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CATEGORIES:Bullard Laboratories Wednesday Seminars
SUMMARY:H\, He\, and seismic evidence for a bilithologic p
 lume-fed asthenosphere - Jason Phipps Morgan\, Ins
 titute of Marine Sciences\, Barcelona
DTSTART;TZID=Europe/London:20250212T140000
DTEND;TZID=Europe/London:20250212T150000
UID:TALK228178AThttp://talks.cam.ac.uk
URL:http://talks.cam.ac.uk/talk/index/228178
DESCRIPTION:Chemical diffusion in the mantle has typically bee
 n viewed to play a negligible role in geodynamic p
 rocesses.  However\, diffusion rates for water (H)
  and helium (He) are large enough that they lead t
 o observable differences between pyroxenite-rich m
 elting associated with ocean island volcanism (OIB
 ) and more peridotite-rich melting associated with
  mid-ocean ridge basalts (MORB). Laboratory measur
 ements of diffusion rates of H and He at ambient m
 antle temperatures in olivine are of order 10 km/1
 .7Gyr for He and 250 km/1.7 Gyr for H. If the mant
 le is an interlayered mixture of recycled oceanic 
 basalts and sediments surrounded by a much larger 
 volume of residual peridotites\, then chemical dif
 fusion can shape the mantle in two important ways.
   Hydrogen will tend to migrate from peridotites i
 nto adjacent pyroxenites\, because clinopyroxene (
 and its high-pressure metamorphs) has a much stron
 ger affinity for water than the olivine and orthop
 yroxene that form the bulk of mantle peridotites. 
 Therefore pyroxenite lithologies will typically ha
 ve twice or more the water content of their surrou
 nding damp peridotites. This will strongly favor t
 he enhanced melting of pyroxenites that is now mos
 tly agreed to be a common feature of the OIB sourc
 e. Radiogenic 4He will have the opposite behaviour
  — it will tend to migrate from where it is produc
 ed in recycled incompatible-element-rich (e.g. U a
 nd Th-rich) pyroxenites into nearby\, larger volum
 e fraction\, but U+Th-poorer peridotites\, while t
 he radioisotopes of Ar and Ne that are also produc
 ed by the decay of the incompatible elements K\, U
 \, and Th will diffuse much less\, and thus remain
  within their original pyroxenite source.  This ef
 fect leads to lower 4He/21Ne and 4He/40Ar ratios i
 n OIB in comparison to the predicted values based 
 on the mantle’s bulk geochemistry\, and complement
 ary higher 4He/21Ne and 4He/40Ar ratios in the MOR
 B source that is formed by the plume-fed asthenosp
 heric residues to OIB melt extraction at plumes.  
 \n\nThe recent observation of a 150-km-deep positi
 ve shear velocity gradient (PVG) beneath non-crato
 nic lithosphere (Hua et al.\, 2023) is further evi
 dence for the initiation of pyroxenitic melting at
  this depth within the asthenosphere. It also impl
 ies that lateral temperature variations at this de
 pth are quite small\, of order ±75°C. This near un
 iformity of temperatures near both mantle plumes a
 nd mid-ocean ridges is\, in turn\, strong evidence
  in favor of the hypothesis that the asthenosphere
  is fed by mantle plumes. We propose that two filt
 ering effects occur as plumes feed the asthenosphe
 re\, removing both the hottest and coldest parts o
 f upwelling plume material. First\, the peridotite
  fraction in the hottest part of upwelling plume m
 aterial melts enough for it to dehydrate\, thereby
  transforming this fraction into a more viscous an
 d buoyant hotspot swell root that moves with the o
 verlying plate\, not as asthenosphere. Second\, si
 nce plume material is warmer than average mantle\,
  it is more buoyant\, creating a natural density f
 ilter that prevents any cooler underlying mantle f
 rom upwelling through it. Preferential melt-extrac
 tion from denser pyroxenites at mantle plumes also
  makes the asthenosphere compositionally buoyant w
 ith respect to its underlying\, more pyroxene-rich
  mantle. These rheological and density filters wil
 l tend to make the asthenosphere sampled by meltin
 g at mid-ocean ridges have a more uniform temperat
 ure than its typical underlying mantle. 
LOCATION:Wolfson Lecture Theatre
CONTACT:Sergei Lebedev
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