Could the effect of order-disorder in garnet be important for upper mantle petrology?

The types of nodules erupted in kimberlite and believed to be of upper mantle origin are divided into five types: ( a ) peridotites and dunites, ( b ) garnet-pyroxenites, ( c ) eclogites and grospydites, ( d ) megacrysts, ( e ) amphibole-bearing and mica-rich types. The value of relative pressure-temperature estimates in determining the conditions of formation of the nodules is noted and such estimates are used to discuss the distribution of rock types and minerals in the upper mantle sampled by kimberlite. Emphasis is placed on garnet—lherzolites and it is found that there is little evidence for the restriction of deformed or undepleted varieties of garnet-lherzolite to particular depths. Phlogopite, apparently in equilibrium with the host garnet—lherzolite assemblage, appears to be restricted to rocks formed in the lower range of pressure and temperature estimates (less than about 1150 °C). Ilmenite-bearing and other megacrysts of relatively high Fe and Ti and low Cr types appear to have pressure-temperature estimates in the intermediate and higher part of the garnet—lherzolite range. The distribution of phlogopite and megacrysts may be related to melting processes. Present information does not suggest extensive changes in the pressure and temperature of formation of nodules as a consequence of diapiric or other activity associated with kimberlite genesis. The pressure-temperature gradients of garnet-lherzolites from individual pipes do not show any inflexions, but appear to be straight or gently curved. Allowing for the uncertainties in absolute pressure and temperature estimates, the pressure-temperature ranges and gradients of nodule suites are roughly in accord with geotherms based on geophysical data for an upper mantle with a convective circulation.

Download Full-text

Upper mantle structure beneath eastern Siberia: Evidence from gravity modeling and mantle petrology

Geochemistry Geophysics Geosystems ◽

10.1029/2002gc000420 ◽

2003 ◽

Vol 4 (7) ◽

Cited By ~ 11

Author(s):

Yvette H. Poudjom Djomani ◽

Suzanne Y. O'Reilly ◽

W. L. Griffin ◽

L. M. Natapov ◽

Y. Erinchek ◽

...

Keyword(s):

Upper Mantle ◽

Gravity Modeling ◽

Eastern Siberia ◽

Mantle Structure ◽

Upper Mantle Structure ◽

Mantle Petrology

Download Full-text

The Earth's Crust and Upper Mantle

10.1029/gm013 ◽

1969 ◽

Cited By ~ 14

Keyword(s):

Upper Mantle ◽

Earth’S Crust ◽

Crust And Upper Mantle ◽

Earth's Crust

Download Full-text

Water Gas May Not Shift in Deep Earth

10.26434/chemrxiv.13489896 ◽

2020 ◽

Author(s):

Nore Stolte ◽

Junting Yu ◽

Zixin Chen ◽

Dimitri A. Sverjensky ◽

Ding Pan

Keyword(s):

Formic Acid ◽

Upper Mantle ◽

Free Energy Calculations ◽

Water Gas Shift ◽

Ab Initio Molecular Dynamics ◽

Water Gas Shift Reaction ◽

Carbon Carrier ◽

Deep Earth ◽

Shift Reaction ◽

Water Gas

The water-gas shift reaction is a key reaction in Fischer-Tropsch-type synthesis, which is widely believed to generate hydrocarbons in the deep carbon cycle, but is little known at extreme pressure-temperature conditions found in Earth’s upper mantle. Here, we performed extensive ab initio molecular dynamics simulations and free energy calculations to study the water-gas shift reaction. We found the direct formation of formic acid out of CO and supercritical water at 10∼13 GPa and 1400 K without any catalyst. Contrary to the common assumption that formic acid or formate is an intermediate product, we found that HCOOH is thermodynamically more stable than the products of the water-gas shift reaction above 3 GPa and at 1000∼1400 K. Our study suggests that the water-gas shift reaction may not happen in Earth’s upper mantle, and formic acid or formate may be an important carbon carrier, participating in many geochemical processes in deep Earth.<br>

Download Full-text