Abstract

The Moses Rock dike, a well-exposed, kimbeilite-bearing breccia intrusion, crops out in gently dipping beds of the Permian Cutler Formation, in eastern Monument Valley, Utah.

Petrographic, bulk chemical, and electron microprobe analyses of kimberiite and its constituent minerals reveal this highly serpentinized microbreccia contains a primary mineral assemblage consisting of olivine (Mg/Mg+Fe), 87 to 93), orthopyroxene and clinopyroxene (falling into two compositional ranges after correction for Na-pyroxene molecules—one with A12O3 between 0.5 and 1 percent another, 2 to 5 percent), spinel, chrome-rich pyrope garnet, ilmenite-geikielite, titanoclinohumite and one or more micas. Diamonds are not known.

We conclude (1) mineral grains in kimberiite are unlike associated dense rock fragments, except rare lherzolite: (2) kimberiite was emplaced as discrete angular mineral clasts, not a silicate melt; (3) P-T assignments based on clinopyroxenes compositions suggest derivation over a depth range in the upper mantle extending to 150 km or more, at temperatures near or below the experimentally determined garnet-lherzolite solidus: (4) the kimberiite was derived by physical disaggregation of both Al-poor and Al-rich pyroxene bearing peridotite in the mantle (garnet- and spinel-lherzolite, respectively); (5) titanoclinohumite is present in both assemblages and may be an important mineralogical site for volatiles in the upper mantle: (6) dense rock fragments (except lherzolite) are unrelated to the kimberiite and are chunks of the vent wall from the crust and possibly the upper mantle sampled during the eruption.

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