• C Radhakrishnamurty

      Articles written in Journal of Earth System Science

    • Susceptibility as a tool for studying magnetic stratigraphy of marine sediments

      B L K Somayajulu C Radhakrishnamurty T J Walsh

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      Eighty six gravity cores collected from the Pacific Ocean by the Scripps Institution of Oceanography have been logged for magnetic susceptibility using a simple and rapid technique. These logs fall into three types: Type 1 showing several highs and lows, Type 2 with a single-broad-hump, and Type 3 showing nearly constant susceptibility with depth.

      Type 1 cores are found to be mainly from sediment-trap (trenches) areas which are close to the active volcanoes and the high peaks probably correspond to a slump or deposition of volcanic material; these events occurred between 0·1 and 2·8 million years ago.

      Type 2 cores are by far the most common, (56 out of 86) and show a maximum deposition of magnetic material (i.e. crest region of the hump) in the range of 0·2 and 1·7 million years. The susceptibility during this period was about a factor of two higher for several cores compared to their respective values during the last 0·1 million years. Oceanwide deposition of volcanic material and/or the atmospherically transported dust rich in magnetic material (cosmic and/or terrestrial) by our planet can account for such an increase. A third possibility may be the change (decrease) in accumulation rates of the sediments during this period.

      In type 3 cores the susceptibility is almost constant with depth and these are randomly distributed (excluding the sediment trap areas) analogous to the case of type 2 cores. A high deposition rate in these areas can alter type 2 into type 3.

      It appears that the maximum of type 2 hump can act as a stratigraphic marker since type 2 cores are the most common ones and are widely distributed over the entire Pacific.

    • Nature of magnetic grains in basalts and implications for palaeomagnetism

      C Radhakrishnamurty S D Likhite E R Deutsch G S Murthy

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      Investigations involving temperature dependence of low-field susceptibility and of low- and high-field hysteresis have been carried out on more than one thousand basalt samples of ages ranging from less than 3 million years to more than 1 billion years. Combined application of these measurements makes it possible to distinguish rapidly the effective particle sizes and oxidation states of the magnetic minerals in the rock.

      One interesting finding is that, in basalts of widely different ages, a strongly cation-deficient phase of magnetite having distinct magnetic properties predominates over other types of magnetic grains inferred to be present. Properties attributable to multidomain magnetite were found often in basalts older than Cretaceous, but seem to be present only occasionally among the younger basalts. Despite the fact of a frequent association of magnetite with titanium in basalts, results of our tests carried out on such basalts do not in general show a magnetic behaviour consistent with the magnetic material being a solid-solution titanomagnetite. Some implications of these results for palaeomagnetism are discussed.

    • In situ magnetic measurements on igneous rock bodies

      C Radhakrishnamurty S D Likhite P W Sahasrabudhe

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      A simple method for an in situ measurement of susceptibility and low-field hysteresis on rock bodies like dykes and flows is described. Extensive use of this method indicates that in some dykes and flows, the nature of the magnetic grains varies rapidly across the bodies. Making use of such observations, it seems possible to select stable rock samples for palaeomagnetic work and also understand to some extent the origin of remanent magnetization in different types of rock bodies.

    • Magnetic studies on the remanence carriers in igneous rocks of different ages

      C Radhakrishnamurty S D Likhite G S Murthy

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      A critical study on the type of the magnetic grains, both in composition and domain state, in rocks of different ages has been carried out. One simple, fast and non-destructive test, which can provide useful information on the nature of the magnetic grains in freshly collected samples, seems to be the ratio of susceptibilities at 77 and 300 K. This ratio, termed relative susceptibility, ranges from 0·1, for samples containing 70% ulvospinel bearing titanomagnetite (TM70) to 1·50 for cation deficient magnetite bearing ones. The results indicate that the value of 0·1 for TM70 is not greatly affected even if some amount of TM80, which is nonmagnetic at 300 K, is present in a rock sample. However, the coercive force at 77 K will increase considerably for such a sample. The effects of mixed compositions and domain states of magnetic grains on the overall behaviour of basalts are discussed.

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