Magnetic mineralogy of the Yaxcopoil‐1 core,Chicxulub

Abstract— Core from the Yaxcopoil‐1 (Yax‐1) hole, drilled as a result of the Chicxulub Scientific Drilling Project (CSDP), has been analyzed to investigate the relationship between opaque mineralogy and rock magnetic properties. Twenty one samples of suevite recovered from the depth range 818–894 m are generally paramagnetic, with an average susceptibility of 2000 times 10^?6 SI and have weak remanent magnetization intensities (average 0.1 A/m). The predominant magnetic phase is secondary magnetite formed as a result of low temperature (<150 °C) alteration. It occurs in a variety of forms, including vesicle infillings associated with quartz and clay minerals and fine aggregates between plagioclase/diopside laths in the melt. Exceptional magnetic properties are found in a basement clast (metamorphosed quartz gabbro), which has a susceptibility of >45000 times 10^?6 SI and a remanent magnetization of 77.5 A/m. Magnetic mafic basement clasts are a common component in the Yax‐1 impactite sequence. The high susceptibility and remanence in the mafic basement clasts are caused by the replacement of amphiboles and pyroxenes by an assemblage with fine <1 μm magnetite, ilmenite, K‐feldspar, and stilpnomelane. Replacement of the mafic minerals by the magnetic alteration assemblage occurred before impact. Similar alteration mechanisms, if operative within the melt sheet, could explain the presence of the high amplitude magnetic anomalies observed at Chicxulub.     

SIZE-DISTRIBUTIONS OF CHONDRULE TYPES IN THE INMAN AND ALLAN HILLS A77011 L3 CHONDRITES

A petrographc study of 9 thin sections of Inman (L3) and 18 thin sections of ALHA77011 (L3) served to determine the size-distributions of different chondrule textural types. Inman chondrules are significantly larger than those in ALHA77011, but in each chondrite, there is no statistically significant difference between the size-distributions of barred olivine and radial pyroxene plus cryptocrystalline chondrules. In ALHA77011, barred olivine chondrules outnumber radial pyroxene plus cryptocrystalline chondrules, whereas in Inman, the reverse is true. Because compound and cratered chondrules were formed by the collision of similarly-sized objects, the dustball precursors of chondrules must have been size-sorted prior to chondrule formation. The region of dustball size-sorting in the solar nebula must have been very large, similarly affecting the physically-separated precursors of different chondrule types. Size-sorting was probably accomplished by aerodynamic particle-gas interactions. Zones of dustball melting (i.e., chondrule formation) were relatively small, generally affecting only dustballs of one compositional type and relatively uniform size. Different chondrule types were then mixed together in somewhat variable ratios. Within the region where chondrites of a particular compositional group agglomerated, there were sub-reservoirs that contained (roughly) uniformly large or uniformly small chondrules with different mixtures of textural types.     

The halite‐bearing Zag and Monahans (1998) meteorite breccias: Shock metamorphism,thermal metamorphism and aqueous alteration on the H‐chondrite parent body

Abstract— Zag and Monahans (1998) are H‐chondrite regolith breccias comprised mainly of light‐colored metamorphosed clasts, dark clasts that exhibit extensive silicate darkening, and a halite‐bearing clastic matrix. These meteorites reflect a complex set of modification processes that occurred on the H‐chondrite parent body. The light‐colored clasts are thermally metamorphosed H5 and H6 rocks that were fragmented and deposited in the regolith. The dark clasts formed from light‐colored clasts during shock events that melted and mobilized a significant fraction of their metallic Fe‐Ni and troilite grains. The clastic matrices of these meteorites are rich in solar‐wind gases. Parent‐body water was required to cause leaching of chondritic minerals and chondrule glass; the fluids became enriched in Na, K, Cl, Br, Al, Ca, Mg and Fe. Evaporation of the fluids caused them to become brines as halides and alkalies became supersaturated; grains of halite (and, in the case of Monahans (1998), halite with sylvite inclusions) precipitated at low temperatures (≤100 °C) in the porous regolith. In both meteorites fluid inclusions were trapped inside the halite crystals. Primary fluid inclusions were trapped in the growing crystals; secondary inclusions formed subsequently from fluid trapped within healed fractures.     

The Galim LL/EH polymict breccia: Evidence for impact-induced exchange between reduced and oxidized meteoritic material

Abstract— Galim is a polymict breccia consisting of a heavily shocked (shock stage S6) LL6 chondrite, Galim (a), and an impact-melted EH chondrite, Galim (b). Relict chondrules in Galim (b) served as nucleation sites for euhedral enstatite grains crystallizing from the impact melt. Many of the reduced phases typical of EH chondrites (e.g., Si-bearing metallic Fe-Ni; Ti-bearing troilite) are absent. Galim (b) was probably shock-melted while in contact with a more oxidized source, namely, Galim (a); during this event, Si was oxidized from the metal and Ti was oxidized from troilite. Galim (a) contains shock veins and recrystallized, unzoned olivine. The absence of evidence for reduction in Galim (a) may indicate that the amount of LL material greatly exceeded that of EH material; shock metamorphism may have taken place on the LL parent body. Shock-induced redox reactions such as those inferred for the Galim breccia appear to be restricted mainly to asteroids because the low-end tail of their relative-velocity distribution permits mixing of intact disparate materials (including accretion of projectiles of different oxidation states), whereas the peak of the distribution leads to high equilibration shock pressures (allowing impact-induced exchange between previously accreted, disequilibrated materials). Galim probably formed by a two-stage process: (1) accretion to the LL parent body of an intact EH projectile at low relative velocities, and (2) shock metamorphism of the assemblage by the subsequent impact of another projectile at significantly higher relative velocities.     

Local mobility and the spatial dynamics of measles in a rural area of Kenya

Population mobility at different scales is important in the analysis of contagious diseases. The role of local mobility among young children is related to spatial and temporal patterns of measles in part of rural Kenya. The dynamics of measles are examined over a period of 3 1/2 years, during which there were two epidemics; the path of the second of these epidemics is studied, using a spatial averaging technique (the centroid). Limited local mobility of children under the age of five, related to low levels of susceptibility provides a probable explanation of the slow diffusion of measles epidemics.This paper is reprinted from Rural Africana, Spring 1981, with the kind permission of the Editors.     

Electromagnetic induction fields in the deep ocean north-east of Hawaii: implications for mantle conductivity and source fields

Summary. We have analysed a thirty-six day recording of the natural electric and magnetic field variations obtained on the deep ocean floor north-east of Hawaii. The electromagnetic fields are dominated by tides which have an appreciable oceanic component, especially in the east electric and north magnetic components. The techniques of data analysis included singular value decomposition (SVD) to remove uncorrelated noise. There are three degrees of freedom in the data set for periods longer than five hours, indicating a correlation of the vertical magnetic field and the horizontal components, suggesting source field inhomogeneity. Tensor response functions were calculated using spectral band averaging with both SVD and least squares techniques and rotated to the principal direction. One diagonal component, determined mainly by the north electric and east magnetic fields, is not interpretable as a one-dimensional induction phenomenon. The other diagonal term of the response function indicates a rapid rise in conductivity to 0.05 mho m⁻¹ near 160 km. No decrease in conductivity below this depth is resolvable. Polarization analysis of the magnetic field indicates moving source fields with a wavelength near 5000 km. Model studies suggest that the two dimensionality in the response function may be caused by motion in the ionospheric current system.     

St. Magnus Bay,Shetland: A probable British meteorite crater of large size

The last two decades have witnessed a continually expanding programme of investigation concerned with the discovery of terrestrial impact craters or astroblemes as Dietz has called them. These are the wounds produced on the Earth's surface by the high speed collisions with giant meteorites which have taken place throughout geological time. With the exception of a few comparatively recent craters, most of the large scars originated tens or hundreds of millions of years ago and have been preserved as fossils in rocks which have since undergone considerable erosion and alteration.Although various workers have now gathered an impressive array of evidence appertaining to the topographical, petrological and mineralogical changes wrought by the sudden release of enormous quantities of energy when collisions occur, it still remains true that most discoveries of craters are made as a result of their quasi-circular appearance on maps or aerial photographs.A search of maps of the British Isles has revealed several possible impact sites, of which the most convincing is the feature known as St. Magnus Bay in the Shetland Islands. No explanation other than massive impact seems able to account for the peculiar shape of the bay and its resemblance to certain ancient Canadian impact crater residuals.In size it ranks among the dozen or so largest terrestrial impact features and is thus an important object for further study; in its own right and in the context of the currently enhanced speculation concerning the origin of craters on such bodies as the Moon and Mars.     

Climatic and human history in Europe and Latin America: An opportunity for comparative study

Motivated by environmental concerns, a few historians have reaffirmed that history is more than relationships among people; it involves human beings interacting with their environment. Some investigators of European history have given consideration to the effects of climate variations on human history. Preliminary examination of historic documents available for Latin America suggest that a time series of climate variations could also be reconstructed for at least the last 400 years. These preliminary data suggest that the general economic and social crises that prevailed in Europe during the Little Ice Age have parallels in Latin America. Such a concurrence of economic and social trends for these regions seems to reflect the influence of the general adverse climate prevailing during this time. Clearly, an opportunity exists for historians and climatologists to reconstruct the past climate of Latin America and compare these records with other parts of the world to establish the timing, length and magnitude of climate fluctuations over periods of human history.     

Partial polaroids in birefringent filters

It is demonstrated that a single partial polaroid in a Lyot filter behaves in much the same manner as a contrast element. Use of a partial polaroid with a transmission ratio of 10 to 1 results in a factor of 10 decrease in the principal secondary maxima. An explanation of the effect of the partial polaroids is presented in terms of the pulse response of the birefringent network.     

The Ideal Resonance Problem: A comparison of the solutions expressed in terms of mean elements and in terms of initial conditions

In an earlier publication (Jupp, 1972), a solution of the Ideal Resonance Problem is exhibited explicitly in terms of the mean elements; to second order in the small parameter in the case of libration, and to first order in the case of deep circulation. Both representations possess a singularity when the mean modulus of the Jacobi elliptic functions is unity; this corresponds to the separatrix of the phase plane of the dynamical system.It is shown here that, provided particular coefficients associated with the problem satisfy specific relations, the singularity is removed, and the resulting solution is applicable throughout the deep resonance region.The solution is then expressed in terms of general initial conditions. Again, in general, the solution has a singularity associated closely with the limiting motion, and the circulation part of the solution is restricted to deep circulation. It is shown that when the previously-mentioned coefficients satisfy particular constraints, the singularity is removed. In addition, with the same constraints, the deep-circulation solution is applicable throughout the circulation region. It is of interest that these constraints are quite different from those associated with the mean, element formulation.