Tracing Ancient Subduction in the Lithospheric Mantle via Traditional and Non-Traditional Stable Isotopes

The outer layer of the Earth is broken into tectonic plates, consisting of the crust and uppermost mantle. Those plates move around producing volcanoes, earthquakes, and mountains. Plate tectonics, the movement of these plates, is unique to Earth and may be key to life on Earth.

Plate tectonics likely plays an important role in maintaining planetary habitability. It helps to moderate the global carbon cycle which in turn affects climate. Plate tectonics affects nutrient fluxes to the oceans through weathering of material both on land where plates come together to build mountains, and on the seafloor where plates spread apart.

There may be a link between the start of or change in plate tectonics between 3 and 2.5 billion years ago and the rise of oxygen in the Earth’s atmosphere. Despite the importance of plate tectonics to Earth processes, the question of when plate tectonics started and whether the start was sudden, gradual, or intermittent, are highly debated. One of the major consequences of plate tectonics is the delivery of surface material to the mantle at subduction zones, where one plate slides under another plate and sinks back into the mantle.

Tracing subducted material in the mantle is one potential way to constrain the presence or absence of plate tectonics at a given time in Earth’s history. This project will use a series of geochemical tracers to identify the presence (or absence) of subducted material in the Earth’s mantle. The team will study pieces of the mantle (called xenoliths) which are brought to the Earth’s surface via volcanoes.

Natural radioactive elements will also be used to constrain the ages of any identified subducted materials, which will allow examination of subduction over time. This work will support the education, research, and scientific training of both undergraduate and graduate students. The graduate student will train for career opportunities in research/curation/outreach at museums via an internship at the Smithsonian National Museum of Natural History (NMNH).

Tracking subducted material in the mantle, and in particular in the sub-continental lithospheric mantle (SCLM), is one potential mechanism for constraining the presence or absence of plate tectonics in Archean and Proterozoic times. The SCLM may provide a more robust chemical record of ancient subduction processes than the convecting mantle because it is isolated from convective mixing, and the oldest cratonic roots have formation ages that extend back into the early Archean.

Although crustal recycling in and of itself does not prove the existence of plate tectonics, it is a necessary consequence of plate tectonics. The research team will use stable isotope geochemistry (18O, 44/40Ca), in combination with trace element and radiogenic isotope tracers, to identify the presence (or absence) of subducted components in the SCLM as sampled by mantle peridotite xenoliths from cratonic (Kaapvaal, Slave, Rae) and non-cratonic (Navajo Volcanic Field) settings.

Subducting lithosphere has distinct 18O and 44/40Ca values compared to normal mantle. Slab-derived melts and fluids that infiltrate the mantle wedge can alter its oxygen and calcium isotope composition, raising or lowering the values depending on the nature of the subducted components. Correlations between 18O and 44/40Ca values and other geochemical tracers will be used to constrain the origin and effects of slab-derived melts and fluids on mantle chemistry.

In addition, correlations between radiogenic isotope systems (e.g., Sm-Nd and Lu-Hf) and stable isotope variations will also be used to constrain the age of any identified subduction components and their relationship to the original stabilization of cratonic lithosphere by melt depletion. Surprisingly, very few studies have examined coupled stable isotope and trace/radiogenic element data in mantle xenoliths and, in particular, no prior study has coupled oxygen and calcium isotope ratios of peridotite xenoliths in the same study.

Determining the mechanism(s) by which continental lithosphere is generated and subsequently modified, and how both of these have varied through time, can thus shed light onto both the timing of plate tectonics onset and on the evolving mechanisms of continental crust production through time.

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.