D. Troy Durant, Douglas R. Toomey and Paul J. Wallace

University of Oregon, Department of Geological Sciences
1272 University of Oregon, Eugene, OR 97403-1272, USA

Segment-scale axial depth variations along fast-spreading mid-ocean ridges like the East Pacific Rise (EPR) have traditionally been associated with magma supply and/or mantle temperature, yet geophysical observations along the northern EPR preclude these hypotheses.  Recent studies show instead that increasing axial depth along the EPR between the Clipperton and Siqueiros transforms correlates with an increase in both crustal density and offset of mantle upwelling with respect to the ridge axis.  The observed increase in axial depth can be explained by an ~0.1 g/cc increase in bulk crustal density.

We use thermodynamic modeling (MELTS) to show how density variations on the order inferred along the northern EPR can result from differences in crystallization depth. Regions where lower crustal formation occurs by deep crystallization have higher average crustal density than those where the lower crust is generated by subsiding cumulates originating within an upper-crustal, axial magma chamber (AMC). Our modeling results demonstrate that the density variations are attributed to differences in the crystallization sequence of major minerals at different pressures.

Because higher crustal density also correlates with regions of off-axis mantle upwelling, we suggest that lateral transport of melt to the rise axis, by up to 20 km or more, generates flow conditions favorable for deep crystallization. Moho-depth crystallization is inferred to occur during and/or post-migration, likely thickening the Moho transition zone. We also suggest that regions of rise-centered upwelling generate flow conditions favorable for shallow crystallization, where mantle melt ascends more efficiently to the AMC before any significant fractionation can occur. Our modeling results quantitatively link segment-scale variations in axial depth and crustal density to segment-scale variations in crystallization depth. In this view, axial highs are associated with magmatic systems that crystallize melt preferentially within upper-crustal magma bodies. Conversely, along-axis deeps are associated with magmatic systems that have significant near-Moho crystallization, a condition that we attribute to off-axis delivery of mantle melt.