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  • 1
    Electronic Resource
    Electronic Resource
    Vertical migration of residual magma in the upper zone of the Bushveld Complex (1994)
    Springer
    Mineralogy and petrology 51 (1994), S. 345-354 
    Details
    ISSN: 1438-1168
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Description / Table of Contents: Zusammenfassung Die Migration von Restschmelzen kann Texturen und Mineralzusammensetzungen in geschichteten Intrusionen beeinflussen, jedoch ist es schwierig, dies eindeutig nachzuweisen. In der Upper Zone des Bushveld-Komplexes bildet ein metabasaltischer Xenolith einen lokal undurchlässigen Horizont, der für vertikal migrierte Residuen als Barriere fungierte. Erhöhte Ab-Gehalte in Plagioklas, sowie erhöhte K2O und Zr-Werte in Gesamtgesteins-Analysen in Anorthosit direkt unterhalb des Xenolithen-verglichen mit der Zusammensetzung des gleichen Horizontes in Streichen des Xenolithen-weisen auf Konzentration von Restschmelzen und/oder Fluiden im Liegenden des Xenolithen hin. Der Vergleich von Cu/Ni und Cu/S Verhältnissen der mineralisierten Anorthosite in der normalen Abfolge mit denen unterhalb des Xenolithen zeigt, daß es sich hier um primäre magmatische Sulfide handelt, die innerhalb des Anorthosits kristallisierten. Diese Sulfide lassen sich nicht auf das Absinken von dichter Sulfidschmelze, die ursprünglich mit den überlagernden Magnetit-Bändern in Zusammenhang standen, und auch nicht auf hydrothermale Zufuhr zurückführen.
    Notes: Summary Migration of residual liquid can potentially affect the textures and mineral compositions in layered intrusions, but is difficult to conclusively demonstrate. In the Upper Zone of the Bushveld Complex a metabasaltic xenolith forms a locally impermeable horizon, which acted as a barrier to vertically migrating residua. Increased Ab content in plagioclase, and K2O and Zr in whole-rock analyses in the anorthosite directly below the xenolith, compared to the same horizon along strike of the xenolith, demonstrate trapping of residual liquid and/or fluid beneath the xenolith. Comparison of Cu/Ni and Cu/S ratios of the mineralised anorthosite in the normal sequence and below the xenolith suggests that these are primary magmatic sulphides which crystallised within the anorthosite and are not derived by sinking of dense interstitial sulphide liquid originally associated with the overlying magnetite layer or introduced hydrothermally.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01159736
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  • 2
    Electronic Resource
    Electronic Resource
    Parental magma to the Bushveld Complex (1980)
    [s.l.] : Nature Publishing Group
    Nature 287 (1980), S. 33-35 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] The layering and mineralogical variation in the Bushveld Complex, shown in Fig. 1, is accepted to be the result of fractional crystallization from a basic magma. However, the exact composition of this magma has been debated. Willemse1 believed it to be similar to the voluminous metamorphosed ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/287033a0
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  • 3
    Electronic Resource
    Electronic Resource
    Nickel in komatiites (1977)
    [s.l.] : Nature Publishing Group
    Nature 266 (1977), S. 716-718 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Fig. 1 Plot of Ni against MgO contents of komatiites analysed in, Table 1. Stars denote proposed parental magma compositions. Table 1 MgO and Ni in komatiites and calculated degrees of fractionation Olivine removed* Sample no. MgO Ni (%) (%by weight) (p.p.m.) Model 1 Model 2 ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/266716a0
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  • 4
    Electronic Resource
    Electronic Resource
    Origin of corundum-normative intrusive and extrusive magmas (1976)
    [s.l.] : Nature Publishing Group
    Nature 259 (1976), S. 102-104 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] A large proportion of granitoid rocks are corundum-normative1 and Chayes2 found that 18% of 1,775 andesite analyses he compiled were also corundum-normative. By contrast, rocks of tholeiitic affinity are virtually always diopside-normative, even for the acid differentiates. This difference in ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/259102a0
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  • 5
    Electronic Resource
    Electronic Resource
    Crystallization of titaniferous chromite, magnesian ilmenite and armalcolite in tholeiitic suites in the Karoo Igneous Province (1993)
    Springer
    Contributions to mineralogy and petrology 114 (1993), S. 221-235 
    Details
    ISSN: 1432-0967
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract Titaniferous chromite (up to 8 wt% TiO2) and magnesian ilmenite (up to 10 wt% MgO) coexist at the base of the differentiated tholeiitic Mount Ayliff Intrusion in the Karoo Province of southern Africa, suggesting that the original magma was TiO2-rich. Picritic lavas with 3% TiO2 from the Lebombo monocline of the Karoo Province also contain microphenocrysts of magnesian ilmenite (up to 6 wt% mgO) and armalcolite (up to 7 wt% MgO). These oxide mineral associations and compositions are atypical of tholeiitic magmas, in which chromite usually has less than 1 wt% TiO2, ilmenite less than 3 wt% MgO and armalcolite is rarely a primary mineral. Experiments have been conducted at one atmosphere pressure on a range of compositions to determine the effect of TiO2 on the crystallization and composition of chromite, ilmenite and armalcolite. The results indicate that increasing the TiO2 content of picritic magmas increases the TiO2 content of the spinel, mainly at the expense of Al2O3, whereas Cr2O3 is not affected. Spinel compositions in the Mount Ayliff Intrusion (with over 45 wt% Cr2O3, less than 10 wt% Al2O3 and 8 wt% TiO2) were duplicated in experiments on a picrite at temperatures of about 1,200°C at the Ni/NiO buffer. Increasing fO2 from fayalite-magnetite-quartz to Ni/NiO buffer is shown to increase the crystallization temperature of armalcolite and to decrease that of ilmenite. The total FeO content of the liquid has little influence on the crystallization temperature of these phases. The TiO2 content of the liquid, when either ilmenite or armalcolite crystallizes, varies inversely with SiO2 content. The MgO content of the liquid at which ilmenite or armalcolite crystallizes depends upon the TiO2 content of the starting composition, with naturally occurring and experimetally determined saturation being demonstrated for liquids with 5 wt% MgO and 5.5 wt% TiO2. The partition coefficent for MgO between armalcolite or ilmenite and liquid is about 1.5. Observed magnesian armalcolite and ilmenite compositions in picrite lavas (both minerals) and in the Mount Ayliff Intrusion (ilmenite only) are consistent with crystallization from a TiO2-rich magma with approximately 5 wt% MgO. The Fe 2 3+ TiO5 component of armalcolite in the picrite lavas matches those formed experimentally at temperatures of 1,150–1,110°C and fO2 of the Ni/NiO to Ni/NiO+1 log unit. Similarities also exist between the compositions of chromite, ilmenite and armalcolite and liquid fraction-ation trends of some Hawaiian high-TiO2 lavas and the experimental studies presented here.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00307757
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  • 6
    Electronic Resource
    Electronic Resource
    Evidence for the trapped liquid shift effect in the Mount Ayliff Intrusion, South Africa (1992)
    Springer
    Contributions to mineralogy and petrology 111 (1992), S. 194-202 
    Details
    ISSN: 1432-0967
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract Postcumulus trapped liquid shift in layered complexes produces cumulate minerals with more fractionated compositions than the original primary phases. This effect is shown by olivine compositions from the base of the Mount Ayliff Intrusion, where varying proportions of olivine to interstitial liquid produce a suite of rocks which define a tight linear trend on a binary whole-rock plot of MgO versus FeO. Extrapolation of this trend constrains the composition of the primary cumulus olivine to the range Fo84–86, whereas olivine actually present have compositions Fo77–83. The magnitude of the discrepancy between the theoretical and observed olivine compositions correlates directly with the weight fraction of interstitial liquid. These observations are quantitatively predicted by the trapped liquid shift model. They also argue against significant migration of residual liquid. Trapped liquid shift is documented over a vertical interval of 60 m. It occurred in rocks lying only 1 m above the basal contact of the intrusion and hence must be a comparatively rapid process.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00348951
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  • 7
    Electronic Resource
    Electronic Resource
    The geochemistry of vanadiferous magnetite in the bushveld complex: Implications for crystallization mechanisms in layered complexes (1983)
    Springer
    Mineralium deposita 18 (1983), S. 505-518 
    Details
    ISSN: 1432-1866
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract Detailed trace-element analyses of pure magnetite from four continuous borehole intersections through the main magnetitite layer from the upper zone of the Rustenburg Layered Suite of the Bushveld Complex are presented. One section has been analysed at one centimetre intervals. Rapid depletion of Cr occurs over short, vertical sequences near the base of the layer, which is due to bottom-crystallization and the resulting chemical depletion of a thin layer of liquid. Sudden increases in Cr content of magnetite are attributed to convection cells which bring undepleted magma into the zone or crystallization. We suggest that these cells individually have lateral extents no greater than hundreds of metres, but collectively may be traced at a specific stratigraphic horizon for several tens of km. This lateral traceability of the effect of convection cells at approximately uniform stratigraphic height demonstrates the long-held implicit assumption that time-planes are in general parallel to the layering, and does not support the hypothesis that layers in the Bushveld Complex grew laterally. The activity of these cells is highly variable, with long periods of quiescence interspersed with periods of rapid, small-scale overturn. Most convection cells do not impinge upon the floor, and the abruptness of the resulting chemical reversal is largely a function of the thickness of the layer of depleted liquid trapped between the cumulates and the sole of the convection cell. Occasionally, these cells do touch the cumulate pile and may even cause erosion. This material may be redeposited elsewhere in the magnetitite layer either as mineralogically distinct fragments if erosion penetrated below the layer or, in the present instance, as a chemically chaotic pile of magnetite. The abruptness of the chemical reversals severely restricts the extent to which post-cumulus redistribution of elements or re-equilibration with percolating trapped liquid (infiltration metasomatism) may have occurred. The appearance of disseminated plagioclase in magnetite layers in variable proportions and in a non-systematic manner in the four profiles is attributed to fluctuations in pressure.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00204494
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  • 8
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    A Model for the Formation and Crystallization of Corundum-Normative Calc-Alkaline Magmas through Amphibole Fractionation (1976)
    University of Chicago Press
    Details
    Publication Date: 1976-07-01
    Print ISSN: 0022-1376
    Electronic ISSN: 1537-5269
    Topics: Geosciences
    Published by University of Chicago Press
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  • 9
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    Possible Liquid Immiscibility Textures in High-Magnesia Basalts from the Ventersdorp Supergroup, South Africa (1979)
    University of Chicago Press
    Details
    Publication Date: 1979-01-01
    Print ISSN: 0022-1376
    Electronic ISSN: 1537-5269
    Topics: Geosciences
    Published by University of Chicago Press
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  • 10
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    The Amphibole Peridotite-Metagabbro Complex, Finero, Northern Italy: Reply (1977)
    University of Chicago Press
    Details
    Publication Date: 1977-07-01
    Print ISSN: 0022-1376
    Electronic ISSN: 1537-5269
    Topics: Geosciences
    Published by University of Chicago Press
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