Dynamics of melting in the Valu Fa-Lau Basin backarc

Schönhofen M, Beier C, Turner SP, Brandl PA (2017)

Publication Language: English

Publication Type: Conference contribution, Abstract of a poster

Publication year: 2017

Event location: Uni­ver­si­tät Bre­men MARUM, Leo­be­ner Stra­ße, 28359 Bre­men, Ger­ma­ny

Abstract

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.

Authors with CRIS profile

Milena Schönhofen-Romer Lehrstuhl für Endogene Geodynamik (LEG) Christoph Beier Lehrstuhl für Endogene Geodynamik (LEG)

Involved external institutions

Macquarie University AU Australia (AU) GEOMAR Helmholtz-Zentrum für Ozeanforschung Kiel DE Germany (DE)

How to cite

APA:

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.

MLA:

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.

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