A new source of Ultra-High Pressure Minerals -- from Ophiolites

Harry W. Green, II

Larissa Dobrzhinetskaya

Department of Earth Science and

Institute of Geophysics and Planetary Physics,

University of California, Riverside, CA, 92521, USA

LuobusaLuobusachromite depositchromite deposit

UHPUHPPelitic gneissPelitic gneiss

Localities of Former StishoviteLocalities of Former Stishovite

Chromite ore bodies shown in yellow: lens- or tabular-shaped, normally 20-300m x 10-100m, enveloped by a thin shell of dunite. High-pressure and reduced minerals are from chromitite.

Slide courtesy J. Yang (2006)

Chromite ore bodies shown in yellow: lens- or tabular-shaped, normally 20-300m x 10-100m, enveloped by a thin shell of dunite. High-pressure and reduced minerals are from chromitite.

Slide courtesy J. Yang (2006)

Very Deep Minerals from Chromitite of Luobusa

Ophiolite

Fe-Ti Metal Pellet

Silicate “rock”Silicate “rock”

Fe/Ti Alloy and Silicate Fragment

Fe/Ti Alloy and Silicate Fragment

b

aaa

bb

Coesite Crystallography

Raman Spectra

Nitride Inclusions in Coesite

Nitride Inclusions in Coesite

EELS Analysis of TiN

EDS of Nitride Inclusions

Osbornite (TiN) onlyOsbornite (TiN) only

Osbornite (TiN) plus BNOsbornite (TiN) plus BN

EELS Analysis of BN

EELS Analysis of BN

EDS Mapping of Nitrides in Coesite

Conclusions• Coesite “crystals” are polycrystalline, implying pseudomorphic replacement of a

previous phase. Only possibility is stishovite, demonstrating minimum depth ~300 km.

• TiO2 II is also present in this rock that must have been hotter than 1300˚C,

confirming depth > 300 km.

• N isotopes are highly negative (15N = -10.4 +/-3.0 per mil) and therefore inconsistent with the source of this material being from a slab graveyard; the only common source of highly negative 15N is iron meteorites with 15N = -60 per mil or higher. Is there a core connection?

• Moreover, the Luobusa ophiolite is not unique; diamond, coesite and many similar materials have now been found in a 2nd ophiolite.

• This rock fragment clearly comes from great depth, implying that at least some of the chromite at Luobusa has come up in the solid state and preserved very reducing conditions within it, despite the high-T oxidizing magmatic environment near the surface.

• Again, does that make this material UHPM? It may have been magmatic at one point but now appears to be metamorphic. What do we call it?

Discussion

• What to you think?

• Are peridotites that show evidence of great depth UHPM? In the case of Alpe Arami, only the eclogites have been shown to have a similar high-P history.

• How about these strange rocks extracted from the chromite of ophiolites? Diamonds, coesite and bizarre reduced phases are now known from at least two ophiolites.

• Does what we call them matter?

• I think it does because these different types of high-P rocks imply very different tectonics -- telling us very different things about Earth.

Summer Solstice Eve in the Summer Solstice Eve in the Western Norway UHPM TerraneWestern Norway UHPM Terrane

THE END