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A Nd and Sr isotopic study of the Trinity peridotite; implications for mantle evolution

Jacobsen, S. B. and Quick, J. E. and Wasserburg, G. J. (1984) A Nd and Sr isotopic study of the Trinity peridotite; implications for mantle evolution. Earth and Planetary Science Letters, 68 (3). pp. 361-378. ISSN 0012-821X. doi:10.1016/0012-821X(84)90122-5.

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Field evidence indicates that the Trinity peridotite was partially melted during its rise as a part of the upwelling convecting mantle at a spreading center. A Sm-Nd mineral isochron for a plagioclase lherzolite yields an age,T = 427 ± 32 Ma and initial ε_(Nd) = + 10.4 ∓ 0.4 which is distinctly higher than that expected for typical depleted mantle at this time. This age is interpreted as the time of crystallization of trapped melt in the plagioclase lherzolite P-T field. This time of crystallization probably represents the time when the massif was incorporated as a part of the oceanic lithosphere. The Sm-Nd model age of the plagioclase lherzolite total rock is T_(CHUR)^(Nd) = 3.4 AE. This suggests that the Trinity peridotite was derived from a mantle that was depleted rather early in earth history. The peridotite contains many generations of pyroxenite dikes and some microgabbro dikes. We report data for two dikes that clearly crosscut the main metamorphic fabric of the peridotite. A microgabbro dike yields a Sm-Nd mineral isochron age of T = 435 ± 21 Ma and ε_(Nd) = + 6.7 ∓ 0.3. A pyroxenite dike yields an initial ε_(Nd) = + 7.3 ± 0.4. The initial ε_(Nd) values for the pyroxenite and gabbro dikes are fairly similar to those for the depleted mantle at this time and are distinct from the lherzolite—demonstrating that they are not genetically related. Rb-Sr data do not give any coherent pattern. However, some bounds can be put on initial Sr values of ε_(Sr) ⩽ −21 for the plagioclase lherzolite and ε_(Sr) ⩽ −8.7 for the microgabbro dike. It is plausible that the dikes represent cumulates left behind from island arc magmas that rose through the the oceanic lithosphere within the vicinity of a subduction zone. Major and trace elements and Sm-Nd isotopic data indicate a multiple stage history for the Trinity peridotite; a small melt fraction was extracted from an undepleted source ∼ 3.4 AE or more ago to produce the proto-lherzolite; a large fraction of melt (∼ 12 to 23%) was extracted from the proto-lherzolite to produce the present rock; the lherzolite was then crosscut by dikes from average depleted mantle ∼ 0.44 AE ago. The data are compatible with the depleted mantle source being formed very early in earth history. Although most available data indicate that the depleted upper mantle has been relatively well stirred through time, the Trinity data suggest that very ancient Nd isotopic values are preserved and thus chemical and physical heteorgeneities are sometimes preserved in the depleted source of mid-ocean ridge basalts as well as the oceanic lithosphere which they intrude.

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Wasserburg, G. J.0000-0002-7957-8029
Additional Information:© 1984 Elsevier Science Publishers B.V. Received September 16, 1983. Revised version received February 27, 1984. This paper was supported by the National Aeronautics and Space Administration grant No. NAG 9-43, and the National Science Foundation grant No. PHY82-15500. We thank A. Zindler for a helpful review of this paper.
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Caltech Division of Geological and Planetary Sciences3824
Lunatic Asylum Lab427
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Official Citation:S.B. Jacobsen, J.E. Quick, G.J. Wasserburg, A Nd and Sr isotopic study of the Trinity peridotite; implications for mantle evolution, Earth and Planetary Science Letters, Volume 68, Issue 3, June 1984, Pages 361-378, ISSN 0012-821X, (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:42013
Deposited By: Ruth Sustaita
Deposited On:22 Oct 2013 22:36
Last Modified:10 Nov 2021 04:37

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