Rock Coatings Research Articles

Geological characterization of remote field sites using visible and infrared spectroscopy: Results from the 1999 Marsokhod field test

Upcoming Mars Surveyor lander missions will include extensive spectroscopic capabilities designed to improve interpretations of the mineralogy and geology of landing sites on Mars. The 1999 Marsokhod Field Experiment (MFE) was a Mars rover simulation designed in part to investigate the utility of visible/near‐infrared and thermal infrared field spectrometers to contribute to the remote geological exploration of a Mars analog field site in the California Mojave Desert. The experiment simultaneously investigated the abilities of an off‐site science team to effectively analyze and acquire useful imaging and spectroscopic data and to communicate efficiently with rover engineers and an on‐site field team to provide meaningful input to rover operations and traverse planning. Experiences gained during the MFE regarding effective communication between different mission operation teams will be useful to upcoming Mars mission teams. Field spectra acquired during the MFE mission exhibited features interpreted at the time as indicative of carbonates (both dolomitic and calcitic), mafic rocks and associated weathering products, and silicic rocks with desert varnish‐like coatings. The visible/near‐infrared spectra also suggested the presence of organic compounds, including chlorophyll in one rock. Postmission laboratory petrologic and spectral analyses of returned samples confirmed that all rocks identified as carbonates using field measurements alone were calc‐silicates and that chlorophyll associated with endolithic organisms was present in the one rock for which it was predicted. Rocks classified from field spectra as silicics and weathered mafics were recognized in the laboratory as metamorphosed monzonites and diorite schists. This discrepancy was likely due to rock coatings sampled by the field spectrometers compared to fresh rock interiors analyzed petrographically, in addition to somewhat different surfaces analyzed by laboratory thermal spectroscopy compared to field spectra.

Rock coatings and aeolian abrasion on Mars: Application to the Pathfinder landing site

Rock coatings can be used to constrain the rate of abrasion by wind on Mars. The susceptibility to abrasion for potential rock coatings on Mars (salt/salt‐cemented coatings, rock varnish, and amorphous silica) were determined experimentally. Rock coatings generally abrade more easily than the host rock, although amorphous silica is an exception. If coatings exist on rocks at the Mars Pathfinder landing site, then the rate of abrasion is extremely low, consistent with previous studies of erosion rates. Alternatively, rock varnish or silica coatings could protect the rocks from sandblasting, although this may require coating formation under current to recent conditions. The Pathfinder area may lack efficient abrasive particles, and the observed duneforms may be composed of sand‐size dust aggregates rather than crystalline particles.

Laser Raman spectroscopy of varnished basalt and implications for in situ measurements of Martian rocks

Earth‐based and orbital spectroscopy and Viking Lander data strongly suggest that rocks on Mars are coated with indurated dust deposits. Rock coatings will influence mineralogical measurements to be acquired in situ from Mars rovers and other surface missions, in that most techniques will detect the coatings and/or coatings plus underlying minerals. In this paper, we present results of experiments on varnished basalt cobbles from the Lunar Crater Volcanic Field, Nevada, using laser Raman spectroscopic microprobe measurements obtained with a 514.5 nm Ar+ laser and irradiation spot sizes from 1 to 20 μm. Measurements from thin sections and fresh, unprepared rock surfaces clearly allow identification of igneous minerals. Varnish spectra acquired from thin sections as well as from unprepared exterior rock surfaces typically show broad peaks consistent with the fine‐grained nature of varnish and the dominance of manganese oxide and clay minerals. Several areas also show sharp Raman peaks due to phosphates and anatase, indicative of the heterogeneity of varnish deposits. Raman spectra taken through varnished rock surfaces reveal the mineralogy of coatings (where varnish is >15 μm in thickness) and a combination of coatings and igneous minerals in areas with thinner varnish. Given the abundance of rock coatings observed at the Viking and Pathfinder landing sites, results indicate that techniques such as Raman spectroscopy (and any others with comparable penetration depths) will sample some combination of coatings and underlying rock materials. The ability to make many in situ measurements across samples, coupled with the ability to expose fresh rock surfaces, will be needed to unambiguously characterize both the coatings and the rocks on Mars from future landers and/or rovers.

Rapid formation of rock varnish and other rock coatings on slag deposits near Fontana, California

AbstractManganiferous rock varnish, silica glaze and iron skins have formed on 20‐ to 40‐year‐old slag piles near Fontana, southern California. Rapid rock‐varnish formation is associated with an unidentified cocci bacterium that grows rapidly in culturing experiments, combined with the likelihood that Mn‐rich solutions flow over slag surfaces. A new model is proposed for the formation of silica glaze, involving soluble Al‐Si complexes and the nature of wetting films. Iron skins occur where runoff is concentrated, and where iron bacteria grow.Observations indicate: (i) substrate can be important in supplying Mn, as well as maintaining a stable surface and channelling runoff to microspots where varnish grows; (ii) ignorance of boundary layer pH/Eh conditions makes chemical‐physical models of varnish formation speculative; (iii) the balance between cementation of recently nucleated varnishes and erosional shear stresses is poorly understood, but is probably an important limiting factor on varnish growth rates; and (iv) there must be at least two steps in biotic varnishing‐accretion of Mn‐Fe in casts, followed by cementation of clays by either in situ cast residue or Mn‐Fe remobilized from casts. Varnish accretion may be limited by the rate at which both steps occur. Rapid development of readily distinguishable manganiferous rock varnishes at this post‐1952 site highlights the danger of speculation on the ages of landforms based solely on the appearance of rock‐varnish coatings.

Digital processing of back-scatter electron imagery: A microscopic approach to quantifying chemical weathering

This paper introduces digital processing of back-scattered electron (BSE) imagery as a microscopic approach to measure porosity from in situ dissolution of minerals. Four case studies exemplify this technique. Cases 1 and 2 explore the initiation and maintenance of weathering forms. In case 1, alveoli start in the Sedona area of Arizona when sandstone porosity exceeds ∼32%. This threshold is probably the point at which intergrain cohesion is reduced enough for shear stresses to erode grains. Case 2 examines the maintenance of gnamma pits and polygonal cracks on a basalt boulder on the island of Maui, Hawaii. Rock dissolution progresses while the surface of the rock is preserved under coatings of silica glaze. When a porosity of ∼37%–47% is reached, the weathering rind loses cohesion and spalls. Then, the protective silica glaze starts to accrete again, and another cycle begins. Cases 3 and 4 involve measuring rates of dissolution over thousands of years. Case 3 concerns rock dissolution in weathering rinds formed on ventifacted aplite boulders. Weathering rinds lost mass for the last 14 k.y. at a rate of 40 ± 15 g/m 2 /k.y., and for the last 17 k.y. at a rate of 43 ± 16 g/m 2 /k.y. Dissolution rates increased over time to 67 ± 23 g/m 2 /k.y. for the last 60–65 k.y. Case 4 addresses the classic topic of which variable is most important in chemical weathering: temperature, precipitation, or microenvironment. In situ measurements of plagioclase dissolution in ∼3-k.y.-old basalt flows reveal that warmer temperatures enhance rates of plagioclase dissolution by about 0.07%/°C, when precipitation and microenvironment are controlled. Plagioclase dissolution increases as precipitation increases at higher elevations, even though temperature decreases. However, microenvironment is a more important control on plagioclase dissolution; organic-rich positions (under lichens) weather two to seven times faster than adjacent organic-poor positions away from epilithic organisms and rock coatings. Rates of rock weathering are often established indirectly, by measuring the erosion of weathered material. In contrast, this microscopic method measures only in situ weathering. Conventional measures of rock weathering usually involve units, for example, depth of weathering pits or thickness of weathering rinds, that are not readily comparable to other data. In contrast, cases 3 and 4 illustrate that in situ measurements of rock and mineral porosity can yield data on mass weathered per unit area over time. This information is comparable to mass balance approaches in watershed- and soil-based weathering research.

Fire, climate, and the persistence of desert varnish near Dampier, Western Australia

In the semi-arid region near Dampier, discontinuous black and dark red coatings (desert varnish) have developed mainly on outcrops of granophyre. Coatings which have persisted occur on the most stable surfaces of large, rather than small, boulders. Rock coatings are absent both from fire-affected outcrop margins and around sediment patches within boulder slopes, and from the spray zone on the coastal margin. Sea level rise between about 17,000 and 6000 yr B.P. would have produced a landward-migrating spray zone within which salt weathering probably destroyed varnish before inundation. The remaining rock coatings have variable chemistry, and include compound coatings with superimpositions of thin Fe-rich, thicker SiAlMnFe layers, and an upper coat of SiAlFe, indicating fluctuatinng environments. Within major outcrops, the best-developed coatings occur on large and stable boulders beyond the reach of fire and salt spray, and often in summit or mid-slope positions. Much varnish loss can be attributed to factors other than climatic change, with coatings presently forming at a slower rate than they are being destroyed.

DISTRIBUTION AND CHEMICAL COMPOSITION OF MICROCOLONIAL FUNGI AND ROCK COATINGS FROM ARID AUSTRALIA

Microcolonial fungi are found on rock coatings at various sites throughout the arid regions of Australia. Fungi tend to concentrate in depressions, although it is uncertain whether they contribute to enlargement of microhollows. As determined by SEM-EDAX, the chemical characteristics of microcolonial fungi are variable but do not reflect those of the surfaces on which they are found. Most fungi contain calcium, sulfur, chlorine and sometimes phosphorus, but not manganese, in quantities greater than those of adjacent rock coatings. Major sources for accumulated elements may be rainfall or dust. Some elements, especially calcium, are apparently concentrated temporarily in microcolonial fungi, with amounts present in dead organisms being relatively low. The incorporation of some dead microcolonial fungi into developing rock coatings is therefore unlikely to lead to anomalously high calcium levels for cation ratio dating. As rock coatings contain trace or undetectable amounts of some other elements concentrat...

Rock weathering in deserts: Mobilization and concentration of ferric iron by microorganisms

Abstract Samples of rock coatings from arid and semi‐arid areas in the continental United States, Hawaii, and Australia were found to contain biochemical compounds (siderophores) that are produced by microorganisms to solubilize otherwise insoluble Fe(III) oxides and oxyhydroxides. Pure cultures of most bacteria and microcolonial fungi isolated from the same coatings produced siderophores (catechols and/or hy‐droxamates) in the laboratory. Isolates of common bacteria from the rock coatings concentrated iron oxides/oxyhydroxides on the exteriors of cell walls when exposed to iron‐bearing solutions, including siderophores. These results suggest a mechanism whereby siderophores mobilize ferric iron from source materials in deserts such as wind‐deposited dust under aerobic and alkaline to neutral pH conditions, and the iron extracted in solution by siderophores is concentrated by precipitation onto bacterial cell walls. Upon death and lysis of the microorganisms the iron oxyhydroxides become attached to the rock or grain substrata in the form of coatings. It is suggested that the biochemical activity of microorganisms may control the iron content of such deposits as clay‐rich desert varnish, iron‐stained siliceous crusts, and grain coatings in desert soils.

SURFACE EXPOSURE DATING: REVIEW AND CRITICAL EVALUATION

The past decade has seen the development and application of over a dozen new methods for quantitative age-determinations of geomorphic surfaces. Some surface exposure dating methods are numerical, including the accumulation of cosmogenic radionuclides 10Be, 14C, 26Al, 36Cl, and 41Ca, accumulation of cosmogenic stable nuclides 3He and 21Ne, 14C dating of organic matter encapsulated in rock coatings, and dendrogeomorphology. Calendar ages are obtained by dendrogeomorphological analysis. Calibrated ages can be obtained by analysis of rock-varnish chemistry, lichenometry, weathering, and soils. Various methods can be used in combination to overcome individual limitations. Whereas conventional methods provide age control on stratigraphic profiles, surface-exposure dating methods are especially suitable for geographic problems, such as analyzing not only temporal, but also spatial variations in the rates of geomorphic processes. [Key words: geomorphology, process, geochronology, cosmogenic nuclides, Quaternary,...

Manganese‐rich rock coatings from Iceland

AbstractManganese‐rich rock coatings are widespread in Iceland, especially in fractures in basalt lavas. Three common types of coating are described. A thick subsurface coating is found on whaleback forms on high plateaux. The most common type is thinner and occurs in hydrothermal clay‐filled fracture systems which are especially well developed in tholeiite lavas. A third type occurs in younger rocks and is associated with tephra‐rich, loessic sediment which fills open fractures and vesicles. There are differences in the chemistry, morphology, and mineralogy of these coatings which are interpreted as being due to the different microenvironments in which they form. The fracture microenvironment is regarded as being of much more importance to coating thickness and chemistry than simple age and in this respect the hydrothermal clays seem to favour rapid manganese concentration. On the other hand, there are variations in coating type, both laterally and vertically through the lava succession, which are related to regional distribution of hydrothermal alteration and rock type. The regional patterns are the result of Iceland's lateral drift and as a result there is an incidental connection between coating type and rock age. There is some evidence to suggest that the manganese coatings may be both forming and being degraded rapidly in the present environment.

Origin, development and chemistry of silica-alumina rock coatings from the semi-arid regions of the island of Hawaii

Exposed rock surfaces in the semi-arid regions of the Island of Hawaii are characterized by an x-ray amorphous silica-alumina surface coating, ranging in thickness from a few to several hundred micrometers. These coatings are composed of clear to translucent white lamellae of amorphous silica with a minor component of some amorphous aluminum phase, and in some cases, interlayered fine-grained clastic material. The coatings are derived from exogenous material. Both the amorphous silica-alumina component and the clastic component of the coating are derived locally. Description of the chemical composition, mineralogy and the morphology of the coating, along with an evaluation of possible sources, enables formulation of a model which describes their development. Because the composition and mineralogy of the coatings is dissimilar from all likely source materials, direct incorporation of the source material could not account for the coating. Instead fine-grained clastic material, locally derived from the soil, is blown onto the rock surface; during a later non-violent wetting event, such as fog or dew, some of this material goes into solution. As this solution is evaporated poorly ordered mineral phases precipitate onto the rock surface forming the coating. Some of the clastic material is incorporated into the coating during the precipitation. Late stage precipitates are removed during later more dynamic wetting events, such as a heavy rainfall, which wash across the rock surface.

Rock coatings in Hawaii

Basaltic lava flows exposed to semiarid weathering on the island of Hawaii develop hydrous silica coatings that thicken and incorporate detrital material with age. The nearly pure silica coatings on the younger rocks are colorless, but basaltic detritus and its weathering products, incorporated into coatings on older flows, produce colors ranging from brown to red. Age, rate of supply of material, and the mechanical stability of flow surfaces dictate the color and thickness of coatings. Fresh, glassy pahoehoe surfaces support thin (as much as 5 µm, clear silica coatings but break down in a few years to expose the vesicular underlying material. At first, coloring is concentrated in the open vesicles, yielding a dark brown color: a mixture of red vesicles and projections of black basalt. With age, thicker accumulations of silica incorporate layers of oxidized basaltic material. The dark basaltic material in Kilauea and Mauna Kea coatings is probably supplied by large explosive eruptions, but at Mauna Loa and Kohala, the source could be locally derived basaltic weathering products. The clear hydrous silica layers are probably the product of leaching of locally derived basaltic dust.