Supplementary Figure 1 Map of the study area Sample locations and main physiographic features of the study area. Contour interval is 200m (a) and 40m

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Bertram Matthews
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$Dashed lines represent the two successive ridge axis jumps from east to west, toward the Mathematician seamounts. Orozco fracture zone is also represented as a thick dashed line.$

1 Supplementary Figure 1 Map of the study area Sample locations and main physiographic features of the study area. Contour interval is 200m (a) and 40m (b). Dashed lines represent the two successive ridge axis jumps from east to west, toward the Mathematician seamounts. Orozco fracture zone is also represented as a thick dashed line. Note that the ULC-influenced basalts are found at the ridge-seamounts connection.

3 Supplementary Figure 2 Isotope comparison to other unradiogenic Pb material Isotope (Sr, Nd, Pb, Hf) compositions of EPR N samples (open circles) together with data from literature (diamonds) for comparison: basalts from the South West Indian Ridge (SWIR), basalts from Pitcairn Island, abyssal peridotites, Horoman peridotites and abyssal peridotite sulphides (see legend for references). Diagrams (a), (b), (c), (d) and (e) clearly illustrate that none of the cited material has a geochemical signature that can account for the unradiogenic lead component (ULC) trends. For example, on diagrams (a) and (c) samples from Garrett FZ have much more depleted Sr and Nd signatures; Pitcairn samples have comparable 206 Pb/ 204 Pb but higher 208 Pb/ 204 Pb and more enriched Nd and Sr signatures. Some of the samples from the SWIR have Pb compositions comparable to ULC (b & d) however, diagrams (a) and (c) highlight the lack of overlap between extreme-ulc and SWIR samples, when Sr and Nd isotopes are considered. The samples that are the closest in composition ( 206 Pb/ 204 Pb, 208 Pb/ 204 Pb) to ULC are the peridotites from Horoman, however the 207 Pb/ 204 Pb data on diagram (d) are clearly different and Nd & Hf isotopes are typically more depleted (c & e).

4 Supplementary Figure 3 Trace element compositions Spidergram of trace element compositions normalized to Primitive mantle 11. The blue line corresponds to ULC-basalt typical pattern. The pink line corresponds to AM-basalt typical pattern.

5 Supplementary Table 1 Trace element modelling Kd Global partition coefficients (calculated) Cpx Opx D P HR-10 gabbro Source compositions Metaggabroic pyroxenite melt Ambient Mantle melt Mixing results (% pyroxenite melt + % ambient mantle melt) (calculated) (measured) Rb Ba Th U Nb La Ce % - 10% 80% - 20%* Sr Nd Zr Hf Sm Eu Gd Tb Dy Ho Y Er Yb Lu % - 50% 20% - 80%

6 Supplementary Table 2 Isotope modelling Most ULCinfluenced mixture Endmember 1 : Endmember 2 : ULC (calculated) 10-PUB16-05 "Ambient Mantle" (measured) (measured) 70%-30% 80%-20%* 90%-10% 87 Sr/ 86 Sr Nd/ 144 Nd Pb/ 204 Pb Pb/ 204 Pb Pb/ 204 Pb Hf/ 177 Hf [C](ppm) (estimated from trace element model table S6) Sr Nd Pb Hf

7 Supplementary Table 3 Isotope compositions Sample name 87 Sr/ 86 Sr 2σ 143 Nd/ 144 Nd 2σ 176 Hf/ 177 Hf 2σ 206 Pb/ 204 Pb 207 Pb/ 204 Pb 208 Pb/ 204 Pb 4 He/ 3 He 2σ R/Ra 2σ 10PUB E E E duplicate E PUB E E E PUB E E E PUB E E E PUB E E E PUB E E E PUB E E E PUB E E E PUB E E E PUB E E E PUB E E E PUB E E E PUB E E E PUB E E E PUB E E E PUB E E E PUB E E E PUB E E E PUB E E E PUB E E E duplicate PUB E E E PUB E E E duplicate E PUB E E E PUB E E E PUB E E E PUB E E E PUB E E E PUB E E E duplicate PUB E E E PUB E E E PUB E E E PUB E E E PUB E E E duplicate E PUB E E E PUB E E E PUB E E E PUB E E E duplicate E PUB E E E PUB E E E PUB E E E PUB E E E duplicate E PUB E E E PUB E E E PUB E E E

8 Sample name 87 Sr/ 86 Sr 2σ 143 Nd/ 144 Nd 2σ 176 Hf/ 177 Hf 2σ 206 Pb/ 204 Pb 207 Pb/ 204 Pb 208 Pb/ 204 Pb 4 He/ 3 He 2σ R/Ra 2σ 10PUB E E E duplicate E PUB E E E PUB E E E PUB E E E PUB E E E PUB E E E PUB E E E PUB E E E PUB E E E PUB E E E PUB E E E PUB E E E PUB E E E duplicate E PUB E E E duplicate E PUB E E E duplicate E E PUB E E E duplicate E E PUB E E E PUB E E E PUB E E E PUB E E E PUB E E E PUB E E E duplicate E PUB E E E duplicate E PUB E E E PUB E E E PUB E E E PUB E E E PUB E E E PUB E E E PUB E E E PUB E E E PUB E E E PUB E E E

9 Supplementary Table 4 Sample locations Sample Latitude ( N) Longitude ( W) Sample Latitude ( N) Longitude ( W) 10PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB

10 Supplementary Table 5 Rock standard BCR-2 repeated measurements (HR ICP MS) through 8 different analytical sessions BCR2 (1) BCR2 (2) BCR2 (3) BCR2 (4) BCR2 (5) BCR2 (6) BCR2 (7) BCR2 (8) average std dev % Certif. Value 21 La Ce Pr Nd Sm Eu Tb Gd Dy Ho Er Tm Yb Lu Hf Pb Th U Li Sc Ti V Co Ga Rb Sr Y Zr Nb Cs Ba U Sc Ti V Co Zn

11 Supplementary Table 6 Major element compositions Sample (wt %) name SiO 2 TiO 2 Al 2 O 3 FeO* MnO MgO CaO Na 2 O K 2 O P 2 O 5 Total 10PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB

12 Sample (wt %) name SiO 2 TiO 2 Al 2 O 3 FeO* MnO MgO CaO Na 2 O K 2 O P 2 O 5 Total 10PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB

13 Supplementary Table 7 Trace element compositions Sample (ppm) name Li Sc Ti V Co Zn Ga Gd Rb Sr Y Zr Nb Cs Ba 10PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB

14 Sample (ppm) name Li Sc Ti V Co Zn Ga Gd Rb Sr Y Zr Nb Cs Ba 10PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB

15 Sample (ppm) name La Ce Pr Nd Sm Eu Tb Gd Dy Ho Er Tb Yb Lu Hf Pb Th U 10PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB

16 Sample (ppm) name La Ce Pr Nd Sm Eu Tb Gd Dy Ho Er Tb Yb Lu Hf Pb Th U 10PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB PUB

17 Supplementary References 1. Meyzen, C. M. et al. Isotopic portrayal of the Earth s upper mantle flow field. Nature 447, (2007). 2. D. Woodhead, J. & T. McCulloch, M. Ancient seafloor signals in Pitcairn Island lavas and evidence for large amplitude, small length-scale mantle heterogeneities. Earth Planet. Sci. Lett. 94, (1989). 3. Eisele, J. et al. The role of sediment recycling in EM-1 inferred from Os, Pb, Hf, Nd, Sr isotope and trace element systematics of the Pitcairn hotspot. Earth Planet. Sci. Lett. 196, (2002). 4. Saal, A. E. et al. Pb isotopic variability in melt inclusions from the EMI EMII HIMU mantle end-members and the role of the oceanic lithosphere. Earth Planet. Sci. Lett. 240, (2005). 5. Wendt, J. I., Regelous, M., Niu, Y. L., Hekinian, R. & Collerson, K. D. Geochemistry of lavas from the Garrett Transform Fault: insights into mantle heterogeneity beneath the eastern Pacific. Earth Planet. Sci. Lett. 173, (1999). 6. Stracke, A. et al. Abyssal peridotite Hf isotopes identify extreme mantle depletion. Earth Planet. Sci. Lett. 308, (2011). 7. Warren, J. M., Shimizu, N., Sakaguchi, C., Dick, H. J. B. & Nakamura, E. An assessment of upper mantle heterogeneity based on abyssal peridotite isotopic compositions. J. Geophys. Res. Solid Earth 114, (2009). 8. Malaviarachchi, S. P. K., Makishima, A., Tanimoto, M., Kuritani, T. & Nakamura, E. Highly unradiogenic lead isotope ratios from the Horoman peridotite in Japan. Nat. Geosci 1, (2008). 9. Warren, J. M. & Shirey, S. B. Lead and osmium isotopic constraints on the oceanic mantle from single abyssal peridotite sulfides. Earth Planet. Sci. Lett. 359, (2012). 10. Burton, K. W. et al. Unradiogenic lead in Earth s upper mantle. Nat. Geosci 5, (2012). 11. McDonough, W. & Sun, S. The composition of the Earth. Chem. Geol. 120, (1995). 12. Paster, T., Schauwec, D. S. & Haskin, L. Behavior of some trace-elements during solidification of skaergaard layered series. Geochim. Cosmochim. Acta 38, (1974).

18 13. Hart, S. & Dunn, T. Experimental cpx melt partitioning of 24 trace-elements. Contrib. Mineral. Petrol. 113, 1 8 (1993). 14. Johnson, KTM. Experimental cpx/and garnet/melt partitioning of REE and other trace elements at high pressures: petrogenetic implications A, (1994). 15. Blundy, J. & Wood, B. Prediction of crystal-melt partition-coefficients from elasticmoduli. Nature 372, (1994). 16. Wood, B. J. & Blundy, J. D. A predictive model for rare earth element partitioning between clinopyroxene and anhydrous silicate melt. Contrib. Mineral. Petrol. 129, (1997). 17. Salters, V. J. M. & Longhi, J. Trace element partitioning during the initial stages of melting beneath mid-ocean ridges. Earth Planet. Sci. Lett. 166, (1999). 18. Hanson, G. Rare-earth elements in petrogenetic studies of igneous systems. Annu. Rev. Earth Planet. Sci. 8, (1980). 19. Kennedy, A. K., Lofgren, G. E. & Wasserburg, G. J. An experimental study of trace element partitioning between olivine, orthopyroxene and melt in chondrules: equilibrium values and kinetic effects. Earth Planet. Sci. Lett. 115, (1993). 20. Malaviarachchi, S. P. K., Makishima, A. & Nakamura, E. Melt-Peridotite Reactions and Fluid Metasomatism in the Upper Mantle, Revealed from the Geochemistry of Peridotite and Gabbro from the Horoman Peridotite Massif, Japan. J. Petrol. 51, (2010). 21. Jochum K. P. & Nehring F. (Max-Plank-Institute fuer Chemie) (2006) USGS BCR-2: GeoReM preferred values (11/2006). GeoReM. Available from:

Effect of tectonic setting on chemistry of mantle-derived melts

Effect of tectonic setting on chemistry of mantle-derived melts Lherzolite Basalt Factors controlling magma composition Composition of the source Partial melting process Fractional crystallization Crustal