The coexistence of recycled and primordial heterogeneity in Earth’s lower mantle: a geodynamical perspective

Abstract:

The nature of compositional heterogeneity in Earth’s lower mantle is a long-standing puzzle that can inform about the long-term evolution and dynamics of our planet. On relatively small scales (<1km), the concept of a “marble cake” mantle has gained wide acceptance, emphasizing that streaks of recycled oceanic crust (ROC) and lithosphere make up much of the mantle. On larger scales (10s-100s km), compositional heterogeneity may be preserved by delayed mixing of this marble cake with either intrinsically-dense or -strong materials of e.g. primordial origin. Intrinsically dense materials may accumulate as piles at the core-mantle boundary, while intrinsically viscous (e.g., enhanced in the strong mineral MgSiO₃ bridgmanite) may survive as “blobs” in the mid-mantle for large timescales (i.e., as plums in the mantle “plum pudding”). So far, only few if any studies have quantified mantle dynamics in the presence of different types of heterogeneity with distinct physical properties.

Here, we address the coexistence of recycled and primordial heterogeneity in Earth’s mantle using state-of-the-art 2D numerical models of global-scale mantle convection in a spherical geometry. We explore the effects of various parameters on mantle dynamics and mixing, including ancient chemical mantle layering and the physical parameters of the primordial materials. Models predict that primordial heterogeneity is preserved in the lower mantle as discrete blobs or streaks for intrinsic viscosity contrasts greater than 30. In turn, ROC heterogeneity takes the form of “marble cake” streaks, an enriched mantle-transition zone and dense piles at the base of the mantle. Importantly, these ROC heterogeneities are robustly predicted to co-exist with primordial blobs, suggesting that the modern mantle may be in a hybrid state between the “marble cake” and “plum pudding” styles. This new hybrid style of mantle convection including preserved MgSiO₃-enriched domains along with recycled piles has the potential of reconciling recent geophysical and geochemical observations of lower-mantle heterogeneity, and may also be linked to geological and geochemical indications of a major change in geodynamic style in the Archean Earth. Our results are therefore relevant for assessing Earth’s bulk composition and long-term thermochemical evolution.