A Partial Late Veneer for the Source of 3.8 Ga Isua Rocks: Evidence from Highly Siderophile Elements and 182W

Abstract:

Highly siderophile elements (HSE) were strongly sequestered into metallic planetary cores, leaving silicate mantles almost devoid of HSE. Late accretion partially replenished HSE in planetary mantles soon after core formation had ceased [1], which for Earth probably postdated the giant Moon-forming impact. Ancient isolated domains in Earth’s mantle – such as the source of 3.8 Ga Isua basalts – might represent mantle isolated from late accreted material, as suggested based on their small ¹⁸²W excesses compared to Earth's present-day mantle [2]. However, such ¹⁸²W excesses may also represent signatures of early differentiation in Earth's mantle, which have been preserved through the giant impact [3]. To assess the origin of ¹⁸²W anomalies and the ¹⁸²W composition of the pre-late veneer mantle, we determined HSE abundances and ¹⁸²W compositions of a suite of mafic to ultramafic rocks from Isua.

Our data show that the Isua source mantle had HSE abundances at ~60% of the present-day mantle, inconsistent with isolation from the late veneer. For the same samples we obtained a 13±4 ppm ¹⁸²W excess over the modern terrestrial mantle, in excellent agreement with previous data [2]. Using a range of possible late veneer compositions and taking into account the recently revised ¹⁸²W value for the Moon [4], we calculate that the Isua mantle source, containing 60% late veneer, would have a ¹⁸²W value of 9±4 ppm, in very good agreement with the measured value for Isua. The combined HSE-W data, therefore, are consistent with only partial addition of the late veneer to the Isua mantle source, and with the interpretation that the 27±4 ppm ¹⁸²W excess of the Moon represents the ¹⁸²W composition of the pre-late veneer Earth's mantle [4].

[1] Dale et al. (2012) Science 336, 72. [2] Willbold et al. (2011) Nature 477, 195. [3] Touboul et al. (2012) Science 335, 1065-1069. [4] Kruijer et al. (2015) Nature 7548, 534