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There Were No Large Volumes of Felsic Continental Crust in the Early Earth GEOSPHERE; V

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There Were No Large Volumes of Felsic Continental Crust in the Early Earth GEOSPHERE; V Research Paper THEMED ISSUE: Subduction Top to Bottom 2 GEOSPHERE There were no large volumes of felsic continental crust in the early Earth GEOSPHERE; v. 13, no. 2 Hugh Rollinson Geoscience, University of Derby, Kedleston Road, Derby DE22 1GB, UK doi:10.1130/GES01437.1 7 figures ABSTRACT Dhuime et al., 2012; Roberts and Spencer, 2015). In the model of Dhuime et al. (2012), the authors calculate that as much as 65% of the present-day volume CORRESPONDENCE: [email protected] New model growth curves for the continental crust based upon Hf iso- of the continental crust had formed by 3.0 Ga. Thus these findings reopen the topes in zircon suggest that large volumes of felsic continental crust were debate initiated by Armstrong (1968, 1991) about the early creation and subse- CITATION: Rollinson, H., 2017, There were no large present in the Hadean and early Archean. These models sit uncomfortably quent destruction of the continental crust. volumes of felsic continental crust in the early Earth: Geosphere, v. 13, no. 2, p. 235–246, doi:10.1130 with estimates of the volume of ancient crust preserved today and imply that The significance of these studies is far reaching because frequently the for- /GES01437.1. the large volumes of crust that were created early in Earth history are now mation of felsic continental crust is linked to the processes of plate tectonics. lost. However, this paper argues that there is no evidence from modern man- So for example Harrison et al. (2005) proposed that because of the large vol- Recieved 30 September 2016 tle geochemistry that very large volumes of early continental crust have been umes of Hadean continental crust that were required by their model, subduc- Revison received 21 November 2016 recycled into the mantle. In contrast, significant volumes of Archean crust may tion and thus plate tectonics were also operating at that time. Dhuime et al. Accepted 20 January 2017 Final version published online 30 March 2017 have been reworked into younger crust, although there is no evidence that this (2012) calculated a crustal growth curve with an inflection at 3.0 Ga. This they process took place in the early Archean and Hadean. interpreted as a fundamental change in geodynamic processes and the start of Geological evidence from the detrital-zircon record does not show evi- plate tectonics on a global scale at 3.0 Ga. dence for large volumes of very early felsic crust, rather, geochemical proxies The purpose of this review is to challenge, on geological grounds, the view for Eo-Archean and Hadean crust strongly suggest that the earliest crusts on that there were large volumes of continental crust in the early Earth. This paper Earth, some of which may have been subaerial, were mafic. A lack of very early explores the geological evidence for the existence of such crust and seeks to felsic crust on Earth calls for a reevaluation of current crustal growth curves understand the implications for present crust and mantle compositions if there and geodynamic models for the start of plate tectonics, the role of supercon- had been large volumes of continental crust that have now been destroyed. In tinents in early continent formation, and the role of the subcontinental litho- addition, the isotope geochemistry community has been quick to comment sphere in continent preservation. The earliest felsic rocks on Earth may have on the veracity of claims for large volumes of continental crust, and this pa- taken the form of oceanic plagiogranites or ocean-island potassic granites as per presents a summary of their critique of this problem (see for example, found in the modern. Guitreau and Blichert-Toft, 2014; Roberts and Spencer, 2015; Payne et al., 2016; Vervoort and Kemp, 2016). INTRODUCTION THE PROBLEM The time at which the first continental crust formed and the rate at which it has subsequently grown have been the subject of scientific debate for several Figure 1 shows some of the recent growth curves calculated for the con- decades. However, in recent years with the advent of precise Hf-isotope mea- tinental crust based upon the study of detrital zircons. These studies have surements in the mineral zircon, new data have been presented that suggest variably used U-Pb ages, Hf-isotope model ages, and oxygen isotopes in their that large volumes of continental crust were present very early in Earth history. construction (Belousova et al., 2010; Dhuime et al., 2012; Roberts and Spencer, Two lines of evidence have been used. Firstly, Harrison et al. (2005) proposed, 2015). An important feature of this figure is the gap between the volumes of on the basis of extreme Hf-isotope mantle depletion in the Hadean, that this continental crust observed on Earth today from the compilation of Goodwin was an indicator that large volumes of continental crust had been extracted (1996) and the volumes estimated in recent growth curves. Goodwin (1996) from the mantle at this time. Secondly, a new generation of crustal growth showed that only ~5% of the present crustal volume is older than 3.0 Ga (as curves have recently been proposed, based upon Hf-isotope model ages for previously indicated by Cawood et al., 2013; Hawkesworth et al., 2013), and For permission to copy, contact Copyright zircon in detrital sediments. These models predict that a large proportion of the while this figure may now need to be revised upwards in the light of more Permissions, GSA, or [email protected]. continental crust had formed very early in Earth history (Belousova et al., 2010; recent geochronology, it still indicates the huge disparity between geological © 2017 Geological Society of America GEOSPHERE | Volume 13 | Number 2 Rollinson | There were no large volumes of felsic continental crust in the early Earth Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/13/2/235/1001654/235.pdf 235 by guest on 26 September 2021 Research Paper 100 4000 A Dhuime et al. (2012) 3500 3000 Roberts & Spencer (2015) y 2500 episodic crustal growth 2000 50 frequenc missing crust 1500 1000 500 Goodwin (1996) Belousova % volume of continental crust baseline crust production et al. (2010) 0 (observed) 1000 2000 3000 4000 U-Pb zircon age (Ma) 0 01000 2000 3000 4000 Age (Ma) Figure 1. The volume of felsic continental crust predicted over geologic time by the recent 4000 growth curves of Belousova et al. (2010), Dhuime et al. (2012), and Roberts and Spencer (2015) high level of crust produc compared with that observed today (Goodwin, 1996) and the resultant “missing crust.” 3500 B 3000 observation and the predictions of recent crustal growth calculations, which suggest that ~65% of the present-day crustal volume was present at 3.0 Ga. y 2500 missing tion This disparity is described as the “missing crust.” crust The problem may be expressed in a slightly different way by examining 2000 missing a frequency histogram of U-Pb zircon ages through time. Figure 2 shows the crust frequenc data of Voice et al. (2011) whose database comprises almost 200,000 U-Pb de- 1500 trital-zircon ages. Plots of this type show time intervals with a high frequency of measured ages separated by time intervals where the number of measured 1000 ages is far fewer. Currently there are different interpretations of these data. missing Some authors argue that there is a baseline of low-level crustal productivity 500 crust (Fig. 2A) upon which there is superimposed a number of episodes of very 0 high crust productivity (Condie and Aster, 2010; Condie, 2014; Parman, 2015). 1000 2000 3000 4000 Other authors argue that there was a high level of crust production throughout U-Pb zircon age (Ma) geologic time but that it has not all been preserved (Fig. 2B). In other words, Figure 2. Two different interpretations of frequency histograms of U-Pb zircon ages for detri- there is “missing crust.” For these authors, the focus is on the time intervals for tal zircons. (A) A relatively low but constant baseline of crust production with superimposed which there are fewer U-Pb zircon ages. These are thought to represent time episodes of crustal growth; (B) a high level of continent crust production showing peaks of intervals when the continental crust was destroyed and so represent periods crust-preserved and troughs of crust-destroyed “missing crust.” Data from Voice et al. (2011). of lack of preservation (Dhuime et al., 2012; Cawood et al., 2013; Condie, 2014). However, in this model there is no simple correlation between the time of crust • Firstly, ancient crust may have been reworked, that is, it now exists as generation and destruction, that is, the crust which is destroyed is not of neces- “younger crust” and has been reconfigured through metamorphism and sity that which has been newly generated (Cawood et al., 2013). partial melting. In this case, of course, the missing crust is not missing in If the “missing crust” is real, there are several possible explanations: an absolute sense, but rather has lost its identity. GEOSPHERE | Volume 13 | Number 2 Rollinson | There were no large volumes of felsic continental crust in the early Earth Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/13/2/235/1001654/235.pdf 236 by guest on 26 September 2021 Research Paper • Secondly, ancient crust may not have been preserved and so has been perhaps synchronized with the super-continent cycle (Dhuime et al., 2012; recycled, i.e., by being returned to the Earth’s mantle and lost from the Hawkesworth et al., 2013). crustal system entirely. This is the process of crustal destruction. Crustal recycling describes the loss of mass from the crust to the mantle.
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