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11.
The ophiolitic sequence which crops out along the Aspropotamos Valley, Northern Pindos, Greece is composed from the bottom to the top of cumulates, dolerites, basaltic lavas, upper pillow lavas with basaltic/andesitic composition, and scarce basaltic dykes. The intrusive sequence, which is the subject of the present paper, exhibits magmatic layering more pronounced at the bottom than at the top where isotropic gabbros occur; they grade into the overlying dolerites. Troctolites with rare ultramafites prevail in the lower section and olivine gabbros in the upper section; at the top two-pyroxene gabbros appear. The rocks are mainly adcumulates and mesocumulates with subordinate heteradcumulates. The cumulus phases separated in the order: olivine and Cr-spinel, plagioclase, clinopyroxene, orthopyroxene. Olivine, plagioclase and pyroxenes frequently exhibit adeumulus overgrowth. Intercumulus phases may be plagioclase, clinopyroxene, orthopyroxene, pale brown amphibole and magnetite. Where pore material is present, it is composed of plagioclase, clinopyroxene, orthopyroxene, hornblende and ores. Cr-spinel occurs mainly at the bottom of the sequence (Cr2O3 between 30·5 and 39·8 per cent), while magnetite appears as a very rare phase in the upper section. Olivine, orthopyroxene, clinopyroxene exhibit slight cryptic variation (Mg × 100/(Mg + Fe) in the range 90–79, 90–70, 93–72 respectively). The investigated dolerites are non-cumulus rocks where clinopyroxene may be more magnesian than in the uppermost gabbros. The cumulate sequence and dolerites underwent variable but generally slight spilitization, in contrast to the overlying lavas. The sequence was generated through crystal accumulation probably from periodic pulses of tholeiitic magma; newly injected magma batches mixing with magma fractions already differentiated in the magma chamber. The high fluid pressure evidenced by the fluid inclusions in plagioclase and the whole chemical trend of the cumulate sequence are consistent with a genesis above a subduction zone, as already hypothesized for the overlying lavas. 相似文献
12.
Cristina P. De Campos Diego Perugini Werner Ertel-Ingrisch Donald B. Dingwell Giampiero Poli 《Contributions to Mineralogy and Petrology》2011,161(6):863-881
Magma mixing is common in the Earth. Understanding the dynamics of the mixing process is necessary for dealing with the likely
consequences of mixing events in the petrogenesis of igneous rocks and the physics of volcanic eruptive triggers. Here, a
new apparatus has been developed in order to perform chaotic mixing experiments in systems of melts with high viscosity contrast.
The apparatus consists of an outer and an inner cylinder, which can be independently rotated at finite strains to generate
chaotic streamlines. The two cylinder axes are offset. Experiments have been performed for ca. 2 h, at 1,400°C under laminar
fluid dynamic conditions (Re ~ 10−7). Two end-member silicate melt compositions were synthesized: (1) a peralkaline haplogranite and (2) a haplobasalt. The viscosity
ratio between these two melts was of the order of 103. Optical analysis of post-experimental samples reveals a complex pattern of mingled filaments forming a scale-invariant (i.e.
fractal) distribution down to the μm-scale, as commonly observed in natural samples. This is due to the development in space
and time of stretching and folding of the two melts. Chemical analysis shows strong non-linear correlations in inter-elemental
plots. The original end-member compositions have nearly entirely disappeared from the filaments. The generation of thin layers
of widely compositionally contrasting interfaces strongly enhances chemical diffusion producing a remarkable modulation of
compositional fields over a short-length scale. Notably, diffusive fractionation generates highly heterogeneous pockets of
melt, in which depletion or enrichment of chemical elements occur, depending on their potential to spread via chemical diffusion
within the magma mixing system. Results presented in this work offer new insights into the complexity of processes expected
to be operating during magma mixing and may have important petrological implications. In particular: (1) it is shown that,
in contrast with current thinking, rheologically contrasting magmas can mix (i.e. with large proportions of felsic magmas
and high viscosity ratios), thus extending significantly the spectrum of geological conditions under which magma mixing processes
can occur efficiently; (2) the mixing process cannot be modeled using the classical linear two-end-member mixing model; and
(3) the chemical compositions on short-length scales represent snapshots within the process of mixing and therefore may not
reflect the final composition of the magmatic system. This study implies that microanalysis on short-length scales may provide
misleading information on the parental composition of magmas. 相似文献
13.
A technique of interpolation based on a stochastic approach and referred to as ‘kriging’ technique has recently been contributed by the French School. A primary feature of the algorithm is its ability to provide an assessment of the predictive reliability. The accuracy of estimate depends on the evaluation of two stochastic quantities: the variogram γ and the main trend m of the hydrologic event z to be reconstructed.For an effective use of the method a correct understanding of the actual role played by m is required. With some ad hoc examples it is shown that using a polynomial trend with unspecified coefficients as suggested by the general theory may lead to paradoxical results whose behaviour is hard to predict a priori. It turns out that increasing the degree of m may yield an increase of the estimation error where one would expect to obtain the opposite.An alternative formulation is suggested which assumes m to be fully known in advance. Its expression is supposed to be derived from both the general behaviour of z as is recognizable from the available records and some extra-amount of information related to the general physical knowledge of the hydrological context. If this extra-amount of information is missing, the use of a constant trend should be recommended. 相似文献
14.
Tourmaline nodules from Capo Bianco aplite (Elba Island,Italy): an example of diffusion limited aggregation growth in a magmatic system 总被引：1，自引：0，他引：1
The morphology of tourmaline nodules occurring in the Capo Bianco aplite (Elba Island, Italy) is studied. Outcrop features
indicate that tourmaline nodules are the product of magmatic crystallization, as they are aligned along flow fields developed
within the magmatic hosting mass. Mesoscopic observations indicate that nodule morphologies are very variable, from rounded
to dendritic. Morphometric analyses show that tourmaline nodules are fractals and that fractal dimension quantifies their
degree of irregularity. Numerical simulations of nodule growth are performed by using a Diffusion-Limited Aggregation process.
The presence in natural samples of nodules with different morphologies is explained by considering a chaotic magmatic system
characterized by a complex interplay between growth rate in different dynamical regions, latent heat of crystallization, and
local convection dynamics. It is suggested that higher growth rates correspond to growth of tourmaline nodules in dynamical
regions where the transfer of nutrients is very efficient. In such conditions, the latent heat released by the growing nodule
is high, inducing strong local convection dynamics, destabilizing the nodule interface, and promoting the formation of dendritic
morphologies. On the contrary, the growth of nodules in dynamical regions characterized by weak transfer of nutrients is inhibited
leading to weak local convection dynamics and, consequently, to the formation of rounded morphologies. 相似文献
15.
Giampiero Iaffaldano Hans-Peter Bunge Martin Bücker 《Earth and Planetary Science Letters》2007,260(3-4):516-523
Past plate motions display a range of variability, including speedups and slowdowns that cannot easily be attributed to changes in mantle related driving forces. One key controlling factor for these variations is the surface topography at convergent margins, as previous modeling shows that the topographic load of large mountain belts consumes a significant amount of the driving forces available for plate tectonics by increasing frictional forces between downgoing and overriding plates. Here we use this insight to pose a new tectonic inverse problem and to infer the growth of mountain belts from a record of past plate convergence. We introduce the automatic differentiation method, which is a technique to produce derivative code free of truncation error by source transformation of the forward model. We apply the method to a publicly available global tectonic thin-shell model and generate a simple derivative code to relate Nazca/South America plate convergence to gross topography of the Andes mountain belt. We test the code in a search algorithm to infer an optimal paleotopography of the Andes 3.2 m.y. ago from the well-known history of Nazca/South America plate convergence. Our modeling results are in excellent agreement with published estimates of Andean paleotopography and support the notion of strong feedback between mountain belt growth and plate convergence. 相似文献
16.
Marco PITTARELLO Massimiliano PROBO Elisa PEROTTI Michele LONATI Giampiero LOMBARDI Simone RAVETTO ENRI 《山地科学学报》2019,(9)
Over the last decades, the reduction of manpower for herd management has led to an increase of continuous grazing systems(CGS) in the Italian Alps, which allow cattle to roam freely. Under CGS, due to high selectivity, livestock exploit grasslands unevenly, over-and under-using specific areas at the same time with negative effects on their conservation. To counteract these effects, a specific policy and management tool(i.e. Grazing Management Plan) has been implemented by Piedmont Region since 2010. The Grazing Management Plans are based on the implementation of rotational grazing systems(RGS), with animal stocking rate adjusted to balance it with grassland carrying capacity. A case study was conducted on alpine summer pastures to test the 5-year effects produced by the implementation of a Grazing Management Plan in grasslands formerly managed under several years of CGS on 1) the selection for different vegetation communities by cattle, 2) the abundance of oligo-, meso-, and eutrophic plant species(defined according to Landolt N indicator value), and 3) forage yield, quality, and palatability. A total of 193 vegetation surveys were carried out in 2011 and repeated in 2016. Cows were tracked yearly with Global Positioning System collars to assess their grazing selectivity, and forage Pastoral Value(PV) was computed to evaluate forage yield, quality, and palatability. Five years after RGS implementation, cow selectivity significantly decreased and the preference for the different vegetation communities was more balanced than under CGS. The abundance of meso-and eutrophic species increased, whereas oligotrophic ones decreased. Moreover, the abundance of moderately to highly palatable plant species increased, whereas non-palatable plant species decreased, with a consequent significant enhancement of the PV. Our findings indicate that the implementation of Grazing Management Plans can be considered a sustainable and effective management tool for improving pasture selection by cattle and forage quality in mountain pastures. 相似文献
17.
Sulphide-bearing Ca-carbonate, Na-carbonate, Na-hydroxide, Na-chloride and Ca-sulphate waters from Northern Apennines were investigated in order to determine their main chemical and isotopic composition and draw inferences on water-rock interaction. 2H and 18O values suggest an origin mostly meteoric for the analysed waters but a well drilled in Miocenic sediments. The Na-carbonate and the Ca-sulphate waters are the most interesting geochemically. Na-carbonate type, which sometimes reaches extreme composition (Na/Ca up to 228, equivalent ratio), may have been derived through prolonged interaction of Ca-carbonate waters with rocks containing feldspar, montmorillonite and illite under calcite saturation/oversaturation; the high F and pH and the very low PCO
2 agree with prograde dissolution of silicates and lasting water-rock interaction. However, Ca–Na ion exchange, involving clays of marine origin, cannot be excluded in addition. The Ca-sulphate waters, occurring in Messinian gypsum-bearing sediments, are saturated in gypsum and calcite and exhibit very high total H2S (up to 219 mg dm-3) and PCO
2 (up to 0.32 bar). Mass balance of sulphate sulphur, sulphide sulphur and delta34S suggests sulphate – derived from gypsum – as source for H2S; CH4 and organic matter generate the reducing conditions and sulphate reduction is mediated by bacteria. One Na-chloride water from a well in Miocenic sediments has unusual composition, containing about 700 mgdm-3 of potential CaCl2 and having 2H and 18O (-47.5 and -4.9 respectively) which plot far from the meteoric water lines; probably it is derived by mixing of meteoric and formation waters. The Na-hydroxide water, with very high pH (11.2), is generated through protracted interaction of meteoric waters with ultramafites. 相似文献
18.
In an attempt to contribute to studies on global climatic change, 110 years of temperature data for Firenze, Italy, were analysed.
Means and trends of annual and monthly temperatures (minimum, maximum and average) were analysed at three different time scales:
short (20 years), medium (36–38 years) and long (55 years). Comparative changes in extreme events viz. frosts in the first and second parts of the 20th century were also analysed. At short time scales, climatic change was found
in minimum and average temperatures but not in maximum temperatures. At all three time scales, the annual means of minimum,
maximum and average temperatures were significantly warmer in the last part than in the early part of the 20th century. The
monthly mean temperatures showed significant warming of winter months. Over the last four decades, minimum, maximum and average
temperatures had warmed by 0.4, 0.43 and 0.4 ∘C per decade, respectively, and if this trend continues, they will be warmer
by 4 ∘C by the end of the 21st century. The significant decline in days with subzero temperatures and frosts in the last half
of the 20th century, further substantiated the occurrence of climate change at this site. 相似文献
19.
20.
Time-scales of recent Phlegrean Fields eruptions inferred from the application of a ‘diffusive fractionation’ model of trace elements 总被引：2，自引：1，他引：1
Diego Perugini Giampiero Poli Maurizio Petrelli Cristina P. De Campos D. B. Dingwell 《Bulletin of Volcanology》2010,72(4):431-447
The variation of chemical element compositions in two pyroclastic sequences (Astroni 6 and Averno 2, Phlegrean Fields, Italy)
is studied. Both sequences are compositionally zoned indicating a variability of melt compositions in the magma chamber prior
to eruption. A clear dichotomy between the behaviour of major vs. trace elements is also observed in both sequences, with
major elements displaying nearly linear inter-elemental trends and trace elements showing a variable scattered behaviour.
Together with previous petrological investigations these observations are consistent with the hypothesis that magma mixing
processes played a key role in the evolution of these two magmatic systems. Recently it has been suggested that mixing processes
in igneous systems may strongly influence the mobility of trace elements inducing a ‘diffusive fractionation’ phenomenon,
whose extent depends on the mixing time-scale. Here we merge information from 1) numerical simulations of magma mixing, and
2) magma mixing experiments (using as end-members natural compositions from Phlegrean Fields) to derive a relationship relating
the degree of ‘diffusive fractionation’ to the mixing time-scales. Application of the ‘diffusive fractionation’ model to the
two studied pyroclastic sequences allowed us to apply the relationship derived by numerical simulations and experiments to
estimate the mixing time-scales for these two magmatic systems. Results indicate that mixing processes in Astroni 6 and Averno
2 systems lasted for approximately 2 and 9 days, respectively, prior to eruption. 相似文献