Xenon isotope geochemistry
[1] The depletion and heavy isotopic enrichment can be explained by hydrodynamic escape to space that occurred in Earth's early atmosphere.
129Xe was derived from the extinct nuclide of iodine, iodine-129 or 129I (with a half-life of 15.7 Million years, or Myr), which can be used in iodine-xenon (I-Xe) dating.
A reservoir that remains an entirely closed system over Earth's history has a ratio of Pu- to U-derived fissiogenic Xe reaching to ~27.
[6] Accordingly, the isotopic composition of the fissiogenic Xe for a closed-system reservoir would largely resemble that produced from pure 244Pu fission.
Iodine-129 decays with a half-life of 15.7 Ma into 129Xe, resulting in excess 129Xe in primitive meteorites relative to primordial Xe isotopic compositions.
When the Earth became closed for the I-Xe system, Xe isotope evolution began to obey a simple radioactive decay law as shown below and became predictable.
The estimated iodine-127 concentration in the Bulk Silicate Earth (BSE) (= crust + mantle average) ranges from 7 to 10 parts per billion (ppb) by mass.
First, in the early solar system, planetesimals collided and grew into larger bodies that accreted to form the Earth.
[2] The second problem is that the total inventory of 129Xe on Earth may be larger than that of the atmosphere since the lower mantle hadn't been entirely mixed, which may underestimate 129Xe in the calculation.
Last but not least, if Xe gas not been lost from the atmosphere during a long interval of early Earth's history, the chronology based on 129I-129Xe would need revising[17] since 129Xe and 127Xe could be greatly altered.
Compared with solar xenon, Earth's atmospheric Xe is enriched in heavy isotopes by 3 to 4% per atomic mass unit (amu).
A possible explanation is that some processes can specifically diminish xenon rather than other light noble gases (e.g. Krypton) and preferentially remove lighter Xe isotopes.
The first assumes that the Earth accreted from porous planetesimals, and isotope fractionation happened due to gravitational separation.
But following research suggested that Xe isotope mass fractionation shouldn't be a rapid, single event.
[21] Research published since 2018 on noble gases preserved in Archean (3.5–3.0 Ga old) samples may provide a solution to the Xe paradox.
[21][22] Isotopically mass fractionated Xe is found in tiny inclusions of ancient seawater in Archean barite[23] and hydrothermal quartz.
The isotopic fractionation gradually increases relative to the solar distribution as Earth evolves over its first 2 billion years.
[21] This two billion-year history of evolving Xe fractionation coincides with early solar system conditions including high solar extreme ultraviolet (EUV) radiation[15][25][26] and large impacts that could energize large rates of hydrogen escape to space that are big enough to drag out xenon.
However, models of neutral xenon atoms escaping cannot resolve the problem that other lighter noble gas elements don't show the signal of depletion or mass-dependent fractionation.
As a result, Xe isotope fractionation may provide insights into the long history of hydrogen escape that ended with the Great Oxidation Event (GOE).
[28] Understanding Xe isotopes is promising to reconstruct hydrogen or methane escape history that irreversibly oxidized the Earth and drove biological evolution toward aerobic ecological systems.
[21][31][32] Other factors, such as the hydrogen (or methane) concentration becoming too low or EUV radiation from the aging Sun becoming too weak, can also cease the hydrodynamic escape of Xe,[28] but are not mutually exclusive.
Therefore, Kr can be rapidly returned to neutral and wouldn't be dragged away by the charged ion wind in the polar region.
During the Great Oxidation Event (GOE), the ozone layer formed when O2 rose, accounting for the end of the MIF-S signature.
[35][36] However, potential memory effects of MIF-S due to oxidative weathering can lead to large uncertainty on the process and chronology of GOE.
[37] Compared to the MIF-S signals, hydrodynamic escape of Xe is not affected by the ozone formation and may be even more sensitive to O2 availability,[32] promising to provide more details about the oxidation history of Earth.
The first explicit recognition of non-atmospheric Xe in terrestrial samples came from the analysis of CO2-well gas in New Mexico, displaying an excess of 129I-derived or primitive source 129Xe and high content in 131-136Xe due to the decay of 238U.
[47][48] Theoretically, the many non-radiogenic isotopic ratios (124Xe/130Xe, 126Xe/130Xe, and 128Xe/130Xe) can be used to accurately correct for atmospheric contamination if slight differences between air and mantle can be precisely measured.
[50] Alternative models for Mars consider that the isotopic fractionation and escape of Mars atmospheric Xe occurred very early in the planet's history and ceased around a few hundred million years after planetary formation rather than continuing during its evolutionary history[21][51] Xe has not been detected in Venus's atmosphere.
The lack also prevents us from checking if the isotopic composition has been mass dependently fractionated, as in the case of Earth and Mars.
