S. Basu Mallik

Characterization of aquifers conducting groundwaters with low and high arsenic concentrations: a comparative case study from West Bengal, India

Abstract Possible relationships between groundwater arsenic concentration and alluvial sediment characteristics in a ~19 km2 area in West Bengal have been investigated using a combination of hydrogeochemical, lithogeochemical and geophysical techniques. Arsenic hotspots, typically associated with elevated groundwater Fe and Mn, were found to be correlated to some extent with old river channels (abandoned meanders, oxbow lakes), where sandy aquifers included intercalated fine-grained overbank deposits, rich in As, Fe, Mn and Corg. Otherwise no demonstrably significant overall differences in any of lithology, grain-size distribution, mineral composition or Fe, Mn and organic C content of the sediments were found between two representative sites with contrastingly low (<50 μg 1-1) and high (>200 μg 1-1) As groundwater contents. Our results are consistent with microbially mediated redox reactions controlled by the presence of natural organic matter within the aquifer and the occurrence of As-bearing redox traps, primarily formed by Fb and Mn oxides/hydroxides, being the most important factors which control the release of As into shallow groundwaters at the study site.

Palaeomagnetic study of Archaean Banded Hematite Jasper Rocks from the Singhbhum-Orissa Craton, India

Abstract The Banded Hematite Jasper Formation within the Iron Ore Supergroup of the Singhbhum Craton in eastern India comprises fine alternating layers of jasper and specularite. It was deposited at ∼3000 Ma and deformed by a mobile episode at ∼2700 Ma. Hematite pigment ( 10 μm) is chiefly responsible for red to brown rhythmic bands in the hematite jasper facies although thermomagnetic study also shows that minor amounts (1–2%) of magnetite are present. Palaeomagnetic study identifies a dual polarity remanence resident in hematite ( D I = 283 60° , α95 = 12°) which predates deformation. Studies of the fabric of magnetic susceptibility and rock magnetic results suggest a diagenetic origin for this magnetisation with the hematite formed from oxidation of primary magnetite. The palaeopole (32°E, 24°N, dp dm = 14 18° ) records the earliest post-metamorphic magnetisation event in the Orissa Craton. A minimum apparent polar wander motion of the Orissa-Singhbhum craton of through 80° is identified during Late Archaean times (2900-2600 Ma).

A palaeomagnetic and rock magnetic study of the northern Rajmahal Volcanics, Bihar, India

Abstract A new palaeomagnetic study of mid Cretaceous lavas from 23 localities in the northern part of the Rajmahal Hills gives a mean virtual geomagnetic pole at 10.4°N, 296.6°E, confirming the results of earlier workers. Stepwise thermal and alternating field demagnetisation indicates that 21 out of 23 sites are normally magnetised, whereas two sites in the northwest of the area have recorded two different transitional directions. We have measured the strong-field thermomagnetic behaviour, DC and AC hysteresis parameters, and the variation at low temperature of low-field susceptibility of these and other samples from the Rajmahal Volcanics. These rock magnetic studies reveal that the basaltic lavas contain titanomagnetites which have hardly been oxidised at high temperature, but have undergone low temperature oxidation. The reddened lavas of Gandeswari Hill, on the other hand, show evidence of extensive high temperature deuteric oxidation of titanomagnetite.

The origin of andesite from Rajmahal traps, eastern India: A quantitative evaluation of a fractional crystallization model

Abstract Major-element mass balance calculation supplemented by trace-element modelling indicates that the andesite and tholeiitic andesites in the northwestern portion of Rajmahal traps, Bihar, eastern India might have originated by about 50% fractional crystallization of a quartz-tholeiitic magma (represented by the Rajmahal traps), leaving a cumulate phase of plagioclase (An 67–74), clinopyroxene (augite), orthopyroxene (hypersthene), and magnetite in the proportion 23.19:20.38:2.67:3.76. Scanty occurrences of coarse-grained gabbroic rocks within the tholeiitic traps of adjacent areas have been noted; modal, major- and trace-element compositions of these rocks show marked similarities with those of the computed cumulate phase and furnish support to the fractionation model. The temperatures and oxygen fugacities estimated from coexisting iron-titanium oxide equilibration range from 920 to 955°C, and from 10−10.85 to 10−10.35 bar, respectively, for the andesites. The temperature estimated from the two-pyroxene geothermometer for the trap-tholeiites is c. 1120°C, while the pressure estimated from the olivine-orthopyroxene-quartz assemblage is of the order of 9 kbar. However, the oxygen fugacities estimated from the two-oxide geothermometer range from 10−11.1 to 10−10.45 bar and the temperatures from 978 to 1015°C, which is possibly the quenching temperature of the tholeiitic lava.

On disturbances in a viscoelastic (three-parameter type) medium acted upon by a magnetic field

SummaryIn this note, the problem of disturbances has been investigated in two perfectly (electrically) conductive media-an ideal liquid and a viscoelastic solid of three-parameter type-acted upon by a magnetic field. The electromagnetic equations of Maxwell and the equations of mechanical motion together with the stress-strain relation of the viscoelastic material of three-parameter type have been used to work out the problem. The results exhibit the predominance of transient behaviour in the displacements as well as in the stresses.

On the deformations in a general linear substance of a layered media subjected to a body force and a deforming magnetic field

SummaryThe propagation of disturbances has been studied in a layered media, comprising a liquid and a general linear substance, subject to a body force and permeated by an initial magnetic field acting normal to the plane of the contact. The effect of the body force due to some internal source has been considered in the problem and a set of results has been obtained under two possible extreme variants of the boundary conditions in the form of integral forms, which can be computed to visualize the displacement—time and displacement—depth variations, both in the presence and absence of the body force. The effect of body force on the substance has been, as a sample case, illustrated in the graphs. The stress-strain relation of the general linear substance and the electromagnetic equations of Maxwell have been used in working out the problem.

On disturbances in an elastic medium in contact with a liquid medium and subjected to a magnetic field

SummaryThe present note makes use of electromagnetic equations of Maxwell and those of mechanical motions to work out the problem of disturbances in two perfectly conductive media—an ideal liquid and a perfectly elastic solid-acted upon by a magnetic field perpendicular to their plane of contact. The displacement suffered by solid medium has been determined when a tangential force, exponentially decaying with time, acts in the plane of contact of the two media. The solution of the problem is achieved by using the method of Laplace transform.

On disturbances in a visco-elastic medium of Maxwell type in contact with a liquid medium and subjected to a magnetic field

SummaryThe problem of disturbances in a visco-elastic medium due to Maxwell in contact with a liquid medium — both being perfectly conductive — has been investigated when an impulsive force acts along the plane of the contact. The electro-magnetic equations of Maxwell, equation of elasticity and the stress-strain relations of the material have been used in the investigation.