Feldspar-bearing Rocks Research Articles

Lead isotope geochemistry of plagioclase in the Skaergaard intrusion by LA-ICP-MS: Assessing the effects of crustal contamination and link with East Greenland flood basalts

The Skaergaard intrusion of East Greenland is one of the best studied layered intrusions on Earth and preserves an exceptional rock record produced during low-pressure, closed-system crystallization of Fe-rich tholeiitic magma. The lead isotope compositions of plagioclase feldspar in cumulates and related igneous rocks of the Skaergaard intrusion were determined directly from single spot analyses in thin section by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) (n = 757). Plagioclase primocrysts, typically with <2 ppm Pb, and feldspar in interstitial pockets from the Layered Series and from most of the Marginal Border Series have the same Pb isotope compositions within analytical uncertainty (208Pb/206Pb = 2.20 ± 0.06; 207Pb/206Pb = 0.889 ± 0.028). There are no significant Pb isotope differences between cores and rims of individual plagioclase primocrysts or between different types of interstitial pockets (e.g., interstitial granophyre, symplectites). These Pb isotope compositions are less radiogenic than those from most of the contemporaneous East Greenland flood basalts indicative of crustal contamination at deeper crustal levels prior to emplacement. Plagioclase from three samples of the Marginal Border Series adjacent to host gneisses (<5 m from the contact) have higher 208Pb/206Pb (2.30 ± 0.06, 2.35 ± 0.02, 2.51 ± 0.21) compared to the rest of the intrusion. These results confirm that crystallization of the Skaergaard intrusion occurred under essentially closed-system conditions with localized minor incorporation of “amphibolitic” gneiss. Hydrothermal alteration occurred at near-solidus to subsolidus conditions by circulating meteoric fluid that had equilibrated with the overlying flood basalts. This alteration resulted in albitized interstitial granophyre and rare spot analyses from plagioclase primocrysts characterized by Pb isotope compositions comparable to those of the flood basalts. Granophyres of different origins (e.g., transgressive granophyre, granophyric segregation) have variable Pb isotope compositions (208Pb/206Pb = 2.09–2.51; 207Pb/206Pb = 0.844–1.013) that are distinct from the Skaergaard cumulates. Based on Pb isotope systematics, combined with published SrNd isotopic compositions and 40Ar/39Ar geochronology from East Greenland and Faroe Island basalts, the Skaergaard intrusion is linked to eruption of the earliest of the Plateau Basalts in East Greenland (Milne Land Formation) during continental breakup and opening of the North Atlantic Ocean. This microanalytical study highlights the ability to conduct rapid Pb isotopic measurements in low-Pb concentration materials (e.g., <2 ppm Pb in plagioclase) at high spatial resolution by LA-ICP-MS with precision sufficient to constrain a range of magmatic processes in layered intrusions and other feldspar-bearing rocks preserved in the geological record.

Bacterial communities in soils as indicators of the potential of syenite as an agromineral

Abstract The objective of this work was to identify microbial communities associated with the surfaces of alkali feldspars and to determine whether these microbes might be involved in the weathering of these rocks for agronomic benefit. Samples were taken from weathering profiles and soils developed on a syenite, considered as a raw material for agromineral production, located in the municipality of Triunfo, in the state of Pernambuco, Brazil. Molecular microbiological techniques (qPCR and 16S rRNA gene sequencing) were used, and data were interpreted by the analysis of variance, hierarchical cluster analysis, and principal coordinates analysis. In addition, scanning electron microscopy was used to image mineral surfaces. Similar bacterial communities were observed in all samples, showing that the bacteria found in soil are present at the earliest stages of rock weathering and are available to play a role in nutrient release. In particular, Actinobacteria and, within this phylum, Actinomycetales were proportionally more abundant than other taxa in rock-dominated soil samples, i.e., in thin soils on or between fractured or broken syenite. The analysis of rock dust used as a remineralizer, crushed with no further treatment, shows that Actinobacteria play a role in the early stages of weathering of feldspar-bearing rocks.

Investigation of the sintering kinetic and development of ceramic masses for obtaining large-format porcelain stoneware in the conditions of high-speed firing

The paper considers the resource and energy saving reserves in the production of large-format stoneware slabs. The possibility of replacing high-quality feldspars with materials of technogenic origin, such as the extraction and processing waste of various feldspar-bearing rocks of domestic deposits has been investigated. The possibility of sintering process intensification has been investigated, as a condition for the use of high-speed firing modes in the stoneware production. As a result of the carried out studies, the kinetic features of ceramic masses sintering with alkaline syenites, granites and pegmatite enrichment wastes as fluxing component were determined. The formation conditions of densely sintered materials with a complex of high operational properties have been determined. The expediency of replacing Turkish feldspars with waste products of alkaline syenites has been established, while the level of physical, mechanical and operational properties of stoneware slabs meets the requirements of international standards. It is shown that, the use of complex flux, including feldspar-bearing rocks waste and dolomite, provides a significant decrease in the firing temperature of products (up to 1150 °C) while maintaining a high level of properties of stoneware slabs of class BІa. The obtained data indicate the possibility of optimizing the heat treatment parameters from the view point of energy saving with the further use of alternative fluxes in the large-format stoneware production. On the example of using the alkaline syenites wastes, the optimal ratios of the clay and fluxing components of ceramic masses have been established to obtain products with a complex of high performance characteristics when burning white clays of different chemical and mineral composition, which is a necessary condition for using a wide range of decorating techniques stoneware slabs. The structural-phase features of materials obtained under the conditions of high-speed firing are established: the formation of finely dispersed acicular mullite, which plays the role of a reinforcing component, as well as high structural homogeneity and maximum compaction, as evidenced by a few individual small pores presence. The combination of excellent technical qualities and large dimensions opens up new possibilities for the use of stoneware slabs as structural elements (countertops, ventilated facades, insulating panels for the installation of electrical systems, etc.).

Ar-Ar ages, Sr-Nd isotope geochemistry, and implications for the origin of the silicate rocks of the Jacupiranga ultramafic-alkaline complex (Brazil)

The Jacupiranga Complex is one of several Meso-Cenozoic alkaline intrusive complexes along the margins of the intracratonic Paraná Basin in southern Brazil. The complex encompasses a wide range of rock-types, including dunites, wehrlites, clinopyroxenites, melteigites-ijolites, feldspar-bearing rocks (diorites, syenites, and monzonites), lamprophyres and apatite-rich carbonatites. While carbonatites have been extensively investigated over the last decades, little attention has been paid to the silicate rocks. This study presents new geochonological and geochemical data on the Jacupiranga Complex, with particular emphasis on the silicate lithotypes. 40Ar/39Ar ages for different lithotypes range from 133.7 ± 0.5 Ma to 131.4 ± 0.5 Ma, while monzonite zircon analyzed by SHRIMP yields a U-Pb concordia age of 134.9 ± 1.3 Ma. These ages indicate a narrow time frame for the Jacupiranga Complex emplacement, contemporaneous with the Paraná Magmatic Province. Most of the Jacupiranga rocks are SiO2-undersaturated, except for a quartz-normative monzonite. Based on geochemical compositions, the Jacupiranga silicate lithotypes may be separated into two magma-evolution trends: (1) a strongly silica-undersaturated series, comprising part of the clinopyroxenites and the ijolitic rocks, probably related to nephelinite melts and (2) a mildly silica-undersaturated series, related to basanite parental magmas and comprising the feldspar-bearing rocks, phonolites, lamprophyres, and part of the clinopyroxenites. Dunites and wehrlites are characterized by olivine compositionally restricted to the Fo83-84 interval and concentrations of CaO (0.13–0.54 wt%) and NiO (0.19–0.33 wt%) consistent with derivation by fractional crystallization, although it is not clear whether these rocks belong to the nephelinite or basanite series. Lamprophyre dikes within the complex are considered as good representatives of the basanite parental magma. Compositions of calculated melts in equilibrium with diopside cores from clinopyroxenites are quite similar to those of the lamprophyres, suggesting that at least a part of the clinopyroxenites is related to the basanite series. Some feldspar-bearing rocks (i.e. meladiorite and monzonite) show petrographic features and geochemical and isotope compositions indicative of crustal assimilation, although this may be relegated to a local process. Relatively high CaO/Al2O3 and La/Zr and low Ti/Eu ratios from the lamprophyres and calculated melts in equilibrium with cumulus clinopyroxene point to a lithospheric mantle metasomatized by CO2-rich fluids, suggesting vein-plus-wall-rock melting mechanisms. The chemical differences among those liquids are thought to reflect both variable contributions of melting resulting from veins and variable clinopyroxene/garnet proportions of the source.

Geochemical Mass-Transfer Patterns as Indicators of the Architecture of a Complete Volcanic-Hosted Massive Sulfide Hydrothermal Alteration System, Panorama District, Pilbara, Western Australia

Outstanding exposure across the Panorama district, in the Archean Pilbara block of Western Australia, reveals a cross section through a complete volcanic-hosted massive sulfide (VHMS) hydrothermal alteration system in an area of low metamorphic grade and very low strain. Geochemical maps, in combination with alteration maps, document mass transfer across this alteration system. Feldspar-bearing rocks at the top of the volcanic pile are enriched in K and Si, but depleted in Na, in contrast to those at the bottom of the volcanic pile that are enriched in Na and depleted in K. Sericite-quartz altered rocks are typically enriched in Si and depleted in Ca, Na, and Fe, whereas chlorite-quartz altered rocks, which dominate transgressive feldspar-destructive alteration zones, are marked by K-Ca-Na depletion and Mg-Fe enrichment. Copper and Zn are strongly depleted in rocks at the base of the volcanic pile and, to a lesser extent, in transgressive feldspar-destructive alteration zones. The amount of metal leached from the volcanic pile is far greater than that contained in known deposits, and thus a magmatic metal source is not necessary to explain the formation of the Panorama VHMS deposits. Mass-transfer patterns at Panorama have similarities to existing models for regional VHMS hydrothermal alteration systems, including the distribution of alkali elements in feldspar-bearing semiconformable alteration zones and the presence of a base metal-depleted zone at the base of the volcanic pile. However, there are significant differences, both in the architecture of the hydrothermal alteration system and in mass-transfer patterns. These include Si enrichment rather than depletion at the top of the volcanic pile, widespread Mg enrichment outside of the upper semiconformable alteration zones, the absence of a silica cap rock above the base metal depleted zone, more hydrous alteration assemblages in the zones of base metal depletion, and extensive zones of transgressive feldspar-destructive alteration.

The causes and petrological significance of cathodoluminescence emissions from alkali feldspars

The cathodoluminescence (CL) of a variety of alkali feldspars from South Greenland has been examined in an attempt to understand the causes of the CL and its petrological significance. Analytical methods have included CL spectroscopy, secondary ion mass spectrometry (SIMS) and electron paramagnetic resonance (EPR) to correlate the presence of certain CL emissions to the presence of certain trace element and point defects. Where possible, blue and red luminescent fractions of the same rock samples have been separated and analysed separately. Blue CL appears to relate to the presence of electron holes on bridging oxygens, particularly on the Al-O-Al bridge, as determined from EPR studies. No correlation with other proposed activators for blue CL such as Eu2+, Ga3+ or Ti4+ was observed. Some blue luminescent feldspars also have an emission in the infra-red (IR), invisible during normal visible CL petrography. The red and IR CL emissions correspond to features in EPR spectra attributed to Fe3+ and support previous suggestions that Fe3+ is related to this emission. However, our studies indicate that the visible red CL relates specifically to Fe3+ on the T1 site, whereas the equivalent CL from disordered feldspars lies in the IR. The difference between red and IR CL emissions therefore relates to the state of Fe3+ order across the tetrahedral sites. These data allow more meaningful interpretations of CL as a petrographic tool in alkali feldspar-bearing rocks.

Atmospheric weathering and silica-coated feldspar: analogy with zeolite molecular sieves, granite weathering, soil formation, ornamental slabs, and ceramics.

Feldspar surfaces respond to chemical, biological, and mechanical weathering. The simplest termination is hydroxyl (OH), which interacts with any adsorption layer. Acid leaching of alkalis and aluminum generated a silica-rich, nanometers-thick skin on certain feldspars. Natural K, Na-feldspars develop fragile surfaces as etch pits expand into micrometer honeycombs, possibly colonized by lichens. Most crystals have various irregular coats. Based on surface-catalytic processes in molecular sieve zeolites, I proposed that some natural feldspars lose weakly bonded Al-OH (aluminol) to yield surfaces terminated by strongly bonded Si-OH (silanol). This might explain why some old feldspar-bearing rocks weather slower than predicted from brief laboratory dissolution. Lack of an Al-OH infrared frequency from a feldspar surface is consistent with such a silanol-dominated surface. Raman spectra of altered patches on acid-leached albite correspond with amorphous silica rather than hydroxylated silica-feldspar, but natural feldspar may respond differently. The crystal structure of H-exchanged feldspar provides atomic positions for computer modeling of complex ideas for silica-terminated feldspar surfaces. Natural weathering also depends on swings of temperature and hydration, plus transport of particles, molecules, and ionic complexes by rain and wind. Soil formation might be enhanced by crushing granitic outcrops to generate new Al-rich surfaces favorable for chemical and biological weathering. Ornamental slabs used by architects and monumental masons might last longer by minimizing mechanical abrasion during sawing and polishing and by silicifying the surface. Silica-terminated feldspar might be a promising ceramic surface.

The Mt. Vulture volcanic complex (Italy): evidence for distinct parental magmas and for residual melts with melilite

New data of mineral compositions, whole-rock major and trace element contents are presented for lava flows and dykes of Mt. Vulture volcano (southeastern Roman Magmatic Province). The dominant rock series ranges from basanite, through tephrite and foidite, to phonolite. Diopsidic to Fe-salitic (hedenbergitic) clinopyroxene is the major phenocryst phase, and shows a very strong Ti- and Al-enrichment trend (up to 5 wt% TiO2 and 12 wt% Al2O3). Phonolites have Fe-rich pyroxenes low in Ti and Al, as a consequence of fractionation of Ti-magnetite and/or melanite garnet. Feldspar ranges in composition from bytownite to Ba- and Sr-rich anorthoclase (up to 7 wt % BaO and 4 wt % SrO); the latter is present in the groundmass of mafic and intermediate rocks. Phonolites are characterized by anorthoclase and sanidine with Ba-rich cores, and with Ba-depleted rims. Subordinate melilite-bearing rocks are also present, and are characterized by melilite with a very large compositional variation, Ca-rich nepheline (up to 8 wt% CaO) and lack of feldspar and amphibole. Some of them also show calcite-rich ocellar structures, possibly due to liquid immiscibility. They have also major and trace element contents different from those of the feldspar-bearing rocks. This all indicates distinct parental magmas for feldspar-bearing and melilite-bearing rocks. These latter were likely generated by differentiation processes starting from olivine- and melilite-bearing ultramafic parental magmas. Mineral compositions and suitable phase diagrams indicate that the Melfi hauynophyre, which is the most MgO-poor melilite-bearing rock of Mt. Vulture, is close to a residual melt analogous to phonolite, but in petrogenetic systems with melilite and without feldspar. The high- to very high-titanium content of most of the ferromagnesian phases, despite a relatively low TiO2 of primitive Mt. Vulture rocks, is absent in other rocks of similar degree of evolution in the Roman Province. It resembles that found in anorogenic magmas and gives further indications for a within-plate signature in this particular volcanic complex.

Evidence for magma mixing processes within the Critical and Lower Zones of the northwestern Bushveld Complex, South Africa

During deposition of the Lower (LZ), Lower Critical (LCZ) and Upper Critical (UCZ) Zones, the course of crystallization was dominated by repeated emplacement of primitive, mafic magma. Evidence for this is provided by textural, chemical and isotopic data. Emplacement of fresh magma into resident residual liquids produced reversal in fractionation trends (i.e. progressive increase in Mg ( Mg + Fe 2+) ratios, and enrichment of Cr in orthopyroxenes) through intervals of 350 m in the LZ, 100–200 m in the LCZ and 10–80 m within the lower part of the UCZ. Reversals culminated with the deposition of olivine cumulates upon pyroxenite within the LZ and LCZ, but a change in the style of reversals led ultimately to ultramafic cumulates (with or without olivine) resting directly upon anorthositic cumulates at the top of the UCZ (Merensky and Bastard Units). These incursions of fresh mafic liquid led to deposition of ultramafic rocks at the base of units constituting the UCZ, whereas the cumulus feldspar-bearing rocks at the tops of such units crystallized from hybrid liquids resulting from the mixing of the new and the resident, residual liquids. There is a general decoupling of trends of cryptic variation of the feldspar and orthopyroxene populations in the UCZ. Wide variations of initial Sr-isotope ratios exist throughout the entire section, within individual units, and within individual members of units. Isotopic disequilibrium also exists between feldspar and pyroxene fractions of some samples, and between individual feldspar grains of different textural habit. The first appearance of cumulus feldspar-bearing cumulates of the UCZ is presaged by the appearance of small, partially resorbed plagioclase inclusions within the orthopyroxene grains of pyroxenites at the top of the LCZ; this texture is conspicuous throughout the lowermost 400 m of the UCZ, but is still sporadically developed within the Bastard Unit at the top of the UCZ. Anorthositic rocks of the lower part of the UCZ are held to have been derived from residual liquids evolved during crystallization of earlier cumulates, but anorthosites with a Main Zone isotopic signature cap the Merensky and Bastard Units.

Rheology of some continental lower crustal rocks

To gain insight into the rheology of selected feldspar-bearing rocks from the continental lower crust (CLC), three granulites from exposed lower crustal terrains and a microgabbro have been deformed to ca. ~10% strain at confining pressures of 0.8–1.0 GPa, temperatures of 600–900°C, and constant strain-rates of 10 −4−10 −7 s −1. The most extensive work was done on the mafic Pikwitonei and felsic Adirondack granulites. The Pikwitonei (Mg-hornblende-plag (An 70−2 px) has only a limited T,ϵ range of apparent steady-state behavior, and strain-softens at T ⩾ 800° C at ϵ = 10 −5−10 −6 s −1, apparently associated with developing shear zones. This strain-softening is pressure-insensitive from P c = 0.8–1.0 GPa at T = 850° C, ϵ = 10 −6 s −1 even though the dominant strain accommodation mechanisms change from semibrittle/cataclastic to crystal plastic (including dynamic recrystallization) with increasing pressure. Plagioclase appears to bear more of the creep strain than amphibole at most experimental conditions except in those runs where both plagioclase and amphibole recrystallization occurs. Kapuskasing (plag-cpx-gt) granulite, deformed at 800°C and at 850°C, 10 −6 s −1 has deformed in the steady-state at stress levels only slightly greater than the Pikwitonei. The Stillwater microgabbro and Adirondack granulite are appreciably weaker and the latter exhibits homogeneous semibrittle steady-state behavior over the entire range of conditions investigated. For the Adirondack (2 felds-2 px-gt-qtz) granulite, quartz (21%) appears to control the creep rate at these low strains, and steady-state mechanical data fit well a creep power law with A = 8 × 10 −3 MPa − n s −1, Q c = 243 kJ/mol and n = 3.1. Extrapolation of this flow law and of preliminary power law creep relations for the Pikwitonei and microgabbro to a range of physical conditions assumed to encompass those at the continental Moho supports geological and geophysical inferences that a mechanical discontinuity occurs at this transition zone, with the lower crust generally being the weaker material. In addition, the stain-softening exhibited by the Pikwitonei may bear directly on the development of detachment zones at mid- to lower-crustal depths during prograde metamorphism.

Crystallographic orientation patterns of myrmekitic quartz: a fabric memory in quartz ribbon-bearing gneisses

Crystallographic orientation patterns are found in myrmekitic quartz inclusions in plagioclase crystals in medium-grade gneisses from a Svecofennian fold belt in SW Finland. The development of crystallographic fabrics in myrmekite is studied in progressively deformed granitic veins. Deformation of these veins is accompanied by partial transformation of K-feldspar to myrmekite, which is a common synkinematic reaction in feldspar-bearing rocks that are deformed in amphibolite facies conditions. Microstructures suggest that myrmekitic quartz nucleated epitaxially on a pre-existing independent quartz crystal adjacent to the feldspar host. An important consequence is that the orientation pattern of myrmekitic quartz reflects that of the preexisting crystals in the host rock at the time of nucleation. As such, the pattern represents a memory of an old fabric in annealed ribbon mylonites.

Discussion

DR. E. A. VINCENT asked whether the author would elaborate on his suggestion that the finer grain of the rocks late in his time-sequence might be due to the concentration of volatiles causing an attenuation of the crystallization range of the minerals. Could the author explain how this was brought about ? Dr. S. I. T omkeieff said that he would like to suggest a different hypothesis in explanation of kaolinite genesis in the St. Austell granite, namely : a biphyletic hypothesis, according to which an exogenetic phase was superimposed on the endogenetic phase. Many years ago, G. Hickling in his study of the Cornish china-clay deposits (1908, Trans. Inst. M. E. 36 , 23) suggested that muscovite was an intermediate product of the transformation of feldspar into kaolinite. The transformation of mica into kaolinite was a well-known sedimentary process. Numerous German kaolinite deposits justified the “moor water” theory, according to which kaolinite and other clay minerals were formed through the action of ground-waters charged with humic substances. The widespread kaolinite deposits of the western Ukraine occurred in granites and other feldspar-bearing rocks of the Pre-Cambrian foundation overlain by Tertiary deposits often containing seams of brown coal. According to I. I. Ginzburg, who studied these deposits (1912, Ann. St. Petersburg Polytechn. Inst 17 , and 1915, ibid 22 ), they were of the nature of residual formations and, according to the evidence, were formed through leaching of underlying rocks by ground-waters in the lower zone of weathering. Dr. Tomkeieff added that he