Pre-subduction mantle noble gas elemental pattern reveals larger missing xenon in the deep interior compared to the atmosphere

Abstract

Understanding volatile evolution on Earth provides critical information on the processes that shape the Earth, and hence the solar system. The noble gas elemental and isotopic composition of the Earth's mantle traces the sources of Earth's volatiles as well as evolution processes, such as mantle degassing and regassing via subduction. However, ubiquitous shallow-level atmospheric contamination of mantle-derived samples has hampered determining the deep mantle heavy noble gas (Kr, Xe) isotopic and elemental composition. Moreover, the present-day elemental composition of the mantle reflects mixing of initial volatiles with atmospheric noble gases recycled through subduction that makes understanding the elemental signatures acquired during accretion difficult. Using a recently-developed protocol, we previously measured the krypton and xenon isotopic and elemental compositions of the Galápagos and Iceland plume sources, which have among the most primitive helium and neon isotopic signatures, sampling one of the least degassed, most primordial mantle reservoirs. Based on these measurements, here we introduce a new approach to correct for recycling and hence determine the initial noble gas elemental ratios of the deep mantle. Our study leaves room for a substantial proportion of 36Ar in the mantle to be primordial in origin. Most importantly, we show that the deep mantle, prior to the injection of any atmospheric volatiles, was depleted in Xe relative to Kr by about two orders of magnitude when compared to chondritic compositions, and the Xe depletion was larger in the mantle than in the modern-day atmosphere. Unlike the atmosphere that has protracted history of Xe depletion through the Archean, the deep mantle xenon depletion was acquired very early in Earth's history, most likely during accretion. Hence, missing xenon in the Earth's interior and in the atmosphere appears to be two distinct problems. Three possible scenarios, potentially acting in sync, may have led to the mantle xenon depletion: Xe partitioning into the core, magma ocean outgassing and a xenon deficit in the Earth's parent bodies, such as comets.