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Radiogenic isotopes document the start of subduction in the Western Pacific

Radiogenic isotopes document the start of subduction in the Western Pacific
Radiogenic isotopes document the start of subduction in the Western Pacific

Subduction initiation is one of the least understood aspects of plate tectonics. In an effort to obtain the first in situ magmatic record of subduction initiation, the International Ocean Discovery Program Expedition 352 drilled at four sites in the inner trench wall of the Bonin Trench to recover 1.22 km of oceanic upper crust accreted within a few m.y. of subduction initiation. The two sites nearer to the trench (U1440 and U1441) yielded axial and off-axis fore-arc basalts (FAB), while those c. 15 km further from the trench (U1439 and U1442) yielded axial low-silica boninites and high-Mg andesites overlain by off-axis high-silica boninites. This study uses Hf–Nd–Sr–Pb isotope analyses from c. 50 stratigraphically representative core samples to trace the evolution of the mantle source during the brief period of FAB-through-boninite magmatism immediately following subduction initiation. Results show that: 1) the FAB have high εHf relative to εNd and were derived from variably depleted mantle of ‘Indian’ provenance with no detectable subduction input; 2) the axial boninites follow mixing trends between a residual FAB mantle source and a subduction component derived from shallow (amphibolite facies) melting of oceanic crust of ‘Pacific’ provenance; and 3) the off-axis boninites define mixing trends between a hybrid mantle wedge (residual mantle + slab melt) and an additional subduction component with lower εNd and higher 207 Pb/ 204 Pb that requires a significant contribution from pelagic sediment. This incoming of pelagic sediments may signify a change from an accretionary to non-accretionary margin as subduction evolves. The results thus indicate a rapidly evolving system in terms of geodynamics, magma genesis and crustal accretion immediately following subduction initiation.

boninites, forearc basalts, International Ocean Discovery Program (IODP) Expedition 352, radiogenic isotopes, subduction initiation
0012-821X
197-210
Li, Hong Yan
6877457e-bf1a-4b93-9619-e66f3abb105c
Taylor, Rex N.
094be7fd-ef61-4acd-a795-7daba2bc6183
Prytulak, Julie
acc25269-e2d2-4724-a319-9eda76e7551e
Kirchenbaur, Maria
284ae0b6-b059-4167-9b77-30f504f658d2
Shervais, John W.
c6470b84-b766-408c-9942-67f96bf59372
Ryan, Jeffrey G.
956b6ae5-3948-4f6d-b824-6f0cf9024956
Godard, Marguerite
0bfe00d4-a306-4519-bce5-2c4305a15e1d
Reagan, Mark K.
2e882b42-aefb-4485-b272-b7bdf6a84e8d
Pearce, Julian A.
f9e0d8d7-1cc4-4d5f-951e-5ae3c5f34d2c
Li, Hong Yan
6877457e-bf1a-4b93-9619-e66f3abb105c
Taylor, Rex N.
094be7fd-ef61-4acd-a795-7daba2bc6183
Prytulak, Julie
acc25269-e2d2-4724-a319-9eda76e7551e
Kirchenbaur, Maria
284ae0b6-b059-4167-9b77-30f504f658d2
Shervais, John W.
c6470b84-b766-408c-9942-67f96bf59372
Ryan, Jeffrey G.
956b6ae5-3948-4f6d-b824-6f0cf9024956
Godard, Marguerite
0bfe00d4-a306-4519-bce5-2c4305a15e1d
Reagan, Mark K.
2e882b42-aefb-4485-b272-b7bdf6a84e8d
Pearce, Julian A.
f9e0d8d7-1cc4-4d5f-951e-5ae3c5f34d2c

Li, Hong Yan, Taylor, Rex N., Prytulak, Julie, Kirchenbaur, Maria, Shervais, John W., Ryan, Jeffrey G., Godard, Marguerite, Reagan, Mark K. and Pearce, Julian A. (2019) Radiogenic isotopes document the start of subduction in the Western Pacific. Earth and Planetary Science Letters, 518, 197-210. (doi:10.1016/j.epsl.2019.04.041).

Record type: Article

Abstract

Subduction initiation is one of the least understood aspects of plate tectonics. In an effort to obtain the first in situ magmatic record of subduction initiation, the International Ocean Discovery Program Expedition 352 drilled at four sites in the inner trench wall of the Bonin Trench to recover 1.22 km of oceanic upper crust accreted within a few m.y. of subduction initiation. The two sites nearer to the trench (U1440 and U1441) yielded axial and off-axis fore-arc basalts (FAB), while those c. 15 km further from the trench (U1439 and U1442) yielded axial low-silica boninites and high-Mg andesites overlain by off-axis high-silica boninites. This study uses Hf–Nd–Sr–Pb isotope analyses from c. 50 stratigraphically representative core samples to trace the evolution of the mantle source during the brief period of FAB-through-boninite magmatism immediately following subduction initiation. Results show that: 1) the FAB have high εHf relative to εNd and were derived from variably depleted mantle of ‘Indian’ provenance with no detectable subduction input; 2) the axial boninites follow mixing trends between a residual FAB mantle source and a subduction component derived from shallow (amphibolite facies) melting of oceanic crust of ‘Pacific’ provenance; and 3) the off-axis boninites define mixing trends between a hybrid mantle wedge (residual mantle + slab melt) and an additional subduction component with lower εNd and higher 207 Pb/ 204 Pb that requires a significant contribution from pelagic sediment. This incoming of pelagic sediments may signify a change from an accretionary to non-accretionary margin as subduction evolves. The results thus indicate a rapidly evolving system in terms of geodynamics, magma genesis and crustal accretion immediately following subduction initiation.

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More information

Accepted/In Press date: 25 April 2019
e-pub ahead of print date: 17 May 2019
Published date: 15 July 2019
Keywords: boninites, forearc basalts, International Ocean Discovery Program (IODP) Expedition 352, radiogenic isotopes, subduction initiation

Identifiers

Local EPrints ID: 434114
URI: http://eprints.soton.ac.uk/id/eprint/434114
ISSN: 0012-821X
PURE UUID: d17c53e6-07fe-4491-b521-71ec73fce884

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Date deposited: 12 Sep 2019 16:30
Last modified: 09 Oct 2020 16:41

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Contributors

Author: Hong Yan Li
Author: Rex N. Taylor
Author: Julie Prytulak
Author: Maria Kirchenbaur
Author: John W. Shervais
Author: Jeffrey G. Ryan
Author: Marguerite Godard
Author: Mark K. Reagan
Author: Julian A. Pearce

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