THE FORMATION OF THE HJALLIN LENS: A QUANTITATIVE STUDY OF IGNEOUS ROCK TEXTURES THROUGH CRYSTAL SIZE DISTRIBUTION (CSD) AND CONDUCTIVE COOLING MODELS, VATNSDALUR, ICELAND

Investigation of flexure zones in northwest Iceland has led to questions of the formation of the Hjallin Lens. The lens is an extremely fine-grained, extrusive igneous body 3 km in length and 150-200 m in height. Fresh samples of basalt were collected from the lens in July of 2008. A quantitative analysis of the rock texture using crystal size distribution (CSD) methods (e.g. Higgins, 2000) was performed on 20 samples to determine the distribution of nucleated plagioclase crystals. Textural results were related to cooling rate to determine if the lens was emplaced by multiple flows or a single eruptive event (Cashman, 1993; Higgins, 2006). Thin sections reveal that plagioclase throughout the Hjallin lens varies in size from 3 micrometers in width and length to 100 micrometers in length and 50 micrometers in width. Backscattered electron composition images were created of each sample on a scanning electron microscope (SEM) at 350X magnification. Length and width of plagioclase crystals on the backscattered electron images were measured using ImageJ by methods previously established by Higgins (Higgins, 1994; 2000). Histograms of the crystal size distribution were produced using CSD Corrections 1.38 (Higgins, 2000). Textural results of the lens basalt reveal a systematic change from glass and small ~3 micrometer length crystals at the base of the lens grading upward into larger ~40 micrometer crystals within 100 meters from the bottom contact. Assuming conductive cooling, the approach modeled by Cashman (1993), and Resmini (2007) was used to determine the cooling rate of the body. The distribution of crystal sizes suggests that the lens was emplaced in multiple events within a quick time frame or in a single eruptive event, cooling similarly to the Makaopuhi Lava Lake discussed by Cashman (1993).