Chlorite-rich ultramafic reaction zones in Colorado Plateau xenoliths: recorders of sub-Moho hydration |
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| Authors: | Douglas Smith |
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| Institution: | (1) Department of Geological Sciences, University of Texas, Austin, TX 78712, USA, US |
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| Abstract: | Three chlorite-rich and one garnet-pyroxenite xenolith from the diatreme at Moses Rock, Utah, document storage and transport
of water and consequent metasomatism in the mantle within the stability field of garnet peridotite, probably at depths of
at least 75 km. Three mineral assemblages are present in zones in one chlorite-rich xenolith: in that xenolith, an assemblage
of chlorite+enstatite+diopside+ ilmenite+titanian chondrodite is separated by diop- side+“talc” from an assemblage of chlorite+diopside+ilmenite+pyrite.
Euhedral grains of enstatite (0.02% Al2O3, 0.05% CaO)+diopside record low temperatures, and high Mn/Fe in these pyroxenes was caused by growth in chlorite-dominated
rock. Derivation from garnet lherzolite is established by relict pyrope (Py71Gr11Alm18). The “talc” has Fe/Mg unusually high relative to that of associated chlorite, and electron probe analyses of the “talc”
sum low, consistent with excess water; the unusual composition may be due solely to alteration and consequent submicroscopic
intergrowths of other phases, but the “talc” could be an analogue of the high-pressure synthetic 10-Å phase. Garnet pyroxenite
has a retrograde assemblage of chlorite-garnet-omphacite. The chlorite-rich rocks formed at contacts between garnet peridotite
and other mantle rock in response to fluid flow. Pressures ≥2.2 GPa are consistent with stability of enstatite + aqueous fluid
and of diopside + talc, with the occurrence of titanian chondrodite, and with the stability of garnet lherzolite. A chlorite
separate has δ18O=6.9, consistent with mantle hydration. The small-scale reaction zones could have formed in a geologically brief time, plausibly
just before eruption at about 25 Ma, and the responsible fluids probably also catalyzed recrystallization of associated eclogites.
The hydration may have been restricted to shear zones that traversed the lower crust and the mantle to at least 75 km depth.
The chlorite-rich rocks may be from the deepest part of the mantle that was sampled by the diatreme eruption. Chlorite-garnet
pairs in garnet pyroxenites and pyrope megacrysts yield temperatures in the range 410–510° C. Low temperatures in the mantle of the Colorado Plateau are consistent with an unusually low mantle heat flux and with
cooling of lithosphere by an underlying subducted slab.
Received: 14 April 1994/Accepted: 23 December 1994 |
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