Projects

How Much Variation in Homogenization Temperatures of Fluid Inclusions is Acceptable for an FIA?

Fluid Inclusion Assemblage (FIA) describes the most finely discriminated fluid inclusion association that can be identified based on petrography. An FIA thus defines a group of fluid inclusions that were trapped at the same time. This also implies that all the inclusions within an FIA were trapped at the same temperature and pressure, and all trapped a fluid of the same chemical composition. If the inclusions represent the original trapping conditions and have not reequilibrated, all inclusions in an FIA should have the same homogenization temperature. However, fluid inclusions presumed to have been trapped at the same time often show a variation in Th. The goal of this project is to identify FIAs in samples from different major geological environments and determine smallest range in homogenization temperature that might be expected under ideal conditions. Several factors might affect the range in Th within a n FIA. The natural temperatyre (and pressure) fluctuations during formation of an FIA will vary depending on the geologic environment, fluid inclusion size may affect the Th (Fig. 1), as well as sample collection and preparation, thermal gradients during Th measurements, etc. Also the factors that influence these variations during and after trapping of fluid inclusions in different environments will be evaluated. In this project we are examining samples of ore and gangue minerals from different types of ore deposit environments. These environments include hydrothermal MVT deposits, deeper magmatic hydrothermal systems, porphyry copper deposits, epithermal deposits, lode-gold deposits in metamorphic environments, and sedimentary (diagenetic) environments.

Relationship between fluid inclusion size and homogenization temperature in fluorite. Fluid inclusions in an FIA have approximately same liquid/vapor ratios, hence using bubble volumes to compare inclusions is appropriate.

Geochemical Evolution of the Campi Flegrei Volcanic System from Studies of Melt Inclusions

Melt inclusion (in the circle) hosted in an olivine crystal (CF-FR-C1 p6).

Silicate-melt inclusions are small droplets (1-several 100 um) of silicate melt entrapped in phenocryst minerals during their growth. 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. 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 system 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 to assess volcanic risks. 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. Campi Flegrei 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 heated inthe Vernadsky Stage in order to produce homogeneous glasses to be analyzed by Electron Microprobe (EMPA), Ion Microprobe (SIMS), Raman Spectroscopy, FTIR and Laser Ablation ICP-MS.

Experimental determination of H2O loss from melt inclusions during laboratory heating: Evidence from Raman spectroscopy

One of the most important assumptions for fluid or melt inclusion studies is that the samples have not gained or lost any material after trapping. However, several workers have suggested that water may be lost from melt inclusions (MI) during laboratory heating. To test this hypothesis, experiments were conducted to quantify H2O loss from MI during laboratory heating - the amount of water in the inclusions was monitored by Raman spectroscopy. Quartz-hosted MI from the early-erupted plinian stage of the Bishop Tuff were heated to 800 C and 1 kbar for 4 to 1512 hours (63 days). Previous studies had shown that unheated MI from this unit of the Bishop Tuff contain 4.8-6.5 wt % H2O. Because many Bishop Tuff MI fluoresce under visible (514 nm) Raman excitation, a method was developed to analyses silicate glasses and MI using an UV (244 nm) excitation source. The Bishop Tuff MI show insignificant H2O loss when heated for less than 12 hours, while up to 75% of the original H2O was lost after 1512 hours of heating. The rate of H2O loss decreases after a few hundred hours, suggesting either a change in H2O speciation or a change in the mechanism of H2O loss. Our results suggest that most silicic MI maintain their original H2O concentration if they are not heated for more than about 12 hours during laboratory studies.

Water content vs. time. Diamond is the average value for each data set, number in parentheses is number of inclusions in that data set, and line represents total range of values.

Determination of REE partitioning behavior between clinopyroxene, orthopyroxene, plagioclase, and silicate melt using melt inclusions from White Island, New Zealand

Clinopyroxene containing multiple "melt inclusion assemblages", containing melt inclusions, plagioclase inclusions, and orthopyroxene inclusions.

One of the most active research areas in igneous and mantle petrology involves the source regions for magmas emplaced into the crust. It is well known that magmas generated in different regions of the mantle and crust show distinct REE patterns. Thus, if the REE concentration of a melt that generated a given igneous rock can be determined, its source region may be inferred. One of the most commonly used methods to determine melt REE concentrations is to measure the REEs in rock minerals and, using "known" partition coefficients, the melt composition can be estimated. Partition coefficients have previously been estimated from both experimental studies and analysis of natural samples. However, many experimental studies involved either unrealistic growth rates or trace element concentrations, or both, or were done on bulk rock samples in which the relationship between phenocrysts and their host is uncertain. This project is examining partitioning behavior of REE and other trace elements between silicate melt and mineral using silicate melt inclusions and mineral inclusions trapped within the same "melt inclusion assemblage" in a host crystal. These contemporaneously trapped inclusions will be analyzed by electron microprobe and laser ablation inductively-coupled mass spectrometry (LA-ICP-MS), and the results will be used to calculate partition coefficients between silicate melt and mineral.

Melt inclusion assemblages: Definition and limits

Melt inclusions have proved to be a successful method of examining magma generation, eruptive styles, and monitors of volatile concentrations in magmas. However, there have been several questions raised about the validity of conclusions based on studies of isolated inclusions in separate crystals. Following the protocol developed for fluid inclusion studies, the melt inclusion assemblage (MIA) concept has been introduced, defined as a group of MI that were trapped contemporaneously. Assuming that the MI in an MIA were trapped at the same time, they should all have the same composition and homogenization temperature, assuming there has been no post-entrapment reequilibration. To test the MIA hypothesis, melt inclusion-bearing pyroxene and plagioclase from White Island, New Zealand are being analyzed using electron microprobe and laser-ablation inductively-coupled mass spectrometry to determine the consistency of data from individual MIAs.

Clinopyroxene containing multiple melt inclusion assemblages. Center of crystal is to the top of the image with growth zones evident by MIA.

Can we determine P-T formation conditions for fluid inclusions that homogenize by halite disappearance?

Fluid inclusions containing liquid, vapor, and a halite crystal that homogenize by halite disappearance are common in magmatic-hydrothermal systems, including porphyry-copper deposits. Previously, only limited experimental data for a composition of 40 weight % NaCl was available to determine the pressure inside such a fluid inclusion at final homogenization (halite dissolution). Using the synthetic fluid inclusion technique, inclusions that homogenize by halite dissapearance were synthesized under halite-saturated conditions at known pressure and temperature. The measured Th L-V (liquid-vapor homogenization) and Tm halite (halite dissappearance) were combined with known pressures to develop a relationship to estimate the minimum trapping pressure and salinity of the fluid. Comparison of experimental results to previous studies reveal an inconsistency between interpreted pressures and known geologic conditions for formation of porphyry systems. These results suggest that most inclusions which homogenize by halite disappearance are the result of either trapping of a solid halite along with the fluid, or reequilibration by necking down, or both.

Temporal and spatial distribution of fluid inclusion characteristics in the porphyry-epithermal environment

Geologic evidence for genetic link between deeper porphyry and shallower epithermal mineralization includes the Lepanto/Far Southeast system in the Phillipines and Red Mountain in Arizona. Geologic evidence and theoretical fluid flow models place reasonable constraints on the temporal and spatial variation of pressure and temperature in these systems. Experimental and theoretical studies of the H2O-NaCl system can be used to predict the fluid properties at the P-T conditions inferred from geological and exeprimental/theoretical studies. The purpose of this study is to combine these data to predict the evolution in fluid properties in time and space in the porphyry-epithermal environment, and to use thse results to predict the temporal and spatial evolution in fluid inclusion characteristics in this environment. These results, in turn, may be used in exploration to predict where in the overall magmatic-hydrothermal system a sample came from, and to provide vectors towards ore.

Oxygen isotope analysis of calcite using laser Raman spectroscopy.

Analysis of the stable oxygen isotopic composition of calcite has many applications in the geosciences. However, traditional techniques to determine d18O require a relatively large amount of sample, which results in an average analysis over a large area, and are destructive. Because 18O is heavier than the more common 16O isotope, 18O-C bonds vibrate at a different frequency then 16O-C bonds within the carbonate group in calcite. Being a vibrational spectroscopic technique, laser Raman analysis is able to resolve the vibrational energy of 18O-C from that of 16O-C bonds. By choosing an arbitrary homogeneous standard with a known d18O value, we can use the ratio of area or height for the 18O and 16O peaks to calculate the d18O value of the calcite at the scale of the size of the laser spot (a few microns), and at a theoretical precision of approximately +/- 1 per mil. The advantage of this technique is the ability to non-destructively analyze d18O values with a spatial resolution of a few microns, which is comparable to the scale of compositional/cathodoluminescent zones observed in many calcites in nature.

Do fluid inclusions in ore-stage gangue minerals record the composition of MVT ore fluids?

Gangue minerals (such as dolomite, calcite, and quartz) associated with Mississippi Valley Type (MVT) base metal deposits are commonly used to study fluid sources, composition and temperature of ore-forming fluids. It is reasonable to assume that primary fluid inclusions hosted in ore-stage gangue minerals which precipitated in the presence of metal-bearing solutions would record the concentrations of metals in this solution. However, previous studies comparing microthermometric data from coeval gangue and ore minerals using visible and infrared microscopy showed distinct and unexpected differences between the two. The purpose of this project is to determine ore-metal contents of fluid inclusions in gangue minerals associated with ore, and to compare these with calculated metal concentrations based on known solubility relationships. Individual fluid inclusions will be analyzed by laser ablation ICP-MS. The results of this study will help us to better undestand which gangue minerals to study to determine sources for ore-forming fluids. These results will also allow us to test the hypothesis that these deposits require at least two separate fluids, one to transport sulfur and another to transport metals. An absence of metals in gangue mineral fluids would not preclude gangue minerals from representing at least one component of the ore-forming solution.