Full Text Available
Note: Clicking the button above will open the full text document at the original institutional repository in a new window.
A study of mantle inclusions in the Koffiefontein Kimberlite Pipe, South Africa
Mantle derived rock and mineral fragments from the Koffiefontein Kimberlite Pipe have been studied in the form of xenoliths, megacrysts, concentrate minerals and inclusions in diamonds. Mantle derived xenoliths are unusually scarce. The predominant xenoliths are pyroxenites. Peridotites are rare and...
Saved in:
| Main Author: | |
|---|---|
| Other Authors: | |
| Format: | Thesis |
| Language: | English |
| Published: |
Department of Geological Sciences
2016
|
| Subjects: | |
| Tags: |
No Tags, Be the first to tag this record!
|
| _version_ | 1867613146241302528 |
|---|---|
| access_status_str | Open Access |
| author | Cardoso, P |
| author2 | Gurney, John |
| author_browse | Cardoso, P Gurney, John |
| author_facet | Gurney, John Cardoso, P |
| author_sort | Cardoso, P |
| collection | Thesis |
| description | Mantle derived rock and mineral fragments from the Koffiefontein Kimberlite Pipe have been studied in the form of xenoliths, megacrysts, concentrate minerals and inclusions in diamonds. Mantle derived xenoliths are unusually scarce. The predominant xenoliths are pyroxenites. Peridotites are rare and eclogites are very rare. The pyroxenites have been assigned to five categories. In general they show a clear metamorphic textural history with widespread evidence of exsolution, polygonisation and deformation. Some of these pyroxenites are interpreted to represent the crystallisation products of basaltic magma at depth. Some have been metasomatised. The pyroxene chemistries suggest that more than one pyroxenite body has been sampled. The peridotites are very similar to the common peridotites of N. Lesotho, and are thought to represent depleted mantle. The Koffiefontein diamonds contain both eclogitic and peridotitic inclusions. The inclusions show evidence for lack of equilibration and one appears to link together the two parageneses mentioned above. The presence of (MgFe)O as an inclusion is reported. The megacrysts studied consisted only of garnets, and a few pyroxenes. They are interpreted to have crystallised from a magma which cooled slowly and was relatively deficient in TiO₂ to account for the scarcity of ilmenite. The heavy mineral concentrate contained garnet, orthopyroxene, opaque minerals and clinopyroxene in order of abundance. The opaque minerals were chromite and ilmenite. The chemical compositions of most of the concentrate grains do not match the mineral compositions found in xenoliths, or megacrysts. A small number of concentrate grains have compositions matched by the diamond inclusions. Attempts to apply geothermobarometry to the pyroxenites and to the diamond inclusions gave conflicting results which are ascribed to lack of equilibration between co-existing mineral phases and/or within individual mineral grains. The Koffiefontein xenolith suite is unusual for the relative abundance of pyroxenite and the scarcity of peridotite. The concentrate is unusual for the relative abundance of orthopyroxene particularly and of chromite. |
| format | Thesis |
| id | oai:open.uct.ac.za:11427/17654 |
| institution | University of Cape Town (South Africa) |
| language | eng |
| last_indexed | 2026-06-10T12:31:30.019Z |
| license_str | Not specified — see source repository |
| provenance_str_mv | Harvested via OAI-PMH from UCTD — University of Cape Town Open Access Repository |
| publishDate | 2016 |
| publishDateRange | 2016 |
| publishDateSort | 2016 |
| publisher | Department of Geological Sciences |
| publisherStr | Department of Geological Sciences |
| record_format | dspace |
| source_str | UCTD — University of Cape Town Open Access Repository |
| spelling | oai:open.uct.ac.za:11427/17654 A study of mantle inclusions in the Koffiefontein Kimberlite Pipe, South Africa Cardoso, P Gurney, John Geochemistry Mantle derived rock and mineral fragments from the Koffiefontein Kimberlite Pipe have been studied in the form of xenoliths, megacrysts, concentrate minerals and inclusions in diamonds. Mantle derived xenoliths are unusually scarce. The predominant xenoliths are pyroxenites. Peridotites are rare and eclogites are very rare. The pyroxenites have been assigned to five categories. In general they show a clear metamorphic textural history with widespread evidence of exsolution, polygonisation and deformation. Some of these pyroxenites are interpreted to represent the crystallisation products of basaltic magma at depth. Some have been metasomatised. The pyroxene chemistries suggest that more than one pyroxenite body has been sampled. The peridotites are very similar to the common peridotites of N. Lesotho, and are thought to represent depleted mantle. The Koffiefontein diamonds contain both eclogitic and peridotitic inclusions. The inclusions show evidence for lack of equilibration and one appears to link together the two parageneses mentioned above. The presence of (MgFe)O as an inclusion is reported. The megacrysts studied consisted only of garnets, and a few pyroxenes. They are interpreted to have crystallised from a magma which cooled slowly and was relatively deficient in TiO₂ to account for the scarcity of ilmenite. The heavy mineral concentrate contained garnet, orthopyroxene, opaque minerals and clinopyroxene in order of abundance. The opaque minerals were chromite and ilmenite. The chemical compositions of most of the concentrate grains do not match the mineral compositions found in xenoliths, or megacrysts. A small number of concentrate grains have compositions matched by the diamond inclusions. Attempts to apply geothermobarometry to the pyroxenites and to the diamond inclusions gave conflicting results which are ascribed to lack of equilibration between co-existing mineral phases and/or within individual mineral grains. The Koffiefontein xenolith suite is unusual for the relative abundance of pyroxenite and the scarcity of peridotite. The concentrate is unusual for the relative abundance of orthopyroxene particularly and of chromite. 2016-03-11T14:26:13Z 2016-03-11T14:26:13Z 1980 Master Thesis Masters MSc http://hdl.handle.net/11427/17654 eng application/pdf Department of Geological Sciences Faculty of Science University of Cape Town |
| spellingShingle | Geochemistry Cardoso, P A study of mantle inclusions in the Koffiefontein Kimberlite Pipe, South Africa |
| thesis_degree_str | Master's |
| title | A study of mantle inclusions in the Koffiefontein Kimberlite Pipe, South Africa |
| title_full | A study of mantle inclusions in the Koffiefontein Kimberlite Pipe, South Africa |
| title_fullStr | A study of mantle inclusions in the Koffiefontein Kimberlite Pipe, South Africa |
| title_full_unstemmed | A study of mantle inclusions in the Koffiefontein Kimberlite Pipe, South Africa |
| title_short | A study of mantle inclusions in the Koffiefontein Kimberlite Pipe, South Africa |
| title_sort | study of mantle inclusions in the koffiefontein kimberlite pipe south africa |
| topic | Geochemistry |
| url | http://hdl.handle.net/11427/17654 |
| work_keys_str_mv | AT cardosop astudyofmantleinclusionsinthekoffiefonteinkimberlitepipesouthafrica AT cardosop studyofmantleinclusionsinthekoffiefonteinkimberlitepipesouthafrica |