Neodymium Isotope Variability at the Grain Scale in the Sub-Continental Lithospheric Mantle: NdO+ Analyses of Individual Clinopyroxene Grains (<5 ng Nd aliquots) from a Kilbourne Hole Harzburgitic Xenolith.

Abstract:

It is evident that at scales of 10² to 10^-2 m there is significant isotopic heterogeneity in the mantle that is not always reflected in primitive melts. The “Os isotopic gap”^[1] is one such manifestation of this phenomenon but a similar offset exists between the Nd isotope composition of abyssal peridotites and the mid-ocean ridge basalts that they are inferred to have produced^[2]. This study takes advantage of recent advances in the analysis of Nd isotopes as NdO^+[3,4] which permit the precise analysis of single clinopyroxene grains (<1 mg mass; <5 ng Nd) from a continental harzburgitic xenolith from Kilbourne Hole, NM. Analyses of aggregates of clinopyroxenes from 5 Kilbourne Hole xenoliths reveal a wide range of ¹⁴³Nd/¹⁴⁴Nd (0.513011 ± 28 to 0.513615 ±19)^[5]. This study demonstrates significant grain-to-grain isotopic heterogeneity at a scale of 10^-2 m (¹⁴³Nd/¹⁴⁴Nd = 0.513089 ± 78 to 0.513364 ± 74) which (i) is equivalent to the range of values for Pacific MORB^[6] and (ii) is more primitive than local basalts with an asthenospheric signature^[7]. This suggests that small-scale refractory domains exist within the mantle which are either not sampled during partial melting or whose presence is obscured by the melting of higher volumes of more fusible material.

Ref:^[1]Alard et al. (2005) Nature 436, 1005-1008 ^[2]Warren et al. (2009) JGR 114, B12203, doi:10.1029/2008JB006186 ^[3]Harvey and Baxter (2009) Chem. Geol. 258, 251-257 ^[4]Honn et al. (2013) AGU Fall abstr. V33-2722 ^[5]Harvey et al. (2012) J. Petrol. 53, 1709-1742 ^[6]Hofmann (1997) Nature 385, 219-229 ^[7]Thompson et al. (2005) J. Petrol. 46, 1603-1643