The vanadium isotope composition of Mars: implications for planetary differentiation in the early solar system

S.G. Nielsen^1,2 ,

¹NIRVANA Laboratories, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA
²Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA

D.V. Bekaert¹ ,

¹NIRVANA Laboratories, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA

T. Magna³ ,

³Czech Geological Survey, Klarov 3, CZ-11821 Prague, Czech Republic

K. Mezger⁴ ,

⁴Institut für Geologie and Center for Space and Habitability, Universität Bern, Baltzerstrasse 1+3, CH-3012 Bern, Switzerland

M. Auro¹

¹NIRVANA Laboratories, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA

Affiliations | Corresponding Author | Cite as | Funding information

Geochemical Perspectives Letters v15 | doi: 10.7185/geochemlet.2032
Received 29 April 2020 | Accepted 29 June 2020 | Published 30 September 2020

Published by the European Association of Geochemistry
under Creative Commons License CC BY-NC-ND 4.0

Keywords: planetary differentiation, vanadium isotopes, Mars, stable isotope fractionation



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Abstract

The V isotope composition of martian meteorites reveals that Bulk Silicate Mars (BSM) is characterised by δ⁵¹V = −1.026 ± 0.029 ‰ (2 s.e.) and is thus ∼0.06 ‰ heavier than chondrites and ∼0.17 ‰ lighter than Bulk Silicate Earth (BSE). Based on the invariant V isotope compositions of all chondrite groups, the heavier V isotope compositions of BSE and BSM relative to chondrites are unlikely to originate from mass independent isotope effects or evaporation/condensation processes in the early Solar System. These differences are best accounted for by mass dependent fractionation during core formation. Assuming that bulk Earth and Mars both have a chondritic V isotopic compostion, mass balance considerations reveal V isotope fractionation factors Δ⁵¹V_core-mantle as substantial as −0.6 ‰ for both planets. This suggests that V isotope systematics in terrestrial and extraterrestrial rocks potentially constitutes a powerful new tracer of planetary differentiation processes accross the Solar System.

Figures and Tables

Table 1 Vanadium isotope data for martian meteorites.	Figure 1 Martian meteorite vanadium isotopic data corrected for GCR effects plotted against V/Ce ratios. Error bars are 2 s.d. as listed in Table 1. Given that V and Ce have different partition coefficients during fractional crystallisation and melting, variations in V/Ce likely reflect a combination of these processes. The lack of systematic V isotope variation over the entire range of V/Ce implies that magmatic processes did not induce detectable V isotope variation on Mars. Ce concentrations from Yoshizaki and McDonough (2020); V concentrations from Table 1. Note log scale on y axis.	Figure 2 Vanadium isotope data for different sample types from the Solar System. Data for terrestrial samples are from Prytulak et al. (2013), Wu et al. (2018) and Qi et al. (2019); chondrite data are from Nielsen et al., (2019) and this study (Table S-2). All data have been corrected for spallation-produced ⁵⁰V. Error bars for each sample group are 2 s.e. weighted by the individual sample error bars shown in Table 1. Shaded areas denote the average for those objects and error weighted 2 s.e. of the entire data population. Chondrite averages have been weighted by individual measurement errors.	Figure 3 Modelled V isotope fractionation factors between the core and mantle of Earth (blue) and Mars (red) as a function of the fraction of total V that entered the core.
Table 1	Figure 1	Figure 2	Figure 3

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Introduction

Vanadium is a refractory and moderately siderophile element with the two naturally occurring isotopes ⁵¹V and ⁵⁰V. Variations of ⁵¹V/⁵⁰V reported for terrestrial and extraterrestrial samples have been attributed to various processes, such as heterogeneous distribution of nucleosynthetically produced V (Nielsen et al., 2019

Nielsen, S.G., Auro, M., Righter, K., Davis, D., Prytulak, J., Wu, F., Owens, J.D. (2019) Nucleosynthetic vanadium isotope heterogeneity of the early solar system recorded in chondritic meteorites. Earth and Planetary Science Letters 505, 131–140.

), kinetic and equilibrium mass dependent isotope fractionation during condensation from the protosolar nebula (Wu et al., 2015

Wu, F., Qin, T., Li, X.F., Liu, Y., Huang, J.H., Wu, Z.Q., Huang, F. (2015) First-principles investigation of vanadium isotope fractionation in solution and during adsorption. Earth and Planetary Science Letters 426, 216–224.

; Nielsen et al., 2019

), early irradiation by solar irradiation (Lee et al., 1998

Lee, T., Shu, F.H., Shang, H., Glassgold, A.E., Rehm, K.E. (1998) Protostellar cosmic rays and extinct radioactivities in meteorites. Astrophysical Journal 506, 898–912.

; Gounelle et al., 2006

Gounelle, M., Shu, F.H., Shang, H., Glassgold, A.E., Rehm, K.E., Lee, T. (2006) The irradiation origin of beryllium radioisotopes and other short-lived radionuclides. Astrophysical Journal 640, 1163–1170.

; Sossi et al., 2017

Sossi, P.A., Moynier, F., Chaussidon, M., Villeneuve, J., Kato, C., Gounelle, M. (2017) Early Solar System irradiation quantified by linked vanadium and beryllium isotope variations in meteorites. Nature Astronomy 1, 0055.

), exposure of meteoroids to galactic cosmic rays (GCR) during their transition to Earth (Hopkins et al., 2019

Hopkins, S.S., Prytulak, J., Barling, J., Russell, S.S., Coles, B.J., Halliday, A.N. (2019) The vanadium isotopic composition of lunar basalts. Earth and Planetary Science Letters 511, 12–24.

), and/or magmatic differentiation (Prytulak et al., 2017

Prytulak, J., Sossi, P.A., Halliday, A.N., Plank, T., Savage, P.S., Woodhead, J.D. (2017) Stable vanadium isotopes as a redox proxy in magmatic systems? Geochemical Perspectives Letters 3, 75–84.

). Since V has only two isotopes, it is difficult to discriminate among the different processes that can lead to V isotope variations in geological material. Therefore, additional constraints are needed to interpret V isotope signatures in cosmochemical and geochemical contexts. To further explore the origin of δ⁵¹V variations (where δ⁵¹V_sample = ((⁵¹V/⁵⁰V)_sample/(⁵¹V/⁵⁰V)_AA − 1) × 1000, with AA being the Alfa Aesar reference solution) among terrestrial planets, a comprehensive set of V isotope data for martian meteorites was obtained. These data are compared with the value for chondrites of δ⁵¹V = −1.089 ± 0.029 ‰ (2 s.e.) (see Supplementary Information) to provide additional constraints on potential accretionary sources for Mars, and to investigate the possible mechanisms of V isotope fractionation during planetary differentiation and igneous processes.

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Results and Discussion

The 24 martian meteorites investigated here represent a petrologic range of shergottites (basaltic, olivine-phyric, lherzolitic), clinopyroxene-rich cumulates (nakhlites) and orthopyroxenite (Table 1). The measured δ⁵¹V for martian meteorites display a rather restricted range of values from −1.25 to −0.89 ‰ (Table 1) with a mean of δ⁵¹V = −1.043 ± 0.034 ‰ (2 s.e.). Due to their young exposure ages and relatively low Fe/V ratios most samples require very minor, if any, correction for spallation by GCR (see Supplementary Information). Corrected δ⁵¹V values are all within error of each other and show no correlation with petrologic type or indicators of melting or fractional crystallisation (Fig. 1). Pooled data for the different martian meteorite groups yield indistinguishable mean δ⁵¹V (Table 1; Supplementary Information), further supporting a limited effect of magmatic processes, such as partial melting and fractional crystallisation, on V isotopes. This inference is consistent with the absence of discernible, systematic δ⁵¹V variations among terrestrial peridotites and basalts (see Supplementary Information), as well as lunar rock data corrected for GCR effects (Hopkins et al., 2019

Hopkins, S.S., Prytulak, J., Barling, J., Russell, S.S., Coles, B.J., Halliday, A.N. (2019) The vanadium isotopic composition of lunar basalts. Earth and Planetary Science Letters 511, 12–24.

Table 1 Vanadium isotope data for martian meteorites.

Sample	Fe^$ (mg/g)	V^$ (μg/g)	CRE age^¶ (Ma)	δ⁵¹V_meas	2 s.d.	splits	n	δ⁵¹V_corr^§
Basaltic shergottites
Los Angeles 001	224	258	3.0	−0.95	0.12	3	8	−0.94
NWA 856	132	273	2.6	−0.94	0.13	1	10	−0.94
NWA 4864	153	325	3.0	−1.11	0.23	3	8	−1.10
Shergotty	173	310	3.0	−0.89	0.19	2	8	−0.89
Zagami	141	312	3.0	−1.15	0.23	2	14	−1.14
				Error weighted group average				−0.97

Olivine-phyric shergottites
EETA 79001 A	143	210	0.7	−1.02	0.19	2	6	−1.02
SaU 005	143	196	1.2	−1.12	0.24	2	14	−1.12
SaU 051	150	204	1.2	−1.03	0.21	2	11	−1.03
SaU 094	149	212	1.2	−1.11	0.20	2	4	−1.11
NWA 1068	155	207	2.8	−1.11	0.16	6	22	−1.10
LAR 06319	159	202	3.3	−1.04	0.16	2	5	−1.03
Y-980459	144	213	1.1	−1.00	0.09	1	4	−1.00
NWA 4925	149	181	0.6	−0.98	0.19	3	9	−0.98
NWA 6162	142	112	1.0	−0.90	0.15	1	4	−0.89
DaG476	122	167	1.0	−1.09	0.13	3	11	−1.09
RBT 04262	201	212	2.1	−1.04	0.06	1	3	−1.04
				Error weighted group average				−1.03

Lherzolitic shergottites
ALH 77005	156	162	4.3	−1.11	0.12	3	11	−1.10
NWA 1950	168	140	4.1	−1.00	0.10	2	9	−0.99
Y-000097	155	180	4.6	−1.01	0.14	1	4	−1.00
				Error weighted group average				−1.03

Nakhlites
Nakhla	160	192	11.6	−1.25	0.19	1	5	−1.23
MIL 03346	137	195	9.5	−1.08	0.14	2	6	−1.07
NWA 817	154	181	10.0	−0.98	0.15	1	2	−0.96
Y-000593	117	125	11.8	−1.03	0.22	1	7	−1.01
				Error weighted group average				−1.06

Orthopyroxenite
ALH 84001	136	201	14.7	−1.09	0.15	1	4	−1.07

					Error weighted average Mars			−1.026
					2 s.e.			0.027
					MSWD			0.991

^$ Iron and V concentrations are either from the literature (in italics; Lodders, 1998Lodders, K. (1998) A survey of shergottite, nakhlite and chassigny meteorites whole-rock compositions. Meteoritics & Planetary Science 33, A183–A190.; Dreibus et al., 2000Dreibus, G., Spettel, B., Haubold, R., Jochum, K.P., Palme, H., Wolf, D., Zipfel, J. (2000) Chemistry of a new shergottite: Sayh al Uhaymir 005. Meteoritics and Planetary Science 35, A49.; Sautter et al., 2002Sautter, V., Barrat, J.A., Jambon, A., Lorand, J.P., Gillet, P., Javoy, M., Loron, J.L., Lesourd, M. (2002) A new Martian meteorite from Morocco: the nakhlite North West Africa 817. Earth and Planetary Science Letters 195, 223–238.; Gillet et al., 2005Gillet, P., Barrat, J.A., Beck, P., Marty, B., Greenwood, R.C., Franchi, I.A., Bohn, M., Cotten, J. (2005) Petrology, geochemistry, and cosmic-ray exposure age of lherzolitic shergottite Northwest Africa 1950. Meteoritics & Planetary Science 40, 1175–1184.; Basu Sarbadhikari et al., 2009Basu Sarbadhikari, A., Day, J.M.D., Liu, Y., Rumble, D., Taylor, L.A. (2009) Petrogenesis of olivine-phyric shergottite Larkman Nunatak 06319: implications for enriched components in martian basalts. Geochimica et Cosmochimica Acta 73, 2190–2214.; Shirai and Ebihara, 2009Shirai, N., Ebihara, M. (2009) Chemical characteristics of the lherzolitic shergottite Yamato 000097: magmatism on Mars inferred from the chemical compositions of shergottites. Polar Science 3, 117–133.; Kuehner et al., 2011Kuehner, S.M., Irving, A.J., Herd, C.D.K., Gellissen, M., Lapen, T.J., Rumble, D. (2011) Pristine olivine-phyric shergottite Northwest Africa 6162: a primitive magma with accumulated crystals derived from depleted martian mantle. 42nd Lunar and Planetary Science Conference, The Woodlands, Texas. 1610.), measured by ICPMS at WHOI (bold), or estimated based on yield during column chemistry.
^¶ CRE ages from Eugster et al. (2002)Eugster, O., Busemann, H., Lorenzetti, S., Terribilini, D. (2002) Ejection ages from krypton-81-krypton-83 dating and pre-atmospheric sizes of martian meteorites. Meteoritics & Planetary Science 37, 1345–1360., Mathew et al. (2003)Mathew, K.J., Marty, B., Marti, K., Zimmermann, L. (2003) Volatiles (nitrogen, noble gases) in recently discovered SNC meteorites, extinct radioactivities and evolution. Earth and Planetary Science Letters 214, 27–42., Christen et al. (2005)Christen, F., Eugster, O., Busemann, H. (2005) Mars ejection times and neutron capture effects of the nakhlites Y000593 and Y000749, the olivine-phyric shergottite Y980459, and the lherzolite NWA1950. Antarctic Meteorite Research 18, 117–132., Nagao and Park (2008)Nagao, K., Park, J. (2008) Noble gases and cosmic-ray exposure ages of two Martian shergottites, RBT 04262 and LAR 06319, recovered in Antarctica. 71st Annual Meteoritical Society Meeting, Matsue, Japan. 5200., Nagao et al. (2008)Nagao, K., Park, J., Choi, H.G. (2008) Noble gases of the Yamato 000027 and Yamato 000097 lherzolitic shergottites from Mars. Polar Science 2, 195–214., Nishiizumi et al. (2011)Nishiizumi, K., Nagao, K., Caffee, M.W., Jull, A.J.T., Irving, A.J. (2011) Cosmic-ray exposure chronologies of depleted olivine-phyric shergottites. 42nd Lunar and Planetary Science Conference, The Woodlands, Texas. 2371. and Wieler et al. (2016)Wieler, R., Huber, L., Busemann, H., Seiler, S., Leya, I., Maden, C., Masarik, J., Meier, M.M.M., Nagao, K., Trappitsch, R., Irving, A.J. (2016) Noble gases in 18 Martian meteorites and angrite Northwest Africa 7812—Exposure ages, trapped gases, and a re-evaluation of the evidence for solar cosmic ray produced neon in shergottites and other achondrites. Meteoritics & Planetary Science 51, 407–428. except for NWA4864, which is assumed similar to the other basaltic shergottites.
^§ Vanadium isotope compositions corrected for spallogenic production of ⁵⁰V using the equation: δ⁵¹V_corr = δ⁵¹V_meas + [Fe] × CRE × 2.1 × 10⁻⁶/[V] (Hopkins et al., 2019Hopkins, S.S., Prytulak, J., Barling, J., Russell, S.S., Coles, B.J., Halliday, A.N. (2019) The vanadium isotopic composition of lunar basalts. Earth and Planetary Science Letters 511, 12–24.).

**Figure 1** Martian meteorite vanadium isotopic data corrected for GCR effects plotted against V/Ce ratios. Error bars are 2 s.d. as listed in Table 1. Given that V and Ce have different partition coefficients during fractional crystallisation and melting, variations in V/Ce likely reflect a combination of these processes. The lack of systematic V isotope variation over the entire range of V/Ce implies that magmatic processes did not induce detectable V isotope variation on Mars. Ce concentrations from Yoshizaki and McDonough (2020)
Yoshizaki, T., McDonough, W.F. (2020) The composition of Mars. *Geochimica et Cosmochimica Acta* 273, 137–162.
; V concentrations from Table 1. Note log scale on y axis.

Invariant V isotope compositions recorded in martian meteorites collectively indicate that BSM (and by analogy BSE) inherited its δ⁵¹V signature during its accretion and/or differentiation history. Here, the GCR corrected and error weighted average V isotope composition of martian meteorites (Table 1) is used to derive the best estimate for the V isotope source signature of BSM, which yields δ⁵¹V = −1.026 ± 0.027 ‰ (2 s.e., MSWD = 0.99). This value is markedly different from the mean δ⁵¹V of BSE (−0.856 ± 0.020 ‰, 2 s.e.). Understanding the cause(s) of V isotope variations among terrestrial planets could potentially provide constraints on the accretion history and building blocks of Earth and Mars, as well as isotopic effects during planetary differentiation. In the following, we explore possible explanations for the V isotope difference between Mars and chondrites, and discuss the implications for how BSM and BSE could have acquired their V isotope compositions.

Whereas stable isotope systems such as Mg, Si, Ca, and Fe in martian samples (Armytage et al., 2011

Armytage, R.M.G., Georg, R.B., Savage, P.S., Williams, H.M., Halliday, A.N. (2011) Silicon isotopes in meteorites and planetary core formation. Geochimica et Cosmochimica Acta 75, 3662–3676.

; Magna et al., 2015

Magna, T., Gussone, N., Mezger, K. (2015) The calcium isotope systematics of Mars. Earth and Planetary Science Letters 430, 86–94.

; Sossi et al., 2016

Sossi, P.A., Nebel, O., Anand, M., Poitrasson, F. (2016) On the iron isotope composition of Mars and volatile depletion in the terrestrial planets. Earth and Planetary Science Letters 449, 360–371.

; Hin et al., 2017

Hin, R.C., Coath, C.D., Carter, P.J., Nimmo, F., Lai, Y.-J., Pogge von Strandmann, P.A.E., Willbold, M., Leinhardt, Z.M., Walter, M.J., Elliott, T. (2017) Magnesium isotope evidence that accretional vapour loss shapes planetary compositions. Nature 549, 511.

; Magna et al., 2017

Magna, T., Hu, Y., Teng, F.-Z., Mezger, K. (2017) Magnesium isotope systematics in Martian meteorites. Earth and Planetary Science Letters 474, 419–426.

) show only limited, if any, deviations from chondritic values, large nucleosynthetic isotope anomalies have been observed for many other elements in chondrites and planetary materials including Mars (e.g., ⁵⁴Cr, ⁵⁰Ti, ¹⁷O, ⁶²Ni; Warren, 2011

Warren, P.H. (2011) Stable-isotopic anomalies and the accretionary assemblage of the Earth and Mars: A subordinate role for carbonaceous chondrites. Earth and Planetary Science Letters 311, 93–100.

). Given that Mars likely accreted very rapidly and early relative to Earth (Dauphas and Pourmand, 2011

Dauphas, N., Pourmand, A. (2011) Hf-W-Th evidence for rapid growth of Mars and its status as a planetary embryo. Nature 473, 489–U227.

), the V isotope difference between Mars and Earth could denote primary spatial variability and/or temporal evolution of the nucleosynthetic V isotope composition of planetary accretion source material in the inner Solar System (e.g., Warren, 2011

). However, although V isotope compositions of bulk carbonaceous chondrites have been proposed to correlate broadly with nucleosynthetic anomalies of ⁵⁴Cr (Nielsen et al., 2019

), subsequent studies have found that the V isotope variation in bulk chondrites can be ascribed to recent production of ⁵⁰V from GCR spallation processes (Hopkins et al., 2019

Hopkins, S.S., Prytulak, J., Barling, J., Russell, S.S., Coles, B.J., Halliday, A.N. (2019) The vanadium isotopic composition of lunar basalts. Earth and Planetary Science Letters 511, 12–24.

). Carbonaceous, ordinary, enstatite and Rumuruti chondrite data corrected for this effect all display uniform δ⁵¹V = −1.089 ± 0.029 ‰ (n = 14, 2 s.e.; see Supplementary Information). The lack of V isotope variation in chondrites compared with the large variations in e.g., ⁵⁴Cr and ⁵⁰Ti implies that nucleosynthetic V isotope anomalies, if they exist, are unlikely to induce planetary scale V isotope heterogeneity. Similarly, irradiation processes are unlikely to account for planetary V isotope variations given the large abundance variations of CAIs, the most likely carriers of irradiation-induced V isotope anomalies (Sossi et al., 2017

), in chondrites that display uniform bulk V isotope compositions. For these reasons, V isotope variations among terrestrial planets most likely do not reflect disparities in the signatures of their accretionary materials, but are the result of planetary processes.

Although BSE has a markedly heavier composition than other solar system bodies, there are still analytically significant differences between BSM and chondrites (Fig. 2). Two sample t tests demonstrate that the datasets of the BSM and chondrites are characterised by statistically distinct means (see Supplementary Information). Monte Carlo simulations using the raw datasets of the BSM and chondrites that include the individual sample errors show that there exists a difference of 0.058 ± 0.051 ‰ (1 s.d.) between these two populations, with a probability of ∼87 % that the V isotope composition of the BSM is heavier than that of chondrites. Using the mean δ⁵¹V compositions of the BSM and chondrites, instead of the raw data, to compute these Monte Carlo simulations yields a systematic difference of 0.067 ± 0.042 ‰ (1 s.d.) between BSM and chondrites. In the likely absence of mass independent V isotope effects (nucleosynthetic or irradiation-related) on the planetary scale, we propose that the most straightforward explanation for the V isotope disparity between differentiated (Earth, Mars) and non-differentiated (chondrites) bodies corresponds to small but non-negligible isotopic fractionation during core formation.

**Figure 2** Vanadium isotope data for different sample types from the Solar System. Data for terrestrial samples are from Prytulak *et al.* (2013)
Prytulak, J., Nielsen, S.G., Ionov, D.A., Halliday, A.N., Harvey, J., Kelley, K.A., Niu, Y., Peate, D.W., Shimizu, K., Sims, K.W.W. (2013) The Stable Vanadium Isotope Composition of the Mantle and Mafic Lavas. *Earth and Planetary Science Letters* 365, 177–189.
, Wu *et al.* (2018)
Wu, F., Qi, Y., Perfit, M.R., Gao, Y., Langmuir, C.H., Wanless, V.D., Yu, H., Huang, F. (2018) Vanadium isotope compositions of mid-ocean ridge lavas and altered oceanic crust. *Earth and Planetary Science Letters* 493, 128–139.
and Qi *et al.* (2019)
Qi, Y.H., Wu, F., Ionov, D.A., Puchtel, I.S., Carlson, R.W., Nicklas, R.W., Yu, H.M., Kang, J.T., Li, C.H., Huang, F. (2019) The vanadium isotopic composition of the BSE: constraints from peridotites and komatiites. *Geochimica et Cosmochimica Acta* 259, 288–301.
; chondrite data are from Nielsen *et al*., (2019)
Nielsen, S.G., Auro, M., Righter, K., Davis, D., Prytulak, J., Wu, F., Owens, J.D. (2019) Nucleosynthetic vanadium isotope heterogeneity of the early solar system recorded in chondritic meteorites. *Earth and Planetary Science Letters* 505, 131–140.
and this study (Table S-2). All data have been corrected for spallation-produced ⁵⁰V. Error bars for each sample group are 2 s.e. weighted by the individual sample error bars shown in Table 1. Shaded areas denote the average for those objects and error weighted 2 s.e. of the entire data population. Chondrite averages have been weighted by individual measurement errors.

Considering that (i) the martian core comprises ∼18 % by mass of the planet (Yoshizaki and McDonough, 2020

Yoshizaki, T., McDonough, W.F. (2020) The composition of Mars. Geochimica et Cosmochimica Acta 273, 137–162.

), (ii) ∼27–60 % of martian V resides in the core (see Supplementary Information), and (iii) Δ⁵¹V_{BSM-chondrites} = 0.067 ± 0.042 ‰, the martian core should be characterised by δ⁵¹V ∼ −1.11 to −1.38 ‰. This requires a metal-silicate isotope fractionation factor of Δ⁵¹V_core-mantle = −0.04 to −0.40 ‰ (Fig. 3). Regarding BSE, it is generally assumed that the main phase of metal segregation during terrestrial core formation readily accounts for the depletion of V in the silicate Earth (O’Neill, 1991

O’Neill, H.S.C. (1991) The origin of the moon and the early history of the earth – A chemical model. Part 2: The earth. Geochimica et Cosmochimica Acta 55, 1159–1172.

; Chabot and Agee, 2003

Chabot, N.L., Agee, C.B. (2003) Core formation in the Earth and Moon: New experimental constraints from V, Cr, and Mn. Geochimica et Cosmochimica Acta 67, 2077–2091.

; Wade and Wood, 2005

Wade, J., Wood, B.J. (2005) Core formation and the oxidation state of the Earth. Earth and Planetary Science Letters 236, 78–95.

), with 40–50 % of terrestrial V now residing in the core (e.g., Wade and Wood, 2005

Wade, J., Wood, B.J. (2005) Core formation and the oxidation state of the Earth. Earth and Planetary Science Letters 236, 78–95.

). If bulk Earth is taken to be chondritic for V isotopes, then mass balance dictates that the core is characterised by δ⁵¹V = −1.39 ± 0.10 ‰, which corresponds to a metal-silicate isotope fractionation factor of Δ⁵¹V_core-mantle = −0.53 ± 0.14 ‰ (using Δ⁵¹V_{BSE-chondrites} = 0.233 ± 0.037 ‰, 2 s.e.). This value is somewhat larger than that required to explain the BSM–chondrite V isotope offset (Fig. 3), which could imply variations in V isotope fractionation during planetary differentiation.

**Figure 3** Modelled V isotope fractionation factors between the core and mantle of Earth (blue) and Mars (red) as a function of the fraction of total V that entered the core.

Hin, R.C., Burkhardt, C., Schmidt, M.W., Bourdon, B., Kleine, T. (2013) Experimental evidence for Mo isotope fractionation between metal and silicate liquids. Earth and Planetary Science Letters 379, 38–48.

, Bonnand et al. (2016)

Bonnand, P., Williams, H.M., Parkinson, I.J., Wood, B.J., Halliday, A.N. (2016) Stable chromium isotopic composition of meteorites and metal-silicate experiments: Implications for fractionation during core formation. Earth and Planetary Science Letters 435, 14–21.

, and Elardo et al., (2019)

Elardo, S.M., Shahar, A., Mock, T.D., Sio, C.K. (2019) The effect of core composition on iron isotope fractionation between planetary cores and mantles. Earth and Planetary Science Letters 513, 124–134.

). A small core–mantle isotope fractionation factor for V is also suggested by the only available investigation of V isotope fractionation during metal–silicate partitioning, where experiments at 1.5 GPa and ∼1900 K revealed no detectable V isotope fractionation outside the analytical uncertainty (∼±0.2 ‰; Nielsen et al., 2014

Nielsen, S.G., Prytulak, J., Wood, B.J., Halliday, A.N. (2014) Vanadium isotopic difference between the silicate Earth and meteorites. Earth and Planetary Science Letters 389, 167–175.

). However, Earth’s core likely formed through a complex, multi-stage process, starting as a reduced body that gradually evolved to more oxidised as Earth grew (e.g., Wade and Wood, 2005

Wade, J., Wood, B.J. (2005) Core formation and the oxidation state of the Earth. Earth and Planetary Science Letters 236, 78–95.

), with a final stage of core segregation in the form of immiscible metal and sulfide melts after the Moon-forming giant impact (e.g., O’Neill, 1991

O’Neill, H.S.C. (1991) The origin of the moon and the early history of the earth – A chemical model. Part 2: The earth. Geochimica et Cosmochimica Acta 55, 1159–1172.

). Additional experiments of V isotopic partitioning at higher pressure-temperature conditions, for variable oxygen fugacities and/or chemical compositions (e.g., presence or not of light elements and other metal alloys) are needed to test the hypothesis of V isotope fractionation during core formation. Such effects have, for example, been observed for Fe isotopes (Elardo et al., 2019

). Notably, the sensitivity of V valence state to the oxygen fugacity during core formation offers a promising alternative for potentially inducing significant V isotope fractionation during metal-silicate partitioning. For example, the oxygen fugacity during core formation on Mars and Earth were likely different (Wood et al., 2006

Wood, B.J., Walter, M.J., Wade, J. (2006) Accretion of the Earth and segregation of its core. Nature 441, 825–833.

) and may have been a contributing factor in the apparent V isotope fractionation difference between these two planets.

One outstanding challenge is to explain the strikingly heavy V isotope composition of BSE compared to BSM. Future experimental investigations may shed light on the possibility for higher temperatures and pressures during terrestrial core formation, and/or late segregation of the Hadean matte (which seemingly did not happen on Mars) to account for this difference. Based on current constraints, it appears that the late stage sulfide segregation on Earth is unlikely to have removed substantial amounts of V to the core (e.g., Wade and Wood, 2005

Wade, J., Wood, B.J. (2005) Core formation and the oxidation state of the Earth. Earth and Planetary Science Letters 236, 78–95.

). We, therefore, consider that the heavy V isotope composition of BSE was most likely established during the main phase of core segregation, before Moon formation. Alternative avenues of investigation to explain the heavy V isotope composition of BSE and BSM may require unusual isotope fractionation processes to operate at the core–mantle boundary, as recently proposed for Fe in the case of thermodiffusion (Lesher et al., 2020

Lesher, C.E., Dannberg, J., Barfod, G.H., Bennett, N.R., Glessner, J.J.G., Lacks, D.J., Brenan, J.M. (2020) Iron isotope fractionation at the core–mantle boundary by thermodiffusion. Nature Geoscience 13, 382–386.

). Nonetheless, we speculate that identifying the process(es) responsible for V isotope variations in terrestrial planets will ultimately allow better understanding the conditions of planetary differentiation in the Solar System.

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Acknowledgements

This work was funded by NASA Emerging Worlds grant NNX16AD36G to SGN. Samples were acquired with funds from the Helmholtz Association through the research alliance HA 203 “Planetary Evolution and Life” to KM. TM contributed through the Strategic Research Plan of the Czech Geological Survey (DKRVO/ČGS 2018-2022). KM acknowledges support through NCCR PlanetS supported by the Swiss National Science Foundation. We thank Jurek Blusztajn for support in the WHOI Plasma Facility.

Editor: Anat Shahar

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References

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