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Rasmus Andreasen

Where on Earth is the enriched hidden reservoir?

Publikation: Bidrag til tidsskrift/Konferencebidrag i tidsskrift /Bidrag til avisTidsskriftartikelForskning

  • M. Sharma, Department Earth Sciences, Dartmouth College, Hanover, NH 03755, USA Mukul.Sharma@Dartmouth.edu
  • ,
  • R. Andreasen
  • K. V. Subbarao, Center for Earth & Space Sciences, University of Hyderabad, Hyderabad, 500 046 India kvsubbarao_iitb@yahoo.co.in
  • ,
  • O. Stecher, Department of Environment, Technology and Social Studies, Roskilde University, Roskilde, 4000 Denmark ostecher@ruc.dk
Recent data for the Sm isotopic composition of meteorites indicate that the excess in ^{142}Nd seen for the terrestrial upper mantle is not caused by nucleosynthetic effects and support the hypothesis of an early terrestrial fractionation event creating the upper mantle as the depleted counterpart to an enriched reservoir. The coupling of the short-lived ^{146}Sm-^{142}Nd and long-lived ^{147}Sm-^{143}Nd systems constrains the fractionation event to within tens of millions of years after planetary accretion. The fractionation of Sm and Nd was large enough to give the upper mantle a ^{142}Nd excess of +17±3 ppm over primitive meteorites but small enough so that the ɛ^{143}Nd value stayed within what is observed in both present day MORB and in basalts through time. So far searches for the enriched reservoir with deficit in ^{142}Nd have yielded negative results. Plume basalts from Deccan and Iceland, some with solar He and Ne isotopic signatures, show no deficits in ^{142}Nd at the 5 ppm (2σ) level. Either the signature is being diluted through entrainment of upper mantle during plume ascent, or perhaps the enriched reservoir is not located within the mantle. The enriched reservoir cannot be located on the moon as the latter is volumetrically too small and analyses of lunar samples suggest a chondritic ^{142}Nd/^{144}Nd ratio for the moon. Is it possible that the enriched reservoir resides in the core? Sm/Nd fractionation and core formation occurred approximately at the same time. However, iron meteorites do not contain significant amounts of REE. Core formation at higher P-T conditions could perhaps allow for a small but significant amount of REE to enter the core with the required fractionation between Sm and Nd. Modeling shows that the entire enriched hidden reservoir could be accommodated in the core with a modest fractionation factor of the corresponding depleted reservoir of 0.07 and a Nd concentration in the core of around 0.7 ppm. Experimental data at adequate P-T conditions are needed to address this issue.
TidsskriftAmerican Geophysical Union, Fall Meeting 2006
StatusUdgivet - 1 dec. 2006
Eksternt udgivetJa

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