Cooling in rifting sequences during increasing burial depth due to heat flow decrease

Two case histories are presented to give evidences for sediment cooling during increasing burial depth due to heat flow decrease at the end of crustal stretching in extensional settings. The first refers to the Lower Cretaceous succession accumulated in a strongly subsiding trough within the Sirt Basin (Libya); the second relates to the Mesozoic succession of the Lombardian Basin (NW Italy) formed during Late Triassic–Early Jurassic rifting of the northern margin of the Adriatic microplate. In both cases, heat flow decreasing at the end of crustal stretching overbalanced the thermal effect of increasing burial depth causing a net cooling of rocks. These examples provide an alternative to exhumation for explaining cooling events recorded by rifting sedimentary sequences.     

Is Nereid a binary satellite?

If real, Nereid's recently observed large photometric variations (Schaefer and Schaefer, 1988) can be explained by modelling this Neptunian satellite as a quasi-contact binary system made of two similar, ellipsoidal components distorted by their mutual tides. Both the amplitude and the likely periodicity of the observed luminosity changes are consistent with the binary model, provided Nereid's density is close to 1 g cm^–3 (similar to that of most outer solar system satellites). The assumed binary would be probably stable against Neptune's gravitational perturbations, and its origin - as already suggested for a few suspected binary asteroids, like 624 Hektor (Weidenschilling, 1980; Farinella et al., 1982) - could be due to rotational fission following a catastrophic impact. During the Neptune encounter due for August, 1989, Voyager 2's cameras will provide images adequate to confirm, or disprove, the binary model.     

Crystal chemistry and evolution of the clinopyroxene in a suite of high pressure ultramafic nodules from the Newer Volcanics of Victoria,Australia

A clinopyroxene suite from lherzolite inclusions associated with the Victorian (Australia) “Newer Volcanics” has been investigated with the aim of understanding the clinopyroxene crystal-chemical response to increasing temperature (e.g. a melting model and/or crystallization processes prevailing at high pressure). The M1 clinopyroxene polyhedron dominates the intracrystalline physical-chemical variations, essentially given by the triple substitution Al^VIFe _M1 ² Ti⁴⁺⇌Cr³⁺ Fe³⁺Mg _M1 ²⁺ corresponding to an increase in the volume of M1 with increasing Mg/Mg+Fe²⁺ (mg) for the clinopyroxene. A relative Ca²⁺ increase in M2 ensures the necessary charge balance. However, Na⁺ occupancy of M2 persists to the highest mg values, i.e. maximum thermal stability, where the volume of M2 is the largest due to Fe _M2 ²⁺ depletion. The variations of M1 and M2 volumes are greater than, and opposite to, the variations in the volume of T (tetrahedron) by factors of ca. 3 and 1.5, respectively. Inclusions with relatively low clinopyroxene content (Mt. Porndon specimens) show distinct intracrystalline variations, essentially reflecting lower Al^VI, i.e. higher volume of M1, and implying a lower pressure regime compared to clinopyroxene-rich analogues (Mt. Leura specimens). The intracrystalline relationships of the Mt. Porndon clinopyroxene suggest that the host peridotite inclusions survived larger degree of mantle melting at shallower depths relative to the Leura analogues.     

K-Rb ratio in high grade metamorphism: A confirmation of the hypothesis of a continual crustal evolution

The K:Rb ratio (R) in a metamorphic sequence with increasing metamorphism of the West Alps has been analysed. By a regression analysis a comparison is made with the trends observed for R byShaw (1968) in many magmatic processes. The K:Rb ratio in the metamorphic rocks shows a marked increase with increasing metamorphism in relation to a removal of the K content. The arithmetic averages of R in the rocks of amphibolite and granulite facies are 231 and 505 respectively. In some samples of granulite facies the R values are higher than those of the continental alkalic basalts and approximate to the oceanic tholeiites. The variations in the K:Rb ratio may be imputed, according toShaw, to an anatexis process which took place under high grade metamorphic conditions. The marked increase of the K:Rb ratio in the deep material would be caused by partial separation between K and Rb in the granulite assemblages and by an upward migration of Rb, relative to K, from the deep crustal levels of granulite facies. These considerations seem to confirmShaw’s hypothesis of the possibility of a marked crustal stratification and consequently of a continual crustal evolution.     

X-ray single-crystal study of spinels: in situ heating

 Two MgAl₂O₄ stoichiometric spinel crystals, one natural and one synthetic, were heated from 25 to 950 °C and studied in situ by single-crystal X-ray diffraction. The natural crystal, quenched from 850 °C, was further heated and cooled. Thermal expansion was characterized, and cation partitioning at the various temperatures was determined according to a model purposely constructed for high-temperature bond lengths. It was found that the structural evolution of the samples with temperature depended on order–disorder at room temperature. At the temperatures lower than the beginning of cation exchange, thermal expansion was completely reversible and the oxygen coordinate remained stable in spite of varying temperatures. At the temperature at which cation exchange starts, the disordered samples first tend to order and then to disorder at higher temperatures, at variance with the ordered sample, which tends to disorder steadily. In general, the evolution of the spinel structural state on cooling and heating over the same temperature range and the same time intervals does not follow the same path. In particular, in the 600–950 °C range, only partially reversible order–disorder processes occurred in the time span used for the experiments. Received: 16 July 2001 / Accepted: 8 January 2002     

Magma contamination by direct wall rock interaction: constraints from xenoliths from the walls of a carbonate-hosted magma chamber (Vesuvius 1944 eruption)

During the 1944 eruption of Vesuvius different types of xenoliths were ejected. They represent fragments of the walls of a low volume (<0.5 km³) shallow (3–4 km depth) magma chamber. The study of these xenoliths enables us to estimate the amount of contamination occurring at the boundary of a high-T alkaline magma chamber hosted in carbonate rocks. The process of contamination of the magma by carbonates can be modelled, using isotopic and chemical data, as a mixing between magma and marbles. Mass exchanges occur at the boundary between the crystallizing magma and marble wall rocks, where endoskarn forms. The contamination of the solidification front of the chamber is very limited. The solidification front and the skarn shell effectively isolate the interior of the magma chamber from new inputs of contaminants from the carbonate wall rocks. Therefore, the main volume of magma, hosted in the magma chamber, did not undergo any significant mass exchange with the wall rocks.     

Recent sea-level changes and related engineering problems in the Lagoon of Venice (Italy)

In the city of Venice, where the average altitude is only a few tens of centimetres, there has been a dangerous increase in the frequency of flooding during the past few decades.Since 1872, the average increase in flooding levels (“acqua alta”) has been about 40cm: 27cm of this is due to the local rise in mean sea-level (of which some 14cm are related to man-induced subsidence of land, and 3–7cm to geological factors), and at least 14cm are caused by hydrodynamical factors, of which about 10cm can be ascribed to man-induced tidal changes. These latter are due above all to the dredging of deep artificial channels, the reclamation of wide areas of tidal flats, and the diking of fish ponds (“valli”), which have changed the ratio between the surface of the lagoon and that of the inlets. Since the latest modification (1963–1969), the lagoon has been open to large oil tankers, thereby allowing an easier entry of storm surges arising out at sea.Indeed an incompatibility exists between the accessibility of very large boats to the lagoon and the safeguard of Venice. Several engineering projects have been proposed with a view to improving the present critical situation. These projects include underground injections to raise the islands, the construction of new embankments, a reduction in the size of the passes, new access to wide areas for the tide, and the construction of flood gates at the passes. The reliability and effects of these projects are analysed and discussed.In conclusion, the removal of the oil terminal from the lagoon and a stricter control of water pollution are considered the most urgent preliminary actions for a long term solution of the flooding problem. This would enable a decrease in the exchanges between the lagoon and the sea, without, however, reducing the depth of the navigation channels to levels inconsistent with most of the present non-oil traffic. A final measure would be the construction of mobile gates at the passes which would permanently protect the lagoon against flooding.The solution put forward here differs however from the project proposed by a group of experts appointed by the Italian government, which aims at keeping the oil terminal inside the lagoon, thus maintaining the navigation channels at depths which increase hydrodynamic effects and make any sedimetological equilibrium in the lagoon impossible.     

Some new results on statistical properties of wind waves

For wind waves modelled by a stationary Gaussian process ζ(t) (ζ = height above m.w.l. of one point of the free surface) it is shown that, in a time interval including an instant t_m where a maximum ζ_m occurs, the ratio between ζ(t) and ζ_m tends with probability approaching one, to the ratio Ψ(t ? t_m)/Ψ(O), as $\text{ζ}{}_{\mathrm{m}}\text{/Ψ}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{(}\text{O}\text{)}$ tends to infinity, Ψ(t) being the autocovariance of the process.Starting from this result, it is possible to find analytically the characteristic periods of the highest waves in a sea with a given energy spectrum. These periods, calculated according to period definitions of three methods of wave record analysis, are found to be in remarkably good agreement with data from Bretschneider¹ and Svasek.²