Poster Abstract Title: 
Mid-Ocean Ridge Dacites
Authors and their affiliations: 
Wanless, V. D. (1, 2), Perfit, M. R. (1), Ridley, W. I. (3), Klein, E. M. (4); (1) University of Florida, (2) Woods Hole Oceanographic Institution, (3) National Science Foundation, (4) Duke University

With increasing exploration of the seafloor, it is becoming apparent that high-silica lavas are an important component of the mid-ocean ridge system.  While volumetrically small compared to mid-ocean ridge basaltic lavas, andesites and dacites have erupted at several different spreading centers; including, the Juan de Fuca Ridge, Galápagos Spreading Center, and East Pacific Rise.  These lavas are typically associated with ridge discontinuities and/or regions of episodic magma supply. Although the high-silica lavas have erupted at different ridges, they show remarkably similar major element trends and incompatible trace element enrichments, suggesting that similar processes control their formation. MOR dacites are geochemically characterized by 1) elevated U, Th, Zr, and Hf; 2) relatively low Nb and Ta; 3) Al2O3, K2O, Cl, H2O concentrations that are higher than expected from fractional crystallization; 4) relatively low δ18O glass values of ~5.6 compared to values ~6.9 ‰ expected from fractional crystallization.  This suggests that crustal assimilation is an important process in their petrogenesis.  Petrologic modeling using AFC formulations suggest that low degree partial melting and assimilation of altered basaltic crust into crystallizing magma chamber can explain the geochemical signatures of the MOR dacites.  The role that this process may have in typical MOR environments and in the formation of MORB lavas, however, is the subject of continuing debate.


Contributions to Integration and Synthesis: 
While the majority of lavas erupted at mid-ocean ridges are basaltic in composition, high-silica lavas have been sampled from numerous different spreading centers; including, East Pacific Rise, Galapagos Spreading Center, Juan de Fuca Ridge, and most recently at Lau Spreading Center. Many of these lavas have similar incompatible trace element patterns on mantle normalized diagrams, suggesting that similar processes control their geochemistry. This study provides a detailed examination of the petrogenesis of dacitic lavas at the 9°N EPR, which is likely applicable to the formation of high-silica lavas at other ridges.