Our solar system doesn't have just one asteroid belt, but two. The one orbiting between Mars and Jupiter is either called "The Main Belt" or occasionally "The Asteroid Belt". The second one lies out past Neptune and is called the Kuiper belt (pronounced Kie-pur) named after Gerard Kuiper, one of the pioneering planetary Scientists of the last century. Lewis' question is what kinds of element you'll find out in the Kuiper belt. It's a great question and I can only give a half-answer. So, first let's make sure we're clear on what elements are.
Everything in the Universe is made from a very small recipe of particles. The same particles, rearranged in different forms, gives us everything we can see. These particles don't exist on their own however, they tend to clump together in stable structures we call atoms. Every atom has a number of particles at its centre called "protons" and the number of protons at the centre of the atom determines what type of atom it is. One proton at the centre is called Hydrogen, for instance. Two protons is called Helium, three is Lithium and so on. Each of these different atom types is called an element.
At the moment there are 118 named elements (118 different types of atom) ranging from 1 proton at the core to 118 protons at the core. 91 of these atom-types occur naturally while the other 27 have to be made artificially and only exist for very brief amounts of time.
As far as we can tell, nature only makes atoms with whole numbers of protons. One proton, two protons, three protons etc. Nature doesn't make fractional-protons very easily (NB: there are unusual and exotic states of matter which could theoretically be made which would fit in between the 118 normal elements, but these haven't been made yet).
What this means is that the 91 occuring elements on Earth are quite likely to be the same 91 occuring elements throughout the Universe because element 1.5 doesn't easily exist. What does change are the abundances. Planet Earth is very rich in Iron and Nickel for instance, while Jupiter seems to be very rich in Hydrogen.
So Lewis' question about the amounts of the different elements is really asking how much of each element type we'd find out in the Kuiper belt. And the answer, for now, is that we don't know. We've only ever sent five probes out to that distance and none of them did any landings or drillings, and none of them were return journeys, so we don't yet know what the rocks out there are like.
We know a little about the gases floating in between the rocks however because gases interact with light in specific ways. We can observe the way the Kuiper belt's gases distort the light of distant stars and that tells us what types of atom we have in the gas (so far a lot of Nitrogen, Hydrogen, Oxygen and Carbon) - very similar to the surface of the Earth in fact.
As to the rocks themselves, we can only make educated guesses. Most of the rocks we've discovered in the solar system are generally based on the elements silicon, carbon, oxygen and various metals, so if I had to place a bet I'd say Kuiper objects are probably the same, but for now we don't really know.