Publication Date:
2021-11-09
Description:
Electrical conductivities of polycrystalline garnets ranging in chemical composition from almandine
(Fe3Al2Si3O12) to pyrope (Mg3Al2Si3O12) were measured at 10 GPa and 19 GPa at temperatures
ranging from 300 to 1700 °C using complex impedance spectroscopy in a multianvil device. Mössbauer
spectroscopy of each sample was carried out both before and after the electrical measurements
to characterize the oxidation state of Fe in the almandine bearing garnets. Similar to the behavior of
other ferromagnesian silicates, the substitution of Fe for Mg along this compositional join dramatically
increases electrical conductivity, but this compositional effect is reduced with increasing temperature.
Conductivities increase with increasing total Fe content, as the average Fe2+-Fe3+ distance decreases.
At 10 GPa, activation energies for conductivity vary smoothly with composition and increase rapidly
toward the pyrope end-member composition, where it reaches a value of 2.5 eV. The results are consistent
with an electrical conductivity mechanism involving small polaron mobility in the Fe-bearing
garnets at 10 GPa. At 19 GPa, however, there is virtually no change in the activation energy as a function
of Fe-Mg substitution for the pyrope-rich garnets. These higher pressure measurements reß ect a
mechanism involving oxygen related point defects, as conductivities increase with pressure at constant
T for each garnet, and the effect of pressure is greater for the more Mg-rich garnets. The data also
allow for a more quantitative evaluation of the effect of chemical composition, speciÞ cally Fe-Mg
substitution, on the electrical conductivity proÞ le of the mantle, using a recently developed laboratory-
derived model. We apply the model using these data to a portion of the transition zone between
520 and 660 km, in which we vary the garnet composition from Py100 to Py85Alm15. Although only
a minor effect on bulk mantle conductivity results, we conclude that the overall garnet composition
may, however, be important in characterizing the magnitude of any EC discontinuity with respect to
the above-lying mantle.
Description:
Published
Description:
1371-1377
Description:
JCR Journal
Description:
reserved
Keywords:
Electrical conductivity
;
pyrope-almandine
;
high pressure
;
cation substitution
;
04. Solid Earth::04.01. Earth Interior::04.01.04. Mineral physics and properties of rocks
;
04. Solid Earth::04.04. Geology::04.04.05. Mineralogy and petrology
Repository Name:
Istituto Nazionale di Geofisica e Vulcanologia (INGV)
Type:
article
Permalink
