This blog has been written by group member Dr Patricia Clay
Reduced salt is key in Earth’s new recipe
How the Earth acquired its volatile elements, like water, has long interested scientists because they are important in influencing not only the chemical evolution of planetary bodies, but also to the development of life at Earth’s surface. In a new study out today in the journal Nature , a University of Manchester and University of Oxford led project investigated this problem by measuring the halogen group elements, chlorine, bromine and iodine, in a diverse group of primitive meteorites. These meteorites may have been some of the building blocks from which Earth formed. The halogens are an important group of volatile elements that are fluid mobile and moderately-highly incompatible. This makes them excellent geochemical tracers. The halogens are also reactive, readily forming naturally occurring salts, which are essential for most life forms. Yet the balance of our planets salt levels needs to be just right, and too much is thought to prohibit life from flourishing. Previous halogen measurements of primitive meteorites showed that their levels were too high in meteorites to account for the levels on Earth today – meaning that somewhere along the way, Earth had to lose a significant portion of its halogens. This turns out to be a difficult thing to do, without also affecting elements of similar volatility.
Using a highly sensitive technique for halogens developed in the Isotope Group at the University of Manchester, NI-NGMS (neutron irradiation noble gas mass spectrometry) we measured low level (parts per million and part per billion) halogens in bulk chondrite (i.e., primitive asteroid) meteorites. We found that halogen levels were far lower than previously thought. These new results mean that we don’t have to look for ways to get rid of the ‘extra’ halogens – there was simply less there to start with. Our measurements also showed remarkable consistency across different types of meteorites – this is important because it means that whatever meteorites formed the Earth, the halogen ingredients for Earth’s recipe remains essentially the same. Crucially, with lower halogen concentrations in primitive meteorites, the halogen group elements behave as expected in line with elements of similar volatility.
The full paper citation is: Patricia L. Clay, Ray Burgess, Henner Busemann, Lorraine Ruzié-Hamilton, Bastian Joachim, James M. D. Day, & Christopher J. Ballentine (2017) Halogens in chondritic meteorites and terrestrial accretion Nature 551, 614–618 doi:10.1038/nature24625