Chemie der Erde 73 (2013) 401–420
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Chemie der Erde
jou rnal homepage: www.elsevier.de/chemer
The bulk composition of Mars
G. Jeffrey Taylor
a r t i c l e i n f o a b s t r a c t
Article history: An accurate assessment of the bulk chemical composition of Mars is fundamental to understanding plan-
Received 16 July 2013
Accepted 11 September 2013
to estimate the bulk chemical composition of Mars: geochemical/cosmochemical, isotopic, and geophys-
ical. The standard model is one developed by Wänke and Dreibus in a series of papers, which is based on
compositions of Martian meteorites. Since their groundbreaking work, substantial amounts of data have
Cosmochemistry become available to allow a reassessment of the composition of Mars from elemental data, including
Geochemistry tests of the basic assumptions in the geochemical models. The results adjust some of the concentrations
depletion of moderately volatile elements such as K (0.6 × CI), and strong depletion of highly volatile ele-
ments (e.g., Tl). The highly volatile elements are within uncertainties uniformly depleted at about 0.06 CI
abundances. The highly volatile chalcophile elements are likewise roughly uniformly depleted, but with
more scatter, with normalized abundances of 0.03 CI. Bulk planetary H2O is much higher than estimated
previously: it appears to be slightly less than in Earth, but D/H is similar in Earth and Mars, indicating a
among the terrestrial planets, a small range compared to CI chondrites (19,000). FeO varies throughout
ferences can be produced by varying oxidation conditions, hence do not suggest the terrestrial planets
were formed from fundamentally different materials. The broad chemical similarities among the terres-
trial planets indicate substantial mixing throughout the inner solar system during planet formation, as
suggested by dynamical models. © 2013 Elsevier GmbH. All rights reserved.
1. Introduction ...... 402
2. Approaches to estimating bulk composition ...... 402
2.1. Models based on geochemistry and nebular components ...... 402
2.1.1. Wänke and Dreibus model ...... 402
2.1.2. Morgan and Anders model ...... 404
2.2. Estimates based on isotopic composition of Martian meteorites ...... 404
2.3. Estimates based on geophysical properties of Mars ...... 405
2.4. Summary of the models ...... 405
3. Datasets ...... 405
4. Complications ...... 406
4.1.1. Highlands megaregolith ...... 406
4.1.2. Lava ﬂows ...... 406
4.1.3. Element ratios ...... 406
4.2. Heterogeneity of the Martian mantle ...... 406
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0009-2819/$ – see front matter © 2013 Elsevier GmbH. All rights reserved. http://dx.doi.org/10.1016/j.chemer.2013.09.006
402 G.J. Taylor / Chemie der Erde 73 (2013) 401–420
5. A reassessment of Martian bulk composition ...... 407
5.1. Estimating uncertainties ...... 407
5.2. Refractory element abundances...... 408
5.3. FeO and MnO ...... 409
5.4. Phosphorous ...... 411
5.5. Moderately volatile elements ...... 411
5.6. Highly volatile elements, including halogens ...... 412
5.7. Ni and Co ...... 412
5.8. Strongly siderophile elements ...... 412
5.9. H2O and D/H ...... 414
6. Discussion ...... 415
6.1. Mars is rich in FeO ...... 416
6.2. Depletion of volatile elements ...... 416
6.3. Water: abundance and source ...... 416
6.4. Highly siderophile elements: implications for accretion ...... 417
6.5. Halogen concentration of the crust ...... 417
6.6. The core ...... 417
6.7. Comparing planet compositions ...... 417
Acknowledgements ...... 418
References ...... 418
1. Introduction geochemistry, isotopic geochemistry, or geophysics, but there is
no reason why a blended model cannot be used.
An accurate assessment of the bulk chemical composition
of Mars is fundamental to the entire geologic history of Mars,
including accretion, differentiation, aqueous alteration to produce
2.1. Models based on geochemistry and nebular components
sediments, and the initial concentrations of important volatile ele-
ments. For example, knowing the bulk composition in principle
2.1.1. Wänke and Dreibus model
allows us to understand crystallization and cumulate overturn in
Wänke (1981, 1987), Dreibus and Wänke (1984, 1987), Wänke
and Dreibus (1988, 1994), Longhi et al. (1992), and Halliday et al.
the crust through intrusion and extrusion of basaltic magmas, and
(2001) estimate the bulk composition from element correlations
the formation of the distinctive source regions (e.g., enriched and
in Martian meteorites, with the assumption that refractory ele-
depleted shergottites) of Martian meteorites. These processes, of
ments are present in chondritic abundances. This model is directly
course, make it tricky to extract the bulk composition from geo-
tied to Mars through element abundances in Martian meteorites,
chemical and geophysical data. An example of the complexity in
and this direct link to samples of Mars has made the model
just modeling magma ocean crystallization and cumulate overturn
exceedingly robust, explaining why it is generally considered to
can be found in Elkins-Tanton et al. (2003, 2005). Fortunately, we
be the standard model for Martian bulk composition. Wänke and
have a solid database for the composition of the surface of Mars
Dreibus assume all elements more refractory than Mn are in chon-
and a good understanding of element behavior during petrologic
dritic proportions. From the cosmochemical viewpoint, refractory
processing. The database includes published meteorite analyses of
and volatile tendencies are related to their condensation tem-
the continuously expanding collection of Martian meteorites, and
perature in the solar nebula. The rough deﬁnitions of refractory
orbital and lander datasets.
and volatile and some gradations are shown in Table 1, using
This paper reviews models for Martian bulk composition and
50% condensation temperatures from Lodders (2003), calculated
makes a complete reassessment in light of the substantial amount −4
at a pressure of 10 atm. The 50% condensation temperature
of data obtained during the past two decades. It begins with
is simply the temperature at which half the mass of an ele-
an overview of the existing bulk composition models and their
ment has condensed (the remainder is in the gas). The standard
approaches, summarizes the datasets available and their util-
Wänke–Dreibus model for Mars bulk composition is shown in
ity, discusses the likelihood that data from the surface provides
Tables 2 and 3.
global information, and provides a complete reassessment of the
As an example of the Wänke–Dreibus approach, consider how
bulk composition of Mars in light of new data. I emphasize the
they derive the FeO content of the mantle from the FeO/MnO ratio
composition of bulk silicate Mars, but brieﬂy discuss models
in meteorites. These oxides do not fractionate from each other
for core composition, which are geochemically less well con-
coefﬁcients are about 1 for both), so igneous processes tend to
preserve their mantle values. MnO values in Martian meteorites
are in the range 0.4–0.6 wt%. The CI chondrite MnO concentration
2. Approaches to estimating bulk composition is 0.46 wt%, implying that Mars is not depleted in MnO. FeO/MnO in
Martian meteorites is 39.1 (but see in Section 5), and FeO/MnO in
Three main approaches have been used to estimate the bulk CI chondrites is 100.6. Thus, if MnO is at CI abundance (0.46), then
chemical composition of Mars. One takes a cosmochemical FeO in the mantle is 39.1 × 0.46 = 17.9 wt%, in reasonable agree-
approach by deﬁning different components and determining their ment with bulk Martian meteorite and GRS orbital data (Taylor
abundances in Mars. Another uses isotopic compositions of Mar- et al., 2006a), and with models derived from the moment of inertia
tian meteorites and chondrite groups to deﬁne the abundances (Bertka and Fei, 1998a,b). Other correlations include, for example,
of the chondritic raw materials that accreted to Mars. A third volatile elements with refractory elements (K/La, K/Th), correla-
uses broad geophysical properties to deﬁne mantle mineralogy. tions among the alkalis (K/Rb, Rb/Cs), and among the halogens
Note that these approaches have tended to be dominated by (Br/Cl). Element correlations tend to be reliable for element pairs
G.J. Taylor / Chemie der Erde 73 (2013) 401–420 403
Elements classiﬁed by geochemical behavior and 50% condensations temperatures at a pressure of 10 atm (Lodders, 2003).
Category Temperature range (K) Lithophiles Siderophiles Chalcophiles
Refractory ≥∼1300 Zr, Hf, Sc, Y, lanthanides, Th, Re, Os, W, Ir, Mo, Ru, Pt, None
U, Al, Ti, Ta, Nb, Ca, Sr, Ba, V, Rh, Ni, Co, (Fe), Pd
Mg, Si, Cr, (Fe)
Moderately volatile 1230–800 P, Mn, K, Ga, Na, Cl , Rb, Cs, Au As, Cu, Ag, Sb, Ge
Highly volatile 750–250 F, Cl , Br, I None Bi, Pb, Zn, Te, Sn, Se, S, Cd, In, Tl, Hg
Ultra volatile <182 H/H2O, N, C None None
The classiﬁcation into lithophile, siderophile, and chalcophile elements is not clear-cut. In the absence of metallic iron or sulﬁdes, for example, elements behave like
Cl condensation temperature might be much lower than calculated by Lodders (2003), similar to those of Br and I.
Major element concentrations in models for bulk silicate Mars.
a b c d e
Wänke and Dreibus Morgan and Anders Sanloup et al. Lodders and Fegley Khan and Connolly
SiO2 44.4 41.6 47.5 45.39 44
TiO2 0.14 0.33 0.1 0.14 –
Al2O 3.02 6.39 2.5 2.89 2.5
Cr2O3 0.76 0.65 0.7 0.68 –
FeO 17.9 15.85 17.7 17.21 17
MnO 0.46 0.15 0.4 0.37 –
MgO 30.2 29.78 27.3 29.71 33
CaO 2.45 5.16 2.0 2.36 2.2
Na2O 0.50 0.1 1.2 0.98 –
K2O 0.04 0.01 – 0.11 –
P2O3 0.16 – – 0.18 –
Total 100.03 100.02 99.4 100.00 98.7
Wänke and Dreibus (1994).
Morgan and Anders (1979).
Sanloup et al. (1999) model EH45:H55.
Lodders and Fegley (1997).
Khan and Connolly (2008).
with similar bulk partition coefﬁcients, such as the incompatible Wänke (1981). Component A is reduced and does not contain ele-
elements K and Th or La, or the compatible elements Mg and Cr. ments more volatile than Mn (this was originally formulated by
Inherent in the Wänke–Dreibus model is the concept that two Wänke and Dreibus as more volatile than Na, but subsequent papers
components combined to form Mars (and the other terrestrial plan- used the slightly more refractory element Mn as the cut-off). The
ets). It stems from ideas developed earlier by Ringwood (1979) and more refractory elements are in CI abundances. Fe and siderophile
Bulk composition of Martian crust + mantle (primitive Martian mantle).
W-Da M-Ab L-Fc W-Da M-Ab L-Fc
Be (ppb) 52 109 – Mo (ppb) 118 – 17
F (ppm) 32 23.6 41 In (ppb) 14 0.10 12
Na (%) 0.37 0.071 0.73 I (ppb) 32 0.59 120
Mg (%) 18.2 18.0 17.8 Cs (ppb) 70 25.9 154
Al (%) 1.60 3.37 1.53 Ba (ppm) 4.5 9.88 5.4
Si (%) 20.6 19.4 21.2 La (ppb) 480 926 400
P (ppm) 700 1985 740 Ce (ppb) 1250 2457 1120
Cl (ppm) 38 0.88 150 Pr (ppb) 180 309 167
K (ppm) 305 76.5 920 Nd (ppb) 930 1704 850
Ca (%) 1.75 3.68 1.68 Sm (ppb) 300 506 250
Sc (ppm) 11.3 23.5 10.5 Eu (ppb) 114 194 99
Ti (ppm) 840 1951 815 Gd (ppb) 400 691 395
Cr (ppm) 5200 4469 4640 Tb (ppb) 76 130 69
Mn (%) 0.36 0.116 0.284 Dy (ppb) 500 877 454
Fe (%) 13.9 12.3 13.4 Ho (ppb) 110 193 98
Co (ppm) 68 – 67 Er (ppb) 325 562 300
Ni (ppm) 400 – 140 Tm (ppb) 47 84 50
Cu (ppm) 5.5 – 2.0 Yb (ppb) 325 557 277
Zn (ppm) 62 41.9 83 Lu (ppb) 50 94 44
Ga (ppm) 6.6 2.43 4.4 Hf (ppb) 230 557 229
Br (ppb) 145 4.73 940 Ta (ppb) 34 56 29
Rb (ppm) 1.06 0.26 3.5 W (ppb) 105 440 80
Sr (ppm) 15.6 35.1 13.5 Tl (ppb) 3.6 0.17 10
Y (ppm) 2.7 6.41 2.8 Th (ppb) 56 125 56
Zr (ppm) 7.2 38.0 8.3 U (ppb) 16 35 16
Nb (ppb) 490 1938 –
Wänke and Dreibus (1994), slightly embellished by Taylor and McLennan (2009).
Morgan and Anders (1979); siderophile and chalcophile elements not included.
Lodders and Fegley (1997); the siderophile/chalcophile elements P, Co, Ni, Cu, Ga, Mo, In, are Tl were calculated from metal-silicate partition coefﬁcients.
404 G.J. Taylor / Chemie der Erde 73 (2013) 401–420
elements are metallic in Component A. Thus, the central assump- 2.2. Estimates based on isotopic composition of Martian
tion in this model is that the refractory elements are in CI relative meteorites
abundances. Component B is oxidized and contains all elements
in CI chondritic abundances. These are useful constructs in under- Lodders and Fegley (1997), Sanloup et al. (1999), and Burbine
standing the components that accreted to the planets, although it and O’Brien (2004) focused on ﬁtting the oxygen isotopic com-
is not certain that the planets were really constructed from known position of Mars, known from Martian meteorites, to mixtures
chondrites or their components. Morgan and Anders (1979) devel- of chondritic meteorites; their results are shown in Table 2 (and
oped a more elaborate multi-component model. Table 3 for Lodders and Fegley’s model). Sanloup et al. (1999) point
out two important features of the isotopic approach. One is that
oxygen is the most abundant element and other models do not
2.1.2. Morgan and Anders model determine it explicitly. The other is that a model based on oxy-
Ganapathy and Anders (1974) and Morgan and Anders (1979) gen isotopes has only one major assumption, in contrast to several
modeled Mars as a mixture of chondritic materials that had when considering assorted components.
been modiﬁed by the same limited set of processes that affected An isotopic estimate can also be tested by data from Mar-
chondrites, such as variations in condensation temperature and tian meteorites, orbiters, and landers. Lodders and Fegley’s (1997)
fractionation of metal from silicate. The model focuses on chon- assessment led to the estimate that Mars was constructed from
dritic components formed in the solar nebula. This may not be a mixture of 85% H-chondrites, 11% CV-chondrites, and 4% CI-
correct: as discussed below, Warren (2011) shows that terrestrial chondrites. In turn, this led to a predicted value of 16,000 for K/Th
planets, differentiated meteorites, and non-carbonaceous chon- in bulk Mars, much higher than GRS data indicate (5300; Taylor
drites are clearly distinguishable from carbonaceous chondrites et al., 2006a). Sanloup et al. (1999) took a similar approach in
(the low-temperature component in the Morgan and Anders, 1979, estimating the composition of Mars, arriving at a best ﬁt being
model). a mixture of 45% EH and 55% H chondrites. They did not esti-
Morgan and Anders (1979) proposed that there are three pri- mate the abundances of K and Th, but judging from their relatively
mary condensates from the solar nebula: a high-temperature, high estimated Na content, Sanloup et al. (1999) composition,
refractory-rich condensate; Fe–Ni metal; and magnesian silicates. appears to be enriched in moderately volatile elements compared
Morgan and Anders (1979) deﬁne a fourth component, FeS and to Mars.
FeO, which they postulate formed by reaction of Fe metal with Burbine and O’Brien (2004) ambitiously examined over 225 mil-
H2S and H2O, respectively, with the FeO ending up in the magne- lion combinations of oxygen isotopic and chemical compositions of
sian silicate component. They also suggest a “remelted” component, 13 chondrite groups, testing reasonableness by comparison with
by which they mean chondrules, hence depleted in volatiles (pre- assumed bulk FeO and Mg/Si and Al/Si for Mars (based on Mar-
sumably lost during high-temperature chondrule formation). Their tian meteorites). They extracted only limited major element data
magnesian silicates and chondrule components are not dramati- as their main goal was to test the feasibility that known mete-
cally different in composition. They further deﬁne a component orite types could be mixed to produce Earth and Mars. Burbine
rich in highly volatile elements, referred to in Morgan and Anders and O’Brien (2004) ﬁnd that the average of reasonable ﬁts to Mars
(1980) as the “unremelted” component. When implemented to bulk composition could involve contributions from all 13 chon-
estimate the planet’s bulk composition, they use four main compo- drite types modeled, but are dominated by enstatite and ordinary
nents, the refractories, magnesian silicates, metallic Fe, the volatile chondrites, which make up about 60% of the contributors (carbona-
component (including both moderately volatile and highly volatile ceous chondrites make up 25% and R chondrites contribute 14%).
elements). One problem with this approach is that Mg/Si and Al/Si are not
A central assumption in the Morgan and Anders approach is that reliable parameters to distinguish Earth and Mars (Filiberto et al.,
elements of similar volatility do not fractionate during nebular pro- 2006; McSween et al., 2009). Martian rocks, for example, range
cesses, allowing them to use four “index” elements (U for refractory from distinctly lower than Earth in Al/Si in Martian meteorites to
elements, Fe for metal, K for moderately volatile elements, and Tl or Earthlike or higher in rocks and soils at the Pathﬁnder and MER
Ar for highly volatile elements) to calculate the abundances of 83 landing sites and in global GRS data (see Fig. 3 in McSween et al.,
elements in the planet. When Morgan and Anders reported their 2009).
work in 1979, the idea of a group of meteorites being from Mars Warren (2011) took a unique approach by using stable isotopes
was just blossoming and quite controversial, so they did not use to evaluate mixing within the early solar system, speciﬁcally the
data from the meteorites. Their estimated Mars bulk composition isotopes of Ti, Cr, and O. His results support the notion that Mars
is given in Table 2. (and Earth) could be mixtures of non-carbonaceous meteorites. He
Morgan and Anders (1979) deﬁned the concentrations of the notes that for Cr and Ti especially there is a dichotomy between
index elements for refractory (U, Th) and moderately volatile materials formed in the outer solar system (mainly carbonaceous
(K) elements in Mars, using gamma-ray data from the Soviet chondrites) and those formed in the inner solar system (ordinary
orbiter Mars 5 and thermal models available at the time to predict chondrites, differentiated meteorites, and the Earth, Moon, and
a value of 620 for K/Th in bulk Mars, much lower than mea- Mars). This important observation is consistent with the results
sured by the Mars Odyssey gamma-ray spectrometer, GRS (5300; of Sanloup et al. (1999) model, which suggests that Mars is made
Taylor et al., 2006a) or in Martian meteorites. In spite of their exclusively of non-carbonaceous meteorites, and with the solutions
estimate for K/Th being far too low, the basic approach is inter- discovered by Burbine and O’Brien (2004), which involve only 25%
esting and it is not signiﬁcantly different from the Wänke–Dreibus carbonaceous chondrites. Lodders and Fegley’s (1997) best esti-
method. The idea that refractory elements are present at CI mate is also consistent as it indicates that the primary ingredients
chondrite relative abundances is common to both compositional of Mars involved 85% H chondrites and 15% carbonaceous chon-
models. The central difference is that Wänke and Dreibus, and my drites; the percentage of the carbonaceous chondrites component
reassessment of the Mars bulk composition (Section 5), use mul- is within the limits Warren (2011) estimates for the carbonaceous
tiple elements determined independently, rather than assuming, contribution to Mars. As noted, the major problem with isotopic
for example, that the highly volatile elements, though depleted, models for Martian bulk composition is that while isotopic com-
have CI (Wänke–Dreibus) or CV3 (Morgan and Anders) relative positions can be matched, the abundances of volatile elements are
abundances. over estimated.
G.J. Taylor / Chemie der Erde 73 (2013) 401–420 405
2.3. Estimates based on geophysical properties of Mars The oxygen isotopic approaches have the great virtue of having
only one parameter to match among mixtures of chondrite groups,
Geophysical properties such as the mean density, moment of and oxygen is the most abundant element. Major element oxide
inertia, tidal Love number and dissipation factor, and radius provide compositions do not differ much from those of the geochemical
independent information about the bulk composition of Mars, models, though the fact is that chondrites, like planetary mantles,
including the size and composition of the core. We do not yet have are all ultramaﬁc rocks. The signiﬁcant difference between isotope-
seismic data or heat ﬂow for Mars, two important measurements based and element-based estimates is the strong enrichment in
that will greatly enhance our knowledge of the Martian interior. volatile elements in the isotope models (Tables 2 and 3). This
However, the InSight (Interior Exploration using Seismic Investiga- enrichment is not seen in the GRS data: K/Th is 5300 for the Martian
tions, Geodesy and Heat Transport) mission, to be launched in 2016, surface versus 16,400 in Lodders and Fegley’s (1997) model. Sim-
will make both seismic and heat ﬂow measurements. Even without ilar differences are seen in the estimated abundances of the other
these data, geophysical studies have contributed to an improved alkalis, halogens, and highly volatile elements such as Tl. Lodders
understanding of the Martian interior and its composition. and Fegley (1997) suggest that aqueous leaching in the mantle and
One geophysical approach is to begin with a compositional hydrothermal alteration in the crust redistributed the volatile ele-
model (usually the Wänke–Dreibus model) and calculate (e.g., ments. Taylor et al. (2006a,b) argue against this concept on the basis
McGetchin and Smyth, 1978; Longhi et al., 1992; Sohl and Spohn, of only modest variations in K/Th in the crust. It is possible that the
1997; Sanloup et al., 1999) or do experiments (e.g., Bertka and Fei, materials that mixed to make Mars were like chondritic meteorites,
1997) to determine how the mineralogy varies with pressure and as Lodders and Fegley (1997) and Sanloup et al. (1999) propose, but
temperature inside the planet. The results can be used to calculate that these components had not yet acquired their full complement
bulk density and moment of inertia for comparison with Martian of volatiles. Alternatively, volatiles could have been lost during the
geophysical properties. accretion process (e.g., O’Neill and Palme, 2008). More likely, the
A second geophysical approach is to invert geophysical data accreting protoplanets were differentiated, hence with the charac-
to construct mineralogical models of the interior, including phase teristics of differentiated meteorites (e.g., lower volatile contents
changes (Khan and Connolly, 2008). This approach has the advan- than chondrites).
tage of determining the bulk chemical composition for major It is reassuring that the estimates based on geophysical proper-
elements from mineralogy and mineralogical variation with depth ties of Mars are similar to those obtained by the other independent
(pressure), with few assumptions. It does not assume CI chondrite methods (Table 2). Although MgO is somewhat higher, it is prob-
abundances, a particular mix of chondrites, or that we can infer ably within error of the other estimates, and FeO (17 wt%) falls in
composition from geochemical correlations. It does not, of course, the narrow range of 15.8–17.9 wt%. The geophysical data almost
give us minor and trace element concentrations. Nevertheless, it is certainly provide the most reasonable estimate for core size and
of great utility and serves as an independent monitor of geochem- composition, as discussed in Section 6.
Khan and Connolly (2008) use Gibbs energy minimization in
the system CaO–FeO–MgO–Al2O3–SiO2 (which make up 98% of 3. Datasets
bulk silicate Mars) to calculate the equilibrium mineralogy as a
function of depth and temperature inside Mars. The minimization Since the groundbreaking work of Wänke and Dreibus, substan-
technique to compute phase equilibria are described in detail by tial amounts of data have become available to allow a reassessment
Connolly (2005). The results of the calculations, constrained by of the composition of Mars from elemental data. Instead of only ∼10
measured geophysical properties (mean density, moment of iner- Martian meteorites that informed Wänke and Dreibus’ work, we
tia, Love number, tidal dissipation factor) are shown in Table 2. now have over 60 distinct Martian meteorites. Chemical, petrologic,
The major element oxide concentrations are quite similar to the mineralogic, and isotopic data are compiled in the Martian Meteorite
Wänke–Dreibus composition, though MgO is distinctly higher in Compendium (Meyer, 2013), a crucially valuable resource. I have
the composition determined by Khan and Connolly (2008), giving used the data reported in the Compendium to compile a database of
Mg/Si higher than in CI chondrites, which have the highest Mg/Si Martian meteorite compositions. For each meteorite, the database
of any chondrite group. consists of averages of multiple analyses of the same meteorite,
but for certain sets of elements, such as many volatile chalcophile
elements, data from only one study are available. A particularly
2.4. Summary of the models useful paper besides the Compendium is the careful summary of
compositional data available through 1997 provided by Lodders
The Wänke–Dreibus geochemical model has become the (1998).
standard for Mars bulk chemical composition. The approach used The meteorite data have the highest ﬁdelity of any data avail-
by Morgan and Anders (1979) is not fundamentally different. Its able, but are restricted to the random collection of Martian
estimate is quite different from that given by Wänke and Dreibus, meteorites. There is a bias toward younger ages in the collection,
particularly for volatile elements for which Morgan and Anders with all shergottites having ages < 500 Ga and the nakhlites and
(1979) values are signiﬁcantly lower. As noted above, for example, Chasigny have ages of only 1.3 Ga. The only older ages reported
Morgan and Anders (1979) estimate K/Th of 620, considerably less thus far are 4.1 Ga for ALH 84001 (Terada et al., 2003; Bouvier
than the global surface value measured by the Mars Odyssey GRS, et al., 2009; Lapen et al., 2010) and 2.1 Ga for NWA 7034 (Agee
5300 (Taylor et al., 2006a). However, if good GRS measurements et al., 2013). Additional data come from orbiting spacecraft and
and Martian meteorites had been available to Morgan and Anders rovers. Of particular importance for understanding Martian bulk
(1979), most of the differences between their estimate and that of chemical properties are the data from the Mars Odyssey gamma-
Wänke and Dreibus would be minimal. The essential point is that ray spectrometer (GRS). This instrument operated for eight years
both geochemical models depict Mars as composed of a refractory in Martian orbit, resulting in global data for K and Th, and equa-
component (condensation temperatures equal to or higher than torial data (from approximately 50 N to 50 S latitude) for Fe,
that of Mn) present in chondritic (speciﬁcally CI) relative abun- Si, and H, with less quantitative information for S, Ca, and Al.
dance, Fe partitioned between FeO and metallic Fe, and volatiles Nominal spatial resolution is 5 , about 500 km. While not impres-
depleted. sive spatial resolution by the standards of imaging spectrometers,
406 G.J. Taylor / Chemie der Erde 73 (2013) 401–420
it is ideal for global data analysis problems. The GRS probes 4.1.3. Element ratios
to a depth of a few tens of centimeters. GRS data are sum- The ratios of elements with very similar geochemical behavior in
marized in Boynton et al. (2007, 2008) and available online at igneous systems will reﬂect their ratio in the mantle. For example,
http://geo.pds.nasa.gov/missions/odyssey/grs.html. K and Th do not readily fractionate, as shown by their similar, and
missions provide excellent data on rocks on the surface. Elemental Borg and Draper, 2003; Hauri et al., 1994). Both elements are incom-
data were obtained by the alpha particle X-ray spectrome- patible and their concentrations in magmas are not greatly affected
ter (APXS). The rocks abraded with the rock abrasion tool by source rock composition or crystallizing phase, even when gar-
(RAT) on the MER missions are particularly useful as they net is involved. There are interesting exceptions, however. Th is
and the abrasion process limits that amount of soil masking form late in the crystallization of a magma and are unlikely to be
the underlying rock. Soil analyses by themselves are useful, retained in a mantle source region, so probably do not play a role in
especially for assessing global average elemental abundances. fractioning K from Th during igneous processes. However, in prin-
Soil data are available online at http://pds-geosciences.wustl.edu/ ciple, it could be signiﬁcant if mantle regions were metasomatized
missions/mer/geo mer datasets.htm. A good compilation and dis- by ﬂuids that contained phosphate components. K is compatible
cussion of the data can be found in Brückner et al. (2008). in phlogopite (Halliday et al., 1995) and somewhat compatible in
amphibole (Halliday et al., 1995), so if these phases were present
in the Martian mantle, it could lead to fractionation of K from Th.
In addition, a rock rich in K (named Jake M) has been analyzed in
Gale Crater, and classiﬁed as a mugearite by Stolper et al. (2013).
4.1. Is the Martian surface composition representative of the
Terrestrial mugerites form by extensive fractional crystallization of
alkaline basalts and are not abundant on Earth, implying that their
formation, if accompanied by fractionation of K from Th, are not
A signiﬁcant uncertainty about estimating the compositions of
of global compositional importance. No Th data are available for
the crust and mantle source regions is the extent to which we
the rock. Nevertheless, in general, K/Th in a lava ﬂow reﬂects the
can use data obtained primarily from surface samples (meteorites,
ratio in its mantle source region. Similar arguments can be made
rocks and soils at the landing sites, orbital data) to determine the
for other element pairs (see Section 5). Studies of terrestrial basalts
composition of the entire crust and mantle. Taylor et al. (2006a)
show that certain elements correlate strongly with one another
argue that we can use the surface value for certain parameters,
(e.g., Jenner and O’Neill, 2012), implying that they have the same
such as the K/Th ratio, to represent the entire crust (and to a great
bulk partition coefﬁcient. Following Wänke and Dreibus, I use such
extent the mantle). I summarize these and additional arguments
correlations as guides to searching for correlations in the Martian
4.1.1. Highlands megaregolith
4.2. Heterogeneity of the Martian mantle
A substantial fraction of the crust (possibly about half;
McLennan and Grotzinger, 2008; Taylor and McLennan, 2009) was
The shergottites exhibit a large range in geochemical and
constructed before the end of the heavy bombardment at ∼3.8 Ga.
isotopic composition. Their rare earth element (REE) patterns
It would have been repeatedly excavated and mixed by impacts,
range from severely light REE-depleted (CI-normalized La/Yb ∼0.1)
especially by basin-forming events, resulting in a thick (10–20 km)
with low abundances through to very slightly LREE-enriched (CI-
megaregolith. This early period of regolith formation may have
normalized La/Yb ∼1.2) with high abundances. The variations in
provided a substantial fraction of the present-day regolith in the
REE patterns correlate with isotopic compositions such as ini-
absence of signiﬁcant crustal recycling on Mars; that is, what goes
87 86 143 144
tial Sr/ Sr and Nd/ Nd (e.g., Norman, 1999, 2002) and
into the Martian crust stays in the Martian crust. Thus, soils and
geochemical parameters such as ratios among incompatible ele-
orbital (GRS) data may be sampling more than just the most recent
ments (e.g., McLennan, 2003) and oxygen fugacity (e.g., Wadhwa,
surface rocks and sediments.
2001; Herd et al., 2002). These variations have been interpreted
to indicate that the shergottites represent a mixture of two dis-
4.1.2. Lava ﬂows tinctive sources. Nakhlites are enriched in incompatible elements
Lava ﬂows visible at the surface of Mars are undoubtedly accom- (CI-normalized La/Yb ∼3) and isotopic data suggest a distinctive
panied by larger volumes of intrusive rock stalled inside the crust. source region (Foley et al., 2005). The origin of these distinctive
On Earth, the ratio of the intrusive to extrusive magma volumes is sources is important. A discussion of how they might have formed
5:1 (Crisp, 1984) in oceanic (basaltic) regions. If this holds for Mars, is beyond the scope of this paper, but just their existence is impor-
the abundant lava ﬂows visible at the surface (themselves forming tant for understanding how to extract the bulk composition of Mars
thick sequences of lavas) are accompanied by ﬁve times as much from element ratios.
intrusive magmas with similar composition. Furthermore, magma In addition, Martian meteorites may not be a representative
compositions reﬂect the compositions of their mantle sources, sample of the Martian crust or representative probes of its man-
assuming we can correct for fractionation processes as the magmas tle. This is made particularly clear in Fig. 1, a plot of the K/Th versus
migrated to the surface or stalled in magma chambers. Of course, K. Virtually all the meteorites are lower in K and in K/Th than the
we see only the uppermost, youngest lava ﬂows, and it is possible GRS global mean. This suggests that we have not sampled a signiﬁ-
that magma compositions changed with time. In fact, on the basis of cant mantle source region that is richer in volatile elements than the
Mars Odyssey GRS data, Hahn et al. (2007) suggested that changes sources represented by the meteorites. The two prominent samples
in surface compositions reﬂect changes in magmatic compositions (Nakhla and lherzolite NWA 1950) with K/Th around 10,000 may
with age. Nevertheless, the compositions of all available lava ﬂows be anomalous or affected by cumulate processes that fractionated
(meteorites, Gusev rocks, inferred from GRS) are highly informative K from Th. Their high K/Th does not appear to be due to terres-
about the compositions of the magmas from which the crust was trial weathering: Nakhla is a fall and both have Th/U > 2, whereas
constructed. samples altered on Earth have Th/U < 0.2. The central point is that
G.J. Taylor / Chemie der Erde 73 (2013) 401–420 407
reservoirs indicates that some caution is advised when extract-
ing planetary bulk compositions from a non-representative set of
samples. This problem is further complicated by the role of vari-
able oxidation state throughout the mantle, which might explain
some differences between the Martian meteorites and the Gusev
rocks (Tuff et al., 2013), but also changes geochemical behavior of
some trace elements, making the link from meteorite to mantle
composition less clear.
5. A reassessment of Martian bulk composition
I take a geochemical approach to reassess the bulk composi-
tion of Mars using the methods outlined in Longhi et al. (1992),
with emphasis on its bulk silicate composition. It is basically the
same approach as reported in the insightful papers by Wänke
Fig. 1. Trace element characteristics of Martian meteorites and the average bulk
surface (Taylor et al., 2006a) suggest diverse sources in the Martian mantle. Por- and Dreibus, but with the addition of estimates of the uncertain-
tions of the mantle must have higher K/Th to counter the lower values in meteorites
ties for each element and to show more of the correlations or
compared to the Mars Odyssey gamma-ray spectrometer (GRS) mean surface com-
lack of correlations than usually given. It also minimizes assump-
position. Similar characteristics are shown by other trace elements (McLennan,
tions about condensation temperatures (e.g., components A and B),
2003; Taylor et al., 2008). This mantle diversity adds an element of uncertainty
to determinations of bulk composition from trace element ratios. except for the unavoidable assumption that refractory elements are
present in CI relative abundances. The new (only slightly modiﬁed)
chemically-determined bulk composition is given in Tables 4 and 5.
Mars may contain a reservoir with higher K/Th than the GRS global
mean, needed to balance the low K/Th of the meteorites (Brandon
et al., 2012). The high K/Th ratio of the Martian surface measured by 5.1. Estimating uncertainties
GRS might be caused by secondary aqueous alteration processes,
implying that this ratio is not reﬂective of the bulk crust. Taylor Estimates of the uncertainties in Martian bulk composition
et al. (2006b) explored this possibility and although plausible, a stem from analytical and sampling uncertainties for each sam-
speciﬁc mechanism giving rise to a planetary-scale change in K/Th ple or the global GRS data, and from variations from sample to
ratio of surﬁcial materials was not identiﬁed. The existence of these sample. These can be quantiﬁed from the data. Uncertainties also
Bulk composition in revised model for bulk silicate Mars.
Conc 2- Method Ratio Conc 2- Method Ratio
Li (ppm) 3.0 1.7 S-B Li/Fe Cd (ppb) 9.6 6.1 A-A Cd/Dy
Be (ppb) 47.7 0.4 S-B Be/Nd In (ppb) 6.9 2.2 S-A In/Y
F (ppm) 21 13 A-B F/K Sn (ppb) 38.5 7.0 A-A Sn/Sm
Na (%) 0.40 0.08 S-A Na/Al I (ppb) 36 22 A-A I/Cl
Mg (%) 18.5 0.7 R – Cs (ppb) 95 37 S-S Cs/La
Al (%) 1.64 0.04 R – Ba (ppm) 4.37 0.21 R –
Si (%) 20.5 0.9 R – La (ppb) 439 48 R –
P (ppm) 675 215 S-A P/Yb Ce (ppb) 1170 110 R –
Cl (ppm) 32 9 S-A Cl/Th Pr (ppb) 176 5.8 R –
K (ppm) 309 36 A(GRS) Nd (ppb) 864 60 R –
Ca (%) 1.74 0.04 R – Sm (ppb) 274 13 R –
Sc (ppm) 11.0 0.4 R – Eu (ppb) 103 6 R –
Ti (ppm) 832 30 R – Gd (ppb) 374 31 R –
Cr (ppm) 4990 420 R – Tb (ppb) 67.3 13.2 R –
Mn (%) 0.34 0.05 S-A, D – Dy (ppb) 450 17 R –
Fe (%) 14.1 0.8 S-A Fe/Mn Ho (ppb) 106 6 R –
Co (ppm) 71 25 S-A Co/Ni Er (ppb) 306 37 R –
Ni (ppm) 330 109 S-A Ni/Mg Tm (ppb) 44.8 6.2 R –
Cu (ppm) 2.0 0.7 S-A Cu/Mg Yb (ppb) 308 26 R –
Zn (ppm) 18.9 2.9 A-A Zn/Sc Lu (ppb) 44.8 1.7 R –
Ga (ppm) 6.6 0.8 S-A Ga/Al Hf (ppb) 217 14 R –
As (ppb) 86 55 A-B As/Ce Ta (ppb) 27.2 1 R –
Se (ppb) 85 36 S-A Se/Yb W (ppb) 74 31 S-A W/Th
Br (ppb) 191 58 S-A Br/Cl Re (ppb) 0.88 0.66 A-A Re/Mg
Rb (ppm) 1.30 0.14 S-S Rb/La Os (ppb) 2.0 0.8 A-A Os/Mg
Sr (ppm) 14.6 0.7 R – Ir (ppb) 2.0 1.0 A-A Ir/Mg
Y (ppm) 2.89 0.52 R – Pt (ppb) 3.1 0.8 A-A Pt/Mg
Zr (ppm) 7.49 0.60 R – Tl (ppb) 1.28 0.71 S-A Tl/Th
Nb (ppb) 501 0.07 R – Bi (ppb) 0.60 0.41 A-A Bi/Th
Ru (ppb) 2.6 0.9 A-A Ru/Mg Th (ppb) 58 12 R –
Pd (ppb) 2.4 0.8 A-A Pd/Mg U (ppb) 16 3 R –
Ag (ppb) 4.2 2.8 A-A Ag/Dy
R: refractory element from volatile-free CI chondrite composition. S-B: slope of correlation line for olivine-phyric and basaltic shergottites. S-S: slope of correlation line
for shergottites, including lherzolitic shergottites. S-A: slope of correlation line for all martian meteorite types. A-B: average of ratio to abundance of another element for
olivine-phyric and basaltic shergottites. A-A: average of ratio to abundance of another element for all martian meteorites. A(GRS): Average Mars Odyssey GRS K, Th analysis
of surface. D: partition coefﬁcient for Mn in basaltic melt divided by Mn in peridotite, using experimental data.
Ratio of element to refractory element used in slope or average methods.
Based on concentrations of element in samples containing ≥15 wt% MgO (Brandon et al., 2012).
408 G.J. Taylor / Chemie der Erde 73 (2013) 401–420
(whose composition is known through the assumption of uniform
Major element concentrations in revised model for bulk silicate Mars.
refractory element abundances) and a volatile one. In most cases,
Concentration (wt%) Uncertainty ±2 sigma Method Ratio I was able to ﬁt a line through the origin, at least to within the
uncertainty of the intercept. That is, I forced the line through a zero
SiO2 43.7 1.0 Refrac –
TiO2 0.14 0.01 Refrac – intercept, allowing the slope to serve as a proxy for the average
Al2O 3.04 0.10 Refrac – ratio of the two elements plotted. This approach assumes that
Cr2O3 0.73 0.04 Refrac –
the scatter about a line is caused by the combination of analytical
FeO 18.1 1.0 Slope Fe/Mn
precision, sampling errors, natural variation within the sample set,
MnO 0.44 0.06 D –
and small differences in bulk partition coefﬁcients (Hanson, 1989).
MgO 30.5 0.05 Refrac –
CaO 2.43 0.01 Refrac – In some cases, R was less than 0.5, but the data did not
Na O 0.53 0.10 Slope Na/Al
2 scatter across a large compositional space (typically a range of
K2O 0.04 0.002 Average K/Th
less than a factor of two of the mean). In those clustered but not
P2O3 0.15 0.047 Slope P/Yb
linearly correlated cases I calculated the standard errors of the
mean of each element. The uncertainty in the ratio is given by
Refrac: refractory element from volatile-free CI chondrite composition. Slope: 2 2 1/2
Ra/b [( a/a) + ( b/b) ] , where Ra/b is the ratio of two elements
slope of correlation line. Average: average of ratio to abundance of another element.
with concentrations a and b, and and are the standard errors
D: partition coefﬁcient for Mn in basaltic melt divided by Mn in peridotite, using a b
experimental data. for elements a and b. In cases where a two-element comparison
Ratio of element to refractory element used in slope or average methods.
was simply too scattered to be meaningful, I did not determine the
concentration of the unknown element.
arise from assumptions made in determining the bulk composi-
tion, such as the assumption that elements more refractory than 5.2. Refractory element abundances
Mn are present in CI relative abundances. I show below that to
the extent we can test this assumption, the relative abundances The cosmochemical approach assumes that refractory elements
of refractory elements are in chondritic proportions. Another are present in Mars at chondritic relative abundances. Below, I
assumption is that a good correlation between two elements is assume that all elements with higher 50% condensation temper-
meaningful. This would seem to be strongest when the corre- atures than that of Cr are present at CI relative abundances. One
lations hold for datasets involving meteorites from all mantle way of testing this crucial, and common, assumption is to exam-
sources. ine the abundances of the refractory lithophile elements in major
I assessed the analytical uncertainties in several ways: For chondrite groups, normalized to CI abundances (Fig. 2). In spite of
the refractory elements, I simply used the 2-sigma uncertain- some irregularities in the abundance patterns, the general impres-
ties determined by Lodders (2003) for CI chondrite analyses. For sion is that the elements are largely unfractionated, as expected.
Mn I used the uncertainty in experimental determinations of the Differences in the compositions of chondrite groups reﬂect differ-
partition coefﬁcients between basaltic melts and peridotites at a ences in the histories and relative abundances of their constituents
range of pressures. These were determined by ﬁtting a line to the in the solar nebula and in their parent bodies (e.g., Huss et al., 2003).
experimental data, using the least squares method outlined by There is a curious increase in relative abundances from Y to the
York (1969). (The least-squares calculations used a spreadsheet more refractory elements, with roughly similar slopes. (Note that
generously provided by Randy Korotev, Washington University the concentration axis in Fig. 2 is linear rather than the conven-
in St. Louis). The calculation ﬁts a line and determines 2-sigma tional logarithmic abundance.) There is also a slight decrease at the
uncertainty in its slope and intercept. Similarly, for FeO, I used least refractory end, from Eu to Cr. The slight depletion in Cr, Si, and
experimental data and ﬁt a line of the crystal/liquid partition coef- Mg is on the order of only 10%, which in principle could translate
ﬁcient for FeO, and its 2-sigma uncertainty. to a 10% uncertainty (or perhaps a systematic over estimate) in the
For trace elements I used linear correlations when the square abundances of these elements in Mars.
of the correlation coefﬁcient (R ) was equal to or greater than I also tested element correlations among the refractory elements
0.5, and applied the York approach to determine the uncertainty in Martian meteorites (Fig. 3), using element pairs with similar
in the slope. Elements usually involved a refractory element geochemical behavior. The incompatible trace element pairs Th/La,
Fig. 2. Abundances of lithophile elements (Lodders and Fegley, 1998) in chondrite groups normalized to CI chondrites (using the mean given by Lodders, 2003). Except for
an increase for the two most refractory elements (Zr and Hf), relative abundances are essentially unfractionated. This gives some conﬁdence that it is reasonable to assume
that these elements are present in chondritic relative abundances in Mars.
G.J. Taylor / Chemie der Erde 73 (2013) 401–420 409
Fig. 3. Correlations among pairs of refractory elements, plotted to test the concept that refractory elements in Mars are present in chondritic proportions. Slopes of the
correlations are within errors of the ratio in carbonaceous chondrites (using values reported by Lodders, 2003).
Zr/Hf, and Zr/Y have reasonably linear slopes with R > 0.7 (Fig. 3). Using the cosmochemical approach, I calculated the concen-
The ratios are within the 2-sigma uncertainties of the CI ratios. The trations of all elements equal to or more refractory than Cr, and
ratio of the compatible major element oxides Cr2O3/MgO forms a renormalized the sum of those elements to 100. It excludes Ni and
tight linear array (R of 0.91) with a statistically signiﬁcant inter- Co, which concentrate in metallic iron during core formation. It
cept. When the line is extrapolated to the bulk planetary MgO of also ignores oxygen at this stage. A complication arises for Fe as it
30.5 (Table 5), the MgO/Cr2O3 ratio is 30.3 ± 4.9, within error of is partitioned between both FeO and metallic Fe in Mars, indicating
the CI value of 26.5. While not overwhelmingly convincing, these that in the calculation of refractory elements in bulk silicate Mars
tests are consistent with the assumption that refractory elements we should use only the Fe in silicate Mars. This is derived from
are present at CI relative abundances in Mars. FeO in the crust (see next section). All elements are then converted
to oxides; oxide concentrations in bulk silicate Mars are given in
Table 4. The uncertainties reﬂect the uncertainties in the average
CI chondrite value and the uncertainty in FeO (see below), which
matters because of the normalization to 100%. Compared to Wänke
and Dreibus (1994), my revision is slightly lower in SiO2.
5.3. FeO and MnO
Wänke and Dreibus derived the FeO concentration in Mars from
the MnO content in Martian meteorites. They noted that MnO has
partition coefﬁcients for major minerals and melts close to 1, so
its abundance in the mantle is the same as in the crust. The set
of meteorites available to them had an average MnO content of
0.46 wt%. Noting that this was the same as in CI chondrites, and that
FeO/MnO in CI chondrites was 100.6, they used the FeO/MnO ratio
in Martian meteorites (39.5 ± 1.2) and the MnO in the meteorites
to derive a bulk FeO of 17.9 ± 0.6.
This approach is rigorous, but there are two problems. One is
Fig. 4. FeO versus MnO in Martian meteorites and rocks at the Gusev landing site
that the partition coefﬁcient (MnO in peridotite/basaltic melt) is
on Mars (Ratted: abraded rock samples; Brushed: less abraded rock samples). Gusev
not exactly 1.0. Using experimental data reported by Takahashi
olivine basalts and alkaline basalts are from a data analysis by McSween et al.
(2006a,b). and Kushiro (1983), Walter (1998), Hertzberg and Zhang (1996),
410 G.J. Taylor / Chemie der Erde 73 (2013) 401–420
Wasylenki et al. (2003), and Le Roux et al. (2011), it is clear that the
crystal/liquid partition coefﬁcient for MnO is less than 1.0. The aver-
age for measurements in the pressure range 10–25 kb (expected
range for magma generation), is 0.93 ± 0.08 (2-). The revised aver-
age MnO in Martian meteorites is 0.48 ± 0.12 (ignoring Governador
Valadares, which has an anomalously high MnO content in the one
analysis available). The partition coefﬁcient and the average MnO
in the crust implies a bulk planetary MnO of 0.44 ± 0.08. Using the
revised FeO/MnO for the larger set of Martian meteorites, 40.3 ± 2.3
(2- ), giving an FeO concentration of 17.7 ± 2.3 wt%, essentially the
same as determined by Wänke and Dreibus (17.9 wt%)
In spite of the agreement between my new estimate using con-
siderably more meteorites in the database, an important caution is
in order. Igneous rocks analyzed at the Gusev landing site have
considerably higher FeO/MnO than that recorded by the mete-
orites (Fig. 4), a point raised by McSween et al. (2009). Almost all
analyses of basaltic rocks at Gusev, whether brushed or abraded
to remove adhering dust and weathering products, have higher
FeO/MnO than do the meteorites. McSween et al. (2006a,b) deter-
mined the compositions of two main suites of rocks at Gusev,
olivine basalts and alkaline basalts, using all available data to
extrapolate measured compositions to the compositions of the
alteration-free igneous rocks. These are also plotted in Fig. 4 and
are slightly (olivine basalts) to signiﬁcantly (alkaline basalts) offset
compared to Martian meteorites. This may reﬂect distinct source
regions or oxidation state of the source regions (Tuff et al., 2013) for
the magmas from which the Gusev rocks formed, and raise ques-
tions about the utility of using FeO/MnO to deduce bulk FeO in
Mars. Fig. 5. Histograms of FeO (wt%) distributions from the Mars Odyssey gamma-ray
spectrometer (GRS, from 5-degree grid points, Taylor et al., 2006a) and (bottom)
An alternative approach is to determine bulk FeO independent
Martian meteorites, abraded (Ratted) rocks from the Gusev landing site, and mean
of its correlation with MnO. All data indicate without question that soils from Gusev, Meridiani, Pathﬁnder, and Viking-1 landing sites. Note the peaks at
18–19 wt%; even the lowest values are higher than typical Mid-Ocean Ridge basalts
the Martian crust is richer in FeO than terrestrial basaltic rocks
∼ on Earth (∼10 wt%).
( 10 wt%) as shown by FeO concentrations measured by GRS, and
in meteorites, soils, and abraded rocks (Fig. 5). FeO peaks at close to
19 wt% with a global mean of 18.4 ± 0.2 (based on summing all spec-
composition used was that of the Homestead chondrite, which
tra, with the 2- uncertainty based on counting statistics (Taylor
is similar to the Wänke–Dreibus bulk Mars composition. They
et al., 2006a), but lower values, down to 13 wt% in some Gusev
found that the Homestead composition produced magmas that
rocks are signiﬁcant. The values signiﬁcantly below the meteorite
contained too much FeO than even the FeO-rich shergottites con-
and GRS distributions may indicate the presence of mantle regions
tain, coupled with the correct CaO/Al2O3 ratio, or had a reasonable
with lower FeO than typical.
FeO concentration coupled with the wrong CaO/Al2O3 ratio. They
Experimental data on FeO partitioning during melting of peri-
concluded that the solution was a polybaric differentiation process,
dotite provide a way to calculate the mantle FeO from the mean
surface FeO. Fig. 6 shows data compiled from several studies (see
caption for references). It assumes that the solid/melt partition
coefﬁcient for FeO is closely approximated by the ratio of FeO in the
original peridotite solid to the FeO in the melt. Data used are only for
cases where the amount of melting is no more than 25%. Note the
trend of decreasing D with increasing pressure. In Mars, magmas
appear to form at pressures of 10–25 kb (Filiberto and Dasgupta,
2011). In this range, the apparent D(FeO) straddle the D = 1 line.
Using the linear ﬁt to the data, at 20 kb, D(FeO) is 0.95 ± 0.06. Using
the mean crustal FeO determined by the GRS, 18.4 wt%, and using
D(FeO) of 0.95, I calculate a mantle FeO of 18.1 ± 2.2 wt% (2);
this is the value used in Table 5. This agrees within uncertainties
with the result obtained from FeO/MnO of the Martian meteorites.
More importantly, the geochemical estimate agrees with the FeO
determined by Khan and Connolly (2008) on the basis of bulk geo-
physical properties of the planet, 17 wt%. This agreement indicates
that lower FeO sources like those giving rise to some Gusev basalts
Fig. 6. Apparent partition coefﬁcient (concentration in initial peridotite divided by
make up a small fraction of the Martian mantle. composition in equilibrium melt) versus pressure in kilobars. Dashed lines bracket
Agee and Draper (2004) made an independent assessment of typical pressures thought to represent source depths for Martian magmas (Filiberto
and Dasgupta, 2011). In this pressure range, the partition coefﬁcient is close to unity,
the FeO concentration in the Martian interior through experi-
implying that surface basalt FeO concentrations are reliable indicators of source
ments on an L-chondrite composition at 5 GPa pressure. Their
region (hence mantle) concentrations.
intent was to try to determine if the mantle source rocks for the
Data are from experimental studies by Falloon and Green (1988), Faloon et al.
shergottite group of Martian meteorites could be formed from a
(1988), Takahashi and Kushiro (1983), Hertzberg and Zhang (1996), Walter (1998),
relatively FeO-rich composition like L chondrites. The L-chondrite Wasylenki et al. (2003), and Agee and Draper (2004).
G.J. Taylor / Chemie der Erde 73 (2013) 401–420 411
Fig. 7. P versus refractory element Yb for olivine phyric and basaltic shergottites.
The correlation coefﬁcient indicates a signiﬁcant correlation between moderately
volatile P and refractory element Yb. Knowing Yb from the assumption of chondritic
refractory abundance allows calculation of the P abundance in bulk Mars.
but with a shergottites source containing an FeO content more like
H-chondrites than L-chondrites, ∼15 wt% FeO.
Concentrations (Table 5) of FeO (18.1 wt%) and MgO (30.5 wt%)
result in an Mg# [100 × molar Mg/(Mg + Fe)] of 75 ± 4. This is in the
middle of the range of Mg# in the most magnesian value in the
cores of olivines in olivine-phyric shergottites, which range from
70 to 86 (Meyer, 2013). Thermal emission spectroscopy show that
magnesian olivine is exposed in places on Mars, the most notable
of which is a ring of the Argyre basin (Koeppen and Hamilton, Fig. 8. Moderately volatile elements Ga and Na plotted against refractory element
Al. Bulk Mars concentrations for Ga and Na can be determined from the Ga/Al and
2008; Lane and Goodrich, 2010). Comparing to laboratory spectra
Na/Al ratios. Data represent analyses of all Martian meteorite types.
of experimentally produced olivine suggests that the Argyre olivine
occurrences are associated with olivine-rich basalts with olivine
Mg# in the range 85 ± 5, within uncertainty of the most magne-
likely to reﬂect the bulk planetary ratio. K/Th varies across the Mar-
sian olivine in olivine-phyric basalts (86, Musselwhite et al., 2006).
tian surface (Fig. 9). Taylor et al. (2006b) examined the possible
Taken together, these data show that the Martian mantle varies in
reasons for this, with no deﬁnitive answers. It is clear, however,
Mg#, but that the bulk composition is close to an Mg# of 75. The
that the distribution is close to Gaussian and well deﬁned. The mean
somewhat lower FeO reported by Agee and Draper (2004) results in
K/Th is 5330 440 (2-). In this case, the uncertainty is calculated
a higher Mg#, ∼79. The geophysically-determined FeO concentra-
from the sum of all the spectra obtained by the GRS, involving over
tion of 17 wt% combined with the geochemically determined MgO × 7
2 10 s of counting time. Thus, it reﬂects the counting statistics,
of 30.5 produces a bulk Mg# of 76.8. Khan and Connolly’s (2008)
not the variation in the data, but does reﬂect the accuracy with
MgO is higher than the geochemically based one (Table 2), and
which we know the mean. Using the K/Th ratio and the Th con-
yields a bulk Mg# of 77.6. Even the highest of these values is lower
centration of 58 ppb (Table 4), I calculate a bulk K concentration of
than the terrestrial bulk Mg# of 89.
309 ± 26 ppm.
I obtain the concentrations of the other alkali elements by the
correlations with La in shergottites only (Fig. 10A, C). Including the
other Martian meteorites renders the correlations much weaker. Rb
Phosphorous is a lithophile element with some siderophile
is particularly well correlated with La (Fig. 10A), revealing a Rb/La
tendencies. It correlates well with Yb (Fig. 7). The slope of the cor-
ratio of 2.91. Using a bulk La content of 439 ppb (Table 4) I esti-
relation line gives the P/Yb ratio in Mars, assuming these elements
mate a bulk Rb concentration of 1.27 ± 0.13 ppm. Rb also strongly
reﬂect the bulk composition. Multiplying by the Martian bulk Yb
correlates with K and although the K/Rb ratio is commonly used to
of 308 ppb (Table 4) gives a P concentration of 675 ppm (0.15 wt%
estimate planetary geochemical reservoirs, it is in principle better
P O ). This is somewhat depleted compared to CI chondrites (P/CI
2 5 to use the ratio to a refractory element (La) rather than to an ele-
is 0.7), perhaps indicating that P was partitioned partially into the
ment (K) that is itself determined by a ratio. Nevertheless, using
core during primary Martian differentiation.
the strong correlation between K and Rb (Fig. 10B), I estimate a Rb
content of 1.45 ± 0.25, within error of the estimate using Rb/La. This
5.5. Moderately volatile elements gives some credence to restricting the Rb calculation to the olivine
phyric and basaltic shergottites only. The Cs (Fig. 10C) data form a
Gallium and sodium correlate well with Al (Fig. 8) with corre- good linear array. Using the bulk La value (Table 4) and a slope of
lation coefﬁcients (R ) of 0.92 (Ga–Al) and 0.75 (Na–Al). Using the 0.180, I estimate a Cs abundance of 79 31 ppb.
slopes, which provide Ga/Al and Na/Al, and the concentration of Other moderately volatile elements include As, Cu, Ag, and Sb, all
Al in Mars (Table 4, 1.63 wt%), I calculate that bulk silicate Mars somewhat chalcophile or siderophile. I found no signiﬁcant corre-
contains 6.6 ppm Ga and 0.40 wt% Na (0.53 wt% Na2O). lation between Sb and any element, and the plots were so scattered
Potassium is best determined in the GRS data as it more likely that an average of Sb with a geochemically similar element would
represents a global average. It correlates well with Th as both not be informative. It is not clear whether the lack of correlation
are strongly incompatible large-ion lithophile elements, hence are is due to analytical issues or complicated geochemical behavior
412 G.J. Taylor / Chemie der Erde 73 (2013) 401–420
Fig. 9. Histogram from Mars Odyssey gamma-ray spectrometer determinations (5-degree grid points) of K/Th ratio of Martian surface (Taylor et al., 2006a,b). Data represent
the ratio in upper few tens of centimeters of surface. Global mean and uncertainty are calculated on the basis of counting statistics of a global spectrum of over 2.4 × 10 s
counting time. Uncertainty represents the uncertainty in our knowledge of the mean and does not reﬂect the natural variation in K/Th.
in Mars. As and Ag also do not form linear arrays when plotted 948 and 1006, respectively (Lodders, 2003). Furthermore, Cl and Br
against suitable elements, but do cluster sufﬁciently to allow esti- are well correlated at a ratio close to chondritic (Fig. 13B), implying
mating their abundance from averages, with Ce for As and Dy for that Br has a normalized abundance of 0.5, too. This is surprising
Ag. Results are given in Table 4. Copper correlates well with Mg in light of low 50% condensation temperature of 546 K for Br. The
(Fig. 11), steadily declining with increasing MgO, indicating less condensation temperatures might be wrong, of course, as they are
compatible behavior for Cu than Mg. I model the bulk Cu as the dependent of what phases are assumed to contain trace elements.
value where MgO equals bulk Mars (30.5 wt%). This results in a Cu Or the chondritic Cl/K is just a coincidence and Cl is actually lower
concentration of 2.0 0.7 ppm. than in bulk Mars than in the uppermost crust.
If we assume that the Martian meteorites did not lose their mag-
matic Cl, then a plot of Th versus Cl (Fig. 13A) allows us to estimate
5.6. Highly volatile elements, including halogens a bulk Cl value of 32 ± 9 ppm. This is lower than Wánke and Dreibus
determined, 38 ppm, but within the uncertainties. Given the good
The highly volatile elements have 50% condensation tempera- correlation between Br and Cl (Fig. 13B), I estimate a Br concentra-
tures less than 750 K (Lodders, 2003), and include Bi, Zn, Sn, Se, tion of 191 ± 58 ppb. The Br/Cl ratio is close to chondritic (Fig. 13B).
Cd, In, and Tl, plus the halogens. (Other elements in this group Iodine was determined from the average I/Cl in Martian meteorites
do not have good correlations with a refractory element. These (data are not well correlated). The normalized abundances of Cl, Br,
include Pb, Te, and Hg.) The two best-determined elements are Tl and I are all in the range 0.05–0.07. Low halogens are supported by
and In (Fig. 12). Both correlate acceptably with a refractory ele- analyses of melt inclusions in olivine phenocrysts in olivine-phyric
ment, from which we can determine their concentrations in bulk shergottite Y 980459 (Usui et al., 2012). They report that only mod-
Mars (Tl is 1.4 0.7 ppb, In is 6.9 2.2 ppb). The other elements are est losses of F and Cl could have taken place because both correlate
determined from average values in Martian meteorites compared well with Na in both melt inclusions and in the glassy groundmass
to the average of an element with similar geochemical behavior: of the rock. Thus, it seems safe to conclude that the Martian mete-
Bi/Th, Zn/Sc, Sn/Sm, Se/Yb (correlates with R of 0.5), and Cd/Dy. orites in general record the halogen concentrations in the mantle
Results are given in Table 4. and the original and Wánke and Dreibus approach is appropriate.
The halogens are potentially valuable as they behave as incom-
patible elements, hence their concentrations in the mantle can be 5.7. Ni and Co
determined from their correlations with refractory incompatible
trace elements. Furthermore, Br and I have low condensation tem- Ni and Co correlate well with Mg (Fig. 14). From the ratio of each
peratures, thus potentially helping us determine the abundance of to Mg and the Mg concentration in Mars (Table 4), we get a Ni con-
all highly volatile elements. However, bulk Martian halogens are centration of 330 ± 109 ppm and a Co concentration of 71 ± 25 ppm.
difﬁcult to assess because, except for F, they are readily lost from These are lower than the bulk planet has (∼2 wt% and ∼0.1 wt%,
lava ﬂow surfaces in gas phases, so our database of mostly extrusive respectively) because most of inventory of these elements is in the
rocks is ambiguous with regards to the magmatic source regions in core.
Because loss is so common for halogens from lavas, Taylor et al. 5.8. Strongly siderophile elements
(2010) suggested that the mean surface Cl/K (1.27, determined by
GRS), which is close to the chondritic value of 1.28, may reﬂect Brandon et al. (2012) present high quality analyses of highly
the bulk composition of silicate Mars. If the chondritic Cl/K is not siderophile elements (Os, Ir, Ru, Pt, and Re) in shergottites. For
a coincidence (a distinct possibility considering that Cl is highly those with MgO greater than about 15 wt%, the siderophiles are
mobile in aqueous ﬂuids and heterogeneously distributed on both in chondritic proportions. If those abundances represent the bulk
rover and GRS scales), then the ratio of these two incompatible composition of Mars, hence assuming that the siderophile elements
elements suggests that the CI-normalized Cl abundance is about the in the other shergottites with lower MgO have been fractionated,
same as that of K, 0.6. This is consistent with the concentration of we can estimate the abundances in bulk silicate Mars from Brandon
Cl on the surface, 0.5 wt%. Such an elevated Cl concentration is also et al.’s (2012) data. I averaged the concentrations of the elements in
consistent with the similar condensation temperatures of Cl and K, shergottites measured by Brandon et al. (2012), and assumed that
G.J. Taylor / Chemie der Erde 73 (2013) 401–420 413
Fig. 12. Correlations of highly volatile elements versus refractory elements, for the
few data available for all Martian meteorite types. (A) Tl versus Th. (B) In versus Y.
Correlations are only modestly strong, but sufﬁcient to allow estimation of In and
Tl in bulk silicate Mars.
Fig. 10. Correlation of moderately volatile alkali elements versus refractory La (A,
C) and for Rb versus K (B). Correlations can be used to determine abundances of the
alkali elements (see text). Data are for shergottites (including lherzolitic shergot-
Fig. 11. Cu versus MgO concentrations correlate reasonably well, for all Martian
meteorite types. A Martian bulk silicate concentration for Cu can be calculated from
Fig. 13. Cl–Th (A) and Br–Cl (B) correlations for all Martian meteorites. Note that
the linear ﬁt for Cu versus MgO at the point where MgO has the bulk silicate Mars
Br/Cl is close to the chondritic ratio.
value of 30.5 wt% (Table 5).
414 G.J. Taylor / Chemie der Erde 73 (2013) 401–420
Fig. 15. H2O versus Cl for 5-degree grid points measured by Mars Odyssey gamma-
ray spectrometer (GRS). GRS actually determines concentration of H, which has been
converted to H2O equivalent (near the surface, H could be bound as OH or H2O). The
correlation coefﬁcient is substantially less than the 0.5 signiﬁcance level required
for other element pairs, but nevertheless informative (see text).
concentrations and an estimate of when apatite crystallized in the
lava, McCubbin et al. (2012) estimate that for these two samples,
their parent magmas contained between 730 and 2870 ppm H2O
(ignoring loss from the lavas). If they were produced by 10% partial
melting, their mantle source regions contained between 73 and
287 ppm H2O. Hallis et al. (2012) measured H2O in apatite crystals
in the unweathered fall Nakhla, ﬁnding H2O contents similar to
those in Shergotty (0.46–0.64 wt%). This suggests a similar water
Fig. 14. Siderophile elements Ni and Co versus Mg. Correlations are reasonable and
content for the mantle source of the nakhlites; using the approach
useful for deriving Ni and Co concentrations in bulk silicate Mars. Most of the Ni and
taken by McCubbin et al. (2010) I estimate a mantle concentration
Co inventories are probably in the core.
of 150–220 ppm. In contrast to these estimates, Usui et al. (2012)
analyzed H2O in melt inclusions in olivine in Y980459. They ﬁnd
their relative and absolute abundances reﬂect their mantle con-
no evidence for degassing of the inclusions, yet ﬁnd an average of
centrations. The results appear in Table 4 and are discussed below.
only 146 ppm in the melt inclusions, hence a pre-degassing magma
This approach is probably sound for Pt because during silicate par-
content with the same value. They calculate that this indicates a
tial melting, Pt has a partition coefﬁcient close to 1 (Jones et al.,
mantle source containing 15–47 ppm H2O, in the range estimated
2003). All these elements have almost identical abundances when
by Dreibus and Wänke (1987). One possible interpretation of
normalized to CI chondrites (using the values for Orgueil, Horan
these results is that water is heterogeneously distributed in the
et al., 2003), ranging from 0.0036 to 0.0045, except for Re (0.22).
mantle. In addition, the water content of the mantle is likely to
Excluding Re, the group of elements averages ∼50% of the value in
have decreased with time: the extensive ﬂuvial alteration of the
the terrestrial primitive upper mantle (Becker et al., 2006).
surface attests to signiﬁcant water release early in Martian history.
Thus, the young igneous rocks on which these mantle water
5.9. H2O and D/H estimates are based may reﬂect a degassed mantle that contained
signiﬁcantly more water initially.
Observations of the Martian surface from spacecraft revealed An independent estimate can be made from the mean concen-
that its crust has been modiﬁed by substantial ﬂuvial activity. trations of H2O and Cl on the surface of Mars as determined by
Landed spacecraft and studies of weathering veins in Martian mete- GRS (Boynton et al., 2008). Both species behave incompatibly dur-
orites have repeatedly conﬁrmed the presence of water in Mars. ing partial melting and both degas from lava ﬂows. If, in spite
However, Dreibus and Wänke (1987) estimate that bulk silicate of their complex behavior during weathering and other aqueous
Mars contains only 39 ppm of H2O, the equivalent of about 15% processing, they maintain an approximately constant ratio and
of a terrestrial ocean-equivalent and 7% of the lower estimate of both concentrate to the same extent in the upper crust, we can esti-
500 ppm in the bulk Earth (see review by Mottl et al., 2007). This mate their abundance from the H2O/Cl ratio and our estimate bulk
seems insufﬁcient considering the evidence for the role of water in Cl content of 32 ppm. GRS data (which measures H in the upper few
the evolution of the Martian crust. tens of centimeters) gives an equatorial (between ∼52.5 degrees
Direct measurements of water in Martian meteorites indicate north and south latitude) H2O concentration (water equivalent of
that the water contents of Martian magmas (e.g., McCubbin measured H) of 3.9 ± 1.9 wt% (2- of total variation; standard error
et al., 2010) are much higher than the venerable Dreibus and of 1500 points gives a 2- of the mean of 0.05 wt%). The large por-
Wänke (1987) estimate. McCubbin et al. (2010) estimate from tions of the crust at higher and lower latitudes have very high H2O
hydrous amphibole in melt inclusions in Chassigny that the mantle concentrations (e.g., Boynton et al., 2008), implying a much higher
source region for the Chassigny magma contained between 130 surface concentration, and higher H2O/Cl. Taking a very conserva-
and 250 ppm H2O, depending on the amount of partial melting tive water concentration of 5 wt% for the entire surface gives H2O/Cl
required to produce the observed magma. Leshin (2000) measured of 10 and implies a bulk Mars concentration of 330 ± 10 ppm. As a
the concentration of H2O in apatite in a depleted shergottite (QUE rough check, a plot of H2O versus Cl, when forced through zero
94201) and McCubbin et al. (2012) measured the concentration (Fig. 15), gives a H2O/Cl ratio of 8.1 ± 0.5, and a bulk Mars H2O of
of H2O in apatite an enriched one (Shergotty). From the observed 260 ± 16 (2-); however, the R is only 0.2. (The correlation line
G.J. Taylor / Chemie der Erde 73 (2013) 401–420 415
ﬁt is essentially the same as using the average equatorial global
Except for the interesting results from Y980459, meteorite data
and the blatant evidence for extensive water action on the surface
support a relatively wet bulk composition for Mars. Geophysical
modeling results seem to support the idea of a wet mantle. Ther-
mal evolution models of crustal evolution (Hauck and Phillips,
2002; Guest and Smrekar, 2005) require water abundances of
100–1000 ppm to produce mantle viscosities low enough to allow
for convection and ductile ﬂow of mantle materials. Combin-
ing all the data, I estimate that the Martian mantle contained
300 ± 150 ppm of H2O, but considering the morphological record
for substantial surface water, the primitive mantle might have con-
tained more than this amount. However, even a minimum value
of ∼300 ppm is about a factor of ten higher than the Dreibus
and Wänke (1987) estimate. Thus, their suggested mechanism for
enriching FeO in Mars, reaction between H2O and metallic iron dur-
ing accretion, was not effective enough to account for the observed
FeO concentration in bulk Mars.
The deuterium/hydrogen ratio might be diagnostic of the
sources of water to Mars (e.g., outer solar system objects such as
comets versus inner solar system objects, adsorbed nebula water
versus water in phyllosilicates in planetesimals). The atmosphere
is enriched by a factor of 5 in D/H (␦D value of a few thousand),
implying a large loss of water through sputtering by the solar wind
and thermal escape of H preferentially to D (Owen et al., 1988).
Weathering products in Martian meteorites also have elevated D/H
( D of a few thousand). To understand the initial D/H in Martian
water we need samples that have not been affected by atmosphere.
Because plate tectonics did not recycle the crust and its modiﬁed
D–H fractionated water, igneous rocks may contain the informa-
tion about the isotopic composition of primary Martian water, if
they have not been altered after emplacement in the crust or while
on Earth. Hallis et al. (2012) measured H isotopes in apatite in
Nakhla, a well-preserved fall. Terrestrial alteration is not detectable
and Martian weathering is identiﬁable. Hallis et al. (2012) show
that the Nakhla parent magma had water with a terrestrial-like
D (−78 to +188); the terrestrial mantle has ␦D of −140 to +60
(Boettcher and O’Neil, 1980; Michael, 1988; Ahrens, 1989; Deloule
et al., 1991; Bell and Rossman, 1992; Thompson, 1992; Graham
et al., 1994; Jambon, 1994; Wagner et al., 1996; Xia et al., 2002).
Usui et al. (2012) measured D/H in melt inclusions in olivine in
Y980459, ﬁnding a ␦D of +275. It appears that bulk Mars began
Fig. 16. Abundances in bulk silicate Mars normalized to CI chondrites for lithophile
with a D/H similar to that of Earth, though it could be slightly (top), chalcophile (middle), and highly siderophile (bottom) elements. Variations
elevated. in chalcophile elements probably reﬂect the extent to which the elements concen-
trate in sulﬁde phases or their behavior when conditions force them to behave as
lithophile elements. Except for Re, highly siderophile elements are present in CI
relative abundances (Brandon et al., 2012) and have abundances about a hundred
times higher than expected if they equilibrated with a metallic phase during core
The composition of bulk silicate Mars derived above is shown
in Tables 4 and 5. Fig. 16 shows the results normalized to CI chon-
drites (Lodders, 2003, except for highly siderophile elements where below with the goal of showing the utility of knowing Martian (and
I use data for Orgueil, Horan et al., 2003), and plotted in order other planetary) compositions.
of decreasing 50% condensation temperature (hence increasing The striking feature of the revised composition is how similar
volatility). Cl is plotted with Br and I, rather than its calculated it is to that derived by Wänke and Dreibus, in spite of anal-
condensation temperature. The pattern for lithophile elements yses of numerous newly found Martian meteorites, orbital and
is the familiar one with uniform refractory element abundances lander geochemical data, and improved global geophysical data.
(assumed, but reasonably so, Figs. 2 and 3). The exceptions are Fe The robustness of the model stems from their geochemical insight
and P, where signiﬁcant fractions were fractionated into the core about element behavior, allowing us to determine volatile ele-
during primary differentiation. Mars has roughly uniform depletion ments from the assumed abundances of refractory elements and
of moderately volatile elements (0.6 × CI), and strong depletion of to assess bulk FeO, and the certainty that we have meteorites from
highly volatile elements. The highly volatile elements are within Mars. It seems remarkable that the conclusions reached by Wänke
uncertainties uniformly depleted at about 0.06 CI abundances. The and Dreibus based on only 10 SNC meteorites stand the test of
highly volatile chalcophile elements (Fig. 16) are likewise roughly abundant new data. The only signiﬁcant difference between my
uniformly depleted, but with more scatter. They have normalized reassessment and the original Wänke–Dreibus model is in the con-
abundances of 0.03 × CI. I discuss these abundances in more detail centration of H2O. Other differences are a matter of degree but
416 G.J. Taylor / Chemie der Erde 73 (2013) 401–420
do not change the basic picture: moderately volatile elements are
depleted by a small amount compared to CI chondrites (factor of
about 0.6), while highly volatile elements are depleted by a factor
6.1. Mars is rich in FeO
There is no doubt that the Martian mantle is much richer in
oxidized iron than is the terrestrial mantle. Both geochemical and
geophysical data conﬁrm it. One might argue that melting of a
hydrous mantle might have enriched magmas in FeO (Nekvasil
et al., 2007), but Taylor et al. (2006a) marshal experimental data to
argue that any magma generated by partial melting of a wet Mar-
tian mantle and subsequent fractionation of that hydrous magma
would lead to lower FeO than in the original mantle peridotite.
Fig. 17. CI-normalized abundances for Earth and Mars compared. Refractory ele-
Orbital measurements of Mercury (e.g., Peplowski et al., 2011)
ments between Cr and Sr are shown for context; the full list of concentrations for
conﬁrm the suggestion (Robinson and Taylor, 2001) that it has a Mars is shown in Table 4. The overall pattern suggests similar compositions for Earth
and Mars, though Mars contains somewhat higher concentrations of moderately
low FeO content (2–4 wt%). Robinson and Taylor (2001) argue on
the basis of surface analyses of Venus (Surkov et al., 1987) that its
bulk FeO is close to that of Earth, about 8 wt% (e.g., McDonough
and Sun, 1995). It is tempting to suggest on the basis of these few Mars and Earth is that Mars has more than double the FeO con-
data points (only four, though they represent 100% of the terrestrial centration and a sulfur-rich core (see below). This emphasizes the
planets!) that there is a gradient in the oxidation state through- similarity among the terrestrial planets. They may have different
out the inner solar system. Another way of looking at this is that mixtures of the ingredients that delivered the refractory elements,
the planetesimals accreting to form the terrestrial planets varied but the sources for the volatile elements may have been quite simi-
in their oxidation state (E chondrites to L chondrites, for exam- lar. The methods that use oxygen isotopes to derive the composition
ple): Mercury received a bigger share of the reduced chondritic of Mars only fail because they add too much of the volatile compo-
raw materials than did Earth than did Mars. This is consistent nent. Perhaps the chondrite groups proposed had similar refractory
with the modeling designed to match oxygen isotopic composi- element abundances, but accretion occurred mostly before the
tions (Sanloup et al., 1999; Lodders and Fegley, 1998; Burbine and volatiles condensed.
O’Brien, 2004). Alternatively, perhaps the FeO was made on Mars by
oxidation of metallic iron. Wänke and Dreibus (1994) hypothesized 6.3. Water: abundance and source
that the higher FeO was caused by metal oxidation by H2O dur-
ing the accretion of Mars. As discussed by Bertka and Fei (1998b), Mars appears to have somewhat less H2O than does Earth,
an oxidation event like this would increase the sulfur content of 300 ± 150 versus 500 ppm; the terrestrial value is a minimum
a metallic core. Whatever the cause, the different FeO concentra- (Mottl et al., 2007). In spite of appearing to have less water
tions of the terrestrial planets must be taken into account when than bulk Earth, Mars has higher concentrations of moderately
modeling planetary accretion. volatile elements and about the same concentrations of highly
volatile elements. This is, of course, convoluted because most highly
6.2. Depletion of volatile elements volatile elements are chalcophiles. If we consider only Cl, Br, and
I, which are lithophile, then Mars appears to be enriched com-
Volatile elements fall into two distinct groups on the abundance pared to Earth, 0.06 versus 0.02 × CI (Fig. 17). Assuming that the
plots (Fig. 16). One involves the moderately volatile (50% condensa- highly volatile elements were delivered to accreting Mars in water-
tion temperatures between ∼800 and 1100 K) lithophile elements bearing planetesimals resembling carbonaceous chondrites, then
K, Ga, Na, Rb, Cs, and F, and the moderately volatile chalcophile those objects contained less water than did those accreting to Earth.
elements As, Cu, and Ag. The lithophiles are depleted by small fac- I estimate that the planetesimals contributing the 0.06 × CI contri-
tors compared to CI chondrites (the mean depletion is 0.6 × CI). In bution of halogens to Mars would have contained on average only
contrast, the moderately volatile chalcophile elements are signif- 0.5 wt% H2O to produce a bulk Martian water content of 300 ppm.
icantly more depleted, average 0.03 × CI. This suggests that their The volatile-bearing planetesimals used in constructing the Earth
depletion is driven by core formation (probably S-rich, see below) would have contained about 3 wt% H2O to produce a bulk water
rather than volatility. Highly volatile elements (50% condensation content of 500 ppm while adding highly volatile elements to bring
temperatures <750 K) are strongly depleted. Lithophile element (Cl, the terrestrial inventory to only 0.02 × CI abundances.
Br, I) abundances are 0.06 × CI and chalcophile elements (Bi, Zn, Sn, The somewhat damp but otherwise volatile-rich planetesimals
Se, Cd, In, and Tl) abundances are 0.03 × CI. The chalcophile ele- are likely to have been formed from typical inner solar system
ments have a large range in depletion factors, from 0.004 (Se) to materials. Otherwise, the D/H of Martian water would not be so sim-
0.09 (In). This range likely reﬂects the combination of formation ilar to the terrestrial value (Section 5.9). Except for Jupiter-family
of a core rich in S and once sulﬁde was depleted in the mantle comets, which have D/H similar to the Earth (Hartough et al., 2011),
subsequent lithophile partitioning of the elements. astronomical measurements of D/H in comets suggest that outer
All the volatile elements are combined in Fig. 17, along with solar system materials have elevated D/H. Thus, it appears that the
a few refractory elements for reference, and compared to terres- inner solar system objects were fed from the same source of H2O,
trial abundances. Moderately volatile elements are somewhat more or at least from sources with the same D/H ratio.
depleted in Earth than in Mars (0.4 versus 0.6 × CI), but on average This analysis of total water in Mars assumes that the planet
overlap for the highly volatile elements. The depletion patterns for did not lose water during accretion or afterwards. It is possible,
the two planets track each other, discrepant most strongly for Br however, that water may have been lost preferentially by impact
and Tl. In spite of the difference for moderately volatile elements, heating during accretion (e.g., Bond et al., 2010), might have reacted
the two bodies are quite similar. The biggest difference between with metallic iron during accretion, accounting for the high FeO
G.J. Taylor / Chemie der Erde 73 (2013) 401–420 417
in Mars (Wänke and Dreibus, 1994), or lost during magma ocean
Comparisons for two important chemical parameters among inner solar system
crystallization (Elkins-Tanton et al., 2005).
K/Th Refs FeO (wt%) Refs
6.4. Highly siderophile elements: implications for accretion
Mercury 5200 a 3 f,g
The signiﬁcance of the concentrations of highly siderophile ele- Venus 3000 b 8 f
Earth 2900 c 8 c
ments in Mars Earth, and the Moon were reviewed in detail by
Moon 360 d 13 h
Walker (2009). New analyses and an updated discussion appear
Mars 5300 e 18 i
in Brandon et al. (2012). The highly siderophile elements are gen-
(a) Peplowski et al. (2011); (b) Surkov et al. (1987); (c) McDonough and Sun (1995);
erally in chondritic relative abundances (except for Re) and are
(d) Warren and Wasson (1979), Warren (1989); (e) Taylor et al. (2006a); (f) Robinson
present at 0.04 × CI concentrations (Fig. 16). The terrestrial prim-
and Taylor (2001); (g) Nittler et al. (2011); (h) Taylor et al. (2006a); (i) Wänke and
itive upper mantle contains about twice this abundance (Becker Dreibus (e.g., 1988) and this paper.
et al., 2006). The abundances in Mars are quite depleted (0.04 × CI),
but if equilibrated with metallic iron during core formation they
a core radius of 1680 km and a composition (derived from calcu-
would be another factor of 40–50 times lower because their low-
lated core density) of 75–78 wt% Fe + Ni and 22–25 wt% S. Using the
pressure metal-silicate partition coefﬁcients are around 10,000. As
new estimated Martian bulk composition, the relative amounts of
explained in detail by Walker (2009) and Brandon et al. (2012),
Fe and FeO (total Fe, as for all refractory elements, is about 1.9 times
three explanations have been advanced to explain the surprisingly
CI), assuming that essentially all the Ni and S are in the core (ignores
high concentrations of highly siderophile elements: (1) inefﬁcient
small fractions in the silicate portion), and assuming an original S
core formation; (2) equilibrium at high pressures where the parti-
abundance similar to the moderately volatile elements (0.6 × CI),
tion coefﬁcients are lower; and (3) late addition of materials rich
I estimate a core composition of 78.6 wt% Fe + Ni and 21.4 wt% S.
in siderophile elements. As Brandon et al. (2012) explained, all
Better estimates await a determination of the core size by seismic
of these ideas have ﬂaws, but noted that late addition may be
measurements to be done by the Interior Exploration using Seis-
the most straightforward explanation. One problem that deserves
mic Investigations, Geodesy and Heat Transport (InSight) mission,
more attention is how a chondritic ratio can be preserved during
scheduled to be launched in 2016.
magma ocean crystallization and subsequent mantle melting.
6.5. Halogen concentration of the crust 6.7. Comparing planet compositions
If the bulk Cl abundance is as low as it appears (Section 5.6), the A central reason for determining planetary compositions is to
high Cl in the uppermost surface has interesting implications for compare them to deduce variations in compositions, conditions
crustal evolution. Half the K in Mars is in the mantle (Taylor et al., in the solar nebula and chemical uniformity of it, accretion pro-
2006a). This should apply to Cl as well because like K it is highly cesses (including the extent of mixing), and differentiation styles
incompatible during igneous processing, implying that the crustal (e.g., ﬂoatation crust or not). Planetary scientists often emphasize
contribution to the total inventory is 16 ppm. Since the crust aver- differences among the terrestrial planets, but the similarities are
ages 57 km thick (Wieczorek and Zuber, 2004), making up about striking (Table 6). The close similarities in the D/H ratios of Mars,
4.6 wt% of bulk silicate Mars, its mean Cl content should be around Earth, carbonaceous chondrites, and Jupiter-family comets sug-
350 ppm. GRS and MER data show that the average surface Cl con- gest a common source of water-bearing material in the inner solar
centration is about 5000 ppm. This indicates that Cl is concentrated system (Alexander et al., 2012). K/Th (Table 6) varies among the
in the uppermost crust. Assuming that the upper few meters are terrestrial planets: Mercury, 5200 ± 1800 (Peplowski et al., 2011);
not exceptionally enriched, mass balances indicate that the entire Venus, ∼3000 (Surkov et al., 1987); Earth, 2900 (Jagoutz et al.,
crustal inventory of Cl could be conﬁned to the upper 4 km. If 1979; McDonough and Sun, 1995; Taylor and McLennan, 2009);
correct, this suggests efﬁcient aqueous transport of Cl from deep in Mars, 5300 ± 220 (Taylor et al., 2006a). However, these variations
the crust to the surface, or continuous aqueous transport to the sur- seem less signiﬁcant when compared to the K/Th ratios of the
face during construction of the crust. This analysis is complicated carbonaceous chondrites (19,000 – McDonough and Sun, 1995).
by the unknown extent to which Cl could have been lost in a gas The terrestrial planets are depleted in volatile elements compared
phase even at depth in the crust; such loss could also fractionate Cl to carbonaceous chondrites, but the depletions are not correlated
from K. Nevertheless, to ﬁrst order, it seems likely that Cl is system- with distance from the Sun. Oxygen isotopes are distinctive among
atically concentrated toward the surface, illustrating the important planets and meteorite groups (Mittlefehldt et al., 2008). The differ-
role of water in the crustal geochemistry of Mars. ence between Earth ( O of 0‰ by deﬁnition), and Mars ( O
It is interesting to consider the ramiﬁcations if Cl is much more of + 0.25‰), is much smaller than the total range observed among
abundant, as argued by Taylor et al. (2010) from the chondritic chondrites O of −4.3 to +2.5‰). Warren (2011) emphasizes the
mean Cl/K ratio of the surface and calculated similar 50% con- similarity among terrestral planets and differentiated meteorites
densation temperatures (Lodders, 2003). Considering that Cl/Br is compared to carbonaceous chondrites in the isotopic compositions
chondritic and Cl/I close to it, this implies that all the halogens have of O, Cr, and Ti.
CI-normalized abundances of 0.6. If the condensations tempera- Some chemical parameters do vary directly with heliocentric
tures of Br and I are as low as Lodders’ (2003) calculations indicate, distance. Bulk silicate FeO (Table 6) increases from ∼3 wt% in
then all the highly volatile elements would have been present in Mercury (Robinson and Taylor, 2001; Nittler et al., 2011), to 8 wt%
Mars at about the same CI-normalized abundance, implying that in Earth (McDonough and Sun, 1995) and Venus (Robinson and
core formation reduced the abundances of the highly volatile chal- Taylor, 2001) to 18 wt% in Mars. FeO and the size of the metallic
cophile elements from 0.6 to the observed ∼0.03. cores are inversely correlated. These trends suggest a range in
oxidation conditions correlated with heliocentric distance, in con-
6.6. The core trast to the apparent weak correlation with heliocentric distance
for D/H, K/Th, and oxygen isotopes. As these differences can be
The most reliable estimate of core composition may be the one produced by varying oxidation conditions, they do not suggest
by Khan and Connolly (2008) based on geophysical data. They ﬁnd the terrestrial planets were formed from fundamentally different
418 G.J. Taylor / Chemie der Erde 73 (2013) 401–420
materials. On the contrary, the broad chemical similarities among Martian Surface: Composition, Mineralogy, and Physical Properties. Cambridge
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