Vanadium concentrations in planetary mantles can provide information about the conditions during early accretion and differentiation. Because V is a slightly siderophile element, it is usually assumed that any depletion would be due to core formation and metal-silicate equilibrium. However, V is typically more compatible in phases such as spinel, magnesiowuestite and garnet. Fractionation of all of these phases would cause depletions more marked than those from metal. In this paper consideration of depletions due to metal, oxide and silicate are critically evaluated

Righter, Kevin

NASA Technical Reports Server

DEPLETION OF VANADIUM IN PLANETARY MANTLES: CONTROLLED BY METAL, OXIDE, OR 
SILICATE?  K. Righter1, 1NASA Johnson Space Center, Mailcode KT, 2101 NASA Parkway, Houston, TX 77058; 
kevin.righter-1@nasa.gov. 
 
Introduction:  Vanadium concentrations in plane-
tary mantles can provide information about the condi-
tions during early accretion and differentiation [1,2].  
Because V is a slightly siderophile element, it is usu-
ally assumed that any depletion would be due to core 
formation and metal-silicate equilibrium (e.g., [3,4]).  
However, V is typically more compatible in phases 
such as spinel, magnesiowüstite and garnet.  Fractiona-
tion of all of these phases would cause depletions more 
marked than those from metal.  In this paper considera-
tion of depletions due to metal, oxide and silicate are 
critically evaluated. 
 
Vanadium compatibility:   
Metal: the partitioning of V between metal and silicate 
has been studied extensively at both low and high pres-
sure and temperature conditions (e.g., [2, 3, 5-8], lead-
ing [3] to propose a predictive expression for D(V) 
met/sil, based on the S and C content of the metal, 
temperature and oxygen fugacity.  This predictive ex-
pression does not include a pressure term because in-
spection of the data reveals no obvious pressure effect.  
This expression can be used to predict D(V) 
metal/silicate as a function of fO2, at fixed T and metal 
composition (Fig. 1).  The significant fO2 effect shows 
that V switches from lithophile (D<1) to siderophile 
(D>1) at approximately 3 log fO2 units below the IW 
buffer.   
Shallow mantle: The partitioning of V during condi-
tions of melting of the shallow terrestrial mantle has 
been studied extensively by [9], and is known to be 
controlled by spinel (see also [10]) and clinopyroxene.  
Similar to metal-bearing systems, V partitioning is 
strongly fO2 dependent, and can switch from compati-
ble to incompatible behavior within a range relevant to 
planetary mantles (Fig. 1).   
Intermediate mantle: Both the martian and terrestrial 
mantles contain a large stability field for majoritic gar-
net and β (Mg,Fe)2SiO4 (e.g., [11]).  There are very 
few data for these phases, and certainly not enough to 
assess whether there is also a dependence upon fO2.  
However, given the available crystal chemical diver-
sity in these phases, the interplay of 2+, 3+, and 4+ 
vanadium is likely to produce fO2 dependent behavior.   
Deep mantle: Several studies have measured the parti-
tioning of V between deep mantle (terrestrial lower 
mantle) phases that may be relevant to early Mars as 
well.  Magnesiowüstite is a significant host phase for 
V and can have D(V) as high as 4 to 6 [7].  Previous 
studies of both Mg- and Ca-silicate perovskite and 
melt have shown that D(V) may be as high as 2-3 [12-
13].  The deep mantle may thus also be host to a sig-
nificant amount of V, especially if there was early frac-
tionation of these phases from a magma ocean. 
Summary of all phases: To illustrate the potential for 
silicate and oxide phases to dominate the V budget, 
two different scenarios are illustrated in Figure 1: 
variation of D(metal/silicate) [3] and D(shallow man-
tle/silicate) [9], as a function of oxygen fugacity (the 
latter based on a mantle consisting of 49% olivine, 
28% opx, 18% cpx and 5% spinel).  Each of these sce-
narios shows that vanadium will be dominated by dif-
ferent phases depending on the environment of melt-
ing. 
Figure 1: comparison of D(V) Metal/silicate liquid [3],  
and D(shallow mantle/silicate) [9], as a function of 
oxygen fugacity.   
 
Summary of depletions 
Depletions of V in planetary mantles are best de-
fined relative to an element of similar lithophile behav-
ior – Al [14].  Correlations of V and Al are evident in 
terrestrial sample suites, but must be interpreted for 
other planetary bodies using data from basaltic rocks.  
As a result, there is some interpretation necessary.  A 
summary of V depletions for Earth, Moon, Mars and 
Vesta is presented in Figure 2. 
Earth: The V content of the primitive terrestrial mantle 
is defined by analyses of komatiites, peridotites and 
basaltic rocks.  All of these samples define a small, but 
significant depletion of V relative to chondritic values.   
Moon: The V content of the lunar mantle can be esti-
mated from the many basalt samples we have from the 
Apollo, Luna and meteorite collections.   Because 
many of these basalts have undergone fractionation, 
Lunar and Planetary Science XXXVII (2006) 2259.pdf
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and some involving the V compatible phase chromite, 
only unevolved samples must be considered in these 
estimates.  Consideration of available data by [10] 
shows that the Moon has only a small depletion of V 
relative to chondritic values.   
Mars: The V content of the primitive martian mantle 
must also be estimated from a few basalt samples we 
have from meteorite collections.  Because many of 
these basalts have also undergone fractionation involv-
ing chromite, only un-evolved samples must be con-
sidered in these estimates.  One of the most primitive 
samples known is the shergottite Y980459 [15]. Con-
sideration of available data by [10] shows that Mars 
has a higher V/Al ratio than chondritic values.   
Vesta: The V content of the HED/Vesta mantle is 
lower than any other body known.  However, our sam-
ples of the eucrites represent a suite of liquids of simi-
lar origin.  These liquids may have been fractionated 
and thus seen the effects of chromite fractionation.  On 
the other hand if they are primitive, unfractionated 
liquids, they may represent a severely depleted mantle 
compared to the Earth, Moon and Mars.  The signifi-
cant depletion of V in the HED mantle in Figure 2 is 
based on the assumption that no chromite has fraction-
ated from the eucritic liquids and that they are primary 
derivatives from the HED mantle. 
 
Figure 2: depletions of V for different planetary man-
tles.  From Righter et al. (2006) [7].  
 
Models 
Earth: The ratios of lithophile elements such as Sm/Nd 
and Lu/Hf, as well and Hf and Nd isotopic composi-
tions, in the primitive mantle can be satisfied by frac-
tionation of 10-15% of an assemblage consisting of 
Mg-perovskite, Ca-perovskite and magnesiowustite 
[16].  Using published D(V) for these phases, and a CI 
chondrite as a starting material, it can be shown that 
the small depletions of V in the primitive terrestrial 
mantle could be caused by fractionation of these 
phases.  This scenario is an alternative to the very high 
temperature and pressure models that have been pro-
posed for V depletions [3,4].    
Moon: Segregation of the small lunar core is unlikely 
to have caused a significant V depletion, as it did for 
other more siderophile elements such as Ni and Re 
[17].  Additionally, the D(V) met/sil for redox condi-
tions relevant to the Moon (IW-2), is too small to have 
caused a depletion (Fig. 2).  As a result, the lunar de-
pletions must be attributed to a previous differentiation 
event, perhaps inherited from the mantle of the impac-
tor that formed the Moon.  Such an impactor could 
have been a large body, such as Mars or Earth, that 
underwent a V depletion due to silicate and oxide frac-
tionation. 
Mars: The depletion of V in the martian mantle, simi-
lar to the Earth and Moon, presents a problem for the  
high PT reduced core formation scenario proposed for 
the Earth [3,4].  A high PT scenario for Mars is incon-
sistent with most siderophile elements (Ni, Co, W, 
[18]).  In addition, the ‘reduced followed by oxidized’ 
accretion scenario [4] won’t work for Mars, since its 
mantle is not deep enough for a significant (or any at 
all) Mg-perovskite stability field [11].  Instead, V de-
pletions produced by intermediate or deep mantle 
phases (oxides or silicates) can easily explain the mar-
tian V data.   
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Depletion of Vandium in Planetary Mantles: Controlled by Metal, Oxide, or Silicate?

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