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Nd and Sr Isotopic Study of the Bay of Islands Ophiolite Complex and the Evolution of the Source of Midocean Ridge Basalts

Jacobsen, Stein B. and Wasserburg, G. J. (1979) Nd and Sr Isotopic Study of the Bay of Islands Ophiolite Complex and the Evolution of the Source of Midocean Ridge Basalts. Journal of Geophysical Research B, 84 (B13). pp. 7429-7445. ISSN 0148-0227. doi:10.1029/JB084iB13p07429.

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Two Sm-Nd internal isochrons for pyroxene gabbros of the Bay of Islands Ophiolite Complex give well-defined ages of 508 ± 6 m.y. and 501 ± 13 m.y. with initial ^(143)Nd/^(144)Nd of ε_(Nd) = +7.7 ∓ 0.1 and ε_(Nd) = +7.5 ∓ 0.2, respectively. Total rock samples from pillow basalts, sheeted dikes, trondhjemites, hornblende gabbros, pyroxene gabbros, and an orthopyroxenite layer from the harzburgite give initial ε_(Nd) in the range from +6.5 to +8.1 with an average value of +7.6. The initial ^(87)Sr/^(86)Sr obtained on a pyroxenegabbro is ε_(Sr) = −19.3 ± 0.3, which is typical of oceanic samples. However, the initial ^(87)Sr/^(86)Sr within the different phases of the complex is found to be highly variable (∼52 ε units) and shows the effect of sea water alteration. The ε_(Nd) values demonstrate a clear oceanic affinity for the Bay of Islands complex and support earlier interpretations made on the basis of structure and geochemistry. The magnitudes of the initial ε_(Nd) values (+7.6) are somewhat smaller than for typical present-day midocean ridge basalts (MORB) (+10). This is most likely due to differential evolution over the past 0.5 aeon of the oceanic mantle relative to the bulk earth. The observed shift is quantitatively what should be expected for a simple single-stage evolution. For a model with a single differentiation event at time T_D to produce the depleted mantle, both Sm-Nd and Rb-Sr data for MORB and the Bay of Islands Complex give T_D ≈ 1.8 aeons. This age is, however, interpreted as the mean age of the MORB source and does not refer to a unique event. The total time for producing this source by a uniform process would be of the order of 3.6 aeons. The Nd isotopic signature of oceanic crust is clearly present in this Paleozoic ophiolite and suggests that typical high-ε_(Nd) reservoirs are sources of oceanic crust through the Phanerozoic. This implies relatively rapid turnover between oceanic crust and mantle sources and a good mixing of oceanic mantle for the past 1.0 aeon, including contributions from recycled continental materials. These data indicate that the major distinction between oceanic basalts and continental basalts observed in recent rocks is also preserved through the Phanerozoic. These isotopic differences clearly imply a long time distinction between the magma sources of these basalt types. It should be possible to apply the distinctive ε_(Nd) values of oceanic crust and mantle to identify old obducted oceanic segments on continental blocks.

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Wasserburg, G. J.0000-0002-7957-8029
Additional Information:© 1979 by the American Geophysical Union. Manuscript Accepted: 31 Jul 1979. Manuscript Received: 10 May 1979. We wish to thank N. Christensen and H. Williams for their advice during the planning of the field work in Newfoundland and H. Williams for his hospitality. We particularly appreciate H. Williams's cleansing of the ophiolite samples from possible tuber malignancies and his help with importing the specimens. N. Christensen also provided one of the samples so that some data could be gathered prior to the field work during the summer of 1978. D. Papanastassiou, M. McCulloch, and the reviewers H. Williams and W. M. White provided numerous helpful comments and suggestions on the manuscript. We also want to thank A. M. Ziegler for stimulating discussions of the ophiolite problem. This work has been supported by NSF grant EAR 76-22494. Division of Geological and Planetary Sciences contribution 3210 (291).
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NSFEAR 76-22494
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Caltech Division of Geological and Planetary Sciences 3210
Lunatic Asylum Lab291
Issue or Number:B13
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Official Citation:Jacobsen, S. B., and G. J. Wasserburg (1979), Nd and Sr isotopic study of the Bay of Islands Ophiolite Complex and the evolution of the source of midocean ridge basalts, J. Geophys. Res., 84(B13), 7429–7445, doi:10.1029/JB084iB13p07429.
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ID Code:42612
Deposited On:21 Nov 2013 21:07
Last Modified:10 Nov 2021 16:25

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