Low-temperature Aqueous Alteration Research Articles

Soluble organic matter Molecular atlas of Ryugu reveals cold hydrothermalism on C-type asteroid parent body

The sample from the near-Earth carbonaceous asteroid (162173) Ryugu is analyzed in the context of carbonaceous meteorites soluble organic matter. The analysis of soluble molecules of samples collected by the Hayabusa2 spacecraft shines light on an extremely high molecular diversity on the C-type asteroid. Sequential solvent extracts of increasing polarity of Ryugu samples are analyzed using mass spectrometry with complementary ionization methods and structural information confirmed by nuclear magnetic resonance spectroscopy. Here we show a continuum in the molecular size and polarity, and no organomagnesium molecules are detected, reflecting a low temperature and water-rich environment on the parent body approving earlier mineralogical and chemical data. High abundance of sulfidic and nitrogen rich compounds as well as high abundance of ammonium ions confirm the water processing. Polycyclic aromatic hydrocarbons are also detected in a structural continuum of carbon saturations and oxidations, implying multiple origins of the observed organic complexity, thus involving generic processes such as earlier carbonization and serpentinization with successive low temperature aqueous alteration.

Extraterrestrial amino acids and amines identified in asteroid Ryugu samples returned by the Hayabusa2 mission

The hot water and acid extracts of two different Ryugu samples collected by the Hayabusa2 mission were analyzed for the presence of aliphatic amines and amino acids. The abundances and relative distributions of both classes of molecules were determined, as well as the enantiomeric compositions of the chiral amino acids. The Ryugu samples studied here were recovered from sample chambers A and C, which were composed of surface material, and a combination of surface and possible subsurface material, respectively. A total of thirteen amino acids were detected and quantitated in these samples, with an additional five amino acids that were tentatively identified but not quantitated. The abundances of four aliphatic amines identified in the Ryugu samples were also determined in the current work. Amino acids were observed in the acid hydrolyzed and unhydrolyzed hot water extracts of asteroid Ryugu regolith using liquid chromatography with UV fluorescence detection and high-resolution mass spectrometry. Conversely, aliphatic amines were only analyzed in the unhydrolyzed hot water Ryugu extracts. Two- to six-carbon (C2-C6) amino acids with individual abundances ranging from 0.02 to 15.8 nmol g−1, and one- to three-carbon (C1-C3) aliphatic amines with individual abundances from 0.05 to 34.14 nmol g−1, were found in the hot water extracts. Several non-protein amino acids that are rare in biology, including β-amino-n-butyric acid (β-ABA) and β-aminoisobuytric acid (β-AIB), were racemic or very nearly racemic, thus indicating their likely abiotic origins. Trace amounts of select protein amino acids that were enriched in the l-enantiomer may indicate low levels of terrestrial amino acid contamination in the samples. However, the presence of elevated abundances of free and racemic alanine, a common protein amino acid in terrestrial biology, and elevated abundances of the predominately free and racemic non-protein amino acids, β-ABA and β-AIB, indicate that many of the amino acids detected in the Ryugu water extracts were indigenous to the samples. Although the Ryugu samples have been found to be chemically similar to CI type carbonaceous chondrites, the measured concentrations and relative distributions of amino acids and aliphatic amines in Ryugu samples were notably different from those previously observed for the CI1.1 carbonaceous chondrite, Orgueil. This discrepancy could be the result of differences in the original chemical compositions of the parent bodies and/or alteration conditions, such as space weathering. In addition to α-amino acids that could have been formed by Strecker cyanohydrin synthesis during a low temperature aqueous alteration phase, β-, γ-, and δ-amino acids, including C3 – C5 straight-chain n-ω-amino acids that are not formed by Strecker synthesis, were also observed in the Ryugu extracts. The suite of amino acids measured in the Ryugu samples indicates that multiple amino acid formation mechanisms were active on the Ryugu parent body. The analytical techniques used here are well-suited to search for similar analytes in asteroid Bennu material collected by the NASA OSIRIS-REx mission scheduled for Earth return in September 2023.

Smooth rims in Queen Alexandra Range (QUE) 99177: Fluid–chondrule interactions and clues on the geochemical conditions of the primordial fluid that altered CR carbonaceous chondrites

Queen Alexandra Range (QUE) 99177 is one of the least altered CR carbonaceous chondrite known, with mineralogical and isotopic characteristics that indicate a high level of pristinity. In this study, we have examined the so-called smooth rims that surround many type I chondrules in QUE 99177, using SEM, EPMA, and FIB-TEM techniques. We have characterized their constituent phases to unravel the precursor material(s) of smooth rims, assess their formation mechanisms, and constrain the conditions of the altering fluid. Smooth rims are the most common type of rims around type I chondrules and exclusively occur around chondrules with Silica-rich Igneous Rims (SIRs). Smooth rims consist of an Fe-rich, hydrous silicate material that is Si- and Fe-rich, with minor Mg, Al, Ca, and Mn, and gives low analytical totals measured by EPMA. TEM observations reveal that the Fe-rich silicate phase is an amorphous gel that contains unaltered crystalline phases and igneous glass. Crystalline phases consist of igneous, unaltered, zoned pyroxenes with compositions consistent with pyroxenes in SIRs, as well as albite and chromite. The amorphous gel preserves previous crystal outlines with morphologies consistent with silica (cristobalite) grains in SIRs and has a composition identical to pseudomorphic silica replacements in SIRs. Based on these observations, we conclude that smooth rims derive from low-temperature aqueous alteration of silica in SIRs by an Fe-rich fluid. We suggest that the Fe was derived by leaching of amorphous silicates in the matrix, which reacted rapidly with melted water ice, although alteration of Fe,Ni metal blebs in SIRs could potentially be an additional source of Fe. Silica underwent dissolution and replacement whereas feldspar and glass remained unaltered because (1) the fluid was slightly alkaline, (2) cristobalite has a reaction rate much higher than quartz and feldspar, and (3) the alteration was very limited and fast, indicating that it was due solely to melting of accreted water ice and there was no introduction of additional fluid from external sources.

Exogenous copper sulfide in returned asteroid Itokawa regolith grains are likely relicts of prior impacting body

Samples from asteroid 25143 Itokawa returned by the Hayabusa mission have been identified as LL4-6 ordinary chondrite materials and have shown it to be a rubble pile that aggregated after break-up of a parent body. Here we investigate particle RB-CV-0038 from the Itokawa regolith using scanning and transmission electron microscopy and energy dispersive spectroscopy. We identify a cubanite-chalcopyrite-troilite-pyrrhotite assemblage, the phases and structure of which are indicative of low-temperature, aqueous alteration. Cubanite is stable only at temperatures below around 250 °C and has thus far only been identified in CI carbonaceous chondrites and the comet 81P/Wild2 sample suite. Chalcopyrite is also very rare in the meteorite record and is found mostly in R chondrites and some CK chondrites. Because the Itokawa parent body experienced significant thermal alteration with little evidence of low-temperature equilibration or aqueous alteration, we propose that the assemblage we identify is most likely exogenous and represents a component of an impacting body.

Characterizing low-temperature aqueous alteration of Mars-analog basalts from Mauna Kea at multiple scales

AbstractWe performed a multi-scale characterization of aqueous alteration of Mars analog basaltic rock from a Mauna Kea drill core using high-resolution visible and short-wave infrared (VIS-SWIR) spectral imaging, scanning electron microscopy, X-ray diffraction, and point VIS-SWIR spectra. Several types of smectites, zeolites, and primary minerals were identified. Mineral classes were mapped in cut sections extracted from the drill core and used to represent the range of alteration products seen in field data collected over 1000 m depth (Calvin et al. 2020). Ten distinct spectral end-members identified in the cut sections were used to map the field point spectra. Trioctahedral Fe- and Mg-rich smectites were present toward the top of the zone of analysis (972 m below the surface) and increased in abundance toward the bottom of the drill core (1763 m depth). The mineralogy demonstrates a general trend of discontinuous alteration that increases in intensity with depth, with less pervasive phyllosilicate alteration at the top, several zones of different mixtures of zeolites toward the center, followed by more abundant phyllosilicates in the lowest sections. Distinctly absent are Fe-Mg phyllosilicates other than smectites, as well as carbonates, sulfates, and Al phyllosilicates such as kaolinite or illite. Furthermore, hematite was only detected in two of 24 samples. The suite of assemblages points to aqueous alteration at low-to-moderate temperatures at neutral to basic pH in low-oxygen conditions, with little evidence of extensive surface interaction, presenting a possible analog for an early Mars subsurface environment. We also present a library of VIS-SWIR spectra of the analyzed cut sections, including both spatial averages (i.e., unweighted linear mixtures) of spectral images of each cut section and single point spectra of the cut sections. This will allow for consideration of nonlinear mixing effects in point spectra of these assemblages from natural surfaces in future terrestrial or planetary work.

Record of low-temperature aqueous alteration of Martian zircon during the late Amazonian

Several lines of evidence support the presence of liquid water on Mars at different times. Among those, hydrated minerals testify to past aqueous weathering processes that can be precisely studied in Martian meteorites such as NWA 7533/7034. Bringing constraints on the timing of weathering of the Martian crust would help understand its evolution, the availability of liquid water, and the habitability of Mars. Here we present a new method based on U–Th–Pb isotope systems to assess if zircon crystals underwent low-temperature aqueous alteration, such as exemplified by Hadean-aged detrital crystals from Western Australia. Data for NWA 7533 zircons show evidence for aqueous alteration and modeling of U–Th–Pb isotope system evolution indicates that the latest alteration event occurred during the late Amazonian (227–56 Ma). This finding largely expands the time duration over which liquid water was available near the Martian surface, thereby suggesting that Mars might still be habitable.

The common origin and alteration history of the hypabyssal and volcanic phases of the Wadi Tarr albitite complex, southern Sinai, Egypt

New data and interpretations are presented for the igneous albitites of the Wadi Tarr area, southern Sinai, Egypt. The albitite masses are isolated in outcrop from any granitic intrusions and have intrusive contacts against the country rocks without any structural control. They have marginal zones of breccias with jigsaw-fit angular clasts suggesting explosive, in-situ formation. The albitites are of two types: the western, medium-grained, hypabyssal albitite and the eastern, fine-grained porphyritic albitite. The field relations suggest emplacement at different levels in a magmatic cupola: the hypabyssal texture and steeply dipping slope of the upper contact of the western albitite imply deeper emplacement whereas the gently dipping contacts and porphyritic texture of the eastern albitite masses indicate that they define the probable location of the cupola apex. Both types of albitites consist of albite (92–97%) with minor amounts of quartz, K-feldspar and biotite. The accessory minerals include Fe-oxides, augite, sulphides, zircon, rutile, xenotime, titanite, allanite and monazite. The whole-rock compositions of the hypabyssal and porphyritic albitites are closely related, but the porphyritic type has lower abundances of Sr, Ba, Y, Nb, Th and Zr. We show that the hypabyssal and porphyritic albitites have a common petrogenetic origin, most likely as late-stage cumulates from a fractionating, strongly alkaline A-type magma, consistent with the compositions of the mafic minerals. The source magma was probably a tephritic liquid; we use MELTS models to show that only a sufficiently alkaline magma follows a differentiation path that both avoids quartz saturation and encounters the alkali feldspar solvus, reaching a residual liquid in equilibrium with highly sodic feldspar. Although the MELTS results show a chemically consistent means of forming igneous albitite, they are incomplete in that physical segregation mechanisms are still required to isolate the albite from mafic minerals and or a low-temperature aqueous alteration stage is needed to leach K from the feldspar. Alteration surrounding the Wadi Tarr albitites is extensive and dominated by alkali metasomatism similar to fenitization. Alteration in the marginal breccia zone of the albitite is dominated by precipitation of amphibole and carbonate in veins and in the breccia matrix, whereas the volcanic country rocks show replacement of feldspars by sericite, carbonate and epidote as well as vein carbonate. The altered volcanic country rocks show lower concentrations of Fe2O3, Sr, Cu, Pb, Ba and Ce, accompanied by higher concentrations of Na2O and MgO compared to unaltered equivalent samples.

COMPOSITIONAL HOMOGENEITY OF CM PARENT BODIES

ABSTRACT CM chondrites are the most common type of hydrated meteorites, making up ∼1.5% of all falls. Whereas most CM chondrites experienced only low-temperature (∼0°C–120°C) aqueous alteration, the existence of a small fraction of CM chondrites that suffered both hydration and heating complicates our understanding of the early thermal evolution of the CM parent body(ies). Here, we provide new constraints on the collisional and thermal history of CM-like bodies from a comparison between newly acquired spectral measurements of main-belt Ch/Cgh-type asteroids (70 objects) and existing laboratory spectral measurements of CM chondrites. It first appears that the spectral variation observed among CM-like bodies is essentially due to variations in the average regolith grain size. Second, the spectral properties of the vast majority (unheated) of CM chondrites resemble both the surfaces and the interiors of CM-like bodies, implying a “low” temperature (<300°C) thermal evolution of the CM parent body(ies). It follows that an impact origin is the likely explanation for the existence of heated CM chondrites. Finally, similarly to S-type asteroids and (2) Pallas, the surfaces of large (D > 100 km)—supposedly primordial—Ch/Cgh-type main-belt asteroids likely expose the interiors of the primordial CM parent bodies, a possible consequence of impacts by small asteroids (D < 10 km) in the early solar system.

Widespread evidence for high-temperature formation of pentlandite in chondrites

By investigating the compositional and textural evolution of sulfides within a wide range of relatively pristine, aqueously altered, and thermally metamorphosed chondrites we constrain the equilibration temperatures of sulfide minerals and compare them to the metamorphic history of their host meteorite. Sulfides in Mighei-like carbonaceous chondrites are complex as they equilibrated mostly between 100 and 135°C, but some may have equilibrated at temperatures up to 600°C. This is consistent with some CM chondrite sulfides forming at high temperature during chondrule cooling and others during low-temperature aqueous alteration and/or annealing. Karoonda-like carbonaceous chondrite sulfides equilibrated between 500 and 230°C, which is consistent with formation during cooling and annealing after thermal metamorphism. Sulfides in the LL chondrites equilibrated between 600 and 230°C, and are consistent with formation during chondrule cooling for Semarkona (LL3.00) and during cooling after thermal metamorphism for the equilibrated samples (types 4–6). Sulfides in the Rumuruti-like (R) chondrites equilibrated between 600 and 500°C, and are consistent with formation after thermal metamorphism. The sulfides within the brachinite equilibrated between 600 and 400°C, consistent with formation during cooling after thermal metamorphism.Contrary to the assertion that pentlandite is solely the product of low-temperature aqueous alteration in many chondrite groups, this study suggests that most sulfides in chondrites are formed at or upon cooling from high-temperature. The evaluation of a single mineral system within samples that retain petrographic context is vital to the interpretation of formation and alteration processes recorded by small extraterrestrial samples, such as those that have been returned by the spacecraft missions Stardust and Hayabusa and will be returned by OSIRIS-REx and Hayabusa2.

The weathering of micrometeorites from the Transantarctic Mountains

Micrometeorites are cosmic dust particles recovered from the Earth’s surface that dominate the influx of extraterrestrial material accreting to our planet. This paper provides the first in-depth study of the weathering of micrometeorites within the Antarctic environment that will allow primary and secondary features to be distinguished. It is based on the analysis of 366 particles from Larkman Nunatak and 25 from the Transantarctic Mountain collection. Several important morphological categories of weathering effects were identified: (1) irregular and faceted cavities, (2) surface etch pits, (3) infilled cavities, (4) replaced silicate phases, and (5) hydrated and replaced metal. These features indicate that congruent dissolution of silicate phases, in particular olivine, is important in generating new pore space within particles. Comparison of the preservation of glass and olivine also indicates preferential dissolution of olivine by acidic solutions during low temperature aqueous alteration. Precipitation of new hydrous phases within cavities, in particular ferrihydrite and jarosite, results in pseudomorph textures within heavily altered particles. Glass, in contrast, is altered to palagonite gels and shows a sequential replacement indicative of varying water to rock ratios. Metal is variably replaced by Fe-oxyhydroxides and results in decreases in Ni/Fe ratio. In contrast, sulphides within metal are largely preserved. Magnetite, an essential component of micrometeorites formed during atmospheric entry, is least altered by interaction with the terrestrial environment. The extent of weathering in the studied micrometeorites is sensitive to differences in their primary mineralogy and varies significantly with particle type. Despite these differences, we propose a weathering scale for micrometeorites based on both their degree of terrestrial alteration and the level of encrustation by secondary phases. The compositions and textures of weathering products, however, suggest open system behaviour and variable water to rock ratios that imply climatic variation over the lifetime of the micrometeorite deposits.

Silicon isotope systematics of acidic weathering of fresh basalts, Kilauea Volcano, Hawai’i

Silicon stable isotopes are fractionated by a host of low-temperature aqueous processes, making them potentially useful as a weathering proxy. Here we characterize the silicon isotope signature of surficial chemical weathering of glassy basaltic lava flows at Kilauea Volcano, Hawaii. Fresh basalt flow surfaces (<40years old) frequently feature opaque amorphous silica surface coatings up to 80μm thick. These silica coatings and associated silica cements are enriched in the heavier isotopes of Si (δ30SiNBS-28=+0.92‰ to +1.36‰) relative to their basaltic substrate (δ30SiNBS-28=−0.3‰ to −0.2‰). Secondary clays and opals are typically depleted in 30Si relative to the dissolved reservoirs from which they precipitated, so this sense of isotopic fractionation is unusual. Mechanisms capable of producing isotopically heavy secondary minerals were explored by conducting batch alteration experiments on fresh basaltic glass. Batch acidic alteration of basalt glass in HCl, H2SO4, and HF produced silica-rich surface layers resembling the Hawaiian surface coatings. Differences in fluid chemical composition affected the direction and magnitude of Si isotope fractionation. Basalt leaching in HCl or H2SO4 produced 30Si-enriched fluids (1000 lnαprecip-Si(aq)≅−0.8‰) and 30Si-depleted secondary silica layers. In contrast, HF-bearing experiments produced highly 30Si-depleted fluid compositions (1000 lnαprecip-Si(aq) up to +8‰). Larger isotopic fractionations were observed in experiments with lower fluid–rock ratios. In Hawaii, where altering fluids contain H2SO4 and HCl but minimal HF, high δ30Si values for the silica coatings were likely achieved by Rayleigh fractionation. Aqueous 30Si-enriched silica was released during incongruent basalt dissolution then subsequently transported and deposited from an evaporating solution at the flow surface. Our results indicate that (1) altering fluid chemistry and fluid–rock ratio impact the Si isotope signature of chemical weathering and (2) δ30Si of solids produced by low temperature aqueous alteration may diverge sharply from watershed- or landscape-scale weathering trends.

Low temperature aqueous alteration of basalt: Mineral assemblages of Deccan basalts and implications for Mars

Al‐rich phyllosilicates (kaolinite, montmorillonite) have been found in layers overlying Fe/Mg‐smectites on Mars, and it has been suggested that this stratigraphy formed through in situ leaching at the surface, similar to terrestrial weathering profiles. We are investigating the remotely sensed signatures of this type of weathering using ten samples from a vertical section of altered Deccan basalts and four samples collected nearby as an analog for leaching resulting in Al‐rich phyllosilicate over Fe/Mg‐smectite stratigraphies. Samples were analyzed with reflectance spectroscopy from 0.28 to 25.0μm, inductively coupled plasma atomic emission spectrometry for 10 major element concentrations (Al, Ca, Fe, K, Mg, Mn, Na, P, Si, Ti), loss on ignition for volatiles, x‐ray diffraction (XRD) for mineralogies, and Mössbauer spectroscopy for Fe redox state. Spectra of basalt samples were dominated by Fe2+ crystal field transitions with weak alteration bands near 1.4 and/or 1.9 μm. Reststrahlen bands in mid‐infrared showed the convolution of plagioclase and pyroxene features typical of basalts. Saprolite samples were incompletely leached, and their spectra were dominated by complex Al‐ and Fe/Mg‐bearing smectite clays and retained no original mafic signatures. XRD and Mössbauer detected pyroxene and plagioclase not visible by reflectance spectroscopy in some saprolite samples. Zeolites were present throughout the saprolite. The laterite was the most leached horizon, and all analyses showed kaolinite and iron oxide assemblages. This kaolinite and hematite association would be expected if kaolinite on Mars formed through leaching under conditions similar to those on Earth and has implications for abundant freshwater on the Martian surface.

Microdenticles on naturally weathered hornblende

Abstract Microdenticles (with lengths in the micron-submicron range rather than tens of microns) are developed on the lateral surfaces of larger “classic” denticles on naturally weathered hornblende from weathered amphibolite of the Carroll Knob Complex in western North Carolina. Microdenticles share the shape and orientation of the larger more typical denticles, producing arrays of microdenticles that give the larger host denticle the appearance of a surface covered with imbricate pointed or rounded scales. The arrays of imbricate microdenticles are formed by low-temperature aqueous alteration during weathering of the Carroll Knob Complex hornblende; they are later-stage corrosion forms on already-corroded surfaces of hornblende that show larger-scale evidence of typical weathering. In the Carroll Knob occurrence, hornblende microdenticles are associated with dilute weathering solutions, suggesting possible control by extreme undersaturation of solutions with respect to hornblende.

Evidence for aqueous activity on comet 81P/Wild 2 from sulfide mineral assemblages in Stardust samples and CI chondrites

The discovery of nickel-, copper-, and zinc-bearing iron sulfides from comet 81P/Wild 2 (Wild 2) represents the strongest evidence, in the Stardust collection, of grains that formed in an aqueous environment. We investigated three microtomed TEM sections which contain crystalline sulfide assemblages from Wild 2 and twelve thin sections of the hydrothermally altered CI chondrite Orgueil. Detailed structural and compositional characterizations of the sulfide grains from both collections reveal striking similarities. The Stardust samples include a cubanite (CuFe 2S 3) grain, a pyrrhotite [(Fe,Ni) 1− x S]/pentlandite [(Fe,Ni) 9S 8] assemblage, and a pyrrhotite/sphalerite [(Fe,Zn)S] assemblage. Similarly, the CI-chondrite sulfides include individual cubanite and pyrrhotite grains, cubanite/pyrrhotite assemblages, pyrrhotite/pentlandite assemblages, as well as possible sphalerite inclusions within pyrrhotite grains. The cubanite is the low temperature orthorhombic form, which constrains temperature to a maximum of 210 °C. The Stardust and Orgueil pyrrhotites are the 4C monoclinic polytype, which is not stable above ∼250 °C. The combinations of cubanite and pyrrhotite, as well as pyrrhotite and pentlandite signify even lower temperatures. The crystal structures, compositions, and petrographic relationships of these sulfides constrain formation and alteration conditions. Taken together, these constraints attest to low-temperature hydrothermal processing. Our analyses of these minerals provide constraints on large scale issues such as: heat sources in the comet-forming region; aqueous activity on cometary bodies; and the extent and mechanisms of radial mixing of material in the early nebula. The sulfides in the Wild 2 collection are most likely the products of low-temperature aqueous alteration. They provide evidence of radial mixing of material (e.g. cubanite, troilite) from the inner solar system to the comet-forming region and possible secondary aqueous processing on the cometary body.

Speciation of sulfur in the insoluble organic matter from carbonaceous chondrites by XANES spectroscopy

Sulfur speciation in a comprehensive set of insoluble organic matter (IOM) samples extracted from 3 CI (Orgueil, Alais, Ivuna), 5 CM (QUE97990, Murchison, Murray, QUE99355, Cold Bokkeveld), 1 CR (Renazzo) and ungrouped C2 Tagish Lake chondrites has been studied by K-edge XANES micro-spectroscopy. Five main sulfur groups were identified: (1) sulfides, (2) aliphatic sulfur, (3) heterocyclic organic sulfur, (4) oxidized organic sulfur and (5) sulfates. The IOM of the 3 CIs and the extensively altered Cold Bokkeveld CM exhibit a higher abundance of heterocyclic versus aliphatic organic sulfur compared to the other CMs and compared to the Renazzo and Tagish Lake chondrites. This suggests greater thermal heating on the parent body of CIs, consistent with the higher temperatures experienced by these chondrites (~ 70 °C for the most altered clasts of Cold Bokkeveld; 100–150 °C for CIs). Alais may have experienced more heating than Ivuna and Orgueil. The IOM of CIs contains oxidized organic sulfur, suggesting the presence of a mild-temperature oxidation process on the parent body. Among type 2 chondrites other than Cold Bokkeveld, no significant variation of the sulfur speciation was detected. This suggests tenuous oxidation processes and a low-temperature aqueous alteration (< ~ 35 °C) on the parent body. The global chemical variations between the CR and CM groups (e.g. H/C elemental ratio, alkyl groups concentration) reported in earlier studies appear to be more the result of chemical variations among the accreted precursors than of post-accretional processes.

Denticles on Chain Silicate Grain Surfaces and Their Utility as Indicators of Weathering Conditions on Earth and Mars

Abstract Denticulated margins (also known in older literature as “sawtooth,” “cockscomb,” or “hacksaw” terminations) are a common feature of pyroxenes and amphiboles, visible in optical and electron microscopes. Denticles are remnants of undissolved material that formerly constituted the walls between elongate etch pits, the characteristic aqueous-dissolution form of chain-silicate minerals. Denticles occur widely in terrestrial near-surface materials that have experienced low-temperature aqueous alteration, including chemically weathered regoliths, soils in a variety of climatic and geomorphic settings, sediments, and sedimentary rocks. Ranges of dissolution forms and dimensions (commonly tens of microns in length) are identical on both pyroxenes and amphiboles in these materials. Denticles on terrestrial chain silicates are formed by low-temperature aqueous alteration. Denticles on a soil mineral grain (of unknown composition) at the Phoenix Mars Lander site and on terrestrial pyroxenes naturally weathered in terrestrial low-temperature aqueous environments are broadly similar in size, shape, and orientation relationships between denticles. However, quantitative morphometric parameters vary among different terrestrially weathered pyroxenes and even between different images of compositionally identical pyroxenes from the same naturally weathered sample; such variations are too large to allow simple identification of minerals of unknown composition from morphology alone. Nevertheless, the overall morphological similarities are consistent with a low-temperature aqueous corrosion origin of denticles at the Phoenix Mars Lander site.

Insights into the formation of Fe- and Mg-rich aqueous solutions on early Mars provided by the ALH 84001 carbonates

The chemical and isotopic pattern of the zoned carbonate globules in the ALH 84001 meteorite reveals a unique aqueous environment on early Mars. If the evolution of the fluid composition was dictated primarily by carbonate precipitation, the zoning pattern of the carbonates can constrain the fluid to have had an Mg/Ca mole ratio > ~5.3 and a Fe/Ca mole ratio > ~1 prior to the formation of the carbonates. Chemical equilibrium modeling of water–rock interactions indicates that low temperatures and low pH favor the formation of an aqueous solution with elevated Mg and Fe concentrations. The modeling shows that a sufficiently Fe- and Mg-rich fluid could have formed through low-temperature (< 100 °C) subsurface aqueous alteration of an ALH 84001-type rock at pH 5–7. This range of pH corresponds to an elevated CO 2 fugacity (~ 0.1–1 bar). Formation of ALH 84001 carbonates could have been driven by degassing of CO 2 and corresponding pH increase in near-surface environments during an upwelling of subsurface CO 2-rich solutions. This scenario is consistent with the unaltered nature of the ALH 84001 rock and with chemical and isotopic composition of its carbonates.

Effects of experimental aqueous alteration on the abundances of argon‐rich noble gases in the Ningqiang carbonaceous chondrite

Abstract— Ar‐rich noble gases, the so‐called “subsolar” noble gases, are a major component of heavy primordial noble gases in unequilibrated ordinary chondrites and some classes of anhydrous carbonaceous chondrites, whereas they are almost absent in hydrous carbonaceous chondrites that suffered extensive aqueous alteration. To understand the effects of aqueous alteration on the abundance of Ar‐rich noble gases, we performed an aqueous alteration experiments on the Ningqiang type 3 carbonaceous chondrite that consists entirely of anhydrous minerals and contains Ar‐rich noble gases. Powdered samples and deionized neutral water were kept at 200 °C for 10 and 20 days, respectively. Mineralogical analyses show that, during the 10‐day alteration, serpentine and hematite formed at the expense of olivine, low‐Ca pyroxene, and sulfide. Noble gas analyses show that the 10‐day alteration of natural Ningqiang removed 79% of the primordial 36Ar, 68% of the 84Kr, and 60% of the 132Xe, but only 45% of the 4He and 53% of the primordial 20Ne. Calculated elemental ratios of the noble gases removed during the 10‐day alteration are in the range of those of Ar‐rich noble gases. These results indicate that Ar‐rich noble gases are located in materials that are very susceptible to aqueous alteration. In contrast, heavy primordial noble gases remaining in the altered samples are close to Q gas in elemental and isotope compositions. This indicates that phase Q is much more resistant to aqueous alteration than the host phases of Ar‐rich noble gases. In the 20‐day sample, the mineralogical and noble gas signatures are basically similar to those of the 10‐day sample, indicating that the loss of Ar‐rich noble gases was completed within the 10‐day alteration. Our results suggest that almost all of the Ar‐rich noble gases were lost from primitive asteroids during early, low‐temperature aqueous alteration.

Post-hydration thermal metamorphism of carbonaceous chondrites

The presence of abundant phyllosilicates in many carbonaceous chondrites indicates a prevailing activity of low-temperature aqueous alteration in primitive asteroids. However, among the hydrous carbonaceous chondrites known, more than 20 samples show evidence of having been heated at elevated temperatures with corresponding phyllosilicate dehydration. The mineralogical features of dehydration suggest that the heating occurred in situ in meteorites, which demonstrates that there are some hydrated asteroids that have been heated at a certain period after aqueous alteration. Recent studies have uncovered details of heating and dehydration processes in hydrous carbonaceous chondrites: step-by-step changes in mineralogy, trace element chemistry, carbonaceous materials, and reflectance spectra have been clarified. Based on data from synchrotron X-ray diffraction analysis of the matrix, heated hydrous carbonaceous chondrites have been classified as Stages I-IV, with the temperature of heating increasing from I to IV. In spite of recent progress, heat sources are poorly defined, mainly due to a lack of chronological information on the timing of the heating, and therefore more data are needed to fully clarify the thermal metamorphism of hydrous carbonaceous chondrites.

Low-temperature aqueous alteration of crystalline pyrochlore: correspondence between nature and experiment

AbstractPyrochlore has been considered as a waste form to immobilize high-level nuclear waste such as excess weapons-grade plutonium. In order to study the aqueous stability of pyrochlore, we have carried out hydrothermal experiments with a natural microlite (Nb + Ta &gt; 2Ti; Ta ⩾ Nb) at 175°C in a neutral and acidic solution for 14 days. The starting material and the experimental products were studied by electron microprobe, backscattered electron (BSE) imaging, powder infrared (IR) and micro-Raman spectroscopy, and powder X-ray diffractrometry (XRD). The microlite has small U (∼200 ppm) and Th (∼800 ppm) contents and is crystalline. The hydrothermal treatment in a solution containing l mol/l HCl and l mol/l CaCl2 causes the partial replacement of the microlite (up to ∼10 μm) by a new pyrochlore phase. This new phase is characterized by a larger unit-cell volume and contains a large number of vacancies at the A site (A = Ca, Na) as well as anion vacancies, molecular water, and possibly OH groups. Analyses of the experimental fluid further revealed that U was also selectively lost to the solution. Treatment in pure water did not produce reaction zones detectable by BSE imaging or powder XRD. However, significant spectral changes in powder IR spectra of the reaction product and the detection of Na and Ca in the solution indicate that the microlite has also reacted in pure water. The experimental alteration features bear a remarkable resemblance to those seen in naturally altered microlite samples, suggesting that short-term experimental results can be applied to natural systems and vice versa in order to evaluate the long-term stability of a pyrochlore waste form.