Km In Radius Research Articles

The Duksung Fan Delta, Se Korea: Growth of Delta Lobes on a Gilbert-Type Topset in Response to Relative Sea-Level Rise

ABSTRACT The Duksung fan delta is located on the strike-slip fault-bounded western margin of Pohang Basin (Miocene), SE Korea. The fan delta is approximately 6 km in radius and consists of alluvial fan, Gilbert-type topset, foreset, and bottomset, and basin plain. This paper concerns the Gilbert-type topset, which is characterized by stacked units (lobes) of small-scale nested foreset (less than a few meters in height) and coarsening (to fining)-upward cycles (a few meters in thickness) of conglomerate and sandstone. Each lobe prograded radially away from the sediment source with a radius of about 2 km and formed thick (> 150 m) shallow marine successions. We suggest that the relative rise in sea level was largely due to marginal normal faulting during expansion of the basin, instead of eus atic sea-level rise.

Modelling Gas And Dust Release From Comet Hale–Bopp

Numerical simulations of the evolving activity of comet Hale-Bopp are presented, assuming a porous, spherical nucleus, 20 km in radius, made of dust and gas-laden amorphous ice. The main effects included are: crystallization of amorphous ice and release of occluded gas, condensation, sublimation and flow of gases through the pores, changing pore sizes, and flow of dust grains. The model parameters, such as initial pore size and porosity, emissivity, dust grain size, are varied in order to match the observed activity. In all cases, a sharp rise in the activity of the nucleus occurs at a large heliocentric distance pre-perihelion, marked by a few orders of magnitude increase in the CO and the CO2 fluxes and in the rate of dust emission. This is due to the onset of crystallization, advancing down to a few meters below the surface, accompanied by release of the trapped gases. A period of sustained, but variable, activity ensues. The emission of water molecules is found to surpass that of CO at a heliocentric distance of 3 AU. Thereafter the activity is largely determined by the behaviour of the dust. If a dust mantle is allowed to build up, the water production rate does not increase dramatically towards perihelion; if most of the dust is ejected, the surface activity increases rapidly, producing a very bright comet.

Thermal and shock history of mesosiderites and their large parent asteroid

To elucidate the geological evolution of the mesosiderite stony-iron meteorites and their 3.4–3.8 Gy 40Ar 39Ar ages, we have investigated their shock and thermal histories. We have studied shock metamorphism in sixteen mesosiderites and find that none have been shocked to more than 10 GPa (shock stage S1–S2). Three mesosiderites contain a tiny fraction (≈0.1 % overall) of mineral fragments that were shocked to shock stage S3–S6 levels. The uniformity of shock features within all fragments shows that these fragments were not shocked in situ. Our shock data for mesosiderites, the absence of evidence of 3.6–3.9 Gy impact melt, the shock history of impact-heated ordinary chondrites, and the difficulty in quantitatively removing Ar in an impact event, all suggest that the mesosiderite parent body did not suffer a major impact event 3.6–3.9 Gy ago. Metallographic cooling rates of ≈0.03°C/My at 400°C were estimated from taenite lamellae in four mesosiderites using the latest diffusion coefficients and FeNiP phase diagram. Cooling rates of 0.01°C / My at 425–325°C were estimated from published compositional data for kamacite grains in four mesosiderites. These two techniques and four other semiquantitative, metallographic cooling rate indicators show that the mesosiderites cooled slower than any iron meteorite. We infer that cooling rates at 400°C were ∼0.02–0.03°C/My and certainly less than 0.5°C/My. The inferred cooling rate is too slow to allow Ar closure before 4 Gy. All of the shock, thermal, and age data for mesosiderites are consistent with slow cooling at depth < 1 My after metal and silicate were mixed around 4.4 Gy ago. Thermal models indicate that the mesosiderites probably cooled in an asteroid some 200–400 km in radius.

Dark clasts in the Khor Temiki aubrite: Not basalts

Abstract— The enstatite achondrite meteorites (aubrites) are ultramafic assemblages with highly variable bulk rare earth element (REE) compositions. An enrichment of REE in a dark clast from the Khor Temiki aubrite led Wolf et al. (1983) to suggest that such dark clasts could be the basaltic (i.e., enstatite‐plagioclase) complements to the ultramafic aubrites, with the relatively high REE contents resulting from the presence of plagioclase, which is a common carrier of the REEs. We have studied several dark clasts from the Khor Temiki aubrite and find no evidence for a basaltic character for such material. The microscopic character of the dark clasts is not significantly different from the main portions of Khor Temiki and consists either of highly brecciated material, containing a fine‐grained matrix, or of enstatite grains with abundant inclusions. We suggest that the dark clasts are shock‐darkened, heterogeneous Khor Temiki material that, by chance, contained variable trace contents of oldhamite (CaS), which has been shown to be a major carrier of REE in aubrites. We find that the REE contents of the clasts range from 0.1 to ∼20× CI. Most have negative Eu anomalies, but one has a small positive anomaly. Extensive searches have failed to identify basaltic material in Khor Temiki and other aubrites. The absence of basaltic material is consistent with, but does not prove, the model of Wilson and Keil (1991). They calculate that, on an asteroidal parent body &lt; ∼100 km in radius, a volatile‐rich basaltic partial melt erupted with a velocity greater than the escape velocity of the asteroid and, thus, was lost into space ∼ 4.55 Ga ago.

Volatiles in unequilibrated ordinary chondrites: Abundances, sources and implications for explosive volcanism on differentiated asteroids

Abstract— Keil and Wilson (1993) proposed that, during partial melting of some asteroidal meteorite parent bodies, explosive pyroclastic volcanism accelerated S‐rich Fe, Ni‐FeS cotectic partial melts into space. These authors argued that this process was responsible for the S‐depletion of many of the magmas from which the magmatic iron meteorites formed. This process only requires the presence of a few hundred to thousand ppm of volatiles in asteroids &lt; ∼100 km in radius. If the precursor materials of these magmatic iron meteorite groups were similar in composition to unequilibrated ordinary chondrites, then the volatile contents of the latter may be a measure of the potential effectiveness of the process. Analysis of volatile contents of seven unequilibrated ordinary chondrite falls by dynamic high‐temperature mass spectrometry revealed that thousands of ppm of indigeneous volatiles, mostly CO, Cl, Na and S, are released at temperatures near the Fe, Ni‐FeS cotectic melting temperature of ∼980 °C. If these volatiles are largely retained in the asteroidal parent bodies until onset of partial melting, S depletion of the residual melt might have been achieved by ejection of S‐rich partial Fe, Ni‐FeS melts by pyroclastic volcanism.

Radioactive Heating of Porous Comet Nuclei

Radioactive heating is the main energy source of comets residing in the distant parts of the Solar System. It should determine whether liquid water could have existed in comets and whether comets, presumably formed of amorphous ice, could have retained the ice, at least partly, in this pristine form. Thermal evolution calculations of relatively large (over 10 km in radius), porous comet nuclei are performed for many different initial parameter combinations. The radioisotopes considered are 40K, 232 Th, 235U, and 238U, in meteoritic abundances, as well as 26Al, in various initial abundances. We allow for heat conduction through the ice-dust matrix, as well as advection by flowing gases. Crystallization of the amorphous ice accompanied by release of occluded gases, and sublimation/condensation from/ onto the pore walls are taken into account. We find that porous comet nuclei may emerge from the long-term evolution in three different configurations, depending on the thermal conductivity, porous structure, radius, etc.: (a) preserving their pristine structure throughout; (b) almost completely crystallized (except for a relatively thin outer layer), and (c) having a crystallized core, a layer of frozen gas (originally occluded in the amorphous ice) and an outer layer of unaltered pristine material. Liquid cores may be obtained only if the porosity is negligible. The extent of such cores and the length of time during which they remain liquid are again determined by initial conditions, as well as by physical properties of the ice. If, in addition to the very low porosity, the conductivity were extremely low, it should be possible to have both an extended liquid core, for a considerable period of time, and an outer layer of significant thickness that has retained its original pristine structure.

The magnetic bode fallacy

The magnetic moments and angular momenta of Solar System bodies are compared to evaluate the statistical reality of the relation that has come to be called the “magnetic Bode law.” Runcorn has suggested that this “law”, giving a slope of about 3/5 on a log‐log plot, is only a geometrical effect of the angular momenta being proportional to the fifth power of radius and the magnetic moment the cube. The best fit line to the six planets with nonzero magnetic fields is log(m/me) = −0.2 (±0.2) + 0.82 (±0.05) log(L/Le), where the subscripts denote the values for Earth. The value 0.82 is shown to be consistent with the 3/5 slope once estimation bias is accounted for. Monte Carlo analysis was used to construct (2500) synthetic solar systems from the variables representing the range of the planet's radii, densities, coefficients of inertia, periods of rotation, and surface poloidal field intensity. If these variables are considered independent, it was found that the probability of obtaining a slope as different from 0.6 as suggested by Runcorn is 34%, but only a 0.2% chance of obtaining values that are so linear as the actual data on this plot. If instead the covariances of the actual planets are included in the analysis, the mean slope is 0.82 with small deviation, but the odds of obtaining such a tight fit as that observed in the actual Solar System becomes 63%. It is concluded that considering the lack of physical plausibility of the correlations between the physical parameters besides field, the strong correlation with rotation as an important factor in computing the angular momentum is spurious. Comparisons are done using both the angular momenta of the whole planet, and the theorized magnetoactive shells and cores. The conducting volumes are also compared with the poloidal dipole fields external to these magnetoactive regions. The studies show that the volumes of the magnetoactive regions appear to be related to the strength of the poloidal field just outside, and that the tighter trend for the log momentum versus log dipole moment plots is mainly due to the geometric factor suggested by Runcorn. Plots of field just outside these regions versus their volumes show that the Earth and Io have significantly higher fields than those predicted by a line extending from the giant planets to Mercury. Such a line also does not miss Dolginov's estimate for Mars by a half order of magnitude. The upper limit of field of Venus falls 3 orders below this trend. If Mars has a core theorized to be some 1780 km in radius, a weak dynamo resulting in a surface field close to the limiting 10 – 100 nT range suggested by earlier spacecraft observations is not inconsistent with the trend of the other planets. The deviation of Earth and Io from a line joining the other planets is thought to be due to the added vigor of convection from their extra sources of energy. For Earth this would likely be caused by freezing of the inner core whereas for Io the enhancement would be from tidal heating as previously suggested.

Environmental assessment of 1991 Hudson volcano eruption ashfall effects on southern Patagonia region, Argentina

The August 1991 eruption of the Hudson volcano in the southern Andean volcanic zone affected an area of 1000 km in radius to the east in the Argentine Patagonic meseta. The thickness of ash ranged from 20 cm in the Andean area to less than 1 mm in the Atlantic coast zone. Wind storms reactivated the ash deposits, together with terrigenous material, more than one year after the eruption. In order to assess the potential risk of the ash, analyses of concentration, size, mineralogical composition, toxic elements, and irritating effects of gases were performed. In all samples, the percentage of particles of the 2-to 5-μm range is below the toxic threshold level. Trace elements are below the toxic threshold concentration for humans and animals. The major impact of the ashfall was on sheep herds; about one third of them were lost in the areas close to the volcano. Soil incorporated the ash layers, and a fast recovery of orchards was reported two years later. Rivers were loaded with sediment in the immediate aftermath, but one year later returned to previous conditions. Along the shores of the Buenos Aires Lake, a fresh tephra layer can be distinguished.

Source parameters of earthquakes in the Aleutian Islands subduction zone

SUMMARY The source parameters of seven moderate-sized earthquakes representing different parts of the Aleutian Islands subduction zone are presented. Fault plane solutions are determined by the relative amplitude method. A simple 2-D kinematic source model is assumed and synthetic broad-band (BB) seismograms of displacement are generated. Comparisons with observed BB records of displacement allow the estimation of source dimensions, focal depth, rupture velocity, mean stress drop and the static seismic moment for each event. Normal faulting associated with extensional stresses generated within the subducting plate is determined for one event. However, the majority of solutions indicate compressive mechanisms consistent with the subduction environment. Focal depth is well resolved as depth phases are apparent on most of the seismograms due to the frequency range (0.05-5.0 Hz) of the broad-band records used. The circular or elliptical faults assumed for the source model vary from 3.5 to 8 km in radius depending on body-wave magnitude. The stress drop values obtained are typical of interplate earthquakes (-30 bar). However, the higher stress drop found for one event at a depth of approximately 180 km in the subducting lithosphere can be attributed to the intraplate character of this earthquake. Seismic moments obtained are similar to those obtained in centroid moment tensor (CMT) solutions, even though the latter are calculated using lower frequency seismic waves. This implies a fairly flat spectral level below the corner frequency.

Explosions of infalling comets in Jupiter's atmosphere

In view of the expected collision of comet Shoemaker-Levy 9 (1993e) with Jupiter in 1994 July, we calculate basic properties of the initial interaction for a simplified Jovian atmosphere. The comet is expected to impact Jupiter at 60 km/sec and at an angle of 45 deg to the zenith. The shock wave generated by the bolide should be optically thick once it has penetrated to an atmospheric density approximately 10(exp -6) gr/cm(exp 3), and we calculate the post-shock conditions assuming local thermodynamic equilibrium (LTE) for shock velocities v(sub sh) in the range 10 to 60 km/sec and preshock densities rho(sub a) = 10(exp -6) to 10(exp -2) gr/cm(exp 3). Our shock calculations include molecular hydrogen, atomic hydrogen, ionized hydrogen, neutral helium, and singly ionized helium. Even at the highest shock velocity, the gas is only partially ionized and the postshock temperature rises with preshock density in order to maintain the ionization. The value of the effective shock adiabatic index gamma(sub sh) varies from 1.17 (at low v(sub sh) and rho(sub a) to 1.40 (at high v(sub sh) and rho(sub a). The ablation rate is limited by the radiative flux that reaches the bolide surface. We argue that the ablated gas does not efficiently transfer its kinetic energy to the atmosphere, and it ultimately slows in a similar fashion to the comet material. As the bolide initially falls through the atmosphere, the character of the shock emission changes. At rho(sub a) approximately 10(exp -8) gr/cm (exp 3), the gas is optically thin and we expect line emission; in the optical spectrum, Balmer emission is expected from the shocked atmosphere and low-ionization metal lines from ablated cometary material. At rho(sub a) approximately 10(exp -6) gr/m(exp 3), the shocked gas is optically thick and the shock front near the bolide produces a blackbody spectrum. The temperature is favorable for ultraviolet (1000 to 3000 A) emission and the luminosity may be approximately 5 x 10(exp 23) ergs/sec for approximately 0.6 sec for a bolide 1 km in radius. At rho(sub a) approximately 10(exp -4) gr/cm(exp 3), the bolide has passed below the ultraviolet photosphere. The shock front emits considerable ionizing radiation, but it is absorbed in a narrow preshock region. The bolometric correction for the optical luminosity is large and we expect a 3000 to 8000 A luminosity of approximately 3 x 10(exp 23) ergs/sec for approximately 1 sec. The optical emission is strongly peaked in the vicinity of the bolide. The bolide does have a somewhat less luminous, optically thick trail extending greater than or equal to 10 km, but the radiation is characterized by a temperature of 4000 to 5000 K. From the fragmentation model of Chyba, Thomas, & Zahnle (1993), the bolide deposits most of its kinetic energy at rho(sub a) approximately 10(exp -3) gr/cm(exp 3) and this is the effective explosion site. The shock wave from such an explosion can move up about one density scale height. We examine the breakout of the shock front from the Jovian atmosphere and find that the shock acceleration in the decreasing density region is slow, so that the energy flux in the shock front is small. Higher velocities might be generated by shock acceleration along the channel left by the bolide if the shock motion can occur before the channel closes off as a result of radiative cooling. Hot gas created by the explosion ultimately rises due to buoyancy on a timescale of a minute. The luminosity is highest when the bubble first rises into the optically thin part of the atmosphere and may be approximately 1 x 10(exp 25) ergs/sec in the near-infrared. Roughly 1% of the initial bolide energy may be radiated in this way; the rest of the energy is lost to sound waves from the initial explosion and to work done by the bubble on the surrounding atmosphere.

Abnormal Lg coda Q in Beijing Area

This paper presents the results of an Lg coda Q study in the Beijing area of China. The data originate from the Beijing Telemetered Network; the Q (=Q0ƒη) measurement technique is the stacked spectral ratio method (SSR). The study area, roughly 250 km in radius, is sampled repeatedly by scattered coda waves recorded at epicentral distances between 190 and 422 km. The results point to 1‐Hz Q (Q0) as low as 150 and η as high as 0.95, in a jarring incongruity with the ambient attenuation characteristics about the study region. The Lg coda Q0 is found varying inversely with η. The unequivocality with which such a relationship becomes unravelled comes about somewhat unexpectedly given the small spatial confine of the study area. If high coda attenuation (low Q0) and high η are taken to correlate with a high level of tectonic activity in a broad sense, a view generally held to be statistically accurate, then the Beijing area belongs to the same league in Eurasia, whose members include such well known “earthquake countries” as the Himalayas, the Tibetan Plateau, the Burmese Arc, and the Tianshan Mountains.

Ureilite smelting

Abstract— Ureilites containing homogeneous Fo76 olivine cores in intimate co‐existence with graphite must have recrystallized at pressures of at least ∼100 bars to suppress smelting of the fayalite component of the olivine to Fe metal. Smelting of olivine and pyroxene‐saturated magmatic liquids produces orthopyroxene‐without‐olivine crystalline derivatives unlike those in ureilites. Thus the Mg# compositional variation within the ureilite suite, which is commonly attributed to partial smelting, cannot plausibly be produced by assemblages rich in liquid. In situ smelting of graphitic olivine + pigeonite crystal mushes can produce the correct crystal assemblage, but fails to provide a plausible account for the removal of metal from ureilites or for the correlation of Mg# with Δ17O. Even if Mg# and Δ17O variations are established in the nebula, ureilite recrystallization with graphite must have occurred at pressures greater than the minima we have experimentally established, corresponding to parent objects not less than ∼100 km in radius.

Modeling gravity variations caused by dilatancies

One postulated earthquake mechanism is the dilatancy model. The dilatations due to deformation, microfracture (fracture), and stress relaxation, either accompanied by fluid invasion or not, change the density, the geometrical configuration in space, and the porosity and the contents of the stressed volume of a tectonically active region. Observed gravity therefore is expected to respond to the sequence of events in the deformation cycle of dilatation. In this paper, a formulation of the gravity variations associated with the dilatation processes based on a combined dilatancy model is given. The combined dilatancy model consists of the dilatancy-diffusion, the dilatancy-instability, and the fault-zone dilatancy models. For numerical calculations, the total tectonically active region involved is represented by a vertical finite, right circular cylinder, within which the fault-zone volume of the potential hypocentral region represented by a disk is embedded. The cylinder has a radius of 40 km and thickness of 15 km with the bottom of the cylinder coinciding with the depth of a tectonically detached zone. The disk is 2 km thick and 11 km in radius with the bottom of the disk at a depth of 15 km. This representation is compatible with that of earthquakes of magnitude 4–5 that frequently occurred through the last decade or so in the Beijing-Tianjin-Tangshan-Zhangjiakou (BTTZ) region in China. The results of the calculations show that the variation forms of gravity changes are a function of space and time, depending upon the size and shape of both the total tectonically active region and the fault-zone volume. Therefore, the calculation of theoretical gravity variations for a specific region comparable to that of the BTTZ region, may be made on the basis of a thorough prior knowledge of the geological setting and the tectonic stress distribution of that region.

Noble gases in ancient asteroidal atmospheres

The atmospheres of rocky asteroids are unlikely to have ever been anything more than tenuous exospheres. However, it is possible that the densities of radiogenic heavy noble gases might have once been high enough to have implanted observable quantities in the regoliths that became meteorites. For this to have happened, a significant fraction of these species must have been photoionized and accelerated by the electromagnetic fields associated with the solar wind, rather than escaping thermally. Then, some fraction of the photoions would be accelerated into the asteroid's surface. Analytical and numerical results presented here suggest that acceleration of photoions by the solar wind motional field is a significant loss process for Xe on asteroids about 200 km in radius or larger, if the Xe is thermalized by its interactions with the surface. For Ar, photoion acceleration can only become important for asteroids nearly 500 km in radius. Thus photoion acceleration, previously invoked for lunar samples, could be responsible for excess fission‐produced Xe found associated with solar wind Xe in howardite meteorites. The lack of such Xe in other types of meteorites may reflect either smaller parent bodies or later times of regolith exposure. Similarly, the failure to observe solar‐wind‐associated radiogenic 40Ar in meteorites is consistent with the much smaller likelihood that Ar will be photoionized.

Mantle heterogeneity beneath oceanic islands: some inferences from isotopes

Radiogenic isotopes in oceanic basalts are extremely useful as tracers of long-lived heterogeneities in the Earth's mantle. Helium isotopes provide unique information in that high 3 He / 4 He ratios are indicative of relatively undegassed mantle reservoirs (i.e. mantle with high time-integrated 3 He/(Th + U) ratios). An alternative hypothesis is that high 3 He / 4 He ratios may have been produced by ancient melting events, if the solid/melt partition coefficient (K d ) for He is greater than that for Th and U (i.e. yielding relatively high He/(Th + U) in the residue of melting). However, the distribution of helium within basaltic phenocrysts, and olivine/glass helium partitioning within mid-ocean ridge basalts, suggest that helium behaves as an incompatible element during melting (K d (olivine/glass) &lt; 0.0055), which strongly supports the hypothesis that high 3 He / 4 He ratios are derived from undegassed mantle reservoirs. Isotopic measurements of He, Sr, and Pb in Hawaiian volcanoes lavas demonstrate that the mantle sources have changed on extremely short timescales, between 100 and 10 000 years before present. The preferred explanation for these variations is that they represent heterogeneities within the Hawaiian mantle plume, combined with late stage melting in the lithosphere for post shield alkali basalts. Helium isotopic data from Kilauea, Hualalai and Mauna Loa suggest that the plume is presently located beneath Kilauea (and Loihi seamount), and constrain the melting zone of the Hawaiian plume to be less than 40 km in radius.

電離層におけるロケット排出物の拡散と電子減少

In coming new space age, we will have frequent trips between the ground and mission orbits by large rockets to realize the large projects such as space stations and solar power satellite systems. In such cases a great deal of rocket effluents should be released in the atomosphere. But, its effects on our environment is still unknown, and therefore various studies on these problems are expected. About such environmental problems, some studies were performed which motivated by so called “ionospheric hole” observed when Skylab-I was launched, May 14, 1973. It was certified by later scientific studies that the phenomenon was caused by the chemical reaction between rocket effluents and ionized particles in the ionosphere, by which the electron density of the ionosphere suddenly decreased to about a half values of that in its normal state in the range of about 1, 000km in radius centering about rocket trajectory and it took about 4 hours to recover. In this study, the results of these fundamental studies are applied to the engineering problem, that is, the numerical simulation of the change in electron density in the ionosphere is carried out in consideration of the diffusion of rocket effluents released along arbitrary trajectory in upper atomosphere and their chemical reactions with the ionospheric constituents.

High-temperature mass spectrometric degassing of enstatite chondrites: Implications for pyroclastic volcanism on the aubrite parent body

Wilson and Keil (1991) argued that a few hundred ppm of expanding volatiles present in early partial (basaltic) melts on the aubrite parent body (or on other differentiated asteroids < 100 km in radius), upon ascent of the magma to the surface of the body, would cause disruption into a spray of droplets moving with velocities in excess of the escape velocities of small asteroidal-sized bodies and thus escape and be lost into space 4.55 Ga ago. Hence, no such basaltic rocks exist as individual meteorites nor as clasts in brecciated aubrites. We report measurements of volatiles in enstatite chondrites, considered to be reasonable analogs of the precursor rocks of the aubrites (Qingzhen: EH3; Indarch, Abee: EH4; St. Mark's: EH5; Hvittis, Pillistfer: EL6, sinoite-bearing; Khairpur: EL6, no sinoite) and address in detail the nature and origin of volatile phases driving this pyroclastic volcanism. We find that 1. (1) Volatiles measured by high-temperature mass spectrometric degassing and released at <780°C are contaminants from terrestrial weathering, handling and cutting; those released at >925°C (principally CO, 1580–2830 ppm; N 2, 110–430 ppm; Cl, 120–450 ppm; S, see discussion for abundance) are indigenous. The precise host phases and host sites of the indigeneous volatiles are uncertain; 2. (2) N 2 release appears to be associated with the melting of the Fe,Ni-sulfide cotectic at ∼950°C. The amounts of gas released are not unlike those previously measured for iron meteorites and sulfides in irons, but exceed by several orders of magnitude the equilibrium N 2 solubility in metallic Fe,Ni for solar nebula pressures. This suggests that either partial pressures during N 2 incorporation were much higher than solar values or N 2 is trapped in lattice defects and microcracks, possibly in sulfides; 3. (3) CO release over a narrow temperature interval of 1025–1255°C is violent and results in pronounced vesiculation of the residue. CO may be trapped in microvesicles and cracks in silicates or, less likely, may form by a reduction reaction during heating between carbon (graphite) and the minor ferrosilite in the enstatite; 4. (4) Cl release beginning at 950 (EH's) and 1025°C (EL's) appears not to be associated with lawrencite, which should sublime at T as low as 325°C; in EH's, some may be released from djerfisherite; 5. (5) Gas release and textural studies suggest that metamorphic reheating of EH's and EL's was to less than about 950°C. EL's did not form during metamorphic reheating of EH's by draining of cotectic Fe,Ni-sulfide liquid; 6. (6) Sinoite present in some EL6's did not form by condensation from the solar nebula, but during parent body metamorphism at T < 950° C via the reaction of SiO 2, gaseous N 2 and Si (in solid solution in the metallic Fe,Ni); 7. (7) As little as 1% by volume partial melting in an asteroid of any size will generate excess pressures in the melt much greater (10's of MPa) than the tensile strength of silicate materials and will initiate formation of cracks. This process will be enhanced by expansion of any exsolved or trapped gas phase in the asteroid and will lead to the formation of pathways (cracks growing into dikes) for melt migration; 8. (8) The pressure gradients implied when these pathways intersect the asteroid surface, coupled with the observed amounts of indigenous volatiles released from EH's and EL's, are easily sufficient to drive the pyroclastic volcanic eruptions envisaged by Wilson and Keil (1991).

Explosive volcanism and the graphite-oxygen fugacity buffer on the parent asteroid(s) of the ureilite meteorites

The ∼39 ureilite meteorites are in many respects primitive objects. They are highly carbonaceous, averaging ∼3.0 wt% C, mostly as graphite. Yet their consistently coarse-grained textures and plagioclase-depleted compositions imply that they are igneous. Despite diverse mafic-silicate Mg/(Mg + Fe) and pyroxene/olivine ratios, plagioclase is totally absent among ∼35 monomict/unbrecciated ureilites; even among 4 polymict/brecciated ureilites the average Al concentration is only 0.28 ± 0.04 wt%. We suggest that the ureilites are residues from partial melting of graphite-rich source materials, which formed sufficiently deep in a relatively large, low- f(O 2) asteroid (or more likely asteroids) for graphite to be stable even at magmatic T. The Al-rich basaltic magmas that ascended from the residues were nearly quantitatively lost from the asteroid during pyroclastic eruptions. Graphite entrained in the ascending magmas was oxidized by an extremely pressure-sensitive reaction into CO x , the expansion of which into the vacuous environment of the asteroid engendered pyroclastic exit velocities greatly in excess of the asteroid's escape velocity. Carbon would oxidize long before the attainment of magmatic temperatures in small asteroids, but we infer that basalt-dissipative explosive volcanism was common on all C-rich asteroids that were roughly 100–200 km in radius and heated to widespread, deep melting; and most unmetamorphosed chondritic meteorites are C-rich. Based on the ureilites' consistently modest depletions of light rare-earth elements, siderophile elements, and FeNi metal, we infer that the melt fraction was removed mostly by equilibrium, “batch” partial melting. Late-stage ureilite evolution involved an abrupt increase in the cooling rate, implying a drop in environmental temperature, and a slight amount of graphite oxidation, implying a drop in pressure. The simplest model to account for this non sequitur (and for the conversion of some of the graphite into diamonds) is catastrophic impact disruption of the primary asteroid. The remarkably efficient removal of the final traces of melt from the ureilites is difficult to explain by any model, but possibly was caused by thermal migration along steep thermal gradients engendered after the catastrophic disruption.

Geophysical models for the formation and evolution of coronae on Venus

Coronae are large circular features on Venus characterized by an annulus of concentric tectonic features, interior fracturing, volcanism, and generally upraised topography. They are suggested to form over sites of mantle upwelling and modified by subsequent gravitational relaxation. We examine this proposition using two geophysical models to determine whether and under what conditions these mechanisms can produce the topography and tectonics exhibited by coronae in the Magellan altimetry data and radar images. Our results show that mantle diapirism can produce the domical topography of novae, which may be coronae in the earliest stage of formation. The model stresses induced at the surface by a mantle diapir imply the formation of radially oriented extensional fracturing as observed in novae. The dimensions of novae indicate that the diapirs responsible for them are smaller than about 100 km in radius and that the elastic lithosphere is less than 32 km thick. Diapirs that have reached the top of the mantle are expected to spread and flatten, producing plateaulike rather than domical topography. We model a flattened diapir at the top of the mantle and show that it will result in plateaulike uplift. The volume of the flattened model diapir is similar to that of the spherical diapirs derived for novae. We model gravitational relaxation of isostatically uncompensated plateaus and show that they relax to the topographic forms associated with coronae and that the model stresses are consistent with the development of the annulus of tectonic features around coronae.

Mylitta Fluctus, Venus: Rift‐related, centralized volcanism and the emplacement of large‐volume flow units

Mylitta Fluctus is a volcanic flow field that covers approximately 300,000 km2 in southern Lavinia Planitia. The flows are typically radar‐bright with uniform surface textures. Central channels are common. Maximum flow lengths range from 400 to 1000 km; flow widths range 30–100 km in the medial and distal portions of the flow field. The total volume of the flow field, based on estimates of flow thickness, is of the order of 2 × 104 km3. The flow field is composed of six smaller flow fields that are interpreted to represent major eruption events in the evolution of Mylitta. An asymmetric shield volcano 200 km in radius with a central caldera has been identified as the single major source. It is located along a possible rift zone at the northern edge of Lada Terra. No evidence for fissure‐fed eruptions is observed, although eruptions may have occurred along fissures within the rift before the main vent centralized. Mylitta is similar in scale to many terrestrial flood basalt provinces, although it is lower in total estimated volume and is distinguished by the presence of a central source edifice. The origin of Mylitta is proposed to be linked to regional extension and possible hotspot activity in a manner similar to that suggested for the origin of terrestrial flood basalts. The lower apparent volume of Mylitta relative to terrestrial flood basalts suggests that the amount of material upwelling from the mantle may have been lower in this case or that the formation of the flow field has not yet finished. Detailed studies of the morphology and distribution of flow fields similar to Mylitta should yield insight into the variation and evolution of hotspot‐related volcanism and the formation of possible flood basalts on Venus.