Decreasing µ142Nd Variation in the Archean Convecting Mantle from 4.0 to 2.5 Ga: Heterogeneous Domain Mixing or Crustal Recycling?

Abstract:

The ¹⁴⁶Sm-¹⁴²Nd (t_1/2=68 Ma) chronometer can be used to examine silicate differentiation in the first 400 Ma of Earth history. Early fractionation between Sm and Nd is recorded in cratonic Archean rocks in their ¹⁴²Nd/¹⁴⁴Nd ratios that that deviate up to ±20 ppm, or µ¹⁴²Nd - ppm deviation relative to the present-day convecting mantle at 0. These values likely record early extraction of incompatible trace element (ITE) enriched material with -µ¹⁴²Nd, either as crust or late stage residual melt from a magma ocean, and resulting in a complimentary ITE depleted residual mantle with +µ¹⁴²Nd. If this early-formed ITE-enriched material was re-incorporated rapidly back into the convecting mantle, both ITE-enriched and ITE-depleted mantle domains would have been established in the Hadean. Alternatively, if it was early-formed crust that remained stable it could have slowly eroded and progressively remixed into the convecting mantle as subducted sediment during the Archean. Each of these scenarios could potentially explain the decrease in the maximum variation in µ¹⁴²Nd from ±20 at 4.0 Ga to 0 at 2.5 Ga [1,2,3]. In the scenario where these variations reflect mixing of mantle domains, this implies long mantle mixing times of greater than 1 Ga in the Archean in order to preserve the early-formed heterogeneities. This can be achieved in a stagnant lid tectonic regime in the Archean with sporadic and short subduction cycles [2]. This scenario would also indicate that mixing times in the convecting mantle were much slower than the previously proposed 100 Ma in the Hadean and Archean. In the alternative scenario, sediment with -µ¹⁴²Nd was progressively mixed into the mantle via subduction in the Archean [3]. This scenario doesn’t require slow mantle mixing times or a stagnant-lid regime. It requires crustal resident times of up to 750 Ma to maintain a steady supply of ancient sediment recycling over the Archean. Each of these scenarios evoke very contrasting conditions for early Earth history and additional examination is needed. We will consider the geochemical consequences of crustal sediment subduction in order to test the hypothesis of crustal recycling as well as provide additional evidence to aid in resolving this issue.

[1] Bennett et al., Science, 318 (2007), p.1907

[2] Debaille et al., EPSL, 373 (2013), p.83

[3] Roth et al., G3, 2014