TY - JOUR
T1 - Barium isotope systematics of subduction zones
AU - Nielsen, Sune G.
AU - Shu, Yunchao
AU - Auro, Maureen
AU - Yogodzinski, Gene
AU - Shinjo, Ryuichi
AU - Plank, Terry
AU - Kay, Suzanne M.
AU - Horner, Tristan J.
N1 - Funding Information:
This study was funded by grant # NSF EAR 1829546 to SGN and TJH. We thank Heye Freymuth and two anonymous reviewers for their helpful comments that allowed us to improve the manuscript. we also thank Baptiste Debret for comments on an earlier version of this manuscript. Fang Huang is acknowledged for efficient editorial handling.
Publisher Copyright:
© 2020 Elsevier Ltd
PY - 2020/4/15
Y1 - 2020/4/15
N2 - Subduction zones are the focal points of mass transfer between the surface and deep Earth. Despite their significance, there remains substantial debate regarding the specific mechanisms of material transport from the slab to the overlying magmatic arc. Broadly, models accounting for slab material transport focus on the relative sequence of events promoting arc volcanism and, in particular, whether mobilization of the down-going slab leads or lags mixing with the mantle wedge. To address these uncertainties, we outline the utility of barium (Ba) isotope mass balance in subduction zones as a means to test different slab material transport models. Barium is a highly fluid-mobile element that is significantly enriched in arc magmas and is thus thought to be a sensitive tracer of slab material transport in arcs. We also present qualitative Ba isotopic mass balances for two well-characterized subduction zones—the Aleutian and Ryukyu magmatic arcs—by analyzing the Ba isotope systematics of their respective subduction inputs and outputs. Despite the narrow (and similar) Ba-isotope range of slab inputs to both systems, we find that erupted magmas exhibit systematic variations indicative of a small negative isotope fractionation during Ba mobilization (≈20–40 ppm AMU−1). We suggest that AOC (altered oceanic crust) is not the principal source of these negative isotope values using other geochemical parameters (e.g., Rb/Ba, Pb isotopes), and infer that the Ba isotope composition of AOC—though contributing a minor amount of Ba in these systems—is isotopically heavier than the overlying sediment package and the depleted mantle. Altogether, these findings are significant as they indicate that the magnitude of isotope fractionation associated with Ba mobilization is small relative to the likely isotopic contrast between subduction inputs in other subduction zones, such as beneath areas of strong ocean upwelling (e.g., South Sandwich, Kamchatka). Thus, we propose that the Ba isotope composition of erupted arc magmas holds great promise for constraining the importance of different slab components, which could help address uncertainties regarding the mechanism of slab material transport in subduction zones.
AB - Subduction zones are the focal points of mass transfer between the surface and deep Earth. Despite their significance, there remains substantial debate regarding the specific mechanisms of material transport from the slab to the overlying magmatic arc. Broadly, models accounting for slab material transport focus on the relative sequence of events promoting arc volcanism and, in particular, whether mobilization of the down-going slab leads or lags mixing with the mantle wedge. To address these uncertainties, we outline the utility of barium (Ba) isotope mass balance in subduction zones as a means to test different slab material transport models. Barium is a highly fluid-mobile element that is significantly enriched in arc magmas and is thus thought to be a sensitive tracer of slab material transport in arcs. We also present qualitative Ba isotopic mass balances for two well-characterized subduction zones—the Aleutian and Ryukyu magmatic arcs—by analyzing the Ba isotope systematics of their respective subduction inputs and outputs. Despite the narrow (and similar) Ba-isotope range of slab inputs to both systems, we find that erupted magmas exhibit systematic variations indicative of a small negative isotope fractionation during Ba mobilization (≈20–40 ppm AMU−1). We suggest that AOC (altered oceanic crust) is not the principal source of these negative isotope values using other geochemical parameters (e.g., Rb/Ba, Pb isotopes), and infer that the Ba isotope composition of AOC—though contributing a minor amount of Ba in these systems—is isotopically heavier than the overlying sediment package and the depleted mantle. Altogether, these findings are significant as they indicate that the magnitude of isotope fractionation associated with Ba mobilization is small relative to the likely isotopic contrast between subduction inputs in other subduction zones, such as beneath areas of strong ocean upwelling (e.g., South Sandwich, Kamchatka). Thus, we propose that the Ba isotope composition of erupted arc magmas holds great promise for constraining the importance of different slab components, which could help address uncertainties regarding the mechanism of slab material transport in subduction zones.
KW - Altered oceanic crust
KW - Arc
KW - Fluids
KW - Magma
KW - Mantle
KW - Sediments
KW - Slab material transport
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U2 - 10.1016/j.gca.2020.02.006
DO - 10.1016/j.gca.2020.02.006
M3 - Article
AN - SCOPUS:85080048812
SN - 0016-7037
VL - 275
SP - 1
EP - 18
JO - Geochmica et Cosmochimica Acta
JF - Geochmica et Cosmochimica Acta
ER -