Different igneous differentiation processes result in various evolution trends in a certain composition space Rn. Almost every genetic process can be described as a single two-pole mixing process, which leads to a straight evolution line in Rn; or as a series of two-pole mixing processes, which in the general case leads to a broken curved evolution line.When a fundamental two-pole mixing process clearly prevails over one or several secondary processes in the genesis of a volcanic suite, a ‘best’ mixing line in Rn can be computed with due attention to experimental uncertainties. A number of statistical tools are available for testing the data-uncertainties-linear model consistency and for pointing out a few dubious data, or the influence of a few secondary processes.The best mixing line and its associated error volumes serve to quantify the trend, and then can be used to get some ‘best’ estimate (or competitive acceptable estimates) of the mineralogical parameters that control the genetic process.Any mineral assemblage defines a continuous composition family in Rn which can be characterized by its gaussian distance to the best mixing line. In the framework of a fractional crystallization model, that distance serves as a test for accepting or rejecting the mineral assemblage as a likely candidate for representing the crystallizing solid.In favorable situations the minimization procedure used to compute the distance also determines both the mineral proportions in the crystallizing solid and their compositions. In less favorable situations it may leave us with some indeterminacy among a small number of acceptable interpretations.The mixing line calculation and the mineralogical inversion procedure are applied to the classical Kilauea Iki Lava Lake (Hawaii) example, where olivine and chromite crystallization with incomplete solid-liquid separation is shown to be a likely explanation for the observations, along with near-surface iron oxidation as a secondary process.The origin of volcanic rocks from Terceira Island (Azores) is less clear. It is shown that the simultaneous crystallization of olivine, Ti-augite and plagioclase can be retained as an acceptable interpretation for the least differentiated rocks, but that substantial secondary processes are also involved. 相似文献
A number of numerical experiments are performed in order to study the role of thermodynamics during the fall of precipitation zones. Thereby the air motion is handled in a Eulerian fashion, using the approximate equations for deep convection, and the precipitation-size particles are treated by a Lagrangian method. The results indicate that the behavior of a precipitation zone is greatly affected by both atmospheric stability and drop evaporation. Stable air strongly opposes downdraft development. Furthermore, air entrained downward by the zone becomes buoyant leading to a considerable updraft aloft. Evaporation, on the other hand, tends to counter the effect of stability and to cause a cool downdraft near the ground and a gust front. In spite of the many simplifications, particularly in the initial conditions, the results as seen by a surface observer show much realism. 相似文献
Rb-Sr whole rock data for gneisses and granites from the oriental Pyrénées yield cadomian ages for the basement gneisses and hercynian ages for the granites. Granites occur in two generations. An alkaline granite dated at 335±15 m.y. is syn.—or slightly post-tectonic, and a later calcalkaline granite dated at 275±12 m.y, is clearly intrusive and marks the end of hercynian tectono-metamorphism. Initial 87Sr/86Sr ratios of the granites put limits on the proportion of juvenile contribution from the mantle to their formation. Petrogenetic models are tested using Rb and Sr as tracers. 相似文献
The mineral separates of a core have been systematically dated by the Io/Th chronology method. The results show that authigenic minerals, though not always completely authigenic, are suitable for dating.We give some evidence against the hypothesis of Io/232Th isotopic equilibrium in marine conditions and suggest that the minerals are mixtures of two components. 相似文献
The xenon isotopic composition measured in samples from various origins shows that variations relative to the atmospheric standard are common. Excesses in129Xe and fissiogenic xenon, derived from the extinct radioactivities129I and244Pu respectively, are characteristic of mid-ocean ridge basalts, whereas238U-fission xenon excesses are only found in granitoid samples or in samples which are contaminated by the continental crust. Hence, the xenon isotopes can be used as tracers in geodynamics. A model for the degassing of the terrestrial mantle is developed and reflections on the time interval between the formation of meteorites and the formation of the Earth are made. 相似文献
Thin levels of amphibolites from the Canigou, Albères and Cap de Creus massifs have been studied in order to investigate their pressure and temperature evolution during time. P and T values have been calculated using the amphibole–plagioclase–quartz thermo-barometer. Si, Al, Mg and Fe of zoned amphiboles have been analysed from core to rim by microprobe. By combining the results obtained from several (or different) crystals, P–T–t paths have been determined using the amphibole-plagioclase-quartz equilibriums. In the Canigou Massif, the amphibolites have recorded anti-clockwise P–T–t paths around a peak of metamorphism located at about 650?°C–6.1 kbar, whereas in the Albères Massif, the calculated P–T–t paths of amphibolites near the paragneisses are retrograde only, from 600?°C–5 kbar to 450?°C–2.5 kbar, but one cummingtonite-bearing amphibolite has also recorded an anti-clockwise evolution around 650?°C–4.5 kbar. The retrograde P–T–t paths recorded for amphibolites from the ‘Cap de Creus’ Massif are retrograde only, from 650?°C–6 kbar and 400?°C–1 kbar. To cite this article: C. Triboulet et al., C. R. Geoscience 337 (2005).相似文献
Most studies of soil organic matter are on humic substances although a substantial contribution of lipids is generally noted for soils. Moreover, soil lipids were shown to have a relatively high resistance to biodegradation. Thus, they could provide information on organic matter sources and diagenetic processes. Soil organic matter is highly heterogeneous and different dynamic pools are in evidenced. However the links between these pools and molecular structure have not yet been established. Soil lipids were investigated in three particle-size fractions of a forest soil. Lipids were shown to be preferentially located in the finest fraction. GC-MS analysis of free lipids has revealed different main sources depending on the particle-size fraction and differential degradation. Furthermore, the results obtained for the free lipids on the one hand, and the saponified lipids on the other hand, have indicated that different preservation mechanisms for lipids occur in this soil. 相似文献
Meteoric smoke particles (MSPs) form through the vaporization of meteoroids and the subsequent re-condensation of metallic species in the mesosphere. Recently, iridium and platinum enrichments have been identified in Greenland ice layers and attributed to the fallout of MSPs supplying polar latitudes with cosmic matter during the Holocene. However, the MSP fallout to Antarctica during the Earth's climatic history remains essentially unknown.
We have determined iridium and platinum in deep Antarctic ice from Dome C and Vostok dated back to 240 kyrs BP. We find high super-chondritic fluxes during warm periods and low meteoric accretion during glacial times, a pattern that is opposite to any known climatic variation in dust fallout to polar regions. The proposed explanation of this accretion regime is a weaker polar vortex during warm periods, allowing peripheral air masses enriched in volcanic iridium and platinum to penetrate inland to Antarctica. The MSP signal emerges only during cold phases and is four times lower than in the Greenland ice cap where more snow accumulates. This suggests that wet deposition is an important route of cosmic material to the Earth's surface. 相似文献
We present here the first available estimations of chemical weathering and associated atmospheric CO2 consumption rates as well as mechanical erosion rate for the Lesser Antilles. The chemical weathering (100–120 t/km2/year) and CO2 consumption (1.1–1.4 × 106 mol/km2/year) rates are calculated after subtraction of the atmospheric and hydrothermal inputs in the chemical composition of the river dissolved loads. These rates thus reflect only the low-temperature basalt weathering. Mechanical erosion rates (approx. 800–4000 t/km2/year) are estimated by a geochemical mass balance between the dissolved and solid loads and mean unaltered rock. The calculated chemical weathering rates and associated atmospheric CO2 consumption rates are among the highest values worldwide but are still lower than those of other tropical volcanic islands and do not fit with the HCO3− concentration vs. 1/T correlation proposed by Dessert et al. (2001). The thick soils and explosive volcanism context of the Lesser Antilles are the two possible keys to this different weathering behaviour; the development of thick soils limits the chemical weathering and the presence of very porous pyroclastic flows allows an important water infiltration and thus subsurface weathering mechanisms, which are less effective for atmospheric CO2 consumption. 相似文献