More on Stable Oxygen Isotopes

The geochemistry of rocks is a study that brings many great tools to the geologist’s tool chest. In the last article from October 2020, “Oxygen Isotope Analysis in Paleoclimatology,” I addressed an aspect of the geochemistry of the ocean and meteoric water on earth (meteoric water being rainwater, snow, water vapor in clouds, etc.) in the form of oxygen isotope distribution. This is a sort of shell game using stable isotopes of oxygen and how they are preferentially distributed in the waters of the earth. I will in this article explore the distribution of oxygen isotopes in rocks of the earth. These types of analyses are commonly used in many aspects of geological research and the reader will undoubtedly run across them in academic papers.

This article is not intended as a mathematical derivation of the equations used in isotope fractionation studies. (Recall that in the October 2020 article the term fractionation was described as a process of concentrating certain types of matter, in this case isotopes, in response to a phase change.) Instead, I will discuss in qualitative terms the principals that dictate how stable isotopes of oxygen distribute themselves in the rocks of the earth, and the resulting ranges of the isotopic ratios one expects to find in the rocks of the world.

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The stable oxygen isotopes were introduced in the October 2020 article and the two studied most often are heavy oxygen, ¹⁸O, and light oxygen, ¹⁶O. The one equation I’ve used is one that defines a comparative ratio of the amount of heavy oxygen, ¹⁸O, in the study substance to the amount of ¹⁸O found in seawater during “normal” climatic conditions:

And, in paleoclimatic studies as discussed in the October 2020 article, it has been determined that δ¹⁸O values are 0 for “normal” seawater, and vary at a ball park range between -5 and +5 ‰ for meteoric water vs. glacially depleted ocean water. Also, the driving force behind the fractionation of oxygen isotopes in water is that the ¹⁸O isotope forms tighter bonds in water molecules than their ¹⁶O counterparts and so are more difficult to vaporize into the atmosphere.

The same principle is true in rocks with oxygen bonds. Quartz is the perfect example and the bonds between silicon (Si) and oxygen in this pure silicate compound (SiO₂ lattice) are strong covalent bonds. In fact, quartz is the mineral with the highest preference for heavy oxygen (8.2 ‰) because the heavy oxygen forms such strong bonds in this substance. Metals tend to form less strong bonds with oxygen than silicon, so minerals with metal-oxygen bonds are less rich in heavy oxygen. Some other values of δ¹⁸O for common rock minerals are 7.5‰ for albite and potassium feldspar, 6.6 ‰ for anorthite (Ca, Na feldspar), 6.4 ‰ for zircon, 6.3 ‰ for pyroxene graduating down in amphibole, biotite, garnet, and olivine to 6.1 ‰, 4.9 ‰ for ilmenite and 3.5 ‰ for magnetite. So we see that felsic minerals (i.e., quartz and feldspars) found in granites and rhyolites are higher in heavy oxygen than mafic minerals (ones containing iron and magnesium) which are abundant in the earth’s mantle.

Figure 1. Oxygen isotope variations in magmas and rocks on Earth.

“Gray band between 5.5 and 5.9 represents “normal- δ18O magmas” for ultramafic and basaltic rocks that constitute predominant mantle and products of is melting; closed system differentiation of such magma into andesite and rhyolite that show normal δ18O values of 5.8 to 6.5 ‰ ... High- δ18O rocks and magmas result from low-temperature precipitation or interaction with sea water... Low- δ18O rocks and magmas result from high-temperature interaction with, or subsequent remelting of, materials that interacted with meteoric waters at high-temperatures.... Data are from both original sources and compilations by Muehlenbachs (1998); Eiler (2001), Valley et al. (2005); Hoefs (2005); Sharp (2006). Selected areas emphasized in this review are shown as open boxes.” Ilya Bindeman, 2008.

The bar graph in Figure 1, reproduced from the Bindeman 2008 article, shows the measured ranges in various categories of rocks. At the top of the figure we see δ¹⁸O ratios for mantle rocks lies between the gray lines of 5.5 to 5.9 ‰. The first two rock types typify some mantle minerals and exhibit this range of values. The next category, basalt, has some variations but the MORB (mid-ocean ridge basalt) and the oceanic IAB (island arc basalt) fall very close to the mantle range, their source of material.

In both mantle and arc derived rocks, the ranges of δ¹⁸O become more variant when the magma source is mixed with felsic continental rocks or hydrated rocks. Due to their own amounts of ¹⁸O, these substances can either increase the δ¹⁸O value (adding seawater, continental crust) or decrease it (with rocks hydrothermally altered by meteoric water). As we move to continental magmas in the andesite and rhyolite categories, we see that most of these exhibit δ¹⁸O values above those of mantle-derived rocks, since they contain abundant quartz and feldspar minerals with a higher δ¹⁸O ratio than the mantle constituents. However, meteoric water in the mix can act to reduce the δ¹⁸O values. The Yellowstone rhyolites have a very low (but still positive) δ¹⁸O value due to the mixing of the magma with meteoric hydrothermal water.

Sedimentary rocks have even more complex sources of δ¹⁸O. Since many sedimentary rocks found on continents were formed in the oceans, we need to look to the source of much of this sediment. Diatoms construct silica shells with very high δ¹⁸O values of 30+ ‰ in their cold oceanic habitats. These tiny creatures are abundant in the ocean and when they die, their shells rain down on the ocean floor creating deep sea sediments of chert. Carbonates formed from coral reefs are likewise high in ¹⁸O. This greatly increases the δ¹⁸O values of the sediments that contain them. This explains the high δ¹⁸O values in the sedimentary and metamorphic rock sections.

Stable isotope geochemistry has been used extensively in the studies of magma and ultimately, continental crust and mantle evolution. These studies have developed over the last 50 years, and until the new millennium, were performed on larger, whole rock sample sizes. Nowadays the sampling process can pinpoint tiny areas on single crystals using laser technology, providing a revolution in accuracy of the measurements. For example, phenocrysts in a magma melt may be quite different in age and creation environment than the liquid melt, and so would yield different results and skew the results found in whole rock analysis. Durable crystals such as zircon can have growth rings and different crystallization environments on a single crystal, necessitating the use of pinpoint accuracy for correct analysis.

I hope that this article helps you to feel a bit more comfortable with stable isotope geochemistry. I have concentrated on the element oxygen because it is the most abundant element in the earth’s crust and atmosphere, and is most extensively used in geochemistry studies. Other traditionally studied elements with multiple stable isotopes include carbon (C), nitrogen (N), hydrogen (H), sulfur (S), lithium (Li), and boron (B), and added to that are newer iron (Fe), molybdenum (Mo), and copper (Cu) stable isotope systems.

This is the second article in the geochemistry series. I’ve got one more in the works for zircon geochronology, or the age dating of zircon crystals. These articles are intended to give our readers a bit of background in the technology that is driving modern geological research.

References

Ilya Bindeman, “Oxygen Isotopes in Mantle and Crustal Magmas as Revealed by Single Crystal Analysis,” Reviews in Mineralogy & Geochemistry, Vol. 69 pp. 445-478, Mineralogical Society of America, 2008. I used the introductory parts of this article to get most of the technical info found in the GSOC article above. The author goes on to discuss stable and unstable equilibrium in magma melts in the meat of the article which gets fairly technical. There is also more info on laboratory techniques.

J. W. Valley, J. S. Lackey, A. J. Cavosie, C. C. Clechenko, M. J. Spicuzza, M. A. S. Basei, I. N. Bindeman, V. P. Ferreira, A. N. Sial, E. M. King, W. H. Peck, A. K. Sinha, C. S. Wei, “4.4 billion years of crustal maturation: oxygen isotope ratios of magmatic zircon,” Contributions in Mineral Petrology, (2005), DOI 10.1007/s00410-005-0025-8. I haven’t completed reading this article, but a key component of its analysis is oxygen isotope research. It discusses earth’s mantle development and continental growth of the early earth. John Valley is a leading expert in the field of geochemistry and is a professor at the University of Wisconsin.

Quartz lattice illustration on the GSOC home page: reference By Andel - Own work, CC0, https://commons.wikimedia.org/w/index.php?curid=79753303

Recommended Videos

There aren’t any cartoonish, lite-technical explanatory videos that I could find specifically for this topic. The reader may want to refer to the videos about stable isotope fractionation referenced in the October 2020 article for some intro concepts.

Ann Bauer, University of Wisconsin, “Chemical Crustal Evolution & Oldest Crust,” PCE3 Prebiotic Chemistry YouTube Channel, October 10, 2020. This is a great educational lecture about the crustal evolution of the earth which addresses the role of the evolution of granitic rocks as a proxy to the existence of surface water, the significance of the oxygen isotope composition of the Jack Hills zircons, the evolution of the chemical composition of mantle and crustal melts, models for crustal growth over earth’s history, crustal composition over time, the inception of plate tectonics, and plate tectonic models of the early earth. Some knowledge of chemistry and isotope analysis is needed, but her explanations are excellent and help the reader get more comfortable with some of the momentous topics she is covering. This type of research is at the heart of modern geology. 48 minutes.

W. M. White, “Introduction to the stable isotope Lecture,” Feb 1, 2016, 38 minutes, and “Stable Isotopes fractionation and use in geosciences,” GeoOceanology YouTube channel, Oct 14, 2016, 61 minutes. These lectures explore the technical details of isotope fractionation geochemistry. Not for the casual reader, but possibly helpful if you’re planning to delve into some of the literature on geochemistry. This author, a professor of geology at Cornell University, has a leading textbook on the subject as well.

John Eiler, Geological and Planetary Sciences, Caltech professor of geology and geochemistry, “Sagan lecture: Isotope Geochemistry and the Study of Habitability and Life on Other Planets,” 2010 AGU Fall Meeting, Dec 22, 2010. John Eiler is clearly a brilliant geochemist and a great speaker. In this lecture he describes the geochemical intricacies of determining if there is life on Mars. Armed with our small smattering of stable isotope geochemistry and a high school chemistry course you can probably make it through this lecture without having your head explode. 74 minutes.

Robert Hazen, Geophysical Laboratory, Carnegie Institution, and Executive Director and PI, Deep Carbon Observatory, “Mineral Evolution and Ecology and the Co-evolution of Life and Rocks,” March 11, 2015, Simons Foundation YouTube channel. This video on mineral evolution is a bit off topic but I found it to be very, very fascinating. Also probably the least technically difficult of the recommended videos. 45 minutes.

Further Reading

These articles give the readers a taste of the range of studies being done using stable isotope geochemistry:

T. C. Feeley, M. A. Clynne, G. S. Winer, and W. C. Grice, “Oxygen Isotope Geochemistry of the Lassen Volcanic Center, California: Resolving Crustal and Mantle Contributions to Continental Arc Magmatism,” Journal Of Petrology, Volume 49, Number 5, pages 971-997, May 2008. doi:10.1093/petrology/egn013.

Richard C. Greenwood, Jean-Alix Barrat, Martin F. Miller, Mahesh Anand, Nicolas Dauphas, Ian A. Franchi, Patrick Sillard, and Natalie A. Starkey, “Oxygen isotopic evidence for accretion of Earth’s water before a high-energy Moon-forming giant impact,” AAAS, Sci Adv. 2018 Mar; 4(3): eaao5928. Published online 2018 Mar 28. doi: 10.1126/sciadv.aao5928.

O'Neil, J. R. & Adami, L. H., “Oxygen isotope analyses of selected Apollo 11 materials,” Geochimica et Cosmochimica Acta Supplement, Volume 1, Proceedings of the Apollo 11 Lunar Science Conference held 5-8 January, 1970 in Houston, TX. Volume 2: Chemical and Isotope Analyses. Edited by A. A. Levinson. New York: Pergammon Press, 1970., p.1425.

D. Rumble, S. Bowring, T. Iizuka, T. Komiya, A. Lepland, M. T. Rosing, Y. Ueno, “The oxygen isotope composition of earth's oldest rocks and evidence of a terrestrial magma ocean,” AGU publication Geochemistry, Geophysics, Geosystems Volume 14 Number 6, June 2013 https://doi.org/10.1002/ggge.20128

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