Parent body histories recorded in Rumuruti chondrite sulfides: Implications for the onset of oxidized,sulfur-rich core formation     

Samuel D. Crossley  Richard D. Ash  Jessica M. Sunshine  Catherine M. Corrigan  Timothy J. McCoy 《Meteoritics & planetary science》2023,58(3):383-404

Models of planetary core formation beginning with melting of Fe,Ni metal and troilite are not readily applicable to oxidized and sulfur-rich chondrites containing only trace quantities of metal. Cores formed in these bodies must be dominated by sulfides. Siderophile trace elements used to model metallic core formation could be used to model oxidized, sulfide-dominated core formation and identify related meteorites if their trace element systematics can be quantified. Insufficient information exists regarding the behavior of these core-forming elements among sulfides during metamorphism prior to anatexis. Major, minor, and trace element concentrations of sulfides are reported in this study for petrologic type 3–6 R chondrite materials. Sulfide-dominated core-forming components in such oxidized chondrites (ƒO₂ ≥ iron-wüstite) follow metamorphic evolutionary pathways that are distinct from reduced, metal-bearing counterparts. Most siderophile trace elements partition into pentlandite at approximately 10× chondritic abundances, but Pt, W, Mo, Ga, and Ge are depleted by 1–2 orders of magnitude relative to siderophile elements with similar volatilities. The distribution of siderophile elements is further altered during hydrothermal alteration as pyrrhotite oxidizes to form magnetite. Oxidized, sulfide-dominated core formation differs from metallic core formation models both physically and geochemically. Incongruent melting of pentlandite at 865°C generates melts capable of migrating along solid silicate grains, which can segregate to form a Ni,S-rich core at lower temperatures compared to reduced differentiated parent bodies and with distinct siderophile interelement proportions.  相似文献   

Reflection and transmission at an interface separating transversely inhomogeneous acoustic half-spaces     

John J. McCoy  Louis Fishman  L. Neil Frazer 《Geophysical Journal International》1986,85(3):543-562

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Genesis of the IIICD iron meteorites: Evidence from silicate-bearing inclusions     

Timothy J. McCoy  Klaus Keil  Edward R. D. Scott  Henning Haack 《Meteoritics & planetary science》1993,28(4):552-560

Abstract— Our studies of the silicate-bearing inclusions in the IIICD iron meteorites Maltahöhe, Carlton and Dayton suggest that their mineralogy and mineral compositions are related to the composition of the metal in the host meteorites. An inclusion in the low-Ni Maltahöhe is similar in mineralogy to those in IAB irons, which contain olivine, pyroxene, plagioclase, graphite and troilite. With increasing Ni concentration of the metal, silicate inclusions become poorer in graphite, richer in phosphates, and the phosphate and silicate assemblages become more complex. Dayton contains pyroxene, plagioclase, SiO₂, brianite, panethite and whitlockite, without graphite. In addition, mafic silicates become more FeO-rich with increasing Ni concentration of the hosts. In contrast, silicates in IAB irons show no such correlation with host Ni concentration, nor do they have the complex mineral assemblages of Dayton. These trends in inclusion composition and mineralogy in IIICD iron meteorites have been established by reactions between the S-rich metallic magma and the silicates, but the physical setting is uncertain. Of the two processes invoked by other authors to account for groups IAB and IIICD, fractional crystallization of S-rich cores and impact generation of melt pools, we prefer core crystallization. However, the absence of relationships between silicate inclusion mineralogy and metal compositions among IAB irons analogous to those that we have discovered in IIICD irons suggests that the IAB and IIICD cores/metallic magmas evolved in rather different ways. We suggest that the solidification of the IIICD core may have been very complex, involving fractional crystallization, nucleation effects and, possibly, liquid immiscibility.  相似文献   

Asteroid Differentiation: Pyroclastic Volcanism to Magma Oceans     

G. Jeffrey Taylor  Klaus Keil  Timothy McCoy  Henning Haack  Edward R. D. Scott 《Meteoritics & planetary science》1993,28(1):34-52

Abstract— Asteroid differentiation was driven by a complex array of magmatic processes. This paper summarizes theoretical and somewhat speculative research on the physics of these processes. Partial melts in asteroids migrate rapidly, taking < 10⁶ years to reach surface regions. On relatively small (<100 km) asteroids with sufficient volatiles in partial melts (<3000 ppm), explosive volcanism accelerated melts to greater than escape velocity, explaining the apparent lack of basaltic components on the parent asteroids of some differentiated meteorites. Partial melting products include the melts (some eucrites, angrites), residues (lodranites, ureilites), and unfractionated residues (acapulcoites). The high liquidus temperatures of magmatic iron meteorites, the existence of pallasites with only olivine, and the fact that enstatite achondrites formed from ultramafic magmas argue for the existence of magma oceans on some asteroids. Asteroidal magma oceans would have been turbulently convective. This would have prevented crystals nucleated at the upper cooling surface (the only place for crystal nucleation in a low-pressure body) from settling until the magma became choked with crystals. After turbulent convection slowed, crystals and magma would have segregated, leaving a body stratified from center to surface as follows: a metallic core, a small pallasite zone, a dunite region, a feldspathic pyroxenite, and basaltic intrusions and lava flows (if the basaltic components had not been lost by explosive volcanism). The pallasite and dunite zones probably formed from coarse (0.5–1 cm) residual olivine left after formation of the magma ocean at >50% partial melting of the silicate assemblage. Iron cores crystallized dendritically from the outside to the inside. The rapid melt migration rate of silicate melts suggests that ²⁶Al could not be responsible for forming asteroidal magma oceans because it would leave the interior before a sufficient amount of melting occurred. Other heat sources are more likely candidates. Our analysis suggests that if Earth-forming planetesimals had differentiated they were either small (<100 km) and poor in volatiles (<1000 ppm) or they were rich in volatiles and large enough (>300 km) to retain the products of pyroclastic eruptions; if these conditions were not met, Earth would not have a basaltic component.  相似文献   

A new class of propagation models based on a factorization of the Helmholtz equation     

Louis Fishman  John J. McCoy 《Geophysical Journal International》1985,80(2):439-461

Summary. A new class of propagation models, which is based on a factorization of the Helmholtz equation, that is, in principle, exact for range-independent environments, is presented. A numerical algorithm which makes use of the forward propagation feature of the propagation models and 'marches' the radiation field over a sequence of incremented range planes is derived and implemented. Numerical results of transmission loss as a function of range are obtained for a number of sample wave speed profiles for two members of the class of propagation models, one being an often-used parabolic wave theory and one being a high-frequency model that is new to our study. These results are compared to each other, to results obtained from an 'exact' numerical integration of the complete Helmholtz theory, and to results obtained using a WKB approximation to the Helmholtz equation formulation. The results demonstrate both the computational viability of the class of propagation models using the derived marching algorithm and very promising numerical agreement between the 'exact' results and those obtained using the high-frequency model.  相似文献   

Classification of five new ordinary chondrites (RC 073, 074, 076–078) from Roosevelt County,New Mexico     

Timothy J. McCoy  Klaus Keil  Ivan E. Wilson 《Meteoritics & planetary science》1993,28(1):120-121

Abstract— Five new ordinary chondrites from Roosevelt County, New Mexico, USA, have been assigned to a chemical group, petrologic type and shock stage. All five are moderately to heavily weathered. They are: RC 073, H5(S2); RC 074, L5(S3); RC 076, L4(S3); RC 077 , L4(S2); RC 078, L4(S2).  相似文献   

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

D. W. Muenow  K. Keil  T. J. McCoy 《Meteoritics & planetary science》1995,30(6):639-645

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 < ～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.  相似文献   

The hydrostatic motor: Utilization of hydrostatic pressure differentials in the deep sea   总被引：1，自引：0，他引：1  

Stephen Selwyn  Floyd W. McCoy 《Geo-Marine Letters》1981,1(3-4):233-236

A hydrostatic motor that converts hydrostatic pressure gradients in the deep sea into harnessable mechanical energy has been developed and successfully tested. Controlled equalization between higher ambient hydrostatic pressures at oceanic depths and lower pressures retained from atmospheric or sea-level conditions—sealed within a chamber as part of the motor assembly—produces reciprocating motion of a piston. This motion is utilized for power or work requirements. Our needs have been directed toward oceanographic instrumentation requirements but many other applications are likely.  相似文献   

Reinterpretation of the exposed record of the last two cycles of Lake Bonneville, Western United States     

William E. Scott  William D. McCoy  Ralph R. Shroba  Meyer Rubin 《Quaternary Research》1983,20(3):261-285

A substantially modified history of the last two cycles of Lake Bonneville is proposed. The Bonneville lake cycle began prior to 26,000 yr B.P.; the lake reached the Bonneville shoreline about 16,000 yr B.P. Poor dating control limits our knowledge of the timing of subsequent events. Lake level was maintained at the Bonneville shoreline until about 15,000 yr B.P., or somewhat later, when catastrophic downcutting of the outlet caused a rapid drop of 100 m. The Provo shoreline was formed as rates of isostatic uplift due to this unloading slowed. By 13,000 yr B.P., the lake had fallen below the Provo level and reached one close to that of Great Salt Lake by 11,000 yr B.P. Deposits of the Little Valley lake cycle are identified by their position below a marked unconformity and by amino acid ratios of their fossil gastropods. The maximum level of the Little Valley lake was well below the Bonneville shoreline. Based on degree of soil development and other evidence, the Little Valley lake cycle may be equivalent in age to marine oxygenisotope stage 6. The proposed lake history has climatic implications for the region. First, because the fluctuations of Lake Bonneville and Lake Lahontan during the last cycle of each were apparently out of phase, there may have been significant local differences in the timing and character of late Pleistocene climate changes in the Great Basin. Second, although the Bonneville and Little Valley lake cycles were broadly synchronous with maximum episodes of glaciation, environmental conditions necessary to generate large lakes did not exist during early Wisconsin time.  相似文献   

Linear magnetization feature associated with Rima Sirsalis     

K.A. Anderson  R.P. Lin  R.E. McGuire  J.E. McCoy  C.T. Russell  P.J. Coleman 《Earth and Planetary Science Letters》1977,34(1):141-151

Measurements of the magnetic fields by the electron reflection method in the neighborhood of the long structural rille Rima Sirsalis show that a magnetic field of strength ? 100 nT (100 γ) is present over a region on the order of 10 km in width and at least 300 km long. The center of the magnetized region closely parallels and is centered on the rille. The linear magnetization feature extends at least to latitude 8°S, 60 km beyond the place where the rille disappears at the edge of Oceanus Procellarum. This extension is coincident with the extrapolation of the rille based on photographs. However, the magnetization is much weaker or entirely absent at 5°S and has vanished at 0° latitude. These results suggest that the rille is indeed a structural feature and has associated with it magnetization, either in the form of intrusive, magnetized rock, or else in the form of a gap in an otherwise more or less uniformly magnetized layer of rock of large extent in two dimensions. Furthermore, the rille structure evidently is present for some distance beneath the lava flows of the Oceanus Procellarum basin.  相似文献