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Volatile evolution of the Phlegrean Volcanic District (Italy) from studies of melt inclusions

Silicate-melt inclusions are small droplets (1-several 100 µm) of silicate melt entrapped in phenocryst minerals during their growth (Fig.1 and 2). Melt inclusions thus provide a sample of the melt that was present in the magma chamber when the phenocryst grew, and offer the possibility of reconstructing the chemical composition of the magma (silicate melt + volatiles) during its evolution from formation at mantle depth to its ascent and eruption at the surface. Moreover, MI provide the only reliable means of determining the volatile evolution before eruption. A basic assumption of melt inclusion studies is that the inclusions behave as closed (= isolated) systems after their formation; that is, after trapping the silicate melt remains isolated from the evolving melt in the magma chamber.
In populated areas in which active volcanoes are present, such as Campi Flegrei (Italy), understanding the role of volatiles in magmas provides important information on volcanic system dynamics. In particular, the volatile content in magmas (e.g. H2O, CO2, Cl, S and F) is of critical importance in determining the eruptive style and magma evolution, because degassing is usually one of the major phenomena before and during an eruption. The Phlegrean eruptive products were selected based on age, eruptive characteristic, mineralogical and chemical compositions, and structural position of the eruptive center to examine possible relationships between magma chemistry, especially the volatile content, and eruptive style. Melt inclusions were analyzed by Electron Microprobe (EMPA), Ion Microprobe (SIMS), Raman Spectroscopy, and Laser Ablation ICP-MS. Our study is a long term project that started in 2004 and is a joint program between Virginia Tech and the Università di Napoli Federico II, (Italy).

(Fig. 1) Melt inclusion hosted in sanidine from the Phlegrean Volcanic Distrist. Note that the MI contain glass plus 5 bubbles. The picture is 50 x 60 μm.
(Fig. 2) Olivine phenocryst containing MI and glass embayments from the Phlegrean Volcanic District. Important to note is that MI are with or without bubble, and the olivine euhedral morphology. The size of the crystal is ~ 2 mm.

Volumetric constraints on CO2 storage in a saline aquifer

 

In the effort to identify potential geological reservoirs for sequestration of CO2, one of the major types of targets is saline aquifers. Whereas there has been significant interest in CO2 solubility constraints, less attention has been given to the volumetric requirements of dissolving CO2 in saline brines in a confined aquifer. Dissolved CO2 has a finite partial molar volume in H2O and H2O-NaCl fluids, and thus injection of CO2 into an aquifer, even if coupled by complete dissolution of the CO2 into the brine, results in increase of the fluid volume, or reservoir pressure, or both. We incorporate the PVTX properties (solubilities, molar volumes and phase relations) of H2O-CO2 and H2O-NaCl-CO2 to investigate the volumetric effect of CO2 injection into a confined saline aquifer. As end member scenarios, we consider the pressure evolution if the pore volume in the aquifer remains constant (isochoric), and the volume evolution if the pore fluid pressure remains constant (isobaric). Under isochoric conditions, the fluid pressure increases whereas under isobaric conditions, fluid volume increases, with CO2 injection. Both scenarios result in breaching of the confining beds and CO2 escape. These results indicate that planning for CO2 storage in saline aquifers must consider not only CO2 solubility constraints and reservoir fluid volume, but must also consider the volumetric effect of dissolving CO2 in the brine, in order to ensure stability of the geologic reservoir.


PT phase diagram for CO2, showing isochores in g/cm3, overlaid by the lithostatic and hydrostatic pressure gradients
 

The fidelity of melt inclusions in recording the pre-eruptive volatile content of magmas

 


Clinopyroxene phenocryst from the White Island volcano (New Zealand). Growth zones of the phenocryst are marked by the presence of solid and/or melt inclusions. MI within a single growth zone are assumed to be entrapped at the same time. The longer side of the crystal is 2.3 mm.

In the last several decades the number of publications describing the use of melt inclusions (MI) to determine the pre-eruptive volatile contents of magmas has increased significantly. In fact, MI provide the only reliable means of determining the volatile evolution before eruption. However, in most MI studies, the volatile contents of the MI vary widely, and it is not possible to assess the reliability of the data. In order for MI to provide reliable information concerning the pre-eruptive volatile content, the MI must obey Roedder’s (Sorby’s) Rules. Namely, the MI must have trapped a single homogeneous melt phase, the volume of the MI must remain constant after trapping, and nothing can be added or lost from the MI after trapping. The constant volume constraint likely does not affect the MI composition (excluding crystallization on the walls), and can usually be ignored.  In fluid inclusion studies, the adherence to Roedder’s Rules is tested by examining two or more fluid inclusions from a Fluid Inclusion Assemblage (FIA), representing a group of FI that were all trapped at the same time. If all of the FI in the assemblage show the same room temperature phase relations and behavior during microthermometry, then it is highly likely that the FI in the assemblage obey Roedder’s Rules.  A similar approach should be used to study MI, but Melt Inclusion Assemblages (MIA) are rarely studied because they are much more difficult to recognize than FIAs and because such studies can be very time consuming and tedious. Moreover, volatiles in MI are often determined by Fourier transform infrared spectroscopy (FTIR), which requires that the MI be exposed on both surfaces of the host phenocryst. That requires the removal of most of the phenocryst (and its contained MI) during polishing and reduces the likelihood that more than one MI will be available for analysis in each crystal. The goal of this project is to asses the fidelity of MI in recording the pre-eruptive volatile content before an eruption using the MIA approach (Fig. 3). For this study we are using mainly phenocrysts from the White Island volcanic products.

 

Application of fluid inclusions and mineral textures in exploration for for epithermal precious metal deposits

During the past two decades considerable advances have been made in our understanding of metal transport and deposition in Au-rich epithermal systems. Less work has been done to understand the genesis of silver-rich epithermal deposits. The search for mineral deposits is a time consuming, risky and very expensive process. Any technique that can help the explorationist to quickly and inexpensively discriminate between areas with high potential for economic mineralization and those with lower potential provides a competitive advantage to those applying the technology. The proposed study is designed to provide such a technique that may be applied in exploration for Ag-rich epithermal deposits.

The Guanajuato mining district is one of the largest silver producing districts in the world. Ore shoots are localized along three major northwest trending vein systems, the La Luz, Veta Madre and Vetas de la Sierra. Mineralization in the district shows much variability between and within deposits, from precious metal-rich to more base-metal-rich deposits, and from gold-rich zones to silver-rich zones. Ore textures also vary from banded quartz veins, to massive quartz veins to stockworks. During January 2010 samples representing all the different mineralization styles were collected from all three vein systems in the Guanajuato mining district. These samples will be examined petrographically, and approximately 100 samples will be selected for detailed microthermometric and microanalytical studies. The final product of this study will be a model that relates host mineral characteristics to fluid inclusion characteristics and ore grade and metal ratios. It is expected that these data will allow the explorationist to predict the likelihood of style of mineralization that occurs at depth (either beneath the surface or beneath the deepest levels of mining) based simply on petrographic examination of the fluid inclusions and the host minerals.


Average grades of Au and Ag (top axis) within each 50m depth increment from the surface to depth in the Veta Madre. Also shown is the percentage of samples (bottom axis) in that same 50m increment that show textural or fluid evidence for boiling.
 

Fluid inclusions that homogenize by halite disappearance: a comprehensive model

PT path followed by a fluid inclusion with a bulk salinity of Xm wt % NaCl during heating from the temperature of bubble disappearance to the halite melting temperature.

 

Fluid inclusions (FI) that homogenize by halite disappearance are common in several geologic settings and ore-deposit types. The temperatures of bubble disappearance (Th) and halite melting (Tm) in such fluid inclusions provide constraints on the pressure and temperature at which the fluid inclusions were entrapped. Th and Tm also constrain the density and salinity of those inclusions. In addition, measuring Th and Tm should allow construction of isochores for those fluid inclusions. Whereas density, composition and isochores of FI that homogenize by bubble disappearance are well defined by Tm and Th, those properties of FI that homogenize by halite disappearance are less well known.

Becker et al. 2008 developed a model to interpret thermometric data from fluid inclusions that homogenize by halite disappearance. At present, we are expanding the range of applicability of that model and complementing it with tools to calculate bulk density, bulk salinity and isochores for this fluid inclusion type without requiring iterative calculations.


 

PVTX constraints on the evolution of mineralizing fluids at the transition between porphyry-copper and high-sulfidation epithermal systems: Red Mountain, AZ

Alteration and mineralization characteristics of porphyry copper systems have been the focus of a wealth of field based and theoretical studies. It is widely recognized that fluids have a significant role in the evolution of porphyry copper deposits. In this study, we model alteration and mineralization at Red Mountain, AZ. The objective of the study is to investigate the role of fluids and their temporal and spatial evolution between the porphyry and high-sulfidation epithermal systems at Red Mountain.

We develop a three-dimensional model of the Red Mountain deposit. We combine this 3-D model with petrographic observations of hand samples and of thin sections. In addition, we conduct detailed ore microscopy and fluid inclusion analysis. Our results provide new insight about how mineralization and alteration have evolved at Red Mountain with time and space. The fluid inclusion record in the samples constrains the PVTX evolution of fluids in given points in the system along time. The combination of different scales of observation leads to a better understanding of the genetic relation between the porphyry and high-sulfidation epithermal systems at Red Mountain, AZ.

Red Mountain, Arizona, viewed from the southeast

Source and evolution of fluids related to rhodochrosite gemstone deposits: a comparative study between Sweet Home Mine (USA) and Wutong (China)

Fluid inclusions in a secondary trail in rhodochrosite gemstone from Wuzhou, Guangxi, China

 

The Wutong (Guanxi Province, China) and Sweet Home (Colorado, USA) mines are Pb-Zn-Ag deposits of similar mineralogy, originally exploited for Ag. Both of these mines are prolific producers of gem-quality rhodochrosite. The Sweet Home Mine is in the Colorado Mineral Belt, which hosts several giant porphyry-Mo deposits, and the genesis of the Sweet Home deposit has been linked to that of the Mo deposits. Conversely, the potential genetic relations of the Wutong Mine are unknown. Comparison between the fluid sources and fluid evolution of these deposits provides a framework to understand which factors are critical in the formation of rhodochrosite gemstones.

Uranium immobilization by autunite-group minerals

Schematic of a permeable reactive barrier.

 

One technology used to inhibit transport of uranium in groundwater is phosphate permeable reactive barriers (PRBs), in which phosphate reacts with dissolved uranium to precipitate uranyl phosphate minerals such as those of the autunite and meta-autunite groups.  These minerals have low solubility and long-term stability and thus offer the possibility of effective, permanent uranium sequestration.  Meta-autunite group minerals are present in the saturated zone above a pitchblende ore at the Coles Hill Uranium Deposit in Pittsylvania County, VA, and low concentrations of uranium in groundwater near the deposit may be an indication of these minerals removing dissolved uranium.  Characterization of how these minerals form could help improve the efficiency of apatite PRBs.  Our research focuses on determining kinetic rates of precipitation for autunite minerals based on measurements from mixed-flow reactor cells.

Quartz dissolution and precipitation in sub-seafloor hydrothermal systems

Model cross section through a ridge axis, with zones of quartz precipitation and dissolution contoured in blue and red, respectively. Arrows show the main direction of fluid flow.

 

Volcanogenic massive sulphide (VMS) deposits represent the fossil analogues of the active hydrothermal vents in the seafloor found at oceanic spreading centres. Fluid inclusion (FI) studies of both active (“black smokers”) and fossil VMS systems have provided much information on the physical and chemical conditions of formation of these deposits. The vast majority of all FI studied from VMS deposits are hosted in quartz. We use numerical fluid-flow modeling in conjunction with the relationship between quartz solubility and pressure-temperature-salinity-density conditions, to predict where, when and how quartz dissolution and precipitation occur in sub-seafloor hydrothermal systems. Results provide insight into the evolution of VMS systems and help to interpret FI measurements from these deposits.

Constraints on the formation conditions of nodules from the Sarno eruption (18.6 ka) of Mt. Somma-Vesuvius

Melt inclusions in clinopyroxenes from the Sarno eruption.

 

The volcanic activity at Mt. Somma-Vesuvius is cyclical; three mega-cycles can be distinguished. They consist of several plinian and/or subplinian eruptions with interplinian episodes characterized by less intense eruptive/effusive activity. The mega-cycles are separated by long repose times.
Nodules are abundant both in the plinian and interplinian records. These subeffusive rocks are interpreted to represent the crystal mush zone along the magma chamber wall. They usually contain abundant melt and related fluid inclusions, hence they represent and provide information on the transitional zone from a magma-dominated system to a hydrothermal system. In addition, fluid inclusions and related melt inclusions are powerful tools to estimate the minimum depth of the magma chamber.
Nodules were collected from the phreato-magmatic phase of the Sarno eruption (18.6 ka). Unlike most of the nodules from the other eruptions, these samples do not have typical cumulate texture, but rather porphyrogranular texture. The mineral phases, especially clinopyroxenes, are abundant in crystallized silicate melt inclusions. Unlike in nodules from other eruptions, all fluid inclusions are single phase and they are secondary, not associated with melt inclusions. The different texture and inclusion population may indicate different pre-eruptive conditions and processes.

The Sarno eruption is one of the biggest eruptions of the Mt. Somma-Vesuvius volcanic complex, but it has been little studied. The aim of this research is to get a better understanding and constrain the pre-eruptive magma chamber processes and the source region of the magmas of this plinian eruption based on melt inclusion studies.

 

H2O in high grade metamorphic rocks using Raman spectroscopy in fluid inclusions

Raman spectrum with laser spot focused on a CO2-rich fluid inclusion, at 150 °C. The Fermi diad of CO2 is between 1200 and 1400 cm-1; inset shows a small H2O peak at 3641 cm-1.

 

Granulite facies metamorphism typically occurs in PT conditions between 650-1000 °C and ≈500-1200 MPa. This type of rock is characterized mostly by anhydrous (low PH2O) mineral assemblages that typically include orthopyroxene, plagioclase and quartz. Fluid inclusions (FI) found in these rocks were thought to be restricted to single-phase, high-density “pure” CO2 FI with traces of CH4 and N2, and with little to no H2O. However, if the H2O content of an H2O-CO2 inclusion is less than 10-20 mol%, the H2O phase is not optically resolvable in most cases.

Using the technique developed by Berkesi et al., 2009 (Journal of Raman Spectroscopy, 2009, 40, 1461-1463) with Raman spectroscopy in fluid inclusions of apparently “pure” CO2, we can determine the presence of H2O by heating fluid inclusion (to +150°C). If CO2-rich inclusions containing small amounts of H2O are heated to a temperature above the one-phase/two-phase boundary, the H2O that occurs as a thin liquid film on the inclusion walls at room temperature will dissolve (evaporate) into the CO2 liquid to produce a homogeneous CO2-rich liquid phase. Raman analysis of this homogeneous phase at elevated temperature by Raman reveals the presence of H2O in the inclusion.

Small amounts of H2O present in FI that are assumed to be pure CO2 inclusions may lead to errors in the determination of pressure (hundreds of bars in some cases). This can be calculated using PVTX data for the system H2O-CO2 and mass balance constraints. At granulite PT conditions the activity of H2O is greater than its mole fraction in the fluid, so small amounts of H2O in CO2-rich fluids results in a fluid with a relatively high aH2O.

 

CO2 sorption on minerals in geologic reservoirs

Excess sorption of supercritical CO2 in CPG-10 materials,
normalized to the specific surface area.

 

Carbon dioxide (CO2) generated in fossil-fuel powered plants is a concern due to its potential contributions to global warming. Large-scale carbon capture and sequestration (CCS) can help to slow the rise of atmospheric CO2 levels. In this process, CO2 is stripped from the plant emissions, compressed and injected into subsurface reservoirs. Directly after injection, the dominant processes to contain the supercritical CO2 in the reservoir are sorption and capillary trapping. Quantification and understanding of these processes is needed to estimate reservoir capacities and model long-term storage security. Our research focuses on understanding what role CO2 sorption plays in the interaction with these geologic units at depth. Using gravimetric sorption experiments we can calculate CO2 excess sorption isotherms for samples with different pore sizes and morphologies. In the future we would like to conduct more of these gravimetric studies as well as some neutron diffraction studies to see where the water is residing in the clay. This work represents an ongoing collaboration with Oak Ridge National Laboratories.

 


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