What are the high field strength and large ion lithophile (HFS or HFSE & LIL or LILE) elements?

Question

There are two groups of elements that are frequently mentioned when discussing incompatible trace elements. They are the high field strength elements (HFS or HFSE) and the large ion lithophile elements (LIL or LILE). What are these groups? How are they used in geochemistry and petrology?

Answer 1

Great question! We tend to split incompatible elements into our LILE's and HFSE's because they can behave differently in different volcanic settings. Let's take a look at what these groups are!

LILE: Large Ion Lithophile Elements

1. As their name indicates these elements are large in size and they also have correspondingly low charge.
2. Good examples of LILE's would be K⁺¹, Rb⁺¹, Cs⁺¹, Ba⁺², Sr⁺².
3. These minerals are often incompatible because they are so BIG and don't fit into the crystal structures of mantle minerals.
4. LILEs are often considered "fluid mobile". This means they want to go wherever water (or other volcanic fluids) are going.

HFSEs: High Field Strength Elements

1. These elements have a high "field strength" which means they have high charge
2. Because of their high charge, there is a stronger attraction between those valence electrons and the positively charged nucleus of that element. That attraction pulls all of the shells in a little closer, making these elements have much smaller radii
3. Good examples of HFSEs include: U⁺⁶, Zr⁺⁴, Hf⁺⁴, Ti⁺⁴, and many of your Rare Earth Elements (REE's)
4. HFSE's are incompatible because of their charge. Your common rock-forming minerals in the mantle do not contain elements that are similar in charge so they can't happily go into those crystal structures.

Both your HFSEs and LILEs are incompatible

1. Because of either their charge (HFSE) or their atomic radius (LILEs), these elements do not easily substitute for the elements in your common mantle minerals.
2. If they can't go into mantle minerals, they're going to get the heck outta dodge when any melting starts occurring!
3. So how do they differ? The most dramatic example is in subduction zone volcanics. Behavior of these elements gives what petrologists call an "arc signature". When oceanic crust subducts, it carries lots of water in that subducting, hydrothermally altered oceanic crust. That water is what causes the mantle to melt in a subduction zone by lowering the melting temperature of the mantle. Remember which one of our LILE's vs HFSE's likes fluid? As soon as this subduction zone asthenosphere starts to melt with some water, our LILEs dive into that magma and eventually erupt at the surface. This means subduction zone volcanoes typically have a notable enrichment in LILE's compared to the HFSEs.