Goldschmidt Conference Abstract, 2007

Ghost primordial He and Ne

FRANCIS ALBAREDE¹ & ICHIRO KANEOKA²

¹Ecole Normale Supérieure, 69007 Lyon, France
albarede@ens-lyon.fr

²Earthquake Research Institute, University of Tokyo, Bunkyo-ku, Tokyo 113, Japan
ikaneoka@aol.com

The canonical view of He isotope geochemistry holds that high ³He/⁴He and solar Ne in oceanic basalts fingerprint hidden undegassed mantle sources. Competing evidence of recycled material processed next to the Earth’s surface being present in the source of Hawaiian basalts is nevertheless very strong, whether it be carried by oxygen or Hf isotopes. We here discuss the marble-cake paradigm in the light of Shuster et al.’s (2003) diffusion data on olivine. The closure temperature for diffusion of ³He is ~200°C and the data show that He moves around remarkably fast, e.g., more than 1 km at 1500 K and more than 5 km at 2000 K within 1 Gy. Ever since the accretion of the Earth, He and Ne originally hosted in primordial material therefore pervaded all the lithologies coexisting on length-scales typical of a marble-cake mantle (1- 100 m), and, in particular, contaminated U- and Th-poor refractory residues tightly folded in with streaks of primitive mantle. Dunite and harzburgite residues left by ridge activity, and their high-pressure equivalents, therefore act as long-term sinks for ‘ghost’ rare gases. Conversely, such restites also act as a source of primordial rare gases for whichever recycled material gets subsequently folded in during the rest of the Earth’s history, even long after most of the primordial material has been removed by processing at mid-ocean ridges. A numerical marble-cake diffusion model with suitable parameters can reproduce the uptake of primordial gases initially present in the primordial lower mantle by background refractory residues and its subsequent transfer to younger ‘layers’ of recycled pyroxenite. Both low- and high-³He/⁴He hotspots may be produced by changing the duration of the diffusion process and the U and Th contents of refolded pyroxenite layers.

Upwelling beneath mid-ocean ridges happens much too fast (<100 My) for the refractory material to be flushed clean of primordial gases by diffusion. However, once back into the deep mantle, primitive-looking He and solar Ne from these refractory layers have plenty of time to permeate into all sorts of newly neighboring layers, whether recycled or primitive. Decoupling by diffusion certainly explains the lack of coherence between ³He/⁴He and lithophile isotopic tracers. Although rare gas isotopes require that the upper mantle is more outgassed than the lower mantle as a whole, neither He with high ³He/⁴He nor solar Ne in basalts are diagnostic of the presence of primordial material in the source itself. Likewise, the missing terrestrial ⁴⁰Ar is likely to be another ghost rare gas hosted largely by refractory residues, thus essentially voiding a widely used constraint on the proportion of undegassed solid mantle.

Reference

Shuster D.L et al. (2003), Earth Planet. Sci. Lett. 217, 19-32.

last updated 20th November, 2007