Where is most of the co2 that has outgassed from earth’s volcanoes?

Locked up in rocks.

Explanation:

The volcano consists of the high melted lava that are present in the rupture of the earth's crust. The disturbances and movement in the tectonic plates might cause volcano eruption.

The volcanic eruption releases large amount of carbon dioxide, ash and harmful chemicals. The carbon dioxide that are erupted is locked in the rocks that are present on the earth's surface. The carbon dioxide is stored in the form of carbonates in the rock.

Thus, the answer is locked up in rocks.