Some aspects of the geochemistry of high-temperature peridotites and megacrysts from the Jagersfontein kimberlite pipe, South Africa

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dc.contributor.advisorGurney, John Jen_ZA
dc.contributor.authorHops, Jennifer Janeen_ZA
dc.date.accessioned2016-11-02T09:07:59Z
dc.date.available2016-11-02T09:07:59Z
dc.date.issued1989en_ZA
dc.descriptionBibliography: pages 188-203.en_ZA
dc.description.abstractThe Jagersfontein kimberlite contains an abundance of both deformed high-temperature peridotites and Cr-poor megacrysts. The Cr-poor megacryst suite is represented by olivine, orthopyroxene, clinopyroxene and garnet. The megacrysts show features which are unique to Jagersfontein, a particularly notable feature being the absence of ilmenite and ilmenite-silicate intergrowths. Major element and REE compositions of the Cr-poor megacryst suite are consistent with a magmatic fractionation sequence. ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd ratios of the Cr-poor clinopyroxene megacrysts indicate a source similar to that of non-DUPAL ocean island basalts. Deformed peridotites at Jagersfontein have high calculated temperatures of equilibration (1132-1361°C), which are slightly lower but which overlap with those of the Cr-poor megacryst suite. Both the high-temperature peridotites and the Cr-poor megacrysts yield similar pressures of equilibration (51±2 kbar), indicating their association with a thermal perturbation and supporting a close spatial association between them. Olivine and pyroxenes in the high-temperature peridotites appear homogeneous, but compositional gradients were observed in several garnet porphyroclasts. These garnets show rim enrichments in TiO₂ and Na₂O. This zonation in the garnets is evidence for enrichment of the deformed peridotites shortly before kimberlite eruption. This enrichment is likely to be due to interaction with the megacryst magma. In addition, the high-temperature peridotites show a general enrichment in Fe, Ti, Na and Al with decreasing Ca/(Ca+Mg). Such features support a magmatic aureole model, in that temperature and degree of enrichment might be expected to increase with proximity to the megacryst magma body. ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd ratios of clinopyroxene separates from the high-temperature peridotites are similar to those from oceanic peridotites. Modal abundances and olivine forsterite contents of the high-temperature peridotites are consistent with an origin as residues of partial melting events involving basalt formation. It is suggested that partial melting events, in the upper mantle beneath Jagersfontein, resulted in the formation of a depleted protolith which underplated the base of the Archaean lithosphere. This depleted protolith was subsequently enriched by interaction with the Cr-poor megacryst magma just prior to kimberlite eruption. The high-temperature peridotites therefore represent samples from the base of the lithosphere rather than from the convecting asthenosphere.en_ZA
dc.identifier.apacitationHops, J. J. (1989). <i>Some aspects of the geochemistry of high-temperature peridotites and megacrysts from the Jagersfontein kimberlite pipe, South Africa</i>. (Thesis). University of Cape Town ,Faculty of Science ,Department of Geological Sciences. Retrieved from http://hdl.handle.net/11427/22415en_ZA
dc.identifier.chicagocitationHops, Jennifer Jane. <i>"Some aspects of the geochemistry of high-temperature peridotites and megacrysts from the Jagersfontein kimberlite pipe, South Africa."</i> Thesis., University of Cape Town ,Faculty of Science ,Department of Geological Sciences, 1989. http://hdl.handle.net/11427/22415en_ZA
dc.identifier.citationHops, J. 1989. Some aspects of the geochemistry of high-temperature peridotites and megacrysts from the Jagersfontein kimberlite pipe, South Africa. University of Cape Town.en_ZA
dc.identifier.ris TY - Thesis / Dissertation AU - Hops, Jennifer Jane AB - The Jagersfontein kimberlite contains an abundance of both deformed high-temperature peridotites and Cr-poor megacrysts. The Cr-poor megacryst suite is represented by olivine, orthopyroxene, clinopyroxene and garnet. The megacrysts show features which are unique to Jagersfontein, a particularly notable feature being the absence of ilmenite and ilmenite-silicate intergrowths. Major element and REE compositions of the Cr-poor megacryst suite are consistent with a magmatic fractionation sequence. ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd ratios of the Cr-poor clinopyroxene megacrysts indicate a source similar to that of non-DUPAL ocean island basalts. Deformed peridotites at Jagersfontein have high calculated temperatures of equilibration (1132-1361°C), which are slightly lower but which overlap with those of the Cr-poor megacryst suite. Both the high-temperature peridotites and the Cr-poor megacrysts yield similar pressures of equilibration (51±2 kbar), indicating their association with a thermal perturbation and supporting a close spatial association between them. Olivine and pyroxenes in the high-temperature peridotites appear homogeneous, but compositional gradients were observed in several garnet porphyroclasts. These garnets show rim enrichments in TiO₂ and Na₂O. This zonation in the garnets is evidence for enrichment of the deformed peridotites shortly before kimberlite eruption. This enrichment is likely to be due to interaction with the megacryst magma. In addition, the high-temperature peridotites show a general enrichment in Fe, Ti, Na and Al with decreasing Ca/(Ca+Mg). Such features support a magmatic aureole model, in that temperature and degree of enrichment might be expected to increase with proximity to the megacryst magma body. ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd ratios of clinopyroxene separates from the high-temperature peridotites are similar to those from oceanic peridotites. Modal abundances and olivine forsterite contents of the high-temperature peridotites are consistent with an origin as residues of partial melting events involving basalt formation. It is suggested that partial melting events, in the upper mantle beneath Jagersfontein, resulted in the formation of a depleted protolith which underplated the base of the Archaean lithosphere. This depleted protolith was subsequently enriched by interaction with the Cr-poor megacryst magma just prior to kimberlite eruption. The high-temperature peridotites therefore represent samples from the base of the lithosphere rather than from the convecting asthenosphere. DA - 1989 DB - OpenUCT DP - University of Cape Town LK - https://open.uct.ac.za PB - University of Cape Town PY - 1989 T1 - Some aspects of the geochemistry of high-temperature peridotites and megacrysts from the Jagersfontein kimberlite pipe, South Africa TI - Some aspects of the geochemistry of high-temperature peridotites and megacrysts from the Jagersfontein kimberlite pipe, South Africa UR - http://hdl.handle.net/11427/22415 ER - en_ZA
dc.identifier.urihttp://hdl.handle.net/11427/22415
dc.identifier.vancouvercitationHops JJ. Some aspects of the geochemistry of high-temperature peridotites and megacrysts from the Jagersfontein kimberlite pipe, South Africa. [Thesis]. University of Cape Town ,Faculty of Science ,Department of Geological Sciences, 1989 [cited yyyy month dd]. Available from: http://hdl.handle.net/11427/22415en_ZA
dc.language.isoengen_ZA
dc.publisher.departmentDepartment of Geological Sciencesen_ZA
dc.publisher.facultyFaculty of Scienceen_ZA
dc.publisher.institutionUniversity of Cape Town
dc.subject.otherGeological Sciencesen_ZA
dc.subject.otherGeochemistryen_ZA
dc.titleSome aspects of the geochemistry of high-temperature peridotites and megacrysts from the Jagersfontein kimberlite pipe, South Africaen_ZA
dc.typeDoctoral Thesis
dc.type.qualificationlevelDoctoral
dc.type.qualificationnamePhDen_ZA
uct.type.filetypeText
uct.type.filetypeImage
uct.type.publicationResearchen_ZA
uct.type.resourceThesisen_ZA
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