Barclays said today it would temporarily stop creating more shares of the iPath Dow Jones Platinum Subindex Total Return ETN (NYSEArca: PGM), effective immediately. PGM tracks the performance of front-month platinum futures contracts traded on the New York Mercantile Exchange.

PGM has just $100 million in assets, far below the levels of other funds that have previously halted creations, such as the $4 billion United States Natural Gas Fund (NYSEArca: UNG). (UNG has since reopened.)

But the platinum futures market is a very thin market, and the New York Mercantile Exchange has tight “accountability” limits in the platinum space. Specifically, the exchange sets its “accountability” limits at 1,500 net futures positions. Above those levels, investors must notify the exchange of their positions, and the exchange can ask them to reduce that position if it believes that the investor is unduly influencing the market.

Not coincidentally, a 1,500 futures position in the platinum market is currently worth $100.5 million: exactly the level of PGM.

We have some indication that NYMEX has started to enforce accountability levels in the commodities space. The PowerShares DB Double Long Crude Oil ETN (NYSEArca: DXO) was shut down and liquidated after Deutsche Bank was notified that it had reached its position limit in the oil market and needed to reduce its position.

PGM is the fifth commodities product to halt creations. In the past, ETPs that halted creations have quickly traded to a premium above their net asset value, as strong investor interest chases a limited number of shares. That is likely to be the case with PGM as well. Investors buying in at inflated levels are taking on significant risk, as that premium can quickly deflate if the fund discovers a way to resume issuing new shares.

There is no word if or when PGM will do that. It is likely to wait until the end of October when the Commodity Futures Trading Commission is expected to announce new federal position limits in the commodity futures space.