This project addresses physical processes associated with melting and melt segregation in the mantle, and how these processes affect the geochemical properties of erupted lavas. It is a natural extension of earlier work on mantle convection in that there is a wealth of geochemical data with which to constrain and test various aspects of mantle dynamics, but in order to effectively use this data one must first understand how melts relate to their mantle source both physically and chemically. A two-dimensional finite element numerical code will be used to study a hierarchy of two-phase flow models involving melting and melt segregation: (i) Melt segregation and trace element fractionation from a chemically uniform source with spatially variable degree of melting. The purpose is to illustrate and understand the complexity, including time dependence, of the trace element concentrations in the "melts" segregating from such a source. (ii) Melt segregation and mixing from a source having heterogeneity in both chemistry and degree of melting. This model will be applied to the time dependent geochemical properties of lavas from Hawaii and also as a framework for modeling melt segregation for a source with ubiquitous heterogeneities, as seems to be required by the geochemistry of near-ridge seamounts. (iii) Models for melting and melt migration under midocean ridges. This type of model will be used to study the implications of crustal thickness and major element chemistry being quite insensitive to spreading rate while the variability of trace element ratios and isotopic ratios is related to the rate of spreading. The questions of how much melt erupts as opposed to underplating the otherwise depleted lithosphere is also considered.
