Making the estimate that the ice is about 3 meters thick in the arctic. (Of course, it is much thicker some places and much thinner in others.) Then the energy of Mount St. Helens would melt about 100 square kilometers of ice in the Arctic. (Assuming the energy were transmitted straight to the ice, without heating the water.)
The bottom line
The Arctic goes through some serious changes in sea ice extent every year as the season change. The sea ice extent changes by about 10 million square kilometers every year. 100 square kilometers is about one hundred thousandth of that. It would take a thousand volcanos the size of Mount St. Helens every year to account for just 1% of the yearly Arctic ice loss.
The numbers for the above conclusion are worked out here
Best Regards,
ClimateSanity