Constraints on the timing of the Moon-forming giant impact from MORB Xe isotopes

Abstract:

As Earth accreted, volatiles were delivered by accreting material and lost by degassing and impact-driven ejection to space. The Moon-forming giant impact initiated the final catastrophic outgassing and bulk volatile ejection event on the early Earth. I-Pu-U-Xe systematics provide a powerful tool to probe degassing of the early Earth. Radiogenic ¹²⁹Xe was produced by β-decay of the extinct nuclide ¹²⁹I (t_1/2 = 15.7 Ma) in the first ~90 Myr of Earth history. Fissiogenic ¹³¹Xe, ¹³²Xe, ¹³⁴Xe, ¹³⁶Xe were produced in distinct, characteristic proportions by the fission of extinct short-lived ²⁴⁴Pu (t_1/2 = 80.0 Myr) and extant long-lived ²³⁸U (t_1/2 = 4.468 Gyr). Here we present radiogenic and fission Xe data in basalts from the Southwest Indian Ridge, and discuss them with other mantle-derived samples to shed light on early Earth volatile accretion and loss.

Based on the ratio of radiogenic ¹²⁹Xe to plutogenic ¹³⁶Xe determined for the MORB source, we calculate an I-Pu-Xe closure age for the upper mantle of ~44-70 Myr after the start of the Solar System. The closure age should correspond to the end of catastrophic mantle outgassing during accretion, and thus constrains the age of the last giant impact (LGI). Our closure age is significantly older than previous Xe closure age determinations of ~100 Myr, and is also older than some direct radiometric ages of lunar crustal samples.

In order to explore the effects of accretion timescales, partial early retention of Xe, and degassing associated with long-term mantle processing on Xe closure age, we develop a new model of I-Pu-U-Xe systematics. We find that for LGI’s between ~35 and 70 Myr after the start of the Solar System, we are able to satisfy constraints on I-Pu-U-Xe systematics simultaneously without invoking partial retention of Xe prior to the last giant impact. For LGI’s after ~80 Myr, partial retention of Xe prior to the LGI is required. Non-zero early retention of Xe is necessary to explain the budgets of primordial ^{124,126,128,130}Xe in the MORB source mantle; however, a high degree of Xe retention in the upper mantle throughout accretion is physically unlikely. We use Monte Carlo analysis to locate the parameter space that satisfies our I-Pu-U-Xe constraints and to thereby improve our age constraint on catastrophic outgassing triggered by the Moon-forming giant impact.