A Quartz-bearing Orthopyroxene-rich Websterite Xenolith from the Pannonian Basin, Western Hungary: Evidence for Release of Quartz-saturated Melts from a Subducted Slab

An unusual quartz-bearing orthopyroxene-rich websterite xenolithhas been found in an alkali basaltic tuff at Szigliget, Bakony–BalatonHighland Volcanic Field (BBHVF), western Hungary. Ortho- andclinopyroxenes are enriched in light rare earth elements (LREE),middle REE and Ni, and depleted in Nb, Ta, Sr and Ti comparedwith ortho- and clinopyroxenes occurring in either peridotiteor lower crustal granulite xenoliths from the BBHVF. Both ortho-and clinopyroxenes in the xenolith contain primary and secondarysilicate melt inclusions, and needle-shaped or rounded quartzinclusions. The melt inclusions are rich in SiO₂ and alkalisand poor in MgO, FeO and CaO. They are strongly enriched inLREE and large ion lithophile elements, and display negativeNb, Ta and Sr anomalies, and slightly positive Pb anomalies.The xenolith is interpreted to represent a fragment of an orthopyroxene-richbody that crystallized in the upper mantle from a hybrid meltthat formed by interaction of mantle peridotite with a quartz-saturatedsilicate melt that was released from a subducted oceanic slab.Although the exact composition of the slab melt cannot be determined,model calculations on major and trace elements suggest involvementof a metasedimentary component. KEY WORDS: quartz; mantle; silicate melt inclusion; SiO₂-rich melt; subduction; Carpathian-Pannonian Region     

Comparison of internal features and microchemistry of ferromanganese crusts from the Central Indian Basin

SEM/probe analyses of two ferromanganese crusts from the Central Indian Basin, formed on basalt substrate, reveal botryoids and cusps. The botryoids composed of δ-MnO₂, are enriched in Co, and the cusps and laminations with todorokite are enriched in Ni and Cu. Microstructural and elemental variations, from the ferromanganese crust outer surface to the basalt substrate, probably reflect changes in the accretion or in the source of metals. The major source of the crust metals is the seawater, as indicated by the Mn/Fe ratio of unity and by the microstructures.     

Distribution and Origin of Seamounts in the Central Indian Ocean Basin

Approximately 200 seamounts of different dimensions have been identified, from multibeam bathymetry maps of the Central Indian Ocean Basin (CIOB) (9°S to 16°S and 72°E to 80°E), of which 61% form eight chains that trend N-S. The seamounts are clustered above and below 12°S latitude. Area II (9°-12°S) shows a concentration of smaller seamounts (≤400 m height), and area I (12°-15°S) has a mixed population (including both less and more than 400 m height). Inspite of the differences in their height, the seamounts of these eight chains are morphologically (slope angle, flatness, basal width) corelatable. Furthermore, we suggest that height-width ratio could be useful in identifying the style of seamount eruption. The seamount chains in the CIOB probably originated from propagative fractures and were produced between 61 and 52 Ma (chrons A26 to A23) as a result of the interaction between the conjugate crusts of the Central Indian and Southeast Indian Ridges during the Indo-Eurasian collision event.     

Characteristics of VHF scintillations in the equatorial anomaly crest region in India

The characteristics of ionospheric scintillations at Rajkot in the equatorial anomaly crest region in India are described for the years 1987–1991 by monitoring the 244-MHz transmission from the satellite FLEETSAT. This period covers the ascending phase of solar cycle 22. Scintillations occur predominantly in the pre-midnight period during equinoxes and winter seasons and in the post-midnight period during summer season. During equinoxes and winter, scintillation occurrence increases with solar activity, whilst in summer it is found to decrease with solar activity. Statistically, scintillation occurrence is suppressed by magnetic activity. The characteristics observed during winter and equinoxes are similar to those seen at the equatorial station, Trivandrum. This, coupled with the nature of the post-sunset equatorial F-region drift and h‘F variations, supports the view that at the anomaly crest station, scintillations are of equatorial origin during equinox and winter, whilst in summer they may be of mid-latitude type. The variations in scintillation intensity (in dB) with season and solar activity are also reported.     

Use of aerial photo-interpretation techniques for land evaluation of a part of Bolangir district,Orissa

The land evaluation has been of paramount importance for the planned development and resource utilization of an area, specially for locating better potential zones for increasing agricultural production. Large areas in the country are yet to be evaluated based on their productivity and potentiality. A case study was undertaken in a part of Bolangir district of Orissa, by using small scale aerial photographs.     

A discussion on the lead isotope geochemistry of galenas from the Bambui group,Minas Gerais — Brazil

Lead isotope data on galenas from the Bambui group, Minas Gerais have been assembled and discussed. The isotope data, when plotted on the Single Stage Model III lead isotope development curve of Cumming and Richards (1975), yield 207/206 ages of 650±50 m.y and a linear array of data points with negative model ages, which intercept the growth curve at 650±60 m.y and 1850±150 m.y. The radiometric dates coincide with the two major orogenic episodes, the Brazilian and the Transamazonian. The data are interpreted in terms of a two stage model, where the source of lead for galenas, formed during Brazilian orogeny, is the basement rocks of Transamazonian age. The calculated Th/U value for the source rock is 2.9±0.3. The geographical distribution of lead isotope ratios shows a zonation in Vazante Paracatu region, an area containing large deposits of zinc. The implication of the distribution on the ore formation is discussed.     

Integrated production and effective loss rates in the ionosphere

The total electron content data obtained at Ahmedabad through the Faraday fading records of the radio beacons abroad the satellites Explorer 22 and 27 are used to determine the overhead integrated production rate (Q ⁰) and integrated loss coefficient (β′) for the epoch 1965–1968. The production rate (Q ₀) is shown to have two peaks during a year around the equinoctal months and for a particular monthQ ₀ increases linearly with the 10·7 cm solar flux. The loss coefficient β′, too, has two equinoctial peaks within a year. The semiannual variations ofQ ₀ and β′ are discussed in relation to similar variation in the [O]/[N₂] ratio.     

Use of aerial photographs for soil erosion studies

Using air photos soil loss from agricultural fields has been estimated in respect of the Rana and granitic areas of south-west of Merida, Spain, applying the universal soil-loss equation. The results of soil-loss have been shown as soil erosion susceptibility and soil erosion hazard classes. This study has shown that large scale aerial photographs (1:12500) are quite useful in evaluating many of the parameters of the universal soil loss equation with limited field checks.