Magmatic processes under mid-ocean ridges: A detailed mineralogic study of lavas from East Pacific Rise 9°30′N, 10°30′N, and 11°20′N

Authors

  • Yucheng Pan,

    1. Institute of Oceanology, Chinese Academy of Sciences, Qingdao, Shandong Province, China
    2. Department of Geology and Geophysics, University of Hawaii, 1680 East West Road, Honolulu, Hawaii, 96822, USA
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  • Rodey Batiza

    1. Marine Geology and Geophysics, National Science Foundation, Arlington, Virginia, USA
    2. Department of Geology and Geophysics, University of Hawaii, 1680 East West Road, Honolulu, Hawaii, 96822, USA
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Abstract

[1] Detailed petrologic study has been made for lavas from the northern East Pacific Rise (EPR) 9°30′N, 11°20′N, and 10°30′N. 9°30′N and 11°20′N have robust magma supply and a shallow melt lens while 10°30′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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