Europium as a universal tracer

In many respects, the rare earth element (REE) europium, an element whose abundance rarely exceeds a few parts per million in crustal rocks of any planet, turns out to be one of the most useful elements in geochemistry and cosmochem-istry. By its enrichment or depletion relative to the other REE, one can trace much of the history of processes in the Solar System and the Earth. But the element is also useful to astrophysicists. Europium is formed in stars almost entirely by the r-process of nucleosynthesis. The atomic emission lines of europium at 4129 and 4205 A are readily measured in stellar spectra, so that its abundance gives a measure of the contribution to the element abundances by the nucleosynthesis of the r-process isotopes, particularly in the earliest stars. By measuring Th/Eu ratios (using the Th line at 4019 A), an estimate of the age of the star can be established by the depletion in thorium (232Th has a half-life of 14.2 Gyr) relative to the abundance of europium [33].

Next, europium records the existence of a diverse series of events in the early Solar System that are recorded in the earliest samples to which we have access: the calcium-aluminium inclusions (CAIs) in primitive meteorites. These include high temperature evaporation and condensation processes that separated europium relative to the more refractory lanthanides on account of its relative volatility.

However, when processes of planetary differentiation begin, europium with its ability to record oxidizing or reducing environments, is of extraordinary usefulness. Thus in basaltic meteorites that are derived from the asteroid 4 Vesta, depletions and enrichments in europium record the details of crystal-liquid fractionation occurring in basaltic lavas.

The depletion of europium in lunar basalts and its complementary enrichment in the feldspar-rich lunar crust was a major key to our understanding that much of the Moon was melted and that a feldspar-rich crust had developed very early in lunar history. Such events did not occur on the Earth, and are probably unique in the Solar System, with a possible exception on Mercury. To account for the amount of feldspar in the lunar crust, close to 50% of the aluminium and europium in the bulk had to be concentrated in the crust, shortly after the formation of the Moon.

Flotation of feldspar crystals in a dry magma ocean is the only viable hypothesis. All basalts derived from the lunar interior display a complementary depletion in europium, due to an overall depletion of their source regions by the prior crystallization of the plagioclase feldspar now in the highland crust. In retrospect, this behavior of europium was one of the most significant geochemical observations on lunar rocks.

The distribution of REE in the crust of the Earth, that has been a major clue to the evolution of the continental crust, further highlights the importance of europium. The uniform REE patterns observed in continental-crustal sediments on the Earth, with the regular depletion in europium, provide not only a key to the problems of estimating the composition of the crust, but provide a crucial piece of evidence about its evolution. Europium is trivalent under the oxidizing conditions at the surface of the Earth, so that the observed depletion of that element in the upper crust records a previous history of that element as a divalent ion under more reducing conditions. This depletion in europium thus provides evidence that the present upper crust of the Earth was formed by the production of granitic melts deep within the crust under reducing conditions; leading to the retention of europium in plagio-clase in the deep crust, with a corresponding depletion at the present surface. The general absence of europium depletion in Archean crustal terrains demonstrates the difference between Archean and Post-Archean crustal development. Overall, an extraordinarily useful element [34].

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