Book Description

Magmatic processes control the chemical compositions of all lavas erupted at mid-ocean ridges. In this thesis, I present studies of magmatic processes on three different mid-ocean ridges to determine which processes are in action and to what extent each has affected the chemistry of mid-ocean ridge basalts at each location. On the Endeavour Segment, Juan de Fuca Ridge, major and trace element data from enriched and depleted lavas and melt inclusions indicate that lavas and melt inclusions are the results of partial melting of a heterogeneous source. Trace element models suggest that depleted lavas are formed from variable degrees of partial melting of a refractory harzburgite source, while enriched lavas result from very small degrees of melting of a clinopyroxenite source. Major and trace element data from axial and seamount lavas and melt inclusions from the northern East Pacific Rise indicate that chemical differences between axial and seamount magmas result from varying exposure to crustal and axial magma chamber processes. Seamount lavas and inclusions are more crystal rich and contain a greater number of inclusions that are generally more primitive and exhibit a larger compositional range in both the incompatible and trace elements. Seamount lavas leave the axial magma chamber before axial lavas, and thereby miss the further fractionation and crystal sorting. Major element data from Southeast Indian Ridge lavas suggest that the dominant control of MORB chemistry is mantle temperature. Lavas from this region range from high Na, low Fe in the east to low Na, high Fe8 in the west, suggestive of higher high Na8, low Fe8 in the east to low Na8, high Fe8 in the west, suggestive of higher pressures and extents of melting in the western part of the study area. Variable degrees and pressures of melting are consistent with a mantle temperature gradient which extends from hot mantle below the Amsterdam-St Paul hotspot to cold mantle below the Australian-Antarctic Discordance.