Misunderstanding what I am getting at.
Uneven heating of "very hard" materials causes one part of the material to expand according to its coefficient of thermal expansion, while the other contracts likewise. This causes internal stresses in the material, which causees it to crack. You are using heat and cold to leverage the "indomitable nature" of the adamantine itself, to destroy the adamantine golem.
Adamantine is hard. VERY VERY hard. That's where it's neigh indistructibility comes from. Hit it with anything, it bounces off and does not even leave a scratch, let alone a ding.
That's also why it would be very susceptible to thermal fracture.
Other, less than orthodox means of defeating the golem:
1) Conjure a gelatinous cube, order it to engulf the golem, then cast Stone on the cube. (This basically embeds the golem in a giant granite block.) This needs to be done in rapid succession before the golem has time to have a turn.
2) If interpreting adamantine faithfully to literature, (hah.) then creative use of Stone to Flesh would turn it into a flesh golem. (The real world legends behind adamantine come from
Adamant, a gemstone.) It would then be MUCH easier to kill. (It would also make for hilarious means of disarming foes weilding adamantine weapons.)
Getting around the thermal expansion/contraction problem by saying something like "Look it has an absurdly high specific heat, OK?" means that wearing adamantine armor is like wearing an asbestos suit, only better. Hit with absurd cold? It stays room temperature to the touch, and anyone inside is fully protected from the cold. Likewise with heat. It also means that if you find adamantine gear on a trip through an elemental plane, and it has been there awhile, it will have become "room temperature" for that plane. So, elemental fire world? You have a permanent cherry red adamantine weapon. Will stay hot for a VERY VERY long time. (basically forever). Likewise with cold from an ice realm. I doubt you want to have to make such concessions
Using straigh tup handwaving to ignore the challenge is being a bad DM in my book. Especially when the degree of fracture can be easily computed.