Distinguishing mantle source heterogeneity from process-based Ca isotope signatures in ocean island basalts

Calcium is linked to the carbon cycle through the precipitation of marine carbonates [e.g., limestone (CaCO3)] in seawater. Carbonate minerals typically sink to the bottom of the oceans and can be transported back into Earth’s mantle at subduction zones. Thus, subducted carbonate minerals are an important link between the carbon cycles at the Earth's surface and in the deep Earth.

It is currently not well understood what happens to carbonate minerals after they are subducted. Researchers use calcium isotope signatures to trace carbonate minerals in the Earth. Ca isotope data have been used to suggest that mantle-derived igneous rocks called ocean island basalts (OIB) represent the return of subducted marine carbonates and/or oceanic crust back to Earth's surface after long-term storage in the mantle.

This project will investigate Ca isotope variations in OIB to refine their potential use as tracers of the deep Earth carbon cycle. Results will help clarify details about how carbonates from Earth's surface are processed during the deep Earth carbon cycle. This project will also provide equity-based research opportunities for students, to help train the next generation of Earth scientists.

Additionally, researchers will develop a publicly available educational video series related to obtaining and interpreting Ca isotope data.

Global OIB compilations suggest that stable Ca isotope ratios (δ44Ca) are inversely correlated with radiogenic isotope tracers sensitive to crustal recycling, implying that mantle source variations likely play a role in generating δ44Ca variability. On the other hand, these relationships often break down when looking at single localities, and similarly strong global correlations exist between δ44Ca and partial melting proxies (e.g., Th, Nb, total alkalinity), suggesting that magmatic processes may instead control the δ44Ca variability.

To address these conflicting observations, this research will analyze stable and radiogenic Ca isotope variations in large suites of previously-collected and well-characterized OIB lavas, cumulates, and phenocrysts from 5 different locations (Samoa, Canary, Reunion, Azores, Mangaia). The δ44Ca data and available major/trace-element and radiogenic isotope data will be used to tune location-specific phase equilibrium models that trace the elemental and δ44Ca evolution of OIB magmas during partial-melting and crystallization.

These models will yield important constraints on (i) magmatic processes, (ii) the source compositions of three important mantle types (PUM, HIMU, and EM-2), and (iii) whether marine carbonates and/or oceanic crust are required to explain the δ44Ca values of OIB sources. Radiogenic Ca isotope data (εCa), which have only rarely been investigated in OIBs, will allow us to (i) better constrain the 40Ca/44Ca of bulk-silicate Earth and (ii) possibly identify ancient pelagic/terrigenous (high K/Ca) sediments in the mantle.

In addition to providing public-facing educational outreach and equitable training opportunities for the next generation of Earth scientists, the results of this project will greatly advance our understanding of how surface-derived carbon behaves in Earth’s mantle – an essential part of the deep Earth carbon cycle.

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.