NSF Org:	EAR Division Of Earth Sciences
Recipient:	WASHINGTON UNIVERSITY, THE
Initial Amendment Date:	July 9, 2019
Latest Amendment Date:	August 22, 2022
Award Number:	1939189
Award Instrument:	Standard Grant
Program Manager:	Jennifer Wade jwade@nsf.gov  (703)292-4739 EAR  Division Of Earth Sciences GEO  Directorate for Geosciences
Start Date:	September 1, 2019
End Date:	August 31, 2023 (Estimated)
Total Intended Award Amount:	$38,549.00
Total Awarded Amount to Date:	$38,549.00
Funds Obligated to Date:	FY 2019 = $38,549.00
History of Investigator:	Rita Parai (Principal Investigator) parai@wustl.edu
Recipient Sponsored Research Office:	Washington University 1 BROOKINGS DR SAINT LOUIS MO  US  63130-4862 (314)747-4134
Sponsor Congressional District:	01
Primary Place of Performance:	Washington University MO  US  63130-4899
Primary Place of Performance Congressional District:	01
Unique Entity Identifier (UEI):	L6NFUM28LQM5
Parent UEI:
NSF Program(s):	Petrology and Geochemistry
Primary Program Source:	01001920DB NSF RESEARCH & RELATED ACTIVIT
Program Reference Code(s):	7916, 9150
Program Element Code(s):	157300
Award Agency Code:	4900
Fund Agency Code:	4900
Assistance Listing Number(s):	47.050

ABSTRACT

Geological processes play an important role in regulating atmospheric oxygen and carbon, and thus surface temperatures, on timescales of ten million years and greater. Carbon is transferred from the surface of the Earth into the mantle by the process of plate subduction, where oceanic plates carrying carbon-rich rocks and sediments sink into the mantle. During the initial stages of subduction, some fraction of the carbon in the sinking plate is removed and transported back to the Earth's surface in the magmas of subduction-zone volcanoes, such as those of the Pacific Ring of Fire. To account for the mass balance of the carbon cycle, and to fully understand variations in Earth's climate on geological time scales, it is therefore necessary to determine how much subducting carbon is rapidly returned back to Earth's surface, and how much is instead transported deep into the mantle. Rates of carbon recycling are difficult to measure directly, however, because carbon dissolved in magmas is sequestered into a separate vapor phase during magmatic ascent, traversing through the crust and into the atmosphere at highly variable rates, and via diffuse pathways. As an alternative to direct measurements of carbon fluxes through subduction zone volcanoes, geochemical proxies can be used to estimate the amount of carbon that was initially present in the magmas. This study will incorporate recent advances in analytical techniques that will use the strontium isotope system as a proxy, with applications to carbonate recycling in particular. The proposed work aims to advance understanding of chemical cycling at subduction zones while promoting teaching, training, and learning. This project involves mentoring of early-career researchers from under-represented groups in the Earth sciences, as well as a first-year Master's student. The proposed work also will build new collaborative relationships among early career faculty members of UMass Amherst and Washington University in St. Louis.

Strontium is a potentially powerful proxy of carbonate recycling though subduction zones because it is typically present at high abundances in carbonates relative to the mantle, and because carbonates have distinct stable Sr isotope compositions. The project team will develop new procedures for the analysis and interpretation of the Sr stable isotope system among igneous rocks from volcanic arcs by building a data framework in the Central American Volcanic Arc (CAVA), where there is a thick layer of subducting sedimentary carbonate, the volcanic rocks and gases have been extensively measured and characterized by previous studies, and there is good geochemical evidence for variable carbonate recycling efficiency. Combined measurements of Sr stable and radiogenic isotope ratios via double-spike TIMS will provide the ability to accurately estimate the Sr flux from the subducting carbonate to the volcanic arc, which can in turn be used to estimate the rates of carbon recycling. If successful, this study will provide novel constraints on the global carbon cycle, establish δ88/86Sr isotope systematics among subducting components and their associated volcanics, and provide the basis for further studies of arc geochemical transport. δ88/86Sr values will be combined with radiogenic Sr isotopes to accurately determine the total carbonate-derived Sr budget of the arc volcanics and whether these systems can be used to assess the efficiency of, and the mechanisms that enable, the recycling of subducting carbonate into volcanic arcs. If the δ88/86Sr system at CAVA provides a benchmark on carbon recycling through volcanic arcs, then these tracers can be used to constrain the fate of subducted carbonate in other arcs around the world. Once modern-day stable Sr isotope arc systematics are established, this proxy can also potentially be used to assess the variability of subducting carbonate in past eras, providing transformative insight into the variability of the global carbon cycle throughout Earth's history. The results of the proposed work will be of interest to the wide geoscience community, including low-temperature geochemistry and Earth history communities interested in long-term variations in surface carbon reservoirs.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

PROJECT OUTCOMES REPORT

Disclaimer

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.

Volatile elements and compounds (like water, carbon, nitrogen and the noble gases) have been transported between the deep Earth and surface reservoirs throughout 4.5 billion years of Earth history. Volatiles are transported out of Earth’s interior into the atmosphere in association with volcanism, and can be transferred from the surface into the interior at subduction zones. The balance between outfluxes and influxes determines how the distribution of volatiles between the surface and interior has changed over time. A critical part of this system is poorly understood: at subduction zones, fluids released from subducting slabs carry some portion of influxed volatiles back to the surface through arc volcanism. To understand how much is transported into the deep mantle at subduction zones, a better understanding of volatile recycling to the surface out of the arc is needed.

Direct measurements of the carbon flux out of arc volcanoes are challenging, as carbon dioxide exsolves from magmas during their ascent towards the surface (a process called “degassing”), and vapor bubbles may ascend rapidly and reach the surface through diffuse pathways. Measurements of carbon dioxide remaining in lavas, or in volcanic gases and hydrothermal fluids from discrete wells or springs, may therefore only capture a portion of the total outflux of carbon dioxide at an arc. Stable strontium (Sr) isotopes (⁸⁸Sr/⁸⁶Sr) have some potential to serve as a tracer of slab carbonate contributions to arc magmatism, as Sr is present in high concentrations in carbonates relative to the mantle and carries a distinct stable Sr isotopic signature.

In this project, we measured Sr stable isotopes in volcanic rocks from the Central American Volcanic Arc, and in sediments and altered oceanic crust similar to what is subducted at this trench. At this location, a thick layer of sedimentary carbonate is present on the subducting Cocos plate. The area has been extensively studied, and there is evidence of variable carbonate contributions to different volcanoes at the arc – making this area well-suited for a test of stable Sr as a carbonate tracer.

We developed methods for purification and analysis of Sr stable isotopes in samples with high Ba concentrations by double-spike thermal ionization mass spectrometry. The project has advanced our understanding of volatile cycling at subduction zones by providing new constraints on Sr contributions to arc magmas from carbonate, which we use to estimate the carbon dioxide flux that would have been carried in these magmas prior to degassing. Measurements of arc lavas and subducting components will provide a basis for comparison for subsequent studies. The project included teaching, training and mentorship of early-career individuals from under-represented groups, including one research lab assistant preparing for graduate study and forming a career plan after completion of an undergraduate degree, and one PhD student in the Earth Sciences. The project also built a collaborative relationship between early career faculty at Washington University in St. Louis and UMass Amherst.

 

Last Modified: 03/12/2024
Modified by: Rita Parai

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