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link.springer.com/content/pdf/10.1007/s00445-021-01434-7.pdf
Typical of the megaspherulite occurrences studied here is the scarcity or absence of small spherulites in the lava host. For the megaspherulites near Silver Cliff, CO, Smith et al2001) assumed “sparse heterogeneous nucleation, under highly non-equilibrium conditions” as the main controlling parameters.
Megaspherulites with diameters up to 4.3 m have been reported from lava and ignimbrites in the Western US (Steens Mountains and Opal Butte in Oregon, Klondyke in Arizona, and Silver Cliff in Colorado), from Argentina and from Mexico (Stirling 1969; Smith et al. 2001; Breitkreuz 2013; Bustos et al. 2020). Ottens and Götze (2016) mentioned meter-sized megaspherulites from various localities in China.
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). Some of the megaspherulites contain cavities and thus should be coined megalithophysae; however, the name megaspherulite is established in literature.
Silver Cliff megaspherulites
Knowledge about the spectacular megaspherulites in a pitchstone quarry north of Silver Cliff (CO) goes back to the end of the nineteenth century (Cross 1891; overview in Smith et al. 2001). The hosting Paleogene lava, 76–106 m thick, covering approximately 3 km2, is vertically subdivided into three zones (Siems 1968; Smith et al. 2001): (i) a lower perlitic vitrophyre, (ii) a middle rhyolitic vitrophyre, and (iii) an upper thick, flow-layered lithoidal rhyolite, with small spherulites at its base. The megaspherulites occur in the upper portion of the middle zone, isolated or in clusters (Fig. 1). Thirty-seven (mega-)spherulites of 0.2 to 4.3 m have been observed/collected by Smith et al. (2001) in the 58-m-wide Black Obsidian Quarry
El Quevar megaspherulites
On the southern flanks of the El Quevar-Azufrero stratovolcano complex in northwest Argentina, the perlitic rhyolitic Quirón lava dome formed during the Miocene (Willson et al. 1999). For this dome, Laurenzi et al. (2007) published a 40Ar/39Ar age of 8.77 ± 0.09 Ma. The EQMs (Fig. 2) occur in the upper part of a > 30-m-thick, fractured green, aphyric perlitic pitchstone, which is covered by < 3 m of white, relatively devitrified, rhyolite with a flow banded structure (Willson et al. 1999). The pitchstone contains small phenocrysts of biotite (2 vol%), plagioclase feldspar (2 vol%), and quartz (l vol%). The ellipsoid lithophysae (10–70 cm) mainly consist of quartz and plagioclase. Willson et al. (1999) also mentioned chevron-shaped cavities in the outer marginal zone. The focus of the contribution by Willson et al. (1999) was the FTIR of lithophysae, hosting perlitic pitchstone and obsidian (Apache’s tears). Next to silicate minerals in the HTCD (feldspar, quartz, cristobalite, biotite), the authors detected the presence of H2O and OH− in both the megaspherulites and the perlitic host.
One of us (CB) sampled fragments of megaspherulites and of decimeter-sized spherulites at the Quirón quarry in 2007.
But missed enclosed spheroids -
res.cloudinary.com/crosbj/image/upload/v1646209168/Geology/1-Igneous/Rhyolite/Lithophadae/MegaSpherolites/ElQuivarArgentina-2_yezml1.jpg
Meissen lava megaspherulites
The Late Carboniferous Meissen lava crops out west of the town of Meissen (Saxony, Germany). It forms part of the 10-km-wide Meissen Volcanic Complex, which, apart from the voluminous rhyolitic lava, comprises pyroclastic deposits and dykes (Hoffmann et al. 2013).
The complex which yielded U/Pb ages of 302.9 ± 2.5 Ma overlies a 330-Ma monzonite with an erosional unconformity (Hoffmann et al. 2013). The Meissen lava consists mainly of flow-foliated, phenocryst-poor lithoidal rhyolite. It contains a number of rounded to elongate domains of pitchstone (≤ 3 km in length) which host the MLMs. Rhyolite and pitchstone contain c. 5 vol% of phenocrysts (quartz, feldspar, biotite; Lange and Heide 1996).
The pitchstone has been quarried since 1868 as a raw material for the glass and ceramic industry because of its high SiO2 (71.6 wt%) and low Fe2O3 contents (1.1wt%; Lange and Heide 1996). The pitchstone has minor domains which contain centimeter-sized spherulites as well as meter-sized crystallization domains (Jentsch 1981). The latter comprise two types:
i) ≤ 7-m ellipsoid, concentrically zoned domains with a perlitic texture, crystallized to quartz, K-feldspar, and zeolite, and
ii) ≤ 1-m concentrically zoned HTCD comprising mainly quartz and K-feldspar.
The former domains, called “Wilde Eier” (“wild eggs”) by the ancient miners, crystallized from perlitic glass, presumably during the final phase of cooling or during later hydrothermal activity. From the latter, the Meissen lava megaspherulites (MLMs) considered here, a 90-cm megaspherulite has been recovered during a construction work at the locality of Hoher Geiger in 2016 (Fig. 3). Smaller MLMs, complete or fragments, have been found earlier. To date, no systematic description of the MLMs has been published.
But failed to notice the enclosed spheroids
Discussion
Based on our results, we propose that the 15 different types of
microtextures and their mineralogies are controlled by whole-rock chemical composition and temperature. Primary crystallization in the cooling melt (mainly sanidine and SiO2 phases) took place above Tg. Compositional differences between ID and OD (in SCMand EQM) and between the megaspherulites and their host are then related to elements expelled during growth, and later H2O intake. Below Tg, and in the course of complete cooling aswell as later during diagenesis, an array of secondary minerals formed. Based on these results and interpretations, we present a model which attempts to describe the complex growth history of the three megaspherulites.
Chemistry and mineralogy of megaspherulites The chemical composition of the investigated megaspherulites illustrates that they formed exclusively in acidic lava with rhyolitic composition (Fig. 6). In general, the mineral composition of the megaspherulites is dominated by K-feldspar (sanidine) and SiO2 phases (quartz, cristobalite), although slight differences in the whole-rock chemistry of the different occurrences are caused by variations in the quantitative mineral abundances (Fig. 7).
The megaspherulite from Triebischtal (MLM) has the highest contents of SiO2 and K2O, and the lowest Al2O3, CaO, and Na2O concentrations (Fig. 7). Accordingly, MLM has a simple mineral composition consisting of the high-temperature K-feldspar phase sanidine and quartz. In contrast, elevated concentrations of CaO and Na2O in SCM and EQM reflect the presence of Na-rich plagioclase (oligoclase-andesine) both as phenocrysts and within the fine-grained matrix. In addition, the marginal zone of SCM reveals distinct changes in chemical composition and mineralogy (Tables 2 and 3). There is an outwards oriented tendency of decreasing SiO2 and increasingAl2O3 concentration resulting in the outer sanidine-quartz rind, which was already reported by Smith et al. (2001).
A peculiar feature of themegaspherulites fromEl Quevar is the dominance of SiO2 modifications such as cristobalite, tridymite, and opal-CT instead of quartz. The low temperature opal-CT indicates formation from an amorphous silica precursor. Cristobalite and tridymite formation in EQM probably was favored by undercooling of the melt preventing the regular crystallization of quartz (Perrotta et al. 1989; Baxter et al. 1999; Zawrah and Hamzawy 2002). Horwell et al. (2013) also discussed the formation of prismatic and platy forms of vapor-phase cristobalite in pores and fractures of volcanic rocks at the Soufrière Hills Volcano, Montserrat.
Ilmenite is a frequent accessory mineral in all megaspherulites as reflected by the measured TiO2 concentration (Table 2). Local chemical analyses of the rock-forming minerals revealed that elevated contents of Ti can also be incorporated in biotite and feldspar (especially sanidine; Table S1). Variations in the iron content of the megaspherulites are often visible by staining effects of the rocks due to iron oxides/hydroxides. However, iron is not only a constituent of iron phases and iron-bearing minerals (e.g., biotite, smectite). Spectral CL measurements also revealed structural incorporation of Fe3+ at Al3+ position in feldspar minerals (Figs. S2 and S4).
The comparison of the chemical composition of the megaspherulites and of the surrounding host (obsidian, perlitic pitchstone) illustrates that during crystallization, there was some movement of elements from the megaspherulite into the cooling melt and vice versa (Fig. 7 and Table 2). The megaspherulites from Silver Cliff show an enrichment of SiO2 and K2Oaswell as depletion in Na2O compared to the surrounding vitrophyre.
Assuming a homogeneous melt during emplacement, the chemical variation must have developed during the crystallization of quartz and sanidine in the megaspherulite, and an exclusion of Na2O into the host. Such chemical fractionation is also documented for EQM (SiO2 enrichment and Al2O3 depletion) and MLM (SiO2 and slight K2O, Na2O enrichment). Chemical differences between megaspherulite and pitchstone host might also be partially attributed to mineralized external water causing secondary compositional alteration of the glass. However, for EQM, a comparison of perlite and obsidian displays pure water input (Table 2).
Our whole-rock analyses of the SCM hosting pitchstone (LOI 5.27–5.69 wt%; Table 2) confirm the high H2O content stated by Smith et al. (2001: 7.6 wt%). Smith et al. (2001) considered the high H2O content as of magmatic origin.
However, Quaternary obsidian lava and domes present in Western US yielded H2O content of ≤ 0.5 wt% (DeGroat-Nelson et al. 1999). Despite the low volatile content, these complexes developed extended vesiculated domains due to supersaturation (Fink 1983) rendering unlikely a formation of a rhyolitic lava flow with 7 wt% H2O without vesiculation or explosive eruption. As a consequence, we assume that the transformation of the SCM glass host to a pitchstone took place late during or after cooling. This conclusion is supported by the analyses of the other megaspherulite occurrences. The obsidian (“Apache’s tears”) surrounding EQM has a LOI of 0.76 wt% pointing to the low water content of the original melt. The LOI of 3.64 wt% of the related perlitic pitchstone can be explained by secondary water uptake during alteration. The elevated LOI of 1.96 wt% (5.6 wt%: Lange and Heide 1996) of the partially crystallized pitchstone of the MLM indicates similar processes.
Primary vs. secondary mineral formation A common feature of both the megaspherulites and the surrounding host rocks is the presence of clay minerals (smectite, Table 3 and SEM-EDX data). Furthermore, accessory minerals were observed such as garnet (Fe-rich spessartine) and fluorite in SCM, or aluminosilicates (likely andalusite) and phosphates in EQM. Accordingly, the question arises concerning primary and secondary
Conclusions
Megaspherulites from Silver Cliff (USA), El Quevar (Argentina), and Meissen (Germany) were investigated by polarizing microscopy, cathodoluminescence microscopy and spectroscopy, and scanning electron microscopy, as well as X-ray diffraction and X-ray fluorescence to obtain detailed information concerning their characteristic textures and whole-rock chemical as well as mineral compositions.
The study aimed to constrain the origin of the megaspherulites and the specific physicochemical conditions for megaspherulite crystallization. Based on the macro- and microscopic texture and composition data of the three megaspherulite occurrences presented here, the following conclusions can be drawn:
* Megaspherulite crystallization occurs only in rhyolitic melts (similar in chemical composition to all investigated samples here).
* A favorable place to grow megaspherulites in a high volume lava is a location close to a large mass of crystallizing lithoidal rhyolite; the emanating latent heat may keep a low supercooling long enough to start megaspherulite growth; megaspherulite growth retards and terminates due to cooling and decreasing diffusion rates.
* Megaspherulite growth starts and ends in melt above Tg as revealed by the presence of crevasses filled with melt, crosscutting the outer domains. & During advancing megaspherulite growth, an increasing amount of interstitial melt and porosity forms between the microcrystals, replaced/filled later by secondary smectite.
* Secondary alteration by fluids may change the composition and texture of the megaspherulites and result in the formation of smectite and zeolite as well as other minerals such as garnet, fluorite, aluminosilicates, or phosphates.
The commonalty of these processes at the three different locations suggests that this could be a global model common to megaspherulite generation in any thick, cooling rhyolitic unit.
Mineralogical and geochemical investigation of megaspherulites from Argentina, Germany, and the USA
Christoph Breitkreuz1 & Jens Götze2 & Alexandra Weißmantel2 Received: 22 July 2020 /Accepted: 5 January 2021
# The Author(s) 2021
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.
Keywords Rhyolitic lava . SEM . CL . XRD . EPMA . Cristobalite . Tridymite
Christoph Breitkreuz1 & Jens Götze2 & Alexandra Weißmantel2 Received: 22 July 2020 /Accepted: 5 January 2021
# The Author(s) 2021
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.
Keywords Rhyolitic lava . SEM . CL . XRD . EPMA . Cristobalite . Tridymite
Typical of the megaspherulite occurrences studied here is the scarcity or absence of small spherulites in the lava host. For the megaspherulites near Silver Cliff, CO, Smith et al2001) assumed “sparse heterogeneous nucleation, under highly non-equilibrium conditions” as the main controlling parameters.
Megaspherulites with diameters up to 4.3 m have been reported from lava and ignimbrites in the Western US (Steens Mountains and Opal Butte in Oregon, Klondyke in Arizona, and Silver Cliff in Colorado), from Argentina and from Mexico (Stirling 1969; Smith et al. 2001; Breitkreuz 2013; Bustos et al. 2020). Ottens and Götze (2016) mentioned meter-sized megaspherulites from various localities in China.
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). Some of the megaspherulites contain cavities and thus should be coined megalithophysae; however, the name megaspherulite is established in literature.
Silver Cliff megaspherulites
Knowledge about the spectacular megaspherulites in a pitchstone quarry north of Silver Cliff (CO) goes back to the end of the nineteenth century (Cross 1891; overview in Smith et al. 2001). The hosting Paleogene lava, 76–106 m thick, covering approximately 3 km2, is vertically subdivided into three zones (Siems 1968; Smith et al. 2001): (i) a lower perlitic vitrophyre, (ii) a middle rhyolitic vitrophyre, and (iii) an upper thick, flow-layered lithoidal rhyolite, with small spherulites at its base. The megaspherulites occur in the upper portion of the middle zone, isolated or in clusters (Fig. 1). Thirty-seven (mega-)spherulites of 0.2 to 4.3 m have been observed/collected by Smith et al. (2001) in the 58-m-wide Black Obsidian Quarry
El Quevar megaspherulites
On the southern flanks of the El Quevar-Azufrero stratovolcano complex in northwest Argentina, the perlitic rhyolitic Quirón lava dome formed during the Miocene (Willson et al. 1999). For this dome, Laurenzi et al. (2007) published a 40Ar/39Ar age of 8.77 ± 0.09 Ma. The EQMs (Fig. 2) occur in the upper part of a > 30-m-thick, fractured green, aphyric perlitic pitchstone, which is covered by < 3 m of white, relatively devitrified, rhyolite with a flow banded structure (Willson et al. 1999). The pitchstone contains small phenocrysts of biotite (2 vol%), plagioclase feldspar (2 vol%), and quartz (l vol%). The ellipsoid lithophysae (10–70 cm) mainly consist of quartz and plagioclase. Willson et al. (1999) also mentioned chevron-shaped cavities in the outer marginal zone. The focus of the contribution by Willson et al. (1999) was the FTIR of lithophysae, hosting perlitic pitchstone and obsidian (Apache’s tears). Next to silicate minerals in the HTCD (feldspar, quartz, cristobalite, biotite), the authors detected the presence of H2O and OH− in both the megaspherulites and the perlitic host.
One of us (CB) sampled fragments of megaspherulites and of decimeter-sized spherulites at the Quirón quarry in 2007.
But missed enclosed spheroids -
res.cloudinary.com/crosbj/image/upload/v1646209168/Geology/1-Igneous/Rhyolite/Lithophadae/MegaSpherolites/ElQuivarArgentina-2_yezml1.jpg
Meissen lava megaspherulites
The Late Carboniferous Meissen lava crops out west of the town of Meissen (Saxony, Germany). It forms part of the 10-km-wide Meissen Volcanic Complex, which, apart from the voluminous rhyolitic lava, comprises pyroclastic deposits and dykes (Hoffmann et al. 2013).
The complex which yielded U/Pb ages of 302.9 ± 2.5 Ma overlies a 330-Ma monzonite with an erosional unconformity (Hoffmann et al. 2013). The Meissen lava consists mainly of flow-foliated, phenocryst-poor lithoidal rhyolite. It contains a number of rounded to elongate domains of pitchstone (≤ 3 km in length) which host the MLMs. Rhyolite and pitchstone contain c. 5 vol% of phenocrysts (quartz, feldspar, biotite; Lange and Heide 1996).
The pitchstone has been quarried since 1868 as a raw material for the glass and ceramic industry because of its high SiO2 (71.6 wt%) and low Fe2O3 contents (1.1wt%; Lange and Heide 1996). The pitchstone has minor domains which contain centimeter-sized spherulites as well as meter-sized crystallization domains (Jentsch 1981). The latter comprise two types:
i) ≤ 7-m ellipsoid, concentrically zoned domains with a perlitic texture, crystallized to quartz, K-feldspar, and zeolite, and
ii) ≤ 1-m concentrically zoned HTCD comprising mainly quartz and K-feldspar.
The former domains, called “Wilde Eier” (“wild eggs”) by the ancient miners, crystallized from perlitic glass, presumably during the final phase of cooling or during later hydrothermal activity. From the latter, the Meissen lava megaspherulites (MLMs) considered here, a 90-cm megaspherulite has been recovered during a construction work at the locality of Hoher Geiger in 2016 (Fig. 3). Smaller MLMs, complete or fragments, have been found earlier. To date, no systematic description of the MLMs has been published.
But failed to notice the enclosed spheroids
Discussion
Based on our results, we propose that the 15 different types of
microtextures and their mineralogies are controlled by whole-rock chemical composition and temperature. Primary crystallization in the cooling melt (mainly sanidine and SiO2 phases) took place above Tg. Compositional differences between ID and OD (in SCMand EQM) and between the megaspherulites and their host are then related to elements expelled during growth, and later H2O intake. Below Tg, and in the course of complete cooling aswell as later during diagenesis, an array of secondary minerals formed. Based on these results and interpretations, we present a model which attempts to describe the complex growth history of the three megaspherulites.
Chemistry and mineralogy of megaspherulites The chemical composition of the investigated megaspherulites illustrates that they formed exclusively in acidic lava with rhyolitic composition (Fig. 6). In general, the mineral composition of the megaspherulites is dominated by K-feldspar (sanidine) and SiO2 phases (quartz, cristobalite), although slight differences in the whole-rock chemistry of the different occurrences are caused by variations in the quantitative mineral abundances (Fig. 7).
The megaspherulite from Triebischtal (MLM) has the highest contents of SiO2 and K2O, and the lowest Al2O3, CaO, and Na2O concentrations (Fig. 7). Accordingly, MLM has a simple mineral composition consisting of the high-temperature K-feldspar phase sanidine and quartz. In contrast, elevated concentrations of CaO and Na2O in SCM and EQM reflect the presence of Na-rich plagioclase (oligoclase-andesine) both as phenocrysts and within the fine-grained matrix. In addition, the marginal zone of SCM reveals distinct changes in chemical composition and mineralogy (Tables 2 and 3). There is an outwards oriented tendency of decreasing SiO2 and increasingAl2O3 concentration resulting in the outer sanidine-quartz rind, which was already reported by Smith et al. (2001).
A peculiar feature of themegaspherulites fromEl Quevar is the dominance of SiO2 modifications such as cristobalite, tridymite, and opal-CT instead of quartz. The low temperature opal-CT indicates formation from an amorphous silica precursor. Cristobalite and tridymite formation in EQM probably was favored by undercooling of the melt preventing the regular crystallization of quartz (Perrotta et al. 1989; Baxter et al. 1999; Zawrah and Hamzawy 2002). Horwell et al. (2013) also discussed the formation of prismatic and platy forms of vapor-phase cristobalite in pores and fractures of volcanic rocks at the Soufrière Hills Volcano, Montserrat.
Ilmenite is a frequent accessory mineral in all megaspherulites as reflected by the measured TiO2 concentration (Table 2). Local chemical analyses of the rock-forming minerals revealed that elevated contents of Ti can also be incorporated in biotite and feldspar (especially sanidine; Table S1). Variations in the iron content of the megaspherulites are often visible by staining effects of the rocks due to iron oxides/hydroxides. However, iron is not only a constituent of iron phases and iron-bearing minerals (e.g., biotite, smectite). Spectral CL measurements also revealed structural incorporation of Fe3+ at Al3+ position in feldspar minerals (Figs. S2 and S4).
The comparison of the chemical composition of the megaspherulites and of the surrounding host (obsidian, perlitic pitchstone) illustrates that during crystallization, there was some movement of elements from the megaspherulite into the cooling melt and vice versa (Fig. 7 and Table 2). The megaspherulites from Silver Cliff show an enrichment of SiO2 and K2Oaswell as depletion in Na2O compared to the surrounding vitrophyre.
Assuming a homogeneous melt during emplacement, the chemical variation must have developed during the crystallization of quartz and sanidine in the megaspherulite, and an exclusion of Na2O into the host. Such chemical fractionation is also documented for EQM (SiO2 enrichment and Al2O3 depletion) and MLM (SiO2 and slight K2O, Na2O enrichment). Chemical differences between megaspherulite and pitchstone host might also be partially attributed to mineralized external water causing secondary compositional alteration of the glass. However, for EQM, a comparison of perlite and obsidian displays pure water input (Table 2).
Our whole-rock analyses of the SCM hosting pitchstone (LOI 5.27–5.69 wt%; Table 2) confirm the high H2O content stated by Smith et al. (2001: 7.6 wt%). Smith et al. (2001) considered the high H2O content as of magmatic origin.
However, Quaternary obsidian lava and domes present in Western US yielded H2O content of ≤ 0.5 wt% (DeGroat-Nelson et al. 1999). Despite the low volatile content, these complexes developed extended vesiculated domains due to supersaturation (Fink 1983) rendering unlikely a formation of a rhyolitic lava flow with 7 wt% H2O without vesiculation or explosive eruption. As a consequence, we assume that the transformation of the SCM glass host to a pitchstone took place late during or after cooling. This conclusion is supported by the analyses of the other megaspherulite occurrences. The obsidian (“Apache’s tears”) surrounding EQM has a LOI of 0.76 wt% pointing to the low water content of the original melt. The LOI of 3.64 wt% of the related perlitic pitchstone can be explained by secondary water uptake during alteration. The elevated LOI of 1.96 wt% (5.6 wt%: Lange and Heide 1996) of the partially crystallized pitchstone of the MLM indicates similar processes.
Primary vs. secondary mineral formation A common feature of both the megaspherulites and the surrounding host rocks is the presence of clay minerals (smectite, Table 3 and SEM-EDX data). Furthermore, accessory minerals were observed such as garnet (Fe-rich spessartine) and fluorite in SCM, or aluminosilicates (likely andalusite) and phosphates in EQM. Accordingly, the question arises concerning primary and secondary
Conclusions
Megaspherulites from Silver Cliff (USA), El Quevar (Argentina), and Meissen (Germany) were investigated by polarizing microscopy, cathodoluminescence microscopy and spectroscopy, and scanning electron microscopy, as well as X-ray diffraction and X-ray fluorescence to obtain detailed information concerning their characteristic textures and whole-rock chemical as well as mineral compositions.
The study aimed to constrain the origin of the megaspherulites and the specific physicochemical conditions for megaspherulite crystallization. Based on the macro- and microscopic texture and composition data of the three megaspherulite occurrences presented here, the following conclusions can be drawn:
* Megaspherulite crystallization occurs only in rhyolitic melts (similar in chemical composition to all investigated samples here).
* A favorable place to grow megaspherulites in a high volume lava is a location close to a large mass of crystallizing lithoidal rhyolite; the emanating latent heat may keep a low supercooling long enough to start megaspherulite growth; megaspherulite growth retards and terminates due to cooling and decreasing diffusion rates.
* Megaspherulite growth starts and ends in melt above Tg as revealed by the presence of crevasses filled with melt, crosscutting the outer domains. & During advancing megaspherulite growth, an increasing amount of interstitial melt and porosity forms between the microcrystals, replaced/filled later by secondary smectite.
* Secondary alteration by fluids may change the composition and texture of the megaspherulites and result in the formation of smectite and zeolite as well as other minerals such as garnet, fluorite, aluminosilicates, or phosphates.
The commonalty of these processes at the three different locations suggests that this could be a global model common to megaspherulite generation in any thick, cooling rhyolitic unit.
