Geochemistry and Geology

Trace element geochemistry, like the entire field of geochemistry, has many applications within the earth sciences, such as understanding the origin and evolution of our planet, understanding rock formation, undertaking provenance studies, constructing geodynamic models, understanding mineralizing processes, predicting volcanic eruptions, studying magma dynamics, determining paleo-ocean conditions, studying soil formation processes, and many more.

X-Ray Fluorescence (XRF) is ideally suited for determining elements in various materials, such as metals, slags and glasses, but also soil, aqueous solutions and air. Elements from boron to uranium can be analyzed with high accuracy, precision and reliability. XRF is a non-destructive analytical technique to determine the chemistry of a sample by measuring the fluorescent (or secondary) X-ray emitted from a sample when it is excited by a primary X-ray source. Each of the elements present in a sample produces a set of characteristic fluorescent X-rays ("a fingerprint") that is unique for that element, making XRF spectroscopy an excellent technology for qualitative and quantitative analysis of material composition.

Another powerful technique is based on inductively coupled plasma (ICP). ICP is a plasma, usually argon, that is partially ionized. It is a very powerful ionization technique and can be combined with various mass spectrometers. ICP-MS is widely used in geochemical applications for performing accurate and reliable quantification of inorganic trace elements in ppm to ppq concentration levels. Combined with laser ablation, ICP enables scientists to perform high spatial resolution analysis. Additionally, coupling ICP with gas chromatography enables speciation studies of elements, such as S, Hg, Pb and Mg.

• ARL OPTIM'X WDXRF Spectrometer
• ARL QUANT'X EDXRF Spectrometer
• Advanced ARL PERFORM'X WDXRF Spectrometer
• Element Series ICP-MS
• iCAP PRO XPS ICP-OES
• Niton XL5 Plus Handheld XRF Analyzer

For more than 150 years – since the advent of light microscopes and petrographic thin sections – geoscientists have been examining and describing rocks. Today, the goal is still to unravel geologic history, including environmental conditions and climate, by making detailed observations of a rock's mineralogy and microstructure.

To this end, geoscientists work on quantification of modal mineralogy (major, minor, trace), grain size estimations of key components, associations between minerals and grains, and various microstructural features that may be present such as fractures, veins, and different types and styles of alteration and weathering.

Electron microscopy provides images containing a wealth of information about the texture and mineral composition of a rock. These images are used by petrologists to augment conventional optical petrographic observations, as well as provide guides for further investigations of specific areas of a sample. Automated data collection and sample holders that can accommodate multiple thin sections and polished blocks, enable highly efficient mineral analysis, with the ability to process several hundred points per second and resultant images typically containing millions of pixels.

Raman spectroscopy is also an established analytical technique for the analysis of geological samples. It not only provides a fast and efficient way of identifying specific materials, but also considerable information on molecular structure and chemical environments. The easy-to-use Thermo Scientific DXR3xi Raman Imaging Microscope and Thermo Scientific OMNICxi Software provide a powerful, research-grade platform to explore geological history.

• Phenom XL G2 Desktop SEM
• DXR3xi Raman Imaging Microscope
• Quattro ESEM

Minerals can form in all geologic environments under a wide range of chemical and physical conditions. They can crystallize from an igneous melt, result from solid state re-crystallization of pre-existing minerals due to changes in temperature and pressure in metamorphic environments or precipitate from ions dissolved in aqueous (surface or hydrothermal) solutions. Each mineral is defined by its unique crystal structure. As a result, looking at the crystal structure of a mineral can tell you a lot about its purity and other properties. Accurate textural analysis and the associated distribution of minerals within the rock texture are key to accurately describing the physical and chemical aspects of a rock system.

Structural analysis of minerals is fundamental to the manufacturing processes of diverse products including metals, cement, ceramics, glasses, chemicals, petrochemicals, semiconductors, and energy. Mineral extraction and its processes have a significant impact on the environment, energy consumption and safety. With ever-increasing demand for high-grade mineral sources, improved productivity and added value is key to meeting that demand while also meeting environmental and quality standards.

X-ray diffraction (XRD) and complementary scanning electron microscopy (SEM) are the preferred techniques used to determine the structure of minerals. This structural determination can help identify and quantify toxic or undesirable elements or compounds that may adversely affect the final product or the environment. The wide-ranging quantification capabilities of Thermo Scientific XRD instruments enable researchers to quantify these elements and compounds with or without reference materials. Additionally, our easy-to-use, versatile SEM imaging platforms, combined with MAPS Mineralogy Software, provide highly accurate EDS-based mineral identification.

• ARL EQUINOX 100 X-ray Diffractometer
• Element Series ICP-MS
• Quattro ESEM
• Apreo SEM for Materials Science
• MAPS Mineralogy Software

In addition to using radiogenic isotopes for absolute age determinations, geoscientists can use them to trace the origin of a component. They make use of the fact that isotope ratios can act as fingerprints that constrain the origin of the studied object. Variations in the abundance of isotopes such as ¹⁴³Nd and ⁸⁷Sr can be used to unravel the origin of a material. A material can be anything from ancient human teeth to an oceanic water mass to rocks and fluids. 

As variations in isotope abundances are very small, highly sensitive and accurate mass spectrometers are required for reliable results. Traditionally, radiogenic isotope ratio analysis is performed by thermal ionization mass spectrometry, a very powerful and accurate technique. However, over recent decades, ICP-based instrumentation has also been developed for this purpose. The advantage of an ICP ionization source that it can ionize almost all elements because the argon plasma has a very high ionization efficiency.

• Triton Series Multicollector Thermal Ionization MS
• Neoma Multicollector ICP-MS

Non-traditional stable isotopes, i.e., the stable isotopes, excluding H, C, N, O and S, are a growing field of research. The development of multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS) at the end of the 1990s enabled non-traditional stable isotope geochemistry to flourish. Light stable isotopes, such as B, Li, Mg, Fe, Ca, Cr, as well as heavy stable isotopes, such as Tl, U, are now routinely measured at a precision that is high enough to resolve natural variations.

Non-traditional stable isotopes have several distinctive geochemical features compared to traditional stable isotopes. Many of these elements are trace elements, redox-sensitive, biologically active and range from highly volatile to refractory (e.g., Ca and Ti). These features, together with the fact that many of them have high atomic numbers with more than two stable isotopes, make the different elements susceptible to different fractionation mechanisms. As such, these non-traditional stable isotopes are unique tracers of different cosmochemical, geological and biological processes.

• Neoma Multicollector ICP-MS
• Triton Series Multicollector Thermal Ionization Mass Spectrometer

In addition to using carbon and oxygen isotopes for paleoclimate research, geoscientists use stable isotopes for a wide variety of applications. Understanding isotopic fingerprints and the sources of fractionation that lead to stable isotope ratio variations can help address a diverse array of questions ranging from ecology and hydrology to geochemistry.

Stable isotope ratio analysis is usually performed by gas isotope ratio mass spectrometry (gas IRMS), where samples are introduced to the mass spectrometer as pure gases. This is achieved through combustion, gas chromatography or chemical trapping. The detected isotope ratios are compared to those of a measured standard, enabling accurate determination of the isotope ratio of the sample. Generally, samples are combusted or pyrolyzed and the desired gas species (e.g., H₂, N₂, CO₂ or SO₂) is purified by means of traps, filters, catalysts and/or chromatography.

• EA IsoLink IRMS System
• Neoma Multicollector ICP-MS
• GC IsoLink II IRMS System

One of the major challenges in geosciences is the analysis of small ion beams. Analysis of ion beams is of particular interest for studies that focus on isotope composition of scarce materials (e.g., dust in ice cores, inclusions in diamonds, components of extraterrestrial material), or materials that have ultra-low concentration of the isotopes of interest (Hf in depleted peridotite, Re and Os in silicate rocks), but also in studies that aim to resolve isotopic variability on a small spatial scale (growth zones in minerals, teeth or hairs). Ultimately, the analytical precision of such studies is limited by the detection system of the mass spectrometer. 

To address this challenge, Thermo Fisher Scientific developed 10¹³ Ω amplifiers, part of a revolutionary new resistor design that enables fast response times and extremely low signal to noise.

• Triton Series Multicollector Thermal Ionization Mass Spectrometer
• Neoma Multicollector ICP-MS
• Helix MC Plus

The basics

When analyzing small ion beams using multicollection mass spectrometry, precision is fundamentally limited by:

1. The error introduced by counting statistics
2. The electrical Johnson-Nyquist noise of the resistor used in the multiplier’s feedback loop.

From the Johnson-Nyquist noise equation, it follows that if the resistor value (and thus the output V) is increased by a factor of 100, as is the case when a 10¹³ Ω amplifier is used, noise will increase by only the square root of 100. As a result, when compared to a default 10¹¹ Ω amplifier, the signal-to-noise ratio is expected to improve by a factor of 10.

The new amplifiers open the door to more applications that were not previously explored as a result of the limited detection technology. With a factor of 4-5 better precision compared to the 10¹¹ Ω current amplifiers, scientists can now analyze sample sizes smaller than ever before. These samples include those with large isotope variations, such as melt inclusions, extraterrestrial material, dust inclusions in ice cores and nuclear safeguards. In these samples, variations can be large, but material is usually very scarce. The large variations make precision in the tens to a couple hundred ppm a requirement.