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The neodymium isotopic compositions and rare earth patterns in highly depleted ultramafic rocks

Sharma, M. and Wasserburg, G. J. (1996) The neodymium isotopic compositions and rare earth patterns in highly depleted ultramafic rocks. Geochimica et Cosmochimica Acta, 60 (22). pp. 4537-4550. ISSN 0016-7037. doi:10.1016/S0016-7037(96)00280-3.

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A large number of terrestrial and extraterrestrial reservoirs are sampled by ultradepleted rocks containing mainly olivine and orthopyroxene. However, very few attempts have been made to determine the REE contents and neodymium isotopic composition of such samples due to substantial analytical problems in handling large sample volumes. We describe a chemical separation technique with high chemical yield and low contamination suitable for extraction of REEs from depleted ultramafic rocks. After sample dissolution, we use the Fe present in the rock for coprecipitation of REEs while keeping Mg, the dominant major element in an ultramafic rock, in solution. This technique reduces the effective sample size by an order of magnitude, thus permitting the use of existing column techniques to separate the lanthanides. Utilizing this procedure, eight harzburgites and a dunite, associated with the mantle sections of ophiolite complexes, were analyzed for REE and neodymium isotopic composition. The results show the LREE concentrations varying from sub parts per billion to less than 10 ppb; the Yb concentrations representing the HREEs vary from 1 to 70 ppb. The rocks have ƒ_(Sm/Nd) values ranging from −0.45 to +3.4 and ϵ_(Nd)(0) values varying from −12.5 to +47.5. The ultramafic rocks can be classified into two groups using their initial ϵ_(Nd)(t) values. The samples with ϵ_(Nd)(t) > +7 are derived from depleted MORB mantle and show extreme LREE depletion and HREE enrichment with no hint of a U-shaped REE pattern. However, samples with ϵ_(Nd)(t) < +7 yield REE patterns that are either U-shaped or flatten towards the LREE end are interpreted as having been contaminated by continental crust. We conclude that the U-shaped REE patterns found in many harzburgites associated with mantle sections of ophiolite complexes are the result of crustal contamination and not the consequence of peculiar fractionation during melting. The HREE depletion shown by the samples with ϵ_(Nd)(t) > +7 can be modeled with equilibrium fractional melting. However, the observed (La/Sm)_N ratios are several orders of magnitude higher than predicted by the melting model. We suggest that the high (La/Sm)_N ratios in the ultradepleted samples may be a result of refertilization by basaltic melts. The chemical procedure developed in this paper permits a full exploration of ultradepleted rocks from the upper mantle as well as some classes of meteorites that are highly depleted.

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Wasserburg, G. J.0000-0002-7957-8029
Additional Information:© 1996 Elsevier Science Ltd. Received March 19, 1996; accepted in revised form July 31, 1996. We thank D. A. Papanastassiou and H. Ngo for generous help in mass spectrometry and chemistry. We are grateful to F. Tera and J. Chen for input on the chemical procedure. E. Moores kindly provided the Vourinos harzburgite sample and A. J. Irving the Twin Sisters dunite. The drill core samples from southern Urals were furnished by E. E. Laz'ko and E. V. Sharkov. The Oman sample was collected by J. Hashimoto and G. J. Wasserburg on a field trip organized by A. Nicolas. The junior author is particularly thankful to J. Hashimoto and the other members of the Oman field party for their great support and aid under difficult circumstances. He also acknowledges the staff of Sultan Qaboug University Hospital and Dr. M. Rangaswamy whose medical competence and personal kindness were a key to what followed. Dr. K. Godjik then got things in hand. The REE patterns in clinopyroxenes were determined using PANURGE with the help of A.J. Fahey and G. Huss. ICP-MS analyses were performed at the Environmental Engineering department at Caltech with the help of P. Green. We thank P. D. Asimow and M. Hirschmann for generous help and discussions and for showing us the workings of Adiabat 2Sh. We also thank E. M. Stolper for supporting the development of Adiabat 2Sh. T. Latourrette, D. A. Papanastassiou, D. Porcelli, and J. E. Quick reviewed an early version of this paper and gave several constructive comments. This paper has also benefited from reviews by J.-L. Bodinier, M. A. Menzies, and D. G. Pearson. Work supported by NASA Grants NAGW-3337 and NAGW-3297. Division Contribution No. 5549 (902). Editorial handling: M. Menzies
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Caltech Division of Geological and Planetary Sciences5549
Lunatic Asylum Lab902
Issue or Number:22
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Official Citation:M. Sharma, G.J. Wasserburg, The neodymium isotopic compositions and rare earth patterns in highly depleted ultramafic rocks, Geochimica et Cosmochimica Acta, Volume 60, Issue 22, November 1996, Pages 4537-4550, ISSN 0016-7037, (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:42556
Deposited By: Ruth Sustaita
Deposited On:19 Nov 2013 17:59
Last Modified:10 Nov 2021 16:24

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