Vikings were the Klingons of the era and loved a good fight. It was probably some sort of a war trophy symbolism, like look what we did to these fockers kinda thing. 

That's what I thought. Probably a gift or war loot.

AO jaan, there is a recent article in The Guardian and the Swedish university site (? Uppsala I believe ?) that I think explains the opinions of the researchers better. Apparently, there is "genetic" evidence from a Viking burial site that 1 or 2 buried individuals are believed to be from Iran. There was contact around 1000 AD or so between Iran and the Vikings that were in Russia at the time (? I seem to recall via the Caspian Sea ?), and coins have been discovered that demonstrate this. There was also a Muslim traveler named Ibn Fadlan who described his encounters with the Vikings in Russia.