Variable refractory Lithophile element compositions of planetary building blocks

Chondrites are undifferentiated sediments of materials left over from the earliest stage of the solar system history, and are widely considered to represent the unprocessed building blocks of the terrestrial planets. Compositional models of the planets generally find chondritic relative abundances of refractory lithophile elements (RLE) in the bulk planets ("constant RLE ratio rule"), based on limited variations of RLE ratios among chondritic meteorites and the solar photosphere. Here, we show that ratios of RLE, such as Nb/Ta, Zr/Hf, Sm/Nd and Al/Ti, are fractionated in chondrules from enstatite chondrites (EC), which provides limitations on the use of the constant RLE ratio rule in the compositional modeling of planets. The fractionated RLE compositions of EC chondrules document a separation of RLE-bearing sulfides before and/or during chondrule formation and different chalcophile affinities of RLE under highly reducing environments. If the Earth’s accretion is dominated by highly reduced EC-like materials, as supported by multiple isotope systematics, the fractionated RLE ratios of the reduced silicates should have been modified during the Earth’s subsequent differentiation, to produce CI-like RLE ratios of the bulk silicate Earth. A lack of Ti depletion in the bulk silicate Earth and the similar chalcophile behavior of Ti and Nb under reducing conditions exclude incorporation of Nb into a core-forming sulfide as the origin of the accessible Earth’s Nb depletion.