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Mantle flow, volatiles, slab-surface temperatures and melting dynamics in the north Tonga arc–Lau back-arc basin

Caulfield, John, Turner, Simon, Arculus, Richard, Dale, Chris, Jenner, Frances, Pearce, Julian A., MacPherson, Colin and Handley, Heather 2012. Mantle flow, volatiles, slab-surface temperatures and melting dynamics in the north Tonga arc–Lau back-arc basin. Journal of Geophysical Research: Solid Earth 117 (B11) , B11209. 10.1029/2012JB009526

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The Fonualei Spreading Center affords an excellent opportunity to evaluate geochemical changes with increasing depth to the slab in the Lau back-arc basin. We present H2O and CO2concentrations and Sr, Nd, Pb, Hf and U-Th-Ra isotope data for selected glasses as well as new Hf isotope data from boninites and seamounts to the north of the Tonga arc. The Pb and Hf isotope data are used to show that mantle flow is oriented to the southwest and that the tear in the northern end of the slab may not extend east as far as the boninite locality. Along the Fonualei Spreading Center, key geochemical parameters change smoothly with increasing distance from the arc front and increasing slab surface temperatures. The latter may range from 720 to 866°C, based on decreasing H2O/Ce ratios. Consistent with experimental data, the geochemical trends are interpreted to reflect changes in the amount and composition of wet pelite melts or super-critical fluids and aqueous fluids derived from the slab. With one exception, all of the lavas preserve both238U excesses and 226Ra excesses. We suggest that lavas from the Fonualei Spreading Center and Valu Fa Ridge are dominated by fluid-fluxed melting whereas those from the East and Central Lau Spreading Centers, where slab surface temperatures exceed ∼850–900°C, are largely derived through decompression. A similar observation is found for the Manus and East Scotia back-arc basins and may reflect the expiry of a key phase such as lawsonite in the subducted basaltic crust.

Item Type: Article
Date Type: Publication
Status: Published
Schools: Earth and Ocean Sciences
Subjects: Q Science > QE Geology
Uncontrolled Keywords: U-Th-Ra disequilibria; backarc; decompression; flux-melting; melting dynamics
Additional Information: Pdf uploaded in accordance with publisher's policy at (accessed 20/02/2014).
Publisher: American Geophysical Union
ISSN: 2169-9356
Date of First Compliant Deposit: 30 March 2016
Date of Acceptance: 3 October 2012
Last Modified: 14 Feb 2019 11:40

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