Abstract. Lavas from the Easter Seamount Chain (ESC) between Salas y Gomez Island and theAhu volcanic field are tholeiitic and alkalic basalts showing regular and systematic chemical variations with longitude. With progressive distance eastward from the east rift of the Easter microplate, the lavas become progressively richer in K20, Na20, FeO, TiO2, and P205, and have higher K20/TiO2 and lower MgO and CaO. These changes reflect differences in the total extent of shallow fractionation and differences in the conditions under which it occurred. Below the Salas y Gomez ridge, where large isostatically compensated volcanoes lead to locally thicker crust, fractionation took place under higher pressure and/or conditions of higher H20, compared with lavas of the Easter ridge and the east rift. Differences in K20, P205, and K20/TiO2 reflect differences in mantle source composition and binary mixing between an enriched plume component and a depleted mid-ocean ridge basalt (MORB)-like component, as indicated by Pb isotopic data. Mixing along the ESC apparently occurred in the solid state prior to melting, whereas mixing below the east rift involved fractionated liquids. We also see evidence for differences in the conditions of melting, using oxide abundances corrected for shallow fractionation and mantle heterogeneity. Melting below the Salas y Gomez region seems to be initially deeper and more extensive, with progressively shallower and less extensive melting toward the east rift. If this model is correct, it implies that some alkali basalts may form by larger extents of melting than previously thought on the basis of trace element modeling. Since independent evidence suggests the Easter plume has a modest to large excess temperature compared with ambient MORB mantle, we conclude the plume is under the Salas y Gomez region.
[1] Lavas from the northern East Pacific Rise with both robust (9°30 0 N) and nonrobust (10°30 0 N) ridge morphologies have compositionally diverse populations of plagioclase and/or olivine due to magma mixing. We interpret zoning in olivines to be primarily due to diffusion after mixing events, which allows us to calculate the residence times of individual crystals in the magma. For the four studied samples, olivine populations exhibit exponential distributions of calculated diffusion times, with short times exponentially more abundant than longer times. We model this distribution as resulting from mixing in an open-system axial magma chamber. Magma residence times in axial chambers at East Pacific Rise 9°30 0 N and 10°30 0 N for the four studied samples are of the order of months. Surprisingly, we find that the differences in mineralogy and magma residence times between robust and nonrobust East Pacific Rise segments are not significant. Taken together, our observations and measurements suggest that the seismically imaged melt lens of the axial magma chamber does not play a significant role in controlling the crystal content and characters of erupted lavas.
[1] Detailed petrologic study has been made for lavas from the northern East Pacific Rise (EPR) 9°30 0 N has nonrobust magma supply and no shallow melt lens. Lavas from all three localities are sparsely phyric and glassy, containing plagioclase ± olivine ± pyroxene. Typically, the lavas contain several to many (up to seven) distinct chemical groups of plagioclase that are not always distinct texturally. The lavas may also contain up to three chemically distinct groups of olivine and two groups of pyroxene. The lavas contain both individual crystals and groups comprising reticulate and dendritic clots that we interpret to represent bits of crystal networks forming in mushy zones of an axial magma chamber. 21 of the 23 samples studied in detail have diverse crystal compositions and require mixing, which most likely occurs when mostly liquid magma passes through mushy zones. We find no significant systematic differences between robust and nonrobust segments in terms of their crystal content, proportion of texturally distinct xenocrysts, crystal size, aspect ratio, roundness, modal abundance, magma residence time, number of diverse mineral-chemical groups, and characteristics of mixing like the total range of composition of disequilibrium minerals present, and the magnitude of the chemical gaps between disequilibrium and equilibrium compositions. This unexpected and remarkable similarity suggests that the presence or absence of a seismically imaged shallow melt lens has essentially no effect on the mineralogy of erupted lavas. We conclude that the fundamentally important magmatic process under mid-ocean ridges, from slow to fast, is the formation of crystal networks and their subsequent compaction; the seismically detected shallow melt lenses likely contain highly evolved magma, formed by expelled interstitial melt during crystal network compaction, and play very little role in crustal accretion. Our conclusion implies that the thick (>3 km) low velocity zone near the base of the crust produces the thin (<30 m) shallow melt lenses, not the other way around.
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