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Letter Doi:10.1038/Nature25031 LETTER doi:10.1038/nature25031 The divergent fates of primitive hydrospheric water on Earth and Mars Jon Wade1, Brendan Dyck2,3, Richard M. Palin4, James D. P. Moore5 & Andrew J. Smye6 Despite active transport into Earth’s mantle, water has been surface basalts6,7 and the widespread distribution of oxidized hydrated present on our planet’s surface for most of geological time1,2. phyllosilicate minerals7. The disassociation of water during such Yet water disappeared from the Martian surface soon after its silicate-mineral hydration reactions and the subsequent loss of hydro- formation. Although some of the water on Mars was lost to space gen to space, for example, may account for the presence of oxidized via photolysis following the collapse of the planet’s magnetic field3–5, magnetite13 and maghemite in the Martian meteorite NWA7034 the widespread serpentinization of Martian crust6,7 suggests that and the Gusev crater rocks it resembles 14, while the source region of metamorphic hydration reactions played a critical part in the Martian SNC (shergottite–nakhlite–chassignite) igneous meteorites sequestration of the crust. Here we quantify the relative volumes is much more reduced14,15. of water that could be removed from each planet’s surface via the Aside from the proportion of iron, the terrestrial and Martian mantles burial and metamorphism of hydrated mafic crusts, and calculate have broadly similar chondritic major-element compositions 16,17. The mineral transition-induced bulk-density changes at conditions abundance of iron in planetary mantles—and hence erupted surface of elevated pressure and temperature for each. The metamorphic material—is primarily controlled by the prevailing oxygen fugacity mineral assemblages in relatively FeO-rich Martian lavas can of planetary accretion, which was higher during the formation of hold about 25 per cent more structurally bound water than those the Martian core than during terrestrial core formation. As a result, in metamorphosed terrestrial basalts, and can retain it at greater terrestrial basalts are relatively FeO-poor (about 7–10 wt%)18 compared depths within Mars. Our calculations suggest that in excess of to more FeO-rich (about 17 wt%)19 basalts derived from the Martian 9 per cent by volume of the Martian mantle may contain hydrous upper mantle. As bulk-rock composition is the primary control on mineral species as a consequence of surface reactions, compared to the mineral assemblages that stabilize during surface hydration and about 4 per cent by volume of Earth’s mantle. Furthermore, neither metamorphism at elevated pressure (P) and temperature (T) condi- primitive nor evolved hydrated Martian crust show noticeably tions, these compositional differences probably played a critical part in different bulk densities compared to their anhydrous equivalents, determining each planet’s bulk geochemistry and surface water budget in contrast to hydrous mafic terrestrial crust, which transforms over geological time. to denser eclogite upon dehydration. This would have allowed Here, we have quantified the balance between mantle hydration efficient overplating and burial of early Martian crust in a stagnant- and density in both terrestrial and Martian interiors using integrated lid tectonic regime, in which the lithosphere comprised a single thermal and petrological modelling. We show that metamorphosed tectonic plate, with only the warmer, lower crust involved in mantle primitive and evolved Martian basalts can hold about 25% more struc- convection. This provided an important sink for hydrospheric water turally bound water in hydrous minerals than terrestrial equivalents, and a mechanism for oxidizing the Martian mantle. Conversely, with the majority stable to greater depths within Mars (> 90 km) than relatively buoyant mafic crust and hotter geothermal gradients on within Earth (about 70 km). In addition, time-integrated mass-balance Earth reduced the potential for upper-mantle hydration early in calculations show that the absolute volume of the Martian mantle that its geological history, leading to water being retained close to its has potentially been hydrated over geological time (> 9%) is at least surface, and thus creating conditions conducive for the evolution twice as large as that of the terrestrial mantle (about 4%). of complex multicellular life. To model the surface evolution of Earth and Mars, we considered a Surface water has existed on Earth for the vast majority of geologi cal mantle potential temperature (TP) of 1,650 °C for both the terrestrial 1 time , with its volume having remained approximately constant since and Martian primitive basalts (although this TP was reached at different the end of the Archaean (2.5 billion years (Gyr) ago) and initiation of times on each planet), and a lower bound of 1,400 °C for evolved rocks. subduction-driven plate tectonics2. However, despite evidence for it SNC meteorites indicate that the rate of secular cooling on Mars was having once possessed liquid water, the modern-day surface of Mars conservatively about three times faster than that of Earth20. We com- is essentially dry and lacks any widespread features of active plate bined these assumptions with a plate-cooling model, taking into tectonics8. The loss of surficial water from Mars since its formation account secular cooling and radiogenic heat production to calculate has been attributed to both photolysis in the upper atmosphere and aerotherms and geotherms through the lithospheres of each planet. sequestration into the crust via metamorphic hydration reactions9. We then investigated the petrological changes that would occur in Although loss to space by atmospheric sputtering and hydro dynamic the end-member cases of primitive and evolved, fully hydrated and escape probably became increasingly viable after cessation of the fluid-undersaturated crust during each planet’s geological evolution Martian magnetic field at about 4.1 Gyr ago3–5, uncertainties in its using thermodynamic phase equilibrium modelling at conditions efficiency, the timing of field collapse, and the initial water inven- defined by these P–T gradients. For Mars, we used bulk compositions tory suggest that this process alone may not have depleted the entire for primitive (Fastball) and evolved (Backstay) basalts, and for Earth volume of the hydrosphere over the lifetime of the planet10–12. Instead, we used a high-MgO Archaean tholeiitic basalt and a modern-day petrological evidence for crustal hydration is present as serpentinized normal mid-ocean-ridge basalt (N-MORB) (see Methods). We 1Department of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, UK. 2Department of Earth Sciences, University of Cambridge, Cambridge CB2 3EQ, UK. 3Department of Earth Sciences, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada. 4Department of Geology and Geological Engineering, Colorado School of Mines, Golden Colorado 80401, USA. 5Earth Observatory of Singapore, Nanyang Technological University, Singapore. 6Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16801, USA. 21/28 DECEMBER 2017 | VOL 552 | NATURE | 391 © 2017 Macmillan Publishers Limited, part of Springer Nature. All rights reserved. RESEARCH LETTER a 2.5 2.5 High-MgO Archaean basalt Fastball 2.0 Modern-day 2.0 terrestrial N-MORB Fastball 1.5 1.5 Backstay 1.0 1.0 Backstay High-MgO 0.5 Archaean basalt 0.5 Modern-day terrestrial Mass of water in hydrated column (wt%) Water content of hydrated protolith (wt%) N-MORB 0 0 35 40 45 50 55 60 65 70 75 80 85 90 12345678910 Depth within planetary interior (km) Absolute volume of hydrated planetary mantle (%) b 10 Figure 2 | Degree of mantle hydration on Earth and Mars as a function t of dewatering of metamorphosed basalts. N-MORB is normal 8 (%) mid-ocean-ridge basalt. U δ 6 4 amphibole to temperatures in excess of 1,200 °C, where they are pre- dicted to become Ti-rich, matching observed compositions from Positively buoyan 2 Martian meteorites22. Although hydration extends to similar absolute depths on both bodies, the shallower Martian aerotherm implies that 0 the volume fraction of silicate Mars that may be hydrated following –2 High-MgO crustal subduction is about double that of Earth (Fig. 2). Additionally, Archaean basalt the faster cooling of Mars20 progressively extends the depth of hydra- –4 Modern-day terrestrial N-MORB tion, exacerbated by the presence of more-evolved basalts over- plating the Martian surface and the negative Clapeyron slope for –6 Fastball Density decit of hydrated column, Backstay Negatively buoyant hornblende dehydration. The effect that hydration has on bulk-rock –8 density heightens this effect; metamorphosed terrestrial basalts are all 35 40 45 50 55 60 65 70 75 80 85 90 significantly less dense—and hence buoyant—when fluid-saturated, Depth within planetary interior (km) and undergo densification upon dehydration (Fig. 1b). The relatively Figure 1 | Calculated petrophysical properties for each modelled Fe-rich Martian materials are, however, calculated to exhibit rela- basaltic protolith. a, Plot of the structurally bound water content tively little volume expansion during hydration, with more-evolved (in weight per cent, wt%) in metamorphosed primitive and evolved basalts showing minimal change in bulk-rock density, and significant basalt on Earth and Mars. b, The relative density deficit between a crustal densification (>5%) of a water-bearing restite, post-melting. Here, the column composed of fully hydrated
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