Consequences for an Alternative Earth Composition: A Decade of Insight

Abstract:

This year marks the 10^th anniversary of the landmark discovery showing that modern terrestrial mantle-derived lavas have 142Nd/144Nd ratios ~18 ppm higher than ordinary chondrites. One interpretation of this discovery is that the accessible Earth has a Sm/Nd ratio that is 5-7% higher than chondrites, which resulted from an early (20-30 Ma after accretion), catastrophic extraction of geochemically-enriched crust from the Earth’s mantle (Boyet and Carlson, Science, 2005). The location of this early-formed enriched reservoir is unknown (it is either hidden in the deep Earth or was lost to space by impact erosion), but is critical, as it hosts the equivalent of the modern continents’ budget of the radioactive heat-producing elements: U, Th and K. If the early-enriched reservoir is no longer in the Earth, there are profound implications for the geochemical and thermal evolution of the planet. First, the bulk silicate Earth would have a present-day 143Nd/144Nd of ~0.5130, and all modern terrestrial mantle and crustal reservoirs were ultimately derived from a non-chondric mantle with superchondritic Sm/Nd. Second, this composition matches the most frequently-occurring 143Nd/144Nd ratio (0.5130, PREMA) in ocean island basalts, including lavas with primitive 3He/4He, and suggests that large portions of the mantle sampled by OIB remain little-modified with respect to 143Nd/144Nd. Third, the modern continents were extracted from a previously depleted mantle, meaning that the modern mantle is pervasively depleted in highly incompatible elements, and that the mantle’s radiogenic heat production is 50% lower than in chondrite-based models. The new bulk silicate Earth composition therefore presents challenges for describing the thermal history of the planet, but may lead to a stable plate tectonic regime over time and could be more conducive to supporting a habitable world.