
NSF Org: |
EAR Division Of Earth Sciences |
Recipient: |
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Initial Amendment Date: | June 22, 2011 |
Latest Amendment Date: | May 23, 2012 |
Award Number: | 1119373 |
Award Instrument: | Continuing Grant |
Program Manager: |
Jennifer Wade
jwade@nsf.gov (703)292-4739 EAR Division Of Earth Sciences GEO Directorate for Geosciences |
Start Date: | July 1, 2011 |
End Date: | June 30, 2015 (Estimated) |
Total Intended Award Amount: | $296,716.00 |
Total Awarded Amount to Date: | $296,716.00 |
Funds Obligated to Date: |
FY 2012 = $169,096.00 |
History of Investigator: |
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Recipient Sponsored Research Office: |
266 WOODS HOLE RD WOODS HOLE MA US 02543-1535 (508)289-3542 |
Sponsor Congressional District: |
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Primary Place of Performance: |
266 WOODS HOLE RD WOODS HOLE MA US 02543-1535 |
Primary Place of
Performance Congressional District: |
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Unique Entity Identifier (UEI): |
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Parent UEI: |
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NSF Program(s): | Petrology and Geochemistry |
Primary Program Source: |
01001213DB NSF RESEARCH & RELATED ACTIVIT |
Program Reference Code(s): | |
Program Element Code(s): |
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Award Agency Code: | 4900 |
Fund Agency Code: | 4900 |
Assistance Listing Number(s): | 47.050 |
ABSTRACT
Intellectual merit: For decades, subduction zones have been studied intensely to understand the physical and chemical mechanisms responsible for the surface volcanism expressed as arcs around the globe. However, there are still large uncertainties about how subducted slab material is discharged into the mantle wedge. For example, despite almost ubiquitous sediment subduction [Plank and Langmuir, 1998], many arc segments appear to have attenuated to negligible sediment signatures [Tera et al., 1986; Morris et al., 1990; Leeman et al., 2005]. Other are inferred to have carried strong chemical signatures from altered ocean crust, even though sediment subduction is clearly taking place [Regelous et al., 1997; Singer et al., 2007]. Are such discrepancies related to the thermal structure of subducting slabs, whereby hotter slabs lose sediment at shallower depths? Or related to sediment accretion in the forearc? Or do we simply not currently have the appropriate means of tracing the different slab fluxes in all arcs? It is proposed to evaluate the utility of novel thallium (Tl) stable isotope systematics in quantifying subduction contributions of pelagic sediments and altered oceanic crust from the Tonga-Kermadec, Aleutian and Central American arcs. The Tl isotope system is unique because the mantle is isotopically homogenous and depleted in Tl, whereas pelagic sediments and altered oceanic crust are enriched in Tl by several orders of magnitude and exhibit highly fractionated Tl isotope compositions. Thallium isotopes appear to be ideally suited to determine slab fluxes to arc lavas. In addition, the three targeted arcs represent different thermal regimes due to the large age range of the subducting oceanic crust. Tl isotopes will be used in conjunction with more conventional trace element and isotope ratios to quantify the role of sediments versus altered oceanic crust and test whether there is a relationship between the age of the subducting oceanic crust and the components incorporated into the arc volcanics.
Broader impacts: The results obtained in this study are expected to be of broad interest to all scientists working on subduction zones. Geodynamicists may, for example, be able to incorporate quantitative fluxes into their models on subduction zone melting processes, which could help resolve if melting is driven by fluid fluxing or anhydrous decompression. Quantification of fluid and/or melt fluxes from the slab in individual arcs may also be used to model the thermal state of the slab/mantle interface, where material is discharged to the mantle wedge. This project will provide funds for a WHOI/MIT Joint Program graduate student and a newly appointed WHOI scientist (PI Nielsen). An undergraduate student will also be involved through the WHOI Summer Student Fellowship Program (funded by the NSF REU program), which provides research funds for undergraduate students from across the country. Even though both the chemical separation and mass spectrometric techniques used to measure Tl isotopes are highly advanced, short projects carried out over a 10?12 week period are possible. The PI has previous experience from four undergraduate students in Oxford, UK, who with close supervision learnt the technique and produced publishable results.
PUBLICATIONS PRODUCED AS A RESULT OF THIS RESEARCH
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PROJECT OUTCOMES REPORT
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This Project Outcomes Report for the General Public is displayed verbatim as submitted by the Principal Investigator (PI) for this award. Any opinions, findings, and conclusions or recommendations expressed in this Report are those of the PI and do not necessarily reflect the views of the National Science Foundation; NSF has not approved or endorsed its content.
This project aimed to use the novel tool of thallium isotope measurements to investigate and quantify the amount of sediment and oceanic crust that is transferred from a subducting plate to the overlying volcanic arc. Sediments are very important vehicles of mass transfer in island arcs because they contain high concentrations of water and other volatiles that are one of the principal causes of the volcanism we observe at plate margins. It is therefore important to characterize and quantify these fluxes in order to understand the conditions that lead to volcansm at plate margins.
In our most complete study to date, we found that lavas from the Aleutian arc, that extends from Alaska into the Pacific, has a small, but detectable sediment component of 0.3-0.6% by weight in islands from the Alaskan peninsula to the island Kanaga. Previous studies have identified sediments as a component in the Aleutian arc, but no reliable estimates of the absolute flux exist. West of Kanaga Island little sediment contributes to the erupted lavas, most likely due to the almost parallel plate motion in this section of the arc, which highlights that plate margin volcanism is not dependent on sediment contributions from the subducting plate.
Using simple models that combine our data with experiments in the literature, we were also able to determine that the sediments in the Aleutian arc lavas likely were transferred directly from the subducted plate into the overlying mantle, where melting takes place. This finding is somewhat controversial as most studies infer that sediments melt before being transferred into the mantle where these melts cause further melting before eruption at the Earth's surface. Our model is the first robust geochemical evidence for bulk sediment transfer in an island arc and supports several recent theoretical papers that suggested such processes likely take place.
Our data and associated models have, therefore, not only provided new valuable information about the Aleutian arc, but also provided evidence in support of new geodynamical models that may change the way we understand physical processes in subduction zones in general.
Last Modified: 08/20/2015
Modified by: Sune G Nielsen
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