Browsing by Author "Fall, Andras"
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- Application of fluid inclusions in geological thermometryFall, Andras (Virginia Tech, 2008-12-10)Many geologic processes occur in association with hydrothermal fluids and some of these fluids are eventually trapped as fluid inclusions in minerals formed during the process. Fluid inclusions provide valuable information on the pressure, temperature and fluid composition (PTX) of the environment of formation, hence understanding PTX properties of the fluid inclusions is required. The most important step of a fluid inclusion study is the identification of Fluid Inclusion Assemblages (FIA) that represent the finest (shortest time duration) geologic event that can be constrained using fluid inclusions. Homogenization temperature data obtained from fluid inclusions is often used to reconstruct temperature history of a geologic event. The precision with which fluid inclusions constrain the temperatures of geologic events depends on the precision with which the temperature of a fluid inclusion assemblage can be determined. Synthetic fluid inclusions trapped in the one-fluid-phase field are formed at a known and relatively constant temperature. However, microthermometry of synthetic fluid inclusions often reveals Th variations of about ± 1- 4 degrees Centigrade, or one order of magnitude larger than the precision of the measurement for an individual inclusion. The same range in Th was observed in well-constrained natural FIAs where the inclusions are assumed to have been trapped at the same time. The observed small variations are the result of the effect of the fluid inclusion size on the bubble collapsing temperature. As inclusions are heated the vapor bubble is getting smaller until the pressure difference between the pressure of the vapor and the confining pressure reaches a critical value and the bubble collapses. It was observed that smaller inclusions reach critical bubble radius and critical pressure differences at lower temperatures than larger inclusions within the same FIA. Homogenization temperature (Th) variations depend on many factors that vary within different geological environments. In order to determine minimum and acceptable Th ranges fro FIAs formed in different environments we investigated several geologic environments including sedimentary, metamorphic, and magmatic hydrothermal environments. The observed minimum Th ranges range from 1-4 degrees Centigrade and acceptable Th range from 5-25 degrees Centigrade. The variations are mostly caused by the fluid inclusion size, natural temperature and pressure fluctuations during the formation of an FIA and reequilibration after trapping. Fluid inclusions containing H₂O-CO₂-NaCl are common in many geologic environments and knowing the salinity of these inclusions is important to interpret PVTX properties of the fluids. A technique that combines Raman spectroscopy and microthermometry of individual inclusions was developed to determine the salinity of these inclusions. In order to determine the salinity, the pressure and temperature within the inclusion must be known. The pressure within the inclusions is determined using the splitting in the Fermi diad of the Raman spectra of the CO₂ at the clathrate melting temperature. Applying the technique with to synthetic fluid inclusions with known salinity suggests that the technique is valid and useable to determine salinity of H₂O-CO₂-NaCl fluid inclusions with unknown salinity.
- Fluid evolution in the nepheline syenites of the Ditrău Alkaline Massif, Transylvania, RomaniaFall, Andras (Virginia Tech, 2005-03-29)The Ditrău Alkaline Massif (Romania) is located in the Eastern Carpathians, as an intrusion in the Bukovina nappe system of the Mesozoic crystalline zone. Nepheline syenites are the most abundant rocks occurring in the central and eastern part of the Massif, and represent the youngest intrusion of the complex. Petrographic observations and fluid inclusion studies were performed on nepheline syenites in order to examine the evolution and the effect of the magmatic fluids on the alteration of nepheline to secondary minerals as sodalite, cancrinite and analcime. Fluid inclusion studies in nepheline, aegirine, albite and cancrinite provide evidence for the role of highly saline fluids in incongruent transformation reactions by which sodalite, cancrinite and analcime crystallize mostly on the expense of nepheline. The fluids, in most cases, can be modeled by the H2O-NaCl system with various NaCl contents; however inclusions with more complex fluid (containing also K, Ca, CO3, etc. besides H2O and NaCl) composition are abundant. Raman spectroscopic studies of daughter minerals in inclusions demonstrate the presence of alkali-carbonatic fluids in some of the earliest inclusions of nepheline, aegirine and albite. The alteration process is supported by the presence of H2O-NaCl fluid inclusions in cancrinite, showing lower salinity compared to those in nepheline. During the crystallization period of the nepheline syenites the rock was in equilibrium with a high salinity, carbonate rich solution that evolved to decreased salinity with time. The following observations support this: • paragenesis of mineral phases and their fluid inclusions: the early phases have high salinity inclusions and the late phases have low-salinity inclusions • the partitioning of chlorine depends on the pressure of the system: at about 2.0 kbars, the fluids coexisting with the melt have a high initial salinity and the salinity decreases with time; inclusions in nepheline show the lowest trapping pressure at ~2.5 kbars, hence the system has a high initial salinity and decreases with time • aH2O increases with time, resulting in the formation of H2O-bearing phases in a late stage of the crystallization of nepheline syenites.
- How Precisely Can the Temperature of a Fluid Event be Constrained Using Fluid Inclusions?Fall, Andras; Bodnar, Robert J. (2018-12)Fluid inclusions in clearly defined fluid inclusion assemblages (FIAs) from various geologic environments were examined to assess the uncertainty associated with determining the temperature of a fluid event based on fluid inclusion homogenization temperatures (T-h). A fluid event is defined as a physical or chemical process such as the healing of a microfracture or the formation of a growth zone in a crystal that occurs in the presence of a fluid phase and results in trapping of fluid inclusions to form an FIA. Examination of data from a large number of fluid events collected within a rigorous temporal and spatial (paragenetic) framework forms the basis for developing a complete fluid pressure-temperature-composition (PTX) history. The range in homogenization temperatures of fluid inclusions within well-constrained FIAs was determined, and the minimum (smallest) range in T-h, the median range in T-h, and the first quartile (Q1 at 25%) and third quartile (Q3 at 75%) of the median T-h ranges were calculated for different fluid environments, including the following: 1. Low-permeability sedimentary environments: 49 out of 144 FIAs show a range in T(h )of <= 1 degrees C; the median range = 2 degrees C (from Q1 = 1 degrees C to Q3 = 3.7 degrees C). 2. Mississippi Valley-type deposits: 11 out of 137 FIAs show a range in T(h )of <= 1 degrees C; the median range = 4.1 degrees C (from Q1 = 2.3 degrees C to Q3 = 8.3 degrees C). 3. Epithermal deposits: 102 out of 923 FIAs show a range in T(h )of <= 1 degrees C; the median range = 9 degrees C (from Q1 = 3.8 degrees C to Q3 = 19 degrees C). 4. Porphyry-type deposits: 24 out of 271 FIAs show a range in T(h )of <= 1 degrees C; the median range = 15 degrees C (from Q1 = 8 degrees C to Q3 = 30 degrees C). 5. Orogenic Au deposits: 21 out of 231 FIAs show a range in T(h )of <= 1 degrees C; the median range = 8.7 degrees C (from Q1 = 4 degrees C to Q3 = 20 degrees C). While all environments show some FIAs in which all the fluid inclusions homogenize at essentially the same temperature (range = <= 1 degrees C), we propose that the median range in T-h reported here represents a reasonable and achievable constraint on the uncertainty associated with the temperature of a fluid event in the environments examined. In summary, the temperature of a fluid event, as represented by the range in T-h of the fluid inclusions within an individual FIA, can be constrained to better than 15 degrees C in all environments examined, and in Mississippi Valley-type and low-permeability (deep) sedimentary basin environments, the range in T-h can be constrained to better than 2 degrees C. The processes that produce variability in T-h of fluid inclusions within an FIA are many and include natural variations in temperature, pressure, or fluid composition during trapping of the FIA, trapping of immiscible fluids, various forms of reequilibration in nature such as necking, stretching, and leakage, and modification of the inclusions during sample preparation and data collection. If the range in homogenization temperature for an individual FIA is found to be greater than the median range determined here for that environment, then assessment of the cause of the variability might provide useful information concerning the trapping and post-trapping history of the sample.