Beda A. Hofmann

Single mineral dating by the PbPb step-leaching method: Assessing the mechanisms

Stepwise PbPb leaching (PbSL) has been successfully used to date rock-forming silicates directly linked to metamorphic reactions defining a PT path. The two features of PbSL are an increase of precision and a control on accuracy: the former, by enhancing the 206Pb/204Pb and 207Pb/204Pb ratios, and the latter, by revealing heterochemical inclusions via the 208Pb/206Pb ratio and checking isotopic equilibrium with the host. The question of the need for inclusions as a prerequisite enabling PbSL dating was investigated on a centimenter-sized single crystal of museum-quality titanite. We obtained petrographic (optical microscope, SEM, electron microprobe, proton microprobe), chemical (ICP-MS), and isotopic (TIMS) data on pristine and increasingly leached splits of different grain sizes, as well as on leach solutions. The PbSL age of 1.00 Ga is identical to the concordant conventional UPb age. By use of isotopic and elemental correlation diagrams, we were able to resolve three isotopically distinct sources of Pb. Visible inclusions of K-feldspars as micro-crack fillings may contribute to the first 50% of common Pb (204Pb) release, and visible 5 μm zircons account, at least in part, for the residue. An additional effect noticed during the first leach steps is related to surface hydrolysis and consequent weakening of metal cation-oxygen bonds, which releases structurally bound common Pb substituting for Ca in the titanite lattice. The radiogenic Pb released during most leach steps has uniform 208Pb/206Pb ratios, suggesting that it is derived from a single mineral phase. Further, as different titanite grain sizes produce a different leach trajectory in Pb isotopic space, it can be concluded that this radiogenic Pb is actually derived from the titanite itself. Therefore, PbSL is indeed capable of discriminating common and radiogenic Pb from a single phase. During leaching, a well defined reaction front is seen to advance into the grains. Inside this front, the titanite is unaltered, and outside it, a silica-gel-like rim is formed, in which electron and proton microprobe data show retention of high field strength (HFS) elements. We propose that radiogenic Pb occurs in the tetravalent state (due to recoil stripping) so that it behaves as an HFS element and is preferentially retained in the gel rim during leaching, relative to common divalent Pb. The mechanism by which a spread in Pb isotope data is obtained during PbSL is, therefore, explained by two processes controlling the release of cations from a mineral grain into a leach solution: (1) an effective surface dependent hydrolysis of metal cations at the inward migrating reaction front whose influence decreases as leaching proceeds and (2) an increasingly dominant remobilization of HFS-elements and radiogenic Pb from the micro-environment of the leached gel-like layer which acts as a selective HFS cation absorber. In conclusion, Pb/Pb dating by stepwise leaching can be effective as a dating tool, even in minerals free from micro-inclusions.

Filamentous fabrics in low-temperature mineral assemblages: are they fossil biomarkers? Implications for the search for a subsurface fossil record on the early Earth and Mars

Abstract The subsurface has been recognized as a possible habitat for microbial life on Mars. An analogous fossil record of subsurface life is nearly missing on Earth. Here we present evidence of the widespread occurrence of such a record: tubular filamentous structures with typical core diameters of 1– 2 μm were found as inclusions in minerals deposited from low-T ( 140 localities worldwide. Filaments are frequently organized in composite structures with architectures similar to microbial mats. Filaments thickly encrusted by minerals exhibit gravity-oriented structures of macroscopic dimensions that are similar to stalactites. Environments of formation are sites of low-T water circulation in macroporous rocks such as volcanics, oxidized ores, limestone solution cavities and cavernous macrofossils. The age of occurrence ranges from Precambrian to Subrecent. We interpret the filamentous structures as permineralized and encrusted microbial filaments based on the following arguments: tubular construction of filaments with constant core diameters typical of microbes ( 1.5±1.1 μm ), coalescence of filaments to form mat-like structures, gravity draping of micron-thin filaments indicating an originally flexible consistency, and restriction to low-T mineral assemblages. These filamentous structures formed in subsurface environments are similar in size, morphology and construction to fossilized microbes observed in modern terrestrial and marine thermal springs. This newly recognized fossil record of subterranean microbial life opens up new perspectives in the study of the paleobiology of the terrestrial subsurface and in the exploration for fossil life on Mars. Potential host rocks on Mars include major nonsedimentary units such as volcanics and impactites.

An ESA study for the search for life on Mars

Similarities in the early histories of Mars and Earth suggest the possibility that life may have arisen on Mars as it did on Earth. If this were the case, early deterioration of the environment on Mars (loss of surface water, decrease in temperature) may have inhibited further evolution of life. Thus, life on Mars would probably be similar to the simplest form of life on Earth, the prokaryotes. We present a hypothetical strategy to search for life on Mars consisting of (i) identifying a suitable landing site with good exobiological potential, and (ii) searching for morphological and biogeochemical signatures of extinct and extant life on the surface, in the regolith subsurface, and within rocks. The platform to be used in this theoretical exercise is an integrated, multi-user instrument package, distributed between a lander and rover, which will observe and analyse surface and subsurface samples to obtain the following information: 1. environmental data concerning the surface geology and mineralogy, UV radiation and oxidation processes; 2. macroscopic to microscopic morphological evidence of life; 3. biogeochemistry indicative of the presence of extinct or extant life; 4. niches for extant life.

Review of occurrences and carbon isotope geochemistry of oxalate minerals: implications for the origin and fate of oxalate in diagenetic and hydrothermal fluids

The known occurrences of oxalate minerals in deep diagenetic and hydrothermal environments are reviewed and new . carbon isotope results are presented that cover, together with previously published data, 22 of 37 known diagenetic ns 22 .

Subsurface Filamentous Fabrics: An Evaluation of Origins Based on Morphological and Geochemical Criteria, with Implications for Exopaleontology

The fossil record of the subsurface biosphere is sparse. Results obtained on subsurface filamentous fabrics (SFF) from >225 paleosubsurface sites in volcanics, oxidized ores, and paleokarst of subrecent to Proterozoic age are presented. SFF are mineral encrustations on filamentous or fibrous substrates that formed in subsurface environments. SFF occur in association with low-temperature aqueous mineral assemblages and consist of tubular, micron-thick (median 1.6 micron) filaments in high spatial density, which occur as irregular masses, matted fabrics, and vertically draped features that resemble stalactites. Micron-sized filamentous centers rule out a stalactitic origin. Morphometric analysis of SFF filamentous forms demonstrates that their shape more closely resembles microbial filaments than fibrous minerals. Abiogenic filament-like forms are considered unlikely precursors of most SFF, because abiogenic forms differ in the distribution of widths and have a lower degree of curvature and a lower number of direction changes. Elemental analyses of SFF show depletion in immobile elements (e.g., Al, Th) and a systematic enrichment in As and Sb, which demonstrates a relation to environments with high flows of water. Sulfur isotopic analyses are consistent with a biological origin of a SFF sample from a Mississippi Valley-Type deposit, which is consistent with data in the literature. Fe isotopes in SFF and active analogue systems, however, allow no discrimination between biogenic and abiogenic origins. The origin of most SFF is explained as permineralized remains of microbial filaments that possibly record rapid growth during phases of high water flow that released chemical energy. It is possible that some SFF formed due to encrustation of mineral fibers. SFF share similarities with Microcodium from soil environments. SFF are a logical target in the search for past life on Mars. The macroscopic nature of many SFF allows for their relatively easy in situ recognition and targeting for more detailed microstructural and geochemical analysis.

Reduction spheroids from northern Switzerland: Mineralogy, geochemistry and genetic models

Abstract Reduction spheroids are small, isolated redox systems occurring in hematite-stained rocks of variable age, origin and provenance. Reduction spheroids from Permo-Triassic continental red-beds and from crystalline rocks of northern Switzerland have been studied mineralogically and geochemically. 48 authigenic minerals were identified in the dark, mineralized cores of the spheroids. Most common are roscoelite, uraninite and Ni-arsenides. Minerals containing Ag, Au, Pd and Pt have been found as well. The bleached spheroids are depleted in ferric iron due to hematite dissolution. Mass-balance calculations show that mobilization of only a minor fraction of the trace elements present in the bulk rock is needed to explain the formation of mineralized spheroids. The relative mobilities were: As > U > Ni > V. The elements precipitated in reduction spheroid cores as mineral phases were probably released from Fe-hydroxides during their alteration to hematite. The reduction spheroid-containing red-beds are unusually rich in As and B compared with underlying organic-rich sediments. Reduction spheroid formation probably took place during Mesozoic time at depths of burial of 400–1100 m. Pore waters were highly saline. Organic matter of unknown origin reacted with dissolved high-valence ions and hematite at discrete sites in the rocks and caused local precipitation of rare elements as mineral phases and dissolution of hematite. The formation of reduction spheroids is not due to detrital organic matter as is demonstrated by their presence in crystalline rocks and by the absence of organic relicts of the needed size. Ore mineral precipitation probably was catalyzed by bacteria because sulfides were formed by in situ, lowtemperature (

The Ordovician chondrite from Brunflo, central Sweden: III. Geochemistry of terrestrial alteration

The fossil H chondrite Brunflo, found in a slab of Ordovician limestone from central Sweden, is pervasively altered to an assemblage dominated by calcite and barite. The meteorite is surrounded by a 15-20 cm wide zone of lighter colors than the unaffected limestone due to dissolution of hematite. Here we present detailed geochemical analyses of two meteorite samples, 14 limestone samples at distances from 0 to 29 cm along two profiles from the meteorite, and a reference sample of Brunflo limestone. Element concentrations in Brunflo and surrounding bleached limestone have been strongly disturbed . during two stages of alteration early oxygenated and deep burial . In the meteorite, the Ni rCo ratio has changed from an initial value of 20 to 0.8 and redox sensitive elements like V, As, Mo, Re and U are strongly enriched. The sulfur isotope 34 . composition of barite from Brunflo d S sq 35‰ indicates initial loss of meteoritic sulfide, followed by later accumulation of sea water sulfate as barite. During deep burial under more reducing conditions, reduction processes supported by an externally derived reductant possibly derived from alum shale underlying the limestone, were largely responsible for the observed redox phenomena. In spite of massive redistribution of many elements, concentrations of Pt, Ir and Au remain at chondritic levels. The geochemistry and mineralogy of alteration determined for Brunflo are similar to .

An integrated exobiology package for the search for life on Mars

Abstract A multi-user integrated suite of instruments designed to optimize the search for evidence of life on Mars is described. The package includes: • ∗ Surface inspection and surface environment analysis to identify the potential Mars landing sites, to inspect the surface geology and mineralogy, to search for visible surficial microbial macrofossils, to study the surface radiation budget and surface oxidation processes, to search for niches for extant life. • ∗ Analysis of surface and subsurface minerals and organics to characterize the surface mineralogy, to analyse the surface and subsurface oxidants, to analyze the mineralogy of subsurface aliquots, to analyze the organics present in the subsurface aliquots (elemental and molecular composition, isotopes, chirality). • ∗ Macroscopic and microscopic inspection of subsurface aliquots to search for lifes indicators (paleontological, biological, mineralogical) and to characterize the mineralogy of the subsurface aliquots. The study is led by ESA Manned Spaceflight and Microgravity Directorate.

Preserved Filamentous Microbial Biosignatures in the Brick Flat Gossan, Iron Mountain, California

A variety of actively precipitating mineral environments preserve morphological evidence of microbial biosignatures. One such environment with preserved microbial biosignatures is the oxidized portion of a massive sulfide deposit, or gossan, such as that at Iron Mountain, California. This gossan may serve as a mineralogical analogue to some ancient martian environments due to the presence of oxidized iron and sulfate species, and minerals that only form in acidic aqueous conditions, in both environments. Evaluating the potential biogenicity of cryptic textures in such martian gossans requires an understanding of how microbial textures form biosignatures on Earth. The iron-oxide-dominated composition and morphology of terrestrial, nonbranching filamentous microbial biosignatures may be distinctive of the underlying formation and preservation processes. The Iron Mountain gossan consists primarily of ferric oxide (hematite), hydrous ferric oxide (HFO, predominantly goethite), and jarosite group minerals, categorized into in situ gossan, and remobilized iron deposits. We interpret HFO filaments, found in both gossan types, as HFO-mineralized microbial filaments based in part on (1) the presence of preserved central filament lumina in smooth HFO mineral filaments that are likely molds of microbial filaments, (2) mineral filament formation in actively precipitating iron-oxide environments, (3) high degrees of mineral filament bending consistent with a flexible microbial filament template, and (4) the presence of bare microbial filaments on gossan rocks. Individual HFO filaments are below the resolution of the Mars Curiosity and Mars 2020 rover cameras, but sinuous filaments forming macroscopic matlike textures are resolvable. If present on Mars, available cameras may resolve these features identified as similar to terrestrial HFO filaments and allow subsequent evaluation for their biogenicity by synthesizing geochemical, mineralogical, and morphological analyses. Sinuous biogenic filaments could be preserved on Mars in an iron-rich environment analogous to Iron Mountain, with the Pahrump Hills region and Hematite Ridge in Gale Crater as tentative possibilities.

The Close-Up Imager Onboard the ESA ExoMars Rover: Objectives, Description, Operations, and Science Validation Activities

Abstract The Close-Up Imager (CLUPI) onboard the ESA ExoMars Rover is a powerful high-resolution color camera specifically designed for close-up observations. Its accommodation on the movable drill allows multiple positioning. The science objectives of the instrument are geological characterization of rocks in terms of texture, structure, and color and the search for potential morphological biosignatures. We present the CLUPI science objectives, performance, and technical description, followed by a description of the instruments planned operations strategy during the mission on Mars. CLUPI will contribute to the rover mission by surveying the geological environment, acquiring close-up images of outcrops, observing the drilling area, inspecting the top portion of the drill borehole (and deposited fines), monitoring drilling operations, and imaging samples collected by the drill. A status of the current development and planned science validation activities is also given. Key Words: Mars—Biosignatures—Plane...The Close-Up Imager (CLUPI) onboard the ESA ExoMars Rover is a powerful high-resolution color camera specifically designed for close-up observations. Its accommodation on the movable drill allows multiple positioning. The science objectives of the instrument are geological characterization of rocks in terms of texture, structure, and color and the search for potential morphological biosignatures. We present the CLUPI science objectives, performance, and technical description, followed by a description of the instruments planned operations strategy during the mission on Mars. CLUPI will contribute to the rover mission by surveying the geological environment, acquiring close-up images of outcrops, observing the drilling area, inspecting the top portion of the drill borehole (and deposited fines), monitoring drilling operations, and imaging samples collected by the drill. A status of the current development and planned science validation activities is also given. Key Words: Mars-Biosignatures-Planetary Instrumentation. Astrobiology 17, 595-611.