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Diamonds and water in the deep Earth: a new scenarioFabrizio Nestolaa & Joseph R. Smythb

a Dipartimento di Geoscienze, Università di Padova, I-35131 Padova, Italyb Department of Geological Science, University of Colorado, Boulder, CO 80309, USAPublished online: 25 Jun 2015.

To cite this article: Fabrizio Nestola & Joseph R. Smyth (2015): Diamonds and water in the deep Earth: a new scenario,International Geology Review, DOI: 10.1080/00206814.2015.1056758

To link to this article: http://dx.doi.org/10.1080/00206814.2015.1056758

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REVIEW ARTICLE

Diamonds and water in the deep Earth: a new scenarioFabrizio Nestolaa and Joseph R. Smythb

aDipartimento di Geoscienze, Università di Padova, I-35131 Padova, Italy; bDepartment of Geological Science, University of Colorado, Boulder,CO 80309, USA

ABSTRACTEarth is a water planet, but how much water exists on and in the Earth? Is the water limited to theEarth’s surface and limited depths of our planet (molecular water of the hydrosphere), or do deepreservoirs of hydrogen and oxygen really exist as proposed in recent works but not yet proven? Dueto the importance of H2O for life and geological processes on the Earth, these questions are amongthe most significant in all of the Earth sciences. Water must be present in the deep Earth as platetectonics could not work without water as a major driving force that lowers both viscosity anddensity of the solid mineral phases of the interior and controls the onset of melting. On subduction,water is returned to the hydrosphere first by dewatering of hydrous phases and second by meltingand arc magmatism in and above the subducting slab. The mantle is composed of oxygen minerals,and the extent to which hydrogen is dissolved in them constitutes the true reservoir of the planet’swater. Are ‘deep water and diamonds’ intimately related as indicated in the title of the presentarticle? What is the connection between these two important terrestrial materials? The necessity toreview this issue arises from the recent discovery of a strongly hydrous ringwoodite in a Braziliandiamond. As ringwoodite constitutes 60% or more of the lower part of the transition zone, between525 and 660 km depth, this could correspond to a huge amount of water in this region, comparableor greater in mass to all of Earth’s hydrosphere. If the water found in this ringwoodite is represen-tative of the water concentrations of the transition zone, then estimates of Earth’s total waterreservoir are in need of major revision. This work is an attempt at such a revision.

ARTICLE HISTORYReceived 5 April 2015Accepted 27 May 2015

KEYWORDSdiamond; water; Earth;reservoir; mantle

Diamond is one of the most familiar of Earth materialsbecause of its ‘special’ physical properties, which makethis high-pressure polymorph of carbon a valuablematerial for industry and jewellery. However, what isless well known is that natural diamond represents areal and unique window into the deep regions of ourplanet. Diamonds are one of the oldest terrestrial mate-rials, commonly older than three billion years. Indeed,diamonds can transport inclusions, which are fragmentsof deep Earth consisting of a wide variety of minerals,from silicates to oxides, to sulphides, carbonates, andmore [see, for instance, inclusions of an important sili-cate such as olivine, (Mg,Fe)2SiO4, Figure 1, which arecommon silicates entrapped in diamond at high-pres-sure and high-temperature conditions, Nestola et al.(2011, 2014)].

Diamond crystallizes at depths greater than130–150 km in the upper mantle (the rocky ‘shell’ ofthe Earth that extends from the base of the crust downto 410 km depth), and it is now widely accepted thatthey can form in the transition zone (between 410 and660 km depth, Pearson et al. 2014) and the lower

mantle (below 660 km depth) (see Shirey et al. 2013for an extensive and recent review of natural diamondsand their mineral inclusions). In general, about 95% ofnatural diamonds have crystallized in the 150–250 kmregion in the upper mantle (so-called ‘lithospheric dia-monds’ formed under tectonically stable cratonic areas),but the remaining 5% are considered to be ‘super-deepdiamonds’ as they are believed to have grown at muchgreater depths in the Earth (i.e. transition zone andlower mantle). Figure 2 is a world map showing thedistribution of diamond occurrences, classifying themas lithospheric, super-deep, and alluvial diamonds (dia-monds that have been naturally extracted from theiroriginal kimberlite and deposited in a different environ-ment commonly far from the original source). FromFigure 2, it is evident that super-deep diamonds arequite rare, but are the only diamonds capable of directlysampling the transition zone and the lower mantle.

A general scheme of the Earth’s interior showingdifferent shells with their relative depth ranges isgiven in Figure 3. In this figure the graphite–diamondboundary and the mantle keel where diamonds most

CONTACT Fabrizio Nestola fabrizio.nestola@unipd.it

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commonly form are also indicated. It should be notedthat this ‘special’ part of the Earth is tectonically stable,and the temperature here may be slightly lower thanthe surrounding mantle material at the same depth. Inthese regions of the Earth, based on the depth, thetemperature ranges between 900 and 1400°C (seeStachel and Harris 2008), the so-called ‘diamond win-dow’. Finally, Figure 3 also shows subduction with thesubducting slab reaching the lower mantle. Indeed, it iswidely accepted that diamonds can also form in thesubducting slab (Shirey et al. 2013), and since subduc-tion is the main geological process capable of hydrating

the mantle (see an extensive and recent review byFaccenda 2014), we can expect that diamonds formedin subduction zones may entrap hydrated minerals fromthis environment.

The most common mineral inclusions found in litho-spheric diamonds have been reviewed by Stachel andHarris (2008), and are listed in Table 1, together with themost common mineral inclusions found in super-deepdiamonds (Kaminsky 2012). From laboratory experi-ments it is well known that some of these mineralscrystallize only at very high-pressure and high-temperature conditions (e.g. bridgmanite, MgSiO3 with

Figure 2. Distribution of diamonds over the world. We decided to show lithospheric, super-deep, and alluvial diamonds. However,two further types of diamonds could be considered, the ultra-high-pressure crustal diamonds and the impact diamonds, which areextremely scarce (see Shirey et al. 2013). The world map is modified after http://www.freeworldmaps.net/.

Figure 1. Different inclusions of Mg-rich olivine (indicated by the arrows) with typical cubo-octahedral shape still trapped in adiamond from Udachnaya kimberlite.

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perovskite-type structure, Tschauner et al. 2014), sowhen we actually find such minerals as inclusions indiamonds, then we can derive direct geological infor-mation of our planet’s interior not obtainable by othermethods. But how can diamond and its mineral inclu-sions also provide direct information about the waterreservoirs in the deep Earth?

The answer is again to be found in the types ofinclusions that occur in diamond: beyond the mostcommon inclusions, we can also find phases that cansometimes be hydrous. It must be remarked here thatwhen we refer to ‘hydrous’, we clearly mean waterbound within the mineral crystal structures. For

instance, one mineral found in diamond is phlogopite,a magnesian mica that contains about 2.5% H2O.Another hydrous mineral found in diamond is theso-called phase ‘Egg’ [(AlSiO3(OH)] (Wirth et al. 2007,Figure 4), which contains even more water, up toabout 7.5% H2O, and is only stable at mantle P and Tconditions as demonstrated by laboratory experiments.Whereas such hydrous minerals are rare in diamonds,the main source of water in diamond is H2O incorpo-rated in trace amounts in the so-called nominally anhy-drous minerals (NAMs) (see a very extensive review ofNAMs published in 2006 and edited by Keppler andSmyth (2006) and Hirschmann (2006)). Numerous stu-dies have shown that minerals that may be completelyanhydrous actually contain small but significantamounts of structurally bound water. It needs to beemphasized strongly that hydrous minerals are not typi-cally found in the mantle, and are only stable in specificregions. Most water in the mantle has to be stored inNAMs as temperatures are typically too high for nomin-ally hydrous minerals to be stable (with the exception ofa few rare phases).

Stable in the Earth’s mantle and belonging to theNAM category, we include olivine, clino and orthopyr-oxenes, ringwoodite (cubic polymorph of olivine), wad-sleyite (orthorhombic polymorph of olivine; bothringwoodite and wadsleyite are the high-pressure poly-morphs of olivine), and garnet. Laboratory experimentshave demonstrated that olivine can host up to 10,000

Figure 4. High-resolution transmission electron microscopyimage of an inclusion of phase Egg in a super-deep diamondfrom Juína kimberlite modified after Wirth et al. (2007). Theinclusion is represented by the darker pseudo-square grain notlonger than about 20 nm.

Figure 3. Approximate scheme of the Earth’s interior from sur-face to about 800 km depth. Such a range of depth allows us toshow the Earth’s different shells including the uppermost partof the lower mantle. In yellow a typical tectonically stablecratonic area with its very old mantle keel (see Shirey et al.2013 for an extensive review). The scheme also shows a possi-ble graphite–diamond boundary and a subduction zone with asubducted slab reaching the lower mantle. Relative to thetransition zone, we decided to subdivide it into upper andlower transition zones based on the wadsleyite–ringwooditetransformation, which occurs at about 525 km depth.

Table 1. Main inclusions found in lithospheric and super-deepdiamonds.Main inclusions in lithospheric diamonds (Stachel and Harris 2008)Olivine (Mg,Fe)2SiO4

Garnet (Mg,Fe,Ca)3(Al,Cr)2Si3O12

Mg-chromite (Mg,Fe)(Cr,Al)2O4

Clinopyroxenes (Ca,Na,Mg,Fe)(Mg,Al,Fe)Si2O6

Orthopyroxene (Mg,Fe)2Si2O6

Coesite SiO2

Sulphides Fe–Ni–Cu–Co

Main inclusions in super-deep diamonds (Kaminsky 2012)Bridgmanite (Mg,Fe)SiO3 (found as back-transformed to low-Ni

enstatite)Ferropericlase (Mg,Fe)OCa-Si-Perovskite CaSiO3 (found as CaSiO3 walstromite-type structure)Stishovite SiO2

Jeffbenite (Mg,Fe)3Al2Si3O12 (also called TAPP, TetragonalAlmandine Pyrope Phase)

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ppm H2O (e.g. Kohlstedt et al. 1996; Smyth et al. 2005,2006; Hushur et al. 2009), orthopyroxene can host atleast up to 8000 ppm (Smyth et al. 2007), and ring-woodite and wadsleyite can host up to 30,000 and33,000 ppm H2O, respectively (Smyth 1987, 1994,2006; Kohlstedt et al. 1996; Ye et al. 2012). Amongclinopyroxenes, natural omphacites [(Ca,Na)(Mg,Al)Si2O6] can host up to 2500 ppm H2O (Smyth et al.1991), but in general calcic and sodic clinopyroxenesshow up to 500 ppm H2O (Skogby 2006), whereas syn-thetic majoritic garnets can incorporate up to700–750 ppm H2O (Bolfan-Casanova et al. 2000;Thomas et al. 2015). The values of water reportedhere, with the exception of natural omphacite, are allexpressed as maximum experimentally observed solubi-lity and, therefore, do not imply that these full amountsof water are actually found in NAMs from natural envir-onments. For example, olivine and orthopyroxenefound in mantle xenoliths do not show more thanseveral hundred ppm H2O, whereas natural omphacitecan show several thousand ppm H2O. Thus, it is stillunknown whether NAMs actually have low water con-centrations in the mantle or whether they lose waterduring the ascent to the Earth’s surface. This clearlydepends on the diffusion coefficients of hydrogen in

different NAMs (see Demouchy and Mackwell 2006),coefficients that are not well documented in the litera-ture. We remind the reader that xenoliths (pre-existingrock fragments embedded inside a magma host) onlyoriginate from the uppermost mantle and are incorpo-rated in, and in direct contact with, these melts (i.e. theyare not protected from them). This means that suchinteraction can cause some reaction and relativechange. Hydrogen is a very fast-moving species, sothis interaction has the most pronounced influence onwater contents and it is unlikely that original watercontents in NAMs will be preserved (Novella et al. 2015).

Hydrous NAMs can be found in diamonds (Kurosawaet al. 1997; Matsyuk and Langer 2004); thus, it is crucialto study such inclusions to understand whether waterexists in the very deep environments of our planet.Pursuing such a possibility, a team of researchers fromseveral countries investigated a series of super-deepdiamonds from Juina (Brazil), and in 2014 found a ter-restrial ringwoodite containing 1.4% H2O (Figure 5). Thisfirst reported terrestrial occurrence of ringwooditeappeared as a tiny inclusion (about 0.05 mm) in alarge (~5 mm) diamond (Pearson et al. 2014). We notehere that ringwoodite is a major part of the expectedanhydrous mantle assemblage, so it is indeed possible

Figure 5. Ringwoodite inclusion investigated by Pearson et al. (2014). The image on the top left side represents the diamondcontaining the ringwoodite inclusion (the image is a modified version after Keppler 2014). The images on the top and bottom rightside represent a magnification of the ringwoodite inclusion and two infrared spectra (measured at two positions at 90°; both imagesare modified versions after Pearson et al. 2014). Finally, the image in the bottom left side is a map of Brazil indicating the MatoGrosso region where the Juína kimberlite is located (image modified after www.freeworldmaps.net).

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that hydrous ringwoodite in this diamond may berepresentative of the actual water content of the partof the mantle in which the diamond formed. If thishydrous ringwoodite does not represent a localizedwater-enrichment but the actual transition zone waterconcentration, this discovery opens new possibilities forthe role of water on global convective circulation (seenext paragraphs). This discovery further demonstratesthat diamond can preserve high-pressure minerals suchas ringwoodite, and therefore it is likely that wadsleyitemay also be found as an inclusion in diamond.Wadsleyite forms between 410 and 525 km depth,slightly shallower than the region at which ringwooditecrystallizes (525–660 km depth), so its finding may pro-vide further constraints on the real water concentrationin this region. The rarity of such inclusions may also bedue to the special ascent rate conditions of diamondsfrom such an extreme depth and to the pressure–temperature conditions to which the diamond is sub-jected during its ascent.

The discovery of ringwoodite not only providesdirect proof of the presence of water in the transitionzone, but also provides crucial constraints on the ascentrate of diamond from a depth of at least 525 km to theEarth’s surface. The occurrence of ringwoodite found byPearson et al. (2014) represents a real natural quenchfrom very high temperature and pressure. However, atthe moment there are no reliable kinetic data to deter-mine the minimum time to preserve ringwoodite fromthe back-transformation to olivine in the upper mantle.It is not unreasonable that the diamond studied byPearson et al. (2014) reached the Earth’s surface fromthe transition zone in about 4–5 hours.

The water reservoirs in Earth

To estimate the total amount of water in the Earth, it isnecessary to first understand the actual reservoirs ofdifferent Earth’s shells and in detail the reservoirs ofbiosphere (including atmosphere), crust, upper mantle,transition zone, lower mantle, and core of our planet.

Recent estimates of interior reservoirs relative to thesurface suggest a surface amount of about 0.024% oftotal mass, and just slightly more, 0.027%, for themantle, for a total estimate of about 0.05% H2O overthe whole planet (Bodnar et al. 2013). (This would be atotal mass of 3.04 × 1024 g H2O out of the entire massof the Earth, which is 5.97 × 1027 g, based on anaverage Earth density of 5.513 g cm–3 and a radius of6371 km.) This is quite small compared with theamount estimated in carbonaceous chondritic meteor-ites of about 10% (Kerridge 1985 and referencestherein), and with comets, which should contain up

to 50% H2O (Delsemme 1988; but we are still waitingfor the results from ESA’s Rosetta mission landed onNovember 2014 to confirm this).

As proposed by Marty and Yokochi (2006), the lowwater content of our planet would not be due to loss tospace during the Earth’s formation because the water’sisotopic composition indicates that the Earth alwayshad a low amount of water. Again, they state thatstrong differences in the water content in planetarybodies could simply be attributed to the differentwater-supplying bodies and/or to the radial distancefrom the Sun (Boss 1998). Such explanations may wellbe reasonable, but we should not exclude the possibi-lity that water in our planet has been severely under-estimated, with large amounts of water still trapped inthe very deep regions of the Earth.

Marty and Yokochi (2006) reviewed the various esti-mates of water contents in the Earth. Whereas there isgeneral agreement on the mass of surface water with anaverage value of 1.5 × 1024 g (corresponding to a con-centration of 250 ppm H2O on the bulk Earth), theactual amount of water in the mantle is still a subjectof vigorous debate. Saal et al. (2002) proposed a bulkamount of 145 ppm of H2O (or, in mass, 1.316 × 1023 g)for the mantle. Based on Saal et al. (2002), summing thesurface and mantle water, the Earth should contain atotal of about 1.63 × 1024 g H2O, which would corre-spond to a concentration of about 273 ppm.

However, different estimates of Earth’s bulk watercontent have been proposed in the literature: Jambonand Zimmermann (1990) proposed 1300 ppm(7.8 × 1024 g) based on Huang et al. (2005), who pro-posed water content in the transition zone between1000 and 2000 ppm (2.14 × 1023 g); the water contentof the mantle would increase by 13% with respect tothe Saal et al. (2002) estimates and by about 25% withrespect to the estimates of Jambon and Zimmermann(1990).

Further work on NAMs proposes 450 ppm (2.7 × 1024 gH2O) for the bulk Earth (Bell and Rossman 1992). A similaramount of water was estimated by Keppler and Bolfan-Casanova (2006). However, the most recent estimate per-formed by Bodnar et al. (2013) considers an average of2000 ppm H2O in the transition zone and 100 ppm in thelower mantle. Based on these assumptions, Bodnar et al.(2013) report a total amount of 3.04 × 1024 g of water inthe Earth.

It must be noted that prior water estimates in thetransition zone and lower mantle were based solely onexperimental data and/or reasonable geochemicalassumptions. Only after the discovery of Pearson et al.(2014) was it possible to demonstrate the actual pre-sence of water in the Earth’s transition zone. Therefore,

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any attempt at calculating the Earth’s bulk water con-tent should take into account such a discovery, whichpredicts for the transition zone an amount of about10,000 ppm H2O. At the same time, such discoveryclearly indicates that any attempts to calculate thetotal amount of water in the Earth cannot ignore therole of diamonds, which provide the only real proof ofwater in the very deep Earth. It is evident that there isno general agreement on mantle bulk water content,and uncertainty of water concentration in the transitionzone is a major part of this. It is even more remarkablethat the most recent estimates by Bodnar et al. (2013)report that, in percentage, the transition zone wouldcontain about 26% of the entire water storage of theEarth and then, together with the amount of waterstored in the oceans (i.e. 45%), the transition zonewater reservoir may play a crucial role in the globalEarth’s water cycle. However, we should not neglect atall the role that could be played by water in the lowermantle and core. Indeed, even if water in the lowermantle and core could be present in very low amounts,the huge mass of these two Earth’s shells would containa very large amount of water.

Water in the various shells of Earth’s interior

In the following section, we revise the estimates ofwater concentrations in the Earth considering the reser-voirs of different geological environments from the bio-sphere (and atmosphere) to the lower mantle and core,considering the possible change in the total reservoir ofthe Earth based on the ringwoodite discovered in thesuper-deep diamond from Brazil.

The lower mantle and the core require special atten-tion because for these very large regions, variousauthors propose very different water contents. As yet,there is no general agreement on H solubility in thenominally anhydrous phases of the lower mantle, whichconstitutes roughly half the mass of the entire planet.However, here we will try to include any possibleamount of water in the lower mantle and core basedon the literature.

Water in the Earth’s biosphere

There is a general agreement of the amount of water inthe Earth’s biosphere. Gleick (1996) took into account theamounts of water from oceans, seas, bays, ice caps,glaciers, permanent snow, groundwater (fresh and sal-ine), soil moisture, ground ice and permafrost, lakes(fresh and saline), atmosphere, swamp water, rivers, andbiological water. The total amount of water in all suchenvironments was estimated to be 1.386 × 1024 g, in

general agreement with Berner and Berner (1996), whoestimate a total amount of water of 1.409 × 1024 g.However, more recently, Berner and Berner (2012) esti-mate a total amount of 1.444 × 1024 g, a difference ofabout 4% with respect to the estimates of Gleick (1996).For all estimates about 97% of such water is stored asseawater (1.401 × 1024 g) in the oceans, whereas theremaining 3% is stored either on the continents or inthe atmosphere. The most recent estimate of water reser-voir for biosphere and atmosphere is that of Bodnar et al.(2013): in this work, the authors propose a value of1.414 × 1024 g. We will use such last estimated value asreliable, as differences are small relative to estimates ofinterior reservoirs.

Water in the Earth’s crust

Continental crustWater content estimates for the Earth’s continental crustare much more complex due to the strong heterogene-ity of this shell of our planet. Wedepohl (1995) carriedout a 3000 km-long refraction seismic profile throughWestern Europe using for upper crust–felsic lowercrust–mafic lower crust a proportion of 1–0.6–0.4.Based on this seismic profile, Wedepohl (1995) pro-posed for the continental crust a bulk water contentof 2% of the total mass. To convert such a percentage,we use an average thickness of continental crust of40 km (e.g. Rudnick and Gao 2003) and consider thatit covers about 40% of the planet’s surface. The Earth’ssurface is about 509,806,000 km2 (using an averageradius of 6371 km); therefore, the continents coverabout 203,922,400 km2. The volume of the continentalcrust could then be approximately 8,156,900,000 km3

and based on a reasonable granitic density of2.67 g cm–3 the continental crust mass could be esti-mated at 2.178 × 1025 g (the mass estimated by Bodnaret al. 2013 is very close to this value of 2.09 × 1025 g).Thus, the water in this shell is estimated at about4.36 × 1023 g. However, if we use the estimate byBurnham (1997), which assumes a mass percentage ofH2O equal to 1.3%, then the total amount in the con-tinental crust would be about 2.8 × 1023 g (as alsoproposed by Bodnar et al. 2013). It is reasonable thenthat the best value would be an average of these twovalues, giving the water content in the continental crustequal to 3.58 × 1023 g.

Oceanic crustTo calculate the water of the oceanic crust, we used thesimplest possible approach assuming an average thick-ness of about 7 km (Staudigel 2003), an areal extensionof oceanic crust equal to 60% of the total surface

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(Bodnar et al. 2013), an average density of about2.96 g cm–3 (Watts 1978; Molnar and Atwater 1978;Pennington 1984; Carlson and Raskin 1984), and calcu-late a surface of 305,883,600 km2. With this approachwe calculate a total volume for the oceanic crust of2.14 × 109 km3 and thus a mass of 6.34 × 1024 g.Assuming a value of water concentration (Alt 2004;such assumption was based on sampling by the OceanDrilling Program, which provided many sections of theuppermost few hundred metres of the ocean crust, seeFigure 15.12 by Alt 2004) equal to about 1.5%, the totalmass of water of the oceanic crust would be9.51 × 1022 g. However, as pointed out by Bodnaret al. (2013), the real content of the oceanic crust musttake into account pore water (Staudigel 2003). Johnsonand Pruis (2003) estimated such pore water in theoceanic crust being about 2.60 × 1022 g (as alsoreported by Bodnar et al. 2013). Summing the twoestimates by Alt (2004), 1.5%, and the Johnson andPruis (2003) on the pore water, we obtain for the ocea-nic crust a total water mass of 1.21 × 1023 g. This valueis not that far from the estimate by Bodnar et al. (2013),who propose 1.38 × 1023 g.

Water in the Earth’s mantle

Water in the Earth’s upper mantleFor the upper mantle, Bodnar et al. (2013) provide agood review of the possible amounts of water andfinally estimate a total amount of 1.2 × 1023 g. Theestimate is only partially based on natural sample mea-surements, and most of the data are retrieved fromlaboratory experiments and various types of calcula-tions. The main problem in calculating water in theupper mantle is to combine the maximum solubility(storage capacity) of the upper mantle phases obtainedin laboratory with water measurements on natural sam-ples. As remarked by Bodnar et al. (2013) based on themaximum solubility of upper mantle phases obtained in

laboratory (see Bolfan-Casanova 2005), they calculate anamount of water = 1.2 × 1024 g, which is an order ofmagnitude larger than that estimated by the sameauthors for the upper mantle. However, studies of nat-ural samples (i.e. xenoliths) report an average value of90–150 ppm of H2O for the upper mantle phases (Mottlet al. 2007; Peslier et al. 2010); however, Keppler andBolfan-Casanova (2006) report an average of 250 ppmH2O for the upper mantle. Bodnar et al. (2013) estimatean average concentration of 200 ppm, which results in atotal amount of water in the upper mantle of1.2 × 1023 g. We used this value in Table 2 of thiswork as a best estimate.

Water in the Earth’s transition zoneThis section may include the most significant result ofthis work. Indeed, in this section, the amount of waterstored in the transition zone will be revised based on thediscovery of Pearson et al. (2014). This ringwooditeshowed a very large amount of water equal to 1.4% byweight H2O. This discovery requires revision of the actualamount of water stored in the transition zone. The max-imum solubility of water in ringwoodite, as obtained bylaboratory experiments, is reported to be between 2.5%and 3% (i.e. 25,000–30,000 ppm; Kohlstedt et al. 1996; Yeet al. 2012). Duffy and Anderson (1989) pointed out thatthe seismic velocities of the anhydrous phases believedto be stable in this region were too fast to fit whole-Earthvelocity models for this region. However, the majorphases can incorporate sufficient hydroxyl to reduceboth P and S velocities to be consistent withPreliminary Reference Earth Model (Dziewonski andAnderson 1981) or other seismic velocity models of theEarth. So the velocity structure of this region is consistentwith significant hydration.

The review by Bodnar et al. (2013) reports for thetransition zone a total amount of H2O equal to7.9 × 1023 g, based on a water concentration of2000 ppm. Such water concentration represents

Table 2. Earth’s water distribution based on hydrous ringwoodite and hydrous wadsleyite and only on hydrous ringwoodite (see thetext for different literature data used in this work for the water distribution calculation of continental and oceanic crusts, transitionzone, and lower mantle).

This work (‘HTZ’ model) This work (‘HR’ model) Bodnar et al. (2013) model

Mass (g) % Mass (g) % Mass (g) %1Biosphere 1.41 × 1024 11.61 1.41 × 1024 13.34 1.41 × 1024 46.5Continental crust 3.58 × 1023 2.95 3.58 × 1023 3.39 2.80 × 1023 9.2Oceanic crust 1.21 × 1023 1 1.21 × 1023 1.15 1.38 × 1023 4.51Upper mantle 1.20 × 1023 0.99 1.20 × 1023 1.14 1.20 × 1023 3.9Transition zone 3.46 × 1024 28.49 1.88 × 1024 17.8 7.9 × 1023 26Lower mantle 6.10 × 1024 50.23 6.10 × 1024 57.74 3.00 × 1023 9.92Core 5.75 × 1023 4.73 5.75 × 1023 5.44 −Total 12.10 × 1024 100 10.06 × 1024 100 3.04 × 1024 100

Notes: 1Data from Bodnar et al. (2013). 2This value is the average of what is estimated in the text.Bodnar et al. (2013) do not include water in the core in their model.

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therefore only 1/15th of the maximum solubility ofringwoodite. We must also consider that, in additionto ringwoodite, the transition zone is composed ofwadsleyite and majoritic garnet, with a proportion,based on laboratory experiments, of majoritic garnets41.1%, ringwoodite 33.5%, wadsleyite 23.1%, and somediopside (about 0.8%) and CaSiO3 (about 1.5%, withperovskite-type or walstromite-type structure) (Frost2008). Wadsleyite can incorporate more water thanringwoodite, with maximum H2O solubility reaching3.3% (i.e. 33,000 ppm, Smyth 1994), whereas majoriticgarnet can only reach a value of 0.07–0.075% H2O(Bolfan-Casanova et al. 2000; Thomas et al. 2015).However, it is well known that the water solubility ofthese phases decreases with increasing temperatureand pressure (Ohtani et al. 2000; Demouchy et al.2005; Litasov and Ohtani 2007), but increases withincreasing iron content (Kawamoto et al. 1996).Bodnar et al. (2013), using the maximum water solubi-lity of the transition zone phases, calculated a storagecapacity of about 8 × 1024 g (20,000 ppm H2O), whichis one order of magnitude larger than the water con-tent reported finally in their Table 2 for the transitionzone. These authors deeply revised the literature data,which reported strongly different values of possiblewater storage in the transition zone (Inoue et al.1995; Williams and Hemley 2001; Jacobsen et al.2005; Huang et al. 2005; Jacobsen and Van Der Lee2006; Smyth 2006; Bolfan-Casanova et al. 2006; Litasovand Ohtani 2007; Mottl et al. 2007). Based on theirestimates and on a stronger lower value of watersaturation, Bodnar et al. (2013) evaluated as reliablewater content a concentration of 2000 ppm, resultingin 7.9 × 1023 g H2O, or about 60% of the total mass ofoceans (see Table 2).

The discovery by Pearson et al. (2014) may comple-tely change the estimates of the total water content ofthe transition zone. These authors found a very hydrousringwoodite still trapped in a super-deep diamond fromBrazil showing a water concentration of about 1.4% wt.H2O (i.e. 14,000 ppm) – a value much higher than thatestimated by any model or assumption in the literaturethus far, but still representing only about 50% of itsmaximum water solubility. If this percentage of max-imum water solubility were applied to wadsleyite andmajoritic garnet as well, then we may assume a similaramount of water incorporated in wadsleyite (1.5–1.6%H2O) and about 0.035% H2O in majoritic garnet.Considering the mineralogical composition of the tran-sition zone proposed by Frost (2008), we also includediopside (CaMgSi2O6) and CaSiO3 perovskite to calculatethe total water content. However, based on the above-mentioned percentages, their possible water content is

negligible, being below the uncertainty level in thecalculation.

An estimate of water content in the transition zonebased on the above-assumed H2O concentrationswould come to about 8640 ppm, resulting in3.46 × 1024 g H2O, which corresponds to about 4.5times the water estimated in the transition zone bythe latest estimates (Bodnar et al. 2013). In terms ofocean water proportions, the transition zone wouldtherefore contain an amount equal to about 2.5 timesof all the Earth’s oceans.

Keppler (2014) noted that the high water contents ofthe transition zone implied by the discovery of Pearsonet al. (2014) might only be a localized phenomenon.However, very recent results performed on the transi-tion of hydrous ringwoodite to bridgmanite and (Mg,Fe)O (transition occurring at about 660 km depth) produceintergranular melt, and this would be consistent withhydration of a large region of the transition zone(Schmandt et al. 2014). In addition, we should notneglect previous works proposing a possible ‘transi-tion-zone water filter’ model, which would well matchwith a significantly hydrous transition zone (i.e.Bercovici and Karato 2003). Although we do not yethave definitive proof that the entire transition zone ishydrous, there is no doubt that future models of thisregion of the Earth must take into consideration ahydrous transition zone.

At the end of the above section on ringwoodite, wepropose in Table 2 a revised version of the total watercontent providing a ‘hydrous transition zone’ (HTZ)model and a ‘hydrous ringwoodite’ (HR) model. Indetail, these two models are different by the watercontent in the transition zone: the HTZ model alsoconsiders hydrous wadsleyite with 50% of its maximumwater solubility, whereas the HR model only considersthe discovery by Pearson et al. (2014), assuming wateronly in the lower part of the transition zone between525 and 660 km depth. Finally, we compared these twomodels, which also include the possible amount ofwater in the lower mantle and core (see the belowsection), with the most recent estimate of Earth’s totalwater by Bodnar et al. (2013).

Water in the Earth’s lower mantle and coreThere is no general agreement on H solubility in thenominally anhydrous phases of the lower mantle, whichconstitutes roughly half the mass of the entire planet.The major mineralogy of this region is thought to bebridgmanite (perovskite-type MgSiO3) plus smalleramounts of ferro-periclase and perovskite-type CaSiO3.Unlike the transition zone, the velocity structure of thisregion is consistent with elastic property measurements

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of the anhydrous mineral phases, and these phases donot appear to be able to accommodate much hydrogen(Sinogeikin et al. 2004). Murakami et al. (2002) report upto 2000 ppmw H2O in (Mg,Fe)SiO3 bridgmanite synthe-sized at 25.5 GPa and 1600ºC in an Al-bearing peridotitecomposition. Litasov et al. (2003) observed only about100 ppm in pure MgSiO3 bridgmanite, but 1400–1800ppmw H2O in Al- and Fe-bearing bridgmanites in ahydrous peridotite system. None of the FTIR or Ramanspectra of silicate bridgmanites in pure MgSiO3 orMgSiO3–Al2O3 systems show sharp absorption bands,so there has been some disagreement as to whetherthese features represent structurally bound hydroxyl(Litasov et al. 2003; Bolfan-Casanova et al. 2003).Bridgmanite samples synthesized in chemically complexsystems show a consistent but broad OH absorptionfeature at about 3397 cm−1, but variable other features.It appears that while H2O solubility in pure Mg bridg-manite is likely negligible (<40 ppmw H2O), bridgma-nites crystallized from more chemically complexsystems may incorporate significant amounts of water,but in reports of higher water contents, the possibilityof hydrous inclusions within the bridgmanite in thesesamples cannot be ruled out. Bodnar et al. (2013) reviewprevious laboratory studies to obtain possible storagecapacity of lower mantle phases such as bridgmanite(MgSiO3 perovskite; water content from sub-ppm toabout 40 ppm), ferropericlase (about 2000 ppm), andCaSiO3 perovskite (a non-quenchable phase; thereforethe data on water on this phase was not considered).Al-bearing stishovite is also a possible hydrous phase,but would likely be restricted to mafic, rather thanultramafic, compositions.

It is likely, then, that minor amounts of nominallyhydrous phases may incorporate the majority of H inthis region of the mantle. Because these phases can beextremely hydrous (>150,000 ppmw H2O), one modalpercentage of these phases can incorporate over 1500ppmw H2O in the region and constitute a significantfraction of the total water in the planet. Nominallyhydrous phases stable at lower mantle conditionsinclude superhydrous phase B, phase Egg [(AlSiO3

(OH)], phase D [(Mg,Al)(Al,Si)2O6H2-3], δ-AlOOH, andphase H [MgSiO2(OH)2]. Of these, phase D may be stableup to 1500 km depth (Frost and Fei 1999), and the lattertwo, which have related crystal structures and may formextensive crystal solution (Bindi et al. 2014), appear tobe stable up to the deepest portions of the lowermantle (Sano et al. 2008; Ohira et al. 2014).

Wirth et al. (2007) report the first occurrence of atetragonal form of the phase Egg as an inclusion in asuper-deep diamond from the same locality where thediamond containing ringwoodite was found (Pearson

et al. 2015). Phase Egg is known to be stable at lowermantle conditions (Ono 1999; Ohtani 2005). Pamatoet al. (2015) published the stability field of the Al-richphase D (an analogue material for dense, hydrous mag-nesium silicate phase D in Earth’s mantle), demonstrat-ing that this phase is stable at temperatures extendingto over 2000°C at 26 GPa. Pamato et al. (2015) show thatphase Egg and their Al-rich phase D can coexist (at 26GPa and 1900 K) at least in the uppermost part of thelower mantle. So it appears that phase Egg and alumi-nous phase D may be significant hosts for H in theupper part of the lower mantle while δ-AlOOH andphase H solid solution may be hosts in the lowerregions of the lower mantle. Two modal percentagesof these phases would not affect the overall density orP and S velocities in the lower mantle, and hence it isdifficult to restrict water contents of the region to below2000 ppmw, which would equal to about 6.1 × 1024 g.For hydrous melting to occur as subducting slabs des-cended below 660 km (Schmandt et al. 2014), bulkwater contents would have to exceed this amount,which is consistent with our estimates of H contents inthe transition zone (Table 2). If whole-mantle convec-tion carries hydrated material from the transition zoneinto the lower mantle, water contents of the lowermantle would be highly dependent on the efficiencyof this process to dehydrate the descending slab andreturn water and perhaps other incompatible elementsto the transition zone.

Hydrogen is highly soluble in both solid and liquidiron metals at the moderate to high pressures of theinterior, and has been considered as a candidate for thelight element in the core by many authors, notablyStevenson (1977, 1981), Fukai (1992), and Yagi andHishinuma (1995). A silicate magma ocean that con-tained significant H2O would necessarily fractionate asignificant amount of its hydrogen into the separatingmetal of the core. Rubie et al. (2015) estimate H con-tents of the core to be between 9 and 58 ppmw (500–3200 ppm atomic) based on accretion models. If oxygenwas also present, this would be 80–500 ppmw H2O. Ifthis amount was present throughout both the inner andouter cores, it would be equivalent to between 1.5 and10 × 1023 g or about 70% of one surface ocean mass.

Water in the Earth: a summary

In this work we have reviewed and revised the recentestimates of total water in the Earth. However, it is clearthat the main changes with respect to previous esti-mates are based on the hydrous ringwoodite discov-ered in a super-deep diamond from Brazil (Pearson et al.2014) and estimates of water in the Earth’s core by

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Rubie et al. (2015). Together with the estimates of waterin the lower mantle and core, if the water measured inthe Brazilian ringwoodite found in diamond is not just alocalized water enrichment, then this discovery wouldopen vast new possibilities for the role of water in thegeological evolution of our planet. If the water found inringwoodite is representative of this region of the man-tle, it would constitute about 2.5 times the total water ofall oceans around the world and this could have a strongimpact on global Earth’s water cycle. In Figure 6 we showan approximate distribution of water in the Earth and,more than single numbers reported in Table 2, it isevident that the Earth has practically two large waterreservoirs, the lower mantle and core and the transitionzone, whereas the surface water and crust and the uppermantle contain much lower amounts of water.

Table 2 compares the results of our review with themost recent estimates by Bodnar et al. (2013). The firstimportant result is the difference between the twomodels, namely the HTZ and HR models. This is due toa very different water content predicted in the transi-tion zone considering that in the HTZ model both ring-woodite and wadsleyite show 50% of their maximumwater solubility as suggested by Pearson et al. (2014),whereas in the HR model wadsleyite is consideredtotally anhydrous. This last assumption is, in our opi-nion, totally unrealistic, as we well know that wadsleyitecan host more water than ringwoodite, and there is noapparent reason why wadsleyite in the transition zone

Figure 6. An approximate representation of the water distribu-tion in the Earth. A simple jug is used showing with differentcolours the Earth’s different shells. Such a simple representationclearly indicates that the transition zone and the surface waterare the main water reservoirs of our planet (Figure 6 is basedon the data reported in Table 2).

Figure 7. A representation of the famous novel by Jules Verne ‘Journey to the Center of the Earth’ of 1864 (handmade by TatianaMantovani).

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should be anhydrous. However, since no wadsleyitefrom transition zone has been found to date, we believethat we must take into account also the HR model inTable 2.

It must be noted that even using the data in the HRmodel the amount of water in the transition zone isconsiderably larger than in previous estimates. In con-clusion, we estimate that the amount of water deepinside our planet has been significantly underestimatedin previous studies. In light of this, Jules Verne, in hisfamous novel ‘Journey to the Centre of the Earth’ dated1864, might have been correct . . . with the only differ-ence that Verne hypothesized ‘liquid water’ (Figure 7)against a less-romantic OH group distributed inside thestructure of a strange mineral that geologists years laterwould have called ‘ringwoodite’.

Acknowledgements

Matteo Chinellato is thanked for the picture in Figure 1. Wethank Tatiana Mantovani for making Figure 7 and StefanoCastelli for technical photographical support.

Disclosure statement

No potential conflict of interest was reported by the authors.

Funding

This work was supported by the European Research CouncilStarting Grant 2012 [grant agreement number 307,322] to FN[project INDIMEDEA] and by US National Science Foundationgrants [EAR 11-13369, EAR 14-16979] to JRS.

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