Origin of High-Al N-MORB by Fractional Crystallization in the Upper Mantle Beneath the Galapagos Spreading Center

The Galàpagos 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 96°W, far from the influence of the Galàpagos hotspot. However, some N-MORB glass samples from the GSC have unusually high Al contents (>16.0 wt.% Al2O3 at >9.0 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 79-81) 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 pMELTS and MELTS algorithms constrained by crustal thickness measurements indicates that normal (low-Al) GSC N-MORB can be produced from a very depleted source (Na2O ~ 0.17 wt.%, K2O ~ 30 ppm, TiO2 ~ 0.10 wt.%) with an Fmax ~ 0.19 and a potential temperature of ~1320°C, followed by shallow crustal fractionation. Although high-Al contents 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. However, models involving significant evolution with up to 20% olivine and clinopyroxene crystallization at pressures of 0.3-0.5 GPa can account for the complete major element compositions of these unusual MORB samples. We suggest that high-pressure fractionation is enhanced by inhibited melt extraction in the uppermost 10-15 km of upper mantle close to fracture zones along slow and intermediate spreading mid-ocean ridges. In such cases, lavas carrying the chemical signature of high-pressure evolution can be erupted where well-developed, steady-state crustal magma chambers are lacking. Although high-pressure fractionation may also occur beneath fast-spreading ridges, steady-state magma chambers at these ridges would act as a compositional buffer and reduce the high-Al chemical signature from this process. The exceptionally depleted nature of the source mantle for Galàpagos N-MORB is consistent with it having undergone prior melting, either beneath the East Pacific Rise or possibly at the Galàpagos hotspot.