C. L. Smith

Tissint Martian Meteorite: A Fresh Look at the Interior, Surface, and Atmosphere of Mars

A New Rock from Mars On 18 July 2011 a meteorite originating from Mars fell on the moroccan desert. Chennaoui Aoudjehane et al. (p. 785, published online 11 October) show that this meteorite was ejected from the surface of Mars 700,000 years ago and contains components derived from the interior, surface, and atmosphere of the red planet. Previous to this fall, only four other martian meteorites have been collected after being witnessed falling to Earth. All the other martian meteorites that are represented in collections around the world, have been found long after their arrival on Earth, and thus have suffered from exposure to the terrestrial environment. A meteorite that fell in Morocco in July 2011 provides a sample to study processes that operated on Mars 700,000 years ago. Tissint (Morocco) is the fifth martian meteorite collected after it was witnessed falling to Earth. Our integrated mineralogical, petrological, and geochemical study shows that it is a depleted picritic shergottite similar to EETA79001A. Highly magnesian olivine and abundant glass containing martian atmosphere are present in Tissint. Refractory trace element, sulfur, and fluorine data for the matrix and glass veins in the meteorite indicate the presence of a martian surface component. Thus, the influence of in situ martian weathering can be unambiguously distinguished from terrestrial contamination in this meteorite. Martian weathering features in Tissint are compatible with the results of spacecraft observations of Mars. Tissint has a cosmic-ray exposure age of 0.7 ± 0.3 million years, consistent with those of many other shergottites, notably EETA79001, suggesting that they were ejected from Mars during the same event.

Characterization of mineral surfaces using FIB and TEM: A case study of naturally weathered alkali feldspars

Abstract Using a focused ion beam (FIB) instrument, electron-transparent samples (termed foils) have been cut from the naturally weathered surfaces of perthitic alkali feldspars recovered from soils overlying the Shap granite, northwest England. Characterization of these foils by transmission electron microscopy (TEM) has enabled determination of the crystallinity and chemical composition of near-surface regions of the feldspar and an assessment of the influence of intragranular microtextures on the microtopography of grain surfaces and development of etch pits. Damage accompanying implantation of the 30 kV Ga+ ions used for imaging and deposition of protective platinum prior to ion milling creates amorphous layers beneath outer grain surfaces, but can be overcome by coating grains with >85 nm of gold before FIB work. The sidewalls of the foil and feldspar surrounding original voids are also partially amorphized during later stages of ion milling. No evidence was found for the presence of amorphous or crystalline weathering products or amorphous “leached layers” immediately beneath outer grain surfaces. The absence of a leached layer indicates that chemical weathering of feldspar in the Shap soils is stoichiometric, or if non-stoichiometric, either the layer is too thin to resolve by the TEM techniques used (i.e., ≤~2.5 nm) or an insufficient proportion of ions have been leached from near-surface regions so that feldspar crystallinity is maintained. No evidence was found for any difference in the mechanisms of weathering where a microbial filament rests on the feldspar surface. Sub-micrometer-sized steps on the grain surface have formed where subgrains and exsolution lamellae have influenced the propagation of fractures during physical weathering, whereas finer scale corrugations form due to compositional or strain-related differences in dissolution rates of albite platelets and enclosing tweed orthoclase. With progressive weathering, etch pits that initiated at the grain surface extend into grain interiors as etch tubes by exploiting preexisting networks of nanopores that formed during the igneous history of the grain. The combination of FIB and TEM techniques is an especially powerful way of exploring mechanisms of weathering within the “internal zone” beneath outer grain surfaces, but results must be interpreted with caution owing to the ease with which artifacts can be created by the high-energy ion and electron beams used in the preparation and characterization of the foils

Sequestration of Martian CO2 by mineral carbonation

Carbonation is the water-mediated replacement of silicate minerals, such as olivine, by carbonate, and is commonplace in the Earth’s crust. This reaction can remove significant quantities of CO2 from the atmosphere and store it over geological timescales. Here we present the first direct evidence for CO2 sequestration and storage on Mars by mineral carbonation. Electron beam imaging and analysis show that olivine and a plagioclase feldspar-rich mesostasis in the Lafayette meteorite have been replaced by carbonate. The susceptibility of olivine to replacement was enhanced by the presence of smectite veins along which CO2-rich fluids gained access to grain interiors. Lafayette was partially carbonated during the Amazonian, when liquid water was available intermittently and atmospheric CO2 concentrations were close to their present-day values. Earlier in Mars’ history, when the planet had a much thicker atmosphere and an active hydrosphere, carbonation is likely to have been an effective mechanism for sequestration of CO2.

Electrochemical studies of iron meteorites: phosphorus redox chemistry on the early Earth

The mineral schreibersite, (Fe,Ni)(3)P, a ubiquitous component of iron meteorites. is known to undergo anoxic hydrolytic modification to afford a range Of phosphorus oxyacids. H-phosphonic acid (H3PO3) is the principal hydrolytic product under hydrothermal conditions, as confirmed here by P-31-NMR spectroscopic studies oil shavings of the Seymchan pallasite (Magadan, Russia, 1967), but in the presence of photochemical irradiation I more reduced derivative, H-phosphinic (H3PO2) acid, dominates. The significance Of Such lower oxidation state oxyacids of phosphorus to prebiotic chemistry upon the early Earth lies with the facts that Such forms Of Phosphorus are considerably more Soluble and chemically reactive than orthophosphate, the commonly found form of phosphorus oil Earth, thus allowing nature a mechanism to circumvent the so-called Phosphate Problem. This paper describes the Galvanic corrosion of Fe3P, a hydrolytic modification pathway for schreibersite, leading again to H-phosphinic acid as the key P-containing product. We envisage this pathway to be highly significant within a meteoritic context as iron meteorites are polymetallic composites in which dissimilar metals, with different electrochemical potentials, are connected by all electrically conducting matrix. In the presence of a Suitable electrolyte medium, i.e., salt water, galvanic corrosion call take place. In addition to model electrochemical studies, we also report the first application of the Kelvin technique to map surface potentials of a meteorite sample that allows the electrochemical differentiation of schreibersite inclusions Within an Fe:Ni matrix. Such experiments, coupled with thermodynamic calculations, may allow LIS to better understand the chemical redox behaviour of meteoritic components with early Earth environments.

Scanning transmission electron microscopy using a SEM: Applications to mineralogy and petrology

Abstract High-resolution imaging of electron-transparent samples using a scanning electron microscope, here termed low voltage (LV) STEM, is a new and valuable technique for studying Earth and planetary materials. The most effective method of LV-STEM imaging uses a pair of electron detectors positioned side-by-side beneath the thin sample. The detector directly underlying the sample forms bright-field images dominated by mass-thickness contrast. Activation of the detector offset from the sample yields dark-field images with a greater component of atomic number contrast. LV-STEM images with significant diffraction contrast can also be obtained, but require careful positioning of the sample relative to the electron detectors. In this study LV-STEM was used successfully to image sub-μm sized kaolinite crystals and tens of nm-sized etch pits on the gold-coated surfaces of weathered feldspar grains. Dark-field LV-STEM was also especially effective for characterizing very fine-scale intergrowths of Mg- and Fe-rich phyllosilicates within uniformly thin samples of the Murchison meteorite prepared using the focused ion beam (FIB) technique. LV-STEM is a quick and easy method for characterizing the morphology and internalstructure of mineraland rock samples and may prove to be especially useful in geomicrobiology research.

Weathering microenvironments on feldspar surfaces: implications for understanding fluid-mineral reactions in soils

Abstract The mechanisms by which coatings develop on weathered grain surfaces, and their potential impact on rates of fluid-mineral interaction, have been investigated by examining feldspars from a 1.1 ky old soil in the Glen Feshie chronosequence, Scottish highlands. Using the focused ion beam technique, electron-transparent foils for characterization by transmission electron microscopy were cut from selected parts of grainsurfaces. Some parts were bare whereas others had accumulations, a few micrometres thick, of weathering products, often mixed with mineral and microbial debris. Feldspar exposed at bare grain surfaces is crystalline throughout and so there is no evidence for the presence of the amorphous ‘leached layers’ that typically form in acid-dissolution experiments and have been described from some natural weathering contexts. The weathering products comprise sub-mm thick crystallites of an Fe-K aluminosilicate, probably smectite, that have grown within an amorphous and probably organic-rich matrix. There is also evidence for crystallization of clays having been mediated by fungal hyphae. Coatings formed within Glen Feshie soils after ~1.1 ky are insufficiently continuous or impermeable to slow rates of fluid-feldspar reactions, but provide valuable insights into the complex weathering microenvironments on debris and microbe-covered mineral surfaces.

Taking the pulse of Mars via dating of a plume-fed volcano

Mars hosts the solar system’s largest volcanoes. Although their size and impact crater density indicate continued activity over billions of years, their formation rates are poorly understood. Here we quantify the growth rate of a Martian volcano by 40Ar/39Ar and cosmogenic exposure dating of six nakhlites, meteorites that were ejected from Mars by a single impact event at 10.7 ± 0.8 Ma (2σ). We find that the nakhlites sample a layered volcanic sequence with at least four discrete eruptive events spanning 93 ± 12 Ma (1416 ± 7 Ma to 1322 ± 10 Ma (2σ)). A non-radiogenic trapped 40Ar/36Ar value of 1511 ± 74 (2σ) provides a precise and robust constraint for the mid-Amazonian Martian atmosphere. Our data show that the nakhlite-source volcano grew at a rate of ca. 0.4–0.7 m Ma−1—three orders of magnitude slower than comparable volcanoes on Earth, and necessitating that Mars was far more volcanically active earlier in its history.Mars hosts the solar system’s largest volcanoes, but their formation rates remain poorly constrained. Here, the authors have measured the crystallization and ejection ages of meteorites from a Martian volcano and find that its growth rate was much slower than analogous volcanoes on Earth.

Chronology of martian breccia NWA 7034 and the formation of the martian crustal dichotomy

The metamorphic history of martian meteorite NWA 7034 suggests that the martian crustal dichotomy may have formed within 100 million years of planetary formation. Martian meteorite Northwest Africa (NWA) 7034 and its paired stones are the only brecciated regolith samples from Mars with compositions that are representative of the average martian crust. These samples therefore provide a unique opportunity to constrain the processes of metamorphism and alteration in the martian crust, which we have investigated via U-Pu/Xe, 40Ar/39Ar, and U-Th-Sm/He chronometry. U-Pu/Xe ages are comparable to previously reported Sm-Nd and U-Pb ages obtained from NWA 7034 and confirm an ancient (>4.3 billion years) age for the source lithology. After almost 3000 million years (Ma) of quiescence, the source terrain experienced several hundred million years of thermal metamorphism recorded by the K-Ar system that appears to have varied both spatially and temporally. Such protracted metamorphism is consistent with plume-related magmatism and suggests that the source terrain covered an areal extent comparable to plume-fed edifices (hundreds of square kilometers). The retention of such expansive, ancient volcanic terrains in the southern highlands over billions of years suggests that formation of the martian crustal dichotomy, a topographic and geophysical divide between the heavily cratered southern highlands and smoother plains of the northern lowlands, likely predates emplacement of the NWA 7034 source terrain—that is, it formed within the first ~100 Ma of planetary formation.

Carbon sequestration on Mars: COMMENT

Using data acquired from the Nili Fossae region of Mars by orbital remote sensing, Edwards and Ehlmann (2015) have demonstrated that CO2 was removed from the planet’s late Noachian–early Hesperian atmosphere by replacement (carbonation) of olivine-enriched basalts. Given the relatively low volumes (~20% at most) of carbonate present in the study area, and the apparent absence of carbonate-rich terranes of similar size elsewhere on Mars, they questioned whether carbonation alone could account for the loss of a putative thick CO2-rich atmosphere. The work of Edwards and Ehlmann is therefore a highly significant contribution to the long-running debate about the nature and evolution of the climate of early Mars. The authors noted that Martian meteorites contain carbonates, and suggested that further evaluation of these rocks could improve our understanding of the planet’s carbon cycle. Here we review published results from Martian meteorites, in particular the nakhlite group, which have already provided important insights into the mechanisms and time scales of mineralization and storage of CO2 in Mars’ crust. This evidence can help to discriminate between the various models for sequestration of Martian carbon that were discussed by Edwards and Ehlmann. The nakhlites were impact-ejected from Mars, probably from one location and in a single event. They are clinopyroxenites, and most contain olivine. The majority of these igneous rocks have been partially aqueously altered to produce a very fine-grained intergrowth of minerals including Fe-rich carbonates (up to ~1 vol%) and hydrous silicates (Gooding et al., 1991; Treiman et al., 1993; Changela and Bridges, 2011; Lee et al., 2013). These alteration products are collectively called ‘iddingsite.’ Water-rock interaction took place at some time between crystallization of the melt (ca. 1300 Ma) and impact-ejection (ca. 11 Ma; Eugster et al., 1997); the best current estimate is ca. 633 Ma (Borg and Drake, 2005). The high deuterium/hydrogen ratio of the iddingsite demonstrates that its parent aqueous fluids had equilibrated with Mars’ atmosphere (Hallis et al., 2012). Mineralogical and crystal-chemical records of atmosphere-water-rock interactions that are contained within the nakhlites support some of the conclusions of Edwards and Ehlmann, but also emphasize the importance of incorporating the meteorite record in any comprehensive model. For example, microstructural evidence demonstrates that the carbonates were formed by replacement of olivine and a plagioclase feldspar-rich mesostasis, whereas the volumetrically dominant augite has remained unaltered (Tomkinson et al., 2013). The differential susceptibility of silicates in these meteorites to carbonation helps to account for distinct contrasts between Nili Fossae rock units in their relative abundances of olivine and carbonate. The occurrence of carbonates within veins in the nakhlites (Tomkinson et al., 2013) is also consistent with the suggestion by Edwards and Ehlmann that fracturing had facilitated the movement of aqueous solutions through the Nili Fossae basalts. However, the partial replacement of nakhlite carbonates by phyllosilicates and Feoxyhydroxides (Tomkinson et al., 2013) demonstrates that this crustal store of carbon was not necessarily permanent. The age of the nakhlite carbonates shows that atmospheric CO2 was being mineralized and stored in the late Amazonian. Scarce 3900-m.y.old carbonates also occur in the Martian orthopyroxenite Allan Hills (ALH)84001 (Borg et al., 1999). The meteorite record therefore supports the ‘deep–diffuse sequestration’ model that was invoked by Edwards and Ehlmann to account for the carbonate that is ‘missing’ from the Martian crust on the assumption that its early atmosphere was thinned mainly by mineral sequestration. The deep-diffuse model proposes that low volumes of finely disseminated carbonate formed within the crust throughout much of Mars’ history, and as Edwards and Ehlmann note, these carbonates would be “undetectable by remote sensing.” Products of aqueous alteration are essentially absent from the shergottite meteorites, which sample younger than ca.600 Ma basalts from several different locations, thus indicating that the drivers of crustal sequestration had effectively ceased within the past few hundred million years. The record of reactions between the Martian atmosphere, hydrosphere, and lithosphere in our meteorite collections is manifestly very patchy as regards age spectrum and rock types (i.e., it is highly biased toward igneous rocks from younger terranes). Additionally, stresses accompanying impact ejection of these rocks, their interplanetary transfer and fall to Earth may militate against the sampling of heavily carbonated lithologies that are likely to be brittle and fractured. Nonetheless, information on the presence/absence and petrographic context of carbonate minerals in Martian meteorites provides very valuable information on the magnitude of CO2 sequestration, petrologic and microstructural controls on the carbonation reaction, and its temporal and geographical range. This record indicates to us that the deep-diffuse model remains viable, and it can be tested further as new meteorites from regions of Mars that have not been previously sampled become available for study.

Provence of water in martian meteorites: D/H ratios of Lafayette using NanoSIMS

Veins of clay and carbonate in the nakhlite meteorite Lafayette formed by dissolution and replacement of olivine.NanoSIMS measurements record δD values up to +4725‰ in Lafayette which reveal martian waters of crustal origin are incorporated into the smectite and adjacent olivine.