ABSTRACT-0751844/0752256 (Langmuir/Michael) Intellectual Merit: This research explores the deep-seated mechanisms that underlie back-arc volcanism and how volcanic chains in these tectonic settings change over time and are influenced by their distance from a subduction trench. A full suite of geochemical data from rocks in the northeastern Lau Basin in the western Pacific Ocean will be analyzed for major and trace elements as well as for the content of various volatile species such as H2O and CO2 and the radiogenic isotopes: Sr, Pb, and Nd. Data will be used to test whether there are systematic variations in water content with distance from the trench and determine how back-arc volcanism is affected by the lack of a volcanic front. Geochemical data will also be used to examine whether subduction of the Louisville Ridge volcanic chain is contributing to regional volcanism. Data will be modeled to determine melting and magmatic source processes. Results of the work will be compared and contrasted to those from the major NSF-funded RIDGE 2000 study site in the Lau Basin to the South.
Broader Impacts: Broader impacts of the work include: collaboration between two institutions, one of which is in an EPSCoR state (Oklahoma), education and training of students and a postdoc, international collaboration, and the use of an NSF-funded multi-user ion probe facility at the Woods Hole Oceanographic Institution. Additional outreach to K-8 school children will also be undertaken.
In Plate Tectonic theory, cold, hydrated oceanic lithosphere is subducted at ocean trenches such as Tonga trench. As it descends into the mantle, the subducted lithospheric slab is dehydrated by metamorphic and igneous processes that lead to the eventual recycling of H2O and chemical elements back to the surface through volcanism at island arcs and submarine back arc spreading centers. Fonualei Rifts Spreading Centers (FRSC) and northeast Lau spreading centers (NELSC) are back arc spreading centers that were discovered recently in the northern part of the Tongan subduction system (Figure 1a). Rock samples were collected from 52 locations along FRSC and NELSC in 2003 and 2004 (Figure 1b). The goal of this project was to use geochemical analyses of the volcanic rocks to examine processes that occur in the subducted slab and the mantle wedge, leading to the formation of magmas in back-arc spreading systems. FRSC extends almost to the volcanic arc front at its southern end, where it appears to have captured the Tofua arc volcanism since there is no volcanic front for more than 150km in front of the FRSC. They join with the NELSC in the north (Fig. 1a). The spreading centers encompass a range of distances from the Tofua arc of 0-15km to 150km, permitting investigation of the change in subduction flux and mantle wedge composition with distance from the island arc and slab. FRSC-NELSC provides a contrasting experiment with the southern Lau Ridges (Eastern Lau Spreading Centers-Valu Fa ridge [ELSC-VFR]) where there is a similar distance range between arc and back-arc (Figure 1a). The contrasts between the systems are: the faster plate convergence rate in the north; lack of a volcanic front for portions of FRSC; and that changes in axial depth and ridge cross-section with spreading rate and distance from the arc are the opposite in the two spreading centers. Latitudinal variations reveal a correspondence in location and composition between back-arc (FRSC) and adjacent arc (Tofua) volcanism. The locations of spikes in subduction input and positive bathymetric anomalies along FRSC correspond to projected locations of arc volcanoes (Fig. 1b), likely reflecting 3-D convective structure of the mantle wedge. Where there is a high subduction input, as revealed by high Ba, the extent of melting becomes very large due to the fluxing effects of H2O. The extent of melting is even higher when the arc input is "captured" by the back arc spreading center (Fig. 2d). FRSC is an end-member for global back-arc lavas in terms of large extents of mantle melting, revealed by low contents of Ti and Na. As a result, a special type of magma: boninite is produced and may erupt (Fig. 2d). Non-mobile trace elements (e.g., Nb) and radiogenic isotopes in FRSC and Tofua arc lavas reveal an increasing proportion northward of a mantle source that was previously melted and then re-enriched by new melts (Fig. 2c). The enrichment continues to increase northwards into NELSC, where it reaches a peak 15.3S. It is likely derived from the Samoan mantle plume, which is a few hundred km from NELSC. Enriched samples have relatively low SiO2, suggesting that enriched magmas like alkali basalts are supplied to NELSC. This would show an important role for melt-melt mixing beneath the ridge. The work at The University of Tulsa also focused on the fate of recycled volatiles (Cl, F, S, H2O) in the subduction system. Our study of FRSC, and comparison with the ELSC-VFR, shows that significant amounts of H2O must be returned to the deep mantle, past the subduction zone. Deeply subducted H2O that gets past the arc and back arc is expected to facilitate global plate tectonics by "softening" mantle olivine, thus allowing the mantle to flow plastically over geological time scales. A further goal was to sample and analyze rare seamounts located on the west side of ELSC, to determine recycling pathways of subducted material to the surface: specifically whether mantle flow to ELSC intercepted input from the subducting slab so that it does not appear west of the axis. We found that ELSC intercepts the subduction-influenced magmas some of the time, but not in every case. An ancillary outcome of the study was a better understanding of the enrichment of economically valuable metals in magmas. Comparison of Sulfur loss (by degassing) and enrichment of Copper in glasses shows that shallow degassing of primitive magmas leads to early sulfur loss and lack of saturation of an immiscible sulfide liquid. This allows enrichment of Copper during the crystallization of magma. Copper could be further concentrated later by hydrothermalism . At least one mining company has contracts with Tonga for seabed mining in Lau Basin. The project supported a graduate student who completed a M.S. thesis. Her academic experience from this project helped her to launch a productive and satisfying career in the energy industry.
