This aspect of our research is focused on the origin and evolution of the highly siderophile elements (HSE) and the moderately siderophile elements (MSE) in the silicate Earth. An emphasis of this work is on the short-lived 182Hf-182W and 146Sm-142Nd isotope systems (182Hf → 182W + ß-; t½ = 8.9 Myr and 146Sm → 142Nd + α; t½ = 68 Myr), and long-lived 190Pt-186Os and 187Re-187Os systems (190Pt → 186Os + α; ≈ 490 Gyr; 187Re → 187Os + ß-; t½ = 42 Gyr). The short-lived systems are particularly exciting for understanding early Earth processes because isotopic variations in these systems could only have been generated during the first few tens (182W) to hundreds (142Nd) of millions of years of Solar System history. Thus, the presence of enrichments or depletions in 182W or 142Nd in Archean or younger rocks requires the long-term preservation and subsequent tapping of one or more primordial reservoirs within the Earth.

The short-lived 182Hf-182W isotope system is particularly important for this work because it can be used to trace the early fractionation of an incompatible lithophile trace element (Hf) from a highly incompatible MSE (W). This very short-lived system records the effects of processes that occurred within the first 60 Ma of solar system history. The observation that the 182W/184W of terrestrial rocks are ~200 ppm more radiogenic than chondrites has led to the interpretation of early formation of the Earth’s core. The isotopic difference between mantle and chondrites, together with mass balance constraints, also implies that the Earth’s core is a W-rich reservoir with 182W/184W that is ~220 ppm lower than terrestrial silicates.

One of two ThermoFisher Triton thermal ionization mass spectrometers at UMd used for high precision isotopic analysis of W, Os, Nd, Sr, Ru and Mo.

One of two ThermoFisher Triton thermal ionization mass spectrometers at UMd used for high precision isotopic analysis of W, Os, Nd, Sr, Ru and Mo.

To examine W isotopes with high precision, we developed a new technique to measure 182W/184W in silicate samples to a precision of ±4.8 ppm (2σ; SD) (Touboul and Walker, 2012). The technique constitutes a high resolution tool for investigating the W isotopic compositions of terrestrial rocks to identify early Earth differentiation processes, heterogeneities related to the late accretion of extraterrestrial materials, and possible chemical interactions between the core and mantle.

As a complement to the short-lived Hf-W isotopic system, variations in initial 186,187Os isotopic compositions of mantle and mantle-derived rocks must reflect moderate to large, long-term fractionations of Pt/Re/Os. Osmium isotopic heterogeneities among young mantle and mantle-derived rocks have, consequently, been interpreted to reflect a variety of processes that can fractionate these elements including core-mantle interactions, derivation of melts from pyroxenite-rich sources, as well as selective melt transport of certain HSE-enriched materials within the mantle. In ancient terrestrial rocks, variations in 186-187Os isotopic compositions could reflect these processes, as well as the end result of metal-silicate equilibration during terrestrial differentiation.

We have focused initial attention on a type of extrusive rock termed komatiite. Komatiite production was common during the Archean, but tapered off afterward, and there are no modern examples. Most of our work has been directed at the 2.8 Ga Kostomuksha suite, from Karelia, Russia. Prior work showed that these rocks were derived from 186Os and 187Os enriched mantle sources, and Puchtel et al. (2005) used this evidence to argue that they included an outer core component. The μ182W values for multiple komatiite flows from Kostomuksha average +14.8±4.4 (2σ) (where μ182W values are the deviation in parts per million from the terrestrial reference standard). This value is well-resolved from terrestrial standards and modern rocks. In comparison, μ182W values for the 3.5 Ga Komati komatiites (southern Africa) show little or no enrichment.  Given the positive enrichment in 182W, it can be concluded that at least this characteristic of the rocks does not reflect core-mantle interaction.

Photo of spinifex texture in Kostomuksha (Russia) komatiite flow. These 2.8 billion year old rocks were analyzed for tungsten isotopic composition. Komatiites like at Kostomuksha were likely generated by high percentages of partial melting of a deep mantle source, and erupted at temperatures >1500°C. Photo is complements of I.Puchtel.

Photo of spinifex texture in Kostomuksha (Russia) komatiite flow. These 2.8 billion year old rocks were analyzed for tungsten isotopic composition. Komatiites like at Kostomuksha were likely generated by high percentages of partial melting of a deep mantle source, and erupted at temperatures >1500°C. Photo is complements of I.Puchtel.

μ182W values for komatiites from the 2.8 billion year old Kostomuksha show well resolved positive offsets from the terrestrial standards and the modern La Palma ocean island basalt. Data for the much older (3.5 billion year old) Komati komatiites are suggestive of a minor enrichment, but it is not presently well resolved. Figure is from Touboul et al. (2012).

μ182W values for komatiites from the 2.8 billion year old Kostomuksha show well resolved positive offsets from the terrestrial standards and the modern La Palma ocean island basalt. Data for the much older (3.5 billion year old) Komati komatiites are suggestive of a minor enrichment, but it is not presently well resolved. Figure is from Touboul et al. (2012).

The figure at right shows the location of a possible lower mantle magma ocean where silicate liquid could equilibrate with liquid metal (either the outer core, or metal diapirs falling through the magma ocean, as shown by the red arrow). Figure is from Labrosse et al. (2007).

The figure at right shows the location of a possible lower mantle magma ocean where silicate liquid could equilibrate with liquid metal (either the outer core, or metal diapirs falling through the magma ocean, as shown by the red arrow). Figure is from Labrosse et al. (2007).

We believe the W isotopic composition of the Kostomuksha komatiites reflects the composition of its mantle source. If so, the enrichment in 182W in the source was likely generated within the Earth during the first 60 Myr of the formation of the Solar System. This domain likely formed when the Earth was considerably less massive than it is at present. One potential mechanism for generating enrichments in 182W, 186Os and 187Os is via metal-silicate interactions in a lower magma ocean. Magma oceans in the lower mantle have been hypothesized to have been generated as a result of accretionary and radioactive heating.

If the 182W enrichment reflects the composition of its mantle source with high fidelity, then the short half-life of 182Hf requires that this primordial component survived convective mixing of the mantle in some form for >1.7 billion years.

Do mantle domains containing primordial material still exist today? Our ongoing analysis of the 182W/184W and 3He/4He ratios in modern ocean island basalts, some of which may sample deep mantle domains, indicates the answer is yes. Data for modern ocean island basalts from Mangaia, La Palma (Canary Islands), and Pitcairn, as well as a mid-ocean ridge basalt from the Central Indian Ridge (CIR) show no signs of anomalous 182W (nor of high 3He/4He). By contrast, 182W is negatively correlated with 3He/4He in basalts from Samoa and Hawaii. This indicates mixing between at least two mantle source materials. One is indistinguishable from ambient upper mantle. The other source is characterized by low μ182W (part per million deviation in 182W/184W ratio from laboratory standards) and high 3He/4He. The anomalous source could be the core, or some other reservoir that formed with anomalous isotopic characteristics early in Earth history.

μ182W versus 3He/4He (R/RA) for ocean island basalts and a mid-ocean ridge basalt from the Cental Indian Ridge. Error bars represent 2 SE of individual analyses or long-term external precision (2 SD) for average analyses of multiple samples, respectively. The black line represents a trendline for all samples. The gray area shows the error envelope for the trend line by using the long-term external precision for μ182W isotopic measurements of ±4 ppm. Figure is from Mundl et al. (2017).

μ182W versus 3He/4He (R/RA) for ocean island basalts and a mid-ocean ridge basalt from the Cental Indian Ridge. Error bars represent 2 SE of individual analyses or long-term external precision (2 SD) for average analyses of multiple samples, respectively. The black line represents a trendline for all samples. The gray area shows the error envelope for the trend line by using the long-term external precision for μ182W isotopic measurements of ±4 ppm. Figure is from Mundl et al. (2017).

For more information about this research, please refer to the following papers:

Touboul M., Puchtel I.S. and Walker R.J. (2012) 182W evidence for long-term preservation of early mantle differentiation products. Science 335, 1065-1069, DOI: 10.1126/science.1216351.

Touboul M., Liu J., O”Neil J., Puchtel I.S. and Walker R.J. (2014) New insights into the Hadean mantle revealed by 182W and highly siderophile element abundances of supracrustal rocks from the Nuvvuagittuq Greenstone Belt, Quebec, Canada. Chem. Geol. 383, 63-75.

Walker R.J. (2014) Siderophile element constraints on the origin of the Moon. Phil. Trans. Roy. Soc. A 372, 20130258, DOI:10.1098/rsta.2013.0258.

Brown S., Elkins-Tanton L. and Walker R.J. (2014) Effects of magma ocean crystallization and overturn on the development of 142Nd and 182W isotopic heterogeneities in the primordial mantle. Earth Planet. Sci. Lett. 408, 319-330.

Touboul M., Puchtel I.S. and Walker R.J. (2015) Tungsten isotopic evidence for disproportional late accretion to the Earth and Moon. Nature 520, 530–533.

Rizo H., Walker R.J., Carlson R.W., Touboul M., Horan M.F., Puchtel I.S., Boyet M., Rosing M.T. (2016) Early Earth differentiation investigated through 142Nd, 182W, and highly siderophile element abundances in samples from Isua, Greenland. Geochim. Cosmochim. Acta 175, 319-336.

Rizo H., Walker R.J., Carlson R.W., Horan M.F., Mukhopadhyay S., Manthos V., Francis D., Jackson M.G. (2016) Memories of Earth formation in the modern mantle: W isotopic compositions of flood basalt lavas. Science 352, 809-812.

Puchtel I.S., Blichert-Toft J., Touboul M., Horan M.F. and Walker R.J. (2016) Coupled 182W-142Nd record of the early differentiation of Earth’s mantle. Geochemistry, Geophysics, Geosystems 17, DOI:10.1002/2016GC006324.

Mundl A., Touboul M., Jackson M.G., Day J.M.D., Kurz M.D., Lekic V., Helz R.T. and Walker R.J. (2017) Tunsten-182 heterogeneity in modern ocean island basalts. Science 356, 66-69.

Last Updated June 2017.