The crystallisation history of normal mid-ocean ridge basalts from the Eastern Pacific Ocean and implications for the composition of primary mid-ocean ridge magmas : evidence from mineralogy, pillow-rim glasses and melt inclusion studies

An understanding of MORB petrogenesis requires compositional data on primary melts, as these provide direct information on the PT-conditions and mechanisms of mantle melting and melt segregation beneath mid-ocean ridges. The reconstruction of primary melts from the compositions of erupted liquids (pillow-rim glasses) requires an understanding of the early crystallisation history of the primary melts. As recent models of MORB petrogenesis imply that not only early crystallisation, but, also extensive mixing of primary melts occurs, information on the early stages of evolution of the primary melts is even more crucial. Such information may be preserved by early formed phenocrysts and can be obtained by studying these phenocrysts and their melt inclusions. However, the ability of melt inclusions to preserve the compositions of trapped liquids needs to be examined (e.g., homogenised melt inclusions in cotectic phenocrysts should have the same compositions, and those trapped during a period of fractionation, as recorded by the pillow-rim glasses, should have compositions similar to the glasses). The three MORB suites described in this thesis are from ODP/DSDP Holes 896A and 504B, Costa Rica Rift, and Sample D9-1 from the Gorda Ridge, in the east Pacific Ocean. Compositional and textural variations of phenocrysts (olivine, plagioclase and spine!) have been used to interpret the crystallisation history for each suite and, together with pillow-rim glass compositions, form the basis for interpreting the compositional variations of melt inclusions. Samples from the Costa Rica Rift and the Gorda Ridge have phenocryst assemblages dominated by calcic plagioclase (>An\\(_{88}\\)) with lesser olivine (