Mantle dynamics, magma and crust formation in an oceanic island arc - back-arc system: a case study from the Tonga arc and Valu Fa Ridge
Final Report Abstract
The 1200 km long Tofua island arc can be divided into four segments based on structural observations of the occurrence of the volcanoes on the upper plate and on the distribution of seismicity. Most volcanoes have relatively homogeneous lava compositions that indicate magma formation from a well-mixed source over the life-time of the volcano. Lavas within each segment typically show distinct geochemical signatures implying distinct effects on magma formation within each segment. The North segment volcanoes erupt lavas depleted in TiO2 and Na2O indicating a highly depleted mantle source due to multiple melting events of the mantle wedge. The magmas in this segment are affected by subduction of pelagic sediments and some show input of a component from subduction of the Louisville Seamount Chain. The northernmost magmas are also affected by melting of Samoan plume mantle and this hot mantle may cause the melting of subducting sediments. The Central and Louisville segment lavas largely reflect mixing of slab component from subducted Pacific MORB, most likely a hydrous fluid, with little evidence of melts from pelagic sediments. In contrast, the South segment shows significant melting of subducted sediments, possibly due to the overlap of the Valu Fa back-arc melting zone with that of the Tofua island arc. The back-arc magmas show no simple binary mixing with the slab components observed in the associated arc front lavas, but rather show less input of the MORB-derived fluid but more influence of the sediment melt. Thus, they are more affected by the deep hot component than by the shallow slab component. The lavas in the island arc front typically show similar 176Hf/177Hf to the associated back-arc lavas but lower Nb/Zr indicating a depletion of the mantle flowing from the west to the east underneath the island arc. This supports the transport of mantle from the back-arc to the island arc melting region. However, incompatible element ratios including fluid-mobile elements and Sr and Pb isotopes differ significantly between lavas from arc and back-arc implying a different input form the slab. The volumes of Tofua island arc volcanoes are between 20 and 170 km3, implying that their mantle source regions may have volumes of 200 to 2000 km3 assuming at least 10% partial melting. The small-scale heterogeneity of the mantle wedge with distinct compositions of lavas on <10 km distance but the homogeneity of lava sequences within single volcanoes suggests very efficient mixing of slab and mantle material on time scales of 100,000 years. Additionally, the Valu Fa back-arc basalts show a similar input of sediment-derived melts into their melting region as indicated, for example, by high (La/Sm)N, Th/Nd, and 208Pb/204Pb, but the lower Ba/Th than adjacent arc front lavas from Ata island indicate less input of MORB-derived fluid. This also indicates that magma ascent as well as the transport of slab-derived components vary on small scales of <10 km, i.e. there is no evidence for ascending melt diapirs (“hot fingers”) with diameters of 50 km in the Tofua island arc, unlike the subduction system in Japan. Rather, the melting regions must be smaller with distinct ascent pathways of the magmas from the mantle to the crust without interaction with neighbouring magma systems. The magmas erupting in the South Tofua island arc segment and associated Valu Fa back-arc range from basalt to rhyolites but they follow different trends indicating less Fe enrichment in the island arc front magma systems. The magmas in the island arc front show crystallization of FeTi-oxides at 2 wt.% MgO, later than those in the back-arc (4 wt.% MgO) which implies a higher fO2 in the island arc magmas. Because the MORB-derived slab fluid apparently has a stronger effect on the island arc magmas than on the back-arc melts, it may be that this fluid largely causes the difference in the oxidation state of the melts.
Publications
- 2021. Effect of fluid boiling on volatile element and Au enrichment in submarine hydrothermal sulphides, Niua South, Tonga arc. Geochimica et Cosmochimica Acta 307, 105-132
Falkenberg, J.J., Keith, M., Haase, K.M., Bach, W., Klemd, R., Strauss, H., Yeo, I.A., Rubin, K.H., Storch, B., and Anderson, M.O.
(See online at https://doi.org/10.1016/j.gca.2021.05.047) - 2022. Effects of the hydrous domain in the mantle wedge on magma formation and mixing at the Northeast Lau Spreading Centre, SW Pacific. Geochemistry, Geophysics, Geosystems 23, e2021GC010066
Haase, K.M., Schoenhofen, M.V., Storch, B., Beier, C., Regelous, M., Rubin, K., and Brandl, P.A.
(See online at https://doi.org/10.1029/2021GC010066) - 2022. Extreme geochemical heterogeneity beneath the North Tonga Arc: interaction of a subduction zone with intraplate seamount chains. Chemical Geology 603, 120903
Storch, B., Regelous, M., Noebel, K.M., Haase, K.M., and Bauer, J.
(See online at https://doi.org/10.1016/j.chemgeo.2022.120903)
