An old Hawaiian lava lake Image:Shiny Things
Atoms are funny things. The tiniest variations can have dramatic macroscopic consequences and, conversely, dramatic macroscopic changes can have subtle influences at the atomic scale. Thus it is that we come to the burgeoning field of stable isotope geochemistry.
Different stable isotopes of the same element behave almost identically chemically. They have the same charge, are essentially the same size, but have slightly different atomic weights. These subtle differences in weights can cause chemical and physical processes to preferentially pick one stable isotope over the other, leading to measurable fractionation of these isotopes.
It was long thought that fractionation of metals in magma was minimal. Theoretically, this makes sense because fractionation decreases with increasing temperature, and it decreases proportional to the atomic mass (m) as 1/m2. Thus heavy, hot materials should not fractionate significantly, and any fractionation that has been observed in mantle derived rocks was assumed to have occurred after the original crystallization. But this isn't always the case, according to a recent paper published in Science.
In this paper, a team from the Fermi Lab in Chicago looked at the isotopic composition of Iron (56Fe/54Fe) in Olivine crystals that formed in a lava lake in Hawaii. Olivine is a good mineral to study because it is the first common mineral to crystallize as a magma body cools, thus its crystallization is unaffected by that of other minerals. They found that Olivine crystals were slightly lighter isotopically than the remainder of the rock. This variation was relatively small (-0.1 vs. 0.2 δ56Fe‰), although individual samples were as low as -1.1 δ56Fe‰.
Why should there be any fractionation? True to theory, other metals such as Magnesium and Lithium do not fractionate during crystallization, so why does iron? Unlike other metals, iron exists in two oxidation states Fe2+ and Fe3+; Olivine will only take Fe2+ in its crystal structure. The authors propose that this dual oxidation state is what leads to the fractionation, but they fail to expound on the idea. It seems that they are proposing that Fe2+ is typically slightly lighter isotopically, which seems like an easy hypothesis to test, but I am unaware of any relevant data. Hopefully, future work will look into the matter.
Aside from helping us to interpret rocks that formed deeper in the earth, understanding iron isotopes tells us about planetary formation. In particular, because it was assumed that iron does not fractionate at high temperatures, variations in iron isotopes seen in meteorites were assumed to be a result of heterogeneities during the formation of our solar system or a result of planetary accretion processes. Now, we have to consider the possibility that fractional crystallization can also affect iron isotopes in these bodies.
Science, 2008. DOI: 10.1126/science.1157166