ALBERT

Description: Detrital zircon spectra reflect the tectonic setting of the basin in which they are deposited. Convergent plate margins are characterized by a large proportion of zircon ages close to the depositional age of the sediment, whereas sediments in collisional, extensional and intracratonic settings contain greater proportions with older ages that reflect the history of the underlying basement. These differences can be resolved by plotting the distribution of the difference between the measured crystallization ages (CA) of individual zircon grains present in the sediment and the depositional age (DA) of the sediment. Application of this approach to successions where the original nature of the basin and/or the link to source are no longer preserved constrains the tectonic setting in which the sediment was deposited.

Description: A global U-Pb and 18 O zircon database shows temporal changes in the magmatic record related to changes in the degree of crustal reworking. The 18 O composition of bulk sediment remains relatively constant through geologic time, with a mean value of 14.9. In contrast, the 18 O values in magmatic zircons vary from relatively low values averaging ~6 in the Archean to increasingly higher and scattered values defining a series of peaks and troughs in post-Archean data. The degree of crustal reworking increases at times of supercontinent assembly. Therefore we attribute the pattern of post-Archean 18 O values recorded by magmatic zircons to a significant increase in the incorporation of high 18 O sediment in response to enhanced crustal thickening and reworking associated with the onset of collisional tectonics, especially during formation of supercontinents.

Description: Most crustal rocks derive from preexisting crust, and so the composition of newly generated (juvenile) continental crust, and hence the tectonic settings of its formation, have remained difficult to determine, especially for the first billion years of Earth’s evolution. Modern primitive mantle–derived magmas have distinct U/Pb ratios, depending on whether they are generated in intraplate (mean U/Pb = 0.37) or in subduction settings (mean U/Pb = 0.10). The U/Pb ratio can therefore be used as a proxy for the tectonic settings in which juvenile continental crust is generated. This paper presents a new way to see back to the U/Pb ratios of juvenile continental crust that formed hundreds to thousands of millions of years ago, based on ion probe analysis of Pb isotopes in alkali feldspar and plagioclase inclusions within well-dated zircons. Pb isotope data are used to calculate the time-integrated U/Pb ratios (i.e., 238 U/ 204 Pb = µ) for the period between the Hf model age and the U-Pb crystallization age of the zircons. These time-integrated ratios reflect the composition of the juvenile continental crust at the time it was extracted from the mantle, and so they can be used as a proxy for the tectonic setting of formation of that crust. Two test samples with Proterozoic Hf model ages and Paleozoic crystallization ages have feldspar inclusions with measured Pb isotope ratios that overlap within analytical error for each sample. Sample Z7.3.1 from Antarctica has Pb isotope ratios (mean 206 Pb/ 204 Pb = 16.88 ± 0.08, 1) that indicate it was derived from source rocks with low U/Pb ratios (~0.11), similar to those found in subduction-related settings. Sample Temora 2 from Australia has more radiogenic Pb isotope ratios (mean 206 Pb/ 204 Pb = 19.11 ± 0.23, 1) indicative of a source with higher U/Pb ratios (~0.36), similar to magmas generated in intraplate settings. Analysis of detrital populations with a range of Hf model ages (e.g., Hadean to Phanerozoic), and for which zircons and their inclusions represent the only archive of their parent magmas, should ultimately open new avenues to our understanding of the formation and the evolution of the continental crust through time.

Description: Most crustal rocks derive from preexisting crust, and so the composition of newly generated (juvenile) continental crust, and hence the tectonic settings of its formation, have remained difficult to determine, especially for the first billion years of Earth’s evolution. Modern primitive mantle–derived magmas have distinct U/Pb ratios, depending on whether they are generated in intraplate (mean U/Pb = 0.37) or in subduction settings (mean U/Pb = 0.10). The U/Pb ratio can therefore be used as a proxy for the tectonic settings in which juvenile continental crust is generated. This paper presents a new way to see back to the U/Pb ratios of juvenile continental crust that formed hundreds to thousands of millions of years ago, based on ion probe analysis of Pb isotopes in alkali feldspar and plagioclase inclusions within well-dated zircons. Pb isotope data are used to calculate the time-integrated U/Pb ratios (i.e., 238 U/ 204 Pb = µ) for the period between the Hf model age and the U-Pb crystallization age of the zircons. These time-integrated ratios reflect the composition of the juvenile continental crust at the time it was extracted from the mantle, and so they can be used as a proxy for the tectonic setting of formation of that crust. Two test samples with Proterozoic Hf model ages and Paleozoic crystallization ages have feldspar inclusions with measured Pb isotope ratios that overlap within analytical error for each sample. Sample Z7.3.1 from Antarctica has Pb isotope ratios (mean 206 Pb/ 204 Pb = 16.88 ± 0.08, 1) that indicate it was derived from source rocks with low U/Pb ratios (~0.11), similar to those found in subduction-related settings. Sample Temora 2 from Australia has more radiogenic Pb isotope ratios (mean 206 Pb/ 204 Pb = 19.11 ± 0.23, 1) indicative of a source with higher U/Pb ratios (~0.36), similar to magmas generated in intraplate settings. Analysis of detrital populations with a range of Hf model ages (e.g., Hadean to Phanerozoic), and for which zircons and their inclusions represent the only archive of their parent magmas, should ultimately open new avenues to our understanding of the formation and the evolution of the continental crust through time.

Description: Earth’s middle age, extending from 1.7 to 0.75 Ga, was characterized by environmental, evolutionary, and lithospheric stability that contrasts with the dramatic changes in preceding and succeeding eras. The period is marked by a paucity of preserved passive margins, an absence of a significant Sr anomaly in the paleoseawater record and in the Hf(t) in detrital zircon, a lack of orogenic gold and volcanic-hosted massive sulfide deposits, and an absence of glacial deposits and iron formations. In contrast, anorthosites and kindred bodies are well developed and major pulses of Mo and Cu mineralization, including the world’s largest examples of these deposits, are features of this period. These trends are attributed to a relatively stable continental assemblage that was initiated during assembly of the Nuna supercontinent by ca. 1.7 Ga and continued until breakup of its closely related successor, Rodinia, ca. 0.75 Ga. The overall low abundance of passive margins is consistent with a stable continental configuration, which also provided a framework for environmental and evolutionary stability. A series of convergent margin accretionary orogens developed along the edge of the supercontinent. Abundant anorthosites and related rocks developed inboard of the plate margin. Their temporal distribution appears to link with the secular cooling of the mantle, at which time the overlying continental lithosphere was strong enough to be thickened and to support the emplacement of large plutons into the crust, yet the underlying mantle was still warm enough to result in widespread melting of the lower thickened crust.

Description: Continental crust is the archive of Earth history. The spatial and temporal distribution of Earth’s record of rock units and events is heterogeneous; for example, ages of igneous crystallization, metamorphism, continental margins, mineralization, and seawater and atmospheric proxies are distributed about a series of peaks and troughs. This distribution reflects the different preservation potential of rocks generated in different tectonic settings, rather than fundamental pulses of activity, and the peaks of ages are linked to the timing of supercontinent assembly. The physio-chemical resilience of zircons and their derivation largely from felsic igneous rocks means that they are important indicators of the crustal record. Furthermore, detrital zircons, which sample a range of source rocks, provide a more representative record than direct analysis of grains in igneous rocks. Analysis of detrital zircons suggests that at least ~60%–70% of the present volume of the continental crust had been generated by 3 Ga. Such estimates seek to take account of the extent to which the old crustal material is underrepresented in the sedimentary record, and they imply that there were greater volumes of continental crust in the Archean than might be inferred from the compositions of detrital zircons and sediments. The growth of continental crust was a continuous rather than an episodic process, but there was a marked decrease in the rate of crustal growth at ca. 3 Ga, which may have been linked to the onset of significant crustal recycling, probably through subduction at convergent plate margins. The Hadean and Early Archean continental record is poorly preserved and characterized by a bimodal TTG (tonalites, trondhjemites, and granodiorites) and greenstone association that differs from the younger record that can be more directly related to a plate-tectonic regime. The paucity of this early record has led to competing and equivocal models invoking plate-tectonic– and mantle-plume–dominated processes. The 60%–70% of the present volume of the continental crust estimated to have been present at 3 Ga contrasts markedly with the 〈10% of crust of that age apparently still preserved and requires ongoing destruction (recycling) of crust and subcontinental mantle lithosphere back into the mantle through processes such as subduction and delamination.