Orolo from Anathem by Neal Stephenson. Orolo sacrifices his life in a really cool and noble way, to help the pursuit of knowledge and to help the people of Arbre to understand the alien ship in orbit. One of the aliens has been shot down on the planet, bringing along blood from all the different humanoids on the ship, and the people on the ship ignite a volcano to destroy her ship. Orolo gives up his spot on the evac helicopter to hand someone the body of the alien and stands there, being engulfed in lava:

"No!" I screamed one more time, as Orolo withered under the pressure wave. He flopped to the ground like a hank of rope. For a moment, smoke shrouded him: radiant heat shining out as a harbinger of the glowing cloud. Our aerocraft rocked and skidded sideways on hard air. The cloud erupted from the gates, vaulted over the rubble of the wall,, and fell onto Orolo. For a fraction of a second he was a blossom of yellow flame in the stream of light, and then he was one with it. All that remained of what he'd been was a wisp of steam coiling above the torrent of fire.