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Chemie der Erde 73 (2013) 401–420

Contents lists available at ScienceDirect

Chemie der Erde

jou rnal homepage: www.elsevier.de/chemer

Invited review

The bulk composition of

G. Jeffrey Taylor

Hawaii Institute of and Planetology, School of and Science and Technology, University of Hawaii, Honolulu, HI 96822, United States

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 is fundamental to understanding plan-

Received 16 July 2013

etary , differentiation, evolution, the of the igneous parent rocks that were altered

Accepted 11 September 2013

to produce on Mars, and the initial concentrations of such as H, Cl and S, important

constituents of the surface. This paper reviews the three approaches that have been used

Keywords:

to estimate the bulk chemical composition of Mars: geochemical/cosmochemical, isotopic, and geophys-

Mars

ical. The standard model is one developed by Wänke and Dreibus in a series of papers, which is based on

Composition

compositions of Martian . Since their groundbreaking work, substantial amounts of data have

Planet formation

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

Terrestrial in the Wänke–Dreibus model, but in general confirm its accuracy. Bulk Mars has roughly uniform

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

common source of -bearing material in the inner . K/Th ranges from ∼3000 to ∼5000

among the terrestrial planets, a small range compared to CI (19,000). FeO varies throughout

the inner solar system: ∼3 wt% in , 8 wt% in Earth and , and 18 wt% in Mars. These dif-

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 formation, as

suggested by dynamical models. © 2013 Elsevier GmbH. All rights reserved.

Contents

1. Introduction ...... 402

2. Approaches to estimating bulk composition ...... 402

2.1. Models based on 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. Is the composition representative of the entire ?...... 406

4.1.1. Highlands megaregolith ...... 406

4.1.2. flows ...... 406

4.1.3. Element ratios ...... 406

4.2. Heterogeneity of the Martian mantle ...... 406

Tel.: +1 808 956 3899; fax: +1 808 956 6322.

E-mail address: [email protected]

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

the ocean (if there was one), to produce

and Dreibus (1988, 1994), Longhi et al. (1992), and Halliday et al.

the crust through intrusion and extrusion of basaltic , 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 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 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-

significantly if the major phases are and (partition

strained.

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 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 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 . 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

Table 1

a −4

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)

b

Moderately volatile 1230–800 P, Mn, K, Ga, Na, Cl , Rb, Cs, Au As, Cu, Ag, Sb, Ge

b

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

a

The classification into lithophile, siderophile, and chalcophile elements is not clear-cut. In the absence of metallic or sulfides, for example, elements behave like

lithophile elements.

b

Cl condensation temperature might be much lower than calculated by Lodders (2003), similar to those of Br and I.

Table 2

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

a

Wänke and Dreibus (1994).

b

Morgan and Anders (1979).

c

Sanloup et al. (1999) model EH45:H55.

d

Lodders and Fegley (1997).

e

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

Table 3

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 –

a

Wänke and Dreibus (1994), slightly embellished by Taylor and McLennan (2009).

b

Morgan and Anders (1979); siderophile and chalcophile elements not included.

c

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 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 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 . 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 , hence depleted in volatiles (pre- assumed bulk FeO and Mg/Si and Al/Si for Mars (based on Mar-

sumably lost during high-temperature 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 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 at the Pathfinder and MER

36

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, , 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 -

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, 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 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.

ical calculations.

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 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 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

The highly successful Pathfinder and very low (1), -melt partition coefficients (Beattie, 1993;

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 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

provide complementary data to those provided by meteorites highly compatible in phosphate (, 1995). Phosphates

and the abrasion process limits that amount of 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 (Halliday et al., 1995) and somewhat compatible in

(Halliday et al., 1995), so if these phases were present

in the Martian mantle, it could lead to fractionation of K from Th.

4. Complications

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

entire crust?

alkaline 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

briefly here.

meteorite dataset.

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 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. 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 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 ) 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, 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 : 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

Table 4

Bulk composition in revised model for bulk silicate Mars.

a b

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

c

Br (ppb) 191 58 S-A Br/Cl Re (ppb) 0.88 0.66 A-A Re/Mg

c

Rb (ppm) 1.30 0.14 S-S Rb/La Os (ppb) 2.0 0.8 A-A Os/Mg

c

Sr (ppm) 14.6 0.7 R – Ir (ppb) 2.0 1.0 A-A Ir/Mg

c

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

c

Ru (ppb) 2.6 0.9 A-A Ru/Mg Th (ppb) 58 12 R –

c

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

a

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 , using experimental data.

b

Ratio of element to refractory element used in slope or average methods.

c

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

Table 5

(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,

a b

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 –

2

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

Total 99.8

mean of each element. The uncertainty in the ratio is given by

a

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

b

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 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.

2

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).

2

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

2

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 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 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,

S/L

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 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 (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 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

5.4. Phosphorous

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 correlate well with Al (Fig. 8) with corre- good linear array. Using the bulk La value (Table 4) and a slope of

2

±

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

7

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-

2

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.

the mantle.

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-

tites).

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 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

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 , 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 (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 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 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

2

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

H2O/Cl ratio.)

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/ ratio might be diagnostic of the

sources of water to Mars (e.g., outer solar system objects such as

versus inner solar system objects, adsorbed nebula water

versus water in phyllosilicates in planetesimals). The

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

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 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

6. Discussion

times higher than expected if they equilibrated with a metallic phase during core

formation.

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

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

approaching 50.

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

volatile elements.

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 , 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 -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., 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

Table 6

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).

objects.

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-

17 17

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

17

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

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