Séminaire ISTeP - Yamirka Rojas-Agramonte

Séminaire de Yamirka Rojas-Agramonte jeudi 26 septembre à 13h en salle Fourcade

Title: Asthenospheric zircon from Galápagos and Pascua dates hundreds million years of plume activity: Implications for upper mantle dynamics

We report the first systematic study of zircon U-Pb geochronology and δ¹⁸O-εHf_(t) isotope geochemistry from the islands of Galapagos and Easter (Rapa Nui) (Rojas-Agramonte et al., 2022, 2024). The data extracted from the zircons allow them to be grouped into three types: 1) young zircons (0 Ma – Mio-Pliocene) with juvenile εHf_(t) and δ¹⁸O isotopic mantle signature and crystallization ages dating the islands, 2) zircons with juvenile εHf_(t) and δ¹⁸O isotopic mantle signature (Mio-Pliocene to mid Jurassic time) which are interpreted to date the time of plume activity below the islands (~164 Ma is the minimum time of impingement of the plumes below the lithosphere), and 3) very old zircons (up to 3000 Ma) with mostly continental (but also juvenile) εHf_(t) isotopic values documenting potential contamination from a number of sources. The time of plume impingement at ~164 Ma in Galapagos and Easter could represent Large Igneous Province (LIP) events whose implications will be discussed in the seminar.

Given lithospheric plate motion, these results imply that zircon predating the Galápagos and Easter lithosphere formed and were stored at sublithospheric depths for extended periods of time. In order to explain these observations, we performed 2D and 3D thermo-mechanical numerical experiments of plume-lithosphere interaction which show that dynamic plume activity gives rise to complex asthenospheric flow patterns and results in distinct long-lasting mantle domains beneath a moving lithosphere. This demonstrates that it is physically plausible that old plume-derived zircons survive at asthenospheric depths below ocean islands due to convective (re)circulation at the scale of the plume head. Our study demonstrates how zircon geochronology and geochemistry provide novel insights into global-scale geodynamics, offering new perspectives on the dynamics of mantle plumes and hotspot activity.

References

Rojas-Agramonte, Y., Pardo, N., van Hinsbergen, D. J., Winter, C., Marroquín-Gómez, M. P., Liu, S., ... & Garcia-Casco, A. (2024). Zircon xenocrysts from Easter Island (Rapa Nui) reveal hotspot activity since the middle Jurassic. In press AGU Advances.

Rojas‐Agramonte, Y., Kaus, B. J., Piccolo, A., Williams, I. S., Gerdes, A., Wong, J., ... & Garcia‐Casco, A. (2022). Zircon Dates Long‐Lived Plume Dynamics in Oceanic Islands. Geochemistry, Geophysics, Geosystems, 23(11), e2022GC010485.