Did natural fission of 235U in the earth lead to formation of the supercontinent Columbia?

Steady decline in the percentage of 235U in terrestrial uranium made natural fission impossible after about 1.8 Ga.Fission before 1.8 Ga disturbed the lead isotope system at various places worldwide,su...     

Response of dry- and liquid-filled porous rocks to static and dynamic loading by variously-shaped projectiles

Summary The present investigation was undertaken to study the response of both dry and liquid-filled porous rocks to static and dynamic loading of penetrators with various tip shapes. Eighteen static and forty-eight dynamic tests were conducted involving 6.35 mm diameter indenters featuring flat, hemispherical and 60° conical tip shapes on three different rocks in the energy range from 1.57 to 5.63 J. Force-penetration data were collected by means of strain gages attached to the penetrators. For all three materials, the magnitude of the resistive forces for a given input energy decreased in the following order: (i) dynamic penetration into a dry sample, (ii) dynamic penetration into a fluid-filled specimen, (iii) static loading of the fluid-filled rock, and (iv) static, loading of the dry rock.An unusual observation for the dynamic force-penetration curves was the temporal disparity between the peak force and maximum deformation, which must be attributed to inertial factors. Based on the experimental data, an analytical rigid-body model was constructed that contained both a static and a dynamic component of the resistive force with the objective of quantifying the effect of dynamic loading and the presence of fluid on the response of the rocks. The dynamic component was portrayed as a viscous resistance proportional to the velocity of the penetrator and the contact surface area which is a function of the indentation. It was found that the model provides a good predictive capability for the dynamic force-indentation relations for prescribed materials and tip geometries upon use of but a single constant for the viscosity.This paper is dedicated to the memory of Corwin O. Rogers.     

Phlogopite in the generation of olivine-melilitites from Namaqualand, South Africa and implications for element fractionation processes in the upper mantle

Major and trace element and Sr, Nd and Pb isotope analyses are presented for thirteen olivine-melilitites from Namaqualand, South Africa. Major element variations are consistent with derivation from carbonated garnet-peridotite at depths of at least 100 km and trace element abundances indicate melt fractions of 4%. Ubiquitous negative K anomalies and low, buffered K₂O concentrations are interpreted to reflect the effect of residual phlogopite during melting. It is suggested that phlogopite stability and low melt potassium saturation concentrations are enhanced by high CO₂/(CO₂ + H₂O) conditions. Residual phlogopite can also account for low measured Rb/Sr, Ba/Sr and Th/U ratios in the melilitites. REE abundances are controlled by residual garnet and hence Sm/Nd ratios are low (0.13–0.18). U/Pb ratios vary from 0.05 to 5 and are a function of Pb concentration which is in turn controlled by residual Pb-rich phase (probably sulphide). Nd and Sr isotopes are comparable with OIB from St. Helena, although two samples extend to higher ⁸⁷Sr/⁸⁶Sr ratios. Present day Pb isotopes are much more variable and partly reflect radiogenic growth since emplacement as a result of the highly variable U/Pb ratios.

Many of the trace element characteristics of the melilitites are distinct from those of within-plate potassic magmas despite both being derived from phlogopite-bearing, enriched mantle source regions. This can be attributed to the depth at which source enrichment occurred and the subsequent control exerted by phlogopite and carbonate during melting. In contrast to melilitites, potassic magmas are derived from shallower depths under low CO₂/(CO₂ + H₂O) conditions and at higher temperatures at which phlogopite melts more readily.

The incompatible element ratios of the melilitites are also similar to those both observed in HIMU ocean island basalts (OIB) and inferred for HIMU OIB source regions from isotope variations (viz, low Sm/Nd, Rb/Sr, K/Nb, Th/U and high U/Pb and Ce/Pb). It is suggested that HIMU OIB's may be derived from sources that have been subject to enrichment by a melt generated in the presence of residual phlogopite.     

Development of fluid conduits in the auriferous shear zones of the Hutti Gold Mine, India: evidence for spatially and temporally heterogeneous fluid flow

The gold mineralization of the Hutti Mine is hosted by nine parallel, N–S trending, steeply dipping, 2–10 m wide shear zones, that transect Archaean amphibolites. The shear zones were formed after peak metamorphism during retrograde ductile D₂ shearing in the lower amphibolite facies. They were reactivated in the lower to mid greenschist facies by brittle–ductile D₃ shearing and intense quartz veining. The development of a S₂–S₃ crenulation cleavage facilitates the discrimination between the two deformation events and contemporaneous alteration and gold mineralization. Ductile D₂ shearing is associated with a pervasively developed distal chlorite–sericite alteration assemblage in the outer parts of the shear zones and the proximal biotite–plagioclase alteration in the center of the shear zones. D₃ is characterized by development of the inner chlorite-K-feldspar alteration, which forms a centimeter-scale alteration halo surrounding the laminated quartz veins and replaces earlier biotite along S₃. The average size of the laminated vein systems is 30–50 m along strike as well as down-dip and 2–6 m in width.Mass balance calculations suggest strong metasomatic changes for the proximal biotite–plagioclase alteration yielding mass and volume increase of ca. 16% and 12%, respectively. The calculated mass and volume changes of the distal chlorite–sericite alteration (ca. 11%, ca. 8%) are lower. The decrease in δ¹⁸O values of the whole rock from around 7.5‰ for the host rocks to 6–7‰ for the distal chlorite–sericite and the proximal biotite–plagioclase alteration and around 5‰ for the inner chlorite-K-feldspar alteration suggests hydrothermal alteration during two-stage deformation and fluid flow.The ductile D₂ deformation in the lower amphibolite facies has provided grain scale porosities by microfracturing. The pervasive, steady-state fluid flow resulted in a disseminated style of gold–sulfide mineralization and a penetrative alteration of the host rocks. Alternating ductile and brittle D₃ deformation during lower to mid greenschist facies conditions followed the fault-valve process. Ductile creep in the shear zones resulted in a low permeability environment leading to fluid pressure build-up. Strongly episodic fluid advection and mass transfer was controlled by repeated seismic fracturing during the formation of laminated quartz(-gold) veins. The limitation of quartz veins to the extent of earlier shear zones indicate the importance of pre-existing anisotropies for fault-valve action and economic gold mineralization.     

Mean meridional circulations of the stratosphere and mesosphere

Observations from the Nimbus 6 pressure modulator radiometer (PMR) have been used to estimate monthly mean planetary wave fluxes of heat and momentum in the stratosphere and mesosphere. While the eddy heat fluxes play an important role in the mean meridional circulation of the winter stratosphere they are shown to be less important in the upper mesosphere. Incorporation of the observed momentum fluxes into the Oxford two-dimensional circulation model has shown that they are incapable of providing the momentum transport necessary to balance the zonal flow accelerations induced by the mean meridional motion. Other unspecified transfer processes represented by Rayleigh frictional damping of the zonal fow are shown to dominate. In contrast the observed fluxes in the stratosphere achieve the necessary redistribution of momentum. Moreover their interannual variability profoundly influences the stratospheric circulation, as demonstrated in the model by the use of two different annual sets of observed momentum fluxes. The desirability of calculating the planetary wave behaviour within the model is indicated.     

Two stages of gold mineralization at Hutti mine, India

The Hutti gold mine is located in a high-angle, NNW–SSE-trending shear zone system, which hosts nine discrete auriferous shear zones (reefs). On a clockwise, retrograde PT path two separate stages of deformation/metamorphism (D₂/M₂ and D₃/M₃) occurred synchronous with two distinct stages of gold mineralization, both of which were associated with different fluid types. Stage 1 mineralization developed during D₂/M₂, where the amphibolite host rocks were altered by a metamorphic fluid with a $ {{\delta }^{{18}}}{{O}_{{{{H}_2}O}}} $ of 7.5–10.1?‰, rich in K, S, As, and Au at pressure and temperature conditions of around 3 kbar and 530?+?20/?30°C, respectively. The stage 1 auriferous shear zones are enveloped by a zoned alteration consisting of a distal biotite–chlorite and proximal biotite–plagioclase assemblage. Subsequently, D₂/M₂ was overprinted by D₃/M₃ deformation and metamorphism at 300–400°C and <2 kbar that formed the stage 2 mineralization. The stage 2 mineralizing fluid which originated from outside the greenstone belt (δ¹⁸O_fluid of 3.2–6.8?‰) was rich in Si, Au, and W. This mineralization stage is distinct by the emplacement of laminated quartz veins central to the shear zone, containing locally visible gold at concentrations of up to 1 kg Au/t. The laminated quartz veins are surrounded by a millimeter-scale chlorite₂–K-feldspar alteration halo, which replaced the stage 1 biotite–plagioclase assemblage. The oxygen isotopic composition of the stage 2 fluid suggests a mixture of a magmatic fluid with an oxygen isotopic composition in the range of 6 to 10?‰ and an isotopically light formation fluid that resulted from fluid–rock interaction in the greenstone pile. The two fluid fluxes at stages 1 and 2 both contributed to the overall gold mineralization; however, it was the second fluid pulse, which gave the Hutti mine its status as the largest gold mine in India. The metamorphic evolution was thereby important for the first stage, whereas the second stage was controlled by tectonism and intrusion of the high-heat production Yellagatti granite that re-established the fluid plumbing and mineralizing system.     

Long-term gain changes in mercury-level tiltmeters of the Stacey type

A uniaxial mercury-level short baselength tilt transducer constructed at the University of Alberta is described. The design closely follows the differential capacitance tiltmeter ofStacey et al. (1969), but differs in that aluminum is used for the beam, and how the mercury cups are supported beneath the beam. It also allows gain verification by comparison of the direct capacitances with a known capacitance. It has been found that the gain factor for the instrument changes with time, because of the relaxation of the mercury surfaces. This has been monitored for several years for four instruments at two sites. The gains change rapidly for several months after installation, but more slowly as time progresses. The magnitudes of such changes in instruments of this type may be as large as 3 percent.