The nature of Earth’s first crust

Abstract

Recycling of crust into the mantle has left only small remnants at Earth's surface of crust produced within a billion years of Earth formation. Few, if any, of these ancient crustal rocks represent the first crust that existed on Earth. Understanding the nature of the source materials of these ancient rocks and the mechanism of their formation has been the target of decades of geological and geochemical study. This traditional approach has been expanded recently through the ability to simultaneously obtain U-Pb age and initial Hf isotope data for zircons from many of these ancient, generally polymetamorphic, rocks. The addition of information from the short-lived radiometric systems 146 Sm-142 Nd and 182 Hf-182 W allows resolution of some of the ambiguities that have clouded the conclusions derived from the long-lived systems. The most apparent of these is clear documentation that Earth experienced major chemical differentiation events within the first tens to hundreds of millions of years of its formation, and that Earth's most ancient crustal rocks were derived from these differentiated sources, not from primitive undifferentiated mantle. Eoarchean rocks from the North Atlantic Craton and the Anshan Complex of the North China Craton have sources in an incompatible-elementdepleted mantle that dates to 4.4-4.5 Ga. Hadean/Eoarchean rocks from two localities in Canada show the importance of remelting of Hadean mafic crust to produce Eoarchean felsic crust. The mafic supracrustal rocks of the Nuvvuagittuq Greenstone Belt are a possible example of the Hadean mafic basement that is often called upon to serve as the source for the high-silica rocks that define continental crust. Many, but not all, ancient terranes show a shift in the nature of the sources for crustal rocks, and possibly the physical mechanism of crust production, between 3.0-3.6 Ga. This transition may reflect the initiation of modern plate tectonics. Eoarchean/Hadean rocks from some terranes, however, also display compositional characteristics expected for convergent margin volcanism suggesting that at least some convergent margin related magmatism began in the Hadean. The persistence of isotopic variability in 142 Nd/ 144 Nd into the mid-Archean, and the eventual reduction in that variability by the end of the Archean, provides new information on the efficiency by which mantle convection recombined the products of Hadean silicate-Earth differentiation. The rate of crust production and recycling in the Hadean/Archean, however, is not resolved by these data beyond the observation that extreme isotopic compositions, such as expected for Hadean evolved, continent-like, crust are not observed in the preserved Eoarchean rock record. The lack of correlation between 142 Nd/ 144 Nd and 182 W/ 184 W variation in Archean rocks suggests that these two systems track different processes; the Sm-Nd system mantle-crust differentiation while Hf-W is dominated by core formation. The major silicate differentiation controlling Sm/Nd fractionation occurred at ~4.4 Ga, po...

Combined Lithophile‐Siderophile Isotopic Constraints on Hadean Processes Preserved in Ocean Island Basalt Sources

“…A potentially more likely candidate for the second required component would be mafic crust that formed during the lifetime of the 146 Sm‐ 142 Nd radiogenic system (component 3 in Figure 6a). Early mafic crust has been evaluated as an explanation for a variety of trace element and isotopic features of ancient crustal rocks (e.g., Carlson et al., 2019; O'Neil & Carlson, 2017; Reimink et al., 2014; Rosas & Korenaga, 2018). Mafic crust with an incompatible trace‐element enriched signature would evolve to negative µ 142 Nd compositions and could also have inherited a positive µ 182 W composition from a mantle domain similar to the source of Archean tonalite‐trondhjemite‐granodiorite (TTG) progenitors.…”

Tungsten Isotope Composition of Archean Crustal Reservoirs and Implications for Terrestrial μ182W Evolution

“…Very early silicate‐silicate differentiation has been proposed as a mechanism to produce coupled anomalies in μ 182 W and μ 142 Nd (Puchtel, Blichert‐Toft, et al, 2016; Touboul et al, 2012) because in both isotopic systems, the daughter element is more incompatible than the parent in most mantle mineral phases. However, this clearly could not have been a globally significant process, as the majority of Archean locations have variable μ 142 Nd yet uniformly positive μ 182 W (Carlson et al, 2019; Rizo et al, 2019). The lack of correlation between μ 142 Nd and μ 182 W shows the two systems are unlikely to be tracking a shared process, at least one that occurred while 182 Hf was still extant.…”

“…Alternatively, these rocks may bear isotopic signatures more similar to modern MORB that document sources characterized by relative depletion in the more incompatible elements. The true timing of the incompatible element depletion that characterizes the modern mantle as sampled by MORB is not well resolved and could occur any time in the Hadean or Archean (e.g., Carlson et al, 2019). Extracting this information from the initial isotopic compositions of whole rocks using long‐lived radiogenic isotope systems, for example, 87 Rb‐ 87 Sr, 147 Sm‐ 143 Nd, and 176 Lu‐ 176 Hf, is compromised by the potential for incorrect initial ratio calculations caused by modification of parent/daughter ratios by metamorphism long after rock formation.…”

“…The two decay schemes have different half‐lives and are sensitive to different chemical fractionation processes; μ 142 Nd and μ 182 W can both record crust‐mantle differentiation, but μ 182 W can track previous silicate‐metal segregation. At a global scale, μ 142 Nd and μ 182 W are not uniformly correlated (Carlson et al, 2019), but regional investigations using the combination of isotopic signatures may elucidate the early differentiation history of the Earth.…”

Titanium isotopes constrain a magmatic transition at the Hadean-Archean boundary in the Acasta Gneiss Complex

“…Ancient rocks are exceedingly rare, with >3.6-Ga outcrops representing only a few parts per million of Earth’s exposed crust ( 20 ), and those that are preserved have often undergone multiple metamorphic events, each with the potential to chemically overprint primary magmatic signatures. This is true for Earth’s oldest presently known rocks, found in the Acasta Gneiss Complex (AGC; Slave craton, Canada).…”