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 = 103 Myr), and long-lived 190Pt-186Os and 187Re-187Os systems (190Pt → 186Os + α; t½ ≈ 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 Earths core. The isotopic difference between mantle and chondrites, together with mass balance constraints, also implies that the Earths core is a W-rich reservoir with 182W/184W that is ~220 ppm lower than terrestrial silicates.
To examine W isotopes with high precision, we developed a new technique to measure 182W/184W in silicate samples to a precision of <5 ppm (external 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. The technique was subsequently refined to also allow us to precisely measure 183W, the abundance of which differs among cosmochemical materials as a result of varying nucleosynthetic inputs (Archer et al., 2017)
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.
Two ThermoFisher Triton thermal ionization mass spectrometers at UMd used for high precision isotopic analysis of W, Os, Nd, Sr, Ru and Mo.
We have focused most of our attention on two broad types of rocks: 1) ancient komatiites and 2) modern ocean island basalts.
Komatiites
Komatiite is a type of high Mg extrusive rock that, in most examples, formed by large extents of partial melting of the mantle, and erupted at temperatures >1400°C. Komatiite production was greatest during the Archean, then tapered off through the Proterozoic and beyond. The youngest known komatiites erupted ~89 million years ago and are accessible on Gorgona Island, Colombia (see below).
Work directed by IGL Laboratory Manager Igor Puchtel has revealed that the 182W isotopic compositions of komatiites were quite variable, with some characterized by enrichments and others characterized by depletions in the isotope. Surprisingly, the type locale Komati komatiites (South Africa) have an isotopic composition that is identical to the modern upper mantle.
μ182W values (deviation in part per million of 182W/184W ratio of sample relative to our laboratory standards) for komatiites from 5 different locations ranging in age from 3.5 Ga to 2.7 Ga. Causes of the isotopic anomalies relative to the modern mantle likely reflect a variety of different processes including silicate-silicate fractionation (Schapenburg), metal-silicate fractionation (Kostomuksha), contamination from radiogenic early crust (Vetreny/Vodla) and grainy late accretion (Boston Creek). The gray band represents the 2SD uncertainty for standards.
Do Phanerozoic komatiites exhibit similar isotopic variability to Archean komatiites? In 2019 we visited Gorgona Island, which is a national park of Colombia, off the Pacific coast of that nation. The trip was sponsored by the National Science Foundation. During the visit we collected~40 diverse rocks for high precision 182W isotopic analysis and other geochemical analysis. The rocks collected include samples of the youngest known komatiites (89 Ma). It will be important to determine if they, or spatially associated basalts, picrites, or gabbros harbor isotopic anomalies, indicating that they may sample an ancient mantle domain similar to the much older komatiites. This work is currently ongoing.
(Left) Photo of spinifex texture in Gorgona Island (Colombia) komatiite flow. These 89 million year old rocks are the youngest known komatiites.
(Right) 2019 Gorgona sampling party from left to right: Bruce Aitken, Charlotte Devitre, Lina Echeverria, Willie Nicklas, Esteban Gazel, Igor Puchtel, Richard Walker.
Ocean Island Basalts
Some ocean island basalt locales are popular vacation destinations. Photo is of the Hanauma Bay, Oahu, Hawaii. Below this rosy view of an idyllic beach and snorkeling locale, hides a sinister tale of 182W isotopic heterogeneity. Hawaiian lavas are characterized by 182W isotopic compositions that range from "normal" to significantly depleted relative to the upper mantle. The 182W depletions correlate with high 3He/4He ratios.
Do portions of the mantle containing primordial material still exist today? To answer this question we study modern ocean island basalts from a number of different locations. Ocean island basalts typically form volcanic islands that, with few exceptions such as Iceland, are found in intraplate settings. Many are believed to be derived from mantle that is hotter than that typically present beneath mid-ocean ridge spreading centers, and so ocean island basalts are synonymous with hotspot volcanism. It has been known since the 1960s that the mantle sources of ocean island basalts harbor a dogs breakfast of diverse recycled components of varying age, resulting from plate tectonic processes. Further, it has been argued in numerous studies that ocean island basalts form from mantle plumes that arise from thermal boundary layers deep inside the Earth. Seismic imaging of mantle underlying some ocean island basalt settings (e.g., Hawaii) suggest that at least some plumes arise from the core-mantle boundary ~2900 km beneath the surface.
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 a number of ocean island basalt systems, including the Hawaiian, Samoan, Icelandic and the Galapagos hotspots all are characterized by negative correlations between 182W and 3He/4He. High ratios for the noble gas system 3He/4He are commonly interpreted as evidence for a component that is primordial and mostly un-degassed.
μ182W versus 3He/4He (R/RA) for basalts from Iceland. Error bars represent 2 SE of individual analyses or long-term external precision (2 SD) for average analyses of multiple samples, respectively. The colored areas represent the error envelope for the trend lines of two correlations delineated by 206Pb/204Pb ratios. This figure illustrates the observation that W-He trends with different slopes exist among different ocean island basalt systems, and in the case of Iceland, even within the same system. The figure is from Mundl-Petermeier et al. (2019)
The widespread occurrence of W-He correlations in ocean island basalt systems likely requires the mixing of at least three distinct portions of the mantle, one of which includes an isotopic signature of an inner Earth domain that formed within the first 60 million years of solar system history. This component may have isotopically equilibrated with Earths metallic core, which based on mass balance comparisons with primitive meteorites is likely strongly depleted in 182W.
Schematic section through Earth's interior (at left) showing mixing of materials from three regions of the mantle to account for the isotopic variations observed in ocean island basalt systems (at right). Schematic shows mantle plumes originating at the core-mantle boundary (CMB) incorporating material from different source regions in variable proportions. A thin molten layer at the CMB that may have equilibrated chemically and isotopically with the Earths core is piling up in areas where mantle plumes form, potentially representing ultra low velocity zones (ULVZs). Its 3He/4He depends on whether the core contributed significant amounts of He during equilibration. Little or un-degassed mantle reservoirs are depicted here as large low shear velocity provinces (LLSVPs), presumably representing a high 3He/4He but normal μ182W reservoir. The purple area represents ambient mantle material that has participated in mantle mixing and mantle convection processes throughout Earths history. Figures are from Mundl-Petermeier et al. (2020).
Kimberlites
Kimberlites are igneous rocks derived from deep mantle sources. Recent study has suggested that certain kimberlites originated from a single homogeneous source with a relatively primitive chemical composition. In Nakanishi et al (2021) we presented W isotope data for a global suite of kimberlites with variable formation ages and found that their mantle sources were characterized by a 182W/184W that is uniform with analytical uncertainties, but lower than the upper mantle ratio. The results are consistent with derivation from a lower mantle reservoir that remained isolated for at least 1000 million years. The 182W/184W of kimberlites is indicative of an ancient mantle source modified by some form of core-mantle interaction, early silicate fractionation, or an overabundance of late accreted materials. This is somewhat similar to the ocean island basalts noted above. But the uniformity of the kimberlite data (except for some kimberlites whose mantle source(s) was likely isotopically modified by ancient, recycled crust) argues for a different source compared with modern ocean island basalts.
The photograph is of a crossed polarised light view of a thin section from the Grizzly kimberlite (Lac de Gras, Canada). The colorful crystals are of the mineral olivine.
μ182W values for primitive (closed symbols) and anomalous kimberlites (open symbols) from Siberia (circles), southern Africa (triangles), and Canada (squares). Light and dark gray areas represent the 2SD (±3.3 ppm) and 2SE (±0.9 ppm) long-term external reproducibility of the Alfa Aesar W laboratory reference standard, respectively. Sample error bars are 2SE of individual analyses. Light and dark blue area represent the 2SD (±3.6 ppm) and 2SE (±1.0 ppm) of the mean of primitive kimberlites, respectively. Results for samples where two separate analyses (including sample digestion) were performed, are shown as the smaller symbols, with their averages and 2SE in the larger symbols (figure from Nakanishi et al., 2021).
For more information about our research on this topic, 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) Preservation of Earth-forming events in the tungsten isotopic composition of modern flood basalts. 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.
Archer G.J., Mundl A., Walker R.J., Worsham E.A. and Bermingham K.R. (2017) High-precision analysis of 182W/184W and 183W/184W by negative thermal ionization mass spectrometry: per-integration oxide corrections using measured 18O/16O. International Journal of Mass Spectrometry 414, 80-86.
Marchi S., Canup R.M. and Walker R.J. (2018) Heterogeneous delivery of silicate and metal to the Earth by large planetesimals. Nature Geoscience 11, 77-81. doi:10.1038/ s41561-017-0022-3.
Puchtel I.S., Blichert-Toft J., Touboul M. and Walker R.J. (2018) Slow mixing of the terrestrial mantle inferred from 182W and HSE systematics of 2.7 Ga komatiites. Geochim. Cosmochim. Acta 228, 1-26.
Horan M.F., Carlson R.W., Walker R.J., Jackson M., Garçon M. and Norman M. (2018) Tracking Hadean processes in modern basalts. Earth Planet. Sci. Lett. 484, 184-191.
Mundl A., Walker R. J., Reimink R. J., Rudnick R. L. and Gaschnig R. M. (2018) Tungsten-182 in the upper continental crust: evidence from glacial diamictites. Chem. Geol. 494, 144-152.
Mundl-Petermeier A., Walker R.J., Jackson M.G., Blichert-Toft J., Kurz M.D., and Halldórsson S.A. (2019) Temporal evolution of primodial tungsten-182 and 3He/4He signatures in the Iceland mantle plume. Chem. Geol. 525, 245-259.
Mundl-Petermeier A., Walker R.J., Fischer R.A., Lekic V., Jackson M.G. and Kurz M.D. (2020) Anomalous 182W in high 3He/4He ocean island basalts: Fingerprints of Earth’s core? Geochim. Cosmochim. Acta 271, 191-214.
Reimink J.R., Mundl-Petermeier A., Carlson R.W., Shirey S.B., Walker R.J., Pearson D.G. (2020) Tungsten isotope composition of Archean crustal reservoirs and implications for terrestrial μ182W evolution. Geochem. Geophys. Geosyst., 10.1029/2020GC009155.
Puchtel I.S., Mundl-Petermeier A., Horan M.F., Hanski E., Blichert-Toft J., Walker R.J. Ultra-depleted 2.05 Ga komatiites of Finnish Lapland: Products of grainy late accretion or core-mantle interaction? Chem. Geol. 554, 119801.
Peters B., Mundl-Petermeier A., Carlson R.W., Walker R.J., Day J.M.D. (2021) Combined lithophile-siderophile isotopic constraints on Hadean processes preserved in ocean island basalt sources. Geochemistry, Geophysics, Geosystems, 10.1029/2020GC009479.
Nakanishi N., Giuliani A., Carlson R.W., Horan M.F., Woodhead J., Pearson D.G. and Walker R.J. (2021) Tungsten-182 evidence for an ancient kimberlite source. PNAS 118, e2020680118.
Nakanishi N., Puchtel I.S., Walker R.J. and Nabelek P.I. (2023) Dissipation of Tungsten-182 Anomalies in the Archean Upper Mantle: Evidence from the Black Hills, South Dakota, USA. Chemical Geology, in press.
Last Updated January 2023