Author: Claude HerzbergSpeaker: Jingyu Li

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

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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

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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

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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?)

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A primary magma: a partial melt of a mantle source

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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

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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

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A source of long-term light rare-earth elements depletion

Peridotite source might also be depleted in CaO and Al2O3

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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)

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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

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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.

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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

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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

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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]

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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

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