The effects of deformation on the kinetics of the net-transfer reaction anorthite + forsterite → cpx + opx + spinel ± gt were
studied using static and shear deformation experiments. Experiments were performed on dry anorthite-olivine (An92–Fo93) samples at 900°C and pressures between 1,000 and 1,600 MPa in a Griggs apparatus. Deformed (‘non-hydrostatic’) and undeformed
(‘static’) samples are compared in terms of phase petrology, reaction rate and reaction mechanisms. Anorthite + olivine reactions
are diffusion-controlled as seen from reaction rim structures. In undeformed samples, delayed reaction onset and low reaction
rates demonstrate sluggish nucleation of reaction products and slow rates of diffusion at dry conditions, even at 700–900 MPa
confining pressure overstepping. The reaction rate is enhanced in deformed An–Fo samples. The higher rate is mainly attributed
to a combination of high stresses and viscous deformation processes of the reactants and products, which cause an increase
in the nucleation rate of products. The results imply that viscous deformation processes alone can be responsible for the
initiation and localisation of metamorphic reactions in dry rocks in the absence of fluid infiltration.
A. A. de Ronde was supported by the Swiss National Fond grants 2100-057092.99 and 2000-065041.01. 相似文献
Zoning patterns of An content and Fe, Mg and Sr concentrations in plagioclase phenocrysts in andesites from Parinacota Volcano (N. Chile) reflect alternating recharge events with two chemically distinct mafic magmas. These magmas are characterized by low and high Sr contents, similar to two recent mafic flank eruptions. One end-member basaltic andesite shows large Sr enrichment and Heavy Rare Earth depletions and thus equilibrated with lower-crustal rocks at depth where plagioclase (high Sr) is unstable, and garnet (high HREE, Y) is stable. A second end-member magma is lower in Sr, Ba contents and has REE patterns typical for parent magmas elsewhere in the Central Andes.
The number of recorded recharge events increases after a catastrophic sector collapse and during the subsequent rebuilding of the stratocone. Variations of An, Fe and Mg contents and morphology of zones suggest also changes in water pressure, including decompression under water under-saturated and water-saturated conditions. Evidence for decompression is more present in post-collapse samples, suggesting that the change in the volcano dynamics involves changes in magma chamber location. This shows the importance of the cone collapse event in the volcano's magmatic evolution. We propose that both end-members are only seen at the surface of Parinacota Volcano because of the particular dynamics of this volcanic system and that similar processes might occur in other volcanic centres of the Central Volcanic Zone. 相似文献
The layered mafic intrusion at Sept Iles, Canada, is one of the largest intrusions in the world. A new interpretation of its structure is proposed, based on a review of its geology and a comparison with the Skaergaard intrusion, Greenland. Several different magmatic components are recognized; hence the name Sept Iles Intrusive suite (SIIS) is proposed. Emplacement of the suite may have been preceded by eruption of flood basalts. The first magmas of the suite rose in the crust to accumulate beneath the density filter afforded by the basalts. The largest component is the Sept Iles Mafic intrusion (SIMI). The Lower series of the SIMI is dominated by leucotroctolites and leucogabbros. Above it lie the Layered series, which is largely comprised of gabbro and troctolite. Both these units are unchanged from earlier interpretations. The anorthosites (s.l.), gabbros and monzogabbros, formerly called the Transitional series, are now considered to be the Upper Border series, developed by floatation of plagioclase. Common autoliths in the Layered series are parts of the hydrothermally altered Upper Border series from towards the interior of the intrusion, which have foundered and settled through the magma. The contamination of the magma that accompanied this event oxidised iron in the magma and led to the precipitation of magnetite around the periphery of the intrusion. The subsequent depletion of Fe3+ and/or increase in SiO2, CaO and P2O5 may have induced apatite saturation and accumulation to form two layers rich in apatite, near the base and at top of the Layered series. Granitic magma was developed by fractional crystallisation and was emplaced along the roof of the chamber, where it acquired large quantities of xenoliths. These were probably derived from the flood basalts, their evolved members and fragments of mafic dykes chilled by the granitic magma. Accumulations of monzonite pillows in this unit testify to another magmatic event and a floor to the granitic magma chamber, indicating lateral transport of magma. Chemically distinct syenites in the upper part of the intrusion are part of the Point du Criade intrusion, a large, late composite sill. Diabase and leucogabbro components show a close link with the SIMI and all the acidic magmas may have originally formed by differentiation of the main magma in cupolas towards the centre of the intrusion. A series of late gabbro intrusions that cut the SIMI may represent a rejuvenation of magmatism. The Border zone is a mass of fine-grained rocks that occurs along the border of the SIMI: it may be another magmatic component, or just the lateral border series of the SIMI. 相似文献
Small amounts of felsic, evolved plutonic rocks, often called oceanic plagiogranites, always occur as veins or small stocks within the gabbroic section of the oceanic crust. Four major models are under debate to explain the formation of these rocks: (1) late-stage differentiation of a parental MORB melt, (2) partial melting of gabbroic rocks, (3) immiscibility in an evolved tholeiitic liquid, and (4) assimilation and partial melting of previously altered dikes. Recent experimental data in hydrous MORB-type systems are used to evaluate the petrogenesis of oceanic plagiogranites within the deep oceanic crust. Experiments show that TiO2 is a key parameter for the discrimination between different processes: TiO2 is relatively low in melts generated by anatexis of gabbros which is a consequence of the low TiO2 contents of the protolith, due to the depleted nature of typical cumulate gabbros formed in the oceanic crust. On the other hand, TiO2 is relatively high in those melts generated by MORB differentiation or liquid immiscibility. Since the TiO2 content of many oceanic plagiogranites is far below that expected in case of a generation by simple MORB differentiation or immiscibility, these rocks may be regarded as products of anatexis. This may indicate that partial melting processes triggered by water-rich fluids are more common in the deep oceanic crust than believed up to now. At slow-spreading ridges, seawater may be transported via high-temperature shear zones deeply into the crust and thus made available for melting processes. 相似文献
Static tests typically are used in the initial stage of environmental assessments to predict the potential for acid mine drainage,
and also are used to monitor waste disposal at active mining operations. The neutralization potential (NP) of 19 sedimentary,
metamorphic, and igneous extrusive rocks was measured by the Sobek method to compare the results with those computed by using
the quantitative mineralogy of the rocks and the NP values that were deemed to be appropriate for the individual minerals.
Most of the computed NP values of the rocks are lower than the measured values, but a close relationship is evident. If the
contents of carbonate minerals are low, the NP in non-ultramafic igneous rocks typically is controlled by the amount of Ca
substitution in plagioclase, whereas in ultramafic rocks the NP contribution is dominated by the amount of olivine and serpentine.
Compositional and NP analyses of plagioclase concentrates indicate that the NP systematically varies from <1 for the Na end-member
to about 14 for compositions near that of the Ca end-member. Among several variables that can affect the Sobek NP values,
the volume and normality of the acid added to the sample are well-known to influence the results; however, the temperature
and duration of the acidification stage can also have a pronounced effect on the measured NP of some rocks, depending on their
mineralogy. Fizz tests to estimate the carbonate content and the consequent acid addition are highly subjective, and the ‘no
fizz’ rating is not to be taken literally. The NP results themselves provide a clear indication of whether a sample has been
over-acidified or under-acidified in the test procedure. 相似文献
The lattice preferred orientation (LPO) of an anorthosite (composed of andesine) sampled from a highly deformed anorthositic mylonite (Grenville Province, Quebec) was measured by TOF neutron diffraction and SEM-EBSD. The quantitative texture analysis of neutron data was accomplished by using the Rietveld texture analysis with the WIMV algorithm, implemented in the program package Materials Analysis Using Diffraction (MAUD). The texture calculations of the EBSD data were performed by using the program BEARTEX. Analyses from neutron and electron diffraction data gave similar results if EBSD data are smoothed to account for grain statistics. The principal pole figures show (010) roughly parallel to the rock foliation, (001) poles exhibiting a low angle (25°) to the pole to foliation, and (100) poles close to the Y-direction (perpendicular to the lineation and foliation pole). The [100] crystallographic direction shows a maximum in the lineation direction, [010] directions concentrate near the foliation pole. The geological deformation conditions and the constructed pole figure patterns indicate that the preferred orientation could be attributed to intracrystalline slip dominantly on (010) with [100] as slip direction. Elastic properties, calculated by averaging, document weak anisotropy that has implications for the seismic structure of the lower crust. 相似文献