Rcrust : a tool for calculating path-dependent open system processes and application to melt loss

Mayne, Matthew Jason (2016-03)

Thesis

ENGLISH ABSTRACT: Earth‘s continental crust is stabilised by crustal differentiation that is driven by partial melting and melt loss: Magmas segregate from their residuum and migrate into the upper crust, leaving the deep crust refractory. Thus, compositional change is an integral part of the metamorphic evolution of anatectic granulites. Current thermodynamic modelling techniques have limited abilities to handle changing bulk composition. New software is developed (Rcrust) that via a path-dependent iteration approach enables pressure, temperature and bulk composition to act as simultaneous variables. Path-dependence allows phase additions or extractions that will alter the effective bulk composition of the system. This new methodology leads to a host of additional investigative tools. Singular paths within Pressure-Temperature-Bulk composition (P-T-X) space give details of changing phase proportions and compositions during the anatectic process, while compilations of paths create path-dependent P-T mode diagrams. A case study is used to investigate the effects of melt loss in an open system for a pelite starting bulk composition. The study is expanded upon by considering multiple P-T paths and considering the effects of a lower melt threshold. It is found that, for the pelite starting composition under investigation, open systems produce less melt than closed systems and that melt loss prior to decompression drastically reduces the ability of the system to from melt upon decompression.

AFRIKAANSE OPSOMMING: Korsdifferensiasie stabiliseer die kontinentale kors van die aarde tydens gedeeltelike smelting. Magma segregeer van hul residuum en migreer in die vlakker kors in. As gevolg daarvan raak die diepkors meer vuurvas. Dus speel komposisionele verandering ‗n belangrike rol in die evolusie van anatektiese granuliete. Hedendaagse termodinamiese modelleringstegnieke het beperkte vermoëns om 'n veranderende grootmaatsamestelling te hanteer. Nuwe sagteware is ontwikkel (Rcrust) wat 'n roete-afhanklike iterasie benadering volg. Hierdie metode laat die druk, temperatuur en grootmaatsamestelling toe om as gelyktydige veranderlikes te funksioneer. Roete-afhanklikheid laat fasetoevoegings of ekstraksies toe om die effektiewe grootmaatsamestelling van die stelsel te verander. Die nuwe metode bied ‗n magdom verskeie maniere aan om ondersoek in te stel. Enkel roetes van P-T-X ruimte beskryf die fase proporsies en komposisies tydens die proses anatektiese, terwyl kombinasies van roetes, roete-akhanklike pseudosections skep. 'n Gevallestudie is op uitgebrei om die gevolge van smeltverlies te ondersoek. Dit is bevind dat oop stelsels minder produktief by smelt vorming is as geslote stelsels, en dat dekompressie smelt minder produktief as verwarming is. Die verlies in smelt produktiwiteit van die oop stelsel impliseer beperkings op die maksimum massa dekompressie smelt wat kan vorm. Tektoniese modelle wat dekompressie smelt as bron van groot volumes smelt gebruik moet dus herevalueer word.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/98358
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