Geochemistry of eastern Pacific MORB: Implications for MORB petrogenesis and the nature of crustal accretion within the neovolcanic zone of two recently active ridge segments

A detailed investigation of two recently volcanically active mid-ocean ridge (MOR) segments in the N.E. Pacific Ocean has resulted in a better understanding of crustal accretion within the neovolcanic zones of medium- to fast-spreading ridges. The use of submersible and other near-bottom systems, in conjunction with rock sampling by wax core, provides excellent spatial control, resulting in a database of precisely located in situ geologic observations and samples. The combination of dense high-resolution sampling, temporal control gained by recent eruptive activity, and multi-year monitoring efforts makes these investigations unique in MOR research.; The CoAxial Segment of the Juan de Fuca Ridge (JdFR) is a medium-spreading rate ridge that has experienced at least three volcanic eruptions between 1981 and 1993. Evidence of amagmatic extension and chemical heterogeneity between different lava flows indicates that the CoAxial Segment has behaved as a magma limited system, despite the occurrence of recent eruptive activity. The current CoAxial Segment neovolcanic zone is greater than 1 km wide, and spatial focussing of eruptive activity is poor relative to the more magmatically robust Cleft Segment of the JdFR and 9--10 N segment of the East Pacific Rise (EPR). A broader survey of the central JdFR reveals that, on a regional scale, geochemically different melt regimes can be associated with distinct structural, morphologic and volcanic provinces.; Detailed study of ridge-axis morphology, structure, and chemistry of the faster spreading, magmatically robust, 9°--10° N segment of the EPR shows a different relationship between ridge-axis structure and magmatic activity than that observed at the CoAxial Segment. Data from a third-order overlapping spreading center (OSC) located at 9° 37'N suggests that there is a close association between magmatic activity and axial discontinuities, and further suggest that this OSC is a magmatic and hydrothermal divide between adjoining third-order ridge segments. Temporal constraints correlate a recent magmatic event to southward propagation of the eastern OSC limb. These data establish a direct causal relationship between magmatic activity and evolution of ridge-axis discontinuities, and confirm that segmentation of melt delivery to the ridge-axis may affect ridge-axis structure along the fast spreading EPR.