REPLACE INTO - how to Not update a field

I dont know if this is possible but I thought someone might know?
I have a REPLACE INTO query working fine but I want a field in the table to indicate the date and time a record was added to the table.

I want a field that indicates when the record was first added and I never want that field updating.

The data is supplied to me via XML and I want to use a REPLACE INTO to update the table as necessary.

Any ideas?