This document summarizes a study that investigated the hydrothermal modification of the Sikhote-Alin iron meteorite under low pH conditions to simulate volcanic environments. Analysis revealed that schreibersite inclusions in the meteorite are more electrochemically noble than the surrounding iron-nickel matrix by around 550 mV. This leads to preferential corrosion of the matrix at inclusion boundaries. The corrosion releases phosphorus from the schreibersite inclusions in the form of H-phosphite, a reactive form of phosphorus with prebiotic chemical potential. Field experiments showed that H-phosphite produced can be further converted to the condensed phosphorus compound pyrophosphite under mild heating, another compound with significance for pre

1. Hydrothermal modification of the Sikhote-Alin iron
meteorite under low pH geothermal environments.
A plausibly prebiotic route to activated phosphorus
on the early Earth
David 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,⇑
a
School of Chemistry, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, UK
b
Centre for Corrosion Technology, Materials and Engineering Research Institute, Sheffield Hallam University, Sheffield S1 1WB, UK
c
Leeds Electron Microscopy and Spectroscopy Centre, University of Leeds, Leeds LS2 9JT, UK
d
Molecular and Nanoscale Physics Group, School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, UK
e
Department of Earth Sciences, Natural History Museum, London SW7 5BD, UK
f
Department of Geology, University of South Florida, Tampa, FL 33620, United States
g
School of Life Science, University of Bradford, Richmond Road, Bradford BD7 1DP, UK
h
Department of Earth and Planetary Sciences, Birkbeck College, University of London, Gower Street, WC1E 6BT, UK
Received 6 August 2012; accepted in revised form 28 December 2012; available online 5 February 2013
Abstract
The 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 would
have added significantly to the inventory of such siderophilic P. Subsequent anaerobic corrosion in the presence of a suitable
electrolyte would produce P in a form different to that normally found within endogenous geochemistry which could then be
released 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 proton
would 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. We
report on both, (i) simulated volcanic conditions using low pH distilled water and (ii) geothermally heated sub-glacial fluids
from the northern Kverkfjo¨ll volcanic region of the Icelandic Vatnajoku¨ll glacier. A combination of X-ray photoelectron
spectroscopy (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 mV
nobler than matrix material. This results in preferential corrosion of the matrix at the matrix-inclusion boundary as confirmed
using 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 are
0016-7037/$ - 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/gca
Available online at www.sciencedirect.com
Geochimica et Cosmochimica Acta 109 (2013) 90–112

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 [H2POÀ
3 , Pi(III)
as determined by 31
P NMR spectroscopy]. This compound has been shown to have considerable prebiotic chemical potential
as 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 [H2P2O2À
5 ,
PPi(III)] by dry-heating under mild (85 °C) conditions. The potential significance of this latter condensed P-compound for
prebiotic chemistry is discussed in the light of its modified chemical properties compared to pyrophosphate [H2P2O2À
7 , PPi(V)].
Ó 2013 Elsevier Ltd. All rights reserved.
1. INTRODUCTION
The 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 demonstrated
to 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). The
chemical behaviour of iron meteorites as a contributor to
the chemical inventory of the early Earth centres not only
on the presence of the potentially catalytically active metals
iron 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 of
siderophilic P-phases such as schreibersite (Fe,Ni)3P affords
water-soluble P-compounds which not only differ to those
normally found within the terrestrial geological record,
but are more chemically reactive (Pasek and Lauretta,
2005, 2008; Bryant and Kee, 2006; Pasek et al., 2007). It
has been suggested that these, more reactive forms of P
could have played a role in the emergence of phosphorus
energy currency molecules (specifically nucleotide triphos-
phates such as ATP) in contemporary biochemistry (Bryant
et al., 2010).
The corrosion of iron is usually a subject of interest to
modern day engineers concerned with the rusting of man-
made artefacts. Naturally occurring iron meteorites ex-
posed to wet, aerobic environments will similarly corrode
as meteorite curators and collectors are well aware. How-
ever, the present atmospheric conditions are very different
to those believed to have existed on the early (Hadean)
Earth when oxygen was present in only trace amounts
(Kasting, 1993), implying that meteoritic corrosion upon
the Hadean Earth would presumably have had to make
use of alternative electron acceptors to dioxygen, a scenario
common to biological systems (Harold, 1986). Galvanic
corrosion occurs when two dissimilar metals are in intimate
contact within an electrolyte and does not require the pres-
ence specifically of dioxygen but only of some electron
acceptor capable of initiating cathodic reactions to balance
the anodic oxidation of metallic iron to ferrous (Fe2+
) or
ferric (Fe3+
) ions. One suitable such acceptor is the proton
(H+
), which would have been an effective electron acceptor
within anoxic, low pH, Hadean environments such as
volcanic 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 iron
and nickel to crystallise into two principal forms, namely
nickel-rich (20–65% Ni) taenite and nickel-poor (5–10%
Ni) kamacite (Hutchison, 2004) and in theory there could
be Galvanic corrosion between these two at the grain
boundaries in the presence of a suitable electrolyte medium.
Indeed Galvanic corrosion is also implicated in oxidative
corrosion of present day samples (Buchwald and Clarke,
1989). Dissolution studies (Tackett et al., 1970; Tackett
and Goudy, 1972) show how kamacite dissolves preferen-
tially from a series of iron–nickel meteorites due to its lower
nickel content and that the overall dissolution rate of the
meteorite depends on its nickel content. Moreover, the
presence 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 the
potential to establish local Galvanic couples. This could
lead to the non-metals such as carbon, sulphur and phos-
phorus being involved in corrosion electrochemistry, ulti-
mately to be released into the corroding fluid
environment and hence become available for early Earth
chemistry.
Schreibersite is an important, if minor, component of
iron meteorites within which it can be found in the 0–
20 vol% range (Geist et al., 2005). Its presence has been
linked to the crystallisation behaviour of kamacite and tae-
nite resulting in the well-known Widmansta¨tten patterns
characteristic of iron meteorites (Goldstein and Hopfe,
2001). The amount of nickel in schreibersite can vary just
as the amount in the matrix can vary and higher overall
nickel contents are frequently associated with higher phos-
phorus (P) contents (Kracher et al., 1980). Schreibersite
inclusions usually have higher nickel content than the sur-
rounding matrix and meteorites of low overall nickel con-
tent are often hexahedrites which have crystallised
primarily as kamacite that has subsequently lost nickel to
adjacent schreibersite inclusions (Mason, 1971). Two co-
existing schreibersite minerals, one a Ni-free variety and
the other containing 23 wt% Ni, were found in Fe–Ni metal
and troilite in lunar rocks (Hunter and Taylor 1982; Scott
et al., 2007; El Goresy et al., 1971), though meteoritic nickel
contents 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 less
D.E. Bryant et al. / Geochimica et Cosmochimica Acta 109 (2013) 90–112 91

3. than 3% (Holtstam, 2006). Therefore, given the electro-
chemical potential differences between meteoritic matrix
and schreibersite inclusions due to varying nickel composi-
tions, it would be expected that local anodes and cathodes
would be more closely identified with matrix and inclusion
regions respectively during hydrothermal modification. In-
deed, such electrochemical potential differences have been
suggested by the scanning Kelvin probe analysis of the
Seymchan pallasite (Bryant et al., 2009), the first time such
techniques have been employed to analyse the surface of a
meteorite. Whilst our studies have focused more closely on
schreibersitic inclusions within iron meteorities, it is worth
noting that phosphorus-bearing Fe and Ni sulphides have
been identified and characterised as primary mineral phases
within type CM carbonaceous chondrites (Nazarov et al.,
2009) thus opening up the potential for chemistries derived
from metal phases to converge with those centred on organ-
ic molecules (vide infra).
Corrosion of iron meteorites in an oxygenic atmosphere
usually 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) can
also be a corrosion intermediate and one which facilitates
corrosion acceleration in the presence of the chloride anion
(Buchwald and Clarke, 1989). The schreibersite inclusions
within iron meteorites also undergo hydrothermal modifi-
cation (Pasek and Lauretta, 2005, 2008; Pasek et al.,
2007) to afford a range of P-oxyanion species including
dihydrogenphosphate (H2POÀ
4 ) and hypophosphate
(H2P2O2À
6 ) but principally the lower oxidation state P-oxy-
anion, H-phosphonate (aka phosphite) [P(III); HPO2À
3 ] as
well as iron nickel hydroxides similar to those derived from
matrix material. Subsequently, it has been demonstrated
that, in the presence of UV light, even lower oxidation state
P species such as H-phosphinate [P(I); H2POÀ
2 ] can be ob-
tained from such inclusions (Bryant and Kee, 2006), and
that this compound may be present even under hydrother-
mal modification at concentrations 10–20 times lower than
phosphite (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 Australian
meteorite samples (Tilley and Bevan, 2010), undoubtedly
the result of oxygenic corrosion processes favouring the
highest, and most thermodynamically stable, oxidation
state of phosphorus. Given that schreibersite has a higher
nickel content than the surrounding matrix it would be ex-
pected to act as the cathode in a Galvanic cell between the
two and this was indeed found to be the case using a scan-
ning Kelvin probe to map the surface of a pallasite (Bryant
et al., 2009).
Described here are our studies on the hydrothermal
modification 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 geothermally
heated 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, geothermally
heated 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. This
region was selected as an ideal low pH geothermal site as
the 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 to
the surface morphology, corrosion and solution chemistry
via a unique combination of elemental (inductively coupled
plasma) analysis, mapping Raman spectroscopy, scanning
electrochemistry, X-ray photoelectron spectroscopy
(XPS), 31
P NMR spectroscopy and infinite focus micros-
copy (IFM) techniques. Collectively, these tools have al-
lowed us to draw important conclusions on the following
problems: (i) where, upon an iron meteoritic surface, anaer-
obic corrosion is most likely to occur; (ii) what P-containing
products result from the hydrothermal modification of P-
containing inclusions within the meteorite Sikhote Alin
and (iii) suggest possible consequences of such corrosion
for primitive Earth solution chemistry. We have focused
on the relative anodic potentials of meteoritic matrix and
schreibersite inclusions which allow us to comment upon,
the nature, location and release of reactive, water-soluble
P during surface corrosion under putative early Earth envi-
ronments. Our key conclusions therefore centre around the
availability of reactive P-species resulting from hydrother-
mal modification of meteoritic surfaces and also how these
P-species can be readily converted to condensed P-oxyacids,
specifically pyrophosphite [H2P2O2À
5 , PPi(III)] which has re-
cently been demonstrated to have properties commensurate
with an ability to act as an energy currency molecule within
putative early Earth environments (Bryant et al., 2010).
2. MATERIALS, LOCATION AND EXPERIMENTAL
METHODS
2.1. Materials and location
2.1.1. Chemicals
Water was purified by ion exchange on a Purite Select
Analyst (PSA) reverse osmosis-deionisation system (Purite
Ltd., Oxford, UK). D2O for NMR analyses was used as re-
ceived from Sigma–Aldrich. Solutions of aqueous HCl were
prepared 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 of
commercial 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. Meteorites
Meteorite samples were provided by the Natural History
Museum, London (BM.1992,M39; BM.1992,M42 and two
separate samples from BM. 1992,M40). Particular emphasis
was 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 was
ca. 10 mm in diameter and the inclusion was ca. 3 mm in
length and 1 mm in width at its widest point.
92 D.E. Bryant et al. / Geochimica et Cosmochimica Acta 109 (2013) 90–112

4. 2.1.3. The Hveradalur geothermal field site; Kverkfjo¨ll
Volcanic System, Iceland
Beneath the northern part of the Vatnajo¨kull glacier in
central Iceland lies the Kverkfjo¨ll volcanic system
(Fig. 14a; Hoskuldsson et al., 2006). There are significant
geothermal 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 and
pH. Of most significance to our investigations here were the
low pH (1–5) geothermal fluids of the Hveradalur geother-
mal area (64° 40.1730
N; 16° 41.1000
W) sampled during the
June 2011 field expedition. A full description of this site,
associated geology and water chemistry will be reported
elsewhere.
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 region
of 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 energies
respectively.
Fig. 2. XPS analyses of both Fe2P and Fe3P surfaces focusing on
binding energies of P (left hand column) and Fe (right hand
column) core 2P3/2 electron binding energies.
D.E. Bryant et al. / Geochimica et Cosmochimica Acta 109 (2013) 90–112 93

5. 2.2. Experimental methods
2.2.1. Scanning electron microscopy and energy dispersive X-
ray (SEM and EDX)
Meteoritic surfaces were examined by scanning electron
microscopy at 20 kV accelerating voltage using a 12 mm
working distance on a Philips XL30 ESEM system fitted
with an Oxford Instruments INCA250 EDX. Images are
routinely acquired in secondary electron mode. Elemental
X-ray data were compiled into 2D maps using the Oxford
Instruments INCA software. Due to the inherently electri-
cally conducting nature of the meteorite sample, carbon
coating was not necessary to prevent charging.
2.2.2. X-ray photoelectron spectroscopy (XPS)
Samples of Fe3P powder, supplied by Alfa-Aesar, were
transferred in a nitrogen filled glove box onto a sample stub
for study within an Escalab 250 XPS instrument (Thermo
Scientific). The surface spectrum was recorded and then
Fig. 3. Fe-2p3/2 electron XPS line scan analysis traversing matrix–schreibersite–matrix regions of sectioned Sikhote-Alin sample (arrow-tip
region in Fig. 1a). Increasing P-composition locates the inclusion region and demonstrates an increasing Fe 2p binding energy within the
inclusion 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). The
inclusion stands out, in red-green, from the predominantly blue coloured matrix. The work-function scale (À500 to À200 mV) is shown
underneath 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 the
web version of this article.)
94 D.E. Bryant et al. / Geochimica et Cosmochimica Acta 109 (2013) 90–112

6. the surface layer removed by ion beam etching (3 kV,
9 mm2
, 1 lA sample current, 480 s etch time) and a second
spectrum recorded. The polished sample of Sikhote-Alin
was cleaned with ethanol (2 mL) and dried (50 °C) to re-
move surface contamination before removing the surface
layer by ion beam etching within the XPS. A series of spec-
tra were obtained at points along a line of length 5.5 mm
which 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 elemental
abundances are a percentage of the total counts adjusted
by a relative sensitivity factor for each element. The control
powders of Fe2P and Fe3P were analysed by immobilising
the powders onto the sample stub using a combination of
carbon tape and indium foil and then analysed as above
with scans of before and after etching. Full spread-sheeted
data are available within the Electronic Annex.
2.2.3. Scanning Kelvin probe (SKP)
The sample was analysed using a Uniscan SKP100
instrument (Buxton, UK). Measurements were acquired
using a 100 lm diameter platinum tip. Scans were carried
out at ambient temperature (ca. 298 K) and all scans were
performed in step-scan mode using the following parame-
ters: electrometer gain setting of 100, full scale sensitivity
of 2.5 mV, output time constant 1.0 s, vibrational ampli-
tude 30 lm. A three-point calibration was carried out to
correlate the SKP output with the redox potential of Zn,
Cu and Pb in equilibrium with saturated solutions of their
Fig. 5. IFM image of the “arrow tip” region of the Sikhote-Alin schreibersite inclusion, pre-corrosion. Dimensions are
2.8461 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). During
corrosion (left) and post-corrosion (right).
D.E. Bryant et al. / Geochimica et Cosmochimica Acta 109 (2013) 90–112 95

7. Fig. 7. (Left) Sikhote-Alin meteorite fragment as per Fig. 1a, post-corrosion but prior to cathodic cleaning. (Right) IFM image in a similar
orientation to that of Fig. 5 displaying post-corrosion surface of the Sikhote-Alin schreibersite inclusion, but prior to cathodic cleaning of the
surface.
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 cleaning
of the surface. The arrow tip is now pointing south. The colour image at right shows the regions where inclusion fragments have been
displaced (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. nitrate salts (Fig. 4). The SKP output (u) can then be con-
verted to a potential value E via the calibration equation
E = f. u; where f = 0.456 from the calibration curve in
Fig. 4. The potential map illustrated in Fig. 4 is a montage
of two separate scans acquired sequentially.
2.2.4. Infinite focus microscopy (IFM)
Topographical analysis was carried out with an Alicona
Infinite 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 cathodic
depolarisation 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. 31
P NMR spectrum (202.456 MHz; D2O) of water soluble extract from Sikhote-Alin following anaerobic digestion in the apparatus
of Fig. 6 (500 cm3
, 10% aqueous HCl; N2; 5 days 50 °C). [HPO4]2À
d 6.63 ppm. [HPO3]2À
d 4.22 ppm; 1
JPH = 566.9 Hz; [DPO3]2À
d 3.89;
1
JPD = 85 Hz (1:1:1 triplet).
D.E. Bryant et al. / Geochimica et Cosmochimica Acta 109 (2013) 90–112 97

9. one above the highest point of interest and one below the
lowest, are registered in the instrumentation and the micro-
scope focuses at a number of incremental points between
the 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 three
dimensional image of the feature being analysed. Once the
data has been collated, it may be used to take a range of
measurements such as depth, volume and surface rough-
ness. The technique is able to measure steep features up
200 300 400 500 600 700 800 900 1000
Raman shift / cm-1
0
100
200
300
400
500
Counts
200 300 400 500 600 700 800 900 1000
Raman shift / cm-1
100
200
300
400
500
600
700
Counts
200 300 400 500 600 700 800 900 1000
Raman shift / cm-1
100
200
300
400
500
600
700
800
Counts
a
b
c
Fig. 12. Raman spectra obtained from different sites of the corroded surface of the Sikhote-Alin meteorite (upper traces) showing mixtures of
iron 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. to 85from vertical and has a maximum theoretical vertical
resolution of 2.3 lm with a 5Â objective lens.
To prepare the post-corrosion specimen for analysis
using the infinite focus microscope (IFM), the sample sur-
face was cleaned of loosely adherent corrosion product by
polarising the sample cathodically to the point where
hydrogen gas was generated on the surface of the metal
according to the reaction 2H+
+ 2e ! H2. The sample
was immersed in a 0.1 M solution of NaOH and its poten-
tial was fixed at À2 V vs standard calomel electrode; the
polarisation continued until the loose material on the sur-
face was removed by the hydrogen bubbles generated on
the metal surface. Once cleaned in this fashion, the topog-
raphy of the sample was examined using IFM.
2.2.5. Raman spectroscopy
Raman spectra were collected using a Renishaw Invia
system. Excitation was achieved using a 633 nm NIR diode
laser (Renishaw), focused through a Â20 objective and fil-
tered to give 100% total laser energy at the sample. Spectra
were collected in static mode centred around 600 cmÀ1
,
with 1 s exposure and 100 accumulations. Raman images
were obtained from two separate sites of the corroded
meteorite surface with a Renishaw InVia micro Raman
spectrometer (Gloucestershire, UK). Spectral arrays of
respectively 50 Â 50 (Site 1) and 50 Â 42 (Site 2) were
obtained with a 20Â objective over areas of 300 Â 300 lm
and 300 Â 250 lm using 6 lm steps. Spectra were obtained
with 633 nm excitation using a static scan centred at
600 cmÀ1
with 10 accumulations of 1 s exposures. Final
images were generated with Direct Classical Least Squares
(DCLS) analysis of the resulting spectral hypercubes with
three iron oxide components (hematite, magnetite and goe-
thite) and presented as false colour 2-D images via merging
of colour components at each spectral pixel and bilinear
interpolation between pixel edges (Renishaw WIRE 3.2
software). Pixels below a nominal correlation threshold
were rendered as transparent. Reference spectra were ob-
tained from the RRUFF database (http://rruff.info/).
2.2.6. Fluid analyses
Fluids 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 remove
suspended particulate matter followed by fine-filtration
using a 0.45 lm filter. Duplicate 30 mL water samples were
taken, one of which was acidified with nitric acid, and these
were 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 Geochemistry
Laboratory 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 light
image, (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 iron
oxide species distribution obtained from DCLS analysis of Raman spectra (blue = hematite, red = magnetite, turquoise = goethite) overlaid
with the white light image. (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 99

11. Fig. 14. (a) Kverfjo¨ll volcanic region at the northern tip of the Vatnajoku¨ll glacier, SE Iceland. (b) Gengissig lake, Hveradalur geothermal
area (64° 40.1730
N; 16° 41.1000
W), 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).
31
P NMR spectrum of post-incubation SA2 (d), SA4 (e), SA5 (f) and SA1 (g) fluids showing presence of both orthophosphate and and H-
phosphite [H2POÀ
4 ; d 0.08 ppm and H-phosphite, H2POÀ
3 ; d 2.71 ppm; 1
JPH 630 Hz for SA1]. (h) 31
P NMR spectrum of post-incubation SA1
fluid, evaporated and dry-heated to 85 °C under flowing dinitrogen atmosphere for 72 h. Present are pyrophosphite [PPi(III), H2P2O2À
5 ; d
À3.35 ppm and À6.64 ppm], Pi(III)-D [d 2.78 ppm; 1
JPD = 88 Hz], PPi(III)-D2 [multiplets at ca. d À4.7; À5.3 and À5.7 ppm], Pi(V) (d
1.85 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. 2.2.7. NMR spectroscopy
31
P NMR analyses were performed on a Bruker Avance
500 MHz instrument operating at 202.634 MHz for 31
P
internally referenced to 85% H3PO4. Iron, principally in
the form of ferrous (Fe2+
) was removed from all samples
prior to NMR analysis to alleviate the problems associated
with paramagnetic broadening. This was done by pH
adjustment, first to ca. 12 by addition of NaOHaq, (1 M)
which leads to precipitation of oxides of iron, followed by
addition 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 water
or D2O, filtered using 0.45 mm syringe filters and analysed.
For those samples run in H2O solvent, D2O inserts were
used to provide a deuterium lock. Samples within which
pyrophosphite, PPi(III), were expected to be present and
analysed were pH adjusted to between 7 and 8 by addition
of NaOHaq, (1 M). NMR spectra from SA 1 (Fig. 14d and
e) were acquired after 8192 scans with a delay time between
pulses of 0.75 s.
2.2.8. Incubation of Sikhote-Alin within Hveradalur
geothermal fluids
Sectioned samples of the Sikhote-Alin IIAB iron mete-
orite (ca. 1 cm3
) were incubated within fluids (30 mL) from
the Hveradalur geothermal area for 4 days at natural tem-
peratures and pH’s (Table 1) followed by a period of
30 days at ambient temperature. After this time, samples
were gravity filtered on Whatman grade 1 filter paper, dried
and analysed by Raman spectroscopy (method outlined
above). Solutions were further filtered using 0.45 lm filters,
adjusted to 30 mL volumes and submitted for ICP-AES
analysis as outlined in Section 2.2.6. Following Raman
analysis, the meteoritic samples were subjected to post-cor-
rosion cathodic depolarisation (vide supra) to remove sur-
face detritus, followed by analysis via infinite focus
microscopy.
2.2.9. Incubation of Sikhote-Alin within simulated, low pH
geothermal fluids
The Sikhote-Alin sample in Fig. 1 was corroded by
suspending it within a plastic-coated drip-tray, above an
aqueous solution of 10% degassed hydrochloric acid
(450 mL) in an atmosphere of dinitrogen for 5 days at
50 °C, exposing only the surface shown (Fig. 1a). This sur-
face was subjected to post-corrosion analysis using Raman
spectroscopy and images were acquired using an infinite fo-
cus microscope (microscopy details above) both before and
after the acid corrosion treatment. The hydrochloric acid
solution was also analysed subsequently for Fe and Ni by
atomic absorption spectrophotometry and for total P using
the phosphomolybdate procedure (American Public Health
Association method 4500-P; calibration details are con-
tained within the Electronic Annex). In addition the solu-
tion was analysed by 31
P NMR to determine the nature
of 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 and
the residues were digested in conc. sulphuric acid (2 mL),
heated to dryness then taken up in sufficient conc. nitric
acid to achieve a clear solution. This solution was then di-
luted in a volumetric flask (100 mL) and analysed for iron
and nickel using a Perkin-Elmer AAnalyst 100 atomic
absorption spectrophotometer with an air/acetylene flame
compared to known standards of 1.0, 3.0 and 5.0 ppm me-
tal content respectively. The second aliquot (225 mL) was
reduced in volume on the rotary evaporator to remove
water and hydrogen chloride. Freshly made aqueous
Na2S solution (1.0 M) was added drop-wise to precipitate
FeS and NiS (vide supra) and the centrifuged and filtered
solution was reduced to dryness with the residues taken
up in D2O (1 mL) and analysed by 31
P NMR spectroscopy.
2.2.10. Dehydration of Ca(H2PO3)2ÁH2O under geothermal
conditions
A 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 and
warming to 60 °C the solution was allowed to stand and
cool. Crystals appeared of Ca(H2PO3)2.H2O. One crystal
was examined by single crystal X-ray diffraction and the unit
cell obtained compared to literature values as confirmation
of the crystals’ identity (Larbot et al., 1984). A sample of this
material (0.1 g; 0.45 mmol) was inserted ca. 2–3 cm beneath
Table 1
Elemental analysis (ICP-AES) and anion (Dionex Ion Chromatograph) measurements on acidified fluids (numbered 1–4; acronyms refer to
field site descriptors) from the Hveradalur geothermal area (in mg LÀ1) along with associated measurements on four Sikhote-Alin samples
(numbered SA1,2,4,5). u
Incubated in fluid 1, LP1.
Incubated in fluid 2, UCL5, LP1. à
Incubated in fluid 3, LP3. –
Incubated in fluid 4, BPR.
Sample Fe Ni P Ca Mg S F Cl pH T (o
C)
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 93.5
Fluid 2 (UCL5) 0.56 – 0.31 29.19 4.48 – 1.69 9.06 4.7 89.2
Fluid 3 (LP3) 31.40 À0.04 0.08 – – – 0.09 – 2.5 79.2
Fluid 4 (BPR) 4.11 À0.03 À0.01 62.59 13.80 141.20 1.39 1.32 4.0 79.5
Sikhote Alin (SA1)u
7.82 0.02 16.78 107.81 11.38 153.48 3.67 8.71 3.1 93.5
Sikhote Alin (SA2)
0.35 0.67 0.17 41.62 7.84 70.00 0.00 0.00 4.7 89.2
Sikhote Alin (SA4)à
68.01 7.52 À0.01 75.69 10.64 150.64 2.94 3.89 2.5 79.2
Sikhote Alin (SA5)–
11.97 1.88 0.06 36.39 5.32 90.58 1.07 2.40 4.0 79.5
D.E. Bryant et al. / Geochimica et Cosmochimica Acta 109 (2013) 90–112 101

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 31
P NMR and 1
H-
spectroscopy upon return to Leeds some 3 weeks later.
3. RESULTS AND DISCUSSION
3.1. SEM and EDX maps of Sikhote-Alin
In Fig. 1a is displayed an optical photograph of the
Sikhote-Alin section used in this study. The sample has a
cleavage plane on its left-hand edge but the important fea-
ture is the arrow tip-shaped inclusion of schreibersite rising
up from the southern edge in a vertical pointing direction
for a distance of 3 mm. In Fig. 1b are collected a secondary
electron SEM spectrum of a portion of the arrow tip region
along with elemental X-ray (EDX) maps at the appropriate
P, Fe, O and Ni Ka energies which reveal the schreibersite
inclusion to be P-rich and, in comparison to the surround-
ing matrix, relatively Ni-rich and Fe-poor. There are small
clumps of oxygen-rich domains which we presume to be
associated with low-iron and low nickel oxides. Due to
the absence of a silicon signature in the EDX map, we sug-
gest these oxides are unlikely to be silicate inclusions but
perhaps localised surface corrosion or possibly alumina
from polishing. Overall the sample shows strong elemental
EDX response for Fe, Ni and P as expected (Fig. 1a).
3.2. X-ray photoelectron spectroscopic (XPS) analysis of
Sikhote-Alin
In Fig. 2 are shown photoelectron spectra of the Fe, 2p3/
2 electrons from a powdered sample of Fe3P, this being a
commonly used proxy for schreibersite (Pasek and Lauret-
ta, 2005), without ion-beam etching. There is a significant
change in trace maxima for Fe, 2p3/2 electrons when the
surface is etched signifying removal of an iron oxide layer
with binding energies centred around 711.4 eV compared
to a underlying matrix with binding energy ca. 708 eV (full
XPS 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 of
706.6 eV and Fe2O3 of 709.8 eV respectively. The implica-
tion is that the etching process removes surface iron oxides
to reveal the pristine sub-surface iron. This effect is illus-
trated again when comparing the binding energies of 2p3/2
electrons 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 and
129.6 eV for Fe2P and Fe3P respectively. For both Fe2P
and Fe3P, the P-2p3/2 electron binding energy curves reveal
a shoulder at the higher energy side ca. 131 eV which we as-
sign to the P-2p1/2 electron as reported for close relative
FeP (Grosvenor et al., 2005). Intriguingly, these authors re-
turned a P-2p3/2 electron binding energy for FeP of
129.3 eV and argued on the basis of a correlation between
core P-2p3/2 binding energies of MP (M = Co, Fe, Mn
and Cr) and electronegativity differences between M and
P, that this was consistent with an approximate charge on
the P atom in FeP of À1 (Grosvenor et al., 2005). From
the NIST XPS data base, P-2p3/2 electron binding energies
are also reported for Fe2P and Fe3P at 129.5 and 129.4 eV
respectively (Nemoshkalenko et al., 1983), commensurate
with those reported here.
Turning our attention to analysis of the schreibersite
inclusions within Sikhote-Alin, the results of an XPS line
scan across the Sikhote-Alin sample are shown in Fig. 3.
The line starts within the matrix material and crosses the
narrow part of the inclusion (arrow-tip in Fig. 1a) to finish
again within matrix material on the other side. There are
practical difficulties in knowing exactly where the beam is
impinging on the sample and for this reason the relative
concentration of phosphorus is calculated from each point
demonstrating that the line does indeed cross the inclusion
as thought, from a relatively low-P region through a region
of relatively high P presence (inclusion) back to low P ma-
trix again. It is clear from the graph that the binding energy
of Fe-2p3/2 electrons closely correlates with the phosphorus
content. However, a comparison of the Fe-2p3/2 binding
energies of Fe2P with Fe3P which are very similar indicates
that the phosphorus content itself may not be the sole fac-
tor responsible for the increased binding energy within the
inclusion compared to matrix Fe. The inclusion is known to
be nickel-rich and iron-poor compared to the matrix as can
be seen from the SEM/EDX pictures of the tip of the
“arrow” in Fig. 1b. An increase of Fe-2p3/2 binding energy
with increasing nickel content is in line with reported stud-
ies of alloys (Nagai et al., 1987); the higher binding energy
suggests 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) analysis
Scanning electrochemical techniques such as the SKP
are ideal methods for probing electrochemical differences
between disparate metal junctions and have been used
widely in the field of applied corrosion research (Williams
et al., 2010). It is a technique that had not been exploited
in the space sciences field prior to our original report of
its 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 corrosion
behaviour at grain boundaries in other materials (Li
et al., 2012). Calibrating the volt potential output of the
SKP against a known set of redox potentials, for Zn, Pb
and Cu (Fig. 4) allowed the output to be expressed in terms
of the electrochemical potential of the specimen being as-
sessed. Also shown in Fig. 4 is a colour-graded SKP graphic
of the arrow-shaped schreibersite inclusion within Sikhote-
Alin (cf: image in Fig. 1a) which clearly distinguishes the
surface potential differences between inclusion and matrix.
Examination of the values of the output in Fig. 4 indicates
a potential difference DE between the inclusion and the sub-
strate to be of the order of 550 mV. This potential difference
acts as the driving force for a galvanic current between
102 D.E. Bryant et al. / Geochimica et Cosmochimica Acta 109 (2013) 90–112

14. inclusion and matrix. The result of this galvanic couple
should be to polarise the matrix anodically, effectively mak-
ing the matrix more susceptible towards oxidation than the
inclusion. Whilst the SKP does not give information
regarding 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 corrosive
attack 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 related
iron 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 to
produce 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 matrix
containing 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 occur
primarily on the matrix material around the interface with
the inclusion. This would result in a potential weakening of
physical attachment of the inclusion within the matrix and
hence ultimately a release of inclusion material which
would subsequently undergo, presumably slower, hydro-
thermal modification to release chemicals to the environ-
ment. To further probe this effect, we designed an
anaerobic hydrothermal reactor within which we could
probe accelerated corrosion of meteoritic fragments in the
presence of simulated low pH water environments. Follow-
ing corrosion and cathodic depolarisation to remove sur-
face oxide detritus, the sample could then be examined by
infinite focus microscopy to assess the validity of the above
electrochemical arguments.
3.4. Hydrothermal modification of Sikhote-Alin under
anaerobic simulated geothermal low pH environments
Infinite focus microscopy (IFM) is a relatively recently
developed technique for analysing surface morphology
and has found significant application in fields as diverse
as engineering corrosion (Jiang and Nesic, 2009) and bio-
materials (Winkler et al., 2010). The Sikhote-Alin sample
in Fig. 1 was analysed by IFM pre and post-acid corrosion
and the solution leachate analysed for Fe, Ni and P levels as
described in Section 2.2.9. Fig. 5 shows the Sikhote-Alin
inclusion imaged prior to the corrosion study. The surface
is essentially flat although a discontinuity can be seen at
the 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 noted
previously through scanning electron microscopy and prob-
ably connected to the fact that schreibersitic inclusions have
increased levels of brittleness compared to the surrounding
matrix as indicated by their raised Vickers hardness num-
bers (Bryant et al., 2009).
The experimental arrangement for simulated anaerobic
corrosion used here is illustrated in Fig. 6, which shows
clearly the plastic coated cage in which the Sikhote-Alin
sample was suspended. Dinitrogen gas was bubbled
through a solution of 10% degassed hydrochloric acid for
a period of 5 days at 50 °C, so that the meteorite sample
would be subjected to condensed, low pH water within a
dynamic hydrothermal environment. Fig. 7 shows the
post-incubation surface of the Sikhote-Alin sample where
the inclusion is now difficult to see being largely obscured
by corrosion products except in the upper left corner where
the crust has flaked off to reveal a step. A line across this
step can be traced by IFM and the topographical change
along this line is plotted in Fig. 8. The topography of the
interface between the inclusion and the matrix shown in
Fig. 8 serves to reinforce the prediction of the SKP analysis
that corrosive attack would be most severe at the matrix-
inclusion boundary where local corrosion due to Galvanic
corrosion 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-inclusion
boundary, with dimensions of ca. 40 lm depth and
120 lm wide, displays clear and preferential dissolution of
matrix material. Should such behaviour continue, one
would envisage weakening of the matrix-inclusion adhesion
to such a point that the inclusion may become sufficiently
weakened to allow it to be released from the surrounding
matrix. Indeed, this appears to be the case in practice as
illustrated by an IFM image of the arrow-tip inclusion
post-cathodic depolarisation to remove surface debris. In
this process the meteorite sample is rendered at a cathodic
potential 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. In
the process, a significant degree of corrosion appears to
have taken place surrounding the inclusion, which has
weakened its attachment to the encompassing matrix to
such an extent that a fragment of inclusion has also been
removed from its pre-incubation position resulting in a gap-
ing crevice (Figs. 9 and 10) visible in the line-scan trace with
dimensions ca. 150 mm deep and 2 mm across. The leachate
solution was analysed for dissolved Fe, Ni and P which
were measured to be present at concentrations of
440 ppm (Fe), 20 ppm (Ni) and 0.7 ppm (P). The latter
compared to a background in the distilled water of
0.007 ppm (see Electronic Annex for calibration and back-
grounds). Further analysis of the Fe-removed (addition of
Na2S) leachate using 31
P NMR spectroscopy identified
the 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-anaerobic
corrosion
The relatively large ‘step’ across the ‘arrow head’ pre-
cluded efficient Raman mapping due to issues relating to
the 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. Site
1 incorporated a corroded ‘pit’, apparently with a relatively
large area of exposed metal surface; in contrast, Site 2 fea-
tured a substantial surface coverage of oxide material. In
D.E. Bryant et al. / Geochimica et Cosmochimica Acta 109 (2013) 90–112 103

15. common with other areas of the corroded meteorite surface,
Raman scattering was relatively weak and the resulting
spectra featured broadened peaks consistent with a largely
amorphous nature for the oxide deposits and/or several
microenvironments of the oxide materials. As iron oxides
have shown an ability to interconvert under laser irradia-
tion, care was taken to minimise the exposure of each site
through the use of relatively short exposure times (De Faria
et al., 1997). Comparison was made with reference spectra
for 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). While
the broadening of the spectra precluded specific identifica-
tion of the different morphological forms, e.g. the FeOOH
species, clear domains for Fe(III), Fe(II,III) oxides and
Fe(III) oxyhydroxides were identifiable in the spectra as
well as substantial areas of mixtures of these species
(Fig. 12). In this context, Raman images were generated
from multivariate analysis using Direct Classical Least
Squares and the best models for the image data were ob-
tained with three components: hematite, magnetite and goe-
thite. Other species were excluded from development
models on the basis of weak or absent correlation between
pixels in the images and the appropriate reference spectra
(Zhang et al., 2005). The resulting images are shown in
Fig. 13.
The distribution of oxide species in the maps are worthy
of note. At Site 1, the metal ‘pit’ appears to be bordered by
a 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 goethite
Fig. 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 oxides
and clay minerals from the geothermal fluids.
104 D.E. Bryant et al. / Geochimica et Cosmochimica Acta 109 (2013) 90–112

16. appears well away from the pit. In contrast, Site 2 appears
to show a much wider expanse of goethite although, again,
magnetite is the dominant species. There are also several
smaller 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 Fe3P
inclusions, hence the formation of hematite in these
environments.
3.6. Hydrothermal modification of Sikhote-Alin under
aerobic geothermal low pH environments
Whilst the above laboratory experiment allowed us to
simulate a low pH geothermal environment our June 2011
field expedition to the Vatnajoku¨ll glacier within SE Ice-
land, afforded us the opportunity to explore hydrothermal
modifications on Sikhote-Alin under bona fide, low pH geo-
thermal field conditions. Our field site was the Hveradalur
geothermal area (64° 40.1730
N; 16° 41.1000
W) within the
Kverkfjo¨ll volcanic range at the northern region of the Vat-
najoku¨ll glacier. A small (ca. 50 m diameter) geothermal
field at the edge of the Gengissig lake containing hydrother-
mal stream waterfalls, streams, hot water, mud pools and
sulphurous 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 samples
of the Sikhote-Alin, each with at least one face displaying
exposed schreibersite mineral were incubated in 50 mL Fal-
con tubes within geothermal fluids at this site. Sikhote-Alin
field sample SA1 was incubated in a hot water pool
(Fig. 14c; pH 3.1; T = 93–94 °C) for 4 days followed by
ambient temperature incubation in the same fluid for a per-
iod of 4 weeks prior to analysis via ICP-AES, SEM-EDX
and IFM techniques. As is clear from the data of Table 1,
all of the Sikhote-Alin samples which were deployed within
geothermal fluids (samples SA1,2,4, and 5) have dissolved
P-levels in the range 0–17 mg LÀ1
significantly higher than
those measured in the blank, distilled water samples (Ta-
ble 1 entries 1–3). Hydrothermal modification of the four
Sikhote-Alin samples results in enhanced P-levels for SA1
and SA5 compared to their fluid hosts but somewhat atten-
uated levels for SA2 and SA4. We suspect that this may be
a result of the greater Fe-levels introduced by the iron mete-
orite (Table 1) leading to precipitation of ferric phosphates.
31
P NMR analysis of the post-incubation fluid from SA1,
following removal of dissolved iron by pH adjustment to
12 and filtration, reveals the presence of both orthophos-
phate (d 0.08 ppm) and H-phosphite (H2POÀ
3 ; d 2.71 ppm;
1
JPH 630 Hz; Fig. 14d), the latter being the expected and
dominant P-oxyacid of hydrothermal schreibersitic modifi-
cation (Pasek and Lauretta, 2005). Similar analyses of each
of the other SA samples SA2, 4 and 5 reveal similar P-spe-
ciation with H-phosphite the dominant component in each
case. Sample SA3 was not deployed in Iceland, but used as
a laboratory reference sample. Together, these data form
Fig. 16. IFM images of the “ducks head” schreibersite inclusion of Sikhote-Alin sample SA1 (a) pre-incubation and (b) post-incubation after
cathodic depolarisation. (c) 2D IFM topological map of the region (highlighted inset) between the “ducks head” and a second schreibersite
domain 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. the thrust of this papers conclusion which illustrates very
nicely the key surface corrosion properties and solution
P-chemistry which afford activated P-compounds under
mild conditions.
Several schreibersite inclusions can be clearly seen on
one of sectioned faces of SA1, and one of these motif’s with
a morphology reminiscent of a “duck’s head” was selected
for closer examination. In Fig. 15 are SEM and EDX map
images of the “duck’s head” post-corrosion and after catho-
dic depolarisation to clean the meteoritic surface, which
demonstrates clearly the differential between nickel-rich,
iron-poor and phosphorus-rich inclusion and surrounding
matrix. 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 principally
corrosion oxides of iron and clay minerals intrinsic to the
geothermal fluid (these and other more specific features will
be discussed in the subsequent field paper). In terms of the
meteoritic surface morphology however, it is most instruc-
tive to compare IFM analyses of both pre-incubated and
post-incubated Sikhote-Alin sample SA1 as in Fig. 16a
and 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 this
point of connection results in the schreibersite standing
proud of the surrounding matrix by ca. 20 lm (Fig. 17).
Our inference is that matrix surrounding the schreibersite
inclusion has been preferentially dissolved during incuba-
tion within geothermal fluids, a result which mirrors that
of our laboratory-based simulations. Sikhote-Alin samples
SA2, 4 and 5 were incubated in geothermal fluids at pH’s
4.7, 2.5 and 4.0 respectively at temperatures of 89.2, 79.2
and 79.5 °C followed by ambient temperature incubation
in the same fluid for 4 weeks in the same manner as SA1,
prior to complementary analysis. In Fig. 18 are reproduced
IFM images of a schreibersite-matrix section (the latter is
the more smooth region of the two) both pre-incubation
(Fig. 18a) and post-incubation (after cathodic depolarisa-
tion; Fig. 18b) on SA2. Comparison of complementary
line-scans across (as far as is possible) the same vector
(red line scanning inclusion-matrix-inclusion-matrix from
top to bottom) in both pre- and post-incubated samples
(Fig. 18c and d respectively) reveals a clear crevice some
60 lm deep within the inter-inclusion matrix region be-
tween 0.8 and 0.9 mm across the scanning vector in the
post-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 the
inclusion-matrix boundary. Samples of SA4 and 5 display
similar behaviour, of which the clearest is found on SA4.
In Fig. 19 are reproduced schreibersite inclusions embedded
within 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 inclusion
stands 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. bated image is a spur of schreibersite clearly visible at the
northern 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 and
some 300 lm wide (Fig. 19d) which is not present in the
pre-incubated sample.
The post-incubation fluid from SA1 above (containing
16.78 mg LÀ1
total P; Table 1) containing both orthophos-
phate and H-phosphite [Pi(III), H2POÀ
3 ; Fig. 14g] was pH
adjusted to 4 by addition of HClaq, followed by evapora-
tion and grinding of the resulting solid evaporate to a fine
powder. This was then heated to 85 °C in a sand bath under
a flowing atmosphere (ca. 1 cm3
sÀ1
flow) of dinitrogen for
a period of 72 h after which time the material was dissolved
in D2O, pH re-adjusted to ca. 7 and the P-components stud-
ies by 31
P NMR spectroscopy. The resulting spectrum
(Fig. 14h) reveals that a significant proportion (40+%) of
the total solution P is now present as the condensed oxy-
acid, pyrophosphite [PPi(III), H2P2O2À
5 ; d À3.35 and
À6.64 ppm] identified by comparison of its AA0
XX0
spin
system to an authentic sample (Bryant et al., 2010). Also
identifiable within this spectrum are products resulting
from H–D exchange within Pi(III) [d 2.78 ppm; 1
JPD = 88 -
Hz] and within PPi(III) [multiplets at ca. d À4.7; À5.3 and
À5.7 ppm] along with smaller signals due to Pi(V) (d
1.85 ppm) and mixed-valent species, isohypophosphate
PPi(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 condensation
of H-phosphite to pyrophosphite is potentially accessible
within a bona fide geological environment is illustrated by
the incubation of a dry sample of Ca(H2PO3)2.H2O, heated
to 94.4 °C in a Falcon tube inserted ca. 3 cm beneath the
sub-surface soil within the geothermal field at the edge of
the Gengissig lake (Fig. 20a). After ca. 3 days exposure,
analysis some 3 weeks later by both 31
P and 1
H NMR spec-
troscopy identified PPi(III) formation which was not pres-
ent in a reference sample of the same compound analysed
Fig. 18. IFM images of a schreibersite-matrix region of SA2; (a) pre-incubation and (b) post-incubation after cathodic depolarisation. (c and
d) Identify (red line) the vector across which IFM topological depth profiles (e) and (f) were recorded for both pre- and post-incubation
samples. (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. without heating (Fig. 20). Pyrophosphite is an intriguing
material as it is structurally and chemically related to the
pyrophosphate [PPi(V)] moiety in nucleotide triphosphates
such as adenosine triphosphate (Fig. 21) the ubiquitous
suite 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) formed
under far milder conditions than the latter and (ii) it is more
chemically reactive in the absence of sophisticated catalysis
(Bryant et al., 2010).
4. CONCLUSIONS
The emergence of phosphate-based biochemistry has
been 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 both
limited solubility in water in the presence of many common
metal 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 10À24
and 9.91 Â 10À16
respectively) and has rela-
tively low chemical reactivity in the absence of activating
agents (Steinman et al., 1965; Beck and Orgel, 1965; Oster-
berg and Orgel, 1972; Hermes-Lima and Vieyra, 1989). The
sophisticated enzymes of contemporary cellular life used to
activate P in energy currency molecules such as nucleoside
triphosphates (e.g. ATP), phosphocreatine and phosphoe-
nol pyruvate (Harold, 1986), are unlikely to have been
available within the Hadean period, however there is con-
siderable support for activated P-chemistry being central
Fig. 19. IFM images of a schreibersite-matrix region of SA4; (a) pre-incubation and (b) post-incubation after cathodic depolarisation with
highlighted 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 represent
the deeper regions of the post-incubated sample. (For interpretation of the references to colour in this figure legend, the reader is referred to
the web version of this article.)
108 D.E. Bryant et al. / Geochimica et Cosmochimica Acta 109 (2013) 90–112

20. to the bioenergetics of the early living organisms (Holm and
Baltscheffsky, 2011). The question then arises, how could
nature 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 Earth
would have been siderophilic phosphide minerals, such as
schreibersite, (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 to
orthophosphate, (Lauretta and Schmidt, 2009) they are
known to occur with natural metallic deposits on Disko Is-
land, Greenland (Klo¨ck et al., 1986) and to be produced by
chemical reduction of phosphates in soils during lightning
strikes (Pasek and Block, 2009). However, significant quan-
tities of schreibersite and related phosphide minerals would
likely 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-flux
rates during the putative late-heavy bombardment (between
ca. 4.0 and 3.8 Ga) and concluded that whilst both IDPs
and iron meteorites would likely have brought similar
quantities of siderophilic P to the early Earth (ca. 108
-
kg yrÀ1
), the far more localised impact events associated
with irons could have afforded very high local concentra-
tions of activated-P, in the region of 105
kg kmÀ2
(Pasek
and Lauretta, 2008). This process would require the interac-
tion of a hydrothermal system with a meteorite small
enough to impact and not destroy the system, yet large en-
ough to add enough reduced phosphorus to influence local
chemistry. The higher fall rate likely present on the early
earth (e.g. Johnson and Melosh, 2012; Bottke et al., 2012)
provided a greater frequency of meteorite falls to the early
earth. As most meteorites do not exceed a mass of about
50 tonnes, and have slowed significantly by ablation during
atmospheric 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 a
hydrothermal 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 these
new 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 electrochemical
differences in composition between matrix Fe–Ni (taenite
and kamacite) and schreibersite inclusions result in prefer-
ential dissolution of matrix material at the matrix-inclusion
boundary leading to weakening of attachment of the inclu-
sion to the meteoritic matrix. This in turn should allow for
detachment of the inclusion with a consequent increase in
the availability of activated P to local water sources. Our re-
port here of the first field studies on low pH hydrothermal
modification of schreibersitic inclusions within Icelandic
Fig. 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.
31
P NMR analysis (202.63 MHz; D2O; 300 K) of the heated solid, after ca. 3 days exposure, identified PPi(III) formation by comparison to an
authentic 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 bc
pairs of P atoms; (b) pyrophosphate, PPi(V), the main energy currency fragment of ATP and (c) pyrophosphite, PPi(III) a related molecular
cousin 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. geothermal fields supports laboratory-based studies that P
in a lower oxidation state than +5, namely H-phosphite
(H2POÀ
3 ; where P is present formally as +3) is the chief
water-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 of
nucleotide triphosphates such as ATP. We propose that
the 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 to
the presence of sophisticated catalysts. Examples of this en-
hanced chemical reactivity will be described in a more spec-
ialised chemistry manuscript.
ACKNOWLEDGEMENTS
The authors are grateful for the financial support received to
support this work specifically, the Engineering and Physical Sci-
ences Research Council (Grant EP/F042558/1 to T.P.K.), the
Leverhulme Trust (Grant F07112AA to I.A.C.), the Science and
Technology Funding Council, and the UK Space Agency for the
award of an Aurora Fellowship (to T.P.K). We thank Dr. Laura
Carmody 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 the
Wolfson geochemistry laboratory at UCL/Birkbeck. The Natural
History Museum, London is thanked for providing samples of
the Sikhote-Akin meteorite. Finally, we thank the reviewers for
their insightful comments and suggestions.
APPENDIX A. SUPPLEMENTARY DATA
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/
j.gca.2012.12.043.
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