Molar tooth structures of the Neoarchean Monteville Formation, Transvaal Supergroup, South Africa. II: A wave-induced fluid flow model

Sedimentological, morphological, and geochemical characteristics of molar tooth (MT) structures in the ca 2·6 Ga Monteville Formation suggest a new fluid flow model for MT formation: (i) intercalated shales and carbonate sands were deposited near to above storm wave base; (ii) sediments cracked, forming an interconnected network of MT cracks that were also open to pores in sand lenses; (iii) storm waves pumped sea water into open MT crack networks, causing rapid microcrystalline carbonate nucleation, Ostwald ripening of nuclei, and growth of granular carbonate cores; some of these cores were transported by water flowing through the cracks; (iv) unfilled MT cracks collapsed, and filled MT ribbons deformed plastically as host sediments compacted and dewatered; (v) carbonate cores were overgrown by polygonal rims; and (vi) MT structures deformed brittlely with additional compaction and produced pebbly lags if reworked. MT cracks may have formed by multiple mechanisms; however, expansion of gas from organic decay and sediment heaving due to wave loading best explain MT crack morphology and are most consistent with the fluid flow model for MT CaCO₃ presented here.     

A Melt Extraction Model Based on Structural Studies in Mantle Peridotites

This study, largely based on field observations in various peridotitemassifs and on basalt xenoliths, traces the successive stagesof melt extraction from mantle diapirs. The first stage, initiatedin the garnet lherzolite field and pursued in the spinel lherzolitefield, creates melt pockets at the sites of the former garnets.During ascent, the melt fraction stable in spinel-lherzolitesis estimated to be 7 per cent. Above this fraction, but dependingupon plastic strain, permeability is obtained and melt extractionstarts. This occurs at 50 km depth. A network of connected meltveins and gashes, opened by fluid assisted shear fracturingin the deforming peridotites, is first created. When its verticalextension attains 10 km. the melt pressure fractures the overlyingperidotites (tensional hydrofracturing) creating a conduit formelt extraction. The buoyant forces generate a negative pressureat the base of the conduit. After communication with the earthsurface is achieved, the melt network surrounding the dyke rootis thus drained. This mechanism explains the remarkable efficiencyof melt extraction in residual harzburgites and dunites. Theconduit is a dyke, with a 20cm width at shallow depth. The meltvelocity through such dykes in shallow mantle is 5 cm s^–1.The rate of extraction of melt is so large that melt extractionis necessarily a discontinuous process even in the case of oceaniccrust generation. Each dyke of the dyke swarm in oceanic crustand ophiolites (and possibly each cumulate layer in the underlyingmafic cumulates) corresponds to a melt extraction event. Thuseach event creates 1 m of crust, during the time lapse of afew weeks. The periodicity of such events (5–50 yr) dependson the spreading rate (10–1 cm yr^–1). Each one corresponds,in the rising diapir, to a hydrofracture produced locally inthe area of the mantle melt network. For spreading rates > 1 cm yr^–1, a 6 km thick oceaniccrust is created with an underlying uppermost mantle composedof residual harzburgites. For smaller rates, the oceanic crustis thinner as a result of smaller degrees of melting, with plagioclaselherzolites as residue. For even smaller rates, no oceanic crustis created (continental rifting) and the residue is a comparativelyfertile spinel lherzolite. This is explained by a direct relationbetween spreading rate and ascent rate of the mantle diapir.For spreading rates < 1 cm yr^–1, the adiabatic meltingin the diapir stops at about 15 km depth in plagioclase lherzolites(except for a final melt extraction just below the Moho) andat > 30km in spinel lherzolites. This model has implications on melting processes (disequilibriummelting), the nature of primary melts and implies a straighterconnection than generally accepted between the physics and chemistryof volcanism and melting processes in the mantle.     

Daily precipitation variability in semiarid agricultural regions in Macedonia,Greece

Abstract

The spatio-temporal variability of daily precipitation series was investigated in a semiarid region of central Macedonia in northern Greece, Ten years of daily rainfall records for seven stations in the region constituted the data base. The spatial characteristics were examined by drawing composite correlation diagrams for the cool (October-March) season and the warm (April-September) season, and the results confirmed the regional homogeneity of the data sets. Furthermore, the temporal analysis indicated that the non-rainy days constituted the major portion of days throughout the year at all the stations. Similarly, light rainfall represented the majority of rainy days. Moreover, the annual rainfall variation showed high values in March, April and November with low values occurring in the summer and autumn. A sharp increase of rainfall between the 185th and the 195th day of the year must be taken into account when the harvest is scheduled. Harmonic and Power Spectrum analyses applied to the annual variation of rain depths using 5-day intervals revealed significant periodicities of 26, 122, 365 and 55 days. Finally the analysis of the annual variation of rain occurrences. revealed periodicities of 365 and 122 days.     

Environmental control on diverse stromatolite morphologies in the 3000 Myr Pongola Supergroup, South Africa

A unique outcrop of partly silicified dolomite in the White Umfolozi section of the Pongola Supergroup, South Africa indicates that stromatolites were diverse and adapted to a range of shallow, tidal depositional settings 3000 Myr ago. Composite columnar stromatolitic bioherms 0.7-1.6m high and 0.4-1.0m in diameter formed along the margins of a tidal channel. They were flanked, away from the channel, by flat stratiform and small domical stromatolites growing in low energy tidal flat environments. Conical stromatolites, 0.05-0.30m high and 0.03-0.10m in diameter, accreted in high-energy coarse-grained carbonate sand along the bottom of the tidal channel. The stromatolites probably formed through the activities of filamentous, oxygen-producing, photoautotrophic cyanobacteria.     

The Role of Laser Ranging for Calibrating Jason-1: The Corsica Tracking Campaign

The French Transportable Laser Ranging System (FTLRS), a highly transportable Satellite Laser Ranging (SLR) instrument, was set up in Corsica (from January to September 2002) for participating to the JASON-1 altimeter verification phase. In addition to the tracking of oceanographic satellite missions and in order to perform an accurate positioning, the FTLRS also acquired laser ranging data on geodetic satellites, STARLETTE and STELLA essentially.

The paper describes the analysis strategy mainly based on the use of a short-arc orbit technique to compute accurate 1 cm local orbits, and then the geocentric positioning (2–3 mm relative to GPS). Finally, we established the JASON-1 absolute calibration value, based on 9 SLR short-arcs (between cycles 1 and 26), at 108.2 ± 8.7 mm; the 10-day repeatability is of 26.1 mm showing that a great accuracy has been reached.