Do aluminium chainrings/cassettes contribute to chain wear? In trying to achieve a long lasting, low maintenance drivetrain, I've read statements about aluminium oxide being the source of the black colour of chain dirt and also being highly abrasive:


  Aluminum oxide from chainrings makes chain dirt black - and it is very hard and abrasive. Grinding wheels are made of aluminum oxide!
  
  &mdash; John Allen on sheldonbrown.com


Others state that pretty much all the abrasives (including the aluminium oxide) come from the road:


  I believe almost all the grit comes from the road. Aluminum is very 
  common in most soils, in the form of aluminum oxide, which is a pretty 
  effective abrasive.
  
  &mdash; Frank Krygowski in rec.bicycles.tech


My question is this:

How much of the dirt/abrasive on a chain comes from the chainrings, and how much does it really affect chain wear?

My feeling is that the aluminium oxide particles ground off the chainrings would be of very low diameter, and thus wouldn't have a significant abrasive effect compared to the larger particles of sand and dirt thrown up from the road/path itself. Obviously this brings up questions as to the lubricant used (as a sticky lube will attract dirt) and riding conditions, but for the sake of this discussion, I'd like to consider a bicycle which is regularly maintained and ridden in all weather, primarily on made roads.

To bring this into the realm of the practical, my reason for asking is that if the cog material truly does affect chain longevity, I would consider changing to steel cogs (not currently considering the other advantages/disadvantages of steel, which have been covered elsewhere).
Interesting question.  Real world conditions are messy with multiple factors impacting any analysis, as such my answer will be speculative, but based on a number of sensible working assumptions. 

Chainring degradation as a source

First lets consider how much material comes off a chainring. Generally speaking, if you replace your chain regularly before it stretches, it should be quite easy to  get 10,000km+ out of a set of chain rings before needing to replace them.  This indicates a slow rate of wear, and therefore a small amount of contaminating/abrasive material.  Furthermore the chain ring touches the rollers of the chain, not the whole chain width.  As such, the small amount of material that does get deposited gets deposited on the roller.  It still needs to make its way to the edge of the roller and then down the side of the roller and into the contact area between the roller and the "bushing-inner plate" before it will start contributing to chain wear.  Given the location of the abrasive material and the limited and slow movement of the rollers (this of course depends on your gearing - e.g., Big-big will have slower roller movement than Big-small), I suspect this is a very slow and low volume process.

Road debris as a source

Now contrast this to road debris.  Even when riding on pristine roads your frame will often get covered in a fine layer of dust.  This dust is being kicked up primarily from the front wheel. Next look at the proximity of the chainring to the front wheel.  It is unfortunately located in very close proximity to the dust plume kicked up by the front wheel.  This will cover the chain with a lot more debris.  It will also force debris into the gap between the roller and the inner plate, which will make it easier for the debris to make its way down into the deep recesses of the chain roller / bushing-inner plate assembly.  Given the volume of debris this will likely be the predominant source of abrasive material and therefore wear.

Lubes

I noticed you mentioned choice of lube


  Obviously this brings up questions as to the lubricant used (as a sticky lube will attract dirt) and riding conditions


This is the common wisdom, but I personally question it.  Over the last 3 years I have been carefully recording chain longevity under different lubes and conditions. Under the same conditions (brand/model of chain and season) a thicker wet lube resulted in longer chain life. Sometimes the difference relative to a light or dry lube was startlingly (e.g., 1000 km more life on an Ultegra chain).  I suspect the thicker/stickier lube remains in place better (some sort of hydrodyamic property that I can't remember the technical term). As for debris, it does have an effect, but I suspect it is somewhat over-rated as I only see a small drop (e.g., ~300-500 km) during the winter under the same lube chain combo and in some pretty nasty muddy conditions as I keep off the main roads and ride on more gravel during the winter.

In the end I suspect choice and frequency and quality of lubing may actually play the biggest role!