Using mineral chemistry to constrain P-T conditions for mantle xenoliths from the Kaapvaal craton, South Africa
1University of Oulu, Oulu Mining School, Geology
|Online Access:||PDF Full Text (PDF, 3.5 MB)|
|Persistent link:|| http://urn.fi/URN:NBN:fi:oulu-201611233107
|Publish Date:|| 2016-11-23
|Thesis type:||Master's thesis
Kimberlites are igneous rocks that originate by small degrees of melting of the mantle. Notably, kimberlites carry large variety of crustal and mantle xenoliths. Geochemical data on xenoliths can provide insights into the processes occurring in the subcontinental lithosphere (SCLM) and deeper.
The Kaapvaal craton in South Africa hosts one of the best-studied kimberlite populations on Earth. In this thesis, a total of 24 thin sections of peridotite xenoliths from Group I Letlhakane, Letseng, Premier and Frank Smith kimberlites were investigated to constrain the pressure, temperature and depth of these mantle xenoliths. To do so, olivine, orthopyroxene, clinopyroxene, garnet and spinel were analyzed for their major element chemistry using electron microprobe analysis (EPMA). P-T calculations were performed using the PTEXL3 spreadsheet program, which contains different geothermobarometers. Depth constraints were fitted to the characteristic Kaapvaal craton geotherm.
According to geochemical results and rough modal mineral estimations, the majority of the mantle xenoliths were identified as depleted harzburgites or lherzolites. Mineral major element compositions show trends of depletion, which correlate with the corresponding mantle xenolith sampling depth. Olivine and orthopyroxene have high average Mg# values of 92.1 and 93.0, respectively, at shallower depth ~70–160 Km. Below ~160 km, Mg# starts to drop rapidly and transition towards a more typical asthenospheric composition. The majority of garnet compositions fall into the G9 classification field. Titanium shows a distinct partition trend that correlates with depletion. Garnets have well developed alteration reaction rims, especially at shallower depths.
Geothermobarometric calculations for four-phase peridotites are comparable with the results from other studies. However, the temperature estimates obtained by T(BKN90) are slightly overestimated and, in contrast, the pressure estimates from P(BBG08) are slightly underestimated. Other assemblages have considerable calculated pressure and temperature conditions and were best fitted for the regional conductive geotherm. The mantle xenoliths show pressures ranging from 22 to 56 kb and temperatures from 753 to 1344 °C that characterize an extensive sampling depth range from 70 to 190 km. Three of the samples extend into the diamond stability field. The obtained P-T data for mantle xenoliths cluster along a 44.0±2.0 mWm⁻² conductive Kaapvaal craton continental geotherm, being slightly higher than that of the average thermal state estimate for the craton.
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