Doug Toomey¹, Emilie Hooft¹, William Wilcock²

¹University of Oregon

²University of Washington

Overview: Tomographic imaging of spreading centers provides insights to mantle flow, the distribution of hot and partially molten rock at mantle and crustal depths, and segment-scale variations in crustal thickness.   Such constraints can be used to study a range of problems, for example:  1) the origin of tectonic, magmatic and hydrothermal segmentation of spreading centers, 2) the nature of melt focusing and storage at mantle and crustal depths, and 3) the location and structure of the thermal boundary layer that separates the magmatic and hydrothermal systems.  Seismic images – in combination with bathymetry, gravity and petrologic data – can also be used to infer the origins of axial depth anomalies, the temporal evolution of segment scale variations in melt production, and the degree to which magmatic differentiation may influence ridge crest processes. 

We have conducted a series of tomographic experiments at fast-, intermediate- and slow-spreading ridges, including the EPR and Endeavour ISSs and the MAR near 35˚N.  The results of these experiments allow us to compare and contrast mantle and crustal structure with spreading rate.  There are some remarkable comparisons:

• Mantle divergence at fast- and slow-spreading ridges is not aligned with the spreading direction. Data from a recent experiment at the intermediate-rate Endeavour segment will further test this unexpected relation.
• Similarly, the axis of mantle upwelling at the EPR, and possibly the MAR, is not aligned with the ridge axis, suggesting that skew of mantle upwelling is related to spreading center segmentation.
• Melt focusing at fast- and intermediate-spreading ridges is not efficient.  At both the EPR and the Endeavour segment, off-axis crustal magma bodies have been detected up to 20 km from the rise axis. In the case of the Endeavour, a preliminary analysis indicates that there are numerous off-axis crustal-level magmatic intrusions that underlie off-axis ridges and constructional volcanic features.
• At fast-spreading ridges, along-axis variations in axial depth are the result of crustal density anomalies which arise from systematic, along-axis variations in crystal fractionation depths.  Whether or not this process is operative at slower spreading rates remains to be examined.

As expected, there are also well known differences in structure of spreading centers as a function of spreading rate; such as segment-scale variations in crustal thickness as well as the depth and shape of crustal magma bodies. 

Integration and Synthesis:  One avenue for integration and synthesis is to develop a series of overview papers that are suitable for a general audience of earth and life scientists, particularly those at the beginning stages of graduate school.  Developing these papers in a way that separates “consensus” from “controversies” will allow the community and its newer members to evaluate progress as well as define directions for future research.  With regards to future research, our ability to fully understand the “mantle to microbe” theme of R2K is limited by a lack of geophysical constraints on mantle structure, particularly at depths associated with primary melting and at a scale that can address the fundamental question of why ridges are segmented.