Abstract
Textures and whole-rock chemistry, as well as mineral composition, were analyzed in megaspherulites (high-temperature crystallization domains [HTCDs]) that formed in different geographical and geotectonic contexts and during different geological periods (Silver Cliff, CO, USA—Paleogene; El Quevar, Argentina—Miocene; Meissen Volcanic Complex, Germany—Late Carboniferous). All of these megaspherulites have formed exclusively in rhyolitic lava, and their mineral composition is dominated by K-feldspar (sanidine) and SiO2 phases (quartz, cristobalite, tridymite). All megaspherulites represent composite HTCDs, comprising three zones: inner domain (ID), outer domain (OD), and a marginal domain (MD). Early evolution of megaspherulites is characterized by either central cavities and sector- to full-sphere spherulites or dendritic quartz-sanidine domains. The latter consist of bundles of fibrils each radiating from a single point reflecting relatively high growth rates. A common feature of OD and MD of all three megaspherulite occurrences is autocyclic banding. It mainly comprises fibrous (≤ 100 μm length), radially oriented sanidine and quartz, which formed at a temperature close to glass transition temperature (Tg). The termination of megaspherulite growth is marked by centimeter-sized sector-sphere spherulites on the surface. Megaspherulite formation requires limited nucleation, which is probably related to the low phenocryst content of the hosting lava. Latent heat from overlying crystallizing lithoidal rhyolite maintained low undercooling conditions keeping nucleation density low and facilitating high diffusion and growth rates. Late megaspherulite growth and its termination under low diffusion conditions is controlled by cooling close to Tg. Calculations based on literature data suggest that the megaspherulite growth presumably lasted less than 60 years, perhaps 30 to 40 years.
Highlights
Spherulites are high-temperature crystallization domains (HTCDs; Breitkreuz 2013; Iddings 1887; Bryan 1941; Lofgren 1971a; McArthur et al 1998; Castro et al 2008) that may form in cooling silica-rich lava, ignimbrite, or subvolcanic bodies
The present study describes and compares megaspherulites from three localities: (i) Silver Cliff megaspherulites (SCM; Smith et al 2001), (ii) El Quevar megaspherulites in northwestern Argentina (EQM; Willson et al 1999), and (iii) Meissen lava megaspherulites (MLM; Jentsch 1981)
Polished rock slabs and thin sections (30 μm thick) have been prepared for photographic documentation and for investigation with polarizing microscopy, cathodoluminescence (CL), and scanning electron microscopy (SEM). Representative aliquots of these samples were further prepared according to the analytical requirements of mineral analysis by X-ray diffraction (XRD) and chemical analysis of major and minor elements by X-ray fluorescence (XRF), respectively
Summary
Spherulites are high-temperature crystallization domains (HTCDs; Breitkreuz 2013; Iddings 1887; Bryan 1941; Lofgren 1971a; McArthur et al 1998; Castro et al 2008) that may form in cooling silica-rich lava, ignimbrite, or subvolcanic bodies. Lithophysae are HTCDs that develop one or more cavities during growth. Textures and composition are here analyzed in detail for megaspherulites that formed in different geographical and geotectonic contexts and during different geological periods (Cenozoic, Late Carboniferous). In contrast to normal HTCD, the studied megaspherulites display a distinct concentric zonation. We combine macro- and microscopic analysis, wholerock and mineral chemistry (XRF, SEM-EDX), X-ray diffraction (XRD), and cathodoluminescence (CL) to document the complex evolution of megaspherulites, and to provide the basis for a generic formation model
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