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TitleMantle transition zone input to kimberlite magmatism near a subduction zone:Origin of anomalous Nd-Hf isotope systematics at Lac de Gras, Canada
AuthorTappe, S; Pearson, D G; Kjarsgaard, B AORCID logo; Nowell, G; Dowall, D
SourceEarth and Planetary Science Letters vol. 371-372, 2013 p. 235-251,
Alt SeriesEarth Sciences Sector, Contribution Series 20130073
PublisherElsevier BV
Mediapaper; on-line; digital
File formatpdf
ProvinceNorthwest Territories
AreaLac de Gras
Lat/Long WENS-112.0000 -110.0000 65.0000 64.0000
Subjectsigneous and metamorphic petrology; tectonics; geochronology; geochemistry; magmatism; mantle; kimberlites; neodymium samarium dates; radiometric dating; radiometric dates; isotopes; isotope ratios; trace element analyses; trace element geochemistry; major element analyses; major element geochemistry; igneous petrology; tectonic interpretations; tectonic setting; Slave Craton; Lac de Gras Kimberlite Field; Precambrian
Illustrationslocation maps; tables; plots
ProgramGEM: Geo-mapping for Energy and Minerals Diamonds
AbstractLate Cretaceous - Eocene kimberlites from the Lac de Gras area, central Slave craton, show the most extreme Nd - Hf isotope decoupling observed for kimberlites worldwide. They are characterized by a narrow range of moderately enriched Nd isotope compositions (ENd(i)=-0.4 to -3.5) that contrasts strongly with their moderately depleted to highly enriched EHf(i) values (+3.9 to -9.9). Although digestion of cratonic mantle material in proto-kimberlite melt can theoretically produce steep arrays in Nd - Hf isotope space, the amount of contaminant required to explain the Lac de Gras data is unrealistic. Instead, it is more plausible that mixing of compositionally discrete melt components within an isotopically variable source region is responsible for the steep Nd - Hf isotope array.
As development of strongly negative dEHf requires isotopic aging of a precursor material with Sm/Nd>Lu/Hf for billion-year timescales, a number of models have been proposed where ancient MORB crust trapped in the mantle transition zone is the ultimate source of the extreme Hf isotope signature. However, we provide a conceptual modification and demonstrate that OIB-type domains within ancient subducted oceanic lithosphere can produce much stronger negative ??Hf during long-term isolation. Provided that these OIB-type domains have lower melting points compared with associated MORB crust, they are among the first material to melt within the transition zone during thermal perturbations. The resulting hydrous alkali silicate melts react strongly with depleted peridotite at the top of the transition zone and transfer negative dEHf signatures to less dense materials, which can be more easily entrained within upward flowing mantle. Once these entrained refertilized domains rise above 300 km depth, they may become involved in CO2- and H2O-fluxed redox melting of upper mantle peridotite beneath a thick cratonic lid.
We argue that incorporation of ancient transition zone material, which includes ultradeep diamonds, into the convecting upper mantle source region of Lac de Gras kimberlites was due to vigorous mantle return flow. This occurred in direct response to fast and complex subduction along the western margin of North America during the Late Cretaceous.
Summary(Plain Language Summary, not published)
The source region of kimberlites from the Ekati and Diavik Mines in the Lac de Gras area of the Nortwest territories is determined to be from the transition zone at ~300 km depth, based on detailed geochemical studies of these rocks.

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