Mantle-Derived Magmas - Roles of Variable Source Peridotite and Variable C-H-O Fluid Compositions

The system forsterite-nepheline-quartz is a useful simple system analogue of melting relations in upper-mantle peridotite. The liquidus phase fields at 28 kbar differ from those at low P by expansion of the enstatite field at the expense of forsterite. The system illustrates a large field of liquid compositions, from model basanites to model quartz tholeiites, which can be derived from one peridotite source. The effect of C-H-O volatiles on melting relationships has been explored with H2O, CH4 and CO2-vapour saturated experiments. The effect of water is to expand the olivine field and depress liquidus T by 350-400oC, but liquids at low degrees of melting of a model peridotite remain Ne- normative. The effect of CO2 is most marked with liquids moving to increasingly undersaturated compositions. Methane saturation produces a similar liquidus depression but results in OH^- solution, low carbon solubility and a reduced melt structure, i.e. Si:O <1:2. The studies in the simple Fo-Ne-Q system are matched by melting studies of several peridotite compositions and by liquidus studies on a variety of magnesian primary magmas from different tectonic settings.-J.M.H.