Early and long-term mantle processing rates derived from xenon isotopes

Abstract:

Noble gases, particularly xenon (Xe), in mantle-derived basalts provide a rich portrait of mantle degassing and surface-interior volatile exchange. The combination of extinct and extant radioactive species in the I-Pu-U-Xe systems shed light on the degassing history of the early Earth throughout accretion, as well as the long-term degassing of the Earth’s interior in association with plate tectonics. The ubiquitous presence of shallow-level air contamination, however, frequently obscures the mantle Xe signal. In a majority of the samples, shallow air contamination dominates the Xe budget. For example, in the gas-rich popping rock 2ΠD43, ¹²⁹Xe/¹³⁰Xe ratios reach 7.7±0.23 in individual step-crushes, but the bulk composition of the sample is close to air (¹²⁹Xe/¹³⁰Xe of 6.7). Thus, the extent of variability in mantle source Xe composition is not well-constrained. Here, we present new MORB Xe data and explore constraints placed on mantle processing rates by the Xe data.

Ten step-crushes were obtained on a depleted popping glass that was sealed in ultrapure N₂ after dredge retrieval from between the Kane-Atlantis Fracture Zone of the Mid Atlantic Ridge in May 2012. 9 steps yielded ¹²⁹Xe/¹³⁰Xe of 7.50-7.67 and one yielded 7.3. The bulk ¹²⁹Xe/¹³⁰Xe of the sample is 7.6, nearly identical to the estimated mantle source value of 7.7 for the sample. Hence, the sample is virtually free of shallow-level air contamination. Because sealing the sample in N₂upon dredge retrieval largely eliminated air contamination, for many samples, contamination must be added after sample retrieval from the ocean bottom.

Our new high-precision Xe isotopic measurements in upper mantle-derived samples provide improved constraints on the Xe isotopic composition of the mantle source. We developed a forward model of mantle volatile evolution to identify solutions that satisfy our Xe isotopic data. We find that accretion timescales of ~10±5 Myr are consistent with I-Pu-Xe constraints, and the last giant impact occurred 45-70 Myr after the start of the solar system. After the giant impact stage, the Pu-U-Xe system indicates that degassing of the planet via solid-state mantle convection and plate tectonics continued to liberate volatiles to the atmosphere and has led to between ~5-8 mantle turnovers over the age of the Earth.