Insights into the Martian mantle: The age and isotopics of the meteorite fall Tissint

The recent witnessed fall of the meteorite Tissint represents the delivery of a pristine new sample from the surface of Mars. This meteorite provides an unprecedented opportunity to study a variety of aspects about the planet's evolution. Using the Rb–Sr and Sm–Nd isotopic systems, we determined that Tissint, a depleted shergottite, has a crystallization age of 574 ± 20 Ma, an initial ε¹⁴³Nd = +42.2 ± 0.5, and an initial ⁸⁷Sr/⁸⁶Sr = 0.700760 ± 11. These initial Nd and Sr isotopic compositions suggest that Tissint originated from a mantle source on Mars that is distinct from the source reservoirs of the other Martian meteorites. The known crystallization ages, geochemical characteristics, ejection ages, and ejection dynamics of Tissint and other similarly grouped Martian meteorites suggest that they are likely derived from a source crater up to approximately 90 km in diameter with an age of approximately 1 Ma that is located on terrain that is approximately 600 million years old.     

Martian cratering 8: Isochron refinement and the chronology of Mars

This paper reviews and refines the technique of dating martian surfaces by using impact-crater isochrons (defined as size distributions of impact craters on undisturbed martian surfaces of specified ages). In the 1970s, this system identified not only abundant ancient martian volcanic surfaces, but also extensive lava plains with ages of a few 10⁸ y-old; this dating was initially controversial but confirmed in the 1980s and 90s by martian meteorites. The present update utilizes updated estimates of the Mars/Moon cratering ratio (the most important calibration factor), improves treatment of gravity and impact velocity scaling effects, combines aspects of the crater size distribution data from earlier work by both Neukum and Hartmann, and for the first time applies a correction for loss of small meteoroids in the martian atmosphere from Popova et al. (2003, Meteorit. Planet. Sci. 38, 905-925). The updated isochrons are not radically different from the previous “2002 iteration” but fit observed data better and give somewhat older model ages for features dated from small craters (diameter D<100 m). Crater counts from young lava flows in various areas give good fits to the new isochrons over as much as 3 orders of magnitude in D, confirming the general isochron shape and giving crater retention ages in the range of some 10⁶ to some 10⁸ y, interpreted as lava flow ages. More complex, older units are also discussed. Uncertainties are greatest if only small craters (D?100 m) are used. Suggestions by other workers of gross uncertainties, due to local secondary craters and deposition/exhumation, are discussed; they do not refute our conclusions of significant volcanic, fluvial, and other geologic activity in the last few percent of martian geologic time or the importance of cratering as a tool for studying processes such as exhumation. Indeed, crater count data suggest certain very recent episodes of deposition, exhumation, and ice flow, possibly associated with obliquity cycles of ∼¹⁰⁷ y timescale. Evidence from ancient surfaces suggests higher rates of volcanism, fluvial activity, glaciation, and other processes in Noachian/Hesperian time than in Amazonian time.     

In situ calibration of the Martian cratering chronology

Crater densities on planetary surfaces allow assessing relative ages but so far firm calibration of so‐called cratering‐chronology models is available only for the Moon and limited to the past 4.1 billion years. Most planetary geological time scales are still model‐dependent, and essentially constrained by meteorite ages or by comparison to (dynamical) solar system evolution models. Here we describe in situ calibration of the Martian cratering chronology using cosmogenic and radiogenic isotope ages obtained by the NASA Curiosity rover. We determined the cratering‐rate ratio between Moon and Mars for recent times, and extended the calibration of cratering rates to earlier times than those based exclusively on lunar data. Our preferred interpretation supports monotonic flux decay since at least 4.24 Ga and likely since about 4.45 Ga, implying orbital migration of the giant planets, and its direct, transient, dynamical effect on the planetesimal populations was initiated early. But only Martian Sample Return will provide strongly needed capability for distinction of the different models currently available.     

Martian cratering 9: Toward resolution of the controversy about small craters

The Malin et al. [Malin, M.C., Edgett, K., Posiolova, L., McColley, S., Noe Dobrea, E., 2006. Science 314, 1573-1577] report of ongoing production of decameter scale primary craters matches within an order of magnitude the crater production curves of Hartmann [Hartmann, W.K., 2005. Icarus 174, 294-320], and appears to contradict models that say our curves grossly overpredict primary production. Nonetheless, such models may still predict correct order-of-magnitude secondary production. The new observations, if correct, require geologically recent martian surface modification in some areas, on timescales comparable to the cycles of obliquity changes, suggesting episodic martian climatic/environmental change driven by the obliquity cycles.     

Martian channels and valleys: Their characteristics,distribution, and age

All Martian channels and valleys visible at a resolution of 125 to 300 meters between 65°N and 65°S were mapped at a scale of 1:5,000,000 and the maps then digitized. Correlations of valley presence with other surface features show that almost all valleys are in the old cratered terrain. preferentially in areas of low albedo, low violet/red ratios, and high elevation. The networks are open, the individual drainage basins are small relative to Earth, and large distances separate the basins, features which all suggest an immature drainage system. The simplest explanation of the correlations and the restriction of valley networks to old terrain is that the channels themselves are old, and that the climatic conditions necessary for their formation did not prevail for long after the decline in the cratering rate around 3.9 billion years ago. Two types of outflow channel are distinguished: unconfined, in which broad swaths of terrain are scoured, and confined, in which flow is restricted to discrete channels. The outflow channels have a wide range of ages and may form under present climatic conditions. Fretted channels are largely restrited to two latitude belts centered on 40°N and 45°S, where relatively rapid erosion along escarpments results from mass wasting. They probably form by enlargement of preexisting channels by escarpment retreat.     

Magmatic cristobalite and quartz in the NWA 856 Martian meteorite

Abstract— Silica‐rich late‐stage crystallization pockets in the Martian meteorite Northwest Africa (NWA) 856 were investigated by transmission electron microscopy (TEM). The pockets occur as wedges between maskelynite laths or between maskelynite and pyroxene. They consist of elongated grains of cristobalite and quartz embedded in a silica‐rich glass. Interstitial to the amorphous phase and silica minerals, a number of small accessory minerals have been identified, typical for late‐stage crystallization products. They are ilmenite, tranquillityite, fayalite, troilite, baddeleyite, apatite, and chloroapatite. Cristobalite and quartz are shocked, as revealed by the occurrence of numerous amorphous lamellae. This assemblage suggests metastable dendritic crystallization under hydrous conditions. Cristobalite crystallization was probably facilitated by the presence of impurities such as Na or H₂O. Our observations show that silica minerals can be formed under magmatic conditions on Mars.     

Hydrogen isotopic composition of the Martian mantle inferred from the newest Martian meteorite fall,Tissint

The hydrogen isotopic composition of planetary reservoirs can provide key constraints on the origin and history of water on planets. The sources of water and the hydrological evolution of Mars may be inferred from the hydrogen isotopic compositions of mineral phases in Martian meteorites, which are currently the only samples of Mars available for Earth‐based laboratory investigations. Previous studies have shown that δD values in minerals in the Martian meteorites span a large range of ?250 to +6000‰. The highest hydrogen isotope ratios likely represent a Martian atmospheric component: either interaction with a reservoir in equilibrium with the Martian atmosphere (such as crustal water), or direct incorporation of the Martian atmosphere due to shock processes. The lowest δD values may represent those of the Martian mantle, but it has also been suggested that these values may represent terrestrial contamination in Martian meteorites. Here we report the hydrogen isotopic compositions and water contents of a variety of phases (merrillites, maskelynites, olivines, and an olivine‐hosted melt inclusion) in Tissint, the latest Martian meteorite fall that was minimally exposed to the terrestrial environment. We compared traditional sample preparation techniques with anhydrous sample preparation methods, to evaluate their effects on hydrogen isotopes, and find that for severely shocked meteorites like Tissint, the traditional sample preparation techniques increase water content and alter the D/H ratios toward more terrestrial‐like values. In the anhydrously prepared Tissint sample, we see a large range of δD values, most likely resulting from a combination of processes including magmatic degassing, secondary alteration by crustal fluids, shock‐related fractionation, and implantation of Martian atmosphere. Based on these data, our best estimate of the δD value for the Martian depleted mantle is ?116 ± 94‰, which is the lowest value measured in a phase in the anhydrously prepared section of Tissint. This value is similar to that of the terrestrial upper mantle, suggesting that water on Mars and Earth was derived from similar sources. The water contents of phases in Tissint are highly variable, and have been affected by secondary processes. Considering the H₂O abundances reported here in the driest phases (most likely representing primary igneous compositions) and appropriate partition coefficients, we estimate the H₂O content of the Tissint parent magma to be ≤0.2 wt%.     

Constraints on the Martian cratering rate based on the SNC meteorites and implications for Mars climatic history

Two constraints placed upon the cratering flux at Mars by the SNC meteorites are examined: crystallization ages as a constraint on surface ages and cosmic ray exposure ages and number of impacts as a constraint on absolute rates. The crystallization ages of the SNC meteorites appear to constrain the Martian cratering rate to be 4xLunar or more if the parent lavas are in the north of Mars and the number of SNC ejecting impacts are small. If the SNCs result from a single impact that formed the Lyot basin then the cratering rate must be at least 7xLunar or higher to produce a basin age less than the SNC crystallization age because the basin ages are themselves determined by crater counting. Assuming multiple uncorrelated impacts for SNC ejection from Mars over 10 million years a cratering rate of approximately 4xLunar is also found for ejecting impacts that form craters over 12km in diameter. Therefore, both crystallization ages and ejection ages and number of impacts appear consistent with a 4xLunar cratering rate at Mars. The effect on Martian chronologies of such a high cratering rate is to place the SNC crystallization ages partly within the epoch of channel formation on Mars and to extend this liquid water epoch over much of Mars history.     

Petrography and composition of Martian regolith breccia meteorite Northwest Africa 7475

The Northwest Africa (NWA) 7475 meteorite is one of the several stones of paired regolith breccias from Mars based on petrography, oxygen isotope, mineral compositions, and bulk rock compositions. Its inventory of lithic clasts is dominated by vitrophyre impact melts that were emplaced while they were still molten. Other clast types include crystallized impact melt rocks, evolved plutonic rocks, possible basalts, contact metamorphosed rocks, and siltstones. Impact spherules and vitrophyre shards record airborne transport, and accreted dust rims were sintered on most clasts, presumably during residence in an ejecta plume. The clast assemblage records at least three impact events, one that formed an impact melt sheet on Mars ≤4.4 Ga ago, a second that assembled NWA 7475 from impactites associated with the impact melt sheet at 1.7–1.4 Ga, and a third that launched NWA 7475 from Mars ~5 Ma ago. Mildly shocked pyroxene and plagioclase constrain shock metamorphic conditions during launch to >5 and <15 GPa. The mild postshock‐heating that resulted from these shock pressures would have been insufficient to sterilize this water‐bearing lithology during launch. Magnetite, maghemite, and pyrite are likely products of secondary alteration on Mars. Textural relationships suggest that calcium‐carbonate and goethite are probably of terrestrial origin, yet trace element chemistry indicates relatively low terrestrial alteration. Comparison of Mars Odyssey gamma‐ray spectrometer data with the Fe and Th abundances of NWA 7475 points to a provenance in the ancient southern highlands of Mars. Gratteri crater, with an age of ~5 Ma and an apparent diameter of 6.9 km, marks one possible launch site of NWA 7475.     

Geochemistry of the Martian meteorite ALH 84001, revisited

Abstract– Major and trace element abundances were determined on powders prepared from four distinct chips from Allan Hills (ALH) 84001 to constrain the bulk rock composition, and to assess the trace element abundances of orthopyroxenes and phosphates. Our new determinations were used to evaluate the composition of the parental melt of this stone. An unrealistic light rare earth element (REE)‐enriched parental melt is calculated from the composition of the orthopyroxene and relevant equilibrium partition coefficients. The involvement of a small amount of trapped melt and subsolidus reequilibrations between orthopyroxene and the interstitial phases can account for this discrepancy. A parental melt that displays a trace element pattern (REE, Zr, and Hf) that closely resembles enriched shergottites such as Zagami or Los Angeles is calculated if these effects are taken into account. These results suggest that some shergottitic melts were already erupted on Mars during the Noachian.     

Martian cratering VI: Crater count isochrons and evidence for recent volcanism from Mars Global Surveyor

Abstract— This paper develops a methodology to establish absolute Martian ages by deriving isochrons on a plot of Martian impact crater density vs. crater diameter, calibrated by lunar crater/age data. The isochrons illustrated here are based on a Mars/Moon cratering ratio of 1.6 at constant size, but there is a factor of 2 to 4 uncertainty in this ratio and the consequent model ages. Martian crater diameter distributions are determined in several areas down to diameter D = 16–45 m; the shapes of the curves in young areas are found to be close to that of the predicted isochrons and close to the standard production function found by Neukum. The youngest areas studied here display the lunar-like production function down to D ～30 m, where saturation equilibrium sets in. Model crater retention ages of several volcanic units are found to be a few hundred million years or less, with estimated uncertainties ranging from a factor of 2 lower to a factor of 4 higher. The results are consistent with Martian meteorite ages. Volcanism on Mars has probably persisted into the last 10 to 15% of the planet's history and is likely ongoing. Because surfaces as young as a few hundred million years have reached crater saturation equilibrium at D < ～60 to 100 m, Mars is likely to have widespread impact-produced regoliths at least a few meters deep, and this may contribute to the widespread mobile dust and boulder fields of Mars.     

Comparative Raman microscopy of a Martian meteorite and Antarctic lithic analogues

The Raman microspectra of the Nakhla SNC meteorite, which probably originates from Mars, are reported here for the first time. The specimen is shown to be heterogeneous, even at a sampling level of 2 μm, but several important mineralogical features have been identified, including clinopyroxene, olivine and plagioclase. In some sampling regions, α-quartz particles are evident and in one sample region the ν(CO²⁻₃) mode of calcite at 1086 cm⁻¹ is found. There is no evidence for organic chemical content detectable in this meteorite specimen, based on an absence of ν(CH) and δ(CH₂) modes near 3000 and 1400 cm⁻¹, respectively. Comparative Raman spectroscopic analyses were made on epilithic examples of Xanthoria elegans from Crater Cirque, northern Victoria Land, Antarctica, and cryptoendoliths from East Beacon, McMurdo Dry Valleys, as positive controls for microbial organics in cold, arid habitats analogous to former Martian conditions. Finally, an assessment is made of the potential role of Raman spectroscopy for extra-terrestrial characterisation of geological specimens.     

Noble gases in the Martian meteorite Northwest Africa 2737: A new chassignite signature

Abstract— We report noble gas data for the second chassignite, Northwest Africa (NWA) 2737, which was recently found in the Moroccan desert. The cosmic ray exposure (CRE) age based on cosmogenic ³He, ²¹Ne, and ³⁸Ar around 10–11 Ma is comparable to the CRE ages of Chassigny and the nakhlites and indicates ejection of meteorites belonging to these two families during a discrete event, or a suite of discrete events having occurred in a restricted interval of time. In contrast, U‐Th/He and K/Ar ages <0.5 Ga are in the range of radiometric ages of shergottites, despite a Sm‐Nd signature comparable to that of Chassigny and the nakhlites (Misawa et al. 2005). Overall, the noble gas signature of NWA 2737 resembles that of shergottites rather than that of Chassigny and the nakhlites: NWA 2737 does not contain, in detectable amount, the solar‐like xenon found in Chassigny and thought to characterize the Martian mantle nor apparently fission xenon from ²⁴⁴Pu, which is abundant in Chassigny and some of the nakhlites. In contrast, NWA 2737 contains Martian atmospheric noble gases trapped in amounts comparable to those found in shergottite impact glasses. The loss of Martian mantle noble gases, together with the trapping of Martian atmospheric gases, could have occurred during assimilation of Martian surface components, or more likely during shock metamorphism, which is recorded in the petrology of this meteorite.     

Linking the Chassigny meteorite and the Martian surface rock Backstay: Insights into igneous crustal differentiation processes on Mars

Abstract— In order to use igneous surface lithologies to constrain Martian mantle characteristics, secondary processes that lead to compositional modification of primary mantle melts must be considered. Crystal fractionation of a mantle‐derived magma at the base of the crust followed by separation and ascent of residual liquids to the surface is common in continental hotspot regions on Earth. The possibility that this process also takes place on Mars was investigated by experimentally determining whether a surface rock, specifically the hawaiite Backstay analyzed by the MER Spirit could produce a known cumulate lithology with a deep origin (namely the assemblages of the Chassigny meteorite) if trapped at the base of the Martian crust. Both the major cumulus and melt inclusion mineral assemblages of the Chassigny meteorite were produced experimentally by a liquid of Backstay composition within the pressure range 9.3 to 6.8 kbar with bulk water contents between 1.5 and 2.6 wt%. Experiments at 4.3 and 2.8 kbar did not produce the requisite assemblages. This agreement suggests that just as on Earth, Martian mantle‐derived melts may rise to the surface or remain trapped at the base of the crust, fractionate, and lose their residual liquids. Efficient removal of these residual liquids at depth would yield a deep low‐silica cumulate layer for higher magmatic water content; at lower magmatic water content this cumulate layer would be basaltic with shergottitic affinity.     

SERS investigation of possible extraterrestrial life traces: Experimental adsorption of adenine on a Martian meteorite

Abstract– The identification of adenine by surface enhanced Raman scattering (SERS) on different mineral phases of a Martian meteorite Dar al Gani (DaG) 670 has been adopted as a test to verify the capability of this technique to detect trace amounts of organic or biological substances deposited over, or contained in, extraterrestrial materials. Raman spectra of different phases of meteorite (olivine, pyroxene, and ilmenite), representative of Martian basaltic rocks, have been measured by three laser sources with wavelengths at 785, 632.8, and 514.5 nm, coupled to a confocal micro‐Raman apparatus. Adenine deposited on the Martian meteorite cannot be observed in the normal Raman spectra; when, instead, meteorite is treated with silver colloidal nanoparticles, the SERS bands of adenine are strongly enhanced, allowing an easy and simple identification of this nucleobase at subpicogram level.     

Theoretical analysis of secondary cratering on Mars and an image-based study on the Cerberus Plains

The issue of crater retention age estimates on planetary surfaces is discussed with an attempt to quantify the effect of overlapping primary and secondary impact crater populations in restricted crater diameter ranges. The approach to this problem is illustrated with a simple model production function where the secondary crater input is artificially enhanced. Extrapolation of such a secondary crater model distribution to a global record results in extraordinarily high crater frequencies that do not exist on Mars, and implies the need of detailed studies of the size-frequency distribution for remote secondary craters, to date poorly known. A key case, the martian crater Zunil and its secondary crater field, illustrate that reasonable predictions for the secondary crater size-frequency distribution at small (<100 m) crater diameters affected the standard model crater retention age for the Cerberus plains less than the statistical uncertainty. These observations show that age determination based on appropriate crater counting statistics is valid in a wide primary crater diameter range.     

K‐Ar dating of rocks on Mars: Requirements from Martian meteorite analyses and isochron modeling

Abstract— Radiometric age dating of Martian rocks and surfaces at known locations for which crater densities can be determined is highly desirable in order to fully understand Martian history. Performing K‐Ar age dating of igneous rocks on Mars by robots, however, presents technical challenges. Some of these challenges can be defined by examining Ar‐Ar data acquired on Martian meteorites, and others can be evaluated through numerical modeling of simulated K‐Ar isochrons like those that would be acquired robotically on Martian rocks. Excess ⁴⁰Ar is present in all shergottites. Thus for Martian rocks, the slopes of K‐Ar isochrons must be determined to reasonable precision in order to calculate reliable ages. Model simulations of possible isochrons give an indication of some requirements in order to define a precise rock age: Issues addressed here are: how many K‐Ar analyses should be made of rocks thought to have the same age; what range of K concentrations should these analyzed samples have; and what analytical uncertainty in K‐Ar measurements is desirable. Meteorite data also are used to determine the D/a² diffusion parameters for Ar in plagioclase and pyroxene separates of several shergottites and nakhlites. These data indicate the required temperatures and times for heating similar Martian rocks in order to extract Ar. Quantitatively extracting radiogenic ⁴⁰Ar could be difficult, and degassing cosmogenic Ar from mafic phases even more so. Considering all these factors, robotic K‐Ar dating of Martian rocks may be achievable, but will be challenging.     

Experimental shock decomposition of siderite and the origin of magnetite in Martian meteorite ALH 84001

Abstract Shock recovery experiments to determine whether magnetite could be produced by the decomposition of iron‐carbonate were initiated. Naturally occurring siderite was first characterized by a variety of techniques to be sure that the starting material did not contain detectable magnetite. Samples were shocked in tungsten‐alloy holders (W = 90%, Ni = 6%, Cu = 4%) to further ensure that any iron phases in the shock products were contributed by the siderite rather than the sample holder. Each sample was shocked to a specific pressure between 30 to 49 GPa. Transformation of siderite to magnetite as characterized by TEM was found in the 49 GPa shock experiment. Compositions of most magnetites are >50% Fe⁺² in the octahedral site of the inverse spinel structure. Magnetites produced in shock experiments display the same range of sizes (?50–100 nm), compositions (100% magnetite to 80% magnetite‐20% magnesioferrite), and morphologies (equant, elongated, euhedral to subhedral) as magnetites synthesized by Golden et al. (2001) and as the magnetites in Martian meteorite Allan Hills (ALH) 84001. Fritz et al. (2005) previously concluded that ALH 84001 experienced ?32 GPa pressure and a resultant thermal pulse of ?100–110°C. However, ALH 84001 contains evidence of local temperature excursions high enough to melt feldspar, pyroxene, and a silica‐rich phase. This 49 GPa experiment demonstrates that magnetite can be produced by the shock decomposition of siderite as a result of local heating to > 470°C. Therefore, magnetite in the rims of carbonates in Martian meteorite ALH 84001 could be a product of shock devolatilization of siderite as well.     

Exsolution and shock microstructures of igneous pyroxene clasts in the Northwest Africa 7533 Martian meteorite

Northwest Africa (NWA) 7533 is a Martian regolith breccia. This meteorite (and its pairings) offers a good opportunity to study (near‐) surface processes that occurred on early Mars. Here, we have conducted a transmission electron microscope study of medium‐ and coarse‐grained (a few tens to hundreds of micrometers) Ca‐rich pyroxene clasts in order to define their thermal and shock histories. The pyroxene grains have a high‐temperature (magmatic) origin as revealed by the well‐developed pigeonite–augite exsolution microstructure. Exsolution lamella characteristics (composition, thickness, and spacing) indicate a moderately slow cooling. Some of the pyroxene clasts display evidence for local decomposition into magnetite and silica at the submicron scale. This phase decomposition may have occurred at high temperature and occurred at high oxygen fugacity at least 2–3 log units above the QFM buffer, after the formation of the exsolution lamellae. This corresponds to oxidizing conditions well above typical Martian magmatic conditions. These oxidizing conditions seem to have prevailed early and throughout most of the history of NWA 7533. The shock microstructure consists of (100) mechanical twins which have accommodated plastic deformation. Other pyroxene shock indicators are absent. Compared with SNC meteorites that all suffered significant shock metamorphism, NWA 7533 appears only mildly shocked. The twin microstructure is similar from one clast to another, suggesting that the impact which generated the (100) twins involved the compacted breccia and that the pyroxene clasts were unshocked when they were incorporated into the NWA 7533 breccia.