Dynamics of melting in the Valu Fa-Lau Basin backarc

Conference contribution
(Abstract of a poster)

Publication Details

Author(s): Schönhofen M, Beier C, Turner SP, Brandl PA
Publication year: 2017
Language: English


Subduction zones are the regions of significant mass

transfer between the Earth’s crust and mantle. The

dynamics of melting and composition of melts in island

arcs differ from those observed at divergent plate

boundaries where melting is solely driven by adiabatic

decompression. Fluids from the subducting slab and

variable depletion of the mantle wedge impact on the

erupting island arc melts. Recent advances in subduction

zone research suggest that a significant portion of melting

in island arcs may however be due to adiabatic

decompression. Uranium series disequilibria provide a

unique tool to test between these competing models.

Adiabatic decompression melting generally results in

230Th-excess while fluid flux melting in arcs displays

excess of 238U.

The Valu Fa-Lau Basin backarc spreading centre is

situated at increasing distance from the trench ranging

from 175-325 km and increasing Benioff zone depth from

40-250 km. Here, we use a new sample set along with

literature data to determine the processes of melting

underneath the backarc. We can show that fluid mobile

element ratios such as Ba/Nb decrease with increasing

distance to the trench while trace element ratios sensitive

to mantle wedge depletion (e.g., Nb/La) suggest a

decreasing degree of depletion with increasing distance. A

single samples more than 300 km distant from the trench

and at Benioff zone depths of ~250 km displays 230Th

excess, low Ba/Nb (<10) and relatively high Nb/La

(~0.55). In contrast, samples closer to the trench (at 170-

270 km) display a significant excess of 238U that is best

explained by addition of fluids from the subducting slab.

We note however, that these are not correlated with fluid

mobile element ratios suggesting that the Uranium series

disequilibria may require an alternative explanation. We

test a model in which the 238U excess in the backarc may

best be explained by adiabatic decompression melting of

an oxidized mantle that has previously been depleted by

melting underneath the arc. This depleted mantle has

subsequently been transported towards the backarc due to

slab rollback of the Pacific Plate.

FAU Authors / FAU Editors

Beier, Christoph PD Dr.
Lehrstuhl für Endogene Geodynamik
Schönhofen, Milena
Lehrstuhl für Endogene Geodynamik

External institutions
GEOMAR Helmholtz-Zentrum für Ozeanforschung Kiel
Macquarie University

How to cite

Schönhofen, M., Beier, C., Turner, S.P., & Brandl, P.A. (2017). Dynamics of melting in the Valu Fa-Lau Basin backarc. Poster presentation at GeoBremen 2017, Uni­ver­si­tät Bre­men MARUM, Leo­be­ner Stra­ße, 28359 Bre­men, Ger­ma­ny.

Schönhofen, Milena, et al. "Dynamics of melting in the Valu Fa-Lau Basin backarc." Presented at GeoBremen 2017, Uni­ver­si­tät Bre­men MARUM, Leo­be­ner Stra­ße, 28359 Bre­men, Ger­ma­ny 2017.


Last updated on 2018-06-08 at 12:09