Compositional relations in minerals from kimberlite and related rocks in the Moses Rock dike, San Juan County, Utah

Creators: McGetchin, Thomas R.; Silver, Leon T.

Abstract

The Moses Rock dike, a well-exposed, kimberlite-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 kimberlite 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 Al_2O_3 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 kimberlite are unlike associated dense rock fragments, except rare lherzolite: (2) kimberlite 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 kimberlite 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 kimberlite and are chunks of the vent wall from the crust and possibly the upper mantle sampled during the eruption.

Additional Information

© 1970 Mineralogical Society of America. Electron microprobe data were obtained with the able assistance of A. A. Chodos. Drs. A. E. Bence, R. S. Naylor and A. L. Albee provided valuable advice and Mrs. Lily Ray assisted in the computer reduction of the microprobe data. We have benefited greatly by discussions with staff and students at the California Institute of Technology and members of the U.S. Geological Survey in Flagstaff, Menlo Park and Washington, D. C. An early version of the manuscript was completed while the senior author was a visitor at the Australian National University, Canberra. Hospitality of Drs. John Lovering, David Green and A. E. Ringwood is gratefully acknowledged. W. G. Ernst, L. S. Hollister and K. D. Watson, U.C.L.A., and Howard Wilshire, U.S.G.S., Menlo Park, California, made criticisms which greatly improved an early version of the manuscript. This report summarizes part of a Ph.D. thesis at the California Institute of Technology. Special thanks are due to Dr. E. M. Shoemaker for orignally suggesting the Moses Rock dike as a thesis topic. Financial support for this research was provided by a Penrose Grant by the Geological Society of America (No. 1040-65), the Atomic Energy Commission Contract AT (04-3)-427, CALT-427-52, and U.S. Geological Survey (Branch of Astrogeology, Flagstaff, Arizona, under contract R-66 from NASA, and Analytical Branch, Denver, Colorado). The senior author had a National Aeronautics and Space Administration Graduate Traineeship from 1965-1967. From 1967 to 1969, he was an active duty Captain, U.S. Air Force, assigned as Assistant Professor in the School of Engineering, Air Force Institute of Technology, Wright-Patterson Air Force Base, Ohio. Funds from AFIT-OAR contract number F-33601-69-C-0568 were used in completion of the research and report. An earlier version appeared as an AFIT Technical Report (AFIT TR 69-12) Publication authorized by the director, U. S. Geological Survey.

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Compositional relations in minerals from kimberlite and related rocks in the Moses Rock dike, San Juan County, Utah

Abstract

Additional Information

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