Aqueous Alteration Processes Research Articles

Mn–Cr relative sensitivity factor in ferromagnesian olivines defined for SIMS measurements with a Cameca ims-1280 ion microprobe: Implications for dating secondary fayalite

The short-lived radionuclide 53Mn, which decays to 53Cr with a half-life of ∼3.7Myr, is useful for sequencing objects that formed within the first 20Myr of Solar System evolution. 53Mn–53Cr relative chronology enables aqueously formed secondary minerals such as fayalite and various carbonates in ordinary and carbonaceous chondrites to be dated, thereby providing chronological constraints on aqueous alteration processes. In situ measurements of Mn–Cr isotope systematics in fayalite by secondary ion mass spectrometry (SIMS) require consideration of the relative sensitivities of the 55Mn+ and 52Cr+ ions, for which a relative sensitivity factor [RSF=(55Mn+/52Cr+)SIMS/(55Mn/52Cr)true] is defined using appropriate standards. In the past, San Carlos olivine (Fa∼10) was commonly used for this purpose, but a growing body of evidence suggests that it is an unsuitable standard for meteoritic fayalite (Fa>90). Natural fayalite also cannot be used as a standard because it contains only trace amounts of chromium, which makes determining a true 55Mn/52Cr ratio and its degree of heterogeneity very difficult.To investigate the dependence of the Mn–Cr RSF on ferromagnesian olivine compositions, we synthesized a suite of compositionally homogeneous Mn,Cr-bearing liquidus-phase ferromagnesian olivines (Fa31–99). Manganese–chromium isotopic measurements of San Carlos olivine and synthesized ferromagnesian olivines using the University of Hawai‘i Cameca ims-1280 SIMS show that the RSF for Fa10 is ∼0.9; it increases rapidly between Fa10 and Fa31 and reaches a plateau value of ∼1.5±0.1 for Fa>34. The RSF is time-dependent: it increases during the measurements of olivines with fayalite content <30 and decreases during the measurements of olivines with fayalite content >50. The RSF measured on ferroan olivine (Fa>90) is influenced by pit shape, whereas the RSF measured on magnesian olivine (Fa10) is less sensitive to changes in pit shape. For these reasons, 53Mn–53Cr systematics of chondritic fayalite (Fa>90) should be determined using standards of similar composition that are measured under the same analytical conditions as the “unknown”.The 53Mn–53Cr ages of secondary fayalites (Fa90–100) in the Elephant Moraine (EET) 90161 (L3.05), Vicencia (LL3.2), Asuka 881317 (CV3) and MacAlpine Hills (MAC) 88107 (C3) chondrites (2.4-1.3+1.8, 4.0-1.1+1.4, 4.2-0.7+0.8 and 5.1-0.4+0.5Myrs after CV CAIs, respectively) are ∼3Myr older when using an RSF measured on a matrix-matched (Fa99) standard, rather than on a San Carlos olivine. The inferred 53Mn–53Cr ages of fayalite formation are consistent with the ages reported for calcites in CM chondrites measured with similarly matrix-matched standards, suggesting an early onset of aqueous alteration on the ordinary and carbonaceous chondrite parent bodies heated by decay of 26Al.

Deuterium- and &lt;sup&gt;15&lt;/sup&gt;N-signatures of organic globules in Murchison and Northwest Africa 801 meteorites

Carbonaceous matter from the matrix of the Murchison (CM2) meteorite and Northwest Africa (NWA) 801 (CR2) meteorite that were extremely rich in deuterium (D) and 15N was studied using in situ isotope imaging. The association of carbon, hydrogen, and nitrogen suggests that the carbonaceous matter was organic in nature, and the maximum magnitudes of D- and 15N-enrichment in the organic matter were δD = 2,880‰ and δ15N = 2,590‰ in Murchison, and δD = 7,500‰ and δ15N = 2,200‰ in NWA 801. The organic matter did not display a simple correlation between the magnitudes of D- and 15N-enrichment, and the isotopically anomalous organic matter was classified into three types based on the H and N isotopic characteristics: extremely 15N-rich without large D-enrichment (15N-rich), extremely D-rich without large 15N-enrichment (D-rich), and highly D- and 15N-rich (D-15N-rich). The occurrence of isotopically anomalous organic matter was attributed to their origin in the molecular clouds and outer solar nebula, ion-molecule reactions at low temperatures, grain surface reactions at low temperatures, and self-shielding effects in gas phase molecules. However, the observed D- and 15N-enrichment of the organic matter is much smaller than predicted by ion-molecule and grain surface reactions. This suggests a secondary modification of the H and N isotopic compositions of the organic matter in the solar nebula and in the parent body. In Murchison, the proportion of D-rich organic matter among the isotopically anomalous organic matter is smaller than in NWA 801, which presumably indicates that the D-enrichments are easily modified by aqueous alteration processes on the parent body. Scanning electron microscopy revealed the morphology of the D- and 15N-rich organic matter as aggregates of globules/particles or non-aggregated globules, where the size of each globule was <1 μm. Although there is no obvious correlation between the morphology of the organic matter and the H and N isotopic compositions, the result that Murchison contains more abundant globule/particle aggregates might suggests formation of aggregated nature due to aqueous activity.

Formation and transformations of Fe-rich serpentines by asteroidal aqueous alteration processes: A nanoscale study of the Murray chondrite

Fe-rich serpentines are an abundant product of the early aqueous alteration events that affected the parent bodies of CM carbonaceous chondrites. Alteration assemblages in these meteorites show a large chemical variability and although water–rock interactions occurred under anoxic conditions, serpentines contain high amounts of ferric iron. To date very few studies have documented Fe valence variations in alteration assemblages of carbonaceous chondrites, limiting the understanding of the oxidation mechanisms. Here, we report results from a nanoscale study of a calcium–aluminum-rich inclusion (CAI) from the Murray chondrite, in which alteration resulted in Fe import and Ca export by the fluid phase and in massive Fe-rich serpentines formation. We combined scanning and transmission electron microscopies and scanning transmission X-ray microscopy for characterizing the crystal chemistry of Fe-serpentines. We used reference minerals with known crystallographic orientations to quantify the Fe valence state in Fe-rich serpentines using X-ray absorption spectroscopy at the Fe L2,3-edges, yielding a robust methodology that would prove valuable for studying oxidation processes in other terrestrial or extra-terrestrial cases of serpentinization. We suggest that aqueous Fe2+ was transported to the initially Fe-depleted CAI, where local changes in pH conditions, and possibly mineral catalysis by spinel promoted the partial oxidation of Fe2+ into Fe3+ by water and the formation of Fe-rich serpentines close to the cronstedtite endmember. Such mechanisms produce H2, which opens interesting perspectives as hydrogen may have reacted with carbon species, or escaped and yield increasingly oxidizing conditions in the parent asteroid. From the results of this nanoscale study, we also propose transformations of the initial cronstedtite, destabilized by later input of Al- and Mg-rich solutions, leading to Fe2+ leaching from serpentines, as well as to random serpentine-chlorite interstratifications. Such transformations towards polysomatic assemblages that are un-equilibrated from the structural, chemical and redox point of views are probably controlled by the various rates of alteration of primary minerals, but also by porosity gradients, as in terrestrial hydrothermal systems. We suggest that the proposed mechanisms may have played a role in the early formation of (Fe2+,Fe3+)-rich serpentines documented in CM chondrites, as well as in their transformation with on-going alteration towards Fe-poorer compositions inferred from previous petrologic, mineralogical and magnetic studies of CM chondrites.

Characteristics and origin of the components of the carbonaceous chondrite NWA 470

For the first time, data are presented from a detailed investigation of all components of the NWA 470 CH-type carbonaceous chondrite. The meteorite’s major characteristics are a high metal content with a solar Co/Ni ratio, a majority of chemically-zoned metal grains, cryptocrystalline chondrules, grossite- and hibonite-bearing refractory inclusions, the presence of SiO2 objects and the lack of a fine-grained matrix. The bulk chemical and isotope composition of oxygen in the meteorite is typical for CH chondrites. However, among all CH chondrites, NWA 470 is the most enriched in iron. Notable among the chondrite’s components are refractory inclusions and chondrules of several different generations, which may have formed under either canonical nebular or dust-rich conditions. In general, the origin of refractory inclusions, SiO2 objects and zoned metal can be explained through fractional condensation with partial isolation of the condensates. Dark inclusions were probably included in the material of CH chondrites from a different parent body, similar in composition to CM chondrites. After mixing of these multiple types of material during accretion, the components of NWA 470 were not further affected by aqueous alteration or thermal metamorphic processes, and the components’ primary characteristics have been preserved.

Aqueous alteration on main belt primitive asteroids: Results from visible spectroscopy

This work focuses on the study of the aqueous alteration process which acted in the main belt and produced hydrated minerals on the altered asteroids. Hydrated minerals have been found mainly on Mars surface, on main belt primitive asteroids and possibly also on few TNOs. These materials have been produced by hydration of pristine anhydrous silicates during the aqueous alteration process, that, to be active, needed the presence of liquid water under low temperature conditions (below 320K) to chemically alter the minerals. The aqueous alteration is particularly important for unraveling the processes occurring during the earliest times of the Solar System history, as it can give information both on the asteroids thermal evolution and on the localization of water sources in the asteroid belt.To investigate this process, we present reflected light spectral observations in the visible region (0.4–0.94μm) of 80 asteroids belonging to the primitive classes C (prevalently), G, F, B and P, following the Tholen (Tholen, D.J. [1984]. Ph.D. Dissertation, University of Arizona, Tucson). classification scheme. We find that about 65% of the C-type and all the G-type asteroids investigated reveal features suggesting the presence of hydrous materials, mainly a band centered around 0.7μm, while we do not find evidence of hydrated materials in the other low albedo asteroids (B, F, and P) investigated.We combine the present observations with the visible spectra of asteroids available in the literature for a total of 600 primitive main belt asteroids. We analyze all these spectra in a similar way to characterize the absorption band parameters (band center, depth and width) and spectral slope, and to look for possible correlations between the aqueous alteration process and the asteroids taxonomic classes, orbital elements, heliocentric distances, albedo and sizes. Our analysis shows that the aqueous alteration sequence starts from the P-type objects, practically unaltered, and increases through the P→F→B→C→G asteroids, these last being widely aqueous altered, strengthening thus the results previously obtained by Vilas (Vilas, F. [1994]. Icarus 111, 456–467). Around 50% of the observed C-type asteroids show absorption feature in the visible range due to hydrated silicates, implying that more than ∼70% of them will have a 3μm absorption band and thus hydrated minerals on their surfaces, based on correlations between those two absorptions (Howell, E.S. et al. [2011]. EPSC-DPS Joint Meeting 2011, vol. 6. Abstracts, 637).We find that the aqueous alteration process dominates in primitive asteroids located between 2.3 and 3.1AU, that is at smaller heliocentric distances than previously suggested by Vilas et al. (Vilas, F., Hatch, E.C., Larson, S.M., Sawyer, S.R., Gaffey, M.J. [1993]. Icarus 102, 225–231). The percentage of hydrated asteroids is strongly correlated with their size. The aqueous alteration process is less effective for bodies smaller than 50km, while it dominates in the 50–240km sized primitive asteroids.No correlation is found between the aqueous alteration process and the asteroids albedo or orbital elements. Comparing the ∼0.7μm band parameters of hydrated silicates and CM2 carbonaceous chondrites, the meteorites that have aqueous altered asteroids as parent bodies, we see that the band center of meteorites is at longer wavelengths than that of asteroids. This difference on center positions may be attributed to different minerals abundances, and to the fact that CM2 available on Earth might not be representative of the whole aqueous altered asteroids population.

Deriving chemical trends from thermal infrared spectra of weathered basalt: Implications for remotely determining chemical trends on Mars

Variations in chemical composition over a planetary surface can be used to study petrologic and aqueous alteration processes. The desire for such data on Mars has prompted investigators to derive chemistry from models of Thermal Emission Spectrometer data. Although chemistry derived from thermal infrared spectral models is reportedly reliable for unaltered igneous rocks, the martian surface has experienced chemical weathering, which can adversely affect models. Here, we examine weathered basalts from Baynton, Australia, for which chemical weathering trends have been previously characterized, to test how well chemistry and chemical trends can be determined from TIR spectra of weathered rocks. The mineralogy of variably weathered rocks was derived from TIR spectra by linear mixing, and major-element chemistry was calculated from those mineral models. Derived chemistries and trends were compared to those measured by X-ray fluorescence. TIR spectroscopy is sensitive to weathering products in weathering rinds because the products are present in a coating geometry, making it a useful technique for remotely detecting weathered surfaces on planetary surfaces such as Mars. This sensitivity results in significant modeled abundances of weathering products (>80% of all phases) from TIR spectra of weathered Baynton surfaces, despite evidence from microscopy and X-ray diffraction showing that igneous minerals dominate the weathering rind. Measured chemical weathering trends show loss of MgO, CaO, Na2O, and K2O and relative enrichment in Al2O3 and FeOT. The modeled trends are similar to the measured trends, but a closer look at the modeled oxide abundances demonstrates that most oxides (i.e., alkalis, SiO2, and FeOT) are not well modeled, especially for weathered surfaces. The reasons for this are: (1) non-linear mixing and the presence of secondary coatings causes the overestimation of secondary phases in spectral models, and (2) spectral libraries generally lack poorly crystalline and amorphous secondary phases that are common in weathering rinds so that crystalline phases such as phyllosilicates are selected. The martian surface has likely been weathered less pervasively than the Baynton rocks and, therefore, weathering products may be dominated by poorly crystalline and amorphous phases, rather than crystalline phyllosilicates. Adding these phases to spectral libraries could improve bulk chemistry derived from the martian surface; however, if the secondary phases are present in a coating geometry, the derived chemistry will reflect the composition of the coating, and it may be difficult to infer the chemistry of the parent rock.

Mid-infrared study of the molecular structure variability of insoluble organic matter from primitive chondrites

Insoluble Organic Matter (IOM) found in primitive meteorites was formed in the Early Solar System and subsequently processed on the parent asteroids. The location, temporal sequence and processes of formation of this IOM are still a matter of debate. In particular, there is no consensus on the actual effect of post-accretional aqueous alteration processes on the chemical composition and structure of IOM. In the most primitive chondrites (types 1 and 2), these alterations have so far been either neglected or generically assigned to oxidation processes induced by fluid circulation.A series of IOM samples extracted from 14 chondrites with extensively documented post-accretional histories have been studied by infrared spectroscopy. Aqueous alteration shows no detectable effect on the chemical composition and structure of IOM within or across chondrite classes. Indeed, the most effective post-accretional process appears to be a high-temperature short-duration heating event and concerns essentially type 2 chondrites. In any case, post-accretional processes cannot account for all the chemical and structural variations of IOM. Chondrites from the CI, CR and CM classes accreted IOM precursors with moderately variable compositions, suggesting a chemical heterogeneity of the protosolar disk. The 3.4μm band, and possibly its overtones and combinations in the near-infrared range, appear to be tracer(s) of the chemical class and possibly of surface heating processes triggered by impacts.

Hydrogen isotopic composition of the water in CR chondrites

The thermal and aqueous alteration experienced by QUE 99177, MET 00426, EET 92042, GRA 95229, Renazzo and Al Rais CR chondrites was assessed through multi-technique characterization of their the carbonaceous matter and hydrated mineral phases. Each of the chondrites escaped long duration thermal metamorphism as reflected by the low structural order of the polyaromatic carbonaceous matter assessed by Raman spectroscopy. The infrared spectra of the matrix grains clearly reveal the presence of hydrated minerals in each of the CR chondrites. In particular, the extent of aqueous alteration experienced by QUE 99177 and MET 00426 may have been previously underestimated. The H isotopic compositions of the altering fluids were measured in situ in fine-grained phyllosilicates and individual coarse-grained hydrated silicates. In the analyzed CR chondrites, the main observations are that (i) the water is systematically enriched in D, and the enrichments (δDwater up to 1600‰) can be even higher than the highest D-enrichments reported for cometary water; (ii) the isotopic composition of the water is highly variable at the micrometer scale; (iii) there is no clear trend observed in the isotopic composition of the water (maximum D-enrichment, range of variation) along the aqueous alteration sequence. The D-enrichments and spatial variability are easier to explain as secondary signatures acquired through parent body processes, rather than as being due to the accretion of at least two ices with distinct isotopic compositions and sources that did not fully mix when they melted at the start of the aqueous alteration process.

Rotational spectra of (162173) 1999 JU3, the target of the Hayabusa2 mission

Context. The Hayabusa2 mission, which will be launched by JAXA in 2014, will return samples from the C-type near-Earth asteroid (162173) 1999 JU3. To better plan the mission, it is important to obtain as many physical characteristics of the asteroid as possible from ground-based observations. Moreover, these can then be calibrated with the in-situ and laboratory studies and be used to better understand other similar objects. Regarding the surface composition of the target asteroid, previous spectroscopic studies in the visible provided conflicting results for the possible presence of a deep absorption band, which is usually related to aqueous alteration processes.Aims. Our goal is to better understand the surface composition of asteroid (162173) 1999 JU3, and how it relates to the spectral differences observed by diverse authors at different epochs and telescopes. Moreover, to support the JAXA mission planning, we aim to constrain the level of aqueous alteration and thermal activity undergone by the object. Methods. The adopted methodology was to observe different regions of the surface of the asteroid that rotates around its axis. Spectroscopic observations that cover about 70% of its surface were therefore obtained at the SOAR telescope in Chile on 2012 July 9−10. Results. Our results indicate that the surface of asteroid (162173) 1999 JU3 presents featureless spectra with very little variation.Conclusions. No sign of an absorption feature that could be related to aqueous alteration processes is detectable in the observed spectra.

Gas–solid carbonation as a possible source of carbonates in cold planetary environments

Carbonates are abundant sedimentary minerals at the surface and sub-surface of the Earth and they have been proposed as tracers of liquid water in extraterrestrial environments. Their formation mechanism is since generally associated with aqueous alteration processes. Recently, carbonate minerals have been discovered on Mars' surface by different orbitals or rover missions. In particular, the phoenix mission has measured from 1% to 5% of calcium carbonate (calcite type) within the soil (Smith et al., 2009). These occurrences have been reported in area where the relative humidity is significantly high (Boynton et al., 2009). The small concentration of carbonates suggests an alternative process on mineral grain surfaces (as suggested by Shaheen et al., 2010) than carbonation in aqueous conditions. Such an observation could rather point toward a possible formation mechanism by dust–gas reaction under current Martian conditions. To understand the mechanism of carbonate formation under conditions relevant to current Martian atmosphere and surface, we designed an experimental setup consisting of an infrared microscope coupled to a cryogenic reaction cell (IR-CryoCell setup). Three different mineral precursors of carbonates (Ca and Mg hydroxides, and a hydrated Ca silicate formed from Ca2SiO4), low temperature (from −10 to +30°C), and reduced CO2 pressure (from 100 to 2000mbar) were utilized to investigate the mechanism of gas–solid carbonation at mineral surfaces. These mineral materials are crucial precursors to form Ca and Mg carbonates in humid environments (0%<relative humidity<100%) at dust–CO2 or dust–water ice–CO2 interfaces. Our results reveal a significant and fast carbonation process for Ca hydroxide and hydrated Ca silicate. Conversely, only a moderate carbonation is observed for the Mg hydroxide. These results suggest that gas–solid carbonation process or carbonate formation at the dust–water ice–CO2 interfaces could be a currently active Mars' surface process. To the best of our knowledge, we report for the first time that calcium carbonate can be formed at a negative temperature (−10°C) via gas–solid carbonation of Ca hydroxide. We note that the carbonation process at low temperature (<0°C) described in the present study could also have important implications on the dust–water ice–CO2 interactions in cold terrestrial environments (e.g. Antarctic).

Reproducing early Martian atmospheric carbon dioxide partial pressure by modeling the formation of Mg‐Fe‐Ca carbonate identified in the Comanche rock outcrops on Mars

The well defined composition of the Comanche rock's carbonate (Magnesite0.62Siderite0.25Calcite0.11Rhodochrosite0.02) and its host rock's composition, dominated by Mg‐rich olivine, enable us to reproduce the atmospheric CO2partial pressure that may have triggered the formation of these carbonates. Hydrogeochemical one‐dimensional transport modeling reveals that similar aqueous rock alteration conditions (including CO2partial pressure) may have led to the formation of Mg‐Fe‐Ca carbonate identified in the Comanche rock outcrops (Gusev Crater) and also in the ultramafic rocks exposed in the Nili Fossae region. Hydrogeochemical conditions enabling the formation of Mg‐rich solid solution carbonate result from equilibrium species distributions involving (1) ultramafic rocks (ca. 32 wt% olivine; Fo0.72Fa0.28), (2) pure water, and (3) CO2partial pressures of ca. 0.5 to 2.0 bar at water‐to‐rock ratios of ca. 500 molH2O mol−1rock and ca. 5°C (278 K). Our modeled carbonate composition (Magnesite0.64Siderite0.28Calcite0.08) matches the measured composition of carbonates preserved in the Comanche rocks. Considerably different carbonate compositions are achieved at (1) higher temperature (85°C), (2) water‐to‐rock ratios considerably higher and lower than 500 mol mol−1 and (3) CO2partial pressures differing from 1.0 bar in the model set up. The Comanche rocks, hosting the carbonate, may have been subjected to long‐lasting (&gt;104 to 105 years) aqueous alteration processes triggered by atmospheric CO2partial pressures of ca. 1.0 bar at low temperature. Their outcrop may represent a fragment of the upper layers of an altered olivine‐rich rock column, which is characterized by newly formed Mg‐Fe‐Ca solid solution carbonate, and phyllosilicate‐rich alteration assemblages within deeper (unexposed) units.

Ancient Impact and Aqueous Processes at Endeavour Crater, Mars

The rover Opportunity has investigated the rim of Endeavour Crater, a large ancient impact crater on Mars. Basaltic breccias produced by the impact form the rim deposits, with stratigraphy similar to that observed at similar-sized craters on Earth. Highly localized zinc enrichments in some breccia materials suggest hydrothermal alteration of rim deposits. Gypsum-rich veins cut sedimentary rocks adjacent to the crater rim. The gypsum was precipitated from low-temperature aqueous fluids flowing upward from the ancient materials of the rim, leading temporarily to potentially habitable conditions and providing some of the waters involved in formation of the ubiquitous sulfate-rich sandstones of the Meridiani region.

Visualisation and quantification of CV chondrite petrography using micro-tomography

Micro-computed tomography is a non-destructive technique that allows the study of 3D meteorite petrography. The technique produces a unique and instructive visualisation of the meteorite for quantifying its components. We studied the overall petrography of the two CV chondrites Allende and Mokoia to constrain their formation histories. A set of movies and stereographic images detail the 3D petrography. Component modal abundances agree with previous reports and modal abundance differences between Allende and Mokoia support the chondrule–matrix complementarity and that chondrules and matrix formed from the same chemical reservoir. We identified two types of chondrules, a normal type and one where a normal type I or II chondrule is almost completely encapsulated by an opaque-rich layer. This layer was probably acquired during a late stage condensation process. The appearance of opaques in chondrules and matrix is different, not supporting a genetic relationships between these. Low abundances of compound chondrules (1.75vol% in Allende and 2.50vol% in Mokoia) indicate low chondrule densities and/or low relative component velocities in chondrule formation regions. Porosities on a scale <10–20μm allowed for only local aqueous alteration processes on the meteorite parent bodies.

Mars’ atmospheric 40Ar: A tracer for past crustal erosion

Noble gas 40Ar may be used as a tracer of the past evolution of volatiles in Mars’ crust, mantle and atmosphere. 40Ar is formed by the radioactive decay of 40K in the mantle and in the crust and is released from the mantle to the atmosphere due to volcanism and from the crust by erosion such as eolian and hydrothermal erosion. Furthermore, 40Ar can escape from the atmosphere into space via atmospheric escape mechanisms. The evolution of the atmospheric abundance of 40Ar thus depends on these three processes whose efficiencies vary with time.In the present study we reconsider atmospheric escape mechanism efficiencies and describe various possible scenarios of the evolution of 40Ar with a model describing the three main reservoirs of 40Ar, the mantle, crust and atmosphere. First, we show that atmospheric escape, which is stronger in the early evolution, does not significantly influence the present abundance of the atmospheric 40Ar. In the early evolution the atmospheric concentration of 40Ar is very low as the outgassing of 40Ar from the mantle occurs relatively late in the martian evolution. Thus, the atmospheric 40Ar concentration is essentially a tracer of Mars’ outgassing history and not of the escape processes. Second, using the results of the most recent published crustal formation models, the calculated present 40Ar atmospheric abundance is smaller than its observed value. This discrepancy may be explained by a significant 40Ar supply from the crust by erosion (16–30% of the 40Ar content of the upper first 10km of crust). The knowledge of the fraction of crustal 40Ar outgassed to the atmosphere is an important constraint for any future global modelling of past Mars’ hydrothermal activity aiming at better characterizing the role of subsurface aqueous alteration processes in Mars climate evolution. One of the main sources of the uncertainty of these results is the present uncertainty in the measured atmospheric 40Ar value (±20%). More precise measurements of 40Ar and 36Ar in the martian atmosphere are therefore required to better constrain the model.

Reproducing hydrogeochemical conditions triggering the formation of carbonate and phyllosilicate alteration mineral assemblages on Mars (Nili Fossae region)

[1] The cooccurrence of Mg-Fe-Ca carbonate and stratigraphically lower, phyllosilicate-bearing rock units, identified in the Nili Fossae region on Mars, may reveal a CO2-driven aqueous alteration process on ancient Mars. We reproduce hydrogeochemical conditions for concurrent carbonate and phyllosilicate formation in rock columns by using hydrogeochemical one-dimensional transport modeling, which is based on chemical equilibrium thermodynamics. Our models assume the isochemical, low-temperature equilibration of ultramafic rocks with pure water at varying atmospheric CO2 partial pressure, water-to-rock ratio, temperature, and rock composition. Equilibration among (1) mineral assemblages of olivine-rich rocks (18–28 wt% MgO), (2) pure water (water-to-rock ratio: 500–50 mol mol−1), (3) CO2 transferred from the atmosphere (CO2 partial pressure lower than ∼2 bars and higher than ∼0.01 bar) at ∼278 K, and (4) 100,000 years of diffusive mass transport create hydrogeochemical conditions enabling the concurrent formation of Mg-rich Mg-Fe-Ca solid-solution carbonate and stratigraphically lower phyllosilicate-rich alteration assemblages in rock columns. The modeled composition of alteration mineral assemblages, their depth distribution, and carbonate composition are similar to the observations from the Nili Fossae region. Higher equilibration temperature (373 K) results in the formation of Ca-dominated Ca-Mg carbonate instead of Mg-rich Mg-Fe-Ca carbonate and in an inverse stratification with phyllosilicate-rich units overlying carbonate-rich units. Modeling results suggest that rocks in the Nili Fossae region, hosting the carbonate and phyllosilicate mineral assemblage, have been exposed to a groundwater alteration process driven by atmospheric CO2 partial pressures ranging between ∼2 bars and ∼0.01 bar at low temperature (∼278 K).

The soluble organic compounds of the Bells meteorite: Not a unique or unusual composition

The Bells meteorite is a CM2 chondrite that has long been considered anomalous for having mineralogical and isotopic differences with CMs together with an overall affinity to CIs in its matrix. We extracted a fragment of the only Bells stone collected unweathered with water and solvents and studied the meteorite’s soluble organic composition. We found Bells to contain abundant organic compounds, which are predominantly O-containing such as hydroxy- and di-carboxylic acids, and a scarcity of amino acids and other N-containing compounds. Amines were not detected and ammonia is less abundant than in both the Murchison and Ivuna meteorites. Overall, Bells’ soluble organic composition is more similar to that of Ivuna than of Murchison. The observation that Bells’ amino acid suite shares a distinct distribution of characteristic molecular species with other stones that are thought to have experienced extensive parent body aqueous alteration, such as the Orgueil, Ivuna and recently analyzed GRO 95577 CR1 meteorites, seems to allow the suggestion that such a composition is secondary to prolonged aqueous alteration processes that superseded some of the initial compositional distinctions determined by the asteroidal environments.

A hydrated clast in the Mokoia CV3 carbonaceous chondrite: Evidence for intensive aqueous alteration in the CV parent body

We report the discovery of a highly hydrated clast in the Mokoia CV3 carbonaceous chondrite that contains eight chondrules and one amoeboid olivine inclusion (AOI), embedded in a matrix dominated by hydrous phyllosilicates. Anhydrous silicates in the chondrules and AOI were extensively replaced by phyllosilicates. The matrix has a composition intermediate between saponite and serpentine and contains abundant framboidal magnetite, being similar to the matrices of the CI chondrites. The mineralogy and texture indicate that the clast resulted from intensive aqueous alteration of a precursor equivalent to the host Mokoia meteorite, and it can be regarded as the CV2-type lithology. The results indicate that there was in fact an extremely wet region within the Mokoia parent body and that this clast escaped subsequent thermal metamorphism. This clast reveals abundant evidence of progressive and dynamic aqueous alteration processes. The chondrules were replaced inward from their margins, and the alteration products were disaggregated and mixed into the matrix; as a result, the chondrules increasingly became smaller in size and irregular in shape. Some of the chondrules were separated into pieces. The results indicate that a major proportion of the matrix materials were produced by the alteration of chondrules and inclusions and that there was a stage in which the alteration products and the original matrix material were extensively and uniformly mixed together. The presence of the clast supports a previously proposed model, which suggests that hydrated chondrules, inclusions, and the rims surrounding them in the host Mokoia chondrite are clasts produced by brecciation within a wet region of the parent body and that they were subsequently transported and incorporated into the dry matrix.

The behavior of the Hf isotope system in radiation-damaged zircon during experimental hydrothermal alteration

The application of the Hf isotope composition of zircon as a geochemical tracer requires the preservation of Lu-Hf systematics within individual grains. We performed hydrothermal experiments on a self-irradiation-damaged zircon to test whether hydrothermal alteration would affect its Hf isotopic composition. Severely radiation-damaged zircon from Sri Lanka was reacted in either Teflon reactors or gold capsules at 200 °C (1080 h, autogeneous pressure), 400 °C (120 h, 1 kbar), and 600 °C (72 h, 1 kbar) in a 1 M HCl-0.2 M HF solution that was spiked with 300 ppm non-natural Hf (98.2% 180Hf) and 970 ppm Yb. Laser ablation inductively coupled plasma mass spectrometry measurements of the Hf- and U-Pb isotope compositions of the altered domains revealed that the U-Pb system of such domains was severely disturbed, resulting in a discordia pointing toward the origin of the concordia diagram, but that the Hf isotope composition was unaffected. In addition, Yb enrichment was observed in the reacted zircon domains, predominantly near the zircon-solution interface. The Yb has apparently diffused into the altered domains. The new data are fully consistent with a diffusion-controlled aqueous alteration process occurring within radiation-damaged zircon.

Exploration of Victoria Crater by the Mars Rover Opportunity

The Mars rover Opportunity has explored Victoria crater, an approximately 750-meter eroded impact crater formed in sulfate-rich sedimentary rocks. Impact-related stratigraphy is preserved in the crater walls, and meteoritic debris is present near the crater rim. The size of hematite-rich concretions decreases up-section, documenting variation in the intensity of groundwater processes. Layering in the crater walls preserves evidence of ancient wind-blown dunes. Compositional variations with depth mimic those approximately 6 kilometers to the north and demonstrate that water-induced alteration at Meridiani Planum was regional in scope.

Composition, Morphology, and Stratigraphy of Noachian Crust around the Isidis basin

Definitive exposures of pristine, ancient crust on Mars are rare, and the finding that much of the ancient Noachian terrain on Mars exhibits evidence of phyllosilicate alteration adds further complexity. We have analyzed high‐resolution data from the Mars Reconnaissance Orbiter in the well‐exposed Noachian crust surrounding the Isidis basin. We focus on data from the Compact Reconnaissance Imaging Spectrometer for Mars as well as imaging data sets from High Resolution Imagine Science Experiment and Context Imager. These data show the lowermost unit of Noachian crust in this region is a complex, brecciated unit of diverse compositions. Breccia blocks consisting of unaltered mafic rocks together with rocks showing signatures of Fe/Mg‐phyllosilicates are commonly observed. In regions of good exposure, layered or banded phyllosilicate‐bearing breccia rocks are observed suggestive of pre‐Isidis sedimentary deposits. In places, the phyllosilicate‐bearing material appears as a matrix surrounding mafic blocks, and the mafic rocks show evidence of complex folded relationships possibly formed in the turbulent flow during emplacement of basin‐scale ejecta. These materials likely include both pre‐Isidis basement rocks as well as the brecciated products of the Isidis basin–forming event at 3.9 Ga. A banded olivine unit capped by a mafic unit covers a large topographic and geographic range from northwest of Nili Fossae to the southern edge of the Isidis basin. This olivine‐mafic cap combination superimposes the phyllosilicate‐bearing basement rocks and distinctly conforms to the underlying basement topography. This may be due to draping of the topography by a fluid or tectonic deformation of a previously flatter lying morphology. We interpret the draping, superposed olivine‐mafic cap combination to be impact melt from the Isidis basin–forming event. While some distinct post‐Isidis alteration is evident (carbonate, kaolinite, and serpentine), the persistence of olivine from the time of Isidis basin suggests that large‐scale aqueous alteration processes had ceased by the time this unit was emplaced.