The Tonga-Kermadec arc – Lau back-arc: recent progress and future directions Simon Turner

Subduction zones and magmatic processes

Tonga–Kermadec arc - Lau back arc

Components (separate sediment & fluid; 10 Be) Bulk chemistry (depleted); IDM source

Fluid addition 50 kyr; cpx-out (& experiments)

Using geochemistry to map mantle flow

Tofua: mostly basaltic andesite, 1 dacite crystallization from ºC at km whilst degassing from 4 wt. % H 2 O

Fonualei: mostly dacitic overlying basaltic andesite & andesite crystallization from ºC at 2-6 km

1. Source IDM or PDM? 2. Fluid < few kyr – islands en-echelon 3. Differentiation in kyr (also crystals)

Image courtesy of Smithsonian West Mata eruption 2009

across-arc traverses afford an opportunity to investigate changes in chemistry and magma genesis with increasing depth to the slab; these should very with slab surface temperature; but could be influenced by the presence of plumes; can we see a change from fluid-fluxed to decompression dominated melting? The Fonualei Spreading Centre

The Fonualei spreading centre extends from the Tonga arc front into the Lau back-arc basin Samples dredged during 2004 Southern Surveyor voyage Analysed for major and trace elements by Keller et al (JGR)

Radiogenic isotopes record a Samoan plume-like influence at least for the MTJ samples Note no such singature is observed in the north Tonga arc lavas

Slab-tracers vary with depth to the slab: Fluid and sediment indicators decrease, plume influence increases

Because most of the lavas are water- undersaturated, we can use H 2 O/Ce ratios to estimate slab-surface temperatures

Results are ºC higher than calculated numerically but increase into the back-arc Experimental pelite melts (at TºC) provide a good approximation of the lava data

Major and trace element data suggest a change from fluid-fluxed melting to melting dominated by decompression with increasing distance from the arc front: Langmuir et al. 2006

This is supported by U-series data

Is there a general shift from fluid-fluxed to decompression dominated melting when the slab surface reaches ~ 900ºC?

Future directions Team Kermadec: (10 Japanese, 11 New Zealanders, 2 Australians, 6 Americans and 4 Germans)