Mineral Modes Research Articles

Boron isotopic constraints on the Nb and Ta mineralization of the syenitic dikes in the ~ 260 Ma Emeishan large igneous province (SW China)

Both Nb–Ta-mineralized and Nb–Ta-poor syenitic dikes in the Panxi region (SW China) are spatially and temporally associated with syenitic plutons, which are part of the ~260Ma Emeishan large igneous province. These syenitic dikes are NW-striking, and have width varying from 1 to 5m and length from 50 to 300m. The dikes are mainly composed of K-feldspar, albite, aegirine and arfvedsonite, however, mineral modes are different in the Nb–Ta-mineralized and Nb–Ta-poor syenitic dikes. The major Nb–Ta-bearing mineral in the dikes is pyrochlore, which is closely associated with albite and occurs in places with intensive albitization. Rocks of the Nb–Ta-mineralized syenitic dikes contain more albite and less K-feldspar than the Nb–Ta-poor dikes, and have compositions more evolved than the Nb–Ta-poor dikes, indicating that the Nb–Ta-mineralized syenitic dikes formed from a highly evolved magma. We analyzed the B concentrations and B isotopic compositions of the samples of both Nb–Ta-mineralized and Nb–Ta-poor syenitic dikes and associated syenitic pluton using a single column purification method and ICP-AES and MC-ICP-MS techniques. The samples of the Nb–Ta-mineralized syenitic dikes have whole-rock B concentrations ranging from 11.4 to 23.9ppm and δ11B values from −17.95 to −14.54‰, whereas the samples of the Nb–Ta-poor dikes and syenitic plutons have B concentrations varying from 3.32 to 16.5ppm and δ11B values from −13.45 to −10.02‰. The high B concentration of the Nb–Ta-mineralized dikes relative to the Nb–Ta-poor dikes is consistent with that B is incompatible and tends to be rich in more evolved magma. The relatively low δ11B values of the Nb–Ta-mineralized dikes indicate that the B isotopes may have fractionated between fluids and rocks in a transitional, magmatic–hydrothermal stage. We propose that the highly evolved magmas in a transitional, magmatic–hydrothermal stage may have Nb– and Ta–fluorine complexes dissolved in the hydrothermal fluids in the presence of Na+. Albite crystallization due to intensive albitization in this stage resulted in the decrease of Na+ in the fluids, decomposing the Nb– and Ta–fluorine complexes. The released Nb and Ta from the complexes were then dissolved in the fluids and finally entered the lattice of pyrochlore crystals in the stage of albitization.

Toxic elements (As, Se, Cd, Hg, Pb) and their mineral and technogenic formations in the snow cover in the vicinity of the industrial enterprises of Tomsk

Snow samples were collected in four industrial areas of Tomsk where brickworks, factories for the production of reinforced concrete structures, machine repair industries and local boilers, petrochemical plant and thermal power station are located. Study of insoluble fraction of aerosols in snow and melted snow water was performed to determine the contents of the emissions from these facilities. The insoluble fraction of aerosols in snow is aerosol particles deposited on snow cover. As, Se, Cd, Hg, Pb concentration was analyzed by inductively coupled plasma mass spectrometry (ICP-MS). Mineral modes of the elements were determined by scanning electron microscope. It was found the snow cover is mainly polluted by As, Se, Cd, Hg, Pb in the thermal power station impact area, by As – in the brickworks impact area, by Se – in the impact area of densely located factories for the production of reinforced concrete structures, machine repair industries and local boilers. The research results show that the mineral modes of As are associated with arsenopyrite, of Pb – with galena in the insoluble fraction of aerosols in snow.

Partial melting control of water contents in the Cenozoic lithospheric mantle of the Cathaysia block of South China

Major and trace element and the H2O contents of minerals in peridotite xenoliths hosted by the Cenozoic basalts of Jiande in the Cathaysia block were evaluated using electron microprobe, laser-ablation ICP-MS and Fourier transform infrared spectroscopy, respectively. The correlations among the major elements of the minerals define a melting trend, and modeling of the Y and Yb contents in the clinopyroxenes indicates that the degree of partial melting ranges from 1% to 15%. Most samples (22 out of 29) show depleted chondrite-normalized rare earth element patterns and a degree of partial melting <4%. The H2O contents (weight in ppm) of the clinopyroxenes, orthopyroxenes and olivines are 390–590ppm, 160–330ppm and ∼0ppm, respectively. Although potential H-loss during xenolith ascent cannot be excluded for olivines, pyroxenes largely preserve the H2O content they have in the mantle prior to sampling by the host basalts, as inferred from (1) the homogenous H2O content within single pyroxene grains, and (2) the equilibrium H2O partitioning between the clinopyroxene and orthopyroxene. Based on the mineral modes and assuming a partition coefficient of 10 for H2O between the clinopyroxene and olivine, the calculated whole-rock H2O contents range from 90 to 220ppm, similar to that of the MORB source. When combined with previously reported data for peridotites hosted by Cenozoic basalts at other localities of the Cathaysia block, the correlation of H2O content with melting index (such as Yb content in cpx, Cr# in spinel) suggests that extent of partial melting is the main factor controlling H2O abundance in these rocks. Nevertheless, the variations in whole-rock H2O contents cannot be perfectly modeled as a simple modal melting process using available experimental partition coefficients of H2O between peridotite and melt. The lack of coherent variations between the H2O contents of the whole rocks and the metasomatic index (La/Yb ratio in clinopyroxene) indicates that mantle metasomatism did not modify the initial H2O contents after the melting event(s). Based on the similarities in major and trace elements, and of H2O contents in the Cathaysia block peridotites and those inferred for the MORB source, we propose that the Cenozoic lithospheric mantle of the Cathaysia block is accreted from the upwelled and cooled asthenospheric mantle. In addition, the H2O contents of the Cenozoic lithospheric mantle of the Cathaysia block are much higher than those of the North China Craton of similar fertility (100ppm vs. 20ppm for whole rock H2O contents), where lithospheric thinning occurred during the Mesozoic. This implies that either the Cathaysia block did not undergo similar lithospheric thinning or that the mechanism of the lithospheric thinning was different.

Determining P–T paths from garnet zoning using a brute-force computational method

Reconstructing metamorphic pressure–temperature (P–T) paths through petrographic textures, mineral assemblages, and compositions is fundamental to the understanding of orogenesis. We present a new method to derive the P–T path of a rock from mineral zoning. Using Perple_X (Connolly in Earth Planet Sci Lett 236(1–2):524–541, 2005) and a Matlab script, we forward model garnet zoning, parageneses, and mineral modes for an arbitrary set of possible P–T paths, and then compare the calculated results to mineral modes, textures, and zoning preserved in rocks. The best-fit P–T path is determined by comparing the calculated zoning to the measured zoning using the sum of squares. The results indicate that combining Mg#, Ca, and Mn zoning in garnet with mineral modes can yield precise constraints on the P–T path of a rock.

Pyroxenes as tracers of mantle water variations

AbstractThe concentration and distribution of volatiles in the Earth's mantle influence properties such as melting temperature, conductivity, and viscosity. To constrain upper mantle water content, concentrations of H2O, P, and F were measured in olivine, orthopyroxene, and clinopyroxene in mantle peridotites by secondary ion mass spectrometry. Analyzed peridotites are xenoliths (Pali Aike, Spitsbergen, Samoa), orogenic peridotites (Josephine Peridotite), and abyssal peridotites (Gakkel Ridge, Southwest Indian Ridge, Tonga Trench). The comparison of fresh and altered peridotites demonstrates that low to moderate levels of alteration do not affect H2O concentrations, in agreement with mineral diffusion data. Olivines have diffusively lost water during emplacement, as demonstrated by disequilibrium between olivine and coexisting pyroxenes. In contrast, clinopyroxene and orthopyroxene preserve their high‐temperature water contents, and their partitioning agrees with published experiments and other xenoliths. Hence, olivine water concentrations can be determined from pyroxene concentrations using mineral‐mineral partition coefficients. Clinopyroxenes have 60–670 ppm H2O, while orthopyroxenes have 10–300 ppm, which gives calculated olivine concentrations of 8–34 ppm. The highest olivine water concentration translates to an effective viscosity of 6 × 1019 Pa s at 1250°C and ~15 km depth, compared to a dry effective viscosity of 2.5 × 1021 Pa s. Bulk rock water concentrations, calculated using mineral modes, are 20–220 ppm and correlate with peridotite indices of melt depletion. However, trace element melt modeling indicates that peridotites have too much water relative to their rare earth element concentrations, which may be explained by late‐stage melt addition, during which only hydrogen diffuses fast enough for reequilibration.

The Lac Des Iles Palladium Deposit, Ontario, Canada part I. The effect of variable alteration on the Offset Zone

The recently discovered Offset Zone of the Mine Block Intrusion of the Lac des Iles Complex hosts palladium mineralization with unusually high Pd/Pt and Pd/Ir ratios in rocks that range from relatively unaltered norite to amphibolites and chlorite–actinolite–talc schist. Quantitative assessment of the effect of progressive alteration using mineral modes was done using total silicate H2O as a monitor of reaction progress (ξ = moles H2O added to form alteration minerals per 100 g of rock). Major mineral modal variations define three reaction regions: (1) ξ = 0.00–0.03, characterized by epidote/clinozoisite formation and some amphibole; (2) ξ = 0.03–0.23, characterized by formation of chlorite, amphibole, quartz muscovite/sericite, and calcite after plagioclase + pyroxene; and (3) ξ = 0.23–0.28, characterized by the formation of talc after earlier formed amphibole. Epidote occurs as an incongruent product from the destruction of plagioclase that is itself lost as the reaction proceeds. Pyroxene is altered at about twice the rate of plagioclase, resulting in pyroxene-rich protoliths to be more altered than those relatively enriched in plagioclase. Major elements variations largely reflect variations in the plagioclase/pyroxene ratio of the protolith, but compositional trends suggest a loss of Na with reaction progress. The base metal sulfides chalcopyrite, pyrrhotite, and pentlandite show decreasing abundance with reaction progress, forming pyrite (± magnetite) as an intermediate reaction product that also is lost as the reaction proceeds. Millerite is overall low but increases slightly. A more limited data set on the platinum-group minerals suggests that platinum-group element (PGE)-arsenides increase whereas PGE-sulfides and PGE-Bi-tellurides decrease with reaction progress. Assuming ore element concentrations in the protolith were constant and similar to relatively fresh norites, Pd increases modestly, by 5 %, whereas Pt decreases by about 65 % in the most altered rocks. Similarly, Cu, Au, and S decrease by 60, 82, and 94 %, respectively, in the most altered rocks. The antithetical behavior of Pd and the fact that Pd enrichment is not seen in altered dikes suggest that the Pd originally was enriched in the more melanocratic protoliths that are most extensively altered and that Pd also was lost in part. These results are consistent with the mineralization having been a high-temperature event that predated the amphibolite/greenschist alteration.

Komsomolskaya diamondiferous eclogites: evidence for oceanic crustal protoliths

The Komsomolskaya kimberlite is one of numerous (>1,000) kimberlite pipes that host eclogite xenoliths on the Siberian craton. Eclogite xenoliths from the adjacent Udachnaya kimberlite pipe have previously been geochemically well characterized; however, data from surrounding diamond-bearing kimberlite pipes from the center of the craton are relatively sparse. Here, we report major- and trace-element data, as well as oxygen isotope systematics, for mineral separates of diamondiferous eclogite xenoliths from the Komsomolskaya kimberlite, suggesting two distinct subgroups of a metamorphosed, subducted oceanic crustal protolith. Using almandine contents, this suite can be divided into two subgroups: group B1, with a high almandine component (>20 mol%) and group B2, with a low almandine component (<20 mol%). Reconstructed REE profiles for B1 eclogites overlap with typical oceanic basalts and lack distinct Eu anomalies. In addition, elevated oxygen isotope values, which are interpreted to reflect isotopic exchange with seawater at low temperatures (<350 °C), are consistent with an upper-oceanic crustal protolith. Reconstructed REE profiles for B2 eclogites are consistent with oceanic gabbros and display distinct Eu anomalies, suggesting a plagioclase-rich cumulate protolith. In contrast to B1, B2 eclogites do not display elevated oxygen isotope values, suggesting an origin deep within the crustal pile, where little-to-no interaction with hydrothermal fluids has occurred. Major-element systematics were reconstructed based on mineral modes; group B1 eclogites have higher MgO wt% and lower SiO2 wt%, with respect to typical oceanic basalts, reflecting a partial melting event during slab subduction. Calculated residues from batch partial melt modeling of a range of Precambrian basalts overlap with group B1 trace-element chemistry. When taken together with the respective partial melt trajectories, these melting events are clearly linked to the formation of Tonalite–Trondhjemite–Granodiorite (TTG) complexes. As a result, we propose that many, if not all, diamondiferous eclogite xenoliths from Komsomolskaya represent mantle ‘restites’ that preserve chemical signatures of Precambrian oceanic crust.

Цеолитный наполнитель, активированный в электромагнитных полях, для производства фанеры

In the course of this work the reduction of emissions of free formaldehyde from the finished plywood is identified, as well as an increase in shear strength to the adhesive layer. Obtained effects in the manufacture of plywood are due to the use of adhesive compositions using natural zeolite sorbent filler – clinoptilolite, previously activated in various electromagnetic fields (fields of over high frequency and weak pulsed magnetic fields). The optimal processing modes of minerals with electromagnetic fields are defined, providing an optimum ratio of values ​​of free formaldehyde emission levels and shear strength to the adhesive layer. The results indicate the possibility of industrial use of the developed techniques.

Hydrogeochemical Relationships between Spring and Subsurface Waters in the Dindefello Area of South Eastern Senegal

The analytical results of water samples collected from the Dindefello Plain area revealed the dominant major ions of bicarbonate, silica, calcium, magnesium and sodium, in order of decreasing relative abundance. Spring water exhibited a very low, i.e. similar to rainwater, mineralization status, unlike groundwater. To better understand the origin of mineralization, the data were further analyzed for interrelationships between parameters and for mineral-water relationships using speciation calculations. This provided various tenuous lines of evidence for speculating various modes of groundwater mineralization. Hydrochemical evaluation of major ions suggested an early stage of mineralization which could be attributed to mineral breakdown driven by pH in the vadose zone under the leaching action of H+. Conceivably the soil in recharge areas supplies CO2 to infiltrating rainwater, thus adding to the amount of aqueous CO2 (H2CO3) already entrapped from the atmosphere. Then, H+ ions, produced from the dissociation of aqueous CO2 ( ), reacts along groundwater flow paths with the carbonaceous-sulfated and silicated minerals, resulting in the observed major ion chemistry. The spatial variations in the groundwater chemistry indicated that the type of bedrock aquifer has an effect on the water chemistry.

Revisiting the late Permian coal from the Huayingshan, Sichuan, southwestern China: Enrichment and occurrence modes of minerals and trace elements

The Late Permian coals from the Huayingshan Coalfield of southwestern China are significantly enriched in Zr (695μg/g), Nb (75.9μg/g), Se (6.99μg/g), Hf (10.1μg/g), and rare earth elements and Y (1423μg/g). Previous studies showed that the sediment-source region for these coals was the Kangdian Upland, which was formed at an early stage of the late Permian Period. The source rocks have a basalt composition, and those studies attributed the enrichment of the above high field strength elements (HFSEs) to derivation from the Kangdian Upland.Geochemical and mineralogical data presented in this study show that the dominant sediment-source regions for the coal and roof strata of the Huayingshan Coalfields are the Dabashan Uplift, Hannan Upland, and Leshan–Longnvsi Uplift. The highly-elevated concentrations of HFSEs in the coals are due to hydrothermal fluids. Three tonstein layers derived from alkali rhyolite were identified. These tonsteins are characterized by highly-enriched HFSEs and by strong negative Eu anomalies in the rare earth element distribution patterns.The major carriers of the rare earth elements in the coal are rhabdophane and silicorhabdophane, the latter of which is also enriched in Zr. Zirconium, however, mainly occurs in zircon. Rhabdophane and silicorhabdophane in the coal are mainly distributed along the bedding planes and occur as cell-fillings. Zircon in the coal occurs as cell-fillings and is of authigenic origin. Anatase in the partings and coals contains Nb, and occurs as fracture-filling and colloidal forms. The modes of occurrence of the above minerals indicate that they were derived from hydrothermal fluids. Mercury and Se mainly occur in sulfide minerals (pyrite and marcasite).

Petrology and geochemistry of the Tertiary Suez rift volcanism, Sinai, Egypt

The Tertiary basaltic rocks of Southwestern Sinai, situated along the Wadi Nukhul–Wadi Matullah–Wadi El Tayiba join, were selected to study the Gulf of Suez rift related-lavas and their geochemical and petrological relation with the rifting process. Whole rock samples were studied petrographically and analysed for major and trace elements. The samples from dykes, sills and flows from multiple magmatic events display a large variety in texture and in modal mineral compositions. They range from olivine dolerites and olivine-bearing basalts to vitrophyric, texturally heterogeneous basalts and crystal lithic tuffs. The transitional tholeiitic basalts display low compatible element concentrations and an enrichment of the whole spectrum of the incompatible elements. Major, trace and Rare Earth Element data suggest that the melts formed by 5% melting of mantle peridotite at the spinel–garnet transition zone (80–90km depth), in the presence of 2–4% residual garnet. During the melt ascent, the fractionating phases were olivine, clinopyroxene and, to a lesser extent, plagioclase. Thermobarometric calculations indicate the presence of two crystallization levels beneath the Gulf of Suez rift: a shallower stage at 15–20km and a deeper stage at depths of 25–30km. The mantle source consists of streaks and blobs of enriched mantle, preserved in the geochemical signatures of these rocks. The enriched mantle sources melted preferentially compared to the surrounding ambient mantle and thus led to a preferential enrichment of the sources of the Gulf of Suez rift.

REE and Hf distribution among mineral phases in the CV–CK clan: A way to explain present-day Hf isotopic variations in chondrites

Chondrites are among the most primitive objects in the Solar System and constitute the main building blocks of telluric planets. Among the radiochronometers currently used for dating geological events, Sm–Nd and Lu–Hf are both composed of refractory, lithophile element. They are thought to behave similarly as the parent elements (Sm and Lu) are generally less incompatible than the daughter elements (Nd and Hf) during geological processes. As such, their respective average isotopic compositions for the solar system should be well defined by the average of chondrites, called Chondritic Uniform Reservoir (CHUR). However, while the Sm–Nd isotopic system shows an actual spread of less than 4% in the average chondritic record, the Lu–Hf system shows a larger variation range of 28% [Bouvier A., Vervoort J. D. and Patchett P. J. (2008) The Lu–Hf and Sm–Nd isotopic composition of CHUR: Constraints from unequilibrated chondrites and implications for the bulk composition of terrestrial planets. Earth Planet. Sci. Lett.273, 48–57]. To better understand the contrast between Sm–Nd and Lu–Hf systems, the REE and Hf distribution among mineral phases during metamorphism of Karoonda (CK) and Vigarano-type (CV) carbonaceous chondrites has been examined. Mineral modes were determined from elemental mapping on a set of five CK chondrites (from types 3–6) and one CV3 chondrite. Trace-element patterns are obtained for the first time in all the chondrite-forming minerals of a given class (CK chondrites) as well as one CV3 sample. This study reveals that REE are distributed among both phosphates and silicates. Only 30–50% of Sm and Nd are stored in phosphates (at least in chondrites types 3–5); as such, they are not mobilized during early stages of metamorphism. The remaining fraction of Sm and Nd is distributed among the same mineral phases; these elements are therefore not decoupled during metamorphism. Of the whole-rock total of Lu, the fraction held in phosphate decreases significantly as the degree of metamorphism increases (30% for types 3 and 4, less than 5% in type 6). In contrast to Lu, Hf is mainly hosted by silicates with little contribution from phosphates throughout the CK metamorphic sequence. A significant part of Sm and Nd are stored in phosphates in types 3–5, and these elements behave similarly during CK chondrite metamorphism. That explains the robustness of the Sm/Nd ratios in chondrites through metamorphism, and the slight discrepancies observed in the present-day isotopic Nd values in chondrites. On the contrary, Lu and Hf are borne by several different minerals and consequently they are redistributed during metamorphism–induced recrystallization. The Lu/Hf ratios are therefore significantly disturbed during chondrites metamorphism, leading to the high discrepancies observed in present-day Hf isotopic values in chondrites.

Metasomatism in the Ultrahigh-pressure Svartberget Garnet-peridotite (Western Gneiss Region, Norway): Implications for the Transport of Crust-derived Fluids within the Mantle

Garnet-peridotites often contain veins or layers of pyroxenite and eclogite of uncertain origin. We investigate the Svartberget garnet-peridotite from the northernmost ultrahigh-pressure domain in the Western Gneiss Region (WGR) in Norway and show that the observed layering represents a sequence of metasomatic reaction zones developed along a fracture system. From the garnet-peridotite wall-rock to the fractures the following sequential reaction zones are recognized: clinohumite bearing garnet-peridotite, olivine–garnet-websterite, garnet-websterite, orthopyroxene–phlogopite–garnet-websterite, coarse-grained phlogopite–garnet-websterite, phlogopite–garnet-websterite, phlogopite-free garnet-websterite, inclusion-rich garnetite, garnetite, eclogite, retrograde omphacitite and felsic amphibole-pegmatite. The MgO, FeO and CaO contents generally decrease from the pristine peridotite towards the most metasomatized samples, with an associated increase in SiO2 and Al2O3. Concentrations of fluid-mobile elements increase from the most pristine peridotite towards the garnetite, whereas Ni and Cr decrease from ∼700 to ∼10 ppm and ∼2600 to ∼25 ppm, respectively. Changes in mineral mode are accompanied by changes in mineral chemistry. All minerals display decreasing Mg# and Cr content with degree of metasomatism, whereas Na2O concentrations in amphibole, and most notably in clinopyroxene, increase from 0·2 to 3·0 and from 0·2 to 8 wt %, respectively. The trivalent ions Cr and Al display complex intra-granular vein-like or patchy zoning in garnet and pyroxenes that may be characteristic of metasomatized peridotites. Dating by the U–Pb method suggests metamorphic growth of zircon in the garnetite at 397·2 ± 1·2 Ma, formation of leucosomes in host-rock gneiss at 391·2 ± 0·8 Ma, and amphibole-pegmatite in the core of a garnetite vein at 390·1 ± 0·9 Ma. Initial 87Sr/86Sr values calculated at 397 Ma are elevated (∼0·723) in the most pristine peridotites and increase to ∼0·743 in the most metasomatized samples. The initial 87Sr/86Sr values of both the host gneiss and its leucosomes are also elevated (0·734–0·776), which suggests that the leucosomes found in the gneisses are the most likely, now solidified, remnants of the reactive agent that metasomatized the Svartberget peridotite. A scenario is envisaged in which material derived from the country rock gneiss was the source of the metasomatic addition of elements to the peridotites and the gneisses acted as the host for all elements removed from the peridotite. The Svartberget peridotite may provide an important analogue of how felsic, slab-derived material interacts with the overlying mantle wedge peridotite in regions of arc magma generation.

Devolatilization history and trace element mobility in deeply subducted sedimentary rocks: Evidence from Western Alps HP/UHP suites

Metapelitic rocks of the Schistes Lustrés in the Cottian Alps, Italy (peak metamorphic conditions of 350–500°C, 1.2–2.0GPa) and at the UHP Lago di Cignana locality (Valtournenche, Italy; ~550°C, 2.5–3.0GPa) preserve records of prograde devolatilization in their mineral modes and chemistry, contents of volatiles and fluid-mobile elements (elements relatively mobile in aqueous fluids), and B and N isotope compositions. This suite allows study of prograde devolatilization history, across a wide range in metamorphic grade, in metasedimentary rocks that experienced high-P/T prograde paths similar to those experienced in most modern subduction zones.Across grade, whole-rock samples are in general uniform in their concentrations of relatively fluid-mobile elements N, B, Li, Cs, Ba, and Rb, normalized to the concentrations of the less mobile K2O and Al2O3, showing only hints of loss in several of the highest-grade samples. With increasing grade, ion microprobe analyses of phengites show subtle decrease in B concentration, uniformity in Ba and Cs concentrations, and increase in Li concentrations, the latter likely due to release from chlorite during its breakdown. In one Cignana sample, phengite inclusions in garnets are enriched in B relative to matrix phengite, consistent with either whole-rock B loss after garnet growth or, more likely, closed-system behavior and partitioning of B into paragonite or tourmaline stabilized after garnet growth. In samples with both paragonite and phengite, paragonite shows relative enrichment in B and Sr, and phengite is enriched in Cs, Ba, and presumably also N and Rb (the latter showing strong whole-rock correlations with K2O).Whole-rock δ15N shows a hint of shift to higher values in the highest grade rocks (Cignana) and, accordingly, calculated prograde dehydration histories for appropriate bulk compositions, using the Perple-X database, indicate that significant (~20%) dehydration would for some rocks occur over the temperature interval of 450 to 550°C, largely related to the breakdown of chlorite (and to a lesser extent carpholite). Small amounts of loss of N into these fluids could have resulted in minor shift in δ15N, with decrease in whole-rock N concentration masked by heterogeneity inherent with the sedimentary protoliths. Partitioning of Cs and Li (possibly also Rb and Ba) from white micas into H2O-rich fluids largely produced by chlorite breakdown could similarly have produced the subtle decreases in the concentrations in these elements noted in several high-grade samples. Neoblastic tourmaline in higher-grade rocks likely sequestered some fraction of the B lost from micas, resulting in a lack of obvious whole-rock B loss to accompany the up-grade trend of decreasing B concentrations in phengite. This tourmaline shows core-to-rim decrease in δ11B consistent with growth during small amounts of progressive B loss from phengites.Taken together, the whole-rock and SIMS data presented here, and the whole-rock dataset of Busigny et al. (2003), demonstrate impressive retention, during prograde forearc devolatilization, of elements thought to be relatively fluid-mobile (particularly H, N, B, Li, Ba, and Cs). Retention of these elements in metasedimentary rocks subducted to depths overlapping those beneath arc volcanic fronts (~90km estimated for subsolidus, peak Cignana metamorphism) implies their availability for transfer into arc source regions, in aqueous fluids or silicate melts, or into the mantle to depths beyond subarc regions.

Paired nakhlites MIL 090030, 090032, 090136, and 03346: Insights into the Miller Range parent meteorite

Abstract– Miller Range (MIL) 03346 is the most oxidized and least equilibrated nakhlite known and displays the highest amount of intercumulus phase. The discovery of three new nakhlites, MIL 090030, MIL 090032, and MIL 090136, in the Miller Range, Antarctica, geographically close to the location of MIL 03346, suggests that they may come from the same parent meteorite. In this study, we investigate the mineralogy and texture of cumulus and intercumulus phases, in situ major and trace element compositions for the cumulus phases, as well as pyroxene crystal size distribution patterns and spatial distribution patterns of MIL 090030, 090032, and 090136. Using these combined results, we conclude that the three nakhlites studied here are paired with MIL 03346. However, modal mineral abundances of MIL 090030, 090032, 090136, and 03346 exhibit variations indicating that a single sample of a Miller Range nakhlite is not modally representative of the parent meteorite and that analyses of multiple samples for a single nakhlite may be necessary to obtain representative modal data for placement in the cumulate pile. Our calculated parental melt composition based on all the paired samples confirms a previous study suggesting that the nakhlite parent melt crystallized as a closed system.

Quantifying crystallization and devitrification of rhyolites by means of X-ray diffraction and electron microprobe analysis

Devitrification of silicic volcanic rocks is a relatively common process, resulting in the production of microcrystalline silica and feldspar components. Here we investigate how the products of pervasive devitrification may be characterized using the combined techniques of X-ray powder diffraction, electron microprobe analysis, and X-ray fluorescence analysis to provide a new calibrated approach to calculating the crystallinity and mineral modes in both glassy vitrophyre and devitrified volcanics. Using the integrated areas of the X-ray diffraction peaks associated with both the crystalline and amorphous components, the relative proportions of groundmass crystallites and amorphous material from both glassy and devitrified material can be calculated. A detailed calibration indicates a linear relationship among the ratio of the integrated counts and bulk crystallinity. Mineral proportions are also calculated from X-ray fluorescence measurements of whole-rock and groundmass separates and are well correlated to crystallinities calculated from both X-ray diffraction and electron microprobe image analysis for vitrophyre samples. Devitrification products in a pervasively devitrified sample are tridymite, quartz, sanidine, and a Ca-rich aluminosilicate component. Mineral analysis and X-ray mapping by electron microprobe analysis indicates that the Ca-rich aluminosilicate component appears to be the dominant metastable or amorphous phase in the devitrified sample with proportions calculated from X-ray mapping (~32%) in reasonable agreement with the calculated proportion of amorphous material determined by means of X-ray diffraction (~38%). These results demonstrate the robustness of this combined X-ray diffraction and electron microprobe imagery technique for quantifying and characterizing crystallization in complex samples.

Calcification and Silicification: Fossilization Potential of Cyanobacteria from Stromatolites of Niuafo‘ou's Caldera Lakes (Tonga) and Implications for the Early Fossil Record

Calcification and silicification processes of cyanobacterial mats that form stromatolites in two caldera lakes of Niuafo'ou Island (Vai Lahi and Vai Si'i) were evaluated, and their importance as analogues for interpreting the early fossil record are discussed. It has been shown that the potential for morphological preservation of Niuafo'ou cyanobacteria is highly dependent on the timing and type of mineral phase involved in the fossilization process. Four main modes of mineralization of cyanobacteria organic parts have been recognized: (i) primary early postmortem calcification by aragonite nanograins that transform quickly into larger needle-like crystals and almost totally destroy the cellular structures, (ii) primary early postmortem silicification of almost intact cyanobacterial cells that leave a record of spectacularly well-preserved cellular structures, (iii) replacement by silica of primary aragonite that has already recrystallized and obliterated the cellular structures, (iv) occasional replacement of primary aragonite precipitated in the mucopolysaccharide sheaths and extracellular polymeric substances by Al-Mg-Fe silicates. These observations suggest that the extremely scarce earliest fossil record may, in part, be the result of (a) secondary replacement by silica of primary carbonate minerals (aragonite, calcite, siderite), which, due to recrystallization, had already annihilated the cellular morphology of the mineralized microbiota or (b) relatively late primary silicification of already highly degraded and no longer morphologically identifiable microbial remains.

Provenance of the Lower Ocoee Supergroup, eastern Great Smoky Mountains

Provenance and tectonic setting of clastic rocks of the Neoproterozoic Ocoee Supergroup were assessed using modes of detrital framework minerals, detrital feldspar chemistry, whole-rock geochemistry, heavy mineral assemblages and chemistry, and detrital zircon U-Pb geochronology. Putative source rocks comprising locally exposed Grenville basement were analyzed for comparison. Samples studied are primarily arkosic to subarkosic siltstones, sandstones, and minor conglomerate. Ternary tectonic discrimination diagrams indicate a continental basement uplift setting, although the mineralogically most mature Longarm quartzite indicates a continental interior setting. The range of detrital feldspar compositions among Ocoee units is comparable to feldspar in basement rocks. Orthoclase-rich detrital alkali feldspar predominates over detrital albitic plagioclase. Whole-rock normative Qz-Or-Pl values and trace-element abundances closely match those of local basement. Snowbird Group siltstones and sandstones contain high abundances of zircon, apatite, titanite, ilmenite, and epidote, which locally occur as concentrations of heavy minerals in discrete laminae. Zircon-tourmaline-rutile indices and titanite compositions are most consistent with a granitic source for these sediments. The majority of detrital zircon U-Pb ages are late Mesoproterozoic, with scattered late Neoproterozoic and Paleoproterozoic grains. All but one of the ∼1200 detrital zircons (at ca. 2000 Ma) can be accounted for by a local Blue Ridge basement magmatic source terrane. Archean detrital zircon ages are absent from the Ocoee detrital zircon age population. This absence is consistent with the general paucity of Archean grains in other Neoproterozoic Southern Appalachian clastic sequences. All data are consistent with a lithologically and geochronologically restricted, proximal, flank source for sediment filling a marginal continental rift. The Ocoee represents the first pulse of sediment dominated by Grenville-age zircon on the eastern Laurentian margin and serves as a major source of recycled zircon for Paleozoic clastic sequences in eastern Laurentia. The youngest detrital zircon age obtained from the basal Ocoee Wading Branch Formation (639 ± 8 Ma) provides a maximum depositional age for the Ocoee Supergroup and onset of rift-filling sedimentation in the intracontinental Ocoee basin.

Multiphase petrography of volcanic rocks using element maps: a method applied to Mount St. Helens, 1980–2005

Quantitative textural analyses including crystal size distributions (CSDs) provide insights into crystallisation kinetics of magmatic systems. Investigations of volcanic crystal textures often rely on greyscale variations on backscattered electron images to identify crystal phases, which must then be thresholded and/or traced manually, a laborious task, and investigations are typically restricted to a single crystal phase. A method is presented that uses energy-dispersive X-ray element maps to generate textural data. Each pixel is identified as a crystal phase, glass or vesicle according to relative chemical composition enabling concurrent acquisition of multiphase CSD, crystallinity and mineral mode data. Data processing is less time intensive for the operator but considerable instrument time is required to generate element maps. The method is applied to 17 dacite samples from the 1980–1986 and 3 from the 2004–2005 eruptive periods of Mount St. Helens volcano (USA) to provide quantitative insights into multiphase textural evolution. All of the CSDs are curved and concave-up in the standard CSD plot with curvature increasing with plagioclase content. To facilitate comparisons with previous studies, CSDs for microlites (<50 μm length crystals) are approximated as straight lines. The line intercepts and slopes provide information on n 0 (nucleation density) and characteristic length or Gτ (the product of growth rate (G) and residence time (τ)), respectively. These parameters, as well as the total groundmass crystallinity, show distinct differences between explosive deposits from summer 1980 and post-summer 1980 domes. Post-summer 1980 microlite n 0 values are mostly at the lower end of the range of those measured for summer 1980 samples. Total groundmass crystallinities during summer 1980 are between 10 and 30 vol.%, whereas post-summer 1980 crystallinity increases to between 39 and 51 vol.%. The range of n 0 values is similar to those previously published for Mount St. Helens but Gτ is consistently higher. Gτ of a May 1985 sample analysed in this study is approximately 2 μm higher compared with previously published data for the same sample when processed using similar methodologies. Groundmass crystallinity data show similar trends to those previously published for the 1980 to 1986 eruption, increasing sharply after summer 1980 then increasing more gradually during the dome-building phase of the eruption. The effects of varying L, the apparent crystal size, and crystal aspect ratio on resultant CSDs are also investigated. Whilst relative temporal variations in CSDs for a given dataset can be investigated, absolute values from different studies cannot be compared unless methods of data acquisition and processing are exactly the same.

The effects of solid solution and diffusion on infiltration-driven metamorphism with application to metaperidotite, Val d’Efra, Central Swiss Alps

The complication introduced by solid solutions in the analysis of infiltration-driven mineral reactions is that the mole fraction of tracer component i in fluid (Xi) changes with reaction progress (ξ). The effect was incorporated into transport models for coupled fluid flow and mineral reaction by parameterizing the relation between Xi and ξ. With specific reference to carbonation and hydration during regional metamorphism of the peridotite body in Val d’Efra, whose constituent minerals are all solid solutions, infiltration of a disequilibrium fluid produces a single sharp reaction front if rock is assumed uniform in composition. The reaction front separates completely unreacted rock downstream from rock upstream with ξ at a steady-state limit (ξss ≤ ξmax) that depends on input fluid composition (ξmax is the maximum possible value). Novel phenomena develop, however, if the flow medium, like the metaperidotite body, is composed of many small domains that differ in initial mineral modes and compositions but with Xi homogenized at a spatial scale larger than the size of the domains (e.g., by diffusion). In this case, infiltration of a disequilibrium fluid produces up to as many different reaction fronts along the flow path as there are domains with 0 ≤ ξ < ξss in all domains except upstream from the slowest moving front where ξ = ξss in all domains. Measured values of ξ in the metaperidotite, (all 0 < ξ < ξmax) are best reproduced by down-temperature infiltration of a disequilibrium fluid with \( X_{{{\text{CO}}_{2} }} = \, 0. 1 9 6 \) into a multi-domain medium with uniform \( X_{{{\text{CO}}_{2} }} \) at each spatial point along the flow path (homogenized across the domains at the m-scale by diffusion), and time-integrated fluid flux ≥1,836 mol fluid/cm2 rock. Results resolve the paradox of the widespread spatial distribution of reactants and products of infiltration-driven decarbonation/dehydration reactions in regional metamorphic terrains (which in the absence of solid solution and compositional domains indicate up-temperature flow) and the prediction of hydrodynamic models that regional metamorphic fluid flow normally is directed vertically upward and down temperature.