Why Primordial Continents Were Recycled to the Deep: Role of Subduction Erosion

Geoscience Frontiers xxx (2016) 1e10

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Research paper Why primordial continents were recycled to the deep: Role of subduction erosion

S. Azuma a,*, S. Yamamoto b, H. Ichikawa a,c, S. Maruyama a a EartheLife Science Institute, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550, Japan b Graduate School of Environment and Information Sciences, Yokohama National University, 79-7 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan c Geodynamics Research Center, Ehime University, 2-5 Bunkyo-cho, Matsuyama 790-8577, Japan article info abstract

Article history: Geological observations indicate that there are only a few rocks of Archean Earth and no Hadean rocks on Received 7 April 2016 the surface of the present-day Earth. From these facts, many scientists believe that the primordial Received in revised form continents never existed during Hadean Earth, and the continental volume has kept increasing. On the 4 August 2016 other hand, recent studies reported the importance of the primordial continents on the origin of life, Accepted 10 August 2016 implying their existence. In this paper, we discussed the possible process that could explain the loss of Available online xxx the primordial continents with the assumption that they existed in the Hadean. Although depending on the timing of the initiation of plate tectonics and its convection style, subduction erosion, which is Keywords: Subduction erosion observed on the present-day Earth, might have carried the primordial continents into the deep mantle. Ó Hadean 2016, China University of Geosciences (Beijing) and Peking University. Production and hosting by Plate tectonics Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ Archean licenses/by-nc-nd/4.0/).

1. Introduction (e.g., Dohm and Maruyama, 2015). Plate tectonics efficiently cools the mantle and core by the subducted cold slab, helping the con- Geological observations indicate that rocks in the Hadean vection of the outer core which produces the magnetic field (4.0e4.6 Ga) do not remain on the current Earth’s surface (Fig. 1). resulting in protection of life from cosmic rays and outer high- From these observations, many scientists believe that primordial energy radiation. Plate tectonics also controls the carbon cycle at continents never existed on Earth (e.g., Condie, 1998; Rino et al., plate boundaries through the subduction and eruption. On the 2008). In principle, however, there are theoretically two possibil- other hand, there is high potential that the existence of primordial ities: (1) the primordial continents once existed, and some continent is directly related to the origin of life (described below). geological processes removed them from the Earth’s surface into In other words, the existence of the primordial continents may be the deep interior, and (2) the primordial continents never existed. one of the constraints to find planets with life. In this paper, we focus on the process of continentalecrust loss and consider the former possibility. From these discussions, we suggest new insight about the Hadean environment. 2. Did primordial continents exist in Hadean? The existence of the primordial continents is considered here to be the key concept of a habitable planet. The general concept of a For the origin of life, the existence of continents is quite habitable planet has primarily focused on the existence of liquid important because many essential elements for life (phosphorus, water, which basically depends on the distance from the stars (sun) potassium, calcium, iron, and others) are supplied only from the (Kasting and Calting, 2003; Ikoma and Genda, 2006). Although this following three-kinds of continental rocks: (1) granite, (2) pri- concept is significant, life is not originated only by liquid water. The mordial continents (anorthosite with KREEP basalts, as well as dike origin of life requires not only liquid water, but also plate tectonics and gabbro as lower mafic crust), and (3) carbonatite (Maruyama and nutrient-enriched continental crust exposed above the ocean et al., 2013)(Fig. 2). In particular, the continental crust above the surface of the ocean plays an important role. Weathering and erosion transforms large rocks into small grains, and these small fi * Corresponding author. grains can ef ciently transport the essential elements into the E-mail address: [email protected] (S. Azuma). ocean and lake where life supposedly originated (e.g., Dohm and Peer-review under responsibility of China University of Geosciences (Beijing). Maruyama., 2015)(Fig. 2). On the other hand, the timing of the http://dx.doi.org/10.1016/j.gsf.2016.08.001 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/).

Figure 1. Global geological map (after Rino et al., 2008).

Weathering Vaporization Pure water

Lake P Periodical Weathering P P K spring K K P K P K P K

Ocean Continental crust P (Granite, anorthosite with KREEP basalts) K

Oceanic plate

Subduction erosion

Figure 2. The roll of continents and essential elements for life. The essential elements such as potassium and phosphorus are supplied only from continental crust. Weathering and erosion transform continental crust into small grains which supply the essential elements for life efﬁciently into the ocean and lakes. The concept suggested to be important for life, which involves the interaction among nutrient-enriched landmass, ocean, and atmosphere through Sun-driven hydrological cycling, is referred to as Habitable Trinity (Dohm and Maruyama, 2015).

Please cite this article in press as: Azuma, S., et al., Why primordial continents were recycled to the deep: Role of subduction erosion, Geoscience Frontiers (2016), http://dx.doi.org/10.1016/j.gsf.2016.08.001 S. Azuma et al. / Geoscience Frontiers xxx (2016) 1e10 3 birth of life is still controversial. It is suggested from the and erosion, subduct along with oceanic plate to the mantle (sedi- carboneisotope composition of the oldest known sediments in Isua ment subduction). Clift and Vannucchi (2004) reported that only supracrustal belt in west Greenland that life appeared on the Earth 17% of sediments at an oceanic trench is incorporated into the at least before 3.8 Ga (Mojzsis et al., 1996). If this suggestion is accretionary prism, and the residual w80% of sediments is carried correct, the primordial continents that supplied the essential ele- into the deep mantle by sediments subduction. Second, the sub- ments to life must have existed in the Hadean. However, the re- ducted oceanic plate erodes the overlying continental crust and mains of a primordial continent have not been found on the surface carries the crustal material into the mantle (subduction erosion). of the present-day Earth (Fig. 1). The oldest dated rock is 4.03 Ga Subduction erosion has been conﬁrmed from deep-sea drilling from the Acasta Gneiss Complex of the Slave Craton, northwestern survey. For instance, Cretaceous granite and accretionary prism are Canada (Bowring and Williams, 1999), and the oldest supracrustal exposed at the fore-arc margin of East Japan (Von Huene et al.,1978). rock is reported from the 3.83 Ga Akilia island supracrustal rock in The landward slope of Peru trench is composed of Cretaceous gneiss the Itsaq Gneiss Complex, southern West Greenland (Mojzsis and rock and the accretionary prism does not develop (e.g., Hussong and Harrison, 2002). Hadean information has been gained only from Wipperman, 1981). Moreover, analysis of foraminifer in the sedi- zircons in some Proterozoic sedimentary rocks (4.0e4.4 Ga) (e.g., ments that cover the fore-arc margin indicates that the fore-arc Wilde et al., 2001; Iizuka et al., 2007). Many scientists have margin subsides (Nasu et al., 1980), suggesting that the subducted researched the geochemical characteristics of the Hadean zircons plate erodes the lower base of the continental crust and carries the (i.e. REE pattern, thermometry, mineral inclusion) to gain Hadean crustal material into the deep mantle (subduction erosion). In environmental information (e.g., Maas et al., 1992; Amelin et al., particular, basal erosion strongly contributes to the subsidence of 1999; Watson and Harrison, 2005). These Hadean zircons suggest fore-arc margin. If both sediment subduction and subduction the derivation from TTG magma, providing the evidence for plate erosion have occurred throughout the Earth’s history, the idea that tectonics and an ocean in Hadean (e.g., Harrison et al., 2007; the volume of continents steadily has increased through time is Hopkins et al., 2008). Plate tectonics with a large volume of water being reconsidered (Armstrong, 1981; Von Huene and Scholl, 1991). would induce remelting of oceanic crust and produce the continental crust (e.g., Campbell and Taylor, 1983). Then, why have Ha- dean rocks not been discovered? In order to answer this question, 3.1. Characteristics of subduction erosion the process of removing continents from the Earth’s surface must ’ be discussed in addition to the process of increasing continents. The present Earth s surface is divided into several plates. These plates collide at the plate margins and one side of the plates subducts into the mantle (plate tectonics). Subduction erosion 3. Process of continental loss occurs at the plate margins where a plate subducts. Accretionary prisms may be also formed at subduction zones instead of sub- There are mainly two processes of removing continental crusts. duction erosion. In present Earth, subduction erosion is dominant First, sediments, which are supplied into the ocean by weathering at 56e75% of the subduction zones (Von Huene and Scholl, 1993;

NE Alaska

Kamchatka SW Alaska Cascadia Aleutians Kurile Nankai Japan S.Oregon Mediterranean Ryukyu Lesser Antilles (Barbados) Makran Manila Izu-Mariana Mexico Philippines Guatemara Costa Rica New Hebrides Andaman Colombia Ecuador Smatra Tonga Java Peru

Kermadec N.Chile

Hikurangi S.Chile

MacQuarie South Sandwitch

Figure 3. Map showing the type of plate margins in the world, which includes the distribution of erosive (6) and accretionary (:) margins in subduction zones (modiﬁed from Von Huene and Scholl, 1993; Clift and Vannucchi, 2004; Scholl and Von Huene, 2009).

Clift and Vannucchi, 2004; Scholl and Von Huene, 2007; Clift factor is the thickness of sediments (e.g., Clift and Vannucchi, et al., 2009; Scholl and Von Huene, 2009; Yamamoto, 2010; 2004). Fig. 4 shows the convergence rate as a function of thick- Stern, 2011)(Fig. 3). At the subducting plate margin, subduction ness of sediments. Open and closed triangles indicate the erosional erosion occurs more universally than the formation of accre- margins and accretionary margins, respectively. It is obvious from tionary prism. Fig. 4 that subduction erosion can be dominant at plate margins Then what is the factor to determine whether subduction where the convergence rate is large (>50 km/my) and the thickness erosion or the formation of an accretion wedge is the dominant of sediments is small (<1 km). Thin sediments cannot fully cover process? Previous study has indicated that the most important the roughness (irregular topography including both concavities and convexities) of an oceanic plate, and therefore the fore-arc wedges can be eroded by the rough surface. This suggests that the surface 120 roughness of the subducting plate is important for subduction erosion. Erosional margins 100 Accretionary margins 3.2. Mechanism of subduction erosion

80 Three models have been proposed to explain the mechanisms of subduction erosion: (1) horst and graben model (Hilde, 1983); (2) model involving subducted seamounts and ridges (Von Huene and 60 Lallemand, 1990); and (3) hydrofracture model (Von Huene and Culotta, 1989; Von Huene et al., 2004)(Fig. 5). These models, which are discussed in the following, are advocated from geological

rate (km/my) 40 survey, with high potential that all three models can be simulta- neously applied to subduction zones. In the horst and graben model, the sediments that are supplied

Orthogonal convergence 20 from continents are trapped in the graben of the subducted slabs and carried into the deep mantle (Hilde, 1983). On the other hand, the horst of the subducted slab erodes the basal crust, and the 0 eroded crustal materials are also trapped in graben (basal erosion) 0123456789 (Tsuru, 2004)(Fig. 5). However, if the volume of sediments exceeds the capacity of the graben, the irregularity of the surface of the Sediment thickness (km) oceanic plate is completely ﬁlled, and the sediments are accreted to continents (forming an accretionary prism). This model can well Figure 4. Orthogonal convergence rate as a function of sediment thickness (after Clift and Vannucchi, 2004) for erosional (open triangles) and accretionary (closed triangles) explain the relationship between the thickness of sediments and margins. erosional margins (or accretionary margins).

Mechanism of subduction erosion in Subduction zone Sediment subduction

Weathering

Upper crust Subsidence Oceanic crust

Mantle lithosphere Subduction erosion

Lower crust t crruss nic OcOceanicceea crust Pore pressure Normal fault Sea mount Horst Graben

Erosion by seamounts Bucket model Hydrofracture model and ridges

Figure 5. Mechanisms-models of subduction erosion suggested in previous studies (Hilde, 1983; Von Huene and Culotta, 1989; Von Huene and Lallemand, 1990).

Subduction erosion by seamounts and ridges of the second erosion has been observed from seismic reflection profiles at the model is considered as a main mechanism to erode the massive Middle America Trench, Costa Rica, and Ecuador Trench (Ranero fore-arc crust (Ranero and Von Huene, 2000). This is supported by and Von Huene, 2000; Zhu et al., 2009). Vannucchi et al. (2008) the observation of the subduction of seamounts at the Middle found a subduction channel, which was a pathway of the frac- America Trench and PerueChile Trench by seismological tech- tured rocks and sediments into the mantle (Cloos and Shreve, niques. This model can explain the “intermittent” basal erosion and 1988a, b; Ichikawa et al., 2013, 2015), at the past plate boundary subsidence of a fore-arc region. However, the subducted seamounts in the Northern Apennines, Italy, suggesting that subduction and ridges may not be able to induce the “continual” basal erosion erosion had occurred in the past. because of their intermittent subduction. The review and consideration of these mechanisms of subduc- To explain the continual subsidence of a fore-arc region (basal tion erosion tell us that the subduction erosion must have occurred erosion), a hydrofracture model was proposed, in which case the throughout the Earth’s history. If plate tectonics works, subduction dehydrated fluids from the subducted slab fracture the overlying erosion is a universal phenomenon. In particular, the basal erosion crust (Von Huene and Culotta, 1989; Von Huene et al., 2004). In this can occur even in accretionary margin. Therefore, the timing of the model, the subducted oceanic crust and sediments dehydrate in initiation of plate tectonics is a key to understand the past sub- high temperature and pressure conditions, and therefore high pore duction erosion and the existence of primordial continents. pressure is produced. The high pore pressure makes the basal portion of continental crusts weak; this includes the promotion of fracturing as indicated both theoretically and experimentally (e.g., 4. Plate tectonics in early Earth Paterson and Wong, 2005). Then the fractured crust is carried into the mantle with the oceanic plate. The high pore pressure in- 4.1. When did plate tectonics begin? fluences the rock strength chemically and physically. In particular, Based on various geological evidences, the timing of the as a physical effect, the pore pressure Pp decreases the effective beginning of plate tectonics has been discussed, while remaining stress Pe, and the rock strength s decreases as follows (Eq. 1): À Á controversial. The geological evidences that have been used as an s s s a indicator of plate tectonics are ophiolite, duplex structure, ultra ¼ 0 þ fcPe ¼ 0 þ fc Pc Pp (1) high pressure (UHP) metamorphism including coesite and dia- where fc is the frictional coefficient, Pc is confining pressure, and s0 mond, geochemical analysis of zircons, and others (e.g., Wilde and a are the constants. This decrease of the effective stress can et al., 2001; Harrison et al., 2005; Stern, 2005; Furnes et al., promote the basal erosion because friction and fracture strength 2007; Hopkins et al., 2008; Hamilton, 2011; Komiya et al., 2015) depends on the effective stress (Eq.1). Actually the progress of basal (Fig. 6). For instance, it has been suggested from the blueschists

Figure 6. The timing of initiation of plate tectonics and the suggested evidences for operating plate tectonics. References cited are: Farquhar et al. (2002), Cawood et al. (2006), Nutman et al. (2002), Furnes et al. (2007), Komiya et al. (2015), Hopkins et al. (2008), Wilde et al. (2001), Harrison et al. (2005), Hamilton (2011), Stern (2005).

Please cite this article in press as: Azuma, S., et al., Why primordial continents were recycled to the deep: Role of subduction erosion, Geoscience Frontiers (2016), http://dx.doi.org/10.1016/j.gsf.2016.08.001 6 S. Azuma et al. / Geoscience Frontiers xxx (2016) 1e10 and UHP metamorphism that plate tectonics originated at 1.0 Ga, Neoproterozoic blueschists indicates higher global rates of subduc- because they are absent before 1.0 Ga (Stern, 2005). However, tion erosion in the remote past because of higher plate convergence blueschists and UHP metamorphism may not be strong evidence rates. On the other hands, some scientists have suggested that the for plate tectonics because the style of plate tectonics was possibly plate tectonics in the early Earth is characterized by a low plate different between the early and present-day Earth. In addition, velocity compared with that of the present-day plate tectonics (e.g., slab melting had occurred at a relatively shallow portion because Korenaga, 2013). However, even if plate velocity in the past Earth the potential temperature of early Earth was higher than that of was lower compared with that of the present-day Earth, the dif- present-day. Even if blueschist-facies metamorphic conditions are ference between them is not expected to be so large. The average attainable in subduction zones of early Earth, blueschists could not velocity of plates during the early Earth has been estimated to be be formed from hot, early Earth’smantle(Palin and White, 2016). 1.75 cm/yr (Foley et al., 2014). In addition, plate size is considered to Palin and White (2016) indicated that oceanic crust formed from be smaller than that of present-day. The potential temperature of the hotter mantle would be rich in magnesium oxide, therefore the ancient mantle was higher than today (Komiya, 2004), and greenschists instead of blueschists would have been formed from therefore, a higher Rayleigh number, resulting in small convection MgO-rich oceanic crust. On the other hand, Furnes et al. (2007) cells and many plate boundaries (e.g., Yanagisawa and Yamagishi, reported the evidence of oceanic crustal accretion in the 2005). Thus, the Archean Earth is inferred to have been covered by w3.80 Ga Isua Supracrustal Belt (ISB) in southwest Greenland and several hundreds of plates instead of dozens of plates of present-day contended that plate tectonics was operating at least before Earth. The several hundreds of plates would have resulted in more 3.80 Ga. This ancient phase of plate tectonics is supported by the active subduction erosion, which includes more active subduction of discovery of oceanic plate stratigraphy (OPS) and duplex struc- island arcs (e.g., Yamamoto et al., 2009a, b). tures within the Saglek Block in northern Labrador, Canada by The number and size of plate boundaries also depends on the Komiya et al. (2015), which indicates that plate tectonics had been plate strength (e.g., Tackley, 1998, 2000). If the plate strength of the in operation before 3.95 Ga. The strongest and oldest evidence for past Earth was smaller than that of the present-day Earth, it would the operation of plate tectonics is Hadean zircons. Mineral in- have been easier to develop the plate boundaries. The plate clusions and oxygen isotopes in these Hadean zircons suggest their strength is strongly dependent on the thermal structure of the derivation from TTG magma, indicating that plate tectonics began plate. We have calculated the thermal structure of the oceanic plate at 4.4 Ga (e.g., Wilde et al., 2001). These previous studies suggested at early Archean using a model of transient half-space cooling by that some kind of recycling process of crustal material had worked Turcotte and Schubert (2002) at given mantle and surface tem- in the Hadean, and that plate subduction, which is similar to plate perature, respectively 1500 C(Komiya, 2004) and 100 C(Komiya tectonics of the present-day Earth, had existed at least before et al., 1999)(Fig. 7b). The corresponding temperatures for present- 3.95 Ga. The operation of subduction erosion strongly depended on the style of plate tectonics in the past Earth. For instance, drip tectonics (e.g., Foley et al., 2014) could cause the subduction of 1600 crustal material, however subduction erosion would not occur in Present day this style of tectonics. 1400 1200 4.2. Convection style in the early Earth 1000 Previous studies have pointed out that the styles of plate tec- 800 tonics are different between the past (HadeaneArchean) and 10 Myr present-day Earth (e.g., van Hunen and van den Berg, 2008; 600

Temperature (ºC) 20 Myr Korenaga, 2011; Foley et al., 2014). This difference could be caused by higher potential temperature of the past Earth (e.g., 400 40 Myr 60 Myr Komiya, 2004), which produced the thicker crust and slab melting. 200 Recent numerical calculation incorporating the grain-damage, a grain size-feedback mechanism (Foley et al., 2014) showed that the 0 0 50 100 150 200 style of subduction process in the Hadean was similar to the drip tectonics, not to the plate tectonics of the present-day Earth. It is Depth (km) inferred that the subduction of supracrustal material was initiated 1600 by drip tectonics w100 Myr later from the solidification of magma Archean ocean. This interpretation is consistent with the recycle of crust in 1400 Hadean (e.g., Watson and Harrison, 2005) supposed by the analysis of Hadean zircons. On the other hand, Komiya et al. (2015) found 1200 that the accretionary prism and the oceanic plate stratigraphy 1000 (OPS) had already formed at 3.95 Ga, suggesting that the style of subduction at that time was similar to that of the present Earth. If 800 the proposed drip tectonics was dominant just after the solidifi- 10 Myr 600 cation of magma ocean in the early Earth, the subduction style Temperature (ºC) 20 Myr should have changed in the HadeaneEoarchean to the style similar 400 40 Myr to that of present-day Earth. In other words, the 60 Myr HadeaneEoarchean Earth satisfied the necessary conditions for 200 subduction erosion. 0 The plate velocity during HadeaneArchean is expected to have 0 50 100 150 200 been larger than that of the present-day Earth because of higher Depth (km) potential mantle temperature (e.g., Komiya, 2004). For instance, Figure 7. Calculated thermal structures of oceanic plate in the past (Archean) and the Stern (2011) suggested that the paucity of ancient pre- present-day Earth.

Present day Present day Moho Moho Mo o

OH=500 ppm H/Si) =0.38, COH=500 ppm H/Si) (λ=0.38, COH=500 ppm H/Si)

Figure 8. Rheological structures of the oceanic plate of the present-day Earth (10, 20, 40 Ma).

Archean

Moho Mo o Mo o

Dry Wet =0.38, COH=500 ppm H/Si) (λ (λ=0.38, COH=500 ppm H/Si)

Figure 9. Rheological structures of oceanic plate of the early Archean Earth (10, 20, 40 Ma). day Earth are 1330 C and 0 C(Iwamori et al., 1995; Komiya et al., of stress, is generally used to infer rheological structure (e.g., 1999). Based on calculated thermal structures, we determined the Kohlstedt et al., 1995); this type of flow law is commonly applied to rheological structure of the past and present-day Earth. In the high-temperature creep. However, the Peierls mechanism becomes calculation of the rheological structure, pressures are assumed to dominant at low temperatures and high stresses (Tsenn and Carter, be equal to the lithostatic pressure determined from a crustal 1987; Katayama and Karato, 2008; Demouchy et al., 2013). In this density of 2900 kg/m3 and a mantle density of 3300 kg/m3. The mechanism, the strain rate is exponentially proportional to the crustal thickness is assumed to be 7 km for present-day Earth and applied stress. In our calculations, the Peierls mechanism is also 20 km for Archean Earth (Komiya, 2004). Whereas the rock applied. The strain rate is assumed to be 10 15 (s 1). Figs. 8 and 9 strength in the brittle-deformation region is calculated from Bye- show the calculated rheological structures of the oceanic plate of rlee’slaw(Byerlee, 1978), the rock strength in the ductile- the present-day Earth and the Archean Earth, respectively. The deformation region is determined by a flow law of each material. development of plate boundaries requires a moderate strength of The flow laws of plagioclase are applied for the crustal strength the oceanic plate (<200e300 MPa) (e.g., Tackley, 1998, 2000). The (Rybacki and Dresen, 2000), and the rock strength of the mantle is plate strength under dry conditions far exceeds the threshold in determined from the flow law of olivine (Karato and Jung, 2003). both Archean and present-day Earth (Figs. 8 and 9), suggesting that Power-law creep, in which the strain rate is proportional to a power the existence of an ocean is the minimum necessary condition for

8E+09

) 7E+09 3 World 6E+09 3 /yr 5 km 5E+09

4E+09 3 /yr 3 km 3E+09 Eurasia

2E+09

Volume of lost continent (km 3 1 km /yr 1E+09 0.5 km3/yr 0 0 0.5 1.0 1.5 Aflica

Time (Gyr)

Figure 10. The accumulated volume of crustal losses as a function of time. Difference of the decrease and increase rate in volume is assumed to be 0.5, 1.0, 3.0, and 5.0 km3/yr. The volumes of the present-day continent are indicated as a rough standard. plate tectonics and therefore subduction erosion. In addition, these et al., 2007; Hopkins et al., 2008; Komiya et al., 2015). Fig. 10 figures clarify that the plate strength of Archean Earth is w70e80% shows the volume of crustal losses as a function of time. In of that of the present-day Earth (The plate strength of Hadean Earth Fig.10, the difference between loss and generation rate in volume of is calculated in Maruyama et al. in this volume). This difference of continental crust is assumed to be 0.5e5.0 km3/yr, and the volumes the plate strength between Archean and present-day Earth sup- of the each present-day continent are indicated as a rough stan- ports a hypothesis that more plate boundaries were produced in dard. In the case of crustal losses of 0.5 km3/yr, the volume of the Archean and Hadean. The greater number of plate boundaries must North America continent would have disappeared from Earth’s have caused extensive subduction erosion and direct subduction of surface in 1.5 billion years. In the case of 1 km3/yr, the volume equal oceanic island arcs. to Eurasia continent would have been eroded. In the case of 3 km3/ yr, all of the continents on the surface of present-day Earth would w 5. Estimated volume of continent losses in Hadean and have been lost during the Archean ( 1.5 Gyr) and the volume of Archean the Eurasia continent would have disappeared during the Hadean (w500 Myr). If the crustal generation rate falls below the loss rate, ’ Although it is expected that extensive subduction erosion all of the continental crust at Earth s surface can be carried into the occurred in the Hadean and Archean, it is difficult to quantitatively mantle by subduction erosion. estimate the volume of crustal losses in these eons. Some previous studies evaluated production and loss rate of continental crust in 6. Conclusion present-day Earth. Clift et al. (2009) and Stern and Scholl (2010) classified geological process into additions or losses of continen- We discussed the plate tectonics and subduction erosion in tal crust, and evaluated the generation and loss in volume of con- Hadean and Archean to consider why the primordial continent tinental crust. Clift et al. (2009) suggested a generation rate of cannot be found on the present-day surface of the Earth. The 3.2e5.0 km3/yr and a loss rate of 4.85 km3/yr in volume of conti- conclusions are as follows: (1) subduction erosion is a natural nental crust. Stern and Scholl (2010) suggested a generation rate of consequence of plate tectonics when the thickness of sediments is 3.2 km3/yr and loss rate of 3.2 km3/yr. In both evaluations, the small (<1 km); (2) plate size during the Hadean and Archean was generation rate balances the loss rate in volume of continental relatively small when compared with present-day Earth, and thus crust. However, the conclusion by Stern and Scholl (2010) is an extensive subduction erosion had occurred; (3) primordial conti- estimate for just Phanerozoic time, and they also pointed out that nents possibly existed and were eroded by subduction erosion. The the volume of continental crust loss due to lower crust delamina- eroded primordial continents were carried into the deep mantle tion and subduction at collision zones are difficult to constrain. (e.g., Kawai et al., 2009; Ichikawa et al., this volume). Therefore, it is possible that the rate of the continental loss can far exceed the rate of continental generation (e.g., the loss and gen- Acknowledgement eration rate of continental crust in the Tethyan region). If the generation falls below the loss, how much volume of continental We thank James Dohm for editing English in this paper. This crust does the Earth’s surface lose? In discussing this topic, plate research was supported by JSPS KAKENHI (Grand-in-Aid for Sci- tectonics that is similar to that of the present-day Earth is assumed entific Research (S)) Grant No. 23224012 (Growth of the second to initiate in HadeaneEoarchean (Harrison et al., 2005; Furnes continent and mantle).

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