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41.
42.
Data on compositions of coexisting minerals in the graphite-bearing carbonatites of the Chernigovka massif are reported. Thermodynamic analysis of these results made it possible to establish that the temperature of equilibrium between graphite, dolomite, calcite, magnetite, and olivine for silica activity buffered by the (zircon + baddeleyite) assemblage is approximately 600°C. The minimal pressure of formation of these mineral assemblages is approximately 0.2 GPa, which is consistent with estimates of the erosion depth for the Chernigovka massif. The oxygen fugacity typical of the graphite-bearing carbonatite is 0.6–0.8 log units below the quartz-magnetite-fayalite buffer. Such values are typical of magmatic systems, e.g., basalts of the mid-ocean ridges (MORB). At 600°C, the gas phase in the C-H-O system equilibrated with the mineral assemblage of the carbonatite studied is dominated by CO2 and H2O, whereas methane-rich fluids appear at lower temperatures. 相似文献
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44.
Cornelius Tschegg Theodoros Ntaflos Vyacheslav V. Akinin Christoph Hauzenberger 《Contributions to Mineralogy and Petrology》2012,164(3):441-455
A suite of mainly spinel peridotite and subordinate pyroxenite xenoliths and megacrysts were studied in detail, enabling us
to characterize upper mantle conditions and processes beneath the modern North American–Eurasian continental plate boundary.
The samples were collected from 37-Ma-old basanites cropping out in the Main Collision Belt of the Chersky Range, Yakutia
Republic (Russian Far East). The spinel lherzolites reflect a mantle sequence, equilibrated at temperatures of 890–1,025 °C
at pressures of 1.1–2 GPa, with melt extraction estimated to be around 2–6 %. The spinel harzburgites are characterized by
lower P–T equilibration conditions and estimated melt extraction up to 12 %. Minor cryptic metasomatic processes are recorded
in the clinopyroxene trace elements, revealing that percolating hydrous fluid-rich melts and basaltic melts affected the peridotites.
One of the lherzolites preserves a unique melt droplet with primary dolomite in perfect phase contact with Na-rich aluminosilicate
glass and sodalite. On the basis of the well-constrained P–T frame of the xenolith suite, as well as the rigorously documented
melt extraction and metasomatic history of this upper mantle section, we discuss how a carbonated silicate melt infiltrated
the lherzolite at depth and differentiated into an immiscible carbonate and silicate liquid shortly before the xenolith was
transported to the surface by the host basalt. Decreasing temperatures triggered crystallization of primary dolomite from
the carbonate melt fraction and sodalite as well as quenched glass from the Na-rich aluminosilicate melt fraction. Rapid entrainment
and transport to the Earth’s surface prevented decarbonatization processes as well as reaction phenomena with the host lherzolite,
preserving this exceptional snapshot of upper mantle carbonatization and liquid immiscibility. 相似文献
45.
Thomas Mutschler Utz Kramar Maximiliano R. Vergara Theodoros Triantafyllidis 《Rock Mechanics and Rock Engineering》2013,46(4):835-847
The fraction of swelling-type clay minerals (smectites) is a first measure to estimate the swelling potential of natural rocks. This swelling potential is only activated if water can be incorporated in the sheet silicates. The samples studied in this work are volcanic rocks from the Southern Andes. According to the petrographic and mineralogical studies, the rocks are characterized as vulcanite with high content of smectite. Undisturbed samples showed minor swelling behaviour in conventional swelling tests, although X-ray diffraction and differential thermal analysis revealed swelling clay contents of more than 70 % in some samples. Powder swelling tests were performed on ground and recompacted rock samples. The results of these tests show a better agreement with those expected according to the mineralogical composition of the samples. The reason for the non-activation of the swelling potential in undisturbed samples was suspected to be the influence of the iron-oxide/hydroxide content, which was on average around 11 %. Scanning electron microscope images showed that the edge terminations of the clay particles are coated with hematite. The hydrophobic properties of hematite impede the access of water between the interlayers of the clay minerals and, in addition, hematite cementation prevents the expansion. In a further series of swelling tests water was replaced by a reducing solution (0.1 molar hydroxylammonium chloride) which removes the Fe-oxide coating. It can be shown that thus the swelling potential of the clay minerals in such undisturbed rocks can be activated. 相似文献
46.
M. Éva Jankovics Gábor Dobosi Antal Embey-Isztin Balázs Kiss Tamás Sági Szabolcs Harangi Theodoros Ntaflos 《Bulletin of Volcanology》2013,75(9):1-23
The last eruptions of the monogenetic Bakony-Balaton Highland Volcanic Field (western Pannonian Basin, Hungary) produced unusually crystal- and xenolith-rich alkaline basalts which are unique among the alkaline basalts of the Carpathian–Pannonian Region. Similar alkaline basalts are only rarely known in other volcanic fields of the world. These special basaltic magmas fed the eruptions of two closely located volcanic centres: the Bondoró-hegy and the Füzes-tó scoria cone. Their uncommon enrichment in diverse crystals produced unique rock textures and modified original magma compositions (13.1–14.2 wt.% MgO, 459–657 ppm Cr, and 455–564 ppm Ni contents). Detailed mineral-scale textural and chemical analyses revealed that the Bondoró-hegy and Füzes-tó alkaline basaltic magmas have a complex ascent history, and that most of their minerals (~30 vol.% of the rocks) represent foreign crystals derived from different levels of the underlying lithosphere. The most abundant xenocrysts, olivine, orthopyroxene, clinopyroxene, and spinel, were incorporated from different regions and rock types of the subcontinental lithospheric mantle. Megacrysts of clinopyroxene and spinel could have originated from pegmatitic veins/sills which probably represent magmas crystallized near the crust–mantle boundary. Green clinopyroxene xenocrysts could have been derived from lower crustal mafic granulites. Minerals that crystallized in situ from the alkaline basaltic melts (olivine with Cr-spinel inclusions, clinopyroxene, plagioclase, and Fe–Ti oxides) are only represented by microphenocrysts and overgrowths on the foreign crystals. The vast amount of peridotitic (most common) and mafic granulitic materials indicates a highly effective interaction between the ascending magmas and wall rocks at lithospheric mantle and lower crustal levels. However, fragments from the middle and upper crust are absent from the studied basalts, suggesting a change in the style (and possibly rate) of magma ascent in the crust. These xenocryst- and xenolith-rich basalts yield divers tools for estimating magma ascent rate that is important for hazard forecasting in monogenetic volcanic fields. According to the estimated ascent rates, the Bondoró-hegy and Füzes-tó alkaline basaltic magmas could have reached the surface within hours to few days, similarly to the estimates for other eruptive centres in the Pannonian Basin which were fed by “normal” (crystal and xenoliths poor) alkaline basalts. 相似文献
47.
48.
M. A. Nazarov L. Ya. Aranovich S. I. Demidova T. Ntaflos F. Brandstätter 《Petrology》2011,19(1):13-25
Fragments of aluminous enstatite from lunar meteorites of highland origin were investigated. It was found that such fragments
usually occur in impact breccias of troctolitic composition. The aluminous enstatite contains up to 12 wt % Al2O3 and shows low CaO (<1 wt %) and almost constant high Mg/(Mg + Fe) ratio (89.5 ± 1.4 at %) identical to that of the Earth’s
mantle. With respect to these parameters, the aluminous enstatites are distinctly different from common orthopyroxene of lunar
rocks. The aluminous enstatite associates with spinel (pleonaste), olivine, anorthite (clinopyroxene was never found), and
accessory minerals: rutile, Ti-Zr oxides, troilite, and Fe-Ni metal. The same assemblage was described in rare fragments of
spinel cataclasites from the samples of the Apollo missions. Thermobarometry and the analysis of phase equilibria showed that
the rocks hosting aluminous enstatite are of deep origin and occurred at depths from 25 km to 130–200 km at T from 800 to 1300°C, i.e., at least in the lower crust and, possibly, in the upper mantle of the Moon. These rocks could form
individual plutons or dominate the composition of the lower crust. The most probable source of aluminous enstatite is troctolitic
magnesian rocks and, especially, spinel troctolites with low Ca/Al and Ca/Si ratios. The decompression of such rocks must
produce cordierite-bearing assemblages. The almost complete absence of such assemblages in the surficial rocks of lunar highlands
implies that vertical tectonic movements were practically absent in the lunar crust. The transport of deep-seated materials
to the lunar surface was probably related to impact events during the intense meteorite bombardments >3.9 Ga ago. 相似文献
49.
Balázs?KissEmail author Szabolcs?Harangi Theodoros?Ntaflos Paul?R.?D.?Mason Elemér?Pál-Molnár 《Contributions to Mineralogy and Petrology》2014,167(3):986
Ciomadul is the youngest volcano in the Carpathian–Pannonian region produced crystal-rich high-K dacites that contain abundant amphibole phenocrysts. The amphiboles in the studied dacites are characterized by large variety of zoning patterns, textures, and a wide range of compositions (e.g., 6.4–15 wt% Al2O3, 79–821 ppm Sr) often in thin-section scale and even in single crystals. Two amphibole populations were observed in the dacite: low-Al hornblendes represent a cold (<800 °C) silicic crystal mush, whereas the high-Al pargasites crystallized in a hot (>900 °C) mafic magma. Amphibole thermobarometry suggests that the silicic crystal mush was stored in an upper crustal storage (~8–12 km). This was also the place where the erupted dacitic magma was formed during the remobilization of upper crustal silicic crystal mush body by hot mafic magma indicated by simple-zoned and composite amphiboles. This includes reheating (by ~200 °C) and partial remelting of different parts of the crystal mush followed by intensive crystallization of the second mineral population (including pargasites). Breakdown textures of amphiboles imply that they were formed by reheating in case of hornblendes, suggesting that pre-eruptive heating and mixing could take place within days or weeks before the eruption. The decompression rim of pargasites suggests around 12 days of magma ascent in the conduit. Several arc volcanoes produce mixed intermediate magmas with similar bimodal amphibole cargo as the Ciomadul, but in our dacite the two amphibole population can be found even in a single crystal (composite amphiboles). Our study indicates that high-Al pargasites form as a second generation in these magmas after the mafic replenishment into a silicic capture zone; thus, they cannot unambiguously indicate a deeper mafic storage zone beneath these volcanoes. The simple-zoned and composite amphiboles provide direct evidence that significant compositional variations of amphiboles do not necessarily mean variation in the pressure of crystallization even if the Al-tschermak substitution can be recognized, suggesting that amphibole barometers that consider only amphibole composition may often yield unrealistic pressure variation. 相似文献
50.
S. I. Demidova M. A. Nazarov M. O. Anosova Yu. A. Kostitsyn Th. Ntaflos F. Brandstaetter 《Petrology》2014,22(1):1-16
Dhofar 1442 is one of the few lunar KREEP-rich meteorites, which contains KREEP norites and KREEP gabbronorite as well as low-Ti basalts and highly evolved granophyres. Zircon is a typical accessory mineral of KREEP rocks. U-Th-Pb dates of 12 zircon grains (four of them were in two lithic clasts, and the others were fragments in the meteorite matrix) indicate that the zircons belong to at least two groups of different age: “ancient” (~4.31 Ga) and “young” (~3.95 Ga), which correspond to two major pulses of KREEP magmatism in the source region of the Dhofar 1442 meteorite. The zircon of the “young” group was most probably related to the crater ejecta of the Mare Imbrium Basin. The rock fragments dated at approximately 3.95 Ga have the composition of KREEP gabbronorite. The parental rocks of the zircon of the “ancient” group in the Dhofar 1442 meteorite are uncertain and could be highly evolved granophyres. This hypothesis is supported by the high Th (100–300 ppm) and U (150–400 ppm) contents. These zircon fragments of the “ancient” group, higher than in the “young” group (<50 ppm Th and <70 ppm U) and are typical of zircon from lunar granitic rocks. The composition of the products of KREEP magmatism in the source region of the Dhofar 1442 meteorite could vary from predominantly granitic to KREEP gabbronoritic at 4.3–3.9 Ga. 相似文献