Looks like it may be a stress crack. Is it possible to post another picture from a greater distance and note where it is located. It may require welding to repair the crack.

Just fill it with some paintable silicone and then paint it with Rust-o-leum enamel paint.

Hello k-rider, I replied to you on Metal forum. Thanks for your response!

Hi Tibbrix : thank you. Any recommendations on paintable silicone materials? Will Home Depot or OSH have those?

I agree that it looks like a stress crack. The stress doesn't have to be from use of the furniture but can be within the metal itself.
If this is not a major brand of patio furniture (you can get matching paint from the dealer) my personal approach would be to buy some acrylic/patio paint from a craft store, such as Michaels, and mix colors until you get something that matches your chair.
Since the metal appears to be weathered, I would use a matte sealer on top of the acrylic paint (also sold at Michaels) to avoid a shiny repair spot. You may find you don't need any sealer, especially if the furniture is protected from the weather. The craft paints come in very small containers so picking up 2-3 colors to mix/match won't break the bank.
Since the crack isn't on a joint I wouldn't worry too much about it. I think a tiny brush and enough paint to cover the exposed aluminum is all you need.