Insights on the process of two-stage coronae formation at olivine-plagioclase contact in mafic dyke from Palghat Cauvery Shear Zone,southern India

Mineralogy and Petrology - Coronae between olivine and plagioclase are a common replacement texture in mafic rocks by magmatic and metamorphic processes. Mafic dykes from Palghat Cauvery Shear Zone...     

Erratum: Properties of a pair of fracture networks produced by triaxial deformation experiments: insights on fluid flow using discrete fracture network models

Results of a series of deformation experiments conducted on gabbro samples and numerical models for computation of flow are presented. Rocks were subjected to triaxial tests (σ1 > σ2 = σ3) under σ3 = 150 MPa confining pressure at room temperature, to generate fracture network patterns. These patterns were either produced by keeping a constant confining pressure and loading the sample up to failure (conventional test: CT), or by building up a high differential stress and suddenly releasing the confining pressure (confining pressure release test: CPR). The networks are similar in overall density but differ primarily in the orientation of smaller fractures. In the case of CT tests, a conjugate fracture set is observed with one dominant fracture zone running at about 20° from σ1. CPR tests do not show such a conjugate pattern and the mean fracture orientation is at around 35° from σ1. Discrete fracture network (DFN) methodology was used to determine the distribution of flow and hydraulic head for both fracture sets under simple boundary conditions and uniform transmissivity values. The fracture network generated by CT and CPR tests exhibit different patterns of flow field and hydraulic head configurations, but convey approximately the same amount of flow at all scales for which DFN models were simulated. The numerical modelling results help to develop understanding of qualitative differences in flow distribution that may arise in rocks of the same mineralogical composition and mechanical properties, but under the influence of different stress conditions, albeit at similar overall stress magnitude.

     

Collapse behaviour of a lateritic soil

Collapsible soils are problematic by nature. They undergo collapse or sudden settlement or subsidence under a given stress when their water content is increased. Collapse is characterised by collapse potential expressed as ΔH/H x 100, where ΔH is collapse compression. The amount of stress applied and the water content at the time of collapse govern the amount of collapse. In other words, collapse potential depends upon the amount of stress and the water content. Loess and other wind-blown silts are examples of collapsible soils. This paper presents a parametric study on the collapse behaviour of a lateritic soil. Remoulded specimens of a lateritic soil passing through a 425 µm sieve were compacted in 1-D consolidometer or oedometer at a density of 13 kN/m³ for studying collapse under varied initial moisture contents and initial surcharge pressures. Collapse compression of the samples was induced by saturating the specimens with water contents corresponding to 100% degree of saturation. After collapse occurred, the 1-D consolidation tests were continued up to an applied stress of 160 kPa. Collapse behaviour was studied for the applied initial pressures (σ_i) of 10, 20 and 40 kPa and for the initial water contents (w_i) of 5%, 10%, 15% and 20%. Collapse compression and collapse potential decreased with increasing w_i for all σ_i.     

Cubic law with aperture-length correlation: implications for network scale fluid flow

Previous studies have computed and modeled fluid flow through fractured rock with the parallel plate approach where the volumetric flow per unit width normal to the direction of flow is proportional to the cubed aperture between the plates, referred to as the traditional cubic law. When combined with the square root relationship of displacement to length scaling of opening-mode fractures, total flow rates through natural opening-mode fractures are found to be proportional to apertures to the fifth power. This new relationship was explored by examining a suite of flow simulations through fracture networks using the discrete fracture network model (DFN). Flow was modeled through fracture networks with the same spatial distribution of fractures for both correlated and uncorrelated fracture length-to-aperture relationships. Results indicate that flow rates are significantly higher for correlated DFNs. Furthermore, the length-to-aperture relations lead to power-law distributions of network hydraulic conductivity which greatly influence equivalent permeability tensor values. These results confirm the importance of the correlated square root relationship of displacement to length scaling for total flow through natural opening-mode fractures and, hence, emphasize the role of these correlations for flow modeling.     

Magmatic history and parental melt composition of olivine‐phyric shergottite LAR 06319: Importance of magmatic degassing and olivine antecrysts in Martian magmatism

Several olivine‐phyric shergottites contain enough olivine that they could conceivably represent the products of closed‐system crystallization of primary melts derived from partial melting of the Martian mantle. Larkman Nunatak (LAR) 06319 has been suggested to represent a close approach to a Martian primary liquid composition based on approximate equilibrium between its olivine and groundmass. To better understand the olivine–melt relationship and the evolution of this meteorite, we report the results of new petrographic and chemical analyses. We find that olivine megacryst cores are generally not in equilibrium with the groundmass, but rather have been homogenized by diffusion to Mg# 72. We have identified two unique grain types: an olivine glomerocryst and an olivine grain preserving a primary magmatic boundary that constrains the time scale of eruption to be on the order of hours. We also report the presence of trace oxide phases and phosphate compositions that suggest that the melt contained approximately 1.1% H₂O and lost volatiles during cooling, also associated with an increase in oxygen fugacity upon degassing. We additionally report in situ rare earth element measurements of the various mineral phases in LAR 06319. Based on these reported trace element abundances, we estimate the oxygen fugacity in the LAR 06319 parent melt early in its crystallization sequence (i.e., at the time of crystallization of the low‐Ca and high‐Ca pyroxenes), the rare earth element composition of the parent melt, and those of melts in equilibrium with later formed phases. We suggest that LAR 06319 represents the product of closed‐system crystallization within a shallow magma chamber, with additional olivine accumulated from a cumulate pile. We infer that the olivine megacrysts are antecrysts, derived from a single magma chamber, but not directly related to the host magma, and suggest that mixing of antecrysts within magma chambers may be a common process in Martian magmatic systems.     

Density perturbations in the earth's core due to hydromagnetic oscillations

Summary The equation for density perturbation induced by plane hydromagnetic waves (responsible for geomagnetic secular variation) has been derived based on -plane formulation. It is deduced that for the usual physical properties of the earth's core such a density perturbation would be small and would not be, in the main, responsible for the recently discovered correlation between gravitation and geomagnetic potentials.     

TransientEM screening in two concentric spherical shell model

Summary Different results have been obtained by Negi and Wait in connection with the effect of overburden in electromagnetic exploration by assuming different definitions of screening factor. Both definitions are based on frequency domain considerations. We shall describe the screening characteristics for a transient field with a two-concentric shell model with screening factor=(magnetic field of target and screen-magnetic field of screen alone)/(field of target alone). It is found that the screening factor does become more than one in some situations which means that the EM response of target is more in some cases when the screen is present than when it is absent.Presented at the fifty-second Annual Meeting of AGU during April 12–16, 1971, Washington, D.C.     

A matrix method for heat conduction in multi-layered media

Summary A matrix formalism is developed to facilitate computation of temperature distribution in the steady state in the interior of a multi-layered earth. The idealized model consists of homogeneous and isotropic layers of constant (but arbitrary) values of heat production as well as thermal conductivities. The known surface data is continued by this method to define the thermal structure at any desired point of the interior for planar and spherical stratification. Extension of the problem to the unsteady flow conditions has been presented.     

A reinterpretation of the linear heat flow and heat production relationship for the exponential model of the heat production in the crust

Summary. The conventional assumption of the equality of the slope of the empirical linear heat flow/heat generation relation and the logarithmic decrement in the exponential model of the heat production is not valid. The rigorous analysis of the heat flow equation shows that, with the increase of the magnitude of the slope, the invalidity becomes significant. In the Indian shield region, the available estimate of the slope is 14.8 km, while the corresponding logarithmic decrement value is 17.2 km, However, it may be emphasized that in the step model the slope is rightly interpreted as the thickness of the radioactive layer.