Unravelling a long-term multi-event thermal record in the cratonic interior of southern Finland through apatite fission track thermochronology

Apatite fission track thermochronology (AFTT) has been applied to the Precambrian basement rocks of southern Finland in an attempt to detect within the long-term thermal history, thermal manifestations in the cratonic interior of tectonic events at the craton margin. The likely subtle magnitude of these manifestations means that AFTT is a useful technique for such a study due to its low temperature sensitivity. A total of 10 samples have been analysed, generating AFTT ages, length statistics and thermal models. Ages range from 313 ± 22 to 848 ± 60 Ma and mean track lengths range from 11.0 ± 1.6 to 13.3 ± 1.8 μm. The data suggests the presence of thermal overprinting of an earlier cooling event. Thermal modelling produces similar results for all samples and typically contains the following major events: (1) two phases of Late-Proterozoic cooling, (2) Late-Silurian re-heating, (3) Cenozoic cooling. The first phase of Late-Proterozoic cooling is interpreted to be due to aulacogen inversion as a result of stress propagation from the collisional tectonics of the Sveconorwegian orogeny. The second phase is discussed in relation to passive margin formation and possible asthenospheric diaper induced relief and exhumation. The Late-Silurian re-heating coincides in time with a proposed Caledonian foreland basin. The Cenozoic cooling is interpreted to represent the latest exposure resulting from North Atlantic Margin formation induced uplift and associated denudation.     

The generation and decay of vorticity

Abstract

Vorticity, although not the primary variable of fluid dynamics, is an important derived variable playing both mathematical and physical roles in the solution and understanding of problems. The following treatment discusses the generation of vorticity at rigid boundaries and its subsequent decay. It is intended to provide a consistent and very broadly applicable framework within which a wide range of questions can be answered explicitly. The rate of generation of vorticity is shown to be the relative tangential acceleration of fluid and boundary without taking viscosity into account and the generating mechanism therefore involves the tangential pressure gradient within the fluid and the external acceleration of the boundary only. The mechanism is inviscid in nature and independent of the no-slip condition at the boundary, although viscous diffusion acts immediately after generation to spread vorticity outward from boundaries. Vorticity diffuses neither out of boundaries nor into them, and the only means of decay is by cross-diffusive annihilation within the fluid.     

Fall,classification and cosmogenic records of the Sabrum (LL6) chondrite

Abstract— The petrographic and chemical characteristics of a fresh Indian meteorite fall at Sabrum are described. Its mean mineral composition is defined by olivine (Fa_31.4), orthopyroxene (Fs_25.1,Wo_2.0), clinopyroxene (Wo₄₅En_45.6Fs_9.4) and plagioclase (An_10.6Ab_83.6Or_5.8). The meteorite shows moderate shock features, which indicate that it belongs to the S4 category. Based on mineralogical and chemical criteria the meteorite is classified as an LL6 brecciated veined chondrite. Several cosmogenic radioisotopes (⁴⁶Sc, ⁷Be, ⁵⁴Mn, ²²Na and ²⁶Al), noble gas (He, Ne, Ar, Kr and Xe), nitrogen isotopes, and particle tracks density have been measured. Concentrations of cosmogenic ²¹Ne and ³⁸Ar indicate that its cosmic‐ray exposure age is 24.8 Ma. Small amounts of trapped Kr and Xe, consistent with petrologic class 5/6, are present. The track density in olivines is found to be (1.3 ± 0.3) × 10⁶/cm². Activities of most of the short‐lived isotopes are lower than those expected from solar cycle variation. ²²Na/²⁶Al (1.12 ± 0.02) is found to be significantly anomalous, being ?25% lower than expected from the Climax neutron monitor data. These results indicate that the cosmic‐ray flux during the terminal segment of the meteoroid orbit was low. The activities of ²⁶Al and ⁶⁰Co and the track density indicate small meteoroid size with a radius ?15 cm.     

The fusion crust of the Winchcombe meteorite: A preserved record of atmospheric entry processes

Fusion crusts form during the atmospheric entry heating of meteorites and preserve a record of the conditions that occurred during deceleration in the atmosphere. The fusion crust of the Winchcombe meteorite closely resembles that of other stony meteorites, and in particular CM2 chondrites, since it is dominated by olivine phenocrysts set in a glassy mesostasis with magnetite, and is highly vesicular. Dehydration cracks are unusually abundant in Winchcombe. Failure of this weak layer is an additional ablation mechanism to produce large numbers of particles during deceleration, consistent with the observation of pulses of plasma in videos of the Winchcombe fireball. Calving events might provide an observable phenomenon related to meteorites that are particularly susceptible to dehydration. Oscillatory zoning is observed within olivine phenocrysts in the fusion crust, in contrast to other meteorites, perhaps owing to temperature fluctuations resulting from calving events. Magnetite monolayers are found in the crust, and have also not been previously reported, and form discontinuous strata. These features grade into magnetite rims formed on the external surface of the crust and suggest the trapping of surface magnetite by collapse of melt. Magnetite monolayers may be a feature of meteorites that undergo significant degassing. Silicate warts with dendritic textures were observed and are suggested to be droplets ablated from another stone in the shower. They, therefore, represent the first evidence for intershower transfer of ablation materials and are consistent with the other evidence in the Winchcombe meteorite for unusually intense gas loss and ablation, despite its low entry velocity.     

Shapes of chondrules determined from the petrofabric of the CR2 chondrite NWA 801

The approximately spherical shapes of chondrules has long been attributed to surface tension acting on ~1 mm melt droplets that formed and cooled in the microgravity field of the solar nebula. However, chondrule shapes commonly depart significantly from spherical. In this study, 109 chondrules in a sample of CR2 chondrite NWA 801 were imaged by X-ray computed tomography and best-fitted to ellipsoids. The analysis confirms that many chondrules are indeed not spherical, and also that the chondrules’ collective shape fabric records a definite 13% compaction in the host meteorite. Dehydration of phyllosilicates within chondrules may account for that strain. However, retro-deforming all chondrules shows that a large majority were already far from spherical prior to accretion. Possible models for these initial shapes include prior deformation of individual chondrules in earlier hosts, and, as suggested by previous authors, rotation of chondrules as they were solidifying, and/or “streaming” of molten chondrules by their differential velocities with their gaseous hosts after melting. More in situ 3-D work such as this study on a variety of unequilibrated chondrites, combined with detailed structural petrography, should help further constrain these models and refine our understanding of chondrite formation.     

Population and geography in India

The field of population geography was first introduced during the 1960s in India and advanced under the direction of Gosal at the Punjab University. Teaching and research in population geography were introduced by Chandigarh at Punjab University, which today is the main center of research activity. Population geography in India has followed the main tenets of geography in general and is based on spatial perspectives. Deficits are apparent in the paucity of research on socioeconomic implications of spatial distributions, but there is infrastructural feedback to support theory development. Theoretical advances moving from theory to fact or from empirical fact to theory are limited. Comprehensive training in methodology and quantitative techniques is needed for further development of population theory: multivariate analysis, factor analysis, principal component analysis, model building, hypothesis testing, and theory formulation. Methodological sophistication will also help in understanding and interpreting the diverse and complex Indian demographic situation. The analysis of population geography in the Indian spatial, cultural, political, and historical context may be applied to other less developed countries of similar sociocultural background. The Indian Census has contributed over the 100 years of its existence reliable and efficiently produced data on a wide variety of measures at assorted scales down to the village level. Field work among geographers has not achieved a level of development commensurate with population censuses. Recent doctoral research has focused on qualitative studies of local situations. Research topics range from the distribution and structure of population, mortality, fertility, and migration to peripheral issues of social segregation. Popular topics include urbanization, labor force, sex composition, literacy, and population growth. Distribution of population and density studies have amounted to only 2 in 30 years. Population texts are in abundance. The Punjab University is the headquarters of the Association of Population Geographers, which publishes the journal "Population Geography." Many advances are still needed in a timely fashion.     

Magmatic evolution of Li–F, rare-metal granites: a case study of melt inclusions in the Khangilay complex, Eastern Transbaikalia (Russia)

We report compositions of homogenized quartz-hosted melt inclusions from a layered sequence of Li-, F-rich granites in the Khangilay complex that document the range of melt evolution from barren biotite granites to Ta-rich, lepidolite–amazonite–albite granites. The melt inclusions are crystalline at room temperature and were homogenized in a rapid-quench hydrothermal apparatus at 200 MPa before analysis. Homogenization runs determined solidus temperatures near 550 °C and full homogenization between 650 and 750 °C. The compositions of inclusions, determined by electron microprobe and Raman spectroscopy (for H₂O), show regular overall trends of increasing differentiation from the least-evolved Khangilay units to apical units in the Orlovka intrusion. Total volatile contents in the most-evolved melts reach over 11 wt.% (H₂O: 8.6 wt.%, F: 1.6 wt.%, B₂O₃: 1.5 wt.%). Concentrations of Rb range from about 1000 to 3600 ppm but other trace elements could not be measured reliably by electron microprobe. The resulting trends of melt evolution are similar to those described by the whole-rock samples, despite petrographic evidence for albite- and mica-rich segregations previously taken as evidence for post-magmatic metasomatism.

Melt variation trends in most samples are consistent with fractional crystallization as the main process of magma evolution and residual melt compositions plot at the granite minimum in the normative Qz–Ab–Or system. However, melts trapped in the highly evolved pegmatitic samples from Orlovka deviate from the minimum melt composition and show compositional variations in Al, Na and K that requires a different explanation. We suggest that unmixing of the late-stage residual melt into an aluminosilicate melt and a salt-rich dense aqueous fluid (hydrosaline melt) occurred. Experimental data show the effectiveness of this process to separate K (aluminosilicate melt) from Na (hydrosaline melt) and high mobility of the latter due to its low viscosity and relatively low density may explain local zones of albitization in the upper parts of the granite.     

27Al, 29Si,and 23Na MAS NMR study of an Al,Si ordered alkali feldspar solid solution series

Solid-state ²⁷Al, ²⁹Si and ²³Na MAS NMR spectra have been obtained for an Al,Si ordered low albite to low microcline ion exchange series for which unit-cell parameters and ²⁹Si NMR data have previously been reported. ²⁷Al δ_i vary continuously with composition from 63.4 (±0.5) ppm for albite to 58.9 (±0.5) ppm for microcline, and parallel the ²⁹Si chemical shifts assigned to the T2m-site. The ²⁷Al and ²⁹Si chemical shifts for this series correlate well with composition-dependent lattice parameters, most notably cell volume and the angle [201]¹b. The linewidths of the ²⁹Si and ²⁷Al resonances indicate a significant amount of structural disorder in the intermediate compositions due to Na, K substitution. The 1 σ width of the distribution of average Si-O-T angles for each T-site is estimated to be about 1° for the Or33 sample. The average ²³Na δ_i varies monotonically from -8.5 (±1) ppm for albite to -24.3 (±1)ppm for Or83. Similarly, the average ²³Na nuclear quadrupole coupling constant decreases from 2.60 to 1.15 (±0.05) MHz and the asymmetry parameter of the electric field gradient increases from 0.25 to 0.6 with increasing K-content from albite to Or83. The observed variations in the quadrupole coupling parameters are consistent with simple electrostatic calculations. Higher resolution ²³Na spectra of the intermediate compositions obtained at 11.7 T indicate the presence of an inhomogeneous linebroadening which is related to the distribution of Na-environments. A model based on a random distribution of local compositions does not simulate the spectra, suggesting that the distribution of Na is skewed toward Na-rich clusters. Observation of the ²³Na NMR lineshape of Or49 after short periods of heat treatment indicate that ²³Na NMR is very sensitive to the changes in the Na, K distribution accompanying the early stages of exsolution. Reversible changes occur after heating at 530° C for 3 h, whereas heating at 600° C produces no changes, possibly bracketing the position of the coherent spinodal for Al, Si ordered alkali feldspars at this composition.     

Strain path partitioning within thrust sheets: microstructural and petrofabric evidence from the Moine Thrust zone at Loch Eriboll,northwest Scotland

Quartz c axis fabrics and microstructures have been investigated within a suite of quartzites collected from the Loch Eriboll area of the Moine Thrust zone and are used to interpret the detailed processes involved in fabric evolution. The intensity of quartz c axis fabrics is directly proportional to the calculated strain magnitude. A correlation is also established between the pattern of c axis fabrics and the calculated strain symmetry.Two kinematic domains are recognized within one of the studied thrust sheets which outcrops immediately beneath the Moine Thrust. Within the upper and central levels of the thrust sheet coaxial deformation is indicated by conjugate, mutually interfering shear bands, globular low strain detrital quartz grains whose c axes are aligned sub-parallel to the principal finite shortening direction (Z) and quartz c axis fabrics which are symmetric (both in terms of skeletal outline and intensity distribution) with respect to mylonitic foliation and lineation. Non-coaxial deformation is indicated within the more intensely deformed and recrystallized quartzites located near the base of the thrust sheet by single sets of shear bands and c axis fabrics which are asymmetric with respect to foliation and lineation.Tectonic models offering possible explanations for the presence of kinematic (strain path) domains within thrust sheets are considered.