Relative rheological evolution of chemically contrasted coeval magmas: example of the Tichka plutonic complex (Morocco)

The Tichka plutonic complex presents many unusual structures related to physical interaction between mafic and felsic coeval magmas. In this paper, a rheological approach is used to attempt to understand the various observed structures. After a general presentation of rheological laws and a discussion about the present state of our understanding of the rheology of suspensions, we analyse the main steps of rheological evolution of magmas during crystallization. Taking into account the effect of cooling, increase of silica content and increase of crystallinity on viscosity during thermal evolution of magma, a general rheological diagram is presented. Two main rheological thresholds at about 35% crystallinity (Th₁) and about 65–70% (Th₂), allow us to distinguish three major steps in the rheological behaviour of magmas during cooling. Comparative study of evolution curves for viscosity of coeval mafic (M) and felsic (F) magmas shows that an inversion temperature Ti exists, at which viscosities of both magmas are the same. This point has major petrologic significance because the viscosity ratio (M/F) inverses during cooling when the temperature of magma falls below the Ti point. A model for the evolution of the Tichka magmas during cooling has been derived from the knowledge of the chemical and mineralogical composition of the different rocks representing the original (M) and (F) magmas as well as some hybrid and differentiated samples. Viscosities and crystal fractions of magmas as functions of temperature have been calculated from chemical and mineralogical data. Curves representing the evolution of apparent viscosity of the three initial magmas of the complex are presented. From these vicosity/temperature curves and from the calculted temperatures corresponding to rheological thresholds Th₁ and Th₂ for the different coeval magmas, five main evolution stages are defined. The different structures observed at the contracts between M and F magmas are developed throughout the entire cooling history of the pluton, each structure being related to one of the main evolution stages.     

Synthesis and stability of the garnet calderite in the system Fe-Mn-Si-O

The rare garnet end member calderite, Mn ₃ ²⁺ Fe ₂ ³⁺ Si₃O₁₂, has been synthesized, under the oxygen fugacity of the hematite/magnetite buffer, at pressures not lower than 22 kbar. The synthetic crystals are generally zoned and may contain up to 3 mol% of the Fe²⁺-(=skiagite-) or 10 mol% of the Mn³⁺-(=blythite-) end members, but, under equilibrium conditions, the stable garnets have a restricted compositional range with about 1–4 mol% skiagite. Mn³⁺ represents only a residue of the Mn₂O₃ used in the starting material. At temperatures above 720° C (at 24 kbar) to 850° C (at 30 kbar) these garnets break down into coexisting pairs of magnetite-jacobsite and pyroxmangite-FeSiO₃ solid solutions. No indication for divariancy of this breakdown reaction could be established so that the observed coexistence of garnet and breakdown products over a PT interval must be due to disequilibrium. Although extrapolations of the high-pressure stability data towards lower pressures are hazardous, it is clear that nearly pure calderite garnets can only form in metamorphic environments characterized by geothermal gradients not exceeding some 10°–15° C/km, that is in subduction zone metamorphism. A low-pressure end of the calderite stability is likely because, at temperatures below 250°–300° C, pyroxmangite probably becomes unstable and hydrous Mn²⁺-silicates appear among the low-temperature breakdown products of calderite. Since the upper temperature stability limits of the common garnet end members spessartine and andradite lie some 800° C above that for calderite, solid solutions with these components will drastically stabilize the garnet phase towards both higher temperatures and lower pressures. This explains why garnets containing around 70 mol% calderite can be formed in amphibolitefacies metamorphism.     

Concentration of gold in in situ laterites from Mato Grosso

The gold concentration studied is located in lateritic soils overlying Precambrian schists of the Cuiaba Group in Mato Grosso, Brazil. The following five horizons may be recognized from bottom to top: (1) a gray-blue altered schist horizon, (2) a red argillaceous alterite, (3) a horizon characterized by iron oxihydroxide-rich pebbles and quartz fragments in an iron oxihydroxide-rich matrix and clays, (4) an iron crust, and (5) the present soil. The most significant gold content is found in the third horizon just below the iron crust. According to geological study and morphological observations of the gold particles, the gold ore mined today is the result of two combined processes, i.e., the ferrallitic alteration of quartz lodes enclosed in schists and the effect of the red argillaceous alterite which acts as an impervious structure preventing the largest gold grains from migrating downward during their mechanical concentration.     

Sterols in interstitial waters of marine sediments

In order to attempt to elucidate the nature of biogeochemical processes occurring at the water-sediment interface, sterols have been analysed in near bottom sea and interstitial waters collected in the eastern and western intertropical Atlantic ocean. Free and esterified sterol concentrations range from 0.2 to 82 μg l^?1 and are much higher than those found in overlying sea water, which range from 0.2 to 1.7 μg l^?1 for the dissolved fraction and from 0.01 to 0.07 μg l^?1 for the particulate fraction. Cholest-5-en-3β-ol and 24-ethylcholest-5-en-3β-ol are the dominant sterols in sea and interstitial waters. The variability encountered for the relative importance of minor sterols such as 24-methylcholesta-5,24(28)-dien-3β-ol and stanols, 5α-cholest-22(E)-en-3β-ol, 5α-cholestan-3β-ol and 24-ethyl-5α-cholestan-3β-ol in interstitial water and their variation with depth is discussed in terms of diversity of inputs and bacterial activity. For sediments cored off the Mauritanian coast, a productive area characterized by an intense upwelling, the chemical signatures observed in interstitial water through stanol/stenol ratios occur at levels of very high heterotrophic aerobic bacterial biomass estimations. The study of the sterol composition of interstitial water could constitute a valuable tool in appreciating the intensity of chemical and biological processes occurring in the first few metres of recent marine sediments.     

Climate change in Thailand and its potential impact on rice yield

In Thailand, the world's largest rice exporter, rice constitutes a major export on which the economy of the whole country depends. Climate change could affect rice growth and development and thus jeopardize Thailand's wealth. Current climatic conditions in Thailand are compared to predictions from four general circulation models (GCMs). Temperature predictions correlate well with the observed values. Predictions of monthly rainfall correlate poorly. Virtually all models agree that significant increases in temperature (from 1 to 7 °C) will occur in the region including Thailand following a doubling in atmospheric carbon dioxide (CO₂) concentration. The regional seasonality and extent of the rise in temperature varies with each model. Predictions of changes in rainfall vary widely between models. Global warming should in principle allow a northward expansion of rice-growing areas and a lengthening of the growing season now constrained by low temperatures. The expected increase in water-use efficiency due to enhanced CO₂ might decrease the water deficit vulnerability of dryland rice areas and could make it possible to slightly expand them.The research described in this article has been funded by the U.S. Environmental Protection Agency (EPA). This document has been prepared at the EPA Environmental Research Laboratory in Corvallis, Oregon, through contract # 68-C8-0006 to ManTech Environmental Technology, Inc. It has been subjected to the Agency's peer and administrative review and approved for publication. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.     

VIRGO: Experiment for helioseismology and solar irradiance monitoring

The scientific objective of the VIRGO experiment (Variability of solar IRradiance and Gravity Oscillations) is to determine the characteristics of pressure and internal gravity oscillations by observing irradiance and radiance variations, to measure the solar total and spectral irradiance and to quantify their variability over periods of days to the duration of the mission. With these data helioseismological methods can be used to probe the solar interior. Certain characteristics of convection and its interaction with magnetic fields, related to, for example, activity, will be studied from the results of the irradiance monitoring and from the comparison of amplitudes and phases of the oscillations as manifest in brightness from VIRGO, in velocity from GOLF, and in both velocity and continuum intensity from SOI/MDI. The VIRGO experiment contains two different active-cavity radiometers for monitoring the solar constant, two three-channel sunphotometers (SPM) for the measurement of the spectral irradiance at 402, 500 and 862 nm, and a low-resolution imager (LOI) with 12 pixels, for the measurement of the radiance distribution over the solar disk at 500 um. In this paper the scientific objectives of VIRGO are presented, the instruments and the data acquisition and control system are described in detail, and their measured performance is given.died 13 October 1994     

Experimental constraints on the origin and evolution of mildly alkalic basalts from the Kerguelen Archipelago, Southeast Indian Ocean

This experimental study examines the role of clinopyroxene fractionation on major element trends and alkalinity variations in mildly alkalic basalts from the Kerguelen Archipelago, Southeast Indian Ocean. Equilibrium crystallization experiments were carried out on a natural basalt (MgO=5 wt.%, alkalinity index=0.10) over a range of pressures (0–1.43 GPa) and water contents (nominally dry to hydrous, 1.2 wt.% H₂O) under relatively oxidizing conditions (Δlog FMQ=+1 to +2) at 0 GPa and relatively reducing conditions (Δlog FMQ=0 to –2) at all higher pressures. The hydrous experiments at 0.93 GPa closely reproduce most of the compositional variations in the 24–25 Ma mildly alkalic lavas from the archipelago, which supports a major role for high-Al clinopyroxene fractionation (5–9 wt.% Al₂O₃) at pressures corresponding to the base of the Northern Kerguelen Plateau (15–20 km). However, clinopyroxene fractionation at depth fails to produce important changes in the alkalinity of the residual melts. The transition from tholeiitic to mildly alkalic basalts on the Kerguelen Archipelago thus reflects primarily changes in melting conditions (lower extents of partial melting at higher pressures), which is related to crustal and lithospheric thickening as distance from the Southeast Indian Ridge increased over time from 43 to 24 Ma.