HR: 0800h
AN: U21B-0407    [Abstracts]
TI: Scale Length of Mantle Heterogeneities: Helium Diffusion Constraints
AU: * Hart, S
EM: shart@whoi.edu
AF: Woods Hole Oceanographic Institution, 360 Woods Hole Road, Woods Hole, MA 02543,
AU: Kurz, M
EM: mkurz@whoi.edu
AF: Woods Hole Oceanographic Institution, 360 Woods Hole Road, Woods Hole, MA 02543,
AU: Wang, Z
EM: zwang@whoi.edu
AF: Woods Hole Oceanographic Institution, 360 Woods Hole Road, Woods Hole, MA 02543,
AB: While Earth's mantle is unequivocally heterogeneous, the size, formation and distribution of these geochemical heterogeneities remain enigmatic. Following the veined mantle proposals of Hanson (1977) and Wood (1979), various postulates of mesoscale lithologic heterogeneities (veins, pods, layers, plums) have been advanced. However, the issue remains contentious, and no smoking gun has survived scrutiny. Do the heterogeneities reflect large scale (tens of km) chemical variability in a lithologically homogeneous (peridotitic) mantle, or smaller mesoscale (less than a few kms) mafic layers or veins embedded in a peridotitic matrix (or both)? We argue that the high diffusion rates of helium preclude survival of He isotope heterogeneities on scales smaller than a few hundred meters, especially if they represent long term in-growth of 4He in the convecting mantle. Using a coupled He diffusion-production model, 1.5 Gy residence times, and a diffusion coefficient of 10^-10 m²/sec (olivine, mid-upper mantle), 1 km slabs or 2.3 km cylinders will lose >80% of in-grown 4He. However, substantial 3/4He signatures may persist in slabs or be induced in adjacent mantle, depending on initial He, U and Th contents. We have modeled three cases of 1.5 Gy 3/4He 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 3/4He of only 3 Ra, and will have infected the surrounding mantle with 4He for >5 km on each side. The average 3/4He of this mixed zone will be <6 Ra even when sampled by melts over a total width of 20 km. For the case of a 1 km OC slab in BSE (50 Ra today), the slab will be 47 Ra today, almost fully equilibrated with ambient mantle. 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 50 km melt sampling width, the average 3/4He will be >20 Ra. In essence, slabs may lose their He signature by diffusion, but it will remain recorded in the surrounding mantle; i.e. veins may run but they can't hide! For both enriched and depleted upper mantle slabs, sampled along a spreading ridge, the 3/4He variability on 10-20 km scale lengths would be easily observed; even massive along-axis melt mixing (50-100 km) would not hide these signatures. We have evaluated 3 extant ridge-crest data sets in this context (MAR 0-47S; EPR 19-23S; SWIR 16- 24E), with a view to defining scale-lengths of He isotope variability. The average 3/4He variability for these 3 areas is 0.47, 0.19 and 0.21 Ra (±1 sigma); a well-sampled sub-area on the MAR (25.7-26.5S) is 0.13 Ra. There is a monotonic variation along the SWIR, from 6.6 to 7.3 Ra; variability about a best fit line is 0.09 Ra (maximum deviation is only 0.20 Ra). At the smallest scale, a single 20 km EPR flow field shows similar variability (0.29 Ra) to the above examples. Since these ridges range from slow 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 any significant size. This is especially clear for the 500 km domain on the SWIR, where very small He variability is observed, superimposed on a large scale He gradient. In particular, the view of the upper mantle as a ubiquitous mixture of veins and depleted matrix, with MORB always representing an averaging of this mixture, appears untenable.
DE: 1025 Composition of the mantle
DE: 1037 Magma genesis and partial melting (3619)
DE: 1038 Mantle processes (3621)
DE: 1040 Radiogenic isotope geochemistry
DE: 1041 Stable isotope geochemistry (0454, 4870)
SC: Union [U]
MN: 2007 Fall Meeting