Nd and Sr isotopic study of the Bay of Islands Ophiolite Complex and the evolution of the source of midocean ridge basalts

Abstract

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 143Nd/144Nd of εNd = +7.7 ∓ 0.1 and εNd = +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 εNd in the range from +6.5 to +8.1 with an average value of +7.6. The initial 87Sr/86Sr obtained on a pyroxenegabbro is εSr = −19.3 ± 0.3, which is typical of oceanic samples. However, the initial 87Sr/86Sr 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 εNd 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 εNd 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 TD to produce the depleted mantle, both Sm‐Nd and Rb‐Sr data for MORB and the Bay of Islands Complex give TD ≈ 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‐εNd 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 εNd values of oceanic crust and mantle to identify old obducted oceanic segments on continental blocks.