Nick Dygert1, Colin Jackson2, and Marc Hesse1
1Jackson School of Geosciences, University of Texas at Austin
2Geophysical Laboratory, Carnegie Institution for Science
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Recent measurements have highlighted that the MORB mantle source has distinctly higher 3He/22Ne compared to primitive mantle (~10 vs. 2, respectively) . We seek to understand the source of this difference by modeling chemical exchange between dunite-channel hosted basaltic liquids and harzburgitic wallrock during the percolation of melts to the surface.
Dunite channels are thought to represent pathways for efficient melt extraction from the upper mantle. Percolation of basaltic melts through dunite channels allows them to retain high-pressure multiple saturation depths and the trace element characteristics of their mantle source. However, diffusive interaction of basaltic melts with harzburgite wallrock has an inevitable effect on the chemistry of the lithospheric mantle. In terms of global geochemical cycles, this effect is inconsequential for slow diffusing elements but can be significant for fast diffusiving, incompatible elements. Helium and neon are highly incompatible during mantle melting [2,3] and He is extremely mobile [e.g., 4]. Measurements of He diffusion in olivine suggest it is orders of magnitude faster than Ne at mantle relevant temperatures. Fast diffusion of He out of dunite channel-hosted basaltic melts and into volatile element depleted harzburgitic wallrock can efficiently fractionate He from Ne. These fractionations can then be imparted onto the depleted mantle by subduction or delamination of lithospheric mantle.
Melt percolation-diffusive interaction calculations suggest that preferential 3He ingassing at dunite channels will significantly increase 3He/22Ne of the depleted mantle. Ingassing of peridotite by dunite-hosted basaltic melts has presumably occurred for most of geologic time. This simple model represents an alternative to the multiple degassed magma oceans invoked by  to increase the 3He/22Ne of the depleted mantle. If kinetic fractionation is the dominant physical process modulating the 3He/22Ne of the mantle, timescales of mantle mixing on the order of billions of years may be required. Despite seismic tomographic evidence for whole mantle convection, the persistence of low 3He/22Ne reservoirs to the present day requires convective isolation of mantle heterogeneities throughout geologic time.
 Tucker & Mukhopadhyay (2014) EPSL 393, 254-265.  Heber et al (2007) GCA 71, 1041-1061.  Jackson et al (2013) EPSL 384, 178-187.  Cherniak & Watson (2012) GCA 84, 269-279.