Petrology And Thermal Structure Of The Hawaiian Plume

Author: Claude Herzberg Speaker: Jingyu Li

Peridotite or pyroxenite/eclogite
Parameterization of melting experiments on peridotite with glass analyses from Hawaii Scientific Deep Project 2 on Mauna Kea volcano

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

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

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?)

A primary magma: a partial melt of a mantle source

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

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

A source of long-term light rare-earth elements depletion
Peridotite source might also be depleted in CaO and Al2O3

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)

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

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.

Imcreasingly similar with increasing temperature
Obvious in glass data
No such behavior in the whole rock

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

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

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]

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

Petrology And Thermal Structure Of The Hawaiian Plume

by lijingyua1 on Dec 11, 2007

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