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PLATE Metamorphic myth Clear evidence for -induced , and thus the operation of on the ancient has been lacking. Theoretical calculations indicate that we may have been looking for something that cannot exist. Jun Korenaga

arth’s surface is divided into a dozen as belts, or orogens, formed compared with the present- or so rigid tectonic plates. Movement when two collided, presumably day oceanic . Using thermodynamic Eand subduction of these plates into after subduction had consumed the seafloor calculations, Palin and White show that the governs nearly all geological that once existed between them. Many of the metamorphism of crust with high processes, such as , mountain these by-products of plate tectonics can be magnesium content is much less likely building and even atmospheric composition. traced back to around 3 billion years ago6. to yield -bearing However, it is unclear when plate tectonics There is also a suggestion for an even earlier assemblages. It is no wonder that we could began. Today, subduction forms - onset of plate tectonics, around 4 billion not find blueschist in those ancient orogens metamorphic rocks — often regarded years ago, based on the mineral of formed when the Earth was much hotter as the hallmark of plate tectonics. These ancient zircons7. than at present — we may have been rocks are found only up to about 800 million One nagging observation that seems looking for rocks that simply could not have years ago1, and their absence prior to this to defy an early onset of plate tectonics is been generated. has been taken as evidence against the absence of ancient blueschist-facies To preserve blueschist at the surface, the plate tectonics on an earlier Earth2,3. Writing metamorphic rocks. These rocks owe their rocks must be rapidly exhumed to Earth’s in Geoscience, Palin and White4 name to the predominance of the blue- surface without undergoing any further use thermodynamic calculations to show coloured mineral glaucophane in their metamorphism8. Given this rather intricate that ancient plate tectonics could not form makeup. form when basaltic scenario required for the surface exposure blueschist-facies rocks to begin with. rocks, which comprise the , of once subducted materials, the absence There is abundant evidence for plate are subject to high pressures and low of blueschists does not completely rule out tectonics during the , up to temperatures at depths of about 15 km the possibility that blueschist rocks did 540 million years ago, but prior to this time, in subduction zones8 (Fig. 1). These form on the younger Earth, but were simply the evidence is less clear5. The very nature metamorphic rocks can be exhumed back to not exhumed. However, Palin and White’s of plate tectonics means that it is difficult Earth’s surface when tectonic plates collide calculations suggest that the metamorphic to find evidence for its onset. Plates are to form orogens. The absence of blueschists products of ancient oceanic crust, though lost into the mantle at subduction zones, in orogens older than 800 million years they are not blueschist, can still be identified with the loss balanced by plate growth at does not necessarily require the absence of by their mineral chemistry, so future mid- ridges. Thus, plate tectonics plate tectonics, but begs the question of why studies will provide the ultimate test for continually rejuvenates Earth’s surface on something that resurfaced so easily in the their hypothesis. a time scale of about a hundred million recent geologic past failed to resurface in Palin and White also note that the years, erasing evidence for its past operation. earlier . metamorphism of magnesium-rich crust We must therefore rely on the subtle by- Palin and White4 have hit this blueschist could have brought more into the products of plate tectonics that may be conundrum from a blind side. Earth’s mantle than that of present-day crust, preserved on the buoyant continents. mantle has been cooling over time at a rate and this prediction is also against the Such by-products include fragments of of about 100 °C per billion years9. A hotter conventional wisdom. The subduction ancient oceanic crust that have been uplifted mantle in the past would have produced of reduces the volume of and exposed on the continents today, as a thicker oceanic crust more enriched in and reduces the viscosity of the

a b c Oceanic crust Blueschist exhumation

Ocean Lithospheric mantle

Blueschist Slab break-o

Figure 1 | Schematic illustration of blueschist formation and exhumation. a, Blueschist-facies metamorphism occurs when modern oceanic crust is subducted to depths of about 15 km. b,c, Some blueschists can detach themselves and migrate upward during slab rollback, and when the subducting slab breaks off b( ), they can be transported further towards Earth’s surface and exposed via during (c). Palin and White4 use thermodynamic calculations to show that blueschist metamorphism would not have occurred in the magnesium-rich oceanic crust that would have characterized the younger, warmer Earth.

NATURE GEOSCIENCE | VOL 9 | JANUARY 2016 | www.nature.com/naturegeoscience 9

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convecting mantle. The new thermodynamic phenomenon of interest takes place. This References calculations therefore have far-reaching secular evolution involves not only internal 1. Maruyama, S. & Liou, J. G. Int. Geol. Rev. 38, 485–594 (1996). 2. Stern, R. J. Geology 33, 557–560 (2005). implications for the coevolution of the temperature, but also a number of other 3. Hamilton, W. B. Lithos 123, 1–20 (2011). surface environment and the dynamics of variables such as chemical composition and 4. Palin, R. M. & White, R. W. Nature Geosci. 9, 60–64 (2016). Earth’s interior5. material strength. When extrapolating our 5. Korenaga, J. Annu. Rev. Earth Planet. Sci. 41, 117–151 (2013). 4 6. Condie, K. C. & Kröner, A. in When Did Plate Tectonics Begin on Palin and White’s study highlights the understanding of contemporary processes Planet Earth? (eds Condie, K. C. & Pease, V.) 281–294 (Geological importance of understanding the secular to deep time, approaches based on basic Society of America, 2008). evolution of the Earth, as well as the physics and chemistry may take precedence 7. Hopkins, M., Harrison, T. M. & Manning, C. E. ❐ Earth Planet. Sci. Lett. 298, 367–376 (2010). advantage of employing first-principles over empirical ones. 8. Agard, P., Yamato, P, Jolivet, L. & Burov, E. Earth Sci. Rev. approaches. The internal state of the planet 92, 53–79 (2009). has been gradually changing, as necessitated Jun Korenaga is in the Department of Geology 9. Herzberg, C., Condie, K. & Korenaga, J. Earth Planet. Sci. Lett. by the of heat-producing and , Yale University, PO Box 208109, 292, 79–88 (2010). elements, so we need to have a clear picture Connecticut 06520-8109, USA. of the global setting in which a particular e-mail: [email protected] Published online: 14 December 2015

PLANETARY SCIENCE on dry Martian have been seen as evidence for past surface water runoff. However, numerical modelling now suggests that accumulation and sublimation of carbon dioxide ice, rather than overland flow of liquid water, may be driving modern formation. Colin Dundas

he search for water has been one higher than today, as expected within the CO2 is the main component of the of the key threads linking decades past few million years3,4. Whatever the water Martian , and condenses Tof Mars science and exploration. source, wet models imply the repeated wherever the surface temperature drops to Water is a dynamic geologic agent, and occurrence of thousands of cubic metres its frost point. If the frost anneals to become its presence affects subjects ranging from of liquid water at each gully1, which would an impermeable slab of ice, such a layer to to . have profound implications for both would be translucent; can penetrate At present, Mars is a cold and dry , and possible biology on Mars. to the base and induce sublimation10. This with most of its water locked away as ice Monitoring of the Martian surface by causes pressure to rise beneath the ice slab, in polar caps or below the surface at lower orbiters over the past two decades has and eventually causes the slab to lift and . However, the discovery of gully shifted the debate by revealing present- break apart, rapidly expelling both gas and less than a million years in age day changes in the morphology of gullies5. entrained material10. Transient dark in images from the Successive images show significant changes spots that are widespread at high latitudes orbiter1 was initially regarded as strong to gullies, including channel erosion and every Martian spring10 — including in some evidence for overland flow of liquid water on the of lobate flows6. The ongoing gully channels9 and in gully-like features Mars in the geologically recent past. Writing activity of Martian gullies is perplexing on dunes11 — have been attributed to in Nature Geoscience, Pilorget and Forget2 because the present-day climate on Mars this process. challenge a wet origin for the Martian gullies is too cold at these times and locations to Pilorget and Forget2 propose that gully and demonstrate with numerical simulations support substantial liquid surface water. activity can be attributed to sublimation that the gullies may instead be formed in the However, there is another volatile species beneath CO2 ice and that this process is present climate due to the sublimation of that may be the culprit in gully formation. common, even in lower- gullies. seasonal CO2 frost (dry ice). CO2 frost covers the Martian poles and They conduct a detailed numerical study At first glance, the strong similarity mid-latitudes every winter. Near the poles, of the condensation of frost on slopes at a between the morphology of the Martian this seasonal cap is around a metre thick, range of latitudes and orientations where gullies (Fig. 1) and terrestrial features and thins towards the mid-latitudes. The gullies are common. In addition, they model formed by streams or wet debris flows points distribution of frost correlates with the temperatures to determine where and when to a wet origin. Much of the debate over regions where gullies are most prominent. CO2 will condense on the Martian surface gully formation has been focused on the Moreover, gully activity appears to be and analyse conditions for the build-up of source of the putative water. Initially, leading seasonal, with a marked preference for the gas pressure and eventual venting. They 1 6,7 models favoured , winter and spring — when CO2 frost is find that the predicted latitudes and slopes but this is inconsistent with the occurrence observed on mid-latitude slopes8. Thus, match the general distribution of gullies, 3 of gullies on sand and isolated peaks . CO2 frost might be responsible for gully which have a marked tendency to face the Other models have proposed melting formation. This was considered shortly poles at lower latitudes. Pilorget and Forget or ground ice during a period when the after gullies were discovered9, but how this find that the debris flows should readily obliquity of Mars (the tilt of its axis) was process works remains unclear. occur at the latitudes and slopes where gully

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