Pyroxene stability within kimberlite magma in the upper mantle : an experimental investigation

Burness, Sara (2015-04)

Thesis (MSc)--Stellenbosch University, 2015.

Thesis

ENGLISH ABSTRACT: Entrainment and assimilation of xenolithic material during kimberlite ascent is considered to be important in shaping the chemistry of the magma and fuelling magma ascent by driving CO2 exsolution. Previous, but as yet unpublished experimental work from Stellenbosch University has demonstrated that orthopyroxene has a key role in this. Orthopyroxene is a very rare xenocrystic constituent of kimberlite but makes up a considerable fraction of the entrained xenolithic material. The initial study used a natural kimberlite composition (ADF1) doped with a peridotite mineral suite (by weight); 88 % ADF1 5% olivine, 5% orthopyroxene and 2% garnet-spinel intergrowth as a starting composition. The subsequent high PT experiments (1100 to 1300°C and 2.0 to 3.5GPa) established that equilibrium orthopyroxene is stable at 1100°C above 2.5GPa, at 1200°C above 2.5GPa and at 1300°C between 2.0 and 3.5GPa. At lower pressures orthopyroxene is completely digested by the experimental melt by the reaction; Mg2Si2O6 (opx) = Mg2SiO4 (ol) + SiO2 (in liquid). In contrast, clinopyroxene is a common phase in kimberlite and often occurs as more than one generation of crystals. Xenocrystic clinopyroxene is dominated by diopside compositions. However, rare omphacite is sometimes also inherited from an eclogite source. The Omphacite, like orthopyroxene, displays textural evidence of severe disequilibrium and may also contribute to the evolution of kimberlitic melt. Thus, a second study produced experiments on the ADF1 kimberlite material at upper mantle PT conditions (1100 to 1300°C and 2.0 to 4.0GPa) as well as an omphacite doped starting material (ADF1+O). These experiments examine the behaviour of pyroxene in kimberlite magma including the influence this may have on magma buoyancy. Within this PT range omphacitic clinopyroxene breaks-down via complex multipart reactions. At 1100°C and 2.0GPa reaction textures around remnant omphacite suggest that omphacite melts incongruently in a complex reaction similar to: Omp + Melt = Ap + Cr-diop + SiO2-enriched Melt. At 1300°C omphacite melts completely and is perceived to produce peritectic Cr-diopside, calcium-rich olivine, carbonate in the melt as well as enrich the melt in SiO2. The melts produced by both the ADF1+O and ADF1 compositions at 1300°C and 4.0GPa are reduced in SiO2 content and have increased TiO2, Cr2O3, Al2O3, MnO, CaO, K2O and P2O5 compared to their respective starting compositions. However, significantly higher proportions of Ca, Na and Fe observed within the ADF1+O melt is a direct consequence of omphacite melting. The ADF1+O starting composition produced equilibrium orthopyroxene above 1100°C and 4.0GPa as well as at 1300°C above 2.0GPa. At lower pressure the orthopyroxene melts incongruently to form peritectic olivine and more silica-rich melt compositions. This digestion favours CO2 exsolution. The effect of orthopyroxene melting can be seen in the melt compositions produced by the peridotite doped starting material (ADF1+P) of the initial study. At 1300°C and 2.0GPa, ADF1+P produced a siliceous melt (37.0 wt.% SiO2) enriched in Al and alkalis compared to the starting ADF1+P composition. This behaviour is directly attributed to xenocrystic orthopyroxene melting at high temperature. In contrast, at the same PT the original kimberlite (ADF1) composition produces a melt with 28.9 wt.% SiO2 and high Ca and Mg contents. Overall, with an increase in pressure the melts become enriched in alkalis and Al2O3 as a direct result of xenocrystic pyroxene melting. In addition, increased pressure allows for a greater solubility of CO2 within the melt. This results in a lower SiO2 melt content and the increased stabilization of equilibrium silica-rich mineral phases (i.e. olivine and equilibrium orthopyroxene). Within the peridotite doped static system (unpublished) the mineral separates with an average crystal size of 115μm ±10μm were all effectively digested in less than 48hours. Similarly, the omphacite doped experiments consumed the 150μm (±10μm) xenocrysts in under 24 hours. Thus, it is suggested that xenocrystic pyroxene is unstable in these experimental kimberlitic melt compositions and is likely to be efficiently assimilated in less than 24 hours. These experimental melts most likely resemble those of natural systems under upper mantle PT conditions. Therefore, pyroxene melting increases the silica content of the melt which in turn drives CO2 exsolution and ascent.

AFRIKAANSE OPSOMMING: Meevoering en assimilasie van xenolitiese materiaal gedurenende kimberliet bestyging is beskou as belangrik in verband met die vorming van die chemie van die magma, en bevorder magma bestyging deur die aandrywing van CO2 ontmenging. Vorige, maar ongepubliseerde eksperimentele werk vanaf Stellenbosch Universiteit het gedemonstreer dat ortopirokseen ‘n sleutelrol hierin het, omrede ortopirokseen ‘n baie skaars xenokristiese bestanddeel van kimberliet is maar ‘n aansienlike fraksie van die meevoerde xenolitiese materiaal moet opmaak. Hierdie studie het ‘n natuurlike primere kimberliet komposisie (ADFI) gedoop met ‘n peridotiet mineraal reeks (per gewig); 88 % ADF1 5% olivien, 5% ortopirokseen en 2% granaat-spinel ingroeiing as begin komposisie gebruik. Die daaropvolgende hoë DT eksperimente (1100 tot 1300°C en 2.0 tot 3.5GPa) het vasgestel dat ewewigsortopirokseen stabiel is teen 1100°C bo 2.5GPa, 1200°C bo 2.5GPa en teen 1300°C vanaf 2.0 tot 3.5Gpa. Teen laer druk word ortopirokseen geheel verteer deur die eksperimentele smelting volgens die reaksie Mg2Si2O6 (opx) = Mg2SiO4 (ol) + SiO2 (in vloeistof). In kontras hiermee is clinopirokseen algemeen in kiemberliet en kom dikwels voor as meer as een generasie se kristalle. Diopsiet komposisies domineer xenokristiese klinopirokseen. Seldsame omfasiet is tog somtyds ook geërf vanaf ‘n eklogiet bron. Die omfasiet, soos ortopirokseen, vertoon teksturuele bewys van ernstige disekwilibrium en mag ook bydra tot die evolusie van kimberlitiese smelt. Dus was daar addisionele eksperimente uitgevoer op die ADF1 kimberliet material teen hoër mantel DT kondisies (1100 tot 1300°C en 2.0 tot 4.0GPa), asook ‘n begin materiaal gedoop met omfasiet (ADF1+O). Hierdie eksperimente ondersoek die gedrag van pirokseen in kiemberliet magma, asook die invloed wat dit sal hê op die dryfvermoë van die magma. Binne hierdie DT reeks breek omfasitiese klinopirokseen af via komplekse multideel reaksie prosesse. Teen 1100°C en 2.0Gpa stel reaksie teksture rondom die oorblywende omfasiet voor dat omfasiet ongelykvormig smelt deur ‘n komplekse reaksie soortgelyk aan: Omp + Smelt = Ap + Cr-diop + SiO2-verrykde Smelt. Teen 1300°C smelt omfasiet volkome en is waargeneem om peritektiese Cr-diopsiet, kalsiumryke olivien en kalsiet te produseer, sowel as dat dit die smelt verryk in SiO2. Die smeltings geprodiseer deur die ADF1+O en ADF1 massa komposisies teen 1300°C en 4.0GPa is verlaag in SiO2 inhoud en bevat verhoogde TiO2, Cr2O3, Al2O3, MnO, CaO, K2O en P2O5 in vergelyking met die onderskeie begin komposisies. Aansienlike hoër proporsies van Ca, Na en Fe is egter waargeneem in die ADF1+O smelt en is ‘n direkte gevolg van die smelting van omfasiet. Die ADF1+O begin samestelling het ewewigsortopirokseen bo 1100°C en 4.0Gpa geproduseer en massa teen 1300°C en 2.0 tot 4.0GPa. Teen laer druk smelt hierdie pirokseen inkongruent om peritektiese olivien en meer silika-ryke smelt samestellings te vorm, en ontmeng CO2. Die effek van ortopirokseen smelting kan aanskou word in die smelt samestellings wat produseer is deur die begin materiaal wat gedoop is in peridotiet (ADF1+P), in die oorspronklike studie. Teen 1300°C en 2.0GPa het ADF1+P ‘n silikahoudende smelt (37.0 wt.% SiO2) produseer wat verryk is in Al en alkalies in vergelyking met die ADF1+P massa samestelling. Hierdie gedrag is direk toegeskryf aan die xenokristiese ortopirokseen wat smelt teen hoë temperatuur. In kontras hiermee, teen dieselfde DT kondisies produseer die oorspronklike kiemberliet (ADF1) massa ‘n smelt met 28.86 gewigspersentasie SiO2 en hoë Ca en Mg inhoud. In die algeheel word die smeltings verryk in alkalies en Al2O3 teen verhoogde druk as ‘n derekte gevolg van xenokristiese pirokseen smelting. Verder laat verhoogde druk toe vir hoër oplosbaarheid van CO2 in die smelt, wat lei tot laer SiO2 inhoud en ‘n toename in stabilisering van ewewigs silika-ryke mineraal fases (dws. olivien en ewewigsortopirokseen). In die peridotiet gedoopde statiese sisteem (ongepubliseerd), was die mineraal skeiding met ‘n gemiddelde kristal grootte van 115μm ±10μm almal effektief verteer in minder as 48 ure. Soortgelyk hieraan het die omfasiet gedoopde eksperimente die 150μm (±10μm) sade onder 24 ure verteer. Dus stel dit voor dat xenokristiese pirokseen in naatuurlike sisteme onstabiel is in kiemberlietiese smelt samestellings en sal waarskynlik geassimileer wees in miner as 24 ure en ‘n meer silica-ryke kiemberlietiese smelt samestelling produseer terwyl dit CO2, ontmenging en bestyging aandryf.

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