Variable refractory lithophile element compositions of planetary building blocks: insights from components of enstatite chondrites

Abstract

Chondrites are sediments of materials left over from the earliest stage of the solar system history. Based on their undifferentiated nature and less fractionated chemical compositions, chondrites are widely considered to represent the unprocessed building blocks of the terrestrial planets and their embryos. Models of chemical composition of the terrestrial planets generally find chondritic relative abundances of refractory lithophile elements (RLE) in the bulk bodies ("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 from the solar value in chondrules from enstatite chondrites (EC). The fractionated RLE ratios of individual EC chondrules document different chalcophile affinities of RLE under highly reducing environments and a separation of RLE-bearing sulfides from silicates before and/or during chondrule formation. In contrast, the bulk EC have solar-like RLE ratios, indicating that a physical sorting of silicates and sulfides was negligible before and during the accretion of EC parent bodies. Likewise, if the Earth's accretion were dominated by EC-like materials, as supported by multiple isotope systematics, the physical sorting of silicates and sulfides in the accretionary disk should not have occurred among the Earth's building blocks. Alternatively, the Earth's precursors might have been high-temperature nebular materials that condensed before the RLE fractionation due to precipitation of the RLE-bearing sulfides...Highly reduced planets that have experienced selective removal or accretion of silicates or metal/sulfide phases, such as Mercury, might have fractionated, non-solar bulk RLE ratios.

arXiv comment73 pages, 16 figures, 3 tables

References in corpus (20)

• Solar System evolution from compositional mapping of the asteroid belt
• Accretion and differentiation of the terrestrial planets with implications for the compositions of early-formed Solar System bodies and accretion of water
• The composition of Mars
• Growing the terrestrial planets from the gradual accumulation of sub-meter sized objects
• Planetary and meteoritic Mg/Si and d30Si variations inherited from solar nebula chemistry
• The Chemical Composition of Mercury
• Thulium anomalies and rare earth element patterns in meteorites and Earth: Nebular fractionation and the nugget effect
• Equilibrium Condensation from Chondritic Porous IDP Enriched Vapor: Implications for Mercury and Enstatite Chondrite Origins
• Photophoretic separation of metals and silicates: the formation of Mercury like planets and metal depletion in chondrites
• Testing the chondrule-rich accretion model for planetary embryos using calcium isotopes
• Earth's chondritic Th/U: negligible fractionation during accretion, core formation, and crust - mantle differentiation
• Global Major-Element Maps of Mercury from Four Years of MESSENGER X-Ray Spectrometer Observations
• Magmatic sulfides in the porphyritic chondrules of EH enstatite chondrites
• Subsolar Al/Si and Mg/Si ratios of non-carbonaceous chondrites reveal planetesimal formation during early condensation in the protoplanetary disk
• Mercury's Internal Structure
• The formation conditions of enstatite chondrites: Insights from trace element geochemistry of olivine-bearing chondrules in Sahara 97096 (EH3)
• The Elusive Origin of Mercury
• Seeding the Formation of Mercurys: An Iron-sensitive Bouncing Barrier in Disk Magnetic Fields
• Composition and Size Dependent Sorting in Preplanetary Growth: Seeding the Formation of Mercury-like Planets
• Nebular history of an ultrarefractory phase bearing CAI from a reduced type CV chondrite