Peridotites from the Mid-Atlantic Ridge at 43° N and their petrogenetic relation to abyssal tholeiites

Tsugio Shibata, Geoffrey Thompson

    Research output: Contribution to journalArticle

    63 Citations (Scopus)

    Abstract

    Whole-rock, major and trace element analyses and microprobe mineral analyses were conducted on serpentinized peridotites recovered from the walls of a MAR (Mid-Atlantic Ridge) 43° N fracture zone. These peridotites are extensively serpentinized; serpentine usually makes up 30-100 vol. percent of the bulk rocks. The relict minerals observed consist mainly of olivine and orthopyroxene with subordinate amounts of clinopyroxene and brown spinel. The range in olivine composition is very limited (Fo91-92). Orthopyroxene forms large, anhedral crystals with clinopyroxene exsolution lamellae and shows undulose extinction with bent cleavages and lamellae. Broad beam microprobe analyses indicate that the composition range of orthopyroxene is also limited (En89.1-87.6Fs8.2-8.0Wo2.7-4.4; Al2O3=1.82-2.64 wt%; Cr2O3=0.63-0.88 wt%). Clinopyroxene tends to fringe large orthopyroxene crystals or fills the interstices between them. The Mg/Fe ratios of clinopyroxene are practically constant; however, the Ca/(Ca + Mg + Fe) ratios range from 0.48 to 0.45. The Cr/(Cr+Al) and Mg/(Mg+ Fe2+) ratios of brown spinel range from 0.57 to 0.36 and 0.69 to 0.56, respectively. The geothermometers utilizing coexisting spinel lherzolite mineral assemblages suggest that the MAR 43° N peridotites attained equilibrium at temperatures from 1100° to 1250° C. Peridotites recovered from the ocean floor are generally considered to have been subjected to partial melting processes and are regarded as residues left after primary magma was removed. Major element chemistry of the MAR 43° N peridotites are compared with those of the ocean-floor ultramafic tectonites reported previously and used together with those published data to demonstrate that the major element abundances of the oceanfloor peridotites define an average trend which is compatible with removal of primary magma from these peridotites at moderate pressures (10-15 kb). Then, the most primitive abyssal tholeiite glasses could be produced by ca. 10% olivine fractionation of such primary magma. Extensive fractionation of olivine and/or orthopyroxene from picritic liquids which are in equilibrium with the lherzolitic or harzburgitic mantle sources at higher pressures (>20 kb) could not yield the majority of the most primitive abyssal tholeiite glasses.

    Original languageEnglish
    Pages (from-to)144-159
    Number of pages16
    JournalContributions to Mineralogy and Petrology
    Volume93
    Issue number2
    DOIs
    Publication statusPublished - Jun 1986

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    orthopyroxene
    olivine
    ridges
    clinopyroxene
    magma
    spinel
    Minerals
    minerals
    lamella
    Fractionation
    fractionation
    tholeiite
    oceans
    Rocks
    rocks
    Glass
    mineral
    interstices
    seafloor
    Crystals

    ASJC Scopus subject areas

    • Geophysics
    • Geochemistry and Petrology
    • Earth and Planetary Sciences(all)
    • Environmental Science(all)

    Cite this

    Peridotites from the Mid-Atlantic Ridge at 43° N and their petrogenetic relation to abyssal tholeiites. / Shibata, Tsugio; Thompson, Geoffrey.

    In: Contributions to Mineralogy and Petrology, Vol. 93, No. 2, 06.1986, p. 144-159.

    Research output: Contribution to journalArticle

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