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1 < Supplementary Information > < Supplementary Tables > < Table 1 Major element and volatile composition of melt inclusions and glasses from host lavas. We present the melt inclusion data corrected and uncorrected for host olivine crystallization, and we report the fraction of olivine crystallized (Xol). Major elements were determined by electron microprobe at the Geophysical Laboratory, Carnegie Institution of Washington and the American Museum of Natural History using a JEOL and a CAMECA microprobe respectively. In both cases, analyses were performed using 15 kV accelerating voltage, 10 nA beam intensity, a 10 µm defocused beam, and ZAF correction procedures. The major element compositions of glass inclusions in olivine were recalculated by reverse modeling of olivine fractional crystallization in steps of 0.1% until the inclusion and host olivine were in equilibrium. This calculation Ol melt assumed a Fe+2/(Fe+3+Fe+2) ratio = 0.9 in the melt and K Fe/Mg = 0.3. The proportion of Fe+3 in the melt inclusions was estimated using of the compositions of syngenetic Cr-spinels inclusions in the samples1. Trace elements and volatiles were determined by ion probe2,3. The major element compositions of the host lava glasses are from Perfit et al.4. K2O and P2O5 contents in inclusions and host glasses were reanalyzed by ion probe. When only electron-probe data is reported for those two elements, the sample name has an asterisk. The precision reported represents the average of the standard deviation (2) of replicated analysis (3 to 4) of several melt inclusions and of different chips from the host glasses during different sessions for a period of one year. The reported pressure during eruption is based on depth of sample collection. Pressures for volatile equilibration and vapor compositions were 2 calculated using the model of Dixon et al.5. The errors are an upper bound based on the uncertainty in depth of sample collection and a propagation of the CO2 content error respectively. Log fS2 values were calculated using the model of Wallace and Carmichael6. However, because the precise Fe+3 content in the melt inclusions and host glasses is not known, there is an uncertainty in the calculated fugacity of S. The estimated fugacity of oxygen is approximately 2 log units below the NNO (Nickel-Nickel Oxide) buffer, consistent with the estimated fugacity of oxygen for MORB7. More oxidizing conditions would make the inclusions more sulfide-undersaturated and vice-versa. In determining volatile contents, we discarded data from melt inclusions and glasses with evidence of devitrification. We also corrected the volatile content of melt inclusions having shrinkage bubbles. This correction considers the volume of the bubble to be spherical and the volume of the melt inclusion to be ellipsoidal. The pressure (P) and the fraction of CO2 in the gas phase were calculated using the model of Dixon et al.5 Assuming the bubbles were shrinkage bubbles rather than syngenetic bubbles, we used P, T (assumed to be the melt-glass transition at 700°C) and the gas and melt volumes, to calculate an upper bound for the number of moles of CO2 and H2O in the bubbles and added them back to the initially measured volatile content of the glass. Thus, the corrected volatile concentrations in Table 1 are strictly upper limits. Table 2 Representative trace element compositions of melt inclusions, and host glasses (corrected and uncorrected for host olivine crystallization). The trace elements of the melt inclusions and host glasses were determined by ion probe2. The precision reported in the table represents the average of the standard deviation (2) of replicated analysis (3 to 4). Average NMORB from 3 the East Pacific Rise is from Donnelly, (personal communication). Values used in the trace element models are also presented: Depleted Mantle composition8,9, bulk partition coefficients (bulk D and P) compiled from the literature, and source mineralogy and melting reaction from Longhi10. The major element model compares temperature and average melt inclusion composition with accumulated melts (1% increments) generated at ~10 kb from a previously depleted mantle (10%)10. The slight difference in the FeO and MgO between the model and the average melt inclusion composition is due to the different melt KOl KOl melt = 0.33 Fe/Mg = 0.3 used in our correction compared with the Fe/Mg used by Longhi melting model10. > < Supplementary Figures > < Figure 5 Major element compositions of Siqueiros picritic glasses and melt inclusions. For comparison we added Mid-Ocean Ridge Basalts (MORB) from the Northern EPR obtained from the MORB database (http://petdb.ldeo.columbia.edu/petdb/) and other Siqueiros glasses that were reported by Perfit et al.4. We also compared the melt inclusion data with two melting models from Longhi10. Most of the major element compositions can be reproduced using polybaric accumulative incremental mantle melting models (Model 1) with small residual porosity (0 to 0.5 %). The calculations indicate that 15% polybaric (20 to 5 Kb) accumulative incremental melting with small residual porosity (0 to 0.5 %) would be required to explain most of the major element data. Model 2 represents a two-stage model of small extent of melting (1 to 3) % melting at low pressure (~10 kbar) of a MORB mantle source after MORB extraction (previously depleted by 10% melting). 4 Figure 6 Primitive mantle normalized plot a, Representative Siqueiros melt inclusions and host glasses showing the observed range in composition. Average normal MORB from the EPR is plotted for comparison (see Table 2 in Supplementary Information). Note depletions in the most incompatible trace elements in the Siqueiros samples compared to those of normal MORB. b, The modeled trace element composition. Two models can explain the trace element data, either 15% batch melting (equivalent to 15% accumulated fractional melting) of a normal MORB source or small extents of melting (0.5 to 5%) of the residual mantle left after MORB extraction (after 10% melting). The modeled trace element abundances bracket the range in compositions for the Siqueiros samples (dark field). Rare earth elements with an asterisk were not measured but calculated using the neighboring rare earth elements. Model parameters are reported in Table 2 (Supplementary Information). Figure 7 Volatile/refractory trace element ratios for elements with similar incompatibility during MORB mantle melting for Siqueiros melt inclusions and glasses: a, CO2/Nb; b, H2O/Ce; c, F/P and d, S/Dy ratios were plotted against Na2O, which can be used as indicator of extent of melting. The volatileundersaturation of many Siquieros samples indicates that the samples were not degassed during eruption. Therefore, the volatile contents, such as CO 2, will depend mainly on their behavior during melting and their content in the mantle source. The similar behaviors of CO2 to Nb, H2O to Ce, F to P and S to Dy in the Siqueiros melt inclusions and glasses demonstrate the incompatible nature of the volatile elements; especially the highly incompatible nature of CO 2, during melting. The data for H2O are of somewhat lesser quality than the data for other 5 volatiles, due to the very low concentrations in the melt inclusions and the higher background of H2O in the ion probe. This is reflected in the poorer reproducibility of H2O compared to other volatiles. Another process that could have affected the H2O/Ce ratios in the melt inclusions is H diffusion through the olivine lattice, which would tend to buffer the H2O contents of the inclusions to that of the host magma. At magmatic temperatures, only a few days will be necessary for H diffusion to have such an effect. Thus, the inclusions with the lowest Ce probably started off with the lowest H2O, but H2O content increased via diffusive exchange with the surrounding magma through the olivine.> < References 1. Danyushevsky, L. V. & Sobolev, A. V. Ferric-ferrrous ratio and oxygen fugacity calculations for primitive mantle-derived melts: calibration and empirical techniques. Mineralogy and Petrology 57, 229-241 (1996). 2. Shimizu, N. & Hart, S. R. Application of the ion probe to geochemistry and cosmochemistry. Annual Review in Earth and Planetary Sciences 10, 483-526 (1982). 3. Hauri, E. H., Newman, S. & Dixon, J. E. SIMS investigations of volatiles in volcanic glasses, 1: calibration, sensitivity and comparison with FTIR. Chemical Geology (in press 2002). 4. Perfit, M. R., Fornari, D. J., Ridley, W. I., Kirk, P. D., Casey, J., Kastens, K.A., Reynolds, J. R., Edwards, M., Desonie, D., Shuster, R. & Paradis, S. Recent volcanism in the Siqueiros transform fault: picritic basalts and implications for MORB magma genesis. 1996). Earth and Planetary Science Letters 141, 91-108 (1996). 5. Dixon, J. E., Stolper, E. M. & Holloway, J. R. An experimental study of water and carbon dioxide solubilities in mid-ocean ridge basaltic liquids. Part I: calibration and solubility models. Journal of Petrology 36, (6) 1607-1631 (1995). 6 6. Wallace, P. & Carmichael, I. S. E. Sulfur in basaltic magmas. Geochemical et Cosmochimica Acta 56, 1863-1874 (1992). 7. Christie, D. M., Carmichael, I. S. E. & Langmuir, C. H. Oxidation state of mid-ocean ridge basalt glasses. Earth and Planetary Science Letters 79, 397-411 (1986). 8. Hofmann, A. W. Chemical differentiation of the Earth: the relationship between mantle, continental crust, and oceanic crust. Earth and Planetary Science Letters 90, 297-314 (1988). 9. Donnelly, K. E. The genesis of E-MORB: extensions and limitations of the hot spot model. Ph. D. Thesis, Columbia University, 310 pp (2002) 10. Longhi, J. Some phase equilibrium systematics of lherzolite melting: I. Geochemistry, Geophysics, Geosystems (in press, 2002).>