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Hematite for Deciphering Past Environments and Climates

Hematite for Deciphering Past Environments and Climates

Editors VoxThis blog is by AGUs Publications Department

Magnetic minerals, mainly iron oxides and some sulfides of iron, preserve a record about the ancient magnetic field and can provide a wealth of geoscientific data. Hematite (ferric Iron Oxide) is a common magnet mineral on Earth and Mars. It not only registers the paleomagnetic fields, but also provides information about ancient climates. The ability to reconstruct the monsoon from well-dated terrestrial hematite allows for detailed reconstructions. Hematite occurrences on Mars are often associated with the former presence water. Therefore, hematite-bearing zones are key to the search for ancient life on Mars.

A Recent articleIn Geophysics ReviewsThis article describes the color and magnetic properties of terrestrial Hematite. These properties are crucial in identifying and interpreting signals due to hematite. The authors provide an overview of the importance and impact of hematite in Earth and Mars.

What is hematite? And where can it be found most often?

HematiteYou can find more information at-Fe2O3) is an iron oxide that derives its name from the Greek haimatiteThe name hematite means “blood-like” and refers to its distinctive red color. Hematite can be found on Mars, Earth, Mars, and some asteroids. Hematite is found in a variety of soils and sediments that are aerobic tropical and subtropical. It is also known as red beds, which refers to its ability to reflect warm and humid climates.

Image of hematite pigment showing its distinctive red color. Credit: Dr. Xiang Zhao

Hematite can also be found in Archean or Paleoproterozoic (from 1.6 billion to 1.6 million years ago) sedimentary bands iron formations that document the evolution of Earth’s early atmosphere.

Hematite is also the dominant pigment in oceanic Red Beds, which are marine sedimentary rocks that have been deposited far from the coast and document global oceanic, climate, and other changes in the Cretaceous greenhouse age.

High pressure and temperature tests on hematite, and its polymorphs, suggest that they can be dominant magnet signal carriers down to depths 600 km (cold portions) of subducted slabs in conditions where the archetypical terrestrial magnet mineral, magnetite (Fe), is present.3O4), has decomposed thermally.

Hematite, the most common pigmenting minerals on Mars’ surface, is responsible for the nickname “The Red Planet”. There are three types of hematite found on Mars: nanophase, red-crystalline, and gray-crystalline hematite. The brighter regions of Mars are dominated by the reddish hue of the former two.

Hematite-rich environments are found on Mars and Earth. Credit: Jiang et al. [2021]Figure 1

How can hematite help us understand the time-varying paleoclimates?

Many geologic processes control the formation of hematite. Hematite is, for example, more abundant in tropical and subtropical soils where there are frequent dry episodes. Variations in hematite may be indicative of paleoclimate changes.

Hematite found in some marine sediments is also transported inland by wind (e.g. the monsoon and westerlies). Stronger winds carry more dust into the oceans and thus more hematite. The hematite in wind-blown sediments can be used to track the evolution of monsoon systems and westerlies.

Also, soils with a low organic content and a pH near neutral tend to favor hematite over goethite formation (a-FeOOH). The hematite-goethite ratio (Hm/Gt), if either phase is not dissolved, can provide important soil moisture information that can be used to monitor climate change.

Hematite abundance is therefore an important proxy for studies of environmental and geologic processes.

What is cation replacement and how does it affect the physical characteristics of hematite.

Comparison of hematite/goethite proportions and marine D18O over different time scales. Credit: Jiang et al. [2021]Figure 26

Cations other than Fe3+(e.g. Al3+Ti4+Mn2+) are always present to some extent in natural hematite. They can be easily incorporated into the hematite crystallitice by substituting Fe.

Al-substituted Hematite, for example, is common in Al-rich subtropical and tropical soils, such as Brazil and South Africa. Al is often associated with soil origin in humid and warm environments because it is easily incorporated into iron oxides by chemical weathering.

Fe3+Al3+Different ionic radius (Fe)3+: 0.65; Al3+: 0.53), so the symmetrical hexagonal octahedral Hematite structure is distorted when Al substitution takes place, which increases internal strain. Al-hematite particle sizes are also smaller. These differences result in Al-substituted Hematite having magnetic properties that differ from unsubstituted.

Non-magnetic Alions are randomly incorporated into the hematite crystal lattice. This reduces the magnetism of the hematite with increasing Al content.cThe magnetic moment of a specimen in a low, Earthlike, magnetic field.MrsThe maximum possible permanent magnet moment after exposure to high magnetic fields) decreases. Furthermore, as Al content increases, the color of Hematite is lighter red. As a result, the characteristic peak position of color reflectance spectra and amplitude of color reflection spectra are affected.

These properties are critical for identifying hematite as well as its cation-substituted counterparts in Earth and Mars.

Trends in color and magnetic parameters Compare Al substitution in hematite. Credit: Jiang et al. [2021]Figure 13 and 16

What is remagnetization, and how does it impact the paleomagnetic record

The natural remanent magnetization (NRM) of a remagnetized stone is of a younger age than the host rock. Remagnetization can complicate the interpretation of paleomagnetic data for rocks, soils and sediments.

Remagnetization mechanisms typically involve one of the following: 1) Magnetic mineral transformations associated to redox processes, i.e. formation of magnetite (Fe3O4), greigite (Fe3S4), or pyrrhotite (Fe7S8); 2) deformation-associated fluid migration and/or pressure solution; 3) chemical weathering in moist, tropical environments; or 4) the resetting of existing NRM by prolonged exposure to a stable geomagnetic field at moderately elevated temperatures (~100-250 C), so called thermoviscous remanent magnetization acquisition.

Secondary NRM acquired through any of these mechanisms can obscure the primary paleomagnetic records and could lead to incorrect interpretations of paleomagnetic data if it is not recognized. To be able to distinguish between primary remanence and secondary remanence is crucial for paleomagnetic research. Hematite has been implicated in widespread remagnetizations, particularly in red beds.

How can terrestrial hematite help us better understand Mars’ geology?

It is crucial to understand the origin of Martian Hematite in order to unravel its geologic history. Also, it will help answer the question whether liquid water existed on Mars. Perhaps hematite will even reveal its climatic past.

Martian samples, which are essentially only from Martian meteorites, are very rare. This makes it difficult to examine the formation of hematite in Mars. Therefore, we must rely on Earth analogs for now.

Hematite is formed by many mechanisms, most of them involving water. It is important to understand the hematite formation pathway in order to interpret information carried by hematite regarding climate, environment, and tectonics as well as planetary evolution.

Although there is no clear path for hematite formation on Mars, it is possible to identify the origin of crystalline Hematite. This can give chemical clues about the Martian environment in the early days of life, particularly regarding the existence of (liquid), water, which is an essential requirement for evolution and survival.

Schematic illustration showing the formation and transformation pathways for common iron oxides. Credit: Jiang et al. [2021]Figure 3

What are some of those unresolved problems that need additional research, data, modeling, or analysis?

Although terrestrial meteorite has been extensively studied, it is difficult for Mars scientists to examine hematite on Mars. To interpret Martian hematite, systematic studies of the properties terrestrial hematite must be done. However, hematite’s properties are complex and depend on the formation conditions.

The key questions are: How similar is hematite on Mars and Earth? Is Mars’ hematite cation-doped or not? How can we quantify the cation content? This could be done by color reflectance measurements. These questions can be answered by a solid database of hematite properties on Earth, Mars, and a thorough understanding of its formation processes.

For future studies of Martian rocks or soils, integrated analysis should provide valuable reference data. Comparing Mars’ hematite with Al-substituted terrestrial hematite may help to determine if the hematite is also Al-substituted. This will provide ground-truthing necessary for remote Martian surface observations.

Zhaoxia Jiang ([email protected], 0000-0003-0860-9753), Ocean University of China; and Qingsong Liu, Southern University of Science and Technology, China

Citation:Jiang, Z., and Q. Liu (2022), Using hematite for past climates and environments to decipherEos, 103, Published on [DAY MONTH] 2022.
This article is not intended to represent the opinions of AGU.Eos,or any of its affiliates. It is the author’s opinion.
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