Mineral Modes Research Articles

Petrology and geochemistry of feldspathic impact‐melt breccia Abar al' Uj 012, the first lunar meteorite from Saudi Arabia

AbstractAbar al' Uj (AaU) 012 is a clast‐rich, vesicular impact‐melt (IM) breccia, composed of lithic and mineral clasts set in a very fine‐grained and well‐crystallized matrix. It is a typical feldspathic lunar meteorite, most likely originating from the lunar farside. Bulk composition (31.0 wt% Al2O3, 3.85 wt% FeO) is close to the mean of feldspathic lunar meteorites and Apollo FAN‐suite rocks. The low concentration of incompatible trace elements (0.39 ppm Th, 0.13 ppm U) reflects the absence of a significant KREEP component. Plagioclase is highly anorthitic with a mean of An96.9Ab3.0Or0.1. Bulk rock Mg# is 63 and molar FeO/MnO is 76. The terrestrial age of the meteorite is 33.4 ± 5.2 kyr. AaU 012 contains a ~1.4 × 1.5 mm2 exotic clast different from the lithic clast population which is dominated by clasts of anorthosite breccias. Bulk composition and presence of relatively large vesicles indicate that the clast was most probably formed by an impact into a precursor having nonmare igneous origin most likely related to the rare alkali‐suite rocks. The IM clast is mainly composed of clinopyroxenes, contains a significant amount of cristobalite (9.0 vol%), and has a microcrystalline mesostasis. Although the clast shows similarities in texture and modal mineral abundances with some Apollo pigeonite basalts, it has lower FeO and higher SiO2 than any mare basalt. It also has higher FeO and lower Al2O3 than rocks from the FAN‐ or Mg‐suite. Its lower Mg# (59) compared to Mg‐suite rocks also excludes a relationship with these types of lunar material.

Chemical and Sr isotopic characterization of North America uranium ores: Nuclear forensic applications

This study reports major, minor, and trace element data and Sr isotope ratios for 11 uranium ore (uraninite, UO2+x) samples and one processed uranium ore concentrate (UOC) from various U.S. deposits. The uraninite investigated represent ores formed via different modes of mineralization (e.g., high- and low-temperature) and within various geological contexts, which include magmatic pegmatites, metamorphic rocks, sandstone-hosted, and roll front deposits. In situ trace element data obtained by laser ablation-ICP-MS and bulk sample Sr isotopic ratios for uraninite samples investigated here indicate distinct signatures that are highly dependent on the mode of mineralization and host rock geology. Relative to their high-temperature counterparts, low-temperature uranium ores record high U/Th ratios (>1000), low total rare earth element (REE) abundances (<1 wt%), high contents (>300 ppm) of first row transition metals (Sc, Ti, V, Cr, Mn, Co, Ni), and radiogenic 87Sr/86Sr ratios (>0.7200). Comparison of chondrite normalized REE patterns between uraninite and corresponding processed UOC from the same locality indicates identical patterns at different absolute concentrations. This result ultimately confirms the importance of establishing geochemical signatures of raw, uranium ore materials for attribution purposes in the forensic analysis of intercepted nuclear materials.

Age and Sr-Nd-Hf isotopes of the sub-continental lithospheric mantle beneath the Cameroon Volcanic Line: Constraints from the Nyos mantle xenoliths

To better constrain the age and composition of the sub-continental lithospheric mantle (SCLM) beneath the Cameroon Volcanic Line (CVL), we present Sr-Nd-Hf-Os isotopes of the Nyos mantle xenoliths (nine lherzolites and four harzburgites). The Nyos lherzolites have fertile compositions with 37.69–41.12wt.% MgO and 2.49–4.38wt.% Al2O3, whereas the harzburgites are more refractory containing 43.2–45.42wt.% MgO and 1.43–2.1wt.% Al2O3. The MgO contents of the Nyos xenoliths overall show negative correlations with CaO, Al2O3, Na2O and TiO2 contents, indicating that they represent mantle residues after melt depletion. Modal minerals like pargasitic amphibole have only been discovered in one Nyos lherzolite, suggesting modal metasomatism locally occurred. Clinopyroxenes in most Nyos xenoliths show enriched trace element compositions, indicating pervasive cryptic metasomatism within the SCLM. Clinopyroxenes in the Nyos lherzolites have less radiogenic Sr and more radiogenic Nd isotopes than the Nyos harzburgites. Clinopyroxene Hf isotopes of the harzburgites are comparable to those of the lherzolites. Although the CVL basalts have similar Sr-Nd isotopes with the Nyos harzburgites, they have different Hf isotopes. Therefore, the CVL basalts could not be sourced solely from the lithospheric mantle. The Nyos lherzolites show consistent HSE patterns, with slight depletion in both Re and Pt over Pd. The harzburgites are remarkably depleted in Pt, Pd and Re relative to Os, Ir and Ru. The Nyos lherzolites have variable 187Os/188Os ratios ranging from 0.11700 to 0.13129, yielding Os model ages (TMA) relative to the primitive upper mantle (PUM) of 0.27–2.18Ga and Re depletion ages (TRD) of 0.12–1.76Ga. In comparison, four harzburgites have more unradiogenic 187Os/188Os ratios of 0.11485–0.11705. Three of the four harzburgites with low Re/Os ratios yield similar TMA and TRD ages, i.e., 1.91–2.43Ga vs. 1.87–2.05Ga, respectively. The 187Os/188Os ratios of the Nyos xenoliths show a good correlation with bulk Al2O3 contents. An initial 187Os/188Os ratio of 0.11064 can be inferred from the 187Os/188Os-Al2O3 correlation, giving an Archean TRD age of ~2.6Ga. This suggests that the Nyos mantle xenoliths have been subjected to melt depletion events during the Paleoproterozoic convergence of the Congo and West Africa cratons, which formed the lithospheric mantle intruded by the CVL. The SCLM beneath the CVL is coeval with or slightly younger than the charnockite and tonalite-trondhjemite-granodiorite in the Ntem Complex of NW Congo Craton. We proposed that the old SCLM was a part of the Congo cratonic mantle. Therefore, our results suggest that old cratonic mantle beneath the CVL has not been completely removed during the Pan-African Orogeny, although the overlying crust might have been strongly deformed and reworked.

The effect of a hydrous phase on P‐wave velocity anisotropy within a detachment shear zone in the slow‐spreading oceanic crust: A case study from the Godzilla Megamullion, Philippine Sea

AbstractWe studied the contributions of plagioclase, clinopyroxene, and amphibole to the P‐wave velocity properties of gabbroic mylonites of the Godzilla Megamullion (site KH07‐02‐D18) in the Parece Vela Rift of the central Parece Vela Basin, Philippine Sea, based on their crystal‐preferred orientations (CPOs), mineral modes, and elastic constants and densities of single crystals. The gabbroic mylonites have been classified into three types based on their microstructures and temperature conditions: HT1, HT2 and medium‐temperature (MT) mylonites. The P‐wave velocity properties of the HT1 mylonite are dominantly influenced by plagioclase CPOs. Secondary amphibole occurred after deformation in the HT1 mylonite, so that its effect on P‐wave velocity anisotropy is minimal due to weak CPOs. Although the HT2 mylonite developed deformation microstructures in the three minerals, the P‐wave velocity properties of the HT2 mylonite are essentially isotropic, resulting from the destructive interference of different P‐wave velocity anisotropy patterns produced by the distinct CPOs of the three constituent minerals (i.e., plagioclase, clinopyroxene, and amphibole). The P‐wave velocity properties of the MT mylonite are influenced mainly by amphibole CPOs, whereas the effect of plagioclase CPOs on P‐wave velocity anisotropy becomes very small with a decrease in the intensity of plagioclase CPOs. As a result, the gabbroic mylonites tend to have weak P‐wave velocity anisotropy in seismic velocity, although their constituent minerals show distinct CPOs. Such weakness in the whole‐rock P‐wave velocity anisotropy could result from the destructive contributions of the different mineral CPOs with respect to the structural framework (foliation and lineation). These results show that amphibole has a high potential for P‐wave velocity anisotropy by aligning both crystallographically and dimensionally during deformation in the hydrous oceanic crust. The results also suggest that the effect of a hydrous phase on P‐wave velocity anisotropy within the detachment shear zone in a slow‐spreading oceanic crust varies depending on the degree of deformation and on the timing of hydrothermal activity.

Noise Simulation and Correction in Synthetic Airborne TIR Data for Mineral Quantification

Rock-forming minerals (such as feldspar and quartz) can be identified and quantified from thermal infrared (TIR) laboratory spectroscopy using spectral models. This paper uses synthetic airborne TIR spectra to test whether the hyperspectral Spatially Enhanced Broadband Array Spectrograph System (SEBASS) would theoretically be able to detect quartz and feldspar minerals and quantitatively predict mineral modes in felsic igneous rocks. Data from a previous laboratory study were used to simulate TIR spectra with band locations and noise levels of the SEBASS sensor. The quantitative partial least squares regression (PLSR) models from that study were applied to newly created synthetic SEBASS data, and results were compared with the predictions from the previous study. Predicted compositions based on SEBASS band positions are nearly identical $(\rho = 0.995)$ to those based on laboratory resolution. Results are still reliable [prediction errors within 0.4% (absolute)] to the original laboratory PLSR predictions when adding up to 1% noise (about five times the SEBASS noise level) to the synthetic data. Prediction errors rapidly increase when noise levels beyond 1% are used. These results show that SEBASS' spectral resolution, spectral coverage, and signal-to-noise levels are sufficient to quantitatively predict quartz and feldspar amounts, and feldspar compositions with models based on PLSR. Spectral distortions, such as reduced spectral contrast, tilts, and vertical shifts, must be compensated for before these quantitative models are applied. A mean and standard deviation (MASD) normalization is proposed using a set of ground data for compensating systematic errors that are common to all image pixels.

Tunability of collagen matrix mechanical properties via multiple modes of mineralization.

Functionally graded, mineralized collagen tissues exist at soft-to-hard material attachments throughout the body. However, the details of how collagen and hydroxyapatite mineral (HA) interact are not fully understood, hampering efforts to develop tissue-engineered constructs that can assist with repair of injuries at the attachments of soft tissues to bone. In this study, spatial control of mineralization was achieved in collagen matrices using simulated body fluids (SBFs). Based upon previous observations of poor bonding between reconstituted collagen and HA deposited using SBF, we hypothesized that mineralizing collagen in the presence of fetuin (which inhibits surface mineralization) would lead to more mineral deposition within the scaffold and therefore a greater increase in stiffness and toughness compared with collagen mineralized without fetuin. We tested this hypothesis through integrated synthesis, mechanical testing and modelling of graded, mineralized reconstituted collagen constructs. Results supported the hypothesis, and further suggested that mineralization on the interior of reconstituted collagen constructs, as promoted by fetuin, led to superior bonding between HA and collagen. The results provide us guidance for the development of mineralized collagen scaffolds, with implications for bone and tendon-to-bone tissue engineering.

Elasticity of plagioclase feldspars

AbstractElastic properties are reported for eight plagioclase feldspars that span compositions from albite (NaSi3AlO8) to anorthite (CaSi2Al2O8). Surface acoustic wave velocities measured using Impulsive Stimulated Light Scattering and compliance sums from high‐pressure X‐ray compression studies accurately determine all 21 components of the elasticity tensor for these triclinic minerals. The overall pattern of elasticity and the changes in individual elastic components with composition can be rationalized on the basis of the evolution of crystal structures and chemistry across this solid‐solution join. All plagioclase feldspars have high elastic anisotropy; a* (the direction perpendicular to the b and c axes) is the softest direction by a factor of 3 in albite. From albite to anorthite the stiffness of this direction undergoes the greatest change, increasing twofold. Small discontinuities in the elastic components, inferred to occur between the three plagioclase phases with distinct symmetry ( , , and ), appear consistent with the nature of the underlying conformation of the framework‐linked tetrahedra and the associated structural changes. Measured body wave velocities of plagioclase‐rich rocks, reported over the last five decades, are consistent with calculated Hill‐averaged velocities using the current moduli. This confirms long‐standing speculation that previously reported elastic moduli for plagioclase feldspars are systematically in error. The current results provide greater assurance that the seismic structure of the middle and lower crusts can be accurately estimated on the basis of specified mineral modes, chemistry, and fabric.

Application of thermodynamic modelling to natural mantle xenoliths: examples of density variations and pressure–temperature evolution of the lithospheric mantle

In this paper, we show how the results of phase equilibria calculations in different mantle compositions can be reconciled with the evidence from natural mantle samples. We present data on the response of bulk rock density to pressure (P), temperature (T) and compositional changes in the lithospheric mantle and obtain constraints on the P–T evolution recorded by mantle xenoliths. To do this, we examine the mantle xenolith suite from the Quaternary alkali basalts of Pali-Aike, Patagonia, using phase equilibria calculation in six representative compositions. The calculations were done subsolidus and in volatile-free conditions. Our results show that the density change related to the spinel peridotite to garnet peridotite transition is not sharp and strongly depends on the bulk composition. In a depleted mantle composition, this transition is not reflected in the density profile, while in a fertile mantle it leads to a relative increase in density with respect to more depleted compositions. In mantle sections characterized by hot geothermal gradients (~70 mW/m2), the spinel–garnet transition may overlap with the lithosphere–asthenosphere boundary. Phase equilibria calculations in peridotitic compositions representative of the Pali-Aike mantle were also used to constrain the origin and evolution of the mantle xenoliths. Our results indicate that the mineral modes and compositions, and the mineral zonation reported for the low-temperature peridotites (spinel and spinel + garnet harzburgites and lherzolites), are linked to a cooling event in the mantle which occurred long before the eruption of the host basalts. In addition, our phase equilibria calculations show that kelyphitic rims around garnets, as those observed in the high-temperature garnet peridotites from Pali-Aike, can be explained simply by decompression and do not require additional metasomatic fluid or melt.

Gold deposition caused by carbonation of biotite during late-stage fluid flow

Alteration reactions associated with gold mineralisation can be used to elucidate the nature of the fluid that transported gold into a deposit. At the Junction gold deposit, Kambalda, Western Australia, gold is hosted in a metamorphosed and hydrothermally altered dolerite. Mineralisation at the deposit scale is associated with zones of K, CO2 and S metasomatism, as is common in many greenstone hosted gold deposits. However, at the thin-section scale gold is not closely associated with sulphide minerals but within zones of carbonate metasomatism and K-loss where pre-existing biotite has reacted to produce chlorite, muscovite and Fe–Mg carbonates. Gold precipitation is intimately associated with biotite breakdown where calcite is locally absent. Quantified mineral modes from detailed microstructural mapping are used to balance reactions describing the breakdown of biotite in the presence and absence of calcite. Using the basic assumption that Al is immobile during metasomatism the reactions are successfully balanced, even in a manifestly open system. Modelling of fluid–rock reactions using HCh constrains the fluid composition (0.11<X(CO2)<0.13) and fluid–rock ratios (<12:1) that can produce the observed mineral assemblage. Additional modelling of solid solution mineral phases using thermocalc estimates alteration conditions of 390°C, 4kbar and also suggests a fluid X(CO2)~0.1. Both these models show that the observed muscovite and chlorite compositions can be produced primarily through the removal of K from the measured precursor biotite. We show that it is not possible to transport and deposit all the gold observed in the alteration zone with the low fluid–rock ratios obtained from modelling of silicate alteration and inferred gold concentrations in these fluids. We suggest that this is typical of greenstone hosted gold deposits and that mechanisms other than aqueous solution, which can transport higher gold concentrations, must be considered.

Chemical interactions between a sedimentary diapir and surrounding magma: Evidence from the Phepane Dome and Bushveld Complex, South Africa

The Phepane Dome is a circular outcrop of metasedimentary rock within the Eastern Lobe of the Bushveld Complex hypothesized to have formed as a diapir, when underlying wallrock rose into the overlying magma body. Interactions between the metasedimentary rock of the Phepane Dome and the magma of the Bushveld Complex were investigated through measurements of oxygen and lithium isotopic compositions, determination of mineral modes and major-element mineral compositions, cathodoluminescence imaging, and dihedral angle analysis. Evidence from cathodoluminescence imaging and dihedral angle analysis suggest that heat transfer during diapirism caused partial melting and complete recrystallization of the Phepane Dome metasedimentary rock. Oxygen isotope analysis of samples from traverses spanning the contact between metasedimentary and igneous rocks demonstrates that relatively minimal exchange of oxygen (over distance ~4 m) occurred across the contact between the Phepane Dome and the surrounding Bushveld magma. The lithium concentrations and isotopic compositions of metasedimentary rock are significantly different from the associated igneous rocks. Lithium isotope analysis of samples from traverses across the contact demonstrates exchange of Li over somewhat greater distances (~60 m) than oxygen, consistent with evidence that suggests a higher diffusivity of Li than most major elements. Models of oxygen diffusion through intergranular melt and aqueous fluid are used to place maximum constraints on the duration of diffusive exchange across the contact, resulting in estimates ranging from 5 kyr to 5 Myr. These values are consistent with previous estimates of the duration of crystallization of the Bushveld Complex and Phepane diapir development.

Pore-scale heterogeneity in the mineral distribution and reactive surface area of porous rocks

The reactive surface area is an important control on interfacial processes between minerals and aqueous fluids in porous rocks. Spatial heterogeneity in the surface area can lead to complications in modelling reactive transport processes, but quantitative characterisation of this property has been limited. In this paper 3D images obtained using X-ray micro-tomography were used to characterise heterogeneity in surface area in one sandstone and five carbonate rocks. Measurements of average surface area from X-ray imagery were 1–2 orders of magnitude lower than measurements from nitrogen BET. A roughness factor, defined as the ratio of BET surface area to X-ray based surface area, was correlated to the presence of clay or microporosity. Co-registered images of Berea sandstone from X-ray and energy dispersive spectroscopy imagery were used to guide the identification of quartz, K-feldspar, dolomite, calcite and clays in X-ray images. In Berea sandstone, clay and K-feldspar had higher average surface area fractions than their volumetric fractions in the rock. In the Edwards carbonate, however, modal mineral composition correlated with surface area. By sub-sampling digital images, statistical distributions of the surface area were generated at various length scales of subsampling. Comparing these to distributions used in published modelling studies showed that the common practice of leaving surface area and pore volume uncorrelated in a pore has lead to unrealistic combinations of surface area and pore volume in the models. We suggest these models adopt a moderate correlation based on observations. In Berea sandstone, constraining ratios of surface area to pore volume to a range of values between that of quartz-lined and five times that of clay-lined spheres appeared sufficient.

Can Fractional Crystallization, Mixing and Assimilation Processes be Responsible for Jamaican-type Adakites? Implications for Generating Eoarchaean Continental Crust

Understanding how the Earth’s first continental land masses were generated is important because the processes responsible directly affected the evolution of the planet’s primordial silicate interior, and also its atmosphere and hydrosphere. Archaean continental crust is dominated by rocks of the trondhjemite–tonalite–granodiorite (TTG) suite. These can be divided into (1) a mid- to late Archaean (∼3·5–2·5 Ga) suite with low SiO2 and high MgO, Sr and transition element contents, and (2) an Eoarchaean (>3·5 Ga) suite with higher SiO2 and lower MgO, Sr and transition element concentrations. Cenozoic adakites are considered to be compositionally similar to mid- to late Archaean (∼3·5–2·5 Ga) TTGs, but not the oldest TTG rocks. Conversely, a suite of Early Eocene adakite-like rhyodacites (Jamaican-type adakites: JTA) from Jamaica are shown to be geochemically similar to the Eoarchaean TTGs. In contrast to newly discovered JTA-like rocks (Ryozen low Sr/Y) in Japan, new trace element and Nd–Hf radiogenic isotope data in this study confirm that the Jamaican JTA cannot be formed by complex mixing, assimilation and fractional crystallization processes. New partial melt models here explore several different source compositions (mid-ocean ridge basalt, ocean island basalt and oceanic plateau), mineral modes, melt modes and partition coefficients. The results of these models clearly demonstrate that the JTA and the Eoarchaean TTG can be generated by partial melting of plagioclase- and garnet-bearing amphibolite source regions with oceanic plateau-like compositions. Further modelling shows that the JTA and Eoarchaean TTG low MgO and transition element abundances can be derived from two dominant processes: (1) relatively shallow partial melting of subducting oceanic crust (compositionally similar to Mesozoic oceanic plateau basalt) whereby the slab melts ascend without interacting with a mantle wedge; (2) partial melting of oceanic plateau-like subducting oceanic crust followed by interaction of the slab melts with a thin and/or discontinuous (boudinage-like?) mantle wedge whereby the expected increase of MgO, Ni, and Cr in the slab melts is obliterated by fractional crystallization of ferromagnesian minerals (mostly amphibole). Consequently, using the JTA as a modern analogue for Eoarchaean TTG production, we propose the existence of subduction zones consuming oceanic plateau-like oceanic crust in Eoarchaean times.

Electrochemical Processes in a Crystal Mush: Cyclic Units in the Upper Critical Zone of the Bushveld Complex, South Africa

The Upper Critical Zone (UCZ) of the Bushveld Igneous Complex displays spectacular layering in the form of cyclic units comprising a basal chromitite layer overlain by a sequence of silicate cumulates in the order, from bottom to top, pyroxenite–norite–anorthosite. Electron microprobe and laser ablation inductively coupled plasma mass spectrometry analyses of chromite and silicate minerals in layers between the UG2 chromitite and the Merensky Reef reveal variations in major and trace element compositions that defy explanation with existing models of cumulate mineral– melt evolution. The anomalous features are best developed at sharp contacts of chromitite with adjacent anorthosite and pyroxenite cumulates. Here, chromite compositions change abruptly from high and constant Mg/(Mg þ Fe 2þ ) and Fe 2þ /Fe 3þ ratios in chromitite layers to variable and generally lower values in chromite disseminated in silicate layers. Furthermore, the composition of disseminated chromites varies depending on the host silicate assemblage; for example, in Ti, V and Zn contents. Importantly, the abrupt change in chromite composition across the chromitite–silicate layer contacts is independent of the thickness of the chromitite layer and the estimated mass proportions of chromite to intercumulus liquid. Chemical variations in plagioclase are also abrupt and some are hard to reconcile with conventional models of re-equilibration with intercumulus liquid. Among those features is the decoupling of alkalis from other incompatible lithophile elements. In comparison with cumulus plagioclase, intercumulus poikilitic plagioclase in chromitite layers is enriched in rare earth elements but strongly depleted in equally incompatible Li, K and Rb. Strong alkali depletion is also observed in intercumulus pyroxene from ultramafic cumulates and chromitite layers. To explain these features, we propose a new model of post-cumulus recrystallization, which intensifies the modal layering in the crystal–liquid mush, producing the observed sequence of nearly monomineralic layers of chromitite, pyroxenite and anorthosite that define the cyclic units. The crucial element of this model is the establishment of redox potential gradients at contacts between chromite-rich cumulates and adjacent silicate layers owing to peritectic reactions between the crystals and intercumulus melt. Because basaltic melts are ionic electrolytes with Na þ as the main charge carrier, the redox potential gradient induces electrochemical migration of Na þ and other alkali ions. Selective mobility of alkalis can explain the enigmatic features of plagioclase composition in the cyclic units. Sodium migration is expected to cause remelting of previously formed cumulates and major changes in modal mineral proportions, which may eventually result in the formation of sharply divided monomineralic layers. The observed variations in ferric/ferrous iron ratios in chromite from the cyclic units and Fe distribution in plagioclase imply a redox gradient of

Building the backbone: the development and evolution of vertebral patterning.

The segmented vertebral column comprises a repeat series of vertebrae, each consisting of two key components: the vertebral body (or centrum) and the vertebral arches. Despite being a defining feature of the vertebrates, much remains to be understood about vertebral development and evolution. Particular controversy surrounds whether vertebral component structures are homologous across vertebrates, how somite and vertebral patterning are connected, and the developmental origin of vertebral bone-mineralizing cells. Here, we assemble evidence from ichthyologists, palaeontologists and developmental biologists to consider these issues. Vertebral arch elements were present in early stem vertebrates, whereas centra arose later. We argue that centra are homologous among jawed vertebrates, and review evidence in teleosts that the notochord plays an instructive role in segmental patterning, alongside the somites, and contributes to mineralization. By clarifying the evolutionary relationship between centra and arches, and their varying modes of skeletal mineralization, we can better appreciate the detailed mechanisms that regulate and diversify vertebral patterning.

Rare earth elements in ferromanganese nodules of the Chukchi Sea

Data on the morphology, microstructure, and chemical and mineral compositions of ferromanganese nodules (FMN) in the Chukchi Sea are presented. Specific features of the distribution and mineral modes of rare earth elements (REE) and yttrium are examined. The studied nodules are characterized by low contents of total REE along with negative Ce and weak positive (virtually neutral) Eu anomalies: Σ(REE) 185.26–299.10 ppm, Cean 0.73–0.96, and Euan 1.01–1.13. The Y/Ho ratio varies from 23.19 to 29.88; the ratio of light and heavy REEs (LREE/HREE)NASC, from 0.74 to 1.50. The presence of La-Ce-Pr-Nd-O and La-Ce-Pr-Nd-Al-O micrograins along with monazite, xenotime, and some other more exotic compounds has been established. The majority of geochemical indicators testify to the diagenetic origin of FMN. The most characteristic property of the studied nodules is high (Fe + Mn)/Ti module (up to 295.05).

Mineralized tissues in dinosaurs interpreted as having formed through metaplasia: A preliminary evaluation

Evolutionary biologists define “metaplasia” as the permanent transformation of a cell identity, and there are many examples of such transformations in living vertebrates (e.g., chondrocytes transforming directly into osteoblasts). These metaplasias have been observed during the mineralization of “ossified” tendons of living birds. In the present study, we examined “ossified” tendons in Bubo and Meleagris and used the characteristics of these metaplastic tissues to recognize them in several non-avian dinosaur taxa. The fossilized skeletal elements that form our sample are varied and include hadrosaurian tendons and a nasal bone, an ankylosaur tail club “handle”, sauropod neural spines, and some dromaeosaur tail rods. The extant avian mineralized tendons were formed of a primary tissue (analogous to primary bone) and secondary reconstructions (SRs; analogous to secondary osteons). Both were composed of fiber bundles (or fascicles) that were closely packed together and separated by arc-shaped spaces in cross-section. When viewed longitudinally, they were arranged in a herringbone pattern. There is no evidence of osteocytes within the primary tendon matrix; what was previously interpreted as osteocyte lacunae are instead arc-shaped spaces between fiber fascicles, and tissue immediately surrounding vascular spaces is dense, avascular and apparently hypermineralized. Mineralization of fibers began centrally and moved in a centrifugal direction. In the non-avian dinosaurs examined, primary and secondary tissue structures were virtually identical to those found within the avian mineralized tendons. Indeed, (1) they were densely fibrous; (2) they showed fiber fascicles separated by arcuate-shaped spaces when viewed transversally, and (3) they were arranged in a herringbone pattern longitudinally. SRs differ from typical Haversian systems in possessing highly irregular borders, suggesting destruction of the fibrous matrix and formation of initial vascular spaces was accomplished perhaps by phagocytosis or enzymatic lysis with subsequent remodeling by fibrocytes, fibroblasts, or an as of yet unknown cell type. Osteocytes with canaliculi were only observed in “mature” SRs, found deep within the elements (and never close to their external borders). Because fossilized primary and secondary tissue structures were identical to those found within the avian mineralized tendons examined, it is likely that identical processes are responsible for their formation. Biomechanical properties were also likely similar, potentially affording carbon fiber-like, trauma-resistant properties to the “ossified” tendons and nasal bones of hadrosaurs, the tail “handles” of ankylosaurs, and the tail rods of dromaeosaurs. In contrast, the primary tissue from a sauropod mineralized nuchal ligament appears to be made of hypermineralized fibrocartilage, but the SRs interdigitating with the hypermineralized fibrocartilage resemble the reconstructions observed in the other fossil skeletal elements and likely formed by the same processes. Since no osteocyte lacunae were observed in any of these dinosaurian primary tissues, we hypothesize that the fossilized cranial and skeletal elements examined here formed through metaplastic transformation (perhaps from fibroblasts) rather than by periosteal and intramembranous ossification. This study suggests that alternative modes of mineralization might be more abundant in non-avian dinosaurs than previously reported.

Study of IR spectra of a polymineral natural association of phyllosilicate minerals

The IR spectrum of a sample from a polymineral natural association of phyllosilicate minerals (clays from the Millerovskoe deposit, Russian Federation) has been analyzed. Calculations are performed by the method of density functional theory with allowance for the real crystallographic features of the compounds entering the sample composition, which were found using methods of structural analysis. The detailed analysis of IR spectra made it possible to find an unambiguous correlation between the absorption bands and corresponding vibrational modes of individual minerals entering the association. It is shown that the widths of vibrational spectral components can be integrally estimated by expanding the IR spectrum of the association in spectra of individual minerals using phase analysis data.

TCInvestigator: automated calculation of mineral mode and composition contours for thermocalc pseudosections

AbstractForward thermodynamic modelling is a commonly used tool for examining the thermobarometric history of a rock. Such models are being applied to a widening range of rock types using increasingly complex model systems. A useful aspect of phase equilibria modelling using pseudosections is the ability to calculate the modal abundance and compositions of phases, in addition to overall phase relations. However, calculating and plotting such data with thermocalc, a popular modelling program, is time consuming. In this article, we provide the methods and a description of a Matlab® computer program, TCInvestigator, which uses thermocalc 3.40 to automatically contour pre‐calculated pressure–temperature pseudosections with modal abundance and mineral composition parameters. The ability to calculate, plot and visualize these parameters automatically and rapidly, allows for a more in‐depth investigation into the metamorphic evolution of a rock. Data can be contoured for all minerals that are present in all fields on the pseudosection, rather than a selection of fields or phases that are a priori considered relevant. Examples of contoured pseudosections for pelitic and greywacke bulk compositions are presented, along with accompanying data sets, to illustrate the applications of this program. The software and example files are available from the CSIRO Data Access Portal at the following address: doi:10.4225/08/54C99E2D47150.

On discrimination between carbonate and silicate inputs to Himalayan rivers

We review new and published analyses of river waters, bedloads and their constituent minerals from the Dhauli Ganga and Alaknanda, headwaters of the Ganges in Garhwal, and the Marsyandi in Nepal and their tributaries. These data are used to discriminate between the inputs of major cations and Sr from silicate and carbonate sources. Methods of estimating the proportion of the carbonate and silicate inputs to river waters using mixing arrays in Sr-Ca-Mg-Na-K-87Sr/86Sr space are shown to suffer from systematic correlations between the magnitude of the precipitation of secondary calcite and the fraction of the silicate component. This results in factor-of-two overestimates of the fractions of silicate-derived Ca, Mg and Sr. To correct for this the magnitude of secondary calcite precipitated and relative fractions of silicate and carbonate-derived cations are instead calculated by modeling the displacement of water compositions from the compositions of the carbonate and silicate components of the bedload in subsets of Sr-Ca-Mg-Na-K-87Sr/86Sr space. The compositions of the carbonate and silicate end-members in the bedload are determined by sequential leaching. The results of this modeling are compared with modeling of the modal mineral inputs to waters where mineral compositions are derived from electron-microprobe analyses of the minerals in the bedload. In the upper Marsyandi catchment, which drains low-grade Tethyan Sedimentary Series formations, a set of mainstem samples collected over a two-year period define tight correlations in Sr-Ca-Mg-Na-K-87Sr/86Sr space. Modeling of the magnitude of secondary carbonate precipitation and fractions of silicate-derived Ca, Mg and Sr in Sr-Ca-Mg-87Sr/86Sr space gives self-consistent results that are compatible with both the calculations of mineral modes and published Mg-isotopic compositions, if the ratio of chlorite to biotite weathering is high or if there is another silicate source of Mg. These calculations imply that between 12 and 31 percent of the Sr and 44 and 72 percent of the Mg is derived from silicate minerals where the range reflects the seasonal change in the ratio of silicate-derived to carbonate-derived cations. Modeling in Sr-Ca-Na and/or K space is inconsistent with the Sr-isotopic and Mg-isotopic constraints and we conclude that in this catchment dissolution of Na and K are incongruent relative to Sr-Ca-Mg. Potassium is preferentially retained in micas whereas the controls on Na are unclear. Modeling of the catchments underlain by High Himalayan Crystalline and Lesser Himalayan Series in Garhwal is complicated by the presence of dolomite as well as calcite in the carbonate and the results imply that dolomite dissolves faster in the acetic acid leaches than in nature. Up to 60 percent of the Sr in the catchment on High Himalayan Crystalline Series and 20 to 30 percent of Sr in the catchments on Lesser Himalayan Series are estimated to be derived from silicates. However it should be noted that the element budgets are not all self-consistent and the use of bedrock-element ratios to model the sources of chemical inputs to river waters remains subject to uncertainties.

Effect of small-scale heterogeneities on interpretation of crustal compositions exemplified by a layered anorthosite

The composition of the lower crust has a significant effect on geodynamic processes because it influences physical rock properties such as densities and seismic velocities. Compositional differences in lower crustal rocks are potentially large and exist on the scales of centimeters up to kilometers resulting in non-unique seismic and gravity data. While larger heterogeneities can be detected as reflections on seismic profiles, irregular small-scale compositional variations are not likely to be discovered, but will influence the averaged seismic velocities and densities of an area. The extent and effects of such small-scale heterogeneities are explored on an exposed high-grade layered anorthositic body by providing a detailed field map, petrological descriptions, pycnometry measurements as well as whole rock and mineral analyses combined with thermodynamic phase equilibria calculations. To evaluate the results of our thermodynamic calculations, densities and mineral modes obtained from the modeled phase equilibria are compared to measured densities and estimated mineral modes from rock samples. The proportion of mafic to ultramafic (plagioclase-poor) rocks in the mapped field area amounts to 10–15% but higher proportions of these rock types in the lower crust are feasible. To further study the effects of compositional variations, we have generated mixtures of mafic to ultramafic and anorthositic/intermediate rocks until the average properties of these mixtures are comparable to those of mafic granulites (3000–3100 kg/m3; 7.1–7.3 km/s). Mixtures of anorthosite with 40–45% and of tonalite with 50–60% high-grade mafic to ultramafic rocks yield average densities and seismic velocities similar to mafic granulites although they still contain 50–60 vol.% plagioclase. Hence small-scale mixing of certain rock types may result in the overestimation of the proportion of mafic (garnet) granulites in the lithologic interpretation of crustal compositions from seismic data. Since the transition to eclogite-facies in plagioclase-rich rocks is shifted to higher pressures and anorthositic/intermediate eclogites yield lower densities, a lower crust with higher modal amounts of plagioclase may not always provide the significant densification needed for certain geodynamic settings (e.g. delamination or subsidence).