Browsing by Author "Morton, Douglas M."

Now showing 1 - 3 of 3
  • Thumbnail Image
    Item
    Episodes of fast crystal growth in pegmatites
    (Springer Nature, 2020) Phelps, Patrick R.; Lee, Cin-Ty A.; Morton, Douglas M.
    Pegmatites are shallow, coarse-grained magmatic intrusions with crystals occasionally approaching meters in length. Compared to their plutonic hosts, pegmatites are thought to have cooled rapidly, suggesting that these large crystals must have grown fast. Growth rates and conditions, however, remain poorly constrained. Here we investigate quartz crystals and their trace element compositions from miarolitic cavities in the Stewart pegmatite in southern California, USA, to quantify crystal growth rates. Trace element concentrations deviate considerably from equilibrium and are best explained by kinetic effects associated with rapid crystal growth. Kinetic crystal growth theory is used to show that crystals accelerated from an initial growth rate of 10−6–10−7 m s−1 to 10−5–10−4 m s−1 (10-100 mm day−1 to 1–10 m day−1), indicating meter sized crystals could have formed within days, if these rates are sustained throughout pegmatite formation. The rapid growth rates require that quartz crystals grew from thin (micron scale) chemical boundary layers at the fluid-crystal interfaces. A strong advective component is required to sustain such thin boundary layers. Turbulent conditions (high Reynolds number) in these miarolitic cavities are shown to exist during crystallization, suggesting that volatile exsolution, crystallization, and cavity generation occur together.
  • Thumbnail Image
    Item
    Geochemical diagnostics of metasedimentary dark enclaves: a case study from the Peninsular Ranges Batholith, southern California
    (Taylor & Francis, 2012) Liao, Kelley Z.; Morton, Douglas M.; Lee, Cin-Ty A.
  • Thumbnail Image
    Item
    Petrogenesis of the cogenetic Stewart pegmatite-aplite, Pala, California: Regional implications
    (The Geological Society of America, 2019) Morton, Douglas M.; Sheppard, J. Blue; Miller, Fred K.; Lee, Cin-Ty A.
    The Stewart pegmatite-aplite dike in Pala, California (USA) is well known as a source of lithium, gem minerals, and unusual phosphate minerals. We reinterpret the petrogenesis of the dike based on a combination of new regional and detailed geochemical isotopic and textural data. The Stewart dike, like other pegmatites in the Pala district and other major pegmatite districts in the northern Peninsular Ranges batholith, is enclosed within gabbro/mafic tonalite. The 40Ar/39Ar method of dating on muscovite from the dike, and U/Pb dating of zircon from the gabbro yield essentially the same age. Initial 87Sr/86Sr is similar for the dike, 0.7042, and the gabbro, 0.7036–0.7037, indicating a juvenile and likely common source for both. The extreme mineralogic, lithologic, and textural variations within the dike are interpreted to have resulted from in situ mineral segregation, autometasomatism, and migration of volatiles within an essentially closed system. Contacts between the pegmatite dike and the host gabbro are diffuse. All previous interpretations of the Stewart pegmatite dike invoked an allogenic origin, formed by fluids derived externally from a nearby or distant granitic body, with the fluids subsequently migrating to, and intruding, their gabbro/mafic tonalite host. These previously proposed sources of pegmatite-forming fluid(s) are ruled out here on the basis of similar ages of the host and pegmatite, similar initial 87Sr/86Sr isotopes, unrealistic 25–40 km distances between any proposed potential source and the host gabbro-tonalite pluton, and mineralogic and textural relationships indicating internal differentiation. For these reasons, we suggest an autogenic origin, whereby the gem-bearing pegmatites are petrogenetically limited to their gabbroic hosts and likely represent late-stage, internal segregation of magmatic fluids. We note that nearly every gem-bearing pegmatite in the northern Peninsular Ranges batholith is hosted within gabbro/mafic tonalite bodies, not within the more silicic plutons.