Source depletion and extent of melting in the Tongan sub-arc mantle

The fluid immobile High Field Strength Elements (HFSE) Nb and Ta can be used to distinguish between the effects of variable extents of melting and prior source depletion of the Tongan sub-arc mantle. Melting of spinel lherzolite beneath the Lau Basin back-arc spreading centres has the ability to fractionate Nb from Ta due to the greater compatibility of the latter in clinopyroxene. The identified spatial variation in plate velocities and separation of melt extraction zones, combined with extremely depleted lavas make Tonga an ideal setting in which to test models for arc melt generation and the role of back-arc magmatism.We present new data acquired by laser ablation-ICPMS of fused sample glasses produced without the use of a melt fluxing agent. The results show an arc trend towards strongly sub-chondritic Nb/Ta (< 17) with values as low as 7.2. Melting models show that large degree melts of depleted MORB mantle fail to reproduce the observed Nb/Ta. Alternatively, incorporation of residual back-arc mantle that has undergone less than 1% melting into the sub-arc melting regime reproduces arc values. However, the extent of partial melting required to produce the composition of the Lau Basin back-arc basalts averages 7%. This apparent discrepancy can be explained if only the lowermost 4 km of the residua from the mantle melt column beneath the back-arc is added to the source of arc magmas. We have identified that the degree of arc/back-arc coupling displayed in the rock record provides an index of the depth of hydrous melting beneath the arc. In this case, this would imply a depth of ~ 75 km for generation of arc magmas, indicating that hydrous melting in the mantle wedge is triggered by the breakdown of hydrous phases in the subducting slab.     

The prospects for modernizing very broadband seismometers

This paper considers the possibilities for modernizing the KSESh broadband seismic station developed earlier at the IPE AS USSR. The goal of the modernization was to extend the frequency and dynamic ranges and to decrease nonlinear distortions. The ways and concrete methods for improving the mechanical construction and electronic components of the station are shown and the intermediate results are described. As preliminary estimates show, there is an opportunity to build devices with the velocimeter response up to 520–540 s periods on the basis of this pendulum.     

Possible contribution of leakage currents from the Atlantic Ocean to the magnetic-field variations observed in Devon

It is suggested that leakage currents flow across Devon, north of Exeter, from large-scale electric-current systems induced in the Atlantic Ocean by the geomagnetic variations incident on the Atlantic Ocean from the ionosphere. This model can account for the variation with period of the contribution of induced current flow to the differences in the magnetic H variations of period > 30 s observed at Exeter and Sidmouth.     

Geothermal energy release at the Solfatara of Pozzuoli (Phlegraean Fields): Phreatic and phreatomagmatic explosion risk implications

The H₂O, CO₂ and H₂S outputs at the Solfatara of Pozzuoli have been measured and a map of the exhaling areas has also been made. The energy released at the surface by the fluids has been estimated to be 10¹⁹ ergs/day.The presence of aquifers at Phlegraean Fields increases the phreatic and phreatomagmatic explosion risk.Our results suggest that even if an uprising magma may interact with water at depth, an explosion could occur only at the shallow levels of a few hundred meters. Since the transfer of energy toward the surface is favoured by the presence of fractures, a detailed analysis of the deep fracture network would help to evaluate the risk levels of the various areas of Phlegraean Fields.     

PHASE DISTORTION DUE TO ABSORPTION IN SEISMOGRAMS AND VSP*

Absorption of seismic energy in the earth reduces amplitudes and changes phases of the propagating seismic waves. Amplitudes are usually recovered according to an estimated exponential decay curve, while phase distortions are generally disregarded. Therefore, accurate processing of seismic data requires a careful investigation of the relationship between absorption and phases. In this paper a procedure is suggested to achieve this goal, and some related topics are worked out. A method is outlined for computing synthetic seismograms and vertical seismic profiles with phase distortion due to absorption. The algorithm works in the frequency domain, and it provides for absorption according to the usual model of exponential decay of amplitude with distance. The absorption coefficient is a linear function of frequency and is related to the quality factor Q of the rocks. Complex seismic velocities are introduced and minimum-phase delay due to absorption is assumed for all cases considered. Methods for estimating Q profiles from seismic well surveys and seismic data are described. Comparison between field and synthetic data shows the effectiveness and benefits of the procedure. Some applications of the method to phase distortion recovery and wavelet processing are presented.     

Observations of vertical ground accelerations exceeding gravity during the 1997 Umbria-Marche (central Italy) earthquakes

We found extensive evidence that the vertical ground accelerations produced during the largest shock (M = 6.0) of the 1997 Umbria-Marche earthquake sequence exceeded 1g in two areas close to the heavily-damaged villages of Annifo and Colle Croce. This evidence comes from the striking observation of thousands of freshly fractured and broken rocks and stones in these areas. Some of the broken stones lie isolated on soft detritic soil while others had been previously piled up, probably a long time agoto clear the fields for farming. The freshness of the cuts and fractures and the consistency of the observations for thousands of rocks and stones in these areas indicate that these rocks were thrown upwards during the earthquake, with breakage occurring at the time of impact. Ground motion calculations consistent with the static deformation inferred from GPS and interferometry data, show that the broken stones and rocks are found in the zone where the strongest shaking took place during the earthquake and that most of the shaking there was vertical.     

Secular changes,polarity epochs and tectonic movements as indicated by palaeomagnetic studies of Hungarian rock samples

Summary The paper contains an outline of the palaeomagnetic investigations in Hungary including some of their geophysical and geological applications. More than 500 samples of volcanic rocks were studied by usual palaeomagnetic techniques. The rocks investigated belong to three different geological periods — the Quaternary — upper Tertiary, the middle Miocene and the lower Cretaceous. The palaeomagnetic data obtained for the individual localities are summarized in the Appendix.Paper presented at the IAGA Symposium Madrid, September 1969.     

Adsorption und Hydrolyse von Methylparathion an Aktivkohle und Adsorberharz Adsorption and Hydrolysis of Methyl Parathion (MPT) on Activated Carbon and Adsorption Resin

Investigating adsorption of methyl parathion on the activated carbons Filtrasorb 400 and F 44 and on the adsorption polymer Wofatit Y 77 we found a stronger adsorption on the carbons at smaller concentration whereas the resin has the larger capacity at higher concentration. Adsorbents were regenerated through hydrolysis of the pesticide at pH = 11.7. The velocity of hydrolysis in the adsorbed state is reduced to a tenth of that in solution. However, a transport resistance in the pores of adsorbents seems to be negligible as may be concluded from the very small measured particle diameter dependence of the velocity constants. In adsorption-regeneration measurements the capacity of the adsorbents stabilized at about 70% of that of the fresh adsorbents.     

No evidence for a 0.8–0.9 m.y. old micro-australite layer in deep-sea cores

Fission-track and⁴⁰Ar—³⁹Ar dating of Australasian tektites indicates that the australites are older than the rest of the Australasian tektites. Some authors have suggested, therefore, that there should be two microtektite layers in the Australasian region: a younger layer approximately synchronous with the Brunhes/Matuyama geomagnetic reversal boundary (previously described in the literature) and an older layer of Australian microtektites (micro-australites) below the Brunhes/Matuyama reversal boundary and perhaps associated with the end of the Jaramillo event. Fifteen cores already known to contain a layer of Australasian microtektites at or slightly above the Brunhes/Matuyama reversal boundary were searched for an 0.8 to 0.9 m.y. old micro-australite layer. No evidence for this layer was found. Thus, the data do not support the conclusion that the Australasian tektites were produced by two impact events at different times.