Why is my older HVAC heat pump freezing up at the heat transfer vents and coils. It is a solid block of ice.?

I have a seventies model FPH closed loop geo h/p. It has worked well for past 7 yrs I've owned the home. The aluminum "fins" below the blower fan are freezing up like really fast after turning the unit on. I thawed it and restarted and it froze in a few hours again . I live on the Gulf coast and it is quite humid. thx.