Zoned minerals in garnet peridotite nodules from the Colorado Plateau: implications for mantle metasomatism and kinetics |
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Authors: | Douglas Smith Stephen N Ehrenberg |
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Institution: | (1) Department of Geological Science, University of Texas, 78712 Austin, Texas, USA;(2) Department of Geological Science, Shell Development Company, Bellaire Research Center, P.O. Box 481, 77001 Houston, Texas, USA |
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Abstract: | Some garnet peridotite nodules from The Thumb, a minette neck on the Colorado Plateau in the southwestern United States, contain
zoned minerals. Zoning does not exceed 1.5 wt.% for any oxide, but some relative changes are large: in one garnet TiO2 and Cr2O3 ranges are 0.05–0.65 and 3.5–5.0 wt.%, respectively. In two porphyroclastic nodules, garnet rims are depleted in Mg and enriched
in Fe, Ti, and Na compared to cores, and one garnet is irregularly zoned in Ti and Cr. Olivine crystals in these rocks are
unzoned, and pyroxene zoning is slight, yet matrix olivine and pyroxene contain more Fe and Ti and less Mg and Cr than inclusions
of these phases in garnet. In three coarse nodules, garnet rims are Ti-rich compared to cores, and Ca, Fe, Mg, and Cr zoning
patterns are complex. Several nodules appear to have partially equilibrated near 1200° C and 35 kb, and under these conditions
cation mobility in pyroxene was greater than in garnet.
The zoning partly reflects Fe and Ti metasomatism in the mantle. Calculations indicate that Fe-Mg gradients in garnet could
have persisted for only a short time in the mantle, perhaps thousands of years or less, so the metasomatism occurred shortly
before eruption. The minette host, a likely source of the Fe and Ti, is rich in light rare earth elements: since the nodules
are much poorer in these elements, little or no infiltrated minette was trapped in them. Diffusion is a possible mechanism
for nodule metasomatism. Some fertile peridotite nodules from kimberlites may have been affected by similar events. Compositional
differences between inclusions in garnet and matrix phases are intriguingly similar to some of the differences between most
peridotite inclusions in diamonds and common lherzolite phases. |
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