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Orthopyroxene stability within Kimberlite magma : an experimental investigation
Thesis (MSc)--Stellenbosch University, 2012.
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| Format: | Thesis |
| Language: | en_ZA |
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Stellenbosch : Stellenbosch University
2012
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| _version_ | 1867614135345217536 |
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| access_status_str | Open Access |
| author | Jacobs, Daniel A. B. |
| author2 | Stevens, Gary |
| author_browse | Jacobs, Daniel A. B. Stevens, Gary |
| author_facet | Stevens, Gary Jacobs, Daniel A. B. |
| author_sort | Jacobs, Daniel A. B. |
| collection | Thesis |
| dc_rights_str_mv | Stellenbosch University |
| description | Thesis (MSc)--Stellenbosch University, 2012. |
| format | Thesis |
| id | oai:scholar.sun.ac.za:10019.1/20211 |
| institution | Stellenbosch University (South Africa) |
| language | en_ZA |
| last_indexed | 2026-06-10T12:47:13.687Z |
| license_str | Other — see source repository |
| provenance_str_mv | Harvested via OAI-PMH from SUNScholar — Stellenbosch University Repository |
| publishDate | 2012 |
| publishDateRange | 2012 |
| publishDateSort | 2012 |
| publisher | Stellenbosch : Stellenbosch University |
| publisherStr | Stellenbosch : Stellenbosch University |
| record_format | dspace |
| source_str | SUNScholar — Stellenbosch University Repository |
| spelling | oai:scholar.sun.ac.za:10019.1/20211 Orthopyroxene stability within Kimberlite magma : an experimental investigation Jacobs, Daniel A. B. Stevens, Gary Le Roex, Anton P. Stellenbosch University. Faculty of Science. Dept. of Earth Sciences. Kimberlite petrogenesis Orthopyroxene stability Incongruent melting Dissertations -- Earth sciences Theses -- Earth sciences Magmas -- South Africa Thesis (MSc)--Stellenbosch University, 2012. ENGLISH ABSTRACT: The common presence of large volumes of coarse-grained olivine in kimberlite magmas has been proposed to attest to the volume of mantle xenolith material that has been disaggregated during the ascent of the magma. Orthopyroxene should constitute 10-50 vol% of mantle xenoliths in kimberlites, some of which must be disaggregated into the kimberlite, yet it is typically absent. This work tests the stability of orthopyroxene in ascending kimberlite magma by conducting experiments at pressures between 2.0 and 3.5 GPa and temperatures between 1100 and 1300°C. The starting material consisted of natural hypabyssal kimberlite that is close in composition to primary group I kimberlite magma with 5wt% orthopyroxene sourced from a natural peridotite added. At higher temperatures and pressures it is seen that orthopyroxene quantities exceed that of the starting material, but at lower temperatures and pressures it is absent. These results indicate that orthopyroxene is not stable in the magma composition investigated within the shallower part of the sub-continental lithospheric mantle. Based on increased olivine volumes in the experiments where orthopyroxene disappeared, as well as textural relationships between olivine and orthopyroxene, it is found that orthopyroxene dissolution is incongruent along the reaction Mg2Si2O6 (opx) = Mg2SiO4 (ol) + SiO2 (in the liquid). It is concluded that this reaction leads to a maximum addition of 5.5 vol% peritectic olivine to the kimberlite as it ascends through the depths equivalent to a pressure window of 2.0 to 3.5 GPa. AFRIKAANSE OPSOMMING: The common presence of large volumes of coarse-grained olivine in kimberlite magmas has been proposed to attest to the volume of mantle xenolith material that has been disaggregated during the ascent of the magma. Orthopyroxene should constitute 10-50 vol% of mantle xenoliths in kimberlites, some of which must be disaggregated into the kimberlite, yet it is typically absent. This work tests the stability of orthopyroxene in ascending kimberlite magma by conducting experiments at pressures between 2.0 and 3.5 GPa and temperatures between 1100 and 1300°C. The starting material consisted of natural hypabyssal kimberlite that is close in composition to primary group I kimberlite magma with 5wt% orthopyroxene sourced from a natural peridotite added. At higher temperatures and pressures it is seen that orthopyroxene quantities exceed that of the starting material, but at lower temperatures and pressures it is absent. These results indicate that orthopyroxene is not stable in the magma composition investigated within the shallower part of the sub-continental lithospheric mantle. Based on increased olivine volumes in the experiments where orthopyroxene disappeared, as well as textural relationships between olivine and orthopyroxene, it is found that orthopyroxene dissolution is incongruent along the reaction Mg2Si2O6 (opx) = Mg2SiO4 (ol) + SiO2 (in the liquid). It is concluded that this reaction leads to a maximum addition of 5.5 vol% peritectic olivine to the kimberlite as it ascends through the depths equivalent to a pressure window of 2.0 to 3.5 GPa. 2012-03-04T14:55:05Z 2012-03-30T10:53:07Z 2012-03-04T14:55:05Z 2012-03-30T10:53:07Z 2012-03 Thesis http://hdl.handle.net/10019.1/20211 en_ZA Stellenbosch University 47 p. : ill. application/pdf Stellenbosch : Stellenbosch University |
| spellingShingle | Kimberlite petrogenesis Orthopyroxene stability Incongruent melting Dissertations -- Earth sciences Theses -- Earth sciences Magmas -- South Africa Jacobs, Daniel A. B. Orthopyroxene stability within Kimberlite magma : an experimental investigation |
| title | Orthopyroxene stability within Kimberlite magma : an experimental investigation |
| title_full | Orthopyroxene stability within Kimberlite magma : an experimental investigation |
| title_fullStr | Orthopyroxene stability within Kimberlite magma : an experimental investigation |
| title_full_unstemmed | Orthopyroxene stability within Kimberlite magma : an experimental investigation |
| title_short | Orthopyroxene stability within Kimberlite magma : an experimental investigation |
| title_sort | orthopyroxene stability within kimberlite magma an experimental investigation |
| topic | Kimberlite petrogenesis Orthopyroxene stability Incongruent melting Dissertations -- Earth sciences Theses -- Earth sciences Magmas -- South Africa |
| url | http://hdl.handle.net/10019.1/20211 |
| work_keys_str_mv | AT jacobsdanielab orthopyroxenestabilitywithinkimberlitemagmaanexperimentalinvestigation |