Hf-Nd-Sr isotopes and incompatible element abundances in island arcs: implications for magma origins and crust-mantle evolution

Details Hf-Nd-Sr isotopes and incompatible element abundances in island arcs: implications for magma origins and crust-mantle evolution Hf-Nd-Sr isotopes and incompatible element abundances in island arcs: implications for magma origins and crust-mantle evolution

Feb 1984

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1984e%26psl..67..167w&link_type=abstract

Earth and Planetary Science Letters, Volume 67, Issue 2, p. 167-185.

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We present Hf, Nd and Sr isotopic data and abundances of K, Rb, Cs, Ba, Sr, Hf and REE for 32 samples from seven intra-oceanic island arcs. Samples from the Marianas, Izu, Aleutian and New Britain arcs have tightly grouped 176Hf/177Hf ~ 0.28320, 143Nd/144Nd ~ 0.51303 and 87Sr/86Sr ~ 0.7035 close to, but distinct from, mid-ocean ridge basalts (MORB) for 143Nd/144Nd and 87Sr/86Sr. In contrast, samples from the Sunda, Banda and Lesser Antilles arcs are much more variable towards lower 176Hf/177Hf and 143Nd/144Nd, and higher 87Sr/86Sr. Isotopically, island arcs on the whole are closely similar to ocean islands. Some commonly-occurring features of the trace element geochemistry of island arcs are apparent in our data: alkali and alkaline-earth elements, particularly Cs, have high abundance relative to LREE compared to oceanic basalts; negative Ce anomalies occur in six out of seven arcs. However, Hf does not appear underabundant relative to REE.
The isotopic data require a continental component in all island arcs, in addition to probable mantle and oceanic crust contributions, even for the arcs with isotope ratios close to MORB. In the absence of continental crust, we can best explain this component by subducted pelagic sediment in the arc magma source region. The involvement of sediments in all arcs implies that there is an inherent recycling of older continent to island arcs, and potentially to new continent, of at least 1%.
Conservative calculations show that the upper subducted slab (basalt + sediment) passes beyond the arc magma genesis zone and enters the deep mantle with a minimum of 500-1000 ppm K, and corresponding amounts of other incompatible elements. If this material is not completely homogenized with the mantle and later becomes part of the source of ocean island magmas, then the ocean island-island arc isotopic similarity is a result of their similar mix of source materials-mantle peridotite with trace element signatures from oceanic crust and sediment.

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