Popocatepetl Research Articles

Plant Colonization of Recent Lahar Deposits on Popocatepetl Volcano, Mexico

This work investigates the initial colonization on recent lahar deposits of the northeast slope of Popocatepetl volcano, Mexico, where 29 circular sample plots (1.57 m2) were established to determine the development stages of colonization in four types of habitats: 1997 lahar, 2001 lahar, margins, and terraces at the channel's bottom. Cluster analysis and the Sørensen Index were used to determine the floristic affinity of these lahars. Richness, frequency, percentage of species contribution, plant cover, density, and stem height were analyzed to determine the composition and structure of plant communities. The distribution of these variables reveals that the communities often have a simple internal structure; however, a relationship has already been established between changes in resilience and the age of the four lahars. Thus, floristic composition develops rapidly (1 sp. in terraces, 11 spp. in the margins, 29 spp. in the 2001 lahar, and 34 spp. in the 1997 lahar). Except for the 1997 lahar, however, structural characteristics are poorly developed in other incipient stages. As colonization advances, the affinity among the various components and sectors of the laharic deposits decreases, thus promoting the gradual incorporation of species found on the adjacent gorge slopes. The Principal Components Analysis used to identify other explanatory factors shows that of 15 variables studied, those associated with the morpho-sedimentology, the hydrovolcanic dynamics and stability of deposits (microtopography, thickness of the deposit, clast shapes, length of the deposit, depth of the gorge, and slope processes) explain a large percentage of variance. Only a few species (Lupinus campestris, Alchemilla procumbens and Penstemon gentianoides), are well adapted to poor soils and the effects of intense erosion caused by the flows.

Isotopic, chemical and dissolved gas constraints on spring water from Popocatepetl volcano (Mexico): evidence of gas–water interaction between magmatic component and shallow fluids

Geochemical research was carried out on cold and hot springs at Popocatepetl (Popo) volcano (Mexico) in 1999 to identify a possible relationship with magmatic activity. The chemical and isotopic composition of the fluids is compatible with strong gas–water interaction between deep and shallow fluids. In fact, the isotopic composition of He and dissolved carbon species is consistent with a magmatic origin. The presence of a geothermal system having a temperature of 80–100° C was estimated on the basis of liquid geothermometers. A large amount of dissolved CO 2 in the springs was also detected and associated with high CO 2 degassing.

Paleomagnetic Study of Lavas from the Popocatepetl Volcanic Region, Central Mexico

Results are presented for a paleodirectional and paleointensity study of 16 Popocatepetl lava flows. Popocatepetl is a tall, active, dacitic-andesitic stratovolcano; it forms the southern end of a N-S-trending Quaternary volcanic range at the eastern edge of the Basin of Mexico. Studied units are mainly andesites and one trachyandestite, and all possess similar trace- and rare-earth-element compositions. Rock-magnetic experiments show that remanence is carried in most cases by Ti-poor titanomagnetite, resulting from oxy-exsolution of original titanomagnetite during flow cooling. Unblocking temperature spectra and high coercivities point to "small" pseudo-single domain magnetic grains for these (titano)magnetites. Single-component, linear demagnetization plots were observed in most cases. A strong, lightning-produced magnetization overprint was detected for one site. Combining the new paleomagnetic data provides a mean paleodirection of I = 35.4°, D = 345.7°, k = 21, a95 = 8.5°, N = 15 for the Popocatepetl volcano region. All studied flows yield normal polarity magnetization, which supports a maximum age within the Brunhes chron. Twenty-five samples were selected for Thellier palaeointensity experiments based on magnetic properties, stable single-component remanent magnetizations, and reasonably reversible continuous thermomagnetic curves. Fourteen samples from four different flows yield reliable paleointensity estimates with the flow-mean virtual dipole moments (VDM) ranging from 5.9 to 9.2 x 1022 Am2. The NRM fractions used for paleointensity determination range from 35 to 96%, and the quality factors vary between 3.4 and 46.9, being normally greater than 6. Mean VDM obtained in this study is 7.2 ± 1.4 x 1022 Am2, slightly lower than the present-day dipolar value.

Models of Ground Deformation and Eruption Magnitude from a Deep Source at Popocatepetl Volcano, Central Mexico

Popocatepetl volcano in Central Mexico entered its latest stage of activity in late 1994. Due to the nature of its eruptive history and its location in a heavily populated area, it constitutes the highest risk in the cuntry. For this reason the volcano is currently under continuous surveillance; yet the interpretation of the information is carried out mostly on empirical basis and an integrating working model is lacking, at the present. In this paper, models of elastic deformation and mass erupted are developed to estimate the mass erupted according to the observed deformation patterns. We present results obtained from input based upon a gravimetric model of the volcano's internal structure and reasonable physical parameters of the volcanic system. These results are helpful in the planning of deformation and gravimetric observations aimed to forecast a major eruption.

Prevention project: a complex geophysical observatory in Mexico as a test facility for lithosphere–atmosphere–ionosphere coupling models

During the recent few years different physical models were developed trying to explain the physical mechanisms of electromagnetic and plasma phenomena observed in the area of anticipated earthquake on different levels: near the ground, in the atmosphere and in the ionosphere. All existing models are based on certain hypothesis because of lack of sufficient experimental data. In the best case, the models had to use the data collected at different places, and in different time, for different earthquakes. The project PREVENTION (PREcursors of Volcano Eruptions and Notable Tremors Integral ObservatioN) intends to establish a complex geophysical observatory in one of the most seismically active and potentially dangerous areas of Mexico – the State of Guerrero. The measurements will be done on all levels from the underground to the space to obtain simultaneous multi-parameter data for direct examination of existing physical models of seismo–ionospheric coupling. The second site of experimental survey will be established in the vicinity of Popocatepetl volcano to study the relation between volcanic activity and the ionospheric effects. The concept of the observatory is presented.

Characterization of size-differentiated inorganic composition of aerosols in Mexico City

Abstract Size-differentiated atmospheric aerosol particles were collected during December 2000–October 2001 in Mexico City (19°N, 99°W) using a micro-orifice uniform deposit impactor. Sulfate and ammonium, which were correlated, were major features of the size distributions. The more predominant mode was at 0.32±0.1 μm, aerodynamic diameter. This peak of concentration is likely the result of condensation of secondary aerosol components from the gas phase. During part of the rainy season (April and June), a larger mode was found at 0.56±0.2 μm, aerodynamic diameter. This peak of concentration, identified as a droplet mode, is probably the result of aqueous-phase reactions (i.e., oxidation of sulfur dioxide in liquid droplets). During August, the peak of concentration was observed at both size ranges of the accumulation mode. Overall, ion balances were achieved with a small deficit of cations, except for the April and June samples, where a significant amount of excess sulfate was present as a result of moderate–high activity of the neighboring volcano Popocatepetl, as well as ambient conditions that favored production of sulfate (moderate–high relative humidity values). Based on the analysis of the ammonia/sulfate molar ratios, the ammonia concentrations were sufficient to fully neutralize sulfate concentrations, except for the April and June samples. During these months, ammonium bisulfate, letovicite and H 2 SO 4(aq) (or a solution of the corresponding ions) were the dominant form of sulfate present in both fine and coarse modes. The acidic nature of these particles (with NH 3 /H 2 SO 4 molar ratio less than 2) is potentially important in assessing health effects of inhaled particles.

Volcanic monitoring for radon and chemical species in the soil and in spring water samples

Soil radon has been monitored at two fixed stations in the northern flank of Popocatepetl Volcano, a high risk volcano located 60 km SE from Mexico City. Water samples from three springs were also studied for radon as well as major and trace elements. Radon in the soil was recorded using track detectors. Radon in the water samples was evaluated using the liquid scintillation method and an Alphaguard. The major elements were determined through conventional chemical methods and trace elements using an ICP-MS equipment. Soil radon levels were low, indicating a moderate diffuse degassing through the flanks of the volcano. Groundwater radon had almost no relation with the eruptive stages. Water chemistry was stable in the reported time (2000–2002).

Timing magma ascent at Popocatepetl Volcano, Mexico, 2000–2001

Magnetic anomalies may be used to constrain magma ascent and are useful as precursors to eruptions especially when correlated with other geophysical and geochemical data. In this paper we present multiparameter data on the magnetics, dome morphology, geochemistry and seismicity associated with the December 2000–January 2001 eruptions, the largest of the recent eruptions at Popocatepetl Volcano. A 6-month data period was studied in order to evaluate the precursors and post-eruption processes. Several cycles of dome construction and destruction occurred from September 2000 through February 2001. In December, large amplitude tremor associated with a higher effusion rate resulted in the formation of a large dome which filled the crater to within about 50 m of the lowest part of the crater rim. Seismic activity in December was marked by many volcanotectonic earthquakes and both high frequency and harmonic tremor. On December 12 and 13, an increase in the tremor amplitude was followed by ash eruptions with 1.7–5-km-high columns. Tremor amplitude increased again on December 15 and oscillated for the next four days. Activity remained high until the end of the month. On January 22, an 18-km-high plume produced ash and pumice fall to the east as well as pyroclastic flows and mudflows which reached 6 km from the crater. The eruption left three concentric explosion pits, partially destroying the December dome. Mixing of a mafic olivine-bearing melt with a more evolved magma triggered the larger eruption on January 22 as can be seen from the higher MgO concentrations in some of the ejecta and the presence of a dark andesitic scoria with lower silica content and a white andesitic pumice with higher silica content. Precursory negative magnetic anomalies up to 5 nT (−3.2 nT, −5 nT, −2.9 nT) were associated with the ascent of the larger batches of magma which preceded the increases in seismicity, before the December 2000–January 22 VEI 3–4 eruptions. No significant increases in seismicity were observed prior to the January 22 eruption, except for a magnitude-2.4 earthquake on the day of the eruption. There was, however, a −0.8-nT magnetic anomaly which lasted from January 10 to 12 and possibly to January 17. Most of the magma ascended in three large batches and several smaller ones from October 22 through December 10. Short-lived crater domes with small volumes were formed at Popocatepetl in September–October and November 2000 and in March 2001. Even the smaller negative anomalies from September to February were associated with the ascent of several batches of magma. They preceded the increases observed in the seismicity and growth of a dome by several days.

Long-period events and tremor at Popocatepetl volcano (1994–2000) and their broadband characteristics

Following an initial phreatic eruption on 21 December 1994, activity at Popocatepetl has been dominated by fumarolic emissions interspersed with more energetic emissions of ashes and gases. A phase of repetitive dome-building and dome-destroying episodes began in March 1996 and is still ongoing at present. We describe the long-period (LP) seismicity accompanying eruptive activity at Popocatepetl from December 1994 through May 2000, using data from a three-component broadband seismometer located 5 km from the summit crater. The broadband records display a variety of signals, with periods ranging in the band 0.04–90 s. Long-period events and tremor with typical dominant periods in the range 0.3–2.0 s are the most characteristic signals observed at Popocatepetl. These signals appear to reflect volumetric sources driven by pressure fluctuations associated with the unsteady transport of gases beneath the crater. Very-long-period (VLP) signals are also observed in association with LP events and tremor. The VLP signals which accompany LP events display Ricker-like wavelets with periods near 36 s, whereas VLP signals associated with tremor waveforms typically show sustained oscillations at periods ranging up to 90 s. The spectra and particle motion patterns remain similar from event to event for the majority of LP and tremor signals analyzed during the time span of this study, suggesting a repeated, non-destructive activation of a common source. Hypocenters determined by phase pick analyses of selected LP events recorded by the seven-station, permanent Popocatepetl short-period network suggest that the majority of these events are confined to a source region in the top 1.5 km below the crater floor. The repetitive occurrences of VLP signals with closely matched waveform characteristics are consistent with a non-destructive reactivation of at least two sources. One source appears to coincide with the main source region of LP seismicity, whereas the second is a deeper source whose activity appears to be intimately linked with episodes of monochromatic tremor.

Principal Component Image Analysis of MODIS for Volcanic Ash. Part I: Most Important Bands and Implications for Future GOES Imagers

In Part I of this paper, the infrared bands of the Moderate Resolution Imaging Spectroradiometer (MODIS) are analyzed for volcanic ash signals using principal component image analysis. Target volcanoes included Popocatepetl volcano near Mexico City and Cleveland volcano in the Aleutian Islands. The analyses were performed to determine the MODIS bands that contribute the most to detecting volcanic ash. Even though the explained variance and signal-to-noise ratio of most of these new images are generally small, several of them provide views of volcanic ash with good contrast to the image background and other image features. Both day and night examples indicate that volcanic ash can be readily detected by combinations of MODIS bands to determine not only the ash extent but also qualitative variations in the concentration and height of the ash. The 36 bands on MODIS give much more flexibility for ash detection than the 4 bands on the current Geostationary Operational Environmental Satellite (GOES) Imager. In particular, MODIS bands 28–32, in the water vapor and longwave infrared portions of the spectrum, contributed most frequently to the detection of airborne volcanic ash. These include bands 28–30 in the 7.3–9.7-μm portion of the spectrum known for volcanic signals. Several of the MODIS bands that proved useful are bands projected for inclusion in the next major upgrade to the GOES Imager (scheduled for 2012). However, band 30 (9.7 μm) is neither available on the current GOES series nor planned for future GOES Imagers. In Part II of this paper, MODIS data for the same volcanic cases examined in Part I are used for specific simulations of current and near-term GOES imagery. The purpose of the simulations is to assess the impact of changes that will occur in the spectral bands of the GOES-M Imager (launched in 2001 and renamed GOES-12) when it becomes operational.

Atmospheric expansion wave simulations of Popocatepetl explosions

We present a compressible gasdynamic numerical model that utilizes a three‐dimensional adaptive grid to provide high spatial resolution of an expanding compression wave. We apply this model to recent explosions of Popocatepetl volcano in Mexico by incorporating both high resolution topography for a lower boundary and atmospheric stratification based upon climatological thermodynamic soundings. Explosions are simulated by the instantaneous release of an overpressure in a prescribed volume over the volcano's crater. The results show a compression wave that propagates as a supersonic shock close to the explosion, transforming into a sonic compression wave at larger distances. We validate the model predictions by comparing them to pressure‐time series measured by a microbarograph located 11 km away from the crater. With the model validation we estimate the equivalent initial internal energy of the explosions to range from 2 × 1013 J to 1.1 × 1015 J.

Influence of volcanic activity on spring water chemistry at Popocatepetl Volcano, Mexico

The results of the 7 years (1994–2000) of monthly monitoring of spring water before and during eruptions show response to volcanic activity. Low salinity and temperature characterize most of the springs, which are located on the flanks of Popocatepetl Volcano. The pH ranges from 5.8 to 7.8 and temperature from 3 to 36 °C. Oxygen and hydrogen isotopic data show that the water is of meteoric origin, but SO 4 2−, Cl −, F −, HCO 3 −, B, and SO 4 2−/Cl − variations precede main eruptive activity, which is considered linked to influx of magmatic gases and acid fluids that react with sublimates and host rock and mix with the large water system. Na +, Ca 2+, SiO 2 and Mg 2+ concentrations in the water also increased before eruptive activity. The computed partial pressure of CO 2 in equilibrium with spring waters shows values higher than air-saturated water (ASW), with the highest values up to 0.73 bar of pCO 2. Boron is detected in the water only preceding the larger eruptions. When present, boron concentration is normally under health standard limits, but in two cases the concentration was slightly above. Other components are within health standard limits, except for F − in one spring.

Aeromagnetic survey over an active stratovolcano in central Mexico

In this paper, we briefly summarize an aeromagnetic survey over a large stratovolcano (Popocatepetl) and surrounding volcanic terrain in central Mexico to show the potential that recent improvements in these methods have in studying active volcanoes. After a decades-long period of quiescence, the Popocatepetl volcano reawakened in 1994. Background seismicity showed a two-fold increase in October 1994. This was followed on 21 December by a swarm of volcano-tectonic events and tremors accompanying a large ash plume. Popocatepetl is an andesitic stratovolcano (Figure 1) in the central sector of the Plio-Quaternary Trans-Mexican volcanic belt (TMVB)—a continental arc associated with subduction of the Cocos and Rivera plates along the Middle Ainerican trench, and oriented almost E-W from the Pacific Ocean to the Gulf of Mexico. The TMVB volcanic-capped plateau is heavily populated. Mexico City is just 70 km away. Figure 1. Landsat thematic mapper image of the Iztaccihuatl-Popocatepetl volcanic range, eastern Mexico. The volcanic range is formed by several stratovolcanoes aligned north-south. Popocatepetl forms the southern end of a north-south trending range of large volcanic structures. The Popocatepetl stratovolcano has evolved through alternate periods of cone formation and destruction, with major cataclysmic events that destroyed ancient cones followed by formation of dome complexes and new summit cones. Recent activity at first was characterized by emission of ash rich in nonjuvenile material and volatiles, marked by intense seismicity with volcano-tectonic earthquakes (VT), long-period (LP) and tremor events. The ash and gas clouds from some events distributed ash over Mexico City and surrounding areas, with lapilli and small blocks falling over the volcano flanks. In March 1996, a dacitic dome built within the summit crater; magnetic activity to the present has been characterized by episodes of dome growth and destruction. Activity during December 2000 was particularly intense and, on 21 January 2001, a major explosive event …

Size of Popocatepetl Volcano explosions (1997–2001) from waveform inversion

Several volcanic explosions have been recorded since April 1997 at broadband seismic stations located around the Popocatepetl volcano, Mexico. We have inverted waveforms of ten of these explosions to estimate the following source parameters: depth, duration, magnitude and direction of the single force, F. The crustal structure used in generating Green's function at nearest stations is derived from the inversion of teleseismic receiver functions at the broadband permanent station PPIG, located 5 km north of the volcano. This inversion reveals a low velocity zone at ∼8 km beneath the summit with high Poisson ratio, possibly related to the magma chamber. We find that F scales with τ, the duration of the source‐time function, as F ∝ tau². Based on this relationship we determine an impulse magnitude scale, Mk. This magnitude is tied to the Mount Saint Helens initial explosive phase of May 18, 1980, whose magnitude is estimated as 4.6. Mk of the ten Popocatepetl explosions ranges between 1.8 and 3.2. Finally, we also propose an equivalent formula for rapid estimation of magnitude of future Popocatepetl explosions, which requires filtered amplitudes at PPIG.

Soil radon response around an active volcano

Soil radon behavior related to the volcanic eruptive period 1997–1999 of Popocatepetl volcano has been studied as a function of the volcanic activity. Since the volcano is located 60 km from Mexico City, the risk associated with an explosive eruptive phase is high and an intense surveillance program has been implemented. Previous studies in this particular volcano showed soil radon pulses preceding the initial phase of the eruption. The radon survey was performed with LR-115 track detectors at a shallow depth and the effect of the soil moisture during the rainy season has been observed on the detectors response. In the present state of the volcanic activity the soil radon behavior has shown more stability than in previous eruptive stages.

The significance of phenocryst diversity in tephra from recent eruptions at Popocatepetl volcano (central Mexico)

Tephra lapilli from six explosive eruptions between April 1996 and February 1998 at Popocatepetl volcano (=Popo) in central Mexico have been studied to investigate the causes of magma diversification in thick-crusted volcanic arcs. The tephra particles are sparsely porphyritic (≈5 vol%) magnesian andesites (SiO2=58–65 wt%; MgO=2.6–5.9 wt%) that contain phenocrysts of NiO-rich (up to 0.67 wt% NiO) magnesian olivine (Fo89–91 cores) with inclusions of Cr-spinel (cr#=59–70), orthopyroxene (mg#=63–76), clinopyroxene (mg#=68–86), intermediate to sodic plagioclase (An33–66), and traces of amphibole. Major and trace element systematics indicate magma mixing. The liquid mg#melt ratios inferred from the ferromagnesian phenocrysts suggest the existence of a mafic (mg#melt ≈ 72–76) and an evolved component magma (mg#melt ≈ 35–40). These component magmas form a hybrid magnesian andesite with an intermediate range of mg#melt=50–72. The mafic end member (mg#melt ≈ 72–75) is saturated with olivine and spinel and crystallizes at temperatures ≈1170–1085 °C with oxygen fugacities close to the fayalite–magnetite–quartz buffer and elevated water contents of several wt% H2O. A likely location of crystallization is at lower crustal levels, possibly at the Moho. Olivine is followed by high-mg# clinopyroxene which could start to crystallize during magma ascent. At depths of ≈4 to 13 km, the mafic magma mixes with an evolved composition containing low-mg# clino- and orthopyroxene and plagioclase at a temperature of ≈950 °C. The repetitive ascent of batches of mafic magmas spaced days to weeks apart implies multiple episodes of crystallization and magma mixing. The tephra is similar to the Popo magnesian andesites, suggesting similar generic processes for the common lavas of the volcano. The advantage of the tephra is that it can be used to reconstruct the composition of the mafic magma. Building on the elemental systematics of the tephra and a comparison to the near-primary basalts from the surrounding monogenetic fields, we infer that the Popo mafic end member is a magnesian andesite with variable, but high SiO2 contents of ≈55–62 wt% and near-primary characteristics, such as high-mg#melt of 72–75, FeO*/MgO ratios <1 (if extrapolated to an mg#melt of 72–75), and high Ni contents (=200 ppm Ni). This model implies that the typical elemental signature of the Popo andesites, such as the low CaO, Al2O3, FeO*, high Na2O contents, and the depletion in high-field strength elements (e.g., P, Zr, Ti), are mantle source phenomena. Thus, determining the elemental budget of the magnesian andesite, as it is prior to the modifications by crustal differentiation, is central to quantifying the subcrustal mass fluxes beneath Popo.

A Seven Months Prospective Study of the Respiratory Effects of Exposure to Ash from Popocatepetl Volcano, Mexico

This paper describes the 7 month follow-up of 35 non-smoker farmers exposed to suspended particles of volcanic ash of Popocatepetl volcano in central Mexico during a series of small explosive eruptions which occurred the last week of 1994 and the first week of 1995 as well as several smaller eruptions until April 1995. There was concern about the effect that the ash might have on the airways. Spirometric parameters and respiratory symptoms were recorded at the baseline and 7 months after the exposure. Analysis of spirometric data showed a significant low forced vital capacity (FVC) and forced expiratory volume in the first second (FEV1) that occurred during the major exposure period. This finding was possibly acute exposure-related because 7 months later, FVC and FEV1 increased to the normal predicted values. Self-reported respiratory and ocular symptoms showed a similar pattern. These findings are consistent with the hypothesis that short exposure to volcanic ash is associated with reversible inflammation of the airways. The reversibility was explained by the small total ash volume as well as by the non-fibrogenic nature of the Popocatepetl ash, because the content of free silica was less than 3.5%.

Temporal and spectral characteristics of seismicity observed at Popocatepetl volcano, central Mexico

Popocatepetl volcano entered an eruptive phase from December 21, 1994 to March 30, 1995, which was characterized by ash and fumarolic emissions. During this eruptive episode, the observed seismicity consisted of volcano-tectonic (VT) events, long-period (LP) events and sustained tremor. Before the initial eruption on December 21, VT seismicity exhibited no increase in number until a swarm of VT earthquakes was observed at 01:31hours local time. Visual observations of the eruption occurred at dawn the next morning. LP activity increased from an average of 7 events a day in October 1994 to 22 events per day in December 1994. At the onset of the eruption, LP activity peaked at 49 events per day. LP activity declined until mid-January 1995 when no events were observed. Tremor was first observed about one day after the initial eruption and averaged 10h per episode. By late February 1995, tremor episodes became more intermittent, lasting less than 5min, and the number of LP events returned to pre-eruption levels (7 events per day). Using a spectral ratio technique, low-frequency oceanic microseismic noise with a predominant peak around 7s was removed from the broadband seismic signal of tremor and LP events. Stacks of corrected tremor episodes and LP events show that both tremor and LP events contain similar frequency features with major peaks around 1.4Hz. Frequency analyses of LP events and tremor suggest a shallow extended source with similar radiation pattern characteristics. The distribution of VT events (between 2.5 and 10km) also points to a shallow source of the tremor and LP events located in the first 2500m beneath the crater. Under the assumption that the frequency characteristics of the signals are representative of an oscillator we used a fluid-filled-crack model to infer the length of the resonator.

EVIDENCE FOR OPEN-SYSTEM BEHAVIOR IN IMMISCIBLE Fe S O LIQUIDS IN SILICATE MAGMAS: IMPLICATIONS FOR CONTRIBUTIONS OF METALS AND SULFUR TO ORE-FORMING FLUIDS

Magmatic sulfides are generally accepted as forming by segregation of an immiscible sulfide liquid from a host silicate melt. Immiscible sulfides have been observed in many types of igneous rocks; however, some types of plutonic and volcanic rocks lack sulfides. We have examined a suite of samples from Mount Pinatubo (Philippines), Volcan Popocatepetl (Mexico), Satsuma–Iwojima (Japan) and Mount St. Helens, Bingham Canyon, Tintic District, and Clear Lake (U.S.A.). The samples reflect a range of crystallization histories and compositions; they range from rhyolite to basalt to trachyandesite, with f (O2) at the time of eruption ranging from below the fayalite – magnetite + quartz (FMQ) buffer to well above the nickel – nickel oxide (NNO) buffer. Textural and chemical evidence from our suite of samples indicate that sulfides initially were present, but were modified prior to complete cooling of the parent melt, giving rise to Fe-oxide globules. The globules formed through: (1) segregation of an immiscible Fe–S–O melt, and possibly, further separation of immiscible Fe–S and Fe–O liquids, and (2) undersaturation with respect to sulfide, causing removal of S from the immiscible sulfide melt. Sulfide undersaturation may have been caused by magma degassing (passively or during eruption), or magma mixing. The recognition of modified magmatic sulfides is important because, with extensive degassing, base and precious metals ( e.g. , Cu, Au) could be stripped from a melt by a S-rich magmatic volatile phase and entrained into a magmatic-hydrothermal fluid, ultimately giving rise to porphyry-type or related mineralization. For a melt containing 0.01 modal % magmatic sulfides, efficient degassing of only 10 km3 of magma could yield enough Cu to form a giant deposit.

Very long‐period signals associated with Vulcanian Explosions at Popocatepetl Volcano, Mexico

Very long period (VLP) seismic signals associated with large summit eruptions of Popocatepetl volcano in the last four years are investigated using data from a broadband seismometer (0.04–120 s) deployed on the north flank of the volcano at a radial distance of 5 km from the active crater. The VLP signals associated with individual eruptions share similar waveforms. Discrete VLP signals accompanying long‐period (LP) events also share similar signatures and have dominant periods that are nearly identical to those observed in the VLP waveforms of explosions. The VLP particle motions for eruption onsets consistently point to a source located a few km beneath the crater. The VLP ground displacement response to each explosion is marked by a compression, followed by a dilatation and terminating with another compression, suggesting a sequence of pressurization‐depressurization‐repressurization of the conduit. The repetitive nature of the waveforms points to a non‐destructive source process which has remained active in the magmatic system of Popocatepetl at least since April 1997.