A phase equilibrium modelling investigation of the consequences of entrainment of components of the source on the crystallisation of mantle-derived magmas in the upper crust, with specific relevance to the petrogenesis of chromitite layers in the Rustenburg Layered Suite of the Bushveld Complex, South Africa

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dc.contributor.advisorStevens, Garyen_ZA
dc.contributor.advisorMoyen, Jean-Francoisen_ZA
dc.contributor.advisorMayne, Matten_ZA
dc.contributor.authorOtto, Tahneeen_ZA
dc.contributor.otherStellenbosch University. Faculty of Science. Dept. of Earth Sciences.en_ZA
dc.date.accessioned2023-03-07T04:23:27Z
dc.date.accessioned2023-05-18T07:08:39Z
dc.date.available2023-03-07T04:23:27Z
dc.date.available2023-05-18T07:08:39Z
dc.date.issued2023-03
dc.descriptionThesis (PhD)--Stellenbosch University, 2023. en_ZA
dc.description.abstractENGLISH ABSTRACT: Thermodynamic modelling techniques offer several advantages over experimental studies in investigations of the partial melting of mafic and ultramafic rocks under upper mantle conditions, but may not be as reliable as experimental studies in accurately predicting rock behaviour. The two main thermodynamic datasets that are in common use for such thermodynamic investigations have different levels of coverage. The Berman (1988) dataset is utilised by the MELTS family of software (Asimow & Ghiorso, 1998; Ghiorso & Sack, 1995; Ghiorso et al., 2002), and along with routines for handling activity-composition relations for solid and liquid solution phases, is embedded within the software. In contrast, the Holland & Powell (2011) dataset is used by a wide range of different software with different computational strategies. Activity-composition relations are handled by published models that are selectable and transparent. One of the programs using the Holland & Powell (2011) dataset, Rcrust (Mayne et al., 2016), has been developed to allow modelling of phase stabilities with changing bulk composition, which makes it particularly powerful for studying processes involving fractionation. In this study, the results of sets of experiments on ultramafic and mafic compositions under upper mantle conditions were compared with outputs from MELTS and Rcrust. Differences between the modelling and experimental results were quantified, and the Holland & Powell (2011) dataset combined with appropriately selected activity-composition models produced the best match with experimental results when melt fraction, melt composition, and the assemblage of coexisting phases were considered. The results demonstrate that modelling is a viable alternative to experimental techniques in investigating melting of the upper mantle. Thermodynamic modelling was used to map the envelope of uncertainty that exists around experimental results due to often unavoidable experimental pitfalls, and demonstrated that factoring in these uncertainties produced a better fit between modelling and experimental results. Next, this study developed a thermodynamic modelling methodology to investigate the stoichiometry of the melting reactions in an eclogite and peridotite as a function of pressure, temperature, and bulk compositional change due to melt loss. Pressure-temperature regions where the partial melting reactions produce peritectic crystals that increased in abundance with melt were identified. The entrainment of these crystals to the magma on segregation produces distinct shifts in the chemistry of the extracted magmas, for both sources. The results illustrate that variable amounts of peritectic crystal entrainment represents a previously unrecognised source of compositional heterogeneity in basaltic and intermediate magmas. The modelled results highlight how peritectic crystal entrainment may be a principal mechanism for creating the magmas that produce layered mafic and ultramafic complexes, because the entrained phases react with melt on ascent to produce magmas with a high crystal load once they reach the upper crust. During the partial melting of peridotite, the most common entrainable peritectic mineral is orthopyroxene, which is Cr2O3- bearing. On ascent, entrained peritectic orthopyroxene reacts to form olivine and chromite, and such magmas will enter high-level magma chambers as a melt-olivine-chromite slurry. Density segregation on intrusion allows the formation of magmatic layering if the magma body has appropriate geometry. Thus, these results provide an explanation for the presence of chromitite seams in layered mafic complexes, as well as average bulk rock Cr2O3 contents well above mantle values in some layered intrusions, such as the Bushveld Complex. Chromitite layers are a consequence of peritectic orthopyroxene entrainment and do not require any complex phase equilibrium magma chamber processes.en_ZA
dc.description.abstractAFRIKAANSE OPSOMMING: Termodinamiese modelleringstegnieke bied verskeie voordele bo eksperimentele studies in ondersoeke van die gedeeltelike smelting van mafiese en ultra-mafiese gesteentes onder boonste manteltoestande, maar is dalk nie so betroubaar soos eksperimentele studies om gesteentegedrag akkuraat te voorspel nie. Die twee hoof termodinamiese datastelle wat algemeen gebruik word vir sulke termodinamiese ondersoeke, het verskillende vlakke van dekking. Die Berman (1988) datastel word gebruik deur die MELTS-familie van sagteware (Asimow & Ghiorso, 1998; Ghiorso & Sack, 1995; Ghiorso et al., 2002), en saam met roetines vir die hantering van aktiwiteit-samestelling-verhoudings vir vaste en vloeibare oplossingsfases, is in die sagteware ingebed. Daarteenoor word die Holland & Powell (2011) datastel gebruik deur 'n wye reeks verskillende sagteware met verskillende berekeningstrategieë. Aktiwiteit-samestelling verhoudings word hanteer deur gepubliseerde modelle wat selekteerbaar en deursigtig is. Een van die programme wat die Holland & Powell (2011) datastel gebruik, Rcrust (Mayne et al., 2016), is ontwikkel om modellering van fasestabiliteite met veranderende massasamestelling moontlik te maak, wat dit besonder geskik maak vir die bestudering van prosesse wat fraksionering behels. In hierdie studie is die resultate van stelle eksperimente op ultra-mafiese en mafiese samestellings onder boonste manteltoestande vergelyk met uitsette van pMELTS en Rcrust. Verskille tussen die modellering en eksperimentele resultate is gekwantifiseer, en die Holland & Powell (2011) datastel gekombineer met toepaslik geselekteerde aktiwiteit-samestelling modelle het die beste vergelyking met eksperimentele resultate gelewer wanneer smeltfraksie, smeltsamestelling en die samestelling van saambestaande fases in ag geneem is. Die resultate toon dat modellering 'n lewensvatbare alternatief vir eksperimentele tegnieke is om die smelting van die boonste mantel te ondersoek. Termodinamiese modellering is gebruik om die omhulsel van onsekerheid wat bestaan rondom eksperimentele resultate as gevolg van dikwels onvermydelike eksperimentele slaggate te karteer, en het getoon dat die inrekening van hierdie onsekerhede 'n beter passing tussen modellering en eksperimentele resultate tot gevolg gehad het. Vervolgens het hierdie studie 'n termodinamiese modelleringsmetodologie ontwikkel om die stoïgiometrie van die smeltreaksies in 'n eklogiet en peridotiet te ondersoek as 'n funksie van druk, temperatuur en massasamestellingsverandering as gevolg van smeltverlies. Druk-temperatuur streke waar die gedeeltelike smeltreaksies peritetiese kristalle produseer wat in oorvloed met smelt toegeneem het, is geïdentifiseer. Die meevoer van hierdie kristalle na die magma tydens segregasie veroorsaak duidelike verskuiwings in die chemie van die onttrekde magmas, vir beide bronne. Die resultate illustreer dat veranderlike hoeveelhede peritetiese kristal-meevoer 'n voorheen onerkende bron van komposisionele heterogeniteit in basaltiese en intermediêre magmas verteenwoordig. Die gemodelleerde resultate beklemtoon hoe peritetiese kristal- meevoer 'n hoofmeganisme kan wees vir die skep van die magma wat gelaagde mafiese en ultra-mafiese komplekse produseer, omdat die meegesleurde fases met smelt reageer op styging om magmas met 'n hoë kristallading te produseer sodra hulle die boonste kors bereik. Tydens die gedeeltelike smelting van peridotiet is die mees algemene meevoerbare peritetiese mineraal ortopirokseen, wat Cr2O3-draend is. Met opstyg reageer meegesleurde peritetiese ortopirokseen om olivien en chromiet te vorm, en sulke magmas sal hoëvlak magmakamers binnegaan as 'n smelt-olivien-chromiet suspensie. Digtheid segregasie op indringing laat die vorming van magmatiese lae toe as die magma liggaam toepaslike geometrie het. Hierdie resultate verskaf dus 'n verduideliking vir die teenwoordigheid van chromietnate in gelaagde mafiese komplekse, sowel as gemiddelde grootmaatgesteentes Cr2O3-inhoud ver bo mantelwaardes in sommige gelaagde indringings, soos die Bosveld Kompleks. Chromitietlae is 'n gevolg van peritektiese ortopirokseen meevoer en vereis geen komplekse fase ewewig magma kamerprosesse nie.af_ZA
dc.description.versionDoctoralen_ZA
dc.format.extentxviii, 159 pages : illustrations (some color), mapsen_ZA
dc.identifier.urihttp://hdl.handle.net/10019.1/127184
dc.language.isoen_ZAen_ZA
dc.publisherStellenbosch : Stellenbosch Universityen_ZA
dc.rights.holderStellenbosch Universityen_ZA
dc.subject.lcshEarth (Planet) -- Mantleen_ZA
dc.subject.lcshThermodynamics modellingen_ZA
dc.subject.lcshThermodynamic equilibriumen_ZA
dc.subject.lcshUltramafic rocks -- South Africa -- Bushveld Complexen_ZA
dc.subject.lcshPetrogenesis -- South Africa -- Bushveld Complexen_ZA
dc.subject.lcshRustenburg layered suite -- South Africa -- Bushveld Complexen_ZA
dc.subject.nameUCTDen_ZA
dc.titleA phase equilibrium modelling investigation of the consequences of entrainment of components of the source on the crystallisation of mantle-derived magmas in the upper crust, with specific relevance to the petrogenesis of chromitite layers in the Rustenburg Layered Suite of the Bushveld Complex, South Africaen_ZA
dc.typeThesisen_ZA
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