Publication Date:
2022-05-25
Description:
© The Author(s), 2016. This is the author's version of the work and is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Geochimica et Cosmochimica Acta 181 (2016): 217-237, doi:10.1016/j.gca.2016.03.010.
Description:
Sediment transport from the subducted slab to the mantle wedge is an important
process in understanding the chemical and physical conditions of arc magma generation. The
Aleutian arc offers an excellent opportunity to study sediment transport processes because the
subducted sediment flux varies systematically along strike (Kelemen et al., 2003) and many
lavas exhibit unambiguous signatures of sediment addition to the sub-arc mantle (Morris et al.,
1990). However, the exact sediment contribution to Aleutian lavas and how these sediments are
transported from the slab to the surface are still debated. Thallium (Tl) isotope ratios have great
potential to distinguish sediment fluxes in subduction zones because pelagic sediments and low
temperature altered oceanic crust are highly enriched in Tl and display heavy and light Tl isotope
compositions, respectively, compared with the upper mantle and continental crust.
Here, we investigate the Tl isotope composition of lavas covering almost the entire Aleutian
arc a well as sediments outboard of both the eastern (DSDP Sites 178 and 183) and central (ODP
Hole 886C) portions of the arc. Sediment Tl isotope compositions change systematically from
lighter in the Eastern to heavier in the Central Aleutians reflecting a larger proportion of pelagic
sediments when distal from the North American continent. Lavas in the Eastern and Central
Aleutians mirror this systematic change to heavier Tl isotope compositions to the west, which
shows that the subducted sediment composition is directly translated to the arc east of Kanaga
Island. Moreover, quantitative mixing models of Tl and Pb, Sr and Nd isotopes reveal that bulk
sediment transfer of ~0.6-1.0% by weight in the Eastern Aleutians and ~0.2-0.6% by weight in
the Central Aleutians can account for all four isotope systems. Bulk mixing models, however,
require that fractionation of trace element ratios like Ce/Pb, Cs/Tl, and Sr/Nd in the Central and
Eastern Aleutians occurs after the sediment component was mixed with the mantle wedge.
Models of Sr and Nd isotopes that involve sediment melting require either high degrees of
sediment melting (〉50%), in which case trace element ratios like Ce/Pb, Cs/Tl, and Sr/Nd of
Aleutian lavas need to be produced after mixing with the mantle, or significant fluid additions
from the underlying oceanic crust with Sr and Nd isotope compositions indistinguishable from
the mantle wedge as well as high Sr/Nd ratios similar to that of low (〈20%) degree sediment
melts.
Thallium isotope data from Western Aleutian lavas exhibit compositions slightly lighter than
the upper mantle, which implies a negligible sediment flux at this location and probably
involvement of low-temperature altered oceanic crust in the generation of these lavas. In general,
the lightest Tl isotope compositions are observed for the highest Sr/Y ratios and most
unradiogenic Sr and Pb isotope compositions, which is broadly consistent with derivation of
these lavas via melting of eclogitized altered oceanic crust.
Description:
This study was
funded by NSF grants EAR-1119373 and -1427310 to SGN and EAR-1456814 to TP
Description:
2017-03-07
Repository Name:
Woods Hole Open Access Server
Type:
Preprint
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