Proceedings of the National Academy of Sciences of the United States of America, Apr 22, 2022
Significance Due to active plate tectonics, there are no direct rock archives covering the first ... more Significance Due to active plate tectonics, there are no direct rock archives covering the first ca. 500 million y of Earth’s history. Therefore, insights into Hadean geodynamics rely on indirect observations from geochemistry. We present a high-precision 182 W dataset for rocks from the Kaapvaal Craton, southern Africa, revealing the presence of Hadean protocrustal remnants in Earth’s mantle. This has broad implications for geochemists, geophysicists, and modelers, as it bridges contrasting 182 W isotope patterns in Archean and modern mantle-derived rocks. The data reveal the origin of seismically and isotopically anomalous domains in the deep mantle and also provide firm evidence for the operation of silicate differentiation processes during the first 60 million y of Earth’s history.
<p>Much of the continental lithosphere developed during the Archean, which ... more <p>Much of the continental lithosphere developed during the Archean, which was an Eon of change in terms of global geodynamics and geochemical cycles. Uncovering the causal links between crust forming processes and prevailing geodynamic mechanisms is crucial for understanding the origins and composition of the present-day continental lithosphere. Pristine Archean crust is scarce yet can be found in cratons worldwide. Many of these occurrences comprise rocks of the tonalite-trondhjemite-granodiorite (TTG) suite, which represent a prevalent component of the Archean continental crust. TTGs are generally considered to have formed by partial melting of amphibolite or eclogite source rocks that had basaltic precursors originally extracted from a depleted mantle (e.g., [1]). The age of the source rocks (i.e., the time between the basalt extraction from the mantle and TTG formation) can be determined from the initial radiogenic isotope compositions of TTGs, provided that the P/D ratio of the source can be reliably estimated and is significantly different from that of the depleted mantle.</p><p>Based on this principle, we estimated the age of basaltic sources of TTGs from cratons of different age and paleogeography from initial <sup>87</sup>Sr/<sup>86</sup>Sr compositions determined by in-situ Sr isotope analysis of primary igneous apatite (LA-MC-ICPMS). The <sup>87</sup>Sr/<sup>86</sup>Sr of these apatites show that prior to 3.4 Ga TTGs were derived from relatively old mafic sources and that the average time between formation of basaltic material from the mantle and subsequent remelting under amphibolite to eclogite facies conditions decreased drastically during the Paleoarchean. This secular change indicates a rapid global increase in the efficiency of TTG production or the emergence of a new TTG-forming process at c. 3.4 Ga [2].</p><p>In this contribution we explore this hypothesis by comparing the <sup>87</sup>Sr/<sup>86</sup>Sr signature of the TTGs with their trace-element compositions, as well as with <sup>176</sup>Hf/<sup>177</sup>Hf zircon data for these rocks and contemporary TTGs from other studies. This combined geochronological, isotope and geochemical analyses will provide new constraints on the age of TTG sources during the Archean and will allow investigation into the nature and probable causes of the apparent rejuvenation at 3.4 Ga, as indicated by Sr isotopes.</p><p>[1] Hoffmann, J.E. et al. (2011) <em>Geochim. </em><em>Cosmochim. Acta </em>75, 4157-4178.</p><p>[2] Caton, S., et al., (in review) <em>Chem. Geol.</em></p>
Abstract We report highly siderophile element data combined with Re–Os isotopes and major and tra... more Abstract We report highly siderophile element data combined with Re–Os isotopes and major and trace elements of the ca. 2.7–3.0 Ga komatiites from the Quebra Osso Group, Minas Gerais, Brazil. These komatiites resemble the rare high Al-type, characterized by high Al2O3/TiO2 ratios (26.7–59.8). These geochemical similarities are shared with the 3.33 Ga Commondale and 3.26 Ga Weltevreden komatiites from the eastern Kaapvaal Craton pointing to a similar origin of these suites. While anhydrous melting in an unusually hot mantle was inferred for the Weltevreden komatiites, the Commondale komatiites were suggested to have formed by hydrous, multi-stage melting. Significant depletion in LREE is coupled with subchondritic Re/Os, unradiogenic to radiogenic 187Os/188Os and fractionated HSE, with enrichments in Ru, Pt, and Pd over Os and Ir. The combination of these signatures suggests minor late-stage crustal influence. Potential late-stage alteration overprint, assimilation of ambient mantle material during magma ascent and complex phase relationships of HSE-hosting phases make it difficult to estimate the composition of the source of the Quebra Osso komatiites and to place constraints on the nature of the late Archean mantle. However, the Quebra Osso komatiites are unlikely to have formed in a single-stage plume setting or in a supra-subduction zone setting. Instead we suggest a multi-stage melting history of the komatiite source to explain the origin of their peculiar geochemical characteristics, as has been suggested for other high-Al2O3/TiO2 komatiite suites.
Earth's lithosphere probably experienced an evolution towards the modern plate tectonic r... more Earth's lithosphere probably experienced an evolution towards the modern plate tectonic regime, owing to secular changes in mantle temperature. Radiogenic isotope variations are interpreted as evidence for the declining rates of continental crustal growth over time, with some estimates suggesting that over 70% of the present continental crustal reservoir was extracted by the end of the Archaean eon. Patterns of crustal growth and reworking in rocks younger than three billion years (Gyr) are thought to reflect the assembly and break-up of supercontinents by Wilson cycle processes and mark an important change in lithosphere dynamics. In southern West Greenland numerous studies have, however, argued for subduction settings and crust growth by arc accretion back to 3.8 Gyr ago, suggesting that modern-day tectonic regimes operated during the formation of the earliest crustal rock record. Here we report in situ uranium-lead, hafnium and oxygen isotope data from zircons of basement rocks in southern West Greenland across the critical time period during which modern-like tectonic regimes could have initiated. Our data show pronounced differences in the hafnium isotope-time patterns across this interval, requiring changes in the characteristics of the magmatic protolith. The observations suggest that 3.9-3.5-Gyr-old rocks differentiated from a >3.9-Gyr-old source reservoir with a chondritic to slightly depleted hafnium isotope composition. In contrast, rocks formed after 3.2 Gyr ago register the first additions of juvenile depleted material (that is, new mantle-derived crust) since 3.9 Gyr ago, and are characterized by striking shifts in hafnium isotope ratios similar to those shown by Phanerozoic subduction-related orogens. These data suggest a transitional period 3.5-3.2 Gyr ago from an ancient (3.9-3.5 Gyr old) crustal evolutionary regime unlike that of modern plate tectonics to a geodynamic setting after 3.2 Gyr ago that involved juvenile crust generation by plate tectonic processes.
Proceedings of the National Academy of Sciences of the United States of America, Apr 22, 2022
Significance Due to active plate tectonics, there are no direct rock archives covering the first ... more Significance Due to active plate tectonics, there are no direct rock archives covering the first ca. 500 million y of Earth’s history. Therefore, insights into Hadean geodynamics rely on indirect observations from geochemistry. We present a high-precision 182 W dataset for rocks from the Kaapvaal Craton, southern Africa, revealing the presence of Hadean protocrustal remnants in Earth’s mantle. This has broad implications for geochemists, geophysicists, and modelers, as it bridges contrasting 182 W isotope patterns in Archean and modern mantle-derived rocks. The data reveal the origin of seismically and isotopically anomalous domains in the deep mantle and also provide firm evidence for the operation of silicate differentiation processes during the first 60 million y of Earth’s history.
<p>Much of the continental lithosphere developed during the Archean, which ... more <p>Much of the continental lithosphere developed during the Archean, which was an Eon of change in terms of global geodynamics and geochemical cycles. Uncovering the causal links between crust forming processes and prevailing geodynamic mechanisms is crucial for understanding the origins and composition of the present-day continental lithosphere. Pristine Archean crust is scarce yet can be found in cratons worldwide. Many of these occurrences comprise rocks of the tonalite-trondhjemite-granodiorite (TTG) suite, which represent a prevalent component of the Archean continental crust. TTGs are generally considered to have formed by partial melting of amphibolite or eclogite source rocks that had basaltic precursors originally extracted from a depleted mantle (e.g., [1]). The age of the source rocks (i.e., the time between the basalt extraction from the mantle and TTG formation) can be determined from the initial radiogenic isotope compositions of TTGs, provided that the P/D ratio of the source can be reliably estimated and is significantly different from that of the depleted mantle.</p><p>Based on this principle, we estimated the age of basaltic sources of TTGs from cratons of different age and paleogeography from initial <sup>87</sup>Sr/<sup>86</sup>Sr compositions determined by in-situ Sr isotope analysis of primary igneous apatite (LA-MC-ICPMS). The <sup>87</sup>Sr/<sup>86</sup>Sr of these apatites show that prior to 3.4 Ga TTGs were derived from relatively old mafic sources and that the average time between formation of basaltic material from the mantle and subsequent remelting under amphibolite to eclogite facies conditions decreased drastically during the Paleoarchean. This secular change indicates a rapid global increase in the efficiency of TTG production or the emergence of a new TTG-forming process at c. 3.4 Ga [2].</p><p>In this contribution we explore this hypothesis by comparing the <sup>87</sup>Sr/<sup>86</sup>Sr signature of the TTGs with their trace-element compositions, as well as with <sup>176</sup>Hf/<sup>177</sup>Hf zircon data for these rocks and contemporary TTGs from other studies. This combined geochronological, isotope and geochemical analyses will provide new constraints on the age of TTG sources during the Archean and will allow investigation into the nature and probable causes of the apparent rejuvenation at 3.4 Ga, as indicated by Sr isotopes.</p><p>[1] Hoffmann, J.E. et al. (2011) <em>Geochim. </em><em>Cosmochim. Acta </em>75, 4157-4178.</p><p>[2] Caton, S., et al., (in review) <em>Chem. Geol.</em></p>
Abstract We report highly siderophile element data combined with Re–Os isotopes and major and tra... more Abstract We report highly siderophile element data combined with Re–Os isotopes and major and trace elements of the ca. 2.7–3.0 Ga komatiites from the Quebra Osso Group, Minas Gerais, Brazil. These komatiites resemble the rare high Al-type, characterized by high Al2O3/TiO2 ratios (26.7–59.8). These geochemical similarities are shared with the 3.33 Ga Commondale and 3.26 Ga Weltevreden komatiites from the eastern Kaapvaal Craton pointing to a similar origin of these suites. While anhydrous melting in an unusually hot mantle was inferred for the Weltevreden komatiites, the Commondale komatiites were suggested to have formed by hydrous, multi-stage melting. Significant depletion in LREE is coupled with subchondritic Re/Os, unradiogenic to radiogenic 187Os/188Os and fractionated HSE, with enrichments in Ru, Pt, and Pd over Os and Ir. The combination of these signatures suggests minor late-stage crustal influence. Potential late-stage alteration overprint, assimilation of ambient mantle material during magma ascent and complex phase relationships of HSE-hosting phases make it difficult to estimate the composition of the source of the Quebra Osso komatiites and to place constraints on the nature of the late Archean mantle. However, the Quebra Osso komatiites are unlikely to have formed in a single-stage plume setting or in a supra-subduction zone setting. Instead we suggest a multi-stage melting history of the komatiite source to explain the origin of their peculiar geochemical characteristics, as has been suggested for other high-Al2O3/TiO2 komatiite suites.
Earth's lithosphere probably experienced an evolution towards the modern plate tectonic r... more Earth's lithosphere probably experienced an evolution towards the modern plate tectonic regime, owing to secular changes in mantle temperature. Radiogenic isotope variations are interpreted as evidence for the declining rates of continental crustal growth over time, with some estimates suggesting that over 70% of the present continental crustal reservoir was extracted by the end of the Archaean eon. Patterns of crustal growth and reworking in rocks younger than three billion years (Gyr) are thought to reflect the assembly and break-up of supercontinents by Wilson cycle processes and mark an important change in lithosphere dynamics. In southern West Greenland numerous studies have, however, argued for subduction settings and crust growth by arc accretion back to 3.8 Gyr ago, suggesting that modern-day tectonic regimes operated during the formation of the earliest crustal rock record. Here we report in situ uranium-lead, hafnium and oxygen isotope data from zircons of basement rocks in southern West Greenland across the critical time period during which modern-like tectonic regimes could have initiated. Our data show pronounced differences in the hafnium isotope-time patterns across this interval, requiring changes in the characteristics of the magmatic protolith. The observations suggest that 3.9-3.5-Gyr-old rocks differentiated from a >3.9-Gyr-old source reservoir with a chondritic to slightly depleted hafnium isotope composition. In contrast, rocks formed after 3.2 Gyr ago register the first additions of juvenile depleted material (that is, new mantle-derived crust) since 3.9 Gyr ago, and are characterized by striking shifts in hafnium isotope ratios similar to those shown by Phanerozoic subduction-related orogens. These data suggest a transitional period 3.5-3.2 Gyr ago from an ancient (3.9-3.5 Gyr old) crustal evolutionary regime unlike that of modern plate tectonics to a geodynamic setting after 3.2 Gyr ago that involved juvenile crust generation by plate tectonic processes.
Uploads
Papers by J. Elis Hoffmann