Edited by Mark Thiemens, University of California San Diego, La Jolla, CA, and approved January 22, 2021 (received for review December 18, 2020)The crusts of the oldest protoplanets are virtually unknown due to the scarcity of samples. Here, we describe the oldest known lava that crystallized ca. 4,565 Ma ago and formed by partial melting of a chondritic parent body. 26Al-26Mg systematics suggest that the elapsed time between melting and crystallization was significant, on the order of several 105 y, probably due to the viscosity of the magma. Although the first protoplanetary crusts were...…Edited by Mark Thiemens, University of California San Diego, La Jolla, CA, and approved January 22, 2021 (received for review December 18, 2020)The crusts of the oldest protoplanets are virtually unknown due to the scarcity of samples. Here, we describe the oldest known lava that crystallized ca. 4,565 Ma ago and formed by partial melting of a chondritic parent body. 26Al-26Mg systematics suggest that the elapsed time between melting and crystallization was significant, on the order of several 105 y, probably due to the viscosity of the magma. Although the first protoplanetary crusts were...WW…