Matthew Steele-MacInnis

Bubbles matter: An assessment of the contribution of vapor bubbles to melt inclusion volatile budgets

Abstract Melt inclusions (MI) are considered the best tool available for determining the pre-eruptive volatile contents of magmas. H2O and CO2 concentrations of the glass phase in MI are commonly used both as a barometer and to track magma degassing behavior during ascent due to the strong pressure dependence of H2O and CO2 solubilities in silicate melts. The often unstated and sometimes overlooked requirement for this method to be valid is that the glass phase in the MI must represent the composition of the melt that was trapped at depth in the volcanic plumbing system. However, melt inclusions commonly contain a vapor bubble that formed after trapping owing to differential shrinkage of the melt compared to the host crystal, and/or crystallization at the inclusion-host interface. Such bubbles may contain a substantial portion of volatiles, such as CO2, that were originally dissolved in the melt. In this study, we determined the contribution of CO2 in the vapor bubble to the overall CO2 content of MI based on quantitative Raman analysis of the vapor bubbles in MI from the 1959 Kilauea Iki (Hawaii), 1960 Kapoho (Hawaii), 1974 Fuego volcano (Guatemala), and 1977 Seguam Island (Alaska) eruptions. We found that the bubbles typically contain 40 to 90% of the total CO2 in the MI. Reconstructing the original CO2 content by adding the CO2 in the bubble back into the melt results in an increase in CO2 concentration by as much as an order of magnitude (thousands of parts per million). Reconstructed CO2 concentrations correspond to trapping pressures that are significantly greater than one would predict based on analysis of the volatiles in the glass alone. Trapping depths can be as much as 10 km deeper than estimates that ignore the CO2 in the bubble. In addition to CO2 in the vapor bubbles, many MI showed the presence of a carbonate mineral phase. Failure to recognize the carbonate during petrographic examination or analysis of the glass and to include its contained CO2 when reconstructing the CO2 content of the originally trapped melt will introduce additional errors into the calculated volatile budget. Our results emphasize that accurate determination of the pre-eruptive volatile content of melts based on analysis of melt inclusions must consider the volatiles contained in the bubble (and carbonates, if present). This can be accomplished either by analysis of the bubble and the glass followed by mass-balance reconstruction of the original volatile content of the melt, or by re-homogenization of the MI prior to conducting microanalysis of the quenched, glassy MI.

Calibration of zircon as a Raman spectroscopic pressure sensor to high temperatures and application to water-silicate melt systems

Abstract The shifts in wavenumber of the ν3(SiO4) (~1008 cm-1) Raman band of fully crystalline synthetic zircon with changing pressure (P) and temperature (T) were calibrated for application as a Raman spectroscopic pressure sensor in optical cells to about 1000 °C and 10 GPa. The relationship between wavenumber (ν) of this band and T from 22 to 950 °C is described by the equation ν (cm-1) = 7.54·10−9·T3 - 1.61·10−5·T2 - 2.89·10−2·T + 1008.9, where T is given in °C. The pressure dependence is nearly linear over the studied range in P. At ~25 °C, the ∂ν/∂P slope to 6.6 GPa is 5.69 cm-1/GPa, and that to 2 GPa is 5.77 cm-1/GPa. The ∂ν/∂P slope does not significantly change with temperature, as determined from experiments conducted along isotherms up to 700 °C. Therefore, this pressure sensor has the advantage that a constant ∂ν/∂P slope of 5.8 ± 0.1 cm-1/GPa can be applied in experiments to pressures of at least about 6.6 GPa without introducing a significant error. The pressure sensor was tested to determine isochores in experiments with H2O+Na2Si3O7 and H2O+NaAlSi3O8 fluids to 803 °C and 1.65 GPa. These pressures were compared to pressures calculated from the equation of state (EoS) of H2O based on the measured vapor dissolution or ice melting temperature for the same experiment. Pressures determined from the zircon sensor in runs in which NaAlSi3O8 melt dissolved in aqueous fluid were close to or lower than the pressure calculated from the EoS of H2O using the vapor dissolution or ice melting temperature. In experiments with H2O+Na2O+SiO2 fluids, however, the pressure obtained from the Raman spectrum of zircon was often significantly higher than that estimated from the EoS of H2O. This suggests that the pressures along some critical curves of water-silicate melt pseudobinary systems should be revised.

Salt precipitation in magmatic-hydrothermal systems associated with upper crustal plutons

Magmatic-hydrothermal systems associated with upper crustal plutons strongly influence volcanic and geothermal processes and form important mineral deposits. Fluids released from plutons are commonly saline and undergo phase separation into high-salinity brines and low-salinity vapors upon ascent. While brine-vapor immiscibility has been extensively studied, precipitation of solid salt during phase separation in magmatic-hydrothermal systems has generally been considered a rare phenomenon. Here we show that most porphyry deposits exhibit fluid inclusion evidence best interpreted by solid salt precipitation from ore-forming solutions. This interpretation naturally links thermodynamics, numerical simulations, and independent estimates of porphyry ore formation depths. Salt precipitation imposes major changes on the permeability of the system. Moreover, salt precipitation has implications for ore formation along the liquid-vapor-halite curve. The recognition of salt-saturated systems is challenging, but very relevant for understanding the evolution of magmatic-hydrothermal systems.

Volumetrics of CO2 Storage in Deep Saline Formations

Concern about the role of greenhouse gases in global climate change has generated interest in sequestering CO(2) from fossil-fuel combustion in deep saline formations. Pore space in these formations is initially filled with brine, and space to accommodate injected CO(2) must be generated by displacing brine, and to a lesser extent by compression of brine and rock. The formation volume required to store a given mass of CO(2) depends on the storage mechanism. We compare the equilibrium volumetric requirements of three end-member processes: CO(2) stored as a supercritical fluid (structural or stratigraphic trapping); CO(2) dissolved in pre-existing brine (solubility trapping); and CO(2) solubility enhanced by dissolution of calcite. For typical storage conditions, storing CO(2) by solubility trapping reduces the volume required to store the same amount of CO(2) by structural or stratigraphic trapping by about 50%. Accessibility of CO(2) to brine determines which storage mechanism (structural/stratigraphic versus solubility) dominates at a given time, which is a critical factor in evaluating CO(2) volumetric requirements and long-term storage security.

Quartz precipitation and fluid inclusion characteristics in sub-seafloor hydrothermal systems associated with volcanogenic massive sulfide deposits

Results of a numerical modeling study of quartz dissolution and precipitation in a sub-seafloor hydrothermal system have been used to predict where in the system quartz could be deposited and potentially trap fluid inclusions. The spatial distribution of zones of quartz dissolution and precipitation is complex, owing to the fact that quartz solubility depends on many inter-related factors, including temperature, fluid salinity and fluid immiscibility, and is further complicated by the fact that quartz exhibits both prograde and retrograde solubility behavior, depending on the fluid temperature and salinity. Using the PVTX properties of H2O-NaCl, the petrographic and microthermometric properties of fluid inclusions trapped at various locations within the hydrothermal system have been predicted. Vapor-rich inclusions are trapped as a result of the retrograde temperature-dependence of quartz solubility as the convecting fluid is heated in the vicinity of the magmatic heat source. Coexisting liquid-rich and vapor-rich inclusions are also trapped in this region when quartz precipitates as a result of fluid immiscibility that lowers the overall bulk quartz solubility in the system. Fluid inclusions trapped in the shallow subsurface near the seafloor vents and in the underlying stockwork are liquid-rich with homogenization temperatures of 200–400°C and salinities close to that of seawater. Volcanogenic massive sulfide (VMS) deposits represent the uplifted and partially eroded remnants of fossil submarine hydrothermal systems, and the relationship between fluid-inclusion properties and location within the hydrothermal system described here can be used in exploration for VMS deposits to infer the direction towards potential massive sulfide ore.

Detection of liquid H2O in vapor bubbles in reheated melt inclusions: Implications for magmatic fluid composition and volatile budgets of magmas?

Abstract Fluids exsolved from mafic melts are thought to be dominantly CO2-H2O ± S fluids. Curiously, although CO2 vapor occurs in bubbles of mafic melt inclusions (MI) at room temperature (T), the expected accompanying vapor and liquid H2O have not been found. We reheated olivine-hosted MI from Mt. Somma-Vesuvius, Italy, and quenched the MI to a bubble-bearing glassy state. Using Raman spectroscopy, we show that the volatiles exsolved after quenching include liquid H2O at room T and vapor H2O at 150 °C. We hypothesize that H2O initially present in the MI bubbles was lost to adjacent glass during local, sub-micrometer-scale devitrification prior to sample collection. During MI heating experiments, the H2O is redissolved into the vapor in the bubble, where it remains after quenching, at least on the relatively short time scales of our observations. These results indicate that (1) a significant amount of H2O may be stored in the vapor bubble of bubble-bearing MI and (2) the composition of magmatic fluids directly exsolving from mafic melts at Mt. Somma-Vesuvius may contain up to 29 wt% H2O.

Quartz-in-garnet inclusion barometry under fire: Reducing uncertainty from model estimates

Quartz inclusions in garnet are suitable for barometry because quartz is highly compressible relative to garnet, and the garnet host can maintain large stress differences generated as pressure-temperature conditions change. However, experimental validation of the quartz-in-garnet approach has been limited, raising questions concerning the accuracy of calculated entrapment pressure. Here we test the results of quartz-in-garnet barometry by conducting in-situ Raman analysis of natural inclusions over a range of temperatures (–175 to 600 °C). We assess the temperature ( T ) dependence of inclusion pressure ( P incl ) at 1 bar and compare calculated entrapment pressures derived from measurements at different temperatures. Experiments used two quartz standards (oriented [c] and ) and fully encapsulated quartz inclusions from three different terranes, retaining different P incl at room T (–444, 296, and 755 MPa). The stretched quartz inclusion ( P incl < 0) had the greatest increase in P incl (+264 MPa, during heating from 25 to 500 °C), whereas the high- P inclusions underwent less change (+138 MPa) in P incl over the same T interval. The greater T sensitivity of inclusions with low P incl reflects the greater thermal expansivity of quartz near the α to β quartz transition. While measured P incl - T trends are consistent with predictions, numerical models tend to overestimate P incl at elevated T , and calculated entrapment pressures show an unrealistic dependence on reference T . Raman spectroscopic measurements conducted in situ at elevated T provide optimal results. In addition, we have recalibrated the thermal portion of the numerical method based on the present results, and provide new empirical expressions for improved quartz-in-garnet barometry.

Ion Association in Hydrothermal Sodium Sulfate Solutions Studied by Modulated FT-IR-Raman Spectroscopy and Molecular Dynamics

Saline aqueous solutions at elevated pressures and temperatures play an important role in processes such as supercritical water oxidation (SCWO) and supercritical water gasification (SCWG), as well as in natural geochemical processes in Earth and planetary interiors. Some solutions exhibit a negative temperature coefficient of solubility at high temperatures, thereby leading to salt precipitation with increasing temperature. Using modulated FT-IR Raman spectroscopy and classical molecular dynamics simulations (MD), we studied the solute speciation in solutions of 10 wt % Na2SO4, at conditions close to the saturation limit. Our experiments reveal that ion pairing and cluster formation are favored as solid saturation is approached, and ionic clusters form prior to the precipitation of solid sulfate. The proportion of such clusters increases as the phase boundary is approached either by decreasing pressure or by increasing temperature in the vicinity of the three-phase (vapor-liquid-solid) curve.

Vibrational mode frequencies of H4SiO4, D4SiO4, H6Si2O7, and H6Si3O9 in aqueous environment, obtained from ab initio molecular dynamics

We report the vibrational properties of H(4)SiO(4), D(4)SiO(4), H(6)Si(2)O(7), and H(6)Si(3)O(9) in aqueous solution at 300 K and 1000 K, obtained from the combination of ab initio molecular dynamics (MD) and a mode-decomposition approach. This combination yields vibrational subspectra for selected vibrational modes at finite temperatures. We also performed normal-mode analysis (NMA) on numerous configurations from the same MD run to sample the effect of the variable molecular environment. We found good agreement between both approaches. The strongest effect of temperature is on the SiOH bending mode δSiOH, which is at about 1145 cm(-1) in solution at 300 K, opposed to about 930 cm(-1) in solution at 1000 K. The frequency of the δSiOH vibration also depends on environment, shifting from 1145 cm(-1) in solution to about 845 cm(-1) in the gas-phase. We found both in the mode-decomposition approach and in multiple-configuration NMA that the H(6)Si(2)O(7) dimer shows a vibrational mode at about 790 cm(-1), which we consider to be responsible for a hitherto unexplained shoulder of the monomer Raman band at 770 cm(-1) in dilute silica solutions. Our results demonstrate the importance of temperature and solvation environment in calculations that aim to support the interpretation of experimental Raman spectra of dissolved silica.

QuIB Calc

Quartz inclusion thermobarometry utilizes the pressure- and temperature-sensitive Raman peak shifts of quartz inclusions in garnet to determine formation pressure and temperature (PT) conditions. The measured Raman shift indicates the pressure currently retained in the inclusions at ambient external conditions, such that entrapment PT conditions (i.e., P and T of garnet growth) can be determined by elastic modeling. Most generally, trapping P is obtained with this method, based on an independent estimate of T. Here we describe QuIB Calc, a MATLAB? program that iteratively solves for garnet growth conditions using the pressure retained in quartz inclusions (as revealed by Raman peak shifts). The program explicitly accounts for the anomalous effects of the quartz lambda transition on the thermal expansivity, and utilizes a mixing subroutine to account for the physical properties of garnet solid solutions. QuIB Calc thus facilitates sophisticated PT calculations using quartz inclusions, and is particularly effective for geobarometry in high pressure terranes. Display Omitted QuIB Calc allows calculation of formation pressure from quartz inclusion in garnet.Effects from lambda transition on quartz thermal expansivity is considered.Linear proportionality for garnet composition are implemented for elastic refinement.