Monitoring Tectonic Cycle and Mantle Reservoirs Using Uranium Isotopic Fractionation

Recycling process of atoms in the tectonic system can bring us some information about geological events occurred in the Earth. In this paper, scientists measured the distribution of surface-processed uranium by using different behavior of uranium oxide to other trace elements, found the residence time of MORB and OIB sources. They also suggested a model of uranium cycle from the ancient Earth. (Th/U anomaly caused by an existence of oxygen in surface is appeared both in upper mantle and deeper mantle reservoirs, and U isotopic composition anomaly caused by an equilibrium fractionation is appeared only in the upper mantle.)

They mainly discussed two major terms of specimens, the Th/U value which is a concentration of thorium divided by a concentration of uranium and which is the parts per thousand difference in a concentration of divided by that of relative to a reference solution standard(CRM 145).

Th and U are both lithophile elements and appear similar behavior in the melting process. In that case, both of the elements exist in tetravalent species. But when the two elements are under some oxidized surface, by oxidation of U, the water solubility of uranium increases while Th tends to remain in solid phases. Therefore the Th/U value increases by weathering and alteration in the surface having oxygen. In ancient history of Earth, the Great Oxygenation Event occurs in 2.4Gya, so the uranium cycle is thought to be started in 2.4Gya. Th/U value was used in explaining recycling of surface U.

In fully oxidized surface, two uranium oxides exists. With some physiochemical backgrounds, the heavier isotope prefers to an oxide that exists in aqueous phase and the lighter isotope prefers to an oxide that wants to remain in solid phase. Thus the U isotopes participate in each of the compounds in different ratio, causing equilibrium fractionation of U. The theory of the equilibrium fractionation also tell us that the fractionation occurs strongly in a low temperature. Therefore the fractionation only occurs in the crustal rocks with fully-oxidized ocean. In other words, a positive (in some parts, also a negative) anomaly of is a signal of surface-processed uranium. Since the ocean was fully oxidized in 600Mya, the fractionation is thought to be started

in only 600Mya.

According to the Th/U- diagram presented by the authors, It is found that MORBs (-0.27~-0.22‰) and altered oceanic crusts (-0.17‰ in average) have high values of , and island arc basalts (~-0.4‰) have low values of , compared to of BHVO-2(-0.314‰). The low value in island arc basalts is made from dehydration of subducting slab. In contrast, values of OIBs did not have a big difference with the values of bulk Earth. This result means that the altered oceanic crusts whose fractionation of U occur are mixed into the upper mantle, but don't affect deeper mantle reservoirs yet.

With prediction, the high Th/U value has to be measured in upper mantle and crusts. However, the measured values are very low and have no anomalies. The result says that the removed surface-processed U was recycled rapidly.

Another result is that the Th/U value of most OIBs are lower then the bulk silicate Earth. It is shown as the effects that surface-produced U spreads to, and changes the composition of the lower part of mantle. The lowering of Th/U increases with the age of the reservoirs. This result says that at least the mantle reservoirs formed between 2.4 and 1.8Gya(the age range of OIB specimens) have been affected by a subducting slab.

A model formed based on the measurements is also shown.(figure 1)

These results give possibility for expecting two rises of oxygen in Earth's early ocean. the results are consistent with history of notable oxygen rise in 2.4 and 0.6Gya(Figure 4).

In 2.4Gya, the Great Oxygenation Event occured and oxygen contents in the Earth's ocean and atmosphere began to increase. Uranium becomes fluid-mobile under oxic conditions and Th/U values start to increase in crust and mantle. However, there was not enough oxygen to make a fractionation of U isotopes in the surface, value was constant until 0.6 Ga.

Once the ocean became fully-oxidized at 0.6 Ga, the fractionation of U isotopes could occur, resulting in high value of the altered oceanic crust. the AOC subducted into the depleted upper mantle, and raised of the upper mantle. During the subduction, the water contents were released into mantle wedge, produced island arc magma. Therefore value of island arc basalts became lower.

The whole process of U cycle is described in figure 1. Furthermore, with the distribution of value, we can infer that the whole mixing of upper mantle occurs in less than 0.6Gyr. The time of the whole mixing of deeper mantle is not clear but the results find that the effect of subducting slab for Th/U value in the OIB reservoirs formed in 2.4-1.8Gyr. Thus the period of the deeper mantle cycle can be investigated.