If you absolutely must have a fire, reset your thinking from "fire pit" to "fire mound"

Creating a fire mound is a great way to enjoy a back-country fire with little to no impact to the ground / vegetation.

1. Carry a small sheet of plastic, burlap, or a section of an old fire shelter, or anything of the like (it shouldn't get hot enough to burn if your mound is constructed properly, though embers might land on the edges)
2. Find a previously-disturbed source of mineral soil (a downed tree root ball, a sandbar in a creek, etc.) near your camp
3. Lay the sheet out on a level bit of ground (rock/bare soil preferred but not necessary)
4. Make a flat "mound" of mineral soil 3-5 inches thick on the sheet, making it a bit wider than your expected fire
5. Build a small twiggy fire on top of the mound
6. When finished, scatter the ashes charcoal twigs (or pack out if required)
7. Return the mineral soil back to its source

BAM! Absolutely no trace of your fire, except maybe flattened vegetation where the was. The mineral soil mound insulates the ground so grass underneath is unharmed.

For areas that don't have available mineral soil (slick rock?), you might consider a small fire-pan (think disposable aluminum turkey roasting pan) to minimize char/scar from the fire.

For more info, check out Leave No Trace principle 5.