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John P. Armstrong Mark Wilson Robert L. Barnett Tom Nowicki Bruce A. Kjarsgaard 《Lithos》2004,76(1-4):415-433
A suite of fresh, Late Cretaceous to Eocene hypabyssal kimberlites from the Lac de Gras field were studied in order to understand better carbonate, silicate and oxide paragenesis. The samples have excellent preservation of textures and primary mineralogy and are archetypal or Group 1 kimberlite. Five kimberlite localities are identified as calcite-bearing based on the presence of high Sr–Ba calcite as phenocrysts, microphenocrysts and in segregations. Three kimberlite localities are identified as dolomite-bearing based on the presence of mixed calcite–dolomite segregations containing oscillatory and banded textures of calcite–dolomite solid solution and dolomite (±magnesite). Sr–Ba calcite are characterized by high XCa (>0.95) and are enriched in Sr (4900–11,100 ppm) and Ba (3200–14,200 ppm). The calcite–dolomite and dolomite–magnesite solid solution compositions span the XCa range from 0.42 to 0.95, and typically have Sr and Ba contents in the range of 1000–4000 ppm. The carbonate, silicate and oxide mineral compositions suggest that the origin of the calcite-bearing versus dolomite-bearing kimberlites studied is related to subtle differences in parent magma composition, in particular, the CO2/H2O ratio. Formation of the carbonates reflects the latter part of a protracted magmatic crystallization sequence, in which Sr–Ba calcite precipitates from an evolved kimberlite melt. Subsequently, calcite–dolomite solid solution and dolomite is precipitated from localized, Mg-rich carbonate fluids at relatively high temperatures (higher than serpentine stability). 相似文献
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Claudia D’Oriano Elisa Poggianti Antonella Bertagnini Raffaello Cioni Patrizia Landi Margherita Polacci Mauro Rosi 《Bulletin of Volcanology》2005,67(7):601-621
The Monte Nuovo eruption is the most recent event that occurred at Phlegrean Fields (Italy) and lasted from 29 September to 6 October 1538. It was characterized by 2 days of quasi-sustained phreatomagmatic activity generating pumice-bearing pyroclastic density currents and forming a 130-m-high tuff cone (Lower Member deposits). The activity resumed after a pause of 2 days with two discrete Vulcanian explosions that emplaced radially distributed, scoria-bearing pyroclastic flows (Upper Member deposits). The juvenile products of Lower and Upper Members are, respectively, phenocryst-poor, light-coloured pumice and dark scoria fragments with K-phonolitic bulk compositions, identical in terms of both major and trace elements. Groundmass is formed by variable proportions of K-feldspar and glass, along with minor sodalite and Fe-Ti oxide present in the most crystallized samples. Investigations of groundmass compositions and textures were performed to assess the mechanisms of magma ascent, degassing and fragmentation along the conduit and implications for the eruptive dynamics. In pumice of the Lower Member groundmass crystal content increases from 13 to 28 vol% from the base to the top of the sequence. Products of the Upper Member consist of clasts with a groundmass crystal content between 30 and 40 vol% and of totally crystallized fragments. Crystal size distributions of groundmass feldspars shift from a single population at the base of the Lower Member to a double population in the remaining part of the sequence. The average size of both populations regularly increases from the Lower to the Upper Member. Crystal number density increases by two orders of magnitude from the Lower to the Upper Member, suggesting that nucleation dominated during the second phase of the eruption. The overall morphological, compositional and textural data suggest that the juvenile components of the Monte Nuovo eruption are likely to record variations of the magma properties within the conduit. The different textures of pumice clasts from the Lower Member possibly reflect horizontal gradients of the physical properties (P, T) of the ascending magma column, while scoriae from the second phase are thought to result from the disruption of a slowly rising plug crystallizing in response to degassing. In particular, crystal size distribution data point to syn-eruptive degassing-induced crystallization as responsible for the transition in eruptive style from the first to the second phase of the eruption. This mechanism not only has been proved to profoundly affect the dynamics of dome-forming calc-alkaline eruptions, but may also have a strong influence in driving the eruption dynamics of alkaline magmas of intermediate to evolved compositions.Editorial responsibility: J. Donnelly-Nolan 相似文献
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We experimentally studied the dacitic magma ejected during the first event in the Usu 2000 eruption to investigate the conditions
of syneruptive magmatic ascent. Geophysical data revealed that the magma reached under West Nishiyama, the location of the
event’s craters, after rising beneath the summit. Prior study of bubble-size distributions of ejecta shows two stages (stage
1 and stage 2) with different magma ascent rates, as the magma accelerated beneath West Nishiyama with the start of the second
stage. To simulate ascent of stage 1 from the main reservoir, which was located at a depth of 4–6 km (125 MPa) to 2 km (50 MPa)
beneath West Nishiyama, decompression experiments were conducted isothermally at 900°C following two paths. Single step decompression
(SSD) samples were decompressed rapidly (0.67 MPa/s) to their final pressure and held for 12 to 144 hours. Multiple step decompression
(MSD) samples were decompressed stepwise to their final pressure and quenched instantly. In MSD, the average decompression
rates and total experimental durations varied between 0.01389 to 0.00015 MPa/s and 1.5 to 144 hours, respectively. Syneruptive
crystallization was confined to stage 1, and the conditions of ascent were determined by documenting similarities in decompression-induced
crystallization between ejecta and experiments. Core compositions, number densities, and shapes of experimental microlites
indicate that ascent to 2 km depth occurred in less than 1.5 h. Volumes and number densities of experimental microlites from
the SSD experiments that best replicate the decompression rate to 2 km indicate that the magma remained at 2 km for approximately
24 h before the eruption. Stagnation at a depth of 2 km corresponds with horizontal transport through a dike from beneath
the summit to West Nishiyama, according to geodetic results. The total magma transport timescale including stage 2 is tens
of hours and is shorter than the timescale of precursory seismicity (3.5 days), indicating that the erupted magma did not
move out of the reservoir for the first 2 days. This is consistent with the temporal change in numbers of earthquakes, which
reached a peak after 2 days. 相似文献
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