Mineral Modes Research Articles

Primary origin of some trachytoid magmas: Inferences from naturally quenched glasses in hydrothermally metasomatized gabbroic xenoliths (Hyblean area, Sicily)

Hydrothermally-modified gabbroic xenoliths from the Hyblean tuff-breccia deposits (Sicily) consist of albitized plagioclase, Fe–Mg-rich clays, aegirine–augite, ±zeolites, titanite, apatite, magnetite, and hydrothermal zircon. Pockets of silicate glass with perlitic cracking occur in some samples forming 15–20% (by volume) of the rock modal assemblage. Electron microprobe analyses show the trachytic composition of the glass, with generally peralkaline sodic affinity [molar Al2O3/(Na2O+K2O)~0.8 (average value); molar Al2O3/(Na2O+K2O+CaO)~0.7 (average value); Na2O/K2O (wt.%)=1.7–2.3]. The glass trace element abundances, obtained by secondary ion mass spectrometry (SIMS) analyses are consistent with those of world-wide trachytes (e.g. Zr/Ti=0.15–018; Nb/Y=0.73–1). Relatively high abundances of Cl (700–1600 ppm) and F (>500 ppm) were also detected in the glass.Careful macroscopic and microscopic observations exclude the possibility that external silicate melt infiltrated the xenolith. The occurrence of glass pockets between the mafic clay assemblages and the feldspar grains, along with comparisons between chemical compositions of the glass and the surrounding minerals, suggest that the glass is due to the melting of a eutectoid system consisting of Na-rich alkali feldspar, Fe–Mg-rich clays and aegirine–augite. Halogens had probably played an important role in the partial melting process by decreasing the melting temperature of modal minerals, especially feldspar.The occurrence of these trachytic glasses lends support to petrologic models suggesting that partial melting of a hydrothermally altered, brine-rich oceanic crust induced by shallow-seated basic intrusions can produce primary trachytoid melts. This may explain the “Daly-gap” characterizing some oceanic within-plate volcanoes.

Feldspar dissolution kinetics and Gibbs free energy dependence in a CO 2-enriched groundwater system, Green River, Utah

Reactions between CO 2-charged brines and reservoir minerals might either enhance the long-term storage of CO 2 in geological reservoirs or facilitate leakage by corroding cap rocks and fault seals. Modelling the progress of such reactions is frustrated by uncertainties in the absolute mineral surface reaction rates and the significance of other rate limiting steps in natural systems. Here we use the chemical evolution of groundwater from the Jurassic Navajo Sandstone, part of a leaking natural accumulation of CO 2 at Green River, Utah, in the Colorado Plateau, USA, to place constraints on the rates and potential controlling mechanisms of the mineral–fluid reactions, under elevated CO 2 pressures, in a natural system. The progress of individual reactions, inferred from changes in groundwater chemistry is modelled using mass balance techniques. The mineral reactions are close to stoichiometric with plagioclase and K-feldspar dissolution largely balanced by precipitation of clay minerals and carbonate. Mineral modes, in conjunction with published surface area measurements and flow rates estimated from hydraulic head measurements, are then used to quantify the kinetics of feldspar dissolution. Maximum estimated dissolution rates for plagioclase and K-feldspar are 2 × 10 − 14 and 4 × 10 − 16 mol m − 2 s − 1 , respectively. Fluid ion-activity products are close to equilibrium (e.g. Δ G r for plagioclase between − 2 and − 10 kJ/mol) and lie in the region in which mineral surface reaction rates show a strong dependence on Δ G r. Local variation in Δ G r is attributed to the injection and disassociation of CO 2 which initially depresses silicate mineral saturation in the fluid, promoting feldspar dissolution. With progressive flow through the aquifer feldspar hydrolysis reactions consume H + and liberate solutes to solution which increase mineral saturation in the fluid and rates slow as a consequence. The measured plagioclase dissolution rates at low Δ G r of 2 × 10 − 14 mol m − 2 s − 1 would be compatible with far-from-equilibrium rates of ~ 1 × 10 − 13 mol m − 2 s − 1 as observed in some experimental studies. This suggests that the discrepancy between field and laboratory reaction rates may in part be explained by the differences in the thermodynamic state of natural and experimental fluids, with field-scale reactions occurring close to equilibrium whereas most laboratory experiments are run far-from-equilibrium.

Differentiation and Compaction in the Skaergaard Intrusion

Igneous differentiation processes are constrained from bulk compositions, densities and mineral modes of 116 cumulate gabbro samples in a new reference profile through the Layered Series of the Skaergaard intrusion, East Greenland. The stratigraphic distribution of P, U and Rb in cumulates and residual magma, modeled by Rayleigh fractionation, constrains the final porosities or trapped liquid contents to 30-52% in LZa troctolites, decreasing to 4-12% at the top of LZb olivine-gabbros and remaining low (1-13%; 4{middle dot}6% on average) in the oxide-gabbros of LZc, MZ and UZ. Local variations in trapped liquid content are associated with modal layering: leucocratic, low-density rocks have higher proportions of trapped melt than adjacent melanocratic, high-density rocks. These observations are explained by varying degrees of compaction. Compaction was most important after the onset of Fe-Ti oxide crystallization because of the high densities of the crystal matrix. Here computed rates of compaction exceed the rate of crystal accumulation in layers that are metres to a few tens of metres thick. In the basal section (LZa) the crystal pile was too thin and the density of the crystal matrix too low to drive the rate of compaction above the high rate of crystal accumulation promoted by initial cooling through the intrusion floor. In the overlying section (LZb) the efficiency of compaction gradually increased as a result of thickening of the crystal pile and lowering of the rate of crystal accumulation. The modeling constrains the P2O5 content of the residual magma to [~]1{middle dot}7 wt % at the level of apatite-in, suggesting that the magma contained [~]49 wt % SiO2 and followed a trend of iron enrichment. Compaction of the uppermost metres to tens of metres of crystal mush at the top of the cumulate pile was an efficient means of differentiation and resulted in layers with variable final porosities and trace element contents depending on the mineralogy and density of the crystal matrix.

Petrology and geochemistry of nepheline syenite and related carbonate-rich rocks in the Pan-African Dahomeyide orogen, southeastern Ghana, West Africa

Nepheline syenite and carbonate-rich rocks known as the Kpong complex occur as deformed layers within the Pan-African Dahomeyide suture zone, in southeastern Ghana. The dominant phases in the nepheline syenite are nepheline and feldspar, whereas the carbonate-rich rock is dominated modally by Sr-rich calcite and annitic biotite. Biotite, calcite and nepheline have similar compositions in the nepheline syenite and the carbonate-rich rock. The rocks of the Kpong complex are enriched in alkalis, Ba, Sr, and Nb, have high LREE/HREE ratios, and show large depletions in Zr and Hf relative to primitive mantle. These features are characteristic of carbonatitic and related alkaline rocks worldwide. Whole-rock major elements in the Kpong complex with respect to SiO 2 in bivariate plots show coherent variations with linear trends. However, based on the low Mg number, low Cr and Ni contents in whole rocks, and on overall high TiO 2, FeO, rocks of the Kpong complex appear to have crystallized from an evolved parent, rather than primary mantle-derived magma. The ubiquitous disaggregated enclaves within the carbonate-rich horizons, the linear compositional arrays defined in binary variation diagrams, and overlaps in major modal mineral compositions all suggest magma assimilation of country rock was involved in the evolution of the Kpong complex.

Mass occurrence of benthic coccoid cyanobacteria and their role in the production of Neoarchean carbonates of South Africa

The sparse Archean fossil record is based almost entirely on carbonaceous remnants of microorganisms cellularly preserved due to their early post-mortem silicification. Hitherto as an exception, sedimentary carbonate rocks from the Neoarchean Nauga Formation of South Africa contain calcified microbial mats composed of microbiota closely resembling modern benthic colonial cyanobacteria (Chroococcales and Pleurocapsales). Their remains, visible under the scanning electron microscope (SEM) after etching of polished rock samples, comprise capsular envelopes, mucilage sheaths, and groups of cells mineralized by calcium carbonate with an admixture of Al–K–Mg–Fe silicates. The capsular organization of the mucilaginous sheaths surrounding individual cells and cell clusters forming colonies and the mode of mineralization are the characteristic common features of the Neoarchean microbiota described and their modern analogues. The new findings indicate massive production of calcium carbonates by benthic coccoid cyanobacteria in the Neoarchean, and offer a solution to the problem of the origin of Archean carbonate platforms, stromatolites and microbial reefs.

Hydrous aluminosilicate metasomatism in an intra-oceanic subduction zone: Implications from the Kurancali (Turkey) ultramafic-mafic cumulates within the Alpine Neotethys Ocean

The Kurancali ultramafic-mafic cumulate body, an allochthonous ophiolitic sliver in central Anatolia, is characterized by the presence of abundant hydrous phases (phlogopite, pargasite) besides augitic diopside, plagioclase, and accessory amounts of rutile, sphene, apatite, zircon, and calcite. Based on modes of the essential minerals, the olivine-orthopyroxene-free cumulates are grouped as clinopyroxenite, hydrous clinopyroxenite, phlogopitite, hornblendite, layered gabbro, and diorite. Petrographical, mineralogical and geochemical features of the rocks infer crystallization from a hydrous magma having high-K calc-alkaline affinity with slightly alkaline character, and point to metasomatised mantle as the magma source. Our evidence implies that the metasomatising component, which modified the composition of the mantle wedge source rock in an intra-oceanic subduction zone, was a H2O, alkali and carbonate-rich aluminosilicate fluid and/or melt, probably derived from a subducted slab. We suggest that the metasomatic agents in the subarc mantle led to the generation of a hydrous magma, which produced the Kurancali cumulates in an island-arc basement in a supra-subduction-zone setting during the closure of the Izmir-Ankara-Erzincan branch of the Alpine Neotethys Ocean.

Systematic distribution of incompatible elements in mantle peridotite: importance of intra- and inter-granular melt-like components

In this paper, we examine the distribution of incompatible elements in Earth’s mantle based on data reported for 20 mantle xenoliths collected from 5 localities worldwide. A structural model combined with an element partitioning model forms the basis for our analyses. The former separates a bulk peridotite into mineral crystal lattices, interfaces (grain and interphase boundaries), and intra- and inter-granular inclusions as sites for incompatible elements. The latter relates the distribution of elements among these sites based on lattice strain theory. By treating both intra- and inter-granular inclusions as a melt-like phase, the combined models successfully reproduce the relative concentrations of incompatible elements among minerals, clean rock (reconstituted from mineral compositions and mineral mode), and whole rock. The analyses reveal common signatures in the rocks: (1) incompatible elements in the crystal lattices of olivine, orthopyroxene and clinopyroxene achieved chemical equilibrium. (2) Olivine, orthopyroxene and clinopyroxene grains contain similar amounts of an intra-granular, melt-like component possibly in the form of sub-micron inclusions with weight (≈volume) fractions between 5 × 10−5 and 1 × 10−2. (3) All rocks contain an inter-granular melt-like component with a fraction between 10−4 and 10−2, well above the amount expected to be stored along interfaces. (4) Fractions of the inter- and intra-granular components are positively correlated, indicating that they were originated from the same process. (5) The inter- and intra-granular melt-like phases are chemically equilibrated with other structural components. Based on plausible upwelling rates for mantle xenoliths, it is unlikely that the melt-like component formed during ascent. Instead, its ubiquitous appearance, its invisibility to optical microscopy, and its absorption of the incompatible elements in a manner similar to a melt phase even at sub-solidus condition, all might be explained by the presence of amorphous silica precipitates such as those observed previously in naturally occurring and experimentally annealed mantle composites. From the mineral mode, grain size, and mineral plus whole rock concentrations of incompatible elements, we can ascribe the chemical signatures of xenoliths to achievement of chemical equilibrium at mantle conditions rather than to a consequence of some disequilibrium (metasomatic) effect as has been done previously. Although it should be tested by additional analytical studies, our model will make it possible to determine whether or not a rock is chemically equilibrated in terms of the distribution of incompatible elements or if a metasomatic (disequilibrium) event is required.

Lunar meteorite LaPaz Icefield 04841: Petrology, texture, and impact‐shock effects of a low‐Ti mare basalt

Abstract— Found during the 2004 U.S. Antarctic Search for Meteorites season, LaPaz Icefield (LAP) 04841 represents an addition to the LaPaz lunar basalts suite and brings the total mass collected to 1.93 kg. The presence of FeNi grains, troilite, and the anorthositic composition of plagioclase are evidence for the lunar origin of this meteorite. Pyroxene and olivine Mn/Fe values plot along the trend set for lunar basalts. Analyses of chromite grains provide a V/(Al + Cr) ratio of 1.33 ± 13, translating to an fO2 one log unit below the IW buffer, in accordance with previous fO2 estimates for lunar basalts. Application of the Zr‐cooling speedometer, for ilmenite and ulvöspinel pairs, gives a cooling rate of 5.2 °C/day, matching previous estimates of cooling rates for the LaPaz lunar meteorites and Apollo mare basalts. Mineral modes and chemistries, as well as trace‐element patterns, provide compelling evidence for pairing of this meteorite to others in the LaPaz lunar basalt suite.

Constraints on the evolutionary history of aluminous mafic rocks in the Ronda peridotite massif (Spain) from trace-element compositions of clinopyroxene and garnet

Multiple trace-element compositions of clinopyroxene and garnet in aluminous mafic layers (±corundum and/or sapphirine) alternating with peridotites from the Ronda massif, Spain, were determined by an ArF 193 nm excimer laser system coupled with a quadrupole ICP-MS. The studied aluminous mafic rock is divided into several sublayers based on the modal mineral variations. Trace-element compositions of minerals vary between sublayers. Primitive mantle-normalized trace-element distribution patterns have more significant positive Eu anomaly for both garnet and clinopyroxene in the corundum-bearing sublayers than for those in the other sublayers whereas corundum-bearing sublayers contain more abundant secondary plagioclase than corundum-free sublayers. These geochemical signatures of the corundum-bearing sublayers intercalated with corundum-free sublayers were acquired as plagioclase-rich portions of gabbroic layered sequence originated at the lower crustal to the uppermost mantle sections rather than selective accumulation of metamorphic plagioclase due to deformation during the late stage of exhumation of the massif. Partition coefficients of trace elements between clinopyroxene and garnet show systematic variations with respect to major element composition, indicating local attainment of chemical equilibrium. The trace element compositions of clinopyroxene and garnet were equilibrated with secondary metamorphic plagioclase, i.e., at granulite-facies conditions, because plagioclase is required for the geochemical balance in Sr and Eu contents of the whole-rock trace element compositions. Positive Eu anomaly of clinopyroxene (±garnet) is also found in mantle-derived aluminous eclogitic xenoliths and eclogitic mineral inclusions in diamond captured in kimberlites, and is thus a useful indicator to explore recycled crustal components in the mantle.

Hydrogen partitioning between nominally anhydrous upper mantle minerals and melt between 3 and 5 GPa and applications to hydrous peridotite partial melting

To extend our knowledge of partitioning of hydrogen between nominally anhydrous mantle minerals and basaltic melts to higher pressures, we conducted 8 experiments at 3–5 GPa and 1350–1440 °C on a modified mid-ocean ridge basalt (MORB) + 2 wt.% H 2O. H contents in olivine, orthopyroxene, clinopyroxene, and garnet were measured by counting 16O 1H ions with low-blank SIMS, and total H 2O concentrations in coexisting glasses were measured by FTIR, SIMS, and electron microprobe. The glasses have water contents ranging from 4.1 to 8.9 wt.%. Eleven measured pyroxenes have H 2O concentrations ranging from 430 to 1430 ppm, two garnets have concentrations of 73 and 260 ppm, and one olivine has a concentration of 29 ppm. Mineral/melt hydrogen partition coefficients are as follows: D H cpx/melt ranges from 0.014–0.021, D H opx/melt ranges from 0.009–0.019, D H gt/melt ranges from 0.001–0.0033, and D H ol/melt has a value of 0.0006. Mineral/mineral hydrogen partitioning data are as follows: D H cpx/opxranges from 1.2–2, D H cpx/gt has values of 5.5 and 14, and a single D H cpx/ol has a value of 27. D H pyx/melt correlates with the concentration of Al 2O 3 in pyroxene and with the concentration of Al on tetrahedral sites. Combining the mineral/melt hydrogen partitioning data from this study, and estimates of mineral modes and compositions along the solidus, we estimate that D H perid/melt increases from 0.006 at 1 GPa up to 0.01 near the garnet lherzolite/spinel lherzolite transition, owing to the maximum in Al 2O 3 concentration in pyroxenes. With increasing pressure D H perid/melt for garnet lherzolite diminishes to 0.005 at 5 GPa, as pyroxenes become less aluminous and less abundant. As a result, incipient hydrous melts beneath oceanic ridges with bulk mantle H 2O contents of 50–200 ppm will have solidus partial melts with H 2O contents between 0.5 and 3.8 wt.%, while oceanic island mantle with 300–1000 ppm will have solidus partial melts with H 2O content between 3 and 20 wt.%.

Emplacement of ultramafic rocks into the continental crust monitored by light and other trace elements: An example from the Geisspfad body (Swiss-Italian Alps)

In order to evaluate the influence of continental crustal rocks on trace element budgets of serpentinized peridotites incorporated into the continental crust, we have analyzed the chemical composition of whole rock samples and minerals of the Geisspfad ultramafic complex (Swiss-Italian Alps). This complex represents a relict oceanic succession composed of serpentinites, ophicarbonates and metabasic rocks, emplaced into crustal gneisses during Alpine collision. Following peak metamorphic amphibolite facies conditions, fluid flow modified some of the trace element contents of ophicarbonates and deformed serpentinites close to the contact with country rocks. The fluid originated from the surrounding continental crustal rocks as documented by the increase of Pb in the serpentinites, and by the strongly negative ∂D values (− 112‰) of some ultramafic rocks close to the contact with surrounding gneisses. Little or no modification of the fluid mobile elements Li, B or U was observed in the serpentinite. In-situ analysis of light elements of serpentinite minerals indicate redistribution of light elements coupled to changes of mineral modes towards the outer 100–150 m of the massif. In the centre of the massif, Li is preferentially concentrated in olivine, while Be and B are hosted by tremolite. In contrast, at the outer rim of the massif, Li and Be are preferentially incorporated into diopside, and B into antigorite. This redistribution of light elements among the different minerals is visible in the serpentinite, at a maximum distance of ~ 100–150 m from the ophicarbonate–metabasite contact. Our results show that interaction of ultramafic rocks and crust-derived fluids can be easily detected by studies of Pb and ∂D in whole rocks. We argue that small ultramafic bodies potentially record an emplacement-related trace element signature, and that crustal light element values in ultramafic rocks are not necessarily derived from a subducting slab.

Mineralogy of volcanic rocks in Gusev Crater, Mars: Reconciling Mössbauer, Alpha Particle X‐Ray Spectrometer, and Miniature Thermal Emission Spectrometer spectra

Complete sets of mineral abundances for relatively unaltered volcanic or volcaniclastic rocks in Gusev Crater have been determined by modeling Mössbauer subspectral areas as mineral weight percentages, and combining those percentages with the proportions of iron‐free minerals not detected by Mössbauer (normative plagioclase, apatite, and chromite, as calculated from Alpha Particle X‐Ray Spectrometer (APXS) chemical analyses). Comparisons of synthetic thermal emission spectra calculated for these mineral modes with measured Miniature Thermal Emission Spectrometer (Mini‐TES) spectra for the same rock classes show either good agreements or discrepancies that we attribute to sodic plagioclase compositions or unmodeled sulfate, glass, or pigeonite. The normative compositions of olivine, pyroxene, and feldspar calculated from APXS data are consistent with spectroscopic constraints on mineral compositions. Systematic variations between olivine abundances in APXS norms (which sample tens of micrometers depth) and olivine proportions measured by Mössbauer (which sample hundreds of micrometers depth) support the hypothesis that dissolution of olivine by acidic fluids has occurred on weathered rock surfaces.

Mössbauer spectroscopy of phyllosilicates: effects of fitting models on recoil-free fractions and redox ratios

AbstractClay minerals are ubiquitous constituents in soils on Earth, are occasionally found in meteorites, and may also occur on planetary surfaces in the presence of water. However, little is known about the fundamental Mössbauer parameters (the intrinsic isomer shift, δI, the characteristic Mössbauer temperature, θM, and the recoil-free fraction, f) that are characteristic of clay minerals and critical to the correct interpretation of the Fe3+/ΣFe ratios as well as the mineral modes. Spectra of well characterized single mineral samples at multiple temperatures may be used for the determinations of f. Hence, measurements of five-layer silicates with a range of layer types are presented here: nontronite, Fe-smectite, glauconite, annite and biotite. The spectra were fitted using three different software packages: WMOSS from Science, Engineering & Education Co. in Minnesota; Recoil, from the University of Ottawa in Canada; and two programs used at the University of Ghent in Belgium. Four different approaches to modelling line shapes were used: (1) Lorentzian; (2) pseudo-Voigt (convolution of Lorentzian and Gaussian curves); (3) quadrupole-splitting distributions (QSD); and (4) a technique that does not assume a particular line shape (subsequently referred to as ‘model-independent’). Values of δI, θM and f were determined using the method of De Grave & Van Alboom (1991).Results show that multiple doublets are routinely required by all models to represent Fe-site occupancy, even when all the Fe atoms of the same valence are in the same site, as is the case for dioctahedral smectite, nontronite, mica and glauconite. Consistent values of centre shift (δ) and quadrupole splitting (Δ) were obtained for the two distributions of M2Fe3+ in the smectites. In glauconite, a single Fe2+ doublet was clearly resolved and gave systematic values for δ, Δ and area, but the two Fe3+ doublets were less defined. In annite, two Fe2+ and two Fe3+ doublets were modelled, while three Fe2+ and one Fe3+ doublet were used for biotite. Three different programs that use Lorentzian line shapes gave very similar results for δ, Δ and area. The two different implementations of QSD line shapes gave similar but sometimes slightly different results, and the pseudo-Voigt and model-independent fits usually fell between the ranges for Lorentzian and QSD results.The value of δI is ~0.58 mm/s for Fe3+ and ~1.31 mm/s for Fe2+ across all models and line shapes, which is expected because the Fe3+ has an additional shielding 3d electron. Values for θM data are nearly identical for Fe3+ in nontronite and Fe-smectite (~450 K), somewhat varied for Fe3+ in glauconite and biotite (θM = ~730 K and ~615 K, respectively), and relatively distinct for Fe2+ (~350 K). Some values for θM and f could not be determined due to the non-monotonic behaviour of the fitted values for δ as a function of temperature. Values of f295 were 0.821–0.917 for Fe3+ and 0.662–0.743 for Fe2+, consistent with previous studies of the recoil-free fraction in micas and other silicates. Calculated scatter in δ, Δ, area and f values as a function of different line shapes and computer software was significantly reduced at lower temperatures. Sources of error in each of the calculated parameters are discussed.

Metamorphosed Ordovician Fe- and Mn-rich rocks in south-central Maine: From peri-Gondwanan deposition through Acadian metamorphism

abstra C t The Wilson Cove Member of the Cushing Formation is a thin (up to 120 m thick) metamorphosed Fe- and Mn-rich unit exposed discontinuously over a distance of >75 km in southern Maine. Cathodoluminescence imaging of zircon grains from the unit reveal texturally isolated detrital cores surrounded by distinct metamorphic overgrowths. U-Pb SHRIMP core ages range from 463 to 2058 Ma with a strong peak in the Neoproterozoic-Early Cambrian. This distribution of ages is consistent with a peri-Gondwanan source region and a Middle to Late Ordovician depositional age. Zircon rims have an age of 373 ± 4 Ma, consistent with growth during late Acadian metamorphism. Whole-rock geochemistry reveals considerable major- and trace-element variability with generally elevated abundances of Fe2O3(t) (15–43 wt%), MnO (0.1–12.1 wt%), Ba (4–2503 ppm), and As (7–1161 ppm). Geochemical discrimination diagrams suggest the protoliths were mixtures of hydrothermal exhalatives and terrigenous clastic sediment, with these materials most likely having been deposited in a marine back-arc basin proximal to a peri-Gondwanan continental source region. Late Devonian low-pressure, amphibolite-facies metamorphism of these bulk compositions produced assemblages dominated by grunerite + garnet + biotite + quartz. These and other mineral assemblages found in the Wilson Cove unit in south-central Maine are consistent with peak metamorphic conditions of 550 to 600 °C and 3–4 kbar previously determined from nearby metapelites. Mineral assemblages, mineral modes, and mineral compositions in these Fe- and Mn-rich rocks are strongly influenced by whole-rock bulk compositional variability. In particular, the compositions of co-existing garnet and grunerite vary systematically as a function of whole-rock MnO concentration.

Decryption of Igneous Rock Textures: Crystal Size Distribution Tools

Research Article| January 01, 2008 Decryption of Igneous Rock Textures: Crystal Size Distribution Tools Pietro Armienti Pietro Armienti Dipartimento di Scienze della Terra, via S. Maria 53, Pisa, Italy, armienti@dst.unipi.it Search for other works by this author on: GSW Google Scholar Author and Article Information Pietro Armienti Dipartimento di Scienze della Terra, via S. Maria 53, Pisa, Italy, armienti@dst.unipi.it Publisher: Mineralogical Society of America First Online: 13 Jul 2017 © The Mineralogical Society Of America Reviews in Mineralogy and Geochemistry (2008) 69 (1): 623–649. https://doi.org/10.2138/rmg.2008.69.16 Article history First Online: 13 Jul 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Pietro Armienti; Decryption of Igneous Rock Textures: Crystal Size Distribution Tools. Reviews in Mineralogy and Geochemistry 2008;; 69 (1): 623–649. doi: https://doi.org/10.2138/rmg.2008.69.16 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyReviews in Mineralogy and Geochemistry Search Advanced Search Abstract In this overview it is planned to show how quantitative determination of mineral modes data, by means of Crystal Size Distribution (CSD), may help in deciphering the kinetics of magmatic crystallization both in natural and experimental conditions. Some stereological considerations will be discussed to provide criteria for the choice of numbers and lengths of size intervals for CSD computations. Equations for the balance of number and size of crystals, in steady state crystallization and in dynamic regime, will be used to define the time frame of magmatic processes, in the context of the case studies of Stromboli and Etna volcanoes.The functional dependence of crystal size (L) and crystal number from time is explicitly derived by interpreting experiments of olivine crystallization that report CSD data, thus obtaining evaluation of thermodynamic parameters from CSD measurements. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Garnet growth at high- and ultra-high pressure conditions and the effect of element fractionation on mineral modes and composition

In this paper we show that thermodynamic forward modelling, using Gibbs energy minimisation with consideration of element fractionation into refractory phases and/or liberated fluids, is able to extract information about the complex physical and chemical evolution of a deeply subducted rock volume. By comparing complex compositional growth zonations in garnets from high-and ultra-high pressure samples with those derived from thermodynamic forward modelling, we yield an insight into the effects of element fractionation on composition and modes of the co-genetic metamorphic phase assemblage. Our results demonstrate that fractionation effects cause discontinuous growth and re-crystallisation of metamorphic minerals in high pressure rocks. Reduced or hindered mineral growth at UHP conditions can control the inclusion and preservation of minerals indicative for UHP metamorphism, such as coesite, thus masking peak pressure conditions reached in subducted rocks. Further, our results demonstrate that fractional garnet crystallisation leads to strong compositional gradients and step-like zonation patterns in garnet, a feature often observed in high-and ultra-high pressure rocks. Thermodynamic forward modelling allows the interpretation of commonly observed garnet growth zonation patterns in terms of garnet forming reactions and the relative timing of garnet growth with respect to the rock's pressure–temperature path. Such a correlation is essential for the determination of tectonic and metamorphic rates in subduction zones as well as for the understanding of trace element signatures in subduction related rocks. It therefore should be commonplace in the investigation of metamorphic processes in subduction zones.

Petrology and geochemistry of a cumulate xenolith suite from Bute: evidence for late Palaeozoic crustal underplating beneath SW Scotland

Xenoliths from Hawk's Nib (Bute, SW Scotland), entrained in alkali basalt of late Carboniferous–early Permian age, generally possess unrecrystallized igneous cumulate textures. Most are inferred to derive from lower crustal (20–30 km) depths, with a small minority from the underlying restitic mantle. Compositions range from ultramafic cumulates (dunite, wehrlite, websterite, clinopyroxenite) to gabbros and anorthosites. The suite is unique in being dominated by poikilitic wehrlites and olivine clinopyroxenites. Major element variation in the cumulates is controlled by modal mineral variations. The ultramafic cumulates all have high REE contents and light REE (LREE)-enriched patterns, with websterites being the least enriched. Gabbroic xenoliths have uniform patterns in which both LREE and heavy REE are depleted relative to the middle REE, with small positive Eu anomalies. Present-day Sr–Nd isotopic data for the suite are the most depleted among Scottish xenoliths. The xenoliths are inferred to be products of young replenishments of the lower crust by basaltic magmas during continental underplating. Orthopyroxene-bearing lithologies crystallized from tholeiitic magmas whereas alkali basalts formed the clinopyroxene-rich samples; both parental magma-types were derived from similar mantle sources.

Petrologia magnética do granito anorogênico Bannach, terreno granitogreenstone de rio Maria, Pará

The Paleoproterozoic Bannach Granite is intrusive in Archean units of the Rio Maria Granite-Greenstone Terrane. The batholith is composed of three group of monzogranites rocks: a coarsegrained amphibole, biotite and sometimes clinopyroxene-bearing facies; a porphyritic facies with biotite; and leucogranitic facies with varied textures. Magnetite, ilmenite, hematite, pyrite, chalcopyrite, sphalerite and goethite, are the opaque oxide minerals observed. Six textural types of ilmenite are distinguished: individual Ilmenite; internal and external composite Ilmenite; sandwich Ilmenite; patch Ilmenite; trellis Ilm. Texture features suggest that titanomagnetite and individual and composite ilmenite crystallized in early magmatic stage. The Ilmenite was destabilized and partially replaced by titanite still in the magmatic stage. During the subsolidus stage,titanomagnetite was transformed by oxidation-exsolution in to intergrowths of almost pure magnetite and ilmenite (sandwich, patch, and trellis ilmenite). Hematization of magnetite and oxidation of the pirite the latter with goethite formation occurred later at lower temperatures. Compared to the average of magnetic susceptibility (MS) for granites, the different facies of the Bannach pluton are characterized by high values of the MS. There is good correlation between MS and magmatic differentiation of the facies forming the batholith. MS decrease from the amphibole+biotite± clinopyroxene-bearing facies to the biotite porphyritic facies, attaining the lowest values in the leucogranites. The modal magnetite contents are high for granites. Opaque minerals, amphibole, total of mafic minerals and biotite modal contents show a positive correlations with MS. Positive correlations among MS and Fe 2 O 3 , FeO, MgO and TiO 2 contents and negative correlations between MS and K 2 O e SiO 2 are consistent with the mentioned relationships between MS and modal mineral content. High MS, significant modal contents of magnetite, the high Fe 2 O 3 /FeO ratios and magmatic assemblage titanite + Mt + quartz demonstrate that the Bannach Granite evolved in relatively oxidizing conditions estimated as situated near those of the NNO buffer.

Petrologia magnética do granito anorogênico Bannach, terreno granitogreenstone de rio Maria, Pará

The Paleoproterozoic Bannach Granite is intrusive in Archean units of the Rio Maria Granite-Greenstone Terrane. The batholith is composed of three group of monzogranites rocks: a coarsegrained amphibole, biotite and sometimes clinopyroxene-bearing facies; a porphyritic facies with biotite; and leucogranitic fácies with varied textures. Magnetite, ilmenite, hematite, pyrite, chalcopyrite, sphalerite and goethite, are the opaque oxide minerals observed. Six textural types of ilmenite are distinguished: individual Ilmenite; internal and external composite Ilmenite; sandwich Ilmenite; patch Ilmenite; trellis Ilm. Texture features suggest that titanomagnetite and individual and composite ilmenite crystallized in early magmatic stage. The Ilmenite was destabilized and partially replaced by titanite still in the magmatic stage. During the subsolidus stage,titanomagnetite was transformed by oxidation-exsolution in to intergrowths of almost pure magnetite and ilmenite (sandwich, patch, and trellis ilmenite). Hematization of magnetite and oxidation of the pirite the latter with goethite formation occurred later at lower temperatures. Compared to the average of magnetic susceptibility (MS) for granites, the different facies of the Bannach pluton are characterized by high values of the MS. There is good correlation between MS and magmatic differentiation of the facies forming the batholith. MS decrease from the amphibole+biotite± clinopyroxene-bearing facies to the biotite porphyritic facies, attaining the lowest values in the leucogranites. The modal magnetite contents are high for granites. Opaque minerals, amphibole, total of mafic minerals and biotite modal contents show a positive correlations with MS. Positive correlations among MS and Fe 2 O 3 , FeO, MgO and TiO 2 contents and negative correlations between MS and K 2 O e SiO 2 are consistent with the mentioned relationships between MS and modal mineral content. High MS, significant modal contents of magnetite, the high Fe 2 O 3 /FeO ratios and magmatic assemblage titanite + Mt + quartz demonstrate that the Bannach Granite evolved in relatively oxidizing conditions estimated as situated near those of the NNO buffer.

An X-Ray Microtomography Study on the Mineral Concentration of Carious Dentine Removed during Cavity Preparation in Deciduous Molars

Dentists use a number of criteria in order to assess when a cavity is caries free, amongst which hardness is probably the most widely used. However, the judgement is subjective. X-ray microtomography (XMT) is a non-destructive microscopic technique that allows in vitro specimens to be scanned, manipulated and then rescanned. In this study, a high-definition XMT scanner was used to determine the mineral distribution of carious dentine in 10 deciduous molars, and the extent of dentine removed by an experienced clinician was investigated. For each tooth, after an initial XMT scan, caries was removed using a steel bur in a slow hand-piece. The tooth was then repositioned and rescanned. Mineral concentrations were calculated from the linear attenuation coefficients assuming the mineral phase to be hydroxyapatite and the organic phase to be collagen. The volume of dentine tissues removed was calculated by subtracting data of the second scan from the first. The results showed that the mean modal mineral concentration for the 10 teeth was 1.42 g · cm^–3 for sound dentine. Because of uncertainty about collagen concentration in carious dentine, the mean modal mineral concentration for the carious dentine had a range of 0.37–0.5 g · cm^–3. It was found that the subjective criteria used by the operator could lead to inconsistency of cavity preparation. The cavities could be overprepared by 8.5–44.3% in volume. However, the overpreparation was not uniform throughout the cavity: residual demineralised dentine could still be detected in the postoperative scan in isolated regions.