I'm guessing Dynamics/Particles are used so you should not be in Maya Software, if so try Maya Hardware in Render Globals. Select .iff as I believe you can only get Z depth in .iff format. Production quality with transparency is what I use as Z depth will give you true transparency around your objects for your composit program. Select the Frames you want rendered. Recently I rendered 1,800 frames in Z depth, opened the sequence in adobe A/E. It does come out beautiful. Just to let you know The frames took up about 1.3 GB. If you could be a little more precise. I'm sure someone here can help you through this Good Luck