Summary
A volumetrically minor Cu−Fe−Ni−S component derived from the uppermost mantle is found within the Beni Bousera (northern Morocco) and Ronda (southern Spain) “Alpine”-type peridotites; it occurs today as inclusions within primary silicates or as assemblages disseminated in the intergranular sites of the host rocks. Detailed microtextural and microprobe data indicate that inclusions and intergranular assemblages behaved as two contrasting systems during the low temperature, incipient serpentinization of the host rocks. The former were equilibrated in closed systems whereas the latter behaved as open systems with respect to hydrothermal fluids; as a result, intergranular assemblages were controlled by redox conditions generated by serpentinization. An early stage of alteration is characterized by a slight decrease of the sulfur content and would be due to the first influx of water inside peridotites; a further transformation was produced by anomalous highly reducing conditions responsible for the crystallization of native iron-bearing alloy assemblages as well as for a preferential partioning of Fe from silicate into pentlandite. The production of anomalous, reducing conditions during incipient serpentinization is ascribed to a low permeability of the host rocks with respect to the diffusion of hydrogen out of the sites of serpentinization. Because of its low temperature behaviour, the intergranular sulfide component would not be of use reconstructing the initial composition of the upper mantle sulfide component; thus, it is concluded that only the sulfide inclusions would provide such informations.
Résumé
Des traces d'un composant sulfuré du système Cu−Fe−Ni−S d'origine mantellique sont présentes dans les péridotites “alpines” de Beni Bousera (Maroc) et Ronda (Espagne); il forme actuellement des inclusions dans les silicates primaires ou des plages intergranulaires disséminées dans les interstices des roches. Une analyse minéralogique et chimique détaillée démontre qu'inclusions et plages intergranulaires sulfurées ont eu un comportement opposé dès les premiers stades de serpentinisation des roches encaissantes. Les premières ont été reéquilibrées en système chimique ouvert vis-à-vis des fluides de serpentinisation; en conséquence les paragenèses intergranulaires résultent d'une suite de transformations du composant sulfuré mantellique, controlée par le degré d'oxydo réduction du fluide de serpentinisation. Dans un premier stade, l'entrée de l'eau dans les péridotites a provoqué une légère baisse de la fugacité du soufre et la cristallisation d'assemblages sulfurés riches en mackinawite. L'augmentation du degré de serpentinisation a ensuite engendré des conditions inhabituellement réductrices responsables d'un important fractionnement du fer libéré par la serpentinisation de l'olivine dans la pentlandite ainsi que de la cristallisation d'une paragenèse complexe de phases métalliques incluant localement le fer natif; ces conditions anormalement reductrices sont imputées à une faible perméabilité des roches encaissantes vi-à-vis de la diffusion de l'hydrogène en dehors dessites de serpentinisation. En raison de leur comportement à basse température, les plages sulfurées intergranulaires sont inutilisables pour reconstituer le composant sulfuré du manteau supérieur; sur ce plan, on conclut que seules les inclusions blindées dans les silicates peuvent apporter des informations.
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Lorand, J.P. The behaviour of the upper mantle sulfide component during the incipient alteration of “Alpine”-type peridotites as illustrated by the Beni Bousera (northern Morocco) and Ronda (southern Spain) ultramafic bodies. TMPM Tschermaks Petr. Mitt. 34, 183–209 (1985). https://doi.org/10.1007/BF01082961
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DOI: https://doi.org/10.1007/BF01082961