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Challenges in Solid Earth Sciences
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ScienceDirect Solid Earth Sciences 1 (2016) 1e4 www.elsevier.com/locate/SESCI
Preface
Challenges in Solid Earth Sciences* “Solid Earth Sciences” is a peer-reviewed open-access journal, publishing research papers on a wide range of subjects in Earth sciences, including 1) geodynamics involved in the physical and chemical evolution of Earth; 2) petrologic processes in igneous to sedimentary and metamorphic rocks; 3) present and past environments relating to the habitable Earth; 4) tectonic processes in the crust and upper mantle, including structural geology to plate tectonics; 5) geochemical behaviors of elements under various conditions and their implications for mineralization and other geological processes; 6) high temperature and high pressure experiments that address chemical and/or physical properties of Earth materials; and 7) new methods, new ideas, and new models that help us to better understand Earth itself as a dynamic system. The journal aims at dealing with major challenges in Earth sciences. On behalf of the “Solid Earth Sciences” editorial board, I call for papers in the field of solid Earth, especially those that address the following major challenges, which, from our perspectives, are: (1) the structure and composition of the Earth's interior; (2) the formation of the Moon; (3) the evolution of the Magma Ocean; (4) the origin of plate tectonics; (5) the nature of mantle plumes; (6) the emergence and evolution of life; (7) the formation of giant ore deposits. The structure and composition of Earth are essential and basic topics for us to understand Earth's dynamic systems. Major challenges in this field are the detailed imaging of Earth's internal boundaries, the heterogeneous structural interior of Earth, the underlying physical properties and compositional and thermal mechanism, and the dynamic processes near the boundaries. The major boundaries in the deep Earth include the inner core boundary (Song and Helmberger, 1998; Zhang et al., 2005), the coreemantle boundary (Lay and Helmberger, 1983); the upper mantle discontinuities (Anderson and Bass, 1986), the lithosphereeasthenosphere boundary (LAB) (Stern et al., 2015) etc. Seismic tomographic images have revealed the large-scale 3D wave velocity structure of the deep mantle (e.g., Grand, 2002), which is characterized by two massive low velocity provinces surrounded by faster material (Kendall and Silver, 1996; Lay et al., 1998; Ni et al., 2002; Wen and Helmberger, 1998). To what degree
those structures can well constrain the deep mantle convection which might involve both chemical and thermal variations depends on interdisciplinary studies of seismology, geodynamics, mineral physics and geochemistry. The properties and structure of the upper mantle transition zone plays an important role in addressing some frontier scientific problems, like the interaction between the subducting lithosphere and mantle discontinuities, the flux exchange between the lower and upper mantle, and the storage of water in the deep mantle (e.g. Huang et al., 2005; Li et al., 2013; Pearson et al., 2014; van der Meijde et al., 2003). The structure and nature of the LAB are keys to a better understanding of the thermalechemical evolution of the continents, coupling between lithosphere, asthenosphere and deeper mantle, the subsurface deformation and so on. Earth's interior boundaries may have major influences on the recycling of water, carbon and metals (e.g. Rohrbach and Schmidt, 2011; Stagno et al., 2013; Thomson et al., 2016). Recent studies further suggest nearly global presence of important boundaries/layers at depth, such as a mid-lithosphere discontinuity at ~100-km depth within continental lithosphere (Karato et al., 2015; Selway et al., 2015), a low-velocity layer above the mantle transition zone (Tauzin et al., 2010), and compositional or rheological layering in the shallow lower mantle (Ballmer et al., 2015; Marquardt and Miyagi, 2015). These features reflect vertical heterogeneities of Earth, and may have resulted from and provided significant feedbacks to the early formation and longterm evolution of Earth. Both are, however, largely unclear or under debate. Overall, detailed information about the structure and properties of major boundaries and layers and associated heterogeneities of Earth's interior are still lacking, which prevent a comprehensive understanding of the dynamics and working mechanism of the entire Earth system. The formation of the Earth and the Moon has been a hot topic of discussions among scientists for years. The currently most popular model is still the “Giant Impact” hypothesis, or variations thereof. According to this model, a Mar-size impactor collided with the proto-Earth and vaporized much of the material. The vaporized particles accreted and made up the Moon, resulting in the Moon being depleted in volatile elements (Benz et al., 1989). This model is challenged by the nearly identical isotope ratios for the non-volatile elements in
Peer review under responsibility of Guangzhou Institute of Geochemistry. http://dx.doi.org/10.1016/j.sesci.2016.06.001 2451-912X/Copyright © 2016, Guangzhou Institute of Geochemistry. 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/).
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Preface / Solid Earth Sciences 1 (2016) 1e4
the Moon and the Earth (e.g, Asphaug, 2014; Stevenson and Halliday, 2014). According to the “Giant Impact” model, lunar samples should be easily distinguishable from those of the Earth, because the Moon is derived mostly from the impacting planetesimal with a small portion from the Earth. Lunar rocks, however, have the same isotopic composition as those from the Earth for O, Ti, Cr, W, K, and other species (Asphaug, 2014). Instead of a Mar-size planetary body, an icy comet has been proposed recently as the Moon-forming impactor (Taylor, 2016). In this scenario, the comet probably collided with and brought water to the Earth about 100 Ma after the formation of the Earth (Albarede, 2009). The major problem for this model is the fact that the Moon is currently very dry. Could volatile loss explain the dry nature of the Moon after such a “wet” impact? How much water in the Earth survived such a giant impact, which has sufficient energy to vaporize silicates? These questions remain to be answered. Another problem directly related to the formation of the Moon is the evolution of the early-Earth Magma Ocean. It has been proposed that the Earth was once totally melted due to energy released from decays of radioactive elements, and also energy from accretion, especially the Giant Impact (Matsui and Abe, 1986). Metals fractionated into the core, whereas silicates stayed in the mantle with kinetics controlled by density segregation. During the solidification of the Magma Ocean, different minerals started to crystallize at different PeT conditions. Crystallization of bridgmanite, ringwoodite, majorite, etc. should have resulted in major chemical fractionations in trace element abundances, e.g. heavy and light rare earth elements (e.g., Albarede et al., 2000; Kato et al., 1988a, 1988b). Xenoliths from the upper mantle and basalts, however, have much less fractionated trace element patterns. It has been proposed that mantle overturns in the Moon (e.g., Ryder, 1991), Mars (e.g., Elkins-Tanton, 2008) and also Earth (e.g., Brown et al., 2014; Debaille et al., 2009; Jackson et al., 2014; O'Neill and Debaille, 2014), might have homogenized the mantle over time. Were there mantle overturns in the early history of the Earth? Could the signatures of the Magma Ocean have survived the mantle overturn? How can such mantle overturn models be tested? Plate tectonics is one of the most important theories for modern Earth sciences (Mckenzie and Parker, 1967). According to the theory of plate tectonics, Earth's surface is separated into major plates by divergent, convergent and transform plate boundaries. The crust, together with lithospheric mantle, floats and drifts on top of the asthenosphere. Two types of crusts are apparent, continental and oceanic. Continental crust is felsic with an average age of ~2.0 Ga. Oceanic crust is mafic with an average age of less than 100 Ma. New oceanic crust forms at spreading ridges, with old oceanic crust being subducted at convergent margins. Earth, however, is probably the only solar terrestrial planet that has plate tectonics. What made Earth so unique? What is the driving force for plate tectonics? When and how did plate tectonics commence? One hypothesis suggests that water is the key factor that makes Earth unique. The Earth is the only rocky planet in the
solar system that has a large amount of liquid water at the surface. Most of the water in the uppermost mantle is hosted in pargasite, which decomposes at pressures greater than 3 GPa (at depths of ~90 km), forming the asthenosphere through water (vapor) induced partial melting (Green et al., 2010). Although this hypothesis plausibly explains the maximum thickness of oceanic lithosphere, it has difficulties to account for the thick cratonic lithosphere. Moreover, the Earth was formed inside the “Snow Line” of the solar system, and as such should not originally have had so much water. It might have received water from an icy comet ~100 Ma after the formation of the Earth (Albarede, 2009). In that case, how did water manage to go deep inside the Earth before plate tectonics to form the asthenosphere, while the planet was progressively degassing? The driving force behind plate tectonics is also obscure. Plate tectonics is overall driven by heat inside the Earth. It has been proposed that well-organized heat loss along spreading ridges controls the engine of plate tectonics (Sun et al., 2007). The energy that drives the Earth's dynamo is still not well constrained (e.g., Arevalo et al., 2009; Gale et al., 2013; O'Rourke and Stevenson, 2016). The main driving forces so far proposed are slab pull, mantle convection, and ridge push/ slab sliding. Slab pull has been considered as the primary driving force. Subducted old oceanic crust forms eclogite that is ~1% denser than mantle peridotite and, when combined with the lower-than-ambient temperature of subducting slabs, pulls the oceanic crust down into the mantle. What then was the driving force before plate subduction started? Also, many plates, e.g. the floor of the Atlantic Ocean, are not subducting, but they move fast, indicating the importance of other driving forces. The initiation of plate subduction (e.g., Gerya et al., 2015; Niu et al., 2003), the chemical and physical processes during plate subduction (e.g., Andersen et al., 2015; Okazaki and Hirth, 2016) and the physical properties and the fate of subducted oceanic slabs (e.g., Hirose et al., 1999; van der Hilst et al., 1997) are also hot topics that need more attentions. The initial concept of mantle plumes was first proposed as a “Hot Spot” to explain the Hawaiian-Emperor Island chain (Wilson, 1963). Wilson's hot spot originated from the convecting mantle, and therefore, it is very difficult to explain its fixed location. In order to solve this problem, the mantle plume hypothesis was proposed (Morgan, 1971). According to this model, a mantle plume originates from the coreemantle boundary. It has been proposed that mantle plumes are initiated by recycling of subducted oceanic slab material based on geochemical data (Hofmann and White, 1982). This model has been challenged because a subducted oceanic crust is denser than the surrounding mantle at lower mantle depths, whereas the subducted oceanic crust may not provide straightforward explanation on the geochemistry of plume magmas (Niu and O'Hara, 2003). From a geophysical perspective, mantle plumes initiate simply as thermal instabilities above the core-mantle boundary. In contrast, others argued that geochemical data favor recycling back of components from subducted oceanic slabs (e.g., Jackson et al., 2007). Such a density trap may be plausibly explained by a mineral
Preface / Solid Earth Sciences 1 (2016) 1e4
segregation model (Sun et al., 2011). Nevertheless, the mantle plume hypothesis still needs further examination. The origin of life became a topic in Earth sciences with the ground breaking paper: “A Production of Amino Acids under Possible Primitive Earth Conditions” (Miller, 1953). Synthesis of organic compounds under primitive Earth conditions shed light on the origin of life (Miller and Parris, 1964). Yet, there are still major issues to bridge the gaps between organic compounds and self-replicating life forms. It is also not clear whether life originated on Earth or has been carried to Earth by a solar/extra-solar system traveler, e.g., a comet or a meteorite (Caro et al., 2002; Steele et al., 2012). There are five major mass extinction events in the Phanerozoic (e.g., Hallam and Wignall, 1999; Raup and Sepkoski, 1984; Sepkoski, 1984; Shen et al., 2011). These mass extinctions have been attributed either to asteroid impact (Alvarez et al., 1980), to large volcanic events (Courtillot and Olson, 2007), or to shoaling of sulfidic waters (Zhang et al., 2015). Determining the causes of mass extinction events is important as the effects of anthropogenic activities to the changes of the Earth's climate become more apparent. How to build/protect the environments of a habitable Earth is a major challenge for human beings. Ore deposits are the result of abnormal enrichments of economically valuable elements by special geological processes. Many mineral resources are dominated by giant deposits/mineralization belts. For example, Bayan Obo (Ling et al., 2013) accounts for 40% of the world light rare earth reserves; Witwatersrand (Kirk et al., 2002) hosts ~40% of the world's gold; over 70% of the world's porphyry Cu deposits are distributed around the circum-Pacific region; most of the world's Mo is hosted in Climax, Qinling-Dabie and northeastern China; whereas the Neotethys hosts over 70% of the world's Sn reserves. The processes that produced these giant ore deposits remain obscure. Solid Earth Sciences publishes the following items: 1) Trends, reporting progress in Earth Sciences during the last 12 months; 2) Review, summarizing major progresses in a special field during the last 5e10 years; 3) Article, reporting a welldeveloped piece of research work; 4) Letter, reporting major progress of an ongoing research; 5) Hypothesis, reporting original, innovative, striking new ideas on phenomena of global significance; 6) Forum, discussing papers published in “Solid Earth Sciences”; and 7) News, reporting major events in Earth sciences. We will also publish a small portion of interesting papers that may have not found support during peer-review, aiming to provide a refuge for challenging new models/hypothesis. References Albarede, F., 2009. Volatile accretion history of the terrestrial planets and dynamic implications. Nature 461 (7268), 1227e1233. Albarede, F., Blichert-Toft, J., Vervoort, J.D., Gleason, J.D., Rosing, M., 2000. Hf-Nd isotope evidence for a transient dynamic regime in the early terrestrial mantle. Nature 404 (6777), 488e490. Alvarez, L.W., Alvarez, W., Asaro, F., Michel, H.V., 1980. Extraterrestrial cause for the cretaceous-tertiary extinction e experimental results and theoretical interpretation. Science 208 (4448), 1095e1108.
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Andersen, M.B., et al., 2015. The terrestrial uranium isotope cycle. Nature 517 (7534), 356eU463. Anderson, D.L., Bass, J.D., 1986. Transition region of the Earths upper mantle. Nature 320 (6060), 321e328. Arevalo, R., McDonough, W.F., Luong, M., 2009. The K/U ratio of the silicate Earth: insights into mantle composition, structure and thermal evolution. Earth Planet. Sci. Lett. 278 (3e4), 361e369. Asphaug, E., 2014. Impact origin of the moon? Annu. Rev. Earth Planet. Sci. 42, 551eþ. Ballmer, M.D., Schmerr, M.C., Nakagawa, T., Ritsema, J., 2015. Compositional mantle layering revealed by slab stagnation at ~1000-km depth. Sci. Adv. 1 e1500815. Benz, W., Cameron, A.G.W., Melosh, H.J., 1989. The origin of the moon and the single-impact hypothesis.3. Icarus 81 (1), 113e131. Brown, S.M., Elkins-Tanton, L.T., Walker, R.J., 2014. Effects of magma ocean crystallization and overturn on the development of Nd-142 and W-182 isotopic heterogeneities in the primordial mantle. Earth Planet. Sci. Lett. 408, 319e330. Caro, G.M.M., et al., 2002. Amino acids from ultraviolet irradiation of interstellar ice analogues. Nature 416 (6879), 403e406. Courtillot, V., Olson, P., 2007. Mantle plumes link magnetic superchrons to phanerozoic mass depletion events. Earth Planet. Sci. Lett. 260 (3e4), 495e504. Debaille, V., Brandon, A.D., O'Neill, C., Yin, Q.Z., Jacobsen, B., 2009. Early martian mantle overturn inferred from isotopic composition of nakhlite meteorites. Nat. Geosci. 2 (8), 548e552. Elkins-Tanton, L.T., 2008. Linked magma ocean solidification and atmospheric growth for Earth and Mars. Earth Planet. Sci. Lett. 271 (1e4), 181e191. Gale, A., Dalton, C.A., Langmuir, C.H., Su, Y.J., Schilling, J.G., 2013. The mean composition of ocean ridge basalts. Geochem. Geophys. Geosystems 14 (3), 489e518. Gerya, T.V., Stern, R.J., Baes, M., Sobolev, S.V., Whattam, S.A., 2015. Plate tectonics on the Earth triggered by plume-induced subduction initiation. Nature 527 (7577), 221eþ. Grand, S.P., 2002. Mantle shear-wave tomography and the fate of subducted slabs. Philos. Trans. R. Soc. Lond. Ser. Math. Phys. Eng. Sci. 360 (1800), 2475e2491. Green, D.H., Hibberson, W.O., Kovacs, I., Rosenthal, A., 2010. Water and its influence on the lithosphereeasthenosphere boundary. Nature 467 (7314), 448-U97. Hallam, A., Wignall, P.B., 1999. Mass extinctions and sea-level changes. Earth Sci. Rev. 48 (4), 217e250. Hirose, K., Fei, Y.W., Ma, Y.Z., Mao, H.K., 1999. The fate of subducted basaltic crust in the Earth's lower mantle. Nature 397 (6714), 53e56. Hofmann, A.W., White, W.M., 1982. Mantle plumes from ancient oceaniccrust. Earth Planet. Sci. Lett. 57 (2), 421e436. Huang, X.G., Xu, Y.S., Karato, S.I., 2005. Water content in the transition zone from electrical conductivity of wadsleyite and ringwoodite. Nature 434 (7034), 746e749. Jackson, C.R.M., Ziegler, L.B., Zhang, H.L., Jackson, M.G., Stegman, D.R., 2014. A geochemical evaluation of potential magma ocean dynamics using a parameterized model for perovskite crystallization. Earth Planet. Sci. Lett. 392, 154e165. Jackson, M.G., et al., 2007. The return of subducted continental crust in Samoan lavas. Nature 448 (7154), 684e687. Karato, S., Olugboji, T., Park, J., 2015. Mechanisms and geologic significance of the mid-lithosphere discontinuity in the continents. Nat. Geosci. 8, 509e514. Kato, T., Ringwood, A.E., Irifune, T., 1988a. Constraints on element partitioncoefficients between MgSiO3 perovskite and liquid determined by direct measurements. Earth Planet. Sci. Lett. 90 (1), 65e68. Kato, T., Ringwood, A.E., Irifune, T., 1988b. Experimental-determination of element partitioning between silicate pervoskites, garnets and liquids e constraints on early differentiation of the mantle. Earth Planet. Sci. Lett. 89 (1), 123e145. Kendall, J.M., Silver, P.G., 1996. Constraints from seismic anisotropy on the nature of the lowermost mantle. Nature 381 (6581), 409e412. Kirk, J., Ruiz, J., Chesley, J., Walshe, J., England, G., 2002. A major archean, gold- and crust-forming event in the Kaapvaal craton, South Africa. Science 297 (5588), 1856e1858.
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Lay, T., Helmberger, D.V., 1983. A lower mantle S-wave triplication and the shear velocity structure of D'. Geophys. J. R. Astron. Soc. 75 (3), 799e837. Lay, T., Williams, Q., Garnero, E.J., 1998. The core-mantle boundary layer and deep Earth dynamics. Nature 392 (6675), 461e468. Li, J., Wang, X., Wang, X.J., Yuen, D.A., 2013. P and SH velocity structure in the upper mantle beneath Northeast China: evidence for a stagnant slab in hydrous mantle transition zone. Earth Planet. Sci. Lett. 367, 71e81. Ling, M.X., et al., 2013. Formation of the world's largest REE deposit through protracted fluxing of carbonatite by subduction-derived fluids. Sci. Rep. 3 http://dx.doi.org/10.1038/srep01776. Marquardt, H., Miyagi, L., 2015. Slab stagnation in the shallow lower mantle linked to an increase in mantle viscosity. Nat. Geosci. 8, 311e314. Matsui, T., Abe, Y., 1986. Evolution of an impact-induced atmosphere and magma Ocean on the accreting earth. Nature 319 (6051), 303e305. Mckenzie, D.P., Parker, R.L., 1967. North Pacific e an example of tectonics on a sphere. Nature 216 (5122), 1276eþ. Miller, S.L., 1953. A production of amino acids under possible primitive Earth conditions. Science 117 (3046), 528e529. Miller, S.L., Parris, M., 1964. Synthesis of pyrophosphate under primitive Earth conditions. Nature 204 (496), 1248e&. Morgan, W.J., 1971. Convection plumes in lower mantle. Nature 230 (5288), 42e&. Ni, S.D., Tan, E., Gurnis, M., Helmberger, D., 2002. Sharp sides to the African superplume. Science 296 (5574), 1850e1852. Niu, Y.L., O'Hara, M.J., 2003. Origin of ocean island basalts: a new perspective from petrology, geochemistry, and mineral physics considerations. J. Geophys. Res. Solid Earth 108 (B4). Niu, Y.L., O'Hara, M.J., Pearce, J.A., 2003. Initiation of subduction zones as a consequence of lateral compositional buoyancy contrast within the lithosphere: a petrological perspective. J. Petrol. 44 (5), 851e866. O'Neill, C., Debaille, V., 2014. The evolution of Hadean-Eoarchaean geodynamics. Earth Planet. Sci. Lett. 406, 49e58. O'Rourke, J.G., Stevenson, D.J., 2016. Powering Earth's dynamo with magnesium precipitation from the core. Nature 529 (7586), 387eþ. Okazaki, K., Hirth, G., 2016. Dehydration of lawsonite could directly trigger earthquakes in subducting oceanic crust. Nature 530 (7588), 81eþ. Pearson, D.G., et al., 2014. Hydrous mantle transition zone indicated by ringwoodite included within diamond. Nature 507 (7491), 221eþ. Raup, D.M., Sepkoski, J.J., 1984. Periodicity of extinctions in the geologic past. Proc. Natl. Acad. Sci. U. S. Am. Biol. Sci. 81 (3), 801e805. Rohrbach, A., Schmidt, M.W., 2011. Redox freezing and melting in the Earth's deep mantle resulting from carbon-iron redox coupling. Nature 472 (7342), 209e212. Ryder, G., 1991. Lunar ferroan anorthosites and mare basalt sources e the mixed connection. Geophys. Res. Lett. 18 (11), 2065e2068. Selway, K., Ford, H., Kelemen, P., 2015. The seismic mid-lithosphere discontinuity. Earth Planet. Sci. Lett. 414, 45e57. Sepkoski, J.J., 1984. A kinetic-model of phanerozoic taxonomic diversity .3. Post-Paleozoic families and mass extinctions. Paleobiology 10 (2), 246e267.
Shen, S.Z., et al., 2011. Calibrating the end-permian mass extinction. Science 334 (6061), 1367e1372. Song, X.D., Helmberger, D.V., 1998. Seismic evidence for an inner core transition zone. Science 282 (5390), 924e927. Stagno, V., Ojwang, D.O., McCammon, C.A., Frost, D.J., 2013. The oxidation state of the mantle and the extraction of carbon from Earth's interior. Nature 493 (7430), 84eþ. Steele, A., et al., 2012. A reduced organic carbon component in martian basalts. Science 337 (6091), 212e215. Stern, T.A., et al., 2015. A seismic reflection image for the base of a tectonic plate. Nature 518 (7537), 85eþ. Stevenson, D.J., Halliday, A.N., 2014. The origin of the moon. Philos. Trans. R. Soc. Math. Phys. Eng. Sci. 372 (2024). Sun, W.D., Ding, X., Hu, Y.H., Li, X.H., 2007. The golden transformation of the cretaceous plate subduction in the west Pacific. Earth Planet. Sci. Lett. 262 (3e4), 533e542. Sun, W.D., et al., 2011. The fate of subducted oceanic crust: a mineral segregation model. Int. Geol. Rev. 53 (8), 879e893. Tauzin, B., Debayle, E., Wittlinger, G., 2010. Seismic evidence for a global low-velocity layer within the Earth's upper mantle. Nat. Geosci. 3, 718e721. Taylor, R., 2016. The moon. Acta Geochim. 35 (1), 1e13. Thomson, A.R., Walter, M.J., Kohn, S.C., Brooker, R.A., 2016. Slab melting as a barrier to deep carbon subduction. Nature 529, 76e79. van der Meijde, M., Marone, F., Giardini, D., van der Lee, S., 2003. Seismic evidence for water deep in earth's upper mantle. Science 300 (5625), 1556e1558. van der Hilst, R.D., Widiyantoro, S., Engdahl, E.R., 1997. Evidence for deep mantle circulation from global tomography. Nature 386 (6625), 578e584. Wen, L.X., Helmberger, D.V., 1998. Ultra-low velocity zones near the coremantle boundary from broadband PKP precursors. Science 279 (5357), 1701e1703. Wilson, J.T., 1963. Evidence from islands on spreading of ocean floors. Nature 197 (486), 536e&. Zhang, G.J., et al., 2015. Widespread shoaling of sulfidic waters linked to the end-Guadalupian (Permian) mass extinction. Geology 43 (12), 1091e1094. Zhang, J., et al., 2005. Inner core differential motion confirmed by earthquake waveform doublets. Science 309 (5739), 1357e1360.
Weidong Sun CAS Key Laboratory of Mineralogy and Metallogeny, Guangzhou Institute of Geochemistry, Guangzhou, 510640, China CAS Center for Excellence in Tibetan Plateau Earth Sciences, Chinese Academy of Sciences, Beijing, 100101, China E-mail address: [email protected].
ScienceDirect Solid Earth Sciences 1 (2016) 1e4 www.elsevier.com/locate/SESCI
Preface
Challenges in Solid Earth Sciences* “Solid Earth Sciences” is a peer-reviewed open-access journal, publishing research papers on a wide range of subjects in Earth sciences, including 1) geodynamics involved in the physical and chemical evolution of Earth; 2) petrologic processes in igneous to sedimentary and metamorphic rocks; 3) present and past environments relating to the habitable Earth; 4) tectonic processes in the crust and upper mantle, including structural geology to plate tectonics; 5) geochemical behaviors of elements under various conditions and their implications for mineralization and other geological processes; 6) high temperature and high pressure experiments that address chemical and/or physical properties of Earth materials; and 7) new methods, new ideas, and new models that help us to better understand Earth itself as a dynamic system. The journal aims at dealing with major challenges in Earth sciences. On behalf of the “Solid Earth Sciences” editorial board, I call for papers in the field of solid Earth, especially those that address the following major challenges, which, from our perspectives, are: (1) the structure and composition of the Earth's interior; (2) the formation of the Moon; (3) the evolution of the Magma Ocean; (4) the origin of plate tectonics; (5) the nature of mantle plumes; (6) the emergence and evolution of life; (7) the formation of giant ore deposits. The structure and composition of Earth are essential and basic topics for us to understand Earth's dynamic systems. Major challenges in this field are the detailed imaging of Earth's internal boundaries, the heterogeneous structural interior of Earth, the underlying physical properties and compositional and thermal mechanism, and the dynamic processes near the boundaries. The major boundaries in the deep Earth include the inner core boundary (Song and Helmberger, 1998; Zhang et al., 2005), the coreemantle boundary (Lay and Helmberger, 1983); the upper mantle discontinuities (Anderson and Bass, 1986), the lithosphereeasthenosphere boundary (LAB) (Stern et al., 2015) etc. Seismic tomographic images have revealed the large-scale 3D wave velocity structure of the deep mantle (e.g., Grand, 2002), which is characterized by two massive low velocity provinces surrounded by faster material (Kendall and Silver, 1996; Lay et al., 1998; Ni et al., 2002; Wen and Helmberger, 1998). To what degree
those structures can well constrain the deep mantle convection which might involve both chemical and thermal variations depends on interdisciplinary studies of seismology, geodynamics, mineral physics and geochemistry. The properties and structure of the upper mantle transition zone plays an important role in addressing some frontier scientific problems, like the interaction between the subducting lithosphere and mantle discontinuities, the flux exchange between the lower and upper mantle, and the storage of water in the deep mantle (e.g. Huang et al., 2005; Li et al., 2013; Pearson et al., 2014; van der Meijde et al., 2003). The structure and nature of the LAB are keys to a better understanding of the thermalechemical evolution of the continents, coupling between lithosphere, asthenosphere and deeper mantle, the subsurface deformation and so on. Earth's interior boundaries may have major influences on the recycling of water, carbon and metals (e.g. Rohrbach and Schmidt, 2011; Stagno et al., 2013; Thomson et al., 2016). Recent studies further suggest nearly global presence of important boundaries/layers at depth, such as a mid-lithosphere discontinuity at ~100-km depth within continental lithosphere (Karato et al., 2015; Selway et al., 2015), a low-velocity layer above the mantle transition zone (Tauzin et al., 2010), and compositional or rheological layering in the shallow lower mantle (Ballmer et al., 2015; Marquardt and Miyagi, 2015). These features reflect vertical heterogeneities of Earth, and may have resulted from and provided significant feedbacks to the early formation and longterm evolution of Earth. Both are, however, largely unclear or under debate. Overall, detailed information about the structure and properties of major boundaries and layers and associated heterogeneities of Earth's interior are still lacking, which prevent a comprehensive understanding of the dynamics and working mechanism of the entire Earth system. The formation of the Earth and the Moon has been a hot topic of discussions among scientists for years. The currently most popular model is still the “Giant Impact” hypothesis, or variations thereof. According to this model, a Mar-size impactor collided with the proto-Earth and vaporized much of the material. The vaporized particles accreted and made up the Moon, resulting in the Moon being depleted in volatile elements (Benz et al., 1989). This model is challenged by the nearly identical isotope ratios for the non-volatile elements in
Peer review under responsibility of Guangzhou Institute of Geochemistry. http://dx.doi.org/10.1016/j.sesci.2016.06.001 2451-912X/Copyright © 2016, Guangzhou Institute of Geochemistry. 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/).
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Preface / Solid Earth Sciences 1 (2016) 1e4
the Moon and the Earth (e.g, Asphaug, 2014; Stevenson and Halliday, 2014). According to the “Giant Impact” model, lunar samples should be easily distinguishable from those of the Earth, because the Moon is derived mostly from the impacting planetesimal with a small portion from the Earth. Lunar rocks, however, have the same isotopic composition as those from the Earth for O, Ti, Cr, W, K, and other species (Asphaug, 2014). Instead of a Mar-size planetary body, an icy comet has been proposed recently as the Moon-forming impactor (Taylor, 2016). In this scenario, the comet probably collided with and brought water to the Earth about 100 Ma after the formation of the Earth (Albarede, 2009). The major problem for this model is the fact that the Moon is currently very dry. Could volatile loss explain the dry nature of the Moon after such a “wet” impact? How much water in the Earth survived such a giant impact, which has sufficient energy to vaporize silicates? These questions remain to be answered. Another problem directly related to the formation of the Moon is the evolution of the early-Earth Magma Ocean. It has been proposed that the Earth was once totally melted due to energy released from decays of radioactive elements, and also energy from accretion, especially the Giant Impact (Matsui and Abe, 1986). Metals fractionated into the core, whereas silicates stayed in the mantle with kinetics controlled by density segregation. During the solidification of the Magma Ocean, different minerals started to crystallize at different PeT conditions. Crystallization of bridgmanite, ringwoodite, majorite, etc. should have resulted in major chemical fractionations in trace element abundances, e.g. heavy and light rare earth elements (e.g., Albarede et al., 2000; Kato et al., 1988a, 1988b). Xenoliths from the upper mantle and basalts, however, have much less fractionated trace element patterns. It has been proposed that mantle overturns in the Moon (e.g., Ryder, 1991), Mars (e.g., Elkins-Tanton, 2008) and also Earth (e.g., Brown et al., 2014; Debaille et al., 2009; Jackson et al., 2014; O'Neill and Debaille, 2014), might have homogenized the mantle over time. Were there mantle overturns in the early history of the Earth? Could the signatures of the Magma Ocean have survived the mantle overturn? How can such mantle overturn models be tested? Plate tectonics is one of the most important theories for modern Earth sciences (Mckenzie and Parker, 1967). According to the theory of plate tectonics, Earth's surface is separated into major plates by divergent, convergent and transform plate boundaries. The crust, together with lithospheric mantle, floats and drifts on top of the asthenosphere. Two types of crusts are apparent, continental and oceanic. Continental crust is felsic with an average age of ~2.0 Ga. Oceanic crust is mafic with an average age of less than 100 Ma. New oceanic crust forms at spreading ridges, with old oceanic crust being subducted at convergent margins. Earth, however, is probably the only solar terrestrial planet that has plate tectonics. What made Earth so unique? What is the driving force for plate tectonics? When and how did plate tectonics commence? One hypothesis suggests that water is the key factor that makes Earth unique. The Earth is the only rocky planet in the
solar system that has a large amount of liquid water at the surface. Most of the water in the uppermost mantle is hosted in pargasite, which decomposes at pressures greater than 3 GPa (at depths of ~90 km), forming the asthenosphere through water (vapor) induced partial melting (Green et al., 2010). Although this hypothesis plausibly explains the maximum thickness of oceanic lithosphere, it has difficulties to account for the thick cratonic lithosphere. Moreover, the Earth was formed inside the “Snow Line” of the solar system, and as such should not originally have had so much water. It might have received water from an icy comet ~100 Ma after the formation of the Earth (Albarede, 2009). In that case, how did water manage to go deep inside the Earth before plate tectonics to form the asthenosphere, while the planet was progressively degassing? The driving force behind plate tectonics is also obscure. Plate tectonics is overall driven by heat inside the Earth. It has been proposed that well-organized heat loss along spreading ridges controls the engine of plate tectonics (Sun et al., 2007). The energy that drives the Earth's dynamo is still not well constrained (e.g., Arevalo et al., 2009; Gale et al., 2013; O'Rourke and Stevenson, 2016). The main driving forces so far proposed are slab pull, mantle convection, and ridge push/ slab sliding. Slab pull has been considered as the primary driving force. Subducted old oceanic crust forms eclogite that is ~1% denser than mantle peridotite and, when combined with the lower-than-ambient temperature of subducting slabs, pulls the oceanic crust down into the mantle. What then was the driving force before plate subduction started? Also, many plates, e.g. the floor of the Atlantic Ocean, are not subducting, but they move fast, indicating the importance of other driving forces. The initiation of plate subduction (e.g., Gerya et al., 2015; Niu et al., 2003), the chemical and physical processes during plate subduction (e.g., Andersen et al., 2015; Okazaki and Hirth, 2016) and the physical properties and the fate of subducted oceanic slabs (e.g., Hirose et al., 1999; van der Hilst et al., 1997) are also hot topics that need more attentions. The initial concept of mantle plumes was first proposed as a “Hot Spot” to explain the Hawaiian-Emperor Island chain (Wilson, 1963). Wilson's hot spot originated from the convecting mantle, and therefore, it is very difficult to explain its fixed location. In order to solve this problem, the mantle plume hypothesis was proposed (Morgan, 1971). According to this model, a mantle plume originates from the coreemantle boundary. It has been proposed that mantle plumes are initiated by recycling of subducted oceanic slab material based on geochemical data (Hofmann and White, 1982). This model has been challenged because a subducted oceanic crust is denser than the surrounding mantle at lower mantle depths, whereas the subducted oceanic crust may not provide straightforward explanation on the geochemistry of plume magmas (Niu and O'Hara, 2003). From a geophysical perspective, mantle plumes initiate simply as thermal instabilities above the core-mantle boundary. In contrast, others argued that geochemical data favor recycling back of components from subducted oceanic slabs (e.g., Jackson et al., 2007). Such a density trap may be plausibly explained by a mineral
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segregation model (Sun et al., 2011). Nevertheless, the mantle plume hypothesis still needs further examination. The origin of life became a topic in Earth sciences with the ground breaking paper: “A Production of Amino Acids under Possible Primitive Earth Conditions” (Miller, 1953). Synthesis of organic compounds under primitive Earth conditions shed light on the origin of life (Miller and Parris, 1964). Yet, there are still major issues to bridge the gaps between organic compounds and self-replicating life forms. It is also not clear whether life originated on Earth or has been carried to Earth by a solar/extra-solar system traveler, e.g., a comet or a meteorite (Caro et al., 2002; Steele et al., 2012). There are five major mass extinction events in the Phanerozoic (e.g., Hallam and Wignall, 1999; Raup and Sepkoski, 1984; Sepkoski, 1984; Shen et al., 2011). These mass extinctions have been attributed either to asteroid impact (Alvarez et al., 1980), to large volcanic events (Courtillot and Olson, 2007), or to shoaling of sulfidic waters (Zhang et al., 2015). Determining the causes of mass extinction events is important as the effects of anthropogenic activities to the changes of the Earth's climate become more apparent. How to build/protect the environments of a habitable Earth is a major challenge for human beings. Ore deposits are the result of abnormal enrichments of economically valuable elements by special geological processes. Many mineral resources are dominated by giant deposits/mineralization belts. For example, Bayan Obo (Ling et al., 2013) accounts for 40% of the world light rare earth reserves; Witwatersrand (Kirk et al., 2002) hosts ~40% of the world's gold; over 70% of the world's porphyry Cu deposits are distributed around the circum-Pacific region; most of the world's Mo is hosted in Climax, Qinling-Dabie and northeastern China; whereas the Neotethys hosts over 70% of the world's Sn reserves. The processes that produced these giant ore deposits remain obscure. Solid Earth Sciences publishes the following items: 1) Trends, reporting progress in Earth Sciences during the last 12 months; 2) Review, summarizing major progresses in a special field during the last 5e10 years; 3) Article, reporting a welldeveloped piece of research work; 4) Letter, reporting major progress of an ongoing research; 5) Hypothesis, reporting original, innovative, striking new ideas on phenomena of global significance; 6) Forum, discussing papers published in “Solid Earth Sciences”; and 7) News, reporting major events in Earth sciences. We will also publish a small portion of interesting papers that may have not found support during peer-review, aiming to provide a refuge for challenging new models/hypothesis. References Albarede, F., 2009. Volatile accretion history of the terrestrial planets and dynamic implications. Nature 461 (7268), 1227e1233. Albarede, F., Blichert-Toft, J., Vervoort, J.D., Gleason, J.D., Rosing, M., 2000. Hf-Nd isotope evidence for a transient dynamic regime in the early terrestrial mantle. Nature 404 (6777), 488e490. Alvarez, L.W., Alvarez, W., Asaro, F., Michel, H.V., 1980. Extraterrestrial cause for the cretaceous-tertiary extinction e experimental results and theoretical interpretation. Science 208 (4448), 1095e1108.
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Weidong Sun CAS Key Laboratory of Mineralogy and Metallogeny, Guangzhou Institute of Geochemistry, Guangzhou, 510640, China CAS Center for Excellence in Tibetan Plateau Earth Sciences, Chinese Academy of Sciences, Beijing, 100101, China E-mail address: [email protected].