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 4 He in the mantle. He 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.

Diffusion of helium in olivine at 1 atm and 2.7 GPa

“…An up-temperature extrapolation of our data also show reasonable agreement with the higher-temperature measurements of Hart (1984); the coincident diffusivities from these studies under the high-temperature conditions used by Hart et al (2008) in their He diffusion/production modeling supports the applicability of their modeling results. Ó 2012 Elsevier Ltd. All rights reserved.…”

“…Helium isotopes in olivine provide constraints on mantle structure and evolution (e.g., Kurz et al, 1982;Allègre et al, 1983;Graham, 2002;Class and Goldstein, 2005;Parman et al, 2005;Hart et al, 2008). Helium isotope measurements of olivine also can provide information in thermochronology Farley, 2004, 2005) cosmic-ray exposure ages for terrestrial samples (e.g., Kurz et al, 1990;Trull et al, 1991;Yokochi et al, 2005;Aciego et al, 2007); paleoclimatology through glacial chronologies determined with the cosmogenic 3 He chronometer (Blard et al, 2007;Licciardi and Pierce, 2008) and ages of meteorites and cosmic dust (Markowski et al, 2006).…”

Noble gas constraints on the origin and evolution of Earth’s volatiles

“…However, helium is certainly one of the most diffusive elements. Helium diffusion from the ambient mantle can reduce the 4 He/ 3 He ratio in the subducted material as suggested by Hanyu and Kaneoka (1997) and Hart et al (2008). Although the diffusion coefficient Hart and collaborators used is relatively high (10 -6 cm 2 s -1 ) compared to the measured helium diffusivity in olivine (~10 -7 cm 2 s -1 at mantle temperatures 22 ; Hart, 1984a), they propose an elegant model that can explain the helium isotopic composition in "low 3 He" hotspots.…”

“…The difference between Hart et al (2008) and Hart (1984a) for the helium diffusivity in the mantle is that Hart et al (2008) propose that the bulk helium diffusivity in the mantle is ~10 times higher than in a single olivine grain due to inter-grain diffusivity.…”

Dismantling the deep earth : geochemical constraints from hotspot lavas for the origin and lengthscales of mantle heterogeneity

“…By contrast, the TITAN elements will be By contrast, subducted oceanic crust that conserves much of its original U and Th budget through the subduction zone will not possess positive TITAN anomalies and will produce more 4 He over time than a U and Th-depleted slab. Consequently, the abundant 4 He produced by an undepleted slab would infect the surrounding mantle with radiogenic 4 He due to rapid diffusion of helium in the mantle (Hart, 1984;Trull and Kurz, 1993;Hart et al, 2007), and is not compatible with the preservation of a high 3 Anderson, 1998;Heber et al, 2007;Jackson et al, 2007a) After processing in subduction zones, the crustal portion of the subducted plate hosts TITAN enrichment, and the asthenospheric DMM coupled to the bottom of the downgoing plate remains unscathed by subduction zone processes. The two components, TITAN-enriched eclogite and high 3 He/ 4 He asthenospheric peridotite, should be intimately associated as a "package" in space and time within a subducted plate, a geometry that is conducive to later mixing in the mantle.…”