Compositional variability and petrogenesis of high-silica lavas at ridge axis discontinuities on EPR ISS: What they can tell us about sub-axial processes at mid-ocean ridges?

D. Wanless¹*, M. Perfit¹, I. Ridley², E. Klein³, & S. White⁴

Corresponding author: dorseyw@ufl.edu
¹University of Florida, Department of Geological Sciences, Gainesville, FL, 32601
²U.S. Geological Survey, Denver, CO, 80225
³Duke University, Nicholas School of the Environment, Durham, NC, 27708
⁴University of South Carolina, Department of Earth and Ocean Sciences, Columbia, SC, 29208

Abstract:
Determining sub-axial magma supply, distribution, and transport beneath mid-ocean ridges (MOR) is important to our overall understanding of heat and magma flux from the mantle to the seafloor at MOR. Of particular interest are ridge discontinuities and how these surficial ridge segmentations translate to mantle processes and sub-axial magma chamber dynamics. While the majority of lavas erupted on-axis at MOR have relatively homogeneous mid-ocean ridge basalt (MORB) compositions, magmas produced at MOR discontinuities can have a range of compositions including both N and E-type MORB and basalts, FeTi basalts, andesites and dacites. Here, we compare lavas from three different ridge discontinuities (8°37’ N, 9° N, 9°37’ N) within the ISS study site to determine how ridge segmentation controls or contributes to magma petrogenesis on MOR. Traditional models suggest that highly evolved lavas on MOR form through extreme crystal fractionation of MORB parent in magma-starved regions or at the cold edges of transform faults. While this process may account for compositions ranging from basalts to basaltic andesites, it is difficult to produce the observed trace element patterns and extreme incompatible element enrichments in the MOR dacites through crystal fractionation alone. Instead, a combination of extreme crystal fractionation and assimilation of crustal material can account for the geochemical variability observed at ridge discontinuities. Furthermore, the presence of a large melt lens beneath at least one of these regions (9° N) could indicate that magmatism is high rather than diminished, suggesting that increased or episodic magmatic flux may be linked to the formation of dacites on spreading centers. Surficial ridge segmentation may also be linked to slight but significant variations in magma source regions. Isotopic evidence suggests that each limb from the large 9° N OSC taps different source compositions. The west limb of the OSC has Pb isotopic ratios intermediate between the 8°37’ N source and the 9°50’ N source, while the east limb has ratios indistinguishable from 9°50’ N. This suggests that large second order ridge discontinuities may reflect both mantle and crustal segregations.

Contributions to Integration and Synthesis:
The combination of several different data sets within the ISS study site has allowed us to both compare magmatic processes at specific ridge discontinuities to determine how sub-axial process affect lava compositions and to examine the link between ridge segmentation and mantle processes. The next step will involve integrating existing geophysical data to understand how high-silica lavas are related to magma flux. At the 9° N site, lava samples were collected in combination with a detailed side-scan sonar survey and can be compared to existing geophysical data from the ISS site to provide an in depth understanding of the magmatic plumbing system associated with dacite formation on MOR. In addition, we plan to compare the ISS study site dacites to other ridges with high-silica lavas. The 9° N lavas show a wide and continuous range of compositions from basalts to dacites, providing the most complete and well constrained data set of high-silica lavas on any MOR. The geochemical similarity of MOR high-silica lavas suggests similar processes are controlling their formation; therefore, inferences made using the combination of several data sets collected within the ISS study site can be applied to the formation of high-silica lavas at other, less studied MOR locations.