The Earth's mantle constitutes over 80% of the planet's volume, and is therefore a key reservoir in global geochemical cycling. The magnitudes and length scales of heterogeneities in the composition of the mantle are an important aspect of this reservoir, but are inaccessible to direct sampling. Mid-ocean-ridge basalt (MORB), the dominant eruptive product of ridges, provides a geographically widespread first-order snapshot of the mantle in terms of the distribution of major and trace elements and its isotopic composition. However, a range of processes occur between melt generation at depth and eruption on the sea floor that modulate the chemical signals of mantle heterogeneity in MORB, making it an imperfect sample. Detailed observations over the past few years have revealed that regional mantle heterogeneity is generally preserved in MORB most accurately where the melt supply is low, and that timescales between melt generation and MORB eruption are relatively short. Nevertheless, because of the variety of volcanic and magmatic processes that act to preserve or destroy signatures of mantle heterogeneity in MORB, a much broader base of observations from different locations will be required to faithfully reconstruct upper mantle heterogeneity.
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This paper is an outgrowth of ideas presented at the 2008 Goldschmidt conference on Ridges and Geochemical Mapping of the Mantle. The authors wish to thank D. Graham and E. Klein for organizing an inspiring session. This work was supported by National Science Foundation grants OCE-9905463 (K.H.R), OCE-0524922 (J.M.S) and EAR-0627991 (E.H.), and Natural Environment Research Council grants NER/I/S/2002/00609/2 and NE/E001254/1 (J.M.). SOEST contribution number 7715.
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