An experimental investigation on the fate of xenocrystic garnet in kimberlitic melts under upper mantle conditions

Grobbelaar, Marelize (2015-04)

Thesis (MSc)--Stellenbosch University, 2015.

Thesis

ENGLISH ABSTRACT: Insets of large anhedral minerals in kimberlites are proposed to mainly have their origin from the disaggregation of mantle-derived xenoliths through mechanical abrasion by the ascending kimberlite magma. Garnet, despite being an important constituent of both mantle-derived peridotite and eclogite, forms only a minor constituent of kimberlites. This suggests that a large proportion of garnet liberated into the kimberlite through the disaggregation of such xenoliths, is consumed before the emplacement of kimberlite. This study records the breakdown mechanism of garnet by the ascending kimberlite magma by conducting experiments between pressures of 2 and 4 GPa and temperatures between 1100 and 1300°C. The starting material used in the experiments was prepared from a natural hypabyssal kimberlite that closely resembles the composition of a proposed primary Group I kimberlite magma. To the kimberlite material 5 wt % garnet, sourced from a natural peridotite, was added. It was found that garnet is not a stable equilibrium phase within the kimberlite magma under the investigated temperature and pressure conditions. Based on large volumes of phlogopite in the experiments it is concluded that garnet melts incongruently in the kimberlite magma to form phlogopite as a peritectic product and melts transitional in composition between silicate and carbonate melts. This is in contrast to more SiO2 -rich melt compositions produced as a consequence of the incongruent breakdown of both orthopyroxene and omphacite within kimberlite magmas. The consequence of the melt compositions produced in the experiments is increased solubility of CO2 in the form of carbonate (CO32-). This finding has implications for both kimberlite ascent mechanisms and the solubility of diamond transported within kimberlite magmas.

AFRIKAANSE OPSOMMING: Dit word voorgestel dat groot oneenvormige mineraalinsluitsels in kimberliet hoofsaaklik afkomstig is van xenolitiese mantelmateriaal wat verbrokkel weens die meganiese werking van die stygende kimberliet magma. Ten spyte daarvan dat granaat ‘n belangrike komponent is van peridotiet en eklogiet xenoliete afkomstig uit die mantel, vorm granaat slegs ‘n geringe deel van die kimberliet mineraalsamestelling. Dit dui daarop dat ‘n beduidende gedeelte van die granaat wat vrygstel word in die kimberliet magma deur die verbrokkeling van xenolitiese materiaal, afgebreek word deur die stygende kimberliet magma voordat dit stol. Hierdie studie ondersoek die afbreekmeganisme van granaat afkomstig van die mantel in die stygende kimberliet magma deur eksperimente uit te voer by drukke tussen 2 en 4GPa en temperature tussen 1100 en 1300°C. Die materiaal met die aanvang van die eksperimente was voorberei uit ‘n natuurlike hipabissale kimberliet wat soortgelyk is in samestelling as ‘n voorgestelde primêre Groep I kimberliet magma se samesteling. Vyf gewigspresent (5 wt %) granaat, verkry van n natuurlike peridotiet, is bygevoeg tot die kimberlietmateriaal. Daar is gevind dat granaat nie ‘n stabiele ewewigsfase is in die kimberliet magma binne die temperatuur-en drukstoestande ondersoek nie. Groot volumes flogopiet teenwoordig in die eksperimente dui daarop dat granaat inkongruent smelt in die kimberliet magma om flogopiet as ‘n peritektiese produk te vorm te same met ‘n smelt wat neig na ‘n karbonatitese smeltsamestelling. Die bevinding is in teenstelling met meer SiO2-ryke smeltsamestellings as gevolg van die inkongruente afbreek van beide ortopirokseen en omfasiet in die kimberliet magma. Die gevolg van die meer karbonatitese smeltsamestellings, is ‘n toename in die oplosbaarheid van CO2 in die smelt in die vorm van karbonaat (CO32-). Die bevinding het nagevolge vir beide kimberliet stygingsmeganismes asook die oplosbaarheid van diamant tydens die vervoer van diamant deur kimberliet magmas.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/96828
This item appears in the following collections: