He was overwhelmed by a proto gravemind that he was fused with. He succumbed to the immense preasure and he was comsumed entirely.
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Close enough, Correct Keyes was not consumed entirely, as Master Chief killed him just before the flood got his memory of Earth, since it was his only memory left (The flood had to systematically discard all his memories since he put his memory of Earth so deep into his subconscious)