Precision analysis of lava samples taken from the Kilauea Iki volcanic crater in Hawaii has given planetary scientists a new tool for reconstructing planetary origins.
As lava cools and solidifies, the content of its iron isotopes evolves, albeit by minute variations in the element at subatomic levels. Kilauea Iki has been sampled multiple times as it cooled over the years, making it the perfect site to track the chemical evolution of the lava over time. The results have enabled scientists to learn more about the formation of the crust than they previously understood.
Hawaiian volcanoes produce basaltic lavas, like that shown here on the flanks of Mount Kilauea. Molten basalt flows relatively smoothly over the surface of the Earth because it is high in iron and magnesium and low in silica. Basalt also makes up much of the Martian crust. Image credit: NASA/JPL
“Magmas are formed by melting of rocks which can then undergo differentiation, a process of chemical evolution driven by crystallisation of minerals and segregation of these minerals,” explains Nicolas Dauphas of the University of Chicago. “In an analogy, it is the same process that would enrich salty water in salt as fresh water ice crystals are formed. This new study gives us a tool for understanding how light colored continental rocks like granites were ultimately formed from dark colored oceanic rocks like basalts.”
The finding contradicts the widely held view that isotopic variations occur only at relatively low temperatures, and only in lighter elements, such as oxygen. But, by using precision instruments in Dauphas’s Origins Laboratory, the research team were able to measure isotopic variations as they occur, in magma at temperatures of 1,100 degrees Celsius.
If applied to a variety of terrestrial and extraterrestrial basalts, including meteorites from Mars and the asteroids, the method could provide more definitive evidence for a scientifically popular idea that the Moon was born from a giant collision between Earth and another large object.
“Some people have suggested that the giant impact vaporised some material, which would have imparted a unique iron isotope signature to the Moon relative to the Earth,” Dauphas tells Astronomy Now. “Our results show that the story may be more complicated than that. If different terrestrial or lunar rocks have different iron isotopic compositions, how do you define the compositions of these two objects?”
The implications of the new work poses some interesting questions that will no doubt be answered in the future. But in the meantime, scientists will be able to use iron isotopes as fingerprints of magma formation and differentiation here on Earth, which played an important role in the formation of our continents.