Early onset of magma ocean crystallization revealed by coupled 146,147Sm-142,143Nd systematics of Nulliak ultramafics (3.78 Ga, Labrador)

Abstract:

Early onset of magma ocean crystallization revealed by coupled ^146,147Sm-^142,143Nd systematics of Nulliak ultramafics (3.78 Ga, Labrador)

Precillia Morino¹, Guillaume Caro¹, Laurie Reisberg ¹

¹CRPG-CNRS, Université de Lorraine, Nancy, France

Coupled ^146,147Sm-^142,143Nd systematics provides constraints on the timing of magma ocean crystallization on Mars, the Moon and Vesta. Estimates for the Earth's mantle, however, are less accurate owing to the sparsity of Eoarchean mantle-derived rocks with undisturbed ¹⁴⁷Sm-¹⁴³Nd systematics. This study attempts to establish a coherent ^142,143Nd dataset for the Eoarchean mantle using well-preserved ultramafic rocks from the Nulliak assemblage (Labrador). Samples include meta-dunites, -pyroxenites and -peridotites exhibiting only minor serpentinization and limited element mobility. The presence of "Barberton type" komatiitic compositions (low Al/Ti, HREE depletion) is suggestive of a deep mantle source. ^146,147Sm-^142,143Nd and ¹⁸⁷Re-¹⁸⁷Os analyses yield a crystallization age of 3.78±0.09 Ga with ε¹⁴³Nd_i=1.5±0.2 and ε¹⁴²Nd=8.6±2 ppm. This ^142,143Nd signature yields a model age of mantle differentiation of 4.43±0.05 Ga (assuming a BSE with chondritic Sm/Nd and ε¹⁴²Nd=0). Superchondritic Sm/Nd compositions for the BSE would translate into older model ages. Irrespective of the choice of primitive mantle composition, Nulliak ultramafics provide differentiation ages 100 Ma older than those estimated from Akilia tonalites but remarkably similar to that estimated from the 2.7 Ga Theo's flow (Abitibi). If Nulliak ultramafics originated from deep melting of a hot plume, their model age could reflect the early onset of magma ocean crystallization in the lowermost mantle.