A Short Survey of Some of Mars Global Geochemical and Tectonic Questons

8/3/2019 A Short Survey of Some of Mars Global Geochemical and Tectonic Questons

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A short survey of some of Mars global geochemicaland tectonic questions

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Introduction

In many ways, Mars global geochemistry is an enigma. On the one hand, S. Taylor and

McLennan[2009] observe that Mars may well have the most completely preserved geological record of

any terrestrial planet, and there indeed exists a wealth of data about Mars from numerous landed and

orbital missions [S. Taylor and McLennan, 2009]. On the other hand, the quest for understanding even

Mars most basic geochemical and related tectonic questions is hampered by the complexity of the

planets history, the individual limitations of the various data sources, and the fundamental difficulty in

synthesizing the varied datasets [S. Taylor and McLennan, 2009]. This paper discusses some of these

basic global questions, their leadings answers, and the relevant evidence.

Why are the bulk characteristics of Earth and Mars different?

Although Mars and Earth both accreted from the early solar nebula, there are some significant

differences in their bulk geochemistry. According to the Wnke-Dreibus model [Wnke and Dreibus,

1988], which is currently the most accepted [S. Taylor and McLennan, 2009], Mars is about twice as richas Earth in moderately volatile elements and is more oxidized, the latter resulting in a core proportionately

smaller than Earths but a primitive mantle twice as enriched in iron. Additionally, the uncompressed

density of Mars is estimated to be ~3.70 g/cm3, 6.5% less than that of Earth [Stacey, 2005]. Moreover, the

planets oxygen isotope ratios differ. Using 17

O = 17

O 0.52 18

O, 17

O = +0.32 for Mars (from SNC

meteorites, that is, those presumed to be from Mars) but 17

O = 0 for Earth (by definition) [e.g., Franchi et

al., 1999].

The oxygen isotope differences in particular strongly suggest that each planet accreted from a

different population of planetesimals [Clayton, 2003]. For oxygen isotopes, both chemical and physical

processes nearly universally obey the mass-dependent relationship d17

O/d18

O 0.52, whereas solar

system meteorites follow the distinctly different trend of d17

O/d18

O 1.0 [e.g., Clayton, 2003]. When the

oxygen isotope concentrations for Earth and Mars are plotted, they predictably have the same 0.52 slope

but the Mars data are offset to higher values (Figure 1) [Clayton, 2003]. This offset practically requires

that the planetesimal population from which Mars accreted supplied oxygen isotope concentrations

distinct from those supplied by the planetesimals that accreted to form Earth [Clayton, 2003].

These distinct inferred populations necessarily imply heterogeneity in the solar nebula.

Unfortunately, interpreting the distribution of this heterogeneity is problematic. Hypothetically, if the Earth

and Mars accreted primarily from planetesimals near each planets respective orbit, these narrow feeding

zones combined with radial heterogeneity could explain the observed oxygen isotope differences

[Clayton, 2003]. However, the numerous dynamical models for solar system accretion universally predict

broadfeeding zones [e.g., Chambers, 1981; Wetherill, 1994]. Moreover, if there were a general

monotonicradial trend based on heliocentric distance, it would not explain why the K/Th ratios of Earth,

Mars, and Vesta (inner asteroid belt) peak at Mars [G. Taylor et al., 2006].


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Halliday et al. [2001] offer a possible resolution to this last inconsistency, suggesting that the

current concentrations of moderately volatile elements (including K) on the terrestrial planets need not be

original but may rather reflect volatile loss as a result of late accretionary events, such as large impacts.

In this interpretation, Mars would be expected to have larger moderately volatile element reservoirs than

the Earth because isotopic evidence indicates that Mars largely avoided the last stage of accretion

[Halliday et al., 2001]. One such line of isotopic evidence involves the short-lived182

Hf-182

W system.

Because W is fractionated into the core but Hf is not, the presence of182

W at super-chondritic levels in all

SNC meteorites suggests that Mars differentiated within the lifetime of182

Hf, likely within 10-15 Myr of the

start of solar system accretion [Yin et al., 2002; Kleine et al., 2002]. Such an early cessation to Mars

accretion is also consistent with the planets mass being approximately one-eighth that of Earth [Halliday

et al., 2001]. Halliday et al. [2001] even speculate that the uncompressed densities and possibly bulk non-

volatile abundances of the planets could be significantly affected by late accretionary events.

What is average martian crust?Although global bulk geochemistry estimates for Mars, such as the favored Wnke-Dreibus

model, must rely on data such as chondritic abundances and the few SNC meteorites, bulk geochemistry

for the martian crust can be constrained by a wealth of surface data from several landed and orbital

missions [S. Taylor and McLennan, 2009]. Nonetheless, even crustal models are subject to a litany of

difficulties. All of the SNC meteorites are basaltic and only one is older than 1.3 Ga [S. Taylor and

McLennan, 2009]. Landed missions are biased by the engineering constraints to which landing site

selection is subject, and their spatial scope is severely limited [S. Taylor and McLennan, 2009]. Orbital

data have global scope, but they also have coarse spatial resolutions of km-100 km scales [S. Taylor and

McLennan, 2009]. In addition, all missions to date have been severely limited in the depth of material

sampled, relying on scoops of surface material (e.g., Phoenix) or electromagnetic penetration (e.g., the

Gamma Ray Spectrometer) to sample only one-half meter, at most, below the surface [S. Taylor and

McLennan, 2009]. Finally, each instrument has its own elemental blind spots, distinct resolution, and

other idiosyncrasies, making synthesizing the resulting datasets especially difficult [e.g., McSween et al.,

2009].

Nonetheless, in 2006, G. Taylor et al. published a set of important constraints for average martian

crustal composition that largely affirm the predictions of the Wnke-Dreibus model. Their approach uses

K, Th, and Fe concentrations measured by the Gamma Ray Spectrometer (GRS) aboard the Mars

Odyssey orbiter [G. Taylor et al., 2006]. Although GRS spatial resolution is very coarse, on the order of

500 km, it samples material down to a depth of approximately one-third meter, considerable for an orbital

instrument [G. Taylor et al., 2006]. In essence, G. Taylor et al. [2006] use K as a proxy for all other

moderately volatile elements, Th as a proxy for all other heat-producing elements, and both Fe

concentration and the K/Th ratio as indices by which to extrapolate some geochemical characteristics and

evaluate the prevailing models for the bulk martian crust.


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G. Taylor et al. [2006] justify their near-surface methodology and emphasis of the K/Th ratio on

several bases. First, K and Th behave very similarly in igneous systems. For example, they are both

highly incompatible, having similar, very low (


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Based on the measured K and Th concentrations, respectively, G. Taylor et al. [2006] interpret

Mars to be approximately twice as rich as Earth in moderately volatile elements and to have about half of

its heat-producing elements concentrated in the crust. This latter interpretation, coupled with the

isotopically-constrained early differentiation of the crust (see below), implies a very low crustal production

rate after 4 Ga [Hauck and Phillips, 2002]. Separately, based on correlation of the La/Yb ratio with K and

Th concentrations in basaltic SNC meteorites, G. Taylor et al. [2006] interpret average martian crust to

have chondritic or slightly super-chondritic La/Yb, be oxidized with about QFM-1.5, and have formed from

undepleted mantle. Some isolated pixels also suggest crust formed from moderately depleted mantle [G.

Taylor et al., 2006].

In addition, G. Taylor et al. [2006] compared average measured K/Th to the average crustal K/Th

predicted by the three major models for Mars bulk geochemistry. Whereas the Ganapathy-Anders-

Morgan model predicts 620 and the Lodders-Fegley model predicts 16,000, the Wnke-Dreibus model

predicts 5450, very close to the measured average of 5300 [G. Taylor et al., 2006]. The Wnke-Dreibus

model also predicts 17.9 wt% FeO, again similar to the measured 18.4 wt% [G. Taylor et al., 2006]. Afterconsidering possible model-specific explanations for the discrepant predictions, G. Taylor et al. [2006]

conclude that the Wnke-Dreibus model is preferred. It should be noted that the Wnke-Dreibus model

(Table 1) was developed assuming chondritic abundances for refractory elements and estimating other

abundances based on elemental relationships in SNC meteorites [Wnke and Dreibus, 1988].

As impressive as the results of G. Taylor et al. and their consistency with the Wnke-Dreibus

model are, there are difficulties. Both approaches are predicated on broad assumptions, several of which

are recognized to be problematic, such as assuming that the upper one-third meter of the crust is

representative of its bulk composition. In addition, despite the prediction that Mars is twice as abundant

as Earth in moderately volatile elements, McSween et al. [2009] report no nepheline-normative rocks

among the SNC meteorites and numerous rover-analyzed rocks (Figure 3) in their recent review.

McSween et al. [2009] also observe that several putative hallmarks of SNC meteorite geochemistry,

including Al-depletion and distinct Fe/Mn and Ni/Mg ratios, are at odds with rover-based ground truth, but

they are unable to conclude which dataset is more likely to be representative. Finally, Nekvasil et al.

[2009] point out that In order to use igneous surface lithologies to constrain Martian mantle

characteristics, secondary processes [e.g., crystal fractionation] that lead to compositional modification of

primary mantle melts must be considered. Similar caution would also apply to constraining bulk crustal

composition based on near-surface abundances if such secondary processes affect crustal magmas.

How did the martian crust form and evolve?

Many aspects of the formation and evolution of the martian crust are poorly constrained. The

correlation of long-lived147

Sm-143

Nd and short-lived146

Sm-142

Nd systems [Harper et al., 1995] do indicate

that silicate mantle reservoirs differentiated within ~30 Myr of Mars accretion [Borg et al., 2003; Foley et

al., 2005], and Pb [Chen and Wasserburg, 1986], Sr [Borg et al., 1997], and Os [Brandon et al., 2000]


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isotopes also indicate little or no mixing after that time. However, these constraints do not necessarily

imply that the crust also formed within ~30 Myr of Mars accretion, though most workers make this

assumption [S. Taylor and McLennan, 2009]. Conversely, the formation of relatively shallow solid material

is strictly constrained by the oldest SNC meteorite, ALH84001, which until recently was thought to have a

crystallization age of 4.51 0.11 Gyr based on a weighted average of Rb-Sr and Sm-Nd radioisotope

data [Nyquist et al., 2001]. However, Lapen et al. [2010] challenge this age, preferring 4.091 0.030 Gyr,

based on Lu-Hf radioisotope data.

In discussing the nature of crustal formation and evolution, consistent terminology is essential. I

use the terminology of S. Taylor[1992]. Primary crust is thus crust crystallized directly from the magma

ocean; secondary crust is crust formed by partial melting of the interior; and tertiary crust is crust formed

by partial melting and differentiation of secondarycrust, thus implying crustal recycling [S. Taylor, 1992].

Within this framework, there are two major schools of thought on the martian crust. One view

stresses the size of the martian crust, which comprises 5% of silicate Mars compared to the 1% of silicate

Earth represented by the terrestrial crust [Wieczorek and Zuber, 2004]. Members of this school findreasonable rates of partial melting insufficient to produce such a significant martian crust by ~30 Myr after

accretion, and therefore interpret the bulk of martian crust to be primary [e.g., S. Taylor and McLennan,

2009]. Nonetheless, S. Taylor and McLennan[2009], who subscribe to this interpretation, estimate that

20 10 % of the crust is secondary on the basis of four earlier estimates of secular magma production,

two of which are photogeology-based and extrapolated to include intrusion [Greeley and Schneid, 1991;

McEwen et al., 1999], and two of which are derived from Ar data and are model-dependent [Tajika and

Sasaki, 1996; Hutchins and Jakosky, 1996].

The second view instead stresses composition, noting that nearly all exposed crust on Mars is

basaltic and distinctly different from the lunar crust [e.g., G. Taylor et al., 2006]. The latter observation is

important because the lunar crust is widely accepted to have formed from a magma ocean [G. Taylor et

al., 2006]. Its strong K depletion and large ranges of K and Th concentrations are indicative of the

formation of buoyant anorthosite cumulates and subsurface residual magma, in stark contrast to Mars K-

rich basaltic crust with comparatively small ranges of K and Th concentrations (Figures 2a, 2b) [G. Taylor

et al., 2006]. G. Taylor et al. [2006], who subscribe to this view, also go so far as to state succinctly,

Secondary crusts are basaltic. Members of this school thus interpret the martian crust to be

predominantly secondary [e.g., G. Taylor et al., 2006]. Nonetheless, G. Taylor et al. [2006] do discuss, at

length, the possibility that the martian crust may at least partially represent a primary crust very different

from that of the Moon, ultimately concluding that this possibility cannot be ruled out.

A third view may also be emerging. In revising the age of ALH84001, Laden et al. [2010]

significantly loosen the strict constraint on the timing of crustal formation. Consequently, they conclude,

albeit without much additional evidence, that The younger age predicts that the primordial martian crust

was likely largely destroyed from intense bombardment at 4.25 to 4.1 Ga [Laden et al., 2010].


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How did the hemispheric dichotomy form?

Preliminary Note:

The multifaceted hemispheric dichotomy of Mars is one of the most prominent features of the

planet and also one of the most enigmatic (see discussion below). A multitude of diverse hypotheses

have been advanced, many predicated on fundamentally different interpretations of the relevant data.

Moreover, few if any hypotheses have been ruled out over time, and the surviving hypotheses have

grown increasingly complex in order to accommodate new data and facilitate numerical modeling.

Consequently, anything but a brief and often superficial examination of the salient data, leading

hypotheses, and major difficulties lies beyond the scope of this paper. The following section should

instead be viewed as a simplified and selective introduction to the issue, and the reader is directed to the

cited material for more detailed analysis.

If one views a global map of martian topography (Figure 4), one pattern is immediately apparent:

the northern hemisphere is dominated by smooth plains whereas the southern hemisphere is dominatedby cratered highlands. Quantitative analysis reveals a distinctly bimodal distribution with ~5.5 km between

maxima [Aharonson et al., 2001]. As the age of a planetary surface is most readily indicated by the

density of the impact crater distribution that has accumulated with time, the dichotomy of terrains also

suggests a young north and an ancient south. In addition, the topographic dichotomy approximately

mirrors a suite of other dichotomies. For example, the south has thicker crust, with mode ~58 km

compared to ~32 km in the north [Neumann et al., 2004]. As mentioned earlier, near-surface FeO is

greater in the north than in the south by 2-3 wt% [G. Taylor et al., 2006]. The north is also dominated by

andesitic composition in Thermal Emission Spectrometer data, which penetrate to


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concentration in the low-lying north [Boynton et al., 2007]. However, although such aqueous transport

would presumably involve brines, the concentration of Cl in the north is not unusually high [Keller et al.,

2006; G. Taylor et al., 2006], rendering the hypothesis problematic.

Of the few constraints on dichotomy formation mechanisms, the age of the crust on either side of

the dichotomy is one of the most important [Watters et al., 2007]. Based on the density of quasi-circular

depressions in the north, which are interpreted as partially buried craters, the crust is estimated to be

similarly ancient in both hemispheres [Frey, 2006a, 2006b]. Frey[2006a, 2006b] assigns nominal ages of

4.04-4.11 Gyr and 4.14-4.32 Gyr to the northern and southern crust, respectively, suggesting that the

north is younger than the south. However, these ages are subject to an array of sampling, crater

detection, and interpretation issues such that the north could even be slightly older than the south [Reese

et al., 2010]. Regardless, the apparent youth of the northern plains is interpreted to reflect a much later

veneer of lava and sediments [e.g., S. Taylor and McLennan, 2009].

Although the above-described age constraint forced modification of some early hypotheses, it

does not rule out any of them at a fundamental level [Watters et al., 2007]. Nonetheless, when this ageconstraint is coupled with the assumed formation of the crust within ~30 Myr of accretion (see above),

endogenic processes unrelated to the magma ocean and/or initial formation of the crust are required to

proceed at rates so high that they may be unreasonable [Solomon et al., 2005; Watters et al., 2007]. In

addition, topographic evidence for subduction appears to be lacking [Pruis & Tanaka, 1995; McGill, 2000].

On the other hand, major objections to an exogenic (impact-related) origin include the noncircular shape

of the dichotomy [Zuber et al., 2000] and the interpretation that the impact melt produced in such an

energetic event would fill the basin, thereby erasing it [Strom et al., 1975]. Although the multiple-impact

hypothesis addresses the first and perhaps the second of these difficulties, topographic and radar

evidence have only revealed one large inset basin within the lowlands and the probability of multiple large

impacts striking only the northern hemisphere is low [McGill and Squyres, 1991; Nimmo and Tanaka,

2005].

In a recent paper, Andrews-Hanna et al. [2008] attempt to answer the noncircularity challenge to

the single impact hypothesis. Historically, tracing the crustal thickness dichotomy beneath regions that

were later tectonically and volcanically active has been problematic because these processes overprint

the primordial crustal thickness [Andrews-Hanna et al., 2008]. In order to accurately resolve the primordial

trace, Andrews-Hanna et al. [2008] model the overprinting crustal thickness signature as reflecting

exclusively flexural support and interpret all remaining, isostatic support as original. The resulting

dichotomy boundary is a line that is very well fit to an ellipse, consistent with a single impact (Figure 5)

[Andrews-Hanna et al., 2008]. Andrews-Hanna et al. [2008] also describe supporting topographic

evidence for this interpretation and explain local decoupling of the topographic and crustal thickness

dichotomies as a consequence of flow in the lower crust. A suite of additional work has successfully

dynamically modeled the single impact hypothesis and demonstrated that such an impact is not self-

erasing [Marinova et al., 2008]. Intriguingly, Reese et al. [2010] also recently proposed a model in which


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the north is not loweredby excavation of a single impact but rather the elevation of the south is increased

by excess melt production at a southernimpact site in an extremely energetic event. Clearly, research

into the martian hemispheric dichotomy is ongoing.

Summary

Few definitive conclusions are available for the global geochemical and related tectonic questions

discussed above. Instead, the questions, corresponding hypotheses, and salient difficulties are

summarized below.

1. The bulk characteristics of Mars, including especially its oxygen isotope ratio, indicate that the

population of planetesimals that accreted to form the planet were distinct from those that accreted

to form the Earth. This isotopic variety implies heterogeneity in the solar nebula which may have

had a radial distribution sampled by narrow accretionary feeding zones for each planet, but

dynamical models universally predict broad feeding zones.

2. G. Taylor et al. [2006] find that K, Th, and Fe concentrations in the upper one-third meter of themartian crust correspond excellently to the Wnke-Dreibus model for Mars bulk geochemistry.

They also separately estimate that Mars is twice as enriched as Earth in moderately volatile

elements, has about half of its heat-producing elements concentrated in the crust, has a crustal

abundance of 18.4 wt% FeO, is oxidized with about QFM-1.5, and has a crust predominantly

formed from undepleted mantle. However, rover-based analyses may not be consistent with

these findings, the inferred martian meteorites on which both the Wnke-Dreibus model and the

separate conclusions of G. Taylor et al. are partly based may not be representative of the crust,

and broad fundamental assumptions inherent to both the Wnke-Dreibus model and G. Taylor et

al. methodology are problematic.

3. The formation and evolution of the martian crust is also problematic. The inferred very early

formation of the crust by ~30 Myr after accretion suggests that the crust mostly crystallized from a

magma ocean, although later intrusive and extrusive additions likely comprise 20 10 % of the

crust. Conversely, the nearly universally basaltic composition of the crust suggests that, unlike

the lunar crust, it likely formed from partial melting of the interior.

4. At the present time, a wide range of both internal and external mechanisms for formation of the

north-south hemispheric dichotomy in topography and crustal thickness are viable, and

correlative geochemical dichotomies may be merely secondary. Age and pacing constraints favor

processes involving one or more large (early) impacts, the primordial magma ocean (e.g.,

fractionation thereof), or primary crustal formation (e.g., mantle convection). A suite of recent

work specifically favors the single impact hypothesis.


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Table 1: Bulk composition of Mars

Values based on the Wnke-Dreibus model [Wnke and Dreibus, 1988] and expanded by S. Taylor and

McLennan[2009]. The Wnke-Dreibus model assumes chondritic abundances for refractory elements

and estimates other abundances based on elemental relationships in inferred martian meteorites [Wnke

and Dreibus, 1988].


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Figure 1

Plot of the oxygen isotope concentrations of inferred martian meteorites (open circles and crosses) and

terrestrial mafic igneous whole-rock samples (solid black circles) [Halliday et al., 2001]. Note the parallel

trend in each dataset, corresponding to d17

O/d18

O 0.52, which is the consequence of a nearly

universal mass-dependent relationship for both chemical and physical processes [e.g., Clayton, 2003].

Conversely, the vertical offset between the datasets is best explained by different oxygen isotope

concentrations in the respective planetesimal populations that accreted to form each body [Clayton,

2003].


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2b

2a


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Figure 2

Results from the G. Taylor et al. [2006] analysis of Gamma Ray Spectrometer data. The mapped

distributions represent the directly measured concentrations in the uppermost one-third meter of the

martian crust [G. Taylor et al., 2006]. Note that K ranges over a factor of ~3 (2a) and Th ranges over a

factor of ~5 (2b), but K/Th ranges over a factor of only ~2 across nearly the entire surface (2c) [G. Taylor

et al., 2006]. Also note that Fe is generally higher by 2-3 wt% north of the topographic dichotomy

boundary, indicated by a thick black line (2d) [G. Taylor et al., 2006].

2d

2c


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Figure 3

Normative compositions for inferred martian meteorites (grayscale symbols) and rover-analyzed rocks

(light blue and dark blue dots) [McSween et al., 2009]. Note that no meteorite or rock is nepheline-

normative, despite estimates that the martian crust is twice as rich as the terrestrial crust in moderatelyvolatile elements [Wnke and Dreibus, 1988; G. Taylor et al., 2006].

Figure 4

Global map of Mars Orbiter Laser Altimeter elevation data in Mercator projection with artificial hillshade;

illumination is from the northeast [Zuber et al., 2000]. The dichotomy of smooth northern plains and

cratered southern highlands is manifest.


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Figure 5

The topographic (a), modern crustal thickness (b), and primordial crustal thickness (c) dichotomies, as

given by Andrews-Hanna et al. [2008]. Figure 5a is generated from laser altimetry data, and 5b is

generated from gravity solutions [Andrews-Hanna et al., 2008]. Figure 5c is the result of the remaining

isostatic support after flexural support is removed [Andrews-Hanna et al., 2008]. Andrews-Hanna et al.

[2008] interpret such flexural support to reflect later crustal thickness overprinting by volcanism andtectonism whereas the isostatic support isolated in 5c is inferred to be primordial. Note how well the

modeled primordial crustal thickness dichotomy (5c, thin black line) matches a best-fit ellipse (5c, thick

black line) [Andrews-Hanna et al., 2008]. Andrews-Hanna et al. [2008] interpret this correspondence to

strongly suggest that both topographic and crustal thickness dichotomies result from a single giant

impact. See text for discussion.

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