Apparatus and methods for gas distribution assemblies are provided. In one aspect, a gas distribution assembly is provided comprising an annular body comprising an annular ring having an inner annular wall, an outer wall, an upper surface, and a bottom surface, an upper recess formed into the upper surface, and a seat formed into the inner annular wall, an upper plate positioned in the upper recess, comprising a disk-shaped body having a plurality of first apertures formed therethrough, and a bottom plate positioned on the seat, comprising a disk-shaped body having a plurality of second apertures formed therethrough which align with the first apertures, and a plurality of third apertures formed between the second apertures and through the bottom plate, the bottom plate sealingly coupled to the upper plate to fluidly isolate the plurality of first and second apertures from the plurality of third apertures.