13We present whole rock Li and Mg isotope analyses of 33 ultramafic xenoliths from 14 the terrestrial mantle, which we compare with analyses of 30 (mostly chondritic) 15 meteorites. The accuracy of our new Mg isotope ratio measurement protocol is 16 substantiated by a combination of standard addition experiments, the absence of mass 17 independent effects in terrestrial samples and our obtaining identical values for rock 18 standards using 2 different separation chemistries and 3 different mass-spectrometric 19 introduction systems. Carbonaceous, ordinary and enstatite chondrites have 20

The Li isotope ratios of four international rock reference materials, USGS BHVO‐2, GSJ JB‐2, JG‐2, JA‐1 and modern seawater (Mediterranean, Pacific and North Atlantic) were determined using multi‐collector inductively coupled plasma‐mass spectrometry (MC‐ICP‐MS). These reference materials of natural samples were chosen to span a considerable range in Li isotope ratios and cover several different matrices in order to provide a useful benchmark for future studies. Our new analytical technique achieves significantly higher precision and reproducibility (< ± O.3%o 2s) than previous methods, with the additional advantage of requiring very low sample masses of ca. 2 ng of Li.

'Recycled' crustal materials, returned from the Earth's surface to the mantle by subduction, have long been invoked to explain compositional heterogeneity in the upper mantle. Yet increasingly, problems have been noted with this model. The debate can be definitively addressed using stable isotope ratios, which should only significantly vary in primitive, mantle-derived materials as a consequence of recycling. Here we present data showing a notable range in lithium isotope ratios in basalts from the East Pacific Rise, which correlate with traditional indices of mantle heterogeneity (for example, 143Nd/144Nd ratios). Such co-variations of stable and radiogenic isotopes in melts from a normal ridge segment provide critical evidence for the importance of recycled material in generating chemical heterogeneity in the upper mantle. Contrary to many models, however, the elevated lithium isotope ratios of the 'enriched' East Pacific Rise lavas imply that subducted ocean crust is not the agent of enrichment. Instead, we suggest that fluid-modified mantle, which is enriched during residency in a subduction zone, is mixed back into the upper mantle to cause compositional variability.

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