Assessing the importance of ultra-depleted domains in Earth’s mantle with Hf-Nd-Os isotope analyses of global abyssal peridotites

• Applicant: Professor Andreas Stracke, Ph.D.
• Subject Area: Mineralogy, Petrology and Geochemistry
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 508386553

Isotopic analyses of submarine mantle rocks, abyssal peridotites, have shown that the composition of Earth’s mantle could differ substantially from previous estimates. If ultra-depleted peridotite, as identified by the Hf-Os isotope ratios in abyssal peridotites, is ubiquitous in the Earth’s mantle, continuous melting must have depleted Earth’s mantle in incompatible elements to a much larger extent than previously thought. The magnitude of incompatible element depletion of Earth’s mantle relates directly to the time-integrated outgoing elemental flux, which is proportional to the rate of oceanic crust generation and the rate of mantle processing through melting regions in the shallow mantle. A greater extent of mantle depletion thus implies a vigorously convecting mantle. This in turn would lead to a higher mass flux between Earth’s mantle and crust, which means that the engine that drives silicate earth differentiation could run faster and crust-mantle cycling could operate at a higher rate than previously thought. Hence, the extent of mantle depletion mirrors the global compositional evolution and dynamics of the Earth. Up to now, however, the scant Hf isotope data on abyssal peridotites –21 Hf isotope analyses on abyssal peridotites compared to >1000 Hf isotope analyses on mid ocean ridge basalts– severely hamper assessing the importance of ultra depleted domains in Earth’s mantle, and thus its overall extent of depletion in incompatible elements. It is therefore proposed to analyze Hf-Nd, but also Os isotope ratios in abyssal peridotites from different global localities representing a broad range of the variability in spreading rate and ocean crust accretion style. The new database will reveal how common, or wide-spread, ultra-depleted mantle is. Careful screening, by petrography and major-trace element chemistry, will identify the most promising samples for the isotope analyses. The new geochemical data, in conjunction with the Hf-Nd-Os isotope analyses, will also identify the samples least affected by reaction with traversing or trapped melts. The Hf-Os isotope data will also yield a range of minimum mantle depletion ages. The distribution of minimum Hf-Os depletion ages per time interval scales with the amount of residual depleted mantle generated by partial melting at ocean ridges. The latter is directly proportional to the mantle processing rate, and will thus give some boundary conditions for the vigor of mantle convection and the rate of silicate earth differentiation. Moreover, our new Hf-Nd-Os isotope data on abyssal peridotites will provide a much needed baseline for interpreting the isotope systematics of peridotites in ophiolites, and could therefore help resolving the nature and timing of the processes involved in formation of the subcontinental lithosphere.

• DFG Programme: Research Grants