White Paper Title: 
Role of Assimilation in Mid-Ocean Ridge Magmatism

Results from our investigations at the 9°N overlapping spreading center may be important to numerous different disciplines within the MOR community and to several different ISS study sites. Although most geochemical variability in MOR basalts is consistent with low-pressure fractional crystallization of various mantle-derived parental melts, our geochemical data from MOR dacitic glasses suggest that assimilation of altered ocean crust is important in their petrogenesis and may be important in the formation of evolved gabbroic rocks and lavas at MOR in general.  Thus, combining and comparing ideas of magma-rock interaction from our studies with observations from typical MORs may provide new insights into ocean spreading center magmatism. 

Assimilation of crustal material requires latent heat from fractional crystallization to partially melt the surrounding wall rock. The heat required to melt the wall rock is highly variable depending on the temperature of the surrounding crust. Therefore, integration of our ideas and petrologic models of combined assimilation –fractional crystallization (AFC) processes with models of hydrothermal circulation on MOR is important in constraining thermal models for cooling of the ocean crust. 

Finally, our study can be directly compared to and integrated with data collected other ISS study sites, particularly the Lau Spreading Center.  High-silica lavas in MOR settings are rare compared to basaltic compositions; however, andesites and dacites have erupted on several ridges.  Our studies suggest the petrogenesis of high-silica lavas at spreading centers requires both partial melting and assimilation of crustal material.  The extensive petrologic and geochemical data from the 9°N OSC can be compared to the data obtained from at Lau Spreading Center to determine the role that assimilation plays in back arc settings and in the development of high-silica lavas in particular.