Geothermometry and kinetics in a two-spinel peridotite nodule, Colorado Plateau

Compositions and zoning of minerals in a two-spinel peridotite from minette in the Navajo volcanic field on the Colorado Plateau provide unusual opportunities to compare geothermometers at low mantle temperature and to study equilibration rates. The xenolith contains pleonaste (Mg/sub .55/Fe/sub .45/Al/sub 1.62/Fe/sub .10/Cr/sub .28/O/sub 4/) and magnetite (Mg/sub. 16/Fe/sub .89/Mn/sub .01/Al/sub .18/Fe/sub 1.38/Cr/sub .32/Ti/sub .06/O/sub 4/) related by granule exsolution, together with olivine (Foyv), orthopyroxene (3.5% Al/sub 2/O/sub 3/), and clinopyroxene. Both two-spinel equilibria and several olivine-pleonaste geothermometers indicate equilibration near or below 700/sup 0/C, confirming the general accuracy and continued equilibration of these geothermometers at low temperatures. Calculated olivine-magnetite temperatures are much too high. Two-pyroxene temperatures are near 800/sup 0/C. Olivine is zoned in Ca, Fe, and Mg within 50 ..mu..m of spinel by exchange with local grain boundary melts. Gradients at pleonaste-magnetite contacts were caused by multicomponent diffusion after heating by minette; effects include slight uphill diffusion of Cr. Effective binary diffusion coefficients near 1100/sup 0/C, estimated by comparison with gradients in olivine, are near 10kaa cm/sup 2//sec for Al in magnetite and 10kab cm/sup 2//sec for Al in pleonaste; an average Mg value is in the same range. The time interval between plucking of the inclusionmore » and minette solidification is calculated as about 60 hours, consistent with ascent times calculated assuming Newtonian viscosity for the minette magmas.« less

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