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681.
José María González-Jiménez Fernando Gervilla Thomas Kerestedjian Joaquín Antonio Proenza 《Resource Geology》2010,60(4):315-334
The Dobromirtsi Ultramafic Massif, located in the Rhodope Mountains (SE Bulgaria), is a portion of a Paleozoic sub-oceanic mantle affected by polyphase regional metamorphism. This massif contains several small, podiform chromitite bodies which underwent the same metamorphic evolution as their host peridotites. Like other ophiolite chromitites, those found in Dobromirtsi carry abundant platinum-group minerals (PGM) and base-metal minerals (BMM). The PGM consist mainly of Ru-, Os-, and Ir-based PGM (laurite RuS2-erlichmanite OsS2, Os-Ru-Ir alloys, irarsite [IrAsS], Ru-rich pentlandite, and an unknown Ir-sulfide) but minor Rh- and Pd-based PGM (hollingworthite [RhAsS] and a series of unidentified stannides and sulfantimonides) are also present. In contrast, the BMM are dominated by pentlandite (Ni,Fe)9S8, followed by heazlewoodite (Ni3S2), breithauptite (NiSb), maucherite (Ni11As8), godlevskite (Ni7S6), gersdorffite (NiAsS), millerite (NiS), undetermined minerals containing Ni, As and Sb, orcelite (Ni5-XAs2), awaruite (Ni3Fe) and chalcopyrite (CuFeS2). The detailed study of the textural relationships, morphology and composition of the PGM and BMM inclusions indicate the existence of two different PGM-BMM assemblages: (i) a primary or magmatic; and (ii) a secondary related with postmagmatic alteration. The PGM and BMM inclusions in unaltered zones of chromite crystals (mainly laurite-erlichmanite and pentlandite) are considered to be primary magmatic minerals formed under variable temperature (1200–1100°C) and sulfur fugacity (between −2 and −0.5 log fS2). In contrast, PGM and BMM located along altered edges of chromite and serpentinised silicate matrix are considered to be secondary, formed from or re-equilibrated with altering fluids. Secondary PGM and BMM assemblages are considered as result of the combination of reducing and oxidising events related with regional metamorphism. Under low fO2 states, fS2 also drops giving place to the formation of S-poor Ni-rich sulfides and secondary Ru-alloys by desulfurisation of primary S-containing minerals. In contrast, predominance of platinum-group elements and/or base-metal arsenides and sulfarsenides associated with the altered edges of chromite (chromite strongly enriched in Fe2O3) is related with the fixing of remobilised PGE (mainly Ir, Rh and Pd) and base-metals (mainly Ni and Fe) when late oxidising fluids supplied As as well as Sb and Sn. 相似文献
682.
Rita M. F. Fonseca Fernando J. A. S. Barriga Patrícia I. S. T. Conceição 《International Journal of Earth Sciences》2010,99(8):1899-1916
The Southern region of Portugal is subjected to several forms of over-erosion. Most leached products, mainly composed of fine
particles containing nutrients, metals or pesticides, are easily transported by river flows. When these are hindered by a
physical barrier such as a dam, the particulate load accumulates on the bottom of the reservoirs, often leading to a pronounced
decrease of water quality. Bottom sediments from three reservoirs were subjected to grain-size analysis and a study of clay
minerals by X-ray diffraction. Most sediments contain a diverse set of clay minerals, mostly illites, smectites, chlorites
and kaolinites. The nature of the clay minerals reflects the nature of the parent rocks. During the cycles of transport and
temporary deposition, they may undergo significant chemical and physical transformations, which lead to an increase of expandable
properties and therefore, to a higher cationic exchange capacity, determining its important role as vehicles of environmental
pollutants. 相似文献
683.
Magmatic source enrichment by slab-derived melts in a young post-collision setting, central Mindanao (Philippines) 总被引:28,自引:0,他引:28
Fernando G. Sajona Ren C. Maury Manuel Pubellier Jacques Leterrier Herv Bellon Joseph Cotten 《Lithos》2000,54(3-4):173-206
Central Mindanao was the locus of a Pliocene (4–5 Ma old) arc–arc collision event followed by basaltic to dacitic magmatism starting at 2.3 Ma, representing the most voluminous volcanic field in the Philippines. Lava compositions range from calc-alkaline to shoshonitic. Adakites and Nb-enriched basalts are among the magmatic products. All the lavas are Na-rich (up to 4.88%), with Na2O/K2O ratios from 2.5 to 6.5. Sr, Nd and Pb isotopic compositions are similar to MORB, except for some shoshonitic lavas that have slightly less radiogenic Nd ratios. K-enrichment in basalts can be related to both fractional crystallization (FC) at moderate pressures and to partial melting of an enriched source. Trace element systematics indicate that the sub-central Mindanao mantle is characterized by the presence of garnet, phlogopite, amphibole, and perhaps some titanate phase. The enrichment of this source is attributed to the interaction of slab-derived melts, i.e., adakites, with the arc mantle. This would explain the presence of Nb-enriched basalts, transitional adakites and high-magnesium andesites, as well as the bulk Na-enrichment and relatively unradiogenic character of the central Mindanao lavas. We envision an ion-exchange type of enrichment, in which the HFSE, LILE and LREE, mobilized during slab melting, are preferentially enriched in the metasomatized mantle, resulting in a diversity of post-collision magma compositions. The MORB-like isotopic signatures of the central Mindanao lavas preclude important contributions of slab-derived hydrous fluids, sediments, continental crust or an OIB-type contaminant. Slab melting after cessation of subduction is deemed possible by thermal rebound of previously depressed geotherms. Initial contributions to mantle enrichment in post-collision sites may thus come from slab melts. In most other cases of post-collision magmatism, however, this signature can be easily masked by enrichments coming from other sources, e.g., the continental lithosphere. 相似文献
684.
J.D. Gulley D. Breecker M. Covington S. Cooperdock J. Banner P.J. Moore A. Noronha C. Breithaupt J.B. Martin J. Jenson 《地球表面变化过程与地形》2020,45(11):2675-2688
Growing evidence suggests microbial respiration of dissolved organic carbon (DOC) may be a principal driver of subsurface dissolution and cave formation in eogenetic carbonate rock. Analyses of samples of vadose zone gasses, and geochemical and hydrological data collected from shallow, uncased wells on San Salvador Island, Bahamas, suggest tidally varying water tables may help fuel microbial respiration and dissolution through oxygenation. Respiration of soil organic carbon transported to water tables generates dysaerobic to anaerobic groundwater, limiting aerobic microbial processes. Positive correlations of carbon dioxide (CO2), radon-222 (222Rn) and water table elevation indicate, however, that tidal pumping of water tables pulls atmospheric air that is rich in oxygen, and low in CO2 and 222Rn, into contact with the tidal capillary fringe during falling tides. Ratios of CO2 and O2 in vadose gas relative to the atmosphere indicate this atmospheric oxygen fuels respiration within newly-exposed, wetted bedrock. Deficits of expected CO2 relative to O2 concentrations indicate some respired CO2 is likely removed by carbonate mineral dissolution. Tidal pumping also appears capable of transferring oxygen to the freshwater lens, where it could also contribute to respiration and dissolution; dissolved oxygen concentrations at the water table are at least 5% saturated and decline to anaerobic conditions 1–2 m below. Our results demonstrate how tidal pumping of air to vadose zones can drive mineral dissolution reactions that are focused near water tables and may contribute to the formation of laterally continuous vuggy horizons and potentially caves. © 2020 John Wiley & Sons, Ltd. 相似文献
685.
Prakash Amol Vijayakumaran Vijith Vijayan Fernando Premanand Pednakar Jai Singh 《Ocean Dynamics》2018,68(12):1607-1623
A linear coastal-trapped-wave (CTW) model is used to examine the effects of large-scale winds, with time scale ranging from a few days to a few weeks, on the West India Coastal Current (WICC), particularly on the shelf off the central west coast of India. We show that unlike the seasonal cycle of WICC, which is primarily forced by the winds along the east coast of India, the high-frequency WICC is mostly driven by the west-coast winds. Nevertheless, the influence of winds as far as Sri Lanka and east coast of India cannot be neglected. Simple numerical experiments with the CTW model show that the strong current observed at Goa (15° N) compared to Bhatkal (13° N) and Jaigarh (17° N) is due to two factors: (1) the superposition of local and remote CTWs and (2) the widening of shelf width north of Goa, which decreases the amplitude of the currents poleward of Goa. If the local winds are weak, the amplitude of current decreases poleward due to friction, and the current at the south leads the north. We also note that the observed phase difference between sea level and alongshore current at Goa could be attributed to the propagation of remotely forced higher-order modes of CTWs. 相似文献
686.
Maia M. Moreira N. Vicente S. Mirão J. Noronha F. Nogueira P. 《Geology of Ore Deposits》2020,62(6):508-534
Geology of Ore Deposits - The Mociços Cu-deposit is part of a cluster of ancient copper mines in the Sousel-Barrancos metallogenic belt in the Ossa-Morena Zone at the SW Iberia. The orebodies... 相似文献
687.