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SUMMARY:H\, He\, and seismic evidence for a bilithologic plume-fed astheno
 sphere - Jason Phipps Morgan\, Institute of Marine Sciences\, Barcelona
DTSTART:20250212T140000Z
DTEND:20250212T150000Z
UID:TALK228178@talks.cam.ac.uk
CONTACT:Sergei Lebedev
DESCRIPTION:Chemical diffusion in the mantle has typically been viewed to 
 play a negligible role in geodynamic processes.  However\, diffusion rates
  for water (H) and helium (He) are large enough that they lead to observab
 le differences between pyroxenite-rich melting associated with ocean islan
 d volcanism (OIB) and more peridotite-rich melting associated with mid-oce
 an ridge basalts (MORB). Laboratory measurements of diffusion rates of H a
 nd He at ambient mantle temperatures in olivine are of order 10 km/1.7Gyr 
 for He and 250 km/1.7 Gyr for H. If the mantle is an interlayered mixture 
 of recycled oceanic basalts and sediments surrounded by a much larger volu
 me of residual peridotites\, then chemical diffusion can shape the mantle 
 in two important ways.  Hydrogen will tend to migrate from peridotites int
 o adjacent pyroxenites\, because clinopyroxene (and its high-pressure meta
 morphs) has a much stronger affinity for water than the olivine and orthop
 yroxene that form the bulk of mantle peridotites. Therefore pyroxenite lit
 hologies will typically have twice or more the water content of their surr
 ounding damp peridotites. This will strongly favor the enhanced melting of
  pyroxenites that is now mostly agreed to be a common feature of the OIB s
 ource. Radiogenic 4He will have the opposite behaviour — it will tend to
  migrate from where it is produced in recycled incompatible-element-rich (
 e.g. U and Th-rich) pyroxenites into nearby\, larger volume fraction\, but
  U+Th-poorer peridotites\, while the radioisotopes of Ar and Ne that are a
 lso produced by the decay of the incompatible elements K\, U\, and Th will
  diffuse much less\, and thus remain within their original pyroxenite sour
 ce.  This effect leads to lower 4He/21Ne and 4He/40Ar ratios in OIB in com
 parison to the predicted values based on the mantle’s bulk geochemistry\
 , and complementary higher 4He/21Ne and 4He/40Ar ratios in the MORB source
  that is formed by the plume-fed asthenospheric residues to OIB melt extra
 ction at plumes.  \n\nThe recent observation of a 150-km-deep positive she
 ar velocity gradient (PVG) beneath non-cratonic lithosphere (Hua et al.\, 
 2023) is further evidence for the initiation of pyroxenitic melting at thi
 s depth within the asthenosphere. It also implies that lateral temperature
  variations at this depth are quite small\, of order ±75°C. This near un
 iformity of temperatures near both mantle plumes and mid-ocean ridges is\,
  in turn\, strong evidence in favor of the hypothesis that the asthenosphe
 re is fed by mantle plumes. We propose that two filtering effects occur as
  plumes feed the asthenosphere\, removing both the hottest and coldest par
 ts of upwelling plume material. First\, the peridotite fraction in the hot
 test part of upwelling plume material melts enough for it to dehydrate\, t
 hereby transforming this fraction into a more viscous and buoyant hotspot 
 swell root that moves with the overlying plate\, not as asthenosphere. Sec
 ond\, since plume material is warmer than average mantle\, it is more buoy
 ant\, creating a natural density filter that prevents any cooler underlyin
 g mantle from upwelling through it. Preferential melt-extraction from dens
 er pyroxenites at mantle plumes also makes the asthenosphere compositional
 ly buoyant with respect to its underlying\, more pyroxene-rich mantle. The
 se rheological and density filters will tend to make the asthenosphere sam
 pled by melting at mid-ocean ridges have a more uniform temperature than i
 ts typical underlying mantle. 
LOCATION:Wolfson Lecture Theatre
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