Bison Designs, which seem to mainly sell key chains and other trinkets, have created a carbon fibre carabiner key chain which can hold around 500lb (227 kg) and weighs 10 grams. According to a description on carbonfibergear.com the gate is heavier than the body of the carabiner. Unfortunately 227 kg is about 1/10th of the strength required for climbing carabiners so it won't be so useful for that. But it will hold a mighty heavy set of keys. The niche for a carbon fibre carabiner for rock climbing still waits to be filled.
Image from Bison Designs