The origin of ultrapotassic igneous rocks

This thesis consists of a review of ultrapotassic igneous rock occurrences and three experimental programs designed to examine the petrogenesis of the lamproites. A definition for ultrapotassic rocks is introduced using the whole-rock chemical screens K20>3 wt%, MgO>3 wt% and K20/Na20>2. Three major end-member groups are recognised; Group I (lamproites) are characterised by low CaO, A1203 and Na 2 0, high K 2 0/A1203 and Mg-number, and extremely high incompatible element contents; Group II have low Si0 2 and high CaO, and lower incompatible elements than group I although they have high relatively Sr; Group III rocks occur in orogenic areas and have high CaO and A1203, and low Ti0 2 , Nb and Ba typical of island arc rocks. Primary magmas-for all three groups probably originate by partial melting of mantle material enriched in incompatible elements. The chemical signatures of the groups indicate differences in (i) source composition prior to enrichment, (ii) the chemical nature of the enriching agent, and (iii) pressure-temperature conditions of melting. The liquldus mineralogy of a pristine, primary leucite lamproite from-- Gaussberg, Antarctica, was studied at .1 atm with controlled f0 2 , oxygen fugacity at the time of crystallisation of the Gaussberg rock is shown by ferric value [10OFe 3 /(Fe 3 +Fe2 )] of spinel, Fe 2 03 content of leucite and Mg-number of olivine, to have been just below NNO. Application of the spinel ferric value calibration to other lamproites indicates that they began to crystallise at f0 2 ranging from MW to above NNO. The ferric value of spinel is very sensitive to changes in oxygen fugacity, and may prove useful as a diamond survivability indicator': diamonds are unlikely to survive in the more oxidised lamproite magmas. The effect of fluorine, an important constituent of ultrapotassic rocks, on phase relationships in the kalsilite-forsterite-quartz system was studied at 28kbar. Fluorphiogopite is found to be stable to 300 0C higher than hydroxyphlogopite, and the peritectic point PHL+EN+F0+L, which can be used to model melting of a mica-harzburgite mantle, lies at an equally magnesian composition. Fluorine acts as a melt polymerising agent as shown by the expansion of the enstatite phase volume relative to forsterite and by FTIR spectroscopic studies. Fluorine forms bonds with network modifying cations and removes KA10 2groups from the aluminosilicate network, causing an increase in Si/(Si+Al) in the network. However, in the presence of water fluorine will appear to depolymerise melts due to the action of OH released by HF dissolution; the viscosity will be lowered by fluorine in either case due to the formation of fluoride complexes. A model is developed for the origin of lamproitic magmas by partial melting of a mica-harzburgite mantle in a reduced environment in the presence offluorine. Lamproitestypically carry depleted mantle nodules and have H20-and F-rich, but CO 2-poor compositions. Primary lamproite magmas appear to range in silica content from around 40 wt% (olivine lamproites) to at least 52 wt% (leucite lamproites). In a reduced mantle (f02 1W to IW+2 log units) CH4 will be the dominant carbon species in fluids, and CO2will be very rare even in a carbon-rich environment. CH4 also acts as a depolymeriser, so that production of silicic melts will be optimised in a reduced, fluorine-rich mantle. Olivine lainproites may be produced by melting of a similar composition at higher pressures. Calculations show that oxidation from the proposed reduced conditions at source to observed surface oxidation states can be achieved by dissociation of only 0.1 wt% H20 driven by diffusive loss of H2. Silica-poor rocks of Group II may originate in an oxidised environment with abundant CO2but little H 2 0. Fluorine will maintain a large phase field for mica in these conditions so that initial melts will be magnesian and strongly silica-undersaturated. A technique is developed for liquidus experiments at high pressures in the presence of reduced H20>CH 4fluids. Two lamproite compositions were studied by this technique to test the hypothesis outlined above. The olivine lamproite has olivine as the liquidus phase at all pressures studied (up to 40 kbar), but the increasing stability of orthopyroxene+ mica with pressure indicates that there may be a OL+OPX+PHL point at the liquidus between 45 and 55 kbar. This is consistent with the occurrence of diamonds in olivine lamproitës. The leucite lamproite has liquidus fields for olivine, mica and orthopyroxene with increasing pressure, but has no point where the three coexist. These phase relationships can be interpreted to fit the mica-harzburgite melting model (with melting at 20 kbar) if minor olivine fractionation occurs at high pressures, or possibly if the water content of the source differs from that of the experiments. Thus, pressure variation may be the principal control of lamproite chemistry. Several experiments with variable CH4 /H2 0 or H20/CO2 fluids enable comparison of melting behaviour at varying f0 2 . At very low f02, melting temperatures are increased due to lowered water activity, but mica stability is increased due to its higher F/OH