Insights into Earth’s Accretion from Deep Mantle Kr Isotopes

Abstract:

Establishing when volatiles, such as carbon, nitrogen and water, were delivered to Earth, as well as fingerprinting their potential sources, is a fundamental objective in understanding the origin and evolution of habitable planets like Earth. Volatile delivery to the early Earth remains controversial with timing ranging from during and after the main phase of accretion, to the majority of the volatile delivery associated with the Moon-forming giant impact.

Krypton isotopes (⁷⁸Kr, ⁸⁰Kr, ⁸²Kr, ⁸³Kr, ⁸⁴Kr, ⁸⁶Kr) provide unique insights into the debate on volatile delivery due to significant isotopic variations among chondritic and solar end-members, although pervasive atmospheric contamination has hampered efforts. Here, we measure, for the first time, the full suite of krypton isotopes from the deep mantle as sampled by the Galápagos and Iceland plumes, which have among the most primitive helium, neon and tungsten isotopic compositions, using a new gas accumulation technique that minimizes atmospheric contamination. We show that the light krypton isotopes (⁷⁸Kr, ⁸⁰Kr) are critical for determining the volatile Earth’s sources.

All krypton isotopes are anomalous with respect to air, with magnitude of the anomalies, except for ⁸⁶Kr, similar to the composition of Phase Q, a poorly characterized carbonaceous phase that hosts the majority of Kr and Xe in achondrites and chondritic meteorites. The anomalies suggest that at most 55 ± 5 % and 69 ± 10 % of Kr in the Galápagos and Iceland sources, respectively, is derived from subducted atmosphere, significantly less than for Xe, for which 90 % of the deep mantle budget is from recycled air. The results also suggest early accretion of carbonaceous material by proto-Earth. Unexpectedly, the Galápagos and Iceland mantle sources show a peculiar ⁸⁶Kr signature. The implications of these new krypton data for the origin of deep mantle volatiles will be discussed during this presentation.