Trace elements
The major and trace elements were measured using a RESOlution-S155 193nm Excimer laser connected to a Thermo iCAP Q ICP-MS which is used only for laser ablation analyses at Harvard University. Two points per chip were obtained, and the values reported are the average of the two analyses. Ca contents of the major element analysis of the same chip were used as an internal standard. Data were then reduced using basalt standard BHVO-2G run in the same run (values in Table S2), which was also used as a drift correction standard run every ten analyses. The Harvard lab basalt glass standard VE32 was present in every probe mount, and was analyzed as an unknown with the other samples to get estimates of between run reproducibility over the course of this study. VE32 data were very consistent between runs (Table S2), and provide good estimates of the errors in the analyses, which generally scale with concentration (Table S2).
Water
Concentrations of H2O dissolved in glasses were analyzed by Fourier Transform Infrared Spectroscopy (FTIR) at the University of Tulsa using published methods and calibrations43,44 with slight modifications15. Doubly polished glass wafers, 100-250μm thick, were placed atop a 2mm thick KBr pellet and analyzed using a NicPlan IR microscope equipped with a HgCdTe detector, attached to a Nicolet 520 FTIR. Thickness was measured by two methods: firstly by digital micrometer, and secondly by focusing the calibrated z-axis of the FTIR microscope stage on the glass wafer and on the adjacent KBr disk using reflected light. Optically clear areas of known thickness (± 2μm), 80 x 80 μm, were analyzed with 256 scans/spot. Absorbance at the broad 3550 cm-1 (combined OH and H2O) and 1630 cm-1 (molecular H2O only) peaks were measured after subtraction of interpolated backgrounds. Density was assumed to be 2.8 g/cm3. Molar absorption coefficients used for all glasses were: 63 l/mol-cm for 3550 and 25 l/mol-cm for e1630. Analyses are the average of 3-4 spot determinations of 3550 cm-1 on two separate wafers. Replicate analyses of different wafers from the same specimen were typically reproducible to ±5%.
Sr-Nd-Pb isotopes
Column chromatographic procedures for Sr-Nd-Pb purifications were carried out in Class 100 or Class 1000 ultra-clean chemistry lab at Lamont-Doherty Earth Observatory following routine procedures45. Hand-selected clean glass chips were sonicated in cold 8N HNO3 in the sonicator for 10 minutes and washed with quartz-distilled water 4 times prior to standard HNO3-HF hotplate digestions. Pb was extracted using BioRad® AG1-X8 resin. Sr was extracted using Eichrom Sr resin. Rare Earth Elements (REE) were extracted using Tru-Spec® resin. Nd was extracted from the REE cut either using alpha-hydroxyisobutyric acid (alpha-HIBA) and BioRad® AG50-X8 resin or using Eichrom® Ln resin and 0.22 N HNO3 acid.
Isotopes were measured on a VG-Axiom multicollector-inductively coupled mass spectrometer (MC-ICPMS). In-run mass fractionations are normalized using 86Sr/88Sr = 0.1194, and 146Nd/144Nd = 0.7219. All samples were further normalized to the 86Sr/88Sr value of 0.71024 for SRM 987, 143Nd/144Nd value of 0.511858 for La Jolla, and 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb values of 16.9356, 15.4891, and 36.7006 for NBS 98146, respectively.
Pb total procedure blank is always below 100 pg. As procedural blanks are negligible given the sample size and current analytical uncertainties, no blank corrections were performed. Measurements of international standard sample BCR-2 (after acid leach) yielded 0.704998 ±0.000013 (2σ, n=6) for 87Sr/86Sr, 0.512609 ± 0.000006 (2σ, n=3) for 143Nd/144Nd, as well as 18.7895±0.0008, 15.6523±0.0007, and 38.8845±0.0019 (2σ, n=1) for 206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb ratios, respectively, which is consistent with the published values of Weis et al.47: 87Sr/86Sr = 0.705019 ±0.000016 (2σ, n=13), 143Nd/144Nd = 0.512634 ±0.000012 (2σ, n=11), 206Pb/204Pb =18.8000±0.0020, 207Pb/204Pb =15.6236±0.0016, and 208Pb/204Pb = 38.8244±0.0034 (2σ, n=2).
The advantage of using Rb/Nb, Ba/Nb, Nb/U, and Ce/Pb to construct the BABB filter
Rb, Ba, Nb, and U as well as Ce and Pb are known to have similar incompatibility during magmatic processes, respectively, thus these element pairs would not be significantly fractionated from each other and these ratios should reflect source characteristics. There are, however, complicating factors. Alteration increases Rb and U contents28 relative to Nb, contamination increases Pb relative to Ce, and mantle metasomatism by very low degree melt would increase Rb and Ba over Nb. Thus, various geological processes would lead to variations in each of the four ratios that would not be caused by subduction influence. Using any three of these four ratios to pass the filter minimizes the possibility of alteration, contamination and source enrichment effect leading to a spurious conclusion with respect subduction influence, and also means that a single anomalous ratio does not rule out that influence. Also, in case a real BABB-like MORB has a bad Pb, or U measurement, using any 3 instead of all four leaves some room for analytical error.
Global BABB-like MORB: additional evidence from H2O/Ce and 87Sr/86Sr vs. 143Nd/144Nd
Enrichments of Rb, Ba, Th, U and Pb in oceanic basalts have been proposed to result from continental crust materials mixed in the convecting mantle48, thus, it is important to distinguish the subduction contribution versus continental crust additions in the source of BABB-like MORB. H2O/Ce ratios are particularly useful as dehydration of the subducted slab would undoubtedly enrich the sub-arc wedge in H2O relative to Ce15, leading to much higher H2O/Ce ratios for rocks with subduction contributions than Pacific MORB average (~20015,16). The continental crust, on the other hand, is dry. The upper continental crust can contain up to 8000 ppm water, mainly in hydrous minerals or as fluid inclusions in felsic minerals such as quartz and feldspars49 and lower continental crust is estimated to contain water of less than 1000 ppm according to studies of granulite xenoliths50. The H2O/Ce ratios, which could then be calculated with average Ce contents of continental crust21, are 127 for upper continental crust, and 50 for lower continental crust. Therefore, contribution of continental crust materials would result in lower H2O/Ce ratios, contrary to the subduction contributions.
Data on water concentrations are far more limited than data for ICP-MS elements. BABB-like MORB, however, have higher H2O/Ce ratios than the Pacific samples without subduction influence (Fig. 3b,c). Michael15 and Dixon, et al.16 noted that higher H2O/Ce ratios in the North Atlantic might be related to a subduction signature. The higher average H2O/Ce ratios for global BABB-like MORB, along with higher Ba/Nb, Rb/Nb and Th/Nb (Fig. 3), as well as lower Nb/U and Ce/Pb ratios, suggesting the Ba, Rb, Th, U, and Pb enrichments in BABB-like MORB mainly result from subduction input, rather than continental crust additions.
Continental crust addition would also lead to higher 87Sr/86Sr coupled with lower 143Nd/144Nd ratios for the derived basalts48 and higher La/Sm ratios. Yet, we observe a shift to higher 87Sr/86Sr with limited variations in 143Nd/144Nd and (La/Sm)N ratios for most BABB-like MORB relative to Pacific MORB (Fig. 3d,e). Pacific MORB can then be used to set up a reference line (PMRL) to calculate the deviation of 87Sr/86Sr relative to 143Nd/144Nd ratios: δSr.
BABB-like MORB show good correlations of δSr with Ba/Nb, Rb/Nb, Ce/Pb, Th/Nb and H2O/Ce ratios, trending towards BABB field (Fig S8), indicating greater mobility of Sr in slab derived fluids than Nd51. Additionally, the majority (60%) of BABB-like MORB are depleted N-MORB with (La/Sm)N<1 (Fig. 3e), also indicating continent crust did not play an important role in the source of BABB-like MORB. All these observations are consistent with a subduction influence for these samples.
It is important to note, however, that the effect of subduction-induced enrichment in fluid-mobile elements (FME: Cs, Rb, Ba, Th, U, Pb and K) and Sr isotopic variations may vary with the original heterogeneity of ambient mantle. When the ambient mantle is rather depleted (high 143Nd/144Nd ratios comparable with Pacific MORB), subduction influence can cause considerable shifts towards higher 87Sr/86Sr ratios as shown by global BABB samples and BABB-like MORB. But as the ambient mantle becomes more enriched, the BABB enrichment will have less effect on the Sr isotopes and FME because enriched ambient mantle would have higher Sr and FME contents (Fig. 3d).