Isotopic Analysis- Rock
Exploration Technique: Isotopic Analysis- Rock
|Exploration Technique Information|
|Exploration Group:||Lab Analysis Techniques|
|Exploration Sub Group:||Rock Lab Analysis|
|Parent Exploration Technique:||Rock Lab Analysis|
|Information Provided by Technique|
|Lithology:||Water rock interaction|
Isotopic analysis is typically conducted by hydrologists, biologists, and geochemists. There are many different isotopes, but they all generally fall into two categories; radioactive or stable. Radioactive isotopes have known decay rates and half-lives, which are very useful for dating particular fluids or materials. Stable isotopes do not decay and are used to measure the ratios of the heavy isotope vs light isotope to reveal general conditions that would lead to this heavy/light isotope ratio. There are many applications of isotope geochemistry, some which have been utilized for geothermal exploration. Isotopic analysis can be used to investigate the thermal history of a reservoir, to determine the degree of water-rock interaction that has occurred in a system, and to date hydrothermal alteration minerals.
At The Geysers vapor-dominated hydrothermal system in California, quartz and calcite veins cutting reservoir rocks showed Δ18O values that record the temperatures and isotopic compositions of fluids present during at least two distinct episodes of rock-fluid interaction. The first episode was recorded by veins of quartz and calcite cutting the host rock that showed Δ18O values around +19 and +16%, respectively. The Δ18O values of greenstones metamorphosed from spilitic basalts during (post-Cretaceous?) burial showed a similar isotopic shift. The D/H ratios of actinolite, chlorite, and micas in host rocks were also strongly altered during this episode. These isotopic results suggest that the vein and alteration minerals formed through interaction of marine silica and carbonate with ocean water entrapped in sediments at about 200°C. The Δ18O-depth distributions of vein minerals was also used to deduce that a paleogeothermal gradient of about 53°C/km existed during the first episode of fluid-rock interaction.
The second episode was recorded by vein quartz with Δ18O values of +4 to +6% and cogenetic vein calcite with Δ18O values of +1 to +3%. This episode began in response to the start of the Pliocene-Pleistocene Clear Lake magmatism, during which time large quantities of meteoric water with temperatures of 160-180°C circulated through fractured host rocks. As temperatures rose and circulation of fluids was restricted, the ancestral hot-water system evolved into the existing active vapor-dominated system. According to the Δ18O values of cogenetic vein quartz and calcite, the modern geothermal system has experienced temperatures as high as 320°C.
- Lab Analysis Techniques
- Fluid Lab Analysis
- Rock Lab Analysis
The two stable isotopes of carbon commonly measured in isotope geochemistry are 12C and 13C. The 13C/12C ratio is used as an indicator of paleoclimate, and records the amount of fractionation that occurs through photosynthetic activity in plants. The radioactive isotope of carbon is 14C, and is routinely used in dating of organic materials. Carbon-14 has a half-life of 5,730 +/- 40 years, and decays to nitrogen-14.
Hydrogen has three naturally occurring isotopes. 1H is the most abundant, accounting for 99.98% of the Earth’s hydrogen, and has a nucleus that consists of a single proton. 2H, also known as deuterium, is a comparatively rare stable isotope, comprising 0.0026-0.0184% (by population) of hydrogen samples on Earth. The lower end of this range is typically encountered in samples of hydrogen gas, whereas ocean water usually shows higher levels of enrichment. The radioactive isotope of hydrogen encountered in nature is 3H, also known as tritium.
Following collection, isotope data of a rock, liquid, or gas sample is analyzed using a mass spectrometer. A mass spectrometer is a device that is able to measure the composition of a sample and the mass-to-charge ratio of a particular element or molecule. First a sample is ionized, converting a fraction of the original sample into ions (charged particles)- there are numerous methods used for this process. The ions are then separated by the mass-to-charge ratio and measured by a sensitive instrument that is capable of detecting varying particle charges by magnetic sector, quadrupole, or time of flight. Finally the data is processed and plotted onto a spectra which displays the masses of the particles of that sample.
- Multicollector-Inductively Coupled Plasma Mass Spectrometer (MC-ICPMS)
- Geothermal Energy Systems: Exploration, Development, and Utilization
- Stable-Isotope Studies of Rocks and Secondary Minerals in a Vapor-Dominated Hydrothermal System at the Geysers, Sonoma County, California
- Gas Source Mass Spectrometry: Stable Isotope Geochemistry
- Elemental Speciation Studies—New Directions for Trace Metal Analysis
- Florida Mountains Area
- Geysers Area
- Newberry Caldera Area
- San Juan Volcanic Field Area
- Seven Mile Hole Area
- U.S. West Region
- Valles Caldera - Redondo Geothermal Area
- Zuni Mountains Nm Area
- Coso Geothermal Area
- Kilauea East Rift Geothermal Area
- Valles Caldera - Sulphur Springs Area
- Valles Caldera - Sulphur Springs Geothermal Area