Complex Phenocryst Textures and Zoning Patterns in Andesites and Dacites: Evidence of Degassing-Induced Rapid Crystallization? (original) (raw)
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Journal of Petrology, 2008
Products of the 1915 Lassen Peak eruption reveal evidence for a magma recharge^magma mixing event that may have catalyzed the eruption and from which four compositional members were identified: light dacite, black dacite, andesitic inclusion, and dark andesite. Crystal size distribution, textural, and in situ chemical (major and trace element and Sr isotope) data for plagioclase from these compositional products define three crystal populations that have distinct origins: phenocrysts (long axis40Á5 mm) that typically have core An contents between 34 and 36 mol %, microphenocrysts (long axis between 0Á1 and 0Á5 mm) that have core An contents of 66^69, and microlites (long axis50Á1mm) with variable An core contents from 64 to 52. Phenocrysts are interpreted to form in an isolated dacitic magma chamber that experienced slow cooling. Based on textural, compositional, and isotopic data for the magma represented by the dacitic component, magma recharge was not an important process until just prior to the 1915 eruption. Average residence times for phenocrysts are in the range of centuries to millennia. Microphenocrysts formed in a hybrid layer that resulted from mixing between endmember reservoir dacite and recharge magma of basaltic andesite composition. High thermal contrast between the two end-member magmas led to relatively high degrees of undercooling, which resulted in faster crystal growth rates and acicular and swallowtail crystal habits. Some plagioclase phenocrysts from the dacitic chamber were incorporated into the hybrid layer and underwent dissolution^precipitation, seen in both crystal textures and rim compositions. Average microphenocryst residence times are of the order of months. Microlites may have formed in response to decompression and/or syn-eruptive degassing as magma ascended from the chamber through the volcanic conduit. Chemical distinctions in plagioclase microlite An contents reveal that melt of the dark andesite was more mafic than the melt of the other three compositions. We suggest that mixing of an intruding basaltic andesite and reservoir dacite before magma began ascending in the conduit allowed formation of a compositionally distinct microlite population. Melt in the other three products was more evolved because it had undergone differentiation during the months following initial mixing; as a consequence, melt and microlites among these three products have similar compositions. The results of this study highlight the integrated use of crystal size distribution, textural, and in situ chemical data in identifying distinct crystal populations and linking these populations to the thermal and chemical characteristics of complex magma bodies.
Abstracts with programs, 2021
This study presents major-and trace-element chemistry of plagioclase phenocrysts from the 1980 eruptions of Mount St. Helens volcano. Despite the considerable variation in textures and composition of plagioclase phenocrysts, distinct segments have been cross-correlated between crystals. The variation of Sr and Ba concentration in the melt, as calculated from the concentration in the phenocrysts using partition coefficients, suggests the cores and rims crystallised from compositionally different melts offset by the plagioclase crystallisation vector. In both of these melts Sr and Ba are correlated despite the abundance of plagioclase in the 1980 dacites. We propose that rapid crystallisation of plagioclase upon magma ascent caused a shift in melt composition towards lower Sr and higher Ba, as documented in the rims of the phenocrysts. Although the cores of the phenocrysts crystallised at relatively shallow depths, they preserve the Sr and Ba of the deep-seated melts as they ascended from a deeper region. Further magma ascent resulted in microlite nucleation, which is responsible for a similar shift to even lower Sr concentration as observed in the groundmass of post-18 May 1980 samples.
Lithos, 2010
A systematic study of textural and compositional zoning (An% and FeO variation) in plagioclase phenocrysts of historic (pre-1971) and recent (post-1971) lavas at Mount Etna was made through back-scattered electron (BSE) images and electron microprobe analyses (EMP). The textures considered include oscillatory zoning and several types of dissolution, resorption and growth textures at the phenocryst cores and/or rims. Two patterns of oscillation were recognized from the combined An–FeO variation: 1) Low Amplitude–High Frequency (LAHF) and 2) High Amplitude–Low Frequency (HALF). The first pattern is interpreted here as due to kinetic effects at the plagioclase/melt interface which developed during crystallization in closed reservoirs. The second, which sometimes involves thin dissolution surfaces marked by irregular edges, angular unconformities and complex dissolution–regrowth patterns, might imply crystallization in a more dynamic regime, probably driven by chemical and physical gradients of the system (e.g., convection in a steadily degassing open-conduit). Dissolution and resorption textures at the core vary from patchy (exclusive to plagioclases within pre-1971 lavas) to strongly sieved, and can be related to increasing rates of decompression under H2O-undersaturated conditions. Thick sieve-textured envelopes at the phenocryst rims, generally coupled with marked An–FeO increase, result from mixing with more primitive and volatile-rich magmas. In the same crystals from recent activity, An and, to a lesser extent, FeO increase, consistent with the mixing of H2O-rich magmas similar in their mafic character to the resident magma (cryptic mixing). Two types of growth textures were also recognized at the crystal rims: 1) stripes of regularly-shaped melt inclusions and 2) swallow-tailed, skeletal crystals. In the first instance, the concordant An–FeO decrease suggests crystallization caused by fast ascent-related decompression accompanied by volatile loss. In the second, An decrease at effectively constant FeO contents may indicate crystallization at a high level of undercooling from already degassed magma, followed by rapid quenching; such a feature might be acquired during syn- or post-eruptive conditions.Although textures found in historic lavas are rather similar to those in the recent ones, some differences occur, such as lack of crystals with patchy cores in recent products and lower An contents in crystals of historic ones. The available data allowed us to obtain information on the dynamics of the feeding system, highlighting their possible modifications over time. In particular, historically erupted magmas, generally acknowledged to be volatile-poor, may have ascended through the deep portions of the plumbing system under H2O-undersaturated conditions at lower rates than the recent ones, recognized as more volatile-rich. Eruption triggering mechanisms from closed reservoirs in the shallow portions of the feeding system are similar for both historic and recent events, and may be generally favoured by a recharging phase of more primitive, undegassed magma or by a few episodes of important fracture opening (e.g., in response to an earthquake swarm).
Chemical Geology, 2002
Melt inclusions in phenocrysts are a potentially powerful tool in petrological research that can provide the only direct information available on the physical parameters (P, T and melt composition) of crystallisation at various stages in the evolution of magmatic systems. However, melt inclusions also differ in principle from other parts of the magmatic system in that their composition, after trapping, may be controlled by the composition of the host phenocryst and therefore the direct application of our understanding of macro-scale magmatic processes to the interpretation of melt inclusion data can lead to erroneous conclusions. Our results indicate that the compositions of melt inclusions in early formed phenocrysts (olivine, pyroxene, plagioclase and spinel), often of most interest in petrological studies, can be affected by processes such as volatile dissociation, oxidation and/or partial re-equilibration with their host, both during natural cooling and homogenisation experiments. In particular, melt inclusions in all minerals are prone to hydrogen diffusion into or out of the inclusions after trapping and prior to eruption, and during homogenisation experiments. If not taken into account, this can significantly affect the crystallisation temperatures derived from the homogenisation experiments. Melt inclusions in highmagnesian olivine phenocrysts commonly have lower Fe contents compared to the initially trapped composition due to reequilibration with the host at lower temperatures. This often leads to the appearance of sulphide globules and in some cases high-magnesian clinopyroxene daughter crystals, and may cause an increase in the oxidation state of the inclusions. Homogenised melt inclusions in plagioclase phenocrysts in MORB usually have lower Ti and Fe, and higher Si contents compared to the melt composition at the moment of trapping. However, homogenisation experiments can provide reliable estimates of trapping temperature and the MgO, Al 2 O 3 , CaO, Na 2 O, and K 2 O contents of the host magma at the moment of trapping. Some of these processes can be identified by observing the behaviour of melt inclusions during homogenisation experiments using low-inertia visually controlled heating stages, and their effects can be minimised by using appropriate experimental conditions as determined by kinetic experiments, ideally completed for each phenocryst type in every sample. We also discuss general aspects of melt inclusion studies aimed at recovering H 2 O content of primary mantle-derived magmas and demonstrate that, in cases of low-pressure crystallisation, it is important to identify the
A systematic study of textural and compositional zoning (An% and FeO variation) in plagioclase phenocrysts of historic (pre-1971) and recent (post-1971) lavas at Mount Etna was made through back-scattered electron (BSE) images and electron microprobe analyses (EMP). The textures considered include oscillatory zoning and several types of dissolution, resorption and growth textures at the phenocryst cores and/or rims. Two patterns of oscillation were recognized from the combined An-FeO variation: 1) Low Amplitude-High Frequency (LAHF) and 2) High Amplitude-Low Frequency (HALF). The first pattern is interpreted here as due to kinetic effects at the plagioclase/melt interface which developed during crystallization in closed reservoirs. The second, which sometimes involves thin dissolution surfaces marked by irregular edges, angular unconformities and complex dissolution-regrowth patterns, might imply crystallization in a more dynamic regime, probably driven by chemical and physical gradients of the system (e.g., convection in a steadily degassing open-conduit). Dissolution and resorption textures at the core vary from patchy (exclusive to plagioclases within pre-1971 lavas) to strongly sieved, and can be related to increasing rates of decompression under H 2 O-undersaturated conditions. Thick sieve-textured envelopes at the phenocryst rims, generally coupled with marked An-FeO increase, result from mixing with more primitive and volatilerich magmas. In the same crystals from recent activity, An and, to a lesser extent, FeO increase, consistent with the mixing of H 2 O-rich magmas similar in their mafic character to the resident magma (cryptic mixing). Two types of growth textures were also recognized at the crystal rims: 1) stripes of regularly-shaped melt inclusions and 2) swallow-tailed, skeletal crystals. In the first instance, the concordant An-FeO decrease suggests crystallization caused by fast ascent-related decompression accompanied by volatile loss. In the second, An decrease at effectively constant FeO contents may indicate crystallization at a high level of undercooling from already degassed magma, followed by rapid quenching; such a feature might be acquired during syn-or post-eruptive conditions. Although textures found in historic lavas are rather similar to those in the recent ones, some differences occur, such as lack of crystals with patchy cores in recent products and lower An contents in crystals of historic ones. The available data allowed us to obtain information on the dynamics of the feeding system, highlighting their possible modifications over time. In particular, historically erupted magmas, generally acknowledged to be volatile-poor, may have ascended through the deep portions of the plumbing system under H 2 O-undersaturated conditions at lower rates than the recent ones, recognized as more volatile-rich. Eruption triggering mechanisms from closed reservoirs in the shallow portions of the feeding system are similar for both historic and recent events, and may be generally favoured by a recharging phase of more primitive, undegassed magma or by a few episodes of important fracture opening (e.g., in response to an earthquake swarm).
Journal of Volcanology and Geothermal Research, 2006
Textural and geochemical characteristics of plagioclase phenocrysts from the eruptive products of Mount Unzen (SW Japan) record repeated intrusions of basaltic magma into a dacitic host magma chamber over the lifetime of the volcano. An important aspect of this mixing style is the exchange of phenocrysts between the intruding basalt magma and host dacite magma, and the effect that this wide-reaching mixing style has on the mineralogical diversity of the erupted products. Plagioclase phenocrysts that originally crystallized from the host dacite magma are identified by oscillatory zoning patterns, low An content cores (An 45 to An 60 ), and low Sr / Ba ratios. Host-derived plagioclase phenocrysts are engulfed during intrusion of basaltic magma, evidenced by their presence in basaltic to andesitic enclaves. In response to changes in temperature and composition of the surrounding melt, the engulfed plagioclases develop resorption zones, which are composed of a densely packed network of micron-sized glass inclusions and high An content plagioclase (An 72 -An 92 ) with high Sr / Ba ratios that match those of plagioclase microphenocrysts inherent to the enclave-forming magma. Over time, host-derived plagioclase phenocrysts that were once engulfed during replenishment events are recycled back to the host as enclaves disaggregate (e.g. . The disaggregation of quenched magmatic inclusions contributes to chemical diversity in silicic lavas of Lassen Peak, California. Bull New Mexico Bureau of Mines and Mineral Resources, 131: 54]). An eruption of andesite lava with no enclaves, something particularly unique for Unzen, occurred in 1663. Similar to enclaves, all plagioclase phenocrysts in this lava flow are surrounded by resorption zones suggesting that the 1663 lava may represent a magma that was erupted after thoroughly mixing with the intruding basaltic. Using experimentally calibrated crystallization rates, we estimate that phenocrysts exist in the Unzen chamber a minimum of 0.5-3 months between the time of their encounter with a basaltic intrusion and eruption.
Contributions To Mineralogy and Petrology, 2002
Textural and compositional zoning in plagioclase phenocrysts in a sample from Parinacota volcano (Chile) was investigated using backscattered electron images and electron microprobe analysis of major and trace elements. Large (2 mm) oscillatory zoned crystals (type I) with resorption surfaces of moderate An discontinuities (⩽10% An) and decreasing trace-element contents (Sr, Mg, Ti) towards the rim reflect melt differentiation and turbulent convection in the main magma body. Early recharge with a low-Sr mafic magma is seen in the core. Small-scale Sr variations in the core indicate limited diffusion and thus residence and differentiation times of the magma shorter than a few thousand years. Smaller crystals (type II) with low trace-element/An ratio reflect the influence of an H2O-rich melt probably from a differentiated boundary layer. Closed-system in-situ crystallisation, mafic magma recharge and the role of a water-rich differentiated boundary layer can be distinguished from the An–trace element relationships. Crystals apparently move relatively freely between different parts and regimes in the magma chamber, evidence for "convective crystal dispersion". High-Sr type II crystals indicate an earlier input of Sr-rich mafic magma. Recharge of two distinct mafic magma types is thus identified (high-Sr and low-Sr), which must have been present – at increasing recharge rates with time – in the plumbing system throughout the volcano's history.
Journal of Petrology, 2017
The residence time of crystals in magmatic systems is an essential parameter to describe the dynamics of these systems and to evaluate the temporal representativeness of the mineral data used to document the physical conditions in the magmas. Uranium-series disequilibria in mineral separates from young volcanic rocks with a known eruption date provide unique insights into these residence times. We present 238 U-230 Th-226 Ra measurements for plagioclase separates and groundmass from magmas erupted at Volc an de Colima, Mexico in 2004 and 2007. The (230 Th/ 232 Th) activity ratios in plagioclase show limited variations within the range measured in groundmasses and previously documented for whole-rocks. The (230 Th/ 232 Th) and (238 U/ 232 Th) activity ratios in plagioclase are predominantly controlled by glass present as inclusions in the crystals or adhering to their rims, even in high-purity crystal separates with less than 2% glass. Variations in these activity ratios are best explained by crustal assimilation during crystallization and do not require ageing of the crystals. One plagioclase separate with very limited contamination by glass impurities has a demonstrable zero 230 Th-238 U crystal age. Precise 226 Ra-230 Th model ages are difficult to obtain owing to the strong influences of uncertainties in the partition coefficients, the temperature of crystallization and the correction for glass impurities in crystal separates. Nevertheless, if these uncertainties are taken into account, 226 Ra-230 Th model ages in the range 0-2000 years are obtained for the plagioclase separates. More complex models elaborated on the basis of the compositional zoning observed in plagioclase phenocrysts suggest that the cores of the crystals are at most 8500 years old. Overall, these results indicate that the plagioclase phenocrysts are relatively young and are not sourced from a long-lived near-solidus crystal mush as is often suggested for arc volcanoes. A more dynamic system in which crystallization predominantly occurs in response to degassing during ascent of the melts is conceivable at Volc an de Colima.
Tertiary andesitic basalts of the Sirjan area, Urmieh-Dokhtar Magmatic Arc, Iran, contain plagioclase phenocrysts enclosed in a matrix of amphibole, clinopyroxene and rare olivine crystals. Textural and compositional evidence suggests two significantly different types of plagioclase phenocrysts occur in the andesitic basalts. The most common type of plagioclase phenocrysts have zoning patterns that display abrupt fluctuations in An content (more than 20 mol %) that correspond to well-developed dissolution surfaces. The less common type of plagioclase phenocrysts is characterised by a core with sieve texture, which is overgrown by oscillatory zoned rims. Changes in temperature, composition and H2O content of the surrounding melt caused the development of resorption zones in the engulfed plagioclases. In addition to the petrographical and mineral chemical evidences, Crystal Size Distribution (CSD) measurements show a kinked plot that reflects the effects of degassing and reveals mixin...
Journal of Petrology, 2009
Plagioclase crystals from the 8 km3 10.5 ka Upper Toluca Pumice (UTP) eruption from Nevado deToluca provide a detailed temporal record of pre-eruptive magmatic processes. The crystals display a range of textures and major and trace element concentrations. A distinct feature of the crystals is the presence of several sharp increases in MgO, FeO and TiO2, which occur at some of the numerous resorp- tion horizons and coincide with increases in XAn and in some cases Ce and La. These abrupt compositional peaks, and the associated textural variations, reflect recharge events of more mafic melt.Three distinct recharge events can be recognized.The general compositional trends suggest that crystallization occurred within a common cham- ber. However, the crystal cores display a range of textures and a wide range of compositions suggesting that they are relics of earlier crystal- lization episodes within the upper crust. The temporal record of Sr and Ba melt contents recorded by the crystals, calculated using partition coefficients, fluctuates significantly within single crystals. Overall, two trends of Sr^Ba variation in the melt are apparent. The first trend involves decreasing Sr and slightly increasing Ba, consistent with plagioclase crystallization.The second trend involves an increase in both Sr and Ba that is not consistent with crystalliza- tion of plagioclase alone. This trend is ascribed to chemical varia- tions within the deeper magma reservoirs from which the various magma batches entering the UTP shallow reservoir were ultimately sourced. The magmatic system under Nevado de Toluca was open and received intermittent, but relatively small, pulses of magma from a deeper source. Dacite was the predominant recharge composi- tion, although some pulses were distinctly more mafic and hotter. These processes led to the accumulation of a large volume of dacitic magma in the upper crust. One of the more mafic pulses appears to have triggered the eruption.