The architecture, eruptive history, and evolution of the Table Rock Complex, Oregon: From a Surtseyan to an energetic maar eruption

The Table Rock Complex (TRC; Pliocene–Pleistocene), first documented and described by Heiken [Heiken, G.H., 1971. Tuff rings; examples from the Fort Rock-Christmas Lake valley basin, south-central Oregon. J. Geophy. Res. 76, 5615-5626.], is a large and well-exposed mafic phreatomagmatic complex in the Fort Rock–Christmas Lake Valley Basin, south-central Oregon. It spans an area of approximately 40 km², and consists of a large tuff cone in the south (TRC1), and a large tuff ring in the northeast (TRC2). At least seven additional, smaller explosion craters were formed along the flanks of the complex in the time between the two main eruptions. The first period of activity, TRC1, initiated with a Surtseyan-style eruption through a 60–70 m deep lake. The TRC1 deposits are dominated by multiple, 1-2 m thick, fining upward sequences of massive to diffusely-stratified lapilli tuff with intermittent zones of reverse grading, followed by a finely-laminated cap of fine-grained sediment. The massive deposits are interpreted as the result of eruption-fed, subaqueous turbidity current deposits; whereas, the finely laminated cap likely resulted from fallout of suspended fine-grained material through a water column. Other common features are erosive channel scour-and-fill deposits, massive tuff breccias, and abundant soft sediment deformation due to rapid sediment loading. Subaerial TRC1 deposits are exposed only proximal to the edifice, and consist of cross-stratified base-surge deposits. The eruption built a large tuff cone above the lake surface ending with an effusive stage, which produced a lava lake in the crater (365 m above the lake floor). A significant repose period occurred between the TRC1 and TRC2 eruptions, evidenced by up to 50 cm of diatomitic lake sediments at the contact between the two tuff sequences. The TRC2 eruption was the last and most energetic in the complex. General edifice morphology and a high percentage of accidental material suggest eruption through saturated TRC1 deposits and/or playa lake sediments. TRC2 deposits are dominated by three-dimensional dune features with wavelengths 200–500 m perpendicular to the flow, and 20–200 m parallel to the direction of flow depending on distance from source. Large U-shaped channels (10–32 m deep), run-up features over obstacles tens of meters high, and a large (13 m) chute-and-pool feature are also identified. The TRC2 deposits are interpreted as the products of multiple, erosive, highly-inflated pyroclastic surges resulting from collapse of an unusually high eruption column relative to previously documented mafic phreatomagmatic eruptions.     

The lower mass function of young open clusters: clues to (sub)stellar formation

We report the present day mass functions (PDMFs) of 4 young open clusters over a mass range from 30 Jupiter masses to 3M_ . Three of these clusters have been chosen to have a similar age of ∼100 Myr. Their PDMFs are remarkably similar and are comparable to the field mass function. This suggests little impact of initial conditions (stellar density, metallicity) on the mass distribution and raises some issues concerning the currently debated star and brown dwarf formation theories. The fourth cluster is older (600 Myr) which allow us to investigate the effect of the cluster dynamical evolution on the shape of the mass function. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A Study of DMS Oxidation in the Tropics: Comparison of Christmas Island Field Observations of DMS, SO2, and DMSO with Model Simulations

This study reports comparisonsbetween model simulations, based on current sulfurmechanisms, with the DMS, SO₂ and DMSOobservational data reported by Bandy et al.(1996) in their 1994 Christmas Island field study. For both DMS and SO₂, the model results werefound to be in excellent agreement with theobservations when the observations were filtered so asto establish a common meteorological environment. Thisfiltered DMS and SO₂ data encompassedapproximately half of the total sampled days. Basedon these composite profiles, it was shown thatoxidation of DMS via OH was the dominant pathway withno more than 5 to 15% proceeding through Cl atoms andless than 3% through NO₃. This analysis wasbased on an estimated DMS sea-to-air flux of 3.4 ×10⁹ molecs cm^-2 s^-1. The dominant sourceof BL SO₂ was oxidation of DMS, the overallconversion efficiency being evaluated at 0.65 ± 0.15. The major loss of SO₂ was deposition to theocean's surface and scavenging by aerosol. Theresulting combined first order k value was estimated at 1.6 × 10^-5 s^-1. In contrast to the DMSand SO₂ simulations, the model under-predictedthe observed DMSO levels by nearly a factor of 50. Although DMSO instrument measurement problems can notbe totally ruled out, the possibility of DMSO sourcesother than gas phase oxidation of DMS must beseriously considered and should be explored in futurestudies.     

Spectral-type simulation of spatially correlated fracture set properties

Fracture set properties such as orientation, spacing, trace length, and waviness tend to be spatially correlated. These properties can be efficiently simulated by spectral analysis procedures that take advantage of the computational speed of the fast Fourier transform. The covariance function of each property to be simulated is obtained from the variogram function estimated from mapped fracture set data and is typically referenced to the mean vector of the set. Simulation procedures for normally and exponentially distributed data involve generating uncorrelated Fourier coefficients that are assigned proper variance according to the spectral density, which is the Fourier transform of the covariance function. These coefficients are then reverse Fourier transformed to produce simulated set properties that have the desired variance and variogram function.     

Watson: A new link in the HE iron chain

Abstract— Watson, which was found in 1972 in South Australia, contains the largest single silicate rock mass seen in any known iron meteorite. A comprehensive study has been completed on this unusual meteorite: petrography, metallography, analyses of the silicate inclusion (whole rock chemical analysis, INAA, RNAA, noble gases, and oxygen isotope analysis) and mineral compositions (by electron microprobe and ion microprobe). The whole rock has a composition of an H-chondrite minus the normal H-group metal and troilite content. The oxygen isotope composition is that of the silicates in the HE iron meteorites and lies along an oxygen isotope fractionation line with the H-group chondrites. Trace elements in the metal confirm Watson is a new HE iron. Whole rock Watson silicate shows an enrichment in K and P (each ～2X H-chondrites). The silicate inclusion has a highly equilibrated igneous (peridotite-like) texture with olivine largely poikilitic within low-Ca pyroxene: olivine (Fa₂₀), opx (Fs₁₇Wo₃), capx (Fs₉Wo₄₁) (with very fine exsolution lamellae), antiperthite feldspar (An_1–Or₅) with <1 μm exsolution lamellae (An_1–3Or_>40), shocked feldspar with altered stoichiometry, minor whitlockite (also a poorly characterized interstitial phosphate-rich phase) and chromite, and only traces of metal and troilite. The individual silicate minerals have normal chondritic REE patterns, but whitlockite has a remarkable REE pattern. It is very enriched in light REE (La is 720X C1, and Lu is 90X C1, as opposed to usual chonditic values of ～300X and 100–150X, respectively) with a negative Eu anomaly. The enrichment of whole rock K is expressed both in an unusually high mean modal Or content of the feldspar, Or₁₃, and in the presence of antiperthite. Whole rock trace element data for the silicate mass support the petrography. Watson silicate was an H-chondrite engulfed by metal and melted at > 1550 °C. A flat refractory lithophile and flat REE pattern (at ～1x average H-chondrites) indicate that melting took place in a relatively closed system. Immiscible metal and sulfide were occluded into the surrounding metal host. Below 1100 °C, the average cooling rate is estimated to have been ～1000 °C/Ma; Widmanstätten structure formed, any igneous zoning in the silicates was equilibrated, and feldspar and pyroxene exsolution took place. Cooling to below 300 °C was completed by 3.5 Ga B. P. At 8 Ma, a shock event took place causing some severe metal deformation and forming local melt pockets of schreibersite/metal. This event likely caused the release of Watson into interplanetary space. The time of this event, 8Ma, corresponds to the peak frequency of exposure ages of the H-chondrites. This further confirms the link between HE irons and the H-chondrites, a relationship already indicated by their common oxygen isotope source. Watson metal structures are very similar to those in Kodaikanal. Watson, Kodaikanal and Netschaëvo form the young group of HE meteorites (ages 3.7 ± 0.2 Ga). They appear to represent steps in a chain of events that must have taken place repeatedly on the HE parent body or bodies from which they came: chondrite engulfed in metal (Netschaëvo); chondrite melted within metal (Watson); and finally melted silicate undergoing strong fractionation with the fractionated material emplaced as globules within metal (Kodaikanal). Watson fills an important gap in understanding the sequence of events that took place in the evolution of the IIE-H parent body(ies). This association of H-chondrite with HE metal suggests a surface, or near surface process-a suggestion made by several other researchers.     

Observations on seismic wave equation and reflection coefficient symmetries in stratified media

Summary. The equations describing seismic waves in a stratified earth have a number of symmetry properties, one of which has recently been used by Garmany to derive a simple expression for the inverse of the matrix of eigenvectors appearing in the solution of the equations. We review these symmetries of the wave equation in several notations to demonstrate that the property used by Garmany is distinct from the usual symmetries found in the seismological literature. Like the others, the new symmetry has implications for the reflection and transmission properties of a medium. These implications are briefly reviewed in order to show how the new symmetry is placed relative to the others. A limited discussion of the physical origins of the symmetries is given and, though the overall picture is incomplete, it is noted that the new symmetry yields conservation of energy for reflection/transmission at a single interface in all types of media (anisotropic, lossy, etc.).     

Full reconstruction of a layered elastic medium from P-SV slant-stack data

Summary. We develop a méthod of reconstructing the elastic paraméters as functions of depth, for a horizontally stratified, isotropic elastic half-space. Unlike previous schemes, which have been able to retrieve the shear wave speed and density from SH seismograms slant stacked at two angles, our méthod makes use of P - SV data at a single stacking paraméter to obtain all three elastic constants. The data required are the elements of the full reflection matrix at the surface, corresponding to measurements of two separate components of the response to two independent sources, one explosive, the other generating shear waves.
In developing this inverse scheme fundamental differences emerge between the acoustic or SH problem, and the coupled P - SV case, the most important being in the nature of the interfacial scattering matrix. We show that it is not possible to make use of the downward reflection data for an interface to determine directly the remaining reflection and transmission coefficients, but that the scattering data may be completed by applying a simple iterative procedure at each interface.
We show the result of applying our inverse scheme to seismograms generated for a six-layered model, including a low-velocity layer. We are able to reconstruct both wave speeds and the density as functions of depth, all quantities being in close agreement with the original model.     

THE VERKHNE DNIEPROVSK IRON METEORITE SPECIMENS IN THE VIENNA COLLECTION AND THE CONFUSION OF VERKHNE DNIEPROVSK WITH AUGUSTINOVKA

It is now a hundred years since a small amount of meteoritic material labelled Verkhne Dnieprovsk was first described. Since then the material has been controversial, due to the corroded character and the very limited amount of material known. Authentic samples, totalling 8 g, have been identified in the Vienna collection, which confirm that Verkhne Dnieprovsk is a unique meteorite, both in its composition, belonging to group II E, and in its heavily shocked and distorted structure. The shock-produced structures include micromelts with a phosphorus gradient, suggesting that the melts originated in situ from mm-sized schreibersite crystals. Unfortunately, no additional information as to location and circumstances of find was discovered at this late date. Further work will probably require field work and interviews on the site and/or studies of Russian archives.