pressure transients in the liquid phase during seismic shaking of a crystal-melt mush, decompression by block subsidence or a combination of both. We find that the window of opportunity for the eruption of large silicic bodies may thus extent to crystallinities beyond 50 vol.% for strain rates on the order of > 10 − 3 to 10 − 4 s − 1 .

Decompression and H2O exsolution driven crystallization and fractionation: development of a new model for low-pressure fractional crystallization in calc-alkaline magmatic systems

Magma ascent, decompression-induced H2O exsolution and crystallization is now recognized as an important process in hydrous subduction zone magmas. During the course of such a process calculations suggest that the ascent rate of a degassing and crystallizing mafic magma will be greater than crystal settling velocities. Thus, any crystals formed as a consequence of volatile exsolution will remain suspended in the magma. If the magma erupts before the percentage of suspended crystals reaches the critical crystallinity value for mafic magma (~55 vol.%) it will produce the commonly observed crystal rich island arc basalt lava. If the magma reaches its critical crystallinity before it erupts then it will stall within the crust. Extension of compaction experiments on a 55 vol.% sand-Karo syrup suspension at different temperatures (and liquid viscosities) to the likely viscosities of interstitial andesitic to dacitic liquid within such a stalled magma suggest that small amounts (up to ~10%) can be expelled on a time scale of 1–10 years. The expelled liquid can create a new intermediate to silicic body of magma that is related to the original mafic magma via fractional crystallization. The short time scale for liquid expulsion indicate that decompression-induced H2O exsolution and crystallization can be an important mechanism for fractional crystallization. Based on this assumption a general model of decompression-induced crystallization and fractionation is proposed that explains many of the compositional, mineralogical and textural features of Aleutian (and other andesites).

Photocatalytic activity for exposed building materials

Photocatalysis is a very promising method to face most of the problems connected with the increasing environmental pollution. Titanium dioxide in its anatase crystallographic phase is the most investigated photocatalytic material and results to be perfectly compatible with silicate body mixes. The possibility to obtain photocatalytic heavy clays materials, to be used for outdoor applications, such as roof tiles, floor tile and outdoor covering tiles, could represent an important goal. In the present work, several titania powders and a titania nanosuspension were added to a body mix used for heavy clay products and the influence of their morphological characteristics on the photoactivity of the fired materials was studied. The titania powders, characterised by the aid of SEM, X-ray diffraction analysis and surface area measurements, BET, presented particles with different aspect ratio and size, ranging from micrometric to nanoparticles. Besides, the nanosuspension was characterised by FEG, TEM, DLS and XRD on the dried suspension. The photocatalytic activity was assessed in aqueous mixtures, by analysing the degradation of an organic dye with a spectrophotometer. The results were explained taking into account the anatase to rutile phase transformation and its possible reaction, during sintering, with the starting raw materials.

When and how did plate tectonics begin? Theoretical and empirical considerations

Plate tectonics is the horizontal motion of Earth’s thermal boundary layer (lithosphere) over the convecting mantle (asthenosphere) and is mostly driven by lithosphere sinking in subduction zones. Plate tectonics is an outstanding example of a self organizing, far from equilibrium complex system (SOFFECS), driven by the negative buoyancy of the thermal boundary layer and controlled by dissipation in the bending lithosphere and viscous mantle. Plate tectonics is an unusual way for a silicate planet to lose heat, as it exists on only one of the large five silicate bodies in the inner solar system. It is not known when this mode of tectonic activity and heat loss began on Earth. All silicate planets probably experienced a short-lived magma ocean stage. After this solidified, stagnant lid behavior is the common mode of planetary heat loss, with interior heat being lost by delamination and “hot spot” volcanism and shallow intrusions. Decompression melting in the hotter early Earth generated a different lithosphere than today, with thicker oceanic crust and thinner mantle lithosphere; such lithosphere would take much longer than at present to become negatively buoyant, suggesting that plate tectonics on the early Earth occurred sporadically if at all. Plate tectonics became sustainable (the modern style) when Earth cooled sufficiently that decompression melting beneath spreading ridges made thin oceanic crust, allowing oceanic lithosphere to become negatively buoyant after a few tens of millions of years. Ultimately the question of when plate tectonics began must be answered by information retrieved from the geologic record. Criteria for the operation of plate tectonics includes ophiolites, blueschist and ultra-high pressure metamorphic belts, eclogites, passive margins, transform faults, paleomagnetic demonstration of different motions of different cratons, and the presence of diagnostic geochemical and isotopic indicators in igneous rocks. This record must be interpreted individually; I interpret the record to indicate a progression of tectonic styles from active Archean tectonics and magmatism to something similar to plate tectonics at ∼1.9 Ga to sustained, modern style plate tectonics with deep subduction—and powerful slab pull—beginning in Neoproterozoic time.

Lava fountains from the 1999 Tvashtar Catena fissure eruption on Io: Implications for dike emplacement mechanisms, eruption rates, and crustal structure

The first direct evidence that patterns of basaltic eruptive activity may be very similar on Io and Earth was seen on November 26, 1999, when the solid‐state imaging system on board the Galileo spacecraft obtained high‐resolution images of a fissure eruption taking place in one of the calderas of the Tvashtar Catena complex. We analyzed the dynamics of the fissure eruption from the morphology of the lava fountains and pyroclastic deposits and derived an estimate of the eruption rate. Our best estimate of the volume eruption rate per unit length of fissure lies in the range 0.7 to 7 m3 s−1 m−1, and multiplying this rate by the fissure length of ∼25 km yields a total volume eruption rate of magma of ∼2×104 to 2×105 m3 s−1. The upper end of the inferred eruption rate is very close to the upper end of the range of eruption rates observed or inferred for fissure eruptions on Earth. Theoretical models of the possible dike systems feeding the fissure show that for eruptions of positively buoyant magma, the elastic properties of rocks are such that a magma flux of 6 to 7 m3 s−1 m−1 is the largest value that can ever be realized in such eruptions, this limiting value being independent of planetary gravity and thus the same on all silicate bodies. The eruption rate inferred for Tvashtar can also be achieved by a negatively buoyant magma, but in that case the source depth cannot be much greater than a few tens of kilometers. We find that some eruptive events are consistent with the effusion of magmas which are buoyant relative to the crust, whereas others appear to involve magmas which are not. This implies that the distribution of crustal volatiles is laterally very inhomogeneous. Our analyses suggest that the depths from which magmas make their final ascent to the surface of Io are generally of order tens, rather than hundreds, of kilometers.

Impact Features on Europa: Results of the Galileo Europa Mission (GEM)

During the Galileo Europa Mission (GEM), impact features on Europa were observed with improved resolution and coverage was compared with Voyager or the Galileo nominal mission. We surveyed all primary impact features >4 km in diameter seen on Europa (through orbit E19). The transition from simple to complex crater morphology occurs at a diameter of about 5 km. We calculated the transient crater dimensions and excavation depths of all craters surveyed. The largest impact feature (Tyre) probably had a transient crater depth between 5 and 10 km and transported material to the surface from a depth of not greater than ∼4 km. Craters <30 km in diameter, such as Manannàn and Pwyll, formed within targets whose immediate subcrater materials exhibited nonfluid behavior on time scales of the impact event, and that are capable, especially in the case of Pwyll, of supporting significant local topographic loads such as a central peak. These craters are nevertheless quite shallow, with very subdued floors, and we suspect that Manannàn and Pwyll's small depth-to-diameter ratios are due to the isostatic adjustment of large-scale topography, facilitated by warm, plastically deformable ice at depth. Morphological similarities between Callanish and Tyre strongly imply that conclusions reached regarding Callanish in J. Moore et al. (1998, Icarus 135, 127–145) also apply to Tyre, which was that Callanish is the consequence of impact into target materials that are mechanically very weak at depth. New evidence that Callanish's circumferential rings formed before the proximal ejecta became immobile implies a low-viscosity substrate at the time of impact. We also report additional evidence that a component of the proximal ejecta of Callanish was emplaced as a fluid. Our observations of Pwyll secondaries support the conclusions stated in Alpert and Melosh (1999) that impacts on icy bodies eject smaller fragments and that fragment size decreases more gradually as velocity increases than observed for impacts on silicate bodies at equivalent ejection velocities. Examination of Pwyll's secondary craters reveals azimuthal variations, with ejecta fragment sizes being larger near the center of a ray than off the ray. Our initial analysis of the characteristic size distribution of Pwyll's secondary craters shows that they form a differential slope slightly shallower than −4. Similar steep slopes for small craters on Ganymede imply that small craters there are mostly formed by secondary impact, and the jovian system may thus be deficient in small impacts relative to the environment of the terrestrial planets.

A New Species of Aristolochia (Aristolochiaceae) from Bahia, Brazil

Aristolochia longispathulata, a new species from the is described and illustrated. The new species belongs to Aristolochia subser. Hexandrae F. Gonzalez. It is easily recognized by flowers that are slightly curved and have a long, spatulate, fimbriate limb, which is pale green on the inner surface. Aristolochia longispathulata is similar to A. arcuata and A. rumicifolia, from eastern and southeastern Brazil. Its floral morphology also resembles that of A. peltata, from the West Indies, and A. sessilifolia, from southern Brazil, Argentina, and Uruguay. RESUMO. Neste trabalho uma nova especie de Aristolochiaceae da Mata Atlantica, Bahia, Brasil, A. longispathulata, e descrita e illustrada. Esta espc Capellari, 1996; Moore, 1915). A new collection of Aristolochia from Bahia, Brazil, has come to my attention because of its long, spatulate, fimbriate flowers (Fig. 1). This collection does not match any species known to occur in northeastern or eastern South America (see Ahumada, 1967, 1975, 1977, 1979, 1989; Feuillet & Poncy, 1998; Hauman, 1923; Hoehne, 1927, 1942; Masters, 1875; Moore, 1915). There is some resemblance with A. rumicifolia Martius & Zuccarini (see Martius & Zuccarini, 1824, tab. 54), from the states of Sao Paulo and Rio de Janeiro, and A. arcuata Masters (Fig. 2), from the states of Minas Gerais, Sao Paulo, and Rio de Janeiro. However, the last two species differ from the Bahian collection in several important diagnostic characters (Table 1), such as the shape and size of the leaves, the length of the peduncle and ovary, and the overall shape of the perianth. The discovery of a new Aristolochia species comes as no surprise as the area of Bahia is well known for endemism not only for plants but also for various groups of animals (see, e.g., Mori et al., 1983). Unfortunately, only a few patches of undisturbed forest remain in that area, which may be the reason for having no other specimens of this new species besides the type collection. Aristolochia longispathulata F. Gonzalez, sp. nov. TYPE: Brazil. Bahia: Jussari, Serra do Teimoso, 7.5 km N of Jussari on road to Palmira, then 2 km W to Fazenda Teimoso, then 45 min. walk W to Reserva da Fazenda Teimoso, 15010'S, 39035'W, 300-450 m, 8 Feb. 1998 (fl), W W Thomas, A. M. Amorim & J. L. Paixao 11741 (holotype, CEPEC; isotype, NY). Figure 1. Aristolochia arcuata et A. rumicifolia affinis, a qua imprimis differt perianthio subrecto, usque 9.5 cm longo, limbo spatulato longiore usque 7 cm longo, fimbriato, pagina interna viridi-glauca. Glabrescent vine. Petioles to 2.5 cm long; blades narrowly ovate, 8-16 cm long, 3-4.5 cm wide, membranaceous, smooth, base slightly cordate, sinus 5-10 mm deep, lobes parallel, apex obtuse, glabrous and with small white, siliceous bodies above, pale green and puberulous especially along the veins below; venation acrodromous, basal primary veins 3, prominent on the lower side, higher order venation densely reticulate, prominulous on NovoN 10: 371-374. 2000. This content downloaded from 157.55.39.211 on Mon, 08 Aug 2016 06:05:20 UTC All use subject to http://about.jstor.org/terms

Species of Thaumatomastix (Thaumatomastigidae, Protista incertae sedis) from the Arctic sea ice biota (North-East Water Polynya, NE Greenland)

The sea ice biota of polar regions contains numerous heterotrophic flagellates very few of which have been properly identified. The whole mount technique for transmission electron microscopy enables the identification of loricate and scaly forms. A survey of Arctic ice samples (North-East Water Polynya, NE Greenland) revealed the presence of ca. 12 taxa belonging to the phagotrophic genus Thaumatomastix (Protista incertae sedis). Species of Thaumatomastix possess siliceous body scales and one naked and one scale-covered flagellum. The presence in both Arctic samples and sea ice material previously examined from the Antarctic indicates that this genus is most likely ubiquitous in polar sea ice and may be an important component in sea ice biota microbial activities.

Catastrophic Impacts on Gravity Dominated Asteroids

We use the smoothed particle hydrodynamics method to simulate catastrophic collisions on silicate bodies whose impact response is dominated by gravity rather than material strength. Encounter speeds of 3, 5, and 7 km sec−1, impact angles of 15°, 45°, and 75°, and target diameters of 10 to 1000 km are investigated. The projectile and target materials are modelled using the Tillotson equation of state for granite. Our model treats gravity rigorously, but neglects strength and fracture effects. We calculate the initial hydrodynamic phase of each event; after the impact shock wave crosses the target, particle motions are nearly ballistic and can be treated analytically. Material that does not escape may reaccrete ∼1 hr to ∼1 yr after the impact. The partitioning of impact energy into heat and motion of projectile and target material favors kinetic energy at higher speeds and larger projectile:target diameter ratios, but does not depend on the absolute size scale of the event. After the impact, most of the kinetic energy is carried by a small amount of fast ejecta. Particle velocity distributions are not sensitive to size scale and have complex, evolving shapes that are poorly represented by simple approximations. The catastrophic threshold (impact energy per unit target mass required to permanently eject 50% of the target against gravity) ranges from 8 × 103J kg−1at 10 km diameter to 1.5 × 106J kg−1at 1000 km, varying as target diameter to the 1.13 ± 0.01 power. Extrapolating these results suggests that gravity dominance extends to stony bodies as small 250 ± 150 m in diameter, smaller than previously believed. This result implies that asteroids as small as a few hundred meters across may be “rubble piles.” Nearly catastrophic impacts can exhume target core material and catapult surface rocks to the antipodes (“scrambling” the target), but selective removal of the outer layers is inefficient. Most material strongly heated in these impacts escapes, limiting globally averaged heating from a single collision to ≤50°C for asteroids ≤1000 km in diameter.

Infrared reflectance spectra (4–12 µm) of typical lunar samples

A laboratory study of the mid‐infrared spectral reflectance properties of typical Apollo lunar rock and soil samples was conducted using a modern interferometric spectrometer. The results show that characteristic spectral features exist in the 4–12 micrometer range that are diagnostic of rock composition and mineralogy, especially for the plagioclase‐rich rocks and soils. This is the first phase of a comprehensive infrared reflectance study underway to better understand the use of infrared emission spectroscopy for mapping the surface composition of the Moon and other silicate bodies.

The ultrastructure of Heleopera petricola an agglutinate soil amoeba; with comments on feeding and silica deposition

The ultrastructure of H. petricola is described from cultured specimens. Siliceous shell plates are bound by a continuous layer of organic cement which is thicker in the posterior half of the body. The cytoplasm is divided into two distinct portions: anteriorly it is vacuolar and composed of numerous pseudopodial strands; posteriorly it is a compact mass containing the organelles, two or three contractile vacuoles, a typical 'ovular' nucleus with several nucleoli that occupies a central position in a region of dense endoplasmic reticulum, and numerous dictyosomes. Prey is captured by a loose arrangement of pseudopodial strands and digestion occurs within specific areas of compact cytoplasmic strands. It is suggested that the cyst wall of naked amoebae is ruptured by specific enzymatic activity and the contents digested after penetration by cytoplasmic strands. Predated siliceous body plates, enclosed in membrane bound vacuoles are modified by the addition of silica to their surfaces. Some organic particles, like bacteria, may also be coated with biogenic silica. Comparisons are made both with the predation of fungal spores by giant naked amoebae and silica biomineralization in another testate amoeba, Netzelia tuberculata.

Ultrastructure of the vegetative organisation and initial stages of silica plate deposition in the soil testate amoebaCorythion dubium

The siliceous body plates ofCorythion dubium are bound by a band of organic cement which is thickest at the lateral margins. The anterior vacuolar cytoplasm is separated by a pigment zone, which forms a dark band in the mid-body region, from the compact posterior region containing a typical vesicular nucleus surrounded by a region of dense endoplasmic reticulum. A pellicular basket of microtubules surrounds the posterior cytoplasm. The large Golgi complex lies between the nucleus and the fundus. Numerous coated and uncoated vesicles from the Golgi cisternae are seen in the peripheral cytoplasm alongside developing plates. These small siliceous plates are enclosed in silicon deposition vesicles lying in surface ruffles of the plasmalemma, often in association with a pair of microtubules. Observations are made on the formation of these vesicles and the early stages of silica deposition. A comparison is drawn between silica deposition inC. dubium and choanoflagellates where there is a similar association between silicon deposition vesicles and microtubules.

Introduction to the Special Section on the Southwestern Nevada Volcanic Field

The southwestern Nevada volcanic field (SWNVF) is a classic area for studying the relationships among ash flow sheets, lava flows, calderas, and high‐silica magma evolution. Ash flow sheets are fundamental to our understanding of the evolution of high‐level silicic bodies because they represent a sample of a large portion of a magma body at an instant in time. This special section is a report of research conducted on four caldera complexes that form a large part of the southwestern Nevada volcanic field. These complexes erupted over a period of 16–6.5 m.y. ago, and their eruptive products cover an area about 160 km long by 110 km wide.