Origin of high-Al N-MORB by fractional crystallization in the upper mantle beneath the Galلpagos Spreading Center

The Galápagos Spreading Center (GSC) includes lavas with chemical compositions ranging from N-MORB to transitional and more enriched MORB. N-MORB dominate the region west of 95.5°W, far from the influence of the Galápagos hotspot. However, some N-MORB glass samples from the GSC have very high Al contents (> 16.0 wt.% Al2O3 at > 8.5 wt.% MgO), similar to high-Al N-MORB from other slow and intermediate spreading ridges and close to fracture zones elsewhere. GSC high-Al N-MORB are dominated mineralogically by uniform plagioclase compositions (An 80–82) with only 1–2% olivine (Fo 85–87), and have glass compositions with higher Al2O3 and lower SiO2 than is predicted by normal MORB fractionation trends. Forward modeling using the MELTS and pMELTS algorithms constrained by crustal thickness measurements indicates that high-Al, low-Si MORB can be produced by high-pressure crystallization in the upper mantle using the same source as normal (low-Al) GSC N-MORB. Although high-Al glasses can be obtained by very low extents of partial melting of this mantle source, such melting models result in significant misfits in other major element oxides, especially SiO2. Models involving significant evolution with up to 20% olivine and clinopyroxene crystallization at pressures of 0.3–0.4 GPa can account for the complete major and selected trace element compositions of these unusual MORB samples. We suggest that high-pressure fractionation is enhanced by conductive cooling of the upper mantle in this area of the GSC, consistent with other recent models correlating mantle crystallization with slow spreading mid-ocean ridges and fracture zones.