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  1. 1. Hydrothermal modification of the Sikhote-Alin ironmeteorite under low pH geothermal environments.A plausibly prebiotic route to activated phosphoruson the early EarthDavid E. Bryant a, David Greenfield b, Richard D. Walshaw c,Benjamin R.G. Johnson d, Barry Herschy a, Caroline Smith e, Matthew A. Pasek f,Richard Telford g, Ian Scowen g, Tasnim Munshi g, Howell G.M. Edwards g,Claire R. Cousins h, Ian A. Crawford h, Terence P. Kee a,⇑aSchool of Chemistry, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, UKbCentre for Corrosion Technology, Materials and Engineering Research Institute, Sheffield Hallam University, Sheffield S1 1WB, UKcLeeds Electron Microscopy and Spectroscopy Centre, University of Leeds, Leeds LS2 9JT, UKdMolecular and Nanoscale Physics Group, School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, UKeDepartment of Earth Sciences, Natural History Museum, London SW7 5BD, UKfDepartment of Geology, University of South Florida, Tampa, FL 33620, United StatesgSchool of Life Science, University of Bradford, Richmond Road, Bradford BD7 1DP, UKhDepartment of Earth and Planetary Sciences, Birkbeck College, University of London, Gower Street, WC1E 6BT, UKReceived 6 August 2012; accepted in revised form 28 December 2012; available online 5 February 2013AbstractThe Sikhote-Alin (SA) meteorite is an example of a type IIAB octahedrite iron meteorite with ca. 0.5 wt% phosphorus (P)content principally in the form of the siderophilic mineral schreibersite (Fe,Ni)3P. Meteoritic in-fall to the early Earth wouldhave added significantly to the inventory of such siderophilic P. Subsequent anaerobic corrosion in the presence of a suitableelectrolyte would produce P in a form different to that normally found within endogenous geochemistry which could then bereleased into the environment. One environment of specific interest includes the low pH conditions found in fumaroles or vol-canically heated geothermal waters in which anodic oxidation of Fe metal to ferrous (Fe2+) and ferric (Fe3+) would be cou-pled with cathodic reduction of a suitable electron acceptor. In the absence of aerobic dioxygen (Eo= +1.229 V), the protonwould provide an effective final electron acceptor, being converted to dihydrogen gas (Eo= 0 V). Here we explore the hydro-thermal modification of sectioned samples of the Sikhote-Alin meteorite in which siderophilic P-phases are exposed. Wereport on both, (i) simulated volcanic conditions using low pH distilled water and (ii) geothermally heated sub-glacial fluidsfrom the northern Kverkfjo¨ll volcanic region of the Icelandic Vatnajoku¨ll glacier. A combination of X-ray photoelectronspectroscopy (XPS) and electrochemical measurements using the scanning Kelvin probe (SKP) method reveals that schreiber-site inclusions are significantly less susceptible to anodic oxidation than their surrounding Fe–Ni matrix, being some 550 mVnobler than matrix material. This results in preferential corrosion of the matrix at the matrix-inclusion boundary as confirmedusing topological mapping via infinite focus microscopy and chemical mapping through Raman spectroscopy. The signifi-cance of these observations from a chemical perspective is that electrochemically noble inclusions such as schreibersite are0016-7037/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.gca.2012.12.043⇑ Corresponding author. Tel.: +44 (0)113 3436421; fax: +44 (0)113 3436565.E-mail addresses: d.greenfield@shu.ac.uk (D. Greenfield), caroline.smith@nhm.ac.uk (C. Smith), mpasek@usf.edu (M.A. Pasek),I.Scowen@bradford.ac.uk (I. Scowen), T.Munshi@bradford.ac.uk (T. Munshi), c.cousins@ucl.ac.uk (C.R. Cousins), i.crawford@ucl.ac.uk(I.A. Crawford), t.p.kee@leeds.ac.uk (T.P. Kee).www.elsevier.com/locate/gcaAvailable online at www.sciencedirect.comGeochimica et Cosmochimica Acta 109 (2013) 90–112
  2. 2. likely to have been released into the geological environment through an undermining corrosion of the surrounding matrix,thus affording localised sources of available water-soluble, chemically reactive P in the form of H-phosphite [H2POÀ3 , Pi(III)as determined by 31P NMR spectroscopy]. This compound has been shown to have considerable prebiotic chemical potentialas a source of condensed P-oxyacids. Here we demonstrate that Pi(III) resulting from the hydrothermal modification of Sikh-ote-Alin by sub-glacial geothermal fluids can be readily dehydrated into the condensed P-oxyacid pyrophosphite [H2P2O2À5 ,PPi(III)] by dry-heating under mild (85 °C) conditions. The potential significance of this latter condensed P-compound forprebiotic chemistry is discussed in the light of its modified chemical properties compared to pyrophosphate [H2P2O2À7 , PPi(V)].Ó 2013 Elsevier Ltd. All rights reserved.1. INTRODUCTIONThe impact of meteoritic material to early planets, espe-cially the Earth, may have played a key role in the emer-gence of life. Meteoritic delivery has been demonstratedto add volatiles such as water (Greenwood et al., 2011;Alexander et al., 2012) and ammonia (Pizzarello et al.,2011) to the early earth chemical inventory as well as organ-ic compounds that are especially enriched within the carbo-naceous chondrite class of impactors (Sephton, 2002). Thechemical behaviour of iron meteorites as a contributor tothe chemical inventory of the early Earth centres not onlyon the presence of the potentially catalytically active metalsiron and nickel (Kress and Tielens, 2001) but through non-metallic elements such as carbon (C), sulphur (S), nitrogen(N) and phosphorus (P) which are widely present as acces-sory phases within those materials (Benedix et al., 2000).The latter have been subject to considerable recent investi-gation as it has been shown that hydrothermal treatment ofsiderophilic P-phases such as schreibersite (Fe,Ni)3P affordswater-soluble P-compounds which not only differ to thosenormally found within the terrestrial geological record,but are more chemically reactive (Pasek and Lauretta,2005, 2008; Bryant and Kee, 2006; Pasek et al., 2007). Ithas been suggested that these, more reactive forms of Pcould have played a role in the emergence of phosphorusenergy currency molecules (specifically nucleotide triphos-phates such as ATP) in contemporary biochemistry (Bryantet al., 2010).The corrosion of iron is usually a subject of interest tomodern day engineers concerned with the rusting of man-made artefacts. Naturally occurring iron meteorites ex-posed to wet, aerobic environments will similarly corrodeas meteorite curators and collectors are well aware. How-ever, the present atmospheric conditions are very differentto those believed to have existed on the early (Hadean)Earth when oxygen was present in only trace amounts(Kasting, 1993), implying that meteoritic corrosion uponthe Hadean Earth would presumably have had to makeuse of alternative electron acceptors to dioxygen, a scenariocommon to biological systems (Harold, 1986). Galvaniccorrosion occurs when two dissimilar metals are in intimatecontact within an electrolyte and does not require the pres-ence specifically of dioxygen but only of some electronacceptor capable of initiating cathodic reactions to balancethe anodic oxidation of metallic iron to ferrous (Fe2+) orferric (Fe3+) ions. One suitable such acceptor is the proton(H+), which would have been an effective electron acceptorwithin anoxic, low pH, Hadean environments such asvolcanic fumaroles and related fluids (Bortnikova et al.,2010; Reigstad et al., 2010).The slow cooling of iron–nickel mixtures within the par-ent body from which meteorites are derived allows the ironand nickel to crystallise into two principal forms, namelynickel-rich (20–65% Ni) taenite and nickel-poor (5–10%Ni) kamacite (Hutchison, 2004) and in theory there couldbe Galvanic corrosion between these two at the grainboundaries in the presence of a suitable electrolyte medium.Indeed Galvanic corrosion is also implicated in oxidativecorrosion of present day samples (Buchwald and Clarke,1989). Dissolution studies (Tackett et al., 1970; Tackettand Goudy, 1972) show how kamacite dissolves preferen-tially from a series of iron–nickel meteorites due to its lowernickel content and that the overall dissolution rate of themeteorite depends on its nickel content. Moreover, thepresence of siderophilic mineral inclusions such as carbides[cohenite, (Fe,Ni,Co)3C], sulphides (troilite, FeS) and phos-phides [such as schreibersite, (Fe,Ni)3P and allabogdanite,(Fe,Ni)2P] (Britvin et al., 2002; Nazarov et al., 2009), lead-ing to further local differences in metal composition has thepotential to establish local Galvanic couples. This couldlead to the non-metals such as carbon, sulphur and phos-phorus being involved in corrosion electrochemistry, ulti-mately to be released into the corroding fluidenvironment and hence become available for early Earthchemistry.Schreibersite is an important, if minor, component ofiron meteorites within which it can be found in the 0–20 vol% range (Geist et al., 2005). Its presence has beenlinked to the crystallisation behaviour of kamacite and tae-nite resulting in the well-known Widmansta¨tten patternscharacteristic of iron meteorites (Goldstein and Hopfe,2001). The amount of nickel in schreibersite can vary justas the amount in the matrix can vary and higher overallnickel contents are frequently associated with higher phos-phorus (P) contents (Kracher et al., 1980). Schreibersiteinclusions usually have higher nickel content than the sur-rounding matrix and meteorites of low overall nickel con-tent are often hexahedrites which have crystallisedprimarily as kamacite that has subsequently lost nickel toadjacent schreibersite inclusions (Mason, 1971). Two co-existing schreibersite minerals, one a Ni-free variety andthe other containing 23 wt% Ni, were found in Fe–Ni metaland troilite in lunar rocks (Hunter and Taylor 1982; Scottet al., 2007; El Goresy et al., 1971), though meteoritic nickelcontents are more typically around 6–7%. The nickel con-tent of terrestrial, natural iron found on Disko Island,Greenland was lower than average meteoritic values at lessD.E. Bryant et al. / Geochimica et Cosmochimica Acta 109 (2013) 90–112 91
  3. 3. than 3% (Holtstam, 2006). Therefore, given the electro-chemical potential differences between meteoritic matrixand schreibersite inclusions due to varying nickel composi-tions, it would be expected that local anodes and cathodeswould be more closely identified with matrix and inclusionregions respectively during hydrothermal modification. In-deed, such electrochemical potential differences have beensuggested by the scanning Kelvin probe analysis of theSeymchan pallasite (Bryant et al., 2009), the first time suchtechniques have been employed to analyse the surface of ameteorite. Whilst our studies have focused more closely onschreibersitic inclusions within iron meteorities, it is worthnoting that phosphorus-bearing Fe and Ni sulphides havebeen identified and characterised as primary mineral phaseswithin type CM carbonaceous chondrites (Nazarov et al.,2009) thus opening up the potential for chemistries derivedfrom metal phases to converge with those centred on organ-ic molecules (vide infra).Corrosion of iron meteorites in an oxygenic atmosphereusually proceeds via goethite (a-FeOOH) through to hae-matite and magnetite (Grokhovsky et al., 2006). Alterna-tively, it has been shown that akaganeite (b-FeOOH) canalso be a corrosion intermediate and one which facilitatescorrosion acceleration in the presence of the chloride anion(Buchwald and Clarke, 1989). The schreibersite inclusionswithin iron meteorites also undergo hydrothermal modifi-cation (Pasek and Lauretta, 2005, 2008; Pasek et al.,2007) to afford a range of P-oxyanion species includingdihydrogenphosphate (H2POÀ4 ) and hypophosphate(H2P2O2À6 ) but principally the lower oxidation state P-oxy-anion, H-phosphonate (aka phosphite) [P(III); HPO2À3 ] aswell as iron nickel hydroxides similar to those derived frommatrix material. Subsequently, it has been demonstratedthat, in the presence of UV light, even lower oxidation stateP species such as H-phosphinate [P(I); H2POÀ2 ] can be ob-tained from such inclusions (Bryant and Kee, 2006), andthat this compound may be present even under hydrother-mal modification at concentrations 10–20 times lower thanphosphite (Pech et al., 2011). Natural weathering of schre-ibersite under aerobic conditions has been shown to pro-duce arupite-vivianite (Fe,Ni)3(PO4)2Á8H2O in Australianmeteorite samples (Tilley and Bevan, 2010), undoubtedlythe result of oxygenic corrosion processes favouring thehighest, and most thermodynamically stable, oxidationstate of phosphorus. Given that schreibersite has a highernickel content than the surrounding matrix it would be ex-pected to act as the cathode in a Galvanic cell between thetwo and this was indeed found to be the case using a scan-ning Kelvin probe to map the surface of a pallasite (Bryantet al., 2009).Described here are our studies on the hydrothermalmodification of the type IIAB iron meteorite Sikhote Alin,which fell in eastern Siberia in 1947, under low pH condi-tions which simulate those prevalent within geothermallyheated volcanic environments (Dessert et al., 2009). In addi-tion to these simulations, we also report here in situ hydro-thermal studies of Sikhote-Alin in sub-glacial, geothermallyheated fluids from the Kverkfjo¨ll volcanic field in the north-ern region of the Icelandic Vatnajoku¨ll glacier during a re-cent field expedition, between 8th and 21st June 2011. Thisregion was selected as an ideal low pH geothermal site asthe region is dominated by basaltic volcanism and near-sur-face hydrothermal activity affording localised, out-of-equi-librium environments which provide low pH, sulphur-rich(to support a sulphuric acid hydrosphere) and high temper-ature fluids. We have explored the changes which occur tothe surface morphology, corrosion and solution chemistryvia a unique combination of elemental (inductively coupledplasma) analysis, mapping Raman spectroscopy, scanningelectrochemistry, X-ray photoelectron spectroscopy(XPS), 31P NMR spectroscopy and infinite focus micros-copy (IFM) techniques. Collectively, these tools have al-lowed us to draw important conclusions on the followingproblems: (i) where, upon an iron meteoritic surface, anaer-obic corrosion is most likely to occur; (ii) what P-containingproducts result from the hydrothermal modification of P-containing inclusions within the meteorite Sikhote Alinand (iii) suggest possible consequences of such corrosionfor primitive Earth solution chemistry. We have focusedon the relative anodic potentials of meteoritic matrix andschreibersite inclusions which allow us to comment upon,the nature, location and release of reactive, water-solubleP during surface corrosion under putative early Earth envi-ronments. Our key conclusions therefore centre around theavailability of reactive P-species resulting from hydrother-mal modification of meteoritic surfaces and also how theseP-species can be readily converted to condensed P-oxyacids,specifically pyrophosphite [H2P2O2À5 , PPi(III)] which has re-cently been demonstrated to have properties commensuratewith an ability to act as an energy currency molecule withinputative early Earth environments (Bryant et al., 2010).2. MATERIALS, LOCATION AND EXPERIMENTALMETHODS2.1. Materials and location2.1.1. ChemicalsWater was purified by ion exchange on a Purite SelectAnalyst (PSA) reverse osmosis-deionisation system (PuriteLtd., Oxford, UK). D2O for NMR analyses was used as re-ceived from Sigma–Aldrich. Solutions of aqueous HCl wereprepared by dilution of commercial samples in PSA deion-ised water. Similarly, aqueous NaOH, Na2S, Zn(NO3)2,Cu(NO3)2 and Pb(NO3)2 were prepared by dissolution ofcommercial solids in PSA water to the appropriate concen-tration. Solution pH measurements were made on a Scho-chem pH meter buffered to pH 4 and 7 with commercial(Fisher Chemicals) standards.2.1.2. MeteoritesMeteorite samples were provided by the Natural HistoryMuseum, London (BM.1992,M39; BM.1992,M42 and twoseparate samples from BM. 1992,M40). Particular emphasiswas placed on a sample of Sikhote-Alin (SA) which con-tains a relatively large, well defined schreibersite inclusion(Fig. 1). This cleaved, circular, polished SA fragment wasca. 10 mm in diameter and the inclusion was ca. 3 mm inlength and 1 mm in width at its widest point.92 D.E. Bryant et al. / Geochimica et Cosmochimica Acta 109 (2013) 90–112
  4. 4. 2.1.3. The Hveradalur geothermal field site; Kverkfjo¨llVolcanic System, IcelandBeneath the northern part of the Vatnajo¨kull glacier incentral Iceland lies the Kverkfjo¨ll volcanic system(Fig. 14a; Hoskuldsson et al., 2006). There are significantgeothermal areas surrounding the rim of the northern cal-dera, some of which have been described previously (O´ lafs-son et al., 2000), which display a range of temperatures andpH. Of most significance to our investigations here were thelow pH (1–5) geothermal fluids of the Hveradalur geother-mal area (64° 40.1730N; 16° 41.1000W) sampled during theJune 2011 field expedition. A full description of this site,associated geology and water chemistry will be reportedelsewhere.Fig. 1. (a) Sikhote-Alin meteorite polished fragment with arrow shaped schreibersite inclusion (highlighted) pointing up from lower edge. (b)Sum EDX compositional spectrum, highlighting strong responses from Fe, Ni and P. (c) SEM image (secondary electron) of arrow tip regionof schreibersite inclusion with energy dispersive X-ray maps at P (2013.7 eV), Fe (6403.84 eV), Ni (7478.15 eV) and O (524.9 eV) Ka energiesrespectively.Fig. 2. XPS analyses of both Fe2P and Fe3P surfaces focusing onbinding energies of P (left hand column) and Fe (right handcolumn) core 2P3/2 electron binding energies.D.E. Bryant et al. / Geochimica et Cosmochimica Acta 109 (2013) 90–112 93
  5. 5. 2.2. Experimental methods2.2.1. Scanning electron microscopy and energy dispersive X-ray (SEM and EDX)Meteoritic surfaces were examined by scanning electronmicroscopy at 20 kV accelerating voltage using a 12 mmworking distance on a Philips XL30 ESEM system fittedwith an Oxford Instruments INCA250 EDX. Images areroutinely acquired in secondary electron mode. ElementalX-ray data were compiled into 2D maps using the OxfordInstruments INCA software. Due to the inherently electri-cally conducting nature of the meteorite sample, carboncoating was not necessary to prevent charging.2.2.2. X-ray photoelectron spectroscopy (XPS)Samples of Fe3P powder, supplied by Alfa-Aesar, weretransferred in a nitrogen filled glove box onto a sample stubfor study within an Escalab 250 XPS instrument (ThermoScientific). The surface spectrum was recorded and thenFig. 3. Fe-2p3/2 electron XPS line scan analysis traversing matrix–schreibersite–matrix regions of sectioned Sikhote-Alin sample (arrow-tipregion in Fig. 1a). Increasing P-composition locates the inclusion region and demonstrates an increasing Fe 2p binding energy within theinclusion compared to matrix.Fig. 4. (Left) SKP (Scanning Kelvin Probe) image of the arrow-shaped schreibersite inclusion within Sikhote-Alin (cf: image in Fig. 1a). Theinclusion stands out, in red-green, from the predominantly blue coloured matrix. The work-function scale (À500 to À200 mV) is shownunderneath the image; red = cathodic, blue = anodic regions. This image is a composite of three separate section maps. (Right) Three-point(Zn, Cu, Pb) calibration curve for SKP Pt tip. (For interpretation of the references to colour in this figure legend, the reader is referred to theweb version of this article.)94 D.E. Bryant et al. / Geochimica et Cosmochimica Acta 109 (2013) 90–112
  6. 6. the surface layer removed by ion beam etching (3 kV,9 mm2, 1 lA sample current, 480 s etch time) and a secondspectrum recorded. The polished sample of Sikhote-Alinwas cleaned with ethanol (2 mL) and dried (50 °C) to re-move surface contamination before removing the surfacelayer by ion beam etching within the XPS. A series of spec-tra were obtained at points along a line of length 5.5 mmwhich traversed the narrow part of the inclusion. The X-ray source provides monochromated aluminium Ka radia-tion (1486.7 eV) with a spot size of 500 lm. Binding ener-gies are referenced to C(1s) at 285.0 eV and elementalabundances are a percentage of the total counts adjustedby a relative sensitivity factor for each element. The controlpowders of Fe2P and Fe3P were analysed by immobilisingthe powders onto the sample stub using a combination ofcarbon tape and indium foil and then analysed as abovewith scans of before and after etching. Full spread-sheeteddata are available within the Electronic Annex.2.2.3. Scanning Kelvin probe (SKP)The sample was analysed using a Uniscan SKP100instrument (Buxton, UK). Measurements were acquiredusing a 100 lm diameter platinum tip. Scans were carriedout at ambient temperature (ca. 298 K) and all scans wereperformed in step-scan mode using the following parame-ters: electrometer gain setting of 100, full scale sensitivityof 2.5 mV, output time constant 1.0 s, vibrational ampli-tude 30 lm. A three-point calibration was carried out tocorrelate the SKP output with the redox potential of Zn,Cu and Pb in equilibrium with saturated solutions of theirFig. 5. IFM image of the “arrow tip” region of the Sikhote-Alin schreibersite inclusion, pre-corrosion. Dimensions are2.8461 Â 2.1587 Â 1.587 mm.Fig. 6. Experimental arrangements for simulated anaerobic corrosion of Sikhote-Alin (500 cm3, 10% aqueous HCl; N2; 5 days 50 °C). Duringcorrosion (left) and post-corrosion (right).D.E. Bryant et al. / Geochimica et Cosmochimica Acta 109 (2013) 90–112 95
  7. 7. Fig. 7. (Left) Sikhote-Alin meteorite fragment as per Fig. 1a, post-corrosion but prior to cathodic cleaning. (Right) IFM image in a similarorientation to that of Fig. 5 displaying post-corrosion surface of the Sikhote-Alin schreibersite inclusion, but prior to cathodic cleaning of thesurface.Fig. 8. Enhanced image of Fig. 7 showing a vector indicated with a red line (left) and also the IFM topographic depth profile across that line(right) revealing a significant crevice of ca. 40 lm depth and 120 lm width at the matrix-inclusion boundary.Fig. 9. IFM image of the “arrow tip” region of the Sikhote-Alin schreibersite inclusion, post-corrosion after cathodic depolarisation cleaningof the surface. The arrow tip is now pointing south. The colour image at right shows the regions where inclusion fragments have beendisplaced (in blue). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)96 D.E. Bryant et al. / Geochimica et Cosmochimica Acta 109 (2013) 90–112
  8. 8. nitrate salts (Fig. 4). The SKP output (u) can then be con-verted to a potential value E via the calibration equationE = f. u; where f = 0.456 from the calibration curve inFig. 4. The potential map illustrated in Fig. 4 is a montageof two separate scans acquired sequentially.2.2.4. Infinite focus microscopy (IFM)Topographical analysis was carried out with an AliconaInfinite Focus Microscope using a 5Â objective lens. Infi-nite focus microscopy (IFM) is a non-contact optical tech-nique that operates using focus-variation: two focal points,Fig. 10. Line-scan IFM image of the “arrow tip” region of the Sihote-Alin schreibersite inclusion, post-corrosion after cathodicdepolarisation cleaning of the surface. The image at right identifies the red line as traversing a crevice ca. 150 lm deep and 2 mm across.Fig. 11. 31P NMR spectrum (202.456 MHz; D2O) of water soluble extract from Sikhote-Alin following anaerobic digestion in the apparatusof Fig. 6 (500 cm3, 10% aqueous HCl; N2; 5 days 50 °C). [HPO4]2Àd 6.63 ppm. [HPO3]2Àd 4.22 ppm; 1JPH = 566.9 Hz; [DPO3]2Àd 3.89;1JPD = 85 Hz (1:1:1 triplet).D.E. Bryant et al. / Geochimica et Cosmochimica Acta 109 (2013) 90–112 97
  9. 9. one above the highest point of interest and one below thelowest, are registered in the instrumentation and the micro-scope focuses at a number of incremental points betweenthe two. Once all the images have been acquired, a proprie-tary software algorithm identifies which regions of each im-age are in focus and combines the layers to produce a threedimensional image of the feature being analysed. Once thedata has been collated, it may be used to take a range ofmeasurements such as depth, volume and surface rough-ness. The technique is able to measure steep features up200 300 400 500 600 700 800 900 1000Raman shift / cm-10100200300400500Counts200 300 400 500 600 700 800 900 1000Raman shift / cm-1100200300400500600700Counts200 300 400 500 600 700 800 900 1000Raman shift / cm-1100200300400500600700800CountsabcFig. 12. Raman spectra obtained from different sites of the corroded surface of the Sikhote-Alin meteorite (upper traces) showing mixtures ofiron corrosion products in comparison to reference spectra (lower traces) (RRUFF) of: (a) hematite, (b) magnetite, (c) goethite.98 D.E. Bryant et al. / Geochimica et Cosmochimica Acta 109 (2013) 90–112
  10. 10. to 85from vertical and has a maximum theoretical verticalresolution of 2.3 lm with a 5Â objective lens.To prepare the post-corrosion specimen for analysisusing the infinite focus microscope (IFM), the sample sur-face was cleaned of loosely adherent corrosion product bypolarising the sample cathodically to the point wherehydrogen gas was generated on the surface of the metalaccording to the reaction 2H++ 2e ! H2. The samplewas immersed in a 0.1 M solution of NaOH and its poten-tial was fixed at À2 V vs standard calomel electrode; thepolarisation continued until the loose material on the sur-face was removed by the hydrogen bubbles generated onthe metal surface. Once cleaned in this fashion, the topog-raphy of the sample was examined using IFM.2.2.5. Raman spectroscopyRaman spectra were collected using a Renishaw Inviasystem. Excitation was achieved using a 633 nm NIR diodelaser (Renishaw), focused through a Â20 objective and fil-tered to give 100% total laser energy at the sample. Spectrawere collected in static mode centred around 600 cmÀ1,with 1 s exposure and 100 accumulations. Raman imageswere obtained from two separate sites of the corrodedmeteorite surface with a Renishaw InVia micro Ramanspectrometer (Gloucestershire, UK). Spectral arrays ofrespectively 50 Â 50 (Site 1) and 50 Â 42 (Site 2) wereobtained with a 20Â objective over areas of 300 Â 300 lmand 300 Â 250 lm using 6 lm steps. Spectra were obtainedwith 633 nm excitation using a static scan centred at600 cmÀ1with 10 accumulations of 1 s exposures. Finalimages were generated with Direct Classical Least Squares(DCLS) analysis of the resulting spectral hypercubes withthree iron oxide components (hematite, magnetite and goe-thite) and presented as false colour 2-D images via mergingof colour components at each spectral pixel and bilinearinterpolation between pixel edges (Renishaw WIRE 3.2software). Pixels below a nominal correlation thresholdwere rendered as transparent. Reference spectra were ob-tained from the RRUFF database (http://rruff.info/).2.2.6. Fluid analysesFluids from a variety of sampling sites from the Hvera-dalur geothermal area were collected for subsequent dis-solved ion chemistry; these were pre-filtered to removesuspended particulate matter followed by fine-filtrationusing a 0.45 lm filter. Duplicate 30 mL water samples weretaken, one of which was acidified with nitric acid, and thesewere analysed with a Dionex Ion Chromatograph and Hor-iba JY Ultima 2C ICP-AES for dissolved anion and for ele-mental analysis respectively, at the Wolfson GeochemistryLaboratory at Birkbeck College – UCL.Fig. 13. Microscope images of two sites studied in the Raman analysis of the corroded Sikhote-Alin meteorite surface. Site 1: (a) white lightimage, (b) a 2-D false colour image of iron oxide species distribution obtained from DCLS analysis of Raman spectra (blue = hematite,red = magnetite, turquoise = goethite) overlaid with the white light image. Site 2: (c) white light image, (d) a 2-D false colour image of ironoxide species distribution obtained from DCLS analysis of Raman spectra (blue = hematite, red = magnetite, turquoise = goethite) overlaidwith the white light image. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of thisarticle.)D.E. Bryant et al. / Geochimica et Cosmochimica Acta 109 (2013) 90–112 99
  11. 11. Fig. 14. (a) Kverfjo¨ll volcanic region at the northern tip of the Vatnajoku¨ll glacier, SE Iceland. (b) Gengissig lake, Hveradalur geothermalarea (64° 40.1730N; 16° 41.1000W), Iceland. The geothermal site used for hydrothermal treatment of Sikhote-Alin is shown as a steaming, ice-exposed region to the north of the lake. (c) Sikhote-Alin field sample SA1 incubating in fluid from liquid pool #1 (LP1; pH 3.1; T = 93–94 °C).31P NMR spectrum of post-incubation SA2 (d), SA4 (e), SA5 (f) and SA1 (g) fluids showing presence of both orthophosphate and and H-phosphite [H2POÀ4 ; d 0.08 ppm and H-phosphite, H2POÀ3 ; d 2.71 ppm; 1JPH 630 Hz for SA1]. (h) 31P NMR spectrum of post-incubation SA1fluid, evaporated and dry-heated to 85 °C under flowing dinitrogen atmosphere for 72 h. Present are pyrophosphite [PPi(III), H2P2O2À5 ; dÀ3.35 ppm and À6.64 ppm], Pi(III)-D [d 2.78 ppm; 1JPD = 88 Hz], PPi(III)-D2 [multiplets at ca. d À4.7; À5.3 and À5.7 ppm], Pi(V) (d1.85 ppm) and PPi(III–V) [multiplets at ca. d À2.6; À5.4 and À5.7 ppm].100 D.E. Bryant et al. / Geochimica et Cosmochimica Acta 109 (2013) 90–112
  12. 12. 2.2.7. NMR spectroscopy31P NMR analyses were performed on a Bruker Avance500 MHz instrument operating at 202.634 MHz for 31Pinternally referenced to 85% H3PO4. Iron, principally inthe form of ferrous (Fe2+) was removed from all samplesprior to NMR analysis to alleviate the problems associatedwith paramagnetic broadening. This was done by pHadjustment, first to ca. 12 by addition of NaOHaq, (1 M)which leads to precipitation of oxides of iron, followed byaddition of aqueous Na2S solution (1.0 M), centrifugation,filtration and re-adjustment back to pH ca. 4 with HCl(1 M). For each sample, 10 mL of fluid were reduced to dry-ness and the residue redissolved in 0.5 mL deionised wateror D2O, filtered using 0.45 mm syringe filters and analysed.For those samples run in H2O solvent, D2O inserts wereused to provide a deuterium lock. Samples within whichpyrophosphite, PPi(III), were expected to be present andanalysed were pH adjusted to between 7 and 8 by additionof NaOHaq, (1 M). NMR spectra from SA 1 (Fig. 14d ande) were acquired after 8192 scans with a delay time betweenpulses of 0.75 s.2.2.8. Incubation of Sikhote-Alin within Hveradalurgeothermal fluidsSectioned samples of the Sikhote-Alin IIAB iron mete-orite (ca. 1 cm3) were incubated within fluids (30 mL) fromthe Hveradalur geothermal area for 4 days at natural tem-peratures and pH’s (Table 1) followed by a period of30 days at ambient temperature. After this time, sampleswere gravity filtered on Whatman grade 1 filter paper, driedand analysed by Raman spectroscopy (method outlinedabove). Solutions were further filtered using 0.45 lm filters,adjusted to 30 mL volumes and submitted for ICP-AESanalysis as outlined in Section 2.2.6. Following Ramananalysis, the meteoritic samples were subjected to post-cor-rosion cathodic depolarisation (vide supra) to remove sur-face detritus, followed by analysis via infinite focusmicroscopy.2.2.9. Incubation of Sikhote-Alin within simulated, low pHgeothermal fluidsThe Sikhote-Alin sample in Fig. 1 was corroded bysuspending it within a plastic-coated drip-tray, above anaqueous solution of 10% degassed hydrochloric acid(450 mL) in an atmosphere of dinitrogen for 5 days at50 °C, exposing only the surface shown (Fig. 1a). This sur-face was subjected to post-corrosion analysis using Ramanspectroscopy and images were acquired using an infinite fo-cus microscope (microscopy details above) both before andafter the acid corrosion treatment. The hydrochloric acidsolution was also analysed subsequently for Fe and Ni byatomic absorption spectrophotometry and for total P usingthe phosphomolybdate procedure (American Public HealthAssociation method 4500-P; calibration details are con-tained within the Electronic Annex). In addition the solu-tion was analysed by 31P NMR to determine the natureof the phosphorus species present as follows. The acid cor-rosion solution was divided in two aliquots of 225 mL each,with the first portion used for metals analysis as follows.The water was removed using a rotary evaporator andthe residues were digested in conc. sulphuric acid (2 mL),heated to dryness then taken up in sufficient conc. nitricacid to achieve a clear solution. This solution was then di-luted in a volumetric flask (100 mL) and analysed for ironand nickel using a Perkin-Elmer AAnalyst 100 atomicabsorption spectrophotometer with an air/acetylene flamecompared to known standards of 1.0, 3.0 and 5.0 ppm me-tal content respectively. The second aliquot (225 mL) wasreduced in volume on the rotary evaporator to removewater and hydrogen chloride. Freshly made aqueousNa2S solution (1.0 M) was added drop-wise to precipitateFeS and NiS (vide supra) and the centrifuged and filteredsolution was reduced to dryness with the residues takenup in D2O (1 mL) and analysed by 31P NMR spectroscopy.2.2.10. Dehydration of Ca(H2PO3)2ÁH2O under geothermalconditionsA solution was prepared of H-phosphonic acid (4.1 g,50 mmol) in deionised water (100 mL) and CaCO3 (1.25 g,12.5 mmol) was added portion-wise. After dissolution andwarming to 60 °C the solution was allowed to stand andcool. Crystals appeared of Ca(H2PO3)2.H2O. One crystalwas examined by single crystal X-ray diffraction and the unitcell obtained compared to literature values as confirmationof the crystals’ identity (Larbot et al., 1984). A sample of thismaterial (0.1 g; 0.45 mmol) was inserted ca. 2–3 cm beneathTable 1Elemental analysis (ICP-AES) and anion (Dionex Ion Chromatograph) measurements on acidified fluids (numbered 1–4; acronyms refer tofield site descriptors) from the Hveradalur geothermal area (in mg LÀ1) along with associated measurements on four Sikhote-Alin samples(numbered SA1,2,4,5). uIncubated in fluid 1, LP1.  Incubated in fluid 2, UCL5, LP1. àIncubated in fluid 3, LP3. –Incubated in fluid 4, BPR.Sample Fe Ni P Ca Mg S F Cl pH T (oC)Blank À0.07 À0.04 À0.04 À2.51 À0.50 0.10 0.28 0.92 – –Blank À0.07 À0.04 À0.05 À2.70 À0.50 0.20 0.27 0.26 – –Blank À0.07 À0.04 À0.02 À2.45 À0.50 2.30 0.16 0.41 – –Fluid 1 (LP1) 11.15 À0.14 À0.14 62.78 7.93 140.47 7.07 – 3.1 93.5Fluid 2 (UCL5) 0.56 – 0.31 29.19 4.48 – 1.69 9.06 4.7 89.2Fluid 3 (LP3) 31.40 À0.04 0.08 – – – 0.09 – 2.5 79.2Fluid 4 (BPR) 4.11 À0.03 À0.01 62.59 13.80 141.20 1.39 1.32 4.0 79.5Sikhote Alin (SA1)u7.82 0.02 16.78 107.81 11.38 153.48 3.67 8.71 3.1 93.5Sikhote Alin (SA2) 0.35 0.67 0.17 41.62 7.84 70.00 0.00 0.00 4.7 89.2Sikhote Alin (SA4)à68.01 7.52 À0.01 75.69 10.64 150.64 2.94 3.89 2.5 79.2Sikhote Alin (SA5)–11.97 1.88 0.06 36.39 5.32 90.58 1.07 2.40 4.0 79.5D.E. Bryant et al. / Geochimica et Cosmochimica Acta 109 (2013) 90–112 101
  13. 13. the surface of the Gengissig lake shore where the tempera-ture was measured at 94.4 °C. After 72 h, the sample was re-moved and analysed by both 31P NMR and 1H-spectroscopy upon return to Leeds some 3 weeks later.3. RESULTS AND DISCUSSION3.1. SEM and EDX maps of Sikhote-AlinIn Fig. 1a is displayed an optical photograph of theSikhote-Alin section used in this study. The sample has acleavage plane on its left-hand edge but the important fea-ture is the arrow tip-shaped inclusion of schreibersite risingup from the southern edge in a vertical pointing directionfor a distance of 3 mm. In Fig. 1b are collected a secondaryelectron SEM spectrum of a portion of the arrow tip regionalong with elemental X-ray (EDX) maps at the appropriateP, Fe, O and Ni Ka energies which reveal the schreibersiteinclusion to be P-rich and, in comparison to the surround-ing matrix, relatively Ni-rich and Fe-poor. There are smallclumps of oxygen-rich domains which we presume to beassociated with low-iron and low nickel oxides. Due tothe absence of a silicon signature in the EDX map, we sug-gest these oxides are unlikely to be silicate inclusions butperhaps localised surface corrosion or possibly aluminafrom polishing. Overall the sample shows strong elementalEDX response for Fe, Ni and P as expected (Fig. 1a).3.2. X-ray photoelectron spectroscopic (XPS) analysis ofSikhote-AlinIn Fig. 2 are shown photoelectron spectra of the Fe, 2p3/2 electrons from a powdered sample of Fe3P, this being acommonly used proxy for schreibersite (Pasek and Lauret-ta, 2005), without ion-beam etching. There is a significantchange in trace maxima for Fe, 2p3/2 electrons when thesurface is etched signifying removal of an iron oxide layerwith binding energies centred around 711.4 eV comparedto a underlying matrix with binding energy ca. 708 eV (fullXPS data are collected in the Electronic Annex accompany-ing this paper). These figures can be compared with litera-ture values from manufacturer’s tables for iron metal of706.6 eV and Fe2O3 of 709.8 eV respectively. The implica-tion is that the etching process removes surface iron oxidesto reveal the pristine sub-surface iron. This effect is illus-trated again when comparing the binding energies of 2p3/2electrons from P in Fe3P and Fe2P; XPS analysis (Fig. 2)reveals an oxide coating with broad binding energies cen-tred around 134 eV commensurate with phosphorus in the+5 oxidation state (Hanawa and Ota, 1991) with the under-lying P material returning binding energies of 129.4 and129.6 eV for Fe2P and Fe3P respectively. For both Fe2Pand Fe3P, the P-2p3/2 electron binding energy curves reveala shoulder at the higher energy side ca. 131 eV which we as-sign to the P-2p1/2 electron as reported for close relativeFeP (Grosvenor et al., 2005). Intriguingly, these authors re-turned a P-2p3/2 electron binding energy for FeP of129.3 eV and argued on the basis of a correlation betweencore P-2p3/2 binding energies of MP (M = Co, Fe, Mnand Cr) and electronegativity differences between M andP, that this was consistent with an approximate charge onthe P atom in FeP of À1 (Grosvenor et al., 2005). Fromthe NIST XPS data base, P-2p3/2 electron binding energiesare also reported for Fe2P and Fe3P at 129.5 and 129.4 eVrespectively (Nemoshkalenko et al., 1983), commensuratewith those reported here.Turning our attention to analysis of the schreibersiteinclusions within Sikhote-Alin, the results of an XPS linescan across the Sikhote-Alin sample are shown in Fig. 3.The line starts within the matrix material and crosses thenarrow part of the inclusion (arrow-tip in Fig. 1a) to finishagain within matrix material on the other side. There arepractical difficulties in knowing exactly where the beam isimpinging on the sample and for this reason the relativeconcentration of phosphorus is calculated from each pointdemonstrating that the line does indeed cross the inclusionas thought, from a relatively low-P region through a regionof relatively high P presence (inclusion) back to low P ma-trix again. It is clear from the graph that the binding energyof Fe-2p3/2 electrons closely correlates with the phosphoruscontent. However, a comparison of the Fe-2p3/2 bindingenergies of Fe2P with Fe3P which are very similar indicatesthat the phosphorus content itself may not be the sole fac-tor responsible for the increased binding energy within theinclusion compared to matrix Fe. The inclusion is known tobe nickel-rich and iron-poor compared to the matrix as canbe seen from the SEM/EDX pictures of the tip of the“arrow” in Fig. 1b. An increase of Fe-2p3/2 binding energywith increasing nickel content is in line with reported stud-ies of alloys (Nagai et al., 1987); the higher binding energysuggests that more energy is required to form the first oxi-dation state and thus the metal behaves as a more “noble”component within a Galvanic couple compared to iron me-tal within the matrix.3.3. Electrochemical mapping of schreibersite inclusions.Scanning Kelvin probe (SKP) analysisScanning electrochemical techniques such as the SKPare ideal methods for probing electrochemical differencesbetween disparate metal junctions and have been usedwidely in the field of applied corrosion research (Williamset al., 2010). It is a technique that had not been exploitedin the space sciences field prior to our original report ofits use to explore differences between matrix and siderophil-ic inclusions within iron meteorites in 2009 (Bryant et al.,2009) but has been used successfully to probe corrosionbehaviour at grain boundaries in other materials (Liet al., 2012). Calibrating the volt potential output of theSKP against a known set of redox potentials, for Zn, Pband Cu (Fig. 4) allowed the output to be expressed in termsof the electrochemical potential of the specimen being as-sessed. Also shown in Fig. 4 is a colour-graded SKP graphicof the arrow-shaped schreibersite inclusion within Sikhote-Alin (cf: image in Fig. 1a) which clearly distinguishes thesurface potential differences between inclusion and matrix.Examination of the values of the output in Fig. 4 indicatesa potential difference DE between the inclusion and the sub-strate to be of the order of 550 mV. This potential differenceacts as the driving force for a galvanic current between102 D.E. Bryant et al. / Geochimica et Cosmochimica Acta 109 (2013) 90–112
  14. 14. inclusion and matrix. The result of this galvanic coupleshould be to polarise the matrix anodically, effectively mak-ing the matrix more susceptible towards oxidation than theinclusion. Whilst the SKP does not give informationregarding the time-evolution of the electrochemical reac-tions that would occur in a corrosive environment, the tech-nique is able to predict the likely location of the corrosiveattack that may occur. Such time-evolution information re-quires linear polarisation resistance measurements (de Cris-tofaro et al., 2012) which have been performed on a relatediron meteorite sample and will be described elsewhere. Con-sequently, the large potential gradient between the two re-gions suggests a strong tendency for the inclusion toproduce accelerated, localised corrosion of the matrix.The ratio of the areas of the two components of this Gal-vanic activity also have an effect upon the nature of the cor-rosion. The consequence of a large cathodic matrixcontaining a relatively small anodic inclusion would be ex-pected to lead to accelerated dissolution of the inclusion,whereas in the case of a comparatively small cathodic inclu-sion, as is the situation with the sample studied, the corro-sion due to the galvanic couple would be expected to occurprimarily on the matrix material around the interface withthe inclusion. This would result in a potential weakening ofphysical attachment of the inclusion within the matrix andhence ultimately a release of inclusion material whichwould subsequently undergo, presumably slower, hydro-thermal modification to release chemicals to the environ-ment. To further probe this effect, we designed ananaerobic hydrothermal reactor within which we couldprobe accelerated corrosion of meteoritic fragments in thepresence of simulated low pH water environments. Follow-ing corrosion and cathodic depolarisation to remove sur-face oxide detritus, the sample could then be examined byinfinite focus microscopy to assess the validity of the aboveelectrochemical arguments.3.4. Hydrothermal modification of Sikhote-Alin underanaerobic simulated geothermal low pH environmentsInfinite focus microscopy (IFM) is a relatively recentlydeveloped technique for analysing surface morphologyand has found significant application in fields as diverseas engineering corrosion (Jiang and Nesic, 2009) and bio-materials (Winkler et al., 2010). The Sikhote-Alin samplein Fig. 1 was analysed by IFM pre and post-acid corrosionand the solution leachate analysed for Fe, Ni and P levels asdescribed in Section 2.2.9. Fig. 5 shows the Sikhote-Alininclusion imaged prior to the corrosion study. The surfaceis essentially flat although a discontinuity can be seen atthe interface of inclusion with matrix. In addition, the inclu-sion appears to show an increased level of roughness com-pared to the matrix, a feature that we have seen and notedpreviously through scanning electron microscopy and prob-ably connected to the fact that schreibersitic inclusions haveincreased levels of brittleness compared to the surroundingmatrix as indicated by their raised Vickers hardness num-bers (Bryant et al., 2009).The experimental arrangement for simulated anaerobiccorrosion used here is illustrated in Fig. 6, which showsclearly the plastic coated cage in which the Sikhote-Alinsample was suspended. Dinitrogen gas was bubbledthrough a solution of 10% degassed hydrochloric acid fora period of 5 days at 50 °C, so that the meteorite samplewould be subjected to condensed, low pH water within adynamic hydrothermal environment. Fig. 7 shows thepost-incubation surface of the Sikhote-Alin sample wherethe inclusion is now difficult to see being largely obscuredby corrosion products except in the upper left corner wherethe crust has flaked off to reveal a step. A line across thisstep can be traced by IFM and the topographical changealong this line is plotted in Fig. 8. The topography of theinterface between the inclusion and the matrix shown inFig. 8 serves to reinforce the prediction of the SKP analysisthat corrosive attack would be most severe at the matrix-inclusion boundary where local corrosion due to Galvaniccorrosion leads to accelerated anodic dissolution of the ma-trix material over the inclusion. The presence of a steep,sharp crevice at the inclusion side of the matrix-inclusionboundary, with dimensions of ca. 40 lm depth and120 lm wide, displays clear and preferential dissolution ofmatrix material. Should such behaviour continue, onewould envisage weakening of the matrix-inclusion adhesionto such a point that the inclusion may become sufficientlyweakened to allow it to be released from the surroundingmatrix. Indeed, this appears to be the case in practice asillustrated by an IFM image of the arrow-tip inclusionpost-cathodic depolarisation to remove surface debris. Inthis process the meteorite sample is rendered at a cathodicpotential in an electrochemical cell with the result that dihy-drogen gas is produced from during reduction at the mete-oritic electrode which serves to remove surface detritus. Inthe process, a significant degree of corrosion appears tohave taken place surrounding the inclusion, which hasweakened its attachment to the encompassing matrix tosuch an extent that a fragment of inclusion has also beenremoved from its pre-incubation position resulting in a gap-ing crevice (Figs. 9 and 10) visible in the line-scan trace withdimensions ca. 150 mm deep and 2 mm across. The leachatesolution was analysed for dissolved Fe, Ni and P whichwere measured to be present at concentrations of440 ppm (Fe), 20 ppm (Ni) and 0.7 ppm (P). The lattercompared to a background in the distilled water of0.007 ppm (see Electronic Annex for calibration and back-grounds). Further analysis of the Fe-removed (addition ofNa2S) leachate using 31P NMR spectroscopy identifiedthe major P-product to be the oxyacid H-phosphite,[H2PO3]À(Fig. 11) as compared against a known standard.3.5. Raman mapping analysis of Sikhote-Alin post-anaerobiccorrosionThe relatively large ‘step’ across the ‘arrow head’ pre-cluded efficient Raman mapping due to issues relating tothe depth of field at the appropriate (Â20) magnification.Instead, two discrete sites of the corroded Sikote-Alin mete-orite were chosen for micro-Raman mapping analysis. Site1 incorporated a corroded ‘pit’, apparently with a relativelylarge area of exposed metal surface; in contrast, Site 2 fea-tured a substantial surface coverage of oxide material. InD.E. Bryant et al. / Geochimica et Cosmochimica Acta 109 (2013) 90–112 103
  15. 15. common with other areas of the corroded meteorite surface,Raman scattering was relatively weak and the resultingspectra featured broadened peaks consistent with a largelyamorphous nature for the oxide deposits and/or severalmicroenvironments of the oxide materials. As iron oxideshave shown an ability to interconvert under laser irradia-tion, care was taken to minimise the exposure of each sitethrough the use of relatively short exposure times (De Fariaet al., 1997). Comparison was made with reference spectrafor several iron oxide species implicated in oxidation,including the Fe(III) oxides–hematite (a-Fe2O3), maghe-mite (c-Fe2O3), the mixed Fe(III,II) oxides–magnetite,and the oxyhydroxide series, goethite [a-Fe(OOH)], akag-aneite [b-Fe(OOH)] and lepidocrocite (Remazeilles and Re-fait, 2007; Re´guer et al., 2007; De Faria et al., 1997). Whilethe broadening of the spectra precluded specific identifica-tion of the different morphological forms, e.g. the FeOOHspecies, clear domains for Fe(III), Fe(II,III) oxides andFe(III) oxyhydroxides were identifiable in the spectra aswell as substantial areas of mixtures of these species(Fig. 12). In this context, Raman images were generatedfrom multivariate analysis using Direct Classical LeastSquares and the best models for the image data were ob-tained with three components: hematite, magnetite and goe-thite. Other species were excluded from developmentmodels on the basis of weak or absent correlation betweenpixels in the images and the appropriate reference spectra(Zhang et al., 2005). The resulting images are shown inFig. 13.The distribution of oxide species in the maps are worthyof note. At Site 1, the metal ‘pit’ appears to be bordered bya preponderance of hematite, around which the lower oxi-dation state magnetite appears. At this site, the major spe-cies identified is Fe(II, III) although a small area of goethiteFig. 15. SEM images and EDX maps of the “ducks head” schreibersite inclusion of Sikhote-Alin sample SA1 clearly displaying Ni-rich, Fe-poor, P-rich nature of the inclusion against matrix. Some oxide materials are also clearly detectable on the surface; a mixture of iron oxidesand clay minerals from the geothermal fluids.104 D.E. Bryant et al. / Geochimica et Cosmochimica Acta 109 (2013) 90–112
  16. 16. appears well away from the pit. In contrast, Site 2 appearsto show a much wider expanse of goethite although, again,magnetite is the dominant species. There are also severalsmaller areas of exposed metal and, again, these are bor-dered by hematite. It is intriguing to speculate that the pit-ted areas observed may have given rise to a more (Galvanic)oxidising environment, perhaps as a result of small Fe3Pinclusions, hence the formation of hematite in theseenvironments.3.6. Hydrothermal modification of Sikhote-Alin underaerobic geothermal low pH environmentsWhilst the above laboratory experiment allowed us tosimulate a low pH geothermal environment our June 2011field expedition to the Vatnajoku¨ll glacier within SE Ice-land, afforded us the opportunity to explore hydrothermalmodifications on Sikhote-Alin under bona fide, low pH geo-thermal field conditions. Our field site was the Hveradalurgeothermal area (64° 40.1730N; 16° 41.1000W) within theKverkfjo¨ll volcanic range at the northern region of the Vat-najoku¨ll glacier. A small (ca. 50 m diameter) geothermalfield at the edge of the Gengissig lake containing hydrother-mal stream waterfalls, streams, hot water, mud pools andsulphurous fumaroles was selected as the site for fluid col-lection and incubation studies on Sikhote-Alin (Fig. 14).Fluid samples from several different sites within this geo-thermal field were analysed for dissolved cations and phos-phorus via ICP-AES (Table 1) and four sectioned samplesof the Sikhote-Alin, each with at least one face displayingexposed schreibersite mineral were incubated in 50 mL Fal-con tubes within geothermal fluids at this site. Sikhote-Alinfield sample SA1 was incubated in a hot water pool(Fig. 14c; pH 3.1; T = 93–94 °C) for 4 days followed byambient temperature incubation in the same fluid for a per-iod of 4 weeks prior to analysis via ICP-AES, SEM-EDXand IFM techniques. As is clear from the data of Table 1,all of the Sikhote-Alin samples which were deployed withingeothermal fluids (samples SA1,2,4, and 5) have dissolvedP-levels in the range 0–17 mg LÀ1significantly higher thanthose measured in the blank, distilled water samples (Ta-ble 1 entries 1–3). Hydrothermal modification of the fourSikhote-Alin samples results in enhanced P-levels for SA1and SA5 compared to their fluid hosts but somewhat atten-uated levels for SA2 and SA4. We suspect that this may bea result of the greater Fe-levels introduced by the iron mete-orite (Table 1) leading to precipitation of ferric phosphates.31P NMR analysis of the post-incubation fluid from SA1,following removal of dissolved iron by pH adjustment to12 and filtration, reveals the presence of both orthophos-phate (d 0.08 ppm) and H-phosphite (H2POÀ3 ; d 2.71 ppm;1JPH 630 Hz; Fig. 14d), the latter being the expected anddominant P-oxyacid of hydrothermal schreibersitic modifi-cation (Pasek and Lauretta, 2005). Similar analyses of eachof the other SA samples SA2, 4 and 5 reveal similar P-spe-ciation with H-phosphite the dominant component in eachcase. Sample SA3 was not deployed in Iceland, but used asa laboratory reference sample. Together, these data formFig. 16. IFM images of the “ducks head” schreibersite inclusion of Sikhote-Alin sample SA1 (a) pre-incubation and (b) post-incubation aftercathodic depolarisation. (c) 2D IFM topological map of the region (highlighted inset) between the “ducks head” and a second schreibersitedomain clearly showing their exposed connectivity in the post-incubation sample.D.E. Bryant et al. / Geochimica et Cosmochimica Acta 109 (2013) 90–112 105
  17. 17. the thrust of this papers conclusion which illustrates verynicely the key surface corrosion properties and solutionP-chemistry which afford activated P-compounds undermild conditions.Several schreibersite inclusions can be clearly seen onone of sectioned faces of SA1, and one of these motif’s witha morphology reminiscent of a “duck’s head” was selectedfor closer examination. In Fig. 15 are SEM and EDX mapimages of the “duck’s head” post-corrosion and after catho-dic depolarisation to clean the meteoritic surface, whichdemonstrates clearly the differential between nickel-rich,iron-poor and phosphorus-rich inclusion and surroundingmatrix. The final EDX map at oxygen Ka frequency iden-tifies significant oxide coverage, the legacy of its recent cor-rosive habitat. These oxide materials are principallycorrosion oxides of iron and clay minerals intrinsic to thegeothermal fluid (these and other more specific features willbe discussed in the subsequent field paper). In terms of themeteoritic surface morphology however, it is most instruc-tive to compare IFM analyses of both pre-incubated andpost-incubated Sikhote-Alin sample SA1 as in Fig. 16aand b respectively. There is a clear region of matrix separat-ing what appears to be two discrete schreibersite domains(Fig. 16a highlighted region) which appears to have disap-peared in the corroded sample (Fig. 16a). A two-dimen-sional IFM topological map of this region (Fig. 16c)between the “ducks head” and a second schreibersite do-main clearly shows they are indeed connected and that thispoint of connection results in the schreibersite standingproud of the surrounding matrix by ca. 20 lm (Fig. 17).Our inference is that matrix surrounding the schreibersiteinclusion has been preferentially dissolved during incuba-tion within geothermal fluids, a result which mirrors thatof our laboratory-based simulations. Sikhote-Alin samplesSA2, 4 and 5 were incubated in geothermal fluids at pH’s4.7, 2.5 and 4.0 respectively at temperatures of 89.2, 79.2and 79.5 °C followed by ambient temperature incubationin the same fluid for 4 weeks in the same manner as SA1,prior to complementary analysis. In Fig. 18 are reproducedIFM images of a schreibersite-matrix section (the latter isthe more smooth region of the two) both pre-incubation(Fig. 18a) and post-incubation (after cathodic depolarisa-tion; Fig. 18b) on SA2. Comparison of complementaryline-scans across (as far as is possible) the same vector(red line scanning inclusion-matrix-inclusion-matrix fromtop to bottom) in both pre- and post-incubated samples(Fig. 18c and d respectively) reveals a clear crevice some60 lm deep within the inter-inclusion matrix region be-tween 0.8 and 0.9 mm across the scanning vector in thepost-incubated sample. The same region within the pre-incubated sample does not show this crevice formation,clear support for the preferential oxidation of matrix mate-rial over inclusion which leads to an undermining of theinclusion-matrix boundary. Samples of SA4 and 5 displaysimilar behaviour, of which the clearest is found on SA4.In Fig. 19 are reproduced schreibersite inclusions embeddedwithin matrix material both pre- (Fig. 19a) and post-incu-bation (Fig. 19b) of SA4. Clearly shown in the post-incu-Fig. 17. Expanded IFM images of the region between “ducks head” and a second schreibersite domain revealing clearly that; (a) the inclusionstands proud of the matrix, (b) a connecting schreibersite bridge exists between the two domains.106 D.E. Bryant et al. / Geochimica et Cosmochimica Acta 109 (2013) 90–112
  18. 18. bated image is a spur of schreibersite clearly visible at thenorthern region of the principal inclusion which is not vis-ible at all in the pre-incubated sample. Furthermore, a false-colour topographic image (Fig. 19e) clearly shows the ma-trix to have been corroded away from the, now sharply-de-fined inclusion, in some regions to a depth of 100 lm andsome 300 lm wide (Fig. 19d) which is not present in thepre-incubated sample.The post-incubation fluid from SA1 above (containing16.78 mg LÀ1total P; Table 1) containing both orthophos-phate and H-phosphite [Pi(III), H2POÀ3 ; Fig. 14g] was pHadjusted to 4 by addition of HClaq, followed by evapora-tion and grinding of the resulting solid evaporate to a finepowder. This was then heated to 85 °C in a sand bath undera flowing atmosphere (ca. 1 cm3sÀ1flow) of dinitrogen fora period of 72 h after which time the material was dissolvedin D2O, pH re-adjusted to ca. 7 and the P-components stud-ies by 31P NMR spectroscopy. The resulting spectrum(Fig. 14h) reveals that a significant proportion (40+%) ofthe total solution P is now present as the condensed oxy-acid, pyrophosphite [PPi(III), H2P2O2À5 ; d À3.35 andÀ6.64 ppm] identified by comparison of its AA0XX0spinsystem to an authentic sample (Bryant et al., 2010). Alsoidentifiable within this spectrum are products resultingfrom H–D exchange within Pi(III) [d 2.78 ppm; 1JPD = 88 -Hz] and within PPi(III) [multiplets at ca. d À4.7; À5.3 andÀ5.7 ppm] along with smaller signals due to Pi(V) (d1.85 ppm) and mixed-valent species, isohypophosphatePPi(III-V) [multiplets at ca. d À2.6; À5.4 and À5.7 ppm],again identified by comparison to authentic samples (Car-roll and Mesmer, 1967). That such a chemical condensationof H-phosphite to pyrophosphite is potentially accessiblewithin a bona fide geological environment is illustrated bythe incubation of a dry sample of Ca(H2PO3)2.H2O, heatedto 94.4 °C in a Falcon tube inserted ca. 3 cm beneath thesub-surface soil within the geothermal field at the edge ofthe Gengissig lake (Fig. 20a). After ca. 3 days exposure,analysis some 3 weeks later by both 31P and 1H NMR spec-troscopy identified PPi(III) formation which was not pres-ent in a reference sample of the same compound analysedFig. 18. IFM images of a schreibersite-matrix region of SA2; (a) pre-incubation and (b) post-incubation after cathodic depolarisation. (c andd) Identify (red line) the vector across which IFM topological depth profiles (e) and (f) were recorded for both pre- and post-incubationsamples. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)D.E. Bryant et al. / Geochimica et Cosmochimica Acta 109 (2013) 90–112 107
  19. 19. without heating (Fig. 20). Pyrophosphite is an intriguingmaterial as it is structurally and chemically related to thepyrophosphate [PPi(V)] moiety in nucleotide triphosphatessuch as adenosine triphosphate (Fig. 21) the ubiquitoussuite of energy currency molecules of contemporary bio-chemistry. Within a prebiotic context however, the advan-tages of PPi(III) over PPi(V) are that it is, (i) formedunder far milder conditions than the latter and (ii) it is morechemically reactive in the absence of sophisticated catalysis(Bryant et al., 2010).4. CONCLUSIONSThe emergence of phosphate-based biochemistry hasbeen a long-recognised problem in the field of abiogenesis(Gulick, 1955). Phosphorus (P) in the fully oxidised +5 oxi-dation state, as in contemporary biochemistry, has bothlimited solubility in water in the presence of many commonmetal ions (solubility products, Ksp at 25 °C for Ca3(PO4)2;Mg3(PO4)2 and Fe(PO4)Á2H2O are 2.07 Â 10À33;1.04 Â 10À24and 9.91 Â 10À16respectively) and has rela-tively low chemical reactivity in the absence of activatingagents (Steinman et al., 1965; Beck and Orgel, 1965; Oster-berg and Orgel, 1972; Hermes-Lima and Vieyra, 1989). Thesophisticated enzymes of contemporary cellular life used toactivate P in energy currency molecules such as nucleosidetriphosphates (e.g. ATP), phosphocreatine and phosphoe-nol pyruvate (Harold, 1986), are unlikely to have beenavailable within the Hadean period, however there is con-siderable support for activated P-chemistry being centralFig. 19. IFM images of a schreibersite-matrix region of SA4; (a) pre-incubation and (b) post-incubation after cathodic depolarisation withhighlighted vector (red line) line scans across which IFM topological depth profiles (c) and (d) were recorded for both pre- and post-incubation samples. False colour image (e) shows a depth profile map where the more blue regions at the inclusion-matrix boundary representthe deeper regions of the post-incubated sample. (For interpretation of the references to colour in this figure legend, the reader is referred tothe web version of this article.)108 D.E. Bryant et al. / Geochimica et Cosmochimica Acta 109 (2013) 90–112
  20. 20. to the bioenergetics of the early living organisms (Holm andBaltscheffsky, 2011). The question then arises, how couldnature have activated geologically available P, predomi-nantly in the form of orthophosphate (Bebie´ and Schoonen,1999), in order to produce primitive energy currency mole-cules? One suitable source of activated P on the early Earthwould have been siderophilic phosphide minerals, such asschreibersite, (Fe,Ni)3P. Whilst such minerals are not com-mon upon the Earth today, mainly due to their thermody-namic instability with respect to oxidation toorthophosphate, (Lauretta and Schmidt, 2009) they areknown to occur with natural metallic deposits on Disko Is-land, Greenland (Klo¨ck et al., 1986) and to be produced bychemical reduction of phosphates in soils during lightningstrikes (Pasek and Block, 2009). However, significant quan-tities of schreibersite and related phosphide minerals wouldlikely have been delivered to the early Earth through mete-oritic impacts and through interstellar dust particles(IDP’s). Pasek and Lauretta have estimated such P-fluxrates during the putative late-heavy bombardment (betweenca. 4.0 and 3.8 Ga) and concluded that whilst both IDPsand iron meteorites would likely have brought similarquantities of siderophilic P to the early Earth (ca. 108-kg yrÀ1), the far more localised impact events associatedwith irons could have afforded very high local concentra-tions of activated-P, in the region of 105kg kmÀ2(Pasekand Lauretta, 2008). This process would require the interac-tion of a hydrothermal system with a meteorite smallenough to impact and not destroy the system, yet large en-ough to add enough reduced phosphorus to influence localchemistry. The higher fall rate likely present on the earlyearth (e.g. Johnson and Melosh, 2012; Bottke et al., 2012)provided a greater frequency of meteorite falls to the earlyearth. As most meteorites do not exceed a mass of about50 tonnes, and have slowed significantly by ablation duringatmospheric entry, and have fragmented before impact,meteorites in general should not destroy hydrothermal sys-tems. An alternative to the random fall of a meteorite into ahydrothermal pool is the de novo generation of hydrother-mal systems after a large impact (Schwenzer and Kring,2009; Osinski et al., in press), and the interaction of thesenew systems with meteorite fragments from the impactor.In this respect, an impact provides both the raw materials(siderophilic phosphorus) and the environment (hydrother-mal system) that has been investigated in the present work.Our studies here on the low pH hydrothermal modifica-tion of iron meteorites reveal that natural electrochemicaldifferences in composition between matrix Fe–Ni (taeniteand kamacite) and schreibersite inclusions result in prefer-ential dissolution of matrix material at the matrix-inclusionboundary leading to weakening of attachment of the inclu-sion to the meteoritic matrix. This in turn should allow fordetachment of the inclusion with a consequent increase inthe availability of activated P to local water sources. Our re-port here of the first field studies on low pH hydrothermalmodification of schreibersitic inclusions within IcelandicFig. 20. Sample of Ca(H2PO3)2ÁH2O, heated to 94.4 °C in a Falcon tube inserted ca. 3 cm beneath the soil at the edge of the Gengissig lake.31P NMR analysis (202.63 MHz; D2O; 300 K) of the heated solid, after ca. 3 days exposure, identified PPi(III) formation by comparison to anauthentic sample [d = À4.4 (AA’XX’, JPH 666 Hz; 0.7 Hz; JPP 17 Hz; (Bryant et al., 2010)].Fig. 21. Molecular structures of (a) adenosine triphosphate (ATP) emphasising the condensed [P–O–P] molecular moieties between ab and bcpairs of P atoms; (b) pyrophosphate, PPi(V), the main energy currency fragment of ATP and (c) pyrophosphite, PPi(III) a related molecularcousin of PPi(V) with two [P–H] bonds replacing two [P–OH] groups of the latter.D.E. Bryant et al. / Geochimica et Cosmochimica Acta 109 (2013) 90–112 109
  21. 21. geothermal fields supports laboratory-based studies that Pin a lower oxidation state than +5, namely H-phosphite(H2POÀ3 ; where P is present formally as +3) is the chiefwater-borne activated P oxyacid. Finally, we have demon-strated that H-phosphite from low pH hydrothermal mod-ification of irons can be readily condensed to pyrophosphite[PPI(III)], a close structural and molecular cousin to pyro-phosphate [PPi(V)], the energy currency component ofnucleotide triphosphates such as ATP. We propose thatthe significance of PPi(III) as a prebiotically plausible en-ergy currency molecule lies in its far greater range of chem-ical reactivity that PPi(V), reactivity that is not limited tothe presence of sophisticated catalysts. Examples of this en-hanced chemical reactivity will be described in a more spec-ialised chemistry manuscript.ACKNOWLEDGEMENTSThe authors are grateful for the financial support received tosupport this work specifically, the Engineering and Physical Sci-ences Research Council (Grant EP/F042558/1 to T.P.K.), theLeverhulme Trust (Grant F07112AA to I.A.C.), the Science andTechnology Funding Council, and the UK Space Agency for theaward of an Aurora Fellowship (to T.P.K). We thank Dr. LauraCarmody for field assistance, Dr. Thorsteinn Thorsteinsson, Mr.Magnus Karlsson and the Icelandic Glaciological Society for logis-tical support and Dr. Karen-Hudson Edwards and Mr. Antony Os-born for assistance with dissolved ion chemistry analysis at theWolfson geochemistry laboratory at UCL/Birkbeck. The NaturalHistory Museum, London is thanked for providing samples ofthe Sikhote-Akin meteorite. Finally, we thank the reviewers fortheir insightful comments and suggestions.APPENDIX A. 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