Use this URL to cite or link to this record in EThOS: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.650892
Title: The chemical and isotopic nature of fluids associated with alkaline magmatism, South Greenland
Author: Finch, Adrian Anthony
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 1990
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Abstract:
Alkaline centres of the Gardar province, South Greenland, show evidence for subsolidus alteration of the consolidated rocks by associated metasomatic fluids. Oxygen isotopic values of metasomatised and unmetasomatised samples alike lie in the range 4-7%, and indicate that metasomatic fluids were juvenile in origin, evolved from the silicate magmas themselves after final crystallisation. The fluid catalysed catastrophic coarsening in alkali feldspars at temperatures below 450oC, and at lower temperatures, also brought about changes in their cathodoluminescence (CL) colours, altering blue luminescing feldspar to a red-luminescent variant. Ratios of blue to red-luminescing alkali feldspar have demonstrated that up to 96% of the volume of some syenitic rocks have undergone reequilibration with the metasomatic fluid. F-contents of biotites from across the Gardar have been examined to determine fluorine contents of the metasomatic fluids using the method of Munoz (1984). Undersaturated magmas were associated with more fluorine-rich fluids than those associated with oversaturated magmas. The most F-rich (Late Igdlerfigsalik) contained approximately four times as much available fluorine as the least F-rich fluid (Younger Giant Dyke Complex). Compositions of fluids associated with centres across the province can be related to the compositions of their original magmas. Oversaturated magmas are associated with CO2-bearing, relatively C- and F-poor fluids with pHs at, or near to, neutrality. Redox potentials are relatively oxidising. Undersaturated miaskitic magmas are associated wih CO2-bearing relatively C- and F-rich fluids: pHs are significantly alkaline and redox potentials are more reducing than in oversaturated magmas. Agpaitic magmatism is associated with CH4-bearing, highly reduced, F-rich fluids and pHs are more alkaline than in miaskitic centres. Low redox potentials are initially brought about by the crystallisation of magmatic aegirine, which removes iron as Fe3+ against the prevailing redox potential in the magma. This process causes progressively reduced redox potentials in the remaining magma and its associated volatiles. Since the oxidation state of the evolved fluid depends on the amount of aegirine crystallised, fluids become more reducing with increasing silica-undersaturation, and are most reducing in fluids from agpaitic magmas. pH and redox potential are related by the equation: CH4 + 8 OH- + 6 H2O + 8e- Zr- and Hf-mobility are observed in some fluids if the activity of F is high enough, due to the formation of polyfluoro-zirconate(IV) and -hafnate(IV) complexes. RE-mobility is observed from all of the fluids studied since the activity of either CO2 or F is sufficient in all cases.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.650892  DOI: Not available
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