DENDRITIC MAGNETITE AND ILMENITE IN 590 Ma GRENVILLE DIKES NEAR OTTER LAKE, QUEBEC, CANADA

Abstract

Northwest of Ottawa, near Otter Lake, Quebec, branching crystals of magnetite occur in gabbro dikes ranging in width from 60 m to 0.2 m, but highly complex dendrites (500 to 40 m across) were found only in nine dikes, 2 to 0.2 m wide. Cooling rates in these small dikes are estimated at 0.6 to 60°C per hour. Complex dendrites are characterized by three intersecting primary plates (parallel to {100}), terminated by secondary arrowhead plates, bound by {111} planes. Many small projections have grown out from the primary plates, thereby isolating small cylindrical volumes of melt. Rare octahedra, ~ 20 m across, and clusters of octahedra also are present. Ilmenite dendrites consist of two or more thin interconnected plates. Dendritic growth of magnetite and ilmenite is attributed to gradients of temperature, resulting from a large heat of crystallization, and of concentration in the melt about growing crystals, and to a tendency for the formation and persistence of {111} planes (magnetite) and {0001} planes (ilmenite).

Magnetic Properties and Paleomagnetism of Zebra Rock, Western Australia: Chemical Remanence Acquisition in Hematite Pigment and Ediacaran Geomagnetic Field Behavior

“…Documented incompatible directions are usually recorded as a thermoremanent magnetization (TRM) in rapidly cooled volcanic rocks (e.g., Halls et al, 2015;Meert et al, 2007;Robert et al, 2017). Dendritic magnetite and ilmenite crystals in 590 Ma Grenville dikes, Canada, that host incompatible directions (Halls et al, 2015) suggest cooling rates as high at 0.6-608C/h (Kretz, 2003). Cooling rates inferred for volcaniclastic deposits range from 1 to 108C/s for Plinian-fall deposits to 30-0.38C/yr for the interiors of thick, pyroclastic-flow deposits (Wallace et al, 2003).…”

Distribuição do tamanho de cristais (DTC) e trama de plagioclásio em diques máficos Mesozóicos das Praias das Conchas e de Lagoinha (Municípios de Cabo Frio e Arraial de Búzios, RJ)

“…A textura é microlítica seriada e intergranular, com os grãos de piroxênio situados entre as (Kretz, 2003). Assim com o avanço de resfriamento, a lenta difusão e perda desses elementos são responsáveis pela forma esquelética no meio da cristalização de piroxênio e plagioclásio em competição.…”