Petrology And Thermal Structure Of The Hawaiian Plume


Published on

The presentation was a failure because I don't know much about the slides. Who can help me understand it? Or should I forget it?

Published in: Business, Technology
  • Be the first to comment

  • Be the first to like this

No Downloads
Total views
On SlideShare
From Embeds
Number of Embeds
Embeds 0
No embeds

No notes for slide

Petrology And Thermal Structure Of The Hawaiian Plume

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