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 of508 ± 6 m.y. and 501 ± 13 m.y. with initial 143 Nd/ 144 Nd of ENd= +7.7 + 0.1 and ENd = +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 ENd 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 Esr = -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 E units) and shows the effect of sea water alteration. The ENd 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 ENd 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 whllt should be expected for a simple single-stage evolution. For a model with a single differentiation event at time T 0 to produce the depleted mantle, both Sm-Nd and Rb-Sr data for MORB and the Bay oflslands Complex give T 0 ::::: 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-ENd 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 ENd values of oceanic crust and mantle to identify old obducted oceanic segments on continental blocks.

Ordovician arc ophiolite, the Hayachine and Miyamori complexes, Kitakami Mountains, Northeast Japan: isotopic ages and geochemistry.

“…4). This trend coincides with that reported from the Bay of Islands Complex, which is attributed to the seawater alteration (Jacobsen and Wasserburg, 1979). The sample with highest as, is amphibolite (+ 31.9), which is supposed to be metamorphosed basalt, diabase dike, or gabbro.…”

“…This correla tion between rock type and asr is consistent with the idea that the wide 87Sr/86Sr ratio is originated from various degree of seawater interaction with a cooling magma system; extrusive or shallow in trusive rocks were highly interacted with seawater and recrystallized into amphibolite with strong lineation, whereas plutonic rocks were not. This is similar to the Bay of Island com plex, in which metagabbro, trondhjemite, and sheeted dikes give higher Es,,, whereas fresh gab bros and peridotites give lower Esr (Jacobsen and Wasserburg, 1979).…”

“…In itial 87Sr/86Sr ratios calculated for the Sm-Nd age of 510 Ma show a wide variation from 0.70361 to 0.70614, corresponding to esr of -4.0 to + 31.9. The pattern of limited ENd and variable Esr is similar to that of the Bay of Islands (Jacobsen and Wasserburg, 1979) and Samail (McCulloch et al, 1980) ophiolites, suggesting the effect of seawater alteration. This problem will be discussed later.…”

Nd and Sr isotopes and K-Ar ages of the Ulreungdo alkali volcanic rocks in the East Sea, South Korea.

“…7 and 8. The zero-slope trend is substantial 87Sr/86Sr shift re sulted with no change in 143Nd/144Nd by a seawater-basaltic rock interaction (Jacobsen and Wasserburg, 1979 In the late Oligocene and early Miocene when the East Sea and Sea of Japan start rifting and spreading, the several tens of extensive explosive volcanism have produced abundant submarine al kali volcanic rocks on the sea floor of the backarc basin. The extensive volcanic activities continued in the Japan and Yamato basins until 2 Ma (Ito and Okuyama, 1989;Usuta, 1989 Genetic interpretations of alkali volcanism in relation to tectonism in terms of Nd, Sr and Pb isotopes and trace element around East Sea and Sea of Japan were presented by many previous workers (Fujibayashi et al, 1988;Nohda et al, 1988;Kaneoka, 1990;Nakamura et al, 1990Nakamura et al, , 1995Tatsumoto and Nakamura, 1991;Shimazu et al, 1993;Morris et al, 1997).…”

Birth, death, and resurrection: The life cycle of suprasubduction zone ophiolites

“…8. Mineral compositions in the lavas and mantle tectonites that are refractory compared to those found in oceanic basalts and abyssal peridotites, e.g., Cr spinels with high Cr/Al ratios and low Mg/Fe ratios [Cameron, 1985;Crawford et al, 1989;Dick, 1989;Dick and Bullen, 1984;Umino et al, 1990] Cohen and O'Nions, 1982;Hamelin et al, 1984Hamelin et al, , 1981Hanan and Schilling, 1989;Ito et al, 1987;Jacobsen and Wasserburg, 1979;McCulloch and Cameron, 1983;McCulloch et al, 1980McCulloch et al, , 1981Noiret et al, 1981;Sun, 1980]. [7] These geochemical signatures result from processes or conditions that are unique to subduction zones or that are enhanced within the subduction zone environment.…”