There is a crack on the other side as well. Looking again at this picture, a section at the base is separating. I appreciate your comments about corrosion. Likely best move would be a plan to replace the mast.You cannot weld corrosion of which there is a significant amount there.
How far up (check the inside) does the corrosion extend ?
Don't think the cost of sleeving or cutting/adding a section would be cost efficient.
The mast is silver or clear aluminum.As a long time Thistler I'm perplexed by the corrosion of the mast butt fitting. I've never seen anything like it. It also raises concern about the mast butt itself which also appears to be quite corroded. To repair it I would remove the cap, remove the corrosion from the mast butt fitting and the interior of the mast with a wire brush, assess the wall thickness after the corrosion removal, and, if I were proceed after this assessment, I would drill small holes at the end the cracks to control spread, coat the interior of the mast butt and the exterior of the mast butt fitting with Boeshield T-9 (or some other corrosion control product), and wrap the exterior of the mast base with a couple of layers unidirectional carbon fiber cloth imbedded in epoxy resin.
I couldn't make out the color of the mast section. Early Thistle masts were spruce. Later masts were gold anodized aluminum, which proved too stiff and heavy. Later alumimum masts were clear or black anodized. Only the clear or black masts are competitive.