Genesis of megaspherulites in El Viejo Rhyolitic Coulee (Pleistocene), Southern Puna, Argentina

BUSTOS, EMILCE; BÁEZ, WALTER ARIEL; BARDELLI, LORENZO; MCPHIE, JOCELYN; SOLA, ALFONSO; CHIODI, AGOSTINA; SIMÓN, VALERIA; ARNOSIO, MARCELO

BULLETIN OF VOLCANOLOGY

SPRINGER

Lugar: Berlin; Año: 2020 vol. 82

0258-8900

Crystalline domains in rhyolitic domes and coulees are commonly characterized by spherulites and lithophysae. Spherulitestypically range from microscopic to a few centimeters in diameter. Larger spherulites, termed ?megaspherulites,? are rare buthave been reported in the USA and Mexico. The uncommon nature of such structures supports the need for a study to explore thefactors and special conditions that allow them to reach such a large size. In the Southern Puna of Argentina, El Viejo Couleeincludes megaspherulites up to 4 m in diameter. We present observations from field work, petrography, scanning electronmicroscopy, bulk geochemistry, and Fourier transform infrared spectroscopy. The megaspherulites occur in obsidian lenses thatdiffer in phenocryst content and composition from the foliated coherent facies that forms the rest of the coulee. The obsidian lacksvesicles and microlites and is unaltered. The megaspherulites comprise growth cones consisting of micropoikilitic texture wherequartz encloses potassium feldspar. The growth cones are separated by interconal areas composed of lithophysae. We proposethat the megaspherulites were formed above the glass transition temperature (Tg) and are the product of primary crystallization ofrhyolitic melt. The exceptionally large size of the megaspherulites implies high diffusion rates which are favored by temperaturesabove Tg during crystallization. The large size also suggests scarcity of nucleation sites, which is consistent with themegaspherulites being hosted by unaltered microlite- and vesicle-free glass. The position of the obsidian lens at the base ofthe coulee may have played a critical role in maintaining the temperature above the Tg long enough to allow the crystallization ofthe megaspherulites. These conditions also favored crystallization in the most advanced stage where micropoikilitic texturereplaced the fans of crystal fibers typical of spherulites. Crystallization of anhydrous quartz and feldspar in the growth conesled to the concentration of volatiles in the melt in the interconal areas, resulting in volatile exsolution and formation of vesiclesthat became nucleation sites for lithophysae. The study advances our understanding of some of the special processes that areinvolved in the cooling and solidification of rhyolitic magmas. Fundamentally, we find that the position of the obsidian at the baseof the coulee was critically important because this position favored maintenance of the temperature above the Tg which, in turn,favored high diffusion rates. Also, the scarcity of nucleation sites in the obsidian melt allowed only a small number of spherulitesto nucleate; those that nucleated therefore grew very large. The meter-scale megaspherulites may have taken ~ 55 years to create.