Boninite petrogenesis and alteration history: constraints from stable isotope compositions of boninites from Cape Vogel,New Caledonia and Cyprus |
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| Authors: | T K Kyser W E Cameron E G Nisbet |
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| Institution: | (1) Department of Geological Sciences, University of Saskatchewan, S7N OWO Saskatoon, Canada;(2) Department of Geology, Australian National University, PO Box 4, 2600 Canberra, ACT, Australia;(3) Department of Geological Sciences, University of Saskatchewan, S7N OWO Saskatoon, Canada |
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| Abstract: | 
18O values of unaltered olivine and pyroxene phenocrysts in boninites from several areas range from 5.8 to 7.4 and indicate that the source for most boninites is more 18O-rich than MORBs and other oceanic basalts. The source for oxygen and other major elements is most likely a refractory portion of the mantle having a
18O value of up to 7.0 to which must be added a small amount of H2O-rich fluid to induce partial melting. This fluid, which is derived from subducted crust, is the vehicle for LREEs including Nd. The variable, normally low 
Nd values typical of boninites do not correlate with the
18O values.Post eruptive exchange of oxygen in the glass of boninites with that of sea water at low temperatures (<150° C) produces
18O values of >10 in optically fresh glass. Hydration of the glass has increased the water contents of most boninites from estimated magmatic values of 1–2 wt% to 2–4 wt% and produced D values of < –80, which may be lower than the original magmatic D values. In contrast to most submarine pillow basalts, the magmatic volatile composition of boninite lavas has been extensively modified as a result of post eruptive interaction with seawater. |
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