Microtextural Features Research Articles

Crystallochemical characterization of the palygorskite and sepiolite from the Allou Kagne deposit, Senegal

AbstractThe Allou Kagne (Senegal) deposit consists of different proportions of palygorskite and sepiolite, and these are associated with small quantities of quartz and X-ray amorphous silica as impurities. No pure palygorskite or sepiolite has been recognized by X-ray diffraction. Textural and microtextural features indicate that fibrous clay minerals of the Allou Kagne deposit were formed by direct precipitation from solution. Crystal-chemistry data obtained by analytical/transmission electron microscopy (AEM/TEM) analyses of isolated fibers show that the chemical composition of the particles varies over a wide range, from a composition corresponding to palygorskite to a composition intermediate between that of sepiolite and palygorskite, but particles with a composition corresponding to sepiolite have not been found. Taking into account the results from selected area electron diffraction and AEM-TEM, fibers of pure palygorskite and sepiolite have been found but it cannot be confirmed that all of the particles analyzed correspond to pure palygorskite or pure sepiolite because both minerals can occur together at the crystallite scale. In addition, the presence of Mg-rich palygorskite and very Al-rich sepiolite can be deduced.It is infrequent in nature that palygorskite and sepiolite appear together because the conditions for simultaneous formation of the two minerals are very restricted. The chemical composition of the solution controls the formation of the Allou Kagne sepiolite and palygorskite. The wide compositional variation appears as a consequence of temporary variations of the chemical composition of the solution.

Metamorphic evolution of migmatites from the deep Famatinian arc crust exposed in Sierras Valle Fértil–La Huerta, San Juan, Argentina

Meta-supracrustal migmatitic packages in the Sierras Valle Fértil–La Huerta of northwestern Argentina occur as wedge- or strip-shaped septa interlayered among mafic to intermediate igneous plutonic rocks. Meta-supracrustal rocks were metamorphosed under granulite-facies conditions during the development of the Famatinian magmatic arc, and are among the structurally deepest rocks exposed within the belt dominated by Ordovician plutonism. Petrographic analysis, mineral chemistry and whole rock geochemistry of granulite-facies migmatites are used to argue that the meta-supracrustal packages comprise a sequence of pelitic to quartzo-feldspathic sedimentary rocks that achieved peak metamorphic P– T conditions of 5.2–7.1 kbar and 770–840 °C. There are no resolvable differences in peak P– T conditions for migmatites separated 70 km along strike of the Sierras Valle Fértil and La Huerta, suggesting that similar levels of the Famatinian paleo-arc crust are currently exposed in these ranges. Idioblastic poikilitic garnets displaying weak to absent chemical zoning profiles developed at or close to the peak metamorphic stage are used in conjunction with petrogenetic grid constraints to interpret the prograde evolution. At the time the supracrustal rocks experienced maximum thermal conditions, they underwent dehydration partial melting. Microtextural features show that felsic melt (leucosome) back reacted with the adjacent coexisting mineral assemblage (mesosome). These observations are interpreted as evidence that the migmatites evolved through a continuous heating–cooling cycle with minor pressure change. This is consistent with the general lack of reaction textures denoting decompression at high temperatures, and with the possibility that in some migmatites retrograde reactions formed staurolite. Collectively, these features indicate that when the migmatites attained peak thermal conditions, the deepest exposed arc crust was about 20–25 km beneath the Ordovician surface. Comparing these results with metamorphic studies elsewhere in the Famatinian arc between 31° and 32°S. indicates that much of the main-arc records primarily prograde P– T trajectories associated with a regional contact metamorphism, but that specific locations in the back-arc, main-arc and accretionary wedge also record post-peak retrogression during crustal exhumation. These differences are attributed to the fact that a collisional orogeny closely followed the cessation of arc magmatism, a collision we infer to be associated with the accretion of a Laurentian terrane to the Gondwana margin. The results of this study therefore provide important insights into the geodynamic context of the formation and closure of the central segment of the Famatinian magmatic arc.

Magma–carbonate interaction: An experimental study on ultrapotassic rocks from Alban Hills (Central Italy)

The Alban Hills ultrapotassic volcanic district is one of the main districts emplaced during Quaternary time along the Tyrrhenian margin of Italy. Alban Hills lava flows and scoria clasts are made up essentially of clinopyroxenes and leucites and their chemical composition is mostly K-foiditic. Differentiated products (MgO < 3 wt.%) are characterised by low SiO 2 concentration (< 50 wt.%) and geochemical features indicate that this unique differentiation trend is driven by crystal fractionation plus carbonate crust interaction. Notably, the Alban Hills Volcanic District was emplaced into thick limestone units. With the aim of constraining the magmatic differentiation, we performed experiments on the Alban Hills parental composition (plagioclase-free phono-tephrite) under anhydrous, hydrous, and hydrous-carbonated conditions. Experiments were carried out at 1 atm, 0.5 GPa and 1 GPa, temperatures ranging from 1050 to 1300 °C, and H 2O and CaCO 3 in the starting material up to 2 and 7 wt.%, respectively. The experiments performed at 0.5 GPa are the most representative of the Alban Hills plumbing system. Clinopyroxene and leucite are the main phases occurring under all the investigated conditions and the liquidus phases. Nevertheless, our experimental results demonstrate that the occurrence of CaCO 3 in the starting material strongly affects phase relations. Experiments performed under hydrous conditions crystallize magnetite and phlogopite at relatively high temperature. This early crystallization drives the glass composition towards a silica enrichment, resulting in a differentiation trend moving from phono-tephritic (Alban Hills parental composition) to phonolitic compositions. This is in contrast with micro-textural evidence showing late crystallization of magnetite and phlogopite in the natural products and with the composition of the juvenile products. On the contrary, in the CaCO 3-bearing experiments (i.e., simulating magma–carbonate interaction) the magnetite and phlogopite stability fields are strongly reduced. As a consequence, the melt differentiation is mainly controlled by the cotectic crystallization of clinopyroxene and leucite, resulting in a differentiation trend moving towards K-foiditic compositions. These experimental results are in agreement with micro-textural features and chemical compositions of Alban Hills natural products and with the magmatic differentiation model inferred by geochemical data. Magma–carbonate interaction is not a rare process and its occurrence has been demonstrated for different plumbing systems. However, the uniqueness of the Alban Hills liquid line of descent suggests that the efficacy of the carbonate contamination process is controlled by different factors, the dynamics of the plumbing system being one of the most important.

Gorceixite from the Upper Cambrian Rocks of the podwiśniówka Mine Pit, Holy Cross Mountains (South-Central Poland)

Gorceixite from the Upper Cambrian Rocks of the podwiśniówka Mine Pit, Holy Cross Mountains (South-Central Poland)This report presents the results of a petrographical, mineralogical (SEM/EDS, XRD) and geochemical (XRF, CV-AAS, ICP-MS) study of gorceixite (barium aluminophosphate) from the abandoned Podwiśniówka mine pit. This site is highlighted by the presence of highly acidic pit pond whose chemistry is strongly affected by the exposed pyrite-bearing zone. The gorceixite occurs in the Upper Cambrian carbonaceous clayey shales, quartzites and tuffs in form of minute accumulations varying from about 0.5 to 100 μm in diameter. These accumulations infill voids, cavities, cracks and partly fissures in the rocks examined. The other minerals of the crandallite series, i.e. florencite and goyazite, can be found only in trace amounts. The gorceixite-bearing rocks, especially carbonaceous clayey shales, are characterized by the highest concentrations of REE reaching 455.09 mg·kg-1. In addition, these rocks are distinctly enriched in light rare earth elements (LREE), with the La/Yb ratio ranging from 24.44 through 36.30. Some of the examined gorceixite accumulations are paragenetically linked to the veined pyrite and nacrite. The latter mineral is indicative of crystallization temperatures of about 200 to 300°C. The coexistence of gorceixite with the veined nacrite or pyrite mineralization and the volcaniclastic rocks, as well as the microtextural features and high concentrations of REE in the gorceixite-bearing parent rocks suggest that this mineral formed as a result of both hydrothermal and volcanic activity in a shallow-marine basin during the late Cambrian.

Origin of the clay mineral assemblages in the Germanic facies of the English Trias: application of the spore colour index method

AbstractThe origin of the regional and stratigraphical variation in the Triassic authigenic clay assemblages of England is discussed in relation to new estimates of the palaeotemperatures experienced by their host sediments and a preliminary study by transmission electron microscopy of their microtextural features. Spore colour index measurements, based on the spore type Deltoidospora s.l. occurring in the sediments (Penarth Group) at the very top of the Triassic sequence, give estimated palaeotemperatures ranging from 60–74°C (south Devon) to 89–97°C (northeast Yorkshire). Calculated palaeotemperatures, based on a gradient of 25°C/km, for the main zone of authigenic clay minerals range from 63–77°C to 89–97°C for the top to 71–85°C to 94–104°C for the base. Irregular mixed-layer smectite-chlorite, corrensite and Mg-rich chlorite are associated with calculated palaeotemperatures of 66–86°C, 66–104°C and 75–104°C respectively. The suggestion that elsewhere in the UK corrensite and Mg-rich chlorite were formed at temperatures in excess of 100°C finds no support. Geothermal gradients would have to have been of the order of at least 100–300°C/km to obtain these temperatures within the Triassic sediments; such values are associated typically with high-level magmatic intrusions or geothermal systems of which there is no geological evidence. The balance of evidence suggests that the Triassic authigenic clay assemblages formed by neoformation during the early stages of sediment diagenesis under the influence of variation in the alkalinity of the depositional environments.

Magmatic and Tectonic Structures from the Chimalpahad Layered Complex, Andhra Pradesh, India

The Chimalpahad Layered Complex (CLC) is the largest (>200 sq km) metamorphosed anorthosite complex in the Precambrian shield of Peninsular India. It exhibits spectacular magmatic layering together with superposed tectonic structures. Modal layering, with primary cumulus texture, is the most prominent magmatic structure displayed by the complex. A variety of magmatic structures (rhythmic mineral-graded layering, cyclic and cross layering, and troughs), practically unaffected (or weakly modified) by later tectonic overprinting, form the relict igneous structures in low strain zones within the CLC; they are described and depicted in sufficient detail, and the possible mechanisms for their formation are enumerated. The structural features in the CLC owe their origin to dual play of magmatic and tectonic processes. The original magmatic structures in high strain zones are variably overprinted by: (1) planar foliation (S1) and mineral lineation (L1) of the D 1 deformation, (2) the F1a and F1b folding event of the D 2 ductile deformation and (3) brittle-ductile fractures/faults of the D 3 deformation that are inferred to have developed during exhumation of the CLC by transpressional tectonism. Micro-textural features of the CLC provide useful criteria for identification of superimposition of solid-state recrystallization on magmatic fabrics. Simultaneous compression and strike-slip shear mechanism (transpression and transtension) have been proposed to account for the magmato-tectonic evolution of the CLC.

Garnet composition as a petrogenetic indicator: An example from a marble -- calc-silicate granulite interface at Kondapalle, Eastern Ghats Belt, India

TheP-T-fluid histories of aluminous garnet+clinopyroxene-bearing calc-silicate granulites are commonly constrained through interpretation of sequential reaction textures based on activity-corrected petrogenetic grids for model systems, for example the widely used CAS(CO2-H2O) topologies. Application of these topologies to calc-silicates of diverse bulk composition is limited by the fact that the role of clinopyroxene and hence of garnet composition in the garnet-forming reactions is ignored or not adequately considered. In this contribution the complexities that arise from the involvement of clinopyroxene in the garnet-forming reactions are critically evaluated. As an example, the paragenetic relations and compositional features of clinopyroxene and coronitic garnet across paired scapolite- and wollastonite-dominated reaction zones (scp+cpx±qtz,cal,qtz — wo+cal+cpx±scp) developed between marble layers (cal±cpx,wo,scp) and their host calc-silicate granulite (cpx+pl±qtz,scp) at an exposure in the high-grade Eastern Ghats Belt (India) are examined. The microtextural features and systematic changes of modal and phase compositions across the profile indicate that clinopyroxene, scapolite, plagioclase, quartz and calcite as well as wollastonite participated in the garnet-forming reactions. Using the sub-assemblage qtz+cal+cpx+scp+grt+v in the system CaO-FeO-Al2O3-SiO2-CO2 it is demonstrated that at the ambient P-T-Xfluid conditions, the reaction coefficients of the participating phases are uniquely defined by the garnet composition. As a result, the P-T slope of a garnet-forming reaction changes with the garnet composition, with sharp changes at “singular points” across which one or more phases change their sides with respect to garnet in a given reaction. To consider the consequences of these complexities on the reaction topology of calc-silicate granulites, a new petrogenetic grid for the system CaO-FeO-Al2O3-SiO2-CO2 involving clinopyroxene has been constructed. The effects of fluid composition (CO2-H2O-O2), fluid pressure (Pfluid

Precise eclogitization ages deduced from Rb/Sr mineral systematics: The Maksyutov complex, Southern Urals, Russia

The Maksyutov complex (Southern Urals, Russia) is a well-preserved example of subduction-related high-pressure metamorphism. One of its two litho-tectonic units consists of rocks that experienced eclogite-facies conditions. Published 40Ar/39Ar data on phengite, U/Pb data on rutile, and Sm/Nd mineral data define a cluster of ages around 370 to 380 Ma. Nevertheless, no consensus exists as to the detailed interpretation of data and the exact age of eclogitization. We present new, high-precision internal mineral Rb/Sr isochrons for eclogite-facies metabasites, felsic eclogites, and eclogite-facies quartz veins. Nine isochrons, mainly controlled by omphacite and white mica phases, give concordant ages with an average value of 375 ± 2 Ma (2σ). Microtextural features, such as prograde growth zoning in eclogite-facies phases, suggest that the assemblages dated formed at a stage of prograde metamorphism. Sr-isotopic equilibria among eclogite-facies phases, and among eclogite-facies fluid veins and the host rocks, indicate that our ages reflect crystallization ages, related to the prograde-metamorphic, probably fluid-mediated eclogitization reactions. This interpretation is reinforced by data from fluid-precipitated quartzitic eclogites, whose modal composition, together with intergrowth relationships, conclusively imply closed-system behavior after crystallization. The possible occurrence of a pre-375 Ma event of ultra-high-pressure metamorphism (UHPM) in the Maksyutov complex is disproved by isotope systematics, microtextures, and mineral zoning patterns.

Primary trapped melt inclusions in olivine in the olivine-augite-orthopyroxene ureilite Hughes 009

We describe the first known occurrence of primary melt inclusions in a ureilite. The ureilite is Hughes 009, one of a small number of ureilites whose primary mineralogy is olivine-augite-orthopyroxene, rather than olivine-pigeonite. Hughes 009 has a coarse-grained, equilibrated texture typical of ureilites, and homogeneous primary mineral compositions: olivine — mg 87.3; augite — mg 89.2, Wo 37.0, Al 2O 3 = 1.6 wt.%; orthopyroxene — mg 88.3, Wo 4.9. It shows only limited secondary reduction effects and no petrographically recognizable carbon phases, which indicates that its original carbon content was lower than in most ureilites. The melt inclusions occur in olivine crystals. They are concentrated in the central regions of their hosts, showing elongate (mostly 20–60 μm in maximum dimension), negative olivine crystal shapes and parallel alignment. These and other features indicate that they were trapped during initial growth of their hosts from a liquid, and are likely to be representative samples of that liquid. They consist of glass and single, subhedral crystals of high-Ca pyroxene, with minor Cr-rich spinel and metal-phosphide-sulfide spherules. They are surrounded by halos of olivine with rounded outlines defined by tiny bits of metal and thin arcs of glass. Pyroxenes within each inclusion show zonation patterns indicating that they nucleated at the olivine/ liquid interface with compositions close to that of the primary augite, and then grew inward with dramatically increasing Al 2O 3 (to 10.8 wt.%), Wo (to ≈50), TiO 2 and Cr 2O 3 contents. Glasses within each inclusion are relatively homogeneous. Glasses from all inclusions show well-defined trends of CaO, TiO 2, Cr 2O 3 , Na 2O and SiO 2 vs. Al 2O 3, (16–23 wt.%) that can be modelled as resulting principally from crystallization of various amounts of the pyroxene. The halos, which represent olivine that grew from the trapped melts, are zoned in Cr and Ca with concentrations decreasing inward, reflecting cocrystallization of pyroxene; they have homogeneous Fe/Mg identical to that of the primary olivine, indicating reequilibration with the host. We develop a petrologic model for the postentrapment history (crystallization, reaction and reequilibration) of the inclusions, based on which we reconstruct the composition of the primary trapped liquid (PTL). The PTL was saturated only with olivine. This result implies that Hughes 009 is a cumulate (consistent with the high Mn/Mg ratio of its olivine and a low abundance of graphite) and that the composition of the PTL is close to that of its parent magma. The low-pressure equilibrium crystallization sequence predicted by MAGPOX calculations for the PTL (olivine → augite → plagioclase → pigeonite) is not, however, consistent with the primary mineralogy of Hughes 009. If the conditions of these calculations are, indeed, appropriate, then complex processes such as magma mixing must have been involved in the petrogenesis of this ureilite. This conclusion is consistent with other evidence that the olivine-augite-orthopyroxene ureilites record a more complex magmatic evolution than is evident in the olivine-pigeonite ureilites. TEM investigations of microtextural features in all phases and XRD determination of Fe 2+-Mg site distribution in orthopyroxene have elucidated the cooling and shock history of this ureilite. Hughes 009 experienced an extremely high cooling rate (7 ± 5°C/h at the closure T of 630°C) late in its evolution, and two distinguishable shock events—the first at peak pressures of 5 to 10 GPa, resulting in mechanical polysynthetic twinning in augite and orthopyroxene and mild undulatory extinction in olivine; and the second at lower pressures, resulting only in brecciation and redistribution of metal. Its late history is similar to that of most ureilites, and probably reflects impact excavation.

Textures, water content and degassing of silicic andesites from recent plinian and dome-forming eruptions at Mount Pelée volcano (Martinique, Lesser Antilles arc)

Previous petrological and phase-equilibrium experimental studies on recent silicic andesites from Mount Pelée volcano have evidenced comparable pre-eruptive conditions for plinian and dome-forming (pelean herein) eruptions, implying that differences in eruptive style must be primarily controlled by differences in degassing behaviour of the Mount Pelée magmas during eruption. To further investigate the degassing conditions of plinian and pelean magmas of Mount Pelée, we study here the most recent Mount Pelée's products (P1 at 650 years B.P., 1902, and 1929 eruptions, which cover a range of plinian and pelean lithologies) for bulk-rock vesicularities, glass water contents (glass inclusions in phenocrysts and matrix glasses) and microtextures. Water contents of glass inclusions are scattered in the plinian pumices but on average compare with the experimentally-deduced pre-eruptive melt water content (i.e., 5.3–6.3 wt.%), whereas they are much lower in the dominant pelean lithologies (crystalline, poorly vesicular lithics and dome samples). This indicates that the glass inclusions of the pelean products have undergone strong leakage and do not represent pre-eruptive water contents. The water content of the pyroclast matrix glasses are thought to closely represent the residual water content in the melt at the time of fragmentation. Determination of the water contents of both the pre-eruptive melt and matrix glasses allows the estimation of the amount of water exsolved upon syn-eruptive degassing. We find the amount of water exsolved during the eruptive process to be higher in the pelean products than in the plinian ones, typically 90–100 and 65–70% of the initial water content, respectively. The vesicularities calculated from the amount of exsolved water compare with the measured vesicularities for the plinian pumices, consistent with a closed-system, near-equilibrium degassing up to fragmentation. By contrast, the low residual water contents, low groundmass vesicularities and extensive groundmass crystallization of the pelean products are direct evidence of open-system degassing. Microtextural features, including silica-bearing and silica-free voids in the pelean lithologies may represent a two-stage vesiculation.

Petrology and Geochronology of Eclogites from the Lanterman Range, Antarctica

The mafic eclogites of the Lanterman Range are the first record of a well-preserved high-pressure assemblage from the Pacific end of the Transantarctic Mountains. They occur among pods and lenses (from <1 to 30 m in size) of mafic and ultramafic metamorphic rocks that constitute a narrow zone intercalated with amphibolite-facies metasediments. This zone extends along the faulted contact between the Wilson Terrane and the Bowers Terrane, in northern Victoria Land. Most of the amphibolites and retrogressed eclogites analysed have geochemical compositions that resemble transitional to E-type mid-ocean ridge basalt (MORB) and Nd isotope data indicative of differentiation from a depleted mantle source. The age of the igneous precursors is not well determined, but Sm–Nd whole-rock data indicate a Neoproterozoic age, most probably around 700–750 Ma. For this group of metabasites a tectonic setting of an incipient ocean basin is proposed on geological and geochemical grounds. In contrast, the well-preserved eclogites are characterized by strong enrichment in more incompatible elements and pronounced negative Ta and Nb anomalies in MORB-normalized element patterns. Geochemical and Nd isotope data suggest that they are derived from a different mantle source with an enriched signature. The age of the protolith of the well-preserved eclogites, however, remains unconstrained. In the well-preserved eclogite samples the reaction textures testify to three main metamorphic states: (1) an eclogite facies stage, (2) a medium-pressure amphibolite facies stage, and (3) a low-pressure amphibolite facies stage. The high-pressure event occurred at temperatures of up to 850°C based on garnet and omphacite thermometry and at a minimum pressure of 15 kbar based on the jadeitic content of omphacites. Internal Sm–Nd isochrons from two well-preserved eclogites are 500±5 Ma (rutile, clinopyroxene, amphibole, whole rock and garnet) and 492±3 Ma (rutile, clinopyroxene, whole rock and garnet). Rutile–whole-rock 238U-206Pb ages ( 500 Ma) overlap the range of the Sm–Nd mineral ages. The inferred P–T path, the microtextural features and the overlap of the Sm–Nd garnet ages with the range of the 238U-206Pb rutile–whole-rock ages indicate fast cooling and suggest that the time of the high-pressure event was 500 Ma. These data place both the formation and exhumation of eclogite within a convergent plate margin setting, thus documenting the subduction–accretional nature of the early Palaeozoic Ross Orogen in northern Victoria Land.

Petrology and Geochronology of Eclogites from the Lanterman Range, Antarctica

The mafic eclogites of the Lanterman Range are the first record of mineral ages. The inferred P–T path, the microtextural features a well-preserved high-pressure assemblage from the Pacific end of and the overlap of the Sm–Nd garnet ages with the range of the the Transantarctic Mountains. They occur among pods and lenses U–Pb rutile–whole-rock ages indicate fast cooling and suggest (from <1 to ~30 m in size) of mafic and ultramafic metamorphic that the time of the high-pressure event was ~500 Ma. These data rocks that constitute a narrow zone intercalated with amphiboliteplace both the formation and exhumation of eclogite within a facies metasediments. This zone extends along the faulted contact convergent plate margin setting, thus documenting the subduction– between the Wilson Terrane and the Bowers Terrane, in northern accretional nature of the early Palaeozoic Ross Orogen in northern Victoria Land. Most of the amphibolites and retrogressed eclogites Victoria Land. analysed have geochemical compositions that resemble transitional to E-type mid-ocean ridge basalt (MORB) and Nd isotope data indicative of differentiation from a depleted mantle source. The age of the igneous precursors is not well determined, but Sm–Nd whole

Scanning electron microscope study of zircons and rutiles from a podzol chronosequence at Cooloola, Queensland, Australia

Summary A chronosequence of four soils derived from aeolian deposited sand at Cooloola, along the eastern coast of southern Queensland, has been studied to document predepositional and pedogenic changes in zircon and rutile grains in the fine sand fraction (53–125μm) with increasing soil age. The soils range in age from Holocene (Typic Quartzipsamment — presently drifting sand) to mid Pleistocene (Spodic Quartzipsamment — 90 ka BP). Parent materials are moderately stratified sands with irregular depositional layers, in places having high concentration of heavy mineral grains. Scanning electron microscopy was used to characterize grain surface microtexture by a semi-quantitative method based on frequency of occurrence of ten classes of surface microtextures. Pedogenic changes were quantified and differences between horizons in the same profile, between different profiles within the same horizon, and between minerals were tested for significance using the χ2-distribution. Zircon grains are less chemically weathered during pedogenesis than rutile possibly due to the higher iron content of the latter. However, zircons tend to show more features which can be associated with physical alteration. Rutile grains in the illuviated B2hs horizons of the Troporthod and Spodic Quartzipsamments are more thickly coated with poorly crystalline iron and alumino-silicate compounds compared with zircon grains, and the presence of grain coatings does not seem to have prevented the surfaces of grains from developing solution pits. With increasing age there is a preponderance of “weathering” features on grains of both minerals. The presence of various shapes and surface microtextures on grains from the different soils suggests a multiple grain source for zircon and rutile, and a complex predepositional history for these minerals. Nevertheless, microtextural features of zircons and rutiles are valuable for the interpretation of the extent and nature of post depositional weathering.

Chemical diagenesis of carbonates in thin-sections: Ion microprobe as a trace element tool

Ion microprobe is a potential tool for determination of trace-element concentrations directly in thin-sections. Such an approach enables direct correlation of chemistry with microtextural features. We demonstrate the utility of this instrument for Sr, Mn, Fe and Mg in calcites and dolomites. At this stage, the sensitivity is ~10 ppm and the reproducibility is usually better than 15 relative percent. The analyzed spots are ~20 μ. We believe that with more homogeneous standards at low concentrtion levels, the technique is potentially capable of sensitivity in the ppb-ppm ranges. Fe may prove an exeption from this prediction. The technique may also prove suitable for determination of Na. Ion microprobe can serve as a valuable tool for diagenetic and petrological studies of carbonates. At this stage, its major disadvantage is the poor optical resolution in transmitted light.