A Planet in Transition: the Onset of Plate Tectonics on Earth Between 3 and 2 Ga?

Geoscience Frontiers 9 (2018) 51e60

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China University of Geosciences (Beijing) Geoscience Frontiers

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Research Paper A planet in transition: The onset of plate tectonics on Earth between 3 and 2 Ga?

Kent C. Condie

Department of Earth and Environmental Science, New Mexico Tech, Socorro, NM 87801, USA article info abstract

Article history: Many geological and geochemical changes are recorded on Earth between 3 and 2 Ga. Among the more Received 5 July 2016 important of these are the following: (1) increasing proportion of basalts with “arc-like” mantle sources; Received in revised form (2) an increasing abundance of basalts derived from enriched (EM) and depleted (DM) mantle sources; 16 August 2016 (3) onset of a Great Thermal Divergence in the mantle; (4) a decrease in degree of melting of the mantle; Accepted 13 September 2016 (5) beginning of large lateral plate motions; (6) appearance of eclogite inclusions in diamonds; (7) Available online 22 September 2016 appearance and rapid increase in frequency of collisional orogens; (8) rapid increase in the production rate of continental crust as recorded by zircon age peaks; (9) appearance of ophiolites in the geologic Keywords: Plate tectonics record, and (10) appearance of global LIP (large igneous province) events some of which correlate with ’ Zircon age peaks global zircon age peaks. All of these changes may be tied directly or indirectly to cooling of Earth s mantle Mantle evolution and corresponding changes in convective style and the strength of the lithosphere, and they may record Stagnant lid the gradual onset and propagation of plate tectonics around the planet. To further understand the Continental crust changes that occurred between 3 and 2 Ga, it is necessary to compare rocks, rock associations, tectonics LIP events and geochemistry during and between zircon age peaks. Geochemistry of peak and inter-peak basalts and TTGs needs to be evaluated in terms of geodynamic models that predict the existence of an episodic thermal regime between stagnant-lid and plate tectonic regimes in early planetary evolution. Ó 2016, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).

1. Introduction To tie geodynamic models to the geologic record, it is necessary to have precise ages of global events in the period of time from 4 to Although many geologic and geochemical changes are reported 2 Ga. We have made progress in this direction in the last 10 years at the end of the Archean (Taylor and McLennan, 1985; Condie and from advances in zircon geochronology, and our zircon U/Pb age O’Neill, 2010; Condie, 2015a), the timing, duration, and causes of database has grown rapidly for both igneous and detrital zircons these changes are not well known. Geodynamic modeling suggests (Condie and Aster, 2010; Voice et al., 2011; Hawkesworth et al., that Earth may have begun as a hot stagnant-lid similar to Io and 2013; Roberts and Spencer, 2015). In this contribution, I review evolved through an episodic transitional state into plate tectonics ten geologic and geochemical changes that occurred on Earth be- over 1e3 Gyr (O’Neill et al., 2016). Perhaps the changes near the end tween 3 and 2 Ga. Any transition of thermal and tectonic regimes of the Archean reflect this transitional state. Modeling has shown during this time interval must accommodate these changes. that under hotter conditions typical of the early Earth, subducted slabs are weaker and more likely to detach and sink into the mantle 2. Transitional events between 3 and 2 Ga (Van Hunen and Moyen, 2012), and this should lead to episodic subduction events in a stagnant lid tectonic regime. Also the lower 2.1. Increasing abundance of basalts with “arc-like” mantle sources viscosity of the Hadean mantle would result in lower lithospheric stress levels preventing the initiation of subduction, which could Because ratios of elements with similar incompatibilities do not promote “bursts” of subduction as mantle cooling progressed change much with degree of melting or fractional crystallization of (O’Neill et al., 2007; O’Neill and Debaille, 2014). mantle-derived magmas, they are useful in characterizing compositions of mantle sources of basalts that have not interacted with E-mail addresses: [email protected], [email protected]. continental crust (Condie, 2003, 2005). Using Nb/Th and Zr/Nb Peer-review under responsibility of China University of Geosciences (Beijing). ratios in young oceanic basalts, three mantle domains can be http://dx.doi.org/10.1016/j.gsf.2016.09.001 1674-9871/Ó 2016, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). 52 K.C. Condie / Geoscience Frontiers 9 (2018) 51e60 identified (Condie, 2015b): enriched (EM), depleted (DM), and hydrated mantle (HM). DM is characterized by high values of both ratios (Nb/Th > 8; Zr/Nb > 20) due to Th depletion relative to Nb, and to Nb depletion relative to Zr. EM has high Nb/Th ratios but typically has Zr/Nb ratios <20. In contrast, HM has very low Nb/Th (<8) and variable Zr/Nb ratios. Similar domains are recognized with Th/YbeNb/Yb and La/SmeNb/Th relationships. DM is most widespread in the asthenosphere tapped at ocean ridges, but also may appear in mantle wedges associated with subduction (Hofmann, 1997; Stracke, 2012). EM is a common mantle source for basalts erupted in oceanic plateaus and islands, where it is thought to occur in mantle plumes and as inhomogeneities in the asthenosphere. As shown in Fig. 1, the proportion of oceanic basalts with HM sources increases with time from values around 20% in the Eoarchean to about 80% by the end of the Archean and remains at the 80%e100% level thereafter. The only exceptions are basalts from Karelia in Finland and Russia around 2 Ga; only 60%e70% of these rocks reflect HM sources. Although it is possible that HM basalts can also be produced in a stagnant lid regime, the rapid increase in the proportion of basalts with an HM source between 3.5 and 2 Ga is consistent with rapid propagation of plate tectonics around the globe during this time interval.

2.2. Increase in abundance of basalts derived from enriched (EM) and depleted (DM) mantle sources

It is possible that comparison of incompatible element ratios in basalts from terrestrial oceanic tectonic settings with mare basalts from the Moon and to shergottites from Mars may be useful in constraining when plate tectonics began on Earth, since these two bodies are examples of stagnant lid planets. Terrestrial oceanic basalts from greenstones and their corresponding mantle sources (EM, DM, HM) can be tracked to 2.5e2.0 Ga with geologic and geochemical characteristics (Condie, 2015b)(Figs. 2 and 3). Prior to this time, however, DM and EM basalts are difﬁcult to distinguish in terms of incompatible element ratios, and the two populations

Figure 2. Zr/NbeNb/Th graph for oceanic non-arc basalts. (a) EM basalts, 2.5e0.15 Ga; Figure 1. Secular change in the percent of oceanic basalts derived from HM (hydrated (b) DM basalts, 2.5e0.15 Ga; (c) EM-DM undifferentiated oceanic basalts 3.4e2.5 Ga. mantle) sources with age. Each point is the median value for a 100-Myr moving PM: primitive mantle from McDonough and Sun (1995); CONT: average continental window. Graph does not include samples that are contaminated with older continental crust from Rudnick and Gao (2014). Data from Condie (2015b) and Condie and Shearer crust. Data from Condie and Shearer (2016). (2016). K.C. Condie / Geoscience Frontiers 9 (2018) 51e60 53

Figure 3. Zr/Nb versus age in oceanic non-arc basalts. Arrowheads are median values with 1s uncertainty as vertical purple bars. DM: depleted mantle; PM: primitive mantle; EM: enriched mantle; undifferentiated basalts include only those with oceanic affinities. Other information given in caption of Fig. 2. seem to collapse into one group with a Zr/Nb ratio of about 20 (Fig. 3). This distribution is similar to the distribution of most mare basalts from the Moon sampled by the Apollo missions, except for the high-Ti basalts from Apollo 11 and 17 both of which are enriched in Nb due to ilmenite fractionation. The primitive mantle grouping is especially evident for Apollo 11, 12 and 15 basalts in which samples cluster tightly around primitive mantle (PM) composition on Zr/NbeNb/Th (Fig. 4), Th/YbeNb/Yb and La/ SmeNb/Th diagrams (Condie and Shearer, 2016). Shergottites fall into three groups (enriched, intermediate and depleted; Greenough and Ya’acoby, 2013), which form a scattered population on the incompatible element ratio diagrams as illustrated by the Zr/ NbeNb/Th diagram (Fig. 4b). Black Beauty is a shergottite breccia that may be a sample of mixed mantle sources and it falls near primitive mantle (PM) composition. The other shergottites have Zr/ Nb ratios both below and above primitive mantle. Even more important is the fact that none of the lunar and very few of the martian basalts fall in the terrestrial DM or EM fields on the incompatible element diagrams. The fact that prior to 2.5 Ga, Earth’s non-arc-like oceanic basalts Figure 4. Zr/NbeNb/Th graph for lunar and martian basalts. (a) Apollo 11, 12 and 15 have similar incompatible element signatures suggests they come lunar basalts; (b) Martian shergottites and nakhilite. Classification of shergottites from a relatively unfractionated mantle source similar to primitive (enriched, intermediate, depleted) from Greenough and Ya’acoby (2013). Data from mantle in composition. Such a source may be characteristic of a Condie and Shearer (2016), other information given in caption of Fig. 2. stagnant lid planet like Mars or a satellite like the Moon, and if so, this suggests that Earth also may have been in a stagnant lid regime from DM and HM basalts, there is very little change in the average prior to 2.5 Ga. An onset of plate tectonics on Earth around 3 Ga Tp of EM basalts, which remains close to 1500 C through time. This may have led to the production and growth of DM and EM mantle means that Tp of EM sources was indistinguishable from ambient sources after this time. From studies of terrestrial mantle xenoliths mantle (DM-HM) before 2.5e2.0 Ga, and during this time interval (Griffin et al., 2003) and from geodynamic modeling (Komiya, the temperature of these sources began to diverge, which is 2004), it has also been shown that the composition of the upper referred to as a “Great Thermal Divergence” in the mantle (Condie mantle changed rapidly during the Proterozoic. et al., 2016a)(Fig. 5). This divergence is interpreted to reflect cooling of ambient mantle from 2.5 Ga onwards, and prior to 2.5 Ga, 2.3. Onset of a Great Thermal Divergence in the mantle EM and DM-HM are indistinguishable. Perhaps this cooling could reflect the onset and propagation of subduction between 3 and Based on major element distributions in oceanic basalts through 2 Ga. DM and HM also show an increase in Mg# after 2.5 Ga, which time, Condie et al. (2016a,b) estimated magma generation tem- is consistent with progressive depletion of ambient mantle after peratures (approximately equal to mantle potential temperatures this time (Condie et al., 2016a). Komatiites come from a high- [Tp]) of the three types of mantle HM, DM and EM. Young EM ba- temperature mantle source (probably from plumes generated in 00 salts yield an average Tp about 150 C higher than the ambient the D layer at the base of the mantle), which may have cooled upper mantle temperature as recorded by DM basalts, whereas in slightly at the end of the Archean. Although this source is recog- the Archean the calculated Tps from DM and EM basalts are nized throughout time, it is rarely tapped after the end of the indistinguishable (ca. 1500 C) (Fig. 5). Unlike the calculated Tps Archean, when komatiites decrease rapidly in abundance. 54 K.C. Condie / Geoscience Frontiers 9 (2018) 51e60

show that this trend cannot be related to changes in the degree of 1700 fractional crystallization, but is probably related to a decreasing KM Komatiite plumes degree of melting of the mantle through time as Earth cooled. Their 1600 results show a progressive decrease in average melt fraction in STAGNANT LID TECTONICS mantle sources from about 0.35 in the Archean to 0.1 today. PLATE TECTONICS Transitional Tectonics Although as pointed out by Keller and Schoene (2012), this rapid EM decrease in degree of melting is at least partly due to non-linear 1500 EM type Plumes melting of the mantle and exhaustion of clinopyroxene in the DM+HM source, the cooling rate might also be enhanced by increasing the volume of cool slabs subducted into the mantle. Most notably is the rapid decrease in cooling rate between 3 and 2 Ga (Fig. 6). Again, Ambient Mantle this is consistent with the rapid spread of plate tectonics during this Mantle Potential Temperature (°C) Temperature Mantle Potential 1350 time interval.

0 2.5 4 2.5. Beginning of large lateral plate motions Age (Ga) Figure 5. Mantle potential temperature as calculated from major element composi- Perhaps the most robust argument for plate tectonics is paleo- tions of oceanic basalts showing a Great Thermal Divergence beginning at the end of magnetic evidence for large lateral plate motions in the geologic the Archean. Mantle sources: KM, komatiite; EM, enriched; DM, depleted; HM, hy- past. Although robust paleomagnetic data clearly indicate that by drated. Modified from Condie et al. (2016a). 3.5 Ga Earth had a stable geomagnetic field probably undergoing magnetic reversals (Usui et al., 2009; Biggin et al., 2011), evidence 2.4. Decrease in degree of melting of the mantle for large lateral plate motions during the Archean remains controversial. To unambiguously support large lateral plate mo- Keller and Schoene (2012) documented a decrease in compat- tions, it is critical to have demagnetization data that includes vector ible element contents and a corresponding increase in incompat- subtraction, precise dating of the magnetization, an adequate ible elements in mafic igneous rocks through time (Fig. 6). They number of samples, field tests to constrain the magnetization age, and tectonic coherence within a given block or craton (Van der Voo, 1993; Pisarevsky et al., 2014). The oldest lateral plate motions of 15 thousands of kilometers for which such high-quality paleomagnetic evidence is available is around 2e2.5 Ga. Mitchell et al. (2014) showed partial apparent polar wander paths for the Slave and Su- 12 perior cratons that indicate large separation of these cratons before final collision and amalgamation around 2.0e2.2 Ga. Likewise, the 1400 paleomagnetic results of Bispo-Santos et al. (2014) showed large lateral motions of the Guiana and West African cratons leading to MgO (wt. %) 9 their linkage into a single craton at 1.98e1.96 Ga. Using angular 1000 speeds of plates calculated from paleomagnetic data, Condie et al. (2015a) showed that average plate speed has increased with time 6 from about 30/100 Myr at 2 Ga to 50/100 Myr in the last 100 Myr 600 Cr (ppm) (Fig. 7). Hence, it would appear that at least by 2.5e2.0 Ga, plate 500 tectonics was operative on Earth, although whether or not it covered the entire planet is still uncertain. 200 2.6. Appearance of eclogite inclusions in diamonds 300

Ni (ppm) Although Archean eclogites are known both in outcrop and as mantle or crustal xenoliths, eclogites do not become common until after the end of the Archean (Rollinson, 1997; Barth et al., 100 2001). Maﬁc crust will invert to eclogite at high pressures, beginning at about 50 km depth and should sink into the mantle, which is less dense. Another way to track recycling of eclogite into 40 the mantle is with mineral inclusions in diamonds that formed in thick Archean lithospheric roots. Diamonds tend to protect the 30 inclusions from later metasomatic effects and alteration. Shirey and Richardson (2011) recognized both peridotitic and eclogitic 20 inclusions in diamonds from kimberlite pipes, the latter of which appear rather suddenly around 3 Ga. This provides compelling 10 evidence that from 3 Ga onwards eclogite was captured and

Apparent Percent Melt preserved in the subcontinental lithosphere, perhaps linked to 0 1000 2000 3000 4000 deep subduction. The absence of eclogite inclusions in Archean Phanero- zoic Proterozoic Archean diamond populations is even more important in than it suggests Age (Ma) that subduction did not occur prior to this time. The major caveat to this conclusion is that most of the data come from the Kaapvaal Figure 6. Secular changes in MgO, Ni and Cr in maﬁc igneous rocks and calculated change in percent melting of the mantle. Error bars are 2 standard errors of the mean. craton in southern Africa, and hence there could be a geographic Modiﬁed from Keller and Schoene (2012), courtesy of Brenhin Keller. bias. K.C. Condie / Geoscience Frontiers 9 (2018) 51e60 55

controversial and uncertain. Because both 3 Hf and 3 Nd distributions with age show prominent vertical data arrays, growth models based on mixing of mantle and crustal end members avoid the model age uncertainties (Condie et al., 2016b). Also, any model for the growth of continental crust must address the episodicity of ages shown by both outcrop and detrital zircon populations (Condie and Aster, 2010, 2013; Hawkesworth et al., 2013). Whole- rock Nd isotopes in convergent-margin-related felsic igneous rocks show the fraction of juvenile continental crust produced during zircon age peaks in the last 2 Gyr is similar to that produced during age valleys (Condie et al., 2016b). This suggests that the mechanisms of continental growth are similar in age peaks and valleys, most probably related to subduction. Unlike crustal growth models based on Hf model ages, the Nd end-member mixing model shows a step-like cumulative growth curve (curve 3, Fig. 8) for continental crust with rapid growth at 3.0e2.4, 2.0e1.3 and 0.6e0.45 Ga. The dominance of juvenile continental crust in accretionary orogens does not require selective preservation during craton-craton collisions (as suggested by Hawkesworth et al., 2009) as an explanation of global zircon age peaks, and thus favors the interpretation of these peaks as crustal production peaks (Condie et al., 2016b). The Nd-based continental growth pattern differs significantly from results based on Hf model ages, which show most continental growth between 4 and 2 Ga and lacks the step-like pattern of the Nd model (Belousova et al., 2010; Dhuime et al., 2012)(curve3,Fig. 8). The Nd model shows about 50% of continental growth by 1.9 Ga, whereas the Hf Figure 7. Secular changes in craton collision frequency and average area-weighted e plate speed (deg/100 Myr) (modified from Condie et al., 2015a). Collision frequency models show 50% growth by 3.5 3.0 Ga. The rapid growth of between cratons is expressed as number of orogen segments per 100-Myr bin moving continental crust based on the Nd isotope model at 0.6e0.45 Ga in 100 Myr increments. Lines are linear regression analysis: n ¼ 8.68 e 0.00224a, corresponds to the assembly of Pangea-Gondwana, the 2.0e1.3 Ga r ¼ 0.287; s ¼ 9.927 0.00223a, r ¼ 0.393 (n, number of orogens; s, plate speed growth partly overlaps the assembly of Nuna at 1.7e1.5 Ga, and fi divided by ve; a, age). Also shown are supercontinent assembly (blue stripes) and the 3.0e2.4 Ga growth parallels supercraton assembly in the breakup (pink stripes) times. Neoarchean at 2.7e2.5 Ga (Condie et al., 2016b). Even detrital zircons preserve a message of change in the Paleoproterozoic. The 2.7. Appearance and rapid increase in frequency of collisional d18O of detrital zircons shows a large increase in the amount of orogens reworked sediment between 2.7 and 2 Ga, probably reflecting the rapid growth of continental crust (Spencer et al., 2014). However, A collisional orogen is an orogen produced by the collision of for all models of continental growth, the amount of material two cratons, such as the modern Himalayan and Alpine orogens. recycled back into the mantle is uncertain, so the cumulative The earliest well documented appearance of collisional orogens is growth curves (Fig. 8) are minima in terms of absolute crustal near the end of the Archean: Majorquq 2.6 Ga in West Greenland growth. (Dyck et al., 2015), MacQuoid 2.56 Ga in Western Canada (Pehrsson Numerous geochemical trends in the continental crust show et al., 2013), Commonwealth Bay 2.5 Ga in Antarctica (Menot et al., increasing rates beginning in the Archean, and these are consistent 2005), Nito Rodrigues 2.48 Ga in Uruguay (Hartmann et al., 2004), and Sleafordian 2.47 Ga in South Australia and the Mawson craton in Antarctica (Hand et al., 2007). Although there are numerous orogens in the Archean, most appear to accretionary orogens, although some investigations have reported terrane collisions within Archean accretionary orogens. However, terrane collisions 80 1 do not require plate tectonics as they may occur in response to fl 2 convective ow in the asthenosphere (Harris and Bedard, 2014). By 3 2.0 Ga, craton-craton collisions are common in the geologic record 60 with a large peak in frequency at about 1.9 Ga (Fig. 7)(Condie et al., 50% 2015a). This dramatic increase in frequency of craton collisions in 40 the Paleoproterozoic would seem to demand widespread propa-

gation of plate tectonics. Volume Continental Crust (%) 20

2.8. Rapid increase in volume of continental crust 1.0 2.0 3.0 4.0 In recent years Hf isotopes have been the preferred method to Age (Ga) constrain continental growth rates (Condie and Aster, 2010, 2013; Dhuime et al., 2011, 2012; Hawkesworth et al., 2013). However, as Figure 8. Models for cumulative growth of continental crust. Curves 1 and 2 are based discussed in detail by Roberts and Spencer (2015), there are many on Hf isotope model ages of zircons and curve 3 is based on whole-rock Nd isotopes uncertain assumptions in using Hf model ages and hence growth and zircon ages. 1, Dhuime et al. (2012);2,Belousova et al. (2010);3,Condie et al. rates based on Hf (or other) isotopic model ages remain (2016b). 56 K.C. Condie / Geoscience Frontiers 9 (2018) 51e60 with both increasing rates of growth of continental crust and with the widespread propagation of plate tectonics between 3 and 2 Ga. For instance, rapid decreases in Ni/Co and Cr/Zn in shales, which track MgO distribution in the sources, suggest that the upper continental crust changed from largely mafic before 3 Ga to felsic by 2.5 Ga (Tang et al., 2016). This compositional change involved a fivefold increase in the mass of the upper continental crust due to addition of felsic igneous rocks, consistent with an onset of global plate tectonics around 3.0 Ga. In another study, Dhuime et al. (2015) show a dramatic increase in Rb/Sr ratio in igneous rocks between 3 and 2 Ga. They interpret this change to reflect rapid growth of continental crust associated with the onset and propagation of plate tectonics. Significant increases in large-ion lithophile and high-field strength elements in continental crust also occur between 3 and 2 Ga (Condie and O’Neill, 2010; Condie, 2015a). De- creases in (La/Yb)n and Sr/Y and an increase in K2O/Na2Oin granitoids after 2.5 Ga reflect greater degrees of fractionation of felsic magmas at shallower depths. Also attesting to a change in the composition of upper continental crust from mafic (and komatiitic) to felsic is a drop in the Ni/Fe ratio in banded iron formations between 2.7 and 2.0 Ga (Konhauser et al., 2009).

2.9. Appearance of ophiolites in the geologic record Figure 9. Secular distribution of LIPs (large igneous provinces). Data from Condie et al. The term ophiolite has been used and misused in the literature (2015b with updates). such that the meaning is ambiguous (Condie, 2015a,b). Some investigators have gone so far as the call almost all Archean green- < stones “ophiolites” (Furnes et al., 2015). Herein, I use ophiolite to or at most regional geographic extent, and of the LIP events 1.9 Ga, refer to a preserved fragment of crust (mantle) formed at an only the 100 Ma event has been widely sampled and discussed. ocean ridge (associated with or not associated with subduction). Why should so many global LIP events occur in the 3 to 2 Ga time This means that arc volcanics without evidence that they interval? Possibly rapid accumulation of subducted slabs at the formed at ocean ridges should not be called ophiolites. Ophiolites base of the mantle was responsible for triggering widespread should preserve one or more of the definitive components mantle plume events. (such as sheeted dykes, podiform chromite, and serpentinized harzburgite). Although ophiolites have been reported in the 3. Discussion Archean, none of them has passed this rigorous definitional filter. The first appearance of ophiolites is around 2 Ga, and we have at Although it has long been recognized that significant changes least six well-described ophiolites ranging in age from 2.1 to occur at the end of the Archean, it is now clear that most, if not all, 1.85 Ga (Table 1). Only two of these contain complete ophiolite of these changes were not sudden, but occurred more gradually stratigraphy: Purtuniq and Jormua (Scott et al., 1992; Peltonen over 1 Gyr. They all seem to be tied directly or indirectly to cooling et al., 1996). A review of the greenstone literature, however, sug- of Earth’s mantle and corresponding changes in convective style gests that many dismembered ophiolites occur in this time range and the strength of the lithosphere, and they may record the possibly extending to 2.5 Ga. Clearly, the first robust evidence we gradual onset and propagation of plate tectonics around the planet. have of ocean ridges appears in the Paleoproterozoic, and thus Although many questions should be addressed to better under- requires plate tectonics. stand these changes, I would like to focus on four questions related to the ten transitions discussed above. 2.10. Appearance of global LIP events 3.1. Can felsic crust be produced in a stagnant lid tectonic regime, and if so, in what volume can it be produced? Global LIP (large igneous province) events, defined as occurring on six or more cratons, are generally thought to be the result of Do large volumes of TTG (tonalite-trondhjemite-granodiorite) widespread mantle plume events (Condie et al., 2015b). They are magma require convergent margins, with either thickened mafic first recorded beginning about 3 Ga with six significant events crust or/and descending slabs as TTG sources? Or can these between 3 and 2 Ga (3.0, 2.7, 2.45, 2.2, 2.05, and 1.9 Ga; Fig. 9). rocks also be produced in a stagnant lid setting by mechanisms Although older LIP events are known, they appear to have only local such as heat-pipe cycling or mantle plumes (Bédard, 2006; Van Kranendonk, 2010; Moore and Webb, 2013; Johnson et al., Table 1 2014)? If we examine modern oceanic plateaus as the closest Examples of Paleoproterozoic ophiolites. analog to a stagnant lid, Iceland is the only example where felsic Location Age (Ma) Reference igneous rocks occur in a significant volume (20% of the igneous Kandra, India 1850 Kumar et al. (2010) component). Most of these rocks are felsic volcanics and do not Nagssugtoquidian, Greenland 1900e1850 Garde and Hollis (2010) show the geochemical characteristics of typical continental crust Amisk, Flin Flon, Canada 1900 Stern et al. (1995) (e.g., Nb-Ta depletion) and even the zircons from such rocks do not Jormua, Finland 1995 Peltonen et al. (1996) have trace element signals of heavy-REE depleted Archean TTG Purtuniq, Eastern Canada 1998 Scott et al. (1992) (Carley et al., 2014). An exception is the 3.9 Ga component in the Central Karelia 2100 Stepanova et al. (2014) Archean Acasta gneisses which resembles some Iceland felsic K.C. Condie / Geoscience Frontiers 9 (2018) 51e60 57 volcanics in terms of heavy REE (Reimink et al., 2016). But perhaps derived plumes (Herzberg and Asimow, 2008; Herzberg and we are just seeing the surficial felsic components in Iceland and at Gazel, 2009). In contrast, in the Archean high-T plumes may greater depths, yet to be exposed, there are better examples of have been widespread. To more fully test this idea, we need to Archean-like TTG. Experimental results indicate that heavy-REE analyze more oceanic basalts especially in the age range of depleted TTGs can be produced by partial melting of wet mafic 2.5e2.0 Ga, when thermal regimes were probably changing in the rocks in oceanic plateaus at depths 50 km (Johnson et al., 2014; mantle, possibly in response to widespread propagation of plate Hastie et al., 2015). But the question remains as to what volume tectonics. In turn, geodynamic modeling of mantle plume pro- of TTG magma can be produced in such a setting (Condie, 2014)? duction needs to focus on this same period of time. One particu- The sources for TTG magmas are wet eclogites and garnet am- larly important question is when did low-T plumes come into phibolites in the deep crust (50 km) or upper mantle (Moyen and existence, and what are the geodynamic changes in the mantle Martin, 2012; Condie, 2014). However, only if wet eclogites that led to their production? Another important question is when continue to sink into the mantle can a continuous supply of TTG be and how rapidly did the LLSVPs grow in the deep mantle (Garnero produced (Vlaar et al., 1994; Zegers and van Keken, 2001; Bédard, et al., 2016)? If they grew as a result of rapid accumulation of 2006; Van Kranendonk, 2010; Johnson et al., 2014). As mafic and subducted slabs, they may record the timing and rate of devel- felsic crust are produced, a growing volume of depleted harzburgite opment of plate tectonics. Clearly we need more geodynamic (olivine þ orthopyroxene) is left behind, and because it lacks models that focus on growth rate of the LLSVPs through geologic garnet, it will not readily sink but accumulates beneath the crust time. (Herzberg and Rudnick, 2012; Sizova et al., 2015). Unless eclogite can sink through the growing harzburgite root, TTG production 3.3. Of what significance are global zircon age peaks and LIP should soon come to an end with only small volumes produced, not events? enough to account for the vast volumes we see in Archean crust often being produced for over 100 Myr in some locations. The Although global age peaks in detrital zircon populations are growing thick harzburgite root may also impede the transfer of new well established, the significance of these peaks is still not well basaltic magma from deep fertile mantle sources to the surface. understood and remains a topic of continued controversy and Another question is that of how water is continuously supplied to discussion (Hawkesworth et al., 2009; Condie et al., 2016b). At the sinking mafic crust, a necessity for continuous production of least part of the answer to this question lies with the interpreta- TTG magma by partial melting (Rapp et al., 1991). If oceans covered tion of the timing of global LIP events, which are probably the a Hadean stagnant lid, a continual supply of sinking hydrated ba- product of widespread mantle plume events. Some major LIP salts could result in prolonged TTG production (Nagel et al., 2012; events correlate well with zircon age peaks (e.g., 2.7, 2.2, and Zhang et al., 2013), as permitted for instance by the heat-pipe 1.9 Ga; Fig. 9), whereas most do not have zircon age peak coun- model of Moore and Webb (2013). Water or not, however, eclo- terparts. Whether the enhanced peak heights of LIP events be- gite would need to continually sink through the thick harzburgite tween 3 and 2 Ga (Fig. 9) are real or a product sampling bias is not root, and we need more models that focus on this question. With yet known. An understanding of these events is also tied to a our current understanding, subduction is the only known way to better understanding of the supercontinent cycle and the age and produce large volumes of TTG over time periods of 100 Myr. origin of the LLSVPs mentioned above. The studies of Jellinek and Manga (2004) suggest that plate tectonic events during which 3.2. Are there two sources for plumes in the mantle after 2.5 Ga? slabs collapse to D00 in short times may initiate mantle plume events. If the collapsing slabs are swept into the LLSVPs by con- As pointed out by Burke (2011),mostLIPs(largeigneous vection as suggested by Burke (2011), the frequency of LLSVP- provinces) and many hotspots <300 Ma correlate with and derived plumes may also increase during the slab collapses. In probably come from LLSVPs (large low S-wave velocity provinces) contrast, Arndt and Davaille (2013) have suggested that pre-1.5 Ga in the deep mantle beneath Africa and the South Pacific. However, zircon age peaks reflect crustal growth peaks caused by global other plumes, and especially those in the Northern Hemisphere, mantle plume events, which are the products of thermal-chemical appear to come from the D00 layer at the base of the mantle. Has instabilities at the base of the mantle. In this scenario, it is the this always been so in earlier periods of time, and when and how mantle plume events that increase the rate of subduction leading did the LLSVPs come into existence? Reexamination of the ther- to increases in continental growth, and thus to zircon age peaks. mal history of the mantle based on basalt major element Geodynamic modeling has not yet yielded an unambiguous so- geochemistry (Condie et al., 2016a) suggests that plumes that give lution to this problem. Although most models agree that a stag- rise to basalts from enriched mantle (EM) sources appeared only nant lid should transition into a plate tectonic regime with bursts after the end of the Archean. Before this time, mantle potential of plate tectonic activity (O’Neill et al., 2007), the timing of these temperatures for oceanic basalts were similar (around 1500 C) bursts in various models ranges from a few million years (Van and incompatible element ratios show that the enriched (EM) and Hunen and Moyen, 2012; Sizova et al., 2015) to billions of years depleted (DM) mantle sources were not yet well developed (Weller et al., 2015). Statistical correlation of global LIP and zircon (Condie, 2015b)(Figs. 2 and 3). It is probable, however, that age peaks shows a strong periodicity in both around 270 Myr Archean komatiites came from plume sources that were consid- (Isley and Abbott, 2002; Puetz et al., 2016), and this agrees well erably hotter (1700 C) than ambient mantle (Herzberg and with the experimental studies of Davaille et al. (2005) which show Asimow, 2008). These observations suggest that prior to 2.5 Ga, destabilization in the D00 layer giving rise to mantle plume events there may have been only one source of mantle plumes (the D00 every 100e300 Myr. The question still remains, however, of what layer), and that it was not until after this time that lower tem- causes destabilizations in D00 and especially what caused the first perature (w1500 C) plumes could survive and make it to the base large destabilization a 2.7 Ga? Do these events require cata- of the lithosphere. If the lower temperature plumes come from strophic collapse of subducted slabs into D00 or could they be the LLSVPs, this means the LLSVPs did not come into existence until result of poorly understood events in the core? If the first “burst” after the Archean. Today, high-T plumes are rare and Gorgona is of plate tectonics began about 3 Ga, perhaps arrival of newly- the only known example producing komatiites (low-Mg types produced slabs in D00 was responsible for the first global mantle only), and its mantle potential temperature is similar to LLSVP- plume event at 2.7 Ga. Alternatively, could catastrophic growth of 58 K.C. Condie / Geoscience Frontiers 9 (2018) 51e60 the inner core or intense turbulence in the outer core (Andrault et al., 2016) have been responsible for destabilization of D00 at 2.7 Ga? In this case, it would have been events in the core that promoted rapid continental growth at 2.7 Ga. Again, better con- strained geodynamic modeling is required to adequately address these questions.

3.4. Did Earth transition from stagnant lid to plate tectonics between 3 and 2 Ga?

Geodynamic modeling indicates that rocky planets should evolve from stagnant lid to plate tectonics through an episodic regime (O’Neill et al., 2007). Can we recognize this episodic regime in the geologic record? This is perhaps the most difficult question to address because we really cannot yet identify stagnant-lid rock packages. Many greenstone belts are characterized by extensive “mafic plains” composed of submarine basalts and associated shallow intrusives in the early stages of development, often over- lain by “arc-like” successions (Wyman, 1999; Benn and Moyen, 2008; Turner et al., 2014). Could these mafic plains represent a return to stagnant-lid tectonics? As pointed out above, incompatible element ratios indicate the DM and EM basalts are rare before 2.5 Ga and thus it is geochemically permissible for the Archean mafic-plain basalts to be erupted in a stagnant-lid regime. The overlying “arc-like” sequences could be the results of short-lived Figure 10. Frequency of ages of detrital zircons from modern rivers in the time period “bursts” in subduction activity. of 2600 to 1950 Ma. Data from Puetz et al. (2016). Evidence clearly shows that recycling of crustal material into the deep mantle occurred prior to 3 Ga. For instance, sulfur isotopes in 3.5. What the future holds? sulfides in young basalts show that crust must have been recycled into the mantle before the Great Oxidation Event at 2.5e2.4 Ga. To make progress in a better understanding of the evolution of Also the preservation of 142Nd and 182W anomalies in some Archean Earth between 4 and 2 Ga, we need to address the four questions (and younger) basalts appears to reflect an early crust-forming discussed above. It is necessary to compare rocks, rock associations event during which the crust was recycled into the deep mantle and tectonics during and between zircon age peaks. In particular, and later, remnants of this crust and/or the depleted restite served incompatible element ratios in age peak and inter-peak basalts and as sources for some basalts (Caro et al., 2003; Bennett et al., 2007; TTGs need to be compared. In particular, we need more geologic Touboul et al., 2012). However, recycling of crust into the mantle and geochemical data in the time window of 1.6e0.8 Ga. One po- does not necessarily require subduction, and it may be possible for tential problem is that the width of zircon age peaks overlaps the such recycling to occur in stagnant-lid regimes such as heat-pipes valleys between the peaks and the position and width of peaks and and mantle plumes (Bédard, 2006; Moore and Webb, 2013; valleys varies with geographic location. We need to know whether Sizova et al., 2015). the changes we see in Earth history between 3 and 2 Ga are due to It has been suggested that the last return to a stagnant-lid the onset and propagation of plate tectonics or to some other cause regime occurred during a possible crustal age gap at 2.4e2.2 Ga that may have followed the onset of plate tectonics. On the other (Condie et al., 2009). The problem with this idea is that the crustal hand, if stagnant-lid tectonics transitioned to plate tectonics be- age gap is rapidly disappearing as more data are becoming tween 4 and 3 Ga, we should find corresponding changes in the available from greatly undersampled geographic regions. We now geologic and geochemical record, which so far have not been have an increasing inventory of both detrital and outcrop zircon identified. The first global pulse of crustal generation is at 2.7 Ga as ages in the 2.4e2.2 Ga “age gap” (Fig. 10). Three orogens contain reflected by the large zircon age peak at this time, whereas earlier exposed TTG-greenstone associations in this time window: the pulses may be of only regional extent. To address and eventually Arrowsmith orogen in Western Canada (Berman et al., 2013; answer these questions, we need to constrain geodynamic models Partin et al., 2014), the orogen(s) around the Sao Francisco such that they better accommodate geologic and geochemical da- craton in Brazil (Klein et al., 2009; Teixeira et al., 2015), and the tabases, especially focusing on the time period of 4 to 2 Ga. This is a Trans-North China orogen (Luo et al., 2008; Yang and Santosh, critical time in Earth history as internal radiogenic heat sources 2015). Other less-well defined orogens that contain igneous decayed and the mantle correspondingly cooled. Many review pa- rocks produced in 2.4e2.2 Ga time window have also been iden- pers have appeared in the last few years on the transition from tified in Australia, Antarctica, West Africa and other locations in stagnant-lid to plate tectonics, and these have served to focus our South America (Gasquet et al., 2003; Swain et al., 2005; Wang attention on specific problems where collaboration is needed be- et al., 2016). There is also evidence in the detrital zircon popula- tween investigators in different disciplines (examples are Cawood tion for an increasing peak at about 2350 Ma (Fig. 10). Hence, if et al., 2006; Korenaga, 2008, 2013; Van Hunen and Moyen, 2012; Earth transitioned from stagnant-lid to plate tectonics between 3 Condie and Aster, 2013; Hawkesworth et al., 2013; O’Neill and and 2 Ga, we need to be able to identify rocks formed during the Debaille, 2014; Roberts and Spencer, 2015; O’Neill et al., 2016). stagnant-lid periods, which means more intense sampling and What we do not need more of at the present time are more review analysis of rocks formed in the valleys between the zircon age papers on this topic; rather we need investigators to return to the peaks. From the information at hand, there is nothing that stands drawing board an come up with some new, innovative models that out as being different in rocks formed between from those formed accommodate existing and new data from a variety of scientific during the age peaks. disciplines. K.C. Condie / Geoscience Frontiers 9 (2018) 51e60 59

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