White Paper Title: 
Geochemical Diversity of Near-Ridge Seamounts: Insights into Oceanic Magmatic Processes and Sources


Geochemical Diversity of Near-Ridge Seamounts: Insights into Oceanic Magmatic Processes and Sources 
N. L. Baxter1; M. R. Perfit1; C. Lundstrom2; D. A. Clague3 
1. Geological Sciences, University of Florida, Gainesville, FL, United States. 
2. Geology, Univeristy of Illinois Urbana-Champaign, Urbana, IL, United States. 
3. Monterey Bay Aquarium Research Institute, Moss Landing, CA, United States. 


Geochemical studies of lavas erupted at near-ridge seamounts may provide an opportunity to better understand the composition of shallow mantle beneath spreading ridges and some of the melting processes that occur to form oceanic crust. While on-axis samples generally reflect homogenization of melts within the axial magma lens, seamount lavas bypass this process, providing a window into the diversity of melts produced in the melting column. By studying near-ridge seamount lavas, we hope to better understand the effects of mantle heterogeneity and melt generation/transport on the geochemistry of lavas erupted in these settings.

We have analyzed lavas from the Lamont Seamount chain adjacent to the EPR (~9° N), the Vance Seamount Chain adjacent to the JdF ridge (~45° N), as well as several individual seamounts/cones located off the JdF ridge. Lavas from these seamounts have incompatible trace element patterns varying from very depleted to moderately enriched (found at the oldest, most distant Vance seamounts) relative to typical mid-ocean ridge basalts (MORB). Overall, the Vance and Lamont seamount lavas are more primitive and diverse than associated ridge samples. These variations can be explained by different degrees of melting and mixing of multiple sources The significant variations in incompatible trace element and isotopic compositions that are somewhat correlated suggest that the mantle underneath the seamounts is heterogeneous on a small scale. Our results support a “veined mantle” model for the Northeastern Pacific mantle and the mantle underneath the EPR.

We are particularly interested in how dissolution-precipitation reactions that are hypothesized to occur during melt transport in the mantle might relate to the diverse trace element compositions found in these seamount chains. We are working on trace element modeling using MELTS and IRIDIUM to see if the creation of dunite conduits through dissolution of CPX and precipitation of olivine in depleted mantle lherzolite can explain the trends seen in these seamounts. By better understanding the processes that are occurring to form these seamounts, we hope to be able to compare them to lavas produced at the ridge, in order to better understand the ridge system as a whole, both on- and off-axis.

Questions and discussion ideas:

Do similar off-axis (vs. back arc) seamounts exist near the Lau spreading center?

There is only minimal evidence for any hydrothermal activity on these seamounts. Is this a consequence of  non-steady state magma chambers and melt production. Is the lack of observed hydrothermalism reflected in the chemistry of the lavas (low Cl?).

What is the geophysical evidence for and constraints on the locations and extents of axial and off-axis melting?

What mechanism can cause these often linear chains to form? How is the “excess” heat generated to cause melting if the mantle has been previously depleted at the ridge crest? What does the significant volume of these features tell us about the melting process?

Recent studies on slow spread ridges (and ophiolites) indicate there is a significant amount of melt rock interaction in the upper mantle and lower crust (including the formation of dunite veins). What evidence (for and against) is there for this at the ISS sites?