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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- Rock:
Isotopes are atoms of the same element that have different numbers of neutrons. An isotopic analysis looks at a particular isotopic element(s) in a given system, while the conditions which increase/decrease the number of neutrons are well understood and measurable.
Other definitions:Wikipedia Reegle

Photo of a Nu plasma Multicollector ICP-MS system used for isotopic analysis. Photo from Vervoot & Mueller (2012).[1]

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.

Use in Geothermal Exploration
The hydrothermal fluids circulating in a geothermal system have a unique signature that can be used to determine where that water came from, how old it is, whether it has mixed with other fluids, and which direction it is moving in. Fractionating elements (H, C, O, and N) are useful for identifying recharge, water/rock ratios, and chemical equilibrium temperatures.[2] Chemical interaction with host rocks will impart a unique isotopic signature similar to that of the reservoir fluid. Isotope ratios in hydrothermal alteration and vein minerals can yield important information regarding modern and fossil hydrothermal systems.

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.[3] 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%.[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.

Field Procedures
Rock samples are collected in the field at the surface or as core cuttings from depth. The Rock Sampling technique page should be consulted for specific details regarding standard rock sampling procedures.

Physical Properties
Variations in bond strength result in mass-dependent isotopic fractionations as phase changes, fluid-rock reactions, and mineral precipitation occur in response to variations in temperature, pressure, and fluid chemistry.[4] The effects of fractionation are more pronounced for lighter elements, in which the mass differences between different isotopes of the same element are relatively large. Different compounds in equilibrium with one another will record different isotopic ratios as a result of mass-dependent fractionation, and the magnitude of these differences depends strongly on temperature. [4] These characteristics provide researchers with useful tools for interpreting isotope data from geothermal systems, based on the flux of different isotopes through various natural geochemical reservoirs. Oxygen has three naturally occurring stable isotopes. 16O is by far the most abundant, followed by 18O, and finally 17O. Measurement of oxygen isotope ratios can be used for a variety of applications. In geothermal exploration and reservoir evaluation, 18O:16O ratios are used to determine origins of fluids (meteoric or magmatic), the degree of mixing with other hydrothermal fluids, distinguish fluid interaction events that caused hydrothermal alteration of host rocks, and constrain the thermal history of the system.

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.

Data Access and Acquisition
Photo of the plasma sampler from an ICP-MS system used by the Atomic Energy and Alternative Energies Commission (CEA), France. Photo from the CEA Website, last updated October 2, 2011.

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.

Schematic diagram of an ICP-MS system with metal speciation capability. Figure 1 from Caruso & Montes-Bayon (2003).[5]

Best Practices
The applications of stable isotope geochemistry span all fields of the geosciences. Understanding of the fluxes of isotopes through different geochemical reservoirs is used to study so many geological processes.[4] Based on these characteristics, and on the strong dependence of isotopic fractionation on temperature, stable isotopes can be used as a geochemical tracer in natural systems. The isotopic ratio of an element in rocks or other material can record the movement of elements through different geochemical reservoirs. If equilibrium conditions can be assumed, stable isotope ratios of two substances can also be used to approximate the temperature of exchange.
Potential Pitfalls
Limitations of isotopic data and pitfalls in data interpretation are specific to the type of isotope data being considered. A qualified geochronologist, geochemist, or geologist with sufficient background in isotope geochemistry should be consulted when interpreting isotope data.

Additional References

Page Area Activity Start Date Activity End Date Reference Material
Isotopic Analysis At Florida Mountains Area (Brookins, 1982) Florida Mountains Area

Isotopic Analysis At Geysers Area (Lambert & Epstein, 1992) Geysers Area

Isotopic Analysis At Newberry Caldera Area (Goles & Lambert, 1990) Newberry Caldera Area

Isotopic Analysis At San Juan Volcanic Field Area (Larson & Jr, 1986) San Juan Volcanic Field Area

Isotopic Analysis At Seven Mile Hole Area (Larson, Et Al., 2009) Seven Mile Hole Area

Isotopic Analysis At U.S. West Region (Krohn, Et Al., 1993) U.S. West Region

Isotopic Analysis At Valles Caldera - Redondo Geothermal Area (Phillips, 2004) Valles Caldera - Redondo Geothermal Area 2004

Isotopic Analysis At Zuni Mountains Nm Area (Brookins, 1982) Zuni Mountains Nm Area

Isotopic Analysis- Rock At Coso Geothermal Area (1984) Coso Geothermal Area 1984 1984

Isotopic Analysis- Rock At Coso Geothermal Area (1997) Coso Geothermal Area 1997 1997

Isotopic Analysis- Rock At Kilauea East Rift Geothermal Area (Quane, Et Al., 2003) Kilauea East Rift Geothermal Area 1989 2000

Isotopic Analysis- Rock At Valles Caldera - Sulphur Springs Area (Ito & Tanaka, 1995) Valles Caldera - Sulphur Springs Area

Isotopic Analysis- Rock At Valles Caldera - Sulphur Springs Geothermal Area (Ito & Tanaka, 1995) Valles Caldera - Sulphur Springs Geothermal Area 1995

Isotopic Analysis- Rock At Valles Caldera - Sulphur Springs Geothermal Area (Musgrave, Et Al., 1989) Valles Caldera - Sulphur Springs Geothermal Area 1989

Isotopic Analysis- Rock At Valles Caldera - Sulphur Springs Geothermal Area (Phillips, 2004) Valles Caldera - Sulphur Springs Geothermal Area 2004

Isotopic Analysis- Rock At Valles Caldera - Sulphur Springs Geothermal Area (WoldeGabriel & Goff, 1992) Valles Caldera - Sulphur Springs Geothermal Area 1992

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