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High ^(143)Nd/^(144)Nd in extremely depleted mantle rocks

Sharma, M. and Wasserburg, G. J. and Papanastassiou, D. A. and Quick, J. E. and Sharkov, E. V. and Laz'ko, E. E. (1995) High ^(143)Nd/^(144)Nd in extremely depleted mantle rocks. Earth and Planetary Science Letters, 135 (1-4). pp. 101-114. ISSN 0012-821X. https://resolver.caltech.edu/CaltechAUTHORS:20131115-092933548

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Abstract

Using an iron hydroxide precipitation to concentrate the rare earth elements it was possible to obtain precise Nd isotopic data on ultramafic rocks. Four samples of unserpentinized harzburgite from the Voykar ophiolite, Polar Urals, are characterized by extreme depletion in fertile components including light rare earth elements down to ∼ 1.7 ppb Nd. These samples have f_(Sm/Nd) values > +1 and ε_(Nd)(O) values ranging from +21 to +47, the highest ever measured in terrestrial whole rock samples. Combined ε_(Nd) and ε_(Sr) values of the ultramafic and mafic rocks and a leach indicate that alteration from seawater and/or groundwater is orders of magnitude less than required to produce a detectable change in the ε_(Nd) values even in samples with > 50% serpentine. Three of the harzburgites plot on a Sm-Nd whole rock array along with samples of dunite, websterite, gabbro and diabase from the ophiolite. The slope of the array corresponds to an age of 387 ± 34 Ma and an initial ε_(Nd) = +8.6 ± 1.8. These results indicate that the oceanic crustal rocks and the harzburgites are complementary and come from a MORB source. While the linear ^(147)Sm/^(144)Nd-^(143)Nd/^(144)Nd correlation is consistent with this model, one sample of harzburgite is far removed from the array, indicating the presence of older, depleted mantle material that is unrelated to the crustal section. This must be locally available in the upper mantle for tectonic occlusion during obduction of the ophiolite. Alternatively, this “aberrant” sample may represent the original ultrabasic section from older basalt extraction that was permeated by basalts derived from much deeper layers that provided the Voykar ophiolite crust. The major-element and low-REE-compositions of the Voykar harzburgites can be formed by progressive extraction of melt from an undepleted mantle protolith with the requirement that melt separation began in the garnet lherzolite stability field. The extreme chemical and isotopic characteristics of the Voykar harzburgites demonstrate that extraction of partial melt can generate an upper mantle with highly variable ε_(Nd) and very low Nd concentrations. The potential presence of large amounts of such material in the upper mantle poses an intriguing geochemical problem in that its bulk isotopic signature may not be detectably expressed in basalts, which are dominated by sources with higher average Nd concentrations and much lower ε_(Nd) and f_(Sm/Nd).


Item Type:Article
Related URLs:
URLURL TypeDescription
http://dx.doi.org/10.1016/0012-821X(95)00150-BDOIArticle
http://www.sciencedirect.com/science/article/pii/0012821X9500150BPublisherArticle
ORCID:
AuthorORCID
Wasserburg, G. J.0000-0002-7957-8029
Additional Information:© 1995 Elsevier Science B.V. Received 19 April 1995; accepted 28 July 1995. We thank Jason Saleeby for suggesting the term "geologic neutrino" to describe highly depleted harzburgite as a potentially significant but difficult to detect mantle component. The fractional melting calculations reported in this study were performed using Paul Asimow's adiabatic melting program which employs MELTS. We are grateful to Marc Ghiorso and Marc Hirschmann who developed MELTS and to Paul Asimow who made the adiabatic fractional melting calculations possible. We also thank Ed Stolper for supporting their development. Henry Ngo generously gave his time and expertise in analyzing the ultramafic rocks. Per Andersson, Tom LaTourrette, David Peate, Matthieu Roy-Barman and Don Porcelli suffered through many coffee discussions and shared our excitement. R.G. Coleman, A.W. Hofmann, A. Nicolas, S.B. Jacobsen, F. Albarooe and an anonymous reviewer are thanked for comments which have substantially improved the manuscript. Work supported by NSF EAR 88-16936 and NASA NAGW-3337. Division Contribution No. 5525(897). [FA]
Funders:
Funding AgencyGrant Number
NSFEAR 88-16936
NASANAGW-3337
Other Numbering System:
Other Numbering System NameOther Numbering System ID
Caltech Division of Geological and Planetary Sciences 5525
Lunatic Asylum Lab 897
Issue or Number:1-4
Record Number:CaltechAUTHORS:20131115-092933548
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20131115-092933548
Official Citation:M. Sharma, G.J. Wasserburg, D.A. Papanastassiou, J.E. Quick, E.V. Sharkov, E.E. Laz'ko, High143Nd/144Nd in extremely depleted mantle rocks, Earth and Planetary Science Letters, Volume 135, Issues 1–4, October 1995, Pages 101-114, ISSN 0012-821X, http://dx.doi.org/10.1016/0012-821X(95)00150-B. (http://www.sciencedirect.com/science/article/pii/0012821X9500150B)
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:42487
Collection:CaltechAUTHORS
Deposited By: Ruth Sustaita
Deposited On:15 Nov 2013 18:20
Last Modified:09 Mar 2020 13:19

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