Stephen C. Schmidt

Passive infrared remote sensing evidence for large, intermittent CO2 emissions at Popocatépetl volcano, Mexico

Abstract Passive infrared (FTIR) and correlation spectrometer (COSPEC) measurements were conducted at Popocatepetl volcano during February 10 to 26, 1998 from sites 4 to 17 km distant from the summit. Volcano behavior was relatively quiet and SO 2 flux averaged 1670±1420 t/day (51 measurements), relatively small for Popocatepetl. Concurrent HCl/SO 2 and HF/SO 2 ratios were 0.17±0.01 and 0.031±0.003, respectively, about the same as ratios measured from 1994 to 1997. The amount of CO 2 in the volcanic plume was quantified using FASCODE in which atmospheric CO 2 is numerically subtracted from the total infrared spectrum to obtain the residual magmatic CO 2 . Surprisingly, CO 2 /SO 2 mass ratios rose dramatically to values as high as 140, about 30 times higher than typical values of 2 to 8 measured from 1994 to 1996. These excursions in high CO 2 /SO 2 ratios were short-lived, lasting no longer than about 0.5 to 3.0 h but CO 2 flux occasionally exceeded 100,000 t/day. We estimate that the average CO 2 /SO 2 ratio for the period was about 23, yielding an average CO 2 flux of roughly 38,000 t/day. Chemical and petrographic analyses of lava and pumice erupted during explosions on June 30, 1997 and January 1, 1998 show conclusively that Popocatepetl produces mixed products formed by injection of mafic magma into a more silicic chamber at temperatures and pressures of roughly 1040°C and 5 kbar. In addition, Popocatepetl eruptive products include xenoliths of metamorphosed carbonate rocks containing wollastonite and other calc-silicate minerals indicating reaction of magma with Cretaceous limestone underlying the volcano. Using a normal CO 2 /SO 2 ratio of 4 for reference, we calculate an average excess CO 2 production of 32,000 t/day for 17 days. This would require assimilation of only 5×10 −4 km 3 of limestone, an amount easily accessible in the 3-km-thick Cretaceous section beneath the volcano. We also examine two scenarios in which excess CO 2 is produced by degassing of subjacent basalt magma, but these explanations seem less plausible to us. Because many other volcanoes are underlain by carbonate sequences, short-duration bursts of CO 2 flux, and increased CO 2 /SO 2 ratio, might be observed at other sites, if simultaneous, real-time measurements of major gas species are made.

VIBRATIONAL SPECTROSCOPY OF MATERIALS UNDER EXTREME PRESSURE AND TEMPERATURE

Abstract We have obtained vibrational spectra of molecular materials at simultaneous high pressure and high temperature using the combined techniques of shock wave compression and coherent Raman spectroscopy. The high pressure/high temperature states produced and investigated are similar to those found in the interior of planets and in detonating high explosives. The molecular structures of simple molecular fluids and fluid mixtures, such as of diatomic and triatomic molecules, are inferred from the spectral parameters — Raman frequencies, linewidths, and peak Raman susceptibilities — of the transitions, which are extracted from the spectral data using a standard semi-classical treatment. For a simple diatomic molecule such as N 2 , CO or O 2 , the measured vibrational frequencies increase with increasing shock pressure. In N 2 , above 17 GPa single-shock pressure the vibrational frequency ceases to rise further and appears to begin to decrease. Observation of a further decrease at higher pressures is prevented by the onset of optical opacity. CO and O 2 become optically opaque at pressures insufficient to observe a similar maximum. The vibrational frequencies of CO and N 2 exhibit distinctly different shock pressure dependencies. The data show a factor of 5 smaller vibrational frequency versus pressure slope in CO as in N 2 . Under shock compression, mixtures of CO and N 2 show a highly non linear dependence of their vibrational frequencies with mixture mole fraction, while at ambient pressure this dependence is linear. We have also estimated vibrational relaxation times for these simple molecules under shock-compression, using a simple population filling argument for the longitudinal relaxation time (T 1 ), and from the measured linewidth for the transverse relaxation time (T 2 ). T 2 shows a strong temperature dependence but little density (pressure) dependence. The intensities of the vibrational hot bands were those expected from a Boltzmann vibrational population distribution, and they were used to estimate vibrational temperatures. These temperatures have been also used to refine the equations of state for nitrogen and argon as well as to understand the effects of non-ideal mixing at these extreme conditions.

Raman spectroscopies in shock-compressed materials

Spontaneous Raman spectroscopy, stimulated Raman scattering and coherent anti-Stokes Raman scattering have been used to measure temperatures and changes in molecular vibrational frequencies for detonating and shocked materials. Inverse Raman and Raman induced Kerr effect spectroscopies have been suggested as diagnostic probes for determining and phenomenology of shock-induced chemical reactions. The practicality, advantages, and disadvantages of using Raman scattering techniques as diagnostic probes of microscopic phenomenology through and immediately behind the shock front of shock-compressed molecular systems are discussed.

Remote Monitoring of Volcanic Gases using Passive Fourier Transform Spectroscopy

Volcanic gases provide important insights on the internal workings of volcanoes and changes in their composition and total flux can warn of impending changes in a volcano’s eruptive state.

Vibrational spectroscopy in high-temperature dense fluids

Coherent anti-Stokes Raman spectroscopy (CARS) in conjunction with a two-stage light-gas gun has been used to obtain vibrational spectra of shock-compressed liquid N2, 02, CO, and their mixtures, as well as liquid N20. The experimental spectra are compared to spectra calculated using a semiclassical model for CARS intensities to obtain vibrational frequencies, peak Raman susceptibilities, and linewidths. The derived spectroscopic parameters suggest thermal equilibrium of the vibrational populations is established in less than a few nanoseconds after shock passage. Vibrational temperatures obtained are compared to those derived from equation-of-state calculations. The variation of the vibrational frequency shift at pressure with species concentration in mixtures is investigated.

Time-resolved coherent anti-Stokes Raman spectroscopy and the measurement of vibrational spectra in shock-compressed molecular materials

We present the use of coherent anti-Stokes Raman scattering (CARS) in conjunctiOn with a two-stage light-gas gun to obtain vibrational spectra of shock-compressed liquid N2, 02, CO. and their mixtures. The experimental spectra are compared to spectra calculated using a semiclassical model for CARS intensities to obtain vibrational frequencies, peak Raman susceptibilities, and linewidths. The derived spectroscopic parameters suggest thermal equilibrium of the vibrational populations is established in less than a few nanoseconds after shock passage. Vibrational temperatures obtained are compared to those derived from equation-of-state calculations. Shifts in the vibrational frequencies reflect the influence of increased density and temperature on the intramolecular motion.

Coherent Raman Scattering Measurements Of Vibrational Frequency Shifts In Shock-Compressed Organic Liquids

The techniques of backward stimulated Raman scattering (BSRS) and reflected broad-band coherent anti-Stokes Raman scattering (RBBCARS) have been used to measure vibrational frequency shifts in shock-compressed liquid benzene and mixtures of liquid benzene and liquid deuterated benzene. BSRS was used only for measurements in neat liquid benzene as it only allows observation of the highest gain vibrational transition. RBBCARS was used to simultaneously measure multiple vibrational modes of multiple species. Accompanying static high pressure Raman experiments in a heated diamond anvil cell were used to establish the phase of the shocked samples. These experiments demonstrate the capabilities of fast non-linear optical techniques in the study of material structure changes and chemical reactions induced by shock-compression.