Petrology And Thermal Structure Of The Hawaiian Plume

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  • 1. Author: Claude Herzberg Speaker: Jingyu Li
  • 2.
    • Peridotite or pyroxenite/eclogite
    • Parameterization of melting experiments on peridotite with glass analyses from Hawaii Scientific Deep Project 2 on Mauna Kea volcano
  • 3.
    • Small population of the core samples had fractioned from a peridotite-source primary lava
      • Deficient in CaO and enriched in NiO
    • Most lavas, by experiment, were produced by melting of garnet pyroxenite
      • Formed in the second stage by reaction of peridotite with partial melts of subducted oceanic crust
  • 4.
    • Pyroxenite occurs in a host peridotite and both contribute to melt production
    • Primary magma compositions vary down the drill core
    • Temperature variations within the underlying mantle plume
  • 5.
    • Low in SiO2
    • High in SiO2
      • (both low in CaO, abundant, in both whole rocks and glasses)
    • Low in SiO2 and high in CaO & K2O
      • (rarely found as glasses at 1800 mbsl)
    • A fourth at 2233 mbsl high in alkalis (not in this picture?)
  • 6.
    • A primary magma: a partial melt of a mantle source
  • 7.
    • Herzberg and O’Hara method
    • Similar to those for other picrites and komatiites, with CaO, MgO, and SiO2
    • But Mauna Kea has more TiO2, K2O, and other incompatible elements
  • 8.
    • CaO contents of accumulated fractional melts of mantle peridotite do not change much over a wide range of initial and final melting pressures within the garnet lherzolite stability field
  • 9.
    • A source of long-term light rare-earth elements depletion
    • Peridotite source might also be depleted in CaO and Al2O3
  • 10.
    • CaO of parental magmas: estimated 8.6~8.9% (ref)
    • Less than 10% CaO in peridotite partial melts
    • A normal peridotite source is inconsistent with these low-CaO contents
    • Augite crystallization (Supports): only when the parental magma evolves by olivine fractionation to about 7.5% MgO
    • A pyroxenite source was an alternative, because the Ni contents are higher than expected for a peridotite source. (ref)
  • 11.
  • 12.
    • HSDP glasses with >7.5% MgO exhibit no signs of augite or plagioclase fractionation
    • Similar to partial melts of Fo90.5 olivine (ref)
    • No change in primary magma composition during transit from the mantle to the crust
  • 13.
    • High-SiO2 : SiO2-rich side of the thermal divide
    • Low-SiO2: olivine-rich side of the thermal divide
    • The seperation indicates that they are partial melts of garnet pyroxenite
    • Models of pyroxenite within a peridotite matrix for Hawaii, and melt production from both sources.
  • 14.
  • 15.
    • Imcreasingly similar with increasing temperature
    • Obvious in glass data
    • No such behavior in the whole rock
  • 16.
    • Hottest magmas: 2100 mbsl, where low- and high- SiO2 are most similar
    • Small variations in Al2O3 are responsible for the variable and noisy MgO signal
  • 17.
    • A constant source temperature at any specific level in the core, but temperate-induced changes with time
    • Peridotite-source melts appear briefly at about 1800 mbsl and 3GPa, liquidus temperature of 1550℃ and potential temperature of 1550 ℃
    • Pyroxenite source temperature variations translate to potential temperatures of 1500~1550℃
    • Temperatures are 1470~1500 ℃ at 3GPa peridotite, and 1560~1580 ℃ near the thermal divide
    • Possible for pyroxenite melts with compositions along the cotectic [] and near the thermal divide to be hotter than peridotite melts
  • 18.
    • The Hawaiian tholeiites cannot be single-stge partial melts of an original basaltic crustal protolith of bimineralic eclogite or quartz/coesite eclogite at 3.0-3.5GPa
      • (Too high in NiO and MgO, and too low in SiO2 and Al2O3)
    • Pyroxenite source forms in a second stage by melt-rock reaction (ref)
      • Quartz eclogite melt at 3GPa and about 1315 ℃
      • SiO2-rich melts can react with the peridotite host to produce opx+cpx+gt
      • Primary magmas will form at contact with cpx and gt where the temperatures are at a minimum on the cotectic [L+opx+cpx+gt]
  • 19.
    • Key for a general understanding of melt production in lithologically heterogeneous mantle
    • Pyroxenite melts with SiO2-rich are unique to the shield-building lavas of Hawaii
    • The phase diagram requires a substantial role for SiO2-rich basaltic oceanic crust
    • Supports the suggestions that recycled crust is organized in large bodies, reconstructed as pyroxenite
    • Oceanic crust has been subducted, stirred, stretched and returned in a plume with its fine structure roughly preserved as geochemical heterogeneities in Hawaiian vocanoes