The conventional way of filling very fine gaps in parquet floors, is to use a resin mixed with the sanding dust that comes off the floor. The sanding dust colours the resin to the same colour of the floor.
Pine Floors & Floorboards
Pine floors and floorboards can be filled with a flexible silicone.
Although with Pine floors, the gaps often help retain their character.
Draughtex is a black or grey rubber strip that can sit in and just beneath the gap of the floor, or level
with the floor.
Visit the site below to see.