The impact of postdepositional alteration on iron- and molybdenum-based redox proxies

Ratios of (un)reactive iron species, authigenic molybdenum contents (Moauth), and molybdenum isotope compositions (δ98Moauth) in sedimentary rocks are geochemical proxies that are widely used to reconstruct past marine redox states, which have been calibrated in modern marine settings covering oxic to euxinic conditions. However, syn- and postdepositional processes can result in alterations and ambiguities of proxy-derived redox signals that can challenge the validity of paleoreconstructions. We present new data from modern organic-rich sediments of two oxygen minimum zone settings in the Gulf of California and the Peruvian margin. The results show that Mo is fully immobilized shortly after deposition by reaction with hydrogen sulfide (H2S) produced during organoclastic sulfate reduction. Thus, any H2S produced deeper in the sediment (e.g., by sulfate reduction coupled to anaerobic methane oxidation) leaves the initially deposited Mo concentrations and δ98Mo signatures unaltered, which supports the robustness of Mo-based redox proxies. In contrast, the Fe speciation data reveal continued pyritization due to constant exposure of Fe minerals to H2S. Importantly, both Fe bound to oxides and carbonates (highly reactive Fe) and also poorly reactive Fe (e.g., sheet silicates) undergo pyritization during early diagenesis. This process generates Fe-based proxy signatures that falsely imply ferruginous or euxinic conditions.

Clay mineral formation in Permian rocks of a geothermal borehole at Northern Upper Rhine Graben, Germany

Hydrothermally altered rhyolite rocks in the Permian Donnersberg Formation of a geothermal borehole in the Northern Upper Rhine Graben (Germany) were investigated to find out answers for the low hydraulic conductivity of the rocks. The composition of clay minerals and the temperature of smectite–illite transformation were carried out using X-ray diffraction, X-ray fluorescence, transmission electron microscopy, Fourier transform infrared spectroscopy, and polarized-light microscopy analyses. Clay mineral (CM) composition includes illite/muscovite (1M and 2M1 polytypes), illite–smectite interstratifications (IS-ml), smectite, and chlorite; and non-clay minerals such as quartz, feldspars, epidote, calcite, dolomite, and hematite were detected. The 2M1-polytype mica might be the only primary sheet silicates from the parent rocks, while the others occur as authigenic neo-formed CMs under heat flow and geothermal gradient. The development of CMs indicates different mechanisms of illitization and smectitization. Based on the texture, morphology, structure/polytype, and chemistry of rocks and minerals, in particular CMs, the study grouped the CM formation into three transformation processes: smectitization during magma cooling and possible contact metamorphisms with decreasing and low temperature, smectite illitization controlled by burial diagenesis and hydrothermal alteration, and illite smectitization followed exhumation and Cenozoic subsidence with decreasing temperature. The rhyolites were altered to all of the orders IS-R0, IS-R1, and IS-R3 by the dissolution-precipitation and layer-to-layer mechanisms. The first one supported small xenomorphic plates and flakes of 1Md, elongated particles of 1M, and pseudo-hexagonal forms of 2M1. The second one could lead to the platy particles of 1Md and 2M1 polytypes. The dominant temperature range for the transformation in the area has been 140–170 °C– ~ 230 °C.

What steers deformation within fold-and-thrust belts: insights from the carbonate multilayer footwall of the Belluno Thrust, Italian eastern Southern Alps

<p>Despite significant recent progress in the understanding and quantification of the parameters controlling deformation modes in carbonate multilayers within fold-and-thrust belts, the details of early deformation and faulting during the initial stages of large-scale thrusting remain poorly documented and understood. Aiming to narrow this knowledge gap, we have chosen to study the relatively low-strain carbonate multilayer footwall of the Belluno Thrust (BT), one of the most external and S-vergent thrusts of the eastern Southern Alps (Italy). The BT footwall is composed of a c. 600 m thick Meso-Cenozoic multilayer succession of shallow water carbonate and pelagic sedimentary units characterized by strong mineralogical heterogeneity, with calcite (32-98%), sheet silicates (1-27%), and quartz (1-37%) as principal components. Its structural framework reflects cumulative strain due to multiple deformation events and is defined by the superposition of different structures such as i) south-verging asymmetric folds, ii) faulted folds, cut by slip planes with centimetric to metric throw, iii) SC-C’ fabrics in the marly layers, and iv) cataclastic domains.  Structures recording the early shortening increments are generally well preserved mesoscopic upright folds. Asymmetric folds with gently N-dipping backlimbs and steeply S-dipping (or even overturned N-dipping) forelimbs, record further shortening of the early upright and symmetrical folds. Strain is strongly partitioned within the marly layers, with discrete faults commonly defined by multiple slip surfaces forming duplex geometries and SC-C’ fabrics and exploiting millimetric to centimetric marly beds as detachment layers. Thrusts and diffuse reverse faults not associated with any cataclasite localise along the backlimbs of the asymmetric folds, suggesting dominant layer-parallel shortening. Cataclasites develop instead along the thrust surfaces that cut across the steeply dipping (locally even overturned) forelimbs, where cataclastic flow becomes the dominant deformation mechanism. On the vertical forelimbs, cataclasis and strain localisation are commonly associated with veins, which contributed to harden the rock system.  </p><p>Based on our systematic observations, we propose that deformation progressively evolved from folding and layer-parallel shortening (initial phases) to faulting and cataclasis (final phases) as a function of the dynamic interplay of the following factors: i) the geometrical relationships between fault orientation, fold attitude (forelimb and backlimb domains) and stress field, ii) the lithotype, which we conveniently account for by referring to the ratio between the cumulative thickness of the outcrop marly layers and the total measured stratigraphic thickness, iii) the involvement of fluids during deformation, iv) the mineral assemblage of the involved layers and v) the geometric framework of the domain localising strain with respect to the principal stress axes orientation. We conclude that these parameters play a major role in guiding strain localisation and partitioning during continuous shortening within fold-and-thrust belts. They also govern the transition from overall aseismic creep to coseismic rupturing at the scale of mesoscopic structures and, possibly, of the entire belt.</p>

Biosignatures in Subsurface Filamentous Fabrics (SFF) from the Deccan Volcanic Province, India

The morphology, chemical, and mineralogical composition of subsurface filamentous fabrics (SFF) from the Deccan Volcanic Province (DVP) were investigated to determine the origin of these spectacular aggregates. SFF occur in a wide variety of morphologies ranging from pseudo-stalactites to irregular fabrics and are classified as SFFIr (irregular) or SFFMa (matted). The SFF samples exhibit a thread-like (or filament-like) center from which mineral precipitation starts to form the final macroscopic morphologies. Detailed investigations revealed organic material (fungal chitin) in the innermost filamentous core, which may have acted as an initial nucleus for the mineralization processes. The morphometric characteristics of certain filamentous fabrics are very similar to those of microbial filaments and the fabrics formed from them but are clearly distinct from similar types of non-biological precipitates (fibrous minerals, speleothems, and “chemical gardens”). These features indicate that the filamentous cores might be products of microbial communities that were active in the basaltic cavities. The SFF cross-sections display similar concentric layers of the mineral succession and reach thicknesses of several centimeters with spectacular lengths up to 100 cm and constant diameters. The typical mineralization sequence points to temporal variation in the chemical composition of the mineralizing fluids from Fe(Mg)-rich (Fe-oxides/-hydroxides, Fe-rich sheet silicates such as celadonite and di-/tri-smectite) to Ca-dominated (Ca-rich zeolites) and finally pure SiO2 (opal-CT, chalcedony, and macro-crystalline quartz). Assuming biological activity at least during the early mineralization processes, circumneutral pH conditions and maximum temperatures of 100–120 °C were supposed. The formation of filamentous cores including Fe-bearing phyllosilicates probably occurred near the surface after cooling of the lava, where the elements necessary for mineral formation (i.e., Si, Mg, Al, Fe) were released during alteration of the volcanic host rocks by percolating fluids.

Asbestiform anthophyllite, tremolite, and related fibrous amphibole chemistry, both as primary and secondary mineralization in metamorphic facies in asbestos occurrences in North American deposit comparisons

<p>As we continue to investigate the asbestos-forming minerals and their associated geology as they occur in North America, we have found that subtle variations can make the standardization of what is and what is not asbestos more difficult. On the other hand, some geochemical trends recently observed have given us significant insight into what we can expect in the ground, which we hope will lend much-needed information to medical investigators to better understand the relationship of mineral morphologic and chemical differences and the ramifications to human health for those potentially exposed. In efforts to understand why certain minerals form in the asbestiform habit, mineralogists still cannot fully explain the cause-and-effect of this phenomenon. Although we know that there are chemical variances and pressure or temperature regimes that are conducive to the formation of asbestos, a complete and absolute picture of how and why amphibole forms fibers, or serpentine forms chrysotile scrolls remains elusive. Research indicates however that there are two primary ways that sheet silicates compensate for the fundamental misfit between their tetrahedral silica layers (T) and their octahedrally-coordinated cation layers (O) that is by either tetrahedral rotation /stretching or by bending or modulation of the layers in concert. Rotation or stretching occurs in both the 1:1 layer silicates (T-O) such as serpentines, and the 2:1 phyllosilicates (T-O-T) such as vermiculite or talc. The other primary means of misfit compensation is structural bending, with the obvious examples of antigorite or chrysotile. Although it was originally hypothesized as early as the 1950s that this curving or bending of the sheet structure was entirely due to the T-O misfit, more recent research points to the importance and variances of hydroxyl bonding in the chrysotile structure. A secondary mode of compensation for the fundamental misfit is by the addition or subtraction of silica tetrahedra or octahedral cations in modulated fashion, which affects the overall chemistry of the mineral as a whole. In polysomatic hydrous biopyriboles we see the importance of hydration alteration reactions in the transformation of chain zippers. Thusly, a wide variety of intergrowth microstructures appear in Mg-rich 1:1 modulated layer silicates, analogous to the hydrous biopyriboles as is common intimate intermixing in a polysomatic series. It is therefore common that the means by which all of our regulated asbestos minerals form is through the combined action of T-O misfit compensation and the action of water in the crystallizing or re-crystallizing process. </p>

A structure hierarchy for silicate minerals: chain, ribbon, and tube silicates

A structure hierarchy is developed for chain-, ribbon- and tube-silicate based on the connectedness of one-dimensional polymerisations of (TO4)n− tetrahedra, where T = Si4+ plus P5+, V5+, As5+, Al3+, Fe3+, B3+, Be2+, Zn2+ and Mg2+. Such polymerisations are described by a geometrical repeat unit (with ng tetrahedra) and a topological repeat unit (or graph) (with nt vertices). The connectivity of the tetrahedra (vertices) in the geometrical (topological) repeat units is denoted by the expression cTr (cVr) where c is the connectivity (degree) of the tetrahedron (vertex) and r is the number of tetrahedra (vertices) of connectivity (degree) c in the repeat unit. Thus cTr = 1Tr12Tr23Tr34Tr4 (cVr = 1Vr12Vr23Vr34Vr4) represents all possible connectivities (degrees) of tetrahedra (vertices) in the geometrical (topological) repeat units of such one-dimensional polymerisations. We may generate all possible cTr (cVr) expressions for chains (graphs) with tetrahedron (vertex) connectivities (degrees) c = 1 to 4 where r = 1 to n by sequentially increasing the values of c and r, and by ranking them accordingly. The silicate (sensu lato) units of chain-, ribbon- and tube-silicate minerals are identified and associated with the relevant cTr (cVr) symbols. Following description and association with the relevant cTr (cVr) symbols of the silicate units in all chain-, ribbon- and tube-silicate minerals, the minerals are arranged into decreasing O:T ratio from 3.0 to 2.5, an arrangement that reflects their increasing structural connectivity. Considering only the silicate component, the compositional range of the chain-, ribbon- and tube-silicate minerals strongly overlaps that of the sheet-silicate minerals. Of the chain-, ribbon- and tube-silicates and sheet silicates with the same O:T ratio, some have the same cVr symbols (vertex connectivities) but the tetrahedra link to each other in different ways and are topologically different. The abundance of chain-, ribbon- and tube-silicate minerals decreases as O:T decreases from 3.0 to 2.5 whereas the abundance of sheet-silicate minerals increases from O:T = 3.0 to 2.5 and decreases again to O:T = 2.0. Some of the chain-, ribbon- and tube-silicate minerals have more than one distinct silicate unit: (1) vinogradovite, revdite, lintisite (punkaruaivite) and charoite have mixed chains, ribbons and/or tubes; (2) veblenite, yuksporite, miserite and okenite have clusters or sheets in addition to chains, ribbons and tubes. It is apparent that some chain-ribbon-tube topologies are favoured over others as of the ~450 inosilicate minerals, ~375 correspond to only four topologically unique graphs, the other ~75 minerals correspond to ~46 topologically unique graphs.

A structure hierarchy for silicate minerals: sheet silicates

The structure hierarchy hypothesis states that structures may be ordered hierarchically according to the polymerisation of coordination polyhedra of higher bond-valence. A hierarchical structural classification is developed for sheet-silicate minerals based on the connectedness of the two-dimensional polymerisations of (TO4) tetrahedra, where T = Si4+ plus As5+, Al3+, Fe3+, B3+, Be2+, Zn2+ and Mg2+. Two-dimensional nets and oikodoméic operations are used to generate the silicate (sensu lato) structural units of single-layer, double-layer and higher-layer sheet-silicate minerals, and the interstitial complexes (cation identity, coordination number and ligancy, and the types and amounts of interstitial (H2O) groups) are recorded. Key aspects of the silicate structural unit include: (1) the type of plane net on which the sheet (or parent sheet) is based; (2) the u (up) and d (down) directions of the constituent tetrahedra relative to the plane of the sheet; (3) the planar or folded nature of the sheet; (4) the layer multiplicity of the sheet (single, double or higher); and (5) the details of the oikodoméic operations for multiple-layer sheets. Simple 3-connected plane nets (such as 63, 4.82 and 4.6.12) have the stoichiometry (T2O5)n (Si:O = 1:2.5) and are the basis of most of the common rock-forming sheet-silicate minerals as well as many less-common species. Oikodoméic operations, e.g. insertion of 2- or 4-connected vertices into 3-connected plane nets, formation of double-layer sheet-structures by (topological) reflection or rotation operations, affect the connectedness of the resulting sheets and lead to both positive and negative deviations from Si:O = 1:2.5 stoichiometry. Following description of the structural units in all sheet-silicate minerals, the minerals are arranged into decreasing Si:O ratio from 3.0 to 2.0, an arrangement that reflects their increasing structural connectivity. Considering the silicate component of minerals, the range of composition of the sheet silicates completely overlaps the compositional ranges of framework silicates and most of the chain-ribbon-tube silicates.

Generating functions for stoichiometry and structure of single- and double-layer sheet-silicates

Two-dimensional nets may be used to generate the stoichiometry and structure of single-layer and double-layer sheet-silicate minerals. Many sheet-silicate minerals are based on the 3-connected plane nets 63, 4.82, (4.6.8)2(6.82)1and (52.8)1(5.82)1, and some more complicated nets, e.g. (5.6.7)4(5.72)1(62.7)1, (4.122)2(42.12)1, (52.8)1(5.62)1(5.6.8)2(62.8)1,have one or two representative structures. Many complicated sheet-silicate minerals are based on sheets of 2-, 3- and 4-connected tetrahedra that may be developed from 3- and 4-connected plane nets by a series of oikodoméic operations on 3- or 4-connected nets that change the topologyof the parent net. There are three classes of oikodoméic operations: (1) insertion of 2- and 3-connected vertices into 3- and 4-connected plane nets; (2,3) replication of single-layer sheets by topological mirror or two-fold-rotation operators, and condensation of the resulting twosingle-layer sheets to form double-layer sheets. The topological aspects of these sheet structures may be described by functions that express stoichiometry in terms of tetrahedron connectivities (formula-generating functions) and functions that associate these formula-generating functionswith specific two-dimensional nets. Using these functions, we may generate formulae and structural arrangements of single-layer and double-layer silicate structures with specific local and long-range topological features.