| Partial melting of the mantle occurs predominantly in the spinel lherzolite stability field, where pyroxenes are consumed to produce olivine and basaltic melt. The phase equilibria of this reaction are relatively well understood but the grain-scale processes through which pyroxenes undergo partial melting are not. With a combination of kinetic experiments and numerical modeling, I demonstrated that during partial melting of lherzolites of varying bulk compositions, lherzolites do not simply melt at equilibrium but go through a series of kinetic process that involve primarily dissolution and reprecipitation. The most important feature of these grain-scale processes is the reduction in equilibration times between melt and minerals by several orders of magnitude, suggesting that melting in the mantle might take place closer to equilibrium conditions than currently believed. These results are presented in Chapters 1, 2, and 4.;In Chapter 3, I explored the nature of the interface between peridotites and pyroxenites in the mantle to understand the mechanisms controlling the style and extent of chemical interaction between these two lithologies. The motivation behind this study is to determine the scale and evolution of heterogeneities in the Earth's mantle and to determine their role in basalt petrogenesis. My dissolution experiments demonstrated the existence of two melt-rock reaction regimes depending on whether the lherzolite is subsolidus or partially molten. At subsolidus conditions, dissolution is extremely slow and equilibration between the pyroxenite-derived melt and peridotite is unlikely. When the lherzolite is partially molten, the dissolution mechanism changes from simple dissolution to reactive dissolution (involving diffusive mixing, dissolution, and reprecipitation of several phases) and resulting in significant increase in equilibration and dissolution rates. Using these experiments I was able to derive kinetic parameters that I subsequently introduced into a reactive porous flow model where I explored the fractionation of incompatible trace elements during melt transport. The results of this simulation suggest that if enriched signals in ocean-floor basalt originate from pyroxenite-derived melts, then the melt migration rate through the uppermost mantle must be very fast (on the order of mm to cm/sec). |
