A crustal-upper-mantle model for the Colorado Plateau based on observations of crystalline rock fragments in the Moses Rock Dike

On the basis of the size, the abundance, and the petrographic character of xenoliths in the Moses Rock dike, a model for the vertical stratigraphy of crystalline rocks beneath the dike is proposed extending from near the surface to a depth of about 200 km. Sedimentary clasts, whose original position in the undisturbed vent walls is known but which are now within the intrusive breccia of the Moses Rock dike, show a decrease in size with distance of upward transport from their original position in the vent walls. This inverse relationship between fragment size and known depth of origin provides an empirical basis for a reconstructed model for the distribution of rocks on the basis of the particle size of fragments in the intrusive breccia. Metabasalt, granite, and granite gneiss are abundant in the upper part of the crust along the dike walls; diorite, gabbro, and amphibole schists of basic composition constitute intermediate layers, and garnet-bearing metagabbro (basic granulite gneiss) and serpentine schist are present in the lower crust. The crustal rock suite is predominantly metavolcanic and metaplutonic and basic in composition. Dense ultramafic rocks, possibly derived from the mantle, constitute about 0.3% of the breccia filling the dike and include jadeite-rich clinopyroxenite, eclogite, spinel-websterite, spinel-lherzolite, and garnet-lherzolite. The M discontinuity appraently occurs within a petrologically complex region and may coincide with phase and compositional transitions, which include hydration. A compositional transition within the upper mantle between spinel- and garnet-peridotite (lherzolite) is inferred. The variety and the abundance of ultramafic and dense types, together with the complexity of their textures, suggest that the mantle may be as complicated as the crust in composition and history

Briefing notes, astronaut reunion

The materials on the following pages are from viewgraphs and information presented at a reunion of former astronauts held at the Johnson Space Center in August 1978. These briefing notes do not constitute a formal publication and should not be cited as such.Based on briefing materials prepared by: Prof. J. W. Head, III, Brown University, Dr. T. R. McGetchin, LPI, Prof. J. J. Papike, SUNY, Stony Broo

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

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

Lightning-Induced Volcanic Spherules

Glass spherules have been documented in many geologic deposits and are formed during high-temperature processes that include cloud-to-ground lightning strikes, volcanic eruptions of low-viscosity magmas, and meteorite impacts. This study reviews the known glass spherule–forming processes and proposes, for the first time, a mechanism induced through the heat generated by volcanic lightning in eruptive columns and plumes (laterally spreading clouds) during explosive eruptions. Ash-fall samples were collected from two eruptions where volcanic lightning was extensively documented: the A.D. 2009 eruption of Mount Redoubt, Alaska (USA), and the 2010 eruption of Eyjafjallajökull, Iceland. These samples reveal individual glass spherules ∼50 μm in average diameter that compose \u3c5% of the examined portion of the deposit. Textures include smooth, hollow, or cracked spherules, as well as aggregates, which suggest melting of ash particles as a result of proximity to the electrical discharge channel and subsequent re-solidification of the particles into spherical morphologies. The natural ash-fall samples are compared with pseudo-ash samples collected from high-voltage insulator experiments in order to test our hypothesis that volcanic ash particles can be transformed into glass spherules through the heat generated by electrical discharge. We refer to this new morphological classification of ash grains as lightning-induced volcanic spherules and hypothesize that this texture not only provides direct physical evidence of lightning occurrence during explosive eruptions, but will also increase settling velocities and reduce aggregation of these particles, affecting ash transport dynamics