(2006) 'Adakites without slab melting : high pressure dierentiation of island arc magma, Mindanao, the Philippines.', Earth and planetary science letters., 243 (3-4). pp. 581-593. Further information on publisher's website:http://dx.doi.org/10.1016/j.epsl.2005.12.034Publisher's copyright statement:Additional information: Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. Pliocene arc lavas generated in the same subduction zone. Excepting the heavy rare earth 27 elements, the adakites and arc lavas also possess similar ratios of incompatible elements 28 suggesting that the adakites were ultimately derived from melting of the mantle wedge. 29The wide range of SiO 2 in the adakites and its strong correlation with trace element 30 concentrations and ratios indicate two possible mechanisms for generating the adakitic 31 signature. (1) Adakitic melt was produced from basaltic arc magma by fractional 32 crystallisation of a garnet-bearing assemblage. (2) Solidified basaltic rock containing 33 garnet melted to yield adakitic magma. In either case the basaltic precursor was generated 34 from fluid-modified mantle then differentiated within the garnet stability field. In Surigao 35 this requires differentiation within mantle. The Surigao example suggests that any 36 subduction zone has the potential to produce adakitic magma if basalt crystallises at 37 sufficient depth. This has important implications for the geodynamics of modern and 38 ancient subduction zones that have generated similar rocks. 39

To explain the elevated osmium isotope (186Os-187Os) signatures in oceanic basalts, the possibility of material flux from the metallic core into the crust has been invoked. This hypothesis conflicts with theoretical constraints on Earth's thermal and dynamic history. To test the veracity and uniqueness of elevated 186Os-187Os in tracing core-mantle exchange, we present highly siderophile element analyses of pyroxenites, eclogites plus their sulfides, and new 186Os/188Os measurements on pyroxenites and platinum-rich alloys. Modeling shows that involvement in the mantle source of either bulk pyroxenite or, more likely, metasomatic sulfides derived from either pyroxenite or peridotite melts can explain the 186Os-187Os signatures of oceanic basalts. This removes the requirement for core-mantle exchange and provides an effective mechanism for generating Os isotope diversity in basalt source regions.

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