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Komatiites II: Experimental and theoretical investigations of post-emplacement cooling and crystallization
- J. Petrol
, 1986
"... When komatiite lavas are emplaced on the sea floor most of the heat transfer occurs through the upper lava-seawater boundary. We have investigated the cooling and crystallization of komatiites using a series of analogue laboratory experiments with aqueous solutions and by theoretical analysis. In ko ..."
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When komatiite lavas are emplaced on the sea floor most of the heat transfer occurs through the upper lava-seawater boundary. We have investigated the cooling and crystallization of komatiites using a series of analogue laboratory experiments with aqueous solutions and by theoretical analysis. In komatiites the viscosity is sufficiently low that convection occurs in the interior of the flow and these motions, due both to thermal and compositional variations, have an important influence on the characteristic features of komatiites such as the strong compositional and textural layering. The experiments have been conducted with crystallizing aqueous solutions which display the same overall dynamical processes as solidifying komatiites. The solutions used are simple eutectic systems having the property that crystallization from a solution which is substantially more concentrated than the eutectic composition leaves behind residual fluid which is less dense than the original fluid. This models the decrease in density of komatiite melts on cooling, due to the crystallization of olivine. Such solutions have been cooled strongly through the metal roof of an otherwise insulated container, using a typical fluid depth of 80 mm. Dendritic crystals grew down vertically from the roof and released light fluid, depleted in solute, which rose to form a zone of stagnant fluid at the top of the container, while the tips of the crystals extended just below the bottom of this light layer. A layer of solid eutectic, with a
Mantle-derived magmas and magmatic Ni-Cu-(PGE) deposits. Econ. Geol., 100th Anniversary volume
, 2005
"... Magmatic Fe-Ni-Cu ± platinum-group element (PGE) sulfide deposits form when mantle-derived mafic and ultramafic magmas become saturated in sulfide and segregate immiscible sulfide liquid, commonly following interaction with crustal rocks. Although the metal contents of primary magmas influence ore c ..."
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Magmatic Fe-Ni-Cu ± platinum-group element (PGE) sulfide deposits form when mantle-derived mafic and ultramafic magmas become saturated in sulfide and segregate immiscible sulfide liquid, commonly following interaction with crustal rocks. Although the metal contents of primary magmas influence ore compositions, they do not control ore genesis because the metals partition strongly into the sulfide liquid and because most magmas capable of segregating sulfide liquid contain sufficient abundances of ore metals. More important controls are the temperature, viscosity, volatile content, and mode of emplacement of the magma, which control the dynamics of magma emplacement and the degree of interaction with crust. By this measure, high-temperature, low-viscosity komatiites and tholeiitic picrites are most capable of forming Ni-Cu-(PGE) deposits, whereas lower-temperature, volatile-rich alkali picrites and basalts have less potential. In most deposits, ore formation is linked directly to incorporation of S-rich country rocks and only indirectly to contamination by granitic crust. However, the geochemical signature of contamination is easily recognized and is a useful exploration guide because it identifies magmas that had the capacity to incorporate crustal material. Several aspects of the ore-forming process remain poorly understood, including the control of mantle melting processes on the PGE contents of mafic-ultramafic magmas, the mechanisms by which sulfur is transferred from wall rocks to ores (bulk assimilation, incongruent melting, and/or devolatilization), the distances and processes by which dense sulfide melts are transported from where they form to where they become concentrated (as finely-dispersed droplets, as segregated layers, or by deformation-driven injection of massive sulfide accumulations), and the dynamic processes that increase the metal contents of the ores.
Investigating the origin of candidate lava channels on Mercury with MESSENGER data: Theory and observations
- J. Geophys. Res. Planets
, 2013
"... [1] Volcanic plains identified on Mercury are morphologically similar to lunar mare plains but lack constructional and erosional features that are prevalent on other terrestrial planetary bodies. We analyzed images acquired by the MESSENGER spacecraft to identify features on Mercury that may have fo ..."
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[1] Volcanic plains identified on Mercury are morphologically similar to lunar mare plains but lack constructional and erosional features that are prevalent on other terrestrial planetary bodies. We analyzed images acquired by the MESSENGER spacecraft to identify features on Mercury that may have formed by lava erosion. We used analytical models to estimate eruption flux, erosion rate, and eruption duration to characterize the formation of candidate erosional features, and we compared results with analyses of similar features observed on Earth, the Moon, and Mars. Results suggest that lava erupting at high effusion rates similar to those required to form the Teepee Butte Member of the Columbia River flood basalts (0.1–1.2 106 m3 s–1) would have been necessary to form wide valleys (>15 km wide) observed in Mercury’s northern hemisphere, first by mechanical erosion to remove an upper regolith layer, then by thermal erosion once a lower rigid layer was encountered. Alternatively, results suggest that lava erupting at lower effusion rates similar to those predicted to have formed Rima Prinz on the Moon (4400 m3 s–1) would have been required to form, via thermal erosion, narrower channels (<7 km wide) observed on Mercury. Although these results indicate how erosion might have occurred on Mercury, the observed features may have formed by other processes, including lava flooding terrain sculpted during the formation of the Caloris basin in the case of the wide valleys, or impact melt carving channels into impact ejecta in the case of the narrower channels.
Compositional cyclicity in a pyroxenitic layer from the Main Zone of the Western Bushveld Complex: evidence for repeated magma influx
"... Lacking the pronounced modal layering of the underlying Critical Zone, the Main Zone of the Bushveld Complex nevertheless displays well-developed cryptic layering, expressed in a series of iron-enrichment trends, each defining a unit of the order of 100 to 200 m thick. At the base of one such unit, ..."
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Lacking the pronounced modal layering of the underlying Critical Zone, the Main Zone of the Bushveld Complex nevertheless displays well-developed cryptic layering, expressed in a series of iron-enrichment trends, each defining a unit of the order of 100 to 200 m thick. At the base of one such unit, 1100 m above the Main Zone- Critical Zone contact, a 10 m thick pyroxenitic layer was intersected in an exploration borehole from the southern sector of the western Bushveld. Within the pyroxenitic layer, mineral chemistry defines a series of five cycles of upward Mg and Cr enrichment in pyroxenes, and Ca enrichment in plagioclase. The mineral chemistry, supported by textural evidence, suggests the influx of successive surges of magma. Sustained streaming of magma gave rise to adcumulate textures in the central portion of each cycle, with orthocumulate xtures at bases and tops of cycles representing waxing and waning stages of magma surges. KZYWORDS: Bushveld Complex, Main Zone, pyroxenite, magma influx, compositional cyclicity.
Econormc Geology
"... The genesis of ko•natiites, basalts, and associated Ni •nineralization i the Dundonald township area, On-tario, is critical to understanding the metallogenic evolution of the Kidd-Munro assemblage, one of the most primitive volcanic assemblages in the world. The 2.5-kin-thick stratigraphic successio ..."
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The genesis of ko•natiites, basalts, and associated Ni •nineralization i the Dundonald township area, On-tario, is critical to understanding the metallogenic evolution of the Kidd-Munro assemblage, one of the most primitive volcanic assemblages in the world. The 2.5-kin-thick stratigraphic succession has a basal calc-alkalic basalt-dacite-rhyolite sequence (2716.8 __ _ 2.1 Ma, U-Pb zircon) cut by the Dundonald peridotite-gabbro sill (27077 • Ma, U-Pb zircon), overlain by komatiitic and basaltic flows containing magmatic Nisulfide mineral-ization, which are, in turn, overlain by chemically distinctive low Ti basalt flows. The Dundonald komatiites correlate with komatiites inthe footwall of the giant Kidd Creek volcanic-associated massive sulfide deposit 40 km to the west, but are slightly older than the komatiites of Munro township which are 40 km to the east. The Dundonald komatiites comprise a Munro-type, Al-undepleted komatiite suite, with liquid compositions having A120:fFiO,o = 18 to 22 and MgO up to 30.3 vet percent (anhydrous, n = 24). Most samples in the ko-matiite suite have slightly depleted light BEE contents, with LaN/SmN = 0.6 to 1.1 and LaN/YbN = 0.4 to 0.9 (n-- 21), and negligible Ti/Ti * and Zr/Zr * anomalies. Their major and trace element geochemistry is consistent with a derivation by sig•fificant partial melting of a chemically primitive mantle with little or no influence of majoritc garnet. Flows that are host o the Dundeal and Alexo Ni deposits have distinctively high ThpM/NbpM ratios (>1.7, n = 3), suggesting crustal contamination, and peperite textures that are consistent with contami-
Geochemistry of Volcanic Rocks Associated with Cu-Zn and Ni-Cu Deposits in the Abitibi Subprovince
"... Volcanogenic massive sulfide (VHMS) deposits in the Abitibi subprovince are preferen-tially associated with volcanic successions containing>150 m thicknesses of felsic volcanic rocks ( • 50 % by area of volcanic terranes) and are found within volcanic sequences ofat least three distinct affinitie ..."
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Volcanogenic massive sulfide (VHMS) deposits in the Abitibi subprovince are preferen-tially associated with volcanic successions containing>150 m thicknesses of felsic volcanic rocks ( • 50 % by area of volcanic terranes) and are found within volcanic sequences ofat least three distinct affinities. Group I, which is host to greater than half of the volcanogenic mas-sive sulfide deposits by tonnage and which comprises only • 10 percent by area of volcanic terranes, is composed of bimodal, tholeiitic basalt-basaltic andesitc, and high silica rhyolite. The basaltic andesites and high silica rhyolites are characterized by high high field strength element and heavy rare earth element (REE) contents, low light to heavy REE ratios (most with LaN/YbN-- 0.8-3), and strong negative Eu anomalies. The Kamiskotia, Matagami, and Chibougamau (Lower cycle) volcanogenic massive sulfide areas, all of which are also under-lain by large, synvolcanic gabbroic omplexes, are associated with group I volcanic se-quences. The Kidd Creek, Potter, Normetal, and Horne deposits are also included in this category. Group II, which is host o one-third of the volcanogenic massive sulfide deposits by tonnage and which is also • 10 percent by area of volcanic terranes, is composed of bimodal,
Interaction between local magma ocean evolution and mantle dynamics on Mars
"... Large scale features of Mars ’ crustal structure including the hemispherical dichotomy and Tharsis were established very early in planetary history. Geodynamical models for origin of the dichotomy and Tharsis, such as lithospheric recycling and a plume from the core mantle boundary, respectively, in ..."
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Large scale features of Mars ’ crustal structure including the hemispherical dichotomy and Tharsis were established very early in planetary history. Geodynamical models for origin of the dichotomy and Tharsis, such as lithospheric recycling and a plume from the core mantle boundary, respectively, involve solid state mantle flow and are difficult to reconcile with timing constraints. An alternative point of view is that the martian crustal asymmetry and Tharsis can be associated with the upwelling and spreading of large, impact induced, melt regions, i.e., local magma oceans. While the local magma ocean induced upwelling model satisfies timing constraints on di-chotomy and Tharsis formation, it neglects any interaction with longer timescale mantle dynamics and cannot explain recent volcanic activity at Tharsis. In this study, fully three-dimensional, spher-ical shell simulations are utilized to investigate coupling between local magma oceans and mantle dynamics with radiogenic heating and core heat flow. For low core heat flux, it is found that upwellings driven by local magma ocean buoyancy are transient features of planetary evolution which is dominated by sublithospheric instabilities. With increasing core heat flux, local magma ocean induced upwellings strongly influence the pattern of thermal plumes from the core mantle
Acquisitions and Acquisitions et Bibliographic Services services bibliographiques
"... The original manuscript received by UMI contains pages with indistinct print. Pages were microfilmed as received. This reproduction is the best copy available UMI ..."
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The original manuscript received by UMI contains pages with indistinct print. Pages were microfilmed as received. This reproduction is the best copy available UMI
[2] Martian outflow channels, first clearly recogni...
"... [1] The widely accepted view that catastrophic flow of liquid water was the dominant process involved in formation of outflow channels on Mars has as part of its foundation the assumption that the flow of lava could not have formed Martian features such as streamlined islands and anastamosing channe ..."
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[1] The widely accepted view that catastrophic flow of liquid water was the dominant process involved in formation of outflow channels on Mars has as part of its foundation the assumption that the flow of lava could not have formed Martian features such as streamlined islands and anastamosing channels. However, lunar and Venusian channels, believed to have formed through volcanic processes in the absence of water, are indeed associated with such features. Additionally, many lunar and Venusian rilles head at topographic depressions, a common characteristic of Martian outflow channels. Lunar rilles typically lack positive relief accumulations of volcanic deposits at their mouths, suggesting that contrary to previous assertions, absence of such accumulations at the mouths of Martian outflow channels is not incompatible with a common mode of formation. Consistent with an igneous origin for outflow channels and certain valley systems on Mars, volcanic processes can produce a wide range of landforms that are similar to those normally associated with the flow of water, including channel terraces and complex channel networks. The simplest interpretation of Martian channels that extend from volcanic source regions onto volcanic plains is as conduits formed by the flow of
