Workshop on 2001 2511.pdf

CHEMICAL COMPOSITION OF THE SURFACE: A SEDIMENTARY PERSPECTIVE. Scott M. McLennan, Department of Geosciences, State University of New York at Stony Brook, Stony Brook, NY, 11794-2100 ([email protected]).

Introduction: The sedimentary rock record is the This distinction is mainly the result of the unique high primary repository of Earth history over the past four standing continental crust on the Earth coupled with billion years [1]. Major and trace element geochem- the presence of low standing basaltic terrains in the istry and radiogenic isotopes are routinely used to in- water covered ocean basins [1]. vestigate the sources of sediment (provenance) and the A second fundamental difference in sedimentation various processes that affect sediments throughout styles between Earth and Mars is the distribution of their history (e.g., weathering, sedimentary transport sediment types. Estimating lithological proportions of and recycling, diagenesis). The most sophisticated sedimentary rocks on Earth is largely model depend- analytical methods that are available have been em- ent and but clastic sediments dominate with the fol- ployed in sedimentary geochemistry and in many lowing approximate proportions: Shale - 59%; Unal- cases include grain by grain analyses of mineralogy, tered volcanogenic sediment - 15%; Carbonate sedi- chemistry and isotopic characteristics. In turn, this ment - 13%; Sandstones - 10%; Evaporites - 2%; Sili- information has been used to address many important ceous biogenic sediment - 1% [e.g., Ref. 8]. On Mars, issues, such as tectonic associations, environments of these proportions almost certainly differ greatly. Al- deposition, paleoclimates, paleohydrology, and though much of the material incorporated into soils crust/mantle evolution [e.g., Ref. 2]. can be considered clastic sediment, the grain size dis- Photographic, spectroscopic and geochemical re- tributions and amount of unaltered volcanogenic de- sults, returned from the surface of Mars over many bris are largely unknown. Carbonate minerals have years and many missions, have increasingly pointed yet to be unambiguously identified on Mars. Assum- towards a wide variety of sedimentary processes play- ing that sulfur and chlorine in Martian soils, amount- ing a dominant role in shaping the Martian surface. ing to as much as 4% and 1% respectively, is associ- Accordingly, there is great potential for applying the ated with evaporitic minerals, evaporite deposits may knowledge that has been learned from studying ter- be far more important than on the Earth. restrial sedimentary rocks towards evaluating Martian Other expected distinctions between terrestrial and geological history. Chemical and mineralogical Martian sedimentary styles center on the role of mete- analyses from the Martian surface, especially those orite impacts as a sedimentary process and the level of from the Viking, Pathfinder, and Mars 2001/2003 meteoritic components within the sediments. On missions, coupled with greater understanding of ba- Earth, the sedimentary mass has grown over geologi- saltic sedimentation on the Earth should provide the cal history and is dynamic, being continuously sedimentological framework within which to study the recycled through cannibalistic processes (i.e., sedi- chemistry and mineralogy of returned Martian sam- ment - sediment recycling), through the continents ples. (e.g., metamorphism and melting), and possibly Mars and Earth - Some Contrasts in Sedimen- through the mantle in association with plate tectonic tary Styles: In spite of the presence of “andesitic” processes (e.g., subduction, delamination of lower rock compositions at the Pathfinder site [3], the crust) [9]. Meteoritic components, especially those chemistry of Viking and Pathfinder soils [3,4], SNC resulting from the intense early bombardment, have meteorites [5,6] and our general understanding of the been largely obscured or lost from the record [1]. chemical evolution of terrestrial planets [7] all lead to Only on relatively localized scales, such as in debris the conclusion that the magmatic history of Mars is resulting from individual impacts (e.g. Cretaceous- probably dominated by . Thus, basaltic sedi- Tertiary boundary layer; tektites) and in slowly accu- mentation should also dominate the surficial proc- mulating deep sea pelagic sediments is evidence of esses. This stands in complete contrast to the Earth meteoritic components clearly present. where purely basaltic sedimentation is very rare and On Mars, such recycling processes are much less restricted to localized parts of volcanic islands (e.g., likely to have occurred in the absence of plate tectonic Hawaii, Iceland), restricted horizons within basaltic processes and the Martian sedimentary mass is proba- constructs (e.g., interflow sediments in flood bly more ancient on average being recycled mainly by provinces and Archean greenstone belts), and associ- cannibalistic processes — that is only through sedi- ated with early phases of oceanic island arc evolution. mentary mixing processes at the surface. In this re- Workshop on Mars 2001 2511.pdf

SEDIMENTARY PERSPECTIVE OF MARTIAN SURFACE CHEMISTRY: S. M. McLennan

gard, the experience gained from study of the lunar distinct possibility that sedimentary silica could be an regolith may prove useful in discriminating the role of important constituent of the near surface environment meteoritic components in Martian soils [7]. [13,17]. Sedimentary Provenance: Although basaltic Mineral Fractionation during Sedimentary Trans- provenance most likely will dominate Martian sedi- port: Sedimentary transport is expected to result in ments, data from SNC meteorites and Pathfinder hydrodynamic separation of heavy minerals. Al- rocks, that cover the range from ultramafic to inter- though no relevant studies have been conducted for mediate, indicate that in detail much variability in terrestrial basaltic sedimentation, it is likely that Fe- source rock compositions is to be expected. Experi- Ti-Cr oxides (e.g., ilmenite, titanomagnetite, magnet- ence from terrestrial studies suggests that the best way ite, chromite) will dominate Martian heavy mineral to unravel provenance components using bulk samples suites. Pathfinder and Viking soils and rocks show is to analyze a diverse suite of samples, maximizing variations in TiO2 contents that are consistent with variations in mineralogy and grain size, as was at- such heavy mineral fractionation [12,13] and further tempted at the Viking and Pathfinder sites. measurements at the Mars 2001 site may confirm or Evidence for Sediment Mixing on Mars: The refute this suggestion. composition of soils at the Viking and Pathfinder sites A second process that is likely to affect surface are broadly similar but significant differences appear chemistry is the transport of fine grained iron-rich to exist for MgO, TiO2, SO3, K2O, and perhaps other dust and deposition on rock surfaces and admixture elements and an important question is how homoge- into soils. Some soils at the Pathfinder and Viking neous are soil compositions on a global scale? Path- sites may be more enriched in iron than predicted finder results provide compelling evidence for physi- from simple soil - rock mixing and such variations are cal mixing between rocks and soils [10,11], however, consistent with a component of iron oxide, presumably some of the geochemical variations are inconsistent fine dust transported by eolian processes [12,13]. with simple two component mixing and accordingly Crust/Mantle Evolution: It is well established on other processes (and/or provenance components) have Earth that the average composition of sedimentary probably affected soil and rock compositions [12,13]. rocks approximates the bulk composition of the upper Sedimentary Processes: An important charac- crust exposed to weathering and erosion and this ob- teristic of Martian soils is their high S and Cl contents servation has been used successfully to constrain the [3,4]. Correlations between these elements and Mg composition and chemical evolution of the Earth’s (and possibly Fe) point to a substantial secondary continental crust [1]. There is good reason to evaluate mineralogy and evaluating the processes that have if such an approach is applicable to Mars, where the given rise to this mineralogy is a critical issue that thermal history of the planet may be especially sensi- will constrain the sedimentary environments that have tive to the transfer of incompatible elements (includ- operated on the Martian surface over its history. ing the heat producing elements) to the crust. Weathering and/or Hydrothermal Alteration: Al- On Earth, mainly shales have been used to evaluate though a great deal of effort has gone into modeling upper crustal evolution due to the relatively homoge- various alteration scenarios for the Martian surface neous compositions of shales, the abundance of shale [e.g., 14-16], such studies are largely in the realm of in the sedimentary record, and the fact that shales speculation until further mineralogical and chemical dominate the mass balance of most elements in the constraints from surface measurements are available. sedimentary record. In detail, however, it is the com- Several lines of evidence point to a sulfur mineralogy position of the entire sedimentary mass that equates to dominated by magnesium sulfates. In addition, there the upper crust, after correction for added CO2, H2O, is abundant evidence for substantial amounts of iron SO3, O2, and so forth [1]. being associated with secondary iron oxide minerals For Mars, there is no a priori reason to assume (e.g., hematite, maghemite). Accordingly, regardless that a homogeneous sedimentary composition exists of the exact mechanism of surface alteration, it is clear [e.g. Ref. 11], analogous to shales on Earth, that can that primary igneous minerals have been altered to a be used for such a purpose. Accordingly, it is impor- secondary ‘sedimentary’ mineralogy. Under a wide tant to evaluate the degree of homogeneity that exists range of hydrological conditions on Earth (e.g., among the soils. Sampling from Viking and Path- weathering, palagonitization), silica is highly mobile. finder does suggest some degree of homogeneity but The fact that up to half of the Mg and Fe contained in some significant differences, notably in Ti, Mg, K and soils, and liberated during alteration processes, may be S, do exist among these sites. Within the uncertain- sequestered in non-silicate minerals leaves open the ties, several authors have noted the broad similarity Workshop on Mars 2001 2511.pdf

SEDIMENTARY PERSPECTIVE OF MARTIAN SURFACE CHEMISTRY: S. M. McLennan

between the average soil composition and the compo- turned samples. sition of basaltic shergottites [e.g., 6, 10-12], although The level of enrichment of incompatible elements the high K2O abundances at the Pathfinder site may in sedimentary rocks is critical for interpreting de- indicate a more incompatible element enriched com- tailed provenance and for constraining geochemical position [12]. models of crust/mantle evolution (the level of heat The Mars 2001 mission should provide especially producing elements in the crust, for example). K2O useful constraints on this problem. The Lander/Rover abundances at the Pathfinder site were unexpectedly will provide additional analyses of surface soils thus high and contrasted with the relatively low levels at further evaluating the level of homogeneity of the soils the two Viking sites. High quality measurements of at four sites. The Orbiter will map the surface chem- potassium in soils and rocks will be especially useful istry on a 300 km resolution using the gamma ray geochemical data. The combination of additional spectrometer [18], thus providing a means of estimat- surface rock and soil APXS data (mainly major ele- ing the bulk surface composition and of testing the ments) coupled with large scale orbital gamma ray degree to which soils provide a meaningful estimate of geochemical data (major and trace elements) promises the upper Martian crust. to provide major advances in our understanding of the Some Priorities for Sedimentary Geochemistry geochemical evolution of Mars. Studies on Mars: Apart from the obvious need for more geochemical data at all scales, an evaluation of References: [1] Taylor, S. R. and McLennan, S. the sedimentary geochemistry of the Martian surface M. (1985) The Continental Crust: Its Composition would benefit from a variety of mineralogical, textural and Evolution (Blackwell, Oxford). [2] Johnsson, M. and chemical data. On the most basic level, distin- J. and Basu, A., eds. (1993) Processes Controlling the guishing sedimentary from igneous origin for ana- Composition of Clastic Sediments. GSA Spec. Paper lyzed rocks is critical, as demonstrated by the uncer- 284. [3] Rieder, R. et al., (1997) Science, 278, 1771- tainty in interpreting the geochemistry of Pathfinder 1774. [4] , B. C. (1982) JGR, 87, 10059-10067. rocks [19]. Distinguishing sedimentary from igneous [5] McSween Jr., H. Y. (1985) Rev. Geophys., 23, rocks on Earth, in situations where it is not obvious 391-416. [6] McSween Jr., H. Y. and Treiman, A. H. such as in metamorphic rocks, tektites, and so forth, it (1998) Rev. Mineral., 36, 6/1-6/53. [7] Taylor, S. R. is necessary to identify geochemical variations among (1982) Planetary Science: A Lunar Perspective (LPI, samples that are uniquely characteristic of sedimen- Houston). [8] Garrels, R. M. and Mackenzie, F. T. tary processes (e.g., weathering, transport, diagenesis) (1971) Evolution of Sedimentary Rocks (Norton, New [1,2]. For basaltic sediments in general, and on Mars York). [9] McLennan, S. M. (1988) Pure Appl. Geo- in particular, such processes have not yet been ade- phys., 128, 683-724. [10] Bell III, J. F. et al. (1999) quately characterized. JGR (in press). [11] McSween Jr., H. Y. and Keil, K. Placing additional constraints on soil mineralogy is (1999) GCA (in review). [12] McLennan, S. M. necessary for understanding the nature of low tem- (1999) LPS XXX, Abst. #1700. [13] McLennan, S. M. perature alteration processes on and near the Martian (1999) 5th Int. Conf. on Mars, Abst. #6148 (LPI Con- surface. Evaluating chemical relationships would trib. No. 972) [14] Burns, R. G. (1993) GCA, 57, especially benefit from constraints on clay mineralogy 4555-4574. [15] Clark, B. C. (1993) GCA, 57, 4575- and non-silicate mineralogy and more precise deter- 4581. [16] Newsom, H. E. et al. (1999) JGR, 104, mination of elements forming anionic components 8717-8728. [17] McLennan, S. M. (1999) 9th Ann. (e.g., P2O5, CO2, SO3). The possibility that sedimen- Goldschmidt Conf., Abst. #7456. [18] Boynton, W. tary silica may be an important constituent has broad V. et al. (1999) LPS XXX, Abst. #1991. implications and evaluating this possibility and the [19] McSween Jr., H. Y. et al. (1999) JGR, 104, 8679- nature of sedimentary silica distribution at or near the 8715. Martian surface are important. In order to evaluate the possible role of sedimen- tary transport on chemical and mineralogical compo- sitions, it is also important to have measurements of the grain size of soils that are analyzed. In due course, setting priorities for selecting material for sample return also may be strongly influenced by grain size distributions, as will setting priorities on developing geochemical analytical approaches to re-