Research in volcanic geology, petrology and planetary science at MIT, 1969 to 1974

The behavior of volcanoes was studied by geologic mapping, petrologic investigations of lava and xenoliths, physical measurements, and theoretical modelling. Field observations were conducted in Alaska (Nunivak Island), Iceland, Hawaii (Mauna Kea), Italy (Etna, Stromboli), and Arizona. The results are discussed and compared with known data for lunar and planetary gelogy. Field methods used for the volcano research are cited and a list is given of all participating scientists and students. Publications and abstracts resulting from the research are also listed

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

Titanoclinohumite: A possible mineralogical site for water in the upper mantle

Titanium-rich clinohumite and layered structure minerals are observed in kimberlite and as inclusions in pyropic garnets from the Moses Rock dike, a kimberlite-bearing breccia dike in San Juan County, Utah. Associated clinopyroxenes observed as inclusions within similar pyropes and also in kimberlite are estimated to have equilibrated at depths ranging from about 50 to 150 km at modest temperatures, generally less than 1000°C. The presence of titanoclinohumite, a high-density hydrous phase, is of considerable interest as a possible site for volatiles in the earth's upper mantle. The dehydration of hydrous phases such as titanoclinohumite within the upper mantle (1) may provide water as a free phase, (2) could be important in the genesis of kimberlite and alkali-basalt magma, and (3) may be one means of producing a low-velocity zone in the upper mantle

Global tectonic studies: Hotspots and anomalous topography

Volcanic activity on Earth and its secular variations are compared with that on other terrestrial planets. Activity at divergent, transform, and convergent plate margins is described with particular emphasis on hot spots and flood basalts. The timing and causing of uplifting above 500 meters, which in not associated with either plate boundaries or the normal nonplate margin edges of continents is considered with particular focus on the Guyana Highlands in southern Venezuela and western British Guiana, and the Brazilian Highlands in the central, eastern, and southern parts of the country. The mode and mechanism of plateau uplifting and the re-elevation of old mountain belts and subsidence of intra-continental basins are also discussed

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