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 |