The influence of primary magma composition,H2O and pressure on mid-ocean ridge basalt differentiation

MORB suites display variations in their chemical differentiation trends which are closely related to the incompatible element enrichment of the basalts. We examine suites of primitive to evolved basalts from the Pacific-Nazca Ridge at 28° S (mostly depleted); from the Juan Fernandez microplate region (depleted) and from the Explorer Ridge, northeast Pacific (mostly enriched). Trends for incompatible element enriched MORBs consistently show less depletion of Al₂O₃ and less enrichment of FeO when plotted on MgO variation diagrams.Least squares modeling indicates that enriched basalts have undergone less plagioclase crystallization than depleted basalts especially in the early stages of differentiation. Using thermodynamic modelling, we show that variations between MORB differentiation trends result largely from differences in the major element chemistry and H₂O content of primary magmas. Our chosen enriched and depleted near-primary magmas are similar in major element chemistry but the enriched near-primary magma has higher H₂O and lower Al₂O₃ than the depleted near-primary magma. The MORB crystallization sequence is: olivineolivine+plagioclase olivine+plagioclase+high-Ca pyroxene; and the separate and combined effects of lower Al₂O₃ and higher H₂O are to cause plagioclase to crystallize later (lower temperature), and to make the interval of olivine+plagioclase crystallization shorter. As a result, enriched differentiates have higher Al₂O₃ and lower FeO than depleted MORBs at a given MgO content, even though their parents' Al₂O₃ is lower. Crystallization of enriched basalts at higher pressure than depleted basalts is not able to account for differences between the differentiation trends because the proportion of plagioclase is higher during three-phase crystallization at high pressure.The variations in trends do not depend on geographic location and thus are superimposed on any regional variations in MORB chemistry or mantle source. Nor are they related to spreading rate. Depleted basalts from the fast-spreading 28° S and Juan Fernandez ridges have differentiation trends similar to depleted basalts from the medium-spreading Galapagos Spreading Center, whereas differentiation trends for enriched basalts from the medium-spreading Explorer Ridge are quite different. Fe³⁺/Fe_total is similar (and quite low) for enriched and depleted basalts, indicating that neither oxidation state nor early magnetite crystallization are important.