Petrology and geochemistry of high degree mantle melts

Abstract

Experimental phase equilibria, whole rock major and trace element concentrations, and mineral major and trace element concentrations are used to constrain the petrogenesis of high degree, hydrous melts of the mantle, with particular focus on komatiites from the Barberton Mountainland, South Africa. Chapter 1 presents experiments on a Barberton komatiite composition under anhydrous and H20 saturated conditions. A comparison of the compositions of augite in the experiments with augite in the samples indicates that at least 4.5 wt.% H20 was present in the komatiite melts prior to emplacement. The presence of H20 in the magmas would allow them to be produced at lower temperatures than required by anhydrous models of komatiite genesis, and would obviate the need for extremely high temperatures in the Archean mantle. In Chapter 2, ion probe analyses of augite in Barberton komatiites are used to quantify the effects that metamorphism has had on the bulk rock compositions. The results indicate that high field strength elements and most rare earth elements were not significantly mobilized by metamorphism, while Eu and Sr were mobilized. Some Barberton magmas were enriched in light rare earth elements and Sr, and depleted in high field strength elements, which are the chemical characteristics of modem subduction related magmas. Chapter 3 presents melting experiments that explore the effect of H20 on melts in equilibrium with olivine and orthopyroxene at 1.2 to 2.4 GPa. The results of the experiments are used to infer the thermodynamic properties of H20 in silicate melts, and to construct a numerical model that predicts the composition of high degree mantle melts. The model is used to estimate the melting conditions that produced high-MgO andesites, boninites, and komatiites. It is shown that Barberton komatiites can be produced by melting at low pressures (2.4-3.0 GPa) and temperatures (1440-1500 °C). Chapter 4 demonstrates that basaltic komatiites overlap the compositions of modem boninites and display nearly identical trace element systematics. Komatiites are also shown to have numerous chemical similarities to boninites as well. It is proposed that komatiites and basaltic komatiites were produced by the same processes that produce modem boninites. The lack of komatiites in modem subduction zones is attributed to -100*C secular mantle cooling that has occurred since 3.5 Ga.