Josephinite: Crystal structures and phase relations of the metals

Josephinite, a complex, metal-bearing rock from the region of the Josephine Peridotite in southwest Oregon, contains FeNiCo metal alloy phases having exsolution textures. Scanning and transmission electron microscopy have revealed Widmanstaetten patterns in which the lamellae are polysynthetically twinned, ordered, face-centered-cubic FeNi₃ surrounded by untwinned, ordered, face-centered-cubic FeNi₃ and body-centered-cubic FeCo. These exsolution textures require a temperature in excess of 500°C for their formation. This is consistent with a mantle derivation of josephinite prior to obduction of the peridotite in one of the Klamath ophiolites.     

Empirical geothermobarometry for garnet peridotites and implications for the nature of the lithosphere,kimberlites and diamonds

As a result of a study on the influence of Cr on geothermometry and geobarometry empirical garnet-orthopyroxene geobarometry has been refined. The applicability to simple and complex systems is demonstrated and the modelling procedure briefly outlined. The suitability of some geothermometers for application to garnet lherzolite xenoliths and their limits of precision are discussed. Fields of pressure-temperature estimates for garnet lherzolites from various provinces are shown. Low-temperature xenoliths (< 1050°C) give estimates in accord with a conductive geotherm for the continental lithosphere. African xenoliths (both South African and “off-craton” Namibian) record a distinctive pattern, exhibiting a “temperature discontinuity”, where high-temperature xenoliths (> 1050°C) give near-isobaric estimates for a range of temperatures. The depth of this “temperature discontinuity” is shallower for off-craton than for on-craton suites. Similar patterns are indicated for suites from U.S.A. and the U.S.S.R.The temperature discontinuity is interpreted as a geologically short-lived transient state, where the deepest parts of the lithosphere are in the process of adjustment to higher heat flux from below. At the highest temperatures recorded by those suites “wet” peridotitic solidi are intersected and thus the xenoliths record events, were P,T conditions may have allowed the formation of kimberlitic or strongly undersaturated magmas at about the depths of this discontinuity. This process is inferred to lead to thermal erosion of the base of the lithosphere.The generation and subsequent crystallization of magmas at the base of the lithosphere is inferred to be the source for diamond formation. The distinctive pattern of high- and low-temperature xenoliths in relation to the diamond stability field shows the existence of a rather small “diamond window” between about 900 and 1300°C at 40–55 kb. This window exists only underneath old, stable continents with a thick lithosphere. If recent age determinations on diamonds are correct, this implies the existence of thick continental lithosphere with similar characteristics to present-day lithosphere in the Archean.A model is presented arguing for hot, upwelling convection streams as a possible source of heat. The model implies differences in lithospheric development according to the speed of plates, carrying a continent: Continuous lithospheric thinning by thermal erosion for steady or very slow moving plates with the break-up of a continent and the formation of an ocean as one extreme, the continuous repetition of thermal erosion and growth on cooling (underplating) of the continental lithosphere as the other extreme for fast moving plates. The latter may be the cause of a number of complexities and heterogeneities both within the deep continental lithosphere as well as in the convecting mantle.     

Volcano spacing,fractures, and thickness of the lithosphere

Both “hot-spot” type and possibly island-arc volcanoes may form at the intersections of fractures whose spacing is near the thickness of the lithosphere and increases with increasing thickness. An approximate equality between layer thickness and spacing of major fractures observed in some sedimentary rocks and clay cake models may thus extend to the “mega-joints” that have fractured the lithosphere and controlled volcano spacing on the earth, and possibly on Mars. If the hot-spot fractures are interpreted as due to shear, many hot-spot fracture systems suggest roughly north-south least principal stress, or, alternatively in some instances, a 90° rotation of this pattern.     

Ionosphere as an Indicator of Processes in the Geospace,Troposphere, and Lithosphere

The possible causes of the strong ionospheric day-to-day variability under the influence of processes in the geospace, troposphere, and lithosphere are considered based on the data of the critical frequency of the F2 layer of the ionosphere at two observation stations. It is shown that even in the absence of powerful events, the ionosphere is influenced both “from above” and “from below”; in this case, the ionosphere can respond to an external action as an open nonlinear dissipative system.     

Inhomogeneity of the lithosphere of the Tibetan Plateau and implications for geodynamics

This paper discusses inhomogeneity in structure of the present lithosphere underneath the Tibetan Plateau, and deduces the P-T-t paths and deep processes during the orogenic process of post India-Asia collision through revealing the nature and sequence of geological events. A three-stage-evolution model for tectonic phases of the Tibetan Plateau has been presented. It is suggested that the formation of Parmirs-type of cool lithosphere roots represents the early phase; Nianqingtanggula-type of thinned lithosphere roots, the middle phase; and Qiangtang-type of “warm” lithosphere roots (formed by cooling of the asthenosphere), the late phase.     

A comparison of the estimated effective elastic thickness of the lithosphere using terrestrial and satellite-derived data in Iran

The effective elastic thickness of the lithosphere has an important role in constraining compositional structure, geothermal gradient and tectonic forces within the lithosphere and the thickness of this layer can be used to evaluate the earthquakes’ focal depth. Hence, assessment of the elastic thickness of the lithosphere by gravitational admittance method in Iran is the main objective of this paper. Although the global geopotential models estimated from the satellite missions and surface data can portray the Earth’s gravity field in high precision and resolution, there are some debates about using them for lithosphere investigations. We used both the terrestrial data which have been provided by NCC (National Cartographic Center of Iran) and BGI (Bureau Gravimetrique International), and the satellite-derived gravity and topography which are generated by EIGEN-GL04C and ETOPO5, respectively. Finally, it is concluded that signal content of the satellite-derived data is as rich as the terrestrial one and it can be used for the determination of the lithosphere bending.     

Dynamics of thinning and destruction of the continental cratonic lithosphere: Numerical modeling

Thinning of the cratonic lithosphere is common in nature, but its destruction is not. In either case, the mechanisms for both thinning and destruction are still widely under debate. In this study, we have made a review on the processes and mechanisms of thinning and destruction of cratonic lithosphere according to previous studies of geological/geophysical observations and numerical simulations, with specific application to the North China Craton (NCC). Two main models are suggested for the thinning and destruction of the NCC, both of which are related to subduction of the oceanic lithosphere. One is the “bottom-up” model, in which the deeply subducting slab perturbs and induces upwelling from the hydrous mantle transition zone (MTZ). The upwelling produces mantle convection and erodes the bottom of the overriding lithosphere by the fluid-melt-peridotite reaction. Mineral compositions and rheological properties of the overriding lithospheric mantle are changed, allowing downward dripping of lithospheric components into the asthenosphere. Consequently, lithospheric thinning or even destruction occurs. The other is the “top-down” model, characterized by the flat subduction of oceanic slab beneath the overriding cratonic lithosphere. Dehydration reactions from the subducting slab would significantly hydrate the lithospheric mantle and decrease its rheological strength. Then the subduction angle may be changed from shallow to steep, inducing lateral upwelling of the asthenosphere. This upwelling would heat and weaken the overriding lithospheric mantle, which led to the weakened lithospheric mantle dripping into the asthenosphere. These two models have some similarities, in that both take the subducting oceanic slab and relevant fluid migration as the major driving mechanism for thinning or destruction of the overriding cratonic lithosphere. The key difference between the two models is the effective depth of the subducting oceanic slab. One is stagnation and flattening in the MTZ, whereas the other is flat subduction at the bottom of the cratonic lithosphere. In the NCC, the eastern lithosphere was likely affected by subduction of the Izanagi slab during the Mesozoic, which would have perturbed the asthenosphere and the MTZ, and induced fluid migration beneath the NCC lithosphere. The upwelling fluid may largely have controlled the reworking of the NCC lithosphere. In order to discuss and analyze these two models further, it is crucial to understand the role of fluids in the subduction zone and the MTZ. Here, we systematically discuss phase transformations of hydrous minerals and the transport processes of water in the subduction system. Furthermore, we analyze possible modes of fluid activity and the problems to explore the applied feasibility of each model. In order to achieve a comprehensive understanding of the mechanisms for thinning and destruction of cratonic lithosphere, we also consider four additional possible dynamic models: extension-induced lithospheric thinning, compression-induced lithospheric thickening and delamination, large-scale mantle convection and thermal erosion, and mantle plume erosion. Compared to the subduction-related models presented here, these four models are primarily controlled by the relatively simple and single process and mechanism (extension, compression, convection, and mantle plume, respectively), which could be the secondary driving mechanisms for the thinning and destruction of lithosphere.     

The activation and cessation of Late Cenozoic extension in the lithosphere at the margin of the Baikal Rift Zone: Alternating sources of volcanism in the Vitim Upland

A comparative analysis of the concentrations of major oxides, trace elements, and the ¹⁴³Nd/¹⁴⁴Nd ratios in representative sequences of volcanic and subvolcanic rocks in the western and eastern Vitim Upland has revealed petrogenetic groups with different relationships among components from lithosphere and sublithosphere sources. It is hypothesized that the initial 16–14-Ma eruptions of picrobasalts and Mg basanites in the east of the upland resulted from high-temperature melting, hence, the melting of sublithospheric peridotite and lithospheric Mg-pyroxenite mantle material with mildly and strongly depleted isotope compositions of Nd relative to the value in the primitive mantle (0.512638). The broad range of varying lava compositions in the 14–9 Ma time span was caused by “passive” rifting in the west of the upland and by “active” rifting in the east. The “passive” rifting manifested itself in the melting of lithospheric material with some admixture of material from the underlying asthenosphere, while the “active” rifting lifted deep-lying mantle material. The structural rearrangement that has been occurring in the Baikal Rift System during the last 9 Ma resulted in stopping the rifting in the area of study. Relaxation, flattening and thinning of the lithosphere beneath the east part of the system during the 1.1–0.6 Ma time span caused magma effusion with values of ¹⁴³Nd/¹⁴⁴Nd that are typical of a moderately depleted asthenospheric source contaminated with deeper mildly depleted mantle material.     

Late Quaternary tephrostratigraphy and pollen stratigraphy of Uwa Formation,Shikoku Island,SW Japan: Reconsidering the MIS 11 super-interglacial horizon

We investigated a late Quaternary terrestrial sedimentary sequence (Uwa Formation) in core IC2, from a site adjacent to that of the reported core IC on NW Shikoku Island, SW Japan, and developed its tephra and pollen stratigraphy to refine the age model of the formation. First, we identified 19 horizons with high glass shard concentrations in the IC2 core sediments as possible tephras or cryptotephras, and correlated them with reported tephras on the basis of the major- and trace-element compositions of their glass shards. All correlated widespread tephras and cryptotephras were products of volcanoes in the Kyushu volcanic zone (Aso, Kakuto, Aira, Ata, and Kikai calderas). Second, we confirmed the presence in core IC2 of two pollen zones dominated by Quercus subgen. Cyclobalanopsis, which is an indicator of very warm interglacial vegetation. In the Japanese Islands, these two vegetation zones have usually been considered to characterize marine isotopic stages (MISs) 1 and 11. A previous study of the Uwa Formation correlated the upper pollen zone to MIS 1, but the lower zone was not correlated to MIS 11; rather, it was inferred to be older than MIS 12 because it was stratigraphically below the “Oda” tephra (equivalent to a distal Kasamori 5 [Ks5] tephra [MIS 12]). In this study, however, noting that the Naruohama-IV tephra (Nh-IV; MIS 10d) and Ks5 cannot be distinguished by their shard chemistries, we inferred that the suggested “Oda” tephra actually correlates to Nh-IV, rather than to the Ks5 tephra. By re-assigning the “Oda” tephra to Nh-IV, we could correlate the underlying Quercus subgen. Cyclobalanopsis-abundant zone to MIS 11 and, consequently, a pair of pollen zones indicating cool and warm conditions below the MIS 11 pollen zone to MISs 12 and 13, respectively. The resulting age model whereby tephra and pollen constraints are integrated showed a roughly constant sedimentation rate from MIS 13, without any long-term gaps; further, our MIS 13 horizon in core IC2 corresponds to the reported 1 Ma tephra horizon in core IC. Therefore, these findings represent a dramatic change in the Uwa Formation age model and validate the Uwa Formation as one of the most useful terrestrial archives of Quaternary tephrostratigraphy and paleoclimatic fluctuation in SW Japan.     

Terrestrial nickel-iron from the Josephine Peridotite,its geologic occurrence,associations, and origin

The distribution of placers containing coarse nuggets of the nickel-iron alloy, josephinite, are found to have a close spatial relationship to a narrow zone of intense shearing, serpentinization, and igneous intrusion within the Josephine Peridotite. These field relations, together with the frequent occurrence of magnetite and serpentine with the alloy, indicate that the mineral is the product of hydrothermal metamorphism and serpentinization of the peridotite. This is confirmed by the discovery of the nickel-iron in serpentine veins cutting moderately altered harzburgite. Andradite garnet, a common skarn mineral produced by contact metamorphism around igneous intrusions, is often intergrown with the nickel-iron. This may indicate that the unusually coarse grain size of the josephinite is the result of the special conditions accompanying igneous intrusion in the serpentine belt.     

Thermal convection in a cylindrical annulus with a non-Newtonian outer surface

This study presents the results of numerical simulations of a model for lithospheremantle coupling in a terrestrial type planet. To first order, a geologically active terrestrial type planet may consist of a metallic core, silicate mantle and lithosphere, with the lithosphere being rheologically different from the mantle. Therefore we have developed a numerical model consisting of a thin non-Newtonian fluid hoop that is dynamically coupled to a thick Newtonian fluid cylindrical annulus. Thus the rheological dichotomy between mantle and lithosphere is built into the model. Time-dependent calculations show the existence of at least two regimes of behaviors. In one regime, the behavior of the hoop switches between periods characterized by low or high speeds, in response to changes in convective vigor and planform. This regime may apply to the planet Venus where the available evidence indicates that prior to 500 myr ago, the planet was resurfaced on a time scale of <100 myr. Since that time, large-scale tectonic activity on Venus has been sharply curtailed. In the other regime, which is more like plate tectonics on Earth, the hoop speeds rise and fall on short time scales.     

Dynamics of unstable continental subduction: Insights from numerical modeling

Numerical experiments are used in this study to systematically investigate the effects of convergence rate, crustal rheological strength, and lithospheric thermal structure on the dynamics of continental collision. The study focuses on the types, conditions and processes of unstable continental subduction. Modelling results suggest that the development of unstable continental subduction can be promoted by conditions that tend to decrease rheological strength of the lithosphere, such as low crustal rheological strength, “hot” thermal structure of the lithosphere, or low convergence rate. Unstable subduction mode can be further categorized into three types: (1) multi-stage slab breakoff, (2) continuously “flowing” of fluid-like slab into the upper mantle, and (3) large-scale detachment of the thickened orogenic root. These three types of unstable continental subduction are respectively associated with (1) a low convergence rate, (2) “hot” thermal structure of the lithosphere with a high convergence rate, and (3) moderate-high crustal rheological strength with a low convergence rate. It is also revealed that the evolution of crustal melting is dominated by the deformation pattern of continental collision, which is mainly controlled by crustal rheological strength. The modelling results have important implications for understanding of continental subduction mode selection under specific geodynamic conditions.     

Density Structure,Isostatic Balance and Tectonic Models of the Central Tien Shan

A new combined satellite-terrestrial model of the gravity field is used together with seismic data for construction of a density model of the lithosphere of the Central Tien Shan and for estimation of its isostatic balance. The Tien Shan is one of the most active intraplate orogens in the world, located about 1,500 km north of the convergence between Indian and Eurasian plate, and surrounded by stable Kazakh platform to the north and the Tarim block to the south. Although this area was extensively studied during recent decades, several principal problems, related to its structure and tectonics, remain unsolved up to now: (1) various geodynamic scenarios have been discussed so far to explain tectonic evolution, such as direct “crustal shortening,” intracontinental subduction and some others, but no definite evidence for any of them has been found. (2) Still, it is not clear why Tien Shan grows so far from the plate boundary at the Himalayan collision zone. Gravity modeling can provide valuable constraints to resolve these questions. The results of this study show that: (1) there exists a very strong deflection of the Tien Shan lithosphere from isostatic equilibrium. At the same time, the patterns of the isostatic anomalies are very different in the Western and Central Tien Shan. The latter one is characterized by much stronger variations. The best fit of the modeling results is found for the model according to which the Tarim plate partially underthrusts the Central Tien Shan; (2) negative density anomalies in the upper mantle under the central block possibly relate to magmatic underplating during the initial stage of the tectonic evolution. Therefore, the weak lithosphere could be the factor that initiates mountain building far away from the collision zone. Alternatively, this might be a gap after detachment of the eclogised lower crust and lithospheric lid, which is filled with the hot asthenospheric material.     

Thermal state, rheology and seismicity in the pannonian basin, Hungary

On the basis of data on crustal structure and terrestrial heat flow, a 3-D geothermal model for the lithosphere in the Pannonian basin, Hungary, has been calculated. This model, together with information on crustal composition, laboratory data on rock friction, and certain assumptions about fluid conditions and strain-rate levels within the lithosphere, has been used to construct a rheological model of the area.The results obtained show a layered rheological structure where an aseismic part of the crust is “sandwiched” between an upper and a lower seismogenic crustal layers. According to the proposed rheological model, seismic activity in the upper crust may be expected down to depths of 10–12 km, which is confirmed well by the observed depth distribution of seismicity. The model also predicts a lower crustal seismogenic layer down to 20–22 km. Because of infrequent occurrences of deep earthquakes and/or a generally small number of reliable hypocenter depth determinations in the study area, this seismogenic zone is less constrained by observations.The depth of the different rheologic horizons within the crust is governed mainly by thermal conditions. The lower boundary of both seismogenic layers appears isothermal. Brittle-ductile transition in the upper crust coincides with the ˜200 °C isotherm, while in the lower crust it coincides with the ˜ 375 °C isotherm. The lowermost crust and the upper mantle beneath Hungary show ductile behavior, thus the possibility of siesmic activity at these horizons can be excluded.     

A test of terminal Mesozoic “catastrophe”

Terminal Mesozoic “catastrophe”-type extinction models that advocate synchronous marine and terrestrial extinctions spanning short time intervals (a few days up to a few millennia) have a common foundation: the simultaneous terminations of geological ranges of some taxa of marine CaCO₃-producing microplankton (and possibly the dinosaurs) at the end of the Cretaceous. Gartner and McGuirk [1] propose a new catastrophe theory that at the end of the Cretaceous fresh-brackish water from the Arctic Ocean spread over the surface of the world's oceans, causing global cooling, aridity, and the extinctions. Like other catastrophe models, this one also fails to address the possibility of hiatus control of ranges at the end of the Cretaceous; a well documented, seemingly nearly universal hiatus of variable and unknown duration separates Cretaceous and Tertiary strata. Documented terminal Cretaceous marine regression (perhaps 10 times more rapid than a typical regression according to Cooper [8] would have caused terrestrial erosion and stripping away of the latest Cretaceous stratigraphic record, thus truncating geological ranges along a seemingly planar datum. The terminal Cretaceous marine CaCO₃ dissolution event would have had the same effect on ranges of marine planktonic CaCO₃-producing microplankton (the event was a shallow-water phenomenon). The simultaneous terminations of geological ranges is thus possibly the result of hiatus control, and the terminal Cretaceous “catastrophe” an illusion. Attempts to use Cretaceous-Tertiary transition floras to support global cooling at the time of the extinctions are not based on sound stratigraphic foundations; realistic paleobotanical-climatic inferences can only be based on the precise correlation of the Cretaceous-Tertiary contact in marine and terrestrial stratigraphic sections, and these correlations have not been made with sufficient precision to support catastrophe theory. The much used “across the Cretaceous-Tertiary boundary” glosses over ignorance of the true terminal Cretaceous scenario, lost forever in most places by the destruction of the terminal Cretaceous stratigraphic record. For now, stable isotope paleotemperature data from marine strata that can be dated radiometrically provide the most reliable estimates of the Cretaceous-Tertiary transition climate; Boersma et al. [5] indicate global warming of deep and shallow oceans “across” the contact (and not surficial cooling only as is required by the spillover model). Older much-cited climate inferences based on leaf physiognomy are suspect in light of Dolph and Dilcher's [23] work that shows little correlation between leaf physiognomy and climate.     

Longshot experiments to study velocity anisotropy in the oceanic lithosphere of the northwestern Pacific

Several long-range explosion seismology experiments have been conducted in the northwestern Pacific basin, where one of the oldest oceanic lithospheres is postulated to exist. The experiments were conducted from 1974 to 1980. Highly sensitive ocean-bottom seismographs which had been developed for longshot experiments were used. The lengths of the profiles ranged from 1000 to 1800 km, and the directions were chosen to provide wide azimuthal coverage. One of the aims of this series of experiments was to test the existence of velocity anisotropy on a large, regional scale.The results show that the oceanic lithosphere has anisotropy wherein the velocity changes by 4–7%. The anisotropy extends from a depth of at least 40 to 140 km beneath the sea bottom; however, the magnitude of the anisotropy may vary with depth. The azimuth of the maximum velocity is 150–160° clockwise from north, and coincides with the “fossil” direction of spreading of the Pacific plate, whereas it differs from the present direction of plate motion by ～ 30°. The azimuth does not seem to depend on depth. In the direction of maximum velocity, the lithosphere is basically two-layered: 8.0–8.2 and 8.6 km s^?1. The depth of the interface is 50–60 km beneath the sea floor.     

Anomalous noble gases in josephinite and associated rocks?

In this paper we report Ne, Ar, Kr and Xe analyses of josephinite, Josephine Peridotite, and serpentinized Josephine Peridotite. In all three samples the elemental abundance patterns resemble patterns associated with surface waters, the Ne data do not exhibit the large²¹Ne enrichments observed earlier, and the Kr and Xe compositions are indistinguishable from atmospheric composition at all isotopes, including¹²⁹Xe. Our data thus offer no significant evidence for isotopic anomalies in the noble gases. We also argue that the previous claims for primordial atmospheric-like Ar, anomalous Kr and Xe, excess¹²⁹Xe, and 4.6 × 10⁹-year age are all questionable interpretations which cannot be defended against more prosaic alternatives. This leaves excess²¹Ne as the only noble gas argument for exotic origin; we suggest that this might be an experimental artifact. Until the²¹Ne question can be settled by more definitive experimentation, we feel that noble gas data cannot be used to support arguments that the origin of josephinite is more exotic than crustal serpentinization.     

Oxygen and hydrogen isotope studies of plutonic granitic rocks

The primary δD values of the biotites and hornblendes in granitic batholiths are remarkably constant at about ?50 to ?85, identical to the values in regional metamorphic rocks, marine sediments and greenstones, and most weathering products in temperate climates. Therefore the primary water in these igneous rocks is probably not “juvenile”, but is ultimately derived by dehydration and/or partial melting of the lower crust or subducted lithosphere. Most granitic rocks have δ¹⁸O = +7.0 to +10.0, probably indicating significant involvement of high-¹⁸O metasedimentary or altered volcanic rocks in the melting process; such an origin is demanded for many other granodiorites and tonalites that have δ¹⁸O = +10 to +13. Gigantic meteoric-hydrothermal convective circulation systems were established in the epizonal portions of all batholiths, locally producing very low δ¹⁸O values (particularly in feldspars) during subsolidus exchange. Some granitic plutons in such environments also were emplaced as low-¹⁸O magmas probably formed by melting or assimilation of hydrothermally altered roof rocks. However, the water/rock ratios were typically low enough that over wide areas the only evidence for meteoric water exchange in the batholiths is given by low D/H ratios (δD as low as ?180); for example, because of latitudinal isotopic variations in meteoric waters, as one moves north through the Cordilleran batholiths of western North America an increasingly higher proportion of the granitic rocks have δD values lower than ?120. The lowering of δD values commonly correlates with re-setting of K-Ar ages, and in the Idaho batholith two broad zones (10,000 km²) can be defined where δD biotite <?100 and K-Ar “ages” have all been re-set to values less than 60 m.y., suggesting that the Ar loss was caused by the meteoric-hydrothermal circulation systems. In certain Precambrian batholiths, a much different type of very low-temperature, regional alteration by surface-derived waters took place over an extended period long after emplacement, producing “brick-red” feldspars and markedly discordant Rb-Sr isochron “ages”.     

Density and composition of interplanetary dust particles

Residual meteoritic material has been detected on the surface of crater interiors on lunar samples 60315,29 and 65315,68. Iron-nickel micrometeoroid residues are present in the form of mettallic spherules embedded in or attached to the crater glass-linings; stony-iron meteorite residual material is homogeneously mixed with the glass-linings.Crater simulation experiments show the dependence of crater diameter to depth ratio on projectile density. On the other hand, the projectile velocities exceeding 4 km/s have no measurable influence on the D/T ratios of microcraters. As a result, diameter-to-depth measurements on lunar microcraters yield three groups of micrometeorites in the size range between 1 μm and 1 mm: iron-nickel, stony-iron and low-density particles. The measured D/T values correspond directly to the kind of determined projectile residues: craters showing residues of iron-nickel meteorites have ratios of D/T = 1.3–1.4 and craters with stony-iron residues have ratios ofD/T = 1.9–2.1.Craters with diameters ?30 μm seem to have been formed predominantly by iron-nickel micrometeorites, whereas craters with diameters ?80 μm predominantly by stony meteorites.     

A thermomechanical model of the formation of a rift valley

A numerical model describing the thermomechanical state of the “cold” upper mantle near a mid-oceanic ridge (MOR) spreading at a moderate rate is constructed in the approximation of the boundary layer theory. The condition of rift valley formation leads to a constraint on the temperature and shows what temperature distribution corresponds to the “cold” upper mantle. Taking into account the dependence of mantle rheology on the pressure, temperature, and viscous stresses, the model distributions of the pressure and normal viscous stresses at the base of the lithosphere result in a bend of the heterogeneous lithosphere near the MOR, producing a seafloor topography typical of a rift valley with a depth of a few hundred meters and a spreading rate of ～2.5 cm/yr, characteristic of the Atlantic Ocean. The model width of the rift valley (～10–15 km) agrees with observations fairly well. The model is consistent with the typical heat-flow values observed in the spreading zone.