Transition element stable isotope studies using plasma mass
spectrometry (MC-ICP-MS)
The advent of multicollector, magnetic sector, inductively coupled mass spectrometry
(MC-ICP-MS) has transformed the scope of research in stable isotope geochemistry by
making possible measurement of the isotopic compositions of the metallic elements at
natural abundance levels (e.g., Mg, Ca, Fe, Cu, Zn, Mo and Cr). Furthermore, radiogenic
isotopic systems that hitherto have been measured with thermal ionization mass
spectrometry (TIMS) can now also be accurately measured at high throughputs, making
MC-ICP-MS one of the most powerful new techniques available to geochemistry. Early
work on transition element stable isotope systems, such as those iron and copper
(57Fe/54Fe; 65Cu/63Cu) have shown them to be potentially valuable proxies for paleoredox
processes – both abiotic and biologically-mediated fractionation phenomena in
sedimentary environments. The installation in 2001 of a Nu Instruments MC-ICP-MS at
the Geological Survey of Israel, partly funded by the Faculty of Sciences and Institute
funds, now allows this technology to be exploited in Israel and consequently the
development of new research programs that hitherto were not feasible. During the last
four years we have been exploring the potential of the iron and copper isotope systems in
various contexts: experimental; organic-rich sediments; soils; the continental sub-surface.
In our latest work we have been applying these isotopic systems to redox processes subsurface transport of metalliferous solutions and the deposition of metallic ores along
faults in the Negev desert and copper ores in the Timna valley. We are also in the process
of developing molybdenum isotope studies Mo-enriched iron oxides along anticlines in
the NE Negev desert in cooperation with Prof. Ariel Anbar at Arizona State University. A
underlying aspect of this research is the aim of integrating the transition metal isotope
systems with other isotope systems, such as those of Sr, Pb and U (see also research page
on radiogenic isotopes, geochronological and trace element studies). The following
abstracts represent a selection of the published work of faculty members in the last few
years. As more studies are published, further abstracts will be included on the webpage
Nu Instruments MC-ICP-MS
1. Mass Fractionation Processes of Transition Metal Isotopes. Earth Planet. Sci. Lett. 200:
47-62 (2002).
X. K. Zhu1)*, Y. Guo1), R. K. O’Nions1), R. J. P. Williams2), A. Matthews1, 3), N. S. Belshaw1), G.
W. Canters4), E. C. de Waal4), U. Weser5), B. K. Burgess6), B. Salvato7)
1
Department of Earth Sciences, University of Oxford2 Department of Chemistry, University of
Oxford,3 Institute of Earth Sciences, Hebrew University of Jerusalem, 4 Department of Chemistry,
Leiden University, 5 Physiologisch-Chemische Institut, Eberhard-Karls-Universität Tübingen, 6
Department of Molecular Biology and Biochemistry, University of California, Irvine,7 CNR
Centre of Metalloproteins, Padua University.
Abstract – Recent advances in mass spectrometry make it possible to utilize isotope variations of
transition metals to address some important issues in solar system and biological Sciences.
Realization of the potential offered by these isotope systems however requires an adequate
understanding
of
the
factors
controlling
their
isotope
fractionations. Here we show the
results of a broadly based study on
copper and iron isotope fractionation
during
various
inorganic
and
biological processes. These results
demonstrate that: (1) naturally
occurring inorganic processes can
fractionate Fe isotopes to a detectable
level even at temperature ~ 1000ºC,
which challenges the previous view
that Fe isotope variations in natural
systems are unique biosignatures; (2)
multiple-step equilibrium processes at
low temperatures may cause large
mass fractionation of transition metal
isotopes even when the fractionation
per single step is small; (3) oxidationreduction is an important controlling
factor of isotope fractionation of
transition metal elements with
multiple valences (See figure showing
copper isotope results in epsilon units
and a Rayleigh model simulating the
isotopic composition of Cu(I) iodide
(curve C) and Cu(II)solution (Curve
A)), which opens up a wide range of
applications of these new isotope
systems, ranging from metal-silicate
fractionation in the solar system to
uptake pathways of these elements in
biological systems; (4) organisms incorporate lighter isotopes of transition metals preferentially,
and transition metal isotope fractionation occurs stepwise along their pathways within biological
systems during their uptake.
2. Controls on iron-isotope fractionation in organic-rich sediments (Kimmeridge Clay,
Upper Jurassic, southern England). Geochim. Cosmochim. Acta 68: 3107-3123 (2004)
Alan Matthews1, Helen S. Morgans-Bell2, Simon Emmanuel1*,Hugh C. Jenkyns2, Yigal Erel1
and Ludwik Halicz3
1
Institute of Earth Sciences, Hebrew University of Jerusalem,2 Department of Earth Sciences,
University of Oxford, 3 Geological Survey of Israel.
depth (m)
Abstract– This study explores the fractionation of iron isotopes ( 57Fe/54Fe) in an organic-rich
mudstone succession, focusing on core and outcrop material sampled from the Upper Jurassic
Kimmeridge Clay Formation type locality in south Dorset, UK. The organic-rich environments
recorded by the succession provide an excellent setting for an investigation of the mechanisms by
which iron isotopes are partitioned among mineral phases during biogeochemical sedimentary
processes.
Two main types of iron-bearing assemblage are defined in the core material: mudstones with
calcite ±pyrite ± siderite mineralogy, and ferroan dolomite (dolostone) bands. A cyclic data
distribution is apparent, which reflects variations in isotopic composition from a lower range of
57Fe values associated with the pyrite/siderite mudstone samples to the generally higher values
of the adjacent dolostone samples (see figure on this page illustrating the iron isotope
compositions). Most pyrite/siderite mudstones vary between -0.4 and 0.1 ‰ while dolostones
range between -0.1 and 0.5 ‰, although in very organic-rich shale samples below 360 m core
depth higher 57Fe values are noted. Pyrite nodules and pyritized ammonites from the type
exposure yield 57Fe values of -0.3 to -0.45 ‰. A fractionation model consistent with the 57Fe
variations relates the lower 57Fe pyrite and siderite ± pyrite mudstones values to the production
of isotopically depleted Fe(II) during biogenic reduction of the isotopically heavier lithogenic
Fe(III) oxides. A consequence of this reductive dissolution is that a 57Fe-enriched iron species
must be produced that potentially becomes available
Stone Bands
for the formation of the higher 57Fe dolostones. An
Siderite-Pyrite-calcite-bearing mudstones
isotopic profile across a dolostone band reveals
Pyrite-Calcite-bearing mudstones
distinct zonal variations in 57Fe, characterized by
-80 Lithogenic Fe oxides (this study)
4
1, 2
3
5
two peaks, respectively located above and below the
120
central part of the band, and decoupling of the
Lithogenic Fe (III) Range
isotopic composition from the iron content. This
(Beard et al, 2003b)
160
form of isotopic zoning is shown to be consistent
with a one-dimensional model of diffusional200
chromatographic Fe-isotope exchange between
dolomite and isotopically enriched pore water. An
240
alternative mechanism envisages the infiltration of
280
dissolved ferrous iron from variable (high and low)
57Fe sources during co-precipitation of Fe(II) ion
320
with dolomite. The study provides clear evidence that
iron isotopes are cycled during the formation and
360
±1
diagenesis of organic carbon-rich sediments.
400
-0.5
0
0.5
57
 Fe (IRMM-Fe) ‰
1
3. Experimental Study of the copper isotope fractionation between aqueous Cu(II) and
covellite, CuS. Chemical Geology 209: 259-269 (2004)
S. Ehrlicha, I. Butlerb, L. Halicza, D. Rickardb A. Oldroydb, Alan Matthewsc
a
Geological Survey of Israel b School of Earth, Ocean and Planetary Sciences, Cardiff University
c
Institute of Earth Sciences, Hebrew University of Jerusalem.
Abstract – We report the results of a study of the geologically important copper isotope
(65Cu/63Cu) fractionation between aqueous Cu(II) and copper sulphide. The experiments were
made in anoxic conditions by precipitating covellite (CuS) from excess Cu as aqueous CuSO4
through the addition of aqueous Na2S. Isotopic measurements were collected on a Nu
Instruments MC-ICP-MS using sample-standard bracketing and mass-bias correction using a
nickel internal standard. The results of a series of experiments at 20°C give a mean fractionation
of 65Cu (Cu(II)aq –CuS) = 3.06 ± 0.14 ‰. Additional experiments made at 2°, 10° and 40°C
show that the fractionation factor varies inversely with temperature according to the relation
(errors at 1 level):
Cu(II) – CuS = 0.26 ± 0.02 x 106T-2 + 0.08 ± 0.25.
Although the question of whether complete
equilibration occurs during the precipitation
reaction cannot be resolved, the measured
fractionation factors provide a proxy for the natural
fractionation processes involving abiogenic
covellite
formation
by
low-temperature
precipitation. The ~ 3‰ Cu(II)aq–CuS fractionation
compares with small experimentally measured
Cu(II)aq-malachite fractionation (0.20 to 0.38 ± 0.04
‰ at 30°C) reported by Maréchal and Sheppard
(2002) and a similar fractionation measured in this
study for Cu(II) hydroxide precipitation from
Cu(II)aq solution. The large Cu(II)aq –CuS isotopic
fractionation supports the conclusion that covellite
is a Cu(I)S(-I) compound and that redox state is
potentially a significant control of abiogenic
65
Cu/63Cu fractionations in low-temperature geological environments.
4. A preliminary mixing model for Fe isotopes in soils. Chemical Geology 222: 23-34 (2005)
S. Emmanuela, Y. Erela, A. Matthewsa, N. Teutschb
a
Institute of Earth Sciences, Hebrew University b Swiss Federal Institute for Aquatic Science and
Technology.
Abstract - Iron partitioning data and whole soil 57Fe values were combined to calculate the
isotopic composition of Fe mixing end-members in these soils. A least squares method was used
to estimate the iron isotope composition of the end-members representing the three main Fe
reservoirs in the Czech soil: (1) silicates (57Fe = -0.02 ± 0.17 ‰), (2) organically bound iron
(57Fe = -0.48 ± 0.27 ‰), and (3) pedogenic Fe oxides (57Fe = -1.07 ± 1.02 ‰). A lack of
variation in the isotopic and chemical partitioning patterns in the Israeli soil prevented the
application of the least squares technique, although an Fe-oxide end member is also proposed
using a similar mixing model (57Fe = -1.72 ± 1.16 ‰). Combination of the isotopic values for
the reservoirs found this study with published isotopic fractionation data suggests that the silicate
fraction in the Israeli soil is dominated by lithogenic sources, whereas the Fe-oxide pool is mainly
influenced by pedogenic precipitation-dissolution processes. The results demonstrate the
potential for Fe isotopes as a tool to quantify Fe cycling in soils