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Invited Review Plate Tectonics, Flood Basalts and the Evolution of Earth's

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Invited Review Plate Tectonics, Flood Basalts and the Evolution of Earth's Invited review doi: 10.1111/j.1365-3121.2008.00843.x Plate tectonics, flood basalts and the evolution of Earth’s oceans Jun Korenaga Department of Geology and Geophysics, Yale University, PO Box 208109, New Haven, CT 06520-8109, USA ABSTRACT The chemical composition of the bulk silicate Earth (BSE) thicker, buoyant basaltic crust, and the subductability of indicates that the present-day thermal budget of Earth is likely oceanic lithosphere is a critical factor regarding the emer- to be characterized by a significant excess of surface heat loss gence of plate tectonics before the Proterozoic. Moreover, over internal heat generation, indicating an important role of sluggish convection in the past is equivalent to reduced secular cooling in Earth’s history. When combined with secular cooling, thus suggesting a more minor role of mantle petrological constraints on the degree of secular cooling, this plumes in the early Earth. Finally, deeper ocean basins are thermal budget places a tight constraint on permissible heat- possible with slower plate motion in the past, and Earth’s flow scaling for mantle convection, along with implications for oceans in the Archean is suggested to have had about twice as the operation of plate tectonics on Earth, the history of mantle much water as today, and the mantle may have started as dry plumes and flood basalt magmatism, and the origin and and have been gradually hydrated by subduction. The global evolution of Earth’s oceans. In the presence of plate tectonics, water cycle may thus be dominated by regassing, rather than hotter mantle may have convected more slowly because it degassing, pointing towards the impact origin of Earth’s generates thicker dehydrated lithosphere, which could slow oceans, which is shown to be supported by the revised down subduction. The intervals of globally synchronous composition of the BSE. orogenies are consistent with the predicted variation of plate velocity for the last 3.6 Gyr. Hotter mantle also produces Terra Nova, 20, 419–439, 2008 2005; Labrosse and Jaupart, 2007), example, are often thought to be more Introduction and a review on so far published active in the past, but such temporal The thermal history of Earth, after the models on Earth’s thermal evolution trend is in direct conflict with what initial period of global magma ocean, has recently been published (Korena- thermal evolution indicates. Also, the is governed by the balance of surface ga, 2008b). Estimating a thermal his- volume of oceans is unlikely to be heat loss and internal heat generation. tory involves both geophysics (the constant, and it is probably decreasing An initially hot Earth has been grad- physics of convective heat loss) and with time. Whereas these topics tend ually cooled down with time because, geochemistry (the abundance of heat- to be considered independently, each for most of Earth’s history, surface producing elements), and many pub- of them represents a different aspect heat loss has been greater than internal lished models do not account for both on the evolution of the integrated heat supply by radiogenic isotopes. aspects simultaneously. Available geo- Earth system, so a proper understand- This simple energy balance places first- chemical and geological data place a ing of Earth’s thermal history is order constraints on various aspects of strict constraint on the permissible essential to make self-consistent pre- physical and chemical processes oper- range of thermal history, and the most dictions about them. ating on the surface of Earth and likely scenario is briefly reviewed in the within its deep interior. There have next section. Thermal history of earth been a number of theoretical studies The purpose of this review article is on Earth’s thermal history (e.g. to discuss some of major geological Global heat balance equations McKenzie and Weiss, 1975; Davies, processes that are directly related to 1980; Schubert et al., 1980; Christen- the thermal evolution of Earth, on the The thermal history of Earth may be sen, 1985; Richter, 1985; Solomatov, basis of this latest understanding. modelled by integrating the following 2001; Korenaga, 2003; Grigne et al., Topics covered include the onset of global energy balance (e.g. Stevenson plate tectonics, the history of mantle et al., 1983): plumes, continental growth, and the Correspondence: Jun Korenaga, Depart- origin and evolution of Earth’s dTmðtÞ ment of Geology and Geophysics, Yale Cm ¼ HmðtÞQðtÞþQCMBðtÞ; University, PO Box 208109, New Haven, oceans. They are difficult problems, dt CT 06520-8109, USA. Tel.: +1 (203) 432 and little can be said with certainty at ð1Þ present. Because they are intimately 7381; fax: +1 (203) 432 3134; e-mail: and [email protected] coupled with thermal evolution, how- ever, we can still derive a few impor- dTcðtÞ dmicðtÞ This article was commissioned by the tant constraints. These constraints C ¼ðL þ E Þ þ H ðtÞ c dt g dt c Editors of Terra Nova to mark the Inter- may be counter-intuitive or not obvi- national Year of Planet Earth. ous at first sight. Mantle plumes, for À QCMBðtÞ; ð2Þ Ó 2008 Blackwell Publishing Ltd 419 Plate tectonics, flood basalts and Earth’s oceans • J. Korenaga Terra Nova, Vol 20, No. 6, 419–439 ............................................................................................................................................................. where the subscripts m and c denote Stevenson et al., 1983). Thus, the first As the physical state of the early the mantle and core components, two terms on the right-hand side of Earth is uncertain, the above differen- respectively, Ci is heat capacity, Ti(t) Eq. (2) may be neglected for simplic- tial equation is integrated backward in is average temperature, and Hi(t)is ity, and Eqs (1) and (2) may be time, starting with the present-day internal heat production. Core heat combined to yield: condition. flux, Q (t), represents heat CMB dT ðtÞ exchange between the mantle and the m C HmðtÞQðtÞ Present-day thermal budget core, and Q(t) is the surface heat flux. dt The mass of the inner core is denoted dDTCMBðtÞ The present-day thermal budget, i.e. þ Cc ; ð3Þ by mic(t), and the first term on the dt Q(0) and Hm(0), may be estimated as right-hand side of Eq. (2) describes the where C is the heat capacity of the follows. The global heat flux is esti- release of latent heat and gravitational mated to be 46 ± 3 TW (Jaupart entire Earth [=Cm+Cc, 7 · energy associated with the growth of 1027 JK)1 (Stacey, 1981)] and et al., 2007), which is the sum of heat the inner core. As mantle evolution is flux due to mantle convection, Q(0), DTCMB(t) is the temperature contrast tracked by a single average tempera- at the core–mantle boundary. The and radiogenic heat production in ture, this formulation corresponds to present-day contrast is estimated to continental crust, Hcc(0). The latter is whole-mantle convection. Though be on the order of 1000 K (e.g. Boeh- estimated to be 7.5 ± 2.5 TW (Rud- layered-mantle convection has been a ler, 1996; Williams, 1998), but how it nick and Gao, 2003), so we may write popular concept (e.g. Jacobsen and should change with time is not under- Hccð0Þ¼7:5 þ 2:5e1 ½TW; ð5Þ Wasserburg, 1979; Richter, 1985; All- stood well. To estimate the temporal egre et al., 1996; Kellogg et al., 1999), variability of the temperature con- and whole-mantle convection is probably trast, we need to calculate the core sufficient to explain available geophys- Qð0Þ¼46 À Hccð0Þþ3e2 ½TW; ð6Þ heat flux QCMB(t), which depends ical and geochemical data if the uncer- critically on the poorly known rheo- where e1 and e2 are random variables tainties of these data and the logy of the lowermost mantle (e.g. following the Gaussian distribution of limitation of theoretical predictions Solomatov, 1996; Karato, 1998; zero mean and unit standard devia- are taken into account (Lyubetskaya Korenaga, 2005a) and also on other tion. The use of two independent and Korenaga, 2007b; Korenaga, complications such as the presence of random variables signifies that the 2008b). Note that layered mantle con- chemical heterogeneities (e.g. Farne- uncertainty of the global heat flux vection is still possible. Because it is tani, 1997; Jellinek and Manga, 2002), and that of continental heat produc- not well defined (Korenaga, 2008b), the post-perovskite phase transition tion are not correlated. Assuming however, exploring its implication for (e.g. Murakami et al., 2004; Oganov whole-mantle convection, heat pro- Earth’s evolution would require us to and Ono, 2004), and drastic changes duction in the bulk silicate Earth deal with more degrees of freedom. in thermal properties (e.g. Badro (BSE) (i.e. crust and mantle) is esti- For simplicity, therefore, we restrict et al., 2004; Lin et al., 2005). We do mated to be 16 ± 3 TW (Lyubetskaya ourselves to whole-mantle convection not even know why the present-day and Korenaga, 2007b), so heat pro- in this review, which should serve as contrast happens to be 1000 K. duction in the convecting mantle may a reference when considering more Thus, it may be reasonable to treat be expressed as: complex convection models (Table 1 dDT (t) ⁄dt as a free parameter and CMB H ð0Þ¼16 À H ð0Þþ3e ½TW; ð7Þ presents a summary of major assump- see how thermal evolution would be m cc 3 tions made in this article). affected by this term. As the simplest where e3 is another random variable Internal heat production in the core option, it is set to zero here, and other following the same Gaussian distribu- is controversial (e.g. Gessmann and possibilities will be explored later (see tion, and the Ôconvective Urey ratio’ Wood, 2002; McDonough, 2003; Core heat flux and the possibility of (Korenaga, 2008b) is defined as: Rama Murthy et al., 2003; Lee et al., superheated core).
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