Supplementary Material Wanless, V. D., M. R. Perfit, E. M. Klein, S. White, and W. I. Ridley (2012), Reconciling geochemical and geophysical observations of magma supply and melt distribution at the 9_N overlapping spreading center, East Pacific Rise, Geochem. Geophys. Geosyst., 13, QXXXXX, doi:10.1029/2012GC004168. S1. Depths of Evolved Basalts Formation at the OSC Ferrobasalts and FeTi basalts at the OSC require 30-55% fractional crystallization from a relatively high-MgO parent (~8.5 wt. % MgO), however, where within the magmatic system this crystallization occurs is uncertain. In principle, it may occur in several locations as discussed by Natland and Dick : 1) in the shallow melt lens; 2) within the uppermost mantle and 3) during magma ascent through a crystal-rich mush zone in the deeper crust or some combination of these. Below we discuss each of these hypotheses with respect to the formation of ferrobasalts and FeTi basalts at the 9° N OSC. The absence of high-MgO lavas at the OSC and the elimination of high-MgO melt as a mixing end-member argue against the idea that all crystallization occurs in the shallow melt lens (note, however, that we are not suggesting that there is no crystallization occurring within this region). This conclusion is supported by petrologic modeling [Danyushevsky and Plechov, 2011] that suggests that several OSC lavas have undergone fractional crystallization at pressures greater than 1 kbar and thus at depths greater than that of the melt lens (Figure 4). It is also clear from the range of compositions of MOR gabbroic rocks (that include gabbronorites and ferrogabbros) and their mineral phases that MORB melts can differentiate to high degrees in the lower parts of the oceanic crust [Hekinian et al., 1993; Coogan, 2007; and Natland and Dick, 2009] and that crystallization is not restricted to the melt lens. The second hypothesis proposes that melts differentiate from primitive high-MgO compositions to more typical MORB compositions (< 9 wt% MgO) in the upper mantle. This may occur in sill-like bodies at the base of the crust below ridge axes [e.g., Natland and Dick, 2009] or during transit to the ridge axis in the mantle [e.g., Toomey et al., 2007]. There is abundant petrologic evidence for the variable degrees of differentiation of primitive or high-MgO MORB melts in the mantle below the ridge axis prior to ascent into the oceanic crust [e.g., Kelemen et al., 1995; 2007; Collier and Kelemen, 2010; Wanless and Shaw, 2012]. However, none of these studies support the formation of evolved ferrobasalt compositions in the mantle or Moho. Instead, examination of exposed sections of oceanic lower crust and mantle in tectonic windows and in ophiolite sections indicate that melts feeding the ridge axis are more primitive than or similar to high-MgO MORB [e.g., Kelemen et al., 2007]. Therefore, there is little evidence to support a hypothesis that crystallization in the mantle alone is effective in producing the moderately to highly evolved ferrobasalts and FeTi basalts erupted at the eastern OSC. The third hypothesis suggests that melts formed in the mantle below the OSC may crystallize within [e.g., Sinton and Detrick, 1992; Wanless and Shaw, 2012] and/or interact with the crystal-rich mush zones [e.g., Coogan et al. 2000; Ridley et al., 2006; Lissenberg and Dick, 2008; Kvassnes and Grove, 2008] in the mid- to lower crust. Such mush zones, postulated to compose most of layer 3 of nascent oceanic crust, have been identified in seismic and compliance data from beneath the east limb melt lens [Crawford et al., 1999; Crawford and Webb, 2002; Canales et al., 2006]. Thus, magmas evolve during ascent through the mush zone either by increasing degrees of fractional crystallization [e.g., Sinton and Detrick, 1992] or by melt-rock reaction [e.g., Lissenberg and Dick, 2008]. This is further supported by petrologic modeling results from the OSC lavas that suggest crystallization occurs at depths greater than that of the melt lens (as discussed above). Thus, we believe that melt differentiation at the OSC likely begins in the lower oceanic crust and continues into the shallow melt lens. Unfortunately, a commonly used geochemical indicator of deeper crystallization (decreasing CaO/Al2O3 with decreasing MgO), cannot be used to support this conclusion because the trends of decreasing CaO/Al2O3 can also be generated by mixing of high-silica magmas that have very low CaO/Al2O3 with a variety of different basaltic melts with higher CaO/Al2O3 (Figure S1). S2. Timescales of Decreased Magma Supply at the OSC In the absence of any data suggesting increased cooling rates, we have argued that melt supply to the shallow melt lens must decrease, at least temporarily, for high-silica lavas to form at the OSC [Wanless et al., 2010]. Placing quantitative constraints on the timescales over which this occurs at the OSC is difficult; however, there are several studies that provide estimates for dacite formation in other settings. At the Juan de Fuca Ridge, crystallization of a melt from a basalt to a dacite was estimated to have occurred over ~10-20 ka at most [Schmitt et al., 2011], thus placing an upper bound on the time required for dacite formation at MORs. In 2005, dacitic melt was discovered during drilling of a hydrothermal well along the lower east rift zone of Kilauea (Puna Ridge) on the Island of Hawai’i [Teplow et al., 2009]. Historically, the lower east rift zone of Kilauea has only erupted basaltic compositions and ~80% of it’s surface is covered by basalt lavas that are <500 years old [Teplow et al., 2009]. The last eruption that occurred along this section of the rift zone was in 1955. Teplow et al.  hypothesized that the dacitic melt evolved via fractional crystallization of the 1955 basaltic magma injected into the rift zone ~50 years ago. While this high-silica melt formed in the rift zone of a subaerial volcano, it may be analogous to a propagating ridge of an OSC, which suggests that very short timescales are possible for dacite formation on MORs. However, the thermal conditions at the OSC may be quite warmer than this subaerial rift zone setting, so this is likely an extreme. At the OSC, the young dacitic pillow mounds on the floor of the east limb axial graben are surrounded by ferrobasalts, suggesting that the magmatic conditions beneath OSC can produce both high- and low silica lavas over relatively short timescales. This is consistent with U-series disequilibria estimates indicating that basaltic and dacitic lavas collected within the axial graben all erupted within the last 8 ka [Waters, 2010]. Additionally, older (based on seafloor imagery and relative positions; Nunnery, 2008) dacitic lavas are located at the top of the axial graben while basaltic lavas were found at the base indicating that high-silica lavas are not restricted to a single time period and that basaltic volcanism commonly occurs between dacitic eruptions. Additionally, the intermittent replenishment of basaltic melts to the system must occur over short enough timescales that the dacitic melts do not completely crystallize prior to mixing with ferrobasaltic melts. 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Klein (2011), Volatile abundances and oxygen isotopes in basaltic to dacitic lavas on midocean ridges: The role of assimilation at spreading centers, Chemical Geology, 287(1-2), 54–65, doi:10.1016/j.chemgeo.2011.05.017. Waters, C. L. (2010), Temporal and Petrogenetic Constraints on Volcanic Accretionary Processes at 9-10 Degrees North East Pacific Rise, Ph.D. Thesis, MIT/WHOI, 201012, 261p. Figure S1: CaO/Al2O3 versus MgO for OSC lavas. EPR lavas from 8° to 10°N are shown for comparison. Dashed lines show calculated [Danyushevsky and Plechov, 2011] fractional crystallization pathways at 0.5, 1 and 2 kbars (see text for details and starting parameters). Solid lines are binary mixing lines between high-silica lavas erupted at the OSC and several different basaltic compositions. The decreasing CaO/Al2O3 ratios may be explained by high-pressure fractional crystallization or by mixing.