New research calibrates the clock for dating Earth evolution and mineral deposit formation

Researchers from the Earth Dynamics Research Group at Curtin University established a new framework for dating Earth’s evolution such as the formation of continents and critical mineral deposits.

The research, published in Earth Science Reviews, studied Australia’s abundant lead-zinc ore deposits along with a vast global database, and determined that 3.2 billion years ago was a critical point in Earth history when Earth changed from a layer-cake structure driven by density differention to a mode of remixing possibly driven by the startup of global-scale plate tectonics, a process that still dominates the Earth system today.

Fig. 1. Giant impact scenario and relative timings for the formation of Earth and the Moon. (a) Accretion of proto-Earth from chondrites with a Canyon Diablo Troilite (CDT) Pb isotopic composition. Proto-Earth started to differentiate just before (b) the collision with giant impactor Theia at ca. 4.52 Ga resulting in mixing and re-homogenization, followed by (c) the formation of BSE by rapid extraction of the core.

The U-Th-Pb system is perhaps one of the most versatile isotopic systems in use by Earth scientists, widely applied to both date and compositionally trace geological events through Earth history. However, the Pb isotope systematics of the Earth are subject to two major paradoxes. Assuming our planet evolved uniformly from a chondritic composition, all the present-day Earth chemical reservoirs should plot on the 4.55 Ga meteorite isochron, also known as the geochron; but in fact, all known reservoirs are more radiogenic (having excess ²⁰⁶Pb and ²⁰⁷Pb) than the carbonaceous chondrites, constituting the first Pb paradox. The second Pb paradox (also called the kappa conundrum) is the apparent difference between the measured ²³²Th/²³⁸U ratio (κ) of oceanic basalts and the time-integrated ²³²Th/²³⁸U ratio (κ_Pb) predicted from the Pb isotope ratios. While significant progress has been made since the realization of the two Pb paradoxes over 50 years ago, the persistence of these issues highlights the limitations in our current understanding of the Earth’s evolution with respect to U, Th and Pb, as we are neither able to ascertain the composition of the present-day bulk silicate Earth (BSE; comprising Earth’s mantle and its crust) nor to determine the starting time(s) of U/Pb and Th/U fractionations in the mantle.

Fig. 2. Lead isotope growth curves for the Bulk Silicate Earth, the continental crust, and complementary mantle reservoirs. (a, b) 206Pb/204Pb vs. 207Pb/204Pb plots, (c, d) 206Pb/204Pb vs. 208Pb/204Pb plots, and (e) 206Pb/204Pb vs. 207Pb/204Pb diagram representing the range for possible Pb growth evolution for both the mantle and the continental crust after the start of rapid crustal formation and mantle remixing at 3.2 Ga. Pb isotope growth curves shown include that for the Bulk Silicate Earth (solid green line), for the average continental crust (solid orange line), for a speculated end member of the upper continental crust (dashed orange line; see data constraints in (b) and in Figs. 14b and 15 in the published article), and for hypothesized notional complementary mantle reservoirs (dashed green lines). The average continental crust model starts at 3.2 Ga and is defined by μ = 10.3 and κ = 4.2 (solid orange line, this study) to fit the upper continental crust (UCC; 206Pb/204Pb = 19.07 207Pb/204Pb = 15.74 and 208Pb/204Pb = 39.35; Millot et al. (2004), Table 6 in the published article). Mass balance calculations predict that the complementary mantle reservoirs have low-μ and low-κ depending on the proportion of the mantle affected by continental extraction (10%, 20%, 30%, and 80% of continental extraction are shown for reference; see Method). The conformable Pb ore deposits (red dots) data in (a) and (c) are from (Cumming and Richards, 1975; Maltese and Mezger, 2020; Stacey and Kramers, 1975). The compilation of Australian Pb ore data (light purple dots) in (b) and (d) are from Geoscience Australia (Huston et al., 2019). The average evolution for the Australian Pb ore data (dark purple dots) were calculated using a bin of 0.25 206Pb/204Pb. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

In this article, we first review the Pb paradoxes and their proposed solutions. We then discuss the previously proposed Pb evolution models and establish a new framework based on a reassessment of global data and current understanding of Earth dynamics. Our model invokes the presence of distinct Pb isotope evolution paths for a diverse range of segregated components of the BSE, with the present-day upper continental crust being one of the end members. The model also features a two-stage Pb evolution for the silicate Earth, with a data-defined ca. 3.2 Ga start time for major compositional differentiation and remixing, possibly due to the initiation of global plate tectonics at that time. We readdress the first Pb paradox through recognizing that, to the first order, the Pb isotopic values of present-day Earth materials lie on a Pb differentiation line defined by our re-estimated present-day BSE and continental crust, and proposing that the data plot mostly to the right of the geochron due to second-order complications caused by both source mixing and fractionation of Earth materials.

Fig. 3. Secular evolution of Pb isotopes on an Albarède-Martine diagram for conformable Pb ores, komatiite, oceanic and continental LIPs, kimberlites, present-day MORBs, and OIBs and LIPs with primordial signature relative to the Bulk Silicate Earth (solid green line), continental crust (solid orange line) and complementary low-μ mantle reservoirs (dashed green lines) growth models: age (as a function of exp.(λt)) vs. Pb 206Pb/204Pb (a), 207Pb/204Pb (b) and 208Pb/204Pb (c) (Albarede and Martine, 1984). The orange-to-green gradient field shows the range for possible Pb growth evolution after 3.2 Ga. The values for λ are the radioactive decay constants for 235U, 238U, 232Th, and t is age in Ga.

We argue that rocks found on Earth’s surface mostly originated from more radiogenic reservoirs (with HIMU being an end member) at shallower levels, where long-term gravitational differentiation and subduction-led mantle remixing preferentially concentrated more radiogenic materials. We also largely mitigated the second Pb paradox through an updated κ vs. κ_pb plot using modern global databases, which shows a general agreement between the mean κ and κ_pb values. We further demonstrate that the choice of Pb evolution models has potentially profound implications when applying non-radiogenic Pb corrections during U-Pb dating of Earth materials.

Fig. 4. Figure illustrating the new Lead isotope evolution framework of this study, emphasizing the importance of Earth’s differentiation (shown in grading colour shades), and remixing and fractionation. The 206Pb/204Pb vs 207Pb/204Pb reference frame for the present-day oceanic and continental igneous rocks (Table 6 in the published article) is compared to the estimates for HIMU, EM1, and EM2 endmembers.

Contact person: Prof Zheng-Xiang Li & Dr Luc Doucet, Earth Dynamics Research Group, Curtin University.

Relevant publication:

Doucet, L.S., Li, Z.-X., Fougerouse, D., Olierook, H.K.H., Gamaleldien, H., Kirkland, C.L., Hartnady, M.I.H., 2023. The global lead isotope system: Toward a new framework reflecting Earth’s dynamic evolution. Earth-Science Reviews, 243, 104483. https://doi.org/10.1016/j.earscirev.2023.104483

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