Scale length of mantle heterogeneities : constraints from helium diffusion

Abstract

A model of coupled He production/diffusion is used to constrain the question of whether Earth’s peridotitic mantle contains ubiquitous mesoscale veins or slabs of other lithologies. The high diffusion rates of helium preclude survival of He isotope heterogeneities on scales smaller than a few tens of meters, especially if they represent long term in-growth of 4He in the mantle. For 1.5 Gy residence times, and a diffusion coefficient of 10-10 m2/sec, 0.5 km slabs or 2 km cylinders will lose >90% of in-grown 4He. However, substantial 3He/4He variations may persist in slabs or be induced in adjacent mantle, depending on initial He, U and Th contents. We have modeled three cases of 3He/4He equilibration between mantle domains: an ocean crust (OC) slab in depleted upper mantle (DMM) or in enriched mantle (BSE), and a BSE slab in DMM. For a 1 km OC slab in DMM (8 Ra today), the slab today will have 3He/4He of only 3 Ra, and will have influenced the surrounding mantle with 4He for >7 km on either side. The average 3He/4He of this mixed zone will be <7 Ra, even when sampled by melts over a total width of 20-50 km. For the case of a 1 km BSE slab in DMM (8 Ra today), the slab will be 37 Ra today, and will have infected a mantle domain >16 km wide. Even with a 60 km melt sampling width, the average 3He/4He will be >15 Ra. Slabs may lose their He signature by diffusion, but their presence will be recorded in the surrounding mantle. We have evaluated 3 along-axis N-MORB ridge-crest data sets in this context (MAR 25.7-26.5°S; EPR 19-23°S; SWIR 16-24°E), with a view to defining scale-lengths of He isotope variability. The average 3He/4He variability for these 3 areas is very small, and independent of spreading rate: 0.13, 0.19 and 0.21 Ra (±1σ). Since these ridges range from ultraslow to very fast-spreading, the variability in size of along-axis magma chambers will lead inevitably to various scales of melt averaging. We conclude that these ridge areas are not sampling mantle that contains enriched veins or recycled oceanic crust slabs of significant size (> tens of meters). It appears difficult to sustain a view of the upper mantle as a ubiquitous mixture of veins and depleted matrix, with MORB always representing an averaging of this mixture.