This is a great question that needs to be researched. Most of the silver that came up to Scandinavia first went through a mint somewhere in the Near East. Each one of these mints would presumably have its own signature alloys, and each of these alloys would presumably have changed over time as different mines went in and out of service within a given mint's catchment area. Trade would also play a role, which would introduce some seemingly random compositions into the minted coins, but there might also have been patterned changes if, for example, a lot of silver from German mines started ending up in North African mints via trade.

In short, you've asked a very insightful question that would tell us a lot not only about who Viking-Age Scandinavians were trading with, but also who those people were trading with. Knowing these things has the potential for radically redefining our understanding of the early middle ages.

Unfortunately, I'm not aware of any major studies being done on silver composition, although scholars and museum curators across Scandinavia are increasingly aware of what these studies might tell us. We're only starting to develop non-destructive means of testing chemical composition of irreplaceable artifacts, so it's becoming a possibility. The critical factor here is money, since not only must the technology be on hand, but there must be people trained and employed in doing the sampling and analysis.

Given the way that these kinds of research projects are currently run in Scandinavia, somebody will need to undertake this research as their PhD thesis, which will establish a baseline survey and thus allow individual museums to begin sampling their artifacts and interpreting them in light of the PhD's results. Only then will we start building the broad regional data set and publishing the scholarly studies needed to answer your question.

Got any plans for the next 3-5 years?