TEXTURAL ANALYSES OF ALKALI FELDSPAR AND QUARTZ IN FELSIC MELTS AT HIGH TEMPERATURE

We conducted a series of crystallization experiments on a granitic melt composition using a piston cylinder apparatus at 500 MPa and temperatures from 400 to 650 °C. The starting material was made from slightly metaluminous rhyolitic obsidian from Lake County, OR with addition of 0.25 wt. % B₂O₃ melted at 500 MPa, 1000 °C for 24 h. Experimental durations varied from 168 to 1464 h.

The grey scale SEM-BSE images were converted to binary images using the Fiji software (Schindelin et al. 2012). We tried to keep image manipulation to the minimum, but when necessary the despeckle tool in Fiji was used. The box-counting fractal dimension (D_B) and lacunarity (Λ) were measured using the FracLac plugin in Fiji (Karperien 1999-2013).

At 500 °C silicate crystallization takes the form of spherulites composed of K-feldspar-quartz intergrowths growing on the capsule walls. In the longer runs (up to 1464 h) the feldspar on the outer rim of the spherulite becomes more sodium rich as potassium becomes depleted in the melt. At 600 °C and 650 °C silicate crystallization also occurs on the capsule walls. At 600 °C feldspar-quartz spherulites form, but the quartz crystals and the spacing between them becomes larger than at 500 °C. At 650 °C the rims of the spherulites are rougher than at lower temperatures and the fan-shaped feldspars forming them become more visible. The intergrowths produced at 650 °C form the graphic texture characteristic of granitic pegmatites. At 600 °C the felspars on the rim of the spherulites, as in longer experiments at 500 °C, become more enriched in sodium. In some places within the capsule feldspars grow as large albite crystals accompanied by quartz of the same size and surrounded by magnetite trapped at the boundary between the spherulite-forming intergrowths and the albite-quartz “pockets”.

The measurements of the textures produced at 500 °C plot close together at relatively high fractal dimensions, and low lacunarity (0.2-0.35), when compared to the results of Baker et al. (2018). The measurements of the textures formed at 650 °C plot at similar fractal dimensions but show much more variation in lacunarity (0.25-0.6). There is some overlap between the data points from experiments at 500 and 650 °C but in general the experiments at higher temperature seem to be characterized by higher lacunarity than those at lower temperature.