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NASA NTRS1988

Growth of continental crust: Clues from Nd isotopes and Nb-Th relationships in mantle-derived magmas

Isotope and trace element geochemistry of Precambrian mantle derived rocks and implications for the formation of the continental crust is discussed. Epsilon Nd values of Archean komatiites are variable, but range up to at least +5, suggesting that the Archean mantle was heterogeneous and, in part, very depleted as far back as 3.4 to 3.5 Ga. This may be taken as evidence for separation of continental crust very early in Earth history. If these komatiite sources were allowed to evolve in a closed system, they would produce modern day reservoirs with much higher epsilon Nd values than is observed. This implies recycling of some sort of enriched material, perhaps subducted sediments, although other possibilities exist. Archean volcanics show lower Nb/Th than modern volcanics, suggesting a more primitive mantle source than that observed nowadays. However, Cretaceous komatiites from Gorgona island have similar Nb/Th to Archean volcanics, indicating either the Archean mantle source was indeed more primitive, or Archean magmas were derived from a deep ocean island source like that proposed for Gorgona.

Arndt, N. T.↗

DOE OSTI2020

Materials Data on Th3Nb by Materials Project

Th3Nb is beta Cu3Ti-like structured and crystallizes in the hexagonal P6_3/mmc space group. The structure is three-dimensional. Th is bonded to eight equivalent Th and four equivalent Nb atoms to form distorted ThTh8Nb4 cuboctahedra that share corners with four equivalent NbTh12 cuboctahedra, corners with fourteen equivalent ThTh8Nb4 cuboctahedra, edges with six equivalent NbTh12 cuboctahedra, edges with twelve equivalent ThTh8Nb4 cuboctahedra, faces with four equivalent NbTh12 cuboctahedra, and faces with sixteen equivalent ThTh8Nb4 cuboctahedra. There are a spread of Th–Th bond distances ranging from 3.19–3.56 Å. There are two shorter (3.37 Å) and two longer (3.45 Å) Th–Nb bond lengths. Nb is bonded to twelve equivalent Th atoms to form NbTh12 cuboctahedra that share corners with six equivalent NbTh12 cuboctahedra, corners with twelve equivalent ThTh8Nb4 cuboctahedra, edges with eighteen equivalent ThTh8Nb4 cuboctahedra, faces with eight equivalent NbTh12 cuboctahedra, and faces with twelve equivalent ThTh8Nb4 cuboctahedra.

36 MATERIALS SCIENCE↗