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SUMMARY:Origin of carbonatites using the triple-isotope approach - Elsa Am
 sellem\, University of Cambridge
DTSTART:20240306T160000Z
DTEND:20240306T170000Z
UID:TALK212917@talks.cam.ac.uk
CONTACT:Megan Holdt
DESCRIPTION:Carbonatites are rare carbonate-rich igneous rocks derived fro
 m carbon and carbonate-rich regions of Earth’s mantle. Although a number
  of igneous processes are recognized to have controlled their compositions
 \, the origin of the carbonate-rich nature of these magmas remains debated
  and has been linked to various mantle-related processes\, including subdu
 ction and plume–lithosphere interaction. High-precision isotope measurem
 ents can provide insights into carbonatite petrogenesis\, including the id
 entification of subducted crustal material in their source region. In part
 icular\, combining mass-dependent and kinetically corrected Mg isotope dat
 a\, also known as the triple-isotope approach\, provides insights into mas
 s fractionation processes driving mass dependent fractionation\, which in 
 turn allows to distinguish between equilibrium and kinetic processes. In t
 his work\, we report high-precision Mg stable isotope data for 59 carbonat
 ites and associated silicate rocks from different localities and ages rang
 ing from 3000 Ma to present-day. In addition\, we also report Mg isotope d
 ata for 17 Phanerozoic carbonate rocks\, with the aim of identifying the i
 sotopic signature of carbonate-rich material potentially recycled to the c
 arbonatite mantle source regions. We observe positive residual deviations 
 after kinetic mass fractionation correction of the Mg isotope data for our
  carbonatites\; an isotope signal that is also present in Phanerozoic carb
 onates. This observation establishes that a component of the Mg present in
  these samples experienced mass-dependent equilibrium isotopic fractionati
 on processes\, which are significantly larger at low temperatures. Given t
 hat the Mg results do not covary with C and O isotopic signals\, the magni
 tude of the fractionation following the equilibrium law observed in carbon
 atites provides strong evidence for recycled material from the Earth’s s
 urface in the mantle source of Ca- and Mg-rich carbonatites. Furthermore\,
  associated silicate rocks present mantle-like Mg isotopic compositions in
  contrast with the genetically linked carbonatites - based on the Sr isoto
 pes - for some complexes\, which is best explained if the carbonatites and
  associated silicate rocks represent distinct generation of partial melts 
 of a mantle source containing recycled carbonate.
LOCATION:Wolfson Lecture Theatre
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