DALLAS (AP) — Oil tycoon T. Boone Pickens and his fourth wife, Madeleine, are filing for divorce.

Pickens spokesman Jay Rosser said Tuesday that the separation is amicable.

Boone Pickens is an Oklahoma native who made much of his fortune in oil drilling. The 84-year-old Pickens has since become an advocate for alternative energy sources. He also is active in politics and has donated millions of dollars to his alma mater of Oklahoma State University and various other causes.

Madeleine Pickens is an animal rights advocate who works to protect horses.

The two married in 2005.

Rosser says in a statement that while both are sad, "they understand" that Madeleine Pickens has family interests and animal welfare projects in California and Nevada, while Boone Pickens primarily works out of Texas.