Nature 362, 530 - 533 (08 April 1993); doi:10.1038/362530a0

Separating mantle from slab signatures in arc lavas using B/Be and radiogenic isotope systematics

C. M. H. Edwards^*†‡, J. D. Morris^*‡ & M. F. Thirlwall^†

^*Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road NW, Washington DC 20015, USA
^†Department of Geology, Royal Holloway College, University of London, Egham, Surrey TW20 OEX, UK
^‡Present addresses: Geological Survey of Canada, 601 Booth Street, Room 473, Ottawa, Ontario, K1A OE8 Canada (C.M.H.E.). Department of Earth Sciences, Washington University, One Bookings Dr CB 1169, St Louis, Missouri 63130-4899, USA (J.D.M.)

AT convergent margins, tectonic processes juxtapose subducted slab, mantle wedge and the crust of the upper plate in a column beneath the overlying arc volcano. As each of these components is expected to be chemically heterogeneous, and as all may contribute to magma chemistry, identifying the different sources of arc magmas has been difficult. A working hypothesis has emerged, in which tholeiitic and calc-alkaline lavas in island arcs are partial melts of the mantle produced by fluxing of the wedge by hydrous fluids from the subducted slab^1,2. Trace-element and radiogenic isotope ratios have been used to define the chemical characteristics of these sources but cannot be unequivocally identified with one source; by contrast, high B/Be and ¹⁰Be/⁹Be ratios in arc lavas uniquely identify the subduction component^3,4, and thus separate chemical variability owing to recent subduction from that reflecting other causes. Here we combine B/Be with Sr, Nd and Pb isotope systematics of alkaline, calc-alkaline and tholeiitic lavas from Java and Flores, Indonesia, to constrain the isotopic composition of their mantle and subduction sources. The alkaline lavas always have low B/Be, from which we conclude that they are derived from mantle that has not been modified by recent subduction (in agreement with refs 5 and 6). We also show that the isotopic composition of the subducted component is relatively homogeneous along the length of the arc, suggesting that the subduction of Australian continental lithosphere in the east started too recently to have changed the nature of the subducted material at present beneath Flores.

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