The stable vanadium isotope composition of the mantle and mafic...

The stable vanadium isotope composition of the mantle and mafic...
Earth and Planetary Science Letters 365 (2013) 177–189
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Earth and Planetary Science Letters
journal homepage: www.elsevier.com/locate/epsl
The stable vanadium isotope composition of the mantle and mafic lavas
J. Prytulak a,b,n, S.G. Nielsen b,c, D.A. Ionov d, A.N. Halliday b, J. Harvey e, K.A. Kelley f, Y.L. Niu g,
D.W. Peate h, K. Shimizu i, K.W.W. Sims j
a
Department of Earth Science and Engineering, Imperial College London, London SW7 2AZ, UK
Department of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, UK
c
Department of Geology & Geophysics, Woods Hole Oceanographic Institute, 266 Woods Hole, MA 02543-1050, USA
d
Université de Saint Etienne & UMR6524-CNRS, F-42023, France
e
School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK
f
Graduate School of Oceanography, University of Rhode Island, RI 02882-1197, USA
g
Department of Earth Science, University of Durham, Durham DH1 3LE, UK
h
Department of Geosciences, University of Iowa, IA 52242, USA
i
Institute for Research on Earth Evolution, Japan Agency of Marine-Earth Science and Technology, 2-15 Natsushima, Yokosuka 237-001, Japan
j
Department of Geology and Geophysics, University of Wyoming, USA
b
a r t i c l e i n f o
abstract
Article history:
Received 10 August 2012
Received in revised form
4 January 2013
Accepted 11 January 2013
Editor: B. Marty
Vanadium exists in multiple valence states under terrestrial conditions (2 þ , 3 þ , 4 þ , 5 þ ) and its isotopic
composition in magmas potentially reflects the oxidation state of their mantle source. We present the
first stable vanadium isotope measurements of 64 samples of well-characterized mantle-derived mafic
and ultramafic rocks from diverse localities. The d51V ranges from " 0.27% to " 1.29%, reported relative
to an Alfa Aesar (AA) vanadium solution standard defined as 0%. This dataset is used to assess the effects
of alteration, examine co-variation with other geochemical characteristics and define a value for the bulk
silicate Earth (BSE). Variably serpentinised peridotites show no resolvable alteration-induced d51V
fractionation. Likewise, altered mafic oceanic crustal rocks have identical d51V to fresh hand-picked
MORB glass. Intense seafloor weathering can result in slightly (# 0.2–0.3%) heavier isotope compositions,
possibly related to late-stage addition of vanadium. The robustness of d51V to common alteration
processes bodes well for its potential application to ancient mafic material. The average d51V of mafic
lavas, including MORB, Icelandic tholeiites and lavas from the Shatsky Rise large igneous province is
" 0.8870.27% 2sd. Peridotites show a large range in primary d51V (" 0.62% to " 1.17%), which covaries positively with vanadium concentrations and indices of fertility such as Al2O3. Although these data
suggest preferential extraction of heavier isotopes during partial melting, the isotope composition of
basalts (d51V¼ " 0.8870.27% 2sd) and MORB glass in particular (d51V¼ " 0.9570.13% 2sd) is lighter
than fertile peridotites and thus difficult to reconcile with a melt extraction scenario. Determination of
fractionation factors between melt and mineral phases such as pyroxenes and garnet are necessary to
fully understand the correlation. We arrive at an estimate of d51VBSE ¼ " 0.770.2% (2sd) for the bulk
silicate Earth by averaging fertile, unmetasomatised peridotites. This provides a benchmark for both high
and low temperature applications addressing planet formation, cosmochemical comparisons of the Earth
and extraterrestrial material, and an inorganic baseline for future biogeochemical investigations. Whilst
d51V could relate to oxidation state and thus oxygen fugacity, further work is required to resolve the
isotopic effects of oxidation state, partial melting, and mineral fractionation factors.
& 2013 Elsevier B.V. All rights reserved.
Keywords:
vanadium isotopes
bulk silicate Earth
high temperature stable isotope
fractionation
1. Introduction
Radiogenic isotopic compositions measured in mantle rocks
and mantle-derived magmas provide compelling evidence for the
n
Corresponding author at: Department of Earth Science and Engineering,
Imperial College London, London, SW7 2AZ, UK. Tel.: þ 44 207 954 6474.
E-mail address: j.prytulak@imperial.ac.uk (J. Prytulak).
0012-821X/$ - see front matter & 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.epsl.2013.01.010
existence of mantle heterogeneity as well as constraints on its
creation and preservation (e.g., Hofmann, 2003; Zindler and Hart,
1986). However, long-lived radiogenic isotope signatures are not
without ambiguity. The variability in isotopic compositions of Sr,
Nd, Pb, and Hf, for example, reflects the time-integrated fractionation of parent from daughter element. The magnitude of this
elemental fractionation, the initial source composition and the
age of the fractionated material all contribute to uncertainty in
interpreting the final isotope composition. Thus, deducing the
178
J. Prytulak et al. / Earth and Planetary Science Letters 365 (2013) 177–189
often multi-stage journey of the mantle and its melts is inherently
non-unique using radiogenic isotopes.
Stable isotopes offer valuable complementary information to
the evidence provided by long-lived radiogenic isotopes; particularly since neither time nor parent–daughter fractionations need
be considered. Instead stable isotope fractionation is driven by
temperature related equilibrium fractionations, relative mass
differences, bond strengths, and kinetic processes (see review by
Schauble, 2004). Of particular interest is the inverse relationship
between temperature and magnitude of isotope fractionation
(Urey, 1947). Since isotope fractionation at high temperatures
was, until recently, considered negligible, stable isotope variability in the mantle or its melts has been attributed to the
recycling and incorporation of surface material. Oxygen and
sulphur isotopes have long been used in this manner to deduce
the presence of sedimentary or hydrothermally altered material
in the source of mantle melts (e.g., Chaussidon et al., 1987; Eiler
et al., 1996). However, oxygen is a major constituent of the
mantle, so large amounts of material are needed to impact oxygen
isotope signatures. Sulphur isotopes are generally measured on
sulphides and can show large isotope fractionations (e.g.,
Chaussidon et al., 1987; Bekker et al., 2009). However, sulphides
are not always present in samples of interest, and the susceptibility of sulphur to volatile, degassing-driven isotope fractionation is a concern in erupted lavas. So-called ‘non-traditional’
stable isotopes have become increasingly prevalent in mantle
studies.
For example, stable isotope systems of elements such as lithium
(e.g., Elliott et al., 2006) and thallium (e.g., Nielsen et al., 2006)
have been employed to trace small contributions of surface
materials into the source of mantle-derived melts.
Advances in multi-collector inductively coupled plasma mass
spectrometry (MC-ICPMS) technology have greatly improved
analytical precision, facilitating the exploration of small variations
in stable isotopes at high temperatures of elements traversing the
entire periodic table (e.g., Halliday et al., 2010). Research has
focused on teasing out the causes of ubiquitous non-traditional
stable isotope fractionation and applying this new information to
geologic questions. This task is aided by resurgence in theoretical
consideration of high temperature stable isotope fractionation
after a long period with little work (e.g., Bigeleisen and Mayer,
1947; Schauble, 2004, Schauble et al., 2001, 2009; Urey, 1947). In
particular, isotope systems of the period four transition metal
elements are being increasingly studied. Multiple oxidation states
of some transition metals and the prediction that changes
between oxidation states can be linked to isotope fractionation
(e.g., Schauble, 2004) means that transition metal stable isotopes
potentially provide key information about, for example, the
physical conditions of a mantle source (e.g., oxygen fugacity)
rather than simply a test for crustal recycling.
Iron isotopes were amongst the first transition metal to have
been investigated for high temperature fractionation (e.g., Zhu
et al., 2000) and thus are perhaps currently the best understood.
High temperature iron isotopic fractionation has been linked to
changing oxygen fugacity (e.g., Dauphas et al., 2009; Williams
et al., 2004, 2005), magmatic differentiation (Hibbert et al., 2012;
Schuessler et al., 2009; Teng et al., 2008; Weyer and Ionov, 2007;
Williams et al., 2004, 2005) and diffusion (Teng et al., 2011;
Weyer and Seitz, 2012). Far fewer data are available for chromium
stable isotopes at high temperatures, with initial results indicating negligible fractionation in major terrestrial reservoirs
(Schoenberg et al., 2008). Hence, it is still unclear if the redox
behaviour of transition metal isotopes provides new, robust
proxies for mantle oxygen fugacity.
We have developed the first method able to measure stable
vanadium isotopes to a precision useful for geologic problems
(Nielsen et al., 2011a; Prytulak et al., 2011). Here we present the
first investigation of vanadium isotopes in mafic and ultramafic
igneous rocks in order to determine the applicability of vanadium
isotopes to mantle processes.
1.1. Vanadium and vanadium isotopes: applications and aims
Vanadium is a moderately incompatible, refractory transition
metal existing in multiple valence states (V2 þ , V3 þ , V4 þ , V5 þ ) at
terrestrial conditions. Many studies have taken advantage of
redox properties of vanadium and the strong relationship
between vanadium partitioning and oxygen fugacity (e.g., Canil,
1997). These include investigation of the oxidation state of the
mantle through time (e.g., Lee et al., 2003; Li and Lee, 2004), the
oxidation state of subduction zones (e.g., Lee et al., 2005), core
formation (e.g., Wood et al., 2008), oceanic anoxia (e.g., Emerson
and Huested, 1991; Tribovillard et al., 2006), hydrocarbon and
crude oil genesis (e.g., Lopez et al., 1995), and nitrogen fixation
(e.g., Bellenger et al., 2008). Whilst useful, elemental studies are
prone to uncertainties such as initial source concentration, degree
of melting and partitioning relationships. Stable isotopes may
provide more straightforward information.
Changes in oxidation state are theoretically predicted, and
experimentally demonstrated to result in fractionation of stable
isotopes (e.g., Schauble et al., 2009; Urey, 1947; Zhu et al., 2000).
Vanadium may be particularly advantageous in this respect given
the number of oxidation states available. Considering the large
array of potential applications to problems in Earth science, it
might be surprising that to date no high precision vanadium
isotope data exist. This lack of vanadium isotope data is due to
two major analytical obstacles. The first is that the 51V/50V of the
only two stable isotopes, 51V (99.76%) and 50V (0.24%), is # 420.
The analytical challenge is compounded by the existence of direct
isobaric interferences from 50Cr and 50Ti on the minor 50V isotope.
The first separation and measurement protocol that overcomes
these difficulties for silicate matrices has been developed (Nielsen
et al., 2011a; Prytulak et al., 2011). This study presents the first
investigation of high temperature fractionation of stable vanadium isotopes in mantle and mantle-derived mafic melts with
three general aims
1) Evaluate the range of natural isotope fractionation in the
mantle and mafic mantle-derived melts
2) Assess the fidelity of the isotope signature to common alteration processes
3) Estimate the stable vanadium isotope signature of the bulk
silicate Earth.
2. Materials
Exploration of new isotope systems is hindered without basic
geochemical context. Therefore bulk rock major elements are the
minimum characterization requirement for the samples in this
study. Trace element and isotopic data are also available in most
cases. Every effort has been made to investigate samples previously studied for other ‘non-traditional’ stable isotope systems
(e.g., Li, Mg, Fe, and Cr). Full major and trace element characterization and GPS locations (where available) for the 64 samples in
this study can be found in the literature, and is also compiled in
the Electronic Appendix. A supplemental figure is included with
the global locations of all samples in this study.
J. Prytulak et al. / Earth and Planetary Science Letters 365 (2013) 177–189
179
2.1. Fresh and altered peridotites
3. Methods
Twenty-four peridotites were chosen from a variety of locations and targeted to include a range of melt depletion and
degrees of alteration. Abyssal peridotites from Ocean Drilling
Program (ODP) Leg 209, Site 1274A in the North Atlantic are
some of the most melt depleted recovered thus far (20–30%, Bach
et al., 2004; Harvey et al., 2006). They are extensively and
systematically altered, with serpentinisation intensity increasing
with depth below the seafloor. Nine dredged abyssal peridotites
from the South West Indian Ridge, American-Antarctic Ridge, and
the Pacific-Antarctic Ridge are presented (Niu, 2004). The dredged
abyssal peridotites have been extensively serpentinised and
subjected to low temperature seafloor weathering.
In an attempt to sample more pristine mantle material, we
measured peridotite xenoliths from several continental localities.
These include spinel and garnet lherzolites from East Africa
(Dawson et al., 1970), Mongolia (Ionov and Hofmann, 2007) and
Siberia (Ionov, 2004; Ionov et al., 1993). The peridotites include
fertile samples previously used to assess the Fe, Li and Mg isotopic
compositions of the bulk silicate Earth (Pogge von Strandmann
et al., 2011; Weyer and Ionov, 2007). Finally, we include analyses
of USGS dunite standard DTS and PCC1. d51V of PCC1 is from
Prytulak et al. (2011).
The vanadium isotope chemical separation and measurement
protocol is fully described in Nielsen et al. (2011a) and Prytulak et al.
(2011). Here we briefly highlight some sample-specific issues.
2.2. Fresh and altered MORB
Six fresh, hand-picked MORB glass samples from the Indian
Ocean (Gannoun et al., 2007) and the Kolbeinsey Ridge (Elkins
et al., 2011) are presented in addition to composite MORB from
ODP Leg 185 Hole 801C in the Pacific Ocean. The composite
samples are physical mixtures of lava from different intervals in
the drill core. They were constructed using visual estimates of
lithologic units, coupled with natural gamma and formation
microscanner logs from shipboard measurements. We present
measurements of MORB composites from different depths in Hole
801C and a ‘SUPER’ composite which was constructed to represent the entire subducting package, including intercalated sediments, at ODP Hole 801C. Detailed discussion of the composite
sampling strategy is given in Plank et al. (2000), and specific core
intervals used for their construction plus major and trace element
data is found in Kelley et al. (2003). Since composites identify
large-scale changes in geochemistry, we complement them with
12 discrete samples from drill sites 801B, C, 1149B, C, and D from
ODP Legs 129 and 185 (Kelley et al., 2003) all outboard of the
Izu–Bonin–Mariana trench on crust ranging from 135 to 170 Ma
in age.
2.3. Other mantle-derived ‘OIB’ mafic lavas
To represent primitive mantle-derived lavas other than MORB,
we include USGS standards BCR2 (Columbia River flood basalt),
BHVO1 and 2 (Hawaiian basalt), BIR1a (Icelandic basalt) that have
previously reported d51V (Prytulak et al., 2011). We augment
these rock standards with mantle-derived material from two
other locations.
The first is a suite of nine historic, mafic, sparsely phyric
( o5%) tholeiitic basalts from the Reykjanes peninsula, Iceland
(Peate et al., 2009). The second suite consists of four wellcharacterized glassy basalts from the Shatsky Rise large igneous
province, drilled by IODP Expedition 324 (Sager et al., 2010).
Four main magma types were recovered on Shatsky: ‘normal’,
low-Ti, high-Nb, and ‘U1349-type’ (Sano et al., 2012). The ‘normal’
lava type is similar to N-MORB in chemical compositions and the
most voluminous on Shatsky, therefore we present two ‘normal’
lavas for comparison with a low-Ti and high-Nb lava.
3.1. Sample digestion
Two types of sample digestion were employed. Peridotites
contain refractory spinel that cannot be dissolved by standard
hotplate techniques. To ensure complete dissolution, peridotites
were dissolved using mini-teflon hexagonal screw-top bombs.
Approximately 70 mg of sample was dissolved in 2:1 mixture of
29 M HF:14 M HNO3 and placed in an oven at 140 1C for 3 days.
Samples were visually assessed for dissolution (spinel appeared
as small black particles). If spinel was visible, then the sample was
bombed again for 4 days in 1:1 10 M HCl þ29 M HF. After this
stage samples always achieved full dissolution.
Standard hotplate methods were sufficient to completely
dissolve mafic lavas. Lava samples were dissolved in a mixture
of 5:1 29 M HF:14 M HNO3 at 160 1C on a hotplate for at least
24 h. They were then evaporated with 14 M HNO3 three times at
160 1C to destroy fluorides formed from the initial dissolution.
3.2. Peridotite considerations
Between 5 and 10 mg of vanadium is required for repeat
measurements. This does not present an issue for the lavas with
V concentrations in excess of 100 mg g " 1. However, depleted
peridotites pose a greater challenge. The chemical separation
procedure is only effective to o100 mg of processed sample.
The low V concentration of peridotites (down to # 10 mg g " 1V)
necessitated several digestions of the same powder to be independently passed through the first three stages of chemical
separation, then re-combined for the final removal of Cr and Ti
(see Nielsen et al., 2011a). Since peridotites contain on the order
of several 1000 mg g " 1 Cr, a Cr cleanup column was repeated
three times to achieve adequate removal of Cr for precise
measurements (see also Prytulak et al., 2011).
3.3. MC-ICPMS
Measurement protocols are fully described in Nielsen et al.
(2011a) and Prytulak et al. (2011), but we briefly recap the salient
details. Measurements were performed on Nu Plasma HR-MCICP-MS (Nu Instruments, Wrexham, UK) instruments at the
University of Oxford and Imperial College London. The Oxford
instrument has a non-standard collector configuration (see
Nielsen et al., 2011a).
The ratio of 51V/50V is #420, therefore V measurements
employ a 109 O resistor on the Faraday cup collecting 51V, with
all other Faraday cups using standard 1011 O resistors. Samples
were measured using a sample-standard bracketing method in
0.1 M HNO3 for one block of 40 ratios. Vanadium isotopes are
reported using standard delta notation:
d51 V ¼ 1000 % ½ð51 V=50 Vsample =51 V=50 VAA Þ"1)
The Alfa Aesar (AA) V standard solution is defined as d51V¼0%
(Nielsen et al., 2011a). A secondary V standard solution from BDH
chemicals is used to evaluate machine performance. The longterm isotope composition of BDH (2009–2012) run at Oxford
University is "1.18 70.17% 2sd (n ¼877) and at Imperial College
London is " 1.19 70.17% 2sd (n¼452). Total procedural blanks
at both Oxford and Imperial were o2 ng, which is negligible
compared to the total amount of V processed (5–25 mg).
180
Table 1
Stable vanadium isotope compositions.
Rock type
V
(lg g " 1)
d51V
2r
Dissolutions
Runs
Sessions
Institution
Sample reference
Peridotites
313-1
313-6
313-102
313-104
313-106
313-112
BD 730
BD 822
314-56
314-58
Mo 101
RC27-9 34-33
PROT 40D-54
PROT 15D-35
PROT 13D-46
PROT 5 38-1
www3 13D-5-2
Vulcan 5 41-29
Vulcan 5 41-55
Vulcan 5 35-3
1274A-5R2-25-35
1274A-11R1-56-65
1274A-27R1-130-140
1274A-5R2-25-35a
1274A-27R1-130-140a
Gnt Lherz
Gnt Lherz
Gnt Lherz
Gnt Lherz
Gnt Lherz
Gnt Lherz
Gnt Lherz
Sp Lherz
Sp Lherz
Sp Lherz
Sp Lherz
Abys. peridotite
Abys. peridotite
Abys. peridotite
Abys. peridotite
Abys. peridotite
Abys. peridotite
Abys. peridotite
Abys. peridotite
Abys. peridotite
Harzburgite
Harzburgite
Harzburgite
Harzburgite
Harzburgite
NA
109
100
88
75
105
41
13
NA
NA
NA
63
55
51
30
46
59
61
59
32
29
26
19
29
19
" 0.72
" 0.62
" 0.58
" 0.83
" 0.75
" 0.70
" 0.99
" 0.78
" 0.77
" 0.81
" 0.64
" 0.84
" 0.27
" 0.69
" 0.66
" 0.78
" 0.80
" 0.74
" 0.44
" 0.84
" 1.17
" 1.17
" 1.08
" 1.32a
" 1.25a
NA
NA
0.07
0.22
0.03
NA
0.20
NA
NA
0.29
0.09
0.14
0.24
0.13
0.24
0.12
0.01
0.15
0.24
0.08
0.07
0.10
0.07
0.14a
0.18a
1
1
1
2
1
1
1
1
1
2
1
1
2
2
2
1
1
1
1
1
1
1
1
1
1
1
1
2
6
2
1
4
1
1
3
3
5
7
8
3
5
2
3
5
5
3
4
3
6
5
1
1
1
2
1
1
1
1
1
1
1
2
4
3
2
2
1
1
2
2
1
1
1
1
1
IC
IC
IC
IC
IC
IC
Oxford
Oxford
IC
IC
IC
IC
Oxford
Oxford
Oxford
Oxford
Oxford
Oxford
Oxford
Oxford
Oxford
Oxford
Oxford
Oxford
Oxford
Ionov et al. (1993)
Ionov et al. (1993)
Ionov (2004)
Ionov (2004)
Ionov (2004)
Ionov (2004)
Dawson et al. (1970)
Dawson et al. (1970)
Ionov et al. (1993)
Ionov et al. (1993)
Ionov and Hofmann (2007)
Niu (2004)
Niu (2004)
Niu (2004)
Niu (2004)
Niu (2004)
Niu (2004)
Niu (2004)
Niu (2004)
Niu (2004)
Harvey et al. (2006)
Harvey et al. (2006)
Harvey et al. (2006)
Harvey et al. (2006)
Harvey et al. (2006)
MORB glass
POS210/1
TR 6D 2g
TR30D 2g
TR 16D 1g
TR 15D 1g
MD57
Basalt
Basalt
Basalt
Basalt
Basalt
Basalt
291
393
291
399
329
316
" 0.97
" 0.92
" 0.96
" 1.04
" 0.84
" 0.99
0.22
0.01
0.05
0.15
0.15
0.09
2
1
1
1
1
1
12
2
3
6
3
6
5
1
3
3
1
3
Oxford and IC
Oxford
Oxford
Oxford
Oxford
Oxford
Elkins et al.
Elkins et al.
Elkins et al.
Elkins et al.
Elkins et al.
Gannoun et
(2011)
(2011)
(2011)
(2011)
(2011)
al. (2007)
251
399
399
338
" 0.90
" 0.97
" 0.97
" 0.89
0.17
0.09
0.20
0.18
1
1
1
1
2
4
5
3
2
2
2
2
Oxford
Oxford
Oxford
Oxford
Kelley
Kelley
Kelley
Kelley
et
et
et
et
al.
al.
al.
al.
(2003)
(2003)
(2003)
(2003)
337
358
64
324
" 0.81
" 0.81
" 1.16
" 0.92
0.04
0.17
NA
0.26
1
2
1
1
3
6
1
6
1
3
1
3
Oxford
Oxford
Oxford
Oxford
Kelley
Kelley
Kelley
Kelley
et
et
et
et
al.
al.
al.
al.
(2003)
(2003)
(2003)
(2003)
MORB composites ODP Hole 801C
MORB 0-110
Composite
MORB 110-220
Composite
MORB 220-420 FLO
Composite
801 SUPER
Composite
Discrete altered oceanic crust samples
1149B 30R2 56-62
Alt. basalt þcc vein and halo
1149C 10R2 47-51
Basalt
1149D 7R1 37-41
Hyaloclastite
1149D 9R3 30-32
Alt. basalt þhalo
and
and
and
and
IC
IC
IC
IC
and IC
and IC
J. Prytulak et al. / Earth and Planetary Science Letters 365 (2013) 177–189
Sample
Breccia of basaltþ cc cement
Basalt
Basaltþ cc veins
Basalt
Basalt
Alt. basalt
Basaltþ FeOx vein and halo
Basaltþ cc veins and haloes
194
307
235
415
173
433
442
410
" 0.78
" 0.71
" 0.93
" 0.83
" 0.65
" 1.04
" 0.93
" 0.82
0.10
0.09
0.10
0.06
NA
0.17
0.23
0.20
1
1
1
1
1
1
1
1
2
2
3
2
1
3
4
3
2
1
2
1
1
1
3
3
Oxford
Oxford
Oxford
Oxford
Oxford
Oxford
Oxford
Oxford
Kelley
Kelley
Kelley
Kelley
Kelley
Kelley
Kelley
Kelley
Recent Icelandic tholeiites
408673
4567 14
4567 22
4567 32
4567 36
4567 40
4567 43
4567 45
4567 49
Tholeiitic
Tholeiitic
Tholeiitic
Tholeiitic
Tholeiitic
Tholeiitic
Tholeiitic
Tholeiitic
Tholeiitic
basalt
basalt
basalt
basalt
basalt
basalt
basalt
basalt
basalt
467
374
423
339
342
359
351
376
279
" 0.94
" 0.82
" 0.75
" 0.88
" 0.80
" 0.85
" 0.82
" 1.00
" 0.93
0.06
0.12
0.26
0.14
0.21
0.10
0.27
0.07
NA
1
1
1
2
1
1
1
1
1
3
4
4
8
5
5
4
3
1
2
1
1
1
1
2
3
2
1
Oxford
Oxford
Oxford
Oxford
Oxford
Oxford
Oxford
Oxford
Oxford
Peate
Peate
Peate
Peate
Peate
Peate
Peate
Peate
Peate
Shatsky rise basalts IODP Exp. 324
U1347A-17R-2-4/8
Low Ti0 basalt
U1350A-17R-2-126/129
High Nb0 basalt
U1350A-22R-2-122/124
‘Normal’ basalt
U1350A-24R-2-110/113
‘Normal’ basalt
428
357
289
274
" 0.69
" 1.29
" 0.66
" 0.70
0.02
0.31
0.09
0.23
1
1
1
1
3
3
3
3
1
1
1
1
IC
IC
IC
IC
Sano
Sano
Sano
Sano
et
et
et
et
USGS basalts and peridotites
BIR1a
BHVO1
BHVO2
BCR2
DTS
PCC1
PCC1a
310
317
317
416
11
31
31
" 0.92
" 0.92
" 0.88
" 0.92
" 0.95
" 1.01
" 1.29a
0.16
0.04
0.10
0.16
NA
0.09
0.21
12
1
7
13
1
2
5
50
4
14
27
1
8
10
8
1
4
7
1
3
3
Oxford
Oxford
Oxford
Oxford
Oxford
Oxford
Oxford
USGS
USGS
USGS
USGS
USGS
USGS
USGS
website
website
website
website
website
website
website
Icelandic basalt
Hawaiian basalt
Hawaiian basalt
CR flood basalt
Dunite
Dunite
Dunite
et
et
et
et
et
et
et
et
et
et
et
et
et
et
et
et
et
al.
al.
al.
al.
al.
al.
al.
al.
al.
al.
al.
al.
al.
al.
al.
al.
al.
al.
al.
al.
al.
(2003)
(2003)
(2003)
(2003)
(2003)
(2003)
(2003)
(2003)
(2009)
(2009)
(2009)
(2009)
(2009)
(2009)
(2009)
(2009)
(2009)
(2012)
(2012)
(2012)
(2012)
‘Dissolutions’ refers to the number of complete repeat digestions and chemical separations of a sample; ‘Runs’ refers to the number of individual sample measurements made for that sample; and ‘Sessions’ refers to the number
of separate mass spectrometry sessions (often separated by weeks), that the same sample was run in. Note: 2s of sample is the internal error if only one dissolution was performed. The 2s of samples with multiple dissolutions is
the external reproducibility. NA¼ data not available, or the sample was only run once and an error of 7 0.17% is used for figures. IC¼ Imperial College London.
a
Samples with residual spinel.
J. Prytulak et al. / Earth and Planetary Science Letters 365 (2013) 177–189
1149D 11R2 86-92
1149D 16R3 2-8
1149D 17R1 92-98
1149D 19R1 85-89
801B 43R1 132-135
801C 15R7 31-34
801C 34R1 93-96
801C 44R3 23-26
181
182
J. Prytulak et al. / Earth and Planetary Science Letters 365 (2013) 177–189
4. Results
20
40
60
mbsf
The range of d51V for the 64 samples is " 0.27% to 1.29%
(Table 1). We consider the long-term reproducibility of our BDH V
solution standard (0.17%) to be the best currently achievable
measurement precision. Therefore, in all subsequent figures,
samples with 2sdo0.17%, or those that could only be run once,
employ error bars of 0.17%, whilst the actual sample 2sd is used
when 40.17%.
Peridotites display the largest isotopic variation, with d51V
from " 0.27% to "1.17% (n ¼23). In contrast, fresh MORB glass
has a more restricted isotopic range of " 0.84% to " 1.04%
(n¼6). Composite samples from altered oceanic crust (AOC) at
ODP Hole 801C are similarly restricted with d51V of " 0.89% to
"0.97% (n ¼4). Discrete AOC samples from ODP Hole 801C do
not differ significantly from MORB glasses and AOC composites
with a range of " 0.65% to "1.16% (n ¼12). The average of all
fresh MORB and AOC composites is " 0.9570.11% 2sd (n¼9).
Other mafic lavas include a suite of tholeiites from the
Rekyjanes peninsula with d51V¼ "0.75 to " 1.00 (n¼8) and an
average of "0.8670.15% 2sd, also similar to MORB. The four
basalts from the Shatsky Rise show more variation, with a range
of d51V from " 0.66% to " 1.29%.
δ51V
-1.5 -1.4 -1.3 -1.2 -1.1 -1.0 -0.9 -0.8 -0.7
0
80
100
120
140
160
Fig. 1. Stable vanadium isotope composition of abyssal peridotites from ODP Hole
1274A (Harvey et al., 2006) versus depth in metres below sea floor (mbsf).
5. Discussion
We first discuss the effects of common alteration processes on
d51V of peridotites and mafic rocks. Secondly, we assess the
impact of partial melting, mineral modes and differentiation.
Lastly, we evaluate the d51V composition of the bulk silicate Earth.
5.1. Alteration of primary d51V signatures
It is desirable to determine how robust d51V is to common
secondary processes such as hydrothermal alteration, serpentinisation and seafloor weathering. Three suites were chosen for
investigation, including (1) systematically serpentinised abyssal
peridotites (Bach et al., 2004; Harvey et al., 2006), (2) abyssal
peridotites that have experienced extensive seafloor weathering
(Niu, 2004), and (3) composite and discrete AOC from ODP Leg
129 and 185 (Kelley et al., 2003).
5.1.1. Serpentinisation of ODP Leg 209 abyssal peridotites
Serpentinisation most drastically affects olivine. Vanadium
does not strongly partition into olivine and therefore we predict
that d51V remains robust to serpentinisation. Furthermore,
serpentinisation is largely isochemical (with the exception of
water, Komor et al., 1985), and thus the opportunity for isotopic
fractionation is limited. To distinguish the effects of serpentinisation versus seafloor weathering (Section 5.1.2) we use drill core
samples without prolonged exposure to seawater. We measured
three abyssal harzburgites from ODP Leg 209, which display
variable degrees of serpentinisation from # 60% to 100% (Bach
et al., 2004; Harvey et al., 2006). We find no d51V fractionation
associated with the degree of serpentinisation (Fig. 1).
5.1.2. Seafloor weathering of dredged abyssal peridotites
‘Seafloor weathering’ can significantly affect the primary
chemical composition of abyssal peridotites. It occurs at temperatures o150 1C, and is typified by Mg loss and alkali element
enrichment, although the two are not necessarily directly linked
(Snow and Dick, 1995). As with serpentinisation, V abundances
are largely immune to seafloor weathering. Such a process may
result in slight V enrichment, possibly explained by artificial
inflation due to Mg loss (Snow and Dick, 1995).
To test the effects of seafloor weathering on d51V, we measured nine extensively altered dredged abyssal peridotites (Niu,
2004). These samples are from various locations in the Pacific and
Indian oceans and therefore can inform on general processes, but
cannot be used to evaluate the extent of alteration in individual
localities. Additionally, the size of the samples is small (1–2 cm),
which amplifies variation due to mineralogical heterogeneity.
Despite these caveats, vanadium and scandium abundances in
the sample set of Niu (2004), which includes # 130 abyssal
peridotites, retain systematic co-variation with MgO, which may
be indicative of primary melt extraction trends (Niu, 2004).
The stable vanadium isotope composition of nine dredged peridotites from Niu (2004) span the largest range within a single suite
("0.29% to " 0.84%). There is no correlation of d51V with Al2O3 or
TiO2 contents that may suggest melt extraction (Fig. 2a and b). If
anything, it appears that the most depleted (i.e. lowest TiO2 and
Al2O3) samples are the most variable, which could be consistent with
refertilisation of a V-depleted peridotite. Furthermore, the rocks with
the highest V contents display the largest range in d51V (Fig. 2c).
However, this relationship could also be a facet of different melt
extraction histories, initial compositions and late stage percolation in
melting residues. Slightly more informative is the diffuse positive
relationship of d51V with Sr and Ba (Fig. 2d and e), elements that are
enriched by seafloor weathering. Therefore, although the effect
remains ambiguous, it appears that extreme seafloor weathering
may drive d51V slightly heavier by 0.2–0.3%. We consider the
heaviest d51V for abyssal peridotite (d51V¼ " 0.2970.24%), which
also displays the highest Ba and Sr contents and the lowest TiO2 and
Al2O3 contents, unlikely to be primary. However, a detailed study
requires a co-genetic suite where the effects of source composition,
melt extraction and post-melting processes can be independently
assessed.
5.1.3. d51VMORB and low temperature alteration of mafic oceanic
crust
Understanding chemical changes occurring in the mafic oceanic crust during hydrothermal alteration is critical to evaluating
chemical fluxes to the ocean and the budgets of elements that are
recycled into the deeper mantle (see review by Staudigel, 2003).
183
J. Prytulak et al. / Earth and Planetary Science Letters 365 (2013) 177–189
δ51V
0
0
TiO2 (wt%)
-0.2
-0.2
-0.4
-0.4
-0.6
-0.6
-0.8
-0.8
-1.0
-1.0
-1.2
0
0.02
0.04
0.06
0.08
0.10
0.12
0.14
Al2O3 (wt%)
-0.2
1
2
3
4
5
6
7
-0.4
-0.6
-0.6
-0.8
-0.8
-1.0
-1.0
0.7
0.9
1.1
1.3
1.5
Ba (µg/g)
-0.2
-0.4
-1.2
0.5
0
0
0
δ51V
-1.2
Sr (µg/g)
1.7
1.9
2.1
2.3
60
65
-1.2
0
500
1000
1500
2000
2500
3000
0
V (µg/g)
-0.2
δ51V
-0.4
-0.6
-0.8
-1.0
-1.2
25
30
35
40
45
50
55
Fig. 2. Stable vanadium isotope composition of dredged abyssal peridotites from Niu (2004) versus (a) TiO2, (b) Al2O3, (c) V, (d) Sr, and (e) Ba.
Furthermore, ancient mafic crust likely underwent similar processes, and therefore we require knowledge of how faithfully d51V
retains its initial signature in order to apply the technique to
ancient samples. To this end, we compare d51V in fresh MORB
glass with composites and discrete samples of altered oceanic
crust (Table 1). As with serpentinisation and seafloor weathering,
there is little evidence for elemental V addition or removal during
low temperature hydrothermal alteration (e.g., Kelley et al.,
2003). Vanadium is removed from seawater by particulate
scavenging in high temperature hydrothermal plumes above
mid-ocean vents (e.g., Elderfield and Schultz, 1996). However,
there is no evidence that seawater alteration introduces vanadium into the ocean crust.
Fig. 3 compares fresh MORB glass from the Indian Ocean and
Kolbeinsey Ridge with altered oceanic crust (AOC) composites
from ODP Hole 801C in the Pacific Ocean including a ‘SUPER’
composite representative of the entire subducting package of
sediments and mafic crust (Kelley et al., 2003). There is remarkably little variation in the basalts, with an average d51V of
" 0.9570.11% 2sd (n¼9) spanning vanadium contents of
# 250–400 mg g " 1.
Analyses of the composites provide averaged chemical compositions over #100 m of drill core. Therefore it is also important
to investigate the d51V of discrete intervals from ODP Holes 801B,
C and Hole 1149B, C, and D. These samples are advantageous in
that some have been studied for Fe isotopes (Rouxel et al., 2003)
and allow initial comparison of d51V with another redox sensitive
transition metal stable isotope system. Kelley et al. (2003)
evaluated chemical enrichments and depletions in drill samples
from Legs 129 and 185. The most significant change was a ninefold enrichment in rubidium (Rb). The lightest d51V value
of " 1.16% is found in a hyaloclastite with significant Rb
184
J. Prytulak et al. / Earth and Planetary Science Letters 365 (2013) 177–189
-0.5
-0.6
5.2. Fractionation during magma generation and evolution
δ51VMORB = -0.95 ± 0.11 ‰ 2sd (n=9)
-0.7
δ51V
-0.8
-0.9
-1.0
-1.1
-1.2
-1.3
200
250
300
350
400
450
V (µg/g)
Fig. 3. Stable vanadium isotope composition of fresh and altered MORB versus
vanadium content. Fresh MORB: green circles; altered MORB composites: brown
circles; SUPER composite (see text): red circle. (For interpretation of the references
to colour in this figure legend, the reader is referred to the web version of this
article.)
enrichment (51 mg g " 1 versus an average of 12 mg g " 1 in the
other discrete samples) and notable V depletion compared to the
other discrete samples (64 mg g " 1 at 8.9 wt% MgO). This sample
clearly invites a more focused study on the extent of high and low
temperature hydrothermally induced V isotope fractionation.
However, even with the inclusion of the hyaloclastite, there is
little overall variation in the discrete AOC suite (Table 1), despite
their differing extents and types of alteration (see Kelley et al.,
2003). The discrete AOC average d51V (excluding the hyaloclastite) is " 0.8470.22% 2sd, overlapping with the range seen in
MORB glass and AOC composites. Thus d51V appears generally
insensitive to common low temperature alteration processes
occurring in the mafic oceanic crust.
Iron isotopes have been touted as a potential tool to examine
past oxidation conditions in igneous materials (e.g., Dauphas
et al., 2009, 2010). It is therefore useful to make some initial
comparisons of the d51V homogeneity with the range of Fe
isotope values documented in the same drill cores. Rouxel et al.
(2003) found a large total range in d57Fe outside their analytical
error of 0.2%. Positive values up to þ2.05% were found in altered
basalts, but only when a significant fraction of Fe had been
leached. Conversely, d57Fe values reaching " 2.49% were documented in hydrothermal deposits, suggesting that Fe isotopes are
susceptible to alteration at high temperature and in samples with
notable Fe-loss. Williams et al. (2009) presented d57Fe compositions of altered dikes from ODP Hole 504B ranging from
0.1070.07% to 0.3370.13%, which do not display the extreme
variation seen by Rouxel et al. (2003), likely because these
samples did not experience significant Fe-loss. The DSDP Hole
504B dikes extend to slightly heavier values than the MORB
average (d57Fe ¼0.1470.06%; Weyer and Ionov, 2007), which
supports the notion that ocean crust alteration can result in
heavier Fe isotope compositions. This is consistent with experimental work indicating that mineral dissolution preferentially
releases light Fe (Brantley et al., 2004; Rouxel et al., 2003;
Wiederhold et al., 2006). Furthermore, a recent study of ancient
komatiites concluded that most of the Fe isotope fractionation
observed was due to alteration (Dauphas et al., 2010) and caution
should therefore be used when interpreting Fe isotope compositions of ancient materials. At first glance, it appears that d51V may
be more robust to alteration versus Fe isotopes. However, a direct
comparison requires d51V measurement of both low and high
temperature alteration products and secondary minerals.
There is conflicting evidence for the existence of redoxsensitive transition metal isotope fractionation due to partial
melting and magmatic differentiation. It is well documented that
heavy Fe isotopes preferentially enter the melt phase and that
melt evolution drives Fe isotope compositions to still heavier
values (e.g., Dauphas et al., 2009; Hibbert et al., 2012; Schoenberg
and von Blanckenberg, 2006; Schuessler et al., 2009; Teng et al.,
2008; Weyer et al., 2005; Weyer and Ionov, 2007; Williams et al.,
2005, 2009). In contrast, there is thus far a lack of resolvable Cr
stable isotope fractionation between peridotite and basalt
(Schoenberg et al., 2008), despite the change in oxidation state
from Cr3 þ in the mantle to Cr2 þ in basaltic melts (e.g., Berry et al.,
2006). We are not aware of any systematic investigation of the
effect of magmatic differentiation on stable Cr isotopes.
We use two sample suites to evaluate the effects of partial
melting and differentiation on vanadium isotopes. (1) We examine the peridotitic residues of melting that have experienced
a range of melt depletion from fertile compositions to # 30%
depletion. (2) We investigate mafic mantle melts including
tholeiites from the Reykjanes peninsula, Iceland, basaltic lavas
from the Shatsky Rise, Pacific Ocean, and previously published
USGS standards BCR2, BIR1a, and BHVO1, 2.
5.2.1. Vanadium isotope composition of peridotites
The peridotites in this study include some of the most meltdepleted material recovered by ocean drilling programs ( # 20–
30%, Bach et al., 2004; Harvey et al., 2006) to fertile continental
xenoliths (Ionov, 2004; Ionov and Hofmann, 2007; Ionov et al.,
1993, 2005) similar in chemical composition to primitive mantle
(e.g., McDonough and Sun, 1995; Palme and O’Neill, 2003). The
challenge in working with peridotites from oceanic settings is
disentangling secondary effects from the consequences of melt
extraction. However, we have demonstrated that d51V is generally
immune to common alteration processes (Section 5.1). The
vanadium contents of our peridotites span an order of magnitude
from 11 to 109 mg g " 1. A conventional way to express fertility is
to use co-variation plots of Al2O3 content as a proxy for melt
depletion because aluminium is relatively immobile during seafloor weathering (Snow and Dick, 1995) and is insoluble in
seawater (Janecky and Seyfried, 1986).
The moderately incompatible behaviour of vanadium is indicated by the significant inverse V–MgO correlation in abyssal
peridotites (Niu, 2004) and is confirmed by the positive correlation of V and Al2O3 (Fig. 4) as well as correlation of V with
different melt extraction indices for residual peridotites in the
literature (e.g. Ionov et al., 2006; Lee et al., 2003). It must be kept
in mind that our samples span global locations and are from
different tectonic settings, which accounts for some scatter.
Vanadium isotopes show a reasonable positive correlation with
V content (Fig. 5a). Our evaluation of seafloor weathering on
highly altered dredged abyssal peridotites (Section 5.1.2) suggested that the heaviest sample (PROT 40D-54) was likely
affected by alteration. The next heaviest dredged peridotite
(Vulcan 5 41-55) also falls off the correlation between V content
and d51V, and we suggest it has experienced secondary modification and do not consider it further. With the exclusion of the two
heaviest dredged abyssal peridotites, there is a positive correlation between d51V and Al2O3 content (Fig. 5b) yielding an r2 of
0.4 for V and 0.5 for Al2O3 which, given the 420 measurements,
equates to less than 8% and 2%, respectively, probability that d51V,
V and Al2O3 are uncorrelated. If the positive correlation between
d51V and Al2O3 is taken as a melt extraction trend, it suggests that,
like Fe, heavy vanadium preferentially enters the melt and
185
J. Prytulak et al. / Earth and Planetary Science Letters 365 (2013) 177–189
120
100
V (!g/g)
80
60
gnt. lherz.
40
spl. lherz.
abys. perid.
20
ODP 209 perid.
dunite
0
0.0
0.5
1.0
1.5
2.0 2.5 3.0
Al2O3 (wt%)
3.5
4.0
4.5
5.0
Fig. 4. Variation of Al2O3 wt% and V mg g " 1 for peridotites from this study. The
dashed lines are representative of primitive mantle V and Al2O3 contents
(McDonough and Sun, 1995; Palme and O’Neill, 2003). (The reader is referred to
the web version of this article for coloured symbols.)
0
V (!g/g)
-0.2
-0.4
"51V
-0.6
-0.8
-1.0
-1.2
-1.4
-1.6
y = 0.0036x - 1.0298; r2 = 0.42
0
20
40
60
80
100
120
0
Al2O3 (wt%)
-0.2
-0.4
"51V
-0.6
difficult to reconcile with current models of the composition of
the MORB sources in the upper mantle (e.g., Salters and Stracke,
2004). Therefore it seems unlikely that partial melting can explain
all the observed d51V variation in unaltered peridotites.
Alternatively, the d51V variation might be explained by equilibrium inter-mineral and mineral–melt isotope fractionation. The
major host phases for vanadium in fertile peridotites are, in order
of importance, clinopyroxene (cpx), orthopyroxene (opx) and
garnet (e.g., Ionov, 2004). There is negligible V in olivine ( # 1–
4 ppm) and it can thus be ignored. Likewise, limited data suggests
that vanadium does not strongly partition into sulphides (e.g.,
Gaetani and Grove, 1997). Vanadium abundance in spinel will be
dependent on composition, but are unlikely to have higher
concentrations than pyroxene and garnet. Considering the low
modal abundance of spinel compared to pyroxene, it follows that
V is largely hosted in pyroxenes in both garnet and spinel
lherzolites.
Table 2 illustrates the range of whole rock (WR) V contents
that can be achieved using the highest and lowest modal
abundances and V concentrations from fertile peridotites.
In particular, spinel lherzolite concentrations can be reproduced
without considering the V budget of spinel. If the relationship
between d51V and Al2O3 (Fig. 5b) is due to equilibrium intermineral fractionations, which may affect residue/melt partition
coefficient during partial melting, then it requires large isotopic
differences between mineral phases. For example, the modal
abundance of cpx versus opx changes with melt extraction.
However, our dataset provides no evidence for significant d51V
differences between residual peridotites within a broad cpx/opx
range, and hence suggests limited d51V fractionation between opx
and cpx (Fig. 6). There is also little evidence suggesting a role for
d51V fractionation due to cpx composition since V concentrations
are similar, for example, in coexisting high and low sodium cpx
(Ionov et al., 2006).
Equilibrium inter-mineral fractionations are theoretically driven by differences in bonding environment. In general, the
stronger bonds formed in lower coordination environments
favour isotopically heavy compositions (e.g., Bigeleisen and
Mayer, 1947; Schauble et al., 2001). For example, recent study
on stable Mg isotopes by Li et al. (2011) use coordination number
to explain isotopically light Mg isotopes in garnet (8) versus
pyroxene (6). However, this is unlikely to be the case with respect
to vanadium in garnet versus pyroxene. Vanadium is octahedrally
coordinated in garnet, entering the B site of the general formula
A3B2Si3O12. There even exists a vanadium end-member of calcium
-0.8
Table 2
Mineral hosts of vanadium in fertile lherzolites.
-1.0
-1.2
Mineral
-1.4
-1.6
Highest
y = 0.0809x - 1.0125; r2 = 0.52
0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
Modal abundance
5.0
"1
Fig. 5. Stable vanadium isotope composition of peridotites versus (a) V mg g and
(b) Al2O3 wt%. Two circled abyssal peridotites are likely altered (see text) and not
included in the linear regression (dashed line). (Symbols as in Fig. 4).
residual peridotites become progressively lighter with increasing
extent of melt depletion. However, MORB glass is significantly
lighter than fertile peridotites (d51VMORB ¼ " 0.9570.11% 2sd,
Fig. 5). A t-test of the two groups of measurements (MORB glass
and fertile peridotites) indicates that there is only a 0.007%
chance that the two groups are from the same population. If the
trends in Fig. 5 are due to melt extraction, this would imply that
MORB is derived from a largely harzburgitic source region. This is
Lowest
V
(mg/g " 1)
Highest
V
(mg/g " 1)
Lowest
Garnet lherzolite
Opx
0.212
Cpx
0.158
Ol
0.633
Garnet
0.134
WR calculated, V (mg g " 1)
0.121
0.111
0.556
0.086
113
350
4
104
82
113
330
1
97
52
Spinel lherzolite
Opx
0.311
Cpx
0.188
Ol
0.777
Spinel
0.022
WR calculated, V (mg g " 1)
0.17
0.016
0.581
0.007
111
281
4
111
225
1
90
24
Modal abundances and concentrations from Ionov (2004), except olivine V
concentration from Ionov et al. (2006).
WR¼ whole rock.
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J. Prytulak et al. / Earth and Planetary Science Letters 365 (2013) 177–189
-0.2
δ51V
-0.4
-0.6
-0.8
-1.0
-1.2
0
0.2
0.4
0.6
0.8
Modal cpx/opx
1
1.2
Fig. 6. Stable vanadium isotope composition versus modal ratio of cpx/opx in
peridotites. Two altered abyssal peridotites (see text) are circled.
garnet, called Goldmandite, that can contain 44–5 wt% V2O3
(e.g., Deer et al., 1966; Moench and Meyrowitz, 1964). Vanadium
is also octahedrally coordinated in both clino- and orthopyroxene,
entering the M1 site in the general formula M2M1Si2O6 and some
varieties of aegirine–augites can contain 43 wt% V2O3 where
V3 þ substitutes for Fe3 þ (e.g., Deer et al., 1966).
Whilst we cannot further evaluate the role of pyroxenes and
garnet with the current whole rock dataset, we can perform a
preliminary evaluation of spinel mineral/melt fractionation. Spinel structures are of interest as vanadium and can be either
octahedrally or tetrahedrally coordinated and exist dominantly as
V4 þ and V5 þ (e.g., Toplis and Corgne, 2002). Prytulak et al. (2011)
documented evidence for spinel mineral/melt fractionation by
employing two different dissolution methods for USGS dunite
standard PCC1, finding that samples with residual spinel were
isotopically lighter than fully dissolved samples. We purposefully
tested this phenomenon by hotplate and bomb dissolutions
(Section 3.1) for two extremely depleted and serpentinised
harzburgites (Table 1). Lighter d51V in samples with residual
spinel was again observed, suggesting that spinel is isotopically
heavy in these samples. This is in general agreement with the
prediction that heavier signatures occur in lower coordination
environments. With only analyses from very depleted material
(i.e. mostly chromite), we cannot evaluate if there is any isotopic
difference linked to spinel composition.
Clearly, pyroxene, garnet and, to a lesser extent, spinel
mineral/melt fractionation factors are needed to understand the
d51V signature of peridotites. The analyses of these mineral
phases is analytically tractable, but beyond the scope of
this paper.
Determination of d51V during evolution to more felsic compositions is beyond the scope of the study. Vanadium correlates with
MgO (12.3–6.5 wt%, not shown) in the Rekyjanes tholeiites,
however, d51V shows no corresponding correlation (Fig. 7a)
indicating that differentiation at high MgO does not impact the
d51V of mafic melts.
Glassy basalts from the Shatsky Rise large igneous province in
the Pacific Ocean may have a more geochemically enriched source
than MORB. Three of the lavas group tightly together to heavier
isotope compositions ( " 0.6870.04%) than MORB (Fig. 7a, b).
However, the remaining Shatsky basalt is the lightest sample in
the study at d51V¼ "1.29%, albeit with a large error of 0.31%
2sd. This lava type was volumetrically minor in the recovered
material and is characterized by significant enrichment in incompatible elements, niobium (Nb) in particular (Sano et al., 2012).
It is difficult to assess the importance of this isotope signature
without further data from similarly enriched lavas. Overall, with
the exception of the high-Nb basalt from the Shatsky Rise, no
resolvable d51V isotope fractionation is seen within all the ‘OIB’
mafic melts presented in this study (Fig. 7a, b) and they yield an
average of d51V0 OIB ¼ " 0.8770.29% 2sd (n¼17).
A few observations on the general d51V homogeneity in
terrestrial basaltic lavas are worth summarizing. (1) There is no
d51V difference between MORB from ridge systems in the Pacific,
Indian and northern Atlantic ocean basins. (2) The admittedly
limited dataset of d51V0 OIB overlaps with MORB, with some
Shatsky Rise basalts displaying slightly heavier values. (3) There
is no d51V fractionation associated with magmatic differentiation
at high ( 45 wt%) MgO contents.
-0.3
-0.5
-0.7
δ51V
0
-0.9
-1.1
-1.3
USGS Stnds
Iceland
-1.5
-1.7
Shatsky Rise
3
4
5
6
7
8
9
10
11
12
13
MgO (wt%)
-0.3
-0.5
-0.7
δ51V
5.2.2. Vanadium isotope composition of ‘OIB’ lavas
We now compare the d51VMORB with mafic material derived
from more geochemically enriched sources. These include a suite
of tholeiites from Iceland, glassy basalts from the Shatsky Rise
large igneous province, and previously published USGS standards
BCR2 (Columbia River Basalt), BIR1a (Iceland), and BHVO1 and 2
(Hawaii) from Prytulak et al. (2011).
We first assess the effects of moderate degrees of magmatic
differentiation with tholeiites from the Reykjanes peninsula, Iceland. We emphasize that, although the Reykjanes samples are not
strictly co-genetic, they are contemporaneous, sparsely phryic
( o5%), and have limited variation in radiogenic Sr and Nd
isotopes (Peate et al., 2009). We focus on high MgO lavas in
keeping with our goal of evaluating d51V in the bulk silicate Earth.
-0.9
-1.1
-1.3
-1.5
-1.7
250
300
350
400
450
500
V (µg/g)
Fig. 7. Stable vanadium isotopes of ‘OIB’ lavas versus (a) MgO wt% and (b) V
mg g " 1. Grey box is the range for d51VMORB.
J. Prytulak et al. / Earth and Planetary Science Letters 365 (2013) 177–189
5.3. d51V in terrestrial reservoirs and the bulk silicate Earth
A ‘Bulk Silicate Earth’ (BSE) value is frequently sought to
characterize stable isotope systems (e.g., Pogge von Strandmann
et al., 2011; Savage et al., 2010; Schoenberg et al., 2008; Teng
et al., 2010; Weyer et al., 2005). The motivation is to pinpoint an
‘Earth’ value that can be used to compare with extra-terrestrial
material, provide information on planetary processes, and define
a baseline for future terrestrial organic and inorganic studies.
We have documented a restricted range of d51V in basaltic
melts and systematic variation in melting residues. Average
values for terrestrial reservoirs and the meteoritic range of
vanadium isotopes (Nielsen et al., 2011b; Prytulak et al., 2011)
are compared in Fig. 8. It should be noted that we do not consider
the continental crust. Simple mass balance calculations performed using a V content of 138 mg g " 1 in the bulk crust
(Rudnick and Gao, 2003) with a mass of 2.25 % 1025 g compared
to 82 mg g " 1 V (McDonough and Sun, 1995) in the primitive
mantle with a mass of 4.00 % 1027 g indicate that less than 1% of
the Earth’s vanadium is housed in the bulk continental crust and
therefore should not significantly impact d51VBSE. Vanadium is
present in the Earth’s core, but it is not relevant for bulk silicate
Earth calculations.
We employ two approaches to determine d51VBSE. (1) Use the
correlation of d51V with V and Al2O3, extrapolating the d51V to
that corresponding to primitive mantle V and Al2O3 values.
(2) Use a fertile peridotite average (e.g., Pogge von Strandmann
et al., 2011). Vanadium in the primitive mantle is estimated at
#82–86 mg g " 1 using a combination of data from peridotites and
komatiites in addition to correlative relationships between MgO,
Ca, Sc, Yb and V (McDonough and Frey, 1989; McDonough and
Sun, 1995; Palme and O’Neill, 2003). Using the relationship in
Fig. 5a, and V¼84 mg g " 1 we extrapolate to d51VBSE ¼ " 0.7%.
Robust estimates for Al2O3 in the primitive mantle are 4.457
0.44 wt%, (McDonough and Sun, 1995), and 4.49 70.37 wt%
Fertile Peridotites
(n=8)
Meteorites
‘OIB’ basalts
(n-17)
Depleted Peridotites
(n=16)
MORB and AOC
(n=9)
-1.9 -1.7 -1.5 -1.3 -1.1 -0.9 -0.7 -0.5 -0.3 -0.1
δ51V
Fig. 8. Stable vanadium isotope composition of terrestrial reservoirs. The grey box
is the range measured in meteorites (d51V¼ " 1.6% to " 1.9%; Nielsen et al.,
2011b; Prytulak et al., 2011).
187
(Palme and O’Neill, 2003). Extrapolating to a primitive mantle
Al2O3 content of 4.47 wt% also yields d51VBSE ¼ " 0.7%. Finally, if
we consider peridotite samples with V475 mg g " 1 and Z3.5 wt%
Al2O3 as ‘fertile’ and average these eight samples (Table 1), we
obtain d51VBSE ¼ "0.7 70.2% 2sd. Given the agreement between
the fertile peridotite average and the extrapolated values from
both Al2O3 and V, we suggest that d51VBSE ¼ " 0.7 70.2% 2sd is
the current best estimate for the bulk silicate Earth.
6. Summary and outlook
We have presented the first precise stable vanadium isotope
measurements of an extensive set of mantle peridotites and
mantle-derived mafic rocks, allowing a glimpse into the magnitude of natural isotope fractionation produced at high temperatures and resulting from common alteration processes. Alteration
of peridotites and basalts does not appear to induce large
vanadium isotope fractionations, although systematic studies of
hydrothermal deposits and secondary minerals are still needed.
Within the dataset, MORB from different ocean basins show no
resolvable vanadium isotopic fractionations, nor are there significant differences between our limited ‘OIB’ and MORB datasets.
However, further investigation of OIB with varying degrees of
geochemical enrichment is needed to substantiate these initial
observations and explore the hints of isotope variation displayed
by, for example, the Shatsky Rise lavas.
The average d51VMORB ( " 0.9570.11% 2sd) and d51V0 OIB
( " 0.8770.29% 2sd) overlap with depleted peridotites (Fig. 8).
However, residual peridotites that have greater extents of melt
depletion display progressively lighter isotope compositions.
Given that d51VMORB is offset to lighter values than fertile
peridotites, it is difficult to reconcile the trend with fractionation
during partial melting. The investigation of mineral/melt fractionation factors is critical to fully understand the isotope signatures
of bulk peridotities.
We use the average of eight fertile unmetasomatized peridotites with Z3.5 wt% Al2O3 and 475 mg g " 1 V as an estimate of
the vanadium isotope composition of the bulk silicate Earth
(d51VBSE ¼ "0.7 70.2%). The d51VBSE value emphasizes and confirms the significant isotope difference between terrestrial and
extra-terrestrial material (Nielsen et al., 2011b; Prytulak et al.,
2011, Fig. 8).
The apparent robustness of vanadium isotopes to common
alteration processes makes it a potentially powerful tool for the
study of ancient mantle and mantle-derived melts. A remaining
fundamental question, however, is how vanadium isotope fractionation is related to oxidation state and thus the oxygen
fugacity of the system. Investigation of subduction-related lavas
and comparison with the tightly defined d51VMORB should yield
some insight. Given the resolvable isotope fractionation observed
at high temperatures, significant potential exists to use stable
vanadium isotopes in low temperature environments where
isotope fractionation is expected to be even larger.
Acknowledgements
This research used samples provided by the Ocean Drilling
Program (ODP) and the Integrated Ocean Drilling Program (IODP).
Shipboard support for JP was provided by NERC grant NE/
H010319/1 and she thanks the IODP and Transocean/Sedco-Forex
staff on board the JOIDES Resolution for their contributions to a
successful expedition. We acknowledge reviews by F.Z. Teng,
O. Rouxel and an anonymous reviewer that improved the manuscript. JP was supported by Petrobras and ERC funding to ANH and
188
J. Prytulak et al. / Earth and Planetary Science Letters 365 (2013) 177–189
NERC postdoctoral fellowship NE/H01313X/1. SGN was supported
by a NERC postdoctoral fellowship. We are grateful to K.W. Burton
for providing the Indian Ocean MORB glass. Barry Coles (Imperial)
and Nick Belshaw (Oxford) are thanked for instrument support.
Appendix A. Supplementary Information
Supplementary data associated with this article can be found in
the online version at http://dx.doi.org/10.1016/j.epsl.2013.01.010.
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